Effects of Relative Abundance on Sexual Isolation and Hybridization Risk in a Naturally Occurring Hybrid Zone of Chrysochus Leaf Beetles

Reinforcement theory is a controversial mechanism by which speciation can occur through reduced hybrid fitness promoting the evolution of reproductive isolating mechanisms. Populations within a hybrid zone may be at differing relative abundances which may affect their risk of hybridization and strength of sexual isolation. The effect of relative abundance has been seldom examined in studies of reinforcement. Chrysochus cobaltinus and C. auratus leaf beetles from a zone of secondary contact in central Washington were run in various multi-choice mating trials to determine if lab results were indicative of those occurring in nature and whether relative abundance had an effect on risk of hybridization, pairwise sexual isolation (PSI) and overall isolation (IPSI). Results from lab experiments were found to be representative of those in nature. Relative abundance had a significant effect on both the risk of hybridization. In addition, our results suggested that overall sexual isolation and that as a species became rarer, they also became choosier. This sets the stage for further research on hybridization risk and relative abundance to be taken into the field setting, and indicates that studies of reinforcement should consider not only relative abundance, but also the relationship between relative abundance and choosiness, to better understand the risk of hybridization

Relative Abundance and the Species-Specific Reinforcement of Male Mating Preference in the Chrysochus (Coleopterachrysomelidae) Hybrid Zone

Most studies of reinforcement have focused on the evolution of either female choice or male mating cues, following the long-held view in sexual selection theory that mating mistakes are typically more costly for females than for males. However, factors such as conspecific sperm precedence can buffer females against the cost of mating mistakes, suggesting that in some hybrid zones mating mistakes may be more costly for males than for females. Thus, the historical bias in reinforcement research may underestimate its frequency. In this study, we present evidence that reinforcement has driven the evolution of male choice in a hybrid zone between the highly promiscuous leaf beetles Chrysochus cobaltinus and C. auratus, the hybrids of which have extremely low fitness. In addition, there is evidence for male choice in these beetles and that male mating mistakes may be costly, due to reduced opportunities to mate with conspecific females. The present study combines laboratory and field methods to quantify the strength of sexual isolation, test the hypothesis of reproductive character displacement, and assess the link between relative abundance and the strength of selection against hybridization. We document that, while sexual isolation is weak, it is sufficient to produce positive assortative mating. In addition, reproductive character displacement was only detected in the relatively rare species. The strong postzygotic barriers in this system are sufficient to generate the bimodality that characterizes this hybrid zone, but the weak sexual isolation is not, calling into question whether strong prezygotic isolation is necessary for the maintenance of bimodality. Growing evidence that the cost of mating mistakes is sufficient to shape the evolution of male mate choice suggests that the reinforcement of male mate choice may prove to be a widespread occurrence