Origin and evolution of endemic plants of the Bonin (Ogasawara) Islands |
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Authors: | Motomi Ito |
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Institution: | (1) Department of Biology, Faculty of Science, Chiba University, 1-33 Yayoi-cho, 263-8522 Inage, Chiba, Japan |
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Abstract: | The Bonin Islands are typical oceanic islands, located at the western part of the North Pacific Ocean and approximately 1,000
km south of mainland Japan. This archipelago consists of about 20 small islands. Although floristic diversity is low due to
the small area and limited environmental diversity, the Bonin Islands harbor unique endemic flora as in other well-known oceanic
islands. This paper presents a brief summary of the results obtained from recent studies on the endemic flora of the Bonin
Islands. The results are reviewed in relation to the four stages of the evolution of endemic flora in the oceanic islands;
migration, establishment, enlargement and diversification. The ancestors of the flora originated mostly from tropical and
subtropical Southeast Asia or mainland Japan by rare events of long distance dispersal. The proportion of bird-dispersed species
is relatively high as for other oceanic islands. Genetic data sets obtained from allozyme variation in some endemic species
suggest that migration occurred several million years ago and genetic diversity is correlated with current population size.
At the time of establishment, self-compatible plants are expected to have an advantage. However, the percentage of dioecious
plants is relatively high. This is partly due to evolutionary changes from hermaphroditic ancestors to dioecy which occurred
in two genera in the Bonin Islands. In addition, there are some examples of evolutionary changes from herbaceous ancestors
to woody endemics. Adaptive radiation is found in some genera, although the number of congeneric endemic species is less than
five. Studies of allozyme variation inPittosporum, Symplocos andCrepidiastrum showed that genetic identity is generally very high between congeneric species in spite of their distinct morphologies. This
result suggests that divergence of these species occurred rather recently and distinct morphological differences are based
on a limited number of genetic changes. |
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Keywords: | adaptive radiation Bonin Islands long distance dispersal oceanic islands |
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