Genetic analysis of the products of a cross involving a suppressive ‘petite’ mutant of S. cerevisiae

Summary A genetically defined highly suppressive petite yeast strain ( ^–cob⁺A^sE^oC^oO^oP^o) was crossed with a grande strain carrying a multiply marked mitochondrial genome ( ⁺A^rE^rC^rO rp^r). Petite diploid progeny, isolated from individual zygotic clones consisting either of wholly petite or mixtures of grande and petite cells, were characterised genetically by crossing to grande haploids. The diploid petites were found to closely resemble the petite parent and in general not to carry mitochondrial markers from the grande parent. In the petites from the mixed clones recombination was detected, but only within the region of homology between the genomes. These observations are inconsistent with models of suppressiveness based on destructive recombination and suggest that the petite genome eliminates the grande genome from zygotic progeny through being preferentially replicated. The most plausible model to explain the observed pattern of zygotic clones postulates a limited number of mDNA replication sites in zygotes, competition for sites between input mDNA molecules and an advantage in this competition for suppressive ^– mDNA.