Expression of the <i>Emx-1</i> and <i>Dlx-1</i> homeobox genes define three molecularly distinct domains in the telencephalon of mouse, chick, turtle and frog embryos: implications for the evolution of telencephalic subdivisions in amniotes

TLDR

Homologies between vertebrate forebrain subdivisions remain uncertain, especially regarding homologs of the mammalian neocortex and the dorsal ventricular ridge of birds and reptiles. The study aimed to map the organization of the main telencephalic subdivisions along the anteroposterior axis by generating a fate map of the dorsal prosencephalon in avian chimeras at the 8‑ to 9‑somite stage. The fate map revealed that the pallium, DVR, and striatum primordia lie dorsally and are caudorostrally ordered, and gene expression of Emx, Dlx, and Pax families was used to mark anteroposterior subdivisions across mouse, chick, turtle, and frog. Expression domains defined three telencephalic subdivisions—pallial, intermediate, and striatal—in all species at neurogenesis onset, with later divergence of intermediate fate across species, leading to proposed homologies based on conserved and divergent gene patterns.

Abstract

ABSTRACT Homologies between vertebrate forebrain subdivisions are still uncertain. In particular the identification of homologs of the mammalian neocortex or the dorsal ventricular ridge (DVR) of birds and reptiles is still a matter of dispute. To get insight about the organization of the primordia of the main telencephalic subdivisions along the anteroposterior axis of the neural tube, a fate map of the dorsal prosencephalon was obtained in avian chimeras at the 8- to 9-somite stage. At this stage, the primordia of the pallium, DVR and striatum were located on the dorsal aspect of the prosencephalon and ordered caudorostrally along the longitudinal axis of the brain. Expression of homeobox-containing genes of the Emx, Dlx and Pax families were used as markers of anteroposterior developmental subdivisions of the forebrain in mouse, chick, turtle and frog. Their expression domains delineated three main telencephalic subdivisions in all species at the onset of neurogenesis: the pallial, intermediate and striatal neuroepithelial domains. The fate of the intermediate subdivisions diverged, however, between species at later stages of development. Homologies between forebrain subdivisions are proposed based on the conservation and divergence of these gene expression patterns.