The evolution of the wnt pathway

Cold Spring Harbor Perspectives in Biology

Volumn 4, Issue 7, 2012, Pages 1-17

  Holstein, Thomas W ^a  

^a UNIVERSITY OF HEIDELBERG   (Germany)

Author keywords

[No Author keywords available]

Indexed keywords

ANIMALIA; EUKARYOTA; FUNGI; HEXAPODA; METAZOA; PROTISTA; VERTEBRATA;

WNT PROTEIN;

ANIMAL; ARTICLE; GENETICS; METABOLISM; MOLECULAR EVOLUTION; MORPHOGENESIS; SIGNAL TRANSDUCTION;

ANIMALS; BODY PATTERNING; EVOLUTION, MOLECULAR; SIGNAL TRANSDUCTION; WNT PROTEINS;

EID: 84862187981     PISSN: None     EISSN: 19430264     Source Type: Journal    
DOI: 10.1101/cshperspect.a007922     Document Type: Article

References (117)

1. Abedin M, King N. 2008. The premetazoan ancestry of cadherins. Science 319: 946-948.
2. Adamska M, Degnan SM, Green KM, Adamski M, Craigie A, Larroux C, Degnan BM. 2007. Wnt and TGF-b expression in the sponge Amphimedon queenslandica and the origin of metazoan embryonic patterning. PLoS ONE 2: e1031.
3. Adamska M, Larroux C, Adamski M, Green K, Lovas E, Koop D, Richards GS, Zwafink C, Degnan BM. 2010. Structure and expression of conserved Wnt pathway components in the demosponge Amphimedon queenslandica. Evol Dev 12: 494-518.
4. Agathon A, Thisse C, Thisse B. 2003. The molecular nature of the zebrafish tail organizer. Nature 424: 448-452.
5. Arendt D, Technau U, Wittbrodt J. 2001. Evolution of the bilaterian larval foregut. Nature 409: 81-85.
6. Augustin R, Franke A, Khalturin K, Kiko R, Siebert S, Hemmrich G, Bosch TC. 2006. Dickkopf related genes are components of the positional value gradient in Hydra. Dev Biol 296: 62-79.
7. Aulehla A, Wehrle C, Brand-Saberi B, Kemler R, Gossler A, Kanzler B, Herrmann BG. 2003. Wnt3a plays a major role in the segmentation clock controlling somitogenesis. Dev Cell 4: 395-406.
8. Baguna J, Riutort M. 2004. The dawn of bilaterian animals: The case of acoelomorph flatworms. Bioessays 26: 1046-1057.
9. Banziger C, Soldini D, Schutt C, Zipperlen P, Hausmann G, Basler K. 2006. Wntless, a conserved membrane protein dedicated to the secretion of Wnt proteins from signaling cells. Cell 125: 509-522.
10. Bartscherer K, Boutros M. 2008. Regulation of Wnt protein secretion and its role in gradient formation. EMBORep 9: 977-982.
11. Bartscherer K, Pelte N, Ingelfinger D, Boutros M. 2006. Secretion of Wnt ligands requires Evi, a conserved transmembrane protein. Cell 125: 523-533.
12. Broun M, Bode HR. 2002. Characterization of the head organizer in hydra. Development 129: 875-884.
13. Broun M, Gee L, Reinhardt B, Bode HR. 2005. Formation of the head organizer in Hydra involves the canonical Wnt pathway. Development 132: 2907-2916.
14. Browne EN. 1909. The production of new hydranths in Hydra by the insertion of small grafts. J Exp Zool 7: 1-37.
15. Brusca RC, Brusca GJ. 2002. Invertebrates. Sinauer, Sunderland, MA.
16. Chapman JA, Kirkness EF, Simakov O, Hampson SE, Mitros T, Weinmaier T, Rattei T, Balasubramanian PG, Borman J, Busam D, et al. 2010. The dynamic genome of Hydra. Nature 464: 592-596.
17. Cho SJ, Valles Y, Giani VC Jr, Seaver EC, Weisblat DA. 2010. Evolutionary dynamics of the wnt gene family: A lophotrochozoan perspective. Mol Biol Evol 27: 1645-1658.
18. Coates JC, Harwood AJ. 2001. Cell-cell adhesion and signal transduction during Dictyostelium development. J Cell Sci 114: 4349-4358.
