BIOLOGICAL CLASSIFICATION  

1.  BIOLOGICAL CLASSIFICATION

• Since the down of civilization, there have been many attempts to classify living organisms. Aristotle was the earliest to attempt a more scientific basis of classification. He used simple morphological characters to classify plants into trees, shrubs and herbs. He also divided animals into two groups, those which had red blood and those that did not.
• The art of identifying distinctions among organisms and placing them into groups that reflect their most significant features and relationship is called biological classification. The purpose of biological classification is to organize the vast number of known plants and animals into categories that could be named, remembered and studied. 

Number of Kingdoms

• Various schemes dividing the organisms into two, three, four, five and six kingdoms have been proposed from time to time.

Two-kingdom system of classification

• This system of classification is the oldest. It was proposed by Carolus Linnaeus in 1758. He divided the living organisms into two kingdoms: Plantae and Animalia on the basis of presence or absence of cell wall.

Drawbacks of two-kingdom classification

• The two-kingdom system of classification worked well for a long time. Now, however, it seems inadequate and unsatisfactory in view of the new information that has come to light about the organisms, particularly the lower forms. Followings are some of the shortcomings of two-kingdom system:
• The two kingdom system takes unicellular and multicellular organisms together. Even unicellular organisms like bacteria were considered as plants.
• Unicellular plants (diatoms, dinoflagellates) and animals (protozoans) resemble each other in level of organization and reproduction by fission but placed in two separate kingdoms.
• Fungi are included in kingdom Plantae inspite of the fact they lack chlorophyll, cellulosic cell wall and are either saprophyte or parasite unlike typical plants.
• Some of the organisms like viruses and lichens can’t be placed in either of these two kingdoms because of peculiar characteristics.
• It puts together eukaryotes with prokaryotes.

Three kingdom system of classification

• Ernst Haeckel (1866), a German zoologist suggested that a third kingdom, Protista be created to include those unicellular eukaryotic microorganisms that are typically neither plants nor animals.
• Three kingdoms according to Haeckel are Protista, Plantae and Animalia.
• This solves the problem of assigning suitable kingdom to the organisms which have similarities with both plants and animals.

Four kingdom system of classification

• Copeland (1956), an American taxonomist, suggested that all prokaryotes. i.e.,bacteria, cyanobacteria, etc., be placed under kingdom Monera (= Mychota). According to Copeland, four kingdoms are Monera ( = Mychota), Protista, Plantae and Animalia.
• Protista are single celled eukaryotic organisms. Fungi continued to remain with plants.
• The main drawback of this system is that fungi are not properly placed.

Five kingdom system of classification

• According to Robert H. Whittaker (1969), an American ecologist, non-chlorophyllous heterotrophic plants to be classified under kingdom Fungi. Five kingdoms in which the living world is divided are Monera, Protista, Fungi, Plantae/Metaphyta (Plants) and Animalia/Metazoa (Animals).
• Five Kingdom: classification is based mainly on following three main criteria:
• Complexity of cell structure: prokaryotic or eukaryotic
• Complexity of cellular organization: unicellular or multicellular
• Mode of nutrition: autotrophic or heterotrophic.
• Other criteria include ecological life style like producers, consumers and decomposers, and the phylogenetic relationships.
• The organisms, according to the Five Kingdom System, are re-distributed into additional three kingdoms while retaining the two kingdoms-Plantae and Animalia. All multicellular, mobile and heterotrophic organisms were assigned the kingdom Animalia. The photosynthetic multicellular organisms were included in the kingdom Plantae. Some of the unicellular algae and protozoans were taken out from plant and animal kingdoms and were included in a separate kingdom Protist Heterotrophic plants were placed into kingdom Fungi. All bacteria and multicellular blue green algae with prokaryotic cells were transferred from kingdom Plantae to a new kingdom Monera.

Characteristics of the Five Kingdoms

Characters	Five Kingdoms
	Monera	Protista	Fungi	Plantae	Animalia
Cell type	Prokaryotic	Eukaryotic	Eukaryotic	Eukaryotic	Eukaryotic
Cell wall	Noncellular (Polysaccharide + amino acid)	Present in some	Present (without cellulose}	Present (cellulose)	Absent
Nuclear membrane	Absent	Present	Present	Present	Present
Body organisation	Cellular	Cellular	Multi cellar/ loose tissue	Tissue/ organ	Tissue/organ/ organ system
Mode of nutrition	Autotrophic (chemosyn- thetic and photosynthetic) and Hetero- trophic (sapro- phyte/para- site)	Autotrophic (Photosyn- thetic) and Hetero- trophic	Heterotrophic (Saprophytic/ Parasitic)	Autotrophic (Photosyn- thetic)	Heterotrophic (H o1ozoIc/ Saprophytic etc.)

• In the five kingdom classification it is thought that the Monera has given rise to the Protista, which gave rise to the remaining three kingdoms of multicellular organism, viz. Fungi, Plantae, and Animalia.

Significance of five-kingdom classification

• This system seems more natural and indicates gradual evolution of early organisms into plants and animals.
• Kingdom Animalia has become more homogeneous by the exclusion of protozoa.
• Kingdom Plantae has become more coherent after exclusion of bacteria, fungi and some unicellular algal forms.
• Creation of kingdom Monera from prokaryotes is fully justified.
• Some organisms like Euglena showing mixotrophic mode of nutrition could be placed either in plant or animal kingdom easily. The creation of kingdom Protista including all unicellular eukaryotes, irrespective of the mode of nutrition, has resolved this problem.
• The fungi, included as sub division of division Thallophyta of two kingdom classification is raised to the rank of a kingdom as they differ morphologically and physiologically from plants with whom they are grouped in old two kingdom classification.
• Five kingdom classification is undoubtedly better than two kingdom classification, resolving many problems, faced in old systems of classification. However, this system is also not perfect. Still it has some drawbacks as briefly discussed below:

1. Kingdoms Monera and Protista still retain heterogeneity, as both heterotroph and autotroph organisms with or without cell wall are included in both these kingdoms. The slime moulds are quite different from the other Protista with which they have been combined.
2. Multicellular green algae can’t be phylogenetically separated from unicellular algae and, thus unicellular algae like Chlamydomonas are placed in kingdom Plantae rather than Protista.
3. Placing algae in three kingdoms seems to be unrealistic.
4. Viruses and lichens do not find any place.
5. Red and brown algae are not related to other members of kingdom Plantae.
6. In spite of its shortcomings, the five-kingdom system of classification is followed by most of the biologists these days, not because it is fully natural but because it is more phylogenetically reasonable.

Six Kingdom System Of Classification

• These six kingdoms are Archaebacteria, Eubacteria, Protista, Fungi (Mycophyta), Plantae and Microorganisms are found in four kingdoms viz. Archaebacteria, Eubacteria, Protista and Fungi.
• Carl Woese (1990) classified organisms on the basis of genetic characters particularly genetic analysis of 16 S rRNA and proposed three domains viz. Archaea, Bacteria and (Domain is a taxonomic group of members of similar kingdoms. It is a category higher than kingdom). All the three domains have evolved from a common ancestor, the progenote.

KINGDOM: MONERA

• The kingdom Monera (monos-single; Dougherty and Allen, 1960) includes all prokaryotes. Monerans are the most primitive forms of life, originating from more ancient living stock termed progenote. The kingdom Monera is divided into two major groups, the Eubacteria(true bacteria) and the Archaebacteria (primitive bacteria). Eubacteria include several sub groups, the most distinctive of which is Cyanobacteria (blue green algae).

Eubacteria

• Antony vonLeeuwenhoek(1675), a Dutch naturalist discovered bacteria from stored rainwater and tartar of teeth and interestingly termed those as tinyanimalcules. Linnaeus (1758) called them
• Ehrenberg (1838) first of all coined the term bacteria(Gk. bakteron = small rod) for these small organisms.
• Louis Pasteur (1822-1895) studied a number of fermentations very carefully and demonstrated that life was not possible without air. He introduced the term ‘aerobic’ and ‘anaerobic’ for the life in the presence or absence of oxygen, respectively.
• Robert Koch (1834-1940), a German doctor, demonstrated that the anthrax disease of sheep was caused by bacteria.

Occurrence

• Bacteria are ubiquitous, i.e., they occur anywhere and everywhere. Some thermophilic bacteria can withstand the temperature up to 78°C while some psychrophilic bacteria occur up to the temperature of -190°C.

Size of bacterial cell

• Bacteria are very small and their size generally ranges from 0.2-1.5 $\mathrm{\mu m}$ in length.
• Dialister pneumonsintes is the smallest bacterium (0.15-0.3 $\mathrm{\mu m}$ long and 100-200 $\mathrm{\mu m}$ diameter).
• Longest bacterium is Spirillum volutans (15 $\mathrm{\mu m}$ ).
• Largest bacterium is Beggiatoa mirabilis.
• A huge bacterium, Epulopscium fishelsoni, which was discovered in the intestine of the brown surgeon fish, is as large as 600 $\mathrm{\mu m}$ and as wide as 80 $\mathrm{\mu m}$.

Shape of bacteria

• Cohn (1872) recognized four types of shapes in bacteria – coccus, bacillus, spirillum and vibrio.

Coccus (Gk. Kokkos = berry):

• Spherical and aflagellate, sub divided into following six groups on the basis of cell arrangement:
• Monococcus: Only single spherical cell represents the bacterium, e.g., Micrococcus luteus.
• Diplococcus: Two cocci divide in one plane and remain attached in pairs, Diplococcus pneumoniae.
• Streptococcus: Several cocci divide in one plane and remain attached to form chains of different lengths, e.g., Streptococcus lactis.
• Tetracoccus: Four cocci divide in two planes at right angles to one another and form groups of four.
• Staphylococcus: Cocci divide in several planes resulting in formation of irregular bunches of cells, sometimes resembling a cluster of grapes, e.g., Staphylococcus aureus.
• Sarcina: Cocci divide in 3 planes at right angles to one another and resemble cubical packets of 8, 27 or more cells, e.g., Sarcina sp.

Bacillus (L. Bacillus – small rod):

• Rod like or cylindrical forms, either singly or may be arranged differently. These are generally flagellated and are of the following types:

◊ Monobacillus : The bacteria occur singly, e.g., Rhodospirillum sodomens.

◊ Diplobacillus: These are arranged in pairs, e.g., Bacillus subtilis.

◊ Palisade Bacillus: These are lined side by side like matchsticks, e.g. Corynebacterium diphtheriae.

◊ Streptobacillus: These form a chain of rods, e.g. Streptomyces, Bacillus anthracis,

Sprillum (Gk. Spira – small coil)

• Coiled forms of bacteria exhibiting twists with one or more turns are called spirilla, g. Spirillum minus.

Vibrio (L. Vibrae – quiver)

• Bacteria with less than one complete twist or turn are called These resemble a comma (,) in appearance, e.g. Vibrio comma. 

GRAM POSITIVE AND GRAM NEGATIVE BACTERIA

• A Danish physician, Christian Gram (1884) developed a stain for staining bacteria, termed Gram stain. On the basis of stainability with Gram stain, bacteria are classified into two groups; Gram positive and Gram negative.
• The procedure involves staining of bacterial smear first with weakly alkaline solution of crystal violet (methyl violet or gentian violet). All the bacterial cells stain blue with this dye. These blue stained cells are then treated with 5% iodine – KI solution (or Lugol’s solution) and washed with 95% alcohol or acetone.
• The bacteria which retain the original blue or purple colour are called Gram positive (+ve) bacteria. Those which lose stain and decolourize after the treatment with alcohol are called Gram negative (-ve) bacteria In Gram (+) bacteria the cell wall has very little lipid content. Therefore, very little stain leaks out of their walls in organic solvent. In Gram (–) bacteria the cell wall has high lipid content. The same dissolves in organic solvent taking out the stain along with.
• The decolorized Gram (–ve) bacteria may, therefore, be stained with safranine or eosine.
• Gram positive bacteria: Pneumococcus, Streptococcus, Staphylococcus, Bacillus, Clostridium, Mycobacterium, Streptomyces.
• Gram negative bacteria: Salmonella, Pseudomonas, Escherichia, Haemophilus, Helicobacter, Vibrio, Rhizobium.

