Siamois cooperates with TGFbeta signals to induce the complete function of the Spemann-Mangold organizer

In Xenopus, the Spemann-Mangold organizer induces and patterns the body axis. Siamois, a Wnt-responsive transcriptional activator, functions to establish and maintain the Spemann-Mangold organizer by regulating organizer gene transcription. While expression of Siamois in marginal blastomeres induces an axis consisting of both head and trunk structures, we show that expression of Siamois in animal blastomeres induces an axis that lacks head structures. Consistent with the absence of head organizer activity in Siamois-expressing animal pole tissue, Siamois did not induce animal expression of Cerberus, Frzb1 and Xlim1, genes implicated in anterior development. A dominant negative form of Siamois inhibited endogenous expression of Cerberus, Frzb1 and Xlim1, indicating that Siamois is necessary for organizer-specific expression of these head organizer genes, but is not sufficient in animal tissue. Siamois induces Cerberus, Frzb1 and Xlim1 in vegetal blastomeres and vegetal induction by Siamois is dependent on endogenous TGFbeta signals. The results provide evidence that Siamois cooperates with TGFbeta signals to activate the expression of organizer genes and to generate an organizer with both head- and trunk-inducing activity.