The cnidarian and the canon: The role of Wnt/β-catenin signaling in the evolution of metazoan embryos

In a recent publication, Wikramanayake and colleagues have implicated the canonical Wnt/β-catenin signaling pathway as a mediator of axial polarity and germ-layer specification in embryos of the cnidarian Nematostella. In this anthozoan, β-catenin is localized in nuclei of blastomeres in one region of the 16- to 32-cell embryo whose descendants subsequently form the entoderm of the embryo. They claim that the pattern of nuclear localization is significant for two reasons: (1) when nuclear localization of β-catenin was inhibited, gastrulation does not occur, and (2) when localization of β-catenin took place in all cells of the pregastrula embryo, the number of entodermal cells increases. Since the Wnt/β-catenin signaling pathway also plays a role in establishing axial polarity and specifying endoderm and mesoderm in a number of bilaterians, Wikramanayake et al. imply that this developmental mechanism is an evolutionary inheritance from a redially symmetrical ancestor. Some of the gaps in the current evidence, which must be filled to evaluate their interpretation, are discussed.