Gram Positive Bacteria	Gram Negative Bacteria
They retain the Gram stain even after washing in organic solvent.	Washing in organic solvent drains away the Stain.
Cell wall is thick, 20-80 nm.	Cell wall is comparatively thin, 7.5-12 nm.
Cell wall is single layered.	It is two layered.
A periplasmic space or gel is less conspicuous.	A periplasmic space or gel lies between cell wall and plasma membrane.
Lipid content is low, 1-5%.	Lipid content is high, 20-30%. .
Peptidoglycan content of wall is 70-80%.	Peptidoglycan content of wall is 10-20%.
The wall contains teichoic acid.	Teichoic acids are absent.
Flagella possess singlepair of rings in the region of basal body.	Flagella have two pairs of rings in the area of their basal body.
Mesosomes are well developed.	Mesosomes are less prominent.
Ratio of DNA to RNA is 1 : 8.	Ratio of DNA to RNA is 1 : 1.
Pathogenic forms are fewer.	Pathogenic forms are more abundant.
They are highly susceptible to antibiotics.	They are less susceptible to antibiotics.

Cell Structure

• A bacterial cell is covered by mucilage. The cell is differentiated into cell wall, plasma membrane, cytoplasm, nucleoid, plasmids, flagella, pili and fimbrie. Mucilage, cell wall and plasma membrane are together called cell envelope.

Mucilage (Glycocalyx)

• It is the outermost coat of bacterial cells which is rich in polysaccharides. A loose sheath is called slime layerwhile a thick (more than 0.2 $\mathrm{\mu m}$ ) and tougher mucilage is called capsule. It protects the cell against the desiccation and viral attack.

Cell wall

• It is present outside the membrane and is a rigid structure.
• The cell walls of almost all the eubacteria (true bacteria) are made up of peptidoglycan, also called murein or mucopeptide.As the name suggests, the peptidoglycan consists of two components; peptide portion and a glycan or sugar portion.
• The glycan portion is composed of alternating units of amino sugar, N-acetyl glucosamine (NAG) and N-­acetyl muramic acid (NAM) joined together by $\mathrm{\beta }$ , 1-4 glycosidic linkage. This forms the backbone structure of cell wall. The mucopeptide chains are laterally joined by amino acids for structural rigidity. In many Gram (+) bacteria and actinomycetes there are four amino acids – L-alanine, D-glutamic, L-lysine and D-alanine. On the other hand, in Gram (–) bacteria, myxobacteria and blue green algae, diaminopimelic acid (DAP) is present in place of lysine.
• In Gram (+) bacteria, the cell wall has a thick peptidoglycan layer (90%) and also contains teichoic acids, formed of glucose, phosphate and alcohol. Teichoic acid has several functions such as binding metals, acting as receptor sites for some viruses, and maintaining cells at low pH to prevent degradation of cell wall by self-produced enzymes.
• The cell wall of Gram –ve bacteria is much more complex. The peptidoglycan layer is very thin making up only 10% or less. However, the most interesting feature is the presence of an outer membrane that covers a thin underlying layer of peptidoglycan. The outer face of outer membrane contains lipopolysaccharides, a part of which is integrated into the membrane lipids. The inner face has a number of proteins, which are anchored into peptidoglycan. The outer membrane of Gram –ve bacteria contains proteins called porin, and these proteins function as channels for the entry and exit of hydrophilic low molecular weight substances.
• In Mycobacterium, Corynebacterium and Noccardia, the wall is that of Gram (+) type but a part of their cell wall is made up of a very long chain of the fatty acid called mycolic acid.
• Due to the presence of the outer membrane, Gram-negative bacteria are rich in lipids that make about 11­-12% of the dry weight of the wall. Teichoic acid is absent in this case.

Protoplast

• Cell wall encloses the protoplast, the living matter. It is differentiated into cell membrane, cytoplasm and nuclear body.

Cell Membrane

• It lies inner to cell wall, actually representing the outermost layer of the protoplast. It is about 80Å thick. The space between cell wall and plasma membrane is referred to as periplasm or periplasmic space. It is about 180Å wide.
• It is living and selectively permeable, controlling the movements of various dissolved substances in and out of cell. It is composed of large amounts of phospholipids, proteins and some amounts of polysaccharides but lacks cholesterol-like sterols of eukaryotic membranes.
• Functionally, the cell membrane resembles mitochondria of eukaryotic cells, as some respiratory enzymes are associated with the membrane. Enzymes for lipid synthesis, permease, and synthesis of cell wall materials are also found.
• The cell membrane gets invaginated and folded to form structure called mesosomein some bacteria, particularly the Gram positive bacteria. Mesosome is of two types, septal and lateral. Septal mesosome is in contact with nucleoid while peripheral or lateral mesosome is not attached to nucleoid. Septal mesosome replicates with the replication of nucleoid, takes part in separation of replicated nucleoids and helps in septum formation. Lateral mesosome contains respiratory enzymes. It is also called chondrioid.

Cytoplasm

• It is homogeneous colloidal mass of carbohydrates, fats, proteins, lipids, nucleic acids, minerals and water. It does not show streaming movement and lacks sap vacuoles. Typical organelles of eukaryotic cell like endoplasmic reticulum, mitochondria, Golgi complex and plastids are absent. The cytoplasm is usually colourless, lacking pigments. However, in photosynthetic bacteria, the cytoplasm contains pigments like bacteriochlorophyll and bacteriopurpurin.
• The various components of cytoplasm are as follows:
  • Ribosomes: A large number of ribosomes (20, 000 to 30, 000) lie scattered freely in the cytoplasm but sometimes may form a small helical chain of 4-6 ribosomes called polyribosomesor polysomes by means of mRNA strands. Ribosomes are of 70s type, the two sub-units being 50s and 30s. Bacterial ribosomes are 14-15 × 20 nm with a molecular weight of 2.7 million daltons.
  • Inclusion bodies: They are nonliving and nonstructural materials which lie free in the cytoplasm. These are of three types,­food reserve (glycogen and proteins granules), inorganic granules (volutin granules, sulphur granules, etc.) and gas vacuoles. The gas vacuoles are useful in buoyancy regulation and protection against harmful radiations.
  • Photosynthetic pigments: They are bacteriochlorophylls, bacteriophaeophytin and carotenoids. In purple sulphur bacteria the pigments are associated with thylakoid membranes that are formed by invagination of plasmalemma. In green sulphur bacteria the photosynthetic pigments occur inside small sacs
  • Nuclear Body: Bacterial cell lacks a well-organized nucleus. The nuclear material, consisting of single naked, circular double stranded DNA molecule, is identified as nuclear body, nucleoidor genophoreor incipient nucleus.It is attached to plasma membrane directly or by means of mesosomes. Circular DNA ring is often termed as bacterial chromosome(prochromosome). It is not associated with histone proteins.
  • Plasmids (Hayes and Lederberg, 1952): They are small, self-replicating, autonomous, extrachromosomal single circular DNA present in the cytoplasm. They endow some extra properties to bacteria which are often useful but not essential for their living. Three forms of plasmids are:
  • F-plasmid: having fertility factor,
  • R-plasmid: having resistance to antibiotics,
  • Col-plasmid: producing colicins or bacteriocins that kill related bacteria.
  • Some plasmids can temporarily integrate with nuclear body and then they are called episomes(Jacob and Wollman, 1958). Formation of episome depends upon chromosome memory.
  • Plasmids are important tool in genetic engineering. Jumping genes or transposons also occur in some plasmids. A common plasmid of E coli is ${\mathrm{pBR}}_{322}$ .
  • Flagella: These are fine protoplasmic threads projecting from the cell wall. Each flagellum is 3-12 in length and 12-18 nm in width. Each flagellum is made up of three parts-basal body, hook and Basal body (basal granule or blepharoplast) is most complex portion of flagellum. It has two pairs of rings (L, P, S and M) in Gram (–) and one pair of rings (S and M) in Gram (+) bacteria around a central rod of 70Å diameter. The outer pair of rings (L and P) lies within the cell wall whereas the inner pair (S and M) is located in the plasma membrane. Hook is the middle curved part of flagellum and is made up of different protein subunits. Filament is the longest and the most obvious portion of flagellum. It is a cylindrical hollow structure made up of protein monomers, flagellin. The flagellin monomers are arranged in 4 + 4.
  • Flagella are made up of specific proteins called flagellin.
  • The 360° rotation of flagellum induces the cell to spin and move forward.

Flagellation

• Depending upon the presence or absence, number and position, following types of flagellar arrangement are observed among bacteria:
• Atrichous: Flagella absent, e.g., Lactobacillus, Pasteurella.
• Monotrichous: A single flagellum present at one pole, e.g. Vibrio cholerae, Thiobacillus.
• Cephalotrichous: A tuft of flagella at one end, e.g. Pseudomonas fluorescens.
• Amphitrichous: One flagellum at both ends, e.g.
• Lophotrichous: Two tufts of flagella at both ends, e.g. Spirillum volutans.
• Peritrichous: A number of flagella, borne all around the bacterium, e.g. Salmonella typhi, E. coli,etc
• Pili: These are hollow, non-helical, filamentous appendages projecting from the walls of some Gram (–) bacteria. These are thinner and shorter than the flagella. They are 1-4 in number and develop only on donor (or male) cells. They are made up of specific proteins called Pili help in the attachment of the bacterial cells during conjugation.

Fimbriae

• They are small bristle like fibres which develop from the surface of bacterial cells. They are quite numerous (300-400 cell). They take part in attachment, mutual aggregation or attachment to host tissue.

 Reproduction

• Bacteria reproduces mainly by vegetative and asexual methods of reproduction. However, they also exhibit some amount of genetic recombination.

Vegetative reproduction

• It takes place by binary fission and budding.

 Binary fission:

• • This is the universal method of multiplication in bacteria in which the cell divides into two equal parts by a transverse constriction of the cytoplasm.
  • Then nucleus divides amitotically and the process of binary fission is completed in 20-30 minutes.

    

  • Binary fission involves following steps:
    Replication of DNA
    Division of mesosome
    Membrane synthesis
    CleavageSeptum formation

     

• Budding: A protuberance develops at one end of bacterial cell. Nuclear body replicates and one of them passes into the protuberance or bud. The bud grows in size, constricts at the base and separates to form a new bacterium, e.g., Bifid bacterium.
• Spore formation: Bacteria produce several types of spores called sporangiospores, arthrospores (oidia), conidia, cysts and Process of sporulation occurs under unfavourable condition. Some rod shaped bacteria like Bacillusand Clostridiumform spores inside the vegetative cell called endospores. Cocci and spirilla bacteria do not produce endospores.
• Endospores are highly thick-walled and resistant spores which are formed in response to adverse environment, presence of harmful waste products, etc

• The endospores may be spherical or oval in shape and are terminal, sub-terminal or central in position. It is covered by a thick wall made of 3-4 layers-exosporium (a loose lipo-protein covering), spore­coat (the actual outer impervious wall layer made up of keratin-like protein), cortex (the middle wall layer formed of peptidoglycan) and core-wall (the innermost wall layer formed of protein. Protoplast of endospore contains nucleoid and cytoplasm having ribosomes, some RNAs and storage proteins. Water content is low. Mainly cortex contains an anticoagulant dipicolinic acid stabilized by $C^{++}$ .
  

• This is mainly a source of perennation and dispersal and not a source of reproduction as one bacterial cell normally produces only a single endospore. It can easily tolerate a temperature of –100°C to 100°C Fortunately, only two of the pathogenic bacteria produce endospores which are tetanus and anthrax bacteria.

Genetic Recombination

• Typical sexual reproduction is absent in bacteria because of absence of haploid-diploid alternation but gene recombination occurs by three methods – conjugation, transformation and transduction. This is also called as parasexual reproduction.

Conjugation

• Conjugation is coming together of two cells and involves transfer of genetic material from one bacterium to another through cell to cell contact. It was first discovered in Escherichia coli by Lederberg and Tatum (1946

Gram Negative-Bacteria:

• Bacteria showing conjugation are dimorphic, i.e., they have two types of cells, donor (male) and recipient (female). The former possesses sex pili and fertility factor in its plasmid and is called ${\mathrm{F}}^{+}$ . The latter is without fertility factor and sex-pili, and is referred to as F^–.
• When two types of cells happen to come nearby, the donor cell extrudes a protein from the tip of its sex pilus. It helps the donor cell to get attached to recipient cell. Wall of the recipient cell dissolves in the region of contact and a conjugation bridge is formed. It takes 6-8 minutes. Gene exchange can occur by two methods:
  • Sterile male method: Fertility factor of the donor replicates and a copy of it gets transferred to the recipient cell through the conjugation bridge. Passage of F⁺ plasmid into the recipient cell changes it into donor cell as well. The phenomenon is called sex-duction. (Jacob and Adelberg) and conjugation of this type is called sterilemale method. Conjugation frequency is 1 : 10⁵.
  • Fertile male method: At times the fertility factor integrates with the nucleoid of the cell, becomes episome and changes the donor cell into Hfr (high frequency of recombination). Hfr is also called super-male or meta-male as its recombination frequency is about 1000 times more than the normal ${\mathrm{F}}^{+}$. Nonintegrated ${\mathrm{F}}^{+}$plasmids disintegrate in Hfr cells. Bacterial nucleoid breaks ahead of episome. It is called zero end. Now the bacterial nucleoid undergoes replication. A copy of freed zero end of bacterial nucleoid straightens and passes into the recipient cell through the conjugation bridge. ${\mathrm{F}}^{+}$factor rarely passes into the recipient as it can do so only after the passage of whole nucleoid. Generally, conjugation persists for a brief period when a few donor genes are transferred, one in seven minutes, two in nine minutes, three in ten minutes, four in eleven minutes, etc. (Wollman and Jacob, 1966). The transferred part of nucleoid is called exogenoteand corresponding segment of recipient cell is called The recipient cell having DNA segment of donor cell is called partial zygote or merozygote. The new genes may replace the genes present in the recipient cells or get added to them. Conjugation of this type is called fertile male method.

