Distribution and Phylogeography of <i>Caryopteris incana</i> (Lamiaceae) Based on Chloroplast DNA Sequences in West Kyushu, Japan

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.