C 2014 Wiley Periodicals, Inc. V
genesis 52:186–192 (2014)
REVIEW
The Echinoderm Larval Skeleton as a Possible Model System for Experimental Evolutionary Biology Hiroyuki Koga,* Yoshiaki Morino, and Hiroshi Wada Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai, Tsukuba, Japan Received 28 November 2013; Revised 12 February 2014; Accepted 14 February 2014
Summary: The evolution of various body plans results from the acquisition of novel structures as well as the loss of existing structures. Some novel structures necessitate multiple evolutionary steps, requiring organisms to overcome the intermediate steps, which might be less adaptive or neutral. To examine this issue, echinoderms might provide an ideal experimental system. A larval skeleton is acquired in some echinoderm lineages, such as sea urchins, probably via the co-option of the skeletogenic machinery that was already established to produce the adult skeleton. The acquisition of a larval skeleton was found to require multiple steps and so provides a model experimental system for reproducing intermediate evolutionary stages. The fact that echinoderm embryology has been studied with various natural populations also presents C 2014 Wiley Periodan advantage. genesis 52:186–192. V icals, Inc.
Key words: echinoderm; larval skeleton; evolution of a novel structure; co-option
Since Aristotle’s description of the incredible variation of animals in his History of Animals (Peck, 1965, 1970, 1991), more than 2000 years had passed before humans began to understand the diversity of life using the concept of evolution. What keeps people from accepting the concept of evolution? Perhaps one obstacle is the existence of large gaps between animal body plans. This issue is now being overcome by recent progress in research on the evolution of development. Based on conserved molecular genetic tools for building animal bodies, much commonality exists among genetic mechanisms for body construction (Carroll et al., 2010; Denes et al., 2007; Sasai and De Robertis, 1997). With the growing understanding of animal development modes as well as increasing paleontological evidence,
we are now filling the gaps and reconstructing the common ancestors of multicellular animals. To fill the gaps completely, we must address the issue of the origin of novel structures. Co-option, the redeployment of an existing gene or organ to a new developmental context, is a key concept that enables simple explanations for the evolution of novelty (True and Carroll, 2002). The Dlx genes have been deployed repeatedly for the evolution of body wall outgrowths (Panganiban et al., 1997). The co-option of a single wingless gene was suggested to be sufficient for a novel wing color pattern in Drosophila species (Werner et al., 2010). As Darwin pointed out, however, understanding how the evolution of novel organs of extreme perfection and complication is achieved is difficult, referring to the vertebrate eye as one of the difficulties with his theory (Darwin, 1859). If the acquisition of a novel structure requires multiple evolutionary steps, how do creatures overcome the intermediate steps, which are apparently less adaptive (or neutral)? This article discusses how the echinoderm pluteus larva is a good system for addressing the issue of the evolution of novel structures. Echinoderms have two types of larva: pluteus and auricularia types. The former, which are seen in sea urchins and brittle stars, possess well-developed skeletons that help to extend the larval arms. The latter type is almost devoid of a larval skeleton, and so the larval arms are not supported by a skeleton. The latter type is seen in starfish and sea cucumbers, and 10 years ago, a species of sea lily was * Correspondence to: Hiroyuki Koga, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai, Tsukuba 305– 8572, Japan. E-mail: [email protected] Published online 18 February 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/dvg.22758
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FIG. 1. Phylogenetic framework of the echinoderm larvae. Sea urchins and brittle stars have pluteus larvae with developed skeletons, whereas starfish and sea cucumbers have auricularia larvae. The basal echinoderm (the sea lily) and the sister group of echinoderms (acorn worms) have auricularia larvae, which are regarded as the ancestral type.
reported to have auricularia-type larva (Nakano et al., 2003). Because the sister group of echinoderms (hemichordates) and the basal group of echinoderms (the sea lily) have auricularia-type larvae, the pluteus type is regarded as a derived state (Fig. 1). One key event in the evolutionary transition from auricularia type to pluteus type is the acquisition of a larval skeleton. CO-OPTION OF THE GENETIC MACHINERY FOR SKELETOGENESIS The larval spicule does not develop, or is possibly degenerative, in auricularia-type larvae. No larval spicule is observed in starfish or hemichordates. Although a small spicule(s) exists in the posterior part of the sea cucumber larvae, it might represent a secondarily derived state from a pluteus form, as noted below. Conversely, all echinoderm species possess a calcitic endoskeleton called the stereom in adults. Among echinoderm characteristics, such as the pentaradial body plan and water vascular system, the endoskeleton of adults is the oldest character shared by extinct species. The basal group of echinoderms (stylophorans) is classified as echinoderms because of their stereom, although they lack the pentaradial body plan and water vascular system (Clausen and Smith, 2005; Smith, 2005). In sea urchins, the larval skeletons are usually derived from primary mesenchyme cells (PMCs), while Yajima (2007) revealed that PMCs do not contribute adult skeletal elements, indicating that larval and adult skeletons are derived from distinct cell populations. Nevertheless, structural and chemical similarities exist between the larval and adult skeletons of sea urchins (Berman et al., 1993; Killian and Wilt, 1989; Killian et al., 2010; Kitajima et al., 1996; Livingston et al., 2006; Mann et al., 2008a, 2008b, 2010; Richardson et al., 1989). Gao and Davidson (2008) showed that several transcription
factors are expressed in common during larval and adult skeletogenesis in sea urchins. These findings suggested that acquisition of the larval skeleton is a product of cooption of the adult skeletogenic machinery into larval cells (Ettensohn, 2009; Gao and Davidson, 2008; Sharma and Ettensohn, 2010). Here, to document the similarity of skeletogenesis between adults and pluteus larvae, we describe adult skeletogenesis in starfish in more detail. SKELETOGENESIS DURING METAMORPHOSIS IN STARFISH The starfish Patiria (Asterina) pectinifera undergoes typical indirect development, undergoing two larval stages: bipinnaria and brachiolaria. Adult spicules begin to form before metamorphosis. At the onset of metamorphosis, larvae already possess 11 large spicules and many small spicules in the adult rudiment (Hamanaka et al., 2011; Hyman, 1955). When the adult rudiment is observed from the future aboral side, the large spicules align in a concentric pattern: five on the outside, another five more internally, and one in the center (Fig. 2e; Hamanaka et al., 2011). In juveniles, the outer spicules compose the tips of the star arms, whereas the inner and center spicules develop into the aboral ossicles, including the hydropore (Hyman, 1955). Here, we briefly describe how this pattern of spicules becomes established. The first sign of spiculogenesis is observed in 1week-old bipinnaria larvae (Fig. 2a). Mineralization is observed beside the left somatocoel as tiny deposits of calcite. As the larva develops, more spicules arise and grow into a mesh-like arrangement along the left side and then the right side of the stomach, resulting in two rows of spicules (Fig. 2b,c). The fact that spicules form on the left first might reflect the developmental progress of the somatocoel; the left somatocoel expands toward
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FIG. 2. Spiculogenesis in the starfish Patiria (Asterina) pectinifera. (a–d) Dorsal confocal images of starfish larva that were raised in artificial seawater containing calcein: (a) tiny spicules (arrowheads) stained by calcein were observed along the left somatocoel of 1-week-old larvae; (b) the spicules grew in a branching manner, and new spicules arose along the right side of the stomach in late bipinnaria; (c) in early brachiolaria, a two-row pattern of major spicules was observed. The arrow indicates spicules located on the oral side; (d) in late brachiolaria, 11 major spicules have formed (eight are seen in the figure: the left three are outer spicules (arrowhead); the right five are inner spicules (arrow) and centric spicule (double arrow)); (e) view from adult aboral side of a late brachiolaria (lipid membranes were stained in magenta). Arrowheads indicate five outer spicules. (f) Round mesenchymal cells crowded around developed spicules in early brachiolaria larva. The scale bars represent 50 mm.
