Immune activation decreases sperm viability in both sexes and influences female sperm storage

All animals are under the constant threat of pathogenic infection. However, little is known regarding the influence of acute infection on sperm viability, particularly in female insects. This information is crucial for our understanding of mating and immune system coevolution, considering that females store sperm and serve as the site of sperm competition. Using the fruitfly, Drosophila melanogaster, we examined the influence of infection on sperm viability and storage. Twenty-four hours after haemocoel inoculation with a pathogen mimic (peptidoglycan, PGN) both sexes exhibited reduced sperm viability, indicating that systemic immune activation played a significant role in gamete survival. Surprisingly, sperm death did not appear to result from a reproductive-immune system trade-off, considering that sperm survived 24 h in vitro once removed from their somatic resources. Instead, our results are most consistent with death owing to immune effector collateral damage. We also examined the potential for sexually transmitted pathogens to influence sperm storage. Females mated with 'infected' males (created by dipping genitalia into a PGN solution) exhibited a higher proportion of empty sperm stores 48 h after mating compared to their controls. Remarkably, these data indicate that females may increase their fitness by removing 'infected' ejaculates from storage over time.