Ascomycetes: Characteristics & Classification

 

Good to know

  • The imperfect stage of Penicillium in which it passes most part of its life cycle is sclerotium.
  • One celled ascomycetes is Saccharomyces cerevisae.
  • One celled lower fungi is Synchytrium endobioticum.
  • Ascomyectes- Powdery mildew.
  • Basidiomycetes- Usually no sexual reproduction.
  • Deuteromycetes- Imperfect fungi.
  • Imperfect stage- Asexual reproduction…anamorphic fungi…mitosporic fungi (e.g. Aspergillus)
  • Perfect stage- Sexual reproduction… teleomorphic fungi…meisosporic fungi (e.g. Eurotium )
  • The septal walls have septal pores which provide cytoplasmic continuity throughout the individual hyphae. Under appropriate conditions, nuclei may also migrate between septal compartments through the septal pores.
  • The sexual stage of Colletotrichum is Glomerella.

 

 

 

Ascomycetes

  • Also called sac fungi because of the way they reproduce.
  • Unifying feature is the presence of a specialized reproductive structure called ascus.
  • Largest phylum (Ascomycota) of fungi.

 

Salient features (by ma’am)

  • Mycelia septate. Exception: Saccharomyces
  • Profusely branched plant.
  • Sexual fruit bodies produced are called ascocarp; 8 ascospores are produced within an ascus.
  • Ascospores are endogenous in origin.

 

General characteristics

Habitat

Most of the members are terrestrial, although a large number lives in fresh and marine waters.

 

Nature of nutrition

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).

 

Structure

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.

 

Cell wall composition

In unicellular forms, the cell wall is composed of glucans and mannans, whereas in septate forms it consists of chitin and glucans.

Reproductive stages

Ascomycetes in general have two distinguished reproductive stages.

  1. Ascus or sexual stage (Perfect or ascigenous stage).
  2. Conidial or asexual stage (Imperfect stage).

 

Asexual reproduction

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. The asexual reproduction differs basing on the species and environmental conditions.

  • Fission and budding are methods of propagation normally encountered in the yeasts and in dimorphic ascomycetes.
  • Conidia often are important in propagating and disseminating species throughout the spring and summer with several generations being produced in a growing season.
  • Chlamydospores and sclerotia have primary function of surviving unfavourable conditions and they are characterised by thick walls. Reproduction is a secondary consequence.

Stromata have a similar function, but rather than initiating mycelial growth directly, they give rise to conidia or ascocarps.

 

Bisexuality

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.

 

Sexual reproduction

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).

 

Ascospore

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.

 

Heterokaryosis

They show the phenomenon of heterokaryosis, i.e., the nuclei of two different genotypes are present in the same mycelium.

 

Fruiting bodies

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.

 

Woronin bodies

A unique character of the Ascomycota (but not present in all ascomycetes) is the presence of Woronin bodies on each side of the septa separating the hyphal segments which control the septal pores. If an adjoining hypha is ruptured, the Woronin bodies block the pores to prevent loss of cytoplasm into the ruptured compartment. The Woronin bodies are spherical, hexagonal, or rectangular membrane bound structures with a crystalline protein matrix.

 

Commercial importance

Many ascomycetes are of commercial importance. Some play a beneficial role, such as the yeasts used in baking, brewing, and wine fermentation, plus truffles and morels, which are held as gourmet delicacies.

 

 

Occurrence

 

Distribution

 

Habitat

  • Saprophytic
  • Pathogenic
  • Parasitic 1.Facultative 2.Obligate
  • Lichen (algae+fungi)
  • Mycorrhiza (Higher plant+fungi)
  • Water (Both freshwater and marine)
  • Terrestrial
  • Air

 

  • Aquatic: In water
  • Coprophilous: In dung
  • Corticolous: In bark, wood and leaves.
  • Lignicolous
  • Folicolous: In leaves.
  • Hypogean: Under soil.
  • Endophyte: Inside plants.
  • In desert: Coccidioides immitis

 

 

Importance

 

Positive importance

  • Yeast is used to produce alcohol and make bread rise, so it is very important to the baking and beverage industries.
  • Ascomycetes can also be directly edible, as in the case of morel mushrooms.
  • Ascomycetes can be used in food production as well. Members of the Penicillium and Aspergillus genii, for example, are used to produce cheese and citric acid.
  • Arguably the most famous edible ascomycete is the truffle. There are several species of truffle typically used as a flavouring agent in cooking.
  • A very famous antibiotic penicillin is produced from members of the genus Penicillium (Penicillium notatum and Penicillium chrysogenum). Flavacin, another antibiotic, is also produced from ascomycetes.
  • Another life-saving drug is ergot, which is produced by Claviceps purpurea. Ergot can reduce bleeding by making vessels narrow. It’s used to stop bleeding during menopause, menstrual cycles, miscarriages, and in childbirth to expel the placenta by contracting the muscles of the uterus.
  • Genetic research: Neurospora crassa.

Negative importance

  • Interestingly, one negative impact comes from Claviceps purpurea. Though it can be used to produce life-saving medicine, ergot can also cause significant problems in wheat and grass crops. It causes the grass to produce fungal spores instead of grains, which makes the crop inedible.

Not only that, but if consumed in high enough amounts it can cause a disease in humans that restricts blood flow, ultimately leading to loss of limbs.

  • Another negative is a powdery mildew caused by members of the Erysiphaceae family (Erysiphae). Erysiphaceae species cover plants in a white coating that makes them inedible.

Powdery mildews such as Erysiphe graminis infect various grains. Erysiphe cichoracearum affects fruits and vegetables causing loss by low production.

Rot disease- Aspergillus

 

 

Classification (as in syllabus)

  • Order 1: Endomycetales
  • Order 2: Eurotiales
  • Order 3: Erysiphales
  • Order 4: Meliolales
  • Order 5: Clavicipitales

 

 

 

 

 

 

Penicillium sp.

Occurrence

Importance

Vegetative structure

Reproduction

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About Abulais Shomrat

Abulais Shomrat
Currently in 4th year (Hons) in Department of Botany, University of Dhaka. Planning to have multiple careers one by one but promised to be with 'Plantlet' as long as it's primary stage remains unfinished.

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