19. Coates JC, Grimson MJ, Williams RS, Bergman W, Blanton RL, Harwood AJ. 2002. Loss of the b-catenin homologue Aardvark causes ectopic stalk formation in Dictyostelium. Mech Dev 116: 117-127.
20. Conway Morris S. 2000. The Cambrian "explosion": Slowfuse or megatonnage? Proc Natl Acad Sci 97: 4426-4429.
21. Croce JC, McClay DR. 2008. Evolution of the Wnt pathways. Methods Mol Biol 469: 3-18.
22. Dann CE, Hsieh JC, Rattner A, Sharma D, Nathans J, Leahy DJ. 2011. Insights into Wnt binding and signalling from the structures of two Frizzled cysteine-rich domains. Nature 412: 86-90.
23. Darras S, Gerhart J, Terasaki M, Kirschner M, Lowe CJ. 2011. B-Catenin specifies the endomesoderm and defines the posterior organizer of the hemichordate Saccoglossus kowalevskii. Development 138: 959-970.
24. Dehal P, Satou Y, Campbell RK, Chapman J, Degnan B, De Tomaso A, Davidson B, Di Gregorio A, Gelpke M, Goodstein DM, et al. 2002. The draft genome of Ciona intestinalis: Insights into chordate and vertebrate origins. Science 298: 2157-2167.
25. De Robertis EM. 2010. Wnt signaling in axial patterning and regeneration: Lessons from planaria. Sci Signal 3: pe21.
26. De Robertis EM, Sasai Y. 1996. A common plan for dorsoventral patterning in Bilateria. Nature 380: 37-40.
27. Dickinson DJ, Nelson WJ, Weis WI. 2011. A polarized epithelium organized by b- and a-catenin predates cadherin and metazoan origins. Science 331: 1336-1339.
28. Dunty WC Jr, Biris KK, Chalamalasetty RB, Taketo MM, Lewandoski M, Yamaguchi TP. 2008. Wnt3a/b-catenin signaling controls posterior body development by coordinating mesoderm formation and segmentation. Development 135: 85-94.
29. Eichinger L, Pachebat JA, Glockner G, Rajandream MA, Sucgang R, Berriman M, Song J, Olsen R, Szafranski K, Xu Q, et al. 2005. The genome of the social amoeba Dictyostelium discoideum. Nature 435: 43-57.
30. Ganguly A, Jiang J, Ip YT. 2005. Drosophila WntD is a target and an inhibitor of the Dorsal/Twist/Snail network in the gastrulating embryo. Development 132: 3419-3429.
31. Gierer A, Meinhardt H. 1972. A theory of biological pattern formation. Kybernetik 12: 30-39.
32. Gierer A, Berking S, Bode H, David CN, Flick K, Hansmann G, Schaller H, Trenkner E. 1972. Regeneration of Hydra from reaggregated cells. Nat New Biol 239: 98-101.
33. Gordon MD, Nusse R. 2006. Wnt signaling: Multiple pathways, multiple receptors, and multiple transcription factors. J Biol Chem 281: 22429-22433.
34. Gordon MD, Dionne MS, Schneider DS, Nusse R. 2005. WntD is a feedback inhibitor of Dorsal/NF-kB in Drosophila development and immunity. Nature 437: 746-749.
35. Grimson MJ, Coates JC, Reynolds JP, Shipman M, Blanton RL, Harwood AJ. 2000. Adherens junctions and b-catenin- mediated cell signalling in a non-metazoan organism. Nature 408: 727-731.
36. Guder C, Philipp I, Lengfeld T, Watanabe H, Hobmayer B, Holstein TW. 2006a. The Wnt code: Cnidarians signal the way. Oncogene 25: 7450-7460.
37. Guder C, Pinho S, Nacak TG, Schmidt HA, Hobmayer B, Niehrs C, Holstein TW. 2006b. An ancient Wnt-Dickkopf antagonism in Hydra. Development 133: 901-911.
38. Gurley KA, Rink JC, Sanchez Alvarado A. 2008. B-Catenin defines head versus tail identity during planarian regeneration and homeostasis. Science 319: 323-327.
39. Gurley KA, Elliott SA, Simakov O, Schmidt HA, Holstein TW, Sanchez Alvarado A. 2010. Expression of secreted Wnt pathway components reveals unexpected complexity of the planarian amputation response. Dev Biol 347: 24-39.
