Adh nucleotide variation in Drosophila willistoni: High replacement polymorphism in an electrophoretically monomorphic protein

Drosophila willistoni was the subject of intensive allozyme studies and the locus coding for alcohol dehydrogenase (Adh) was found to be virtually monomorphic. DNA sequence analysis of 18 alleles throughout the distribution of the species has revealed six replacement polymorphisms. The ratio of replacement to silent polymorphisms is higher in D. willistoni than in any other Drosophila species studied for Adh nucleotide variation. Also in contrast to other species, the variation in introns and noncoding DNA is about the same as in the coding region. We speculate that both these differences indicate D. willistoni has historically had a small population size possibly related to Pleistocene refugia in the Neotropics.