THE INDUCTION OF MITOTIC GENE CONVERSION BY X-IRRADIATION OF HAPLOID SACCHAROMYCES CEREVISIAE

Mitotic recombination in Saccharomyces cerevisiae was examined by means of experiments in which one of the haploid parents was X-irradiated prior to zygote formation. By this method radiation-induced lesions are restricted to only one of the two non-sister chromatids that may be expected to undergo mitotic exchange in the diploid. The principal results of this work are: (1) X-irradiated haploid cells that are incapable of further vegetative growth (colony formation) are efficiently rescued into viable diploids by mating with unirradiated haploid cells. (2) X-rays delivered to only one of the two haploid parents are recombinogenic in the resultant diploid. The frequency of detected recombinational events increases as a probable linear function of the X-ray dose. (3) A majority of the induced recombinational events are nonreciprocal in nature (mitotic gene conversion). These results complement those obtained from X-irradiation of the vegetative diploid itself, where the induced genetic exchanges are principally reciprocal.