A simple plan — cnidarians and the origins of developmental mechanisms


Comparisons with cnidarians, long considered to be 'simple' animals, are providing crucial insights into the origins of conserved developmental mechanisms and the nature of the common metazoan ancestor. Traditionally, an extra germ layer and a second axis of body symmetry are the features that distinguish 'higher' Metazoa from lower animals such as cnidarians. Moreover, it was expected that 'lower' animals would have a simple gene set that corresponds to their simple morphology. Now, molecular genetic approaches are blurring the developmental divide between cnidarians and bilateral animals, and cnidarian sequencing projects are showing that the common metazoan ancestor was more genetically complex than was previously assumed.

Key Points

  • Cnidarians lack true mesoderm, in the sense of a third germ layer that arises as a direct result of gastrulation. However, at the molecular level, muscle development in the medusa of the hydrozoan Podocoryne carnea seems to mirror its differentiation as a mesodermal lineage in 'higher' animals.

  • Molecular evidence indicates that the Anthozoa are the basal cnidarian group, but they lack both the medusa stage and well-developed muscle. So, there is active debate as to whether the molecular similarities between P. carnea muscle development and that in Bilateria indicate convergent evolution or the loss of the medusa stage in the Anthozoa.

  • Many of the same 'mesodermal marker' genes are expressed during gastrulation in Anthozoa and Bilateria. This might indicate that the primitive function of these genes was in such fundamental processes as cell proliferation, adhesion and motility, and indicates the possibility that as true mesoderm evolved from endomesoderm, these genes became associated with the former.

  • During early coral and anemone development, a second axis is defined by the expression of a gene related to DPP/BMP4 — the key determinant of the dorsal–ventral axis in higher animals — and this asymmetrical expression is maintained in the sea anemone polyp.

  • Molecular analyses establish the bilateral anthozoans, rather than the radial hydrozoans, as the basal cnidarians. Anthozoans have several of the regulatory genes that are responsible for patterning both of the principal axes of higher animals (for example, Emx and dpp/Bmp4), and express them in ways that are inconsistent with the assumption of a single body axis. EST projects on several cnidarians consistently highlight the complexity of the cnidarian gene complement and imply that, at the genetic level, the common ancestor of bilateral animals was more complex than has previously been assumed.

  • The blurring of the triploblast/diploblast divide, the confirmation that basal cnidarians are bilateral and the complexity of their gene set lead us to suggest rethinking the entire notion of cnidarians as 'lower' animals.

  • Several key developmental genes have been independently duplicated in cnidarians, including snail and Hox-related genes, and, even at the genus level, large changes in genome size have occurred. These factors complicate the understanding of the evolution of specific gene families and will need to be considered in selecting representative cnidarians for genomic sequencing.

  • Cnidarians are not a genetically uniform group. Their diversity and genetic complexity indicate that their apparently simple morphology might mask considerable functional complexity, providing great scope for evolutionary studies.

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Figure 1: Muscle development in the medusa of Podocoryne carnea.
Figure 2: Expression of the mesodermal marker snail during early gastrulation in Acropora millepora and Drosophila melanogaster.


  1. 1

    Goldstein, B. & Freeman, G. Axis specification in animal development. Bioessays 19, 105–116 (1997).

  2. 2

    Willmer, P. Invertebrate Relationships: Patterns in Animal Evolution (Cambridge Univ. Press, Cambridge, UK, 1990).

  3. 3

    Martindale, M. Q., Finnerty, J. R. & Henry, J. Q. The Radiata and the evolutionary origins of the bilaterian body plan. Mol. Phylogenet. Evol. 24, 358–365 (2002).

  4. 4

    Hayward, D. C., Miller, D. J. & Ball, E. E. snail expression during embryonic development of the coral Acropora: blurring the diploblast/triploblast divide? Dev. Genes Evol. 214, 257–260 (2004).

  5. 5

    Müller, P., Yanze, N., Schmid, V. & Spring, J. The homeobox gene Otx of the jellyfish Podocoryne carnea: role of a head gene in striated muscle and evolution. Dev. Biol. 216, 582–594 (1999).

  6. 6

    Schierwater, B., Dellaporta, S. & DeSalle, R. Is the evolution of Cnox-2 Hox/ParaHox genes 'multicolored' and 'polygenealogical?' Mol. Phylogenet. Evol. 24, 374–378 (2002).

