Two locally acting genetic controls of gene conversion,ccf-5andccf-6, inAscobolus immersus

Abstract

SUMMARYTwo new conversion control factors (ccfs),ccf-5andccf-6, have been characterized in the Pasadena strains ofAscobolus immersus. Both are monogenic, with two known allelic forms (calledAandB) of each factor, and affect the frequency of meiotic gene conversion at awhite(w) ascospore locus closely linked to it,ccf-5affectingw-9andccf-6affectingw-BHj. Theccfs appear to be specific to their own target site, with no effect on at least nine unlinkedwmutations. Conversion of thewlocus affected was studied in + ×wcrosses with all four possibleccfarrangements: for example, for + ×w-9, withccf-5(A) in both parents, withccf-5(B) in both parents, withccf-5(A) in +,Binw-9, and withccf-5(B) in +,Ainw-9. For bothccfs, there were slight differences between crosses homozygous forAand those homozygous forB, and also slight differences between the two forms of heterozygous cross,A/BandB/A, but the major effect was for heterozygosity for the control factor to depress conversion frequency of thewlocus, compared with either homozygous state. These twoccfs are compared with other sites affecting recombination in fungi and higher eukaryotes. Two possible modes of action ofccfs5and6are (i) on pairing closeness before hybrid DNA initiation, and (ii) on later stages such as the spread of hybrid DNA from an initiation point.