 Gram Positive Bacteria

• They don’t have sex pili. However, donor and recipient cells occur. A donor cell secretes a protein, adhesin over it. The protein helps the donor cell in sticking to a recipient cell for conjugation. In Streptococcus faecalis,the recipient cells produce a pheromone like chemical for attracting the donor cells.
• The conversion of F^– bacterium into ${\mathrm{F}}^{+}$ following the infection by episome DNA provides another evidence that DNA is the genetic material.

Transformation

• It is the change in genetic constitution of a bacterium by picking genes of its dead relatives present in outside medium and integrating the same in its nucleoid.
• Transformation was discovered by Griffith (1928) when he found that nonvirulent living bacteria of Pneumococcus could become virulent by picking up the trait from heat-killed virulent strain. Avery et al (1944) found that the trait was carried by a DNA segment of the dead bacteria. Ability to pick up genes from outside is called It is present generally towards the end of growth period.

Transduction

• It is the process of carrying over genes from one bacterium to another through the agency of viruses.
• Transduction was discovered by Zinder and Lederberg (1952) in case of Salmonella typhimurium. In transduction, a virus picks up one or more bacterial genes. As it attacks to a new host, the genes are passed into the new host. The transducing virus is seldom harmful because it does not carry its full gene complement.
• Transduction is of two types, generalised and restricted or specialised. Any gene is transferred in generalised transduction while the particular gene is passed on in case of restricted or specialised transduction. It is successful if the new gene integrates with the host gene or abortive if the new gene fails to do so.

Economic Importance of Bacteria

• Bacteria play significant role in day-to-day activities of human beings.

Beneficial activities
Role in agriculture

• Decay and decomposition of organic matter: They are termed as Nature’s scavengersas they help in preventing the accumulation of dead remains of plants and animals.
• Sewage disposal: The bacteria decompose the organic matter present in the sewage, converting into simpler inorganic substances. The inorganic substances thus formed, being soluble pass through filter along with water that is highly useful for irrigation purpose.
• Nitrogen cycle: Nitrogen is important constituent of all living organisms. Amino acids are converted into ammonia by ammonifying bacteria (Bacillus vulgaris. B. ramosus). Ammonia reacts with other inorganic substances present in the soil to form ammonium salts. Nitrifying bacteria convert ammonium salts first into nitrites (Nitrosomonas, Nitrosococcus) that are subsequently converted into nitrates (Nitrobacter). Nitrates and nitrites and to some extent ammonium salts are absorbed by plants to be converted into proteins again.
• Nitrogen fixation: It is the biological process in which atmospheric nitrogen is converted into nitrogenous compounds by nitrogen fixing bacteria. These bacteria are of two types:
• Free living: Free-living bacteria are Azotobacter (aerobic) and Clostridium (anaerobic). These convert molecular nitrogen into ammonia that combines with water to form ammonium salts. Ammonium salts are eitherdirectly absorbed by certain plants or converted into nitrites and nitrates by nitrifying bacteria.
• Symbiotic: Common symbiotic bacteria are Rhizobium leguminosorum associated with root nodules of leguminous plants. These convert nitrogen into ammonia that is directly converted into amino acids by plants.
• Manure preparation: Saprophytic bacteria help in preparation of farmyard manure by converting farm refuse, dung and other organic waste into humus, a partially decomposed organic matter. Humus is further decomposed to liberate minerals essential for growth of crop plants.
• Gobar gas plants: In gobar gas plants, animal dung and other organic wastes are converted into manure along with the production of fuel gas with the help of saprophytic bacteria.

Role in industry

• Man has utilized the metabolic activities of bacteria in preparation of a number of industrial products as described below:
• Butter milk and sour cream: Butter is usually manufactured by churning cream that has been soured by bacteria. Initial souring is carried out by the bacteria Streptococcus cremoris or S. lactis while Leuconostoc citrovorum produces the necessary flavour.
• Yogurt: Yogurt is made by carrying out fermentation of milk with a mixture of Lactobacillus vulgaricus and Streptococcus thermophilus at 40°C. The characteristic flavour of yogurt is due to the accumulation of lactic acid and acetaldehyde produced by L vulgaricus.
• Cheese: The natural production of cheese involves lactic acid fermentation with various mixtures of Streptococcus and Lactobacillus species used as starter cultures to initiate the fermentation.
• Vinegar: Alcohol obtained by initial fermentation of carbohydrates by Saccharomyces cerevisiae is followed by a secondary oxidative transformation of alcohol into acetic acid by Acetobacter and Gluconobacter for the production of vinegar.
• Retting: Retting is a controlled microbial decomposition of lint materials to liberate fibres. Clostridium butyricum is useful in this process.
• Leather industry: The decomposing and fermentation capability of bacteria is utilized for removing fats, hairs and other tissues from the hides. The cleaned hides are then tanned to prepare leather.

Role in medicine

• Bacteria have been used extensively in preparation of antibiotics, vaccines, serums and vitamins:
• Antibiotics: These are the organic substances produced by microorganisms that inhibit the growth of other organisms (mostly pathogens) but not affecting the growth of organisms secreting these. The first antibiotic was, extracted a few years earlier by Gratia and Dash (1924) from a mycelial bacterium. Most common antibiotic in medicine are produced by filamentous bacteria, known as

Table: Antibiotics obtained from bacteria

Antibiotic	Source
Streptomycin	Streptomyces griseus
Terramycin or oxytetracycline	S. ramosus
Erythromycin	S. erthryeus
Chloromycetin or chloramphcnicol	S. venezualae and S. lavendulae
Neomycin	S. fradiae
Viomycin	S. puniceus
Novabiocin	S. niveus
Nystatin	S. noursae

• Vaccines: Vaccine is made by suspension of non virulent or weakened strains of bacterium causing the disease. When the vaccine is injected into the body, antibodies are produced which provide immunity against the virtual strains of the bacterium that produce disease. Vaccines are prepared against the diseases like cholera, typhoid, small pox, etc.
• Serum: Serum is solution in which the antibodies are present for a particular pathogenic bacterium. Serum, when injected into the body, provides immunity by destroying the invading pathogenic bacteria. Serum is prepared by injecting virulent bacteria into blood of animals like horse at regular intervals in small doses. Subsequently the antibodies are separated from the plasma to get serum.
• Vitamins:Escherichia coli present in human intestine produces large quantities of vitamin B complex and vitamin K. Bacteria are utilized in industrial production of a number of vitamins like riboflavin from Clostridium butylicum, cobalamin (B₁₂) from Bacillus megatherium. Acetobacter aceti is used in some steps during the preparation of vitamin C. 

Harmful activities

• Spoilage of food: Saprophytic bacteria cause decay of vegetables, fruit, meat, cheese, butter, jelly, jams, pickles and making them unfit for human consumption. Some bacteria even produce strong toxins in the infected foodstuffs, which cause food poisoning.
• Deterioration of domestic articles: Some saprophytic bacteria like Spirochaete cytophaga cause deterioration of domestic articles of daily use such as leather, woolens, canvas articles, etc.
• Denitrification of soils: Denitrifying bacteria like Thiobacillus denitrificans and Micrococcus denitrificans convert nitrates and nitrites present in the soil into gaseous nitrogen and thus depleting the soil of its nitrogen, thereby decreasing soil fertility. Fortunately these bacteria don’t thrive well in well aerated soils.
• Diseases: About 90% of the human diseases and many plant diseases are caused by bacteria.

Human disease
Disease	Casual bacterial species
Cholera	Vibrio cholerae
Typhoid	Salmonella typhi
Diptheria	Corynebacterium diptheriae
Whooping cough	Bordetella pertussis
Pneumonia	Diplococcus pneumoniae
Tuberculosis	Mycobacterium tuberculosis
Leprosy	Mycobacterium leprae
Bubonic plague	Mycobacterium leprae

Plant disease
Disease	Casual bacterial species
Angular leaf spot of cotton	Xanthomonas malvacearum
Blight of beans	Pseudomonas phaseolicola
Blight of paddy	Xanthomonas oryzae
Canker of citrus	Xanthomonas citri
Wild of maize	Xanthomonas stewartii
Crown gall of apple	Agrobactrium tumaefaciens
Leaf spot of cucumber	Xanthomonas cucurbitae

Cyanobacteria

• In two kingdom system, cyanobacteria were included in class Cyanophyceae or Myxophyceae of sub­division Phycophyta (Alga) of division Thallophyta. But now it is included in kingdom Monera because they are prokaryotes.

• Cyanobacteria (Gk. Cyano = blue, bact = rod) or blue green algae are Gram negative photosynthetic prokaryotes, being the most primitive organisms to have oxygenic photosynthesis. They are the most successful and self-dependent organisms on the earth and survived successfully for more than three billion years. They were the first to add oxygen to the atmosphere, which is indispensable for the existence of aerobic forms of living organisms.

Occurrence

• They are mainly fresh water forms, though few are marine. A few species grow in hot water springs having a temperature range 70°C – 75°C (e.g., Phormidium, Mastigocladus) and others grow at very low temperature in the polar regions (e.g., Nostoc, Schizothrix, Microcoleus, ,). Red sea is named because of abundant occurrence of a planktonic cyanobacterium, Trichodesmium erythraeum, which imparts red coloration to water. They occur in symbiotic association with almost every group of eukaryotes.

Structural Organization

• They may be unicellular and multicellular. The latter may be colonial or filamentous. Unicellular forms may be spherical or oval in shape. Unicellular forms have single celled body, e.g., Synechococcus, Chroococcus, Anacystis, The colonial forms are of two types, dendroid (e.g., Chamaesiphon) and coccoid (e.g. Microcystis). In dendroid colony, the unicells are connected by gelatinous stalk while in coccoid colony, a number of cells is occur inside a common mucilage. .
• Filamentous form consists of one or more cellular strands, called trichomes, surrounded by mucilaginous sheath. A filament may contain one (e.g., Oscillatoria) to many trichomes (e.g., Schizothrix). Filament may have false-branching (e.g., Scytonema) or true branching (e.g. Hapalosiphon, Stigonema, ). Uniseriate filaments may be homocystous (e.g., Oscillatoria, Arthrospira) or heterocystous (e.g., Nostoc, Rivularia).
• Cyanobacteria are characterized by the absence of flagellum even in motile forms, although most of these are motile. Some filamentous cyanobacteria show gliding and oscillatory movement. 

Cell Structure

• The cell structure in cyanobacteria is basically similar to that in bacteria. The cell lacks a well defined nucleus and the chromatin material is centrally located resembling the bacterial chromosome. Like bacteria, small circular DNA segments may also occur in addition to nucleoid. They are known as plasmidshaving
• The cell wall is bilayered and invariably covered by mucilaginous sheath, composed largely of mucopeptide (peptidoglycan).
• The cell wall is followed by plasma membrane made up of lipid and protein.
• A coiled membranous ingrowth of plasmalemma, called lamellasome is present.
• Cell protoplast is often differentiated into outer pigmented chromoplasm and inner colourless
• The protoplast lacks membrane bound cell organelles and contains 70S ribosomes.
• The sap vacuoles are absent. Instead, the cell may contain gas filled vacuoles (pseudovacuole or gas vacuole) that help to regulate the buoyancy of the organism in water. Each gas vacuole consists of a number of sub microscopic units called gas vesicles, which function as light screen and provide pneumatic strength.
• The characteristic feature of a cyanobacterial cell is the presence of a system of photosynthetic lamellae called thylakoids, which make the  structure more elaborate in comparison to that in bacteria. The characteristic photosynthetic pigments present in the thylakoids are chlorophyll a and phycobilins e. phycocyanin (blue coloured) allophycocyanin(light blue coloured) and phycoerythrin (red coloured). The thylakoid has both PS-I and PS-II.
• The cell contains reserve food material in the form of special starch called cyanophycean starch. Other granules present in a cyanobacterial cell are volutin granules, polyhedral bodies, -granules (lipid droplets).
• Surrounded by the nuclear material in the centre of the cell, there is a structure known as carboxysome, which contains enzymes for the dark reaction of photosynthesis.

