Org Divers Evol DOI 10.1007/s13127-015-0239-1 REVIEW Acoelomorpha: earliest branching bilaterians or deuterostomes? Iñaki Ruiz-Trillo1,2,3 & Jordi Paps4 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 this long-standing issue. Specifically, we argue for the benefits by nearly 400 species of misleadingly inconspicuous flat- of (1) obtaining additional genomic data from acoelomorphs, worms. Despite this, acoelomorphs have been at the centre in particular from taxa with slower evolutionary rates; (2) the of a heated debate about the origin of bilaterian animals for development of new tools to analyse the data; and (3) the use 150 years. The animal tree of life has undergone major chang- of metagenomics or metatranscriptomics data. We believe the es during the last decades, thanks largely to the advent of combination of these three approaches will provide a defini- molecular data together with the development of more rigor- tive answer as to the position of the acoelomorphs in the an- ous phylogenetic methods. There is now a relatively robust imal tree of life. backbone of the animal tree of life. However, some crucial nodes remain contentious, especially the node defining the Keywords Acoela . Metazoa . Phylogeny . Acoelomorpha . root of Bilateria. Some studies situate Acoelomorpha (and Bilateria 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 ances- tor directly gave rise to deuterostomes and protostomes. The resolution of this node will have a profound impact on our Historically—and even today—the Turbellaria domi- understanding of animal/bilaterian evolution. In particular, if nate much of our phylogenetic thinking on the lower acoelomorphs are the sister group to Bilateria, it will point to a Metazoa; hardly any other group of invertebrates has simple nature for the first bilaterian. Alternatively, if been accorded a position of comparable importance or acoelomorphs are deuterostomes, this will imply that they been subjected to so many different interpretations are the result of secondary simplification. Here, we review (Peter Ax, Relationships and phylogeny of the the state of this question and provide potential ways to solve Turbellaria. In The Lower Metazoa. University of Cali- fornia 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, Acoelomorpha is a group of bilaterally symmetric animals 08003 Barcelona, Catalonia, Spain with an apparent morphological simplicity: they lack body 2 Departament de Genètica, Universitat de Barcelona, Av. Diagonal, cavities, corporal segmentation, circulatory and respiratory 645, 08028 Barcelona, Catalonia, Spain systems, nephridia or protonephridia and larval stages, and 3 Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig their digestive system only has one opening to the exterior. Lluís Companys, 23, 08010 Barcelona, Catalonia, Spain The Acoelomorpha have been included in the phylum 4 School of Biological Sciences, University of Essex, Colchester, UK Platyhelminthes since it was created (Gegenbaur 1859)and I. Ruiz-Trillo, J. Paps are divided in two major clades: acoels and nemertodermatids. Molecular phylogenies situate Acoelomorpha Platyhelminthes were, at the same time, split in three major as the sister-group to the rest of Bilateria lineages: Acoelomorpha, Catenulida and Rhabditophora and Platyhelminthes as lophotrochozoans (Ehlers 1985). However, it was already pointed out that there were no clear, unequivocal synapomorphies to unite the three The first molecular phylogenies, based on the small subunit groups of Platyhelminthes (for a review, see Haszprunar 1996; ribosomal gene or 18S (18S rRNA), rearranged the main lin- Julian et al. 1986). The sister group relationship of Acoela and eages of the animal tree of life and pointed to a Bnew animal Nemertodermatida (that together form the clade phylogeny^ (Adoutte et al. 2000;PhilippeandTelford2006). Acoelomorpha) was based on somehow stronger arguments. A major change within the Bilateria was the definition of the These included the ciliary rootlet system, a peculiar duet superclades Ecdysozoa (Aguinaldo et al. 1997)and spiral cleavage during the early stages of development, Lophotrochozoa (Halanych et al. 1995). Ecdysozoa included lack of nephridia, well-defined guts and through-gut, all animals whose body is encapsulated by an external cuticle even though the lack of complex structures could be a that undergoes moulting (ecdysis) (Aguinaldo et al. 1997). plesiomorphic feature (Achatz et al. 2012). There are, as Lophotrochozoa united animals with no clear morphological