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Greg W. Rouse - One of the best experts on this subject based on the ideXlab platform.
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New deep-sea species of Xenoturbella and the position of Xenacoelomorpha
Nature, 2016Co-Authors: Greg W. Rouse, Nerida G. Wilson, Jose I. Carvajal, Robert C. VrijenhoekAbstract:The discovery of four new Xenoturbella species from deep waters of the eastern Pacific Ocean is reported here. The genus and two nominal species were described from the west coast of Sweden^ 1 , 2 , but their taxonomic placement remains unstable^ 3 , 4 . Limited evidence placed Xenoturbella with molluscs^ 5 , 6 , but the tissues can be contaminated with prey^ 7 , 8 . They were then considered deuterostomes^ 9 , 10 , 11 , 12 , 13 . Further taxon sampling and analysis have grouped Xenoturbella with acoelomorphs (=Xenacoelomorpha) as sister to all other Bilateria (=Nephrozoa)^ 14 , 15 , or placed Xenacoelomorpha inside Deuterostomia with Ambulacraria (Hemichordata + Echinodermata)^ 16 . Here we describe four new species of Xenoturbella and reassess those hypotheses. A large species (>20 cm long) was found at cold-water hydrocarbon seeps at 2,890 m depth in Monterey Canyon and at 1,722 m in the Gulf of California (Mexico). A second large species (~10 cm long) also occurred at 1,722 m in the Gulf of California. The third large species (~15 cm long) was found at ~3,700 m depth near a newly discovered carbonate-hosted hydrothermal vent in the Gulf of California. Finally, a small species (~2.5 cm long), found near a whale carcass at 631 m depth in Monterey Submarine Canyon (California), resembles the two nominal species from Sweden. Analysis of whole mitochondrial genomes places the three larger species as a sister clade to the smaller Atlantic and Pacific species. Phylogenomic analyses of transcriptomic sequences support placement of Xenacoelomorpha as sister to Nephrozoa or Protostomia. Description of four new species of Xenoturbella and phylogenomic analyses, aligning Xenacoelomorpha as sister group to the rest of Bilateria, or as sister to Protostomia. Lacking a centralized nervous system, coelom, anus and reproductive organs, the deep-sea flatworm Xenoturbella presents problems when it comes to its classification and teasing out its evolutionary history. Despite its simplicity, some of Xenoturbella 's features appear to align it among the deuterostomes, the group of animals that includes ourselves. If true, this implies either a radical simplification of the body plan or the acquisition of many key deuterostome features independently by the various deuterostome groups. Two papers in this issue tackle different aspects of Xenoturbella , but together, move the field on a notch. Greg Rouse et al . add four new deep-sea species of Xenoturbella from the eastern Pacific Ocean to the two already known from the Atlantic. Phylogenomic analysis aligns them at the base of the Protostomia or even as basal bilaterians — much as would be expected from their simple morphology and not invoking radical simplification. Andreas Hejnol and colleagues come to a broadly similar conclusion based on robust phylogenetic analysis using eleven transcriptomes of Xeonturbella and acoel worms.
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New deep-sea species of Xenoturbella and the position of Xenacoelomorpha
Nature, 2016Co-Authors: Greg W. Rouse, Nerida G. Wilson, Jose I. Carvajal, Robert C. VrijenhoekAbstract:The discovery of four new Xenoturbella species from deep waters of the eastern Pacific Ocean is reported here. The genus and two nominal species were described from the west coast of Sweden, but their taxonomic placement remains unstable. Limited evidence placed Xenoturbella with molluscs, but the tissues can be contaminated with prey. They were then considered deuterostomes. Further taxon sampling and analysis have grouped Xenoturbella with acoelomorphs (=Xenacoelomorpha) as sister to all other Bilateria (=Nephrozoa), or placed Xenacoelomorpha inside Deuterostomia with Ambulacraria (Hemichordata + Echinodermata). Here we describe four new species of Xenoturbella and reassess those hypotheses. A large species (>20 cm long) was found at cold-water hydrocarbon seeps at 2,890 m depth in Monterey Canyon and at 1,722 m in the Gulf of California (Mexico). A second large species (~10 cm long) also occurred at 1,722 m in the Gulf of California. The third large species (~15 cm long) was found at ~3,700 m depth near a newly discovered carbonate-hosted hydrothermal vent in the Gulf of California. Finally, a small species (~2.5 cm long), found near a whale carcass at 631 m depth in Monterey Submarine Canyon (California), resembles the two nominal species from Sweden. Analysis of whole mitochondrial genomes places the three larger species as a sister clade to the smaller Atlantic and Pacific species. Phylogenomic analyses of transcriptomic sequences support placement of Xenacoelomorpha as sister to Nephrozoa or Protostomia.
