Isamu Takano

An ? mating-type allele insensitive to the mutagenic action of the homothallic gene system in Saccharomyces diastaticus

SummaryTwo mating-type alleles, a and α, are interchangeable with each other due to the specific mutagenic action of the homothallic genes in Saccharomyces. However, a haploid segregant having the α mating-type potency but inconvertible to homothallism by the mutagenic action of the homothallic genes was segregated from a strain of S. diastaticus. The inconvertibility was strictly specific to the α mating-type clone in its pedigree. The genetic analyses of the inconvertible α clones indicated that the inconvertibility was not due to the loss of the specific homothallic genes nor to a specific cytoplasmic inhibitor for the mating-type conversion. The most possible explanation is the presence of an α mating-type allele which is insensitive or resistant to the specific mutagenic action of the homothallic genes.

Multiple fusion of protoplasts in Saccharomyces yeasts.

SummaryFusion of protoplasts prepared from haploid strains of Saccharomyces yeasts having identical mating type was induced with the aid of polyethylene glycol. Stable fusion products were isolated by complementation of the auxotrophic markers. Of 64 isolates derived by protoplast fusion between two different haploid strains having α mating type, 35 fusion products were estimated from their cell volumes to be diploid, 13 to be triploid and 16 to be tetraploid. The isolates showing tetraploid cell size were thought to have resulted from fusion of three protoplasts of one strain and one protoplast of the other (three-to-one fusion) or from two-to-two fusion. In protoplast fusion of three different haploid strains having a mating type, all four possible phenotypes of fusion product were recovered. Fusion products of three different protoplasts were obtained in much lower frequency (2.1×10-6) than those of two different protoplasts (1.2×10-5 to 1.4×10-4) in the three other combinations. Genetic analyses revealed that triploid fusion products were formed by protoplast fusion of two different strains as well as of three different strains.