well, some important morphological differences be- synapomorphies (as the name indicates, some sport a feeding tween these two groups of worms, the acoels and the structure called lophophore, others a trochophore larvae, nemertodermatids. For example, acoel sperm has two others none of these) (Halanych et al. 1995). Other authors flagella and their statocyst bear single lithocyte, while have used the term Spiralia to group some or all the members nemertodermatid sperm has a single flagellum and their of the Lophotrochozoa, based on the idea that the last common statocysts hold two lithocytes. An in depth description ancestor or the group likely displayed spiral cleavage during of the morphology of acoelomorphs has been recently development; however, this trait is not present in all the mem- published by Achatz et al. (2012). bers of the clade and displays homoplasy as result of second- Despite the absence of clear synapomorphies to unite ary simplification (for a review, see (Giribet 2008). acoelomorphs and the rest of flatworms, most morphological The first 18S rRNA molecular phylogeny including a phylogenetic studies positioned Acoelomorpha within flatworm situated Platyhelminthes as the earliest branching Platyhelminthes, which appeared in different positions within bilaterians (Field et al. 1988). This result was reproduced in Metazoa, usually within Protostomia (Fig. 1a, see references more recent analyses (Winnepenninckk et al. 1995). This in Baguñà and Riutort 2004a). However, Haszprunar placed supported the classical morphological view placing Acoelomorpha as the sister lineage to the rest of Bilateria Platyhelminthes as the sister group to the rest of Bilateria, (Haszprunar 1996) (Fig. 1b). In particular, Platyhelminthes and suggested an acoelomate-to-coelomate ladder-like evolu- was suggested to be a paraphyletic assemblage with tion in bilaterians. The results, however, had to be taken with Acoelomorpha as the earliest branching clade, followed by caution, since the 18S rRNA sequences of Platyhelminthes Rhabditophora, then Catenulida, and then the rest of showed remarkably long-branches, indicating that they had Bilateria. This scenario is reminiscent of the planuloid- higher rates of nucleotide substitution than other metazoans acoeloid hypothesis supported by Von Graff (Graff 1882) (Winnepenninckk et al. 1995). At that time, the Blong-branch and Hyman (Hyman 1940), in which an acoel-like flatworm attraction artefact^ (LBA) had been already described, in was suggested as the first bilaterian animal. In any case, the which taxa with longer branches tend to artifactually group dominating hypotheses situated Acoelomorpha either within together, usually incorrectly, because they cluster closer to the Platyhelminthes or as sister to Rhabditophora, Catenulida and outgroup (Felsenstein 1978). LBA is a pervasive problem in the rest of Bilateria. molecular phylogenies, not just in the ribosomal rRNA genes The possibility that Platyhelminthes, and specially Acoela but also in large phylogenomic datasets, where it often ob- and Nemertodermatida are an offshoot of the first bilaterians, scures the relationships between key taxa. Different ap- is crucial to our view of animal evolution (Baguñà and Riutort proaches to overcome such systematic problems have been 2004b). The planuloid-acoeloid proposal supports a simple suggested in the literature (Anderson and Swofford 2004; last common ancestor of the bilaterians, similar to the planula Bergsten 2005;Papsetal.2009a). Interestingly, in the first larvae of cnidarians, that lacked coelom, segmentation, phylogenetic tree inferred with a wide taxon sampling of though-gut, larval stages and many organ systems. The Platyhelminthes and other animal phyla, Platyhelminthes competing hypothesis is the archicoelomate theory (Sedgwick (without Acoela) appeared related to the Protostomia and not 1884;Jagersten1955), which suggests a rather complex as the earliest-branching bilaterians (Carranza et al. 1997). last bilaterian ancestor with a coelom, through-gut and However, the phylogenetic position of acoels was considered a complex nervous system. Thus, molecular data ap- unreliable by the authors (Carranza et al. 1997), due to LBA. peared as an ideal, independent dataset in which to test Indeed, the only two acoel sequences available at that time had these contrasting hypotheses. even longer branches than those from other Platyhelminthes. Acoelomorpha: earliest branching bilaterians or deuterostomes? Fig. 1 Diverse phylogenetic hypotheses on the