Data from: Intraspecific genetic admixture and the morphological diversification of an estuarine fish population complex

The North-east American Rainbow smelt (Osmerus mordax) is composed of two glacial races first identified through the spatial distribution of two distinct mtDNA lineages. Contemporary breeding populations of smelt in the St. Lawrence estuary comprise contrasting mixtures of both lineages, suggesting that the two races came into secondary contact in this estuary. The overall objective of this study was to assess the role of intraspecific genetic admixture in the morphological diversification of the estuarine rainbow smelt population complex. The morphology of mixed-ancestry populations varied as a function of the relative contribution of the two races to estuarine populations, supporting the hypothesis of genetic admixture. Populations comprising both ancestral mtDNA races did not exhibit intermediate morphologies relative to pure populations but rather exhibited many traits that exceeded the parental trait values, consistent with the hypothesis of transgressive segregation. Evidence for genetic admixture at the level of the nuclear gene pool, however, provided only partial support for this hypothesis. Variation at nuclear AFLP markers revealed clear evidence of the two corresponding mtDNA glacial races. The admixture of the two races at the nuclear level is only pronounced in mixed-ancestry populations dominated by one of the mtDNA lineages, the same populations showing the greatest degree of morphological diversification and population structure. In contrast, mixed-ancestry populations dominated by the alternate mtDNA lineage showed little evidence of introgression of the nuclear genome, little morphological diversification and little contemporary population genetic structure. These results only partially support the hypothesis of transgressive segregation and may be the result of the differential effects of natural selection acting on admixed genomes from different sources.

北美东北部彩虹胡瓜鱼（Osmerus mordax）最初可通过两种独特的线粒体DNA（mtDNA）谱系的空间分布格局，被划分为两个冰川宗。圣劳伦斯河口现存的彩虹胡瓜鱼繁殖种群均包含两种谱系的混合个体，这表明两个冰川宗在此河口发生了二次接触。本研究的核心目标是探究种内遗传混合在河口彩虹胡瓜鱼种群复合体的形态分化中所发挥的作用。具有混合祖先的种群的形态特征会随两个冰川宗对河口种群的相对贡献程度发生变化，这一结果支持了遗传混合假说。同时携带两种线粒体DNA冰川宗的种群，相较于纯系种群并未呈现中间形态，反而表现出诸多超出亲本性状值的特征，这与超亲分离假说相符。然而，针对核基因库层面的遗传混合证据仅为该假说提供了部分支持。核扩增片段长度多态性（Amplified Fragment Length Polymorphism，AFLP）标记的变异清晰地揭示了对应两种线粒体DNA冰川宗的遗传特征。两个冰川宗在核基因组层面的混合仅在以其中一种线粒体DNA谱系为主导的混合祖先种群中表现显著，这类种群同时呈现出最高程度的形态分化与种群遗传结构。与之相反，以另一种线粒体DNA谱系为主导的混合祖先种群几乎未表现出核基因组的基因渐渗迹象，形态分化程度极低，且当代种群遗传结构不明显。上述结果仅部分支持超亲分离假说，其成因可能是自然选择对不同来源的混合基因组施加了差异化的选择压力。