Wnt signaling and the evolution of a functional gut. (A) In Nematostella, initial archenteron invagination is regulated by bottle cell induction by NvStbm, and this can occur independently of endoderm cell fate specification mediated by Wnt/ß-catenin signaling. Both Wnt pathways are required for completion of gastrulation. It is possible that a single upstream ligand (X) or receptor (Fz) coordinates the activation of both branches of Wnt signaling during embryogenesis. (B) Model indicating the co-option of a polarity found in the oocytes of the last common ancestor to the eumetazoa to locally activate Wnt signaling at the animal pole. The centrosomes associated with the oocyte nucleus at meiosis served as a scaffold to localize critical Wnt pathway components to the apical pole (a). These components would be inherited by blastomeres at the animal pole (b), and their activation would drive apical constriction of these cells to form bottle cells (c), leading to the initial invagination of an archenteron (d). Endoderm specification mediated by Wnt/ß-catenin could have been coordinated with primary archenteron invagination by a single upstream ligand (A). Alternatively, endoderm specification could have occurred after the evolution of the primary invagination by the localized activation of the Wnt/ß-catenin pathway at the animal pole.