Constructing Different Ways with the Same Means: The Biochemical Basis of Within-Species Color Polymorphism

Abstract

How are different carotenoid color phenotypes produced on the same biochemical network? Plumage of house finches (Haemorhous mexicanus) varies from light yellow to purple among individuals within and across populations. By mapping the network locations of carotenoids that make up yellow and red morphs, we can examine the mechanistic differences that underlie these color polymorphisms. Carotenoid color in birds is produced by dietary carotenoids that can be internally modified into other derived carotenoids through enzymatic reactions. Variation in color could be due to distinct diets, use of different enzymatic pathways, or differences in the amounts of compounds. I extracted carotenoids and constructed biochemical networks from the feathers of 67 red and 66 yellow individuals from populations in Arizona and Montana. I quantified the color of each individual's feathers and compared the network structure and the concentrations of the compounds between red and yellow morphs. Both differences in diet and which enzymatic pathways are used contribute to the color polymorphism in house finch plumage. Both morphs utilize the same dietary compounds, with yellow individuals producing distinct derived compounds and lower derived carotenoid concentrations overall. These results could shed light on mechanisms of population divergence and local adaptations in phenotypic traits.