Identification of a neural crest rudiment in a non-vertebrate chordate

Neural crest arises at the neural plate border, expresses a core set of regulatory genes, and produces a diverse array of cell types including ectomesenchyme derivatives that elaborate the vertebrate head(1,2). The evolution of neural crest has been postulated as a key event leading to the appearance of new cell types that fostered the transition from filter feeding to active predation in ancestral vertebrates(3). However, the origin of neural crest remains controversial, since homologous cell types have not been unambiguously identified in non-vertebrate chordates(1,4). Here we show that the tunicate Ciona intestinalis possesses a cephalic melanocyte lineage (a9.49) similar to neural crest that can be reprogrammed into migrating ectomesenchyme by the targeted misexpression of Twist. Our results suggest that the neural crest melanocyte regulatory network predated the divergence of tunicates and vertebrates. We propose that the co-option of mesenchyme determinants, such as Twist, into the neural plate ectoderm was crucial for the emergence of the vertebrate “new head”(3).