Data from: One species for one island? Unexpected diversity and weak connectivity in a widely distributed tropical hydrozoan

Isolation by distance (IBD) is one of the main modes of differentiation in marine species, above all in species presenting low dispersal capacities. This paper reports the genetic structuring in the tropical hydrozoan Lytocarpia brevirostris α (sensu Postaire et al. 2016b), a brooding species, from 13 populations in the Western Indian Ocean and one from New Caledonia (Tropical Southwestern Pacific). At the local scale, populations rely on asexual propagation at short distance, which was not found at larger scales; identical genotypes were restricted to single populations. After the removal of repeated genotypes, all populations presented significant positive FIS values (between 0.094 and 0.335). Gene flow was extremely low at all spatial scales, between sites within islands (< 10 km distance) and among islands (100 to > 11 000 km distance), with significant pairwise FST values (between 0.012 and 0.560). A general pattern of IBD was found at the Indo-Pacific scale, but also within sampled ecoregions of the Western Indian Ocean province. Clustering analyses identified each sampled island as an independent population, while analysis of molecular variance indicated that population genetic differentiation was significant at small (within island) and intermediate (among islands within province) spatial scales. The high population differentiation might reflect the life cycle of this brooding hydrozoan, possibly preventing regular dispersal at distances more than a few kilometres and probably leading to high cryptic diversity, each island housing an independent evolutionary lineage.

距离隔离（Isolation by Distance, IBD）是海洋物种分化的核心模式之一，对于扩散能力较弱的物种尤为典型。本研究报道了热带水螅虫Lytocarpia brevirostris α（按照Postaire等人2016b的界定）的遗传结构特征，该物种为育幼型生物，其采样种群覆盖西印度洋的13个种群以及新喀里多尼亚（热带西南太平洋）的1个种群。在局域尺度上，种群依赖短距离无性繁殖，但该繁殖模式在更大空间尺度下并未被观测到；相同基因型仅局限于单个种群内部。去除重复基因型后，所有种群均呈现显著为正的种群内近交系数（FIS）值，区间为0.094至0.335。所有空间尺度下的基因流均处于极低水平，无论是岛屿内部样点间（距离<10km）还是岛屿间（100km至>11000km），两两种群间的遗传分化系数（FST）值均呈显著差异，区间为0.012至0.560。研究在印度-太平洋尺度下发现了普遍的IBD模式，同时在西印度洋生物地理省的采样生态区内也观测到该模式。聚类分析将每个采样岛屿识别为独立种群，而分子方差分析（Analysis of Molecular Variance, AMOVA）结果表明，种群遗传分化在小尺度（岛屿内部）和中尺度（省内岛屿间）空间尺度下均具有统计学显著性。较高的种群分化程度可能反映了该育幼型水螅虫的生活史特征：其可能阻碍了数公里以上距离的常规扩散，并可能催生了极高的隐存多样性，每个岛屿均拥有独立的进化谱系。