A ROLE FOR SIAMOIS IN SPEMANN ORGANIZER FORMATION

The vertebrate body plan is specified in the early embryo through the inductive influence of the organizer, a special region that forms on t he dorsalmost side of the embryo at the beginning of gastrulation, In Xenopus, the homeobox gene Siamois is activated prior to gastrulation in the area of organizer activity and is capable of inducing a seconda ry body axis when ectopically expressed, To elucidate the function of endogeneous Siamois in dorsoventral axis formation, we made a dominant repressor construct (SE) in which the Siamois homeodomain was fused t o an active repression domain of Drosophilia engrailed, Overexpression of 1-5 pg of this chimeric mRNA in the early embryo blocks axis devel opment and inhibits activation of dorsal, but not ventrolateral, margi nal zone markers, At similar expression levels, SE proteins with alter ed DNA-binding specificity do not have the same effect, Coexpression o f mRNA encoding wild-type Siamois, but not a mutated Siamois, restores dorsal development to SE embryos. Furthermore, SE strongly blocks axi s formation triggered by beta-catenin but not by the organizer product noggin, These results suggest that Siamois function is essential for beta-catenin-mediated formation of the Spemann organizer, and that Sia mois acts prior to noggin in specifying dorsal development.