Ascomycetes: The Sac Fungi | Structure & Characyeristics

 

1. The mycelium consists of septate hyphae. (Yeasts are an exception in that they are basically unicellular)
2. They are saprophytic, decomposers, parasiuc or copropnilous (grOwing on dung).
3. The septa possess central pores called septal pores. The pores allow communication and transport between adjacent cells.
4. Cell wall contains chitin.
5. Motile structures do not occur in the life cycle.
6. In majority of ascomycetes, the common mode of asexual reproduction is through the formation of conidia. Conidia are borne on branched or unbranched hyphae called conidiophores, e.g., Penicilium, Aspergillus.
7. Female sex organ is called ascogonium.
8. Plasmogamy occurs by means of
   (i) Gametangial contact (e.g., Pyronema)
   (ii) Conjugation (e.g., Yeast)
   (iii) Spermatization (e.g., Ascobolus)
   (iv) Somatogamy (e.g., Peziza) (v) Autogamy (e.g., Morchella).
9. Karyogamy is delayed after plasmogamy. A new transitional phase appears in the life cycle. It is called dikaryophase. The cells of dikaryophase are called dikaryotic cells. Each such cell possesses two different nuclei (Dikaryon). This forms a shorter phase of life cycle.
10. Once a cell becomes dikaryotic, it transfers the nucleus to other cells by the crozier method (method of dikaryotization) to make them dikaryotic.
11. Some dikaryotic cells function as ascus mother cells. This converts the cells into asci (singular ascus). Ascus is a sporangial sac peculiar to Ascomycetes. Ascus is the site of karyogamy and meiosis. 4 to 8 haploid meiospores named ascospores are produced endogenously in eacn ascus. In most of the cases, half the number of ascospores belong to one mating type(+) while the other half belong to the second mating type (–).
12. Ascospores may be arranged linearly (Neurospora) or unorderly (yeast)
13. The asci may occur freely or get aggregated into specific fructifications called ascocarps. Ascocarps are of many types: cup-like (apothecium, e.g., Peziza), flask-shaped (perithecium, e.g., Neurospora, Claviceps), elongated with a slit (hysterothecium), closed (cleistothecium, e.g., Penicillium) cushion like, chambered (Ascostroma, e.g., Pleospora). The fructifications of some ascomycetes are edible, e.g., morels, truffles.

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Members of this class are said to be our worst fungal enemies. (i) Morels (sponge mushroom) are Ascomycetes with edible ascocarps having fleshy sponge-like conical cap or pileus and a stalk like stipe, e.g., Morchella esculenta (vern. Guchhi), M. deliciosia. (ii) Truffles: They are edible tuber-like subterranean ascocarps, e.g., Tuber aestivum. (iii) Claviceps purpurea causes ergot of rye and C. microcephala causes ergot of bajra in which ears are filled with sclerotia of the fungus. Eating of infected cereals produces ergotism. (t produces a alkaloid called ergotine which causes abortion, if eaten, unknowingly). This is used as a drug to promote expulsion of foetus). (iv) Neurospora crassa (Pink bread mould), is often employed in biochemical and genetic work (experimental genetics), so is called Drosophila of plant kingdom. (v) Erisyphe: The fungus produces powdery mildew (fungal disease in which pathogen results in a powdery coating of spores on the surface of the host), e.g., Erysiphe graminicola. (vi) Penicilium chrysogenum is used in commercial production of the antibiotic penicillin. The later was the first commercial antibiotic. It was discovered from P. notatum. The fungus is employed in ripening of cheese, e.g., P roqueforti and P camemberti. (vii) Aspergillus: It is common green smoky mould which not only contaminates laboratory cultures (hence called weed of laboratory), but also various food stuffs including bread, butter etc. Aspergillus flavus is highly poisonous due to the presence of aflatoxins. A. oryzae is the source of diastase enzyme. (viii) Brewing Industry: Under anaerobic conditions, sugary solutions inoculated with yeasts are converted into alcoholic beverages, e.9., beer, Wine, cider, toddy. The two common yeasts used by brewing industry are Saccharomyces cerevisiae (Beer or Baker’s yeast) and S. ellipsoidens (Wine yeast). (ix) Gibberellins: They were first discovered in the extracts of Gibberella fujikuroi growing on rice (bakanae disease of rice). It is the perfect stage of fungus Fusarium monilifome (Deuteromycete). Gibberellins dre natural plant growth hormones.