Dev112029 1..9

Bilaterian neurogenesis is characterized by the generation of diverse neural cell types from dedicated neural stem/progenitor cells (NPCs). However, the evolutionary origin of NPCs is unclear, as neurogenesis in representatives of the bilaterian sister group, the Cnidaria, occurs via interstitial stem cells that also possess broader, non-neural, developmental potential. We address this question by analysing neurogenesis in an anthozoan cnidarian, Nematostella vectensis. Using a transgenic reporter line, we show that NvSoxB(2) – an orthologue of bilaterian SoxB genes that have conserved roles in neurogenesis – is expressed in a cell population that gives rise to sensory neurons, ganglion neurons and nematocytes: the three primary neural cell types of cnidarians. EdU labelling together with in situ hybridization, and within the NvSoxB(2)::mOrange transgenic line, demonstrates that cells express NvSoxB(2) before mitosis and identifies asymmetric behaviours of sibling cells within NvSoxB(2) lineages. Morpholino-mediated gene knockdown of NvSoxB(2) blocks the formation of all three neural cell types, thereby identifying NvSoxB(2) as an essential positive regulator of nervous system development. Our results demonstrate that diverse neural cell types derive from anNvSoxB(2)-expressing population of mitotic cells in Nematostella and that SoxB genes are ancient components of a neurogenic program. To our knowledge this is the first description of a lineage-restricted, multipotent cell population outside the Bilateria and we propose that neurogenesis via dedicated, SoxB-expressing NPCs predates the split between cnidarians and bilaterians.