Differential intraspecific genetic variations of the closely related, wide-ranged freshwater copepods Cyclops vicinus Uljanin, 1875 and C. kikuchii Smirnov, 1932

Although Cyclops vicinus Uljanin, 1875 and C. kikuchii Smirnov, 1932 are broadly distributed in Eurasia, the genetic divergences between European and Asian populations have been investigated in neither species. In Japan, the nomenclature and distinction between these two congeners are still questionable (thus, hereafter C. vicinus s.l.). We determined the taxonomic position of the Japanese C. vicinus s.l. by morphological analyses and by sequencing nuclear internal transcribed spacer 1(ITS-1) and the mitochondrial 12S (mt12S) genes. The ITS-1 phylogeny separated Japanese C. vicinus s.l. into two genetic groups; one was clustered with European C. kikuchii (thus, CkikJPN), while the other was clustered with European C. vicinus (thus, CvicJPN). Morphological analyses using the lengths of terminal furcal setae revealed that CkikJPN and CvicJPN correspond to C. kikuchii and C. vicinus, respectively. These results revealed that C. vicinus and C. kikuchii in Japan are confidently the same species as those in Europe. The degree of divergence in the mt12S between Japanese and European populations varied largely between the species, as it was nearly at the interspecific level in C. kikuchii, while it was at the intraspecific level in C. vicinus; it is the first case in which the genetic divergence between the European and Asian population is greatly different among closely related species of freshwater copepods.