FGF signaling is required for β-catenin-induced organizer formation. (A,B) Distribution of phenotypes of ichabod embryos injected with RNAs for β-catenin alone (red) or co-injected with RNAs for β-catenin and the inhibitor of FGF signaling (yellow) XFD (A), MKP3 (B) or the inactive FGF receptor, d50 (light blue, A). Uninjected ichabod embryos were severely ventralized (dark blue). β-catenin can partially or completely rescue most embryos, but both the dominant negative FGF receptor, XFD (A) or MKP3(B) inhibit this rescue. The non-functional FGF receptor, d50, had no effect on β-catenin rescue (A). (C-G) Typical representative embryos are shown for each condition of treatment. (H-M) FGF signaling is essential for formation of the organizer, as assayed by gsc expression. At 50% epiboly, wild-type embryos robustly express this gene in the shield region (H), but ichabod embryos are devoid of expression (I). β-catenin induction of gsc in ichabod embryos (J) is abolished by coexpression of XFD (K) or by injection of SU5402 (M) but not d50(L). Embryos in H-M are shown in animal pole views.
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