Xenacoelomorpha is the sister group to Nephrozoa
Xenacoelomorpha is the sister group to Nephrozoa
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DOI：
10.1038/nature16520
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发表时间：
2016-02-04
期刊：
影响因子：
64.8
通讯作者：
Hejnol, Andreas
中科院分区：
文献类型：
--
作者：
Cannon, Johanna Taylor;Vellutini, Bruno Cossermelli;Hejnol, Andreas
在动物深层亲缘关系的研究中，异种胚胎在生命树中的位置仍然是一个尚未解决的主要问题(1)。异形线虫是一种两侧对称的海洋蠕虫，由Acoela、Nmertodermatda和Xenoturella组成，缺乏大多数其他双边动物的一些共同特征，例如肛门、肾脏和循环系统。两种相互矛盾的假说正在争论中：异种异形亚纲是所有剩余的双口亚纲的姊妹群(=Nephrozoa，即原口和后口亚纲)(2，3)，或者是后口亚纲中的一个分支(4)。因此，确定这一分支的系统发育位置对于理解双边特征的早期进化或作为复杂性的急剧二次丧失是至关重要的。在这里，我们展示了强大的系统基因组支持异形目为肾形动物的姐妹分类群。我们的系统发育分析，基于11个新的异形体转录本，并使用最大似然和贝叶斯推断分析下的不同进化模型，有力地证实了这一结果。对25个实验数据集的严格测试表明，长分支吸引、饱和和丢失数据不会影响这些结果。由我们的系统发育分析支持的两种动物的姐妹群关系表明，双边动物的最后一个共同祖先可能是一种底栖、纤毛虫，只有一个通向上皮肠道的开口，排泄器官、体腔腔和神经索是在异种形态从肾动物的干血统中分离出来后进化出来的。
The position of Xenacoelomorpha in the tree of life remains a major unresolved question in the study of deep animal relationships(1). Xenacoelomorpha, comprising Acoela, Nemertodermatida, and Xenoturbella, are bilaterally symmetrical marine worms that lack several features common to most other bilaterians, for example an anus, nephridia, and a circulatory system. Two conflicting hypotheses are under debate: Xenacoelomorpha is the sister group to all remaining Bilateria (= Nephrozoa, namely protostomes and deuterostomes)(2,3) or is a clade inside Deuterostomia(4). Thus, determining the phylogenetic position of this clade is pivotal for understanding the early evolution of bilaterian features, or as a case of drastic secondary loss of complexity. Here we show robust phylogenomic support for Xenacoelomorpha as the sister taxon of Nephrozoa. Our phylogenetic analyses, based on 11 novel xenacoelomorph transcriptomes and using different models of evolution under maximum likelihood and Bayesian inference analyses, strongly corroborate this result. Rigorous testing of 25 experimental data sets designed to exclude data partitions and taxa potentially prone to reconstruction biases indicates that long-branch attraction, saturation, and missing data do not influence these results. The sister group relationship between Nephrozoa and Xenacoelomorpha supported by our phylogenomic analyses implies that the last common ancestor of bilaterians was probably a benthic, ciliated acoelomate worm with a single opening into an epithelial gut, and that excretory organs, coelomic cavities, and nerve cords evolved after xenacoelomorphs separated from the stem lineage of Nephrozoa.