Data from: Large-scale genetic panmixia in the blue shark (Prionace glauca): a single worldwide population, or a genetic lag-time effect of the “grey zone” of differentiation?

The blue shark Prionace glauca, among the most common and widely studied pelagic sharks, is a top predator, exhibiting the widest distribution range. However, little is known about its population structure and spatial dynamics. With an estimated removal of 10 to 20 million individuals per year by fisheries, the species is classified as “Near Threatened” by International Union for Conservation of Nature. We lack the knowledge to forecast the long-term consequences of such a huge removal on this top predator itself and on its trophic network. The genetic analysis of more than 200 samples collected at broad scale (from Mediterranean Sea, North Atlantic and Pacific Oceans) using mtDNA and nine microsatellite markers allowed to detect signatures of genetic bottlenecks but a nearly complete genetic homogeneity across the entire studied range. This apparent panmixia could be explained by a genetic lag-time effect illustrated by simulations of demographic changes that were not detectable through standard genetic analysis before a long transitional phase here introduced as the “population grey zone”. The results presented here can thus encompass distinct explanatory scenarios spanning from a single demographic population to several independent populations. This limitation prevents the genetic-based delineation of stocks and thus the ability to anticipate the consequences of severe depletions at all scales. More information is required for the conservation of population(s) and managements of stocks, which may be provided by large scale sampling not only of individuals worldwide, but also of loci genome-wide.

大青鲨（Prionace glauca）是最为常见且被广泛研究的大洋性鲨鱼之一，作为顶级捕食者，其拥有全球最广的分布范围。然而，目前学界对其种群结构与空间动态的了解仍十分有限。据估算，每年因渔业捕捞导致的大青鲨死亡个体数量达1000万至2000万头，世界自然保护联盟（International Union for Conservation of Nature, IUCN）已将该物种列为“近危”等级。我们尚且无法预测如此大规模的捕捞死亡对该顶级捕食者自身及其营养网络所带来的长期影响。研究团队对全球大范围采集的200余份样本（取自地中海、北大西洋及太平洋海域）采用线粒体DNA（mtDNA）与9个微卫星标记开展遗传分析，不仅检测到了遗传瓶颈的信号，还发现整个研究区域内几乎完全的遗传同质性。这种看似的泛交现象（panmixia）可通过遗传滞后效应得到解释：本研究提出的“种群灰色地带（population grey zone）”指的是一段长期过渡阶段，在此阶段之前，常规遗传分析无法检测到种群数量变化的信号，本研究通过种群动态模拟验证了这一效应。因此本研究所得结果可涵盖多种不同的解释场景，其范围从单一种群到多个独立种群均有涉及。这一局限性使得研究无法基于遗传学特征划定渔业管理种群，进而无法预判不同规模下种群严重枯竭所带来的后果。要实现该物种种群的保护与渔业管理种群的有效管理，亟需获取更多相关信息；而这类信息可通过两方面的大范围采样获得：一是在全球范围内采集个体样本，二是开展全基因组位点的采样分析。