Unidad Académica de Sistemas Arrecifales, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Apdo. Postal 1152, Cancún, Quintana Roo 77500, Mexico.
Mol Phylogenet Evol. 2010 Dec;57(3):1072-90. doi: 10.1016/j.ympev.2010.09.010. Epub 2010 Sep 21.
Introgressive hybridization is described in several phylogenetic studies of mass-spawning corals. However, the prevalence of this process among brooding coral species is unclear. We used a mitochondrial (mtDNA: nad5) and two nuclear (nDNA: ATPSα and SRP54) intron markers to explore species barriers in the coral genus Madracis and address the role of hybridization in brooding systems. Specimens of six Caribbean Madracis morphospecies were collected from 5 to 60 m depth at Buoy One, Curaçao, supplemented by samples from Aruba, Trinidad & Tobago and Bermuda. Polymerase chain reaction and denaturing gradient gel electrophoresis were coupled to detect distinct alleles within single colonies. The recurrent nDNA phylogenetic non-monophyly among taxa is only challenged by Madracis senaria, the single monophyletic species within the genus. nDNA AMOVAs indicated overall statistical divergence (0.1% significance level) among species but pairwise comparisons of genetic differentiation revealed some gene exchange between Madracis taxa. mtDNA sequences clustered in two main groups representing typical shallow and deep water Madracis species. Madracis pharensis shallow and deep colonies (with threshold at about 23-24 m) clustered in different mtDNA branches, together with their depth-sympatric congenerics. This divergence was repeated for the nDNA (ATPSα) suggestive of distinct M. pharensis depth populations. These matched the vertical distribution of the dinoflagellate symbionts hosted by M. pharensis, with Symbiodinium ITS2 type B7 in the shallows but type B15 in the deep habitats, suggesting symbiont-related disruptive selection. Recurrent non-monophyly of Madracis taxa and high levels of shared polymorphism reflected in ambiguous phylogenetic networks indicate that hybridization is likely to have played a role in the evolution of the genus. Using coalescent forward-in-time simulations, lineage sorting alone was rejected as an explanation to the SRP54 genetic variation contained in Madracis mirabilis and Madracis decactis (species with an old fossil record), showing that introgressive hybridization has taken place between these species, either directly or through the gene pool of other Madracis taxa. Madracis widespread non-monophyly and the absence of statistical divergence between some species suggest that introgressive hybridization plays an important role in the evolution of the genus. Different reproductive traits and symbiont signatures of taxa forming distinct genetic clusters also point to the same conclusion. We suggest that Madracis morphospecies remain recognizable because introgressive hybridization is non-pervasive and/or because disruptive selection is in action.
内侵杂交在大规模产卵珊瑚的几个系统发育研究中有所描述。然而，这种过程在亲代珊瑚物种中的普遍程度尚不清楚。我们使用线粒体（mtDNA：nad5）和两个核（nDNA：ATPSα和 SRP54）内含子标记来探索珊瑚属 Madracis 中的物种障碍，并解决杂交在亲代系统中的作用。从库拉索岛的 Buoy One 收集了来自 5 至 60 米深度的六个加勒比海 Madracis 形态种的标本，并补充了来自阿鲁巴岛、特立尼达和多巴哥岛和百慕大的样本。聚合酶链反应和变性梯度凝胶电泳相结合，以检测单个菌落中的独特等位基因。在分类群中反复出现的 nDNA 系统发育非单系性仅受到属内唯一单系种 Madracis senaria 的挑战。nDNA AMOVAs 表明种间存在总体统计差异（0.1%显著水平），但遗传分化的成对比较显示 Madracis 分类群之间存在一些基因交换。mtDNA 序列聚类为两个主要群组，代表典型的浅水区和深水区 Madracis 物种。浅水区和深水区 Madracis pharensis （约 23-24 米的阈值）与共生的同种异体聚类在不同的 mtDNA 分支中，与共生的同种异体聚类在一起。这一分化在 nDNA（ATPSα）中重复出现，表明 M. pharensis 存在不同的深度种群。这些与 M. pharensis 所容纳的虫黄藻共生体的垂直分布相匹配，浅水区共生体为 Symbiodinium ITS2 型 B7，而深水区共生体为 B15，表明共生体相关的破坏性选择。Madracis 分类群的反复非单系性和模糊系统发育网络中反映的高水平共享多态性表明，杂交可能在属的进化中发挥了作用。使用合并正向时间模拟，谱系排序被单独拒绝作为 Madracis mirabilis 和 Madracis decactis（具有古老化石记录的物种）中包含的 SRP54 遗传变异的解释，表明种间内侵杂交已经发生，或者通过其他 Madracis 分类群的基因库发生。Madracis 广泛的非单系性和一些物种之间缺乏统计差异表明，内侵杂交在属的进化中起着重要作用。形成不同遗传聚类的分类群的不同繁殖特征和共生体特征也指向了相同的结论。我们认为，Madracis 形态种仍然是可识别的，因为内侵杂交是非弥漫性的，或者因为破坏性选择在起作用。
