Different Forces Drive the Evolution of the Acp26Aa and Acp26Ab Accessory Gland Genes in the Drosophila melanogaster Species Complex

The Acp26Aa and Acp26Ab genes that code for male accessory gland proteins are tandemly arranged in the species of the Drosophila melanogaster complex. An ∼1.6-kb region encompassing both genes has been sequenced in 10, 24, and 18 lines from Spain, Ivory Coast, and Malawi, respectively; the previously studied 10 lines from North Carolina have also been included in the analyses. A total of 110 nucleotide and 4 length polymorphisms were detected. Silent variation for the whole Acp26A region was slightly higher in African than in non-African populations, while for both genes nonsynonymous variation was similar in all populations studied. Based on Fst estimates no major genetic differentiation was detected between East and West Africa, while in general non-African populations were strongly differentiated from both African populations. Comparison of polymorphism and divergence at synonymous and nonsynonymous sites revealed that directional selection acting on amino acid replacement changes has driven the evolution of the Acp26Aa protein in the last 2.5 myr.