Speciation in the ocean could differ from terrestrial environments due to fewer barriers to gene flow. Hence, sympatric speciation might be common, with American and European eel being candidates for exemplifying this. They show disjunct continental distributions on both sides of the Atlantic, but spawn in overlapping regions of the Sargasso Sea from where juveniles are advected to North American, European and North African coasts. Hybridization and introgression are known to occur, with hybrids almost exclusively observed in Iceland. Different speciation scenarios have been suggested, involving either vicariance or sympatric ecological speciation. Using RAD sequencing and whole-genome sequencing data from parental species and F1 hybrids, we analysed speciation history based on the joint allele frequency spectrum (JAFS) and pairwise sequentially Markovian coalescent (PSMC) plots. JAFS supported a model involving a split without gene flow 150,000-160,000 generations ago, followed by secondary contact 87,000-92,000 generations ago, with 64% of the genome experiencing restricted gene flow. This supports vicariance rather than sympatric speciation, likely associated with Pleistocene glaciation cycles and ocean current changes. Whole-genome PSMC analysis of F1 hybrids from Iceland suggested divergence 200,000 generations ago and indicated subsequent gene flow rather than strict isolation. Finally, simulations showed that results from both approaches (JAFS and PSMC) were congruent. Hence, there is strong evidence against sympatric speciation in North Atlantic eels. These results reiterate the need for careful consideration of cases of possible sympatric speciation, as even in seemingly barrier-free oceanic environments palaeoceanographic factors may have promoted vicariance and allopatric speciation.

中文翻译：

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使用基因组数据分析了欧洲鳗（Anguilla anguilla）和美洲鳗（A. rostrata）的物种形成史。

由于对基因流动的障碍较少，海洋中的物种可能与陆地环境不同。因此，同养物种可能很常见，美洲鳗和欧洲鳗是例证。它们在大西洋两岸显示出不连续的大陆分布，但在萨尔加索海的重叠区域中产卵，在那里，幼体被平移到北美，欧洲和北非海岸。已知发生杂交和渗入，在冰岛几乎只观察到杂种。已经提出了不同的物种形成场景，涉及到变异性或同胞生态物种形成。利用RAD测序和亲本物种和F1杂种的全基因组测序数据，我们根据联合等位基因频谱（JAFS）和成对的顺序马尔可夫聚结（PSMC）图分析了物种形成历史。JAFS支持的模型涉及150,000-160,000代前无基因流分裂，然后87,000-92,000代前二次接触，其中64％的基因组经历了受限的基因流。这支持变异而不是同族物种形成，这可能与更新世冰川期和洋流变化有关。对来自冰岛的F1杂种的全基因组PSMC分析表明，在200,000代前存在分歧，并表明随后的基因流动而不是严格的分离。最后，仿真表明两种方法（JAFS和PSMC）的结果是一致的。因此，有强有力的证据反对北大西洋鳗鱼的同胞物种形成。