  7. 7

    Nieto, M. A. The snail superfamily of zinc-finger transcription factors. Nature Rev. Mol. Cell Biol. 3, 155–166 (2002).

  8. 8

    Technau, U. & Scholz, C. B. Origin and evolution of endoderm and mesoderm. Int. J. Dev. Biol. 47, 531–539 (2003).

  9. 9

    Haeckel, E. Die gastrea theorie, die phylogenetische classification des thierreiches und die homologie der keimblatter. Jenaische Zeischrift fur Naturwissenschaft 8, 1–55 (1874).

  10. 10

    Baylies, M. K. & Michelson, A. M. Invertebrate myogenesis: looking back to the future of muscle development. Curr. Opin. Genet. Dev. 11, 431–439 (2001).

  11. 11

    Tavares, A. T., Izpisuja-Belmonte, J. C. & Rodriguez-Leon, J. Developmental expression of chick twist and its regulation during limb patterning. Int. J. Dev. Biol. 45, 707–713 (2001).

  12. 12

    Sefton, M., Sanchez, S. & Nieto, M. A. Conserved and divergent roles for members of the Snail family of transcription factors in the chick and mouse embryo. Development 125, 3111–3121 (1998).

  13. 13

    Schmid, V., Alder, H., Plickert, G. & Weber, C. Transdifferentiation from striated muscle of medusae in vitro. Cell Differ. Dev. 25 (Suppl.), 137–146 (1988).

  14. 14

    Schuchert, P., Reber-Müller, S. & Schmid, V. Life stage specific expression of a myosin heavy chain in the hydrozoan Podocoryne carnea. Differentiation 54, 11–18 (1993).

  15. 15

    Gröger, H., Callaerts, P., Gehring, W. J. & Schmid, V. Gene duplication and recruitment of a specific tropomyosin into striated muscle cells in the jellyfish Podocoryne carnea. J. Exp. Zool. 285, 378–386 (1999).

  16. 16

    Spring, J. et al. The mesoderm specification factor twist in the life cycle of jellyfish. Dev. Biol. 228, 363–375 (2000). Together with the companion paper reference 17, on Brachyury, Mef-2 and Snail, this paper first established the existence of genes in Cnidaria thought to be markers of mesoderm and muscle in higher animals.

  17. 17

    Spring, J. et al. Conservation of Brachyury, Mef2, and Snail in the myogenic lineage of jellyfish: a connection to the mesoderm of bilateria. Dev. Biol. 244, 372–384 (2002).

  18. 18

    Müller, P. et al. Evolutionary aspects of developmentally regulated helix-loop-helix transcription factors in striated muscle of jellyfish. Dev. Biol. 255, 216–229 (2003).

  19. 19

    Boero, F. et al. The cnidarian premises of metazoan evolution: from triploblasty, to coelom formation, to metamery. Ital. J. Zool. 65, 5–9 (1998).

  20. 20

    Martindale, M. Q., Pang, K. & Finnerty, J. R. Investigating the origins of triploblasty: 'mesodermal' gene expression in a diploblastic animal, the sea anemone Nematostella vectensis (phylum, Cnidaria; class, Anthozoa). Development 131, 2463–2474 (2004). Describes the expression patterns of seven 'mesodermal' genes during embryonic development of a sea anemone, all but one of which are restricted to the endodermal layer.

  21. 21

    Stathopoulos, A. & Levine, M. Dorsal gradient networks in the Drosophila embryo. Dev. Biol. 246, 57–67 (2002).

  22. 22

    Rodaway, A. & Patient, R. Mesendoderm. an ancient germ layer? Cell 105, 169–172 (2001).

  23. 23

    Savagner, P. Leaving the neighborhood: molecular mechanisms involved during epithelial-mesenchymal transition. Bioessays 23, 912–923 (2001).

  24. 24

    Collins, A. G. Phylogeny of Medusozoa and the evolution of cnidarian life cycles. J. Evol. Biol. 15, 418–432 (2002). Uses a large set of molecular data as key supporting evidence for the modern cnidarian phylogeny shown in box 1, and concludes that the polyp probably preceded the medusa in cnidarian evolution.

