FGF Signaling Emerged Concomitantly with the Origin of Eumetazoans

Complex metazoan bodies require cell-to-cell communication for development, a process often mediated by signaling molecules binding to specific receptors. Relatively few signaling pathways have been recruited during evolution to build multicellular animals from unicellular zygotes. Of these few signaling pathways, one of particular importance is the receptor tyrosine kinase (RTK) pathway. In metazoans, fibroblast growth factors (FGFs) bind to receptors in the RTK family, but the origin of the FGF gene family has so far remained a mystery. Here we show that extant bona fide FGFs most likely originated from proteins bearing an FGF-like domain that arose in a choanoflagellate/metazoan ancestor. We found orthologous genes closely related to FGF in choanoflagellates as well as in many metazoans such as sponges, acoels, protostomes, or nonvertebrate deuterostomes. We also show that these genes have a common evolutionary history with Retinitis Pigmentosa 1 (RP1). Even if some metazoan signaling pathways emerged long before multicellularity, we show that FGFs, like their receptors, originated in a eumetazoan ancestor.