Coincidence relations between gene conversion and mitotic recombination in Saccharomyces.

major attributes of mitotic gene conversion in Saccharomyces have been TEai led by ROMAN (1956a) and by KAKAR ( 1961) for heteroallelic diploids, while LINDEGREN (1953) , WINGE (1955) , and PAPAZIAN and LINDEGREN (1960) have investigated instances of aberrant segregation ratios in meiosis. Heteroallelic repair, or gene conversion, is essentially non-reciprocal in character. Three lines of evidence presented by ROMAN (1956a) indicate that crossing-over in the conventional sense is not implicated in the conversional process. These are: (1) double mutants are not recovered from heteroallelic revertants; (2) either input allele may be recovered from a given convertant, although not necessarily with equal frequencies; ( 3 ) heteroallelic repair is not correlated with the occurrence of mitotic recombination for linked outside marker genes. While conventional crossing-over per se may not be associated with the mechanism responsible for conversional revertants in heteroallelic diploids, it is significant that conversion rates for specific allelic combinations are enhanced following meiosis (ROMAN, 1956a). MAGNI and VON BORSTEL (1962) have also reported that mutation rates are significantly higher among meiotic products compared to mitotically dividing cells. JAMES (1955) , and JAMES and LEE-WHITING (1955), reported that ultraviolet irradiation significantly increased the frequency of mitotic recombination, as evidenced by the occurrence of visibly sectored clones. ROMAN (1956a) confirmed this finding and also demonstrated ultraviolet stimulation of gene conversion in heteroallelic diploids. Moreover, ROMAN and JACOB (1958) found that increasing ultraviolet dosage yielded a persistent increase in conversion rates, with no concomitant increase in homozygosis rates for outside linked markers. This finding, like MITCHELL’S (1957), suggests that crossing-over and conversion are separable events. However, the present study reports evidence for the coincident association of heteroallelic repair with mitotic recombination for both linked and unlinked markers within a given diploid nucleus during the course of a single DNA replication cycle.