Sorbonne Universités, UPMC Univ Paris 06, CNRS, Evolution Paris-Seine UMR7138, Institut de Biologie Paris-Seine, Case 05, 7 quai St Bernard, 75005 Paris, France.
Centre de Théorisation et de Modélisation de la Biodiversité, Station d'Ecologie Théorique et Expérimentale, UMR CNRS 5321, Moulis 09200, France; Département de Biochimie, Centre Robert-Cedergren, Université de Montréal, Montréal, QC H3C 3J7, Canada.
Curr Biol. 2017 Apr 3;27(7):958-967. doi: 10.1016/j.cub.2017.02.031. Epub 2017 Mar 16.
Resolving the early diversification of animal lineages has proven difficult, even using genome-scale datasets. Several phylogenomic studies have supported the classical scenario in which sponges (Porifera) are the sister group to all other animals ("Porifera-sister" hypothesis), consistent with a single origin of the gut, nerve cells, and muscle cells in the stem lineage of eumetazoans (bilaterians + ctenophores + cnidarians). In contrast, several other studies have recovered an alternative topology in which ctenophores are the sister group to all other animals (including sponges). The "Ctenophora-sister" hypothesis implies that eumetazoan-specific traits, such as neurons and muscle cells, either evolved once along the metazoan stem lineage and were then lost in sponges and placozoans or evolved at least twice independently in Ctenophora and in Cnidaria + Bilateria. Here, we report on our reconstruction of deep metazoan relationships using a 1,719-gene dataset with dense taxonomic sampling of non-bilaterian animals that was assembled using a semi-automated procedure, designed to reduce known error sources. Our dataset outperforms previous metazoan gene superalignments in terms of data quality and quantity. Analyses with a best-fitting site-heterogeneous evolutionary model provide strong statistical support for placing sponges as the sister-group to all other metazoans, with ctenophores emerging as the second-earliest branching animal lineage. Only those methodological settings that exacerbated long-branch attraction artifacts yielded Ctenophora-sister. These results show that methodological issues must be carefully addressed to tackle difficult phylogenetic questions and pave the road to a better understanding of how fundamental features of animal body plans have emerged.
解决动物谱系早期多样化的问题一直很困难，即使使用基因组规模的数据集也是如此。几项系统基因组学研究支持了经典的观点，即海绵动物（多孔动物门）是所有其他动物的姐妹群（“多孔动物姐妹”假说），这与后生动物（两侧对称动物+栉水母动物+刺胞动物）的主干谱系中肠道、神经细胞和肌肉细胞的单一起源相一致。相比之下，其他几项研究恢复了另一种拓扑结构，即栉水母动物是所有其他动物（包括海绵动物）的姐妹群。“栉水母姐妹”假说意味着神经元和肌肉细胞等后生动物特有的特征，要么沿着后生动物主干谱系进化一次，然后在海绵动物和扁盘动物中丢失，要么在栉水母动物和刺胞动物+两侧对称动物中独立进化至少两次。在这里，我们报告了我们使用 1719 个基因数据集重建深时后生动物关系的结果，该数据集对非两侧对称动物进行了密集的分类采样，这些数据是使用半自动化程序组装的，旨在减少已知的误差源。与之前的后生动物基因超级比对相比，我们的数据集在数据质量和数量方面表现更好。使用最合适的位置异质性进化模型进行分析，为将海绵动物作为所有后生动物的姐妹群提供了强有力的统计支持，栉水母动物作为第二早分支的动物谱系出现。只有那些加剧长枝吸引伪影的方法学设置才会产生栉水母姐妹假说。这些结果表明，必须仔细解决方法学问题，以解决困难的系统发育问题，并为更好地理解动物体节的基本特征是如何出现的铺平道路。