the right side to surround the stomach. Note that the collinear expression of sea urchin Hox genes was observed along the somatocoel from the left-hand side (ArenasMena et al., 2000). Clumps of round mesenchymal cells are observed around the growing spicules (Fig. 2f; Hamanaka et al., 2011), which is reminiscent of the larval skeletogenic mesenchymal cells in the sea urchin. Similar clumps of mesenchymal cells were observed surrounding the spicules in juvenile sea urchins and adult sea cucumbers (MacBride, 1903; Smith et al., 2008; Woodland, 1906). Finally, spicules from the left row become the five outer ones and spicules from the right row become the inner ones and center one (Fig. 2d,e). From a molecular perspective, some homologs of sea urchin skeletogenic genes have been reported to mark these skeletogenic cells in adult juvenile starfish, including Ets1, Alx1, Hex (Gao and Davidson, 2008), and vegfr (Fig. 3c–e; Morino et al., 2012). Perhaps under control of these genes, the effector genes of skeletogenesis, such as the carbonic anhydrase gene ApCA1, show specific expression in skeletogenic cells (Fig. 3a,b; Morino et al., 2012). COMPARISON OF GENE REGULATORY NETWORKS BETWEEN SEA URCHIN SKELETOGENESIS AND STARFISH MESODERM The above comparison of the gene regulatory machinery supports the idea that acquisition of the pluteus
larval skeleton was achieved via co-option of the genetic machinery for adult skeletogenesis. Therefore, to search for the key molecular events in co-option, we focused on the genes that are involved in both adult and pluteus larval skeletogenesis of the sea urchin, but not in the mesoderm differentiation of starfish larvae. Several research groups, including ours, have searched for the key molecules. Unexpectedly, however, most of the transcription factors involved in sea urchin larval skeletogenesis are also expressed in the mesoderm cells of starfish larvae, which do not develop a skeleton (Hinman and Davidson, 2007; Hinman et al., 2009; Koga et al., 2010; McCauley et al., 2010; Shoguchi et al., 2000). Vascular endothelial growth factor (VEGF) signaling is the only potential candidate responsible for the co-option so far (Morino et al., 2012). MOLECULAR ASPECTS OF CO-OPTION OF THE SKELETOGENIC MACHINERY In sea urchin larvae, the VEGF receptor is expressed in skeletogenic mesenchymal cells; the ligand is expressed mainly in epidermal cells adjacent to the skeletogenic mesenchymal cells and later in the tips of the larval arms toward which the skeletal rods elongate. Inhibition of VEGF signaling by either VEGF ligand or receptor led to a loss of skeleton (Duloquin et al., 2007). A detailed study by Adomako-Ankomah and Ettensohn
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FIG. 3. Gene expression correlated with spicules. (a) Gene expression pattern of ApCA1 from the aboral side of brachiolaria. The arrowheads indicate the staining along left side of stomach, whereas the arrows indicate the staining along right side of stomach. The double arrowhead indicates staining at oral side. The two-row pattern of spicules was detected consistently. Asterisks indicate nonspecific signals by dusts. (b–d) Gene expression pattern from the left lateral side of brachiolaria: (b) ApCA1 was expressed in clumps of mesenchymal cells located between the coelom and epidermis in a pentaradial pattern (arrowheads). (c) Apets1/2 was also expressed in mesenchymal cells within the adult rudiment. (d) Apalx1 was expressed in mesenchymes of the adult rudiment with a slightly more restricted manner. (e) Apvegfr was expressed in mesenchymal cells located between the coelom and epidermis. (f) Apvegf was expressed in the epidermis surrounding the stomach. The scale bars represent 100 mm.
(2013) provided evidence that VEGF signaling is required not only for initiating skeletogenesis but also for elongation of the skeletal rods toward animal poles. Morino et al. (2012) examined the expression of ligand and receptors in starfish and found that both were expressed during adult skeletogenesis in starfish, but not in larval development (Fig. 3e,f). VEGF expression was also observed in sea urchin adult skeletogenesis (Gao and Davidson, 2008). Therefore, VEGF is likely one of the key factors responsible for the acquisition of a larval skeleton. Notably, both the VEGF ligand and receptor are expressed during larval skeletogenesis in brittle stars (Morino et al., 2012). Because brittle stars are not phylogenetically close to sea urchins within echinoderms, as sea urchins are perhaps more closely related to sea cucumbers (Fig. 1; Janies, 2001; Littlewood et al., 1997; Paul and Smith, 1984; Wada and Satoh, 1994), the larval skeleton is thought to have been acquired independently in sea urchins and brittle stars (Smith, 1984). Consequently, the co-option of VEGF signaling must have occurred independently in brittle stars and sea urchins. Alternatively, the common ancestor of brittle stars and sea urchins acquired a larval skeleton, which was reduced secondarily in sea cucumbers. This alternative hypothesis is equally parsimonious with the hypothesis of independent acquisition. The hypothesis of secondary loss in sea cucumbers is favored if the activation of
VEGF signaling requires activation of receptors in skeletogenic cells as well as a ligand in adjacent ectoderm cells. This idea is more consistent with the discovery that Alx1 expression in larval mesenchyme cells is shared between sea urchins and sea cucumbers, but not starfish (McCauley et al., 2012). FROM “ESSENTIAL” TO “SUFFICIENT” We argue that the genetic regulatory network is similar between the skeletogenic mesoderm of sea urchins and the nonskeletogenic larval mesoderm of starfish. This similarity encourages us to perform trials to reproduce the evolutionary process by inducing development of a larval skeleton in starfish larvae. Using ascidians, Abitua et al. (2012) recently reported a notable study in which they induced a motile neural crest-like cell by ectopically expressing a single transcription factor, twist, in melanocytes. Freitas et al. (2012) succeeded in inducing an autopod-like structure in the zebrafish fin via the forced expression of Hoxd13. Similarly, we can induce the ectopic expression of genetic material lacking for skeletogenesis in the starfish embryo. In this case, the VEGF ligand and receptor is an immediate candidate for this strategy. A series of comparative developmental studies provided a list of essential conditions for acquisition of the novel structure, but this “synthetic experimental evolution” (Erwin and Davidson, 2009)
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FIG. 4. Evolutionary scheme for the activation of VEGF signaling. To activate VEGF signaling, the simultaneous expression of ligand and receptor genes is required. (a) Assuming that ectopic expression of the ligand or receptor alone is neutral, it can be retained as a variant in a population by chance. Sexual reproduction might create a combination of VEGF- and VEGFr-expression variants, so that individuals in which VEGF signaling is activated can occur. (b) If the ectopic expression of either the ligand or receptor is adaptive, it can spread among a population by selection. After fixation of the ligand or receptor, VEGF signaling can be activated by a change in the other.