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Higher-level metazoan relationships: recent progress and remaining questions
Organisms Diversity & Evolution, 2011Co-Authors: Gregory D. Edgecombe, Reinhardt M. Kristensen, Ricardo C. Neves, Greg W. Rouse, Katrine Worsaae, Andreas Hejnol, Casey W. Dunn, Gonzalo Giribet, Martin V. SørensenAbstract:Metazoa comprises 35–40 phyla that include some 1.3 million described species. Phylogenetic analyses of metazoan interrelationships have progressed in the past two decades from those based on morphology and/or targeted-gene approaches using single and then multiple loci to the more recent phylogenomic approaches that use hundreds or thousands of genes from genome and transcriptome sequencing projects. A stable core of the tree for bilaterian animals is now at hand, and instability and conflict are becoming restricted to a key set of important but contentious relationships. Acoelomorph flatworms (Acoela + Nemertodermatida) and Xenoturbella are sister groups. The position of this clade remains controversial, with different analyses supporting either a sister-group relation to other bilaterians (=Nephrozoa, composed of Protostomia and Deuterostomia) or membership in Deuterostomia. The main clades of deuterostomes (Ambulacraria and Chordata) and protostomes (Ecdysozoa and Spiralia) are recovered in numerous analyses based on varied molecular samples, and also receive anatomical and developmental support. Outstanding issues in protostome phylogenetics are the position of Chaetognatha within the protostome clade, and the monophyly of a group of spiralians collectively named Platyzoa. In contrast to the broad consensus over key questions in bilaterian phylogeny, the relationships of the five main metazoan lineages—Porifera, Ctenophora, Placozoa, Cnidaria and Bilateria—remain subject to conflicting topologies according to different taxonomic samples and analytical approaches. Whether deep bilaterian divergences such as the split between protostome and deuterostome clades date to the Cryogenian or Ediacaran (and, thus, the extent to which the pre-Cambrian fossil record is incomplete) is sensitive to dating methodology.
Hiroaki Nakano - One of the best experts on this subject based on the ideXlab platform.
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Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha
Nature Communications, 2013Co-Authors: Hiroaki Nakano, Maximilian J. Telford, Kennet Lundin, Sarah J. Bourlat, Peter Funch, Jens R. Nyengaard, Matthias Obst, Michael C. ThorndykeAbstract:Xenoturbella bocki, a marine animal with a simple body plan, has recently been suggested to be sister group to the Acoelomorpha, together forming the new phylum Xenacoelomorpha. The phylogenetic position of the phylum is still under debate, either as an early branching bilaterian or as a sister group to the Ambulacraria (hemichordates and echinoderms) within the deuterostomes. Although development has been described for several species of Acoelomorpha, little is known about the life cycle of Xenoturbella. Here we report the embryonic stages of Xenoturbella, and show that it is a direct developer without a feeding larval stage. This mode of development is similar to that of the acoelomorphs, supporting the newly proposed phylum Xenacoelomorpha and suggesting that the last common ancestor of the phylum might have been a direct developer.
