The decay of premating reproductive isolation via hybridization in Ohomopterus ground beetles

Given that a balance between selection and gene flow mediates the maintenance of reproductive isolation (RI) at secondary contact, evaluation of their relative contributions is important for understanding the origin of RI. We examined two hypotheses related to selection and gene flow at secondary contact by focusing on variation in male conspecific mate choice in the ground beetle Carabus maiyasanus. This species includes populations with varied degrees of genetic admixture via hybridization with parapatric Carabus iwawakianus, which may impair the adaptive set of genes for conspecific mate choice (the genetic admixture hypothesis). This species co-occurs with other related species in some areas, where conspecific mate choice could be enhanced (the reproductive character displacement hypothesis). Comparative analysis of mate choice experiments indicated that conspecific mate choice weakened with increasing hybridization, supporting the genetic admixture hypothesis. An experimental analysis revealed the contributions of both the male preference and female recognized traits. These results indicate that premating RI by conspecific mate choice decayed via hybridization, while this species exhibits reproductive character displacement in species-specific genital morphologies as an agent of postmating, prezygotic RI, providing insights into how a balance between selection and gene flow mediates the evolution and maintenance of RI at secondary contact.