Metabolism

• They are self-dependent organisms, because most of these are capable of converting atmospheric nitrogen into ammonium compounds besides utilizing atmospheric $C{O}_2$ for synthesis of organic food during photosynthesis. Nitrogen fixation under anaerobic conditions mainly occurs in specialized cells called Heterocysts are large sized pale coloured mucilage free thick walled cells which are impermeable to oxygen. Each heterocyst has one or two terminal depressions with pores called polar nodules. Heterocysts contain nitrogen fixing enzyme called nitrogenase.

 Reproduction

• Cyanobacteria reproduce vegetatively and asexually. Typically sexual reproduction is absent. Gene recombination is, however, reported to occur.

Vegetative reproduction:

• Binary fission: It occurs in unicellular forms. The daughter cells formed by amitotic division, separate immediately after the division. This is the most common method of reproduction, e.g.,
• Fragmentation: It occurs in filamentous forms. The filament breaks up into short pieces or fragments that grow to form new filaments, e.g.
• Hormogones: They are small trichome segments which separate from the parent due to death of intervening cells (necridia), e.g., Nostoc, Anabaena,

 Asexual reproduction

• Asexual reproduction takes place by means of akinetes, endospores, exospores or nannocytes.

Genetic recombination.

• Genetic recombination has been reported by H. D. Kumar of B.H.U. Varanasi in Anacystis nidulans, Anabaena sp., etc. All the three kinds of bacterial parasexuality, i.e., conjugation, transformation and transduction have been reported in cyanobacteria.

Importance Of Cyanobacteria
Useful activities

• They play significant role in evolution of aerobic forms of life. They provide suitable conditions for the growth of other organisms in the hostile environment.
• They convert atmospheric nitrogen into ammonium compounds and excess of these compounds is excreted out enriching the soil. Nitrogen fixation is done by about 50 species of cyanobacteria. On the basis of nitrogen fixation, they are of two types:

◊ Free living N₂-fixing cyanobacteria (e.g., Anabaena, Nostoc, Aulosira, Scytonema. Tolypothrix. Stigonema, Gloetrichia, etc.)

◊ Symbiotic N₂-fixing cyanobacteria (e.g. Nostoc and Anabaena are common in lichens, Anthoceros, Azolla and Cycas root; Azollapinnata has Anabaena azollae in its fronds).

• Cyanobacteria like Nostoc and Anabaena have been used for reclaiming usar soils. They secrete acidic chemicals, which counteract the alkalinity of the usar soil.
• Some cyanobacteria serve as food to several aquatic animals. Spirulina is being used as a source of protein rich supplement to diet (single cell protein; SCP).
• They are used as bio-fertilizers.
• Anabaena and Aulosira do not allow mosquito larvae to grow.
• Extract of Lyngbia, a cyanobacterium is used for manufacture of antibiotic.

Harmful activities

• Some cyanobacteria like Microcystis aeruginosa, Anabaena flosaquae, Aphanizomenon flos-aquae secrete toxins into the surroundings, which are harmful to aquatic animals and even to human beings.
• They cause depletion of O₂ supply to aquatic animals by the formation of algal bloom by Microcystis, Anabaena and
• Some cyanobacteria cause damage by growing on textile, cordage and tents.
• Anacystis causes corrosion of metallic water pipe.

 Archaebacteria (Gk. archae = ancient; bact = rod)

• They are ancient, and the most primitive prokaryotes. They are living under extremely adverse conditions like very high temperature (hot water springs) and high salt concentration (salt marshes). Very few other organisms can survive under such environmental conditions. They are termed oldest living fossilsas they have survived the geological changes successfully.
• The cell wall in archaebacteria contains proteins (pseudomurein) and non-cellulosic polysaccharides. It lacks peptidoglycan, the characteristic cell wall material in bacteria and cyanobacteria. The cell membrane contains branched chain lipids. This chemical composition of the cell membrane enables these organisms to withstand extremes of temperature and pH.
• They are grouped into obligate anaerobes and facultative anaerobes.

Obligate or Strict Anaerobes

• They live under anaerobic conditions only and get killed in presence of oxygen.
• The mode of nutrition is chemosynthetic.
• These are further divided into two sub groups, the methanogens and halophiles.

Methanogens

• They are obligate anaerobes occurring in marshy habitats. They are capable of converting CO₂ and formic acid into methane and hence the name This property is exploited commercially in the production of fuel gas and methane in gobar gas plants the biogas fermenters.
• Some of the methanogens live in rumen of herbivorous animals like buffalo, cow, etc. (ruminants) that chew their cud. These microorganisms assist in fermentation of cellulose in such animals.
• Methanogens have been grouped under four genera Methanobacterium, Methanobacillus, Methnosarcina and

Halophiles

• They live in habitats having high salinity and high light intensity. The minimum salt concentration required for growth of halophiles is 2–2.5M while optimum is 4-5M. They possess red carotenoids in their cell membrane. They protect them from intense harmful radiations.
• They may be cocci (e.g. Halococcus) or bacilli with polar flagellum (e.g. Halobacterium) in shape.
• Under anaerobic conditions, Halobacterium develops purple membrane having photoreceptor pigment In light, it acts as a proton pump and helps to synthesize ATP by chemiosmosis.
• The unique property of halophiles to withstand high salt concentration is due to:

◊  Unique composition of cell membrane containing branched chains of lipids,

◊  Mucilage covering around the cell wall,Absence of sap vacuoles.

◊  The cell can’t be plasmolysed even in high salt concentration due to absence of sap vacules.

 Facultative anaerobes

• They normally respire aerobically but are capable of withstanding anaerobic conditions as well quite comfortably. The archaebacteria in this category are

Thermoacidophiles

• These are capable of tolerating high temperature as well as high acidity and hence the name They often live in hot water springs where the temperature is as high as 80°C and the pH as low as 2.
• They oxidize sulphur to sulphuric acid under aerobic conditions and the energy obtained in these reactions is utilized for synthesis of organic food. The medium becomes highly acidic due to production of sulphuric acid. Under anaerobic conditions sulphur is reduced to H₂
• Thermoacidophiles are capable of withstanding extremely low and high temperature due to membrane containing branched chains of lipids.
• The cell the presence of resistant enzymes that can operate under acidic conditions.
• Examples of thermoacidophiles are Thermoplasma, Thermoproteus,

■    Neisseria gonorrhoea attaches to urinary tract by means of fimbriae. ■    The fragrance, which emanates from the soil after first rain or freshly ploughed soil, is due to growth of bacteria, Streptomyces in the soil. An oil geosmin is responsible for the characteristic smell. ■   Mycobacterium leprae (Hensen’s bacillus) causes leprosy and cannot be cultured in vitro, therefore, eyes of Armadillo are used to prepare vaccine. ■    Antibiotic penicillin and cephalosporin inhibit cross-linking of peptidoglycan strands while lysozyme (present in tears, saliva and other body secretions) hydrolyse peptidoglycan. ■   Teichoic acid, present in the cell wall of gram (+) bacteria, binds Mg++ and protects bacteria from thermal injuries. ■    Robert Koch identified the protein tuberculin derived from Mycobacterium tuberculosis. ■    The effective drugs against tuberculosis are streptomycin, p-amino-salicylic acid (PAS) and rifamycin. ■    Waksman coined the term antibiotic and streptomycin. ■    Bdellovibrio bacteriovirus is a very small monotrichous predatory bacterium which purifies the water of river Ganges. ■    At university of Illinois, Prof. Anand Mohan Chakraborty an Indian born molecular biologist) and his coworkers (1979) developed a superstrain of Pseudomolnas which can degrade oil. Cyanobacteria associated with protists are called Cynellae. Many cyanobacteria show chromatic adaptation (Gaidukov phenomenon), i.e. change in pigmentation with change in wavelength of light, e.g. Oscillatoria. Maximum protein content (71%) on dry weight basis is found in Spirulina, a cyanobacterium. It is also the only vegetable source of vit. B12. Nostoc sp. occur in the petiole of Gunnera, an angiosperm. ■    Heterocyst does not perform the photosynthesis like other vegetative cells. ■    Auosira fertilissima is the most active nitrogen fixer in rice fields. ■    Cylindrospermum is an active nitrogen fixer of sugarcane and maize fields. ■    Tolypothrix is used as N2-fixer in experimental fields.

3.  KINGDOM: PROTISTA

• All unicellular eukaryotic organisms, irrespective of their mode of nutrition, are included in the kingdom Protista. This kingdom forms a connecting link between the prokaryotic kingdom Monera on one hand and other three eukaryotic kingdoms, Fungi, Plantae and Animalia on the other hand. Protists were the first eukaryotes to evolve on this earth some 1600 million years ago. Protists may be considered as the ancestors of all multicellular eukaryotes. Ernst Haeckel created the kingdom Protista for those organisms, which do not have tissue.

General characteristics

• They are solitary unicellular or colonial unicellular eukaryotic organisms.
• Mostly they are aquatic organisms. Many protists are planktons. Some forms are parasites.
• Protists have different body shapes. The cell wall, when present, contains cellulose,pellicle, shell, etc. may be present.
• Cytoplasm contains membrane bound cell organelles; many have centrioles also.
• The protoplasm is surrounded by a distinct plasma membrane made of lipoprotein.
• Cytoplasm shows streaming movement.
• Photosynthetic forms contain chloroplasts with internal thylakoids and act as chief producers of food in the oceans and fresh waters.
• Nucleus has typical structure porous nuclear envelope, chromatin material, nucleoplasm and nucleolus.
• A mesokaryon occurs in dinoflagellates.
• They commonly move with the help of pseudopodia, flagella or cilia. Flagella have (9 + 2) organization of microtubules that are composed of a protein named
• Nutrition may be photosynthetic, holozoic, saprobic or parasitic. Some protists have mixotrophic nutrition (photosynthetic and saprobic) as in euglenoids.
• Food reserve can be starch, paramylum. chrysolaminarin, glycogen and fat.
• Reproduction occurs by both asexual and sexual methods. An embryo stage is absent.
• In protists asexual reproduction is a rapid method of multiplication and occurs by binary fission, multiple fission, spore formation, budding, cyst formation, etc.
• Sexual reproduction is believed to have originated in primitive protists. It involves meiosis and syngamy.
• Sexual reproduction may be isogamous (e.g., dinoflagellates and slime moulds) or anisogamous (e.g. Ceratium, a dianoflagellate) or oogamy (e.g. protozoans).
• Life cycle may show zygotic meiosis (e.g. dinoflagellates and cellular slime moulds) or gametic meiosis (e.g., diatoms and acellular slime moulds).
• Parasitic protists may cause diseases such as dysentery, malaria, sleeping sickness, etc.

 Major groups of protists

• The kingdom Protista has been broadly divided into three main groups.
• Photosynthetic protists or protistan algae (Plant Protista)
• Consumer or decomposer protists: slime moulds (Advanced Protista).
• Protozoan protists (Animal Protista)

Photosynthetic protists or protistan algae

• Photosynthetic protists are mainly unicellular eukaryotic algae, called protistan algae.They constitute the main portion of the They include dinoflagellates, diatoms and euglenoids.

 Dinoflagellates

They are a group of about 1,000 species of golden brown photosynthetic protists. The class of dinoflagellates is known as Dinophyceae (of phylum or division Pyrrophyta). The important characters of dinoflagellates are:

• Dinoflagellates are basically unicellular motile and biflagellate, golden brown, photosynthetic protists.
• Most of them are marine but some occur in fresh water. Some dinoflagellates such as Gymnodinium and Gonyaulax grow in large number in the sea and make the water look red and cause the so-called red tide.
• Nutrition is holophytic or photosnythetic. Symbiotic forms are called zooxanthellae.The mode of nutrition of Noctiluca is holozoic and that of Ceratium is mixotrophic. A colourless Blastodinium is parasite on animals.
• Cells are generally covered by a rigid coat, the theca or lorica of articulated and sculptured plates of cellulose and pectin.
• The theca contains two grooves, the longitudinal groove called the sulcus and the circular groove known as the cingulum or annulus or
• The two flagella are different (heterokontae), one transverse flagellum and other longitudinal flagellum.
• Due to presence of two flagella at right angle to each other, the dinoflagellates show peculiar spinning movement. Hence they are called whorling whips.
• Food reserve is stored in the form of starch in fresh water forms and oils in marine forms.
• A noncontractile vacuole called pusuleis present near the flagellar base. Pustule is an osmoregulatory organelle.

•  Varieties of eye spots occur in dinoflagellates.

•  Cells possess a relatively large and prominent nucleus known as The interphase nucleus has condensed chromosomes which lack histones.

