Multigene analysis of lophophorate and chaetognath phylogenetic relationships

Maximum likelihood and Bayesian inference analyses of seven concatenated fragments of nuclear-encoded housekeeping genes indicate that Lophotrochozoa is monophyletic, i.e., the lophophorate groups Bryozoa, Brachiopoda and Phoronida are more closely related to molluscs and annelids than to Deuterostomia or Ecdysozoa. Lophophorates themselves, however, form a polyphyletic assemblage. The hypotheses that they are monophyletic and more closely allied to Deuterostomia than to Protostomia can be ruled out with both the approximately unbiased test and the expected likelihood weights test. The existence of Phoronozoa, a putative clade including Brachiopoda and Phoronida, has also been rejected. According to our analyses, phoronids instead share a more recent common ancestor with bryozoans than with brachiopods. Platyhelminthes is the sister group of Lophotrochozoa. Together these two constitute Spiralia. Although Chaetognatha appears as the sister group of Priapulida within Ecdysozoa in our analyses, alternative hypothesis concerning chaetognath relationships could not be rejected.     

The phylogenetic position of Brachiopoda—a comparison of morphological and molecular data

Analyses of rRNA and rDNA among Metazoa result in a hypothesis of a sistergroup relationship of Brachiopoda and certain spiralian taxa, whereas analyses of morphological data imply that Brachiopoda show affinities to Deuterostomia within the Radialia. Regarding Brachiopoda as a derived spiralian taxon must be followed by a reinterpretation of the evolution of distinct brachiopod morphological characters—like cleavage pattern, coelom or larva. The experimental insertion of a monophyletic taxon consisting of Brachiopoda and Phoronida into a widely accepted phylogenetic tree of Spiralia leads to the hypothesis that at least trimeric organization, mesosomal tentacular apparatus and heterogeneously assembled metanephridia are products of convergent evolution in Brachiopoda plus Phoronida and Deuterostomia. The hypothesis of a radialian nature of Brachiopoda and Phoronida, as implied by morphological data, remains as the most parsimonious possibility to explain the evolution of seven regarded characters (cleavage pattern, larva, tentacular apparatus, coelom, metameric segmentation, metanephridia and chaetae) in Brachiopoda. Due to the conflicting results of both methods a hitherto undetected systematical problem is discussed possibly hindering data comparability. If the course of evolution can principally be inferred from the information preserved in recent and fossil animals, the results should be congruent in the analyses of both, molecular and morphological data.     

R-spondins are involved in the ovarian differentiation in a teleost,medaka (Oryzias latipes)

Background

Among the four major bilaterian clades, Deuterostomia, Acoelomorpha, Ecdysozoa, and Lophotrochozoa, the latter shows an astonishing diversity of bodyplans. While the largest lophotrochozoan assemblage, the Spiralia, which at least comprises Annelida, Mollusca, Entoprocta, Platyhelminthes, and Nemertea, show a spiral cleavage pattern, Ectoprocta, Brachiopoda and Phoronida (the Lophophorata) cleave radially. Despite a vast amount of recent molecular phylogenetic analyses, the interrelationships of lophotrochozoan phyla remain largely unresolved. Thereby, Entoprocta play a key role, because they have frequently been assigned to the Ectoprocta, despite their differently cleaving embryos. However, developmental data on entoprocts employing modern methods are virtually non-existent and the data available rely exclusively on sketch drawings, thus calling for thorough re-investigation.

Results

By applying fluorescence staining in combination with confocal microscopy and 3D-imaging techniques, we analyzed early embryonic development of a basal loxosomatid entoproct. We found that cleavage is asynchronous, equal, and spiral. An apical rosette, typical for most spiralian embryos, is formed. We also identified two cross-like cellular arrangements that bear similarities to both, a "molluscan-like" as well as an "annelid-like" cross, respectively.

Conclusions

A broad comparison of cleavage types and apical cross patterns across Lophotrochozoa shows high plasticity of these character sets and we therefore argue that these developmental traits should be treated and interpreted carefully when used for phylogenetic inferences.     

