Polymorphism and Divergence in Regions of Low Recombination in Drosophila

Abstract

Surveys of DNA sequence variation within Drosophila populations were initially aimed at describing standing levels of substitutional and insertion-deletion polymorphism in different regions and in different species. While the general goal was a broad picture of variation, the next goal was to try to infer the mechanisms responsible for patterns of variation. For example, variation at loci on the sex chromosomes was found to be comparable to that at autosomal loci, i. e. no evidence for any reduction of variation was seen. While there is little power in these studies to test the expected 3/4 reduction in heterozygosity expected under pure selective neutrality, they do rule out recessive, mildly deleterious mutations as a major component of DNA sequence polymorphisms. In surveys of restriction-map variation at the white 28 and zeste-tko 1 regions in 64 lines of D. melanogaster, the level of variation attributable to nucleotide substitutions or the estimated heterozygosity per nucleotide (0.009 and 0.004, respectively) was similar to that previously reported for autosomal regions. These estimates provided no support for models that predicted less polymorphism on the X chromosome.