Contrasting genetic structure of closely related giant water bugs: Phylogeography of Appasus japonicus and Appasus major (Insecta: Heteroptera, Belostomatidae)

• A. japonicus and A. major are very similar in morphological characteristics.
• Furthermore, distributional ranges of them are widely overlapping.
• Japanese A. japonicus is paraphyly with respect to the continental linage.
• Whilst, Japanese A. major is monophyly with respect to the continental linage.
• These differences are due to evolutionally history of both species.

Appasus japonicus and A. major, two belostomatid species of the giant water bug found in parts of East Asia, have very similar morphological characteristics and ecological niches, and also overlapping habitats. However, the results of our previous published study utilizing molecular phylogenetic analyses of mitochondrial and nuclear DNA revealed extensive genetic differences, which indicated that the possibility of inter-specific hybridization was extremely unlikely. We collected A. japonicus and A. major from the Japanese Archipelago, Korean Peninsula, and Russian Far East, and conducted molecular analyses of mitochondrial DNA COI and 16S rRNA to compare phylogenetic relationships between these species. Three major clades were recognized within A. japonicus. Specimens from the Korean population constituted a monophyletic clade, and were a sister group of the western region of the Japanese Archipelago. The Eastern Japanese clade was clearly differentiated. Four major clades were recognized within A. major. Specimens from the Japanese and Korean populations revealed two distinct monophyletic clades. Significant differentiation was clearly observed between their genetic structures. Furthermore, the results of mismatch distribution and Bayesian skyline plot analyses suggested the possibility of a bottleneck effect or founder effect in two of the A. major clades. Collectively, these results demonstrated both similarities and differences in these two species even though their distribution widely overlaps in East Asia, with their morphological characteristics and ecological niches being very similar. These differences in genetic structures are considered to be due to their evolutionary history.

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