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Is the fast/slow allozyme variation at the Adh locus of Drosophila melanogaster an ancient balanced polymorphism?

Any general model of population genetics must be able to accommodate observations both of standing variation (polymorphism) and of evolutionary history (divergence). Interpretation of the connection (or lack thereof) between these two kinds of data impinges directly on our ideas of evolutionary mechanism. Studies of the alcohol dehydrogenase locus (Adh) in Drosophila melanogaster and its close relatives provide what is perhaps our most detailed description of variation and evolution of a gene at the molecular and phenotypic levels. There is abundant evidence that two common electrophoretic variants in D. melanogaster populations caused by a threonine (Fast)/lysine (Slow) polymorphism at codon 192 have different biochemical properties and are currently under some form of variable selection (reviewed in Powell 1997). Other polymorphisms at the locus may also be current targets of natural selection (Berry and Kreitman 1993), and epistatic effects among some of these polymorphisms must be considered (Stam and Laurie 1996). Molecular population genetic studies of Adh have revealed at least four interesting patterns. First, some fraction of the amino acid evolution in the melanogaster subgroup has been caused by natural selection (McDonald and Kreitman 1991). Second, a peak of excess silent polymorphisms is found at nucleotide sites that are closely linked to the Fast/ Slow polymorphism in D. melanogaster. The excess polymorphisms are incompatible with neutral molecular evolution; however, a model of balancing selection fits the data well (Kreitman and Hudson 1991). Third, the Fast electromorph in D. melanogaster appears to have increased in frequency in the recent past, as Fast haplotypes harbor little silent polymorphism and show little sequence divergence from Slow haplotypes (Kreitman 1983; Aquadro et al. 1986). Finally, correlated evolution of particular sites among different lineages suggests that epistatic selection acts on some property of Adh transcripts (Parsch, Tanda, and Stephan 1997). One message from these molecular analyses is that the evolutionary history of Adh is as complex as it is interesting. Despite the wealth of accumulated knowledge about Adh polymorphism and divergence, several questions remain. One of the mysteries associated with Adh nucleotide variation in D. melanogaster is the observation by Kreitman and Aguade (1986) and Kreitman and Hudson (1991)