Deciphering deuterostome phylogeny: molecular, morphological, and palaeontological perspectives

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CHAPTER 9

Deciphering deuterostome phylogeny: molecular, morphological, and palaeontological perspectives
Andrew B. Smith and Billie J. Swalla

9.1

Introduction

Deuterostomes form one of the three major divisions of the Bilateria and are sister group to the Lophotrochozoa plus Ecdysozoa (Eernisse and Peterson, 2004; Philippe et al., 2005a; Telford et al., 2005; Dunn et al., 2008).Traditionally the group was recognized on the basis of a shared embryonic development pattern: gastrulation occurs at the vegetal pole and the blastopore becomes the anus, while the mouth forms secondarily (Chea et al., 2005). Analysis of molecular data has consistently found the deuterostome grouping, with generally high levels of support (Turbeville et al., 1994; Wada and Satoh, 1994; Halanych etal., 1995; Cameron et al., 2000). Five major clades make up the Deuterostomia: craniates, cephalochordates, echinoderms, hemichordates, and tunicates (see Figure 9.1). Because vertebrates, including ourselves, belong to the craniates, there has long been a fascination about their invertebrate origins. Theories of chordate evolution have abounded for over 100 years, but it is only in the last 10 to15 years that deuterostome relationships have come into sharp focus, driven largely by new data from molecular genetics and the fossil record, and new analyses of traditional morphological and ontogenetic data. From this plethora of information, some complementary, others supporting contradictory conclusions, a more coherent picture of the phylogeny and early evolution of deuterostomes is startingto emerge. Here we review four key areas where there has been the most heated debate in the last 5 years: phylogenetic relationships of the major deuterostome groups; the earliest fossil record and
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divergence times of deuterostome groups; the evolution of body axes; and the characteristics of the ancestral deuterostome body plan.

9.2 Deuterostome phylogenetic relationships
Until 10years ago there was little consensus about the relationships of the major deuterostome groups (see reviews by Gee, 1996, and Lambert, 2005). Depending upon whether emphasis was given to comparative adult morphology, embryology, or the fossil record, different sister-group relationships could be argued. Larval traits provided support for a grouping of echinoderms and hemichordates (Hara et al., 2006;Swalla, 2006), adult traits provided support for a grouping of hemichordates and chordates (Cameron et al., 2000) while palaeontological data were used to support an echinoderm– chordate pairing (Gee, 1996). Probably the most widely accepted view in the mid-1990s was that echinoderms were sister group to the rest and that chordates and hemichordates were sister taxa [i.e. (echinoderms{hemichordates [tunicates (cephalochordates craniates)]})]. With the arrival of molecular data the problem of deuterostome relationships seemed to be solved. Early results were based on analyses of ribosomal gene sequences and pointed to echinoderms and hemichordates as sister groups (Turbeville et al., 1994; Wada and Satoh, 1994) and to a monophyletic Chordata comprising tunicates, cephalochordates, andcraniates (Turbeville et al., 1994). Within the chordates, tunicates were identi ed as sister group

DEUTEROSTOME PHYLOGENY

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Xenoturbellida Crinoidea Echinodermata Asteroidea Ophiuroidea Holothuroidea I Echinoidea Pterobranchia

Hemichordata

Harrimaniidae Ptychoderidae Cephalochordata Phlebobranchia Thaliacea II

Tunicata

Aplousobranchia Appendicularia Stolidobranchia MolgulidaeVertebrata

Figure 9.1 Current deuterostome phylogeny, according to available molecular and morphological data. Dotted lines show clades of uncertainty where conflicting data have been obtained; I and II mark clades where the evidence for a monophyletic group is very high. I, Ambulacraria is made up of hemichordates and echinoderms. Mitochondrial, ribosomal and genomic evidence are in agreement...
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