Patterns of DNA sequence variation suggest the recent action of positive selection in the janus-ocnus region of Drosophila simulans

Levels of nucleotide polymorphism in three paralogous Drosophila simulans g enes, janusA (janA), janusB (janB), and ocnus (ocn), were surveyed by DNA s equencing. The three genes lie in tandem within a 2.5-kb region of chromoso me arm 3R. In a sample of eight alleles from a worldwide distribution we fo und a significant departure from neutrality by several statistical tests. T ire most striking feature of this sample was that in a 1.7-kb region contai ning the janA and janB genes, 30 out of 31 segregating sites contained vari ants present only once in the sample, and 29 of these unique variants were found in the same allele. A restriction survey of an additional 28 lines of D. simulans revealed strong linkage disequilibrium over the janA-janB regi on and identified six more alleles matching the rare haplotype. Among the r are alleles, the level of DNA sequence variation was typical for D. simulan s autosomal genes and showed no departure from neutrality. In addition, the rare haplotype was more similar to the D. melanogaster sequence, indicatin g that it was the ancestral form. These results suggest that the derived ha plotype has risen to high worldwide frequency relatively recently, most lik ely as a result of natural selection.