approach tells us that sufficient steps exist for the novel structure. A MODEL FOR MULTISTEP EVOLUTION Determining how many elementary steps are sufficient for a certain form of morphological evolution would be insightful. However, the final goal is not to list the required steps for morphological evolution, but to resolve Darwin’s dilemma: how to overcome an intermediate stage that is considered to be less adaptive. When thinking about the larval activation of VEGF signaling in sea urchin larvae, one may reasonably assume that an intermediate stage occurred in which either only the ligand was expressed in the epidermis or only the receptor was expressed in mesenchyme cells, so that VEGF signaling was not activated. Two possible explanations can be considered. First, the expression of either the ligand or receptor is almost neutral or at least not seriously deleterious and can consequently be retained for some duration to await activation of the counterpart. Most echinoderm species have large populations, which should harbor genetic variation, as reported in the sea urchin (Garfield et al., 2012, 2013; Pespeni et al., 2012, 2013a,b). In addition, the activation of the counterpart did not have to occur in the same individuals, as the activation of VEGF signaling can be achieved via sexual reproduction (Fig. 4a). Second, if either the ligand or receptor is adaptive, it can spread in a population more rapidly (Fig. 4b). This idea is testable in echinoderm species in which mRNA is easily injected, so an intermediate stage can be
mimicked relatively easily. Recent advances in nextgeneration sequencing will enable us to detect fractional gene expression, even if no morphological phenotype is observed. ECHINODERMS AS A MODEL FOR EVOLUTION Finally, we discuss the advantageous features of echinoderms for studying evolutionary development, such as their utility in studying experimental embryology, various morphologies, and availability of natural populations. In terms of embryology, echinoderms, especially sea urchins and starfish, are among the best-studied marine invertebrates. Therefore, we benefit from detailed information and elaborate techniques of experimental embryology. On this basis, studies have been conducted using various classes of echinoderm (Dupont et al., 2009; Hara et al., 2006; Hirokawa et al., 2008; Koga et al., 2010; McCauley et al., 2012; Morino et al., 2012). Furthermore, echinoderm embryology has been investigated using individuals from natural populations, while studies of other model animals have mostly involved laboratory strains. This allows us to combine population genetics and embryology readily (Garfield et al., 2013; Pespeni et al., 2013a). As we discussed above, the variation in gene expression within a population might be an important condition for achieving multistep evolution. To test this idea, echinoderms offer an excellent system. In Ontogeny and Phylogeny, Gould (1977) wrote, “There may be nothing new under the sun, but permutation of the old within complex systems can do
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wonders. . . This is the chief joy of our science.” How does nature allow permutations? Perhaps the key issue is how creatures overcome transitory, intermediate stages. To study this issue, we believe that echinoderms provide a model experimental system. LITERATURE CITED Abitua PB, Wagner E, Navarrete IA, Levine M. 2012. Identification of a rudimentary neural crest in a non-vertebrate chordate. Nature 492:104–107. Adomako-Ankomah A, Ettensohn CA. 2013. Growth factor-mediated mesodermal cell guidance and skeletogenesis during sea urchin gastrulation. Development 140:4214–4225. Arenas-Mena CS, Cameron AR, Davidson EH. 2000. Spatial expression of Hox cluster genes in the ontogeny of a sea urchin. Development 127:4631–4643. Berman A, Hanson J, Leiserowitz L, Koetzle TF, Weiner S, Addadi L. 1993. Biological control of crystal texture: A widespread strategy for adapting crystal properties to function. Science 259:776–779. Carroll SB, Grenier JK, Weatherbee SD. 2010. From DNA to diversity: Molecular genetics and the evolution of animal design. Malden: Blackwell. Clausen S, Smith AB. 2005. Palaeoanatomy and biological affinities of a Cambrian deuterostome (Stylophora). Nature 438:351–354. Darwin C. 1859. On the origin of species by means of natural selection: Or the preservation of favored races in the struggle for life. London: John Murray. Denes AS, Jekely G, Steinmetz PR, Raible F, Snyman H, Prud’homme B, Ferrier DE, Balavoine G, Arendt D. 2007. Molecular architecture of annelid nerve cord supports common origin of nervous system centralization in bilateria. Cell 129:277–288. Duloquin L, Lhomond G, Gache C. 2007. Localized VEGF signaling from ectoderm to mesenchyme cells controls morphogenesis of the sea urchin embryo skeleton. Development 134:2293–2302. Dupont S, Thorndyke W, Thorndyke MC, Burke RD. 2009. Neural development of the brittlestar Amphiura filiformis. Dev Genes Evol 219:159–166. Erwin DH, Davidson EH. 2009. The evolution of hierarchical gene regulatory networks. Nat Rev Genet 10:141–148. Ettensohn CA. 2009. Lessons from a gene regulatory network: Echinoderm skeletogenesis provides insights into evolution, plasticity and morphogenesis. Development 136:11–21. Freitas R, G omez-Marın C, Wilson Jonathan M, Casares F, G omez-Skarmeta Jose L. 2012. Hoxd13 contribution to the evolution of vertebrate appendages. Dev Cell 23:1219–1229. Gao F, Davidson EH. 2008. Transfer of a large gene regulatory apparatus to a new developmental address in
