THE ACTION OF HOMOTHALLISM GENES IN SACCHAROMYCES DIPLOIDS DURING VEGETATIVE GROWTH AND THE EQUIVALENCE OF hma and HM LOCI FUNCTIONS

Amar J. S. Klar ¹ and Seymour Fogel ¹

¹ Department of Genetics, University of California, Berkeley, California 94720

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 /. Homothallism genes can switch a/a or / vegetative diploid cells, generated from a strain with genotype a/ HO/ho HM/HM HMa/HMa, to a/ diploids or a/a// tetraploids during a given mitotic division cycle. We found that both a/a and / sectors generated from a strain with genotype a/ HO/HO hm/hm hma/HMa switch to a/ diploids or a/a// tetraploids. This finding supports Naumov and Tolstorukov's suggestion (1973) that the hm a allele provides for the same functions as the HM allele, namely, a switch at the mating-type locus from 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 HM loci (55cM), confirming Harashima, Nogi and Oshima (1974) data, was observed.