Cymbella janischii (A. Schmidt) De Toni 1891, an endemic diatom of the Pacific Northwest, was found in 2006 in Japan, and since then, its distribution has been expanding. Here, we analyzed rbcL, psaB, psbA, 18S rRNA, and 28S rRNA gene sequences (6526 bp in total) of the C. janischii specimens from several locations in Japan and explored their genetic relatedness with C. janischii from its country of origin (the United States) and its closely related species. We showed that all Japanese specimens had the same sequences, regardless of geographical distance, and formed a clade with the US C. janischii. The identities and the pairwise distance between the sequences of the Japanese and the US diatoms were 99.937% and 0.0003, respectively, indicating that these diatoms are extremely similar. These results provide potential genetic evidence of the recent invasion and rapid spread of C. janischii from the US in Japan.
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