EXPRESSION OF THE EMX-1 AND DIX-1 HOMEOBOX GENES DEFINE 3 MOLECULARLYDISTINCT DOMAINS IN THE TELENCEPHALON OF MOUSE, CHICK, TURTLE AND FROG EMBRYOS - IMPLICATIONS FOR THE EVOLUTION OF TELENCEPHALIC SUBDIVISIONS IN AMNIOTES

\omologies between vertebrate forebrain subdivisions are still uncerta in, In particular the identification of homologs of the mammalian neoc ortex or the dorsal ventricular ridge (DVR) of birds and reptiles is s till a matter of dispute, To get insight about the organization of the primordia of the main telencephalic subdivisions along the anteropost erior axis of the neural tube, a fate map of the dorsal prosencephalon was obtained in arian chimeras at the 8- to 9-somite stage. At this s tage, the primordia of the pallium, DVR and striatum were located on t he dorsal aspect of the prosencephalon and ordered caudorostrally alon g the longitudinal axis of the brain. Expression of homeobox-containin g genes of the Ems, Dir and Pax families were used as markers of anter oposterior developmental subdivisions of the forebrain in mouse, chick , turtle and frog. Their expression domains delineated three main tele ncephalic subdivisions in all species at the onset of neurogenesis: th e pallial, intermediate and striatal neuroepithelial domains. The fate of the intermediate subdivisions diverged, however, between species a t later stages of development. Homologies between forebrain subdivisio ns are proposed based on the conservation and divergence of these gene expression patterns.