•  Cell division occurs through dinomitosis in which the nuclear membrane persists. Microtubular spindle is not formed. Chromosomes are acentric and move while attached to inner membrane of nuclear envelope.

•  Chromatophores contain chlorophyll a, chlorophyll c, a-carotene (fucoxanthin rich) and xanthophylls (including dinoxanthin and peridinin).

•  Some marine dinoflgellates show bioluminescence, i.e., they emit light, e.g., Gonyaulax, Noctiluca, Pyrocystis, Pyrodinium,

•  Some dinoflagellates (e.g., Gonyaulax catenella) are poisonous to vertebrates. When they are in large number, they produce the toxin called saxitoxin.

•  Asexual reproduction occurs by cell division, spores and cysts.

•  Sexual reproduction is isogamous and anisogamous.

•  The life cycle involves zygotic meiosis. Gametic meiosis occurs in

Diatoms (Gk. dia – through, temnein – to cut)

• Diatoms comprise the phylum Chrysophyta of protistan algae. They constitute the class Bacillariophyceae.
• Bacillariophyceae contains about 200 genera and 5000 species. The important characters of diatoms are:
• Diatoms are microscopic, variously coloured and of diverse forms which do not possess flagella except in the reproductive state.
• Diatoms occur in all aquatic and moist terrestrial habitats. They may be free floating or bottom dwellers.
• Diatoms may show gliding type of movement with the help of mucilage.
• The body is covered by a transparent siliceous shell known as frustule. The frustule is made of two valves, epitheca and hypotheca.
• Each cell has a large central vacuole. The single large nucleus is commonly suspended in the central vacuole by means of cytoplasmic strands.
• Photosynthetic pigments contain chlorophyll-a, chlorophyll-c, $\beta$-carotene, fucoxanthin, diatoxanthin, diadinoxanthin, etc.
• The food reserve is in the form of oils and leucosin or chrysolaminarin (polysaccharide, $\beta$-1, 3 glucan), Volutin globules (proteinaceous in nature) are also present.
• The common mode of multiplication is by binary fission. Each daughter retains one valve of the parent as epitheca and secretes a new hypotheca. As a result, one of the two daughters is slightly smaller than the parent. Over the generations there would be considerable reduction In size. The normal size is restored by the formation of rejuvenescent cells (auxospores).
• Sexual reproduction varies from isogamy to oogamy. Meiosis is gametic.
• Diatoms are unusual in that their vegetative cells are typically diploid.
• Diatoms resemble dinoflagellates in having fucoxanthin.
• Diatoms are very important photosynthesizers. A 60 tonne blue whale may have 2 tonnes of planktons in the gut, which is mostly diatoms.
• The siliceous frustules of diatoms do not decay easily. They pile up at the bottom of water reservoirs and form big heaps called diatomite or diatomaceous earth.

  

Euglenoid flagellates(Euglena like flagellates)

• Euglenoids, are group of both chlorophyllous and non-chlorophyllous flagellate protists. They belong to class Euglenophyceae and phylum or division Euglenophyta. Euglenoids include about 36 genera and 800 species. The important characteristics are:
• Euglenoids are unicellular flagellate protists of fresh water and damp soils.
• These protists are devoid of cell wall.
• The body is covered by thin and flexible pellicle (periplast). The pellicle has oblique but parallel stripes called
• The apical end bears an invagination having three parts-cytostome, cytopharynx and reservoir. A large contractile vacuole lies at the anterior end near the base of reservoir. An orange-red eye spot or stigma contains red pigment astaxanthin, found elsewhere only in crustacea. The euglenoids have two flagella, usually one short.
• They can also perform creeping movements by expansion and contraction of their body. The phenomenon is called metaboly

• Nutrition is mixotrophic (holophytic + saprobic).

• Photosynthetic pigments are chlorophyll-a, chlorophyll-b, $\beta$-carotene and xanthophylls.

• They store their carbohydrates as paramylum bodies.

• Under favourable conditions, they multiply by longitudinal binary fission.

• Sexual reproduction has not yet been definitely proved.

• They perennate during unfavourable periods as cyst.

• They are regarded as connecting link between plants and animals.

  Examples: Euglena, Phacus, Peranema, Astasia, etc.

Consumer-decomposer protists:
Slime moulds

They were formerly included amongst mycetozoaor fungus animalsor myxomycota. Slime moulds are included in the division of Gymnomycota by mycologists. Because of their protistan nature, they are also called protistan fungi. They have about 600 species. The slime moulds have the following characters:

• The slime moulds live usually amongst decaying vegetation.
• They do not have chlorophyll.
• They are surrounded by the plasma membrane only (somatic parts are without cell walls). However, the spores have the cellulose cell wall.
• At one stage of the life cycle they have amoeboid
• They have phagotrophic or saprotrophic nutrition.
• Both asexual and sexual modes of reproduction are found. They produce spore within sporangia.
• Asexual reproduction takes place through binary fission, spores, cyst and sclerotium.
• The slime moulds resemble both protozoa and the true fungi. They are like protozoa in their amoeboid plasmodial stage and similar to true fungi in spore formation.
• Slime moulds are of two types: acellularand

Acellular slime moulds:
Plasmodial slime moulds

Acellular slime moulds exhibit the following characters:

• Their somatic bodies are free living multinucleate, naked diploid protoplasmic masses called the plasmodia.
• The plasmodium forms a number of sporangia (fruiting bodies). Each sporangium is present on a stalk.
• The sporangium usually contains a network of fine threads called capillitium.
• The spores are formed by meiosis from the diploid protoplast or the sporangium.
• They move with the help of pseudopodia like the amoebae.
• The spores germinate to produce biflagellate swarm cells that function as gametes.
• The sexual reproduction is isogamous.
• The diploid zygote forms the plasmodium that becomes multinucleate by repeated mitotic division.
  Examples: Physarum, Physarella, Fuligo, Dictydium, etc.

Cellular Slime Moulds

Cellular slime moulds have the following characters:

• Complete absence of flagellated cells in their life cycle.
• Presence of wall less uninucleate myxamoebae.
• Formation of pseudoplasmodium.
• Presence of naked sporangia (without sporangial cover).
• Presence of cellulose wall around spores.
• Anisogamous sexual reproduction, etc.
• Examples: Dictyostellum, Polysphondylium, Protostelium, Acytostelium,

  

4.  KINGDOM: FUNGI

Definition

• Alexopoulos (1952) defined fungi as nucleated, achlorophyllous organisms, which typically reproduce sexually and asexually and whose usually filamentous, branched somatic structures are surrounded by walls containing cellulose or chitin or both.

Distribution

• There are more than 1,00,000 species of fungi, which are cosmopolitan in distribution.

HABITAT

• Fungi are ubiquitous i.e., found in almost every habitat. They flourish well in moist, dark and warm conditions. The most usual habitat of fungi is wet soil rich in humus. A few forms are aquatic, e.g., Saprolegnia, Allomyces, Achlya,

Nutrition

• Fungi lack chlorophyll and are unable to synthesize their own food by the process of photosynthesis. Therefore, they obtain their nutrition from the external source by the process of extra cellular digestion and absorption of the digested material. Such mode of nutrition is called heterotrophicand absorptive,and the organisms are called
• According to their mode of nutrition, fungi are of two types: parasites and saprophytes.
• Parasites: They obtain their food from living hosts. Parastitic fungi are of two types, obligate parasite (e.g. Puccina) and facultative saprophyte (e.g. Ustilago).
• Saprophytes: They derive their food from dead and decaying organic matter. Saprophytes are of two types, obligate saprophyte (e.g. Mucor, Rhizopus, ) and facultative parasite (e.g. Fusarium).
• Some fungi are symbiotically associated with algae to form lichens. A few form has symbiotic association with roots of higher plants (e.g.. Pinus) called mycorrhiza.

Thallus Organization

• The plant body of fungi is thallus, which may be acellular and multicellular. Acellular thallus may be motile (e.g., Synchytrium) or non-motile (e.g., Saccharomyces). Multicellular thallus is tubular, filamentous, branched and is called mycelium.The unit of mycelium is hypha.
• The mycelium may be aseptate and septate. Aseptate mycelium lacks septa. It is multinucleate and is called coenocytic,g., members of Phycomycetes. In aseptate mycelium, septa formation occurs at the time of injury and during separation of reproductive structures. Septate mycelium is partitioned into separate compartments by means of cross walls (septa). Individual cell may be uninucleate, binucelate or multinucleate.
• Each septum is perforated by a central pore. The pore may be either simple or dolipore. The septal pores may, however, remain partially plugged by woronin bodies.
• Mycelium may be eucarpic(only a part forms the reproductive body) or holocarpic(the whole mycelium is transfomed into reproductive body). The eucarpic forms may be monocentric (having a single sporangium or polycentric (having many sporangia).

Fungal Tissues

• In some higher fungi, the mycelium gets organised into loosely or compactly woven structure, which looks like a tissues called plectenchyma (Gk. plekein = to weave, enchyma = tissue). It is of two types:
• Prosenchyma (Gk. pros = towards, enchyma = tissue): It consists of loosely woven hyphae which he almost parallel to each other and the cells and hyphae are easily distinguishable.
• Pseudoparenchyma (Gk. pseudo = false, enchyma = tissue): It consists of compact mass of parenchyma like tissues where the hyphae are closely packed and interwoven so that they lose their individuality and cannot be easily distinguishable from one another.
• In addition to above, the fungal mycelium may form some specialized structures like rhizomorphs, sclerotia and stroma.

Cell Structure

• Fungi are eukaryotic and possess true nuclei. Fungal cells are bounded by definite cell wall. The cell wall is made up of fungal cellulose/chitin (nitrogen containing polysaccharide or heteropolymer of N-acetyl glucosamine which are also found in insects). True cellulose is found in cell wall of some Phycomycetes (e.g. Phytophthora).
• The cell wall encloses protoplast, which is differentiated into plasma membrane, cytoplasm, nucleus and vacuoles. Cytoplasm has all the eukaryotic cell organelles, except plastids. However, a reddish pigment, neocercosporin has been isolated from the fungus Cercospora kikuchii. The dictyosomes are also not typical. Lomasome is a convoluted complex of membranous outgrowth of plasmalemma.
• Vacuoles are many but small. Near the hyphal tip, the cytoplasm contains small vesicles called chitosomes. They contain cell wall materials.
• Nuclei are smaller as compared to those of higher plants. Nuclei undergo intranuclear spindle formation (karyochorisis).
• Food reserve is glycogen (animal starch) and oils.

Reproduction

• Reproduction can be vegetative, asexual and sexual.

Vegetative Reproduction

Vegetative reproduction occurs by the following methods:

• Fragmentation: Mycelium may break accidentally or due to decay into two or more parts. Each segment develops into complete mycelium.
• Fission: It occurs in unicellular forms where cell division produces two daughter cells g., yeasts.
• Budding: The protrusion grows out into a bud. The bud constricts at the bases and separates into a new individuals g., yeasts. Sometime a number of buds may be seen attached to the parent cell, e.g., Torula.
• Chlamydospores: They are thick walled, black, multinucleate resting spores formed by the collection of protoplasm at one or many places from the adjacent places. They are mostly intercalary and after the separation, germinate to produce new mycelia, e.g., Rhizopus, Mucor, Ustilago,
• Oidia (Arthrospores): Oidia are thin walled structures produced in chain. They are formed by the segmentation of hypha in the presence of excess water, sugar and salts. They germinate immediately after liberation and produce new mycelia, e.g., The oidia formation in Mucor represents the torula stage.

Asexual Reproduction

• Asexual reproduction is accomplished by means of asexual spores formed by mitosis, so also called Asexual spores are of two types, motile (zoospores) and non-motile (aplanospores).

Zoospores

• The zoospores are thin walled flagellate spores produced inside a sac like structure known as zoosporangium.
• Zoospores may be uniflagellate or biflagellate. The flagella occur anteriorly, laterally or posteriorly, e.g., aquatic fungi.
• The biflagellate zoospore is of two types, primary and secondary. The primary zoospore is pyriform and has two anteriorally attached and forwardly directed flagella. The secondary zoospore, formed by the germination of primary zoospore, is reniform and has two unequal laterally attached and oppositely directed flagella. The phenomenon of the occurrence of two types of zoospores in the same asexual life cycle of a fungus is called diplantismand the fungus is called diplantic,e.g.,Saprolegnia.