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Acoelomorph flatworms are deuterostomes related to Xenoturbella
Nature, 2011Co-Authors: Hervé Philippe, Hiroaki Nakano, Kevin J. Peterson, Albert J. Poustka, Henner Brinkmann, Richard R. Copley, Leonid L. Moroz, Andreas Wallberg, Maximilian J. TelfordAbstract:Xenoturbellida and Acoelomorpha are marine worms with contentious ancestry. Both were originally associated with the flatworms (Platyhelminthes), but molecular data have revised their phylogenetic positions, generally linking Xenoturbellida to the deuterostomes^ 1 , 2 and positioning the Acoelomorpha as the most basally branching bilaterian group(s)^ 3 , 4 , 5 , 6 . Recent phylogenomic data suggested that Xenoturbellida and Acoelomorpha are sister taxa and together constitute an early branch of Bilateria^ 7 . Here we assemble three independent data sets—mitochondrial genes, a phylogenomic data set of 38,330 amino-acid positions and new microRNA (miRNA) complements—and show that the position of Acoelomorpha is strongly affected by a long-branch attraction (LBA) artefact. When we minimize LBA we find consistent support for a position of both acoelomorphs and Xenoturbella within the deuterostomes. The most likely phylogeny links Xenoturbella and Acoelomorpha in a clade we call Xenacoelomorpha. The Xenacoelomorpha is the sister group of the Ambulacraria (hemichordates and echinoderms). We show that analyses of miRNA complements^ 8 have been affected by character loss in the acoels and that both groups possess one miRNA and the gene Rsb66 otherwise specific to deuterostomes. In addition, Xenoturbella shares one miRNA with the Ambulacrarians, and two with the acoels. This phylogeny makes sense of the shared characteristics of Xenoturbellida and Acoelomorpha, such as ciliary ultrastructure and diffuse nervous system, and implies the loss of various deuterostome characters in the Xenacoelomorpha including coelomic cavities, through gut and gill slits. The acoel flatworms are among the simplest animal forms, so simple that they have neither a through-gut nor a body cavity. But new molecular research has pulled them from their basal position in animal evolution, uniting them with creatures such as echinoderms (starfish, sea urchins and the like) and placing them much closer to the chordates, the group that includes humans. This follows previous revelations that Xenoturbella , a simple flatworm with mysterious evolutionary connections, also belonged to this group. The research implies that acoels are not primitively simple, as had been thought, but have become simpler with time, losing features such as a body cavity, anus and gill slits. New molecular research has pulled acoel flatworms from their basal position in animal evolution, uniting them with creatures such as echinoderms (starfish, sea urchins and allies) — indeed, very much closer to the chordates, the group that includes ourselves. The work follows previous revelations that Xenoturbella , a simple flatworm of mysterious evolutionary connections, also belonged to this group. The research implies that acoels are not primitively simple, as had been thought, but have lost features such as a body cavity, anus and gill slits.
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Acoelomorph flatworms are deuterostomes related to Xenoturbella
Nature, 2011Co-Authors: Hervé Philippe, Hiroaki Nakano, Kevin J. Peterson, Albert J. Poustka, Henner Brinkmann, Richard R. Copley, Leonid L. Moroz, Andreas Wallberg, Maximilian J. TelfordAbstract:Xenoturbellida and Acoelomorpha are marine worms with contentious ancestry. Both were originally associated with the flatworms (Platyhelminthes), but molecular data have revised their phylogenetic positions, generally linking Xenoturbellida to the deuterostomes and positioning the Acoelomorpha as the most basally branching bilaterian group(s). Recent phylogenomic data suggested that Xenoturbellida and Acoelomorpha are sister taxa and together constitute an early branch of Bilateria. Here we assemble three independent data sets-mitochondrial genes, a phylogenomic data set of 38,330 amino-acid positions and new microRNA (miRNA) complements-and show that the position of Acoelomorpha is strongly affected by a long-branch attraction (LBA) artefact. When we minimize LBA we find consistent support for a position of both acoelomorphs and Xenoturbella within the deuterostomes. The most likely phylogeny links Xenoturbella and Acoelomorpha in a clade we call Xenacoelomorpha. The Xenacoelomorpha is the sister group of the Ambulacraria (hemichordates and echinoderms). We show that analyses of miRNA complements have been affected by character loss in the acoels and that both groups possess one miRNA and the gene Rsb66 otherwise specific to deuterostomes. In addition, Xenoturbella shares one miRNA with the Ambulacrarians, and two with the acoels. This phylogeny makes sense of the shared characteristics of Xenoturbellida and Acoelomorpha, such as ciliary ultrastructure and diffuse nervous system, and implies the loss of various deuterostome characters in the Xenacoelomorpha including coelomic cavities, through gut and gill slits.
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Nervous system development of two crinoid species, the sea lily Metacrinus rotundus and the feather star Oxycomanthus japonicus
Development Genes and Evolution, 2009Co-Authors: Hiroaki Nakano, Yoko Nakajima, Shonan AmemiyaAbstract:Nervous system development in echinoderms has been well documented, especially for sea urchins and starfish. However, that of crinoids, the most basal group of extant echinoderms, has been poorly studied due to difficulties in obtaining their larvae. In this paper, we report nervous system development from two species of crinoids, from hatching to late doliolaria larvae in the sea lily Metacrinus rotundus and from hatching to cystidean stages after settlement in the feather star Oxycomanthus japonicus . The two species showed a similar larval nervous system pattern with an extensive anterior larval ganglion. The ganglion was similar to that in sea urchins which is generally regarded as derived. In contrast with other echinoderm and hemichordate larvae, synaptotagmin antibody 1E11 failed to reveal ciliary band nerve tracts. Basiepithelial nerve cells formed a net-like structure in the M. rotundus doliolaria larvae. In O. japonicus , the larval ganglion was still present 1 day after settlement when the adult nervous system began to appear inside the crown. Stalk nerves originated from the crown and extended down the stalk, but had no connections with the remaining larval ganglion at the base of the stalk. The larval nervous system was not incorporated into the adult nervous system, and the larval ganglion later disappeared. The aboral nerve center, the dominant nervous system in adult crinoids, was formed at the early cystidean stage, considerably earlier than previously suggested. Through comparisons with nervous system development in other Ambulacraria, we suggest the possible nervous system development pattern of the echinoderm ancestor and provide new implications on the evolutionary history of echinoderm life cycles.
