Comparative aspects of forebrain organization in the ray-finned fishes: touchstones or not?

The comparative interpretation of forebrain organization of the ray-finned fishes presents a number of challenges. The telencephalon develops by an eversion process, and much of its surface is ventricular rather than pial. A topological map of the ventricular surface shows that the area ventralis (presumed subpallium) is bordered dorsally by the dorsomedial (DM) zone of the area dorsalis, which is, in turn, bordered by the olfactory-recipient dorsoposterior (DP) zone of the area dorsalis. Within the diencephalon small dorsal and ventral thalami and a large group of migrated nuclei assigned to the posterior tuberculum (but of uncertain status) are present. Both the dorsal thalamus and the migrated nuclei of the posterior tuberculum project to the telencephalon. A review of the known connections of the four major zones of the area dorsalis together with their topological positions leads to the following tentative interpretations. Zone DP is homologous to the primary olfactory cortex. There is not enough information concerning the dorsodorsal zone (DD) to speculate on its comparative relationships. Zone DM is a limbic-like area lying between the subpallium and the olfactory pallium and is possibly homologous to the pallial amygdala. The dorsolateral zone (DL) displays a pattern of connectivity with the cerebellum and the optic tectum suggesting a possible homology with non-olfactory, non-limbic pallial areas. Caveats are raised concerning the above interpretations of DM and DL. One or both of them may have evolved independently of the telencephalic zones in tetrapods and may thus represent examples of homoplasy.