• We are sorry, but NCBI web applications do not support your browser and may not function properly. More information
Logo of procbhomepageaboutsubmitalertseditorial board
Proc Biol Sci. May 22, 2003; 270(1519): 1077–1083.
PMCID: PMC1691347

Combined large and small subunit ribosomal RNA phylogenies support a basal position of the acoelomorph flatworms.

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.

Full Text

The Full Text of this article is available as a PDF (173K).

Selected References

These references are in PubMed. This may not be the complete list of references from this article.
  • Adoutte A, Balavoine G, Lartillot N, Lespinet O, Prud'homme B, de Rosa R. The new animal phylogeny: reliability and implications. Proc Natl Acad Sci U S A. 2000 Apr 25;97(9):4453–4456. [PMC free article] [PubMed]
  • Berney C, Pawlowski J, Zaninetti L. Elongation factor 1-alpha sequences do not support an early divergence of the Acoela. Mol Biol Evol. 2000 Jul;17(7):1032–1039. [PubMed]
  • Castresana J. Selection of conserved blocks from multiple alignments for their use in phylogenetic analysis. Mol Biol Evol. 2000 Apr;17(4):540–552. [PubMed]
  • Erwin Douglas H, Davidson Eric H. The last common bilaterian ancestor. Development. 2002 Jul;129(13):3021–3032. [PubMed]
  • Giribet G, Distel DL, Polz M, Sterrer W, Wheeler WC. Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology. Syst Biol. 2000 Sep;49(3):539–562. [PubMed]
  • Henry JQ, Martindale MQ, Boyer BC. The unique developmental program of the acoel flatworm, Neochildia fusca. Dev Biol. 2000 Apr 15;220(2):285–295. [PubMed]
  • Hooge MD. Evolution of body-wall musculature in the Platyhelminthes (Acoelomorpha, Catenulida, Rhabditophora). J Morphol. 2001 Sep;249(3):171–194. [PubMed]
  • Huelsenbeck JP, Ronquist F. MRBAYES: Bayesian inference of phylogenetic trees. Bioinformatics. 2001 Aug;17(8):754–755. [PubMed]
  • Jeanmougin F, Thompson JD, Gouy M, Higgins DG, Gibson TJ. Multiple sequence alignment with Clustal X. Trends Biochem Sci. 1998 Oct;23(10):403–405. [PubMed]
  • Katayama T, Yamamoto M, Wada H, Satoh N. Phylogenetic position of Acoel turbellarians inferred from partial 18S rDNA sequences. Zoolog Sci. 1993 Jun;10(3):529–536. [PubMed]
  • Littlewood DT, Olson PD, Telford MJ, Herniou EA, Riutort M. Elongation factor 1-alpha sequences alone do not assist in resolving the position of the acoela within the metazoa. Mol Biol Evol. 2001 Mar;18(3):437–442. [PubMed]
  • Mallatt Jon, Winchell Christopher J. Testing the new animal phylogeny: first use of combined large-subunit and small-subunit rRNA gene sequences to classify the protostomes. Mol Biol Evol. 2002 Mar;19(3):289–301. [PubMed]
  • Medina M, Collins AG, Silberman JD, Sogin ML. Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA. Proc Natl Acad Sci U S A. 2001 Aug 14;98(17):9707–9712. [PMC free article] [PubMed]
  • Peterson KJ, Eernisse DJ. Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences. Evol Dev. 2001 May-Jun;3(3):170–205. [PubMed]
  • Robinson M, Gouy M, Gautier C, Mouchiroud D. Sensitivity of the relative-rate test to taxonomic sampling. Mol Biol Evol. 1998 Sep;15(9):1091–1098. [PubMed]
  • Ruiz-Trillo I, Riutort M, Littlewood DT, Herniou EA, Baguña J. Acoel flatworms: earliest extant bilaterian Metazoans, not members of Platyhelminthes. Science. 1999 Mar 19;283(5409):1919–1923. [PubMed]

Articles from Proceedings of the Royal Society B: Biological Sciences are provided here courtesy of The Royal Society

Formats:

Related citations in PubMed

See reviews...See all...

Cited by other articles in PMC

See all...

Links

Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...