Fig. 9. Roles of FGF signaling in ß-catenin-dependent dorsal axis formation. Two versions of the pathways (A,B) are consistent with experimental data. In both versions, ß-catenin induces chd by two separate pathways (solid arrows), one comprising sequential induction of sqt and genes for FGF ligands, the second dependent on boz expression and involving FGF signaling in the accumulation of boz transcript and in Boz induction of chd. In A, FGFs induced by Sqt signal to repress BMP gene expression, and in B the FGFs act more directly on chd activation and BMP gene repression is a consequence of Chd expression. In both models, Boz acts to induce signaling by an FGF-dependent mechanism and to repress BMPs in an FGF-independent manner, and ß-catenin-induced boz transcript is maintained by FGF signaling. As Boz acts as a repressor (Leung et al., 2003a; Solnica-Krezel and Driever, 2001), it is diagrammed to repress unknown factors, indicated with a generic `X', which in turn repress chd and sqt. Dotted lines show pathways demonstrated in other studies (reviewed by Schier and Talbot, 2005). Cross-activation of boz and sqt has previously been demonstrated in ichabod embryos (Kelly et al., 2000).