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Xenacoelomorpha is the sister group to Nephrozoa

2016; Nature Portfolio; Volume: 530; Issue: 7588 Linguagem: Inglês

10.1038/nature16520

1476-4687

Johanna T. Cannon, Bruno C. Vellutini, Julian P. Smith, Fredrik Ronquist, Ulf Jondelius, Andreas Hejnol,

Marine Biology and Ecology Research

Robust phylogenetic analysis based on transcriptomes of Xenoturbella and acoelomorph worms shows that Xenacoelomorpha is an early bilaterian lineage forming the sister group to Nephrozoa. Lacking a centralized nervous system, coelom, anus and reproductive organs, the deep-sea flatworm Xenoturbella presents problems when it comes to its classification and teasing out its evolutionary history. Despite its simplicity, some of Xenoturbella's features appear to align it among the deuterostomes, the group of animals that includes ourselves. If true, this implies either a radical simplification of the body plan or the acquisition of many key deuterostome features independently by the various deuterostome groups. Two papers in this issue tackle different aspects of Xenoturbella, but together, move the field on a notch. Greg Rouse et al. add four new deep-sea species of Xenoturbella from the eastern Pacific Ocean to the two already known from the Atlantic. Phylogenomic analysis aligns them at the base of the Protostomia or even as basal bilaterians — much as would be expected from their simple morphology and not invoking radical simplification. Andreas Hejnol and colleagues come to a broadly similar conclusion based on robust phylogenetic analysis using eleven transcriptomes of Xeonturbella and acoel worms. The position of Xenacoelomorpha in the tree of life remains a major unresolved question in the study of deep animal relationships1. 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 Deuterostomia4. 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.