Contrasting histories of three gene regions associated with In(3L)Payne of Drosophila melanogaster.

In the present report, we studied nucleotide variation in three gene regions of Drosophila melanogaster, spanning > 5 kb and showing different degrees of association with the cosmopolitan inversion In(3L)Payne. The analysis of sequence variation in the regions surrounding the breakpoints and the heat shock 83 (Hsp83) gene locus, located close to the distal breakpoint, revealed the absence of shared polymorphisms and the presence of a number of fixed differences between arrangements, indicating absence of genetic exchange. In contrast, for the esterase-6 gene region, located in the center of the inversion, we observed the presence of shared polymorphisms between arrangements suggesting genetic exchange. In the regions close to the breakpoints, the common St arrangement is 10 times more polymorphic than inverted chromosomes. We propose that the lack of recombination between arrangements in these regions coupled with genetic hitchhiking is the best explanation for the low heterozygosity observed in inverted lines. Using the data for the breakpoints, we estimate that this inversion polymorphism is around 0.36 million yr old. Although it is widely accepted that inversions are examples of balanced polymorphisms, none of the current neutrality tests including our Monte Carlo simulations showed significant departure from neutral expectations.