Evolution of the Multicellular Animals

SYNOPSIS. Molecular sequence analysis is providing new insightsinto the study of metazoan relationships. The use of ribosomalRNA sequences is revising many of the metazoan phylogenies thathave been established traditionally with anatomical and embryologicaldata. Four new findings that seem to be well supported by moleculardata, both from the authors' laboratories and from others, aredescribed and discussed. First, the arthropods are members ofa deep primary clade within the protostomes and are not thesister taxa of either the annelids or the mollusks. Second,the lophophorate animals are clearly protostomes and are containedwithin a lophotrochozoan superclade including the mollusks,annelids, and many other phyla. Third, the arthropods togetherwith all other molting animals comprise a second monophyleticsuperclade within the protostomes, the ecdysozoa. Fourth, theplatyhelminthes are contained within the lophotrochozoan superclade.     

Hox genes and the phylogeny of the arthropods

The arthropods are the most speciose, and among the most morphologically diverse, of the animal phyla. Their evolution has been the subject of intense research for well over a century, yet the relationships among the four extant arthropod subphyla - chelicerates, crustaceans, hexapods, and myriapods - are still not fully resolved. Morphological taxonomies have often placed hexapods and myriapods together (the Atelocerata) [1, 2], but recent molecular studies have generally supported a hexapod/crustacean clade [2-9]. A cluster of regulatory genes, the Hox genes, control segment identity in arthropods, and comparisons of the sequences and functions of Hox genes can reveal evolutionary relationships [10]. We used Hox gene sequences from a range of arthropod taxa, including new data from a basal hexapod and a myriapod, to estimate a phylogeny of the arthropods. Our data support the hypothesis that insects and crustaceans form a single clade within the arthropods to the exclusion of myriapods. They also suggest that myriapods are more closely allied to the chelicerates than to this insect/crustacean clade.     

Palaeontological and Molecular Evidence Linking Arthropods, Onychophorans, and other Ecdysozoa

Membership of Arthropoda in a clade of molting animals, the Ecdysozoa, has received a growing body of support over the past 10 years from analyses of DNA sequences from many genes together with morphological characters involving the cuticle and its molting. Recent analyses based on broad phylogenomic sampling strengthen the grouping of cycloneuralian worms and arthropods as Ecdysozoa, identify the velvet worms (Phylum Onychophora) as the closest living relatives of arthropods, and interpret segmentation as having separate evolutionary origins in arthropods and annelid worms. Determining whether the water bears (Phylum Tardigrada) are more closely related to onychophorans and arthropods or to unsegmented cycloneuralians such as roundworms (Nematoda) is an open question. Fossil taxa such as the Cambrian anomalocaridids provide a combination of arthropod and cycloneuralian characters that is not observed in any living ecdysozoan. Fossils break up long branches and help to resolve the sequence of character acquisition at several critical nodes in the arthropod tree, notably in a suite of Cambrian lobopodians that may include the stem groups of each of the major panarthropod lineages.     

The phylogenetic status of arthropods, as inferred from 18S rRNA sequences

Partial 18S rRNA sequences of five chelicerate arthropods plus a crustacean, myriapod, insect, chordate, echinoderm, annelid, and platyhelminth were compared. The sequence data were used to infer phylogeny by using a maximum-parsimony method, an evolutionary-distance method, and the evolutionary-parsimony method. The phylogenetic inferences generated by maximum-parsimony and distance methods support both monophyly of the Arthropoda and monophyly of the Chelicerata within the Arthropoda. These results are congruent with phylogenies based on rigorous cladistic analyses of morphological characters. Results support the inclusion of the Arthropoda within a spiralian or protostome coelomate clade that is the sister group of a deuterostome clade, refuting the hypothesis that the arthropods represent the "primitive" sister group of a protostome coelomate clade. Bootstrap analyses and consideration of all trees within 1% of the length of the most parsimonious tree suggest that relationships between the nonchelicerate arthropods and relationships within the chelicerate clade cannot be reliably inferred with the partial 18S rRNA sequence data. With the evolutionary-parsimony method, support for monophyly of the Arthropoda is found in the majority of the combinations analyzed if the coelomates are used as "outgroups." Monophyly of the Chelicerata is supported in most combinations assessed. Our analyses also indicate that the evolutionary-parsimony method, like distance and parsimony, may be biased by taxa with long branches. We suggest that a previous study's inference of the Arthropoda as paraphyletic may be the result of (a) having two few arthropod taxa available for analysis and (b) including long-branched taxa.      

