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Philippe H, Brinkmann H, Martinez P, Riutort M, Baguñà J. Acoel flatworms are not platyhelminthes: evidence from phylogenomics. PLoS One 2007; 2:e717. [PMID: 17684563 PMCID: PMC1933604 DOI: 10.1371/journal.pone.0000717] [Citation(s) in RCA: 122] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2007] [Accepted: 07/04/2007] [Indexed: 11/24/2022] Open
Abstract
Acoel flatworms are small marine worms traditionally considered to belong to the phylum Platyhelminthes. However, molecular phylogenetic analyses suggest that acoels are not members of Platyhelminthes, but are rather extant members of the earliest diverging Bilateria. This result has been called into question, under suspicions of a long branch attraction (LBA) artefact. Here we re-examine this problem through a phylogenomic approach using 68 different protein-coding genes from the acoel Convoluta pulchra and 51 metazoan species belonging to 15 different phyla. We employ a mixture model, named CAT, previously found to overcome LBA artefacts where classical models fail. Our results unequivocally show that acoels are not part of the classically defined Platyhelminthes, making the latter polyphyletic. Moreover, they indicate a deuterostome affinity for acoels, potentially as a sister group to all deuterostomes, to Xenoturbellida, to Ambulacraria, or even to chordates. However, the weak support found for most deuterostome nodes, together with the very fast evolutionary rate of the acoel Convoluta pulchra, call for more data from slowly evolving acoels (or from its sister-group, the Nemertodermatida) to solve this challenging phylogenetic problem.
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Affiliation(s)
- Hervé Philippe
- Canadian Institute for Advanced Research, Centre Robert-Cedergren, Département de Biochimie, Université de Montréal, Montréal, Québec, Canada
- * To whom correspondence should be addressed. E-mail: (HP); (JB)
| | - Henner Brinkmann
- Canadian Institute for Advanced Research, Centre Robert-Cedergren, Département de Biochimie, Université de Montréal, Montréal, Québec, Canada
| | - Pedro Martinez
- Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Barcelona, Spain
| | - Marta Riutort
- Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Barcelona, Spain
| | - Jaume Baguñà
- Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Barcelona, Spain
- * To whom correspondence should be addressed. E-mail: (HP); (JB)
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Tekle YI, Raikova OI, Justine JL, Jondelius U. Ultrastructure and tubulin immunocytochemistry of the copulatory stylet-like structure in Childia species (Acoela). J Morphol 2007; 268:166-80. [PMID: 17236190 DOI: 10.1002/jmor.10502] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
One of the main characters used in acoel taxonomy is the male copulatory organ. Despite this, ultrastructural studies of this structure are scarce. We studied the ultrastructure of the copulatory organ in eight species of acoels belonging to the taxon Childia. Members of Childia possess a well-developed conical or cylindrical stylet-like structure composed of needles. Immunogold cytochemistry of tubulin was used to determine the composition of the needles. Stylet-like structures of Childia species at the ultrastructural level are basically similar. Stylet needles show intracellular differentiations. As shown both by ultrastructural and immunocytochemical methods, the stylet needles, in all species studied, are composed of long, parallel microtubules, either tightly packed or polymerized. We report unusual polymerization of microtubules, resulting in formation of a honeycomb-like structure in cross section. Variations of ultrastructure among Childia species include numbers and arrangement of stylet needles, shape of needles, needle compactness, microtubule polymerization, direction of stylet growth, and presence/absence of different types of granules. The stylet-like structures are homologous within Childia, but are likely to prove nonhomologous with the other needle-like structures found in acoel copulatory organs. Stylets in Platyhelminthes are not homologous with stylet-like structures in acoels.
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Affiliation(s)
- Yonas I Tekle
- Department of Systematic Zoology, Evolutionary Biology Centre, Uppsala University, 75236 Uppsala, Sweden.