40. Halanych KM, Bacheller JD, Aguinaldo AM, Liva SM, Hillis DM, Lake JA. 1995. Evidence from 18S ribosomal DNA that the lophophorates are protostome animals. Science 267: 1641-1643.
41. Hardin J, King RS. 2008. The long and the short of Wnt signaling in C. elegans. Curr Opin Genet Dev 18: 362-367.
42. Harwood AJ. 2008. Dictyostelium development: A prototypic Wnt pathway? Methods Mol Biol 469: 21-32.
43. Harwood AJ, Plyte SE, Woodgett J, Strutt H, Kay RR. 1995. Glycogen synthase kinase 3 regulates cell fate in Dictyostelium. Cell 80: 139-148.
44. Heisenberg CP, Tada M, Rauch GJ, Saude L, Concha ML, Geisler R, Stemple DL, Smith JC, Wilson SW. 2000. Silberblick/ Wnt11 mediates convergent extension movements during zebrafish gastrulation. Nature 405: 76-81.
45. Heuberger J, Birchmeier W. 2010. Interplay of cadherin-mediated cell adhesion and canonical Wnt signaling. Cold Spring Harb Perspect Biol 2: a002915.
46. Hobmayer B, Rentzsch F, Kuhn K, Happel CM, von Laue CC, Snyder P, Rothbächer U, Holstein TW. 2000. WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra. Nature 407: 186-189.
47. Holstein TW, Watanabe H, Ozbek S. 2011. Signaling pathways and axis formation in the lower metazoa. Curr Top Dev Biol 97: 137-177.
48. Iglesias M, Gomez-Skarmeta JL, Salo E, Adell T. 2008. Silencing of Smed-b-catenin1 generates radial-like hypercephalized planarians. Development 135: 1215-1221.
49. Iglesias M, Almuedo-Castillo M, Aboobaker AA, Salo E. 2011. Early planarian brain regeneration is independent of blastema polarity mediated by the Wnt/b-catenin pathway. Dev Biol 358: 68-78.
50. Imai K, Takada N, Satoh N, Satou Y. 2000. b-Catenin mediates the specification of endoderm cells in ascidian embryos. Development 127: 3009-3020.
51. Ingham PW, Nakano Y, Seger C. 2011. Mechanisms and functions of Hedgehog signalling across the metazoa. Nat Rev Genet 12: 393-406.
52. Janssen R, Le Gouar M, Pechmann M, Poulin F, Bolognesi R, Schwager EE, Hopfen C, Colbourne JK, Budd GE, Brown SJ, et al. 2010. Conservation, loss, and redeployment of Wnt ligands in protostomes: Implications for understanding the evolution of segment formation. BMC Evol Biol 10: 374.
53. Kadowaki T, Wilder E, Klingensmith J, Zachary K, Perrimon N. 1996. The segment polarity gene porcupine encodes a putative multitransmembrane protein involved inWingless processing. Genes Dev 10: 3116-3128.
54. King N, Westbrook MJ, Young SL, Kuo A, Abedin M, Chapman J, Fairclough S, Hellsten U, Isogai Y, Letunic I, et al. 2008. The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans. Nature 451: 783-788.
55. Kusserow A, Pang K, Sturm C, Hrouda M, Lentfer J, Schmidt HA, Technau U, von Haeseler A, Hobmayer B, Martindale MQ, et al. 2005. Unexpected complexity of the Wnt gene family in a sea anemone. Nature 433: 156-160.
56. Lapebie P, Gazave E, Ereskovsky A, Derelle R, Bezac C, Renard E, Houliston E, Borchiellini C. 2009. WNT/b-catenin signalling and epithelial patterning in the homoscleromorph sponge Oscarella. PLoS ONE 4: e5823.
57. Lee PN, Pang K, Matus DQ, Martindale MQ. 2006. AWNT of things to come: Evolution of Wnt signaling and polarity in cnidarians. Semin Cell Dev Biol 17: 157-167.
58. Lee PN, Kumburegama S, Marlow HQ, Martindale MQ, Wikramanayake AH. 2007. Asymmetric developmental potential along the animal-vegetal axis in the anthozoan cnidarian, Nematostella vectensis, is mediated by Dishevelled. Dev Biol 310: 169-186.
