Ascomycotina: Classification, Characteristics and Reproduction

Contents:

General Characteristics
Thallus Structure
Classification
Ascomycetes Examples
Reproduction
- Asexual Reproduction
- Sexual Reproduction
Life cycle
Economic Importance

ASCOMYCOTINA

Subkingdom: Mycota
Division: Eumycota
Sub-division: Ascomycotina

It is the largest group of fungi having more than 32,000 species in 3,400 genera
it is also known as Sac- Fungi
askos – bag or bladder and mykes – fungus
It is characterized by the formation of an ascus
This consists of molds that have septated hyphae and some yeasts

General Characteristics

The spore formed by these fungi is known as ascospores which are sexually produced and are present within a sac
They consist of both parasitic and saprophytic species
They may be symbiotic
The majority species are terrestrial but few are aquatic ie. marine and freshwater
It is responsible for a few diseases like powdery mildew.
They show both sexual and asexual reproduction.
They produce the sexual ascospores and asexual conidiospores

Thallus Structure

It may grow either as yeasts, i.e. unicells multiplying by budding or fission or as mycelia which consist of septate hyphae
Mycelium is well developed with branched and septate hyphae
Mycelium may be homokaryotic or heterokaryotic
Some fungi may be dimorphic ie. they can switch from the yeast to the filamentous state or vice versa. Ex- Candida

Classification

It is subdivided into 6 classes-

Hemiascomycetes- Mycelium is poorly developed or absent, No ascocarp formation
Plectomycetes- Mycelium is well developed, branched and septate, Ascocarp is formed
Pyrenomycetes- Mycelium well developed and septate, ascocarp are produced
Discomycetes- Well developed mycelium, Sex organs are lacking
Laboulbeniomycetes- Thallus is reduced, Ascocarp is formed
Loculoascomycetes- Thallus is mycelial, Ascocarp is formed

Ascomycetes Examples

Saccharomyces
Xylaria
Peziza
Phyllachora

Reproduction

Reproduction may occur sexually or asexually.

Asexual Reproduction

1. By Formation of Conidia-

These are exogenous and non-motile spores which are produced on the tip of conidiophores. They may be branched or unbranched and unicellular or multicellular
Conidiogenesis occurs in two ways which appear to be distinct: blastic and thallic
Blastic– The conidium develops by the blowing-out of the wall of a cell, mostly from the tip of a hypha
Thallic- This occurs by conversion of a pre-existing hyphal element in which terminal or intercalary cells of a hypha become cut off by septa.

2. Other spore formations like Oidia or Chlamydospores- This may occur in some species.

Sexual Reproduction

Ascomycota have male and female gametangia in their haploid stage ie. antheridium and ascogonium respectively.

Gametangial copulation
Gametangial contact
Plasmogamy– A single cell with two haploid nuclei. This creates a binucleate, dikaryotic condition in the ascogonium leading to karyogamy
karyogamy– At the tip of the hyphae the nuclear fusion takes place

Diploid nucleus immediately undergoes meiosis followed by mitosis resulting in 8 haploid nuclei. Each nuclei accumulate cytoplasm, secrete a wall around it develop into an ascospore.

Life cycle

Economic Importance

The species Neurospora crassa has been the subject of intensive genetical research
Alcoholic fermentation by yeasts as the basis of the wine and brewing industries
Antibacterial antibiotics such as penicillin from Penicillium chrysogenum and cephalosporin from Acremonium spp.
Food production as in bread-making by yeast, cheese ripening by Penicillium roqueforti and P. camemberti
Food spoilage may result from ascomycete contamination. ex- Claviceps purpurea

Reference

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Introduction to Fungi, Third Edition
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https://link.springer.com/chapter/10.1007%2F978-94-011-1496-7_4

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