Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria

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.

Sialic Acids as Receptors for Pathogens

“…According to previous analyses [2,25], CMAH was acquired by a deuterostome via lateral gene transfer from a microalga. Notwithstanding the question of deuterostome monophyly [26], we performed BLAST searches using an extensive database of eukaryotic protein sequences and recovered a full sequence of putatively functional CMAH from a sponge Oscarella pearsei and partial sequences from sponges Oscarella carmela and Corticium candelabrum. All of these sponges belong in the class Homoscleromorpha; no CMAH sequences were detected in any of the three other sponge classes.…”

The Enigmatic Reissner’s Fiber and the Origin of Chordates

“…Most phylogenetic analyses place Deuterostomia as a monophyletic group (as depicted here), with Chordata and ambulacria as sister groups. However, some recent findings using large scale genetic databases claim that there is no evidence for Deuterostomia as a monophyletic group ( Kapli et al, 2021 ). Hence the interrogation mark shown at this node.…”

Exploring genome gene content and morphological analysis to test recalcitrant nodes in the animal phylogeny

“…Bilateria have long been split into two lineages, the Protostomia (Ecdysozoa + Lophotrochozoa) and the Deuterostomia (traditionally: Chordata + Ambulacraria [= Hemichordata + Echinodermata]), based on the different origins of the mouth and other features during development (e.g., Hyman, 1959). However, recent phylogenomic studies challenged the monophyly of Deuterostomia (Kapli et al, 2021;Marlétaz et al, 2019) and recovered paraphyletic deuterostomes in conjunction with Xenambulacraria. This combination of results, if confirmed, would have substantial implications for our understanding of the last common ancestor of all Bilateria, which might then have been a fairly large organism, with pharyngeal gill slits and other traits previously thought to represent apomorphies of Deuterostomia (see Kapli et al, 2021 for an in-depth discussion).…”

“…However, recent phylogenomic studies challenged the monophyly of Deuterostomia (Kapli et al, 2021;Marlétaz et al, 2019) and recovered paraphyletic deuterostomes in conjunction with Xenambulacraria. This combination of results, if confirmed, would have substantial implications for our understanding of the last common ancestor of all Bilateria, which might then have been a fairly large organism, with pharyngeal gill slits and other traits previously thought to represent apomorphies of Deuterostomia (see Kapli et al, 2021 for an in-depth discussion).…”