The Action of Homothallism Genes in Saccharomyces Diploids during Vegetative Growth and the Equivalence of hma and HMalpha Loci Functions

The action of homothallism genes in vegetatively growing diploid cells was examined. The results demonstrate that homothallism genes function during regular vegetative growth cycles as well as during the first few divisions after spore germination. A procedure based on ultraviolet-induced reciprocal mitotic recombination monitored by homozygosity for cryptopleurine resistance (a recessive marker closely linked to the mating-type locus) allowed us to identify and recover Saccharomyces cerevisiae colonies sectored for the mating-type locus i.e., a/a and alpha/alpha. Homothallism genes can switch a/a or alpha/alpha vegetative diploid cells, generated from a strain with genotype a/alpha HO/ho HMalpha/HMalpha HMa/HMa, to a/alpha diploids or a/a/alpha/alpha tetraploids during a given mitotic division cycle. We found that both a/a and alpha/alpha sectors generated from a strain with genotype a/alpha HO/HO hmalpha/hmalpha hma/HMa switch to a/alpha diploids or a/a/alpha/alpha tetraploids. This finding supports Naumov and Tolstorukov's suggestion (1973) that the hm a allele provides for the same functions as the HMalpha allele, namely, a switch at the mating-type locus from alpha to a. The HO allele is dominant to ho but hma and HMa alleles are codominant. A loose linkage between the mating-type and the HMalpha loci ( approximately 55cM), confirming Harashima, Nogi and Oshima (1974) data, was observed.