Neurophylogeny: Architecture of the nervous system and a fresh view on arthropod phyologeny

The phylogenetic relationships within the Arthropoda have beencontroversial for more than a century. Today, comparative studieson the structure and development of the nervous system contributeimportant arguments to this discussion, so that the term "neurophylogeny"was coined for this discipline. The large number of recent studieson the nervous system in various nonmodel arthropods indicatesthat we are far advanced in the process of analyzing the cellulararchitecture of the arthropod nervous system in a depth thatwill ultimately provide characters at a level of resolutionequal or even superior to that of characters traditionally usedin morphological phylogenetic studies. This article sets outto summarize the current state of the discussion on arthropodphylogeny and briefly evaluates the morphological charactersthat have been used as arguments in favor of the traditionalTracheata hypothesis. Then, a thorough overview is given ofcharacters derived from structure and development of the arthropodbrain and the ventral nerve cord from the cellular level tothe level of larger neuropil systems. These characters supportthe new Tetraconata hypothesis suggested by Dohle and provideevidence for a clade that unites malacostracan and remipedecrustaceans with the Hexapoda.     

Systematic searches for molecular synapomorphies in model metazoan genomes give some support for Ecdysozoa after accounting for the idiosyncrasies of Caenorhabditis elegans

There has been broad acceptance among evolutionary biologists of the Ecdysozoa hypothesis that, based principally on molecular phylogenetic studies of small and large subunit ribosomal RNA sequences, postulates a close relationship between molting taxa such as arthropods and nematodes. On the other hand, recent studies of as many as 100 additional genes do not support the Ecdysozoa hypothesis and instead favor the older Coelomata hypothesis that groups the coelomate arthropods with the coelomate vertebrates to the exclusion of the nematodes. Here, exploiting completely sequenced genomes, we examined this question using cladistic analyses of the phylogenetic distribution of 1712 orthologous genes and 2906 protein domain combinations; we found stronger support for the Coelomata hypothesis than for the Ecdysozoa hypothesis. However, although arrived at by considering very large data sets, we show that this conclusion is unreliable, biased toward grouping arthropods with chordates by systematic high rate of character loss in the nematode. When we addressed this problem, we found slightly more support for Ecdysozoa than for Coelomata. Our identification of this systematic bias even when using entire genomes has important implications for future phylogenetic studies. We conclude that the results from the intensively sampled ribosomal RNA genes supporting the Ecdysozoa hypothesis provide the most credible current estimates of metazoan phylogeny.     

Phylogenetic position of the Tardigrada based on the 18S ribosomal RNA gene sequences

The phylogenetic position of the Tardigrada remains uncertain. This is due to the limited information available, and the uncertainty of whether some characters are homologous or analogous with other taxa. Based on some morphological characters, current discussion centres on whether the taxon branches from the annelid-arthropod lineage, or lies within the arthropod complex. The molecular data presented here from an analysis of the 18S rRNA gene sequences are used to test the validity of these two hypotheses. Phylogenetic inference by the maximum parsimony and distance (neighbour-joining) methods suggests that the Tardigrada is a sister group of the major protostome eucoelomate assemblage that emerged before the arthropods, annelids, molluscs, and sipunculids evolved. The tardigrade clade also appears as an independent lineage separate from the nematode clade, thus supporting the current idea that tardigrades do not have a close aschelminth relationship. The molecular data also imply that several morphological features, considered significant in determining the phylogenetic relationships of tardigrades, are not synapomorphic characters.     

Congruence between nuclear and mitochondrial genes in Demospongiae: a new hypothesis for relationships within the G4 clade (Porifera: Demospongiae)

The current morphological classification of the Demospongiae G4 clade was tested using large subunit ribosomal RNA (LSU rRNA) sequences from 119 taxa. Fifty-three mitochondrial cytochrome oxidase 1 (CO1) barcoding sequences were also analysed to test whether the 28S phylogeny could be recovered using an independent gene. This is the largest and most comprehensive study of the Demospongiae G4 clade. The 28S and CO1 genetrees result in congruent clades but conflict with the current morphological classification. The results confirm the polyphyly of Halichondrida, Hadromerida, Dictyonellidae, Axinellidae and Poecilosclerida and show that several of the characters used in morphological classifications are homoplasious. Robust clades are clearly shown and a new hypothesis for relationships of taxa allocated to G4 is proposed.     

