Connections of the Pallial Telencephalon in the Senegal Bichir, Polypterus

Abstract

The connections of the dorsal telencephalon (pallium) of the Senegal bichir (Polypterus senegalus) were investigated in order to test the hypothesis that the simple everted pallium of polypterid fishes is directly comparable to the evaginated pallium of most vertebrates. Neuroanatomical tracers (DiI or biotinylinated dextran amines) were injected into the three traditionally recognized divisions of the bichir pallium to determine their afferent and efferent connections. The connections were corroborated with injections into regions identified as projecting to, or receiving input from, one or more pallial divisions. The results suggest that the bichir pallium consists not of three, but only two primary divisions: a dorsomedial one and a dorsolateral one. This reinterpretation refutes the hypothesis of a direct one-to-one comparison with the tripartite pallium of most vertebrates. Homologues of the bichir dorsomedial and dorsolateral pallium are recognized in the lateral and medial pallium of anuran amphibians, respectively. The pallium of bichirs is compared to that of derived ray-finned fishes as a link between derived ray-finned fishes and other vertebrates. The available information on the connections of the pallium of teleosts suggests that only the olfactory recipient (pars posterior) of the pallium can be directly compared to bichirs and amphibians and that the remaining divisions of the pallium in teleosts are uniquely derived features of the teleost telencephalon.