Metazoan relationships on the basis of 18S rRNA sequences: A few years later...

The 18S rRNA database is continuously growing and new tree construction met hods are being developed, The present study aims at assessing what effect t he addition of recently determined animal 18S rRNA sequences and the use of a recently developed distance matrix calculation method have on the result s of some previously published case studies on metazoan phylogeny. When re- assessing three case studies, part of their conclusions was confirmed on th e basis of the present 18S rRNA data set: 1) the monophyly of Arthropoda; 2 ) the monophyly of the Vestimentifera-Pogonophora and their protostome char acter; 3) the doubt about the monophyletic origin of Echiura-Sipuncula and 4) the coelomate character of Nemertea, Yet, it is also demonstrated that s ome of the previous results are no longer warranted when updating the analy ses: 1) the monophyly of both the Annelida and the Eutrochozoa; 2) the sist er-group relationship of Echiura to Vestimentifera-Pogonophora and 3) the p olyphyly of the Mesozoa and their close relationship to Myxozoa and Nematod es, In addition, some new very preliminary evidence is provided for: 1) a c ommon ancestry of Platyhelminthes and Mesozoa and the monophyly of the latt er group and 2) the monophyly of Clitellata, Hirudinida and Oligochaeta. Fi nally, doubt is casted on the monophyly of the Polychaeta and the polychaet e orders Spionida, Phyllodocida, and Sabellidae, Of course, these hypothese s also need further testing.