The Acoela: on Their Kind and Kinships, Especially with Nemertodermatids and Xenoturbellids (Bilateria Incertae Sedis)

Overview

Journal Org Divers Evol

Date 2013 Oct 8

PMID 24098090

Citations 20

Authors

Johannes G Achatz

Marta Chiodin

Willi Salvenmoser

Seth Tyler

Pedro Martinez

Affiliations

Soon will be listed here.

Abstract

Acoels are among the simplest worms and therefore have often been pivotal in discussions of the origin of the Bilateria. Initially thought primitive because of their "planula-like" morphology, including their lumenless digestive system, they were subsequently dismissed by many morphologists as a specialized clade of the Platyhelminthes. However, since molecular phylogenies placed them outside the Platyhelminthes and outside all other phyla at the base of the Bilateria, they became the focus of renewed debate and research. We review what is currently known of acoels, including information regarding their morphology, development, systematics, and phylogenetic relationships, and put some of these topics in a historical perspective to show how the application of new methods contributed to the progress in understanding these animals. Taking all available data into consideration, clear-cut conclusions cannot be made; however, in our view it becomes successively clearer that acoelomorphs are a "basal" but "divergent" branch of the Bilateria.

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References

Ruiz-Trillo I, Riutort M, Littlewood D, Herniou E, Baguna J . Acoel flatworms: earliest extant bilaterian Metazoans, not members of Platyhelminthes. Science. 1999; 283(5409):1919-23. DOI: 10.1126/science.283.5409.1919. View

De Robertis E . Evo-devo: variations on ancestral themes. Cell. 2008; 132(2):185-95. PMC: 2280037. DOI: 10.1016/j.cell.2008.01.003. View

Richter S, Loesel R, Purschke G, Schmidt-Rhaesa A, Scholtz G, Stach T . Invertebrate neurophylogeny: suggested terms and definitions for a neuroanatomical glossary. Front Zool. 2010; 7:29. PMC: 2996375. DOI: 10.1186/1742-9994-7-29. View

Sikes J, Bely A . Making heads from tails: development of a reversed anterior-posterior axis during budding in an acoel. Dev Biol. 2009; 338(1):86-97. DOI: 10.1016/j.ydbio.2009.10.033. View

Bery A, Cardona A, Martinez P, Hartenstein V . Structure of the central nervous system of a juvenile acoel, Symsagittifera roscoffensis. Dev Genes Evol. 2010; 220(3-4):61-76. PMC: 2929339. DOI: 10.1007/s00427-010-0328-2. View

Hejnol A, Obst M, Stamatakis A, Ott M, Rouse G, Edgecombe G . Assessing the root of bilaterian animals with scalable phylogenomic methods. Proc Biol Sci. 2009; 276(1677):4261-70. PMC: 2817096. DOI: 10.1098/rspb.2009.0896. View

De Mulder K, Kuales G, Pfister D, Willems M, Egger B, Salvenmoser W . Characterization of the stem cell system of the acoel Isodiametra pulchra. BMC Dev Biol. 2009; 9:69. PMC: 2806412. DOI: 10.1186/1471-213X-9-69. View

Ruiz-Trillo I, Riutort M, Fourcade H, Baguna J, Boore J . Mitochondrial genome data support the basal position of Acoelomorpha and the polyphyly of the Platyhelminthes. Mol Phylogenet Evol. 2004; 33(2):321-32. DOI: 10.1016/j.ympev.2004.06.002. View

Holland N . Early central nervous system evolution: an era of skin brains?. Nat Rev Neurosci. 2003; 4(8):617-27. DOI: 10.1038/nrn1175. View

10.

Bourlat S, Juliusdottir T, Lowe C, Freeman R, Aronowicz J, Kirschner M . Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Nature. 2006; 444(7115):85-8. DOI: 10.1038/nature05241. View

11.

Marlow H, Srivastava M, Matus D, Rokhsar D, Martindale M . Anatomy and development of the nervous system of Nematostella vectensis, an anthozoan cnidarian. Dev Neurobiol. 2009; 69(4):235-54. DOI: 10.1002/dneu.20698. View

12.

Marcus E, MACNAE W . Architomy in a species of Convoluta. Nature. 1954; 173(4394):130. DOI: 10.1038/173130a0. View

13.

Bourlat S, Nielsen C, Lockyer A, Littlewood D, Telford M . Xenoturbella is a deuterostome that eats molluscs. Nature. 2003; 424(6951):925-8. DOI: 10.1038/nature01851. View

14.

Mallatt J, Craig C, Yoder M . Nearly complete rRNA genes assembled from across the metazoan animals: effects of more taxa, a structure-based alignment, and paired-sites evolutionary models on phylogeny reconstruction. Mol Phylogenet Evol. 2009; 55(1):1-17. DOI: 10.1016/j.ympev.2009.09.028. View

15.

Moreno E, Permanyer J, Martinez P . The origin of patterning systems in bilateria-insights from the Hox and ParaHox genes in Acoelomorpha. Genomics Proteomics Bioinformatics. 2011; 9(3):65-76. PMC: 5054442. DOI: 10.1016/S1672-0229(11)60010-7. View

16.

Ladurner P, Pfister D, Seifarth C, Scharer L, Mahlknecht M, Salvenmoser W . Production and characterisation of cell- and tissue-specific monoclonal antibodies for the flatworm Macrostomum sp. Histochem Cell Biol. 2004; 123(1):89-104. DOI: 10.1007/s00418-004-0722-9. View

17.

Gschwentner R, Ladurner P, Nimeth K, Rieger R . Stem cells in a basal bilaterian. S-phase and mitotic cells in Convolutriloba longifissura (Acoela, Platyhelminthes). Cell Tissue Res. 2001; 304(3):401-8. DOI: 10.1007/s004410100375. View

18.

Todt C . Structure and evolution of the pharynx simplex in acoel flatworms (Acoela). J Morphol. 2008; 270(3):271-90. DOI: 10.1002/jmor.10682. View

19.

Webster B, Copley R, Jenner R, Mackenzie-Dodds J, Bourlat S, Rota-Stabelli O . Mitogenomics and phylogenomics reveal priapulid worms as extant models of the ancestral Ecdysozoan. Evol Dev. 2006; 8(6):502-10. DOI: 10.1111/j.1525-142X.2006.00123.x. View

20.

Petrov A, Hooge M, Tyler S . Comparative morphology of the bursal nozzles in acoels (Acoela, Acoelomorpha). J Morphol. 2006; 267(5):634-48. DOI: 10.1002/jmor.10428. View