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54
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Dunn EF, Moy VN, Angerer LM, Angerer RC, Morris RL, Peterson KJ. Molecular paleoecology: using gene regulatory analysis to address the origins of complex life cycles in the late Precambrian. Evol Dev 2007; 9:10-24. [PMID: 17227363 DOI: 10.1111/j.1525-142x.2006.00134.x] [Citation(s) in RCA: 78] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
Molecular paleoecology is the application of molecular data to test hypotheses made by paleoecological scenarios. Here, we use gene regulatory analysis to test between two competing paleoecological scenarios put forth to explain the evolution of complex life cycles. The first posits that early bilaterians were holobenthic, and the evolution of macrophagous grazing drove the exploitation of the pelagos by metazoan eggs and embryos, and eventually larvae. The alternative hypothesis predicts that early bilaterians were holopelagic, and new adult stages were added on when these holopelagic forms began to feed on the benthos. The former hypothesis predicts that the larvae of protostomes and deuterostomes are not homologous, with the implication that larval-specific structures, including the apical organ, are the products of convergent evolution, whereas the latter hypothesis predicts homology of larvae, specifically homology of the apical organ. We show that in the sea urchin, Strongylocentrotus purpuratus, the transcription factors NK2.1 and HNF6 are necessary for the correct spatial expression profiles of five different cilia genes. All of these genes are expressed exclusively in the apical plate after the mesenchyme-blastula stage in cells that also express NK2.1 and HNF6. In addition, abrogation of SpNK2.1 results in embryos that lack the apical tuft. However, in the red abalone, Haliotis rufescens, NK2.1 and HNF6 are not expressed in any cells that also express these same five cilia genes. Nonetheless, like the sea urchin, the gastropod expresses both NK2.1 and FoxA around the stomodeum and foregut, and FoxA around the proctodeum. As we detected no similarity in the development of the apical tuft between the sea urchin and the abalone, these molecular data are consistent with the hypothesis that the evolution of mobile, macrophagous metazoans drove the evolution of complex life cycles multiple times independently in the late Precambrian.
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Affiliation(s)
- Ewan F Dunn
- Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA
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55
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Evolution of a novel subfamily of nuclear receptors with members that each contain two DNA binding domains. BMC Evol Biol 2007; 7:27. [PMID: 17319953 PMCID: PMC1810520 DOI: 10.1186/1471-2148-7-27] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/22/2006] [Accepted: 02/23/2007] [Indexed: 11/11/2022] Open
Abstract
Background Nuclear receptors (NRs) are important transcriptional modulators in metazoans which regulate transcription through binding to the promoter region of their target gene by the DNA binding domain (DBD) and activation or repression of mRNA synthesis through co-regulators bound to the ligand binding domain (LBD). NRs typically have a single DBD with a LBD. Results Three nuclear receptors named 2DBD-NRs, were identified from the flatworm Schistosoma mansoni that each possess a novel set of two DBDs in tandem with a LBD. They represent a novel NR modular structure: A/B-DBD-DBD-hinge-LBD. The 2DBD-NRs form a new subfamily of NRs, VII. By database mining, 2DBD-NR genes from other flatworm species (Schmidtea mediterranea and Dugesia japonica), from Mollusks (Lottia gigantean) and from arthropods (Daphnia pulex) were also identified. All 2DBD-NRs possess a P-box sequence of CEACKK in the first DBD, which is unique to 2DBD-NRs, and a P-box sequence of CEGCKG in the second DBD. Phylogenetic analyses of both DBD and ligand binding domain sequences showed that 2DBD-NR genes originate from a common two DBD-containing ancestor gene. A single 2DBD-NR orthologue was found in Arthropoda, Platyhelminths and Mollusca. Subsequent 2DBD-NR gene evolution in Mollusks and Platyhelminths involved gene duplication. Chromosome localization of S. mansoni 2DBD-NR genes by Fluorescent in situ hybridization (FISH) suggests that 2DBD-NR genes duplicated on different chromosomes in the Platyhelminths. Dimerization of Sm2DBDα indicates that 2DBD-NRs may act as homodimers, suggesting either that two repeats of a half-site are necessary for each DBD of 2DBD-NRs to bind to its target gene, or that each 2DBD-NR can recognize multiple sites. Conclusion 2DBD-NRs share a common ancestor gene which possessed an extra DBD that likely resulted from a recombination event. After the split of the Arthropods, Mollusks and Platyhelminths, 2DBD-NR underwent a recent duplication in a common ancestor of Mollusks, while two rounds of duplication occurred in a common ancestor of the Platyhelminths. This demonstrates that certain NR gene underwent recent duplication in Prostostome lineages after the split of the Prostostomia and Deuterostomia.
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Affiliation(s)
- Yonas Isaak Tekle
- a Department of Systematic Zoology , Evolutionary Biology Centre, Uppsala University , Norbyvägen 18D, SE‐752 36, Uppsala, Sweden
- b Department of Systematic Zoology , Evolutionary Biology Centre, Uppsala University , Norbyvägen 18D, SE‐752 36, Uppsala, Sweden E-mail:
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Turbeville JM, Smith DM. The partial mitochondrial genome of the Cephalothrix rufifrons (Nemertea, Palaeonemertea): characterization and implications for the phylogenetic position of Nemertea. Mol Phylogenet Evol 2006; 43:1056-65. [PMID: 17210260 DOI: 10.1016/j.ympev.2006.10.025] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2006] [Revised: 10/08/2006] [Accepted: 10/23/2006] [Indexed: 11/22/2022]
Abstract
A continuous 10.1kb fragment of the Cephalothrix rufifrons (Nemertea, Palaeonemertea) mitochondrial genome was sequenced and characterized to further assess organization of protostome mitochondrial genomes and evaluate the phylogenetic potential of gene arrangement and amino acid characters. The genome is A-T rich (72%), and this biased base composition is partly reflected in codon usage. Inferred tRNA secondary structures are typical of those reported for other metazoan mitochondrial DNAs. The arrangement of the 26 genes contained in the fragment exhibits marked similarity to those of many protostome taxa, most notably molluscs with highly conserved arrangements and a phoronid. Separate and simultaneous phylogenetic analyses of inferred amino acid sequences and gene adjacencies place the nemertean within the protostomes among coelomate lophotrochozoan taxa, but do not find a well-supported sister taxon link.
