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In bilaterians and cnidarians, embryonic and epithelial cell-polarity are regulated by the interactions between Par proteins, Wnt/PCP signaling pathway, and cell-cell adhesion. Par proteins are highly conserved across Metazoa, including ctenophores. But strikingly, ctenophore genomes lack components of the Wnt/PCP pathway and cell-cell adhesion complexes; raising the question if ctenophore cells are polarized by mechanisms involving Par proteins. Here, by using immunohistochemistry and live-cell imaging overexpression of specific mRNAs, we describe for the first time the subcellular localization of selected Par proteins in blastomeres and epithelial cells during the embryogenesis of the ctenophore Mnemiopsis leidyi. We show that these proteins distribute differently compared to what has been described for other animals, even though they segregate in a host-specific fashion when expressed in cnidarian embryos. This differential localization might be related to the emergence of different junctional complexes during metazoan evolution. Data obtained here challenge the ancestry of the apicobasal cell polarity and raise questions about the homology of epithelial tissue across the Metazoa.

In triploblastic animals, Par proteins regulate cell-polarity and adherens junctions of both ectodermal and endodermal epithelia. But, in embryos of the diploblastic cnidarian Nematostella vectensis, Par proteins are eliminated altogether in the bifunctional endomesodermal epithelia. Using immunohistochemistry, CRISPR/Cas9, and overexpression of specific mRNAs, we describe the functional association between Par proteins, s- catenin, and snail genes in N. vectensis embryos. We demonstrate that the aPKC/Par complex regulates the localization of s-catenin in the ectoderm by directing its role in cell adhesion, and that endomesodermal epithelia are organized by a different cell adhesion system. We also show that snail genes, which are mesodermal markers in bilaterians, are sufficient to downregulate Par proteins and translocate s-catenin from the junctions to the cytoplasm in N. vectensis. These data provide insight into the evolution of epithelial structure and the evolution of mesoderm in metazoan embryos.