Elongation factor 1-alpha sequences do not support an early divergence of the acoela

The phylogenetic position of the Acoela is a key problem in the understandi ng of metazoan evolution. Recent studies based on 18S ribosomal DNA (rDNA) sequences have placed the Acoela in an extremely basal position as the sist er group to all other extant triploblastic animals, suggesting that the phy lum Platyhelminthes is polyphyletic. In order to test the results obtained with 18S rDNA, we sequenced elongation factor 1-alpha (EF1a) for the acoel Convoluta roscoffensis and five species of Turbellaria (two polyclads, Lept oplana tremellaris, and Prostheceraeus vittatus, and three triclads, Crenob ia alpina, Schmidtea polychroa and Girardia tigrina). Phylogenetic analyses of EF1a sequences show that the acoel sequences branch within the Platyhel minthes, in opposition to the 18S rDNA data. Moreover, comparison of the ce ntral variable region of EF1a shows similar sequence signatures between C. roscoffensis and the three triclad species. Although EF1a sequences fail to prove the monophyly of the phylum Platyhelminthes, they do not confirm the early divergence of the Acoela.