Carotenoid pigments in a mutant cardinal: Implications for the genetic and enzymatic control mechanisms of carotenoid metabolism in birds

Birds that use carotenoids to color their feathers must ultimately obtain these pigments from the diet, but they are also capable of metabolically transforming dietary carotenoids into alternate forms that they use as plumage colorants. The genetic and enzymatic control mechanisms underlying carotenoid metabolism are poorly understood. We investigated carotenoid pigments present in the feathers of an aberrantly colored yellow Northern Cardinal (Cardinalis cardinalis) to determine how metabolic pathways may have been altered. Normal red cardinals display four primary keto-carotenoids in plumage that are endogenously derived from a series of common food carotenoids. We found that the yellow feathers of this mutant lacked all four of the typical red pigments, and instead contained a yellow dietary carotenoid (lutein) and three yellow metabolic derivatives (3′-dehydrolutein and canary-xanthophylls A and B). Because yellow metabolites appear to be manufactured via a different metabolic process (dehydrogenation) than the usual red forms (oxidation at the C-4 or -4′ positions), it seems that this genetic mutation did not broadly disrupt all metabolic activity, but specifically impaired only one class of metabolic reactions and its associated enzymes.