The genus, Ochrosia, is widely distributed from the West Indian Ocean throughout tropical Asia to the Middle Southern Pacific region. Ochrosia comprises many island-endemic species, suggesting that long-distance dispersal and isolation after migration are key factors for clarifying the diversification process. However, the phylogeny and biogeography of endemic Ochrosia species have not been evaluated well due to the difficulty of adequate sampling from the entire distribution range of the genus. In this study, we focused on two Ochrosia species endemic to the Bonin (Ogasawara) Islands in the northwest Pacific. The Bonin Islands are of volcanic origins and consist of two islands groups, the Ogasawara and Volcano Islands groups, approximately 300 km apart. Ochrosia nakaiana is endemic to the Ogasawara Islands group, whereas O. hexandra is endemic to the Volcano Islands group. To elucidate the phylogenetic positions of these two endemic Ochrosia species, we conducted molecular phylogenetic studies with dating and biogeographic analyses including other Ochrosia species. The phylogenetic trees showed that the two endemic species had distinct origins; O. nakaiana was closely related to O. oppositifolia and O. iwasakiana, whereas O. hexandra was related to O. mariannensis. Based on the chloroplast DNA phylogeny, the genus, Ochrosia, divided into two major lineages 36.6 million years ago. Further, the two endemic species of the Bonin Islands were independently derived approximately 1-2 million years ago. Ochrosia nakaiana originated from the Southeast Asia, New Caledonia, or other Pacific Islands, while O. hexandra derived from O. mariannensis in Micronesia. We demonstrated different origins of the two endemic Ochrosia species on the Bonin Islands. This study provided an excellent example of the complex origins and speciation of flora in the oceanic islands.
Keywords: Apocynaceae; Biogeography; Conservation; Dispersal; Endemics; Oceanic islands.
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