Descending Telencephalic Information Reaches Longitudinal Torus and Cerebellum via the Dorsal Preglomerular Nucleus in the Teleost Fish, Pantodon buchholzi: A Case of Neural Preaptation?

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Abstract
The weakly electric mormyrids are known to have an ascending neuronal pathway that reaches the diencephalon and carries information concerned with electrolocation. The recipient diencephalic center, the dorsal preglomerular nucleus, receives a massive telencephalic input and projects to the corpus and valvula cerebelli. This circuitry has been interpreted as a uniquely derived (autapomorphic) feature for mormyrids. In the present study, we demonstrate with the fluorescent neuronal tracer DiI that the closely related, but non-electroreceptive, teleost Pantodon buchholzi possesses a dorsal preglomerular nucleus with similar telencephalo-cerebellar circuitry. The projection to the cerebellum only reaches the corpus, however, not the valvula cerebelli. Further, the dorsal preglomerular nucleus of Pantodon displays a descending pathway via the torus longitudinalis. Two phylogenetic interpretations for the presence of telencephalo-cerebellar pathways in both mormyrids and Pantodon are possible: if such a pathway existed as a preaptation in the common ancestor of mormyrids and Pantodon, it must be an exaptation for electroreception in mormyrids, since this sensory modality evolved anew in this teleost group; alternatively, the pathway evolved in parallel homoplasy, once in Pantodon, as part of a descending premotor pathway, and independently in mormyrids, where the system gains access to ascending electrosensory information.

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