Fgf signaling depends on Twist1 and regulates a subset of ectomesenchyme gene expression.

<p>(A,B) In situs at 18 hpf show that expression of the Fgf target gene <i>pea3</i> is reduced in the arches (numbered) of <i>twist1a/1b</i>-MO embryos (n = 8/8) compared to un-injected controls (n = 0/12). (C–L) <i>sox10</i>:Gal4VP16; <i>UAS</i>:dnFgfr1a embryos show ectopic arch expression of <i>sox10</i> at 18 hpf (n = 8/8) versus controls (n = 0/8), reduction of <i>dlx2a</i> at 24 hpf (n = 3/3) versus controls (n = 0/3), but no change in <i>fli1a</i> at 24 hpf (n = 0/7) versus controls (n = 0/5). <i>twist1a</i> expression was unchanged at 24 hpf in dnFgfr1a embryos (n = 10) compared to controls (n = 7), as was <i>twist1b</i> expression in dnFgfr1a embryos (n = 8) compared to controls (n = 8). Black arrows indicate the second arch, white arrowheads the ear, and red arrowheads the vasculature. Scale bar = 50 µm.