Eoandromeda and the origin of Ctenophora

The Ediacaran fossil Eoandromeda octobrachiata had a high conical body with eight arms in helicospiral arrangement along the flanks. The arms carried transverse bands proposed to be homologous to ctenophore ctenes (comb plates). Eoandromeda is interpreted as an early stem‐group ctenophore, characterized by the synapomorphies ctenes, comb rows, and octoradial symmetry but lacking crown‐group synapomorphies such as tentacles, statoliths, polar fields, and biradial symmetry. It probably had a pelagic mode of life. The early appearance in the fossil record of octoradial ctenophores is most consistent with the Planulozoa hypothesis (Ctenophora is the sister group of Cnidaria + Bilateria) of metazoan phylogeny.     

Investigation of molluscan phylogeny using large-subunit and small-subunit nuclear rRNA sequences

The Mollusca represent one of the most morphologically diverse animal phyla, prompting a variety of hypotheses on relationships between the major lineages within the phylum based upon morphological, developmental, and paleontological data. Analyses of small-ribosomal RNA (SSU rRNA) gene sequence have provided limited resolution of higher-level relationships within the Mollusca. Recent analyses suggest large-subunit (LSU) rRNA gene sequences are useful in resolving deep-level metazoan relationships, particularly when combined with SSU sequence. To this end, LSU (approximately 3.5 kb in length) and SSU (approximately 2 kb) sequences were collected for 33 taxa representing the major lineages within the Mollusca to improve resolution of intraphyletic relationships. Although the LSU and combined LSU+SSU datasets appear to hold potential for resolving branching order within the recognized molluscan classes, low bootstrap support was found for relationships between the major lineages within the Mollusca. LSU+SSU sequences also showed significant levels of rate heterogeneity between molluscan lineages. The Polyplacophora, Gastropoda, and Cephalopoda were each recovered as monophyletic clades with the LSU+SSU dataset. While the Bivalvia were not recovered as monophyletic clade in analyses of the SSU, LSU, or LSU+SSU, the Shimodaira-Hasegawa test showed that likelihood scores for these results did not differ significantly from topologies where the Bivalvia were monophyletic. Analyses of LSU sequences strongly contradict the widely accepted Diasoma hypotheses that bivalves and scaphopods are closely related to one another. The data are consistent with recent morphological and SSU analyses suggesting scaphopods are more closely related to gastropods and cephalopods than to bivalves. The dataset also presents the first published DNA sequences from a neomeniomorph aplacophoran, a group considered critical to our understanding of the origin and early radiation of the Mollusca.     

Dendrogramma,New Genus,with Two New Non-Bilaterian Species from the Marine Bathyal of Southeastern Australia (Animalia,Metazoa incertae sedis) – with Similarities to Some Medusoids from the Precambrian Ediacara

A new genus, Dendrogramma, with two new species of multicellular, non-bilaterian, mesogleal animals with some bilateral aspects, D. enigmatica and D. discoides, are described from the south-east Australian bathyal (400 and 1000 metres depth). A new family, Dendrogrammatidae, is established for Dendrogramma. These mushroom-shaped organisms cannot be referred to either of the two phyla Ctenophora or Cnidaria at present, because they lack any specialised characters of these taxa. Resolving the phylogenetic position of Dendrogramma depends much on how the basal metazoan lineages (Ctenophora, Porifera, Placozoa, Cnidaria, and Bilateria) are related to each other, a question still under debate. At least Dendrogramma must have branched off before Bilateria and is possibly related to Ctenophora and/or Cnidaria. Dendrogramma, therefore, is referred to Metazoa incertae sedis. The specimens were fixed in neutral formaldehyde and stored in 80% ethanol and are not suitable for molecular analysis. We recommend, therefore, that attempts be made to secure new material for further study. Finally similarities between Dendrogramma and a group of Ediacaran (Vendian) medusoids are discussed.     

