Phylogeography of the copepod Tigriopus japonicus along the Northwest Pacific rim

Copepod genus Tigriopus is distributed worldwide in coastal splash-pools, and is recognized as a suitable taxon for testing evolutionary and environmental hypotheses that are also relevant to water column research. Here, we present the complete DNA sequence (7709 bp) of a single ribosomal DNA (rDNA) unit of a Tigriopus species distributed along the coastlines of the Northwest Pacific ocean. The genomic organization of the Tigriopus japonicus (T. japonicus) rDNA was observed to be 18S (1817 bp)-ITS1 (465 bp)-5.8S (165 bp)-ITS2 (226 bp)-28S (3628 bp)-IGS (1417 bp). The relationship of populations was studied using rDNA ITS (internal transcribed spacer) sequences from different geographical areas. The phylogenetic analysis of 18S rDNAs showed a single monophyletic clade of the Copepoda with an unresolved polytomy of the Harpacticoida, Monstrilloida and Siphonostomatoida. In Harpacticoida phylogeny, three species of Tigriopus were monophyletic; T. japonicus and Tigriopus californicus as sister species formed a cluster with Tigriopus brevicornis. Furthermore, phylogeographic analysis of ITS-5.8S sequences showed that Tigriopus from 15 Asian locations formed four distinct clades according to their geographical origins. When ITS-5.8S of those Asian specimens was compared with the corresponding mitochondrial COI (cytochrome c oxidase subunit I) gene data, a high similarity was found in the identical genotypes. However, they were different between the localities. This suggests that four groups identified from Asian Tigriopus would be different (sub)species, and the pattern of Asian Tigriopus genetic relatedness is due to their geographical separation.