Nodal-Activin pathway is a conserved neural induction signal in chordates

Neural induction is the process through which pluripotent cells are committed to a neural fate. This first step of central nervous system formation is triggered by the 'Spemann organizer' in amphibians and by homologous embryonic regions in other vertebrates. Studies in classical vertebrate models have produced contrasting views about the molecular nature of neural inducers and no unifying scheme could be drawn. Moreover, how this process evolved in the chordate lineage remains unresolved. Here we show, by using graft and micromanipulation experiments, that the dorsal blastopore lip of the cephalochordate amphioxus is homologous to the vertebrate organizer and is able to trigger the formation of neural tissues in a host embryo. In addition, we demonstrate that Nodal-Activin is the main signal eliciting neural induction in amphioxus, and that it also functions as a bona fide neural inducer in the classical vertebrate model Xenopus. Together, our results allow us to propose that Nodal-Activin was a major factor for neural induction in the ancestor of chordates. This study further reveals the diversity of neural inducers used during chordate evolution and provides support against a universally conserved molecular explanation for this process.