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Ascomycetes

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.

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