Urochordates are monophyletic within the deuterostomes

Understanding the phylogenetic relationships of the three major urochordate groups within the deuterostomes is central to understanding the evolution of the chordates. We have prepared a detailed phylogenetic analysis of uroc hordates based on comparisons of 10 new urochordate 18S ribosomal DNA seque nces with other urochordate sequences in GenBank. Maximum parsimony, neighb or-joining, minimum evolution, and maximum likelihood analyses of this larg e urochordate data set are consistent with a topology in which the urochord ates are monophyletic within the deuterostomes and there are four separate clades of urochordates. These four distinct clades-styelid + pyurid ascidia ns, molgulid ascidians, phlebobranch ascidians + thaliaceans, and larvacean s-are mostly consistent with traditional morphological hypotheses and class ifications. However, we find that the ascidians may not be a monophyletic g roup (as they have been considered traditionally) but instead appear paraph yletic. Another disparity with traditional classification is that the thali aceans do not form a separate urochordate clade but rather cluster with the phlebobranch ascidians. Larvaceans have long branch lengths, which can be problematic for molecular phylogenetic methods, and their position within t he urochordates cannot be unequivocally determined with 18S rDNA. This is i mportant because the tadpole morphology of larvacean and ascidian larvae is the key trait of interest that distinguishes urochordates as chordates. Ne vertheless, the present data set resolves at least three clades of urochord ates and suggests strongly that urochordates form a monophyletic clade with in the deuterostomes.