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Affiliation(s)
- J M Turbeville
- Department of Biology and Center for the Study of Biological Complexity, Virginia Commonwealth University, Richmond, VA 23284, USA.
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58
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Park JK, Rho HS, Kristensen RM, Kim W, Giribet G. First Molecular Data on the Phylum Loricifera – An Investigation into the Phylogeny of Ecdysozoa with Emphasis on the Positions of Loricifera and Priapulida. Zoolog Sci 2006; 23:943-54. [PMID: 17189906 DOI: 10.2108/zsj.23.943] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Recent progress in molecular techniques has generated a wealth of information for phylogenetic analysis. Among metazoans all but a single phylum have been incorporated into some sort of molecular analysis. However, the minute and rare species of the phylum Loricifera have remained elusive to molecular systematists. Here we report the first molecular sequence data (nearly complete 18S rRNA) for a member of the phylum Loricifera, Pliciloricus sp. from Korea. The new sequence data were analyzed together with 52 other ecdysozoan sequences, with all other phyla represented by three or more sequences. The data set was analyzed using parsimony as an optimality criterion under direct optimization as well as using a Bayesian approach. The parsimony analysis was also accompanied by a sensitivity analysis. The results of both analyses are largely congruent, finding monophyly of each ecdysozoan phylum, except for Priapulida, in which the coelomate Meiopriapulus is separate from a clade of pseudocoelomate priapulids. The data also suggest a relationship of the pseudocoelomate priapulids to kinorhynchs, and a relationship of nematodes to tardigrades. The Bayesian analysis placed the arthropods as the sister group to a clade that includes tardigrades and nematodes. However, these results were shown to be parameter dependent in the sensitivity analysis. The position of Loricifera was extremely unstable to parameter variation, and support for a relationship of loriciferans to any particular ecdysozoan phylum was not found in the data.
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Affiliation(s)
- Joong-Ki Park
- Department of Parasitology, College of Medicine, Chungbuk National University, Cheongju, Chungbuk, Korea
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59
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Raikova OI, Tekle YI, Reuter M, Gustafsson MKS, Jondelius U. Copulatory organ musculature in Childia (Acoela) as revealed by phalloidin fluorescence and confocal microscopy. Tissue Cell 2006; 38:219-32. [PMID: 16901523 DOI: 10.1016/j.tice.2006.04.001] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2005] [Revised: 04/05/2006] [Accepted: 04/12/2006] [Indexed: 12/01/2022]
Abstract
Copulatory organs of eight species of the monophyletic taxon Childia were investigated in detail, using phalloidin fluorescence method and confocal microscopy. Childia species were shown to have one, two or several tubular stylets, conical to cylindrical in shape, composed of few to numerous needles. The musculature varied greatly, from the absence of seminal vesicle to extensively developed seminal vesicles with several additional types of specialized muscles. Ten copulatory organ characters were coded and mapped on the total evidence tree. The data obtained permitted to follow the evolution of the Childia stylet and to demonstrate that the structure of the stylet apparatus is largely consistent with the phylogeny of the group (CI=0.75). Possible function of different muscle specializations was discussed.