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PCR survey of Xenoturbella bocki Hox genes.
Journal of experimental zoology. Part B Molecular and developmental evolution, 2008Co-Authors: Guido Fritzsch, Martin Schlegel, Thomas Stach, Michael C. Thorndyke, Hiroaki Nakano, Thomas Hankeln, Olle Israelsson, Manja U. Böhme, Peter F. StadlerAbstract:Xenoturbella bocki has recently been identified as one of the most basal deuterostomes, although an even more basal phylogenetic position cannot be ruled out. Here we report on a polymerase chain reaction survey of partial Hox homeobox sequences of X. bocki. Surprisingly, we did not find evidence for more than five Hox genes, one clear labial/PG1 ortholog, one posterior gene most similar to the PG9/10 genes of Ambulacraria, and three central group genes whose precise assignment to a specific paralog group remains open. We furthermore report on a re-evaluation of the available published evidence of Hox genes in other basal deuterostomes.
Marian Y Hu - One of the best experts on this subject based on the ideXlab platform.
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alkaline guts contribute to immunity during exposure to acidified seawater in the sea urchin larva
The Journal of Experimental Biology, 2020Co-Authors: Meike Stumpp, Inga Petersen, Femke Thoben, Matthias Leippe, Marian Y HuAbstract:ABSTRACT Larval stages of members of the Abulacraria superphylum including echinoderms and hemichordates have highly alkaline midguts. To date, the reason for the evolution of such extreme pH conditions in the gut of these organisms remains unknown. Here, we test the hypothesis that, analogous to the acidic stomachs of vertebrates, these alkaline conditions may represent a first defensive barrier to protect from environmental pathogens. pH-optimum curves for five different species of marine bacteria demonstrated a rapid decrease in proliferation rates by 50–60% between pH 8.5 and 9.5. Using the marine bacterium Vibrio diazotrophicus, which elicits a coordinated immune response in the larvae of the sea urchin Strongylocentrotus purpuratus, we studied the physiological responses of the midgut pH regulatory machinery to this pathogen. Gastroscopic microelectrode measurements demonstrate a stimulation of midgut alkalization upon infection with V. diazotrophicus accompanied by an upregulation of acid–base transporter transcripts of the midgut. Pharmacological inhibition of midgut alkalization resulted in an increased mortality rate of larvae during Vibrio infection. Reductions in seawater pH resembling ocean acidification conditions lead to moderate reductions in midgut alkalization. However, these reductions in midgut pH do not affect the immune response or resilience of sea urchin larvae to a Vibrio infection under ocean acidification conditions. Our study addressed the evolutionary benefits of the alkaline midgut of Ambulacraria larval stages. The data indicate that alkaline conditions in the gut may serve as a first defensive barrier against environmental pathogens and that this mechanism can compensate for changes in seawater pH.
Robert C. Vrijenhoek - One of the best experts on this subject based on the ideXlab platform.
