MULTILOCUS SELECTION AND THE STRUCTURE OF VARIATION AT THE WHITE GENEOF DROSOPHILA-MELANOGASTER

We surveyed sequence variation and divergence for the entire 5972-bp t ranscriptional unit of the white gene in 15 lines of Drosophila melano gaster and one line of D. simulans. We found a very high degree of hap lotypic structuring for the polymorphisms in the 3' half of the gene, as opposed to the polymorphisms in the 5' half. To determine the evolu tionary mechanisms responsible for this pattern, we sequenced a 1612-b p segment of the white gene from an additional 33 lines of D. melanoga ster from a European and a North American population. This 1612-bp seg ment encompasses an 834-bp region of the white gene in which the polpm orphisms form high frequency haplotypes that cannot be explained by a neutral equilibrium model of molecular evolution. The small number of recombinants in the 834-bp region suggests epistatic selection as the cause of the haplotypic structuring, while an investigation of nucleot ide diversity supports a directional selection hypothesis. A multi-loc us selection model that combines features from both hypotheses and tak es the recent history of D. melanogaster into account may be the best explanation for these data.