Positive selection drives the evolution of the Acp29AB accessory gland protein in Drosophila

Nucleotide sequence variation at the Acp29AB gene region has been surveyed in Drosophila melanogaster from Spain (12 lines), Ivory Coast (14 lines), a nd Malawi (13 lines) and in one line of D. simulans. The similar to 1.7-kb region studied encompasses the Acp29AB gene that codes for a male accessory gland protein and its flanking regions. Seventy-seven nucleotide and 8 len gth polymorphisms were detected. Nonsynonymous polymorphism was an order of magnitude lower than synonymous polymorphism, but still high relative to o ther non-sex-related genes. In D. melanogaster Variation at this region rev ealed no major genetic differentiation between East and West African popula tions, while differentiation was highly significant between the European an d the two African populations. Comparison of polymorphism and divergence at synonymous and nonsynonymous sites showed an excess of fixed nonsynonymous changes, which indicates that the evolution of the Acp29AB protein has bee n driven by directional selection at least after the split of the D. melano gaster and D. simulans lineages. The pattern of variation in extant populat ions of D. melanogaster favors a scenario where the fixation of advantageou s replacement substitutions occurred in the early stages of speciation and balancing selection is maintaining variation in this species.