CORTICAL ROTATION IS REQUIRED FOR THE CORRECT SPATIAL EXPRESSION OF NR3, SIA AND GSC IN XENOPUS EMBRYOS

beta-catenin, a component of the wnt-signal-transduction pathway, is e ssential for the formation of the dorsal axis in Xenopus laevis embryo s. On the dorsal side of the embryo, beta-catenin is translocated into the nuclei via a process linked to cortical rotation. When cortical r otation is blocked by UV-irradiation, nuclear beta-catenin is found in the vegetal pole of the embryo. Here we show that overexpression of b eta-catenin in animal cap explants, in the absence of mesoderm inducti on, is sufficient to activate the expression of genes with dorsalizing activity such as siamois (sia) and nodal-related 3 (nr3) but not goos ecoid (gsc). In embryos ventralized by UV-treatment, the expression of the dorsal-specific genes sis, nr3 and gsc is induced at the vegetal pole after the Mid-Blastula-Transition (MBT). While nr3 and sie expres sion continues in these embryos until gastrula stages, gsc transcripti on cannot be maintained. We propose that the spatial separation of the expression domains of genes with dorsalizing activities and the prosp ective mesodermal region results in the loss of dorsal structures in t he embryo. The role of cortical rotation is to generate an overlap of the region with dorsal axis-forming activity, indicated by nuclear tra nslocation of b-catenin, and the prospective mesoderm in the marginal zone to assure the correct positioning of the Spemann organizer.