Data from: Demographic histories and genetic diversities of Fennoscandian marine and landlocked ringed seal subspecies

Island populations are on average smaller, genetically less diverse, and at a higher risk to go extinct than mainland populations. Low genetic diversity may elevate extinction probability, but the genetic component of the risk can be affected by the mode of diversity loss, which, in turn, is connected to the demographic history of the population. Here, we examined the history of genetic erosion in three Fennoscandian ringed seal subspecies, of which one inhabits the Baltic Sea ‘mainland’ and two the ‘aquatic islands’ composed of Lake Saimaa in Finland and Lake Ladoga in Russia. Both lakes were colonized by marine seals after their formation c. 9500 years ago, but Lake Ladoga is larger and more contiguous than Lake Saimaa. All three populations suffered dramatic declines during the 20th century, but the bottleneck was particularly severe in Lake Saimaa. Data from 17 microsatellite loci and mitochondrial control-region sequences show that Saimaa ringed seals have lost most of the genetic diversity present in their Baltic ancestors, while the Ladoga population has experienced only minor reductions. Using Approximate Bayesian computing analyses, we show that the genetic uniformity of the Saimaa subspecies derives from an extended founder event and subsequent slow erosion, rather than from the recent bottleneck. This suggests that the population has persisted for nearly 10,000 years despite having low genetic variation. The relatively high diversity of the Ladoga population appears to result from a high number of initial colonizers and a high post-colonization population size, but possibly also by a shorter isolation period and/or occasional gene flow from the Baltic Sea.

相较于大陆种群，岛屿种群的平均种群规模更小、遗传多样性更低，且灭绝风险更高。低遗传多样性可能会提升种群灭绝概率，但种群灭绝风险的遗传组分会受到多样性丧失模式的影响，而该模式又与种群的动态历史密切相关。本研究针对3个芬诺斯堪的亚环斑海豹（Fennoscandian ringed seal）亚种的遗传侵蚀历史展开了分析：其中一个亚种栖息于波罗的海‘大陆’生境，另外两个亚种则栖息于由芬兰塞马湖（Lake Saimaa）与俄罗斯拉多加湖（Lake Ladoga）构成的‘水生岛屿’生境。两处湖泊均于约9500年前形成后被海洋海豹定居，但拉多加湖的面积更大、生境连通性也更强。20世纪期间，3个种群均遭遇了剧烈的种群规模缩减，其中塞马湖种群的遗传瓶颈（bottleneck）尤为严重。通过对17个微卫星位点（microsatellite loci）以及线粒体控制区序列（mitochondrial control-region sequences）的数据分析，本研究发现塞马湖环斑海豹已丢失了其波罗的海祖先种群中的绝大多数遗传多样性，而拉多加湖种群的遗传多样性仅出现了小幅下降。借助近似贝叶斯计算（Approximate Bayesian Computing）分析，本研究证实塞马湖亚种的遗传均一性源于一次持续时间较长的奠基者事件（founder event）以及后续缓慢的遗传侵蚀过程，而非近期的种群瓶颈。这一结果表明，尽管遗传多样性水平较低，该种群仍存续了近1万年。拉多加湖种群相对较高的遗传多样性，可能源于初始定居者数量较多、定居后种群规模较大，同时也可能与其较短的隔离时长以及/或偶尔与波罗的海种群间的基因流（gene flow）有关。