FUNCTIONAL EQUIVALENCE AND CO-DOMINANCE OF HOMOTHALLIC GENES HM/hm AND HMa/hma IN SACCHAROMYCES YEASTS

Satoshi Harashima ¹ and Yasuji Oshima ¹

¹ Department of Fermentation Technology, Osaka University, Yamadakami, Suita-shi, Osaka 565, Japan

The specificity of mating type in Saccharomyces yeasts is controlled by a pair of alleles, a and , on chromosome III. They are mutually interconverted by the function of three kinds of homothallic genes, each consisting of a single pair of alleles, HO/ho, HM/hm and HMa/hma. For the a to conversion, HO HM HMa, HO hm HMa and HO hm hma genotypes are effective; whereas, the to a conversion occurs in HO HM HMa, HO HM hma and HO hm hma cells. To explain these observations, Naumov and Tolstorukov (1973) and Harashima, Nogi and Oshima (1974) suggested that hma and HM are functionally equivalent and effective for the to a conversion in combination with HO; whereas, hm and HMa are functionally equivalent and effective for the a to conversion with the function of HO. To test this idea and to compare it with two other possible mechanisms, some of the tetrad segregants from four kinds of a/a / tetraploids homozygous for the HO allele and for one of the HM/hm and HMa/hma loci, while heterozygous for the other one with +/+/-/- configuration, were investigated with respect to their thallism by self-sporulation. Results indicated the functional equivalence of both the HM and hma alleles and the hm and HMa alleles in mating-type conversion, and the co-dominance of the alleles of each locus. From these findings and other data, we agree with the revision of the nomenclature of the HM/hm and HMa/hma genes to HMRa/HMR and HML/HMLa, respectively.