Molecular Phylogeny of Insular Endemics of Pittosporum (Pittosporaceae) on the Ogasawara Islands

Abstract

The Ogasawara Islands are oceanic islands that harbor a unique flora with many endemic taxa. Four endemic species of Pittosporum inhabit the islands: P. boninense Koidz., P. beecheyi Tuyama, P. chichijimense Nakai ex Tuyama, and P. parvifolium Hayata. Each species has been considered to have rapidly diverged within the islands. To investigate this issue, we assessed the phylogenetic relationships among 55 species of Pittosporum, including the four Ogasawara endemics and their close relatives occurring in Japan and other countries, based on nuclear ribosomal internal transcribed spacer (ITS) sequences. The four endemics formed a robust monophyletic group, and their sister groups in East Asia consist of P. tobira (Thunb.) W. T. Aiton, P. illicioides Makino, and P. glabratum Lindl. ITS sequence differentiation among the four endemics was quite low. These findings suggest their recent and rapid speciation, derived from a single ancestor.