Carcass Scavenging Relaxes Chemical-Driven Female Interference Competition in Flour Beetles.

AbstractFemale-female nonsexual interference competition is a major fitness determinant of biased sex ratio groups with high female density. What strategies can females use to overcome the negative impact of this competition? To answer this question we used flour beetles (Tribolium castaneum) where competing females from female-biased groups were already known to suppress each other's fecundity by secreting toxic quinones from their stink glands, indicating a unique chemical-driven interference competition. Surprisingly, increasing resources did not alleviate these fitness costs. Females also did not disperse more from the site of interference competition. Hence, the competition was influenced by neither the total resource availability nor the lack of opportunity to avoid chemical interference. Instead, protein sequestered via scavenging of nutrient-rich carcasses relaxed female competition by increasing fecundity and reducing the quinone content. Finally, stink gland components themselves triggered carcass scavenging and increased fecundity, indicating the possibility of a novel chemical-driven feedback loop to reduce the competition. In the present work we provide the rare analyses where multiple competing hypotheses were jointly tested to establish carcass scavenging as an important potential strategy to overcome the fitness costs of intrasexual female interference competition.

Sexual conflict drives micro- and macroevolution of sexual dimorphism in immunity.

BACKGROUND: Sexual dimorphism in immunity is believed to reflect sex differences in reproductive strategies and trade-offs between competing life history demands. Sexual selection can have major effects on mating rates and sex-specific costs of mating and may thereby influence sex differences in immunity as well as associated host-pathogen dynamics. Yet, experimental evidence linking the mating system to evolved sexual dimorphism in immunity are scarce and the direct effects of mating rate on immunity are not well established. Here, we use transcriptomic analyses, experimental evolution and phylogenetic comparative methods to study the association between the mating system and sexual dimorphism in immunity in seed beetles, where mating causes internal injuries in females. RESULTS: We demonstrate that female phenoloxidase (PO) activity, involved in wound healing and defence against parasitic infections, is elevated relative to males. This difference is accompanied by concomitant sex differences in the expression of genes in the prophenoloxidase activating cascade. We document substantial phenotypic plasticity in female PO activity in response to mating and show that experimental evolution under enforced monogamy (resulting in low remating rates and reduced sexual conflict relative to natural polygamy) rapidly decreases female (but not male) PO activity. Moreover, monogamous females had evolved increased tolerance to bacterial infection unrelated to mating, implying that female responses to costly mating may trade off with other aspects of immune defence, an hypothesis which broadly accords with the documented sex differences in gene expression. Finally, female (but not male) PO activity shows correlated evolution with the perceived harmfulness of male genitalia across 12 species of seed beetles, suggesting that sexual conflict has a significant influence on sexual dimorphisms in immunity in this group of insects. CONCLUSIONS: Our study provides insights into the links between sexual conflict and sexual dimorphism in immunity and suggests that selection pressures moulded by mating interactions can lead to a sex-specific mosaic of immune responses with important implications for host-pathogen dynamics in sexually reproducing organisms.