Data from: Assessing pre- and post-zygotic barriers between North Atlantic eels (Anguilla anguilla and A. rostrata)

Elucidating barriers to gene flow is important for understanding the dynamics of speciation. Here we investigate pre- and post-zygotic mechanisms acting between the two hybridizing species of Atlantic eels: Anguilla anguilla and A. rostrata. Temporally varying hybridization was examined by analyzing 85 species-diagnostic single-nucleotide polymorphisms (SNPs; FST greater than or equal to0.95) in eel larvae sampled in the spawning region in the Sargasso Sea in 2007 (N=92) and 2014 (N=460). We further investigated whether genotypes at these SNPs were nonrandomly distributed in post-F1 hybrids, indicating selection. Finally, we sequenced the mitochondrial ATP6 and nuclear ATP5c1 genes in 19 hybrids, identified using SNP and restriction site associated DNA (RAD) sequencing data, to test a previously proposed hypothesis of cytonuclear incompatibility leading to adenosine triphosphate (ATP) synthase dysfunction and selection against hybrids. No F1 hybrids but only later backcrosses were observed in the Sargasso Sea in 2007 and 2014. This suggests that interbreeding between the two species only occurs in some years, possibly controlled by environmental conditions at the spawning grounds, or that interbreeding has diminished through time as a result of a declining number of spawners. Moreover, potential selection was found at the nuclear and the cytonuclear levels. Nonetheless, one glass eel individual showed a mismatch, involving an American ATP6 haplotype and European ATP5c1 alleles. This contradicted the presence of cytonuclear incompatibility but may be explained by that (1) cytonuclear incompatibility is incomplete, (2) selection acts at a later life stage or (3) other genes are important for protein function. In total, the study demonstrates the utility of genomic data when examining pre- and post-zyotic barriers in natural hybrids.阐明基因流障碍对于理解物种形成动态具有重要意义。本研究针对两种杂交大西洋鳗鲡——欧洲鳗鲡（Anguilla anguilla）与美洲鳗鲡（Anguilla rostrata）——之间发挥作用的合子前与合子后生殖隔离机制展开探究。我们通过分析2007年（样本量N=92）与2014年（N=460）在马尾藻海产卵场采集的鳗鲡幼体样本中的85个物种诊断性单核苷酸多态性（single-nucleotide polymorphisms, SNPs，FST≥0.95），考察了杂交事件的时间动态变化。进一步，我们针对这些SNP位点的基因型在F1代之后的杂种个体中的非随机分布特征进行分析，以判断是否存在选择作用。此外，我们对19株经单核苷酸多态性与限制性酶切位点相关DNA（restriction site associated DNA, RAD）测序鉴定的杂种个体的线粒体ATP6基因与核编码ATP5c1基因进行测序，以此检验此前提出的核质不相容性假说：该假说认为核质不相容会导致三磷酸腺苷（adenosine triphosphate, ATP）合酶功能异常，并对杂种产生选择淘汰作用。2007年与2014年的马尾藻海采样中均未发现F1代杂种，仅检测到晚期回交个体。这表明两种鳗鲡的种间杂交仅在部分年份发生，其调控因素可能为产卵场的环境条件；或是由于产卵亲体数量下降，种间杂交随时间推移逐渐减少。此外，在核基因与核质互作层面均检测到潜在的选择信号。值得注意的是，1尾玻璃鳗幼体出现了基因型不匹配现象：其携带美洲型ATP6单倍型与欧洲型ATP5c1等位基因。这一结果与核质不相容性假说相悖，但其可通过以下三种途径解释：（1）核质不相容性并不完全；（2）选择作用发生于更晚的生命阶段；（3）其他基因对蛋白质功能更为关键。综上，本研究证实了基因组数据在检测自然杂种合子前与合子后生殖隔离障碍中的应用价值。