MICROBIOLOGY Study Guide - 2012

CHAPTER 5. FUNGI

5.6. YEAST REPRODUCTION

Yeasts can reproduce asexually or sexually. Asexual reproduction involves budding and fission, whereas sexual reproduction entails spore formation.

Yeasts of the family Saccharomycetaceae reproduce by budding, those of Schizosaccharomycodaceae by fission, and yeasts of the family Saccharomycodaceae reproduce by budding that culminates in fission.

The following forms of asexual Yeast reproduction are distinguished:

✵ multilateral budding (as in Saccharomyces) — a bud forms at various sites On the surface of the yeast Cell;

✵ bipolar budding (as in Hanseniaspora) — budding occurs sequentially from each pole of The Cell in an alternating fashion;

✵ binary fission (as in Schizosacch) — the mother cell elongates, The Nucleus divides, the new nuclei migrate toward the cell poles, these two buds are separated by a septum, followed by The formation of cell walls and their division into two independent Cells (Fig. 17).

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Fig. 17. Vegetative Reproduction of yeasts (F. Priest, I. Campbell, 2005):

a — multilateral budding; b — bipolar budding; c — binary fission

Budding — the time required for a cell to produce a similar offspring is called the Cell Cycle. It comprises four MAIN STAGES OF approximately equal duration.

G₁ — presynthetic phase. It begins with the synthesis of Enzymes required for bud formation and DNA duplication. Bud formation is preceded by complex intracellular processes: the Hydrolysis of Cell wall Polysaccharides and its softening, the activation of the enzyme Chitin synthase, and The Biosynthesis of chitin, which is a component of the bud septum. The phase concludes with The Emergence of the bud.

S — interphase (synthetic phase). Where the bud joins the cell, a constriction gradually forms. Once the bud reaches approximately 1/3 the size of the mother cell, the nucleus migrates into the constriction.

G₂ — postsynthetic phase. During this phase, the bud continues to increase in size. Nuclear material is distributed between the mother and daughter cells.

M — mitosis. The nucleus divides; one nucleus remains in the mother cell, while the other migrates into the bud.

(C) — cytokinesis (Cell Division). The constriction gradually narrows, the Formation of the chitin layer is completed, and after approximately 2 hours, the young daughter cell separates from the mother cell. Separation of the daughter cell occurs only after the layers of the septum part, leaving a bud scar on the mother cell.

During reproduction, up to 25–30 bud scars appear on a yeast cell. Daughter cells do not always separate from their mother cells; they may also reproduce by budding, resulting in the formation of multicellular clusters.

Fission in yeast cells occurs similarly to bacterial division — following nuclear division, a transverse septum forms, leading to The production of two daughter cells identical to the mother cell.

Sexual reproduction. Yeasts of the species Saccharomyces cerevisiae possess a well-characterized life cycle, with cells capable of reproducing by budding in both Haploid and Diploid phases (Fig. 18). Haploid saccharomycete yeasts occur in two mating types: a and α. Cells of different mating types are able to recognize each other and form a/α diploids capable of undergoing Meiosis to produce four haploid ascospores. In some homothallic strains, efficient mating type switching occurs: a-type yeasts can transform into α-type yeasts, and vice versa. Such switching is associated with a specialized Homologous Recombination system. Yeast strains in which the haploid phase is stable and can be maintained through numerous generations are termed heterothallic.

Fig. 18. Life Cycle of Saccharomyces yeasts (F. Priest, I. Campbell, 2005)

Vegetative cells of opposite mating types (a and α) fuse, undergoing successive processes of plasmogamy (fusion of the Cytoplasm of two cells) and karyogamy (fusion of nuclei). This results in the formation of a zygote — a diploid cell with a double set of Chromosomes. Under favorable conditions, diploid cells are stable and can reproduce by budding indefinitely. They are larger and more active than haploid cells.

In the event of starvation or other unfavorable conditions, the cell initiates sporulation.

The diploid nucleus divides by meiosis into 4–8 parts, after which ascospores are formed. The formation and maturation of ascospores takes 15–48 hours. The yeast cell wall changes, and the cell turns into an ascus. Typically, an ascus contains four ascospores, although asci with two and three spores are also found. Yeast spores have a thicker and more durable wall than vegetative cells. They are quite resistant to unfavorable external factors (Temperature, desiccation, pH, etc.), but are less thermostable compared to bacterial spores and are destroyed at a temperature of 80 °C.

Under favorable conditions, ascospores germinate into haploid cells, which then reproduce by budding. Rapid germination of ascospores is observed only in media containing readily available carbon sources, such as glucose or fructose.

In cultivated yeasts, whose development takes place on carbohydrate-rich media, The ability to sporulate is significantly weakened or completely lost. The ability of yeasts to form spores can be artificially restored by cultivating them on nutrient-depleted media.