Phenotypic expression of primary lesions of genetic material in <i>Saccharomyces yeasts</i>

"Illegitmate" mating of yeasts (α × α), either spontaneous or induced by uv light or ethyl methanesulfanate, in a selective system for "cytoduction" revealed that about 95% of cytoductants expressed their original (α) mating type. Inducing the mating by treating the recipient of cytoplasm with uv light reached two orders of magnitude. An additional copy of MATα in the α recipient almost completely eliminated the effect, which means that nonheritable mating type changes observed are formally recessive and are localized within MATα complex. About 1% of cytoductants obtained were nonmating types and some of them were identified as matα1 mutants. Rad18 mutant as a recipient showed a considerably elevated spontaneous frequency of illegitimate hybridization and cytoduction. The cytoductants also preserved the original mating type. These facts suggest that nonheritable changes of mating type are due to repairable primary (premutational) lesions in MATα genetic material. The significance of these results for understanding the mechanism of nonheritable variability is discussed.Key words: mating type, nonheritable changes, primary lesions, premutational events, Saccharomyces cerevisiae, repair. </jats:p