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Mutation of the two homothallic genes, HMLα/HMLa and HMRa/HMRα, in homothallic strains of Saccharomyces cereuisiae was studied. Of 11 mutants of the HMLa gene, eight were due to a phenotypic mutation from HMLα to HMLa, i.e., a mutation causing a change in function of the original HML allele to that of the other HML allele (functional mutation), and three were due to a defective mutation at the HMLα gene, i.e., a mutation causing a non-functional allele (nonfunctional mutation). All 14 mutants of the HMRa gene, on the other hand, were due to a phenotypic mutation from HMRa to HMRα i.e., a functional mutation. Phenotypic reverse mutations, i.e., HMLa to HMLα and HMRα to HMRa, were also observed in the cultivation of EMS (ethyl methanesulfonate) treated spores having the HO HMRα HMLa genotype. Mutation from heterothallic cells to homothallism was observed in a nonfunctional mutant of the HMLα gene, by mutagenesis with EMS, but not in the functional mutants of the HMLα and HMRa genes or in the authentic strains having the α HO HMRα HMLα (α Hp) and a HO HMRa HMLa (a Hq) genotypes. These observations suggest that the functional mutation is not caused by the direct mutation from a homothallic allele to the opposite, but by replacement of a transposable genic element produced from a homothallic locus with a region of a different homothallic locus. These observations also support the controlling-element model and the cassette model, which have been proposed to explain the mating-type differentiation by the homothallic genes.

DUPLICATED GENES PRODUCING TRANSPOSABLE CONTROLLING ELEMENTS FOR THE MATING-TYPE DIFFERENTIATION IN SACCHAROMYCES CEREVISIAE