Evolutionary effects of geographic and climatic isolation between Rhododendron tsusiophyllum populations on the Izu Islands and mainland Honshu of Japan

Geographic and environmental isolations of islands and the mainland offer excellent opportunity to investigate colonization and survival dynamics of island populations. We inferred and compared evolutionary processes and the demographic history of Rhododendron tsusiophyllum, in the Izu Islands and the much larger island Honshu, treated here as the mainland, using thousands of nuclear SNPs obtained by ddRAD-seq from eight populations of R. tsusiophyllum and three populations of R. tschonoskii as an outgroup. Phylogenetic relationships and their habitats suggest that R. tsusiophyllum had evolved and migrated from cold north to warm south regions. We detected clear genetic divergence among populations in three regions of Honshu and the Izu Islands, suggesting restricted migration between them due to isolated habitats on mountains even in the mainland. The three regions have different changes in effective population size, especially, genetic diversity and population size of the Izu Islands are small compared to the others. Further, habitats of populations in the Izu Islands are warmer than those in Honshu, suggesting that they have undergone adaptive evolution. Our study provides evidences of montane rather than insular isolation on genetic divergence, survival of populations and significance of adaptive evolution for island populations with small population size and low genetic diversity, despite close proximity to mainland populations.

岛屿与大陆之间的地理及环境隔离，为探究岛屿种群的定殖与存续动态提供了绝佳契机。本研究以伊豆诸岛及本州岛（本文中将本州岛视为大陆）上的杜鹃属物种Rhododendron tsusiophyllum为研究对象，利用基于双酶切限制性位点相关DNA测序（ddRAD-seq）获得的数千个核单核苷酸多态位点（SNPs），对其演化过程及种群动态历史进行推断与比较；其中R. tsusiophyllum共设8个种群，外类群为3个R. tschonoskii种群。系统发育关系及其生境特征表明，R. tsusiophyllum起源于寒冷北部区域，并向温暖的南部区域扩散迁移。本研究在本州岛三个区域及伊豆诸岛的种群中检测到显著的遗传分化，表明即便在大陆本州岛上，山地生境的隔离也导致种群间的基因交流受限。上述三个区域的有效种群大小动态变化存在显著差异，其中伊豆诸岛种群的遗传多样性与种群规模均低于本州岛其他区域种群。此外，伊豆诸岛种群的生境温度高于本州岛种群，提示其经历了适应性演化过程。尽管伊豆诸岛种群与本州岛大陆种群地理距离较近，本研究结果证实山地隔离而非岛屿隔离是引发遗传分化、影响种群存续的关键因素，并揭示了适应性演化对于种群规模小、遗传多样性低的岛屿种群的重要意义。