Aplanospores

• Aplanospores are of two types, sporangiosporesand conidia.
• Sporangiospores are thin walled, non-motile, uni-or multinucleate spores produced endogenously in sporangium. After dispersal and germination, they produce new mycelia, e.g., Rhizopus, Mucor, etc.
• Conidia (Gk. konis – dust, idion – diminutive) are non-motile, thick walled asexual spores produced exogenously at the tip of special hypha, called conidiophores, in a basipetal succession. They are produced either singly (e.g. Pythium, Phytophthora) or in chain (e.g. Penicillium, Aspergillus, etc). Conidia are dispersed by wind. On falling on suitable substratum, each conidium produces a new mycelium e.g., Penicillium, Aspergillus, etc.
• 

Sexual Reproduction

• True fungi reproduce by sexual method, but in the members of the fungi imperfecti(Deuteromycetes), sexual reproduction is not seen.
• The sexual stages are either absent or unknown. The sexual reproduction in true fungi is affected by the fusion of two nuclei of different parentage.

Process of Sexual Reproduction
During sexual reproduction three processes occur:

• Plasmogamy: Fusion of cytoplasm of two sex units of opposite strains (+ and –).
• Karyogamy: Fusion of nuclei to form synkaryon (2n).
• Meiosis: Reductional division of synkaryon to form haploid meiospores.
• Inhigher fungi, plasmogamy is not soon followed by karyogamy rather it gets delayed. Before fusion, they get paired to form dikaryon (n + n).
• In some fungi, these processes do not occur at specific time in the life cycle. This phenomenon is called parasexuality. It was first recorded by Pontecorvo and Roper (1952) in Aspergillus nidullans.

Methods of Sexual Reproduction
The various methods by which the compatible nuclei are brought together through plasmogamy are as follows:

• Gametangial copulation: Inthis method, the sexual fusion occurs between the entire contents of two gametangia in any of the following two ways:
• In some true fungi, particularly in the holocarpic forms, the gametangial copulation is affected by the transfer of entire contents of one gametangium into the other through a pore developed in the gametangial walls at the point of contact.
• In some true fungi, the two gametangia fuse with each other by the dissolution of contact walls, leading to the formation of a common cell in which the contents of the two gametangia mix.
• Gametangial contact:
  In many true fungi, the male and female gametangia come into contact with each other. The male nucleus or nuclei from male gametangium (antheridium) are directly transferred into the female gametangium (oogonium), either through a pore formed in the common wall at the point of contact, or through one or more fertilization tubes that arise from the antheridium. The fertilization then takes place by the fusion of male nucleus with the egg inside the oogonium. After the fertilization, either both the gametangia (i.e., antheridium and oogonium) or only the male gametangium (antheridium) may eventually collapse.

      •  Spermatization: In some true fungi, numerous uninucleate, unicellular, non-motile            male cells called spermatia(sign. spermatium), are borne externally in various ways             on the spermatiophores.

• Somatogamy: In most of the higher true fungi, the sex organs are completely lacking. In such true fungi, the sexual reproduction consists in the anastomosis of somatic hyphae bearing the nuclei of different parentage.

Types of sexual reproduction

There are three types of sexual reproduction-isogamy, anisogamy and oogamy.

• Isogamy: In isogamy, both fusing gametes are morphologically and physiologically similar.
• Anisogamy: In anisogamy, the fusing gametes are morphologically dissimilar and physiologically similar.
• Oogamy: In oogamy, the fusing gametes are both morphologically and physiologically dissimilar.

Sex organs

• There are two types of sex organs (gametangia) male and female. The male sex organ is called antheridiumwhile the female sex organ is called oogonium(in the members of Oomycetes) or ascogonium(in the members of Ascomycetes). There is no development of sex organ in the members of Basidiomycetes, though they reproduce sexually by somatogamy. There is no sexual reproduction at all in the members of Deuteromycetes, so they are called Fungi Imperfecti.

Classification of Fungi

• A number of criteria are used for classification of fungi. The important ones are morphology of vegetative structure, morphology of reproductive structure, types of meiospores, types of mitospores, life cycle, physiology, biochemistry, etc.
• The major groups of fungi are as follows:

CHARACTERISTIC FEATURE OF MAJOR GROUPS

Phycomycetes (the algal fungi)

• They are found on decaying leaves and damp places and also as parasites.
• Hyphal wall contains cellulose and other glycans in many members. In some cases chitin or fungus cellulose is also present.
• The mycelium is aseptate coenocytic (multinucleate).
• Asexual reproduction is through zoospores as well as aplanospores. Both the kinds of spores are produced in sporangia.
• Zoospores are generally biflagellate.
• The sexual reproduction takes place by fusion of similar (isogamy) and dissimilar (oogamy) gametes. The method of sexual reproduction is gametangial contact.
• Gametes are usually non-flagellate.
• The common members are Albugo, Phytophthora,

Zygomycetes (the conjugation fungi)

• It is a class of terrestrial fungi which are mostly saprotrophic and rarely parasitic.
• Hyphal wall contains chitin or fungus cellulose.
• The mycelium is coenocytic (multinucleate, aseptate) like the one found in phycomycetes.
• Motile cells (zoospores or planogametes) are absent.
• Mitospores are non motile.
• The method of sexual reproduction is gametangial copulation.
• The gametes are commonly multinucleate and are called
• Sexual reproduction produces a resting diploid spore called Because of the presence of zygospore, the group of fungi is called Zygomycetes.
• Zygospore does not give rise to new mycelium directly. Instead it produces a new sporangium called germ sporangium(previously called zygosporangium). Germ sporangium forms meiospores called germ spores, which later develop new mycelia.
• The common members are Rhizopus, Mucor,

Ascomycetes (the sac fungi)

• They include pigmented moulds (brown, green, blue, pink), powdery mildews, yeasts, cup fungi, morels and truffles.
• The mycelium consists of separate hyphae. Yeasts are an exception in that they are basically unicellular. Cell wall contains
• Motile structures do not occur in the life cycle.
• In majority of Ascomycetes, the common mode of asexual reproduction is through the formation of conidia, formed in chains exogenously.
• Conidia are borne on branched or unbranched hyphae called They detach from the parent and form new mycelia.
• Sexual reproduction takes place through fusion of sex cells, gametangial contact between an antheridium and ascogonium, and autogamy.
• Sexual reproduction is through (ascospore) which are formed endogenously in a sac like structure called ascus(pl. asci).
• The gametes involved in sexual reproduction are non-motile, compatible and are generally represented as (+) and (–) The fusion of gametes is followed by reductional division that produces ascospores, which on germination gives rise to new mycelia.
• The asci may occur freely or get aggregated into specific fruiting bodies called Ascocarps are of many types: apothecium (cup like, e.g., Peziza), perithecium (flask shaped e.g., Neurospora) or cleistothecium (closed e.g., Penicillium). The fructifications of some Ascomycetes are edible, e.g., morels, truffles, etc.
• Yeast, Penicillium, Aspergillus and Claviceps are common examples.

Basidiomycetes (the club fungi)

• They are the most advanced and most commonly seen fungi as their fructifications are often large and conspicuous, g., mushrooms, toadstools, puffballs, bracket fungi, etc.
• Basidiomycetes are among the best decomposers of wood. They are able to decompose both cellulose and lignin.
• Motile structures or cells are absent.
• Mycelia are of two types, primary and secondary. Primary mycelium contains monokaryotic cells, i.e., cells with single haploid nuclei.
• Monokaryotic phase or primary mycelium may multiply by oidia, conidia like spores and pycniospores.
• Sexual reproduction does not involve sex organs. Instead fusion occurs between guminating basidiospores and other monokaryotic spores, between a spore or spermatium and a receptive hypha (spermatisation) or between two hyphal cells of primary mycelia (somatogamy).
• There is often differentiation of two mating types, (+) and (–).
• Sexual reproduction actually causes plasmogamy or fusion of protoplasts without fusion of their nuclei.
• Karyogamy is delayed for long. The intervening phase is called dikaryophase. It produces a new mycelium called secondary mycelium.
• Secondary mycelium is long lived. It consists of profusely branched septate hyphae in which the septa possess dolipore or central pore with barrel shaped outgrowths.
• Dikaryophase or secondary mycelium may multiply by different types of spores chlamydospores, aecidiospores, uredospores, teleutospores, etc.
• Karyogamy and meiosis occur in club shaped structures known as basidia(singular: basidium). The name of the class is given after them.
• A basidium commonly produces four meiospores or basidiospores exogenously at the tips of fine outgrowths called sterigmata.

Deuteromycetes (the fungi imperfectii)

• Deuteromycetes is an artificial class of fungi that has been created to include all those fungi in which sexual stage is not known or they have not been classified and placed anywhere else as yet.
• The mycelium is usually septate. Coenocytic forms are not known.
• Asexual reproduction often occurs by conidia along with some other types of spores. In some cases even asexual spores are absent.
• It is believed that most members of deuteromycetes are actually ascomycetous fungi in which sexual reproduction is either absent or yet to be discovered.
• Common examples are Fusarium, Colletotrichum, Alternaria,

Economic importance of fungi

• Mushroom: Agaricus campestris is a common mushroom with edible basidiocarps. Agaricus brunnescens (= bisporus) is cultivated. Honey mushroom (Armillari ella= Armillaria mellea) is edible but is a root parasite on many trees. Psilocybe mexicana (sacred mushroom) is hallucinating like LSD. It is used by Mexican Indians for certain religious ceremonies.
• Toadstools: A poisonous mushroom is called It often possesses white basidiospores, e.g., Amanita palloides/A. caesarea(death cup/Caesar’s mushroom). The latter was used for killing Roman emperor Claudius Caesar by his wife.
• Bracket or shelf fungi: They are Basidiomycetes whose basidiocarps appear on tree trunks, logs, lumber, just as bracket or shelves (lignocolous or epixylic) e.g., Ganoderma, Fames applantus (perennial), Polyporus sulphureus (annual).
• Puffballs: They are edible in the young stage but send puffs of spores on ripening, g., Lycoperdon, Clavatia. The latter has anticancer properties.
• Smuts: Smuts are pathogenic basidiomycetes that possess thick walled, black coloured resting spores called chlamydospores, teleutospores or smut spores. Smuts are of two types, loose and covered. In loose smuts, the spores are exposed from the beginning, g., loose smut of wheat (Ustilago tritici). In covered smuts, the spores remain covered till before liberation, e.g., com smut (Ustilago maydis, grains become very large gall like), bunt of wheat (Tilletia tritici), whip smut of sugarcane (U. scitaminae).
• Rusts: The pathogenic basidiomycetes produce rusty pustules, g., Puccinia graminis tritici (black rust of wheat). Puccinia completes its life cycle on two hosts, wheat and barberry, Such a parasite is called heteroecious.Indian scientist K.C. Mehta is famous for his work on black rust of wheat. The spores produced on wheat are uredospores (stage II) and teleutospores (stage III). Uredospores can re-infect wheat but teleutospores cannot do it. Instead they give rise to basidia (stage IV). Basidiospores infect barberry. Pycnidia (stage I) develop on the upper surface of barberry leaves where dikaryotization occurs. It gives rise to aecidial stage (stage 0). Aecidia develop on the lower surface of barberry leaves. They form aecidiospores that infect wheat.
• Predator fungi: The fungi trap small nematodes and other animals to obtain nourishment from them, g., Dactyella bembicoides, D. ellipsospora.
• Stinkhorn:Phallus impudicus (dead man’s finger). Stinking odour is due to spore mass that is attractant to flies.

Other Economic Importance

• Vitamins: Many fungus metabolites are the rich sources of vitamins that are used as nutritional supplement and in medical therapy.
• Medicines: Fungi have explored new field in medicine by producing antibiotics and several other useful components.
• Antibiotics: Antibiotics are defined as substance of biological origin that at low concentration can inhibit the growth of bacteria and other microorganisms. In 1991, Sir Alexander Fleming, for the first time, discovered the antibiotic penicillin from the fungal colony of Penicillium notatum. Raper (1952) also extracted the same antibiotic from P chrysogenum. Since then, many compounds with antibiotic actions have been isolated. A list of some common antibiotics obtained from fungi is given in table.

Table: Antibiotics produced by fungi

Antibiotics	Source	Biological activity
Penicillin	Penicillium notatum	Active against Gram (+)ve bacteria
Griseofulvin	Penicillium griseofulvum	Antifungal
	P. nigricans
ephalosporin	Emericellopsis minimum	Antibacterial
Viridin	Gliocladium vilens	Antifungal
Fumagillin	Aspergillus fumigatus	Amoebocide
Fumigatin	A. fumigatus	Antibacterial
Giiotoxin	Gliocladium virens	Antibacterial and antifungal

• Disease of plants: Fungi destroy many agricultural crops, fruits, nut plants and shade trees. When a disease spreads in epidemic form, it wipes out the whole crop and causes severe losses. Some of the important plant diseases are given in table.

