Genetic diversity and structure of the tropical seagrass Cymodocea serrulata spanning its central diversity hotspot and range edge

Persistence of populations at their distributional ranges relies on local population dynamics and the fitness of species with low dispersal potential. We analyzed the population genetic diversity and structure of a tropical seagrass species, Cymodocea serrulata , at 34 sites spanning Philippine (diversity hotspot) and Ryukyu Islands (northern limit of distribution) populations using microsatellite (SSR) markers. Seagrass populations in the diversity hotspot are hypothesized to contain higher genetic diversity and clonal richness than those resulting from expansion or geographic range limits. We tested this hypothesis by comparing the genetic diversity, genetic structure and clonal richness of C. serrulata populations in the Philippines and Ryukyu Islands. C. serrulata populations showed decreased genetic diversity and clonal richness at their northern limit. Clonal reproduction predominated at the northern limit, while sexual reproduction prevailed in the diversity hotspot. Decreased genetic diversity and clonal richness at the northern limit may be the consequence of drift resulting from founder effect, reduced habitat, sea surface temperature and low gene flow and/or natural selection across life stages, wherein clonal reproduction confers greater environmental fitness. Analysis of molecular variance (AMOVA) and the fixation index, F ST , showed significant genetic differentiation within and among geographic populations. STRUCTURE analysis revealed that the Ryukyu Islands populations were mosaics of genets from the eastern Philippines, likely carried by the Kuroshio Current.