Direct benefits of ejaculate-derived compounds on female reproduction and immunity in the field cricket, Gryllus texensis
Anne M. Bronikowski
Sex consumes energy and drastically increases rates of disease, yet many female animals mate more than is necessary to ensure reproductive success. Surprisingly, this trend exists even in species in which the only resource that females receive from mating is the ejaculate itself. In this dissertation, I use the Texas field cricket, Gryllus texensis, to investigate how high mating rates enhance female fecundity and immune response, resulting in increased lifetime fitness. Using laboratory experiments, I examine the effect that mating multiple times and with multiple males has on female fecundity and survival. My results demonstrate that although the risks associated with mating are significant, the immediate fecundity benefits that females receive from high mating rates likely outweigh the costs.
Next, I investigate the sexual transfer and storage of prostaglandin E2 (PGE2), a component of the ejaculate that has long been hypothesized to be responsible for the increases in fitness that female crickets exhibit after multiple mating. My results confirm that PGE2 fits the predictions required of it to be a likely candidate, as it is 1) transferred to the female spermatheca during copulation, 2) ephemerally available after mating, and 3) replenished and accumulated by females that mate multiple times. If ejaculate-derived PGE2 truly acts as a fitness-enhancing compound providing direct benefits to females, it provides a proximate mechanism for why mated females experience increased fecundity and survival.
In the final portion of this dissertation, I experimentally test the effects of sexual maturity, copulation, accessory fluids, and testes-derived compounds on female fecundity and survival of a bacterial pathogen. My results demonstrate that testes-derived compounds are responsible for both inducing egg laying and up-regulating immune response in mated female crickets. We posit that the acquisition of this limiting compound positively impacts female cricket physiology, thereby driving repeated mating events despite the energetic costs and disease risks of mating. These data reveal a potential biochemical mechanism to the end result of increased fitness achieved by females that mate frequently, providing a deeper understanding of how widespread multiple mating could evolve.