FGF is required for post-gastrula mesoderm induction and repression of the NMP state. (A-E) Zebrafish embryos from an HS:dnfgfr1 hemizygous outcross were heat shocked at the 12- or 18-somite stage and analyzed for myoD expression at 24 hpf. Wild-type siblings exhibit ∼31 somites (A,C,E), whereas HS:dnfgfr1 embryos heat shocked at 12 somites have 16 somites and those heat shocked at 18 somites have 22 somites (B,D,E). The number of embryos showing the illustrated phenotype among the total number examined is indicated. Error bars indicate s.d. *P<0.001 using unpaired t-test. (F-I') Expression of ntla (arrowheads) indicates that there is no posterior loss of notochord after FGF inhibition at the 12- or 18-somite stage. (J,K) Embryos heat shocked at 50% epiboly (just at the start of gastrulation) and fixed 3 h after heat shock show a near complete loss of ntla expression. (L,M) Embryos heat shocked at the 8-somite stage and fixed 3 h later exhibit an expansion of ntla into lateral and anterior domains (arrowhead). (N,O) Inhibition of FGF signaling at the 8-somite stage also leads to an expansion of sox2 3 h after the heat shock. (A-D,F,G,H,I) Lateral view, anterior left; (F′,G′,H′,I′,L-O) dorsal view, anterior left; (J,K) vegetal view, dorsal top.
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