For most birds, the redder the better. Having that vivid hue in their bills, feathers, and even on their bare skin may help males attract mates and ward off would-be rivals. In a pair of Current Biology papers published this week, two teams have identified a gene cluster that codes for enzymes that allow red siskins and zebra finches to convert yellow pigments they get from their diets into red ones. The genes may also play a role in color vision and detoxification.
"Nobody knows for sure why red color is associated with reproductive success," Joseph Corbo from the Washington University School of Medicine said in a statement. "We thought that if we could figure out how they produced that red color, that would help us understand the advantage to being red." Birds obtain yellow pigments (called carotenoids) from their diet, which includes seeds and fruits. Previous studies found that birds with red feathers must synthesize red pigments (called ketocarotenoids) from yellow carotenoids. But exactly how they do that remained a mystery.
Corbo’s team, together with Auburn University’s Geoffrey Hill and Miguel Carneiro of Universidade do Porto, sequenced the genomes of red siskins (Spinus cucullata), common yellow canaries (Serinus canaria), and a century-old hybrid of the two called "red factor" canaries. Then, by comparing their genomes, the team found the gene responsible for color differences among the birds: CYP2J19, the gene that codes for a cytochrome P450 enzyme. This "redness gene" is highly expressed in the skin, feather, liver, and eyes of the red birds, but only in the eyes of the yellow birds.
"Diurnal birds appear to use this gene to produce red pigments in the retina to enhance color vision," Corbo explained. "However, only birds with red feathers additionally express the gene in their skin. These findings suggest that nearly all birds have the latent capacity to make red feathers, but in order to actually do so, they must evolve the means of expressing [this gene] in the skin in addition to the retina."
Another team, led by University of Cambridge’s Nicholas Mundy and University of Gothenburg’s Staffan Andersson, also arrived at the cytochrome P450 gene cluster. They compared zebra finches (Taeniopygia guttata), which have a red beak, with mutant zebra finches that have yellow beaks (the two are pictured to the right). The yellow-beaked birds have multiple mutations in the region containing three related cytochrome P450 genes. And the enzyme is expressed in near-undetectable levels in yellow beaks.
Additionally, the cytochrome P450 enzyme plays a key role in breaking down and metabolizing toxic compounds in the liver – suggesting that heightened redness may be a sign of quality. "Our results, which link a detoxification gene to carotenoid metabolism, shed new light on this old hypothesis about the honesty of signaling," Andersson added. That’s when a physical trait is a genuine signal of good genes, and something females consider when picking their mates.
Image in the text: On the left, a wildtype male zebra finch with red beak, and on the right, a male zebra finch with the mutant yellowbeak gene. Stuart Dennis