Modeling the evolution of the MC2R and MC5R genes: Studies on the cartilaginous fish, Heterondotus francisci

Comparative studies support the hypothesis that the proliferation of melanocortin receptor genes (MCRs) in gnathostomes corresponds to the 2R hypothesis for the radiation of gene families in Phylum Chordata. This mini-review will initially focus on the distribution of MCRs in cartilaginous fish and the relationship between the shark MC5R gene and the proposed ancestral MC5R/2R gene. This section will be followed by the results of recent studies on the features of the ligand binding site common to all melanocortin receptors. These data will provide the background for a set of hypotheses to explain the unique ligand selectivity of the MC2 receptor in teleosts and tetrapods.