Fig. 6. Xnr3 function in convergent extension requires maternal FGFR1 receptor. (A) Animal caps dissected from FGFR1^- late blastulae were unable to elongate in the presence of basic FGF, and this defect was specifically rescued by the injection of 75 pg of synthetic FGFR1 mRNA at the 2-cell stage. (B) Convergent extension movement in Xnr3-overexpressing animal caps was inhibited by the depletion of maternal FGF receptor, FGFR1. FGFR1^– caps were also inhibited from responding to basic FGF but not activin. The experiment was repeated with the same result. (C) FGFR1 depletion repressed the induction of MyoD (upper histogram) but not of NCAM (lower histogram) in Xnr3 over-expressing animal caps. In contrast, FGFR1 depletion did not prevent the induction of MyoD by activin. Gene expressions were analyzed by real-time RT-PCR system at stage 20. In each case, ornithine decarboxylase (ODC) was used as a loading control (data not shown), and each bar was normalized to the level of ODC expression. (D) The phenotype of FGFR1^– embryos. Oocytes were injected with 3 or 4 ng of antisense FGFR1 oligo, fertilized by the host transfer technique and photographed at the tailbud stage. There was a dose response of gastrulation and convergent extension abnormalities, with dorsally curved axes and open neural folds. (E) Histogram of RT-PCR analyses for Xbra and chordin in sibling embryos of the embryos shown in D, frozen at the early (left histogram) and mid-gastrula (right histogram) stages. FGFR1 depletion (FGFR1^– high=4 ng dose of oligo) prevents the expression of Xbra and this was rescued by the re-introduction of FGFR1 mRNA. In contrast, dorsal mesodermal markers such as chordin were little affected by FGFR1 depletion (upper histogram). (F) Xnr3 induces activation of ERK2. Animal caps were isolated from stage 9.5 embryos injected with eFGF (5 pg), FRL1 (2 ng), Xnr3 (500 pg) or Xnr1 (500 pg), cultured until stage 10 and subjected to immunoblotting for phosphorylated (activated) ERK2. -tubulin was used as a loading control. Oocytes, untreated or incubated in progesterone (+Prog.), were included as negative and positive controls, respectively.

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