Systematic study on the process of speciation, infra-specific differentiation and genomic differentiation of higher plants on the island system with special reference to the Ryukyu Archipelago

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The Ryukyu Archipelago is the product of the repeated formation and division of a landbridge by means of transgression and regression during the Pliocene and Quaternary periods. To reveal the distribution pattern and the degree of specialization, we examined the intraspecific diversification of nucleotide sequences of organelle DNA in Lysimachia mauritiana (Primulaceae). The sequences of two spacer regions distinguish four major haplotypes on the Ryukyu Archipelago, Kyushu and Honshu of Japan. The distribution range of the four haplotypes is well concordant with the geographical boundaries, Kerama and Tokara Gaps on the Ryukyu Islands, the Pacific Ocean side and the Sea of Japan side. The two tectonic gaps have been recognized to be major geographic gaps of the landbridge through the mainland of China, and Taiwan and Kyushu during the Pleistocene. We also examined the somatic chromosome numbers and morphology. The chromosome number from the southern and middle Ryukyu was mainly 2n=18 with six large metacentric chromosomes while 2n=20 from Taiwan and Okinawa Isl., 2n=17 from Okinawa Isl. and 2n=16 from Amamioshima Isl. were also reported. On the other hand, islands of Northern Ryukyu exclusively comprise 2n=20 with four large metacentric chromosomes. Any correlation between cpDNA haplotypes and chromosome numbers was not recognized.