β-catenin has an ancestral role in cell fate specification but not cell adhesion

The {beta}-catenin protein has two major known functions in animal cells. It keeps epithelial tissue homeostasis by its connection with Adherens Junctions (AJ), and it serves as a transcriptional cofactor along with Lef/Tcf to enter the nucleus and regulate target genes of the Wnt/{beta}-catenin (cWnt) signaling pathway. To assess the ancestral role of {beta}-catenin during development we examined its distribution and function in the ctenophore Mnemiopsis leidyi (one of the earliest branching animal phyla) by using ctenophore-specific antibodies and mRNA injection. We found that {beta}-catenin protein never localizes to cell-cell contacts during embryogenesis as it does in other metazoans, most likely because ctenophore-cadherins do not have the cytoplasmic domain required for interaction with the catenin proteins. Downregulation of zygotic Ml{beta}-catenin signaling led to the loss of endodermal and mesodermal tissues indicating that nuclear {beta}-catenin may have a deep role in germ-layer evolution. Our results indicate that the ancestral role for {beta}-catenin was in the cell-fate specification and not in cell adhesion and also further emphasizes the critical role of this protein in the evolution of tissue layers in metazoans.