Fig. 2

A transverse diagrammatical representation of Apteronotus leptorhynchus and Brachyhypopomus gauderio at various stages of development. In A. leptorhynchus, a neurogenic electric organ (nEO) (e) develops during the larval stage on the ventral side of the vertebrae (v), as two pear-shaped structures located on either side of the aorta (a) [31]. By the adult stage, this nEO has undergone a massive expansion. In B. gauderio, we have recently discovered that a pair of pear-shaped nerve cords (PSNCs) (p) develop in an equivalent position at the embryonic stage [1]. However, their level of expansion by the adult stage is highly limited compared to that of the nEO in A. leptorhynchus [1, 31]. These findings suggest that the PSNCs in B. gauderio may represent the evolutionary origin of the nEO in A. leptorhynchus