Data from: Range-edge genetic diversity: locally poor extant southern patches maintain a regionally diverse hotspot in the seagrass Zostera marina

Refugial populations at the rear edge are predicted to contain higher genetic diversity than those resulting from expansion, such as in post-glacial recolonizations. However, peripheral populations are also predicted to have decreased diversity compared to the center of a species’ distribution. We aim to test these predictions by comparing genetic diversity in populations at the limits of distribution of the seagrass Zostera marina, with populations in the species' previously described central diversity 'hotspot'. Zostera marina populations show decreased allelic richness, heterozygosity and genotypic richness in both the 'rear' edge and the 'leading' edge compared to the diversity 'hotspot' in the North Sea / Baltic region. However, when populations are pooled, genetic diversity at the southern range is as high as in the North Sea / Baltic region while the 'leading edge' remains low in genetic diversity. The decreased genetic diversity in these Southern Iberia populations compared to more central populations is possibly the effect of drift due to small effective population size, as a result of reduced habitat, low sexual reproduction and low gene flow. However, when considering the whole southern edge of distribution rather than per population, diversity is as high as in the central 'hotspot' in the North Sea / Baltic region. We conclude that diversity patterns assessed per population can mask the real regional richness that is typical of rear edge populations, which have played a key role in the species biogeographic history and as marginal diversity hotspots have very high conservation value.学界预测，分布后缘的避难所种群（refugial populations）所携带的遗传多样性，要高于扩张过程中形成的种群——比如冰期后重新定居所产生的种群。不过，相较于物种分布的核心区域，外围种群的遗传多样性也被预测会更低。本研究旨在通过对比海草大叶藻（Zostera marina）分布极限处的种群与该物种此前被报道的核心遗传多样性热点区域的种群的遗传多样性，来验证上述两项预测。相较于北海/波罗的海区域的多样性热点，大叶藻在分布后缘与前缘的种群均表现出更低的等位基因丰富度（allelic richness）、杂合度（heterozygosity）与基因型丰富度（genotypic richness）。但如果将种群合并统计，南部分布区的遗传多样性水平与北海/波罗的海区域相当，而分布前缘的遗传多样性仍处于较低水平。相较于核心区域种群，伊比利亚南部种群的遗传多样性降低，可能是由于生境缩减、有性繁殖率低下与基因交流不足导致有效种群大小（effective population size）偏小，进而引发遗传漂变所致。不过，若从整个南部分布边缘而非单一种群的角度考量，其遗传多样性水平与北海/波罗的海区域的核心热点区域相当。本研究得出结论：仅基于单一种群评估的多样性模式，可能会掩盖分布后缘种群真实的区域丰富度特征——这类种群在物种的生物地理历史中发挥了关键作用，且作为边际多样性热点具有极高的保护价值。