59. Lengfeld T, Watanabe H, Simakov O, Lindgens D, Gee L, Law L, Schmidt HA, Ozbek S, Bode H, Holstein TW. 2009. Multiple Wnts are involved inHydra organizer formation and regeneration. Dev Biol 330: 186-199.
60. Lessing D, Nusse R. 1998. Expression of wingless in the Drosophila embryo: A conserved cis-acting element lacking conserved Ci-binding sites is required for patchedmediated repression. Development 125: 1469-1476.
61. Logan CY, Miller JR, Ferkowicz MJ, McClay DR. 1999. Nuclear b-catenin is required to specify vegetal cell fates in the sea urchin embryo. Development 126: 345-357.
62. Lorenowicz MJ, Korswagen HC. 2009. Sailing with the Wnt: Charting the Wnt processing and secretion route. Exp Cell Res 315: 2683-2689.
63. Martin BL, Kimelman D. 2008. Regulation of canonical Wnt signaling by Brachyury is essential for posterior mesoderm formation. Developmental cell 15: 121-133.
64. Martin BL, Kimelman D. 2009. Wnt signaling and the evolution of embryonic posterior development. Curr Biol 19: R215-R219.
65. Martindale MQ, Hejnol A. 2009. A developmental perspective: Changes in the position of the blastopore during bilaterian evolution. Dev Cell 17: 162-174.
66. McGregor AP, Pechmann M, Schwager EE, Feitosa NM, Kruck S, Aranda M, Damen WG. 2008. Wnt8 is required for growth-zone establishment and development of opisthosomal segments in a spider. Curr Biol 18: 1619-1623.
67. McMahon AP, Moon RT. 1989. Ectopic expression of the proto-oncogene int-1 in Xenopus embryos leads to duplication of the embryonic axis. Cell 58: 1075-1084.
68. Meinhardt H. 2002. The radial-symmetric Hydra and the evolution of the bilateral body plan: An old body became a young brain. Bioessays 24: 185-191.
69. Meinhardt H. 2004. Different strategies for midline formation in bilaterians. Nat Rev Neurosci 5: 502-510.
70. Meinhardt H. 2006. Primary body axes of vertebrates: Generation of a near-Cartesian coordinate system and the role of Spemann-type organizer. Dev Dyn 235: 2907-2919.
71. Meinhardt H. 2012. Modeling pattern formation in Hydra- a route to understand essential steps in development. Int J Dev Biol doi: 10.1387/ijdb.113483hm.
72. Momose T, Houliston E. 2007. Two oppositely localized Frizzled RNAs as axis determinants in a cnidarian embryo. PLoS Biol 5: e70.
73. Momose T, Derelle R, Houliston E. 2008. A maternally localized Wnt ligand required for axial patterning in the cnidarian Clytia hemisphaerica. Development 135: 2105-2113.
74. Nakamura Y, Tsiairis CD, Ozbek S, Holstein TW. 2011. Autoregulatory and repressive inputs localize Hydra Wnt3 to the head organizer. ProcNatl Acad Sci 108: 9137-9142.
75. Niehrs C. 2010. On growth and form: A Cartesian coordinate system of Wnt and BMP signaling specifies bilaterian body axes. Development 137: 845-857.
76. Nusse R. 2001. An ancient cluster of Wnt paralogues. Trends Genet 17: 443.
77. Onai T, Lin HC, Schubert M, Koop D, Osborne PW, Alvarez S, Alvarez R, Holland ND, Holland LZ. 2009. Retinoic acid and Wnt/b-catenin have complementary roles in anterior/posterior patterning embryos of the basal chordate amphioxus. Dev Biol 332: 223-233.
78. Pang K, Ryan JF, Mullikin JC, Baxevanis AD, Martindale MQ. 2010. Genomic insights into Wnt signaling in an early diverging metazoan, the ctenophore Mnemiopsis leidyi. Evodevo 1: 10.
79. Petersen CP, Reddien PW. 2008. Smed-b-catenin-1 is required for anteroposterior blastema polarity in planarian regeneration. Science 319: 327-330.
