Do Candidate Genes Mediating Conspecific Sperm Precedence Affect Sperm Competitive Ability Within Species? A Test Case in Drosophila

When females mate to multiple males, the last male to mate fathers the majority of progeny. When males of different species inseminate a female, the sperm of the male conspecific to the female is favored in fertilization in a process known as conspecific sperm precedence (CSP). A large number of studies in Drosophila have assayed the genetic basis of sperm competition, with a main focus on D. melanogaster and accessory gland protein genes. Only a few studies have attempted to disentangle the genetic basis of CSP between related species of Drosophila. Although there is no a priori reason to believe that genes influencing intraspecific sperm competitive ability might also mediate conspecific sperm precedence, no study has addressed the question. Here, we test a group of candidate CSP genes between D. simulans and D. mauritiana for their effect on sperm competition in D. melanogaster. The use of P-element insertion lines identified CG14891 gene disruption as the only one causing a significant decrease in second male paternity success relative to wild-type and ebony tester males. The gene disruption affected both sperm displacement and the sperm fertilizing ability. Out of five genes tested using RNA interference, only gene knockdown of CG6864 (Mst89B) significantly reduced the male’s ability to father progeny when second to mate. Our results suggest that CG14891 and CG6864 might have been co-opted from an intraspecies gene function (i.e., sperm competition) into an interspecies avoidance phenotype (i.e., CSP). Alternatively, the dual role of these genes could be a consequence of their pleiotropic roles.