GENERAL MICROBIOLOGY - T.P. Pirog - 2004

9. YEASTS

9.4. YEAST REPRODUCTION

9.4.2. Sexual Reproduction in Yeasts

During sexual reproduction, meiotic division of the diploid Yeast Nucleus yields haploid spores, namely ascospores (endogenous) and sporidia (exogenous). Yeasts are heterothallic or homothallic organisms; thus, a distinction is made between homothallism (fusion of offspring originating from a single spore or Cells derived from the same haploid Cell) and heterothallism (fusion, or conjugation, between cells that are descendants of different haploid cells).

Ascospores are produced by ascomycetous yeasts. Typically, an ascus contains 1 to 4–8 spores (Saceharomyces_y Saccharomycoides), and occasionally several dozen (Lipomyces, Kluyveromyces). The Morphological diversity of ascospores serves as a taxonomic feature.

The asexual (budding) and sexual pathways of reproduction in the ascomycetous yeast Saccharomyces cerevisiae are illustrated in Fig. 9.5. In the sexual process, a normal diploid cell (a cell containing two sets of Chromosomes and, consequently, two sets of genes) undergoes Meiosis (reduction division) to form an ascus containing four haploid ascospores (cells with a single set of chromosomes and genes). These ascospores exist in two mating types: a and α. Budding allows cells of each mating type to generate further haploid cells. The subsequent mating of a and α haploid cells yields a normal diploid a/α cell. Haploid Cells of the same mating type may also occasionally conjugate, forming abnormal diploid cells (a/a or α/α) that reproduce exclusively via asexual budding.

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Fig. 9.5. Reproduction of Ascomycetes yeasts

Exogenous sexual spores, or sporidia, are formed in basidiomycetous yeasts. Following the fusion of haploid cells in these yeasts, a dikaryotic mycelium develops, upon which thick-walled, round (oval, dumbbell-shaped, or pear-shaped) cells containing lipid inclusions—known as teliospores or teleutospores—are formed. Teliospores can germinate to produce either a septate (2-to-4-celled) mycelium or a non-septate (single-celled) promycelium (germ tube) that bears sporidia.