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echinoid evolution. Proc Natl Acad Sci USA 105: 6091–6096. Garfield D, Haygood R, Nielsen WJ, Wray GA. 2012. Population genetics of cis-regulatory sequences that operate during embryonic development in the sea urchin Strongylocentrotus purpuratus. Evol Dev 14:152–167. Garfield DA, Runcie DE, Babbitt CC, Haygood R, Nielsen WJ, Wray GA. 2013. The impact of gene expression variation on the robustness and evolvability of a developmental gene regulatory network. PLoS Biol 11:e1001696. Gould SJ. 1977. Ontogeny and phylogeny. Cambridge: Belknap Press of Harvard University Press. Hamanaka G, Hosaka E, Kuraishi R, Hosoya N, Matsumoto M, Kaneko H. 2011. Uneven distribution pattern and increasing numbers of mesenchyme cells during development in the starfish, Asterina pectinifera. Dev Growth Differ 53:440–449. Hara Y, Yamaguchi M, Akasaka K, Nakano H, Nonaka M, Amemiya S. 2006. Expression patterns of Hox genes in larvae of the sea lily Metacrinus rotundus. Dev Genes Evol 216:797–809. Hinman VF, Davidson EH. 2007. Evolutionary plasticity of developmental gene regulatory network architecture. Proc Natl Acad Sci USA 104:19404–19409. Hinman VF, Yankura KA, McCauley BS. 2009. Evolution of gene regulatory network architectures: Examples of subcircuit conservation and plasticity between classes of echinoderms. Biochim Biophys Acta 1789:326–332. Hirokawa T, Komatsu M, Nakajima Y. 2008. Development of the nervous system in the brittle star Amphipholis kochii. Dev Genes Evol 218:15–21. Hyman LH. 1955. The invertebrates: Echinodermata: The coelomate bilateria, Vol. IV. New York: McGraw-Hill Book C. Janies D. 2001. Phylogenetic relationships of extant echinoderm classes. Can J Zool 79:1232–1250. Killian CE, Croker L, Wilt FH. 2010. SpSM30 gene family expression patterns in embryonic and adult biomineralized tissues of the sea urchin, Strongylocentrotus purpuratus. Gene Expr Patterns 10:135–139. Killian CE, Wilt FH. 1989. The accumulation and translation of a spicule matrix protein mRNA during sea urchin embryo development. Dev Biol 133:148– 156. Kitajima T, Tomita M, Killian CE, Akasaka K, Wilt FH. 1996. Expression of spicule matrix protein gene SM30 in embryonic and adult mineralized tissues of sea urchin Hemicentrotus pulcherrimus. Dev Growth Differ 38:687–695. Koga H, Matsubara M, Fujitani H, Miyamoto N, Komatsu M, Kiyomoto M, Akasaka K, Wada H. 2010. Functional evolution of Ets in echinoderms with focus
192
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on the evolution of echinoderm larval skeletons. Dev Genes Evol 220:107–115. Littlewood DTJ, Smith AB, Clough KA, Emson RH. 1997. The interrelationships of the echinoderm classes: Morphological andmolecular evidence. Biol J Linn Soc 61:409–438. Livingston BT, Killian CE, Wilt F, Cameron A, Landrum MJ, Ermolaeva O, Sapojnikov V, Maglott DR, Buchanan AM, Ettensohn CA. 2006. A genome-wide analysis of biomineralization-related proteins in the sea urchin Strongylocentrotus purpuratus. Dev Biol 300:335–348. MacBride E W. 1903. The development of Echinus esculentus, together with some points in the development of E. miliaris and E. acutus. Philos Trans R Soc Lond B Biol Sci 195:285–327. Mann K, Poustka AJ, Mann M. 2008a. In-depth, highaccuracy proteomics of sea urchin tooth organic matrix. Proteome Sci 6:33.. Mann K, Poustka AJ, Mann M. 2008b. The sea urchin (Strongylocentrotus purpuratus) test and spine proteomes. Proteome Sci 6:22. Mann K, Wilt FH, Poustka AJ. 2010. Proteomic analysis of sea urchin (Strongylocentrotus purpuratus) spicule matrix. Proteome Sci 8:33. McCauley BS, Weideman EP, Hinman VF. 2010. A conserved gene regulatory network subcircuit drives different developmental fates in the vegetal pole of highly divergent echinoderm embryos. Dev Biol 340:200–208. McCauley BS, Wright EP, Exner C, Kitazawa C, Hinman VF. 2012. Development of an embryonic skeletogenic mesenchyme lineage in a sea cucumber reveals the trajectory of change for the evolution of novel structures in echinoderms. EvoDevo 3:1–11. Morino Y, Koga H, Tachibana K, Shoguchi E, Kiyomoto M, Wada H. 2012. Heterochronic activation of VEGF signaling and the evolution of the skeleton in echinoderm pluteus larvae. Evol Dev 14:428–436. Nakano H, Hibino T, Oji T, Hara Y, Amemiya S. 2003. Larval stages of a living sea lily (stalked crinoid echinoderm). Nature 421:158–160. Panganiban G, Irvine SM, Lowe C, Roehl H, Corley LS, Sherbon B, Grenier JK, Fallon JF, Kimble J, Walker M, Wray GA, Swalla BJ, Martindale MQ, Carroll SB. 1997. The origin and evolution of animal appendages. Proc Natl Acad Sci USA 94:5162–5166. Paul CRC, Smith AB. 1984. The early radiation and phylogeny of echinoderms. Biol Rev 59:443–481. Peck AL. 1965. Aristotle history of animals books I–III. Cambridge: Harvard University Press. Peck AL. 1970. Aristotle history of animals books IV–VI. Cambridge: Harvard University Press.
Peck AL. 1991. Aristotle history of animals books VII–X. Cambridge: Harvard University Press. Pespeni MH, Barney BT, Palumbi SR. 2013a. Differences in the regulation of growth and biomineralization genes revealed through long-term common-garden acclimation and experimental genomics in the purple sea urchin. Evolution 67:1901–1914. Pespeni MH, Garfield DA, Manier MK, Palumbi SR. 2012. Genome-wide polymorphisms show unexpected targets of natural selection. Proc Biol Sci 279:1412–1420. Pespeni MH, Sanford E, Gaylord B, Hill TM, Hosfelt JD, Jaris HK, LaVigne M, Lenz EA, Russell AD, Young MK, Palumbi SR. 2013b. Evolutionary change during experimental ocean acidification. Proc Natl Acad Sci USA 110:6937–6942. Richardson W, Kitajima T, Wilt F, Benson S. 1989. Expression of an embryonic spicule matrix gene in calcified tissues of adult sea urchins. Dev Biol 132: 266–269. Sasai Y, De Robertis EM. 1997. Ectodermal patterning in vertebrate embryos. Dev Biol 182:5–20. Sharma T, Ettensohn CA. 2010. Activation of the skeletogenic gene regulatory network in the early sea urchin embryo. Development 137:1149–1157. Shoguchi E, Satoh N, Maruyama YK. 2000. A starfish homolog of mouse T-brain-1 is expressed in the archenteron of Asterina pectinifera embryos: Possible involvement of two T-box genes in starfish gastrulation. Dev Growth Differ 42:61–68. Smith AB. 1984. Classification of the echinodermata. Palaeontology 27:431–459. Smith AB. 2005. The pre-radial history of echinoderms. Geol J 40:255–280. Smith MM, Cruz Smith L, Cameron RA, Urry LA. 2008. The larval stages of the sea urchin, Strongylocentrotus purpuratus. J Morphol 269:713–733. True JR, Carroll SB. 2002. Gene co-option in physiological and morphological evolution. Annu Rev Cell Dev Biol 18:53–80. Wada H, Satoh N. 1994. Phylogenetic relationships among extant classes of echinoderms, as inferred from sequences of 18s rdna, coincide with relationships deduced from the fossil record. J Mol Evol 38: 41–49. Werner T, Koshikawa S, Williams TM, Carroll SB. 2010. Generation of a novel wing colour pattern by the Wingless morphogen. Nature 464:1143–1148. Woodland W. 1906. The Scleroblastic Development of the Spicules in Cucumariidae. Q J Microsc Sci 49: 533–559. Yajima M. 2007. A switch in the cellular basis of skeletogenesis in late-stage sea urchin larvae. Dev Biol 307: 272–281.