  25. 25

    Rodaway, A. et al. Induction of the mesendoderm in the zebrafish germ ring by yolk cell-derived TGF-β family signals and discrimination of mesoderm and endoderm by FGF. Development 126, 3067–3078 (1999).

  26. 26

    Maduro, M. F., Meneghini, M. D., Bowerman, B., Broitman-Maduro, G. & Rothman, J. H. Restriction of mesendoderm to a single blastomere by the combined action of SKN-1 and a GSK-3β homolog is mediated by MED-1 and-2 in C. elegans. Mol. Cell 7, 475–485 (2001).

  27. 27

    Ruiz-Trillo, I., Riutort, M., Littlewood, D. T., Herniou, E. A. & Baguna, J. Acoel flatworms: earliest extant bilaterian metazoans, not members of Platyhelminthes. Science 283, 1919–1923 (1999).

  28. 28

    Martindale, M. Q. & Henry, J. Q. Intracellular fate mapping in a basal metazoan, the ctenophore Mnemiopsis leidyi, reveals the origins of mesoderm and the existence of indeterminate cell lineages. Dev. Biol. 214, 243–257 (1999).

  29. 29

    Slack, J. M., Holland, P. W. & Graham, C. F. The zootype and the phylotypic stage. Nature 361, 490–492 (1993).

  30. 30

    Finnerty, J. R. & Martindale, M. Q. Homeoboxes in sea anemones (Cnidaria: Anthozoa): a PCR-based survey of Nematostella vectensis and Metridium senile. Biol. Bull. 193, 62–76 (1997).

  31. 31

    Martinez, D. E., Bridge, D., Masuda-Nakagawa, L. M. & Cartwright, P. Cnidarian homeoboxes and the zootype. Nature 393, 748–749 (1998).

  32. 32

    Gauchat, D. et al. Evolution of Antp-class genes and differential expression of Hydra Hox/paraHox genes in anterior patterning. Proc. Natl Acad. Sci. USA 97, 4493–4498 (2000).

  33. 33

    Finnerty, J. R., Pang, K., Burton, P., Paulson, D. & Martindale, M. Q. Origins of bilateral symmetry: Hox and dpp expression in a sea anemone. Science 304, 1335–1337 (2004).

  34. 34

    De Robertis, E. M. & Sasai, Y. A common plan for dorsoventral patterning in Bilateria. Nature 380, 37–40 (1996).

  35. 35

    Holley, S. A. & Ferguson, E. L. Fish are like flies are like frogs: conservation of dorsal–ventral patterning mechanisms. Bioessays 19, 281–284 (1997).

  36. 36

    Cornell, R. A. & Ohlen, T. V. Vnd/nkx, ind/gsh, and msh/msx: conserved regulators of dorsoventral neural patterning? Curr. Opin. Neurobiol. 10, 63–71 (2000).

  37. 37

    Hayward, D. C. et al. Localized expression of a dpp/BMP2/4 ortholog in a coral embryo. Proc. Natl Acad. Sci. USA 99, 8106–8111 (2002). Demonstrates that the expression of a molecule that is closely related to those responsible for establishing the dorsal–ventral axis in bilaterians defines a second axis during early cnidarian development.

  38. 38

    Hayward, D. C. et al. Gene structure and larval expression of cnox-2Am from the coral Acropora millepora. Dev. Genes Evol. 211, 10–19 (2001).

  39. 39

    Sprecher, S. G. & Reichert, H. The urbilaterian brain: developmental insights into the evolutionary origin of the brain in insects and vertebrates. Arthropod Struct. Dev. 32, 141–156 (2003).

  40. 40

    Smith, K. M., Gee, L., Blitz, I. L. & Bode, H. R. CnOtx, a member of the Otx gene family, has a role in cell movement in hydra. Dev. Biol. 212, 392–404 (1999).

  41. 41

    Mokady, O., Dick, M. H., Lackschewitz, D., Schierwater, B. & Buss, L. W. Over one-half billion years of head conservation? Expression of an ems class gene in Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa). Proc. Natl Acad. Sci. USA 95, 3673–3678 (1998).

  42. 42

    Yanze, N., Spring, J., Schmidli, C. & Schmid, V. Conservation of Hox/paraHox-related genes in the early development of a cnidarian. Dev. Biol. 236, 89–98 (2001).

