Skip to main content
NCBI home page
Proceedings of the Royal Society B: Biological Sciences logoLink to Proceedings of the Royal Society B: Biological Sciences
. 2003 May 22;270(1519):1077–1083. doi: 10.1098/rspb.2003.2342

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

Maximilian J Telford 1, Anne E Lockyer 1, Chloë Cartwright-Finch 1, D Timothy J Littlewood 1
PMCID: PMC1691347  PMID: 12803898

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 (172.7 KB).

Selected References

These references are in PubMed. This may not be the complete list of references from this article.

  1. 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. doi: 10.1073/pnas.97.9.4453. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. 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. doi: 10.1093/oxfordjournals.molbev.a026384. [DOI] [PubMed] [Google Scholar]
  3. Castresana J. Selection of conserved blocks from multiple alignments for their use in phylogenetic analysis. Mol Biol Evol. 2000 Apr;17(4):540–552. doi: 10.1093/oxfordjournals.molbev.a026334. [DOI] [PubMed] [Google Scholar]
  4. Erwin Douglas H., Davidson Eric H. The last common bilaterian ancestor. Development. 2002 Jul;129(13):3021–3032. doi: 10.1242/dev.129.13.3021. [DOI] [PubMed] [Google Scholar]
  5. Giribet G., Distel D. L., Polz M., Sterrer W., Wheeler W. C. 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. doi: 10.1080/10635159950127385. [DOI] [PubMed] [Google Scholar]
  6. Henry J. Q., Martindale M. Q., Boyer B. C. The unique developmental program of the acoel flatworm, Neochildia fusca. Dev Biol. 2000 Apr 15;220(2):285–295. doi: 10.1006/dbio.2000.9628. [DOI] [PubMed] [Google Scholar]
  7. Hooge M. D. Evolution of body-wall musculature in the Platyhelminthes (Acoelomorpha, Catenulida, Rhabditophora). J Morphol. 2001 Sep;249(3):171–194. doi: 10.1002/jmor.1048. [DOI] [PubMed] [Google Scholar]
  8. Huelsenbeck J. P., Ronquist F. MRBAYES: Bayesian inference of phylogenetic trees. Bioinformatics. 2001 Aug;17(8):754–755. doi: 10.1093/bioinformatics/17.8.754. [DOI] [PubMed] [Google Scholar]
  9. Jeanmougin F., Thompson J. D., Gouy M., Higgins D. G., Gibson T. J. Multiple sequence alignment with Clustal X. Trends Biochem Sci. 1998 Oct;23(10):403–405. doi: 10.1016/s0968-0004(98)01285-7. [DOI] [PubMed] [Google Scholar]
  10. 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] [Google Scholar]
  11. Littlewood D. T., Olson P. D., Telford M. J., Herniou E. A., 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. doi: 10.1093/oxfordjournals.molbev.a003821. [DOI] [PubMed] [Google Scholar]
  12. 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. doi: 10.1093/oxfordjournals.molbev.a004082. [DOI] [PubMed] [Google Scholar]
  13. Medina M., Collins A. G., Silberman J. D., Sogin M. L. 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. doi: 10.1073/pnas.171316998. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Peterson K. J., Eernisse D. J. Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences. Evol Dev. 2001 May-Jun;3(3):170–205. doi: 10.1046/j.1525-142x.2001.003003170.x. [DOI] [PubMed] [Google Scholar]
  15. 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. doi: 10.1093/oxfordjournals.molbev.a026016. [DOI] [PubMed] [Google Scholar]
  16. Ruiz-Trillo I., Riutort M., Littlewood D. T., Herniou E. A., Baguña J. Acoel flatworms: earliest extant bilaterian Metazoans, not members of Platyhelminthes. Science. 1999 Mar 19;283(5409):1919–1923. doi: 10.1126/science.283.5409.1919. [DOI] [PubMed] [Google Scholar]

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

RESOURCES