Ascomycetes

Members of the phylum Ascomycota are known as ascomycetes. Ascomycota is the largest phylum of the kingdom fungi and has around 64000 species.

They come under the subkingdom Dikarya (presence of dikaryon).

They produce sexual non-motile spores known as ascospores. They are produced in a sac-like structure known as an ascus. Each ascus contains 4-8 ascospores. They are commonly known as sac-fungi.

Ascomycetes have a diverse habitat and feeding habit, some are saprophytes whereas others are pathogen causing various diseases in animals and plants. Some of the ascomycetes are edible as mushrooms. Some of them live in a symbiotic association as lichens and mycorrhiza.

Common examples of ascomycetes include yeast, powdery mildews, cup fungi, morels, truffles, Neurospora, Aspergillus, Cladonia, Penicillium, Candida, Claviceps, etc.

Ascomycetes are economically very important. We get fermented food (bread, cheese, alcoholic beverages), antibiotics and various chemicals (Penicillin). Many species are used in biological studies and research (yeast, Neurospora). Morels and truffles are used as delicacies.

Ascomycetes are further classified based on the different structures of asci bearing structures and methods of the release of ascospores.

General Characteristics of Ascomycetes

  • One character that is present is most of the ascomycetes is a reproductive structure known as ascus or asci
  • Mostly they are terrestrial, parasitic or coprophilous
  • They are unicellular or multicellular fungi
  • The mycelium is made up of septate and branched hyphae
  • The cell wall is made up of chitin or ꞵ-glucans
  • There is cytoplasmic continuity due to septal pores
  • Asexual reproduction is by the formation of conidia exogenously on conidiophores
  • Yeast reproduces asexually by budding
  • Sexual reproduction is by conjugation between two gametangia. They are either homothallic or heterothallic
  • The fruiting body is known as ascocarp. There are four types of ascocarps:
    1. Cleistothecium- The fruiting body is spherical and remains tightly closed, e.g. Aspergillus
    2. Perithecium- The fruiting body is flask-shaped with one external opening, e.g. Neurospora
    3. Apothecium- The fruiting body is cup-shaped and asci are present in hymenium, e.g. Peziza
    4. Ascostroma- There is no differentiated fruiting body. Asci are present in the stroma, e.g. Mycosphaerella
  • Ascospores are produced endogenously in asci

Sexual Reproduction in Ascomycetes

  • Two different mating types hyphae come together and fuse
  • Plasmogamy takes place but it does not follow karyogamy immediately
  • The fused structure contains two haploid nuclei from each parent, i.e. dikaryon
  • New hyphae are produced with dikaryotic cells
  • At the tip of the hyphae, asci develop in the ascocarp
  • In each ascus, two nuclei fuse together (karyogamy) to form a diploid zygote
  • Formation of Ascospores: The diploid zygote undergoes meiosis to form 4 haploid nuclei, which undergo mitotic division to form 8 haploid nuclei. Each of the nuclei accumulates cytoplasm and a thick cell wall surrounds it. These are known as ascospores
  • Ascospores are released from asci through pore, slit or hinged lid and dispersed by air currents
  • Under favourable conditions, ascospores germinate to form new mycelia

Economic Importance of Ascomycetes

Ascomycota is the biggest group of fungi having diverse habitat. They are cosmopolitan and we derive many useful products from them. We get many foods, medicines and chemical products from them.

  • Entire brewing, bread and cheese making industry depends on yeast for fermented products
  • Who doesn’t know about antibiotic, Penicillin, which we get from Penicillium chrysogenum
  • Ciclosporin, an immunosuppressor is derived from the fungus Tolypocladium niveum. It is used in organ transplants and autoimmune diseases
  • Many organic acids and enzymes are produced by ascomycetes, e.g. citric acid, gluconic acid, amylases, proteases, etc.
  • Claviceps purpurea (Ergot) is used as medicine to stop excessive bleeding during menstrual periods and to speed up labour
  • Different kinds of cheese are prepared from different Penicillium species, e.g. Camembert, Brie, Roquefort, etc.
  • Aspergillus is used to prepare soy sauce and to prepare other Asian alcoholic beverages
  • Morels, Truffles and lobster mushroom are used as fungal delicacies
  • Neurospora, Saccharomyces, etc. are widely used to study genetics

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