Cytoplasmic mixing in Saccharomyces cerevisiae: Studies on a grande X neutral petite mating and implications for the mechanism of neutrality.

We have studied zygotic cytoplasmic mixing in two neutral petite by grande matings. Cytoplasmic mixing in zygotes and zygotic buds was followed histochemically and genetically. Both techniques showed that in most zygotes rapid cytoplasmic mixing occurred after early zygote formation and before first bud completion. The progeny mitochondrial genotypes produced from each cross were compatible with a model assuming random segregation of petite and grande inputs between the zygote and its first bud.While both crosses produced only grande zygotic colonies and grande diploid progeny when assayed by classical genetic methods, a class of events producing petite zygotes, retaining petite mitochondrial DNA, with grande first diploid buds was discovered. The results show that the expression of neutrality in our strains requires efficient cytoplasmic mixing during zygote maturation. The results suggest that the expression of mitochondrial genome neutrality requires competition or interaction between petite and grande parental mitochondrial DNA molecules. The possible nature of these phenomena is discussed.