Histochemical, connectional and cytoarchitectonic evidence for a secondary reduction of the pretectum in the European eel, Anguilla anguilla: a case of parallel evolution.

There are at least three different patterns of pretectal organization in teleost fishes: a simple pattern observed in cyprinids, an elaborate pattern present in percomorphs, and an intermediately complex pattern seen in many other teleost groups. The taxonomic distribution of the pretectal patterns indicates that the simple and the elaborate patterns are both evolutionarily derived (apomorphic) from the primitive (plesiomorphic) intermediately complex one. In anguillids, the pretectal pattern observed cytoarchitectonically has an anatomical configuration similar to that of the simple pattern in cyprinids. The distribution of acetylcholinesterase positivity in the pretectum (namely acetylcholinesterase positivity in the parvo- and magnocellular superficial and posterior pretectal nuclei, and acetylcholinesterase negativity in the pretectal cell plate and the ovoid preglomerular cell aggregate), as well as the retinal projections (namely retinal terminals in the parvocellular superficial and central pretectal nuclei, and absence of such terminals in the magnocellular superficial and posterior pretectal nuclei and the pretectal cell plate), strongly supports the interpretation suggested by the cytoarchitectonic analysis. As anguillids (elopomorpha) and cyprinids (ostariophysi) are related only distantly, this secondary simplification in the pretectum likely occurred independently, i.e. this simplification represents a case of parallel reduction.