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Affiliation(s)
- O I Raikova
- Department of Systematic Zoology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, SE-75236 Uppsala, Sweden
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60
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Tekle YI, Raikova OI, Justine JL, Hendelberg J, Jondelius U. Ultrastructural and immunocytochemical investigation of acoel sperms with 9 + 1 axoneme structure: new sperm characters for unraveling phylogeny in Acoela. ZOOMORPHOLOGY 2006. [DOI: 10.1007/s00435-006-0023-z] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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61
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CASU M, CURINI-GALLETTI M. Genetic evidence for the existence of cryptic species in the mesopsammic flatworm Pseudomonocelis ophiocephala (Rhabditophora: Proseriata). Biol J Linn Soc Lond 2006. [DOI: 10.1111/j.1095-8312.2006.00588.x] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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62
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Sempere LF, Cole CN, McPeek MA, Peterson KJ. The phylogenetic distribution of metazoan microRNAs: insights into evolutionary complexity and constraint. JOURNAL OF EXPERIMENTAL ZOOLOGY PART B-MOLECULAR AND DEVELOPMENTAL EVOLUTION 2006; 306:575-88. [PMID: 16838302 DOI: 10.1002/jez.b.21118] [Citation(s) in RCA: 247] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
How complex body plans evolved in animals such as fruit flies and vertebrates, as compared to the relatively simple jellyfish and sponges, is not known, given the similarity of developmental genetic repertoires shared by all these taxa. Here, we show that a core set of 18 microRNAs (miRNAs), non-coding RNA molecules that negatively regulate the expression of protein-coding genes, are found only in protostomes and deuterostomes and not in sponges or cnidarians. Because many of these miRNAs are expressed in specific tissues and/or organs, miRNA-mediated regulation could have played a fundamental evolutionary role in the origins of organs such as brain and heart--structures not found in cnidarians or sponges--and thus contributed greatly to the evolution of complex body plans. Furthermore, the continuous acquisition and fixation of miRNAs in various animal groups strongly correlates both with the hierarchy of metazoan relationships and with the non-random origination of metazoan morphological innovations through geologic time.
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Affiliation(s)
- Lorenzo F Sempere
- Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755, USA
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63
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Willems WR, Wallberg A, Jondelius U, Littlewood DTJ, Backeljau T, Schockaert ER, Artois TJ. Filling a gap in the phylogeny of flatworms: relationships within the Rhabdocoela (Platyhelminthes), inferred from 18S ribosomal DNA sequences. ZOOL SCR 2006. [DOI: 10.1111/j.1463-6409.2005.00216.x] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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65
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Ladurner P, Scharer L, Salvenmoser W, Rieger RM. A new model organism among the lower Bilateria and the use of digital microscopy in taxonomy of meiobenthic Platyhelminthes: Macrostomum lignano, n. sp. (Rhabditophora, Macrostomorpha). J ZOOL SYST EVOL RES 2005. [DOI: 10.1111/j.1439-0469.2005.00299.x] [Citation(s) in RCA: 112] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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66
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Tekle YI, Raikova OI, Ahmadzadeh A, Jondelius U. Revision of the Childiidae (Acoela), a total evidence approach in reconstructing the phylogeny of acoels with reversed muscle layers. J ZOOL SYST EVOL RES 2005. [DOI: 10.1111/j.1439-0469.2004.00293.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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67
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Glenner H, Hansen AJ, Sørensen MV, Ronquist F, Huelsenbeck JP, Willerslev E. Bayesian Inference of the Metazoan Phylogeny. Curr Biol 2005. [DOI: 10.1016/j.cub.2005.02.024] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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68
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69
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Nielsen C. Trochophora larvae: cell-lineages, ciliary bands and body regions. 2. Other groups and general discussion. JOURNAL OF EXPERIMENTAL ZOOLOGY PART B-MOLECULAR AND DEVELOPMENTAL EVOLUTION 2005; 304:401-47. [PMID: 15915468 DOI: 10.1002/jez.b.21050] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The embryology of sipunculans, entoprocts, nemertines, platyhelminths (excluding acoelomorphs), rotifers, ectoprocts, phoronids, brachiopods, echinoderms and enteropneusts is reviewed with special emphasis on cell-lineage and differentiation of ectodermal structures. A group Spiralia comprising the four first-mentioned phyla plus annelids and molluscs seems well defined through the presence of spiral cleavage with early blastomere specification, prototroch with characteristic cell-lineage, cerebral ganglia developing from cells of the first micromere quartet (i.e., the episphere) and a ventral nervous system developing from the hyposphere. The planktotrophic trochophore was probably the larval type of the ancestor of this group. Another group comprising phoronids, brachiopods, echinoderms and enteropneusts appears equally well delimited. It is characterized by radial cleavage with late blastomere specification, possibly by the presence of a neotroch consisting of monociliate cells, by the absence of cerebral ganglia and of a well-defined brain and paired longitudinal nerve cords developing in connection with the blastopore, and by coelomic organization. Its ancestral larval type was probably a dipleurula. Several characters link rotifers with the spiralians, although they do not show the spiral pattern in the cleavage. Ectoprocts are still a problematic group, but some characters indicate spiralian affinities.
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Affiliation(s)
- Claus Nielsen
- Zoological Museum (University of Copenhagen), Universitetsparken 15, DK-2100 Copenhagen, Denmark.
