Postnatal brain development of the pulse type, weakly electric gymnotid fish Gymnotus omarorum

• Gymnotus omarorum total brain showed a negative allometric growth with body length.
• Mes- and rhombencephalon growth were negative and positive allometric with brain.
• Cerebellum and electrosensory lateral line lobe showed a positive allometric growth.
• Extraventricular–cisternal proliferation may subserve rombencephalic relative growth.
• The fast electrosensory pathway is already ensembled and functional in late larvae.

Teleosts are a numerous and diverse group of fish showing great variation in body shape, ecological niches and behaviors, and a correspondent diversity in brain morphology, usually associated with their functional specialization. Weakly electric fish are a paradigmatic example of functional specialization, as these teleosts use self-generated electric fields to sense the nearby environment and communicate with conspecifics, enabling fish to better exploit particular ecological niches.We analyzed the development of the brain of the pulse type gymnotid Gymnotus omarorum, focusing on the brain regions involved directly or indirectly in electrosensory information processing. A morphometric analysis has been made of the whole brain and of brain regions of interest, based on volumetric data obtained from 3-D reconstructions to study the growth of the whole brain and the relative growth of brain regions, from late larvae to adulthood. In the smallest studied larvae some components of the electrosensory pathway appeared to be already organized and functional, as evidenced by tract-tracing and in vivo field potential recordings of electrosensory-evoked activity. From late larval to adult stages, rombencephalic brain regions (cerebellum and electrosensory lateral line lobe) showed a positive allometric growth, mesencephalic brain regions showed a negative allometric growth, and the telencephalon showed an isometric growth. In a first step towards elucidating the role of cell proliferation in the relative growth of the analyzed brain regions, we also studied the spatial distribution of proliferation zones by means of pulse type BrdU labeling revealed by immunohistochemistry. The brain of G. omarorum late larvae showed a widespread distribution of proliferating zones, most of which were located at the ventricular–cisternal lining. Interestingly, we also found extra ventricular–cisternal proliferation zones at in the rombencephalic cerebellum and electrosensory lateral line lobe. We discuss the role of extraventricular–cisternal proliferation in the relative growth of the latter brain regions.

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