ISEM, Univ Montpellier, CNRS, EPHE, IRD, Montpellier, France.
Faculty of Sciences of Bizerta, Laboratory of Environment Bio-monitoring, University of Carthage, 7021, Zarzouna, Bizerta, Tunisia.
Heredity (Edinb). 2019 Jun;122(6):770-784. doi: 10.1038/s41437-018-0174-y. Epub 2019 Jan 23.
The Almeria-Oran Front (AOF) is a recognised hotspot of genetic differentiation in the sea, with genetic discontinuities reported in more than 50 species. The AOF is a barrier to dispersal and an ecological boundary; both can determine the position of these genetic breaks. However, the maintenance of genetic differentiation is likely reinforced by genetic barriers. A general drawback of previous studies is an insufficient density of sampling sites at the transition zone, with a conspicuous lack of samples from the southern coastline. We analysed the fine-scale genetic structure in the mussel Mytilus galloprovincialis using a few ancestry-informative loci previously identified from genome scans. We discovered a 600-km-wide mosaic hybrid zone eastward of the AOF along the Algerian coasts. This mosaic zone provides a new twist to our understanding of the Atlantic-Mediterranean transition because it demonstrates that the two lineages can live in sympatry with ample opportunities to interbreed in a large area, but they hardly do so. This implies that some form of reproductive isolation must exist to maintain the two genetic backgrounds locally cohesive. The mosaic zone ends with an abrupt genetic shift at a barrier to dispersal in the Gulf of Bejaia, Eastern Algeria. Simulations of endogenous or exogenous selection in models that account for the geography and hydrodynamic features of the region support the hypothesis that sister hybrid zones could have been differentially trapped at two alternative barriers to dispersal and/or environmental boundaries, at Almeria in the north and Bejaia in the south. A preponderantly unidirectional north-south gene flow next to the AOF can also maintain a patch of intrinsically maintained genetic background in the south and the mosaic structure, even in the absence of local adaptation. Our results concur with the coupling hypothesis that suggests that natural barriers can explain the position of genetic breaks, while their maintenance depends on genetic barriers.
阿尔梅里亚-奥兰前缘（AOF）是海洋中遗传分化的公认热点，据报道，超过 50 个物种存在遗传不连续性。AOF 是扩散的障碍和生态边界；两者都可以决定这些遗传断裂的位置。然而，遗传分化的维持可能受到遗传障碍的加强。先前研究的一个普遍缺点是过渡带采样点的密度不足，南部海岸线的样本明显缺乏。我们使用先前从基因组扫描中确定的少数祖先信息丰富的基因座，分析了贻贝 Mytilus galloprovincialis 的精细遗传结构。我们发现，在 AOF 以东的阿尔及利亚海岸线上，存在一个 600 公里宽的马赛克杂交区。这个马赛克区域为我们理解大西洋-地中海过渡提供了一个新的视角，因为它表明两个谱系可以在一个大区域内共生并进行大量杂交，但它们几乎没有这样做。这意味着必须存在某种形式的生殖隔离来维持两个遗传背景的局部一致性。马赛克区在阿尔及利亚东部贝贾亚湾的扩散障碍处突然发生遗传转变。在考虑到该地区地理和水动力特征的模型中，对内生或外生选择的模拟支持了以下假设：姐妹杂交区可能在两个替代的扩散障碍和/或环境边界处被不同程度地困住，一个在北部的阿尔梅里亚，另一个在南部的贝贾亚。在 AOF 旁边主要是单向的南北基因流也可以维持南部的一个固有遗传背景斑块和马赛克结构，即使没有局部适应。我们的结果与耦合假说一致，该假说表明自然障碍可以解释遗传断裂的位置，而其维持则取决于遗传障碍。