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70
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Olson PD, Tkach VV. Advances and Trends in the Molecular Systematics of the Parasitic Platyhelminthes. ADVANCES IN PARASITOLOGY 2005; 60:165-243. [PMID: 16230104 DOI: 10.1016/s0065-308x(05)60003-6] [Citation(s) in RCA: 114] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
The application of molecular systematics to the parasitic Platyhelminthes (Cestoda, Digenea and Monogenea) over the last decade has advanced our understanding of their interrelationships and evolution substantially. Here we review the current state of play and the early works that led to the molecular-based hypotheses that now predominate in the field; advances in their systematics, taxonomy, classification and phylogeny, as well as trends in species circumscription, molecular targets and analytical methods are discussed for each of the three major parasitic groups. A by-product of this effort has been an ever increasing number of parasitic flatworms characterized genetically, and the useful application of these data to the diagnosis of animal and human pathogens, and to the elucidation of life histories are presented. The final section considers future directions in the field, including taxon sampling, molecular targets of choice, and the current and future utility of mitochondrial and nuclear genomics in systematic study.
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Affiliation(s)
- Peter D Olson
- Division of Parasitology, Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
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71
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72
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Wallberg A, Thollesson M, Farris JS, Jondelius U. The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling. Cladistics 2004; 20:558-578. [DOI: 10.1111/j.1096-0031.2004.00041.x] [Citation(s) in RCA: 119] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
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Ruiz-Trillo I, Riutort M, Fourcade HM, Baguñà J, Boore JL. Mitochondrial genome data support the basal position of Acoelomorpha and the polyphyly of the Platyhelminthes. Mol Phylogenet Evol 2004; 33:321-32. [PMID: 15336667 DOI: 10.1016/j.ympev.2004.06.002] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2003] [Revised: 03/02/2004] [Indexed: 10/26/2022]
Abstract
We determined 9.7, 5.2, and 6.8 kb, respectively, of the mitochondrial genomes of the acoel Paratomella rubra, the nemertodermatid Nemertoderma westbladi, and the free-living rhabditophoran platyhelminth Microstomum lineare. The identified gene arrangements are unique among metazoans, including each other, sharing no more than one or two single gene boundaries with a few distantly related taxa. Phylogenetic analysis of the amino acid sequences inferred from the sequenced genes confirms that the acoelomorph flatworms (acoels+nemertodermatids) do not belong to the Platyhelminthes, but are, instead, the most basal extant bilaterian group. Therefore, the Platyhelminthes, as traditionally constituted, is a polyphyletic phylum.
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Affiliation(s)
- Iñaki Ruiz-Trillo
- Departament de Genètica, Universitat de Barcelona, Av. Diagonal, 645, 08028 Barcelona, Spain
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74
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Abstract
The typical sperm is comprised of a head, midpiece and flagellum. Around this theme there is an enormous diversity of form--giant sperm, multi-flagellate sperm and also sperm that lack flagella entirely. Explaining this diversity in sperm morphology is a challenging question that evolutionary biologists have only recently engaged in. Nonetheless, one of the selective forces identified as being an important factor in the evolution of sperm form is sperm competition, which occurs when the sperm of two or more males compete to fertilize a female's ova. In species with a truly monandrous mating system, the absence of sperm competition means that the selection pressure on males to produce motile sperm may be relaxed. Potentially aflagellate sperm are less costly to produce, both in terms of energy and time. Thus, selection may therefore favour the loss of the sperm flagellum and any other motile mechanisms in monandrous taxa. A review of the literature revealed that 36 taxonomic groups, from red algae to fish, were found independently to have evolved aflagellate sperm. I review what is known about the mating systems of each of these taxa and their nearest sister taxa. A sister-group analysis using this information provided weak evidence suggesting that the evolution of aflagellate sperm could be linked to the removal of selective pressures generated by sperm competition.
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Affiliation(s)
- Edward H Morrow
- Department of Animal Ecology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden.
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75
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Jondelius U, Larsson K, Raikova O. Cleavage in Nemertoderma westbladi (Nemertodermatida) and its phylogenetic significance. ZOOMORPHOLOGY 2004. [DOI: 10.1007/s00435-004-0105-8] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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76
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Abstract
The origin of the bilaterian metazoans from radial ancestors is one of the biggest puzzles in animal evolution. A way to solve it is to identify the nature and main features of the last common ancestor of the bilaterians (LCB). Recent progress in molecular phylogeny has shown that many platyhelminth flatworms, regarded for a long time as basal bilaterians, now belong to the lophotrochozoan protostomates. In contrast, the LCB is now considered a complex organism bearing several features of modern bilaterians. Here we discuss an alternative view, in which acoelomorph (Acoela + Nemertodermatida) flatworms, which do not belong to the Platyhelminthes, represent the earliest extant bilaterian clade. Sequences from ribosomal and other nuclear genes, Hox cluster genes, and reinterpretation of some morphological features strongly support the basal position of acoelomorphs arguing against a complex LCB. This reconstruction backs the old planuloid-acoeloid hypothesis and may help our understanding of the evolution of body axes, Hox genes and the Cambrian explosion.
