Exploring the Evolutionary History of Melanin-Concentrating and Melanin-Stimulating Hormone Receptors on Melanophores: Neopterygian (Holostean) and Chondrostean Fishes

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Abstract
The occurrence of melanin-concentrating hormone (MCH) receptors on integumental melanophores was found to extend back in the evolutionary line of ray-finned bony fishes (Actinopterygii) to the group ancestral to modern teleosts, the Holostei. The two species of holosteans studied, Amia calva and Lepisosteus platyrhincus, exhibited changes of melanophore index (melanosome aggregation), indicating responses to MCH and to melatonin but no response to norepinephrine (NE). Polyodon spathula, a species of chondrostean (an older group of bony fishes ancestral to holosteans), failed to respond to MCH, to melatonin, or to NE. Nevertheless, Polyodon skin darkened (melanosome dispersion) in response to melanocyte-stimulating hormone (MSH). The preliminary implication of these observations is that the mechanism of physiological color change involving MCH and its melanophore receptors evolved near the end of the Paleozoic or during the early Mesozoic, just before or early in the evolution of neopterygian (holostean and teleostean) fishes.