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.
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