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Affiliation(s)
- Jaume Baguñà
- Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain.
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77
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Chen JY, Bottjer DJ, Oliveri P, Dornbos SQ, Gao F, Ruffins S, Chi H, Li CW, Davidson EH. Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian. Science 2004; 305:218-22. [PMID: 15178752 DOI: 10.1126/science.1099213] [Citation(s) in RCA: 80] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Ten phosphatized specimens of a small (<180 micrometers) animal displaying clear bilaterian features have been recovered from the Doushantuo Formation, China, dating from 40 to 55 million years before the Cambrian. Seen in sections, this animal (Vernanimalcula guizhouena gen. et sp. nov.) had paired coeloms extending the length of the gut; paired external pits that could be sense organs; bilateral, anterior-posterior organization; a ventrally directed anterior mouth with thick walled pharynx; and a triploblastic structure. The structural complexity is that of an adult rather than a larval form. These fossils provide the first evidence confirming the phylogenetic inference that Bilateria arose well before the Cambrian.
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Affiliation(s)
- Jun-Yuan Chen
- Nanjing Institute of Geology and Paleontology, Nanjing 210008, China.
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78
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79
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Peterson KJ. Isolation of Hox and Parahox genes in the hemichordate Ptychodera flava and the evolution of deuterostome Hox genes. Mol Phylogenet Evol 2004; 31:1208-15. [PMID: 15120410 DOI: 10.1016/j.ympev.2003.10.007] [Citation(s) in RCA: 94] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2003] [Revised: 09/25/2003] [Indexed: 11/19/2022]
Abstract
Because of their importance for proper development of the bilaterian embryo, Hox genes have taken center stage for investigations into the evolution of bilaterian metazoans. Taxonomic surveys of major protostome taxa have shown that Hox genes are also excellent phylogenetic markers, as specific Hox genes are restricted to one of the two great protostome clades, the Lophotrochozoa or the Ecdysozoa, and thus support the phylogenetic relationships as originally deduced by 18S rDNA studies. Deuterostomes are the third major group of bilaterians and consist of three major phyla, the echinoderms, the hemichordates, and the chordates. Most morphological studies have supported Hemichordata+Chordata, whereas molecular studies support Echinodermata+Hemichordata, a clade known as Ambulacraria. To test these competing hypotheses, complete or near complete cDNAs of eight Hox genes and four Parahox genes were isolated from the enteropneust hemichordate Ptychodera flava. Only one copy of each Hox gene was isolated suggesting that the Hox genes of P. flava are arranged in a single cluster. Of particular importance is the isolation of three posterior or Abd-B Hox genes; these genes are only shared with echinoderms, and thus support the monophyly of Ambulacraria.
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Affiliation(s)
- Kevin J Peterson
- Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA.
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80
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Abstract
Several molecular data sets suggest that acoelomorph flatworms are not members of the phylum Platyhelminthes but form a separate branch of the Metazoa that diverged from all other bilaterian animals before the separation of protostomes and deuterostomes. Here we examine the Hox gene complement of the acoel flatworms. In two distantly related acoel taxa, we identify only three distinct classes of Hox gene: an anterior gene, a posterior gene, and a central class gene most similar to genes of Hox classes 4 and 5 in other Bilateria. Phylogenetic analysis of these genes, together with the acoel caudal homologue, supports the basal position of the acoels. The similar gene sets found in two distantly related acoels suggest that this reduced gene complement may be ancestral in the acoels and that the acoels may have diverged from other bilaterians before elaboration of the 8- to 10-gene Hox cluster that characterizes most bilaterians.
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Affiliation(s)
- Charles E Cook
- University Museum of Zoology, Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK.