THE PINGUIOPHYCEAE CLASSIS NOVA, CHROMOPHYTE ALGAE PRODUCING LARGE AMOUNTS OF OMEGA-3 FATTY ACIDS

The streptophytes comprise the Charophyceae sensu Mattox and Stewart (a morphologically diverse group of fresh-water green algae) and the embryophytes (land plants). Several charophycean groups are currently recognized. These include the Charales, Coleochaetales, Chlorokybales, Klebsormidiales and Zygnemophyceae (Desmidiales and Zygnematales). Recently, SSU rRNA gene sequence data allied Mesostigma viride (Prasinophyceae) with the Streptophyta. Complete chloroplast sequence data, however, placed Mesostigma sister to all green algae, not with the streptophytes. Several morphological, ultrastructural and biochemical features unite these lineages into a monophyletic group including embryophytes, but evolutionary relationships among the basal streptophytes remain ambiguous. To date, numerous studies using SSU rRNA gene sequences have yielded differing phylogenies with varying degrees of support dependent upon taxon sampling and choice of phylogenetic method. Like SSU data, chloroplast DNA sequence data have been used to examine relationships within the Charales, Coleochaetales, Zygnemophyceae and embryophytes. Representatives of all basal streptophyte lineages have not been examined using chloroplast data in a single analysis. Phylogenetic analyses were performed using DNA sequences of rbc L (the genes encoding the large subunit of rubisco) and atp B (the beta-subunit of ATPase) to examine relationships of basal streptophyte lineages. Preliminary analyses placed the branch leading to Mesostigma as the basal lineage in the Streptophyta with Chlorokybus , the sole representative of the Chlorokybales, branching next. Klebsormidiales and the enigmatic genus Entransia were sister taxa. Sister to these, the Charales, Coleochaetales, embryophytes and Zygnemophyceae formed a monophyletic group with Charales and Coleochaetales sister to each other and this clade sister to the embryophytes.     

Molecular phylogeny of the Haplosporidia based on two independent gene sequences

The phylogenetic position of the Haplosporidia has confounded taxonomists for more than a century because of the unique morphology of these parasites. We collected DNA sequence data for small subunit (SSU) ribosomal RNA and actin genes from haplosporidians and other protists for conducting molecular phylogenetic analyses to help elucidate relationships of taxa within the group, as well as placement of this group among Eukaryota. Analyses were conducted using DNA sequence data from more than 100 eukaryotic taxa with various combinations of data sets including nucleotide sequence data for each gene separately and combined, as well as SSU ribosomal DNA data combined with translated actin amino acids. In almost all analyses, the Haplosporidia was sister to the Cercozoa with moderate bootstrap and jackknife support. Analysis with actin amino acid sequences alone grouped haplosporidians with the foraminiferans and cercozoans. The haplosporidians Minchinia and Urosporidium were found to be monophyletic, whereas Haplosporidium was paraphyletic. "Microcell" parasites, Bonamia spp. and Mikrocytos roughleyi, were sister to Minchinia, the most derived genus, with Haplosporidium falling between the "microcells" and the more basal Urosporidium. Two recently discovered parasites, one from abalone in New Zealand and another from spot prawns in British Columbia, fell at the base of the Haplosporidia with very strong support, indicating a taxonomic affinity to this group.     

The 18S ribosomal RNA sequence of the sea anemone Anemonia sulcata and its evolutionary position among other eukaryotes

Evolutionary trees based on partial small ribosomal subunit RNA sequences of 22 metazoa species have been published [(1988) Science 239, 748-753]. In these trees, cnidarians (Radiata) seemed to have evolved independently from the Bilateria, which is in contradiction with the general evolutionary view. In order to further investigate this problem, the complete srRNA sequence of the sea anemone Anemonia sulcata was determined and evolutionary trees were constructed using a matrix optimization method. In the tree thus obtained the sea anemone and Bilateria together form a monophyletic cluster, with the sea anemone forming the first line of the metazoan group.     

