[Modification changes of the genetic material in Saccharomyces yeasts]

The problem of mating-type switches in heterothallic yeast cells was investigated. In selective system for cytoduction in alpha x alpha crosses alpha-cytoductants were predominantly obtained. Thus matings in alpha x alpha crosses can proceed through non-heritable changes (modifications) of the mating type alpha----a. The frequency of alpha-cytoductants after UV-irradiation of the recipient cells exceeded the control value 50-90 times. The extra copy of MAT alpha dramatically decreased the frequency of cytoductants in alpha x alpha crosses, either spontaneously or after UV-irradiation. The rad18 recipient defective in postreplication repair had 70-times increased level of mating-type modifications, as compared with isogenic Rad+ strain. An explanation consistent with these data is that mating-type modifications are due to phenotypic expression of primary lesions of MAT alpha locus. Such lesions might be expressed as transient a-mating type. After the mating event, these lesions can be repaired or turned to true mutations within the MAT locus. In fact, approximately half of non-mating cytoductants from alpha x alpha crosses had the phenotype of mat alpha 2 mutants.