Data from: Spatial patterns of AFLP diversity in Bulbophyllum occultum (Orchidaceae) indicate long-term refugial isolation in Madagascar and long-distance colonization effects in La Réunion

Bulbophyllum occultum, an epiphytic orchid mainly distributed in the rainforests of (north)eastern Madagascar and La Réunion, represents an interesting model case for testing the effects of anthropogenic vs historical (e.g., climate induced) habitat isolation and long-distance colonization on the genetic structure of plant species with disjunct distributions in the Madagascan region. To this aim, we surveyed amplified fragment length polymorphisms (AFLPs) across 13 populations in Madagascar and nine in La Réunion (206 individuals in total). We found overall high levels of population subdivision (ΦPT=0.387) and low within-population diversity (HE, range: 0.026–0.124), indicating non-equilibrium conditions in a mainly selfing species. There was no impact of recent deforestation (Madagascar) or habitat disturbance (La Réunion) detectable on AFLP diversity. K-means clustering and BARRIER analyses identified multiple gene pools and several genetic breaks, both within and among islands. Inter-island levels of population genetic diversity and subdivision were similar, whereby inter-individual divergence in flower colour explained a significant part of gene pool divergence in La Réunion. Our results suggest that (i) B. occultum persisted across multiple isolated (‘refugial’) regions along the eastern rainforest corridor of Madagascar over recent climatic cycles and (ii) populations in La Réunion arose from either single or few independent introductions from Madagascar. High selfing rates and sufficient time for genetic drift likely promoted unexpectedly high population genetic and phenotypic (flower colour) differentiation in La Réunion. Overall, this study highlights a strong imprint of history on the genetic structure of a low-gene-dispersing epiphytic orchid from the Madagascan region.

暗鳞石豆兰（Bulbophyllum occultum）是一种附生兰（epiphytic orchid），主要分布于马达加斯加东北部与留尼旺的热带雨林中，是检验人为活动与历史（如气候驱动的）生境隔离、长距离定殖对马达加斯加地区间断分布植物物种遗传结构影响的极具研究价值的模型案例。为达成该研究目标，我们对马达加斯加的13个种群及留尼旺的9个种群（总计206个个体）开展了扩增片段长度多态性（amplified fragment length polymorphisms, AFLPs）分析。研究结果显示，该物种种群整体分化水平较高（ΦPT=0.387），种群内遗传多样性偏低（期望杂合度HE，取值范围为0.026–0.124），表明这一主要以自交为主的物种处于遗传非平衡状态。未检测到马达加斯加近期的森林砍伐或留尼旺的生境干扰对AFLP多样性产生显著影响。K均值聚类（K-means clustering）与BARRIER分析鉴定出岛屿内部及岛屿间存在多组基因库与多处遗传断裂带。岛屿间的种群遗传多样性与分化水平相近，而留尼旺种群内个体间的花颜色分化可解释基因库分化的显著部分。本研究结果表明：其一，在近期气候循环过程中，暗鳞石豆兰沿马达加斯加东部雨林走廊的多个孤立“避难所”区域存续；其二，留尼旺的种群源自马达加斯加的单次或少数几次独立引种事件。高自交率与充足的遗传漂变时间，可能推动了留尼旺种群中出乎意料的高水平种群遗传与表型（花颜色）分化。总体而言，本研究凸显了历史因素对马达加斯加地区低基因扩散能力附生兰遗传结构的强烈塑造作用。