  43. 43

    Hobmayer, B. et al. WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra. Nature 407, 186–189 (2000).

  44. 44

    Angier, N. With new fly, science outdoes Hollywood. New York Times (Print) A1, A15 (24 Mar 1995).

  45. 45

    Plaza, S., De Jong, D. M., Gehring, W. J. & Miller, D. J. DNA-binding characteristics of cnidarian Pax-C and Pax-B proteins in vivo and in vitro: no simple relationship with the Pax-6 and Pax-2/5/8 classes. J. Exp. Zoolog. Part B Mol. Dev. Evol. 299, 26–35 (2003).

  46. 46

    Kozmik, Z. et al. Role of Pax genes in eye evolution: a cnidarian PaxB gene uniting Pax2 and Pax6 functions. Dev. Cell 5, 773–785 (2003). This paper, together with reference 45, demonstrates that the involvement of true Pax6 genes in photoreceptor specification might post-date the cnidarian/higher Metazoa divergence.

  47. 47

    Piatigorsky, J., Horwitz, J., Kuwabara, T. & Cutress, C. E. The cellular eye lens and crystallins of cubomedusan jellyfish. J. Comp. Physiol. 164, 577–587 (1989).

  48. 48

    Martin, V. J. Photoreceptors of cnidarians. Can. J. Zool. 80, 1703–1722 (2002).

  49. 49

    Harrison, P. L. & Wallace, C. C. in Ecosystems of the World 25 (ed. Dubinsky, Z.) 133–207 (Elsevier, Amsterdam, 1990).

  50. 50

    Miller, D. J. et al. Pax gene diversity in the basal cnidarian Acropora millepora (Cnidaria, Anthozoa): implications for the evolution of the Pax gene family. Proc. Natl Acad. Sci. USA 97, 4475–4480 (2000).

  51. 51

    Gröger, H., Callaerts, P., Gehring, W. J. & Schmid, V. Characterization and expression analysis of an ancestor-type Pax gene in the hydrozoan jellyfish Podocoryne carnea. Mech. Dev. 94, 157–169 (2000).

  52. 52

    Callaerts, P., Halder, G. & Gehring, W. J. PAX-6 in development and evolution. Annu. Rev. Neurosci. 20, 483–532 (1997).

  53. 53

    Cvekl, A. & Piatigorsky, J. Lens development and crystallin gene expression: many roles for Pax-6. Bioessays 18, 621–630 (1996).

  54. 54

    Kortschak, R. D., Samuel, G., Saint, R. & Miller, D. J. EST analysis of the cnidarian Acropora millepora reveals extensive gene loss and rapid sequence divergence in the model invertebrates. Curr. Biol. 13, 2190–2195 (2003). Establishes that there has been considerable gene loss in the model invertebrates D. melanogaster and C. elegans by the discovery in corals of genes that were previously thought to be vertebrate inventions.

  55. 55

    Ohno, S. Evolution by Gene Duplication (Springer, Heidelberg, 1970).

  56. 56

    Lundin, L. G. Evolution of the vertebrate genome as reflected in paralogous chromosomal regions in man and the house mouse. Genomics 16, 1–19 (1993).

  57. 57

    Human Genome Sequencing Consortium. Initial sequencing and analysis of the human genome. Nature 409, 860–921 (2001).

  58. 58

    Li, W. H., Gu, Z., Wang, H. & Nekrutenko, A. Evolutionary analyses of the human genome. Nature 409, 847–849 (2001).

  59. 59

    Venter, J. C. et al. The sequence of the human genome. Science 291, 1304–1351 (2001).

  60. 60

    Mochizuki, K., Sano, H., Kobayashi, S., Nishimiya-Fujisawa, C. & Fujisawa, T. Expression and evolutionary conservation of nanos-related genes in Hydra. Dev. Genes Evol. 210, 591–602 (2000). Elegantly demonstrates the functional divergence of independently duplicated cnidarian genes.

  61. 61

    Zacharias, H., Anokhin, B., Khalturin, K. & Bosch, T. C. Genome sizes and chromosomes in the basal metazoan Hydra. Zoology (in the press).

