Strong genetic structure revealed by microsatellite variation in Callicarpa species endemic to the Bonin (Ogasawara) Islands.

Adaptive radiation is the diversification of a founding population into multiple taxa that are differentially adapted to diverse ecological niches. The three Callicarpa (Lamiaceae) species endemic to the Bonin Islands are considered to represent an example of adaptive radiation on oceanic islands. All three species are distributed in the Chichijima Island Group and grow in different habitats, while only one species, C. subpubescens, is distributed among other island groups. Particularly, in the Hahajima and Mukojima Island Groups, C. subpubescens grows in various habitats and shows relatively high morphological variation. We investigated genotypes of the three Callicarpa species at 14 microsatellite markers to elucidate genetic differentiation within and between species or island groups and between different habitats or morphologies. We found that genetic differentiation within C. subpubescens in the Hahajima and Mukojima Island Groups was equally as high as that between the three species in the Chichijima Island Group, while differentiation within C. subpubescens in the Chichijima Island Group was much lower. Analyses such as a Bayesian clustering analysis showed that genetically distinct groups were associated with the three species in the Chichijima Island Group, whereas they showed strong genetic structure within C. subpubescens in the Hahajima and Mukojima Island Groups among different habitats and morphologies. These results indicated that ecological diversification occurred in the Hahajima and Mukojima Island Groups. Meanwhile, high genetic differentiation among different island groups was also observed, reflecting isolation by distance. It implies that non-ecological factors such as geographic isolation also played important roles in genetic differentiation in Callicarpa species in the Bonin Islands. These findings suggest that the Callicarpa species in the Bonin Islands are differentiated into multiple genetic groups by both ecological and non-ecological factors.

Genetic population structure of Osmunda japonica, rheophilous Osmunda lancea and their hybrids.

Rheophilous Osmunda lancea often hybridizes with a dryland ally, Osmunda japonica, to produce O. x intermedia, forming zonation in riverbanks and the adjacent dryland along flooding frequency clines. This study examined the genetic structure of populations consisting of O. x intermedia and the two parental species by analyzing ten nuclear DNA markers [six cleaved amplified polymorphic sequence (CAPS) markers and three simple sequence repeat (SSR) markers developed from an expressed sequence tag (EST) library, and the sequence of the glyceraldehyde-3-phosphate dehydrogenase gene GapCp] and chloroplast DNA sequences. The results suggest that the nuclear genes of O. japonica and O. lancea are genetically differentiated despite shared polymorphism in their chloroplast DNA sequences. This discrepancy may be attributable to natural selection and recent introgression, although it is not evident if introgression occurs between O. japonica and O. lancea in the examined populations. Our findings of putative F2 hybrids in O. x intermedia support its partial reproducibility, and also suggest that formation of later-generation hybrids generates morphological variation in O. x intermedia. O. lancea plants collected from geographically distant localities were genetically very similar, and it is suggested that O. lancea originated monotopically.