Open Access

Marjorie Crandall, Richard Egel, Vivian L. Mackay

• Sexual reproduction in yeasts has a survival function by providing an alternative to the vegetative processes when conditions are no longer conducive for growth. If both sexes are in the correct physiological state (usually under starvation conditions), then initiation of copulation involves the mutual induction of a sexual response. This response is mediated by diffusible compounds and by physical contact. Initial cell contacts between opposite mating types can be disrupted easily, but stronger intercellular bonds form later that result in the fusion of two cells into one. Union between mates involves mixing of parental gene pools. The new diploid organism or its subsequent offspring might be better equipped to survive in a new environment because they may contain new combinations of parental genes. Hence, sex is more advantageous to the survival of the species than it is to the individual organism. The purpose of this review is to compare the steps in the mating process in three species of yeasts. The various physiological factors, events and regulatory phenomena that are part of the mating process will be described for Hansenuta wingei, Schizosaccharomyces pombe and Saccharomyces cerevisiae in Sections II, III and IV, respectively. Then, in Section V, the similarities and differences among these yeast systems will be discussed. Emphasis in this article will be on recent observations since reviews of earlier work are available for each mating system (for H. wingei: Crandall and Brock, 1968; Crandall and Caulton, 1975; for Schizosacch. pombe: Leupold, 1970; Gutz et at., 1974; for Sacch. cerevisiae: Fowell, 1969a, b; Bilinski et at., 1975; Sena et at., 1975). For a comprehensive review of conjugation in all yeasts, fungi and other micro-organisms, consult Crandall (1977). The three yeasts to be reviewed here are quite diverse in terms of their ecological niches, metabolism, morphology and life cycles. Therefore, for a better understanding of the physiology of sexual reproduction in these organisms, it is necessary first to consider these characteristics. A more detailed description of each yeast is given in Lodder (1970).