Constitutive activation of MC1R in the large-billed crow (Corvus macrorhynchos) and its potential role in black plumage

Abstract

Melanin-based plumage coloration in birds is largely regulated by the melanocortin 1 receptor (MC1R), a G protein–coupled receptor that promotes eumelanin synthesis via cAMP signaling. In domestic chickens, constitutively activating mutations such as the MC1R^E (E92K) allele cause melanistic phenotypes, demonstrating that persistent MC1R activation can drive generalized darkening. However, to our knowledge, no experimental study has directly demonstrated constitutive MC1R activation in wild birds exhibiting uniformly black plumage. We investigated the sequence and signaling properties of MC1R from the Large-billed Crow (Corvus macrorhynchos), a species with strongly eumelanin-dominant plumage. Crow MC1R exhibited elevated basal cAMP signaling and minimal responsiveness to α-melanocyte-stimulating hormone (α-MSH) in both stable Chinese hamster ovary (CHO-K1) cells and transient CRE-luciferase assays in HEK293T cells, demonstrating ligand-independent activation comparable to that observed in the melanizing chicken MC1R^E (E92K) allele. Comparative sequence analysis identified multiple substitutions conserved across Corvus species. Among these, E12K and E18K were functionally evaluated based on prior associations with melanism in other birds. Although E12K modestly increased basal signaling in chicken MC1R, E18K alone or in combination with E12K did not reproduce crow-level constitutive activity, and reciprocal substitutions in crow MC1R failed to abolish ligand-independent activation. These findings demonstrate that crow MC1R possesses constitutive activity and suggest that this phenotype reflects lineage-specific modifications rather than a single activating substitution. Our results provide experimental evidence that constitutive MC1R activation is a plausible molecular mechanism that may contribute to the black plumage in the Large-billed Crow, although a direct causal relationship remains to be established.