The Mating System in Saccharomyces

Many, but not all, Saccharomyces species are heterothallic. The mating types in heterothallic species are determined by two allelic genes. The mating-type genes occasionally mutate from one to the other. Vegetative cells of opposite mating type were found in some single ascospore colonies. They show conjugation tubes, stimulated by the proximity of cells of the other mating type. The cultures alao show zygotes, which on sporulation yield plus(+) and minus(−) progenies. The zygotes bud off diploid vegetative cells which grow faster than the original haploid cells and so tend to replace them. If mutation occurs early, replacement is complete and the culture gives no mating reaction but sporulates. If it occurs late, the culture is a mixture of haploid cells giving a mating reaction and diploid cells that will sporulate.