Not All Roads Lead to Red: The genetics of sexual dichromatism in the northern cardinal ( Cardinals cardinalis )

Abstract Sexual dichromatism, characterized by sex-specific differences in coloration, is widespread among birds and often involves carotenoid-based pigmentation. Despite extensive research on the social and ecological environments favoring sexual dichromatism, the molecular mechanisms underlying its development and evolution remain largely unexplored. In this study, we investigated the genetic and molecular processes giving rise to sexual dichromatism in the red ketocarotenoid-based plumage of northern cardinals (Cardinalis cardinalis). We quantified carotenoid concentrations in plasma and feather follicles, confirmed that homologs of CYP2J19, BDH1L, and TTC39B catalyze the production of C-4 ketocarotenoids, and performed gene expression analyses across tissues. Males showed significantly higher plasma and feather ketocarotenoid concentrations, upregulated CYP2J19 and TTC39B expression in liver and feather tissues, and upregulated carotenoid transport gene expression in the gut and feather follicles. Females exhibited a dramatic upregulation of BCO2 in their feather follicles, facilitating carotenoid degradation and attenuating red pigmentation. Additionally, sex-biased expression of hormonal regulators such as HSD17B4 and ZNF131 in the feather follicle suggests hormonal modulation influences dichromatism. These findings indicate that sex-specific regulation of carotenoid processing genes underpins the vivid red coloration in males and the drab phenotype in females, likely maintained by a balance between natural and sexual selection, with mechanisms of sexual antagonism affecting divergent gene expression. This work advances understanding of the molecular basis of avian sexual dimorphism, highlighting key genetic pathways involved in carotenoid-based coloration and providing a foundation for further research into the evolution of sexually dichromatic traits.