Resource-dependent evolution of female resistance responses to sexual conflict

Sexual conflict can promote the evolution of dramatic reproductive adaptations as well as resistance to its potentially costly effects. Theory predicts that responses to sexual conflict will vary significantly with resource levels-when scant, responses should be constrained by trade-offs, when abundant, they should not. However, this can be difficult to test because the evolutionary interests of the sexes align upon short-term exposure to novel environments, swamping any selection due to sexual conflict. What is needed are investigations of populations that are well adapted to both differing levels of sexual conflict and resources. Here, we used this approach in a long-term experimental evolution study to track the evolution of female resistance to sexual conflict in the fruit fly Drosophila melanogaster. In resource-rich regimes, high-conflict females evolved resistance to continual exposure to males. There was no difference in baseline survival, consistent with the idea that responses evolving under nutritional abundance experienced no trade-offs with resistance. In the poor resource regimes, the ability of high-conflict females to evolve resistance to males was severely compromised and they also showed lower baseline survival than low-conflict females. This suggested high-conflict females traded off somatic maintenance against any limited resistance they had evolved in response to sexual conflict. Overall, these findings provide experimental support for the hypothesis that evolutionary responses to sexual conflict are critically dependent upon resource levels.