Shades of meaning: the pigment-type switching system as a tool for discovery.

The pigment-type switching system, which controls whether melanocytes produce black/brown eumelanin or yellow/red pheomelanin, is responsible for many familiar coat coloration patterns in both domestic and wild mammals. In conjunction with the accessory proteins attractin and mahogunin ring finger 1, endogenous agonists and antagonists modulate signaling by the melanocortin 1 receptor to determine pigment type. Mutations in pigment-type switching genes can cause a variety of pleiotropic phenotypes, and these are often similar between mutants at different loci because the proteins encoded by these genes act together as part of conserved molecular pathways that are deployed in multiple biological contexts. When this is the case, pigment-type switching provides a powerful model system for elucidating the shared molecular mechanisms underlying the pigmentary and non-pigmentary phenotypes. This review outlines the current understanding of the pigment-type switching pathway and discusses the opportunities that exist for exploring the molecular basis of pleiotropic phenotypes using this model system.