Phylogenomic analyses of Echinodermata support the sister groups of Asterozoa and Echinozoa.

Adrian Reich,Casey Dunn,Gary Wessel,Koji Akasaka,Hector Escriva

doi:10.1371/journal.pone.0119627

Abstract

Echinoderms (sea urchins, sea stars, brittle stars, sea lilies and sea cucumbers) are a group of diverse organisms, second in number within deuterostome species to only the chordates. Echinoderms serve as excellent model systems for developmental biology due to their diverse developmental mechanisms, tractable laboratory use, and close phylogenetic distance to chordates. In addition, echinoderms are very well represented in the fossil record, including some larval features, making echinoderms a valuable system for studying evolutionary development. The internal relationships of Echinodermata have not been consistently supported across phylogenetic analyses, however, and this has hindered the study of other aspects of their biology. In order to test echinoderm phylogenetic relationships, we sequenced 23 de novo transcriptomes from all five clades of echinoderms. Using multiple phylogenetic methods at a variety of sampling depths we have constructed a well-supported phylogenetic tree of Echinodermata, including support for the sister groups of Asterozoa (sea stars and brittle stars) and Echinozoa (sea urchins and sea cucumbers). These results will help inform developmental and evolutionary studies specifically in echinoderms and deuterostomes in general.

Highlights

The funders had no role in study design, data collection and Echinoderms are closely related to chordates and have been an invaluable model system for developmental biology for over one hundred fifty years [1,2]
The adult body plan is unique among bilaterians as echinoderms demonstrate pentameric symmetry, while the larvae are bilaterally symmetrical
The comparable number of SwissProt sequences and comparable size of the N50s of the de novo transcriptomes in relation to the RefSeq datasets both suggest that the assembled transcriptomes are of high quality; especially compared with the S. purpuratus RefSeq dataset, an echinoderm with a well sequenced and annotated genome [8]

Summary

Introduction

Echinoderms are closely related to chordates and have been an invaluable model system for developmental biology for over one hundred fifty years [1,2]. Analyses of mitochondrial genomes recovered two additional hypotheses for echinoderm relationships (Fig. 1B), placing Ophiuroidea as the sister group to all other echinoderms [19] or in a separate study, Asteroidea as sister to Echinozoa [13,20]. Our study differs from other recent phylogenetic analyses of transcriptomes [18,24] with respect to depth of sequencing, number of sampled taxa, and species analyzed.

Results

Conclusion