Defective FGF-dependent mesodermal gene expression and aberrant splicing of fgf8 and fgfr2. (A–H) Activin can induce eomes but not bra in fus morphants. (A,B) Normal expression of bra and eomes at stage 10.5. (C,D) activin mRNA injection induces ectopic bra and eomes at the site of injection (red staining) in normal embryos. (E,F) fusMO alone represses bra and, at this stage, modestly enhances eomes expression. (G,H) activin mRNA in combination with fusMO fails to induce ectopic bra expression, whereas activin potently induces ectopic eomes in combination with fusMO. (I) Defective splicing of fgf8 transcripts in fus morphants. RT–PCR showing fgf8a and fgf8b transcripts. In the fusMO lane, an aberrant transcript, fgf8b-in1ret, retains intron 1. (J) Selective decrease of fgfr2 transcript in fusMO morphants. RT–PCR using primers to amplify full-length fgfr1–4 transcripts. (K) RT–PCR using primers for individual splice forms of fgfr2 shows selective absence of the fgfr2c splice form in fus morphants, whereas fgfr2b is unaffected. odc was used as RT–PCR control. (L–N) Rescue of bra expression pattern by fgf8b and fgfr2c mRNA injection in fus morphants. (L) Normal bra expression at stage 10.5. (M) fus knockdown represses bra expression. (N) Coinjection of fgf8b and fgfr2c mRNA into two blastomeres at the two-cell stage of fus morphants partly restores endogenous bra expression.