Further support for the clades obtained by multiple molecular phylogenies in the acanthomorph bush

Several recent molecular studies have begun to clarify the phylogeny of Acanthomorpha (Teleostei), a wide clade of teleost fishes. However, different molecular datasets do not agree on a single history of the taxa, probably because of marker-specific biases. The 'total-evidence' approach maximizes character congruence, but may be biased by a single robust, but non-phylogenetic constraint from one dataset. We have therefore taken the approach to analyse also each dataset separately prior to their combination, and detect repeated groups: signal common to markers is more probably a reflection of shared ancestry than marker-specific signal. Partial sequences (678+527 base pairs) of exons of the MLL gene (Mixed Lineage Leukaemia-like) gene were used, as well as the datasets of Chen et al. (ribosomal 28S, rhodopsin gene, mitochondrial 12S and 16S). Most of the repeated clades of Chen et al. are supported by the new dataset. Some new groups were repeatedly found: a Scarus-Labrus group (clade M), the presence of Gasterosteidae as a sister taxon or within the clade Zoarcoidei-Cottoidei (clade Is), Polymixia as a sister-group to the clade Zeoidei-Gadiformes (clade O), the clade Q grouping Mugiloidei, Cichlidae, Atherinomorpha, Blennioidei and Gobiesocoidei; and the interesting clade N, reducing potential sister-groups to Tetraodontiformes to either Caproidei, Lophiiformes, Acanthuroidei, Drepanidae, Chaetodontidae, and Pomacanthidae.     

Ecdysozoa: The Relationship between Cycloneuralia and Panarthropoda

The hypothesis that molting protostomes such as nematodes and arthropods form a monophyletic group known as Ecdysozoa is directly opposed to Articulata, in which some segmented protostomes such as annelids and arthropods form a monophyletic taxon. Ultrastructural and cladistic studies have led to the widely accepted hypothesis that nematodes belong among the protostomes. While early molecular studies suggested that nematodes were basal triploblasts, more recent molecular evidence suggests that this was an artifact of ‘long branch attraction’ and 18S rRNA gene, total evidence and hox gene studies all support the placement of nematodes within Ecdysozoa. The branching pattern within Ecdysozoa has been difficult to elucidate, but it now appears that priapulids and kinorhynchs form the earliest branching clade, followed by nematodes + nematomorphs, and finally the panarthropods. This suggests that Cycloneuralia is paraphyletic and that arthropods are the most derived of the ecdysozoans.     

Phylogenetic relationships of annelids,molluscs, and arthropods evidenced from molecules and morphology

Annelids and arthropods have long been considered each other's closest relatives, as evidenced by similarities in their segmented body plans. An alternative view, more recently advocated by investigators who have examined partial 18S ribosomal RNA data, proposes that annelids, molluscs, and certain other minor phyla with trochophore larva stages share a more recent common ancestor with one another than any do with arthropods. The two hypotheses are mutually exclusive in explaining spiralian relationships. Cladistic analysis of morphological data does not reveal phylogentic relationships among major spiralian taxa but does suggest monophyly for both the annelids and molluscs. Distance and maximum-likelihood analyses of 18S rRNA gene sequences from major spiralian taxa suggest a sister relationship between annelids and molluscs and provide a clear resolution within the major groups of the spiralians. The parsimonious tree based on molecular data, however, indicates a sister relationship of the Annelida and Bivalvia, and an earlier divergence of the Gastropoda than the Annelida–Bivalvia clade. To test further hypotheses on the phylogenetic relationships among annelids, molluscs, and arthropods, and the ingroup relationships within the major spiralian taxa, we combine the molecular and morphological data sets and subject the combined data matrix to parsimony analysis. The resulting tree suggests that the molluscs and annelids form a monophyletic lineage and unites the molluscan taxa to a monophyletic group. Therefore, the result supports the Eutrochozoa hypothesis and the monophyly of molluscs, and indicates early acquisition of segmented body plans in arthropods. Received: 25 September 1995 / Accepted: 15 March 1996     

