EVOLUTION & DEVELOPMENT 2:2, 93–101 (2000) The A/P axis in echinoderm ontogeny and evolution: evidence from fossils and molecules Kevin J. Peterson,a,b César Arenas-Mena,a,c and Eric H. Davidsona,* aDivision of Biology, California Institute of Technology, Pasadena, CA 91125, USA; bDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA; cStowers Institute for Medical Research, Kansas City, MO 64110, USA *Author for correspondence (email: [email protected]) SUMMARY Even though echinoderms are members of the such that there is but a single plane of symmetry dividing the Bilateria, the location of their anterior/posterior axis has re- animal into left and right halves. We tentatively hypothesize mained enigmatic. Here we propose a novel solution to the that this plane of symmetry is positioned along the dorsal/ven- problem employing three lines of evidence: the expression of tral axis. These axis identifications lead to the conclusion that a posterior class Hox gene in the coeloms of the nascent the five ambulacra are not primary body axes, but instead are adult body plan within the larva; the anatomy of certain early outgrowths from the central anterior/posterior axis. These fossil echinoderms; and finally the relation between endo- identifications also shed insight into several other evolutionary skeletal plate morphology and the associated coelomic tis- mysteries of various echinoderm clades such as the indepen- sues. All three lines of evidence converge on the same answer, dent evolution of bilateral symmetry in irregular echinoids, but namely that the location of the adult mouth is anterior, and the do not elucidate the underlying mechanisms of the adult co- anterior/posterior axis runs from the mouth through the adult elomic architecture. coelomic compartments. This axis then orients the animal INTRODUCTION not be homologous to the radial symmetry of cnidarians. In- stead radial symmetry in echinoderms is an apomorphy of at Except for sponges, cnidarians, and their allies, almost all least the crown-group members of the phylum, and presum- animals have conspicuous anterior/posterior (A/P), dorsal/ ably, of some of its stem lineages as well. ventral (D/V), and left/right (L/R) axes. These axes are de- Although this much is well accepted, one of the many as- fining characters of the Bilateria, and because the bilaterians pects of echinoderm anatomy “designed to puzzle the zoolo- are monophyletic (Adoutte et al. 1999), bilateral symmetry gist” (Hyman 1955), is the location in the adult echinoderm must have emerged during the initial evolution of this great body plan of primary axes homologous to those of hemichor- clade of animals (Peterson et al. 2000). Within the Bilateria, dates, chordates, and other bilaterians. This difficulty stems the echinoderms secondarily evolved radially symmetric in part from the lack of a clearly defined head region, and the body plans. Their radial symmetry confounded early at- absence of a brain or other concentration of neurons and sen- tempts to classify echinoderms, and they were often allied sory structures. Moreover, although axes of bilateral symme- with cnidarians as “Radialia,” although their larval forms are try appear in some groups of echinoderms, their homology is clearly bilaterally symmetrical (see Hyman 1955; Winsor equivocal, because key landmarks such as the position of the 1976 for historical discussions). It is now clear that echino- anus or hydropore vary in position among them (Hotchkiss derms are the sister taxon of the hemichordates, and these to- 1998). As we show in this paper, the apparent planes of bi- gether constitute the sister taxon of the chordates, the three lateral symmetry in some of these forms are of no relevance taxa together comprising the deuterostomes. This phyloge- to the problem of identifying fundamental morphological netic topology is supported by mitochondrial DNA (Castre- relationships of the echinoderm body plan. We propose here sana et al. 1998a, 1998b), 18S rRNA (Turbeville et al. 1994; a new interpretation of axial homologies in echinoderms, Wada and Satoh 1994), and cladistic morphological analyses based on three separate kinds of evidence: the pattern of ex- (Peterson and Eernisse, unpublished data). Because both pression of a posterior class Hox gene in the coeloms of the hemichordates and chordates are clearly bilaterally symmet- nascent adult body within a sea urchin larva; the anatomy of rical, it follows that the radial symmetry of echinoderms can- certain early fossil echinoderms; and the relation between © BLACKWELL SCIENCE, INC. 93 94 EVOLUTION & DEVELOPMENT Vol. 2, No. 2, March–April 2000 endoskeletal plate morphology and the associated coelomic sacs which have grown down the sides of the stomach. The tissues, as analyzed by Mooi and David (1997, 1998). A left somatocoel adjoins the rudiment which forms where the plausible pathway for evolution of the radially symmetric vestibule invaginates and makes contact with the left hydro- echinoderm body plan