genesis 52:186–192 (2014)
REVIEW
The Echinoderm Larval Skeleton as a Possible Model System for Experimental Evolutionary Biology Hiroyuki Koga,* Yoshiaki Morino, and Hiroshi Wada Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai, Tsukuba, Japan Received 28 November 2013; Revised 12 February 2014; Accepted 14 February 2014
Summary: The evolution of various body plans results from the acquisition of novel structures as well as the loss of existing structures. Some novel structures necessitate multiple evolutionary steps, requiring organisms to overcome the intermediate steps, which might be less adaptive or neutral. To examine this issue, echinoderms might provide an ideal experimental system. A larval skeleton is acquired in some echinoderm lineages, such as sea urchins, probably via the co-option of the skeletogenic machinery that was already established to produce the adult skeleton. The acquisition of a larval skeleton was found to require multiple steps and so provides a model experimental system for reproducing intermediate evolutionary stages. The fact that echinoderm embryology has been studied with various natural populations also presents C 2014 Wiley Periodan advantage. genesis 52:186–192. V icals, Inc.
Key words: echinoderm; larval skeleton; evolution of a novel structure; co-option
Since Aristotle’s description of the incredible variation of animals in his History of Animals (Peck, 1965, 1970, 1991), more than 2000 years had passed before humans began to understand the diversity of life using the concept of evolution. What keeps people from accepting the concept of evolution? Perhaps one obstacle is the existence of large gaps between animal body plans. This issue is now being overcome by recent progress in research on the evolution of development. Based on conserved molecular genetic tools for building animal bodies, much commonality exists among genetic mechanisms for body construction (Carroll et al., 2010; Denes et al., 2007; Sasai and De Robertis, 1997). With the growing understanding of animal development modes as well as increasing paleontological evidence,
we are now filling the gaps and reconstructing the common ancestors of multicellular animals. To fill the gaps completely, we must address the issue of the origin of novel structures. Co-option, the redeployment of an existing gene or organ to a new developmental context, is a key concept that enables simple explanations for the evolution of novelty (True and Carroll, 2002). The Dlx genes have been deployed repeatedly for the evolution of body wall outgrowths (Panganiban et al., 1997). The co-option of a single wingless gene was suggested to be sufficient for a novel wing color pattern in Drosophila species (Werner et al., 2010). As Darwin pointed out, however, understanding how the evolution of novel organs of extreme perfection and complication is achieved is difficult, referring to the vertebrate eye as one of the difficulties with his theory (Darwin, 1859). If the acquisition of a novel structure requires multiple evolutionary steps, how do creatures overcome the intermediate steps, which are apparently less adaptive (or neutral)? This article discusses how the echinoderm pluteus larva is a good system for addressing the issue of the evolution of novel structures. Echinoderms have two types of larva: pluteus and auricularia types. The former, which are seen in sea urchins and brittle stars, possess well-developed skeletons that help to extend the larval arms. The latter type is almost devoid of a larval skeleton, and so the larval arms are not supported by a skeleton. The latter type is seen in starfish and sea cucumbers, and 10 years ago, a species of sea lily was * Correspondence to: Hiroyuki Koga, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai, Tsukuba 305– 8572, Japan. E-mail: [email protected] Published online 18 February 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/dvg.22758
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FIG. 1. Phylogenetic framework of the echinoderm larvae. Sea urchins and brittle stars have pluteus larvae with developed skeletons, whereas starfish and sea cucumbers have auricularia larvae. The basal echinoderm (the sea lily) and the sister group of echinoderms (acorn worms) have auricularia larvae, which are regarded as the ancestral type.
reported to have auricularia-type larva (Nakano et al., 2003). Because the sister group of echinoderms (hemichordates) and the basal group of echinoderms (the sea lily) have auricularia-type larvae, the pluteus type is regarded as a derived state (Fig. 1). One key event in the evolutionary transition from auricularia type to pluteus type is the acquisition of a larval skeleton. CO-OPTION OF THE GENETIC MACHINERY FOR SKELETOGENESIS The larval spicule does not develop, or is possibly degenerative, in auricularia-type larvae. No larval spicule is observed in starfish or hemichordates. Although a small spicule(s) exists in the posterior part of the sea cucumber larvae, it might represent a secondarily derived state from a pluteus form, as noted below. Conversely, all echinoderm species possess a calcitic endoskeleton called the stereom in adults. Among echinoderm characteristics, such as the pentaradial body plan and water vascular system, the endoskeleton of adults is the oldest character shared by extinct species. The basal group of echinoderms (stylophorans) is classified as echinoderms because of their stereom, although they lack the pentaradial body plan and water vascular system (Clausen and Smith, 2005; Smith, 2005). In sea urchins, the larval skeletons are usually derived from primary mesenchyme cells (PMCs), while Yajima (2007) revealed that PMCs do not contribute adult skeletal elements, indicating that larval and adult skeletons are derived from distinct cell populations. Nevertheless, structural and chemical similarities exist between the larval and adult skeletons of sea urchins (Berman et al., 1993; Killian and Wilt, 1989; Killian et al., 2010; Kitajima et al., 1996; Livingston et al., 2006; Mann et al., 2008a, 2008b, 2010; Richardson et al., 1989). Gao and Davidson (2008) showed that several transcription
factors are expressed in common during larval and adult skeletogenesis in sea urchins. These findings suggested that acquisition of the larval skeleton is a product of cooption of the adult skeletogenic machinery into larval cells (Ettensohn, 2009; Gao and Davidson, 2008; Sharma and Ettensohn, 2010). Here, to document the similarity of skeletogenesis between adults and pluteus larvae, we describe adult skeletogenesis in starfish in more detail. SKELETOGENESIS DURING METAMORPHOSIS IN STARFISH The starfish Patiria (Asterina) pectinifera undergoes typical indirect development, undergoing two larval stages: bipinnaria and brachiolaria. Adult spicules begin to form before metamorphosis. At the onset of metamorphosis, larvae already possess 11 large spicules and many small spicules in the adult rudiment (Hamanaka et al., 2011; Hyman, 1955). When the adult rudiment is observed from the future aboral side, the large spicules align in a concentric pattern: five on the outside, another five more internally, and one in the center (Fig. 2e; Hamanaka et al., 2011). In juveniles, the outer spicules compose the tips of the star arms, whereas the inner and center spicules develop into the aboral ossicles, including the hydropore (Hyman, 1955). Here, we briefly describe how this pattern of spicules becomes established. The first sign of spiculogenesis is observed in 1week-old bipinnaria larvae (Fig. 2a). Mineralization is observed beside the left somatocoel as tiny deposits of calcite. As the larva develops, more spicules arise and grow into a mesh-like arrangement along the left side and then the right side of the stomach, resulting in two rows of spicules (Fig. 2b,c). The fact that spicules form on the left first might reflect the developmental progress of the somatocoel; the left somatocoel expands toward
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FIG. 2. Spiculogenesis in the starfish Patiria (Asterina) pectinifera. (a–d) Dorsal confocal images of starfish larva that were raised in artificial seawater containing calcein: (a) tiny spicules (arrowheads) stained by calcein were observed along the left somatocoel of 1-week-old larvae; (b) the spicules grew in a branching manner, and new spicules arose along the right side of the stomach in late bipinnaria; (c) in early brachiolaria, a two-row pattern of major spicules was observed. The arrow indicates spicules located on the oral side; (d) in late brachiolaria, 11 major spicules have formed (eight are seen in the figure: the left three are outer spicules (arrowhead); the right five are inner spicules (arrow) and centric spicule (double arrow)); (e) view from adult aboral side of a late brachiolaria (lipid membranes were stained in magenta). Arrowheads indicate five outer spicules. (f) Round mesenchymal cells crowded around developed spicules in early brachiolaria larva. The scale bars represent 50 mm.
