The evolution of mating cues in a beetle hybrid zone: causes of geographic variation in cuticular hydrocarbon profiles

The reinforcement of pre-mating barriers in a hybrid zone often leads to reproductive character displacement. However, it can be difficult to link evidence for reproductive character displacement with specific traits important to mate choice. I analyzed the cuticular hydrocarbon (CHC) profiles of Chrysochus cobaltinus and C. auratus to assess whether these pheromones are responsible for the previously observed pattern of reproductive character displacement in the Chrysochus hybrid zone. I found significant CHC divergence between the species, but overall CHC divergence was not higher among hybrid zone populations. However, CHC profiles of sympatric C. cobaltinus were more homogenous and were significantly different from CHC profiles of conspecific allopatric populations near the hybrid zone. Allozyme based analyses of population structure indicated that genetic relatedness is not a likely explanation for C. cobaltinus CHC variation. When controlling for genetic distance, there was greater CHC divergence between sympatric populations and allopatric populations near the hybrid zone, compared to divergence between sympatric populations and allopatric populations far from the hybrid zone. This result indicates that populations in the vicinity of the hybrid zone may be responding to unique selection pressures. In addition, species-specific differences in CHC variation support the hypothesis that reinforcing selection on females is species-specific. Overall results provide partial support for the hypothesis that previously observed reproductive character displacement is due to changes in Chrysochus CHC profiles. Further research on the compounds governing mate choice will provide more information on the evolution of reproductive barriers in this system.