Figure 3.

The evolutionary assembly of the neural crest gene regulatory network (GRN). Across the tips of the tree we have indicated by the extent of the box, enveloping more or less branches, the evolutionary addition of the various elements of the neural crest GRN as identified by Sauka-Spengler and colleagues(16). This clearly shows that although functional integration of neural crest specifiers and effectors into the GRN occurs only in vertebrates, the early patterning and neural plate border specification network components have a much more protracted evolutionary history, extending at least to the last common ancestor of triploblastic bilaterian eumetazoans. The integration of the key signalling molecules and transcription factors into the neural crest GRN should not, however, be confused with the evolutionary origin of these factors, the majority of which were present in the last common ancestor of metazoans, as indicated by their point of origin in the internal branches of the tree. Finally, the apparent point of evolutionary origin of major neural crest cell phenotypes is also indicated on the internal branches of the tree.