Abstract
The Notch signaling pathway plays a major role in embryological development and in the ongoing life processes of many animals. Its role is to provide cell-to-cell communication in which a Sender cell, bearing membrane-embedded ligands, instructs a Receiver cell, bearing membrane-embedded receptors, to adopt one of two available fates. Elucidating the evolution of this pathway is the topic of this paper, which uses an orthologs approach, providing a comprehensive basis for the study. Using BLAST searches, orthologs were identified for all the 49 components of the Notch signaling pathway. The historical time course of integration of these proteins, as the animals evolved, was elucidated. Insofar as cell-to-cell communication is of relevance only in multicellular animals, it is not surprising that the Notch system became functional only with the evolutionary appearance of Metazoa, the first multicellular animals. Porifera contributed a quarter of the Notch pathway proteins, the Cnidaria brought the total to one-half, but the system reached completion only when humans appeared. A literature search elucidated the roles of the Notch system's components in modern descendants of the ortholog-contributing ancestors. A single protein, the protein tyrosine kinase (PTK) of the protozoan Ministeria vibrans, was identified as a possible pre-Metazoan ancestor of all three of the Notch pathway proteins, DLL, JAG, and NOTCH. A scenario for the evolution of the Notch signaling pathway is presented and described as the co-option of its components, clade by clade, in a repurposing of genes already present in ancestral unicellular organisms.