MOLECULAR SYSTEMATIC ANALYSES INDICATE THAT THE ENIGMATIC APOPHLAEA IS A MEMBER OF THE HILDENBRANDIALES (RHODOPHYTA, FLORIDEOPHYCIDAE)

Nuclear small-subunit (SSU) ribosomal DNA sequences were determined for Apophlaea lyallii Hooker f. et Harvey from New Zealand and an isolate of Hildenbrandia rubra (Sommerfelt) Meneghini from France. These data, in addition to SSU sequences representative of all the major florideophyte lineages, were used in molecular systematic investigations to confirm inclusion of Apophlaea in the Hildenbrandiales. Anatomical features that serve to unite Apophlaea and Hildenbrandia are discussed to buttress this result. Furthermore, our analyses support recognition of the Hildenbrandiales as sister to all other extant florideophyte lineages studied to date. An anomalous result positioning the SSU sequence generated herein for Hildenbrandia rubra closer to Apophlaea lyallii than to a previously published sequence for Hildenbrandia rubra requires further investigation.     

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.     

Validation of a universal set of primers to study animal-associated microeukaryotic communities

The application of metabarcoding to study animal-associated microeukaryotes has been restricted because the universal barcode used to study microeukaryotic ecology and distribution in the environment, the Small Subunit of the Ribosomal RNA gene (18S rRNA), is also present in the host. As a result, when host-associated microbial eukaryotes are analysed by metabarcoding, the reads tend to be dominated by host sequences. We have done an in silico validation against the SILVA 18S rRNA database of a non-metazoan primer set (primers that are biased against the metazoan 18S rRNA) that recovers only 2.6% of all the metazoan sequences, while recovering most of the other eukaryotes (80.4%). Among metazoans, the non-metazoan primers are predicted to amplify 74% of Porifera sequences, 4% of Ctenophora, and 15% of Cnidaria, while amplifying almost no sequences within Bilateria. In vivo, these non-metazoan primers reduce significantly the animal signal from coral and human samples, and when compared against universal primers provide at worst a 2-fold decrease in the number of metazoan reads and at best a 2800-fold decrease. This easy, inexpensive, and near-universal method for the study of animal-associated microeukaryotes diversity will contribute to a better understanding of the microbiome.     

PHYLOGENY OF THE BASAL LINEAGES OF STREPTOPHYTA BASED ON RBCL AND ATPB GENE SEQUENCE DATA

The streptophytes comprise the Charophyceae sensu Mattox and Stewart (a morphologically diverse group of fresh‐water green algae) and the embryophytes (land plants). Several charophycean groups are currently recognized. These include the Charales, Coleochaetales, Chlorokybales, Klebsormidiales and Zygnemophyceae (Desmidiales and Zygnematales). Recently, SSU rRNA gene sequence data allied Mesostigma viride (Prasinophyceae) with the Streptophyta. Complete chloroplast sequence data, however, placed Mesostigma sister to all green algae, not with the streptophytes. Several morphological, ultrastructural and biochemical features unite these lineages into a monophyletic group including embryophytes, but evolutionary relationships among the basal streptophytes remain ambiguous. To date, numerous studies using SSU rRNA gene sequences have yielded differing phylogenies with varying degrees of support dependent upon taxon sampling and choice of phylogenetic method. Like SSU data, chloroplast DNA sequence data have been used to examine relationships within the Charales, Coleochaetales, Zygnemophyceae and embryophytes. Representatives of all basal streptophyte lineages have not been examined using chloroplast data in a single analysis. Phylogenetic analyses were performed using DNA sequences of rbcL (the genes encoding the large subunit of rubisco) and atpB (the beta‐subunit of ATPase) to examine relationships of basal streptophyte lineages. Preliminary analyses placed the branch leading to Mesostigma as the basal lineage in the Streptophyta with Chlorokybus, the sole representative of the Chlorokybales, branching next. Klebsormidiales and the enigmatic genus Entransia were sister taxa. Sister to these, the Charales, Coleochaetales, embryophytes and Zygnemophyceae formed a monophyletic group with Charales and Coleochaetales sister to each other and this clade sister to the embryophytes.     