Table: Fungal disease of plants

Disease	Casual organism
Black or stern rust of wheat	Puccinia graminis tritici
Brown or leaf rust of wheat	Puccinia recondita tritici
Yellow or stripe rust of wheat	Puccinia striiformis
Loose smut of wheat	Ustilago nuda tritici
Flag smut of wheat	Phythium graminicolum
Powdery mildew of wheat	Erysiphe graminis tritici
Bunt of wheat	Tilletia foetida
Leaf smut of rice	Entyloma oryzae
Covered smut of oat	Ustilago kolleri
Loose smut of barley	Ustilago nuda hordei
Loose smut of oat	Ustilago avenae
Downy mildew of jowar	Sclerospora sorghii
Green ear of bajra	Sphacelotheca sorghii
Covered smut of jowar	Sphacelotheca cmenta
Green smut of jowar	Sclerospora graminicola
Red rot of sugarcane	Colletrotrichum falcatum
Cane smut	Ustilago scitaminae
Wilt of arhar	Fusarium udum
Early blight of potato	Alternaria solani
Late blight of potato	Phytophthora infestans
Wart disease of potato	Synchytrium endobioticum
Onion smut	Urocystis cepulae
Anthracnose of mango	Glomerella psidii
Blight of garlic	Alternaria palwldi
Leaf rust of coffee	Haemlia vestarix
Leaf rust of tea	Pestalozzia theae
White rust of crucifers	Albugo candida (=Cystpous candidus)
Root rot of pine	Poria monticola
Wilt of Acacia	Fusarium vasinfectum
Wilt of cotton	Fusarium spp.
Stem rot of jute	Macrophoma corchori
Leaf spot of rice	Helminthosporium oryzae

• Disease of fishes:Saprolegnia and Achlya are the common parasites of fishes. Saprolegnia ferax and S. parasitica infect the fishes of the domestic aquaria.
• Hallucinogenic fungi: A number of fungi affect the nervous system in such a manner that the consumer has hallucinations. Amanita muscaria and Psilocybe mexicana are the common hallucinogenic fungi.
• Fungi as allergens: A large number of fungi, e.g., Aspergillus, Penicillium, Cladosporium, Alternaria, Fusarium, Cephalosporium, Trichoderma, Chaetomium, Mucor, Rhizopus, Claviceps, the yeasts, the rusts and smuts are responsible for causing various types of allergic disease in man.

Rhizopus: Life Cycle

Occurrence

• Rhizopus is a common and widely distributed saprophytic fungus belonging to the family Mucoraceae. The genus includes about 35 species of which Rhizopus stolonifer is one of the most common representative. It is commonly called bread mould.

Vegetative structure

• The vegetative body is a coenocytic mycelium that forms white cottony growth on the substratum. The mycelium is differentiated into three kinds of hyphae- rhizoidal hyphae, stolons and The mycelium is multinucleate and non septate. These septa are, however, formed in the older hyphae and at the time of formation of reproductive structures. The cells of the young hyphae contain granular protoplasm in which are embedded numerous nuclei, glycogen and oil globules. The hyphal wall is composed of fungal chitin.

Reproduction

• It reproduces asexually as well as sexually.

Asexual Reproduction

• Asexual reproduction takes place by means of non motile spores that are produced within sporangia, borne at the apex of unbranched erect hyphae called sporangiophores.
• Under suitable conditions, certain vertical hyphae grow up in clusters from the mycelium. These unbranched upright hyphae are called The tip of each sporangiophore swells to produce a globose sporangium. A considerable amount of cytoplasm, with nuclei and food materials migrate into this enlarged tip. The tip of the sporangiophore enlarges and the nuclei undergo repeated mitotic divisions. The protoplasmic contents accumulate densely in the peripheral portion of the developing sporangium, leaving the central much less dense portion with few nuclei and vacuolated protoplasm. A layer of vacuoles now appears between the peripheral and central portions. These vacuoles unite laterally with one another to form a dome shaped septum.

• This separates the sporangium into a peripheral fertile portion and an inner dome shaped sterile portion, known as the columella. Within the protoplasm of the peripheral fertile portion, cleavage furrows begin to start. These cleavage furrows divide the protoplasm into numerous small pieces, each with several nuclei. Each piece secretes a wall and becomes a spore.
• At maturity, the wall of sporangium becomes fragile and is disintegrated by the slightest disturbance of air current. The spores, which form a dry powdery mass, are scattered by air currents as the sporangium raptures. The columella usually persists at the end of the sporangiophore even after the bursting of the sporangium.
• The spores are small, dark coloured and globose to ellipsoidal in shape. Upon reaching a suitable substratum, each spore germinates by a germ tube, which gives rise to a new mycelium.

Sexual reproduction

• Sexual reproduction occurs by the conjugation of two gametangia. Some species of Rhizopus are homothallic, requiring hyphae of only one mating type (e.g., Rhizopus sexualis) while other are heterothallic, requiring opposite mating types (e.g., R.stolonifer).
• In heterothallic species, g., Rhizopus stolonifer, the sexual reproduction occurs when two hyphae of opposite strains come in close proximity to each other. From these hyphae, develop short lateral branches, the progametangia, the tips of which grow and ultimately come in contact. A considerable amount of cytoplasm with numerous nuclei, mitochondria and food materials migrate into these progametangia. At the end of each progametangium, a multinucleate segment is cut off by a cross wall from the remainder of the branch. Each such segment that has densely multinucleate protoplast is called gametangium, and the remainder of the branch that has more vacuolate protoplast is called suspensor. The wall between the two gametangia dissolves, their protoplasmic contents mingle and the opposite nuclei fuse in pairs to form a multinucleate zygospore.

• The nuclei that fail to fuse become disorganized. The resulting zygospore enlarges and develops a dark, thick waited wall around itself to become a resting structure. It undergoes a period of dormancy for many months.
• During germination, the outer wall splits and the inner wall grows out to form a hyphae like germ tube or promycelium. It grows erect to form a sporangiophore, bearing a terminal-sporangium. Such a sporangium is also known as germ sporangiumor zygosporangium. Meiosis occurs during the germination of zygospore so that the germ spores, which are produced in the germ sporangium, are all haploid.
• The germ spores that are produced in the germ sporangia are either all (+), or all (–), or a mixture of both types (+) and (–). The germ spores on reaching a suitable substratum germinate to produce new hyphae.
• Sometimes, zygospores are produced parthenogenetically. Such parthenogenetically produced zygospores are called azygospores. Occasionally, the gametangia fail to fuse. The gametangia become surrounded by a thick wall resulting in the formation of azygospore.

The Yeast: Life Cycle

Occurrence

• The yeasts are widespread in their distribution. They are frequently found growing saprophytically on substrates that contain sugar, such as decaying vegetables, ripe fruits and grains, sugary exudates of trees and nectar of flowers. They are also found in soil containing abundance of humus, in decaying organic matter, and milk product. Some of the important genera of yeast are Saccharomycoides. The yeasts are well known for their ability to ferment sugar with the production of carbon dioxide and alcohol. The beer, wine and alcohol yeasts are included in the germs Saccharomyces. Yeasts are also important as a valuable source of vitamins. A few species are parasitic and cause disease of plant, animals and human beings. The genus includes a number of species, of which, S. cerevisiae is the most important. Some common Indian species are: S. cerevisiae, S. apiculatus. S. fructuum and Schizosaccharomyces octosporus.

Vegetative structure

• The vegetative body is not composed of hyphae. It is a unicellular fungus, but the cells may often be united in chains, thus forming filaments or pseudomycelium. The yeast cells are very polymorphic and are capable of assuming different form and shape, depending upon the medium in which they grow.

• Single yeast cells are colourless, but when grown on solid media, colonies are formed by yeast cells that may be white, cream coloured. They are normally spherical, oval or shortly cylindrical. Each yeast cell is small and ranges in diameter from 5-10 . The cell wall is thin, delicate and is composed of chitin in combination of other compounds such as carbohydrates, mannan and glucan. The cell wall of S. cerevisiae is made up of two layers an outer dense layer, about 0.05 . thick, and an inner dense layer, about 0.2 thick, containing microfibrils. Enclosed within cell wall, is the granular cytoplasm that in the old cells can be differentiated into an outer ectoplasm and inner endoplasm. Embedded in the cytoplasm, are the mitochondria, endoplasmic reticulum and ribosomes. The reserve food materials in the cytoplasm are in the form of glycogen, oil globules and protein particles. The cell contains a large central hyaline area and small deeply staining body on one side of it. These two structures have been differently interpreted by different scientists.

Reproduction

• Yeasts reproduce both asexually as well as sexually.

Asexual reproduction

• Asexually, they reproduce either by budding, in which numerous buds are given off by mother cells, or by fission, in which daughter cells of equal size are produced by divisions of mother cells. On this basis, the yeasts are grouped as: Zygosaccharomyces, the budding yeasts, and Schizosaccharomyces, the fission yeasts.

Budding

• When food supply is abundant, the yeast cells reproduce rapidly by a peculiar process, known as During this process, the nucleus of mother cell divides mitotically, but according to some scientists, the process is amitotic. Soon, a small protuberance appears on the surface of the vegetative cell in the form of a bud, and into this, passes one of the nuclei. It is then separated from mother cell by the formation of a wall between the two cells, thus forming a new individual, and the process may be repeated indefinitely. Under conditions of rapid growth, the daughter cell may itself begin to bud while still attached to the parent cell. In this way short chains of cells loosely jointed together may be formed. These chains of cells are called pseudomycelium. Sometimes, the pseudomycelium is also branched.

Fission

• In Schizosaccharomyces, during reproduction the parental cell elongates and its nucleus divides to produce two daughter nuclei. Gradually, a partition wall is formed just in the middle of cell, dividing it into equal or nearly equal halves. The two daughter nuclei so formed may remain linked together for some time and repeat the process or they may separate soon and then divide.

Sexual Reproduction

• The sexual reproduction takes place when the food supply is scanty. For the purpose of sexual reproduction, no specialized sex organs are formed. It occurs by conjugation of either two similar or dissimilar haploid vegetative cells or gametangia – the process is called This result in the formation of diploid zygote which functions directly as an ascus and produce ascospores.
• The number of ascospores per ascus varies with the species. The shape of the ascospore is also variable. They are generally globose or ovoid as in Saccharomyces, Schizosaccharomyces and

Life Cycle

• There is considerable variation in the overall life cycle pattern. There may be long haploid stage with a transient diploid stage, a long diploid stage with a transient haploid stage, or a haploid and diploid stage of about equal length. These variations can be observed by studying the life cycles of three yeasts. The three types of life cycle are referred to as: (i) Haplontic, (ii) Diplontic, and (iii) Haplodiplontic.
• Haplontic life cycle: This type of life cycle is represented by Schizosaccharomyces octosporus. In sexual reproduction, two haploid somatic cells behave as potential gametangia. Two adjacent cells of similar size put out short beak like protuberances that come in contact and fuse forming a continuous passage. This passage is called as ‘conjugation tube. The nucleus from each cell passes into the conjugation tube so formed, and there they fuse. Gradually, the two uniting cells along with the conjugation tube form zygote.

• The diploid zygote nucleus undergoes three successive divisions, of which the first is meiotic, producing usually eight haploid nuclei, around which ascospores become organized. The zygote cell at this stage becomes the ascus.
• The ascospores are liberated by the rupture of the ascus wall. These ascospores behave as somatic cells and begin to reproduce by budding or fission, producing indefinite numbers of cell generations. In this type of life cycle, the diploid phase is very short and is confined only to zygote that undergoes meiosis immediately after karyogamy, while haploid phase is very much elaborated.
• Diplontic: In this type of life cycle that is represented by Saccharomyces ludwigii, the diploid phase is long while haploid phase is very short..

• The vegetative cells are diploid in these forms which multiply extensively by budding, producing number of diploid cells. These diploid somatic cells enlarge and function as asci. After some time, the diploid cell divides meiotically to produce four haploid nuclei around which four ascospores are developed. These ascospores are the only haploid cells in the life cycle. The four ascospores do not come out of the ascus, but fuse in pairs to produce two diploid zygotic cells. Each diploid cell germinates by producing a germ tube that pushes out through the ascus wall and acts as a sprout mycelium. From sprout mycelium diploid cells are budded off.
• Haplodiplobiontic: In this type of life cycle, the haploid and diploid phases are equally well represented showing distinct alternation of generations. This type of life cycle is represented by Saccharomyces cerevisiae

• During the life cycle, the two haploid cells unite to form a diploid cell. The diploid cells multiply by budding and produce a large number of diploid cells. Each of the diploid cells behaves as an ascus; its nucleus divides meiotically to produce four haploid nuclei. Each of these nuclei organizes into an ascospore. Ascospores on liberation from ascus multiply by budding to produce haploid cells.