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81
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Raikova OI, Reuter M, Gustafsson MKS, Maule AG, Halton DW, Jondelius U. Basiepidermal nervous system in Nemertoderma westbladi (Nemertodermatida): GYIRFamide immunoreactivity. ZOOLOGY 2004; 107:75-86. [PMID: 16351929 DOI: 10.1016/j.zool.2003.12.002] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2003] [Accepted: 12/23/2003] [Indexed: 11/24/2022]
Abstract
The Nemertodermatida are a small group of microscopic marine worms. Recent molecular studies have demonstrated that they are likely to be the earliest extant bilaterian animals. What was the nervous system (NS) of a bilaterian ancestor like? In order to answer that question, the NS of Nemertoderma westbladi was investigated by means of indirect immunofluorescence technique and confocal scanning laser microscopy. The antibodies to a flatworm neuropeptide GYIRFamide were used in combination with anti-serotonin antibodies and phalloidin-TRITC staining. The immunostaining revealed an entirely basiepidermal NS. A ring lying outside the body wall musculature at the level of the statocyst forms the only centralisation, the "brain". No stomatogastric NS has been observed. The GYIRFamide immunoreactive part of the "brain" is formed of loosely packed nerve fibres with multiple small neurones and a few large ones. The peptidergic and aminergic patterns of the NS do not correspond to each other: the former is more developed on the ventral side, the latter is more pronounced on the dorsal side. A pair of GYIRFamide immunoreactive nerve cords innervates the ventral side of the animal, the mouth and the male genital opening. The nemertodermatids studied to-date display no common NS pattern. Possible synapomorphies of the Acoelomorpha are discussed. The study demonstrates that the nemertodermatid NS possesses a number of plesiomorphic features and appears more primitive than the NS in other worms, except the Xenoturbellida. The bilaterian ancestor supposedly possessed only a basiepidermal nerve net and had no centralised brain-like structures and no stomatogastric NS.
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Affiliation(s)
- Olga I Raikova
- Zoological Institute, Russian Academy of Sciences, St Petersburg, Russia
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82
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Giribet G, Sorensen MV, Funch P, Kristensen RM, Sterrer W. Investigations into the phylogenetic position of Micrognathozoa using four molecular loci. Cladistics 2004; 20:1-13. [DOI: 10.1111/j.1096-0031.2004.00004.x] [Citation(s) in RCA: 123] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022] Open
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83
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Tyler S, Hooge M. Comparative morphology of the body wall in flatworms (Platyhelminthes). CAN J ZOOL 2004. [DOI: 10.1139/z03-222] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The soft-bodied nature of the platyhelminths is due largely to the structure of the body wall and its lack of sclerotic elements such as cuticle. Free-living members, i.e., most turbellarians, show considerable variety, but the basic form of the body wall comprises a simple ciliated epithelium overlying a network of muscles. We illustrate this body wall structure in a representative typhloplanoid rhabditophoran and discuss variations in representatives of the Acoela, Catenulida, and other free-living rhabditophorans. The major parasitic groups of platyhelminths, the rhabditophoran Neodermata, follow a developmental pattern that replaces a similar ciliated epidermis in a larval stage with a specialized epidermis called a neodermis, which is assumed to be key to their success as parasites. This neodermis consists of a syncytium that covers the body in a continuous sheet connected to perikarya that lie below the body wall musculature. The neodermis can be seen as a special adaptation of a developmental mechanism common to all platyhelminths, in which epidermal growth and renewal are accomplished by replacement cells originating beneath the body wall. The cell type responsible for all cell renewal, including body wall renewal, in platyhelminths is the neoblast, and its presence may be the one autapomorphic character that unites all taxonomic groups of platyhelminths.
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84
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Abstract
The phylum Platyhelminthes has traditionally been considered the most basal bilaterian taxon. The main difficulty with this placement is the lack of convincing synapomorphies for all Platyhelminthes, which suggest that they are polyphyletic. Recent molecular findings based on 18S rDNA sequence data and number and type of Hox genes strongly suggest that the majority of Platyhelminthes are members of the lophotrochozoan protostomes, whereas the Acoelomorpha (Acoela + Nemertodermatida) fall outside of the Platyhelminthes as the most basal bilaterian taxon. Here we review phylum-wide analyses based on complete ribosomal and other nuclear genes addressed to answer the main issues facing systematics and phylogeny of Platyhelminthes. We present and discuss (i) new corroborative evidence for the polyphyly of the Platyhelminthes and the basal position of Acoelomorpha; (ii) a new consensus internal tree of the phylum; (iii) the nature of the sister group to the Neodermata and the hypotheses on the origin of parasitism; and (iv) the internal phylogeny of some rhabditophoran orders. Some methodological caveats are also introduced. The need to erect a new phylum, the Acoelomorpha, separate from the Platyhelminthes (now Catenulida + Rhabditophora) and based on present and new morphological and molecular characters is highlighted, and a proposal made.