Phylogenomic analyses of lophophorates (brachiopods, phoronids and bryozoans) confirm the Lophotrochozoa concept

Based on embryological and morphological evidence, Lophophorata was long considered to be the sister or paraphyletic stem group of Deuterostomia. By contrast, molecular data have consistently indicated that the three lophophorate lineages, Ectoprocta, Brachiopoda and Phoronida, are more closely related to trochozoans (annelids, molluscs and related groups) than to deuterostomes. For this reason, the lophophorate groups and Trochozoa were united to Lophotrochozoa. However, the relationships of the lophophorate lineages within Lophotrochozoa are still largely unresolved. Maximum-likelihood and Bayesian analyses were performed based on a dataset comprising 11,445 amino acid positions derived from 79 ribosomal proteins of 39 metazoan taxa including new sequences obtained from a brachiopod and a phoronid. These analyses show that the three lophophorate lineages are affiliated with trochozoan rather than deuterostome phyla. All hypotheses claiming that they are more closely related to Deuterostomia than to Protostomia can be rejected by topology testing. Monophyly of lophophorates was not recovered but that of Bryozoa including Ectoprocta and Entoprocta and monophyly of Brachiozoa including Brachiopoda and Phoronida were strongly supported. Alternative hypotheses that are refuted include (i) Brachiozoa as the sister group of Mollusca, (ii) ectoprocts as sister to all other Lophotrochozoa including Platyzoa, and (iii) ectoprocts as sister or to all other protostomes except chaetognaths.     

Phylogenetic context and Basal metazoan model systems

In comparative studies using model organisms, extant taxa areoften referred to as basal. The term suggests that such taxaare descendants of lineages that diverged early in the historyof some larger taxon. By this usage, the basal metazoans comprisejust four phyla (Placozoa, Porifera, Cnidaria, and Ctenophora)and the large clade Bilateria. We advise against this practicebecause basal refers to a region at the base or root of a phylogenetictree. Thus, referring to an extant taxon or species as basal,or as more basal than another, can be misleading. While muchprogress has been made toward understanding some of the phylogeneticrelationships within these groups, the relationships among themare still largely not known with certainty. Thus, sound inferencesfrom comparative studies of model organisms demand continuedillumination of phylogeny. Hypotheses about the mechanisms underlyingmetazoan evolution can be drawn from the study of model organismsin Cnidaria, Ctenophora, Placozoa, and Porifera, but it is clearthat these model organisms are likely to be derived in manyrespects. Therefore, testing these hypotheses requires the studyof yet additional model organisms. The most effective testsare those that investigate model organisms with phylogeneticpositions among two sister groups comprising a larger cladeof interest.     

Assessing the effects of a sequestered germline on interdomain lateral gene transfer in Metazoa

A sequestered germline in Metazoa has been argued to be an obstacle to lateral gene transfer (LGT), though few studies have specifically assessed this claim. Here, we test the hypothesis that the origin of a sequestered germline reduced LGT events in Bilateria (i.e., triploblast lineages) as compared to early‐diverging Metazoa (i.e., Ctenophora, Cnidaria, Porifera, and Placozoa). We analyze single‐gene phylogenies generated with over 900 species sampled from among Bacteria, Archaea, and Eukaryota to identify well‐supported interdomain LGTs. We focus on ancient interdomain LGT (i.e., those between prokaryotes and multiple lineages of Metazoa) as systematic errors in single‐gene tree reconstruction create uncertainties for interpreting eukaryote‐to‐eukaryote transfer. The breadth of the sampled Metazoa enables us to estimate the timing of LGTs, and to examine the pattern before versus after the evolution of a sequestered germline. We identified 58 LGTs found only in Metazoa and prokaryotes (i.e., bacteria and/or archaea), and seven genes transferred from prokaryotes into Metazoa plus one other eukaryotic clade. Our analyses indicate that more interdomain transfers occurred before the development of a sequestered germline, consistent with the hypothesis that this feature is an obstacle to LGT.     

Old trees, new trees--is there any progress?

