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.

中文翻译：

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全基因组分子系统发育分析和交配实验揭示了巨型水虫的进化历史和物种形成的中间阶段

物种形成的中间阶段对于理解创造生物多样性的过程很重要，并且还包含许多有趣的现象。然而，困难与划分明确的物种形成阶段有关，因为物种形成是一个连续的过程。因此，阐明物种形成是进化生物学中一项有趣且重要的任务。我们在此展示了一个处于物种形成中间阶段的物种的例子，该物种使用了使用交配实验和 mtDNA COI的系统发育分析进行研究的巨型水虫Appasus japonicus (Heteroptera, Belostomatidae)(658 bp) 和 16S rRNA (435 bp) 区域，以及 nDNA SSR (13 位点) 及其全基因组 SNP (11,241 SNP)。我们基于他们的 mtDNA 数据集和全基因组 SNPs 数据集的系统发育分析结果强烈支持日本人群的副系。因此，推测其分布于日本列岛的祖先谱系随后迁移至欧亚大陆（即发生了反向扩散）。此外，交配实验的结果表明，在A . 刺参, 即使在密切相关的谱系之间, 交配器官形态的分化也已经建立了前交配生殖隔离。相反，在没有交配器官形态分化的情况下，即使遗传分化很突出，也不会建立前交配生殖隔离。这些结果表明，它们的系统发育距离并不能预测交配前的生殖隔离。此外，在本研究中，我们提出了一个明确的例子，即在密切相关的谱系之间进行生殖隔离驱动物种形成。