In the frog Xenopus, the Spemann organizer expresses Wnt and BMP antagonists 3, whose microinjection in single vegetal blastomeres can induce ectopic rostral or dorsal structures correspondingly 4, 5. Whether this similarity reflects the homologous or convergent origin of the cnidarian and vertebrate blastoporal axial organizers remained unclear, since the molecular nature of the signal conveying axial organizer properties to an embryonic tissue has not been determined outside deuterostomes. Although axis-forming blastoporal organizers were long thought to be a chordate-specific feature, transplantation of a fragment of the mid-gastrula blastopore lip in the sea anemone Nematostella vectensis, a member of the early branching non-bilaterian phylum Cnidaria (which, in addition to sea anemones, includes corals, hydroids and jellyfish), also resulted in the formation of an ectopic body axis 2, just like the Mangold-Spemann organizer in amphibians. The ability of the amphibian dorsal blastopore lip to induce ectopic body axes upon transplantation has fascinated biologists for nearly a century 1. By combining molecular analysis with experimental embryology, we provide evidence that the emergence of the Wnt/β-catenin driven blastopore-associated axial organizer predated the cnidarian-bilaterian split over 600 million years ago. We also demonstrate that the establishment of the secondary, directive axis in Nematostella by BMP signaling is sensitive to an initial Wnt signal, but once established the directive axis becomes Wnt-independent. Here we show that the blastopore lip of a non-bilaterian metazoan, the anthozoan cnidarian Nematostella vectensis, possesses the same capacity and uses the same molecular mechanism for inducing extra axes as chordates: Wnt/β-catenin signaling. However, the axis-inducing potential of the blastopore-associated tissue is commonly regarded as a chordate feature. The startling capacity of the amphibian Spemann organizer to induce naïve cells to form a Siamese twin embryo with a second set of body axes is one of the hallmarks of developmental biology.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |