Current Biology
Volume 29, Issue 11, 3 June 2019, Pages 1818-1826.e6
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Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria

https://doi.org/10.1016/j.cub.2019.04.009Get rights and content
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Highlights

  • Model violations hinder inference of the phylogeny of Bilateria

  • Methods predicted to cause errors place Xenacoelomorpha outside Nephrozoa

  • Xenacoelomorpha is the sister group of Ambulacraria (hemichordates and echinoderms)

  • Monophyletic deuterostomes (Chordata plus Xenambulacraria) are not clearly supported

Summary

Xenoturbella and the acoelomorph worms (Xenacoelomorpha) are simple marine animals with controversial affinities. They have been placed as the sister group of all other bilaterian animals (Nephrozoa hypothesis), implying their simplicity is an ancient characteristic [1, 2]; alternatively, they have been linked to the complex Ambulacraria (echinoderms and hemichordates) in a clade called the Xenambulacraria [3, 4, 5], suggesting their simplicity evolved by reduction from a complex ancestor. The difficulty resolving this problem implies the phylogenetic signal supporting the correct solution is weak and affected by inadequate modeling, creating a misleading non-phylogenetic signal. The idea that the Nephrozoa hypothesis might be an artifact is prompted by the faster molecular evolutionary rate observed within the Acoelomorpha. Unequal rates of evolution are known to result in the systematic artifact of long branch attraction, which would be predicted to result in an attraction between long-branch acoelomorphs and the outgroup, pulling them toward the root [6]. Other biases inadequately accommodated by the models used can also have strong effects, exacerbated in the context of short internal branches and long terminal branches [7]. We have assembled a large and informative dataset to address this problem. Analyses designed to reduce or to emphasize misleading signals show the Nephrozoa hypothesis is supported under conditions expected to exacerbate errors, and the Xenambulacraria hypothesis is preferred in conditions designed to reduce errors. Our reanalyses of two other recently published datasets [1, 2] produce the same result. We conclude that the Xenacoelomorpha are simplified relatives of the Ambulacraria.

Keywords

Xenoturbella
Acoelomorpha
Ambulacraria
Nephrozoa
phylogenomics
systematic error
phylogeny
Metazoa

Cited by (0)

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Present address: Bioscience Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

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These authors contributed equally

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