5. VIRUSES & VIROIDS

Viruses

• Viruses are the most primitive, non-cellular and non-cytoplasmic infectious agents. Adolf Meyer (1886) of Germany discovered the viral disease, tobacco mosaic. J. Ivanowski (1892) discovered the viruses in an extract of tobacco plant infected with tobacco mosaic virus (TMV) and found that this extract could infect healthy plants even after it was passed through a filter that checks bacteria. M. W. Beijerinck (1898) named it “contagium vivum fluidum“, i.e., living infectious fluid. Louis Pasteur coined the tum Virus.

• M. Stanley (1935) isolated pure crystals of tobacco mosaic virus. Bawden and Pirie (1937) first of all studied the chemical nature of viruses and concluded that they were nucleoprotein.
• The electron microscopic structure of viruses revealed the fact the crystals of viruses are composed of numerous individual complex units referred to as virions. Virions are the basic structural unit of a virus that is capable of infecting a specific host.

General characteristics of virus

• Viruses are non-cellular, non cytoplasmic, infectious agents.
• They are smaller than bacteria, and thus can pass through bacteria proof filter.
• Viruses are transmissible from diseased to healthy organisms.
• All viruses are obligate parasites and can multiply only within the living host cells.
• Viruses contain only a single type of nucleic acid, either DNA or RNA.
• Viruses are host specific in that they infect only a single species and definite cells of the host organisms.
• Viruses are effective in very small doses. Most of them are highly resistant to germicides and extremes of physical conditions.

General structure of viruses

Shape and size

• The shape of different types of viruses varies considerably. They may be spherical or golf ball like (poliovirus, herpes virus), rod shape (tobacco mosaic virus, TMV), tadpole like (bacteriophages), helical (influenza virus) or polyhedral (adenovirus).
• The size ranges from 10 nm (virus of foot and mouth diseases of cattle) to 300 nm (smallpox virus: Variola and TMV). Plant viruses, in general, are smaller than bacterial or animal viruses.

Chemical structure and composition

• Viruses have a very simple structure. A virus is made of a nucleic acid core and protein coat. A fully assemble particle, i.e., a virion, is capable of infecting the host.

Nucleic acid

• A virion always contains only a single kind of nucleic acid, i.e., either DNA or RNA. The nucleic acid may occur as single or double stranded. Generally in plant viruses, RNA is present but in cauliflower mosaic virus (CFMV), DNA is present. Generally in animal viruses, DNA is present, except influenza virus, rous sarcoma virus, AIDS virus, mumps virus, poliomyelitis virus, reovirus, etc., where RNA is present. The genomes of viruses may have anything from three to over 200 genes.
• The infectious property of a virion is due to its nucleic acid. A host cell can synthesize complete virion if only free viral nucleic acid is injected within the cytoplasm of a living host cell.

Capsid or protein coat

• The protein coat is called capsid. It is present generally when the virus is outside host cells. It is made up of many identical protein sub units called capsomeres. The capsomeres are composed of either one or several types of proteins. Host specificity of viruses is due to the proteins of the capsid. In a virus particle, the capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus.
• Some large virus particles (i.e., virions) have an additional covering of lipids or lipoproteins outside the capsid. Such virions are called enveloped(e.g., influenza virus, mumps virus) and those without this additional covering are referred to as naked(e.g., TMV). Envelope is not made by the virus itself. It is a modified fragment of plasma membrane of the last cell invaded by the virus.

Virus	Type of nucleic acid
Herpes	Double stranded DNA
Chicken pox	Double stranded DNA
Hepatitis B	Double stranded DNA
Cyanophages	Single stranded DNA
	Single stranded DNA
Influenza virus	Single stranded DNA
HIV	Single stranded RNA
Polio	Single stranded RNA
TMV	Single stranded RNA
Mycophages Reovirus	Double stranded RNA
Wound tumour virus	Double stranded RNA
Rice dwarf virus	Double stranded RNA
Cauliflower Mosaic virus	Double standed DNA

International Committee to Virus Nomenclature has given a system of naming the virus. The system consists of two parts. The first part is the common name of virus and second part contains the coded information about the virus. The second part is known as cryptogram(Gibbs and Harrison, 1968). It contains following four pairs:

• First pair: Represents type of nucleic acid/number of strands in nucleic acid.
• Second pair: Represents molecular weight of nucleic acid (in millions daltons)/amount of nucleic acid expressed as percentage.
• Third pair: Denotes the shape of virus/shape of nucleoprotein.
• Fourth pair: Denotes the type of host/carrier used in the transmission of virus, e.g. cryptogram of TMV is R/l : 2/5 : E/E : S/O.It means it contains RNA single stranded (ssRNA); molecular weight of RNA is 2 millions, and it makes 5% of the viral particle and nucleocapsid are elongated with parallel sides; it infects seed plants and vector or carrier is absent.

Structure of Tobacco Mosaic Virus (TMV-A Plant Virus)

• Tobacco mosaic virus is the best-known and most thoroughly studied plant virus. It was discovered by Ivanowski (1892) and crystallized by W. M. Stanley (1935).
• TMV is rod shaped, measuring approximately 3000Å in length and 180Å in diameter; total molecular weight being 39.4×10⁶. It is made up of RNA and proteins. The RNA is single stranded and is helically coiled around the central hollow axis of the rod. It extends throughout the length of the virus particle. The single stranded RNA is 330 nm in length and has 7300 nucleotides. RNA alone is capable of causing infection. The protein coat or capsid is made up of approximately 2130 identical sub units, the capsomeres. These are arranged in a compact helical manner around the central axis. The ratio of nucleotides : capsomeres is 3 : 1.
• The TMV is of naked type, i.e. the protein coat (capsid) is not surrounded by an additional covering.
• TMV infects a wide range of flowering plants. Symptoms vary with host species and include stunted growth, mottling and distortion of leaves. Infections spread through the plant. It can be transmitted by aphids and other insects, which feed on phloem sap.

Structure Of Bacteriophage (The Bacterial Virus)

• Viruses that grow as intracellular parasites of bacteria are called bacteriophages, which literally mean eaters of bacteria. These were discovered independently by Frederick W. Twort (1915) and Felix deHerelle (1917).
• They are also composed of protein and a nucleic acid, but differ from other viruses in having bacteria as their host cells.
• Of all the bacteriophages, T series (characterized by the presence of tail) is the most extensively studied. It infects Escherichia coli. It has tadpole like shape with a hexagonal head and tail. The length of head and tail is almost equal (approximately 950Å each). The head is 650Å in diameter; it has a protein in which a double stranded DNA is tightly packed. The cylindrical tail is made entirely of proteins. It has a core tube (80Å in diameters) filled with lysozyme type of enzymes that probably help in the penetration of tail in the susceptible cell. Basal end of the tail has a hexagonal spiked end plate. At each corner of the hexagonal plate there is a thin tail fibre that measures 1500Å in length. The tail fibres and the end of plate help in the attachment of phage particle to the bacterial cell. The upper end of the tail is attached to the head by a narrow neck.
• The core tube of tail is surrounded by a contractile sheath. It is attached to the neck by a. collar. A complete phage particle has almost equal amounts of DNA (40-50%) and proteins (50-60%).

Influenza Virus (An Animal Virus)

• Influenza viruses are spherical in shape measuring 800-1200Å in diameter. The protein capsid encloses a single stranded RNA. Proteins constitute approximately 90% of the virus particle while the rest (10%) is RNA.
• The nucleocapsid (nucleic acid + protein coat) is enclosed in an envelope of lipoproteins. The envelope of influenza virus is characterized by the presence of a projection, called spikes, on its surface, made of haemoagglutinin, a protein that agglutinates the red blood corpuscels (RBC) of the host cell.
• It can withstand low temperature and is killed at 65°C. Infection causes fever, sneezing and cough. The particles usually multiply in respiratory tract.

Some other types of viruses

Cyanophages

• These are viruses that parasitize a wide range of blue green algae (cyanobacteria). Cyanophages were first discovered by Safferman and Morris (1963). The nucleic acid of all the cyanophages investigated were found to be double stranded DNA. They have a head similar to T -phages and a long helical tail. The phage described by Safferman and Morris is found as parasites in three algal forms namely Lyngbya, Phormidium and Therefore, this cyanophage has been named as LPP-l.

Mycophage

• Viruses parasitizing fungi (mycophages) are also common. These were first discovered in Agaricus bisporus (mushroom) by Sinden (1957). The mycophages are polyhedral or spherical and contain double stranded RNA. .

Reproduction and Transmission in Viruses

• Viruses do not have their own metabolic activity and, hence, use cellular ATP, ribosomes, tRNA and certain biosynthetic products of the host cell. Thus, viruses multiply only inside the host cell. During multiplication, nucleic acid of the virus is replicated and the proteins required for the viral coat are also formed. The process of multiplication can be explained by bacteriophages with the following major steps:
• The phage attaches itself to the wall surface of the bacterium with the help of tail fibres and end plate.
• The tail sheath contracts and the phage DNA is injected into the bacterial cell. The empty protein coat, called ghost,remains attached to the bacterium for some time.
• Phage DNA now takes over the protein synthesis machinery of bacterial host and inactivates the DNA of the bacterium.
• Phage DNA replicates and also synthesizes new proteins for its capsid. The protein coats are assembled spontaneously around phage DNA. This process continues till a large number of phage particles are formed.
• Ultimately the bacterial cell bursts by the process called lysis and new phage particles (200 to 1000) are liberated. These particles are capable for infecting other bacteria of the same type.
• The entire process of multiplication takes about 30 minutes. It is called latent period.
• The viruses are transmitted through various agencies, such as seeds, grafting, vegetative propagation, insects, mechanical means, nematodes, etc.

Alternative lytic and lysogenic pathways followed by bacteriophage after infecing a host

Viroids

Viroids are extremely simple infectious agents discovered in 1967 by Diener and Raymer. These consist of only very small RNA genomes 240-350 nucleotides long and no other structure (no protein coat). They are smallest known agents of infectious plant diseases.

• Diener and Raymer (1967) found that causal agent of potato spindle tuber disease (PSTD) was a free RNA (no nucleoprotein). Diener (1971) called it viroid. Since then a number of diseases like cadangof coconut and cucumber pale fruit have been found to be due to viroids.
• These viroids (RNA macromolecules) contain sufficient information needed to direct their own replication but totally depend upon host’s metabolic machinery.

Prions

• Prusiner al(1982) discovered infectious agents which they called prions. These are made of proteins only (no nucleic acid). These can multiply themselves and are infectious also. Their discovery has raised big question mark on the commonly accepted view that nucleic acids are the hereditary material. Professor Stanley Prusiner has been awarded 1997 Nobel Prize for Medicine for the study of Prions.
• Prions are the causal agents of scrapie disease (a degenerative disorder of the central nervous system) of sheep and goat. Affected animals lose coordination of their movements, are irritated to rub (scrap/scratch) their skin, and ultimately cannot walk. It develops slowly. Therefore, previously it was called slow virus disease.
• Kuru (a degenerative disease of central nervous system found in few tribes of New Guinea) and Creutzfeldt­ Jakob Disease (CJD, another such disease, cosmopolitan among middle-aged people) are caused by prions.

Economic importance of viruses

• The plant and animal viruses are economically important because of the diseases they cause.

Plant diseases.

• A variety of symptoms are caused by viral infections. These include local lesions, clearing of veins, mosaic formation, ring spotting, chlorosis, distortion, necrosis, breaking of blossoms, stunning and premature defoliation.

Table: Viral diseases of plants

Disease	Host
Rosette disease	Ground nut (Arachis hypogea)
Little leaf of urinal	Brinjal (Solanum melongena)
Yellow vein mosaic	Lady’s finger (Abelmoschus esculentus)
Potato leaf roll	Potato (Solanum tuberosum)
Leaf curl of papaya	Papaya (Carica papaya)
Bunchy top	Banana (Musa indica)
Grassy shoot	Sugarcane (Saccharum officinarum)
Tobacco mosaic	Tobacco (Nicotiana sp.)

Animal disease

• Diseases caused in mammals include small pox, chicken pox, poliomyelitis, influenza, measles, yellow fever, encephalitis, infectious hepatitis, viral bronchitis, viral enteritis, common cold, mumps, AIDS, etc.

Table: Viral diseases of man

Diseases	Virus
Influenza	A myxovirus, (DNA virus) three types-A, B and C, of varying severity.
Common cold	Large variety of viruses, most commonly rhinovirus (RNA virus).
Small pox	Variola virus (DNA virus), pox virus
Mumps	A paramyxovirus (RNA virus)
Measles	A paramyxovirus (RNA virus)
Yellow fever	An arbovirus that is arthropod borne virus (RNA virus).
Poliomyelitis	Poliovirus (RNA virus)