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

The control of gene conversion properties and corresponding-site interference: the effects of conversion control factor 5 on conversion at locus w9 in Ascobolus immersus

The controls of various aspects and parameters of gene conversion at locus w-9 in the fungus Ascobolus immersus were investigated, along with positive and negative corresponding-site interference in meiotic chromatid pairing. When conversion control factor 5 alleles A and B were heterozygous, conversion frequencies at the closely linked target locus w9 were reduced to 3%, compared with 10.7% when A or B was homozygous, through effects on hybrid-DNA (h-DNA) formation parameters gamma1 and gamma2. In different ways, not related to whether ccf-5 alleles were homozygous or heterozygous, ccf-5 also affected parameters relating to the relative frequencies of asymmetric and symmetric h-DNA, the frequency with which the chromatid bearing the wild-type allele was the invading chromatid in asymmetric h-DNA, and h-DNA correction parameters for the frequency and direction of correction of mispairs. Corresponding-site interference is interference between the two pairs of non-sister chromatids of a bivalent in h-DNA formation at exactly corresponding sites. This interference was positive in the high conversion frequency crosses homozygous for ccf-5 alleles but was strongly negative in the low conversion frequency crosses heterozygous for ccf-5 alleles, through differential effects on parameters gamma1 and gamma2. Models of chromatid pairing are discussed.

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

The extent of disparity in gene conversion direction in yeast (Saccharomyces cerevisiae) is important for recombination mechanisms and for effects of conversion on allele frequencies in populations. An analysis of published and unpublished data demonstrates that yeast frequently shows significant and extensive conversion disparity, contrary to many published statements. All types of mutation--base-substitutions, frameshifts and longer deletions and additions--can show significant 6:2/2:6 and/or 5:3/3:5 disparity. There was little correlation between the occurrence of 6:2/2:6 and 5:3/3:5 disparities; when both were significant, they were more often in opposite directions than in the same direction. Surprisingly, there was little correlation between a mutation's molecular nature and its disparity properties, which generally seem unpredictable. Disparity in yeast has multiple origins. From the equations discussed, all disparity types can be explained by one or more of: correction direction disparity, chromatid invasion disparity (including cases caused by different frequencies of double-strand breaks or gaps in nonsister homologous chromatids), strand invasion disparity, and different correction frequencies for the two types of mispair for a heterozygous mutation. Levels of overall disparity and of conversion frequency mean that conversion must often change allele frequencies in sexually reproducing yeast populations.