The evolution of the Ecdysozoa

Abstract

Ecdysozoa is a clade composed of eight phyla: the arthropods, tardigrades and onychophorans that share segmentation and appendages and the nematodes, nematomorphs, priapulids, kinorhynchs and loriciferans, which are worms with an anterior proboscis or introvert. Ecdysozoa contains the vast majority of animal species and there is a great diversity of body plans among both living and fossil members. The monophyly of the clade has been called into question by some workers based on analyses of whole genome datasets. We review the evidence that now conclusively supports the unique origin of these phyla. Relationships within Ecdysozoa are also controversial and we discuss the molecular and morphological evidence for a number of monophyletic groups within this superphylum.

Figure 1

Different out-groups support different positions of the root of the Euarthropoda. (a) Support for Myriochelata (Myriapoda plus Chelicerata) is strong using mitochondrial when the Euarthropod tree is rooted using phylogenetically distant lophotrochozoans (Paradoxopoda). (b) Support is equivocal for either rooting using long branch but phylogenetically closer ecdysozoan nematodes (unresolved Euarthropoda). (c) The tree switches to supporting a monophyletic Mandibulata (Myriapoda with Crustacea and Hexapoda) when using the phylogenetically close and short-branched priapulid as an out-group (O. Rota-Stabelli & M. J. Telford 2007, unpublished results).

Figure 2

Phylogeny of the Ecdysozoa. Bayesian analysis using SSU and LSU ribosomal RNA sequences, complete mitochondrial genomes and eight nuclear protein-coding genes (Vacuolar ATP-synthase, enolase, Glyceraldehyde 3-phosphate dehydrogenase, carnitine palmitoyltransferase, Na/K ATPase, RNA Pol II, Dyskerin and EF 1-alpha). Some taxa with missing data have been merged into composite sequences. Support values shown as Bayesian posterior probabilities and non-parametric bootstrap.

Figure 3

The phylogeny of the Ecdysozoa espoused in this manuscript. Names of probable monophyletic groups are given for each box. Unresolved portions of the tree are shown as multifurcations. We have shown Mandibulata (including Myriapoda) as monophyletic and Panarthropoda (including Tardigrada) as monophyletic groups despite some uncertainty over these groups as we feel the morphological evidence particularly convincing for these clades.