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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.
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A Review and Assessment of the Shared-Pathway Hypothesis for the Maintenance of Signal Honesty in Red Ketocarotenoid-Based Coloration
Synopsis For decades, scientists have noted connections between individual condition and carotenoid-based coloration in terrestrial and aquatic animals. Organisms that produce more vibrant carotenoid-based coloration tend to have better physiological performance and behavioral displays compared with less colorful members of the same species. Traditional explanations for this association between ornamental coloration and performance invoked the need for color displays to be costly, but evidence for such hypothesized costs is equivocal. An alternative explanation for the condition-dependence of carotenoid-based coloration, the Shared-Pathway Hypothesis (SPH), was developed in response. This hypothesis proposes that red ketocarotenoid-based coloration is tied to core cellular processes involving a shared pathway with mitochondrial energy metabolism, making the concentration of carotenoids an index of mitochondrial function. Since the presentation of this hypothesis, empirical tests of the mechanisms proposed therein have been conducted in several species. In this manuscript, we review the SPH and the growing number of studies that have investigated a connection between carotenoid-based coloration and mitochondrial function. We also discuss future strategies for assessing the SPH to more effectively disentangle evidence that may simultaneously support evidence of carotenoid-resource tradeoffs.
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- Award ID(s):
- 1754152
- PAR ID:
- 10332121
- Date Published:
- Journal Name:
- Integrative and Comparative Biology
- Volume:
- 61
- Issue:
- 5
- ISSN:
- 1540-7063
- Page Range / eLocation ID:
- 1811 to 1826
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/icb/icab056
