Data from: Hidden diversity in the freshwater planktonic diatom Asterionella formosa

Many freshwater and marine algal species are described as having cosmopolitan distributions. Whether these widely distributed morphologically similar algae also share a similar gene pool remains often unclear. In the context of island biogeography theory, stronger spatial isolation deemed typical of freshwater lakes should restrict gene flow and lead to higher genetic differentiation among lakes. Using nine microsatellite loci, we investigate the genetic diversity of a widely distributed freshwater planktonic diatom, Asterionella formosa, across different lakes in Switzerland and the Netherlands. We applied a hierarchical spatial sampling design to determine the geographical scale at which populations are structured. A subset of the isolates was additionally analysed using amplified fragment length polymorphism (AFLP) markers. Our results revealed complex and unexpected population structure in A. formosa with evidence for both restricted and moderate to high gene flow at the same time. Different genetic markers (microsatellites and AFLPs) analysed with a variety of multivariate methods consistently revealed that genetic differentiation within lakes was much stronger than among lakes, indicating the presence of cryptic species within A. formosa. We conclude that the hidden diversity found in this study is expected to have implications for the further use of A. formosa in biogeographical, conservation and ecological studies. Further research using species-level phylogenetic markers is necessary to place the observed differentiation in an evolutionary context of speciation.

诸多淡水与海洋藻类物种被报道具有广布性分布范围。这些形态相似且分布广泛的藻类是否共享同一基因库，目前仍不甚明确。在岛屿生物地理学理论（island biogeography theory）的框架下，淡水湖泊普遍被认为具有更强的空间隔离性，这应当会限制基因流，并导致不同湖泊间出现更高的遗传分化。本研究依托9个微卫星位点（microsatellite loci），针对瑞士与荷兰境内多个湖泊中广泛分布的淡水浮游硅藻——美丽星杆藻（Asterionella formosa）的遗传多样性展开探究。我们采用分层空间采样设计，以明确种群结构形成的地理尺度。此外，我们还利用扩增片段长度多态性（amplified fragment length polymorphism, AFLP）标记对部分分离株进行了分析。

研究结果揭示了美丽星杆藻复杂且出乎意料的种群结构，同时存在受限基因流与中等至高强度基因流的现象。通过多种多元分析方法对不同遗传标记（微卫星位点与AFLP）进行分析后，结果均一致表明：湖泊内部的遗传分化程度远高于湖泊之间，这暗示美丽星杆藻类群中存在隐存物种。

综上，本研究发现的隐藏多样性，预计将对后续在生物地理学、保护生物学及生态学研究中使用美丽星杆藻产生重要影响。未来需借助物种水平的系统发育标记开展进一步研究，以将观测到的遗传分化置于物种形成的进化背景中进行解读。