Genetic diversity, structure, and demography of Pandanus boninensis(Pandanaceae) with sea drifted seeds, endemic to the Ogasawara Islands of Japan: Comparison between young and old islands

Pandanus boninensis, endemic to the Ogasawara Islands, Japan, is distributed on both the older Bonin and younger Volcano Islands. In this study, we conducted population genetic analyses of P. boninensis on these islands to examine the population diversity and structure across old and young islands, to assess potential differences in population demography with island age, and to collect any evidence of migration between old and young islands. We found that the genetic diversity of expressed sequence tag (EST)–based microsatellite (SSR) markers, the nucleotide diversity of nuclear DNA sequences, and the haplotype diversity of chloroplast DNA on young islands were lower than those on old islands. Clustering analyses of EST-SSR indicated that populations on old islands were strongly diverged from those on young islands. Approximate Bayesian computation analysis of EST-SSR suggested that population expansion occurred on old islands while population reduction occurred on young islands. We also found evidence of migration among old islands (mostly from south to north), while it appears that there have been very few migration events between old and young islands. These differences could be due to the fact that young islands tend to be geographically isolated and support smaller populations that began a shorter time ago from limited founders. The P. boninensis populations on the Volcano Islands are interesting from an evolutionary perspective as they constitute a classic example of the early stages of progressive colonization on oceanic islands with small effective population sizes and low genetic diversity.

日本小笠原群岛（Ogasawara Islands）特有物种小笠原露兜树（Pandanus boninensis），分布于该群岛内形成年代更久的主岛群与相对年轻的火山列岛之上。本研究针对该物种在两类岛屿上的种群开展群体遗传学分析，旨在探究新旧岛屿间的种群多样性与群体结构，评估不同岛屿年代对应的种群历史动态差异，并搜集新旧岛屿间存在基因流的相关证据。研究结果显示，年轻岛屿上基于表达序列标签（expressed sequence tag, EST）开发的微卫星（simple sequence repeat, SSR）标记的遗传多样性、核DNA序列的核苷酸多样性，以及叶绿体DNA的单倍型多样性，均低于古老岛屿上的对应水平。基于EST-SSR的聚类分析表明，古老岛屿与年轻岛屿上的种群存在显著遗传分化。针对EST-SSR数据的近似贝叶斯计算（approximate Bayesian computation, ABC）分析显示，古老岛屿种群发生了扩张，而年轻岛屿种群则出现了缩减。此外，本研究还观测到古老岛屿间存在基因流（主要由南向北），但新旧岛屿间的基因交流事件则极为罕见。上述差异可能源于年轻岛屿地理隔离程度更高、种群规模更小，且其奠基者群体规模有限、定植历史更短。小笠原露兜树在火山列岛上的种群，因兼具海洋岛屿渐进定植早期阶段的典型特征——有效种群规模小且遗传多样性低，从演化生物学视角来看极具研究价值。