Brain Distribution of Orthopedia (Otp) Transcription Factor in Bony Fish: A Comparative Neuroanatomical Perspective.

The transcription factor Orthopedia (Otp) plays a key role in neuronal differentiation, regional specification, and long-term maintenance of neuronal identity across vertebrates. Although Otp expression is widely used as a landmark for hypothalamic regionalization, its distribution in bony fishes has remained poorly characterized. To assess the evolutionary conservation of Otp expression, we analyzed its immunohistochemical distribution in the central nervous system of representative species from all major clades of bony fishes: cladistians, chondrosteans, holosteans, teleosts, and lungfish. To refine anatomical localization and evaluate potential cellular coexpression, we additionally examined markers including calbindin, tyrosine hydroxylase (TH), and the transcription factors Islet-1, Pax7, and Satb1/2. Otp-immunoreactive neurons were consistently observed in the medial amygdala, preoptic area, paraventricular region, all basal hypothalamic domains except the mamillary region, as well as in a population in alar rhombomere 1, the dorsal interpeduncular nucleus, central gray, reticular formation, and the dorsal horn of the spinal cord. Notably, Otp-positive cells were detected in the posterior tubercle only in actinopterygian fishes, in contrast to lungfish. Overall, this Otp expression pattern is highly conserved in relation to that reported in cartilaginous fishes and tetrapods. This study closes a major phylogenetic gap and demonstrates that the brain-wide expression pattern of Otp is largely conserved across vertebrates, supporting its fundamental and ancient role in neuronal differentiation and cell-type specification in forebrain and hindbrain.