Fibroblast growth factor receptors function redundantly during zebrafish embryonic development

Fibroblast growth factor (Fgf) signaling regulates many processes during development. In many cases, one tissue layer secretes an Fgf ligand that binds and activates an Fgf receptor (Fgfr) expressed by a neighboring tissue. Although many Fgf ligands have known requirements in development, less is known about the requirements for the receptors. We have generated null mutations in each of the five fgfr genes in zebrafish. Considering the many requirements for Fgf signaling throughout development and that null mutations in the mouse Fgfr1 and Fgfr2 genes are embryonic lethal, it was surprising that all zebrafish homozygous mutants are viable and fertile, with no discernable embryonic defect. Instead, we have discovered surprising complexity of the Fgf pathway, where multiple receptors are involved in coordinating developmental processes. For example, mutations in the ligand fgf8a cause loss of the midbrain-hindbrain boundary, whereas in the fgfr mutants, this phenotype is only seen in embryos that are triple mutant for fgfr1a;fgfr1b;fgfr2, but not in any single and double mutant combinations. We show that this apparent fgfr redundancy is also seen during the development of several other tissues, including posterior mesoderm, pectoral fins, viscerocranium, and neurocranium. These data therefore begin to define the Fgfrs that function with a particular Fgf ligand to regulate early development in zebrafish.