Double assurance in the induction of axial development by egg dorsal determinants in <i>Xenopus</i> embryos

Abstract

We recently reported that microinjection of Xenopus nodal-related ( xnr ) mRNAs into β-catenin-depleted Xenopus embryos rescued a complete dorsal axis. Xnrs mediate the signal of the Nieuwkoop center that induces the Spemann–Mangold organizer in the overlying mesoderm, a process inhibited by the Nodal antagonist Cerberus-short (CerS). However, β-catenin also induces a second signaling center in the dorsal prospective ectoderm, designated the Blastula Chordin and Noggin Expression (BCNE) center, in which the homeobox gene siamois ( sia ) plays a major role. In this study, we asked whether the Xnrs and Sia depend on each other or function on parallel pathways. Expression of both genes induced β-catenin-depleted embryos to form complete axes with heads and eyes via the activation of similar sets of downstream organizer-specific genes. Xnrs did not activate siamois , and, conversely, Sia did not activate xnrs , although both were induced by β-catenin stabilization. Depletion with morpholinos revealed a robust role for the downstream target Chordin. Remarkably, Chordin depletion prevented all ectopic effects resulting from microinjection of the mRNA encoding the maternal cytoplasmic determinant Huluwa , including the radial expansion of brain tissue and the ectopic expression of the ventral gene sizzled . The main conclusion was that the BCNE and Nieuwkoop centers provide a double assurance mechanism for axial formation by independently activating similar downstream transcriptional target gene repertoires. We suggest that Siamois likely evolved from an ancestral Mix-type homeodomain protein called Sebox as a Xenopus -specific adaptation for the rapid differentiation of the anterior neural plate in the ectoderm.