Pax6 and Pax7 in the Central Nervous System of Cladistian Fishes: A Comprehensive Expression Analysis

Among actinopterygian fishes, cladistians stand as the more basal extant species in the group, holding a key phylogenetic position close to the common ancestor of Osteichthyes. Despite the recent publication of studies regarding the neurochemical organization of their central nervous system (CNS), there is still a significant lack of genoarchitectonic data that may prove essential to fully understand the patterning of the brain of these fishes. The paired box genes Pax6 and Pax7 are known to determine several boundaries in the CNS and are indispensable, for instance, for the survival of neurons and the change from cell proliferation to cell differentiation. By means of immunohistofluorescence methods, we analyzed the expression patterns of the transcription factors Pax6 and Pax7 in the CNS of three representative species of cladistian fishes, with a particular focus on their evolutionary implications. Thus, conserved Pax6 immunoreactive cell groups were present in the olfactory bulb, subpallial areas, the prethalamus, the basal prosomere 3, the pretectum, the mesencephalic tegmentum, the cerebellum, the basal rhombencephalon, the spinal cord, and the retina. A number of exclusive features were identified, including the almost total absence of expression in the pallium, which was observed only in cladistians, and its absence in the hypothalamus, which is a primitive anamniote trait. Likewise, the Pax7 expression pattern was generally conserved, with traits like the absence of labeling in the telencephalon and the expression in the retromamillary hypothalamic domain, the basal prosomere 3, the pretectum, the optic tectum, and the alar part of the first rhombomere. Additionally, no Pax7 labeling was detected in the spinal cord, comprising a specific cladistian feature.