Ascomycetes-The sac fungi ”Biological Classification” Botany Notes Chapter 2, for Class 12-CBSE

Ascomycetes : The sac fungi

  1. The mycelium consists of septate hyphae (Yeasts are an exception in that they are basically unicellular).
  2. They are saprophytic, decomposers, parasitic or coprophilous (growing on dung).
  3. The septa possess central pores called septal pores. The pores allow communication and transport between adjacent cells.
  4. Cell wall contain 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., Penicillium Aspergillus.
  7. Female sex organ is called ascogonium.

8.Plasmongamy occurs by means of –

  1. 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).
  2. Karyogamy is delayed after plasmongamy. 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.
  3. Once a cell becomes dikaryotic, it transfers the nucleus to other cells by the crozier method (method of dikaryotization ) to make them dikaryotic.
  4. 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 each 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 (-).
  5. Ascopores may be arranged linearly (Neurospora) or unorderly (yeast).
  6. 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), cloased 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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