Current Affairs 11th Class

Blakeslee, (1904) while working with Mucor sp. observed that in some species sexual union was possible between two hyphae of the same mycelium, in others it occured between two hyphae derived from 'different' spores. He called the former phenomenon as homothallism and the latter as heterothallism. Thus, the homothallic species are self-fertile whereas the heterothallic are self sterile. In heterothallic species the two 'thalli' are sexually incompatible. They are said to belong to opposite strains. Blakeslee designated them as + and ? i.e., belonging to opposite strains or mating types. Bipolar heterothallism found in Mucor and Rhizopous.

(1) Phycomycetes (Oomycetes/Algal fungi) : It is also called lower fungi, mycelium is coenocytic. Hyphal wall may contain chitin or cellulose (e.g., Phytophthora). Asexual reproduction occurs with the help of conidio-sporangia. Under wet conditions they produce zoospores. Under dry conditions, the sporangia directly function as conidia. Zoospores have heterokont flagellation (one smooth, other tinsel). Sexual reproduction is oogamous. It occurs by gametangial contact where male nucleus enters the oogonium through a conjugation tube. The fertilized oogonium forms oospore. e.g., Saprolegnia, Albugo (Cystopus), Phytophthora, Phythium, Sclerospora, Peronospora, Plasmopara. (2) Zygomycetes (Conjugation fungi) : Mycelium is coenocytic. Hyphal wall contains chitin or fungal cellulose. Motile stage is absent. Spores (Sporangiospores/aplanospores) are born inside sporangia. Sexual reproduction involve fusion of coenogametes through conjugation (Gametangial copulation). It produces a resting diploid Zygospore. On germination, each zygospore forms a germ sporangium at the tip of a hypha called promycelium e.g., Mucor, Rhizopus, Pilobolus, Abscidia, Saksenea. (3) Ascomycetes (Sac fungi) : These are unicellular as well as multicellular fungi. In the latter, mycelium is septate. The asexual spores formed in chains are called conidia. The spores are formed exogenously, i.e., outside sporangium. They detach from the parent and form new mycelia. Sexual reproduction is through ascospores, which are formed endogenously (within the mycelium) in a sac like structure called ascus (pl. asci). The gametes involved in sexual reproduction are nonmotile compatible and are generally represented as + and –. The fusion of gametes is followed by reductional division that produces haploid ascospores. The fruiting body called ascocarp. The ascocarp are of four types : (i) Cleistothecium : It is an ovoid or spherical fruiting body which remains completely closed e.g., Aspergillus. (ii) Perithecium : It is a flask shaped fruiting body which opens by a single pore called ostiole. e.g., Neurospora. (iii) Apothecium : It is a saucer-shaped fruiting body. The asci constitute the fertile zone called hymenium e.g., Peziza. (iv) Ascostroma : It is not a distinct fruiting body. It lacks its own well defined wall. The asci arise directly with a cavity (locule) of stroma. It is also called as pseudothecium e.g., Mycosphaerella. (4) Basidiomycetes (Club fungi) : They are the most advanced fungi and best decomposers of wood. These are called club fungi because of a club shaped end of mycelium known as basidium. They have septate multinucleated mycelium. Septa possess central dolipores and Lateral clamp connections. The sexual spores called basidiospores are generally four in number. They are produced outside the body (exogenuous) unlike ascomycetes where they are endogenous. Two compatible nuclei fuse to form zygote, which undergoes meiosis and forms four basidiospores. The fruiting body containing basidia is a multicelular structure called basidiocarp. The common members are edible mushrooms (Agaricus), smut (Ustilago) and rust (Puccinia). (5) Deuteromycetes (Fungi imperfecti) : The group include all those fungi in which sexual or perfect stage is not known. Mycelium is made of septate hyphae. Asexual reproduction commonly occur by means of conidia. e.g., Alternaria solani, Fusarium, Gibberella, Colletotrichum falcatum, Helminthosporium oryzae, Cercospora personata.

