Genetic Mechanisms of Rare Matings of the Yeast SACCHAROMYCES CEREVISIAE Heterozygous for Mating Type.

Yeast heterozygous for mating type lacks the ability to conjugate as judged by the mass-mating technique and accordingly is designated "non-mater". However, the non-mater shows rare mating ability with a frequency of less than 10(-6). In the present study, the RD auxotroph mating method was mainly employed with the intention of examining the rare mating ability of various non-maters, using lactate ethanol minimal medium as a selective medium for hybridization. Crosses of aalphaxa, aalphaxa, aaalphaxa, aalphaalphaxa, etc. resulted in the production of respective hybrids with a relatively high frequency of about 10(-6) to 10(-7), whereas crosses of aaalphaxa, aalphaalphaxalpha, aaalphaalphaxa, aaalphaalphaxalpha, etc. resulted in hybrids with an extremely low frequency of about less than 10(-8). Genetic analyses revealed that the rare matings were mostly caused by the presence of cells derived from the non-maters in which mating type had converted to a homozygous genotype. Mitotic recombination was shown to be a likely explanation for most of the conversion, judging from associated exchange of an outside marker, thr4. By successive employment of the RD auxotroph mating method, it was possible to produce a series of polyploid yeasts, triploids to octoploids. The DNA content and the cell volume were observed to increase parallel to the elevated ploidy states.