Highly differentiated population structure of a Mangrove species, Bruguiera gymnorhiza (Rhizophoraceae) revealed by one nuclear GapCp and one chloroplast intergenic spacer trnF–trnL
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To evaluate the genetic diversity of a mangrove species and clarify the genetic structure of its populations, we studied nucleotide polymorphism in two DNA regions of Bruguiera gymnorhiza collected from the southern islands of Japan, Thailand, Malaysia, Indonesia, Micronesia, and India. The two DNA sequences were the chloroplast (cp) intergenic spacer between trnL and trnF genes (ca. 300 bp), and a part (ca. 550 bp) of the nuclear gene coding for glyceraldehyde-3-phosphate dehydrogenase (GapCp). Little polymorphism was found within each of the three geographical regions, Pacific Ocean, Bay of Bengal and Arabian Sea. Throughout the vast regions east of the Malay peninsula including Indonesia, Thailand, Micronesia and the southern islands of Japan (Pacific Ocean), essentially only one haplotype (apart from variation in number of a T repeat) was present. A second haplotype was present on the western coast of Malay Peninsula and the eastern coast of India (Bay of Bengal). On the southwest of Malay Peninsula both of these haplotypes were present. Finally a third haplotype was found only on the western coast of India (Arabian Sea). When taken over all geographic populations, total nucleotide variation within the species was large (μ = 0.006, average of the two genes). Our results are consistent with the hypothesis that this low genetic diversity within any local population and differentiation between the different oceans or regions are caused by very low gene flow between each of the different oceans coupled with frequent fluctuation of population sizes due to the change in sea level. The significance of these results is discussed from evolutionary point of the mangrove forests.
KeywordsIntraspecific diversity Population structure Mangrove conservation Chloroplast DNA GapCp Bruguiera gymnorhiza
We thank Dr. T. Miyagi, Tohoku Gakuin University, Dr. T. Kikuchi, Gifu University, Dr. Y. Mochida, Yokohama National University and other members of the mangrove research group with whom we worked in Thailand and Malaysia in 1996 for providing opportunity for sampling in the mangrove forests and helping with identification. Thanks are also to Dr. K. Ogino, Ehime University, Dr. Mahani Mansor Clyde, Universiti Kebangsaan Malaysia, Dr. Mashhor Mansor, Universiti Sains Malaysia, Dr. M. Lokman Husain, Universiti Kolej Terengganu and many other members of laboratories which we visited in Malaysia in 1997 for providing opportunities for sampling and a place for DNA extraction, and helping with sampling and DNA extraction. We also thank Dr. K. Harada, Dr. E. Nitasaka for help in sampling of the material. Discussion with Dr. A. Szmidt of Kyushu University was helpful. We also thank Dr. Richard Frankham of Macquarie University and Dr. Peter Saenger of Southrn Cross University for careful reading of the manuscript and valuable suggestions.
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