Fine structure of the epistome in Phoronis ovalis: significance for the coelomic organization in Phoronida

Abstract. The hypothesis of a common ancestry of the lophophorate taxa Brachiopoda, Bryozoa, Phoronida, and the Deuterostomia can be traced back to the late 19th century when Masterman recognized a tripartite organization of the body consisting of pro-, meso-, and metasome, along with coelomic body cavities in each compartment, as characteristic for Echinodermata, Pterobranchia, Phoronida, and Brachiopoda. This idea became quite popular under the name "archicoelomate" concept. The organization of the phoronids, and especially of their transparent actinotroch larva, has for a long time been used as a touchstone for the validity of this concept. As a coelomic lining can reliably be recognized only on the ultrastructural level, this technique has been applied for adults of Phoronis ovalis , which is assumed to be a sister species to all other phoronids. Phoronis ovalis contains only two coelomic compartments, a posterior coelom inside the trunk (metasoma), occupying the space between the trunk epidermis and the digestive epithelium, and an anterior lophophoral coelom inside and basal to the tentacular crown (mesosoma). There is no coelomic cavity inside the epistome (prosoma). This part of the body is filled with myoepithelial cells, which are continuous with the epithelial lining of the lophophore cavity. These cells form a lumenless bilayer and possess long, tiny myofilamentous processes, which are completely embedded in an extracellular matrix. A comparison with data on P. muelleri shows that there is no need to assume three different coelomic cavities in Phoronida, in contrast to the predictions of the archicoelomate concept. At least for this taxon, a correspondence to the situation in deuterostomes can hardly be found.     

18S rRNA suggests that Entoprocta are protostomes,unrelated to Ectoprocta

The Ento- and Ectoprocta are sometimes placed together in the Bryozoa, which have variously been regarded as proto- or deuterostomes. However, Entoprocta have also been allied to the pseudocoelomates, while Ectoprocta are often united with the Brachiopoda and Phoronida in the (super)phylum Lophophorata. Hence, the phylogenetic relationships of these taxa are still much debated. We determined complete 18S rRNA sequences of two entoprocts, an ectoproct, an inarticulate brachiopod, a phoronid, two annelids, and a platyhelminth. Phylogenetic analyses of these data show that (1) entoprocts and lophophorates have spiralian, protostomous affinities, (2) Ento- and Ectoprocta are not sister taxa, (3) phoronids and brachiopods form a monophyletic clade, and (4) neither Ectoprocta or Annelida appear to be monophyletic. Both deuterostomous and pseudocoelomate features may have arisen at least two times in evolutionary history. These results advocate a Spiralia-Radialia-based classification rather than one based on the Protostomia-Deuterostomia concept. Correspondence to: J.R. Garey     

Gastrotricha and metazoan phylogeny

The phylogenetic position of the Gastrotricha within Bilateria and relationships among gastrotrich subgroups are reanalysed using morphological, developmental, nonsequence molecular, and ecological characters, together with the conserved regions of small-subunit ribosomal RNA genes (SSU rDNA). The analysis shows that traditional 'Macrodasyida' is a paraphyletic stemline of Chaetonotida, with Dactylopodolida, Redudasys , and Turbanellida as the basalmost gastrotrich groups. The 'Cycloneuralia hypothesis', which assumes sister group relationships between Gastrotricha and Ecdysozoa is supported. The sensitivity analysis of the combined dataset yields the following scheme of relationships of the main bilaterian clades: (1) Acoelomorpha is a basalmost bilaterian clade; (2) both Deuterostomia and Protostomia (less Acoelomorpha) are monophyletic; (3) the phylogenetic position of Ectoprocta, Brachiopoda + Phoronida, and Cycloneuralia within Protostomia is unstable; (4) Trochozoa (incl. Entoprocta, Nemertea, Lobatocerebrum , and possibly Jennaria ), Platyhelminthes s.s ., and Gnathifera-Myzostomida form a clade ('Spiralia'); (5) Cycliophora and possibly also Chaetognatha may be close to the gnathiferans. Evolution of metazoan ciliation and cycloneuralian cuticle is discussed. It is concluded that cycloneuralian and gastrotrich ancestors were multiciliate and had epidermal cilia covered by cuticular sheaths.     

