Pax6 expression highlights regional organization in the adult brain of lungfishes, the closest living relatives of land vertebrates

Abstract

The Pax6 gene encodes a regulatory transcription factor that is key in brain develop-ment. The molecular structure of Pax6, the roles it plays and its patterns of expres-sion in the brain have been highly conserved during vertebrate evolution. Asneurodevelopment proceeds, the Pax6 expression changes from the mitotic germinalzone in the ventricular zone to become distributed in cell groups in the adult brain.Studies in various vertebrates, from fish to mammals, found that the Pax6 expressionis maintained in adults in most regions that express it during development. Specifi-cally, in amphibians, Pax6 is widely expressed in the adult brain and its distributionpattern serves to highlight regional organization of the brain. In the present study, weanalyzed the detailed distribution of Pax6 cells in the adult central nervous systemof lungfishes, the closest living relatives of all tetrapods. Immunohistochemistryperformed using double labeling techniques with several neuronal markers of knowndistribution patterns served to evaluate the actual location of Pax6 cells. Our resultsshow that the Pax6 expression is maintained in the adult brain of lungfishes, in dis-tinct regions of the telencephalon (pallium and subpallium), diencephalon, mesen-cephalon, hindbrain, spinal cord, and retina. The pattern of Pax6 expression is largelyshared with amphibians and helps to understand the primitive condition that wouldhave characterized the common ancestors to all sarcopterygians (lobe-finned fishes and tetrapods), in which Pax6 would be needed to maintain specific entities of sub-populations of neurons.