Parasites physically block host copulation: a potent mechanism of parasite-mediated sexual selection

Research on the role of parasites in sexual selection has focusedmainly on host mate choice favoring relatively unparasitizedmales. But parasites can also generate variance in host reproductivesuccess by influencing the ability of individual hosts to directlycompete among themselves for mates or fertilizations, a subjectarea that has received far less attention. We demonstrate experimentallythat parasitism by mites can drive sexual selection by way ofa novel mechanism involving male competition: physical inhibitionof host copulation. Mite resistance in natural populations isheritable, emphasizing the evolutionary potential of parasite-mediatedsexual selection in this system and indicating that femalesshould be receiving indirect fitness benefits as a result ofthis process. We show that parasitism by mites, Macrochelessubbadius, reduces mating success of male Drosophila nigrospiracula.Smaller males were more strongly compromised, identifying hostbody size as a tolerance trait. As parasite load increased,the rate at which males attempted to copulate but failed becauseof obstruction by mites increased. When mites were removed frominfested males, host mating success was restored. Thus, thephysical presence of the mites per se generates differentialmating success, in this case by interrupting the normal flowof mating behaviors. This study elucidates a potent mechanismof parasite-mediated sexual selection in a system wherein parasiteresistance is demonstrably heritable, and as such expands ourunderstanding of the evolutionary potential of sexual selection.