Ascomycetes

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Introduction

Fungi are divided into two broad subgroups based on their sexual spore formation, they are ascomycetes and basidiomycetes. As in all macrofungi, there is the formation of sexual spores, which lead to a combination of genetic information from two parents. The presence of fruiting bodies tells us about the types of fungi section that produce and dispersal of sexual spores. Whereas some macrofungal species may also produce asexual spores.


What is Ascomycetes?

Define Ascomycetes - A fungi which develop inside the asci means cylindrical sac is known as ascomycetes. Cylindrical sacs include moulds, mildews, and yeasts, the fungal component of most lichens, and a few large forms such as morels and truffles.

Phylum Ascomycota is also called ascomycetes and it is considered as the largest phylum of the fungi kingdom with approx 64000 species. All these come under the subkingdom Dikarya due to the presence of dikaryon. Ascomycetes produce a sexual non-motile spore called ascospores and the sac-like structures are known as an ascus. Every ascus contains nearly four to eight ascospores and they are commonly known as the sac fungi.

The living habitat and the feeding habits of the ascomycetes are very diverse, like some of them are saprophytes and the others are pathogens causing various diseases in plants and animals. Some of the ascomycetes are edible in nature like mushrooms and some of them live in a symbiotic association like lichens and mycorrhiza.  Some of the ascomycetes examples are yeast, powdery mildews, cup fungi, morels, truffles, Neurospora, Aspergillus, Cladonia, Penicillium, Candida, Claviceps, etc.

Ascomycetes are very important economically like in the form of fermented food (bread, cheese, alcoholic beverages), antibiotics (Penicillin) and various chemicals. And many species of ascomycetes are used in biological studies and research (yeast, Neurospora), morels and truffles are used as delicacies. Ascomycetes are classified based on the different structures of asci and depending on the method of releasing ascospore.


General Characteristics of Ascomycetes 

Some of the characteristics of ascomycetes are given below.

  • The reproductive structure of the ascomycetes is known as ascus or asci 

  • Most of the ascomycetes are terrestrial, parasitic or coprophilous.

  • It might be unicellular or multicellular fungi 

  • The mycelium of the ascomycetes is made up of septate or branched hyphae and the cell wall is made up of chitin or  ꞵ-glucans.

  • Ascomycetes have cytoplasmic continuity due to septal pores.

  • In ascomycetes, asexual reproduction is by the formation of conidia exogenously on conidiophores and Sexual reproduction is by conjugation between two gametangia. They can be either homothallic or heterothallic

  • In ascomycetes, the fruiting body is called ascocarp and there are mainly four types of ascocarps. Ascomycetes examples of ascospores are cleistothecium, perithecium, apothecium, and ascostroma. 

  • Ascospores are produced endogenously in ascii. 

Ascomycetes Reproduction Sexually 

There are two different mating types of hyphae that come together and fuse. After fusion Plasmogamy takes place, but it does not follow karyogamy immediately. According to dikaryon, the fused structures of ascomycetes contain two haploid nuclei, one from each parent to form a diploid zygote.

Formation of Ascospore: The diploid zygote undergoes meiosis to form 4 haploid nuclei, which undergoes mitotic division to form 8 haploid nuclei and each of the nuclei accumulates cytoplasm and a thick wall surrounds it. They are known as ascospores. These ascospores germinate to form new mycelia under favourable conditions. 


Ascomycetes Life Cycle

Asexual Reproduction Life Cycle 

  • Most of the species undergo asexual reproduction by the formation of multinucleated conidia.

  • Conidia are formed from the conidiogenous cells.

  • These cells are formed at the tip of modified hyphae called conidiophores or conidia bearers.

Sexual Reproduction Life Cycles 

The sexual life cycle of ascomycetes involves nuclear fusion and meiosis, which occurs only in those possessing ascii, because it is within the young ascus where the events occur. Ascospores include one or more haploid nuclei and therefore most of the ascomycetes have a haploid vegetative mycelium. Mycelium is also capable of sexual reproduction, which occurs by fragmentation, budding or formation of conidia, chlamydospore, and sclerotia.

The mating behaviour of the ascomycetes may be homothallic or heterothallic in nature. In homothallic the mycelium derived from a single ascospore, which is capable of reproducing sexually. By developing the asci, some of the examples are Pyronema domesticum and Sordaria fimicola.


Economic Importance of Ascomycetes 

They are one of the biggest groups of fungi having diverse habitats. Many useful products are derived from them like foods, medicines, and some chemical products. 

  • Many food industries depend on the yeast for fermentation like brewing, bread and cheese making. 

  • They are used to make antibiotics like penicillin, which is derived from  Penicillium chrysogenum.

  • Ascomycetes produce many organic acids and enzymes like citric acid, gluconic acid, amylases, proteases, etc.

  • Ciclosporin derived from the fungus Tolypocladium niveum is used to transplant organs and treat autoimmune diseases. It is used as an immunosuppressant.

  • It is used to prepare different types of cheese using different penicillium species like camembert, brie, and Roquefort.

  • Ascomycetes like Claviceps purpurea (Ergot) are used as medicine to stop excessive bleeding during menstrual periods and to speed up labour.

  • They are one of the delicacies like morels, truffles and lobster mushroom. Aspergillus is used to prepare soya sauce and also other Asian alcoholic beverages. 

  • Some of the ascomycetes are widely used to study genetics Neurospora, Saccharomyces, etc.

Conclusion 

After going through what is ascomycetes? Ascomycetes life cycle, we understood that it involves two types of reproduction sexual and asexual, due to the small size of the spore they can easily spread through the wind. Also, we got to know ascomycetes are very important for economic development and it also has many medicinal uses and importance in the food industry.  

FAQ (Frequently Asked Questions)

1. Why is Yeast Classified as Ascomycetes?

Answer. Yeasts are classified as ascomycetes because they are non-motile single cells with chitinous cell walls, so they are classified under fungi. Ascomycetes mainly reproduce by budding and fission, yeasts also engage in sexual reproduction that results in the production of an ascus, which makes them classify under Ascomycota.

2. Which is the Member of Ascomycetes?

Answer. The members of the Ascomycota include well-described filamentous fungi Hypocrea jecorina (anamorph Trichoderma reesei), Aspergillus niger, and Neurospora crassa and the yeast Saccharomyces cerevisiae.

3. Where are Ascomycetes Found?

Answer. The species of the ascomycetes are found in dryland around the world. In habitat ranging from tropical and temperate forests to grasslands and beyond. Some of the species of Ascomycota also live in extreme environments, one of the examples is coccidioides immitis.