Wood warblers, also called New World warblers, are some of the most colorful birds in North America, with more than a hundred species in the family ranging in color from yellow, orange and red to blue, green and pink. A new study led by researchers at Penn State has uncovered several instances of the birds passing color-related genes to other species of wood warblers, including those that are not closely related. This glimpse into the hidden evolutionary dynamics of these songbirds may help explain why some species display certain colors as well as how the group diversified into so many different species in such a relatively short evolutionary time, the researchers said.
A paper describing the research published today (Dec. 11) in the journal PLOS Biology.
"Wood warblers are beloved by birders in part because of their wide variety of plumage colors, and it's only natural to wonder how such diversity of colors arose in these beautiful songbirds," said David Toews, Louis Martarano Career Development Professor of Biology in the Penn State Eberly College of Science and an author of the paper. "It turns out that some of these birds may have borrowed colors from their neighbors, rather than evolving them independently. We previously found evidence that a particular color-related gene had been shared between species within the same genus, which is one notch up on the taxonomic ladder, but now we show that there is gene movement from species in one genus to another. That means some genetic funny business was going on with these warblers millions of years ago."
In addition to the pigment melanin, which produces brown and black plumage, the colors of wood warblers are influenced by carotenoid pigments, which are responsible for bright yellow, red and orange plumage. The research team collected DNA from 400 warblers across 100 species and six subspecies, focusing on three main genes related to carotenoid pigments.
The team compared an evolutionary tree of the warblers - built using their entire genomes and representing the overall relationships among the birds - with trees they created independently for each of the three carotenoid-related genes. These gene-specific trees show similarities and differences of the individual gene across the family, and discrepancies between the trees suggest where gene exchanges may have occurred. Statistical follow-up tests confirmed the gene exchanges, which the researchers said resulted from a process called introgression.
"When birds of two different species mate, their hybrid offspring inherit genes from both parents," said Kevin Bennett, postdoctoral scholar in biology in the Penn State Eberly College of Science and first author of the paper. "When that hybrid goes on to mate with an individual of one of its parent species, it can pass on genes from the other. Over several generations, the genetic material from one species can be incorporated into the other, which is called introgression. Our research group previously identified introgression of a gene called BCO2 among several species of wood warblers within the genus Setophaga, but it seemed like the version of the gene that was getting passed around might have originated from outside that group. Here, we confirmed that this version of the gene came from outside the genus and found that it has been exchanged between species of a different genus on multiple occasions."
The gene beta-carotene oxygenase 2 (BCO2), when turned on, produces a protein that breaks down yellow carotenoids, resulting in more white or gray coloration. When turned off, BCO2 results in the accumulation of yellow carotenoids and thus yellow plumage. The researchers found evidence of introgression of BCO2 among several wood warbler species, including from a species within the genus Leiothlypis to multiple Setophaga species as well as to multiple species in the genus Cardellina. They also observed introgression of BCO2 from the genus Vermivora to Geothlypis, though the order and exact timing of these exchanges remain unclear.
"We think the initial introgression events from Leiothlypis occurred between half a million to two million years ago - while the donor and recipient species themselves diverged several million years before that," Toews said. "But the 'borrowed' version of this gene has stuck around all that time, even as the species themselves evolved and split. We believe this version of the gene subtly alters the BCO2 protein itself, which likely impacts the yellow coloration of the birds. Because warblers use color to choose a mate, this might play into who the bird mates with, helping to dictate whether the gene gets passed on. We plan to further investigate the functional implications of this version of BCO2."
While introgression of BCO2 from Leiothlypis to other species may have occurred millions of years ago, the researchers suggested that introgression into one species, the red-faced warbler, is not only more recent, but is currently in progress, as not all members of the species they sampled contained the borrowed version of the gene.
"We are sampling additional red-faced warblers from across a broader geographic range to try to catch this evolutionary process in the act," Bennett said. "We hope to get a better sense of where this version of the gene has come from, how far into its geographic range it has reached and how it impacts the ecology of the birds."
The research team also investigated a pair of genes called BDH1L and CYP2J19 that work together to convert yellow carotenoids into a type of carotenoid that can result in red plumage. They found evidence of introgression of BDH1L from a red Cardellina species into two red species in the genus Myioborus and CYP2J19 between two species of Myioborus.
"Only five of the more than hundred species of wood warblers are red, and one of these recipient Myioborus species has populations that are red and some that are yellow," Bennett said. "We only found evidence of introgression into the red population, which provides strong evidence that introgression is directly causing the color difference of these birds. We think it is possible that wood warblers had a single evolutionary origin of red plumage and that it was passed on to the other red species through introgression from there."
Ultimately, the researchers uncovered at least nine instances of introgression with BCO2 and at least one instance for each of the other genes. Their findings suggest that introgression - which has not yet been documented to this scale in vertebrates - may be more common than previously thought. Borrowing genes from evolutionary neighbors, they said, may be an easier way to evolve new traits compared to random mutations.
According to the researchers, the spread of carotenoid genes via introgression could also help explain how this group of warblers rapidly diversified into so many species in evolutionary time.
"Without modern tools, like high-throughput DNA sequencing, the intertwined history of these birds would remain hidden," Bennett said. "It's really exciting to uncover the hidden stories of the birds and to see how genes from independent evolutionary lineages can come together to solve evolutionary problems."
In addition to Toews and Bennett, the team at Penn State includes Lan-Nhi Phung, who graduated with a doctoral degree in biology in 2024 and is now at the University of Rochester in New York, and former postdoctoral scholars Andrew Wood, now at the University of Minnesota Duluth, and Marcella Baiz, now at the University of Buffalo. The research team also includes Irby Lovette at Cornell University.
Funding from the U.S. National Science Foundation, the Penn State Eberly College of Science and the Penn State Huck Institutes of the Life Sciences supported this research.
