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

Abstract

ABSTRACT 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 aα × a, aα × a, aaα × a, aαα × a, etc. resulted in the production of respective hybrids with a relatively high frequency of about 10−6 to 10−7, whereas crosses of aaα × a, aαα × α, aaαα × a, aaαα × α, 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.