Molecular population genetics of structural variants of esterase 6 in Drosophila melanogaster

Several lines of evidence indicate that natural selection operates between the major EST6-F and EST6-S allozymes of Drosophila melanogaster. In particular, consistent latitudinal clines and seasonal variation in their relative frequencies strongly suggest that they are not selectively equivalent in field populations. Several laboratory studies have found frequency-dependent fitness differences among the Est6-F and Est6-S genotypes. Moreover, the purified EST6-F and EST6-S allozymes differ in biochemical properties and the physiology of the enzyme, as a major component of the seminal fluid, suggests that these differences could affect reproductive aspects of fitness. However, molecular analyses reveal high levels of variation in the EST6 protein both within and between the EST6-F and EST6-S allozymes. Limited thermostability and more sensitive electrophoretic analyses reveal at least 17 variants of the two allozymes and sequence comparisons among 13 isolates of the Est6 gene reveal 16 nucleotide polymorphisms that would lead to amino acid differences. Two closely linked amino acid differences are strongly associated with the major difference between EST6-F and EST6-S; either or both of these are likely to cause the observed biochemical differences between EST6-F and EST6-S and may be the primary targets for the selection between these allozymes. The functional and adaptive significance of the other amino acid polymorphisms is unclear, although the data suggest that the EST6-8 haplotype within EST6-S has both arisen and proliferated relatively recently.