Magazine R575

Metazoa. Each of these three (100% posterior probability), Correspondences datasets is broadly sampled as well as (88%) across the Metazoa. We have and (100%). also sequenced ~ 5000 ESTs Chaetognaths are placed as Broad taxon and from the pelagic chaetognath sister to lophotrochozoans Flaccisagitta enflata, recovering (100%) (Figure 1A). gene sampling 72 of the genes used by Philippe Bayesian analyses of the indicate that et al. [4]. Preliminary analyses of 72 genes in the EST dataset SSU and LSU ribosomal RNAs (11,650 amino acids) positioned chaetognaths are showed chaetognath sequences the chaetognaths as sister clustering with taxa that also to the lophotrochozoan taxa show long phylogenetic branches ( and molluscs) and a higher than average GC (Figure 1B; Supplemental data), David Q. Matus1, Richard R. content (see Supplemental data while maximum likelihood Copley2, Casey W. Dunn1, published with this article online). analyses placed them within Andreas Hejnol1, Heather In order to address the possibility the protostomes but did not Eccleston3, Kenneth M. that this grouping was an artifact resolve their relationship to other Halanych3, Mark Q. Martindale1, of GC content, we recoded all taxa (Supplemental and Maximilian J. Telford4,* nucleotides as purines (A,G = R) data). Most features of this tree and pyrimidines (C,T/U = Y). were consistent with the ‘new’ Despite advances in phylogenetic Maximum likelihood and Bayesian metazoan phylogeny [4,7–9], methods, there are still a number analyses of this recoded dataset with the notable exception that of enigmatic phyla whose grouped the chaetognaths the platyhelminths grouped with affinities remain poorly resolved. with the priapulids within the . To eliminate long One of the most recalcitrant Ecdysozoa (Supplemental branch attraction artefacts, we of these is a group of small data). However, with priapulids analyzed subsets of taxa to predatory , excluded, chaetognaths grouped determine the effects of taxon the chaetognaths (arrow ). with the Lophotrochozoa (100% sampling (Supplemental data). Resolution of the phylogenetic posterior probability), without Regardless of the taxa removed position of the chaetognaths resolving their position within the (nematodes, platyhelminths or is key for reconstructing the group (Supplemental data). The ) the chaetognaths evolutionary history of some association with the Ecdysozoa, remained as sister to annelids of the most fundamental therefore, depends on a single and molluscs with varying features of , including taxon and is not likely to reflect a degrees of support in Bayesian those that have been used to general affinity for the group. analyses and as sister to delineate two major clades of Furthermore, we analyzed the Ecdysozoa (, animals — the protostomes and two complete chaetognath nematodes and tardigrades) . An affiliation of mitochondrial genomes in with weak bootstrap support in chaetognaths to deuterostomes combination that previously were likelihood analyses (Figure 1B; was inferred from a number analyzed independently with also see Supplemental data). of shared embryological and differing results [5,6]. Bayesian Topological tests on all four morphological features including analysis of the nucleotide datasets did not reject the radial, indeterminate cleavage, matrix places chaetognaths hypotheses that chaetognaths a posterior position of the within the Lophotrochozoa form a clade with ecdysozoans. blastopore (deuterostomy), allied to molluscs, and Bayesian Only the Tropomyosin dataset enterocoelous formation, analysis of the amino acid matrix rejected the positioning of and a tripartite adult body places them as sister to the chaetognaths as sister to the plan with a post-anal tail [1,2]. Lophotrochozoa (Supplemental protostomes as significantly Various other features (e.g., a data). Maximum likelihood worse. (Shimodaira-Hasegawa ventrally positioned nervous analyses of nucleotide and test using RELL bootstrapping; system, ecdysozoan-like spines, amino acid datasets support the Supplemental data). and a lack of circular body wall hypothesis that chaetognaths All of our analyses indicate muscle) have been interpreted as are sister to lophotrochozoans that chaetognaths are evidence of a protostome albeit incongruously allied with protostomes, consistent with affinity [3]. the long branched other recent studies [5,6,10] and Here, we analyse the Ornithoctonus huwena using the accompanying paper [11]. phylogenetic position of the nucleotide data (Supplemental This has implications for our chaetognaths using existing data). Bayesian and likelihood understanding of the evolution datasets, i.e. small and large analyses of tropomyosin from of morphological characters and subunit nuclear ribosomal RNAs 56 taxa (Supplemental data) for the reconstruction of the last (SSU and LSU), and complete show unequivocal support for common ancestor of protostomes mitochondrial genomes. We the new metazoan phylogeny and deuterostomes. The early also report a new dataset from within the , recovering a development of chaetognaths the tropomyosin gene of many monophyletic Protostomia clade resembles a radial cleavage Current Biology Vol 16 No 15 R576

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Trochophore concepts: ciliary bands and the evolution of larvae suggest that larvae with ciliary EST analyses also support a in spiralian Metazoa. Biol. J. Linn. Soc. bands evolved more than once, Lophotrochozoan relationship; 66, 411–464. reinforcing doubts over the however, the sensitivity of these 14. Kapp, H. (2000). The unique of Chaetognatha. Zoologischer Anzeiger between dipleurula and analyses to the removal of taxa 239, 263–266. trochophora larvae [13]. suggests that broader taxon 1Kewalo Marine Lab, University of The deuterostomy and sampling is required – particularly Hawaii, 41 Ahui St., Honolulu, Hawaii enterocoely of chaetognaths of the poorly represented 96816, USA. 2Wellcome Trust Centre could be convergent on that Lophotrochozoa. for Human Genetics, Roosevelt of the deuterostomes. For Drive, Oxford OX3 7BN, UK. 3Life example, coelom formation Sciences Department, Rouse 101, in chaetognaths has been Auburn University, Auburn, Alabama Supplemental data 36849, USA. 4Department of Biology, described as fundamentally Supplemental data including experi- University College London, Darwin different from that seen in the mental procedures are available at Building, Gower Street, London WC1E Deuterostomia [14] and the http://www.current-biology.com/cgi/ 6BT, UK. site and fate of the blastopore content/full/16/15/R575/DC1/ *E-mail: [email protected]