Functional equivalence and co-dominance of homothallic genes HM alpha/hm alpha and HMa/hma in Saccharomyces yeasts.

The specificity of mating type in Saccharomyces yeasts is controlled by a pair of alleles, a and alpha, 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 alpha/hma alpha and HMa/hma. For the a to alpha conversion, HO HM alpha, HMa, HO hm alpha HMa and HO hma alpha hma genotypes are effective; whereas the alpha to a conversion occurs in HO HM alpha HMa, HO HM alpha hma and HO hm alpha hma cells. To explain these observations, NAUMOV and TOLSTORUKOV (1973) and HARASHIMA, NOGI and OSHIMA (1974) suggested that hma and HM alpha are functionally equivalent and effective for the alpha to a conversion in combination with HO; whereas, hm alpha and HMa are functionally equivalent and effective for the a to alpha 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/alpha/alpha tetraploids homozygous for the HO allele and for one of the HM alpha/hm alpha 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 alpha and hma alleles and the hm alpha and HMa alleles in mating-type conversion, and the co-dominance of the alleles of each locus. From the findings and other data, we agree with the revision of the nomenclature of the HM alpha/hm alpha and HMa/hma genes to HMRa/HMR alpha and HML alpha/HMLa, respectively.