The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes

Recent hypotheses on metazoan phylogeny have recognized three main clades of bilaterian animals: Deuterostomia, Ecdysozoa and Lophotrochozoa. The acoelomate and 'pseudocoelomate' metazoans, including the Platyhelminthes, long considered basal bilaterians, have been referred to positions within these clades by many authors. However, a recent study based on ribosomal DNA placed the flatworm group Acoela as the sister group of all other extant bilaterian lineages. Unexpectedly, the nemertodermatid flatworms, usually considered the sister group of the Acoela together forming the Acoelomorpha, were grouped separately from the Acoela with the rest of the Platyhelminthes (the Rhabditophora) within the Lophotrochozoa. To re-evaluate and clarify the phylogenetic position of the Nemertodermatida, new sequence data from 18S ribosomal DNA and mitochondrial genes of nemertodermatid and other bilaterian species were analysed with parsimony and maximum likelihood methods. The analyses strongly support a basal position within the Bilateria for the Nemertodermatida as a sister group to all other bilaterian taxa except the Acoela. Despite the basal position of both Nemertodermatida and Acoela, the clade Acoelomorpha was not retrieved. These results imply that the last common ancestor of bilaterian metazoans was a small, benthic, direct developer without segments, coelomic cavities, nephrida or a true brain. The name Nephrozoa is proposed for the ancestor of all bilaterians excluding the Nemertodermatida and the Acoela, and its descendants.     

Lophotrochozoan phylogeny assessed with LSU and SSU data: evidence of lophophorate polyphyly

Of the three major bilaterian clades, Lophotrochozoa has the greatest diversity and disparity of body forms and is the least understood in terms of phylogenetic history. Within this clade, small nuclear ribosomal subunit (SSU or 18S) studies have failed to provide resolution and other molecular markers have insufficient taxon sampling. To examine relationships within Lophotrochozoa, we collected and complied complete SSU data and nearly complete (>90%) large nuclear ribosomal subunit (LSU or 28S) data totaling approximately 5kb per taxon, for 36 lophotrochozoans. Results of LSU and combined SSU+LSU likelihood analyses provide topologies more consistent with morphological data than analyses of SSU data alone. Namely, most phyla recognized on morphological grounds are recovered as monophyletic entities when the LSU data is considered (contra SSU data alone). These new data show with significant support that "Lophophorata" (traditionally recognized to include Brachiopoda, Phoronida, and Bryozoa) is not a monophyletic entity. Further, the data suggest that Platyzoa is real and may be derived within lophotrochozans rather than a basal or sister taxon. The recently discovered Cycliophora are allied to entoprocts, consistent with their initial placement based on morphology. Additional evidence for Syndermata (i.e., Rotifera+Acanthocephala) is also found. Although relationships among groups with trochophore-like larvae could not be resolved and nodal support values are generally low, the addition of LSU data is a considerable advance in our understanding of lophotrochozoan phylogeny from the molecular perspective.     

Testing the new animal phylogeny: a phylum level molecular analysis of the animal kingdom

The new animal phylogeny inferred from ribosomal genes some years ago has prompted a number of radical rearrangements of the traditional, morphology based metazoan tree. The two main bilaterian clades, Deuterostomia and Protostomia, find strong support, but the protostomes consist of two sister groups, Ecdysozoa and Lophotrochozoa, not seen in morphology based trees. Although widely accepted, not all recent molecular phylogenetic analyses have supported the tripartite structure of the new animal phylogeny. Furthermore, even if the small ribosomal subunit (SSU) based phylogeny is correct, there is a frustrating lack of resolution of relationships between the phyla that make up the three clades of this tree. To address this issue, we have assembled a dataset including a large number of aligned sequence positions as well as a broad sampling of metazoan phyla. Our dataset consists of sequence data from ribosomal and mitochondrial genes combined with new data from protein coding genes (5139 amino acid and 3524 nucleotide positions in total) from 37 representative taxa sampled across the Metazoa. Our data show strong support for the basic structure of the new animal phylogeny as well as for the Mandibulata including Myriapoda. We also provide some resolution within the Lophotrochozoa, where we confirm support for a monophyletic clade of Echiura, Sipuncula and Annelida and surprising evidence of a close relationship between Brachiopoda and Nemertea.     

