Positive selection at a seminal fluid gene within a QTL for conspecific sperm precedence

The role of sexual selection in driving the rapid evolution of male reproductive proteins has been tested in a wide variety of organisms. Sperm competition is a form of postmating sexual selection that can contribute to reproductive isolation between species by biasing the proportion of progeny fathered by conspecific over heterospecific males. This phenomenon is known as conspecific sperm precedence (CSP). A previous quantitative trait loci study between Drosophila simulans and D. sechellia identified a locus associated with CSP within the second chromosome centered at the 53 cytogenetic map position. Male accessory gland proteins (ACPs) are associated with triggering postmating physiological responses in D. melanogaster females that can contribute to differential male reproductive success. Moreover, a large number of ACPs evolve rapidly and under positive selection among closely-related species of Drosophila. Here we have sequenced five candidate Acp genes (Acp53C14a, Acp53C14b, Acp53C14c, Acp53Ea and Acp54A1) within the previously mapped D. simulans-D. sechellia CSP locus from different D. simulans and D. sechellia strains. Polymorphism data analysis shows evidence of a selective sweep at Acp53Ea within D. simulans. In the context of CSP, the combined use of polymorphism and interspecies sequence divergence shows that Acp53C14c gene tree topology separates D. simulans and D. sechellia. Moreover, Acp53C14c is the only gene showing evidence of positive selection with five fixed amino acid substitutions between species. Our results highlight Acp53C14c as a candidate gene for future gene targeting studies to elucidate its role in CSP between D. simulans and D. sechellia.