Female birds often sport less colorful plumage than males of the same species. Some birds evolved this sexual dichromatism as an adaptation related to mating and survival, but scientists still don’t understand how two birds of the same species, with almost identical genomes, can look so different. New research suggests that in canaries and some related birds, the difference may involve a single gene that codes for an enzyme that destroys red and yellow pigment molecules (Science 2020, DOI: 10.1126/science.aba0803).
To investigate the mechanism behind this plumage phenomenon, Miguel Carneiro of the University of Porto and colleagues bred red siskins, which are sexually dichromatic, with common canaries, which look the same male or female, to create birds that were nearly identical to canaries but displayed sexual dichromatism. When they compared the hybrids’ genomes to that of canaries, they found just one region containing three genes that differed. Only one of those genes was expressed differently in the male and female hybrids: BCO2. BCO2 codes for the enzyme β-carotene oxygenase 2, which breaks down the reddish-orange pigment β-carotene. On parts of the body where males have redder plumage, female hybrid canaries expressed BCO2 at higher levels than males did. The researchers propose estrogen may affect BCO2 expression, explaining the difference between males and females, but they did not investigate that mechanism further.
Other researchers had hypothesized that sexual dichromatism resulted from differences between males and females in how pigment molecules were taken up or metabolized. The discovery that increased pigment degradation in females is the cause in hybrid canaries “is counter to traditional thought,” says bird hybrid expert Scott A. Taylor of the University of Colorado Boulder, who was not involved in the study.
When Carneiro’s group looked at wild birds’ genomes, they found that BCO2 didn’t always play a role in color differences between sexes. For example, the gene was involved in plumage differences in European serins but not house finches. But Carneiro says the simplicity of the BCO2 mechanism may be one reason that carotenoid red and orange pigments are found in so many dichromatic bird species compared with other pigments.