The amount of comparative data for phylogenetic analyses is constantly increasing. Data come from different directions such as morphology, molecular genetics, developmental biology and paleontology. With the increasing diversity of data and of analytical tools, the number of competing hypotheses on phylogenetic relationships rises, too. The choice of the phylogenetic tree as a basis for the interpretation of new data is important, because different trees will support different evolutionary interpretations of the data investigated. I argue here that, although many problematic aspects exist, there are several phylogenetic relationships that are supported by the majority of analyses and may be regarded as something like a robust backbone. This accounts, for example, for the monophyly of Metazoa, Bilateria, Deuterostomia, Protostomia (= Gastroneuralia), Gnathifera, Spiralia, Trochozoa and Arthropoda and probably also for the branching order of diploblastic taxa (“Porifera”, Trichoplax adhaerens, Cnidaria and Ctenophora). Along this “backbone”, there are several problematic regions, where either monophyly is questionable and/or where taxa “rotate” in narrow regions of the tree. This is illustrated exemplified by the probable paraphyly of Porifera and the phylogenetic relationships of basal spiralian taxa. Two problems span wider regions of the tree: the position of Arthropoda either as the sister taxon of Annelida (= Articulata) or of Cycloneuralia (= Ecdysozoa) and the position of tentaculate taxa either as sister taxa of Deuterostomia (= Radialia) or within the taxon Spiralia. The backbone makes it possible to develop a basic understanding of the evolution of genes, molecules and structures in metazoan animals.     

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.     

Are the Platyhelminthes a monophyletic primitive group? An assessment using 18S rDNA sequences

In most zoological textbooks, Platyhelminthes are depicted as an early- emerging clade forming the likely sister group of all the other Bilateria. Other phylogenetic proposals see them either as the sister group of most of the Protostomia or as a group derived from protostome coelomate ancestors by progenesis. The main difficulty in their correct phylogenetic placing is the lack of convincing synapomorphies for all Platyhelminthes, which may indicate that they are polyphyletic. Moreover, their internal phylogenetic relationships are still uncertain. To test these hypotheses, new complete 18S rDNA sequences from 13 species of "Turbellaria" have been obtained and compared to published sequences of 2 other "Turbellaria," 3 species of parasitic Platyhelminthes, and several diploblastic and deuterostome and protostome triploblastics. Maximum-parsimony, maximum-likelihood, and neighbor-joining methods were used to infer their phylogeny. The results show the order Catenulida to form an independent early- branching clade and emerge as a potential sister group of the rest of the Bilateria, while the rest of Platyhelminthes (Rhabditophora), which includes the parasites, form a clear monophyletic group closely related to the protostomes. The order Acoela, morphologically considered as candidates to be ancestral, are shown to be fast-clock organisms for the 18S rDNA gene. Hence, long-branching of acoels and insufficient sampling of catenulids and acoels leave their position still unresolved and call for further studies. Within the Rhabditophora, our analyses suggest (1) a close relationship between orders Macrostomida and Polycladida, forming a clear sister group to the rest of orders; (2) that parasitic platyhelminthes appeared early in the evolution of the group and form a sister group to a still-unresolved clade made by Nemertodermatida, Lecithoepitheliata, Prolecithophora, Proseriata, Tricladida, and Rhabdocoela; and (3) that Seriata is paraphyletic.      

Phylogeny of Rhigonematomorpha based on the complete mitochondrial genome of Rhigonema thysanophora (Nematoda: Chromadorea)

We determined the complete mitochondrial genome sequence of Rhigonema thysanophora, the first representative of Rhigonematomorpha, and used this sequence along with 57 other nematode species for phylogenetic analyses. The R. thysanophora mtDNA is 15 015 bp and identical to all other chromadorean nematode mtDNAs published to date in that it contains 36 genes (lacking atp8) encoded in the same direction. Phylogenetic analyses of nucleotide and amino acid sequence data for the 12 protein‐coding genes recovered Rhigonematomorpha as the sister group to the heterakoid species, Ascaridia columbae (Ascaridomorpha). The organization of R. thysanophora mtDNA resembles the most common pattern for the Rhabditomorpha+Ascaridomorpha+Diplogasteromorpha clade in gene order, but with some substantial gene rearrangements. This similarity in gene order is in agreement with the sequence‐based analyses that indicate a close relationship between Rhigonematomorpha and Rhabditomorpha+Ascaridomorpha+Diplogasteromorpha. These results are consistent with certain analyses of nuclear SSU rDNA for R. thysanophora and some earlier classification systems that asserted phylogenetic affinity between Rhigonematomorpha and Ascaridomorpha, but inconsistent with morphology‐based phylogenetic hypotheses that suggested a close (taxonomic) relationship between rhigonematomorphs and oxyuridomorphs (pinworms). These observations must be tempered by noting that few rhigonematomorph species have been sequenced and included in phylogenetic analyses, and preliminary studies based on SSU rDNA suggest the group is not monophyletic. Additional mitochondrial genome sequences of rhigonematids are needed to characterize their phylogenetic relationships within Chromadorea, and to increase understanding of mitochondrial genome evolution.     