The fungi are achlorophyllous organisms and hence they can not prepare their food. They live as heterotrophs i.e., as parasites and saprophytes. Some forms live symbiotically with other green forms. Parasites : They obtain their food from a living host. A parasite may be obligate or facultative. The obligate parasites thrive on a living host throughout their life. The facultative parasites are infact saprophytes which have secondarily become parasitic. Some forms produce rhizoids for absorbing food. The parasitic fungi produce appressoria for adhering to the host. For absorbing food, the obligate parasites produce haustoria. The haustoria may be finger-like, knob-like or branched. Each haustorium is distinguishable into a base, stem and body. Saprophytes : They derive their food from dead and decaying organic matter. The saprophytes may be obligate or facultative. An obligate saprophyte remains saprophytic throughout it's life. On the other hand, a facultative saprophyte is infect a parasite which has secondarily become saprophytic. Symbionts : Some fungal forms grow in symbiotic association with the green or blue-green algae and constitute the lichen. A few fungal forms grow in association with the roots of higher plants. This association is called as mycorrhiza. They are two types – Ectotrophic mycorrhiza and Endotrophic mycorrhiza e.g., (VAM).

(1) Vegetative reproduction Fragmentation : Some forms belonging to Ascomycotina and Basidiomycotina multiply by breakage of the mycelium. Budding : Some unicelled forms multiply by budding. A bud arises as a papilla on the parent cell and then after its enlargement separates into a completely independent entity. Fission : A few unicelled forms like yeasts and slime molds multiply by this process. (2) Asexual reproduction Oidia : In some mycelial forms the thallus breaks into its component cells. Each cell then rounds up into a structure called oidium (pl. oidia). They may germinate immediately to form the new mycelium, e.g., Rhizopus. Chlamydospores : Some fungi produce chlamydospores which are thick walled cells. They are intercalary in position. They are capable of forming a new plant on approach of favourable conditions, e.g., Rhizopus, Saprolengnia Ustilago. Sporangiospores : These are thin-walled, non-motile spores formed in a sporangium. They may be uni-or multinucleate. On account of their structure, they are also called as aplanospores. e.g., Rhizopus, Mucor. Zoospores : They are thin-walled flagellate spores which occur in aquatic fungi. These spores formed in a zoosporangium. The zoospores are of several types:

• Uniflagellate with whiplash type flagellum e.g., Allomyces.
• Uniflagellate with tinsel type flagellum e.g., Rhizidiomyces.
• Biflagellate with a tinsel type and a whiplash type flagella e.g., Saprolegnia.
• Biflagellate with two whiplash type flagella e.g., Plasmodiophora.

Conidia : In some fungi the spores are not formed inside a sporangium. They are born freely on the tips of special branches called conidiophores. The spores thus formed are called as conidia. e.g., Penicillium. Bi-celled conidia are formed in Trichothecium. In Fusarium it is possible to differentiate smaller microconidia from larger macroconidia. (3) Sexual reproduction : With the exception of Deuteromycetes (fungi imperfecti), the sexual reproduction is found in all groups of fungi. During sexual reproduction the compatible nuclei show a specific behaviour which is responsible for the onset of three distinct mycelial phases. The three phases of nuclear behaviour are as under : Plasmogamy : Fusion of two protoplasts. Karyogamy : Fusion of two nuclei. Meiosis : The reduction division. These three events are responsible for the arrival of the following three mycelial phases : Haplophase : As a result of meiosis the haploid (n) or haplophase mycelium is formed. Dikaryotic phase : The plasmogamy results in the formation of dikaryotic mycelium (n + n). Diplophase : As a result of karyogamy the diplophase mycelium (2n) is formed. In some fungi plasmogamy, karyogamy and meiosis do occur in a regular sequence but not at specified time or points in life cycle. Such a cycle is described as parasexual cycle and phenomena celled parasexuality recorded by Pontecorvo and Roper.  The fungi reproduce sexually by the following methods

• Planogametic copulation : In this type of copulation fusion occur between flagellate gamete. It is of three types :

(i) Isoplanogametic copulation : Both the fusion gametes are similar and flagellate. e.g., Synchytrium. (ii) Anisoplanogametic copulation : The fusing gametes are flagellate, more...