A NEW GENE AFFECTING THE EFFICIENCY OF MATING-TYPE INTERCONVERSIONS IN HOMOTHALLIC STRAINS OF SACCHAROMYCES CEREVISIAE

James E. Haber ¹ and Barbara Garvik ²

¹ Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02154
² Department of Biology, Brandeis University, Waltham, Massachusetts 02154

Homothallic strains of Saccharomyes cerevisiae are able to switch efficiently from one mating genotype to another. From a single haploid spore arise both a and mating type cells, which then self-mate to produce a colony consisting almost exclusively of nonmating a/ diploid cells. We have isolated a mutant homothallic strain that gives rise to colonies that show bisexual mating behavior. The mating reaction is always asymmetric, that is, in some colonies a mating is much stronger than mating, while others show greater than a mating.—This mating phenotype arises from the presence of three cell types in a colony: some a/ nonmating diploids and an unequal number of a and haploid cells. The predominant haploid type is that of the original cell that gives rise to the colony. This mixture of cell types arises from a very reduced efficiency of homothallic mating-type interconversions in the mutant strain.—The mutation, designated switch (swi1–1), behaves as a single genetic locus. The mutation is centromere linked, but not linked to the mating type locus or to any of the homothallism genes: HO, HMa and HM. The switch mutation does not affect the efficiency of self-mating, but rather directly affects the frequency of interconversion of mating types.

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