Acoelomorpha: earliest branching bilaterians or deuterostomes?
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REVIEW
Acoelomorpha: earliest branching bilaterians or deuterostomes? Iñaki Ruiz-Trillo 1,2,3 & Jordi Paps 4
Received: 22 July 2015 / Accepted: 29 September 2015 # The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract The Acoelomorpha is an animal group comprised by nearly 400 species of misleadingly inconspicuous flatworms. Despite this, acoelomorphs have been at the centre of a heated debate about the origin of bilaterian animals for 150 years. The animal tree of life has undergone major changes during the last decades, thanks largely to the advent of molecular data together with the development of more rigorous phylogenetic methods. There is now a relatively robust backbone of the animal tree of life. However, some crucial nodes remain contentious, especially the node defining the root of Bilateria. Some studies situate Acoelomorpha (and Xenoturbellida) as the sister group of all other bilaterians, while other analyses group them within the deuterostomes which instead suggests that the last common bilaterian ancestor directly gave rise to deuterostomes and protostomes. The resolution of this node will have a profound impact on our understanding of animal/bilaterian evolution. In particular, if acoelomorphs are the sister group to Bilateria, it will point to a simple nature for the first bilaterian. Alternatively, if acoelomorphs are deuterostomes, this will imply that they are the result of secondary simplification. Here, we review the state of this question and provide potential ways to solve
this long-standing issue. Specifically, we argue for the benefits of (1) obtaining additional genomic data from acoelomorphs, in particular from taxa with slower evolutionary rates; (2) the development of new tools to analyse the data; and (3) the use of metagenomics or metatranscriptomics data. We believe the combination of these three approaches will provide a definitive answer as to the position of the acoelomorphs in the animal tree of life. Keywords Acoela . Metazoa . Phylogeny . Acoelomorpha . Bilateria
Historically—and even today—the Turbellaria dominate much of our phylogenetic thinking on the lower Metazoa; hardly any other group of invertebrates has been accorded a position of comparable importance or been subjected to so many different interpretations (Peter Ax, Relationships and phylogeny of the Turbellaria. In The Lower Metazoa. University of California Press, Berkeley, 1963, ed. E. C. Dougherty).
* Iñaki Ruiz-Trillo [email protected]; [email protected]
The phylum Acoelomorpha 1
Institut de Biologia Evolutiva (CSIC-Universitat Pompeu Fabra), Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Catalonia, Spain
2
Departament de Genètica, Universitat de Barcelona, Av. Diagonal, 645, 08028 Barcelona, Catalonia, Spain
3
Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys, 23, 08010 Barcelona, Catalonia, Spain
4
School of Biological Sciences, University of Essex, Colchester, UK
Acoelomorpha is a group of bi
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