Large Heterologies Impose Their Gene Conversion Pattern onto Closely Linked Point Mutations.

We have studied the meiotic non-Mendelian segregation (NMS) pattern of seven large heterologous combinations located in the b2 ascospore gene of Ascobolus. The NMS patterns of these aberration heterozygotes widely differ from each other and from those of point mutations located in the same genetic region. They give lower gene conversion frequencies than point mutations, no postmeiotic segregations (PMS), and either parity or disparity that favors the wild type allele. Two related deletions, G234 and G40, were studied for their effects on the conversion behavior of closely linked point mutations. We found that, when heterozygous, the deletions impose their own NMS pattern onto close mutations. These effects occur on both sides of the heterologies. The effects upon PMS and disparity of linked point mutations gradually disappear as point mutations become more distant. The effects on NMS frequencies and on aberrant 4:4 are polar. They persist for all mutations located downstream from the high conversion end of the gene. This last effect can reflect a blockage of symmetric hDNA formation by large heterologies, whereas the epistasis of the NMS pattern of large heterologies over that of closely linked point mutations suggests that large heterologies and point mutations undergo conversion by means of distinct pathways.