Manuzzi et al_Population genomics and phylogeography of a benthic coastal shark (Scyliorhinus canicula) using 2b-RAD SNPs

The existence of strong genetic structure is expected in species with limited ability to disperse and philopatric behaviour. These life-history traits are found in many small benthic elasmobranchs, such as in the small-spotted catshark (Scyliorhinus canicula). However, no evidence of genetic structure was found across its northeastern Atlantic (NEA) range using traditional molecular markers. Here, fine-scale genetic differentiation was detected between the British Isles and southern Iberia using 2674 single nucleotide polymorphism loci generated using 2b-Restriction site Associated DNA (2b-RAD). Geographical distance and historical demography were two major drivers shaping the distribution of genetic diversity of S. canicula along the NEA. Significant positive spatial autocorrelation of allelic frequencies was detected, with genetic differentiation generally increasing with geographical distance. However, marked genetic divergence of the Celtic Sea and South Portugal collections from their closest neighbours resulted in geographically constrained genetic breaks south of the British Isles and off southwestern Iberia. Historical demographic reconstruction of population pairs across these genetic breaks suggested a scenario of historical isolation before secondary contact, probably related to distinct northern and southern glacial refugia. These results provide new insights into the population structure of S. canicula along the NEA and serve as a reference for benthic elasmobranchs.

对于扩散能力有限且具有恋栖习性（philopatric behaviour）的物种而言，较强的遗传结构被认为是普遍存在的。这类生活史特征在诸多小型底栖板鳃亚纲鱼类中均有发现，例如小斑猫鲨（Scyliorhinus canicula）。然而，此前利用传统分子标记对其东北大西洋（Northeastern Atlantic, NEA）分布范围开展的研究，并未检测到遗传结构存在的相关证据。本研究借助2b限制性位点相关DNA（2b-Restriction site Associated DNA, 2b-RAD）技术获取的2674个单核苷酸多态性位点，在不列颠群岛与伊比利亚南部之间检测到了精细尺度的遗传分化。地理距离与历史种群动态是塑造小斑猫鲨在东北大西洋分布区内遗传多样性格局的两大主要驱动因素。研究检测到等位基因频率存在显著的正空间自相关，遗传分化程度总体上随地理距离增加而升高。不过，凯尔特海与葡萄牙南部的采样群体与其近邻群体间存在显著的遗传分化，进而在不列颠群岛以南及伊比利亚西南部海域形成了受地理限制的遗传断裂带。对这些遗传断裂带两侧种群对的历史种群动态重建结果显示，其经历了次生接触前的历史隔离过程，这一过程或与南北迥异的冰期避难所（glacial refugia）相关。本研究结果为小斑猫鲨在东北大西洋的种群结构研究提供了新的视角，同时也可为底栖板鳃亚纲鱼类的相关研究提供参考。