Notch expression during ctenophore development gives insight into its ancestral function

The canonical Notch pathway is a juxtacrine signaling module with widely conserved roles maintaining progenitor cell populations, promoting binary cell fate decisions, and establishing tissue boundaries. This pathway emerged in Metazoan lineages, although many molecular components and regulators existed prior to that divergence. Given that the vast majority of Notch studies focus on bilaterians, it is unclear when or how Notch gained its developmental signaling functions. To clarify the ancestral function of Notch, we turned to an early branching Metazoan -- the ctenophore Mnemiopsis leidyi -- and conducted structural analyses of putative Notch components to evaluate predicted signaling potential. We characterized gene expression of these components with in situ hybridization and show they are expressed during late embryogenesis. Using hybridization chain reaction (HCRTM), we examine MlNotch expression relative to canonical transcriptional targets and putative stem cell markers and identify differential expression patterns at a cellular resolution. Pharmacological inhibition reveals zones of active and inactive MlNotch during late embryonic development. Our results suggest that Notch evolved its signaling potential by the Metazoan divergence and is likely involved in regulating progenitor cell populations and coordinating developmental fate decisions.