Genome-wide molecular phylogenetic analyses and mating experiments which reveal the evolutionary history and an intermediate stage of speciation of a giant water bug

The intermediate stages of speciation are important for understanding the processes involved in the creation of biodiversity, and also comprise a number of interesting phenomena. However, difficulties are associated with dividing clear speciation stages because speciation is a continuous process. Therefore, the elucidation of speciation is an interesting and important task in evolutionary biology. We herein present an example of a species in an intermediate stage of speciation using the giant water bug Appasus japonicus (Heteroptera, Belostomatidae) that was investigated using mating experiments and phylogenetic analyses of the mtDNA COI (658 bp) and 16S rRNA (435 bp) regions, and nDNA SSR (13 loci) and its genome-wide SNPs (11,241 SNPs). The results of our phylogenetic analyses based on their mtDNA data set and the genome-wide SNPs data set strongly supported the paraphyly of the Japanese populations. Therefore, it is suggested that their ancestral lineage which being distributed in the Japanese Archipelago subsequently migrated to the Eurasian Continent (i.e., back-dispersal occurred). Furthermore, the results of the mating experiments suggested that among A. japonicus, even between closely related lineages, premating reproductive isolation has been established by the differentiation of copulatory organ morphologies. In contrast, premating reproductive isolation is not established in the absence of the differentiation of copulatory organ morphologies, even if genetic differentiation is prominent. These results suggested that their phylogenetic distance does not predict premating reproductive isolation. Furthermore, in the present study, we present a clear example of premating reproductive isolation driving speciation between closely related lineages.