The complete mitochondrial genome of the onychophoran Epiperipatus biolleyi reveals a unique transfer RNA set and provides further support for the ecdysozoa hypothesis

Onychophora (velvet worms) play a crucial role in current discussions on position of arthropods. The ongoing Articulata/Ecdysozoa debate is in need of additional ground pattern characters for Panarthropoda (Arthropoda, Tardigrada, and Onychophora). Hence, Onychophora is an important outgroup taxon in resolving the relationships among arthropods, irrespective of whether morphological or molecular data are used. To date, there has been a noticeable lack of mitochondrial genome data from onychophorans. Here, we present the first complete mitochondrial genome sequence of an onychophoran, Epiperipatus biolleyi (Peripatidae), which shows several characteristic features. Specifically, the gene order is considerably different from that in other arthropods and other bilaterians. In addition, there is a lack of 9 tRNA genes usually present in bilaterian mitochondrial genomes. All these missing tRNAs have anticodon sequences corresponding to 4-fold degenerate codons, whereas the persisting 13 tRNAs all have anticodons pairing with 2-fold degenerate codons. Sequence-based phylogenetic analysis of the mitochondrial protein-coding genes provides a robust support for a clade consisting of Onychophora, Priapulida, and Arthropoda, which confirms the Ecdysozoa hypothesis. However, resolution of the internal ecdysozoan relationships suffers from a cluster of long-branching taxa (including Nematoda and Platyhelminthes) and a lack of data from Tardigrada and further nemathelminth taxa in addition to nematodes and priapulids.     

Phylogenetic position of cetaceans relative to artiodactyls: reanalysis of mitochondrial and nuclear sequences

By a maximum likelihood analysis of mitochondrial DNA sequences, we examine Graur and Higgins' hypothesis of the Ruminantia/Cetacea clade with Suiformes as an outgroup. Graur and Higgins analyzed these sequences by the neighbor-joining and parsimony methods, as well as by the maximum likelihood method under the assumption that the substitution rate is the same for all sites. The Ruminantia/Suiformes clade assumed by the traditional taxonomy was rejected strongly by this analysis and the Ruminantia/Cetacea clade was supported. Adoption of a more realistic model distinguishing among rates at different codon positions in the maximum likelihood analysis of the same data, however, grossly reduces the significance level on the Graur-Higgins hypothesis. Thus, although the Ruminantia/Suiformes grouping is indeed least likely from Graur and Higgins' data set of mitochondrial DNA, this traditional tree cannot be rejected with statistical significance under the new analysis, and more data are needed to settle the issue. In the same way, we examine Irwin and Arnason's suggestion of the Hippopotamus/Cetacea clade by using cytochrome b and hemoglobins alpha and beta, and it turn out that their suggestion is also fragile. This analysis demonstrates the importance of selecting an appropriate model among the alternatives in the maximum likelihood analysis and of using many different genes from many relevant species in order to make reliable phylogenetic inferences.      

Evolution of syncarpy and other morphological characters in African Annonaceae: a posterior mapping approach

The congenital fusion of carpels, or syncarpy, is considered a key innovation as it is found in more than 80% of angiosperms. Within the magnoliids however, syncarpy has rarely evolved. Two alternative evolutionary origins of syncarpy were suggested in order to explain the evolution of this feature: multiplication of a single carpel vs. fusion of a moderate number of carpels. The magnoliid family Annonaceae provides an ideal situation to test these hypotheses as two African genera, Isolona and Monodora, are syncarpous in an otherwise apocarpous family with multicarpellate and unicarpellate genera. In addition to syncarpy, the evolution of six other morphological characters was studied. Well-supported phylogenetic relationships of African Annonaceae and in particular those of Isolona and Monodora were reconstructed. Six plastid regions were sequenced and analyzed using maximum parsimony and Bayesian inference methods. The Bayesian posterior mapping approach to study character evolution was used as it accounts for both mapping and phylogenetic uncertainty, and also allows multiple state changes along the branches. Our phylogenetic analyses recovered a fully resolved clade comprising twelve genera endemic to Africa, including Isolona and Monodora, which was nested within the so-called long-branch clade. This is the largest and most species-rich clade of African genera identified to date within Annonaceae. The two syncarpous genera were inferred with maximum support to be sister to a clade characterized by genera with multicarpellate apocarpous gynoecia, supporting the hypothesis that syncarpy arose by fusion of a moderate number of carpels. This hypothesis was also favoured when studying the floral anatomy of both genera. Annonaceae provide the only case of a clear evolution of syncarpy within an otherwise apocarpous magnoliid family. The results presented here offer a better understanding of the evolution of syncarpy in Annonaceae and within angiosperms in general.     

