Xenoturbellida: The fourth deuterostome phylum and the diet of worms

Abstract

Since the discovery of the marine worm Xenoturbella bocki in 1915 by Sixten Bock and its first published description by Einar Westblad (Westblad,1949, Arkiv Zoologi 1:3–29), Xenoturbella was generally allied to the turbellarian flatworms, perhaps most closely to acoelomorphs. In 1997, however, analyses of ribosomal DNA (Norén and Jondelius, 1997, Nature 390:31–32) and developing oocytes (Israelsson, 1997, Nature 390:32) [and, subsequently, embryos (Israelsson, 1999, Proc R Soc Lond B 266:835–841)] recovered from Xenoturbella specimens led to the surprising conclusion that it was in fact a highly degenerate bivalve mollusc. Bourlat et al. showed in 2003 that this result was due to contamination from bivalves in its diet (Bourlat et al.,2003, Nature 424:925–928). Our analyses showed Xenoturbella is a deuterostome, related to the Ambulacraria (echinoderms and hemichordates). Subsequent work has shown that Xenoturbellida is a separate lineage from the Ambulacraria and therefore constitutes the fourth deuterostome phylum (Bourlat et al.,2006, Nature 444:85–88). I consider this phylogenetic position in the light of what is known of its genetics, morphology, and ontogeny. I examine what this phylogenetic position for Xenoturbella can tell us about its own evolution and what light this might shine on the common ancestor of the deuterostomes and hence on the origins of the chordates. genesis 46:580–586, 2008. © 2008 Wiley‐Liss, Inc.

Chordate origins and evolution

“…Importantly, although chordate topologies may have changed, integrity of the subphylum has been reaffirmed, albeit, perhaps, at the surprising price of destabilizing deuterostomes as a natural group (Delsuc et al, 2008). This point is discussed in this special issue along with the account of challenging morphological and molecular similarities observed between Xenoturbella and Acoelomorph flatworms (Bourlat et al, 2003;Cameron et al, 2000;Delsuc et al, 2008;Philippe et al, 2007;Telford, 2008).…”

“…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).…”

Serotonin and its metabolism in basal deuterostomes: insights fromStrongylocentrotus purpuratusandXenoturbella bocki

“…The sediment was sieved through a millimetre sieve and individual specimens collected as described previously (Stach et al, 2005;Telford, 2008). Unless otherwise stated, all reagents were obtained from Sigma-Aldrich at analytical grade or higher, without additional purification.…”

“…This marine worm-like organism has many features that can be described as ancestral characteristics for bilaterial animals. For example, it has no through gut, gonads or anus (Telford, 2008) and its nervous system is composed of a nerve net without a distinct brain (Raikova et al, 2000;Westblad, 1949). Originally classified within flatworms, a relationship with bivalve mollusks was subsequently suggested (reviewed in Bourlat et al, 2008;Bourlat et al, 2003).…”

“…Later it was found that samples were contaminated with molluskan eggs and the organism was placed in a phylum of its own, Xenoturbellida, within the superphylum Deuterostomia (Bourlat et al, 2006). More recently, a genome-wide survey confirmed this distinct phylogenetic position of Xenoturbellida (Bourlat et al, 2009;Dunn et al, 2008;Philippe et al, 2009;Telford, 2008); however, some studies imply that it may be an even more basal lineage within the bilaterian superclade (Hejnol et al, 2009).…”