The study of vascular plant diversity in the Bonin Islands including Mukozima-Retto and Kazan-Retto

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The Bonin Islands consist of about 20 small islands, however, floristic and evolutionary studies had been conducted mainly in Chichijima and Hahajima islands, because of the difficulty to visit the other remote islands of Mukojima-Retto and Kazan-Retto. The aim of this study is to get in touch with the flora of these remote islands and to estimate the processes and factors of diversification for native plant species.The vascular flora of Mukojima and Nakodojima in Mukojima-Retto, which had been caused a great deal damage by a large number of feral goats, was confirmed. All of the goats were eradicated in recent years, however, many taxa reported in previous flora list were missing, and several non-native plant species have been irrupting. The flora of Kitaiojima in Kazan-Retto has been well preserved in comparison with Mukojima-Retto, but some invasive non-native plant species such as Ficus microcarpa have been spreading. We have studied about the impact caused by non-native species, in order to address the conservation problems for oceanic island biotas.Morphological and molecular studies to infer the speciation in oceanic islands have been progressed for genus Syzygium, Hibiscus and Boninia. These studies suggested that population structures of the plant species have been derived from ecological divergence associated with. vegetation types, mode of seed dispersal and restricted gene flow by shift of flowering periods respectively.The sexual differantations were studied for the Bonin endemics and the relatives. The Bonin endemic Wikstroemia pseudoretusa was founded to be dioecious, and it seems to evolve from the hermaphroditic ancestor within the Islands. Furthermore, we also observed that Morinda umbellata subsp. boninensis and Psychotria homalosperma are probably androdioecious and heterostylous respectively.