Ascomycetes (Yeast)

Ascomycetes

General characteristics of ascomycetes

  • Members of the ascomycetes occur as saprophytes or parasites.
  • Mycelium is well developed and branched except yeasts (unicellular).
  • The species of it are called Sac fungi because they produce sexual spores within the sac like ascus.
  • Sexual reproduction takes place by the fusion of somatic hyphae of opposite strains (gametangia).
  • The 8 ascospores are formed in the ascus i.e. endogenous in origin.
  • Some ascomycetes like cup fungi (Peziza) have specialized fructifications, known as ascocarp.

Yeast

Systematic position

Division: Mycota

Class: Ascomycetes

Order: Endomycetales

Family: Saccharomycetaceae

Genus: Saccharomyces

Yeast is a unicellular fungus. Thallus is a non-mycelial, consist of single minute oval or spherical shape. Each cell consist of 2 layered cell wall surrounds the protoplast. The cell wall is thin and delicate. The protoplast consist of granular dense cytoplasm a minute nucleus, a large vacuole and mitochondria.

Reproduction of Yeast

Yeast reproduces both sexually and asexually.

Asexual reproduction

It reproduces asexually by means of budding, fission and endospores formation.

Budding: Under favorable conditions the yeast cell produces one or more towards the bud and finally the bud separate and becomes independent.

Fission: In this process yeast cell elongates and its nucleus divides into 2 which is separated by transverse wall. Finally divides into 2 cells and becomes independent.

Endospores formation: Under unfavourable conditions the protoplasm divides and each part is surrounded by a thick wall and becomes endospores which on favourable condition germinate to produce the chain of cells.

Sexual reproduction

It occurs by conjugation between 2 haploid somatic cells or gamentangium. As a result diploid zygote cell is formed which directly behave as ascus to form 4 or 8 haploid ascospores.

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Haplobiontic life cycle

In this type haploid phase is prolonged while diploid stage is very short representing by zygote only. During the process 2 somatic cells fuses by means of conjugation tube. As a result zygote is formed which undergoes 3 successive divisions to form 8 haploid nuclei and finally 8 ascospores are formed in an ascus. The ascospores rupture the ascus and behave as independent yeast cell.

Diplobiontic life cycle

In this diploid phase is prolonged while haploid phase is represented by ascospores. During the process diploid somatic cell directly behaves as ascus. The nucleus of it undergoes meiotic division to form 4 haploid ascospores. These ascospores fuse in pairs to produce 2 diploid zygotic cells. The zygotic cell germinate by producing germ tube and cut off the diploid cells from sprout mycelium.

Haplodiplobiontic life cycle

In this type a diploid zygote is formed from the fusion of 2 haploid cells. These diploid cells multiply by budding and produce a large number of diploid cells. Each diploid cell behaves as ascus. The nucleus of it undergoes meiotic division to produce 4 haploid nuclei. Each of these nuclei organized into an ascospore. The ascospores released from ascus and multiply by budding to produce haploid cells.

Economic importance of Yeast

  • It helps on oxidation of sugar to CO2 and alcohol in the presence of O2 (alcoholic fermentation).
  • Yeast cells are pressed into cakes with starch which are used in baking industries and home.
  • A few species are parasites on plants and causes diseases.
  • Several species are pathogenic to man causing number of disease.
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