Many species of birds use red to signal mates or ward off rivals, with the special color typically added to their beaks, feathers and skin. Now, two separate studies have pinpointed the gene that encodes the enzyme responsible for converting yellow pigments obtained in bird diets into red.
"To produce red feathers, birds convert yellow dietary pigments known as carotenoids into red pigments and then deposit them in the feathers," said Miguel Carneiro of the Universidade do Porto in Portugal and senior author of the first study. "Birds also accumulate these same red pigments in one of the cone photoreceptor types in their retina to enhance color vision. We discovered a gene that codes for an enzyme that enables this yellow-to-red conversion in birds."
"It was known that some birds have the ability to synthesize red ketocarotenoids from the yellow carotenoids that they obtain in their diet, but the gene or enzyme involved, and its anatomical location, have been obscure," added Nick Mundy of the University of Cambridge and first author of the second study. "Our findings fill this gap and open up many future avenues for research on the evolution and ecology of red coloration in birds."
Carneiro and his team analyzed the genome sequences of yellow and red canaries and compared the data to red siskins, revealing that the expression of the cytochrome P450 enzyme - also referred to as gene CYP2J19 - is connected to red coloration.
In Mundy's experiment, the team found the cytochrome P450 gene cluster through the examination of standard zebra finches with red beaks and mutant zebra finches with yellow beaks. Zebra finches have a total of three cytochrome P450 genes, and multiple mutations of this region were discovered in yellowbeak birds.
Interestingly, CYP2J19 belongs to a family of genes that play an important role in detoxification, raising the question of how it evolved into its current role.
"In sexual selection, red color is thought to signal individual quality, and one way it can do this is if the type or amount of pigmentation is related to other physiological processes, like detoxification," said Staffan Andersson of the University of Gothenburg and co-author of the second study. "Our results, which link a detoxification gene to carotenoid metabolism, may shed new light on the debated honesty of carotenoid-based signals."
Both studies were published in the journal Current Biology.