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New deep-sea species of Xenoturbella and the position of Xenacoelomorpha
Nature, 2016Co-Authors: Greg W. Rouse, Nerida G. Wilson, Jose I. Carvajal, Robert C. VrijenhoekAbstract:The discovery of four new Xenoturbella species from deep waters of the eastern Pacific Ocean is reported here. The genus and two nominal species were described from the west coast of Sweden^ 1 , 2 , but their taxonomic placement remains unstable^ 3 , 4 . Limited evidence placed Xenoturbella with molluscs^ 5 , 6 , but the tissues can be contaminated with prey^ 7 , 8 . They were then considered deuterostomes^ 9 , 10 , 11 , 12 , 13 . Further taxon sampling and analysis have grouped Xenoturbella with acoelomorphs (=Xenacoelomorpha) as sister to all other Bilateria (=Nephrozoa)^ 14 , 15 , or placed Xenacoelomorpha inside Deuterostomia with Ambulacraria (Hemichordata + Echinodermata)^ 16 . Here we describe four new species of Xenoturbella and reassess those hypotheses. A large species (>20 cm long) was found at cold-water hydrocarbon seeps at 2,890 m depth in Monterey Canyon and at 1,722 m in the Gulf of California (Mexico). A second large species (~10 cm long) also occurred at 1,722 m in the Gulf of California. The third large species (~15 cm long) was found at ~3,700 m depth near a newly discovered carbonate-hosted hydrothermal vent in the Gulf of California. Finally, a small species (~2.5 cm long), found near a whale carcass at 631 m depth in Monterey Submarine Canyon (California), resembles the two nominal species from Sweden. Analysis of whole mitochondrial genomes places the three larger species as a sister clade to the smaller Atlantic and Pacific species. Phylogenomic analyses of transcriptomic sequences support placement of Xenacoelomorpha as sister to Nephrozoa or Protostomia. Description of four new species of Xenoturbella and phylogenomic analyses, aligning Xenacoelomorpha as sister group to the rest of Bilateria, or as sister to Protostomia. Lacking a centralized nervous system, coelom, anus and reproductive organs, the deep-sea flatworm Xenoturbella presents problems when it comes to its classification and teasing out its evolutionary history. Despite its simplicity, some of Xenoturbella 's features appear to align it among the deuterostomes, the group of animals that includes ourselves. If true, this implies either a radical simplification of the body plan or the acquisition of many key deuterostome features independently by the various deuterostome groups. Two papers in this issue tackle different aspects of Xenoturbella , but together, move the field on a notch. Greg Rouse et al . add four new deep-sea species of Xenoturbella from the eastern Pacific Ocean to the two already known from the Atlantic. Phylogenomic analysis aligns them at the base of the Protostomia or even as basal bilaterians — much as would be expected from their simple morphology and not invoking radical simplification. Andreas Hejnol and colleagues come to a broadly similar conclusion based on robust phylogenetic analysis using eleven transcriptomes of Xeonturbella and acoel worms.
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New deep-sea species of Xenoturbella and the position of Xenacoelomorpha
Nature, 2016Co-Authors: Greg W. Rouse, Nerida G. Wilson, Jose I. Carvajal, Robert C. VrijenhoekAbstract:The discovery of four new Xenoturbella species from deep waters of the eastern Pacific Ocean is reported here. The genus and two nominal species were described from the west coast of Sweden, but their taxonomic placement remains unstable. Limited evidence placed Xenoturbella with molluscs, but the tissues can be contaminated with prey. They were then considered deuterostomes. Further taxon sampling and analysis have grouped Xenoturbella with acoelomorphs (=Xenacoelomorpha) as sister to all other Bilateria (=Nephrozoa), or placed Xenacoelomorpha inside Deuterostomia with Ambulacraria (Hemichordata + Echinodermata). Here we describe four new species of Xenoturbella and reassess those hypotheses. A large species (>20 cm long) was found at cold-water hydrocarbon seeps at 2,890 m depth in Monterey Canyon and at 1,722 m in the Gulf of California (Mexico). A second large species (~10 cm long) also occurred at 1,722 m in the Gulf of California. The third large species (~15 cm long) was found at ~3,700 m depth near a newly discovered carbonate-hosted hydrothermal vent in the Gulf of California. Finally, a small species (~2.5 cm long), found near a whale carcass at 631 m depth in Monterey Submarine Canyon (California), resembles the two nominal species from Sweden. Analysis of whole mitochondrial genomes places the three larger species as a sister clade to the smaller Atlantic and Pacific species. Phylogenomic analyses of transcriptomic sequences support placement of Xenacoelomorpha as sister to Nephrozoa or Protostomia.
Yoko Nakajima - One of the best experts on this subject based on the ideXlab platform.
