Abstract
Caryopteris incana is a continental plant, transferred to Japan from continental Asia via a land bridge between the Korean Peninsula and Tsushima Islands during a glacial period. It currently grows wild in West Kyushu, Japan. In a previous study, we investigated the distribution of C. incana in the Tsushima Islands and confirmed the genetic structure of populations by using chloroplast DNA sequence analysis, suggesting that different haplotypes were distributed in the same area. Thus, it seemed that populations of C. incana throughout the Tsushima Islands colonized at different times; each haplotype had remained within its population without mixing. In this study, we conducted fieldwork to construct a detailed distribution map in West Kyushu excluding the Tsushima Islands. Additionally, we confirmed genetic structure of the C. incana population in these areas by using chloroplast DNA sequence analysis to study the intraspecific phylogenetic relationship of C. incana in Japan. We confirmed 37 natural populations in 257 locations throughout West Kyushu excluding the 72 natural populations in the Tsushima Islands. We also confirmed a recent decreasing trend in the number of natural populations in the Nagasaki Mainland. Using the leaves of individuals cultivated from seeds collected from each natural population, we analyzed the chloroplast DNA sequence variations. Among the investigated populations, sequence variations were confirmed in six regions of chloroplast DNA, and those haplotypes were mainly classified into two groups distributed in different areas on the phylogenetic tree. This finding revealed that the common ancestor of C. incana in Japan diverged early into two groups, followed by a fragmentation in population distribution for each area. The haplotype network almost reflected the geographical distribution on haplotypes. However, several haplotypes that were distributed in other areas were confirmed in the Nagasaki Mainland, suggesting a complicated distribution formation in the past.
Highlights
Biogeography, especially phylogeography, investigates the history of organisms by focusing on generalized information and using genetic variation as an index by employing improved gene technologies [1]
Mainland China has repeatedly connected with the Korean Peninsula by a land bridge over the East China Sea (ECS) basin during the Tertiary period and the late Pleistocene age [6] [13]
Many species of the continental plants, emigrating from Mainland China to Japan through the Korean Peninsula, have extended distribution through the land bridge, which is made by falling sea levels during the glacial periods
Summary
Biogeography, especially phylogeography, investigates the history of organisms by focusing on generalized information and using genetic variation as an index by employing improved gene technologies [1]. In East Asia, fluctuations in sea levels produce dynamic changes in land configurations during the quaternary glacial period [5]-[9] This has affected the fragmentation and connection of natural habitats within this area, which result in the plant species richness and endemism in East China, Southern Japan, and the Korean Peninsula [10] [11]. The Korean Peninsula has been repeatedly connected with Northwest Kyushu and Honshu by a land bridge between the South Korean Peninsula and Tsushima from the early Pleistocene age to the late Pleistocene age [8] In these areas, West Kyushu, located near many continental islands, has had repeated immigration of continental species and emigration of endemic species, by multiple routes during different periods, and these areas have received attention owing to their unique floral composition [14]. Caryopteris incana (Thunb.) Miq. is one such species of a continental plant growing wild to the islands of Nagasaki and Kagoshima in Japan
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