Phylogenetic Relationships of Acanthocephala Based on Analysis of 18S Ribosomal RNA Gene Sequences

Acanthocephala (thorny-headed worms) is a phylum of endoparasites of vertebrates and arthropods, included among the most phylogenetically basal tripoblastic pseudocoelomates. The phylum is divided into three classes: Archiacanthocephala, Palaeacanthocephala, and Eoacanthocephala. These classes are distinguished by morphological characters such as location of lacunar canals, persistence of ligament sacs in females, number and type of cement glands in males, number and size of proboscis hooks, host taxonomy, and ecology. To understand better the phylogenetic relationships within Acanthocephala, and between Acanthocephala and Rotifera, we sequenced the nearly complete 18S rRNA genes of nine species from the three classes of Acanthocephala and four species of Rotifera from the classes Bdelloidea and Monogononta. Phylogenetic relationships were inferred by maximum-likelihood analyses of these new sequences and others previously determined. The analyses showed that Acanthocephala is the sister group to a clade including Eoacanthocephala and Palaeacanthocephala. Archiacanthocephala exhibited a slower rate of evolution at the nucleotide level, as evidenced by shorter branch lengths for the group. We found statistically significant support for the monophyly of Rotifera, represented in our analysis by species from the clade Eurotatoria, which includes the classes Bdelloidea and Monogononta. Eurotatoria also appears as the sister group to Acanthocephala. Received: 12 October 1999 / Accepted: 8 February 2000     

A divergent Cardinium found in daddy long-legs (Arachnida: Opiliones)

Recent studies indicate that a newly described bacterial endosymbiont, Cardinium, is widespread in arthropods and induces different reproductive manipulations in hosts. In this study, we used a portion of the 16S rRNA gene of the Cardinium to screen 16 Opilionid species from the suborder Palptores. We found the incidence of Cardinium in these Opiliones was significantly higher than in other pooled arthropods (31.2% versus 7.2%, P = 0.007). Phylogenetic analyses using maximum parsimony (MP) and Bayesian analysis revealed two distinct clades in Opiliones. One is a divergent monophyletic clade with strong support that has so far not been found in other arthropods, and a second one contains Cardinium both from Opiliones and other arthropods. There is not complete concordance of the Cardinium strains with host phylogeny, suggesting some horizontal movement of the bacteria among Opiliones. Although the divergence in the sequenced 16S rRNA region between the Cardinium infecting Opiliones and Cardinium from other arthropods is greater than among Cardinium found in other arthropods, all are monophyletic with respect to the outgroup bacteria (endosymbionts of Acanthamoeba). Based on high pairwise genetic distances, deep branch, and a distinct phylogenetic grouping, we conclude that some Opiliones harbor a newly discovered Cardinium clade.     

Molecular phylogenetic analysis of Passifloraceae sensu lato (Malpighiales) based on plastid and nuclear DNA sequences

A phylogenetic analysis of Passifloraceae sensu lato was performed using rbcL, atpB, matK, and 18S rDNA sequences from 25 genera and 42 species. Parsimony analyses of combined data sets resulted in a single most parsimonious tree, which was very similar to the 50% majority consensus tree from the Bayesian analysis. All nodes except three were supported by more than 50% bootstrap. The monophyly of Passifloraceae s.l. as well as the former families, Malesherbiaceae, Passifloraceae sensu stricto, and Turneraceae were strongly supported. Passifloraceae s.s. and the Turneraceae are sisters, and form a strongly supported clade. Within Passifloraceae s.s., the tribes Passifloreae and Paropsieae are both monophyletic. The intergeneric relationships within Passifloraceae s.s. and Turneraceae are roughly correlated with previous classification systems. The morphological character of an androgynophore/gynophore is better used for characterizing genera grouping within Passifloraceae s.s. Other morphological characters such as the corona and aril are discussed.     

