In this article we will discuss about the features and significance of ascomycetes.
Salient Features of Ascomycetes:
(i) Most of the members are terrestrial, although a large number lives in fresh and marine waters. The majority of ascomycetes are saprophytic, some are parasites of insects and other animals, and some are responsible for causing destructive plant diseases. Some ascomycetes characteristically grow on dung and are popularly called coprophilous fungi (Peziza).
(ii) The members vary in their form and structure. Yeasts and other a few members (e.g. Taphrina) are unicellular, but almost all other members of this group have a well-developed, profusely branched, and septate mycelium with uni- or multinucleate cells and perforated septa.
(iii) In unicellular forms, the cell wall is composed of glucans and mannans, whereas in septate forms it consists of chitin and glucans.
(iv) Asexual reproduction takes place by various types of non-motile spores, such as oidia, chlamydospores, and conidia. In unicellular forms, fission, fragmentation, and budding are the most common methods of propagation.
(v) They are homothallic or heterothallic. In some heterothallic species, though male (antheridium) and female (ascogonium) sex organs develop on the thallus of the same strain, they are self-incompatible. In these species, male gamete of one mating type fertilises ascogonium of other mating type. This process is known as physiological heterothallism.
(vi) Sexual reproduction takes place by gametangial copulation (e.g., yeast), gametangial contact (e.g., Aspergillus, Penicillium, Erysiphe), somatogamy (e.g., Peziza, Morchella) or spermatization (e.g., Polystigma).
(vii) The sexual spore is haploid called ascospore, which is formed endogenously by free cell formation after karyogamy and meiosis within a sac-like or cylindrical structure referred to as an ascus. If the sexual spore of a fungus is an ascospore, the fungus is an Ascomycete regardless of any other character. This one character distinguishes Ascomycetes from all other fungi.
(viii) They show the phenomenon of heterokaryosis, i.e., the nuclei of two different genotypes are present in the same mycelium.
(ix) The fruiting bodies are known as ascomata (sing. ascoma; earlier called ascocarp). The ascomata are of four types cleistothecium (cleistothecial ascoma), perithecium (perithecial ascoma), apothecium (apothecial ascoma), and ascostroma (stromatic stroma) or pseudothecium.
Significance of Ascomycetes:
(i) Some Ascomycetes cause destructive plant diseases such as Dutch elm disease (Ceratocystis ulmi), ergot of rye (Claviceps purpurea), apple scab (Venturia inaequalis), powdery mildews of various crop plants (Erysiphe, Phyllactinia, Uncinula, etc.), and many others.
(ii) The most interesting Ascomycetes are the yeasts which are variously significant because they are involved in the production of many products of bakery and brewing industry. To appreciate their significance in human’s welfare, one must only smell and taste newly baked bread or an alcoholic drink.
(iii) Claviceps purpurea causes ergot disease. Ergot contains a variety of alkaloids including LSD (lysergic acid diethylamide), the well-known hellucinogenic drug of the day.
(iv) Penicillium spp. are involved in production of penicillin, the wonder drug.
(v) Many ascomycetous fungi are used in production of cheese. The Japanese and Chinese often use the members of ‘Aspergillus flavus-oryzae group’ to produce foods and important industrial alcohol.
(vi) Neurospora genetics laid down by Dodge provides a new approach of haploid genetics and biochemical genetics. This became possible only because the fungus Neurospora opened up a new way in the study of heredity.