Evolutionary inferences from DNA variation at the 6-phosphogluconate dehydrogenase locus in natural populations of drosophila: selection and geographic differentiation.

Several allozyme-coding genes in Drosophila melanogaster show patterns suggesting that polymorphisms at these loci are targets of balancing selection. An important question is whether these genes have similar distributions of underlying DNA sequence variation which would indicate similar evolutionary processes occurring in this class of loci. One such locus, 6-phosphogluconate dehydrogenase (Pgd), has previously been shown to exhibit clinal variation for Fast/Slow electromorph variation in the United States and Australia, unusually large electromorph frequency differences between the United States and Africa, and other patterns indicative of selection. We measured four-cutter DNA restriction site and allozyme variation at Pgd among 142 D. melanogaster X chromosomes collected from several geographic regions including North Carolina, California, and Zimbabwe (Africa). We also sequenced a representative sample of 13 D. melanogaster Pgd genes collected in North Carolina and a single copy of Pgd from the sibling species, Drosophila simulans. While some population genetic models predict excess DNA polymorphism in genes which are targets of balancing selection, the D. melanogaster samples from the United States had significantly reduced levels of DNA polymorphism and extraordinarily high levels of linkage disequilibrium, providing evidence of hitchhiking effects of advantageous mutants at Pgd or at linked sites. Therefore, while selection has probably influenced the distribution of DNA variation at Pgd, the precise nature of these selective events remains obscure. Since the Pgd region appears to have low rates of crossing over, the reduced level of variation at this locus supports the idea that recombination rates are important determinants of levels of DNA polymorphism in natural populations. Furthermore, while patterns of allozyme variation are very similar at Pgd and Adh, the DNA data show that the evolutionary histories of these genes are dramatically different. We observed extensive differences in the amount and distribution of variation in D. melanogaster Pgd samples from the United States and Zimbabwe which cannot be explained by differential selection on the Fast/Slow polymorphism in these two geographic regions. Thus, genetic drift among partially isolated populations has also been an important factor in determining the distribution of variation at Pgd in D. melanogaster. Finally, we assayed four-cutter variation at Pgd in a sample of 19 D. simulans X chromosomes and observed reduced levels of DNA variability and high levels of linkage disequilibrium. These patterns are consistent with predictions of some hitchhiking models.