Is carotenoid ornamentation linked to the inner mitochondria membrane potential? A hypothesis for the maintenance of signal honesty.

Several mechanistic hypotheses have been proposed for how carotenoid pigmentation of integumentary structures can serve as an honest signal of individual quality. These hypotheses are founded on proposed links between carotenoids, immuno responsiveness, and oxidative stress, but an absence of biochemical information on the oxidative pathways of carotenoids has limited the sophistication of such hypotheses. Based on published evidence, we propose that the oxidation of carotenoids for the purpose of ornamentation in birds and reptiles is coupled to the inner mitochondria membrane. We predict that several carotenoid oxidation reactions yielding ornamental pigments occur on the inner mitochondrial membrane. Three of these reactions are proposed to occur within the ubiquinone biosynthesis cluster known as the Coq cluster consisting of approximately a dozen Coq members, tightly integrated and intimately associated with Complex I and III of the electron transport system. Ubiquinone and highly oxidized ornamental carotenoids share a stereochemically-conserved binding region suggesting that these two molecules may have shared similar pathways in the past. Carotenoids and ubiquinones may cooperate as redox participants in anti-radical reactions or independently in helping to maintain membrane or supra-complex stabilization during times of high-energy demand. Under this hypothesis, oxidation of carotenoids is coupled to the inner mitochondria membrane potential such that ornamental coloration reflects the efficiency of cellular respiration.