Shrunk and scattered black spots turn out due to MC1R knockout in a white-black Oujiang color common carp (Cyprinus carpio var. color)

In teleosts, the chromatophore distribution pattern could result in their skin countershading. Previous observations suggested that Melanocortin receptor-1 (MC1R) plays roles in melanin modulation and their patterning processes in many mammals and some fish species. Using CRISPR-Cas9 techniques, we engineered the seven-transmembrane MC1R knockout mutants in Oujiang color common carp to test the hypothesis that the gene might have a role in melanogenesis in this fish. The loss-of-function (MC1R) fishes lost normal melanophore production and their subsequent growth, even while exposed in sunlight for a long time. The usual pathway of melanogenesis in white and black (WB) color carps was altered by this process. Deletion, substitution, and insertions were noticed and frameshift mutation was obtained. Disruptions in a shorter (10 bp-30 bp) fragment produced shrunken and underdeveloped melanophores on the fish skin instead of black patches and the mutant fishes appeared as grayish in color; and a higher rate of modification (215 bp deletion) resulted in remarkable reduction in melanin formation on the skin, and the albinic skin with quite shrunken melanophores appeared. Our present study found that a disruption in MC1R could change the formation pattern of black patches, which suggested that MC1R plays an important role in melanogenesis in Oujiang color common carp. Further evaluation of the relative expression level of some associated genes on the skin of both wild type and mutants can draw a clear scenario of MC1R pathways in melanogenesis in Oujiang color common carp.