The role of copulatory plugs in the mosquito-parasitic nematode Strelkovimermis spiculatus.

The mosquito-parasitic nematode, Strelkovimermis spiculatus (Mermithidae: Nematoda) emerges from hosts and aggregates to form mating clusters characterized by intense male-male competition for females. Successful males deposit an adhesive copulatory plug over the female vulva after insemination. In choice experiments, males strongly preferred virgin females, whereas plugged females were ignored. Males made no attempt to displace copulatory plugs deposited by previous males. Plugged females repelled males without the need for physical contact. The observed chemical repellency was independent of females, since excised plugs alone showed the same negative male response. Removal of the plug shortly after mating reduced fecundity by 90%, presumably due to spermatids leaking from the vulva. The plug as a nutritional gift hypothesis was rejected because there was no post-mating reduction in plug size that would have indicated absorption. Despite being a male adaption to sperm competition, we conclude that the copulatory plug serves the fitness needs of both males and females in multiple roles that include paternity preservation, reduced male harassment, improved male search for mates, increased fecundity, and sperm retention.