Fission yeast is a haploid ascomycete that switches mating-type every few cell divisions during vegetative growth (Egel 1977; Beach 1983). Under conditions of nutrient deprivation, cells of opposite mating-type (h+ or h−), which have become arrested in the G1 phase of the cell cycle, conjugate to form a diploid zygote (Egel 1971; Nurse and Bissett 1981). The zygote, now heterozygous at the mating-type locus (h+/h−), enters the premeiotic S phase and then undergoes two meiotic divisions that lead to the formation of a four-spored ascus.
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David Beach (1985) studied this question.