Independent pseudogenization of CYP2J19 in penguins, owls and kiwis implicates gene in red carotenoid synthesis
Carotenoids have important roles in bird behavior, including pigmentation for sexual signaling and improving color vision via retinal oil droplets. Yellow carotenoids are diet-derived, but red carotenoids (ketocarotenoids) are typically synthesized from yellow precursors via a carotenoid ketolase. Recent research on passerines has provided evidence that a cytochrome p450 enzyme, CYP2J19, is responsible for this reaction, though it is unclear if this function is phylogenetically restricted. Here I provide evidence that CYP2J19 is the carotenoid ketolase common to Aves using the genomes of 65 birds and the retinal transcriptomes of 15 avian taxa. CYP2J19 is functionally intact and robustly transcribed in all taxa except for several species adapted to foraging in dim light conditions. Two penguins, an owl and a kiwi show evidence of genetic lesions and relaxed selection in their genomic copy of CYP2J19, and six owls show evidence of marked reduction in CYP2J19 retinal transcription compared to nine diurnal avian taxa. Notably, none of these taxa are known to use red carotenoids for sexual signaling and several species of owls and penguins represent the only birds known to completely lack red retinal oil droplets. The remaining avian taxa belong to groups known to possess red oil droplets, known or expected to deposit red carotenoids in skin and/or plumage, and/or frequently forage in bright light. The loss and reduced expression of CYP2J19 is likely an adaptation to maximize retinal sensitivity, given that oil droplets reduce the amount of light available to the retina.