Intralocus sexual conflict over immune defense, gender load, and sex-specific signaling in a natural lizard population.

In species with separate sexes, antagonistic selection on males and females (intralocus sexual conflict) can result in a gender load that can be resolved through the evolution of sexual dimorphism. We present data on intralocus sexual conflict over immune defense in a natural population of free-ranging lizards (Uta stansburiana) and discuss the resolution of this conflict. Intralocus sexual conflict arises from correlational selection between immune defense and orange throat coloration in these lizards. Males with orange throats and high antibody responses had enhanced survival, but the same trait combination reduced female fitness. This sexual antagonism persisted across the life cycle and was concordant between the juvenile and adult life stages. The opposing selective pressure on males and females is ameliorated by a negative intersexual genetic correlation (r(m,f)=-0.86) for immune defense. Throat coloration was also genetically correlated with immune defense, but the sign of this genetic correlation differed between the sexes. This resulted in sex-specific signaling of immunological condition. We also found evidence for a sex-specific maternal effect on sons with potential to additionally reduce the gender load. These results have implications for signaling evolution, genetic integration between adaptive traits, sex allocation, and mutual mate choice for indirect fitness benefits.