Dlx3 is expressed in the ventral forebrain of chicken embryos: implications for the evolution of the Dlx gene family

The archetypal genomic arrangement of vertebrate Dlx genes is as three bigene clusters (Dlx1/2, Dlx3/4, Dlx5/6). Phylogenetic sequence analysis of mouse and zebrafish Dlx clusters supports the notion that the Dlx3/4 cluster is more derived and the absence of expression of either Dlx3 or Dlx4 in the central nervous system, as reported to date, is consistent with this. Together, these observations have prompted a model in which cis-regulatory elements, responsible for directing Dlx gene transcription in the forebrain, were lost from the Dlx3/4 bigene cluster prior to the divergence of tetrapods from fish. Here, we describe Dlx3 expression in the forebrain of chicken embryos; this constitutes the first documented evidence of expression of either Dlx3 or Dlx4 in the central nervous system of a vertebrate. Our observations have implications for models of the evolutionary history of the Dlx gene family, for the genomic organization of Dlx genes in birds and for functional redundancy of Dlx gene function during avian forebrain development.