Inferring the Evolutionary Histories of the Adh and Adh-dup Loci in Drosophila Melanogaster from Patterns of Polymorphism and Divergence

The DNA sequences of 11 Drosophila melanogaster lines are compared across three contiguous regions, the Adh and Adh-dup loci and a noncoding 5' flanking region of Adh. Ninety-eight of approximately 4750 sites are segregating in the sample, 36 in the 5' flanking region, 38 in Adh and 24 in Adh-dup. Several methods are presented to test whether the patterns and levels of polymorphism are consistent with neutral molecular evolution. The analysis of within- and between-species polymorphism indicates that the region is evolving in a nonneutral and complex fashion. A graphical analysis of the data provides support for a hypothesized balanced polymorphism at or near position 1490, site of the amino acid replacement difference between Adhf and Adhs. The Adh-dup locus is less polymorphic than Adh and all 24 of its polymorphisms occur at low frequency--suggestive of a recent selective substitution in the Adh-dup region. Adhs alleles form two distinct evolutionary lineages that differ one from another at a total of nineteen sites in the Adh and Adh-dup loci. The polymorphisms are in complete linkage disequilibrium. A recombination experiment failed to find evidence for recombination suppression between the two allelic classes. Two hypotheses are presented to account for the widespread distribution of the two divergent lineages in natural populations. Natural selection appears to have played an important role in governing the overall patterns of nucleotide variation across the two-gene region.