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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).
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