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β-catenin-dependent endomesoderm specification appears to be a Bilateria-specific co-option

Endomesoderm specification based on a maternal β-catenin signal and axial patterning by interpreting a gradient of zygotic Wnt/β-catenin signalling was suggested to predate the split between Bilateria and their evolutionary sister Cnidaria. However, in Cnidaria, the roles of β-catenin signalling in both these processes have not been proven directly. Here, by tagging the endogenous β-catenin protein in the sea anemone Nematostella vectensis, we show that the oral-aboral axis in a cnidarian is indeed patterned by a gradient of β-catenin signalling. Unexpectedly, in a striking contrast to Bilateria, Nematostella endoderm specification takes place opposite to the part of the embryo, where β-catenin is translocated into the nuclei. This suggests that β-catenin-dependent endomesoderm specification is a Bilateria-specific co-option, which may have linked endomesoderm specification with the subsequent posterior-anterior patterning.