Ascomycetes

Sac fungi or Ascomycetes are the members of phylum Ascomycota of subkingdom Dikarya and Kingdom fungi. Resembling sacs, their reproductive structure, ascus, carry four to eight ascospores. Housing around 64,000 species, the morphological, habitat and feeding diversity of these organisms is vast.

Table of Contents

Characteristics of Ascomycetes
Examples of Ascomycetes
Reproduction in Ascomycetes
Industrial Usage of Ascomycetes
Practice Problems
Frequently Asked Questions

Characteristics of Ascomycetes

Habitat: Parasitic, terrestrial and coprophilous
Cellularity: Unicellular and multicellular
Morphology: Septate and branched hyphae, septal pores contributing to cytoplasmic continuity
Cell Wall: Chitin or beta glucan
Reproduction: Both sexual and asexual
Human welfare: For food production, study and research and economic benefits
Fruiting body: Ascocarp that is of four types:
Cleistothecium: Spherical and closed fruiting body. Example, Aspergillus
Perithecium: Flask-shaped and one opening on the exterior. Example, Neurospora
Apothecium: Cup-shaped with asci in hymenium. For example, Peziza.
Ascostroma: Lack of distinguishable fruiting body. Example, Mycosphaerella

Examples of Ascomycetes

The examples of specific fungi are categorised as follows:

Common Ascomycetes

Penicillium species found in soil, decaying matter and air
Cladosporium species found in indoor and outdoor environments
Xylaria polymorpha, or dead man’s fingers found in decaying woods

Rare Ascomycetes

Cordyceps species found only in insect hosts
Gaestrum species found only in star-shaped fruiting bodies
Microstoma species

Pathogenic Ascomycetes

Candida albicans infects the oral system
Aspergillus fumigatus affects the respiratory system in people with the weak immune system

Non-Pathogenic Ascomycetes

Morchella species of edible nature found in forests
Neurospora crassa used for genetic and biochemical studies
Saccharomyces cerevisiae used in brewing and baking

Reproduction in Ascomycetes

The ascomycetes can reproduce through both sexual and asexual modes. The insights into both modes are as follows:

Sexual Reproduction

Sexual reproduction in ascospores occurs through the fusion of two different types of hyphae. It results in the event of plasmogamy, followed by karyogamy in a later stage. The haploid nuclei fuse together to form a diploid zygote that goes through meiosis to form 4 haploid nuclei followed by mitotic divisions. The nucleus gathers the cytoplasm along with a thick wall resulting in the structure named ascospores. The stated structures grow to form new mycelia under favourable conditions.

The reproduction behaviour is either homo or heterothallic. The mode of sexual reproduction also encompasses different methods, such as budding or conidia formation, sclerotia, fragmentation or chlamydospore.

Asexual Reproduction

Asexual reproduction occurs through the development of multinucleated conidia that result from conidiogenous cells. The site of cell formation is the tip of modified hyphae, where the structure is referred to as conidia bearers or conidiophores.

Industrial Usage of Ascomycetes

Humans use Ascomycetes for their welfare via numerous industrial processes. It generates loads of economic benefits. Here are some important usages of the same:

Food production via yeast fermentation. It involves bread and cheese making and brewing. Lobster mushrooms, morels and truffles are widely consumed. They are also a source of soy sauce and alcoholic beverage production.
Usage of their acids and enzymes such as citric acid, amylases, gluconic acid, proteases and others.
Medicinal uses such as antibiotics and medication prevent excess loss of blood during the menstrual cycle. It is also used to stimulate the speed of labour.
Ciclosporin usage for organ transplantation and as an immunosuppressant for the treatment of autoimmune diseases.
For genetic studies.

Practice Problems

Q1. What is the ploidy of ascospores?

A. Haploid
B. Diploid
C. Triploid
D. 2n

Ans. A. Haploid - Ascospores are haploid sexual spores.

Q2. What is the composition of the Ascomycetes cell wall?

A. Chitin
B. Chitin and beta-glucans
C. Cellulose
D. Peptidoglycan

Ans. B. Chitin and beta-glucans

They have chitinous cell walls along with the presence of beta-glucans.

Q3. Penicillin is derived from which Ascomycete?

A. Penicillium chrysogenum
B. Aspergillus niger
C. Candida albicans
D. Saccharomyces cerevisiae

Ans. A. Penicillium chrysogenum

Penicillin serves as an antibiotic derived from Penicillium chrysogenum.

Frequently Asked Questions

Q1. Which cells are responsible for hyphae formation?
Ans. The dikaryotic cells are responsible for hyphae formation.

Q2. Which are the characteristic features of Ascomycetes with respect to the environment?
Ans. Ascomycetes are decomposers that help in providing nutrition back to the soil. They are also found in mutualistic relationships with plants.

Q3. Are lichens ascomycetes?
Ans. Lichen is the association between fungus and bacteria. Ascomycetes are also fungus and hence are capable of forming lichens.