PROJECT SUMMARY The sea squirt Ciona intestinalis is member of the urochordates, the closest living relatives of the vertebrates. Ciona tadpoles provide a favorable system for studying the evolutionary origins of key vertebrate innovations such as cranial placodes and neural crest. There are four major sensory cell types that arise from the neural plate border, which is the source of placodes and neural crest in vertebrates. The Ciona system is ideally suited to elucidate the gene networks and developmental mechanisms underlying the specification of all four sensory cell types: hair cells (head), GnRH-expressing aATENs (trunk), pATENs and BTNs (tail). The detailed understanding of these cell types should provide new insights into the evolutionary origins and developmental mechanisms underlying the specification of comparable cell types derived from placodes and neural crest in vertebrates, particularly placodes. During the preceding funding period we obtained evidence that the hair cells and aATENs arise from a proto-placodal territory straddling the anterior border of the presumptive neural tube. The aATENs possess dual chemosensory and neurosecretory properties, suggesting they represent an ancestral cell type that produced specialized cell types through a process of “cellular subfunctionalization” during the evolution of the vertebrates. The pATENs and BTNs possess similarities with derivatives of the neural crest in vertebrates, such as dorsal root ganglia. There are two over-arching goals of the proposed study: (i) explore the specification, differentiation, and relatedness of hair cells, aATENs, pATENs, and BTNs in the Ciona tadpole; and, (ii) explore the process of cellular subfunctionalization, whereby ancestral sensory cell types in Ciona produce multiple specialized cell types forming neuronal circuits in vertebrates.