the right side to surround the stomach. Note that the collinear expression of sea urchin Hox genes was observed along the somatocoel from the left-hand side (ArenasMena et al., 2000). Clumps of round mesenchymal cells are observed around the growing spicules (Fig. 2f; Hamanaka et al., 2011), which is reminiscent of the larval skeletogenic mesenchymal cells in the sea urchin. Similar clumps of mesenchymal cells were observed surrounding the spicules in juvenile sea urchins and adult sea cucumbers (MacBride, 1903; Smith et al., 2008; Woodland, 1906). Finally, spicules from the left row become the five outer ones and spicules from the right row become the inner ones and center one (Fig. 2d,e). From a molecular perspective, some homologs of sea urchin skeletogenic genes have been reported to mark these skeletogenic cells in adult juvenile starfish, including Ets1, Alx1, Hex (Gao and Davidson, 2008), and vegfr (Fig. 3c–e; Morino et al., 2012). Perhaps under control of these genes, the effector genes of skeletogenesis, such as the carbonic anhydrase gene ApCA1, show specific expression in skeletogenic cells (Fig. 3a,b; Morino et al., 2012). COMPARISON OF GENE REGULATORY NETWORKS BETWEEN SEA URCHIN SKELETOGENESIS AND STARFISH MESODERM The above comparison of the gene regulatory machinery supports the idea that acquisition of the pluteus
larval skeleton was achieved via co-option of the genetic machinery for adult skeletogenesis. Therefore, to search for the key molecular events in co-option, we focused on the genes that are involved in both adult and pluteus larval skeletogenesis of the sea urchin, but not in the mesoderm differentiation of starfish larvae. Several research groups, including ours, have searched for the key molecules. Unexpectedly, however, most of the transcription factors involved in sea urchin larval skeletogenesis are also expressed in the mesoderm cells of starfish larvae, which do not develop a skeleton (Hinman and Davidson, 2007; Hinman et al., 2009; Koga et al., 2010; McCauley et al., 2010; Shoguchi et al., 2000). Vascular endothelial growth factor (VEGF) signaling is the only potential candidate responsible for the co-option so far (Morino et al., 2012). MOLECULAR ASPECTS OF CO-OPTION OF THE SKELETOGENIC MACHINERY In sea urchin larvae, the VEGF receptor is expressed in skeletogenic mesenchymal cells; the ligand is expressed mainly in epidermal cells adjacent to the skeletogenic mesenchymal cells and later in the tips of the larval arms toward which the skeletal rods elongate. Inhibition of VEGF signaling by either VEGF ligand or receptor led to a loss of skeleton (Duloquin et al., 2007). A detailed study by Adomako-Ankomah and Ettensohn
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FIG. 3. Gene expression correlated with spicules. (a) Gene expression pattern of ApCA1 from the aboral side of brachiolaria. The arrowheads indicate the staining along left side of stomach, whereas the arrows indicate the staining along right side of stomach. The double arrowhead indicates staining at oral side. The two-row pattern of spicules was detected consistently. Asterisks indicate nonspecific signals by dusts. (b–d) Gene expression pattern from the left lateral side of brachiolaria: (b) ApCA1 was expressed in clumps of mesenchymal cells located between the coelom and epidermis in a pentaradial pattern (arrowheads). (c) Apets1/2 was also expressed in mesenchymal cells within the adult rudiment. (d) Apalx1 was expressed in mesenchymes of the adult rudiment with a slightly more restricted manner. (e) Apvegfr was expressed in mesenchymal cells located between the coelom and epidermis. (f) Apvegf was expressed in the epidermis surrounding the stomach. The scale bars represent 100 mm.
(2013) provided evidence that VEGF signaling is required not only for initiating skeletogenesis but also for elongation of the skeletal rods toward animal poles. Morino et al. (2012) examined the expression of ligand and receptors in starfish and found that both were expressed during adult skeletogenesis in starfish, but not in larval development (Fig. 3e,f). VEGF expression was also observed in sea urchin adult skeletogenesis (Gao and Davidson, 2008). Therefore, VEGF is likely one of the key factors responsible for the acquisition of a larval skeleton. Notably, both the VEGF ligand and receptor are expressed during larval skeletogenesis in brittle stars (Morino et al., 2012). Because brittle stars are not phylogenetically close to sea urchins within echinoderms, as sea urchins are perhaps more closely related to sea cucumbers (Fig. 1; Janies, 2001; Littlewood et al., 1997; Paul and Smith, 1984; Wada and Satoh, 1994), the larval skeleton is thought to have been acquired independently in sea urchins and brittle stars (Smith, 1984). Consequently, the co-option of VEGF signaling must have occurred independently in brittle stars and sea urchins. Alternatively, the common ancestor of brittle stars and sea urchins acquired a larval skeleton, which was reduced secondarily in sea cucumbers. This alternative hypothesis is equally parsimonious with the hypothesis of independent acquisition. The hypothesis of secondary loss in sea cucumbers is favored if the activation of
VEGF signaling requires activation of receptors in skeletogenic cells as well as a ligand in adjacent ectoderm cells. This idea is more consistent with the discovery that Alx1 expression in larval mesenchyme cells is shared between sea urchins and sea cucumbers, but not starfish (McCauley et al., 2012). FROM “ESSENTIAL” TO “SUFFICIENT” We argue that the genetic regulatory network is similar between the skeletogenic mesoderm of sea urchins and the nonskeletogenic larval mesoderm of starfish. This similarity encourages us to perform trials to reproduce the evolutionary process by inducing development of a larval skeleton in starfish larvae. Using ascidians, Abitua et al. (2012) recently reported a notable study in which they induced a motile neural crest-like cell by ectopically expressing a single transcription factor, twist, in melanocytes. Freitas et al. (2012) succeeded in inducing an autopod-like structure in the zebrafish fin via the forced expression of Hoxd13. Similarly, we can induce the ectopic expression of genetic material lacking for skeletogenesis in the starfish embryo. In this case, the VEGF ligand and receptor is an immediate candidate for this strategy. A series of comparative developmental studies provided a list of essential conditions for acquisition of the novel structure, but this “synthetic experimental evolution” (Erwin and Davidson, 2009)
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FIG. 4. Evolutionary scheme for the activation of VEGF signaling. To activate VEGF signaling, the simultaneous expression of ligand and receptor genes is required. (a) Assuming that ectopic expression of the ligand or receptor alone is neutral, it can be retained as a variant in a population by chance. Sexual reproduction might create a combination of VEGF- and VEGFr-expression variants, so that individuals in which VEGF signaling is activated can occur. (b) If the ectopic expression of either the ligand or receptor is adaptive, it can spread among a population by selection. After fixation of the ligand or receptor, VEGF signaling can be activated by a change in the other.
