Intralocus Sexual Conflict

Intralocus sexual conflict arises when there are sex-specific optima for a trait that is expressed in both sexes and when the constraint of a shared gene pool prevents males and females from reaching their optima independently. This situation may result in a negative intersexual correlation for fitness. Here I first discuss key differences between intra- and interlocus conflict, the type of sexual conflict that arises in mating interactions between males and females. I then review the experimental evidence for the existence of genomewide sexually antagonistic variation and discuss how intralocus conflict can be resolved. Substantial genomewide sexually antagonistic variation exists in Drosophila melanogaster lab populations. Yet, in the same species, sex-specific gene regulation appears to evolve rapidly, suggesting that the obstacles to the resolution of intralocus conflict are minor. The fact that negative intersexual correlations for fitness are observed even if sexual dimorphism can evolve rapidly suggests that intralocus conflict is highly dynamic. The final part of this review examines the evolutionary consequences of intralocus sexual conflict for the evolution of the sex chromosomes, sexual selection, and sex determination. Intralocus conflict helps to explain many of the peculiar features of the sex chromosomes and has shaped the functional bias and expression biases of sex-linked genes. The genomic distribution of sexually selected genes, in particular, affects sexual selection in various ways. The presence of sexually antagonistic variation can strongly interfere with the good genes' process of sexual selection and erode the genetic benefits of mate choice. Regarding sex determination, this review concentrates on evolutionary transitions between different sex determination mechanisms. Such transitions have occurred frequently in several taxa. Theory and empirical data suggest an important role for intralocus conflict in triggering switches between sex determination systems.