Does sexual dimorphism reflect sexual antagonism? Covariation of female fitness with brothers' sexual traits and their female homologues in neriid flies.

Alleles favoured by sexual selection in males can reduce fitness when expressed in females, generating intralocus sexual conflict. It remains unclear whether such conflict is fully resolved by the evolution of sexual dimorphism. If conflict persists, then female fitness might covary negatively with secondary sexual trait expression in male relatives and with sexually homologous trait expression in females themselves. However, because secondary sexual traits often exhibit strong condition dependence, a resource-poor developmental environment could weaken these covariances. We tested these predictions by manipulating larval nutrition of neriid flies (), generating high (rich diet) and low (poor diet) adult sexual dimorphism in head elongation. Consistent with predictions, in rich-diet families where male head elongation was relatively high, females produced low-viability offspring. Moreover, high female head elongation was associated with delayed oviposition. By contrast, in poor-diet families, we found no evidence of negative covariation between female fitness measures and male head elongation, while female head elongation covaried positively with some measures of female fitness. Our results confirm that sexually dimorphic morphology can reflect sexually antagonistic fitness variation, indicating that intralocus sexual conflict remains unresolved in this species. Our results also suggest that the nutritional environment can modulate the signal of sexual antagonism.