![]() ![]() Characterization of amphioxus amphivent, an evolutionarily conserved marker for chordate ventral mesoderm. Initial patterning of the central nervous system: how many organizers? Nature Rev. An anterior signalling centre in Xenopus revealed by the homeobox gene XHex. Amphioxus Evx genes: implications for the evolution of the midbrain–hindbrain boundary and the chordate tailbud. The ventral and posterior expression of the zebrafish homeobox gene eve1 is perturbed in dorsalized and mutant embryos. S., Joly, C., Schulte-Merker, S., Boulekbache, H. Left–right asymmetry in embryonic development: a comprehensive review. ![]() An amphioxus nodal gene ( AmphiNodal) with early symmetrical expression in the organizer and mesoderm and later asymmetrical expression associated with left-right axis formation. Homologues of Twisted gastrulation are extracellular cofactors in antagonism of BMP signalling. Twisted gastrulation can function as a BMP antagonist. Regulation of ADMP and BMP2/4/7 at opposite embryonic poles generates a self-regulating morphogenetic field. Regionally specific induction by the Spemann–Mangold organizer. The establishment of Spemann's organizer and patterning of the vertebrate embryo. Dorsal–ventral patterning and neural induction in Xenopus embryos. Experimental studies on the neural induction in amphioxus. Topographic changes in nascent and early mesoderm in amphioxus embryos studied by DiI labeling and by in situ hybridization for a Brachyury gene. Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Multigene analyses of bilaterian animals corroborate the monophyly of Ecdysozoa, Lophotrochozoa, and Protostomia. Molecular phylogeny and divergence times of deuterostome animals. Did the first chordates organize without the organizer? Trends Genet. Evolution of the organizer and the chordate body plan. In light of recent phylogenetic analyses placing cephalochordates basally in the chordate lineage, we propose that separate signalling centres for patterning the D/V and A/P axes may be an ancestral chordate character. These results suggest evolutionary conservation of the mechanisms for both D/V and A/P patterning of the early gastrula. In addition, Wnt-antagonists (for example, Dkks and sFRP2-like) are expressed anteriorly, whereas Wnt genes themselves are expressed posteriorly, consistent with a role for Wnt signalling in anterior/posterior (A/P) patterning. ![]() Here we show that the gastrula of the cephalochordate amphioxus expresses dorsal/ventral (D/V) patterning genes (for example, bone morphogenetic proteins ( BMPs), Nodal and their antagonists) in patterns reminiscent of those of their vertebrate orthlogues, and that amphioxus embryos, like those of vertebrates, are ventralized by exogenous BMP protein. Although a topic of long-standing interest, the evolutionary origin of the organizer remains unclear. © 2004 Wiley-Liss, Inc.The organizer of the vertebrate gastrula is an important signalling centre that induces and patterns dorsal axial structures. Loss of function experiments using SpNodal antisense morpholinos also support a role for SpNodal in the establishment of the AO axis. The expression pattern of SpNodal is consistent with a role for nodal signaling in the establishment of fates along the AO axis. To further examine TGF-β signaling in these embryos, we cloned an endogenous TGF-β from sea urchin embryos that is a member of the activin subfamily, SpNodal, and show through gain of function analysis that it recapitulates results obtained with exogenous activins and NiCl2. These results argue that specification of fates along the embryonic AO axis involves TGF-β signaling. Sensitivity to exogenous activin occurs between fertilization and the late blastula stage, which is also the time of highest NiCl2 sensitivity. The phenotypes observed resemble the ventralized phenotype previously reported for NiCl2, a known disrupter of AO patterning. Injection of activin mRNAs into fertilized eggs or treatment with exogenously applied recombinant activin altered the allocation of ectodermal fates and ventralized the embryo. In an analysis of signaling molecules that influence patterning along the sea urchin embryonic axes, we found that members of the activin subfamily of transforming growth factor-β (TGF-β) signaling molecules influence the establishment of AO polarities in the early embryo. Components of the Wnt signaling pathway are involved in patterning the sea urchin primary or animal–vegetal (AV) axis, but the molecular cues that pattern the secondary embryonic axis, the aboral/oral (AO) axis, are not known. ![]()
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