Evolution of regional identity in the vertebrate nervous system.

When and how did the mechanisms controlling regional identity in the vertebrate neural tube arise during evolution? The anatomy and embryology of the major deuterostome phyla (echinoderms, hemichordates, chordates) suggest that a true neural tube with dorsoventral and mediolateral regionalization arose with the chordates. We suggest that this was intimately associated with the origin of the notochord; this leads us to propose a modification of Garstang's century-old scenario for origins of the chordate neural tube. Differences along the rostrocaudal axis are seen in all chordates, but became particularly pronounced with the origin of a brain in craniates. Recent molecular data are starting to give insights into these evolutionary transitions. Here we review how Hox gene expression patterns are giving clues to brain origins and we examine the role of molecular phylogenetics in these analyses. We also ask whether the molecular evolution of genes such as noggin, Brachyury, Sonic hedgehog, Wnt, and En may have played direct or permissive roles in the origins of the neural plate, notochord, floor plate, and brain.