Urochordates Are Monophyletic Within the Deuterostomes

Abstract

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 urochordates based on comparisons of 10 new urochordate 18S ribosomal DNA sequences with other urochordate sequences in GenBank. Maximum parsimony, neighbor-joining, minimum evolution, and maximum likelihood analyses of this large urochordate data set are consistent with a topology in which the urochordates are monophyletic within the deuterostomes and there are four separate clades of urochordates. These four distinct clades--styelid + pyurid ascidians, molgulid ascidians, phlebobranch ascidians + thaliaceans, and larvaceans--are mostly consistent with traditional morphological hypotheses and classifications. However, we find that the ascidians may not be a monophyletic group (as they have been considered traditionally) but instead appear paraphyletic. Another disparity with traditional classification is that the thaliaceans 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 the urochordates cannot be unequivocally determined with 18S rDNA. This is important because the tadpole morphology of larvacean and ascidian larvae is the key trait of interest that distinguishes urochordates as chordates. Nevertheless, the present data set resolves at least three clades of urochordates and suggests strongly that urochordates form a monophyletic clade within the deuterostomes.

Additional molecular support for the new chordate phylogeny

“…The sister-group relationship between tunicates and vertebrates (Olfactores) observed in phylogenomics is in conflict with most (if not all) analyses of rRNA which favor cephalochordates as the closest relatives of vertebrates (Euchordates) Mallatt and Winchell, 2007;Swalla et al, 2000;Wada and Satoh, 1994;Winchell et al, 2002). However, the statistical support for Euchordates in rRNA-based phylogenetic studies is moderate.…”

Phylogeny of the families Pyuridae and Styelidae (Stolidobranchiata, Ascidiacea) inferred from mitochondrial and nuclear DNA sequences

“…Within the Ascidiacea, the Order Stolidobranchiata is one of the most important groups as it is species and exhibits high morphological plasticity and complexity. To date, molecular and morphological data support the monophyly of the Stolidobranchiata uniting the traditionally recognized families Pyuridae, Styelidae and Molgulidae (Berrill, 1950;Kott, 1985;Monniot et al, 1991;Swalla et al, 2000;Zeng et al, 2006). However, the internal classification of this order remains under discussion.…”

“…Turbeville et al, 1994;Cameron et al, 2000;Swalla et al, 2000;Bourlat et al, 2003;Blair and Hedges, 2005). The phylogeny of the Phylum Chordata, originally divided into three subphyla, Vertebrata, Cephalochordata and Urochordata, has also been intensely studied in order to clarify the mechanisms of chordate evolution (Winchell et al, 2002;Zeng and Swalla, 2005).…”

“…There are strong morphogical evidences that Pyuridae and Molgulidae are related, with the latter having probably originated from the former (Berrill, 1950). However, Swalla et al (2000) showed that the families Pyuridae and Styelidae formed a robust clade, separated from Molgulidae, but with both families being either paraphyletic or polyphyletic. In the molecular phylogeny reconstructed by Zeng et al (2006), including five pyurids and 12 styelids, the family Pyuridae appeared either as a paraphyletic or a monophyletic group depending on the algorithms of reconstruction applied.…”

“…Wada et al, 1992;Wada, 1998;Swalla et al, 2000;Stach and Turbeville, 2002;Turon and López-Legentil, 2004;Pérez-Portela et al, 2007;Zeng et al, 2006). However, only rarely have both types of marker been combined in a study (Stach and Turbeville, 2002;Zeng et al, 2006).…”