Intercellular signaling underpins metazoan development by mediating the induction, organization, and cooperation of cells, tissues, and organs. Herein, the origins of the four major signaling pathways used during animal development and differentiation-Wnt, Notch, transforming growth factor-beta (TGF-beta), and Hedgehog-are assessed by comparative analysis of genomes from bilaterians, early branching metazoan phyla (poriferans, placozoans, and cnidarians), and the holozoan sister clade to the animal kingdom, the choanoflagellates. On the basis of the incidence and domain architectures of core pathway ligands, receptors, signal transducers, and transcription factors in representative species of these lineages, it appears that the Notch, Wnt, and TGF-beta pathways are metazoan synapomorphies, whereas the Hedgehog pathway arose in the protoeumetazoan lineage, after its divergence from poriferan and placozoan lineages. Examination of the binding domains and motifs present in signaling pathway components of nonbilaterians reveals cases in which signaling interactions are unlikely to be operating in accordance with bilaterian canons. Overall, this study highlights the stability and antiquity of the core cytosolic components of each pathway, juxtaposed with the more variable and recently evolved molecular interactions taking place at the cell surface.