On the origin systematic position of the calcareous-shelled inarticulate brachiopods

Available data on the anatomy, ontogeny embryology of Brachiopoda ( sensu lato ) suggest that this Phylum in the traditional view is in fact a clade of organization includes two stocks of lophophorate organisms of quite different origins. Their rank does not correspond to existing Class divisions. The phosphatic-shelled in articulates are regarded as a separate Class Lingulata. The Phylum Brachiopoda ( sensu stricto ) is restricted to the calcareous-shelled inarticulate articulate lineages. Ancestors of the calcareous-shelled brachiopods probably differentiated from the protolophophorates before the radiation of the other lophophorate stocks took place. The articulation of the valves appeared independently in several lineages during the early stages of brachiopod evolution.     

On 20 years of Lophotrochozoa

Lophotrochozoa is a protostome clade that includes disparate animals such as molluscs, annelids, bryozoans, and flatworms, giving it the distinction of including the most body plans of any of the three major clades of Bilateria. This extreme morphological disparity has prompted numerous conflicting phylogenetic hypotheses about relationships among lophotrochozoan phyla. Here, I review the current understanding of lophotrochozoan phylogeny with emphasis on recent insights gained through approaches taking advantage of high-throughput DNA sequencing (phylogenomics). Of significance, Platyzoa, a hypothesized clade of mostly small-bodied animals, appears to be an artifact of long-branch attraction. Recent studies recovered Gnathifera (Syndermata, Gnathostomulida, and Micrognathozoa) sister to all other lophotrochozoans and a clade called Rouphozoa (Platyhelminthes and Gastrotricha) sister to the remaining non-gnathiferan lophotrochozoans. Although Bryozoa was traditionally grouped with Brachiopoda and Phoronida (Lophophorata), most molecular studies have supported a clade including Entoprocta, Cycliophora, and Bryozoa (Polyzoa). However, recent phylogenomic work has shown that entoprocts and bryozoans have compositionally heterogeneous genomes that may cause systematic artifacts affecting their phylogenetic placement. Lastly, relationships within Trochozoa (Mollusca, Annelida, and relatives) largely remain ambiguous. Recent work has shown that phylogenomic studies must identify and reduce sources of systematic error, such as amino acid compositional heterogeneity and long-branch attraction. Still, other approaches such as the analysis of rare genomic changes may be needed to overcome challenges to standard phylogenomic approaches. Resolving lophotrochozoan phylogeny will provide important insight into how these complex and diverse body plans evolved and provide a much-needed framework for comparative studies.     

Myoanatomy and serotonergic nervous system of plumatellid and fredericellid phylactolaemata (lophotrochozoa,ectoprocta)

The phylogenetic position of the Ectoprocta within the Lophotrochozoa is discussed controversially. For gaining more insight into ectoproct relationships and comparing it with other potentially related phyla, we analysed the myoanatomy and serotonergic nervous system of adult representatives of the Phylactolaemata (Plumatella emarginata, Plumatellavaihiriae, Plumatella fungosa, Fredericella sultana). The bodywall contains a mesh of circular and longitudinal muscles. On its distal end, the orifice possesses a prominent sphincter and continues into the vestibular wall, which has longitudinal and circular musculature. The tentacle sheath carries mostly longitudinal muscle fibres in Plumatella sp., whereas F. sultana also possesses regular circular muscle fibres. Three groups of muscles are associated with the lophophore: 1) Lophophoral arm muscles (missing in Fredericella), 2) epistome musculature and 3) tentacle musculature. The epistome flap is encompassed by smooth muscle fibres. A few fibres extend medially over the ganglion to its proximal floor. Abfrontal tentacle muscles have diagonally arranged muscle fibres in their proximal region, whereas the distal region is formed by a stack of muscles that resemble an inverted ‘V’. Frontal tentacle muscles show more variation and either possess one or two bases. The digestive tract possesses circular musculature which is striated except at the intestine where it is composed of smooth muscle fibres. The serotonergic nervous system is concentrated in the cerebral ganglion. From the latter a serotonergic nerve extends to each tentacle base. In Plumatella the inner row of tentacles at the lophophoral concavity lacks serotonergic nerves. Bodywall musculature is a common feature in many lophotrochozoan phyla, but among other filter feeders like the Ectoprocta is only present in the ‘lophophorate’ Phoronida. The longitudinal tentacle musculature is reminiscent of the condition found in phoronids and brachiopods, but differs to entoproct tentacles. Although this study shows some support for the ‘Lophophorata’, more comparative analyses of possibly related phyla are required. J. Morphol., 2011. © 2011 Wiley Periodicals, Inc.     

