Evidence for Co-Dominance of the Homothallic Genes, HMalpha/hmalpha and HMa/hma, IN SACCHAROMYCES YEASTS.

To investigate the dominance and recessiveness of the homothallism genes, HMalpha/hmalpha and HMa/hma, for mating-type conversion, we constructed hybrids with various configurations of the homothallic genes by fusion of protoplasts prepared from haploid strains having identical mating types. Eight different combinations of the homothallic genes were tested for their function by observing the mating and sporulation abilities of the fusion products. With few exceptions, nonmating and sporogenous fusion products were obtained from the following combinations: alpha HO hmalpha HMa + alpha ho hmalpha hma, alpha HO hmalpha HMa + alpha ho HMalpha hma, alpha HO hmalpha HMa + alpha ho HMalpha HMa, aHO HMalpha hma + aho hmalpha hma, aHO HMalpha hma + aho hmalpha HMa and aHO HMalpha hma + aho HMalpha HMa. All the fusion products from the alpha HO hmalpha HMa + alpha ho hmalpha HMa and aHO HMalpha hma + aho HMalpha hma combinations showed mating types identical to those of the respective haploid strains. These results clearly support the co-dominance of the HMalpha/hmalpha and HMa/hma alleles and indicate that the hmalpha allele has the same function as the HMa allele and that the hma allele has the same function as the HMalpha allele.