Evolutionarily conserved and divergent expression of members of the FGF receptor family among vertebrate embryos, as revealed by FGFR expression patterns in Xenopus

Fibroblast growth factors (FGFs) mediate many cell-cell signaling events du ring early development. While the actions of FGFs have been well-studied, t he roles played by specific members of the FGF receptor (FGFR) family are p oorly understood. To characterize the roles played by individual FGFRs we c ompared the regulation and expression of the three Xenopus FGFRs described to date (XFGFR-1, XFGFR-2, and XFGFR-4). First, we describe the expression of Xenopus FGFR-4; XFGFR-4 is present as a maternal mRNA and is found in th e embryo through at least the tadpole stage. XFGFR-4 and XFGFR-1 mRNAs are present at comparable levels, arguing that both mediate FGF signaling durin g early development. Second, the expression of XFGFR4 in animal caps differ s from the expression of XFGFR-1 and XFGFR-2, suggesting that the FGFRs are independently regulated in ectoderm. Third, using whole-mount in situ hybr idization, we show that XFGFR-1. XFGFR-2, and XFGFR-4 are expressed in dram atically different patterns, arguing that specific FGF signaling events are mediated by different members of the FGFR family. Among these, FGF signali ng during the induction of neural crest cells is likely to be mediated by X FGFR4. Comparison of our results with previously reported FGFR expression p atterns reveals that FGFR-1 expression is highly conserved among vertebrate embryos, and FGFR-2 expression shows many features that are conserved and some that are divergent. In contrast, the expression pattern of FGFR-4 is h ighly divergent among vertebrate embryos.