GENETIC MECHANISMS OF RARE MATINGS OF THE YEAST SACCHAROMYCES CEREVISIAE HETEROZYGOUS FOR MATING TYPE

Norio Gunge ¹ and Yasuo Nakatomi ¹

¹ Dai-Nippon Sugar Mfg. Co., Ltd., Moriya-cho, Kanagawa-ku, Yokohama, Japan

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 a x a, a x a, aa x a, a x a, etc. resulted in the production of respective hybrids with a relatively high frequency of about 10^-6 to 10^-7, whereas crosses of aa x a, a x , aa x a, aa x , 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.

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