Extreme population genetic differentiation and secondary contact in the freshwater copepod Acanthodiaptomus pacificus in the Japanese Archipelago



    1. Department of Ecosystem Ecology, Graduate School of Life Sciences, Tohoku University, 6-3 Aramaki aza aoba, Sendai, Miyagi 980-8578, Japan
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      Present address: Institute of Biological Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan

  • Wataru Makino and Akifumi S. Tanabe (at the time of this research) belong to the Laboratory of Community and Ecosystem Ecology, Tohoku University. This laboratory explores a wide range of research topics in aquatic and terrestrial ecology. WM is interested in all aspects of copepods, especially phylogeography and ecological stoichiometry. AST works on molecular phylogenetics and its application to research on evolutionary biology.

Wataru Makino, Fax: +81-22-795-6686; E-mail: wmakino@mail.tains.tohoku.ac.jp


We investigated the sequence variation in the mitochondrial cytochrome c oxidase subunit 1 (mtCOI) gene and the nuclear ribosomal internal transcribed spacers (ncITS) of the calanoid copepod Acanthodiaptomus pacificus in Japan. A. pacificus individuals were divided into three divergent mtCOI lineages (mt-A, -B and -C). mt-A was distributed in the northernmost part of Japan, from Hokkaido to the northern part of Honshu Island, whereas mt-C was the southernmost lineage, distributed from central Honshu to Shikoku and Kyushu Islands. mt-B was distributed between these former two lineages, resulting in parapatry with mt-C and mt-A. In all lineages, 80% of the localities were fixed for a single haplotype, and different localities tended to have different haplotypes. The degree of genetic differentiation among these lineages (15–22%) was at an interspecific level, according to the criteria of the DNA barcode technique. However, the topology of ncITS was not congruent with that of mtCOI, as the reciprocal monophyly was not observed within mt-B and mt-C. Therefore, we merged them into the Southern Lineage and separated it from the Northern Lineage (i.e. mt-A). Evidence of introgression was found within the Southern Lineage, while gene flow was not observed between the Northern and Southern Lineages, suggesting that A. pacificus is a cryptic species complex. We also argue that genetic differentiations of A. pacificus in Japan may reflect the history of separation, transgression and regression of the landmass during the formation of current Japanese Archipelago.