Genetic structure of Cochlodium polykrikoides populations revealed by microsatellite polymorphism

To reveal the process of population expansion and ascertain the origin of the Sea of Japan population of the noxious red tide forming dinoflagellate Cochlodinium polykrikoides, 13 samples, isolated from 11 different localities along the Japanese and Korean coasts, were analyzed using ten polymorphic microsatellites. Population genetic analyses such as population differentiation and assignment tests identified three clusters, i.e. 1) the Sea of Japan samples, 2) Yatsushiro Sea (Kumamoto Pref.) samples and 3) the Seto Inland Sea with pacific samples, indicating the genetic structuring of 13 C. polykrikoides samples into three distinct populations. The proportion of shared alleles in paired individuals (P_SAxy) between samples from the Sea of Japan and other samples was very low, indicating that a large genetic barrier has occurred between these two populations. No significant relationship between genetic distance versus geographical distance and almost no genetic distance was detected between paired samples within the Sea of Japan, although there is a distance of >600 km at maximum. In addition, the P_SAxy data values for samples within the Sea of Japan, samples were much higher than for those in other areas clearly showing a strong gene flow from west to east along the Japanese coast, i.e. a large scale transfer of Cochlodinium, has occurred by the Tushima Warm Current. For the P_SAxy, data for the Seto Inland Sea and Pacific samples, individuals showing relatively high P_SAxy, were concentrated in the three areas of Nagasaki (NAG), Hyogo (HYO) and Mie (MIE) Prefecture, suggesting that frequent gene flow by human-assisted dispersal through the translocation of pearl oyster spats may have occurred between these locations, despite NAG being separated from MIE by a long distance (ca. 700 km). In this study, significant population differences suggested that genetically distinct populations of C. polykrikoides have been locally established in the Seto Inland Sea from the pacific populations. No significant differences and a markedly low genetic distance within Yatsushiro Sea samples collected at the same location but in different years, indicated the stability of the genetic structures. Considering these data, C. polykrikoides has over wintered and survived as vegetative populations in the water columns of each local water mass, where the lowest water temperature is perhaps >10°C, implying possible settlement in these coastal areas after new introductions. This scenario may point to one of the processes of population expansion and globalization in C. polykrikoides in temperate regions. In addition, rising water temperatures by global warming effects would also help their survival during cold winters and further population expansions.