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Synopsis In many species of birds, red carotenoid coloration serves as an honest signal of individual quality, but the mechanisms that link carotenoid coloration to animal performance remain poorly understood. Most birds that display red carotenoid coloration of feathers, bills, or legs ingest yellow carotenoids and metabolically convert the yellow pigments to red. Here, we review two lines of investigation that have rapidly advanced understanding of the production of red carotenoid coloration in birds, potentially providing an explanation for how red coloration serves as a signal of quality: the identification of the genes that enable birds to be red and the confirmation of links between production of red pigments and core cellular function. CYP2J19 and BDH1L were identified as key enzymes that catalyze the conversion of yellow carotenoids to red carotenoids both in the retinas of birds for enhanced color vision and in the feathers and bills of birds for ornamentation. This CYP2J19 and BDH1L pathway was shown to be the mechanism for production of red coloration in diverse species of birds and turtles. In other studies, it was shown that male House Finches (Haemorhous mexicanus) have high concentrations of red carotenoids within liver mitochondria and that redness is positively associated with mitochondrial function. These observations suggested that the CYP2J19 and BDH1L pathway might be tightly associated with mitochondrial function. However, it was subsequently discovered that male House Finches do not use the CYP2J19 and BDH1L pathway to produce red pigments and that both CYP2J19 and BDH1L localize in the endoplasmic reticulum, not the mitochondria. Thus, we have the most detailed understanding of links between cellular function and redness in a bird species for which the enzymes to convert yellow to red pigments remain unknown, while we have the best understanding of the enzymatic pathways to red in species for which links to cellular function are largely unstudied. Deducing whether and how signals of quality arise from these distinct mechanisms of ornamental coloration is a current challenge for scientists interested in the evolution of honest signaling. 

The light environment underwater can vary dramatically over space and time, challenging the visual systems of aquatic organisms. To meet these challenges, many species shift their spectral sensitivities through changes in visual pigment chromophore composition and opsin expression. The red shiner (Cyprinella lutrensis) is a North American cyprinid minnow species that inhabits waters ranging widely in turbidity and temperature. We hypothesized that the visual system of the red shiner is plastic with chromophore composition and opsin expression varying in response to the environment. To test this hypothesis, we collected red shiners throughout the year from three Oklahoma creeks that vary in turbidity. We characterized the light environment by spectroradiometry, measured chromophore composition of the eyes with high performance liquid chromatography, characterized the mechanisms of chromophore metabolism, and examined ocular gene expression by RNA sequencing and de novo transcriptome assembly. We observed significantly higher proportions of the long-wavelength shifted A2 chromophore in the eyes of fish from the turbid site and in samples collected in winter, suggesting that there may be a temperature-dependent trade-off between chromophore-based spectral tuning and chromophore-related noise. Opsin expression varied between turbid and clear creeks, but did not align with light environment as expected, and the magnitude of these differences was limited compared to the differences in chromophore composition. We confirmed that red shiner CYP27C1 catalyzes the conversion of A1 to A2, but the ocular expression of CYP27C1 was not well correlated with A2 levels in the eye, suggesting conversion may be occurring outside of the eye. 

ABSTRACT The carotenoid‐based colours of birds are a celebrated example of biological diversity and an important system for the study of evolution. Recently, a two‐step mechanism, with the enzymes cytochrome P450 2J19 (CYP2J19) and 3‐hydroxybutyrate dehydrogenase 1‐like (BDH1L), was described for the biosynthesis of red ketocarotenoids from yellow dietary carotenoids in the retina and plumage of birds. A common assumption has been that all birds with ketocarotenoid‐based plumage coloration used this CYP2J19/BDH1L mechanism to produce red feathers. We tested this assumption in house finches (Haemorhous mexicanus) by examining the catalytic function of the house finch homologues of these enzymes and tracking their expression in birds growing new feathers. We found that CYP2J19 and BDH1L did not catalyse the production of 3‐hydroxy‐echinenone (3‐OH‐echinenone), the primary red plumage pigment of house finches, when provided with common dietary carotenoid substrates. Moreover, gene expression analyses revealed little to no expression ofCYP2J19in liver tissue or growing feather follicles, the putative sites of pigment metabolism in moulting house finches. Finally, although the hepatic mitochondria of house finches have high concentrations of 3‐OH‐echinenone, observations using fluorescent markers suggest that both CYP2J19 and BDH1L localise to the endomembrane system rather than the mitochondria. We propose that house finches and other birds that deposit 3‐OH‐echinenone as their primary red plumage pigment use an alternative enzymatic pathway to produce their characteristic red ketocarotenoid‐based coloration.