Single cell atlas of Xenoturbella bocki highlights the limited cell-type complexity of a non-vertebrate deuterostome lineage

Evidence for a sister relationship between Xenacoelomorpha and Ambulacraria (Xenambulacraria) has revived the longstanding debate surrounding the complexity of the Urdeuterostomian and Urbilaterian ancestors and has led to a reassessment of early bilaterian features. Employing whole organism single-cell RNA-seq in the marine xenacoelomorph worm Xenoturbella bocki, we show that Xenambulacrarian nerve nets share regulatory features and a peptidergic identity with those found in cnidarians and protostomes. We also suggest that Xenacoelomorpha muscles are likely to have evolved their "smooth" phenotype convergently. Furthermore, we identify pigmented cells as a potential synapomorphy of the Xenambulacraria. Taken together, these data are consistent with a simplification from an ancestral urbilaterian/urdeuterostomian body plan and a non-centralized nerve net, minimally differentiated contractile cells and evidence of deuterostome cell-type synapomorphies.