Notch, β-catenin and MAPK signaling segregate endoderm and mesoderm in the diploblast Nematostella vectensis

Cnidaria are typically considered diploblastic (i.e. consisting of two germ layers) in contrast to their triploblastic sister clade, the Bilateria. However, a recent study suggested that sea anemones and other cnidarians have three segregated germ layer identities, corresponding to the bilaterian germ layers ¹ . Here, we investigated, how the three germ layer identities are specified during early development of the sea anemone Nematostella vectensis . First, the mesodermal territory is specified at the animal pole at 6 hours postfertilization, followed by the specification of a ring of endodermal territory between mesoderm and ectoderm. We then assessed the role of β-catenin, MAPK and Notch signaling pathways during mesoderm and endoderm formation. Our results show that the mesoderm is initiated by MAPK signaling and simultaneously restricted to the future oral side by mutually exclusive nuclear β-catenin signaling. The mesodermal cells then express the Delta ligand, while the ectodermal cells express the Notch receptor. Inhibition of Notch signaling as well as ectopic expression of the Notch intracellular domain showed that endodermal tissue identity is induced by Notch signaling at the boundary between mesoderm and ectoderm. We propose a new model that outlines the different steps leading to the segregation of mesoderm and endoderm identities in Nematostella , confirming the presence of 3 distinct germ layer identities in this cnidarian. Notably, the observed crosstalk of MAPK, β-catenin and Notch signaling in the specification of three germ layers in Nematostella is highly reminiscent to early stage gastrulae of sea urchins suggesting that triploblasty may predate the split of cnidarians and bilaterians.