approach tells us that sufficient steps exist for the novel structure. A MODEL FOR MULTISTEP EVOLUTION Determining how many elementary steps are sufficient for a certain form of morphological evolution would be insightful. However, the final goal is not to list the required steps for morphological evolution, but to resolve Darwin’s dilemma: how to overcome an intermediate stage that is considered to be less adaptive. When thinking about the larval activation of VEGF signaling in sea urchin larvae, one may reasonably assume that an intermediate stage occurred in which either only the ligand was expressed in the epidermis or only the receptor was expressed in mesenchyme cells, so that VEGF signaling was not activated. Two possible explanations can be considered. First, the expression of either the ligand or receptor is almost neutral or at least not seriously deleterious and can consequently be retained for some duration to await activation of the counterpart. Most echinoderm species have large populations, which should harbor genetic variation, as reported in the sea urchin (Garfield et al., 2012, 2013; Pespeni et al., 2012, 2013a,b). In addition, the activation of the counterpart did not have to occur in the same individuals, as the activation of VEGF signaling can be achieved via sexual reproduction (Fig. 4a). Second, if either the ligand or receptor is adaptive, it can spread in a population more rapidly (Fig. 4b). This idea is testable in echinoderm species in which mRNA is easily injected, so an intermediate stage can be
mimicked relatively easily. Recent advances in nextgeneration sequencing will enable us to detect fractional gene expression, even if no morphological phenotype is observed. ECHINODERMS AS A MODEL FOR EVOLUTION Finally, we discuss the advantageous features of echinoderms for studying evolutionary development, such as their utility in studying experimental embryology, various morphologies, and availability of natural populations. In terms of embryology, echinoderms, especially sea urchins and starfish, are among the best-studied marine invertebrates. Therefore, we benefit from detailed information and elaborate techniques of experimental embryology. On this basis, studies have been conducted using various classes of echinoderm (Dupont et al., 2009; Hara et al., 2006; Hirokawa et al., 2008; Koga et al., 2010; McCauley et al., 2012; Morino et al., 2012). Furthermore, echinoderm embryology has been investigated using individuals from natural populations, while studies of other model animals have mostly involved laboratory strains. This allows us to combine population genetics and embryology readily (Garfield et al., 2013; Pespeni et al., 2013a). As we discussed above, the variation in gene expression within a population might be an important condition for achieving multistep evolution. To test this idea, echinoderms offer an excellent system. In Ontogeny and Phylogeny, Gould (1977) wrote, “There may be nothing new under the sun, but permutation of the old within complex systems can do
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wonders. . . This is the chief joy of our science.” How does nature allow permutations? Perhaps the key issue is how creatures overcome transitory, intermediate stages. To study this issue, we believe that echinoderms provide a model experimental system. LITERATURE CITED Abitua PB, Wagner E, Navarrete IA, Levine M. 2012. Identification of a rudimentary neural crest in a non-vertebrate chordate. Nature 492:104–107. Adomako-Ankomah A, Ettensohn CA. 2013. Growth factor-mediated mesodermal cell guidance and skeletogenesis during sea urchin gastrulation. Development 140:4214–4225. Arenas-Mena CS, Cameron AR, Davidson EH. 2000. Spatial expression of Hox cluster genes in the ontogeny of a sea urchin. Development 127:4631–4643. Berman A, Hanson J, Leiserowitz L, Koetzle TF, Weiner S, Addadi L. 1993. Biological control of crystal texture: A widespread strategy for adapting crystal properties to function. Science 259:776–779. Carroll SB, Grenier JK, Weatherbee SD. 2010. From DNA to diversity: Molecular genetics and the evolution of animal design. Malden: Blackwell. Clausen S, Smith AB. 2005. Palaeoanatomy and biological affinities of a Cambrian deuterostome (Stylophora). Nature 438:351–354. Darwin C. 1859. On the origin of species by means of natural selection: Or the preservation of favored races in the struggle for life. London: John Murray. Denes AS, Jekely G, Steinmetz PR, Raible F, Snyman H, Prud’homme B, Ferrier DE, Balavoine G, Arendt D. 2007. Molecular architecture of annelid nerve cord supports common origin of nervous system centralization in bilateria. Cell 129:277–288. Duloquin L, Lhomond G, Gache C. 2007. Localized VEGF signaling from ectoderm to mesenchyme cells controls morphogenesis of the sea urchin embryo skeleton. Development 134:2293–2302. Dupont S, Thorndyke W, Thorndyke MC, Burke RD. 2009. Neural development of the brittlestar Amphiura filiformis. Dev Genes Evol 219:159–166. Erwin DH, Davidson EH. 2009. The evolution of hierarchical gene regulatory networks. Nat Rev Genet 10:141–148. Ettensohn CA. 2009. Lessons from a gene regulatory network: Echinoderm skeletogenesis provides insights into evolution, plasticity and morphogenesis. Development 136:11–21. Freitas R, G omez-Marın C, Wilson Jonathan M, Casares F, G omez-Skarmeta Jose L. 2012. Hoxd13 contribution to the evolution of vertebrate appendages. Dev Cell 23:1219–1229. Gao F, Davidson EH. 2008. Transfer of a large gene regulatory apparatus to a new developmental address in
191
echinoid evolution. Proc Natl Acad Sci USA 105: 6091–6096. Garfield D, Haygood R, Nielsen WJ, Wray GA. 2012. Population genetics of cis-regulatory sequences that operate during embryonic development in the sea urchin Strongylocentrotus purpuratus. Evol Dev 14:152–167. Garfield DA, Runcie DE, Babbitt CC, Haygood R, Nielsen WJ, Wray GA. 2013. The impact of gene expression variation on the robustness and evolvability of a developmental gene regulatory network. PLoS Biol 11:e1001696. Gould SJ. 1977. Ontogeny and phylogeny. Cambridge: Belknap Press of Harvard University Press. Hamanaka G, Hosaka E, Kuraishi R, Hosoya N, Matsumoto M, Kaneko H. 2011. Uneven distribution pattern and increasing numbers of mesenchyme cells during development in the starfish, Asterina pectinifera. Dev Growth Differ 53:440–449. Hara Y, Yamaguchi M, Akasaka K, Nakano H, Nonaka M, Amemiya S. 2006. Expression patterns of Hox genes in larvae of the sea lily Metacrinus rotundus. Dev Genes Evol 216:797–809. Hinman VF, Davidson EH. 2007. Evolutionary plasticity of developmental gene regulatory network architecture. Proc Natl Acad Sci USA 104:19404–19409. Hinman VF, Yankura KA, McCauley BS. 2009. Evolution of gene regulatory network architectures: Examples of subcircuit conservation and plasticity between classes of echinoderms. Biochim Biophys Acta 1789:326–332. Hirokawa T, Komatsu M, Nakajima Y. 2008. Development of the nervous system in the brittle star Amphipholis kochii. Dev Genes Evol 218:15–21. Hyman LH. 1955. The invertebrates: Echinodermata: The coelomate bilateria, Vol. IV. New York: McGraw-Hill Book C. Janies D. 2001. Phylogenetic relationships of extant echinoderm classes. Can J Zool 79:1232–1250. Killian CE, Croker L, Wilt FH. 2010. SpSM30 gene family expression patterns in embryonic and adult biomineralized tissues of the sea urchin, Strongylocentrotus purpuratus. Gene Expr Patterns 10:135–139. Killian CE, Wilt FH. 1989. The accumulation and translation of a spicule matrix protein mRNA during sea urchin embryo development. Dev Biol 133:148– 156. Kitajima T, Tomita M, Killian CE, Akasaka K, Wilt FH. 1996. Expression of spicule matrix protein gene SM30 in embryonic and adult mineralized tissues of sea urchin Hemicentrotus pulcherrimus. Dev Growth Differ 38:687–695. Koga H, Matsubara M, Fujitani H, Miyamoto N, Komatsu M, Kiyomoto M, Akasaka K, Wada H. 2010. Functional evolution of Ets in echinoderms with focus
192
KOGA ET AL.
