Evolution of the melanocortin system.

The melanocortin system is one of the most complex of the hormonal systems. It involves different agonists encoded in the multiplex precursor proopiomelanocortin (POMC) or in different genes as β-defensins, endogenous antagonist, like agouti-signalling protein (ASIP) or agouti-related protein (AGRP), and five different melanocortin receptors (MCRs). Rounds of whole genome duplication events have preceded the functional and molecular diversification of the family in addition some co-evolutionary and tandem duplication processes have been proposed. The evolutionary patterns of the different partners are controversial and different hypotheses have emerged from a study of the sequenced genomes. In this review, we summarize the different evolutionary hypotheses proposed for the different melanocortin partners.