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Color-related genes help explain how there are so many species of small colorful birds called warblers.
According to a new study, two genes important for the different colors and plumage patterns of torpedoes have evolved through two very different processes.
“Woodlands are an incredibly colorful and diverse group of birds, with more than a hundred species in total,” says Marcella Baiz, a postdoctoral researcher at Penn State and first author of the paper. “These species emerged very quickly over evolutionary time in what biologists call a radiation species. To better understand this radiation, we studied the genetic regions related to plumage coloring within a particularly colorful subset of warblers. “
The research team sequenced the genomes of all 34 species within the Setophaga genus of wood torpedoes and created a phylogenetic tree that elucidates how each species relates to each other. Then, they focused on nine closely related pairs of “sister species”. Each pair is the result of a divergent species in two. Seeing if similar evolutionary processes are at play in each of the pairs allowed the researchers to gain a better understanding of the overall radiation.
A findings paper appears in the journal Current biology.
“In most cases, it is difficult to reach the genes behind the diversification process because it can be difficult to link specific genes to specific traits, such as colors,” says David Toews, assistant professor of biology at Penn State and leader of the research team. . “But many reed species readily interbreed, producing hybrid offspring with a mix of traits from the parent species, so previously we were able to link certain color patterns with their underlying genetic regions.
‘In this study, we focused on two coloring genes, but we were able to study them on all species of this large genus, to give us a window into the rest of the radiation.
The two genes in question
The first gene, the Agouti signaling protein (ASIP), is involved in the production of the pigment melanin, which underlies the brown and black plumage in these warblers. Within each pair of sister species where there were differences in the amount or location of black or brown in their plumage, the team predictably found genetic differences close to ASIP.
“We created an evolutionary tree based solely on the region of the ASIP gene, which more clearly shows how the gene has changed across the genus,” says Baiz. “The patterns in this gene tree mirror the patterns in the phylogenetic tree based on what we see in the whole genome. This implies that the differences we see in ASIP result from mutations that have arisen independently in different species. However, the genetic tree of the second gene, BCO2, showed a very different pattern that did not match the entire genome tree, which suggests that different processes are at play. “
The second gene, oxygenated beta-carotene 2 (BCO2), is involved in the production of carotenoid pigments, which result in bright yellow, red, and orange plumage. The researchers suggest that a process called introgression – the exchange of genes between species that evolved separately – could explain why the pattern of genetic changes in BCO2 did not align with the group’s overall radiation.
“Introgression can occur when two separate species hybridize and the hybrid offspring continue to mate with one of the original species,” says Baiz. “After several generations, the genetic material of one species can be incorporated into the other. The signal of this type of ancient introgression can be kept in the genomes of living individuals, as can the way ancestry tests can reveal how many Neanderthal genes you have. In this case, we unexpectedly found evidence of an ancient introgression to BCO2 in two otherwise distantly related warblers in this genre. “
The researchers found evidence of introgression involving the yellow bird and the magnolia bird and involving the prairie bird and the calf bird, all species with colorful carotenoid coloring in their feathers. However, they note that with current data it is difficult to establish the directionality of gene transfer.
“One possibility is that the magnolia bird’s version of BCO2 got into the yellow beak, and this ‘new to them’ version produced a larger deposition of carotenoids in the yellow bird’s feathers,” Toews says. “It’s funny to think that the ancient introgression is what made the yellow bird so yellow!”
110 species of warbler
This is one of the earliest examples of carotenoid gene transfer between vertebrates. Collectively, the findings of this study suggest that both introgression and a more standard mode of evolution, where mutations occur and are passed from parent to offspring, played a role in generating color diversity in this genus and could have help enable extreme diversification of warblers.
In the future, researchers hope to link specific mutations in these genes to changes in plumage coloration and to map the molecular pathways involved in pigment production. They would also like to expand their study to all 110 species of warblers.
“There is a possibility that there may be introgression from another genre entirely,” says Toews. “Expansion to other warblers would allow us to explore this possibility and to understand more fully the radiation from these fascinating birds.”
Other co-authors are from Penn State; the University of California, Riverside; and the Cornell Lab of Ornithology. The Cornell Lab of Ornithology, Penn State Eberly College of Science, and Penn State Huck Institutes of the Life Sciences supported this work.
Source: Penn State
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