on the evolution of echinoderm larval skeletons. Dev Genes Evol 220:107–115. Littlewood DTJ, Smith AB, Clough KA, Emson RH. 1997. The interrelationships of the echinoderm classes: Morphological andmolecular evidence. Biol J Linn Soc 61:409–438. Livingston BT, Killian CE, Wilt F, Cameron A, Landrum MJ, Ermolaeva O, Sapojnikov V, Maglott DR, Buchanan AM, Ettensohn CA. 2006. A genome-wide analysis of biomineralization-related proteins in the sea urchin Strongylocentrotus purpuratus. Dev Biol 300:335–348. MacBride E W. 1903. The development of Echinus esculentus, together with some points in the development of E. miliaris and E. acutus. Philos Trans R Soc Lond B Biol Sci 195:285–327. Mann K, Poustka AJ, Mann M. 2008a. In-depth, highaccuracy proteomics of sea urchin tooth organic matrix. Proteome Sci 6:33.. Mann K, Poustka AJ, Mann M. 2008b. The sea urchin (Strongylocentrotus purpuratus) test and spine proteomes. Proteome Sci 6:22. Mann K, Wilt FH, Poustka AJ. 2010. Proteomic analysis of sea urchin (Strongylocentrotus purpuratus) spicule matrix. Proteome Sci 8:33. McCauley BS, Weideman EP, Hinman VF. 2010. A conserved gene regulatory network subcircuit drives different developmental fates in the vegetal pole of highly divergent echinoderm embryos. Dev Biol 340:200–208. McCauley BS, Wright EP, Exner C, Kitazawa C, Hinman VF. 2012. Development of an embryonic skeletogenic mesenchyme lineage in a sea cucumber reveals the trajectory of change for the evolution of novel structures in echinoderms. EvoDevo 3:1–11. Morino Y, Koga H, Tachibana K, Shoguchi E, Kiyomoto M, Wada H. 2012. Heterochronic activation of VEGF signaling and the evolution of the skeleton in echinoderm pluteus larvae. Evol Dev 14:428–436. Nakano H, Hibino T, Oji T, Hara Y, Amemiya S. 2003. Larval stages of a living sea lily (stalked crinoid echinoderm). Nature 421:158–160. Panganiban G, Irvine SM, Lowe C, Roehl H, Corley LS, Sherbon B, Grenier JK, Fallon JF, Kimble J, Walker M, Wray GA, Swalla BJ, Martindale MQ, Carroll SB. 1997. The origin and evolution of animal appendages. Proc Natl Acad Sci USA 94:5162–5166. Paul CRC, Smith AB. 1984. The early radiation and phylogeny of echinoderms. Biol Rev 59:443–481. Peck AL. 1965. Aristotle history of animals books I–III. Cambridge: Harvard University Press. Peck AL. 1970. Aristotle history of animals books IV–VI. Cambridge: Harvard University Press.
Peck AL. 1991. Aristotle history of animals books VII–X. Cambridge: Harvard University Press. Pespeni MH, Barney BT, Palumbi SR. 2013a. Differences in the regulation of growth and biomineralization genes revealed through long-term common-garden acclimation and experimental genomics in the purple sea urchin. Evolution 67:1901–1914. Pespeni MH, Garfield DA, Manier MK, Palumbi SR. 2012. Genome-wide polymorphisms show unexpected targets of natural selection. Proc Biol Sci 279:1412–1420. Pespeni MH, Sanford E, Gaylord B, Hill TM, Hosfelt JD, Jaris HK, LaVigne M, Lenz EA, Russell AD, Young MK, Palumbi SR. 2013b. Evolutionary change during experimental ocean acidification. Proc Natl Acad Sci USA 110:6937–6942. Richardson W, Kitajima T, Wilt F, Benson S. 1989. Expression of an embryonic spicule matrix gene in calcified tissues of adult sea urchins. Dev Biol 132: 266–269. Sasai Y, De Robertis EM. 1997. Ectodermal patterning in vertebrate embryos. Dev Biol 182:5–20. Sharma T, Ettensohn CA. 2010. Activation of the skeletogenic gene regulatory network in the early sea urchin embryo. Development 137:1149–1157. Shoguchi E, Satoh N, Maruyama YK. 2000. A starfish homolog of mouse T-brain-1 is expressed in the archenteron of Asterina pectinifera embryos: Possible involvement of two T-box genes in starfish gastrulation. Dev Growth Differ 42:61–68. Smith AB. 1984. Classification of the echinodermata. Palaeontology 27:431–459. Smith AB. 2005. The pre-radial history of echinoderms. Geol J 40:255–280. Smith MM, Cruz Smith L, Cameron RA, Urry LA. 2008. The larval stages of the sea urchin, Strongylocentrotus purpuratus. J Morphol 269:713–733. True JR, Carroll SB. 2002. Gene co-option in physiological and morphological evolution. Annu Rev Cell Dev Biol 18:53–80. Wada H, Satoh N. 1994. Phylogenetic relationships among extant classes of echinoderms, as inferred from sequences of 18s rdna, coincide with relationships deduced from the fossil record. J Mol Evol 38: 41–49. Werner T, Koshikawa S, Williams TM, Carroll SB. 2010. Generation of a novel wing colour pattern by the Wingless morphogen. Nature 464:1143–1148. Woodland W. 1906. The Scleroblastic Development of the Spicules in Cucumariidae. Q J Microsc Sci 49: 533–559. Yajima M. 2007. A switch in the cellular basis of skeletogenesis in late-stage sea urchin larvae. Dev Biol 307: 272–281.
