The elaboration of the complex vertebrate brain is thought to depend partly on a unique signalling centre known as the midbrain-hindbrain boundary (MHB)organiser. But now (on ), Kaoru Imai and colleagues report that an MHB-like organising structure patterns the central nervous system (CNS) of the invertebrate sea squirt, Ciona intestinalis. Ciona is a tunicate, which, like vertebrates, belong to the chordate phylum. Its larvae closely resemble frog tadpoles and possess a simplified CNS with at least four morphologically distinct compartments. By characterising gene expression and function in the Ciona larvae CNS, the researchers assemble a provisional gene regulatory network (GRN) for CNS development in Ciona. This GRN reveals that an FGF, called FGF8/17/18, has a central role in CionaCNS compartmentalisation, similar to that of FGF8 in patterning the vertebrate MHB. Based on these findings, the researchers propose that FGF8-mediated CNS patterning was present in the last common ancestor of tunicates and vertebrates, and served to delineate two regions of the chordate brain.
