Environmental and genetic effects on phenotypic differences between Elaeocarpus photiniifolia (Elaeocarpaceae) ecotypes in dry and mesic habitats on a Japanese oceanic island

Ecological speciation is a consequence of phenotypic and genetic differentiation, which is driven by divergent selection and reproductive isolation between different environments. In Elaeocarpus photiniifolia, a plant species endemic to the Bonin Islands, genetic differentiation between a shrub ecotype in dry habitats and a tree ecotype in mesic habitats occurs in the Chichijima Island Group, while gene flow between the ecotypes and migration between the habitats are feasible. To clarify environmental and genetic effects on traits different between the ecotypes, we examined the soil moisture content, nuclear microsatellite genotypes, plant height, and flowering phenology in dry and mesic habitats in two areas on Chichijima Island. The soil moisture content decreased from June to July in both areas and tended to be lower in dry habitats than in mesic habitats in July. Genetic structure indicated not only genetic differentiation between the ecotypes but also admixture between them and migration between the habitats. Both environmental and genetic effects were detected in plant height and flowering phenology: lower height and earlier flowering of the shrub ecotype in the dry habitats, indicating phenotypic plasticity in response to drought, and genetic variation between ecotypes in both traits. These findings indicate that phenotypic plasticity in response to different environments results in different flowering phenology, which facilitates reproductive isolation and genetic differentiation, leading to genetic variation in adaptive traits associated with the environments.