An Evolutionary Interpretation of Teleostean Forebrain Anatomy

During the past few years, our investigations of the forebrain in the zebrafish (a teleost fish) have shown that its molecular anatomy and expression patterns of genes involved in the regulation of neuronal transmitter phenotypes, such as gamma-aminobutyric acid- (GABA-)ergic neurons, are very similar to those seen in mammalian model organisms such as mouse and rat. For example, we have been able to identify previously undiscovered homologies, such as subpallial regions in the zebrafish that are homologous to the medial and lateral ganglionic eminences in mammals, as well as regions homologous to the larval eminentia thalami and its adult derivative, the bed nucleus of the stria medullaris. Furthermore, in what we term the partial eversion model of the telencephalon in teleosts, we propose homologies to all four mammalian pallial areas and conclude that the posterior zone of the dorsal telencephalic area in teleosts is homologous to the piriform cortex and is formed by a migratory stream of cells originating in a dorsomedial zone of the pallium (the primordial medial zone of area dorsalis telencephali). In this review we critically discuss and justify these findings in the context of forebrain evolution in fishes.