Excess polymorphism at the Adh locus in Drosophila melanogaster.

The evolutionary history of a region of DNA encompassing the Adh locus is studied by comparing patterns of variation in Drosophila melanogaster and its sibling species, D. simulans. An unexpectedly high level of silent polymorphism in the Adh coding region relative to the 5' and 3' flanking regions in D. melanogaster is revealed by a populational survey of restriction polymorphism using a four-cutter filter hybridization technique as well as by direct sequence comparisons. In both of these studies, a region of the Adh gene encompassing the three coding exons exhibits a frequency of polymorphism equal to that of a 4-kb 5' flanking region. In contrast, an interspecific sequence comparison shows a two-fold higher level of divergence in the 5' flanking sequence compared to the structural locus. Analysis of the patterns of variation suggest an excess of polymorphism within the D. melanogaster Adh locus, rather than lack of polymorphism in the 5' flanking region. An approach is outlined for testing neutral theory predictions about patterns of variation within and between species. This approach indicates that the observed patterns of variation are incompatible with an infinite site neutral model.