To reveal the role of diverged body size and genital morphology in reproductive isolation among closely related species, we examined patterns of, and factors limiting, introgressive hybridization between sympatric Ohomopterus ground beetles in central Japan using mitochondrial NADH dehydrogenase subunit 5 (ND5) gene sequences. We sampled 17 local assemblages that consisted of two to five species and estimated levels of interspecific gene flow using the genetic distance, D(A), and maximum-likelihood estimates of gene flow. Sharing of haplotypes or haplotype lineages was detected between six of seven species that occurred in the study areas, indicating mitochondrial introgression. The intensity and direction of mitochondrial gene flow were variable among species pairs. To determine the factors affecting introgression patterns, we tested the relationships between interspecific D(A) and five independent variables: difference in body size, difference in genital size, phylogenetic relatedness (nuclear gene sequence divergence), habitat difference, and species richness of the assemblage. Body and genital size differences contributed significantly to preventing gene flow. Thus, mechanical isolation mechanisms reduce the chance of introgressive hybridization between closely related species. Our results highlight the role of morphological divergence in speciation and assemblage formation processes through mechanical isolation.

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Species composition and long-term fluctuation of ground beetles attracted to a light trap operated at Yamaguchi University Farm for 10 years were analyzed. A total of 77 species of Omophronidae, Carabidae, and Brachinidae were caught. Three species, Stenolophus dijj'icilis, Anoplogenius cyanescens, and Acupalpus inornatus, were most frequently caught. The fauna found was compared with those in nearby agroecosystems, and the original habitats from whence the ground beetles flew were inferred. The annual activity of the beetles during the 10 years is illustrated. Increasing, decreasing, or periodically varying abundance during the 10 years were found for different beetle species. Environmental factors seems to have no influence on the catch.

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