Genetic consequences of anagenetic speciation in Acer okamotoanum (Sapindaceae) on Ullung Island, Korea

Background and Aim Anagenesis (also known as phyletic speciation) is an important process of speciation in endemic species of oceanic islands. We investigated genetic variation in Acer okamotoanum, an anagenetically derived species endemic to Ullung Island, South Korea, to infer genetic consequences of anagenesis in comparison with other groups that have undergone cladogenesis (and adaptive radiation). Methods We examined genetic variation based on eight polymorphic microsatellite markers from 145 individuals of A. okamotoanum and 134 individuals of its putative progenitor A. mono. We employed standard population genetic analyses, clustering analyses, Bayesian clustering analyses in STRUCTURE and bottleneck analyses. Key Results Based on both the Neighbor-Joining tree and Bayesian clustering analyses, clear genetic distinctions were found between the two species. Genetic diversity in terms of allelic richness and heterozygosity shows slightly lower levels in A. okamotoanum in comparison with A. mono. Bayesian clustering analyses showed a relatively high F-value in the cluster of A. okamotoanum, suggesting a strong episode of genetic drift during colonization and speciation. There was no clear evidence of a bottleneck based on allelic frequency distribution and excess of observed heterozygotes, but the M-ratio indicated a historical bottleneck in several populations of A. okamotoanum. No geographical genetic structure within the island was found, and the genetic variation among populations of A. okamotoanum was quite low. Conclusions We hypothesized that genetic consequences of oceanic-endemic plants derived via anagenesis would be quite different from those derived via cladogenesis. Populations of A. okamotoanum form a cluster and are clearly differentiated from A. mono, which suggests a single origin for the anagenetically derived island endemic. No pattern of geographical differentiation of populations occurs in A. okamotoanum, which supports the concept of initial founder populations diverging through time by accumulation of mutations in a relatively uniform environment without further specific differentiation.