FUNCTIONAL EQUIVALENCE AND CO-DOMINANCE OF HOMOTHALLIC GENES HMα/hmα AND HM  a/hm  a IN SACCHAROMYCES YEASTS

ABSTRACT The specificity of mating type in Saccharomyces yeasts is controlled by a pair of alleles, a and α, 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, HMα/hmα and HMa/hma. For the a to α conversion, HO HMα HMa, HO hmα HMa and HO hmα hma genotypes are effective; whereas, the α to a conversion occurs in HO HMα HMa, HO HMα hma and HO hmα hma cells. To explain these observations, Naumov and Tolstorukov (1973) and Harashima, Nogi and Oshima (1974) suggested that hma and HMα are functionally equivalent and effective for the α to a conversion in combination with HO; whereas, hmα and HMa are functionally equivalent and effective for the a to α 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/α/α tetraploids homozygous for the HO allele and for one of the HMα/hmα 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 HMα and hma alleles and the hmα and HMα 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 HMα/hmα and HMa/hma genes to HMRa/HMRα and HMLα/HMLa, respectively.