Abstract
The bilaterally symmetric animals (Bilateria) are considered to comprise two monophyletic groups, Protostomia (Ecdysozoa and the Lophotrochozoa) and Deuterostomia (Chordata and the Xenambulacraria). Recent molecular phylogenetic studies have not consistently supported deuterostome monophyly. Here, we compare support for Protostomia and Deuterostomia using multiple, independent phylogenomic datasets. As expected, Protostomia is always strongly supported, especially by longer and higher-quality genes. Support for Deuterostomia, however, is always equivocal and barely higher than support for paraphyletic alternatives. Conditions that cause tree reconstruction errors-inadequate models, short internal branches, faster evolving genes, and unequal branch lengths-coincide with support for monophyletic deuterostomes. Simulation experiments show that support for Deuterostomia could be explained by systematic error. The branch between bilaterian and deuterostome common ancestors is, at best, very short, supporting the idea that the bilaterian ancestor may have been deuterostome-like. Our findings have important implications for the understanding of early animal evolution.
Highlights
The bilaterally symmetric animals (Bilateria) are widely held to be composed of two major monophyletic groups, Protostomia and Deuterostomia
We investigate the support for different topologies relating the Chordata, Xenambulacraria, Ecdysozoa, and Lophotrochozoa
While the classic phylogeny suggests that protostomes and deuterostomes originated at the same point (Urbilateria), there is no expectation that the branches leading to the common ancestors of Protostomia and Deuterostomia should be of the same length
Summary
The bilaterally symmetric animals (Bilateria) are widely held to be composed of two major monophyletic groups, Protostomia and Deuterostomia. The inconsistent support seen for Deuterostomia in recent studies might be evidence that a rapid and ancient radiation in the Precambrian produced the two deuterostome subclades. Cases of rapid radiations imply a limited time between speciations (short internal branches) that can lead to the accumulation of minimal phylogenetic signal, and this signal can be blurred by the numerous substitutions occurring later (long terminal branches). Such problems are exacerbated when heterogeneities in the evolutionary process cause model violations producing systematic errors [1].
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