  62. 62

    Hyman, L. H. The Invertebrates: Protozoa Through Ctenophora (McGraw–Hill, New York, 1940).

  63. 63

    Hyman, L. H. The Invertebrates: Smaller Coelomate Groups (McGraw–Hill, New York, 1959).

  64. 64

    Bridge, D., Cunningham, C. W., Schierwater, B., DeSalle, R. & Buss, L. W. Class-level relationships in the phylum Cnidaria: evidence from mitochondrial genome structure. Proc. Natl Acad. Sci. USA 89, 8750–8753 (1992).

  65. 65

    Medina, M., Collins, A. G., Silberman, J. D. & Sogin, M. L. Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA. Proc. Natl Acad. Sci. USA 98, 9707–9712 (2001). The most extensive study of relationships among groups of basal animals based on a large molecular data set.

  66. 66

    Peterson, K. J. & Eernisse, D. J. Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences. Evol. Dev. 3, 170–205 (2001).

  67. 67

    Odorico, D. M. & Miller, D. J. Internal and external relationships of the Cnidaria: implications of primary and predicted secondary structure of the 5′-end of the 23S-like rDNA. Proc. R. Soc. Lond. B 264, 77–82 (1997).

  68. 68

    Hand, C. & Uhlinger, K. R. The culture, sexual and asexual reproduction, and growth of the sea-anemone Nematostella vectensis. Biol. Bull. 182, 169–176 (1992).

  69. 69

    Fritzenwanker, J. H. & Technau, U. Induction of gametogenesis in the basal cnidarian Nematostella vectensis (Anthozoa). Dev. Genes Evol. 212, 99–103 (2002).

  70. 70

    Miller, D. J. & Ball, E. E. The coral Acropora: what it can contribute to our knowledge of metazoan evolution and the evolution of developmental processes. Bioessays 22, 291–296 (2002).

  71. 71

    Fröbius, A. C., Genikhovich, G., Kurn, U., Anton-Erxleben, F. & Bosch, T. C. Expression of developmental genes during early embryogenesis of Hydra. Dev. Genes Evol. 213, 445–455 (2003).

  72. 72

    Ruppert, E. E., Fox, R. S. & Barnes, R. D. Invertebrate Zoology (Brooks/Cole–Thomson Learning, Belmont, 2004).

  73. 73

    Steele, R. E. Developmental signaling in Hydra: what does it take to build a 'simple' animal? Dev. Biol. 248, 199–219 (2002).

  74. 74

    Bosch, T. C. & Fujisawa, T. Polyps, peptides and patterning. Bioessays 23, 420–427 (2001).

  75. 75

    Martin, V. J., Littlefield, C. L., Archer, W. E. & Bode, H. R. Embryogenesis in Hydra. Biol. Bull. 192, 345–63 (1997).

  76. 76

    Galliot, B. & Schmid, V. Cnidarians as a model system for understanding evolution and regeneration. Int. J. Dev. Biol. 46, 39–48 (2002).

  77. 77

    Gröger, H. & Schmid, V. Nerve net differentiation in medusa development of Podocoryne carnea. Sci. Mar. 64 (Suppl. 1), 107–116 (2000).

  78. 78

    Beddington, R. S., Rashbass, P. & Wilson, V. Brachyury — a gene affecting mouse gastrulation and early organogenesis. Development (Suppl), 157–165 (1992).

  79. 79

    Technau, U. & Bode, H. R. HyBra1, a Brachyury homologue, acts during head formation in Hydra. Development 126, 999–1010 (1999).

  80. 80

    Technau, U. Brachyury, the blastopore and the evolution of the mesoderm. Bioessays 23, 788–794 (2001).

  81. 81

    Scholz, C. B. & Technau, U. The ancestral role of Brachyury: expression of NemBra1 in the basal cnidarian Nematostella vectensis (Anthozoa). Dev. Genes Evol. 212, 563–570 (2003).

  82. 82

    Wikramanayake, A. H. et al. An ancient role for nuclear β-catenin in the evolution of axial polarity and germ layer segregation. Nature 426, 446–450 (2003).

  83. 83

    Primus, A. & Freeman, G. The cnidarian and the canon: the role of Wnt/β-catenin signaling in the evolution of metazoan embryos. Bioessays 26, 474–478 (2004).

