Segregation of endoderm and mesoderm germ layer identities in the diploblast Nematostella vectensis

A recent study suggested that morphologically diploblastic sea anemones (Cnidaria) have three segregated germ layer identities corresponding to the bilaterian germ layers. Here, we investigated how these germ layer identities are specified during early development of the sea anemone Nematostella vectensis. Our gene expression analysis shows that the mesodermal territory is specified at the animal pole at 6 h postfertilization, followed by the specification of the definitive endoderm between mesoderm and ectoderm. We then assessed the role of β-catenin, MAPK, and Notch signaling during mesoderm and endoderm formation. We show that the mesodermal marker genes are activated by MAPK signaling while being repressed elsewhere by β-catenin signaling. Delta-expressing mesoderm then signals to Notch-expressing ectoderm, inducing the definitive endoderm domain at the mesoderm/ectoderm interface. Gain- and loss-of-function experiments showed that Notch signaling is sufficient for endoderm induction. Based on our results, we propose a model of germ layer specification in Nematostella defined by a crosstalk of MAPK, β-catenin, and Notch signaling. Given the similarity of the germ layer specification between the sea anemone and echinoderms, we propose that triploblasty may have predated the split of cnidarians and bilaterians.