Mutational nature of an allele-specific conversion of the mating type by the homothallic gene HO alpha in Saccharomyces.

G E N E T I C analyses of the controlling systems for homothallism uersus heterothallism in Saccharomyces revealed several striking features, i.e., the epistatic effect of a homothallic gene in relation to the heterothallic mating type alleles ( WINGE and ROBERTS 1949), the complementary interaction of homothallic genes ( TAKAHASHI 1958) and the mating type allele-specific conversion of heterothallism to homothallism (HERMAN and ROMAN 1966). In a previous publication (TAKANO and OSHIMA 1967), evidence was presented of a genetic controlling system for homothallism which is composed of two unlinked genes originated from S. ouiformis. One of them is an 01 mating type allele-specific gene HO, and the other, H M , is complementary to HO, in conversion of a mating type to homothallism. Spore cultures having the 01 HO, hm genotype produce zygotes after the first few mitotic divisions, in general after the second division. The diploid cultures thus obtained are easily sporulated and yield four-spored asci in which there is a 2 homothallic : 2 a segregation. The homothallic segregants again diploidize during early cell division and again 2 homothallic : 2 a segregations are obtained after sporulation. The segregants of a mating type in these asci produce stable haploid clones and are supposed to be produced from the 01 mating type allele by the effect of the HO, gene. This type of an a mating type culture, phenotypically indistinguishable from the normal a mating type culture, is designated d. The HO, gene in combination with another complementary homothallic gene HM shows a phenomenon quite similar to the D gene, in which the epistatic diploidization occurred by directed mutation as described by WINGE and ROBERTS