Tests of double-strand gap repair as a major source of meiotic gene conversion in fungi

Abstract

In current recombination theory, meiotic gene conversions may arise from hybrid-DNA (hDNA), often with correction of mispaired or unpaired bases, or from repair of double-strand gaps. The two types of model have quite different predictions for:- the occurrence and proportions of different classes of aberrant segregations; relationships between conversion properties and the molecular origin of a mutation; the extent of disparity in conversion direction; and whether a heterozygous mutation can affect the conversion properties of other nearby heterozygous mutations. These criteria are used to study the origin of meiotic gene conversions in the fungi Ascobolus, Sordaria and Saccharomyces. Novel tests are described, one based on comparisons of observed conversion frequencies for very close sites within a locus, and one based on possible correlations of 6:2/2:6 disparity with the proportion of aberrant segregations having postmeiotic segregation. From Ascobolus, Sordaria and yeast, most data are consistent with most or all meiotic conversions coming from hDNA; they are not consistent with all or most conversions arising directly from double-strand gap repair. Differences in frequencies of postmeiotic segregation between fungi with four-spored asci and ones with eight-spored asci may reflect a shorter period for hDNA correction in the latter type.