The organizer in vertebrate embryos is responsible for the formation of the primary body axis. In amphibian embryos, the organizer forms in the dorsal marginal zone (prospective dorsal mesoderm) at a location determined by the point of sperm entry. Using inducible versions of axis-inducing proteins, it has been shown that, irrespective of the mode of secondary axis induction, organizer formation in the ventral marginal zone is temporally restricted from the midblastula transition to the onset of gastrulation. Here, we show that the competence of marginal zone cells to respond to organizer-inducing signals is under temporal control, one of the regulators being the homeobox transcription factor Xcad2. Overexpression of Xcad2 restricts the temporal competence for axis induction, whereas partial loss of function expands this competence, supporting our suggestion. We propose that Xcad2 competes with putative axis-inducing signals within the marginal zone to prevent expression of organizer-specific genes. Elimination of endogenous Xcad2 allows for the activation of organizer genes beyond the normal competence window during early/mid-gastrulation. We conclude that Xcad2, through its early expression in the ventrolateral marginal zone, terminates the competence of this embryonic region to respond to organizer-inducing signals by preventing the activation of organizer-specific genes.
Bibliographical noteFunding Information:
We thank Martin Blum, Herbert Steinbeisser, and Rolando Rivera-Pomar for support and discussions throughout the course of the work and their comments regarding the manuscript. We thank Dale Frank, Ralph Rupp, Eddy De Robertis, Marc Kirschner, David Kimelman, Sergei Sokol, Christof Niehrs, and Hazel Sive for plasmids. We are thankful to Aharon Razin, Graciela Pillemer, and Ayala Frumkin for critically reading the manuscript. This work was supported in part by grants of The Israel Science Foundation founded by The Israel Academy of Sciences and Humanities and Volkswagen Stiftung (to A.F.).
- Axis induction
- Caudal genes
- Transcription factor