A University of Tulsa researcher has helped discover the gene responsible for creating sexual dimorphism in birds. Assistant Professor of Biological Sciences Matthew Toomey and an international team of biologists published the article “A genetic mechanism for sexual dichromatism in birds” this week in the prestigious research journal Science. 

Sexual dichromatism is a term describing the phenomenon observed in many bird species, in which males and females exhibit striking differences in coloration. Typically, male birds display flashy, colorful feathers, while females tend to be drab. Scientists have proposed that the explanation for this difference is that male birds are competing among other males for the attention of females. Although outwardly males and females can display very different color patterns, their genomes are nearly identical. How then do these color differences between the sexes arise? Toomey’s research collaboration explains for the first time how changes in the expression of a single gene can generate dramatic coloration differences between male and female birds. 

To investigate the mechanisms of sexual dichromatism, Toomey and his colleagues studied the mosaic canary (pictured). This breed of canary was created by bird fanciers decades ago by breeding the yellow canary species, where males and females are the same color (monochromatic), with a sexually dichromatic species, the red siskin. The initial goal of the bird fanciers was to produce a red canary. In 2016, Toomey and the team found that these red canaries carry red siskin genes for an enzyme that converts yellow pigments to red. Along with monochromatic red canaries, bird breeders also produced mosaic canaries that carry the red siskin genes for both redness and sexual dichromatism. To identify the gene for dichromatism, Toomey and the group sequenced the genomes of the dichromatic mosaic canaries, compared them to the typical monochromatic canary and identified differences associated with the gene for enzyme β-carotene oxygenase 2 (BCO2). 

Toomey and his colleagues recently discovered that BCO2 plays a key role in breaking down pigments controlling the coloration of the beaks and legs of birds by studying another oddly colorful canary, the urucum breed. “We compared the urucum canary breed, which has uniquely colorful beaks and legs, to typical canaries, with drab beaks and legs,” he explained. “We found that the urucum birds have a mutation in BCO2 that renders it non-functional. The urucum birds become colorful in the beak and legs because they are not able to break down pigments the way a typical canary does. This result suggests that bright beak and leg coloration might be easily switched on and off through the course of evolution with simple changes in the expression of BCO2.” 

In the dichromatic mosaic canaries, the research team showed that the expression of BCO2 differs between sexes. Female mosaic canaries express higher levels of the enzyme than males, which destroys colorful pigments in developing feathers and leads to the relatively drab appearance of females. The research team also observed female-biased expression of BCO2 in other dichromatic bird species suggesting that may be a common mechanism of dichromatism amongst birds. 

This result paves the way for deeper investigations into how other factors such as mating systems, nesting behaviors, predation pressures and light environments affect bird coloration. Toomey explains, “We now have an unprecedented opportunity to trace how coloration has evolved in response to these evolutionary pressures, through specific genetic regulatory changes.”    

See the published research in Science.