Molecular paleobiological insights into the origin of the Brachiopoda

Most studies of brachiopod evolution have been based on their extensive fossil record, but molecular techniques, due to their independence from the rock record, can offer new insights into the evolution of a clade. Previous molecular phylogenetic hypotheses of brachiopod interrelationships place phoronids within the brachiopods as the sister group to the inarticulates, whereas morphological considerations suggest that Brachiopoda is a monophyletic group. Here, these hypotheses were tested with a molecular phylogenetic analysis of seven nuclear housekeeping genes combined with three ribosomal genes. The combined analysis finds brachiopods to be monophyletic, but with relatively weak support, and the craniid as the sister taxon of all other brachiopods. Phylogenetic-signal dissection suggests that the weak support is caused by the instability of the craniid, which is attracted to the phoronids. Analysis of slowly evolving sites results in a robustly supported monophyletic Brachiopoda and Inarticulata (Linguliformea+Craniiformea), which is regarded as the most likely topology for brachiopod interrelationships. The monophyly of Brachiopoda was further tested with microRNA-based phylogenetics, which are small, noncoding RNA genes whose presence and absence can be used to infer phylogenetic relationships. Two novel microRNAs were characterized supporting the monophyly of brachiopods. Congruence of the traditional molecular phylogenetic analysis, microRNAs, and morphological cladograms suggest that Brachiopoda is monophyletic with Phoronida as its likely sister group. Molecular clock analysis suggests that extant phoronids have a Paleozoic divergence despite their conservative morphology, and that the early brachiopod fossil record is robust, and is not affected by taphonomic factors relating to the late-Precambrian/early-Cambrian phosphogenic event.     

Phylogeny of the Metazoa Based on Morphological and 18S Ribosomal DNA Evidence

Cladistic analysis of traditional (i.e. morphological, developmental, ultrastructural) and molecular (18S rDNA) data sets (276+501 informative characters) provides a hypothesis about relationships of all meta-zoan higher taxa. Monophyly of Metazoa, Epith-eliozoa (= -03non-Porifera), Triploblastica, Mesozoa, Eutriploblastica (=Rhabditophora+Catenulida+“higher triploblasts”=Neotriploblastica, including Xeno- turbellida and Gnathostomulida), Rhabditophora, Syndermata (=“Rotifera”+Acanthocephala), Neotrichozoa (=Gastrotricha+Gnathostomulida), Nematozoa (=Nematoda+Nematomorpha), Panarthropoda (=Onychophora+Tardigrada+ Arthropoda), Cephalorhyncha, Deuterostomia, Ambulacralia (=Hemichordata+Echinodermata), Chordata, Phoronozoa (=Phoronida+“Brachiopoda”), Bryozoa, Trochozoa (=Eutrochozoa+Entoprocta+ Cycliophora), Eutrochozoa, and Chaetifera (=Annelida+ Pogonophora+Echiura) is strongly supported. Cnidaria (including Myxozoa), Ecdysozoa (=Cepha- lorhyncha + Nematozoa + Chaetognatha + Panarthropoda), Eucoelomata (=Bryozoa+Phoronozoa+Deuterostomia+Trochozoa, possibly including also Xenoturbellida), and Deuterostomia+Phoronozoa probably are monophyletic. Most traditional “phyla” are monophyletic, except for Porifera, Cnidaria (excluding Myxozoa), Platyhelminthes, Brachiopoda, and Rotifera. Three “hot” regions of the tree remain quite unresolved: basal Epitheliozoa, basal Triploblastica, and basal Neotriploblastica. A new phylogenetic classification of the Metazoa including 35 formally recognized phyla (Silicispongea, Calcispongea, Placozoa, Cnidaria, Ctenophora, Acoela, Nemertodermatida, Orthonecta, Rhombozoa, Rhabditophora, Catenulida, Syndermata, Gnathostomulida, Gastrotricha, Cephalorhyncha, Chaetognatha, Nematoda, Nematomorpha, Onychophora, Tardigrada, Arthropoda, Echinodermata, Hemichordata, Chordata, Phoronozoa, Bryozoa s. str., Xenoturbellida, Entoprocta, Cycliophora, Nemertea, Mollusca, Sipuncula, Echiura, Pogonophora, and Annelida) and few i ncertae sedis g roups (e.g. Myzostomida and Lobatocerebromorpha) is proposed.     

