Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida

Abstract

Deuterostomes comprise vertebrates, the related invertebrate chordates (tunicates and cephalochordates) and three other invertebrate taxa: hemichordates, echinoderms and Xenoturbella. The relationships between invertebrate and vertebrate deuterostomes are clearly important for understanding our own distant origins. Recent phylogenetic studies of chordate classes and a sea urchin have indicated that urochordates might be the closest invertebrate sister group of vertebrates, rather than cephalochordates, as traditionally believed. More remarkable is the suggestion that cephalochordates are closer to echinoderms than to vertebrates and urochordates, meaning that chordates are paraphyletic. To study the relationships among all deuterostome groups, we have assembled an alignment of more than 35,000 homologous amino acids, including new data from a hemichordate, starfish and Xenoturbella. We have also sequenced the mitochondrial genome of Xenoturbella. We support the clades Olfactores (urochordates and vertebrates) and Ambulacraria (hemichordates and echinoderms). Analyses using our new data, however, do not support a cephalochordate and echinoderm grouping and we conclude that chordates are monophyletic. Finally, nuclear and mitochondrial data place Xenoturbella as the sister group of the two ambulacrarian phyla. As such, Xenoturbella is shown to be an independent phylum, Xenoturbellida, bringing the number of living deuterostome phyla to four.

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

“…New phylogenomic approaches have recently overturned conventional thinking about the relationships within chordates (Philippe et al, 2005;Bourlat et al, 2006;Delsuc et al, 2006;Dunn et al, 2008;Blair and Hedges, 2005). One of the most recent molecular phylogenies has suggested that the Subphylum Urochordata (Tunicata), represented by three different classes, Ascidiacea, Thaliacea and Larvacea, should be raised to the phylum level (Zeng and Swalla, 2005) but the subject is still under discussion since there are discrepancies between phylogenomic analyses and results from mitochondrial and rRNA data.…”

Chordate origins and evolution

“…Fortunately, the exponential accumulation of molecular data from genomes and animal models comes to the rescue. Molecular phylogenies are continually being constructed, recently incorporating multiple genes on a genomic scale, hopefully reducing sources of systematical errors (Bourlat, 2006;Delsuc et al, 2005;Dunn et al, 2008). The interested developmental biologist is also compelled to understand how fossils can and do change our views on the evolution of development (Conway-Morris, 1994;Hall, 2002).…”

“…In the last 4 years new phylogenetic ideas have been proposed, a new phylum, Xenoturbellida, has joined deuterostomes and been assigned to a position basal to ambulacraria (Bourlat et al, 2006), some proposed deuterostomes such as the lophophorates (Halanych et al, 1995) and enigmatic chaetognaths (Marletaz et al, 2006b) have now been firmly displaced to join protostomes and some paradigms such as the calcichordate hypothesis have been revisited (Delsuc et al, 2006 and references therein).…”

“…Max Telford has pioneered the molecular work that placed Xenoturbellids as deuterostomes (Bourlat et al, 2006) and brings us up to date on how this worm may answer questions of chordate origins (Telford, 2008). Then Atsuko Sato, working with John Bishop and Peter Holland, contributes a paper with photos of larvae of Rhabdopleura normani that summarizes the importance that pterobranchs have played in classic views of deuterostome evolution (Sato et al, 2008), due to the extensive fossil record of Pterobranchs or graptolites (Swalla and Smith, 2008).…”

“…These novel ideas have been subjected to thorough testing and the Olfactores hypothesis has received further support (Bourlat et al, 2006;Delsuc et al, 2006Delsuc et al, , 2008Dunn et al, 2008;Swalla and Smith, 2008). In contrast, the unorthodox assembly of cephalochordates and echinoderms, which initially raised suspicions about chordate monophyly (Delsuc et al, 2006), was not confirmed (Bourlat et al, 2006;Delsuc et al, 2008).…”