The structure and evolution of the melanocortin and MCH receptors in fish and mammals.

Zebrafish are an excellent genetic model system for studying developmental and physiological processes. Pigment patterns in zebrafish are affected by mutations in three types of chromatophores. The behavior of these cells is influenced by alpha-melanocyte-stimulating hormone (alphaMSH) and melanin-concentrating hormone (MCH). Mammals have five alphaMSH receptors (melanocortin receptors) and one or two MCH receptors. We have identified the full complement of melanocortin and MCH receptors in both zebrafish and the pufferfish, Fugu. Zebrafish have six melanocortin receptors, including two MC5R orthologues, while Fugu, lacking MC3R, has only four. We also demonstrate that Fugu and zebrafish have two and three MCHR genes, respectively. MC2R and MC5R are physically linked in all species examined. Unlike other species, we find the Fugu genes contain introns, one of which is in a conserved location and is probably ancestral. We also detail the differential expression of the zebrafish genes throughout development.