Evolutionary Comparison of Genes Involved in Brain Development: Insights into the Innovation of the Mammalian Neocortex

Little is known about the mechanism underlying the innovation of the mammalian neocortex. To address this issue, we first performed comparative analysis of genes vital for the development of the pallium (Emx1/2 and Pax6) and subpallium (Dlx2 and Nkx1/2) among 500 vertebrate species. We found that these genes have no clear variations in chromosomal duplication/loss, gene locus synteny or Darwinian selection. However, there is an additional fragment of approximately 20 amino acids in mammalian Emx1 and a poly-(Ala)6–7 in Emx2. An innovated method was first employed for gene transfer into the avian brain: Lentiviruses expressing mouse or chick Emx2 (m-Emx2 or c-Emx2 Lv) were injected into the ventricle of the chick telencephalon at embryonic Day 3 (E3), and the embryos were allowed to develop to E12-14 or to posthatchling. After transfection with m-Emx2 Lv, the cells expressing Reelin, Vimentin or GABA increased, and neurogenesis of calbindin cells changed to the mammalian inside-out pattern in the mesopallium (M). In addition, a behavior test for posthatched chicks indicated that the passive avoidance ratio increased significantly. The study suggests that the acquisition of an additional fragment in mammalian Emx2 is associated with the innovation of the mammalian neocortex.