Divergence revealed by population crosses in the red flour beetle Tribolium castaneum

There is growing interest in the potential for population divergence (and hence speciation) to be driven by co-evolutionary arms races due to conflicts of interest between the sexes over matings and investment in offspring. It has been suggested that the signature of sexually antagonistic co-evolution may be revealed in crosses between populations through females showing the weakest response to males from their own population compared with males from other populations. The rationale behind this prediction is that females will not have been able to evolve counter-adaptations to manipulative signals from males with which they have not co-evolved. Recent theoretical treatments suggest that this prediction is not strictly exclusive to the sexual conflict theory, but it remains the case that population crosses can provide insights into the evolution of mate choice within populations. We describe crosses between six populations of the red flour beetle Tribolium castaneum. Although successful matings are no more or less likely between populations compared to within populations, females do increase their oviposition rate in response to males from other populations, relative to males from their own population. Our results are therefore consistent with the proposition that sexual conflict has driven population divergence in this species. However, we argue that the available evidence is more supportive of the hypothesis that increased female investment in response to males from other populations is a side-effect of inbreeding avoidance within populations.