Latitudinal clines for nucleotide polymorphisms in the Esterase 6 gene of Drosophila melanogaster

Previous studies have found non-neutral patterns of nucleotide polymorphism in the promoter and coding regions of Est6 in D. melanogaster. Coding region polymorphism peaks around two closely linked replacement differences associated with the EST6-F/EST6-S allozyme polymorphism. The promoter contains two common, highly diverged haplotype groups, P1 and P7, that differentially affect Est6 expression. Allozyme studies have also revealed latitudinal clines in EST6-F and EST6-S frequencies that recur across continents. Here we analyse nucleotide polymorphisms across the promoter and the region of peak coding sequence polymorphism in 10 Australian populations along a 25 degrees latitudinal gradient in order to examine the basis for the allozyme clines. As with the earlier studies, we find an excess of intermediate to high frequency variants in both the P1/P7 region and around the two EST6-F/EST6-S replacements in some populations. The two EST6-F/EST6-S replacement polymorphisms show latitudinal clines whereas the P1 and P7 groups of promoter haplotypes do not. However the strongest clines are for three co-segregating silent site polymorphisms in a 4 bp stretch at the 3' end of the sequenced region. Monte Carlo simulations show that the clines for those three sites can explain all others in the data but none of the others can explain those three. Thus the allozyme clines may not reflect selection on either the P1/P7 polymorphism or the two replacements previously associated with the EST6-F/EST-S difference.