A phylogenetic reconsideration of suctorian ciliates (Protista,Ciliophora, Phyllopharyngea) based on small subunit rRNA gene sequences

Compared with other ciliated protozoa, molecular studies of phylogenetic relationships within the subclass Suctoria are rare. In this work, phylogenetic analyses focusing on this group were performed based on all data available. In addition, the small subunit ribosomal RNA (SSU rRNA) genes of three suctorian ciliates (Acineta compressa, Acineta tuberosa and Paracineta limbata) were newly sequenced. Furthermore, the putative secondary structures of the variable region 2 of the SSU rRNA gene were predicted and compared within the Suctoria. Our results show that (i) there is support for the monophyly of the subclass Suctoria, which is a sister clade to the cyrtophorids; (ii) based on combined morphologic and molecular features, we propose the following evolutionary routine within the Suctoria: Exogenina – Evaginogenina – Endogenina; (iii) the similarities of the secondary structures of the V2 region and the SSU rRNA gene sequences within the subclass Suctoria are consistent with the branching of the phylogenetic lineages.     

Early evolution of the bilateria

The phylogeny of the Bilateria and especially the early steps in the evolution of the bilaterian bauplan are still a controversial topic. In this context the relationships of the platyhelminths and the nematodes play a crucial role. Previous molecular studies of the relationships of these groups, which were based on 18S ribosomal DNA sequences, yielded conflicting results. In the present study a new framework is developed for the phylogenetic analysis of bilaterian relationships, using concatenated amino acid sequences of several nuclear genes. In this analysis, the rhabditophoran platyhelminths are probably the sister group of all other analyzed Bilateria, the Eubilateria, which are characterized by a one-way intestine with an anus. The Eubilateria are split into the nematode lineage and the coelomates. The phylogenetic results of the present study indicate that genetic features found in the model organisms Caenorhabditis and Drosophila might be found in all Eubilateria. Estimations of the divergence times show that the major bilaterian phyla did not originate in an explosive radiation during the Cambrian but rather that the Bilateria have a several hundred million years long Precambrian history.     

Phylogenetic analysis of the Metastrongyloidea (Nematoda: Strongylida) inferred from ribosomal RNA gene sequences

Phylogenetic relationships among nematodes of the strongylid superfamily Metastrongyloidea were analyzed using partial sequences from the large-subunit ribosomal RNA (LSU rRNA) and small-subunit ribosomal RNA (SSU rRNA) genes. Regions of nuclear ribosomal DNA (rDNA) were amplified by polymerase chain reaction, directly sequenced, aligned, and phylogenies inferred using maximum parsimony. Phylogenetic hypotheses inferred from the SSU rRNA gene supported the monophyly of representative taxa from each of the 7 currently accepted metastrongyloid families. Metastrongyloid taxa formed the sister group to representative trichostrongyloid sequences based on SSU data. Sequences from either the SSU or LSU RNA regions alone provided poor resolution for relationships within the Metastrongyloidea. However, a combined analysis using sequences from all rDNA regions yielded 3 equally parsimonious trees that represented the abursate Filaroididae as polyphyletic, Parafilaroides decorus as the sister species to the monophyletic Pseudaliidae, and a sister group relationship between Oslerus osleri and Metastrongylus salmi. Relationships among 3 members of the Crenosomatidae, and 1 representative of the Skrjabingylidae (Skrjabingylus chitwoodorum) were not resolved by these combined data. However, members of both these groups were consistently resolved as the sister group to the other metastrongyloid families. These relationships are inconsistent with traditional classifications of the Metastrongyloidea and existing hypotheses for their evolution.