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Open Access Highly Accessed Research article

An updated 18S rRNA phylogeny of tunicates based on mixture and secondary structure models

Georgia Tsagkogeorga12, Xavier Turon3, Russell R Hopcroft4, Marie-Ka Tilak12, Tamar Feldstein5, Noa Shenkar56, Yossi Loya5, Dorothée Huchon5, Emmanuel JP Douzery12 and Frédéric Delsuc12*

Author Affiliations

1 Université Montpellier 2, Institut des Sciences de l'Evolution (UMR 5554), CC064, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France

2 CNRS, Institut des Sciences de l'Evolution (UMR 5554), CC064, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France

3 Centre d'Estudis Avançats de Blanes (CEAB, CSIC), Accés Cala S. Francesc 14, 17300 Blanes (Girona), Spain

4 Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska, USA

5 Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, 69978, Israel

6 Department of Biology, University of Washington, Seattle WA 98195, USA

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BMC Evolutionary Biology 2009, 9:187  doi:10.1186/1471-2148-9-187

Published: 5 August 2009

Abstract

Background

Tunicates have been recently revealed to be the closest living relatives of vertebrates. Yet, with more than 2500 described species, details of their evolutionary history are still obscure. From a molecular point of view, tunicate phylogenetic relationships have been mostly studied based on analyses of 18S rRNA sequences, which indicate several major clades at odds with the traditional class-level arrangements. Nonetheless, substantial uncertainty remains about the phylogenetic relationships and taxonomic status of key groups such as the Aplousobranchia, Appendicularia, and Thaliacea.

Results

Thirty new complete 18S rRNA sequences were acquired from previously unsampled tunicate species, with special focus on groups presenting high evolutionary rate. The updated 18S rRNA dataset has been aligned with respect to the constraint on homology imposed by the rRNA secondary structure. A probabilistic framework of phylogenetic reconstruction was adopted to accommodate the particular evolutionary dynamics of this ribosomal marker. Detailed Bayesian analyses were conducted under the non-parametric CAT mixture model accounting for site-specific heterogeneity of the evolutionary process, and under RNA-specific doublet models accommodating the occurrence of compensatory substitutions in stem regions. Our results support the division of tunicates into three major clades: 1) Phlebobranchia + Thaliacea + Aplousobranchia, 2) Appendicularia, and 3) Stolidobranchia, but the position of Appendicularia could not be firmly resolved. Our study additionally reveals that most Aplousobranchia evolve at extremely high rates involving changes in secondary structure of their 18S rRNA, with the exception of the family Clavelinidae, which appears to be slowly evolving. This extreme rate heterogeneity precluded resolving with certainty the exact phylogenetic placement of Aplousobranchia. Finally, the best fitting secondary-structure and CAT-mixture models suggest a sister-group relationship between Salpida and Pyrosomatida within Thaliacea.

Conclusion

An updated phylogenetic framework for tunicates is provided based on phylogenetic analyses using the most realistic evolutionary models currently available for ribosomal molecules and an unprecedented taxonomic sampling. Detailed analyses of the 18S rRNA gene allowed a clear definition of the major tunicate groups and revealed contrasting evolutionary dynamics among major lineages. The resolving power of this gene nevertheless appears limited within the clades composed of Phlebobranchia + Thaliacea + Aplousobranchia and Pyuridae + Styelidae, which were delineated as spots of low resolution. These limitations underline the need to develop new nuclear markers in order to further resolve the phylogeny of this keystone group in chordate evolution.