Speciation on a conveyor belt: sequential colonization of the hawaiian islands by Orsonwelles spiders (Araneae,Linyphiidae)

Spiders of the recently described linyphiid genus Orsonwelles (Araneae, Linyphiidae) are one of the most conspicuous groups of terrestrial arthropods of Hawaiian native forests. There are 13 known Orsonwelles species, and all are single- island endemics. This radiation provides an excellent example of insular gigantism. We reconstructed the cladistic relationships of Orsonwelles species using a combination of morphological and molecular characters (both mitochondrial and nuclear sequences) within a parsimony framework. We explored and quantified the contribution of different character partitions and their sensitivity to changes in the traditional parameters (gap, transition, and transversion costs). The character data show a strong phylogenetic signal, robust to parameter changes. The monophyly of the genus Orsonwelles is strongly supported. The parsimony analysis of all character evidence combined recovered a clade with of all the non-Kauai Orsonwelles species; the species from Kauai form a paraphyletic assemblage with respect to the latter former clade. The biogeographic pattern of the Hawaiian Orsonwelles species is consistent with colonization by island progression, but alternative explanations for our data exist. Although the geographic origin of the radiation remains unknown, it appears that the ancestral colonizing species arrived first on Kauai (or an older island). The ambiguity in the area cladogram (i.e., post-Oahu colonization) is not derived from conflicting or unresolved phylogenetic signal among Orsonwelles species but rather from the number of taxa on the youngest islands. Speciation in Orsonwelles occurred more often within islands (8 of the 12 cladogenic events) than between islands. A molecular clock was rejected for the sequence data. Divergence times were estimated by using the nonparametric rate smoothing method of Sanderson (1997, Mol. Biol. Evol. 14:1218-1231) and the available geological data for calibration. The results suggest that the oldest divergences of Orsonwelles spiders (on Kauai) go back about 4 million years.     

Repeatability of clades as a criterion of reliability: a case study for molecular phylogeny of Acanthomorpha (Teleostei) with larger number of taxa

Although much progress has been made recently in teleostean phylogeny, relationships among the main lineages of the higher teleosts (Acanthomorpha), containing more than 60% of all fish species, remain poorly defined. This study represents the most extensive taxonomic sampling effort to date to collect new molecular characters for phylogenetic analysis of acanthomorph fishes. We compiled and analyzed three independent data sets, including: (i) mitochondrial ribosomal fragments from 12S and 16s (814bp for 97 taxa); (ii) nuclear ribosomal 28S sequences (847bp for 74 taxa); and (iii) a nuclear protein-coding gene, rhodopsin (759bp for 86 taxa). Detailed analyses were conducted on each data set separately and the principle of taxonomic congruence without consensus trees was used to assess confidence in the results as follows. Repeatability of clades from separate analyses was considered the primary criterion to establish reliability, rather than bootstrap proportions from a single combined (total evidence) data matrix. The new and reliable clades emerging from this study of the acanthomorph radiation were: Gadiformes (cods) with Zeioids (dories); Beloniformes (needlefishes) with Atheriniformes (silversides); blenioids (blennies) with Gobiesocoidei (clingfishes); Channoidei (snakeheads) with Anabantoidei (climbing gouramies); Mastacembeloidei (spiny eels) with Synbranchioidei (swamp-eels); the last two pairs of taxa grouping together, Syngnathoidei (aulostomids, macroramphosids) with Dactylopteridae (flying gurnards); Scombroidei (mackerels) plus Stromatoidei plus Chiasmodontidae; Ammodytidae (sand lances) with Cheimarrhichthyidae (torrentfish); Zoarcoidei (eelpouts) with Cottoidei; Percidae (perches) with Notothenioidei (Antarctic fishes); and a clade grouping Carangidae (jacks), Echeneidae (remoras), Sphyraenidae (barracudas), Menidae (moonfish), Polynemidae (threadfins), Centropomidae (snooks), and Pleuronectiformes (flatfishes).     

Genome-scale evidence of the nematode-arthropod clade

Background  

The issue of whether coelomates form a single clade, the Coelomata, or whether all animals that moult an exoskeleton (such as the coelomate arthropods and the pseudocoelomate nematodes) form a distinct clade, the Ecdysozoa, is the most puzzling issue in animal systematics and a major open-ended subject in evolutionary biology. Previous single-gene and genome-scale analyses designed to resolve the issue have produced contradictory results. Here we present the first genome-scale phylogenetic evidence that strongly supports the Ecdysozoa hypothesis.