Disparity of Gene Conversion in Frameshift Mutants Located in Locus b2 of Ascobolus immersus

The frequency of conversion and the disparity in the direction of conversion were studied for six frameshift mutants lying in locus b2 of Ascobolus immersus and giving more 2 wild type:6 mutant (2+6m) than 6 wild type:2 mutant (6+2m) aberrant asci (type B). The frequency of conversion decreased from left to right in the locus. The disparity steadily increased from left to right and then reached a plateau. Twenty-two frameshift mutants giving more 6+2m than 2+6m aberrant asci (type A) and closely linked to three type B mutants were also studied; they showed the same frequency of conversion and the same disparity (but in the opposite direction) as the type B mutant to which they are linked. The polar variation of both the frequency of conversion and the disparity as a function of position were expected on the basis of a previous study of 15 mutants giving postmeiotic segregations and located in locus b2. This variation is assumed to reflect the existence of a preferential region for the initiation of hybrid DNA (HDNA) during recombination and a duality in the distribution of this HDNA, with preponderant asymmetrical HDNA near the starting point and preponderant symmetrical HDNA farther from it.