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85
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Raikova OI, Reuter M, Gustafsson MKS, Maule AG, Halton DW, Jondelius U. Evolution of the nervous system in Paraphanostoma (Acoela). ZOOL SCR 2004. [DOI: 10.1111/j.1463-6409.2004.00137.x] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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86
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Telford MJ, Lockyer AE, Cartwright-Finch C, Littlewood DTJ. Combined large and small subunit ribosomal RNA phylogenies support a basal position of the acoelomorph flatworms. Proc Biol Sci 2003; 270:1077-83. [PMID: 12803898 PMCID: PMC1691347 DOI: 10.1098/rspb.2003.2342] [Citation(s) in RCA: 123] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
The phylogenetic position of the phylum Platyhelminthes has been re-evaluated in the past decade by analysis of diverse molecular datasets. The consensus is that the Rhabditophora + Catenulida, which includes most of the flatworm taxa, are not primitively simple basal bilaterians but are related to coelomate phyla such as molluscs. The status of two other groups of acoelomate worms, Acoela and Nemertodermatida, is less clear. Although many characteristics unite these two groups, initial molecular phylogenetic studies placed the Nemertodermatida within the Rhabditophora, but placed the Acoela at the base of the Bilateria, distant from other flatworms. This contradiction resulted in scepticism about the basal position of acoels and led to calls for further data. We have sequenced large subunit ribosomal RNA genes from 13 rhabditophorans + catenulids, three acoels and one nemertodermatid, tripling the available data. Our analyses strongly support a basal position of both acoels and nemertodermatids. Alternative hypotheses are significantly less well supported by the data. We conclude that the Nemertodermatida and Acoela are basal bilaterians and, owing to their unique body plan and embryogenesis, should be recognized as a separate phylum, the Acoelomorpha.
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Affiliation(s)
- Maximilian J Telford
- Laboratory for Development and Evolution, Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK.
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87
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Lundin K, Schander C. Epidermal ciliary ultrastructure of adult and larval sipunculids (Sipunculida). ACTA ZOOL-STOCKHOLM 2003. [DOI: 10.1046/j.1463-6395.2003.00136.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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88
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Giribet G. Molecules, development and fossils in the study of metazoan evolution; Articulata versus Ecdysozoa revisited. ZOOLOGY 2003; 106:303-26. [PMID: 16351916 DOI: 10.1078/0944-2006-00131] [Citation(s) in RCA: 107] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Two conflicting hypotheses of protostome relationships, Articulata and Ecdysozoa, are reviewed by evaluating the evidence in favor and against each one of them. Understanding early embryonic development and segmentation in non-arthropod non-annelid protostomes seems crucial to the debate. New ways of coding metazoan matrices, avoiding ground-patterns and higher taxa, and incorporating fossil evidence seems the best way to avoid circular debates. Molecular data served as the catalyzer for the Ecdysozoa hypothesis, although morphological support had been implicitly suggested. Most molecular analyses published so far have shown some support for Ecdysozoa, whereas none has ever supported Articulata. Here, new analyses of up to four nuclear loci, including 18S rRNA, myosin heavy chain II, histone H3 and elongation factor 1-alpha are conducted to test the molecular support for Ecdysozoa, and, at least under some parameter sets, most data sets show a clade formed by the molting animals. In contrast, support for Articulata is not found under any analytical conditions.
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Affiliation(s)
- Gonzalo Giribet
- Museum of Comparative Zoology, Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
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89
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Ruiz-Trillo I, Paps J, Loukota M, Ribera C, Jondelius U, Baguna J, Riutort M. A phylogenetic analysis of myosin heavy chain type II sequences corroborates that Acoela and Nemertodermatida are basal bilaterians. Proc Natl Acad Sci U S A 2002; 99:11246-51. [PMID: 12177440 PMCID: PMC123241 DOI: 10.1073/pnas.172390199] [Citation(s) in RCA: 193] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2002] [Accepted: 06/28/2002] [Indexed: 11/18/2022] Open
Abstract
Bilateria are currently subdivided into three superclades: Deuterostomia, Ecdysozoa, and Lophotrochozoa. Within this new taxonomic frame, acoelomate Platyhelminthes, for a long time held to be basal bilaterians, are now considered spiralian lophotrochozoans. However, recent 18S rDNA [small subunit (SSU)] analyses have shown Platyhelminthes to be polyphyletic with two of its orders, the Acoela and the Nemertodermatida, as the earliest extant bilaterians. To corroborate such position and avoid the criticisms of saturation and long-branch effects thrown on the SSU molecule, we have searched for independent molecular data bearing good phylogenetic information at deep evolutionary nodes. Here we report a phylogenetic analysis of DNA sequences from the myosin heavy chain type II (myosin II) gene from a large set of metazoans, including acoels and nemertodermatids. Our study demonstrates, both for the myosin II data set alone and for a combined SSU + myosin II data set, that Platyhelminthes are polyphyletic and that acoels and nemertodermatids are the extant earliest bilaterians. Hence, the common bilaterian ancestor was not, as currently held, large and complex but small, simple, and likely with direct development. This scenario has far-reaching implications for understanding the evolution of major body plans and for perceptions of the Cambrian evolutionary explosion.
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Affiliation(s)
- I Ruiz-Trillo
- Departament de Genètica and Departament de Biologia Animal, Universitat Barcelona, Av. Diagonal, 645 08028 Barcelona, Spain
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