80. Petersen CP, Reddien PW. 2009. Wnt signaling and the polarity of the primary body axis. Cell 139: 1056-1068.
81. Petersen CP, Reddien PW. 2011. Polarized notum activation at wounds inhibits Wnt function to promote planarian head regeneration. Science 332: 852-855.
82. Philipp I, Aufschnaiter R, Ozbek S, Pontasch S, Jenewein M, Watanabe H, Rentzsch F, Holstein TW, Hobmayer B. 2009. Wnt/b-catenin and noncanonical Wnt signaling interact in tissue evagination in the simple eumetazoan Hydra. Proc Natl Acad Sci 106: 4290-4295.
83. Prabhu Y, Eichinger L. 2006. The Dictyostelium repertoire of seven transmembrane domain receptors. Eur J Cell Biol 85: 937-946.
84. Prud'homme B, Lartillot N, Balavoine G, Adoutte A, Vervoort M. 2002. Phylogenetic analysis of the Wnt gene family. Insights from lophotrochozoan members. Curr Biol 12: 1395.
85. Putnam NH, Srivastava M, Hellsten U, Dirks B, Chapman J, Salamov A, Terry A, Shapiro H, Lindquist E, Kapitonov VV, et al. 2007. Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization. Science 317: 86-94.
86. Reddien PW, Sanchez Alvarado A. 2004. Fundamentals of planarian regeneration. Annu Rev Cell Dev Biol 20: 725-757.
87. Rentzsch F, Fritzenwanker JH, Scholz CB, Technau U. 2008. FGF signalling controls formation of the apical sensory organ in the cnidarian Nematostella vectensis. Development 135: 1761-1769.
88. Riddiford N, Olson PD. 2011. Wnt gene loss in flatworms. Dev Genes Evol 221: 187-197.
89. Rigo-Watermeier T, Kraft B, Ritthaler M, Wallkamm V, Holstein TW, Wedlich D. 2011. Functional conservation of Nematostella Wnts in canonical and noncanonical Wntsignaling. Biol Open doi: 10.1242/bio.2011021.
90. Rokas A, Kruger D, Carroll SB. 2005. Animal evolution and the molecular signature of radiations compressed in time. Science 310: 1933-1938.
91. Ruiz-Trillo I, Riutort M, Littlewood DT, Herniou EA, Baguna J. 1999. Acoel flatworms: Earliest extant bilaterian Metazoans, not members of Platyhelminthes. Science 283: 1919-1923.
92. Ruiz-Trillo I, Roger AJ, Burger G, Gray MW, Lang BF. 2008. A phylogenomic investigation into the origin of metazoa. Mol Biol Evol 25: 664-672.
93. Schierwater B, Eitel M, Jakob W, Osigus HJ, Hadrys H, Dellaporta SL, Kolokotronis SO, Desalle R. 2009. Concatenated analysis sheds light on early metazoan evolution and fuels a modern "urmetazoon" hypothesis. PLoS Biol 7: e20.
94. Schilde C, Araki T, Williams H, Harwood A, Williams JG. 2004. GSK3 is a multifunctional regulator of Dictyostelium development. Development 131: 4555-4565.
95. Schubert M, Holland LZ, Stokes MD, Holland ND. 2001. Three amphioxus Wnt genes (AmphiWnt3, AmphiWnt5, and AmphiWnt6) associated with the tail bud: The evolution of somitogenesis in chordates. Dev Biol 240: 262-273.
96. Sebe-Pedros A, de Mendoza A, Lang BF, Degnan BM, Ruiz-Trillo I. 2011. Unexpected repertoire of Metazoan transcription factors in the unicellular Holozoan Capsaspora owczarzaki. Mol Biol Evol 28: 1241-1254.
97. Shimizu T, Bae YK, Muraoka O, Hibi M. 2005. Interaction of Wnt and caudal-related genes in zebrafish posterior body formation. Dev Biol 279: 125-141.
98. Sodergren E, Weinstock GM, Davidson EH, Cameron RA, Gibbs RA, Angerer RC, Angerer LM, Arnone MI, Burgess DR, Burke RD, et al. 2006. The genome of the sea urchin Strongylocentrotus purpuratus. Science 314: 941-952.
99. Srivastava M, Begovic E, Chapman J, Putnam NH, Hellsten U, Kawashima T, Kuo A, Mitros T, Salamov A, Carpenter ML, et al. 2008. The Trichoplax genome and the nature of placozoans. Nature 454: 955-960.