Complete nucleotide sequences of mitochondrial genomes of two solitary entoprocts, Loxocorone allax and Loxosomella aloxiata: implications for lophotrochozoan phylogeny

The complete nucleotide sequences of the mitochondrial (mt) genomes of the entoprocts Loxocorone allax and Loxosomella aloxiata were determined. Both species carry the typical gene set of metazoan mt genomes and have similar organizations of their mt genes. However, they show differences in the positions of two tRNA(Leu) genes. Additionally, the tRNA(Val) gene, and half of the long non-coding region, is duplicated and inverted in the Loxos. aloxiata mt genome. The initiation codon of the Loxos. aloxiata cytochrome oxidase subunit I gene is expected to be ACG rather than AUG. The mt gene organizations in these two entoproct species most closely resemble those of mollusks such as Katharina tunicata and Octopus vulgaris, which have the most evolutionarily conserved mt gene organization reported to date in mollusks. Analyses of the mt gene organization in the lophotrochozoan phyla (Annelida, Brachiopoda, Echiura, Entoprocta, Mollusca, Nemertea, and Phoronida) suggested a close phylogenetic relationship between Brachiopoda, Annelida, and Echiura. However, Phoronida was excluded from this grouping. Molecular phylogenetic analyses based on the sequences of mt protein-coding genes suggested a possible close relationship between Entoprocta and Phoronida, and a close relationship among Brachiopoda, Annelida, and Echiura.     

Structure and Function of Metazoan Ciliary Bands and Their Phylogenetic Significance

Ciliated epithelia, especially the ciliary bands used in swimming and filter feeding, of representatives of the following phyla have been investigated: Porifera, Cnidaria, Annelida, Mollusca, Sipuncula, Nemertini, Platyhelminthes, Entoprocta, Ectoprocta, Rotifera, Pterobranchia, Phoronida, Brachiopoda, Echinodermata and Enteropneusta. The trochaea theory predicts that Porifera and Cnidaria have only monociliate cells and lack ciliary bands used in filter-feeding, that the gastroneuralian larvae have downstream-collecting ciliary bands with prototroch and metatroch of compound cilia on multiciliate cells, and that notoneuralian larvae have an upstream-collecting neotroch on monociliate cells. The observations generally fit these predictions and the exceptions are discussed. In all the ciliated epithelia, except that of the sponge larva, each ciliated cell has an accessory centriole perpendicular to the basal body of the cilium and situated on its downstream side.     

Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida,Cycliophora, Plathelminthes,and Chaetognatha: a combined approach of 18S rDNA sequences and morphology

Triploblastic relationships were examined in the light of molecular and morphological evidence. Representatives for all triploblastic "phyla" (except Loricifera) were represented by both sources of phylogenetic data. The 18S ribosomal (rDNA) sequence data for 145 terminal taxa and 276 morphological characters coded for 36 supraspecific taxa were combined in a total evidence regime to determine the most consistent picture of triploblastic relationships for these data. Only triploblastic taxa are used to avoid rooting with distant outgroups, which seems to happen because of the extreme distance that separates diploblastic from triploblastic taxa according to the 18S rDNA data. Multiple phylogenetic analyses performed with variable analysis parameters yield largely inconsistent results for certain groups such as Chaetognatha, Acoela, and Nemertodermatida. A normalized incongruence length metric is used to assay the relative merit of the multiple analyses. The combined analysis having the least character incongruence yields the following scheme of relationships of four main clades: (1) Deuterostomia [((Echinodermata + Enteropneusta) (Cephalochordata (Urochordata + Vertebrata)))]; (2) Ecdysozoa [(((Priapulida + Kinorhyncha) (Nematoda + Nematomorpha)) ((Onychophora + Tardigrada) Arthropoda))]; (3) Trochozoa [((Phoronida + Brachiopoda) (Entoprocta (Nemertea (Sipuncula (Mollusca (Pogonophora (Echiura + Annelida)))))))]; and (4) Platyzoa [((Gnathostomulida (Cycliophora + Syndermata)) (Gastrotricha + Plathelminthes))]. Chaetognatha, Nemertodermatida, and Bryozoa cannot be assigned to any one of these four groups. For the first time, a data analysis recognizes a clade of acoelomates, the Platyzoa (sensu Cavalier-Smith, Biol. Rev. 73:203-266, 1998). Other relationships that corroborate some morphological analyses are the existence of a clade that groups Gnathostomulida + Syndermata (= Gnathifera), which is expanded to include the enigmatic phylum Cycliophora, as sister group to Syndermata.     

