Data from: Extensive mitochondrial introgression in North American Great Black-backed Gulls (Larus marinus) from the American Herring Gull (Larus smithsonianus) with little nuclear DNA impact

Recent genetic studies have shown that introgression rates among loci may greatly vary according to their location in the genome. In particular, several cases of mito-nuclear discordances have been reported for a wide range of organisms. In the present study, we examine the causes of discordance between mitochondrial (mtDNA) and nuclear DNA introgression detected in North American populations of the Great Black-backed Gull (Larus marinus), a Holarctic species, from the Nearctic North American Herring Gull (Larus smithsonianus). Our results show that extensive unidirectional mtDNA introgression from Larus smithsonianus into Larus marinus in North America cannot be explained by ancestral polymorphism but most likely results from ancient hybridization events occurring when Larus marinus invaded the North America. Conversely, our nuclear DNA results based on 12 microsatellites detected very little introgression from Larus smithsonianus into North American Larus marinus. We discuss these results in the framework of demographic and selective mechanisms that have been postulated to explain mito-nuclear discrepancies. We were unable to demonstrate selection as the main cause of mito-nuclear introgression discordance but cannot dismiss the possible role of selection in the observed pattern. Among demographic explanations, only drift in small populations and bias in mate choice in an invasive context may explain our results. As it is often difficult to demonstrate that selection may be the main factor driving the introgression of mitochondrial DNA in natural populations, we advocate that evaluating alternative demographic neutral hypotheses may help to indirectly support or reject hypotheses invoking selective processes.近期的遗传学研究表明，不同基因座（locus，复数形式loci）的基因渐渗（introgression）率可能因其在基因组中的位置差异而呈现显著变化。具体而言，已有多项研究在广泛类群的生物中报道了线粒体-核基因不协调（mito-nuclear discordance）现象。本研究旨在探究：在环北极分布的大黑背鸥（Larus marinus）北美种群中，检测到的、源自北美银鸥（Larus smithsonianus）的线粒体DNA（mtDNA）与核DNA基因渐渗之间存在不一致性的成因。研究结果显示，北美地区从北美银鸥（Larus smithsonianus）向大黑背鸥（Larus marinus）发生的大规模单向mtDNA基因渐渗，无法通过祖先多态性（ancestral polymorphism）解释，而极有可能源于大黑背鸥入侵北美大陆时发生的古老杂交事件。与之相反，基于12个微卫星标记（microsatellite）的核DNA分析结果显示，北美大黑背鸥从北美银鸥处获得的核DNA基因渐渗程度极低。我们基于已被提出用于解释线粒体-核基因不协调现象的种群统计学（demographic）机制与选择机制框架，对上述结果展开讨论。我们无法证明选择是导致线粒体-核基因渐渗不一致的主要原因，但也不能排除选择在本次观测结果中可能发挥的作用。在种群统计学解释框架中，仅小种群遗传漂变（genetic drift）与入侵背景下的配偶选择偏好能够解释本研究的观测结果。由于在自然种群中，证明选择是驱动线粒体DNA基因渐渗的主要因素往往十分困难，我们主张通过评估替代的种群统计学中性假说，来间接支持或驳斥援引选择过程的相关假说。