Phylogenetic position of nemertea derived from phylogenomic data

Abstract

Nemertea and Platyhelminthes have traditionally been grouped together because they possess a so-called acoelomate organization, but lateral vessels and rhynchocoel of nemerteans have been regarded as coelomic cavities. Additionally, both taxa show spiral cleavage patterns prompting the placement of Nemertea as sister to coelomate Protostomia, that is, either to Neotrochozoa (Mollusca and Annelida) or to Teloblastica (Neotrochozoa plus Arthropoda). Some workers maintain a sister group relationship of Nemertea and Platyhelminthes as Parenchymia because of an assumed homology of Gotte's and Muller's larvae of polyclad Platyhelminthes and the pilidium larvae of heteronemerteans. So far, molecular data were only able to significantly reject a sister group relationship to Teloblastica. Although phylogenomic data are available for Platyhelminthes, Annelida, Mollusca, and Arthropoda, they are lacking for Nemertea. Herein, we present the first analysis specifically addressing nemertean phylogenetic position using phylogenomic data. More specifically, we collected expressed sequence tag data from Lineus viridis (O.F. Muller, 1774) and combined it with available data to produce a data set of 9,377 amino acid positions from 60 ribosomal proteins. Maximum likelihood analyses and Bayesian inferences place Nemertea in a clade together with Annelida and Mollusca. Furthermore, hypothesis testing significantly rejected a sister group relationship to either Platyhelminthes or Teloblastica. The Coelomata hypothesis, which groups coelomate taxa together to the exclusion of acoelomate and pseudocoelomate taxa, is not congruent with our results. Thus, the supposed acoelomate organization evolved independently in Nemertea and Platyhelminthes. In Nemertea, evolution of acoely is most likely due to a secondary reduction of the coelom as it is found in certain species of Mollusca and Annelida. Though looking very similar, the Gotte's and Muller's larvae of polyclad Platyhelminthes are not homologous to the pilidium larvae of heteronemerteans. Finally, the convergent evolution of segmentation in Annelida and Arthropoda is further substantiated.