Hemichordates: Development


Hemichordates are marine invertebrates consisting of two distinct groups: the solitary enteropneusts, or acorn worms (comprising a few hundred species), and the colonial and tube‐dwelling pterobranchs (comprising 20 or so species). Phylogenetically, hemichordates are the sister group to echinoderms, together composing the Ambulacraria, and share some features with chordates. Hemichordates are thus considered key organisms for addressing the origins of deuterostome and chordate body plans. Unlike the other deuterostome models including echinoderms and chordates, however, information about the developmental biology of this group is limited. Recent improvements in the accessibility of embryos, functional tool development and genomic resources from several model organisms have yielded important information on the cellular and genetic mechanisms of embryogenesis and organogenesis in hemichordates. Comparisons of hemichordates and other deuterostomes permit identification of the common ancestor of deuterostomes and help elucidate the origin of the chordate body plan.

Key Concepts

  • Hemichordate developmental biology is key to elucidating the origin of chordates and the early evolution of deuterostomes.
  • Descriptive and comparative studies using novel model species provide new insights into hemichordate evolution and diversity.
  • Establishment of genetic resources and experimental methods drive the molecular developmental biology of hemichordates.
  • Hemichordates and chordates show a conserved anteroposterior patterning mechanism.
  • Hemichordate research indicates that inversion of the dorsoventral axis occurred in the chordate lineage.
  • Gill slits of hemichordates and chordates are almost certainly homologous.
  • The stomochord of hemichordates and the notochord of chordates share some genetic features, but are unlikely to be homologous.
  • The origin of the tubular nervous system predates the diversification of hemichordates and chordates, but the homology of the collar nerve cord and neural tube still requires resolution.

Keywords: hemichordate; development; chordate evolution; body plan; nervous system

Figure 1. General morphology of an enteropneust (a) and a pterobranch (b) showing tripartite body plans. (c) Phylogeny of bilaterians showing the hemichordate relationship based on recent molecular phylogenies (Cannon ., , ; Peterson ., ).
Figure 2. Embryonic development of a direct developer, . (a–c) side views, (d–h) sections. (a) Fertilised egg. (b) Fertilised egg after the vegetal contraction. (c) Eight‐cell stage. (d) Blastula. Arrows indicate signalling by the endomesoderm that posteriorises the ectoderm. (e) Gastrula. (f) Longitudinal section of gastrula showing expression of the dorsoventral axis patterning genes. (g) enterocoely. (h) Neurula. (i) Neurula showing three signalling centres. The signalling molecules are expressed in the ectoderm. p, c and t are presumptive proboscis, collar and trunk, respectively. vc, ventral contraction.
Figure 3. (a) Diagram of an enteropneust indicating the positions of the sections in (b)–(d). (b) A longitudinal section of the proboscis stem and collar showing the position of the stomochord. (c) A cross‐section of the dorsal portion of the collar showing the collar nerve cord and stomochord. (d) A cross‐section of the posterior part of the trunk. (e) A high magnification of the boxed area in (d) indicating the topological relationship among the gut, pygochord, blood vessel and ventral nerve cord. (f) Diagram showing a cross‐section of an enteropneust. (g), (h) Cross‐sections of the dorsal portion of the collar (g) and the ventral portion of the posterior trunk (h). bv, blood vessel; cc, collar nerve cord; g, gut; ps, proboscis skeleton; py, pygochord; st, stomochord; vnc, ventral nerve cord. Scale bars: (b), (d) = 200 µm; (c), (e) = 50 µm.
Figure 4. Schematic illustration of chordate and hemichordate neurulation. The hemichordate neurulation takes place in the collar region.
Figure 5. Predicted bilaterian evolutionary scenario. A dorsoventral patterning process predates the diversification of deuterostomes and protostomes. The tubular nervous system evolved in the deuterostome ancestor. Inversion of the dorsoventral axis (BMP/chordin gradients) occurred in the chordate lineage. Illustration of the protostome is based on annelid worms (Denes ., ; Lauri ., ). Note that the gene has not yet been found in annelids. D, dorsal; V, ventral.


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Miyamoto, Norio, and Wada, Hiroshi(Apr 2015) Hemichordates: Development. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0004110.pub2]