A genetic mechanism for sexual dichromatism in birds

Sexual dichromatism — a difference in coloration between males and females — is central to the study of ornamentation, mate choice, and sexual selection. Here, we show that carotenoid-based dichromatism in mosaic canaries, a hybrid product between the sexually dichromatic red siskin and monochromatic canaries, is controlled by the carotenoid-cleaving enzyme Beta-Carotene Oxygenase 2 (BCO2). Dichromatism in mosaic canaries is explained by differential carotenoid degradation in the integument of each sex, rather than sex-specific variation in physiological functions such as pigment uptake or transport. Transcriptome analyses suggest that carotenoid degradation in the integument might be a common mechanism contributing to sexual dichromatism in other finches. Our findings demonstrate how differences in ornamental coloration between sexes can evolve through simple molecular mechanisms controlled by genes of major effect.