from a bilaterally symmetric ancestor coel. Expression of Hox11/13b confirms that the somato- can thus be derived. These ideas also illuminate some of the coels are the posterior coeloms. Note that there is no expres- enigmatic aspects of echinoderm anatomy, and as we show, sion in the vestibule, the anlagen of the adult nervous system, they provide a useful framework for interpretation of gene nor in any other mesodermal cells, though the gene is also expression patterns during the ontogeny of the adult echino- expressed in the larval anus (a detailed description of poste- derm body plan. rior Hox gene expression in larval development will appear elsewhere; Arenas-Mena et al. unpublished data). Because the mesodermal somatocoels are the primary sites of Hox11/13b EXPRESSION OF THE Hox11/13b GENE IN AN expression in tissues that will give rise to the adult body ECHINOID LARVA parts, we rely on the architecture of the mesoderm to identify axial homologies, rather than on the adult nervous system, A cluster of ten Hox genes has recently been isolated from which has been used in the past to address this same question the genome of Strongylocentrotus purpuratus (Martinez et (e.g., Raff and Popodi 1996; Morris 1999). al. 1999; see also Popodi et al. 1996 for the Hox cluster of Heliocidaris erythrogramma). This cluster includes paralogs of almost all vertebrate Hox genes. Quantitative measure- HOMOLOGIES BETWEEN ECHINODERM AND ments of transcript concentration, using probes specific to HEMICHORDATE COELOMS each Hox gene, have shown that most of these genes are not expressed at all during embryogenesis, but that they are tran- The disposition of the anlagen of the mesodermal compo- scribed copiously once adult body plan formation is initiated nents is a fundamental feature in both echinoderm and hemi- in the feeding larva (Arenas-Mena et al. 1998). Thus, as in chordate development (Peterson et al. 1997, 1999a, 1999b). all Bilateria, the Hox gene complex would appear to be uti- Figure 2 shows what we envision as the primitive arrange- lized in the development of the phylotypic echinoderm body ment of the coeloms in the common ancestor of these two plan. Where they are utilized is a particularly interesting sister phyla. There is an anterior coelom called the protocoel question, given their radially symmetric character (Raff and in hemichordates, and the axocoel in echinoderms that may Popodi 1996). Here we consider the locus of expression of or may not have been paired. An anterior coelom is usually the most posterior gene of the cluster examined, Hox11/13b connected to the exterior via the hydropore. The middle pair (as indicated by its name, the homeobox of this gene displays of coeloms, i.e., the mesocoel or hydrocoel, is found in both diagnostic features shared specifically with vertebrate phyla, but the right hydrocoel is greatly reduced in echino- Hox11, 12, and 13 genes; Martinez et al. 1999). Expression derms; its only contribution to the adult body plan is the of Hox genes of this class at the posterior end of the anterior/ madreporic vesicle. The most posterior pair of coeloms are posterior axis is a bilaterian synapomorphy. In the following the metacoels or somatocoels, that is the loci of SpHox11/13b argument we use the pattern of expression of the Hox11/13b expression in the sea urchin larva. Thus, the latest common gene early in adult body plan development as a marker of ancestor of echinoderms and hemichordates can be pre- posterior homology, with respect to the body plans of other sumed to have had at least five coeloms arranged in a clear deuterostomes. anterior/posterior fashion with the protocoel(s)/axocoel(s) Hox11/13b is one of the two Hox genes expressed in the anterior, and the metacoels/somatocoels posterior. S. purpuratus embryo, where it is transcribed in many em- Although the initial organization of these mesodermal an- bryonic regions (Dobias et al. 1996; see also Ishii et al. lagen is similar in the larvae of hemichordates and echino- 1999). However, the embryonic expression is likely to be ir- derms, the final disposition of the coelomic sacs (or their de- relevant to adult body plan formation, because the transcripts rivatives) in the adult forms becomes markedly different. In are largely in embryonic and larval structures which are not the hemichordate, the mesoderm, because it is derived from included in the radially symmetric juvenile which emerges at the gut, forms in the apical/blastoporal axis, the primitive metamorphosis. After about 2 weeks of feeding, the Hox11/ axis of symmetry among bilaterian larvae (Nielsen 1995). In 13b gene is activated in the coelomic mesoderm of the rudi- the adult the anterior/posterior axis runs in the same plane as ment from which the adult body plan derives. The pattern of the apical/blastoporal axis of the embryo/larva such that the expression is illustrated in the whole mount in situ hybridiza- apical plate of the larva is found at the tip of the proboscis at tion shown in Fig.