  84. 84

    Gerhart, J. & Kirschner, M. Cells, Embryos and Evolution (Blackwell Science, Malden, 1997).

  85. 85

    Reichert, H. & Simeone, A. Developmental genetic evidence for a monophyletic origin of the bilaterian brain. Philos. Trans. R. Soc. Lond. B 356, 1533–1544 (2001).

  86. 86

    Gauchat, D., Kreger, S., Holstein, T. & Galliot, B. prdl-a, a gene marker for Hydra apical differentiation related to triploblastic paired-like head-specific genes. Development 125, 1637–1645 (1998).

  87. 87

    Grasso, L. C. et al. The evolution of nuclear receptors: evidence from the coral Acropora. Mol. Phylogenet. Evol. 21, 93–102 (2001).

  88. 88

    Samuel, G., Miller, D. & Saint, R. Conservation of a DPP/BMP signaling pathway in the nonbilateral cnidarian Acropora millepora. Evol. Dev. 3, 241–250 (2001).

  89. 89

    Miller, S. W. et al. A DM domain protein from a coral, Acropora millepora, homologous to proteins important for sex determination. Evol. Dev. 5, 251–258 (2003).

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We thank the many cnidarian workers mentioned in the text for their help and cooperation in the production of this paper and apologize to all those whose work has gone unmentioned owing to space limitations; for example, the elegant work of T. Leitz and co-workers on the control of metamorphosis in Hydractinia, which has led to important insights into metamorphosis. We gratefully acknowledge the support of the Australian Research Council (both directly and through the Centre for the Molecular Genetics of Development, Canberra, Australia), and the contributions made both by various external collaborators and by members of our laboratories, past and present. Part of the title is a reference to the excellent eponymous film by Sam Raimi, the plot of which suggested to us some analogies between human behaviour and cnidarian evolution.

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Correspondence to Eldon E. Ball or David C. Hayward.

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A monophyletic group of metazoan animals that is characterized by bilateral symmetry. This group comprises all of the Metazoa except for the Radiata (Ctenophores and Cnidaria) and the Parazoa (sponges).


(EST). A nucleic acid sequence that is derived from cDNA, usually from the ends of cDNA clone inserts as part of high-throughput sequencing projects.


(Higher Metazoa). We use these terms as synonyms of Bilateria.


(Lower Metazoa). Here used to refer to Cnidaria, Ctenophora and Parazoa. Other authors include the Platyhelminthes (flatworms).


The opening of the archenteron in the gastrula.


A clade comprising three of the four cnidarian classes, which produce a sexually reproducing medusa (jellyfish) as part of the life cycle.


Animals that are traditionally considered to have radial symmetry. This group includes the ctenophores and cnidarians, and, according to some authors, the sponges.


A means of asexual reproduction seen in some sea anemones that involves division of the polyp into two or more parts with cleavage occurring in the transverse plane.


The early stage of animal development in which a single layer of cells surrounds a fluid-filled cavity, forming a hollow ball.


The free-swimming, ciliated larva of a cnidarian.


An individual specialized unit of a colonial cnidarian.


The terminal region of a polyp, on which the mouth is situated.


Longitudinal sheets of tissues that extend radially from the body wall into the body cavity.


The sessile form of life history in cnidarians; for example, the freshwater Hydra.


The mass of cells on the end of a medusa bud that becomes the velum and subumbrella surface.


The oral surface of a medusa.


(Also known as mesogloea). The body layer between ectoderm and endoderm in cnidarians, ctenophores and acoelomates, which is traditionally distinguished from mesoderm on the basis of the former being acellular and the latter cellular. However, in reality, enormous variation is seen across the Cnidaria in the extent to which the matrix of the mesoglea is invaded by various cellular and fibrillar components, and only in some hydrozoans does it approach true acellularity.


Cells at the dorsal midline of the vertebrate neural tube, which undergo an epithelial-to-mesenchymal transition and migrate to many locations, contributing to the development of a wide variety of structures, including the peripheral nervous system and craniofacial features, therefore essentially enabling a second wave of development.


A derived state that is shared by several taxa.


Two genes are paralogous if they were a result of a duplication event.


Any marine colonial hydrozoan of the order Siphonophora, including the Portuguese man-of-war.

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