100. Srivastava M, Simakov O, Chapman J, Fahey B, Gauthier ME, Mitros T, Richards GS, Conaco C, Dacre M, Hellsten U, et al. 2010. The Amphimedon queenslandica genome and the evolution of animal complexity. Nature 466: 720-726.
101. Steiner AB, Engleka MJ, Lu Q, Piwarzyk EC, Yaklichkin S, Lefebvre JL, Walters JW, Pineda-Salgado L, Labosky PA, Kessler DS. 2006. FoxD3 regulation of Nodal in the Spemann organizer is essential for Xenopus dorsal mesoderm development. Development 133: 4827-4838.
102. Sullivan JC, Ryan JF, Mullikin JC, Finnerty JR. 2007. Conserved and novel Wnt clusters in the basal eumetazoan Nematostella vectensis. Dev Genes Evol 217: 235-239.
103. Tamai K, Semenov M, Kato Y, Spokony R, Liu C, Katsuyama Y, Hess F, Saint-Jeannet JP, He X. 2000. LDL-receptorrelated proteins in Wnt signal transduction. Nature 407: 530-535.
104. Technau U, Steele RE. 2011. Evolutionary crossroads in developmental biology: Cnidaria. Development 138: 1447-1458.
105. Technau U, Cramer von Laue C, Rentzsch F, Luft S, Hobmayer B, Bode HR, Holstein TW. 2000. Parameters of self-organization in Hydra aggregates. Proc Natl Acad Sci 97: 12127-12131.
106. Thorpe CJ, Weidinger G, Moon RT. 2005. Wnt/b-catenin regulation of the Sp1-related transcription factor sp5l promotes tail development in zebrafish. Development 132: 1763-1772.
107. van Amerongen R, Nusse R. 2009. Towards an integrated view of Wnt signaling in development. Development 136: 3205-3214.
108. van Amerongen R, Mikels A, Nusse R. 2008. Alternative wnt singnaling is initiated by distinct receptors. Sci Signal 1: re9.
109. van de Wetering M, Cavallo R, Dooijes D, van Beest M, van Es J, Loureiro J, Ypma A, Hursh D, Jones T, Bejsovec A, et al. 1997. Armadillo coactivates transcription driven by the product of the Drosophila segment polarity gene dTCF. Cell 88: 789-799.
110. Vorwald-Denholtz PP, De Robertis EM. 2011. Temporal pattern of the posterior expression ofWingless in Drosophila blastoderm. Gene Expr Patterns 11: 456-463.
111. Wikramanayake AH, Huang L, Klein WH. 1998. b-Catenin is essential for patterning the maternally specified animal-vegetal axis in the sea urchin embryo. Proc Natl Acad Sci 95: 9343-9348.
112. Wikramanayake AH, Hong M, Lee PN, Pang K, Byrum CA, Bince JM, Xu R, Martindale MQ. 2003. An ancient role for nuclear b-catenin in the evolution of axial polarity and germ layer segregation. Nature 426: 446-450.
113. Wikramanayake AH, Peterson R, Chen J, Huang L, Bince JM, McClay DR, Klein WH. 2004. Nuclear b-catenindependent Wnt8 signaling in vegetal cells of the early sea urchin embryo regulates gastrulation and differentiation of endoderm and mesodermal cell lineages. Genesis 39: 194-205.
114. Windsor PJ, Leys SP. 2010. Wnt signaling and induction in the sponge aquiferous system: Evidence for an ancient origin of the organizer. Evol Dev 12: 484-493.
115. Xiao S, Laflamme M. 2009. On the eve of animal radiation: Phylogeny, ecology and evolution of the Ediacara biota. Trends Ecol Evol 24: 31-40.
116. Yamaguchi TP, Takada S, Yoshikawa Y, Wu N, McMahon AP. 1999. T (Brachyury) is a direct target of Wnt3a during paraxial mesoderm specification. Genes Dev 13: 3185-3190.
117. Yu JK, Satou Y, Holland ND, Shin IT, Kohara Y, Satoh N, Bronner-Fraser M, Holland LZ. 2007. Axial patterning in cephalochordates and the evolution of the organizer. Nature 445: 613-617.