Phylogenèse des Phoronida

Phylogenese of Phoronida. Lophophorata and the Archimerata concept The main phylogenetic characteristics of Phoronida and other Lophophorates are discussed: 1. Archimeric segmentation of the body; 2. Egg cleavage of radial (or biradial) type, coeloblastula and gastrulation by invagination (emboly); 3. Mesoderm formation by a derived enteroccelous method (primitive stage of enteroc? ly); 4. Bringing of the anus anteriorly to lie rocoelous method (primitive stage of enterocoely); 4. Bringing of the anus anteriorly to lie sence of a true lophophore; 6. Larva not of trochophoral type, but actinotroch related to Tornaria-Dipleurula; 7. Nervous system basi-epithelial with primitive neurulation in Phoronopsis, without any orthogons; 8. Adult nervous ganglion neo-formed, not issuing from the apicale plate; in Phoronida this ganglion is located in the prosome and not in the mesosome; 9. Lack of cephalization. All these characteristics are closely related to that of the primitive phyla of the Chordata assemblage. The only exception is the presence of protonephridia with solenocytes in actinotroch, but such organs are also described in Cephalochordata. The Lophophorata (Phoronida, Brachiopoda, Bryozoa) are undoubtedly a primitive group on the Chordata trend and the Phoronida appear to be the most phylogenetically evolved phylum of this group with predominating position. The validity of placing the Lophophorata within the Echinoderm-Hemichordata assemblage is demonstrated. The term Archic?lomata appears not suitable and its substitution by Archimerata, assemblage at the base of the Chordata, is here proposed. The Archimerata concept brings together the Lophophorata, Echinodermata and Hemidiordata and is considered as a phylogenetic stage and a natural systematic unit.     

The Essential Role of "Minor" Phyla in Molecular Studies of Animal Evolution

SYNOPSIS. Molecular studies have revealed many new hypothesesof metazoan evolution in recent years. Previously, using morphologicalmethods, it was difficult to relate "minor" animal groups representingmicroscopic metazoans to larger, more well known groups suchas arthropods, molluscs, and annelids. Molecular studies suggestthat acanthocephalans evolved from rotifers, that priapulidsshare common ancestry with all other molting animals (Ecdysozoa),and that flatworms, gnathostomulids and rotifers form a sistergroup to the remaining non-molting protostomes (Lophotrochozoa),together forming Spiralia. The lophophorate phyla (phoronids,brachiopods and bryozoans) appear as protostomes, allied withannelids and molluscs rather than with deuterostomes. Thesefindings present a very different view of metazoan evolution,and clearly show that small and simple animals do not necessarilyrepresent ancestral or primitive taxa.     

Spiralian phylogenomics supports the resurrection of Bryozoa comprising Ectoprocta and Entoprocta

Phylogenetic analyses based on 79 ribosomal proteins of 38 metazoans, partly derived from 6 new expressed sequence tag projects for Ectoprocta, Entoprocta, Sipuncula, Annelida, and Acanthocephala, indicate the monophyly of Bryozoa comprising Ectoprocta and Entoprocta, 2 taxa that have been separated for more than a century based on seemingly profound morphological differences. Our results also show that bryozoans are more closely related to Neotrochozoa, including molluscs and annelids, than to Syndermata, the latter comprising Rotifera and Acanthocephala. Furthermore, we find evidence for the position of Sipuncula within Annelida. These findings suggest that classical developmental and morphological key characters such as cleavage pattern, coelomic cavities, gut architecture, and body segmentation are subject to greater evolutionary plasticity than traditionally assumed.