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Nervous system development of two crinoid species, the sea lily Metacrinus rotundus and the feather star Oxycomanthus japonicus
Development Genes and Evolution, 2009Co-Authors: Hiroaki Nakano, Yoko Nakajima, Shonan AmemiyaAbstract:Nervous system development in echinoderms has been well documented, especially for sea urchins and starfish. However, that of crinoids, the most basal group of extant echinoderms, has been poorly studied due to difficulties in obtaining their larvae. In this paper, we report nervous system development from two species of crinoids, from hatching to late doliolaria larvae in the sea lily Metacrinus rotundus and from hatching to cystidean stages after settlement in the feather star Oxycomanthus japonicus . The two species showed a similar larval nervous system pattern with an extensive anterior larval ganglion. The ganglion was similar to that in sea urchins which is generally regarded as derived. In contrast with other echinoderm and hemichordate larvae, synaptotagmin antibody 1E11 failed to reveal ciliary band nerve tracts. Basiepithelial nerve cells formed a net-like structure in the M. rotundus doliolaria larvae. In O. japonicus , the larval ganglion was still present 1 day after settlement when the adult nervous system began to appear inside the crown. Stalk nerves originated from the crown and extended down the stalk, but had no connections with the remaining larval ganglion at the base of the stalk. The larval nervous system was not incorporated into the adult nervous system, and the larval ganglion later disappeared. The aboral nerve center, the dominant nervous system in adult crinoids, was formed at the early cystidean stage, considerably earlier than previously suggested. Through comparisons with nervous system development in other Ambulacraria, we suggest the possible nervous system development pattern of the echinoderm ancestor and provide new implications on the evolutionary history of echinoderm life cycles.
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Development of the nervous system in the brittle star Amphipholis kochii
Development Genes and Evolution, 2008Co-Authors: Taiji Hirokawa, Miéko Komatsu, Yoko NakajimaAbstract:There are several studies of neural development in various echinoderms, but few on ophiuroids, which develop indirectly via the production of pluteus larvae, as do echinoids. To determine the extent of similarity of neuroanatomy and neural development in the ophiuroids with other echinoderm larvae, we investigated the development of the nervous system in the brittle star Amphipholis kochii (Echinodermata: Ophiuroidea) by immunohistochemistry. Immunoreactive cells first appeared bilaterally in the animal pole at the late gastrula stage, and there was little migration of the neural precursors during A . kochii ontogeny, as is also the case in echinoids and holothuroids. On the other hand, neural specification in the presumptive ciliary band near the base of the arms does occur in ophiuroid larvae and is a feature they share with echinoids and ophiuroids. The ophiopluteus larval nervous system is similar to that of auricularia larvae on the whole, including the lack of a fine network of neurites in the epidermis and the presence of neural connections across the oral epidermis. Ophioplutei possess a pair of bilateral apical organs that differ from those of echinoid echinoplutei in terms of relative position. They also possess coiled cilia, which may possess a sensory function, but in the same location as the serotonergic apical ganglia. These coiled cilia are thought to be a derived structure in pluteus-like larvae. Our results suggest that the neural specification in the animal plate in ophiuroids, holothuroids, and echinoids is a plesiomorphic feature of the Ambulacraria, whereas neural specification at the base of the larval arms may be a more derived state restricted to pluteus-like larvae.
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Apical organs in echinoderm larvae: insights into larval evolution in the Ambulacraria.
Evolution & Development, 2007Co-Authors: Maria Byrne, Yoko Nakajima, Francis C. Chee, Robert D. BurkeAbstract:SUMMARY The anatomy and cellular organization of serotonergic neurons in the echinoderm apical organ exhibits class-specific features in dipleurula-type (auricularia, bipinnaria) and pluteus-type (ophiopluteus, echinopluteus) larvae. The apical organ forms in association with anterior ciliary structures. Apical organs in dipleurula-type larvae are more similar to each other than to those in either of the pluteus forms. In asteroid bipinnaria and holothuroid auricularia the apical organ spans ciliary band sectors that traverse the anterior-most end of the larvae. The asteroid apical organ also has prominent bilateral ganglia that connect with an apical network of neurites. The simple apical organ of the auricularia is similar to that in the hemichordate tornaria larva. Apical organs in pluteus forms differ markedly. The echinopluteus apical organ is a single structure on the oral hood between the larval arms comprised of two groups of cells joined by a commissure and its cell bodies do not reside in the ciliary band. Ophioplutei have a pair of lateral ganglia associated with the ciliary band of larval arms that may be the ophiuroid apical organ. Comparative anatomy of the serotonergic nervous systems in the dipleurula-type larvae of the Ambulacraria (Echinodermata+Hemichordata) suggests that the apical organ of this deuterostome clade originated as a simple bilaterally symmetric nerve plexus spanning ciliary band sectors at the anterior end of the larva. From this structure, the apical organ has been independently modified in association with the evolution of class-specific larval forms.
