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 doublestrand 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.

New equations and a method for finding nine parameter values for two alleles at one locus to study gene conversion using Ascobolus immersus

“…The tests and results for that were discussed by Lamb (1987). The steps in the double-strand gap repair model (Szostak et al 1983;Orr-Weaver and Szostak 1985) are as follows: intimate pairing of homologous nonsister chromatids; a double-strand gap; enlargement of the gap to a doublestrand break; and repair of the gap off the paired nonsister chromatid.…”

“…The two major explanations for meiotic gene conversion are hybrid DNA (hDNA) models and double-strand gap repair models. Results from Ascobolus, Sordaria, and Saccharomyces were analyzed by Lamb (1987), who concluded that all those data were consistent with hDNA as the sole origin of gene conversion and that most data were not consistent with all or most conversions arising from doublestrand gap repair. The present equations are therefore based on hDNA models of recombination, but how these equations can be adapted to include double-strand gap repair is discussed.…”

The nine-parameter gene conversion model: simpler equations, validity tests, and multiple fits

“…Gene conversion studies have provided much important information on the mechanism of homologous meiotic recombination, as discussed by Orr-Weaver and Szostak (1985), Lamb (1987) and Stahl (1994). One approach is to use octads of fungal spores from meiosis for detailed analysis of gene conversion events.…”

“…Their scheme included symmetric and asymmetric hybrid-DNA (hDNA), as in the Meselson and Radding (1975) model, although it is not tied to the particular details of that model. The relevance of double-strand gap-repair as a source of gene conversion in such fungi as Ascobolus was assessed by Lamb (1987), with a quantitative treatment by Lamb and Zwolinski (1992).…”

Gene conversion disparity in yeast:its extent, multiple origins, and effects onallele frequencies

“…Hybrid DNA could arise as a result of initiation by double-strand breaks or gaps, or by other means, as in the Meselson & Radding (1975) model. Although double-strand gap-repair can cause conversion in yeast, conversion will usually be discussed as arising from hDNA, because results from yeast and filamentous fungi fitted expectations of conversion from hDNA, not from double-strand gaps (Lamb, 1987;Detloff et al, 1991), although double-strand breaks probably initiate the hDNA. On the modified DSBR model (Sun et al, 1991), most conversions arise from hDNA, not from double-strand gap-repair.…”