Nutrient limitation during development can restrict the ability of adults to invest in costly fitness traits, and genotypes can vary in their sensitivity to developmental nutrition. However, little is known about how genotype and nutrition affect male ability to maintain ejaculate allocation and achieve fertilization across successive matings. Using 17 isogenic lines of Drosophila melanogaster, we investigated how variation in developmental nutrition affects males' abilities to mate, transfer sperm, and sire offspring when presented with successive virgin females. We found that, with each successive mating, males required longer to initiate copulation, transferred fewer sperm, and sired fewer offspring. Males reared on a low-nutrient diet transferred fewer sperm than those reared on nutritionally superior diets, but the rate at which males depleted their sperm, as well as their reproductive performance, was largely independent of diet. Genotype and the genotype × diet interaction explained little of the variation in these male reproductive traits. Our results show that sperm depletion can occur rapidly and impose substantial fitness costs for D. melanogaster males across multiple genotypes and developmental environments.

Rapid exaggeration of host and pathogen traits via arms race dynamics is one possible outcome of host–pathogen coevolution. However, the exaggerated traits are expected to incur costs in terms of resource investment in other life-history traits. The current study investigated the costs associated with evolved traits in a host–pathogen coevolution system. We used the Drosophila melanogaster (host) –Pseudomonas entomophila (pathogen) system to experimentally derive two selection regimes, one where the host and pathogen both coevolved, and the other, where only the host evolved against a non-evolving pathogen. After 17 generations of selection, we found that hosts from both selected populations had better post-infection survivorship than controls. Even though the coevolving populations tended to have better survivorship post-infection, we found no clear evidence that the two selection regimes were significantly different from each other. There was weak evidence for the coevolving pathogens being more virulent than the ancestral pathogen. We found no major cost of increased post-infection survivorship. The costs were not different between the coevolving hosts and the hosts evolving against a non-evolving pathogen. We found no evolved costs in the coevolving pathogens. Thus, our results suggest that increased host immunity and pathogen virulence may not be costly.

Post‐copulatory sexual selection (PSS) is an important selective force that determines fitness in polyandrous species. PSS can be intense in some cases and can drive the evolution of remarkable ejaculate properties. In males, investment in ejaculate plays an important role in the outcome of PSS. Thus, males are expected to adaptively tailor their ejaculate according to the perceived competition in their vicinity. Plastic responses in ejaculate investment to variation in intrasexual competition are disparate and widespread in males. We investigated the evolution of plasticity in reproductive traits using Drosophila melanogaster populations evolving for more than 150 generations under male‐ or female‐biased sex ratios. When exposed to different numbers of competitors early in their life, males from these two regimes responded differently in terms of their copulation duration and sperm competitive ability. In addition, the effect of this early life experience wore off at different rates in males of male‐biased and female‐biased regimes with increasing time from the removal of competitive cues. Furthermore, our study finds that males change their reproductive strategies depending upon the identity of rival males. Together, our results provide evidence of the evolution of male reproductive investment that depends on socio‐sexual cues experienced early in life.