Integrated brain diversification along the early neuraxes

Brains develop under the influence of signaling centers that link major dorsal/ventral (DV) and anterior/posterior (AP) axes. Over ontogeny, these 'developmental neuraxes' progress from near global signaling gradients into more localized gene expression domains separated by molecular boundaries. Therefore, developmental changes along a neuraxis can have major consequences across the brain, or more precise effects on a specific structure, depending upon the time during ontogeny in which change occurs. It is well known from mammalian systems how evolution has acted later in development, via differential neurogenesis, to reshape the cortex. Recent studies in fishes show how early changes in DV and AP patterning result in divergence of integrated brain regions that ultimately define visual versus olfactory ecotypes. We explore the generality of this trend and suggest that such early developmental differences integrating brain diversification along the neuraxes may be a common theme in vertebrates. Early differences in brain patterning among species imply that adult variation in sensory function and behavior manifests in the embryo.