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Genes controlling warbler colors evolved through two separate processes

Genes controlling warbler colors evolved through two separate processes
New research suggests distinct evolutionary processes explain the diversity of two color-related genes that account for diversity of color and pattern among wood warblers. Photo by Darrell Cochran/Penn State

Nov. 30 (UPI) -- Two genes that dictate the unique colors and plumage patterns of New World warbler species evolved via two distinct processes, researchers have found.

These divergent pathways, described Monday in the journal Current Biology, could explain with this group of passerine birds evolved so rapidly.

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"Wood warblers are an incredibly colorful and diverse group of birds, with more than a hundred species in total," lead study author Marcella Baiz said in a news release.

"These species arose very quickly in evolutionary time in what biologists call a species radiation. To better understand this radiation, we studied genetic regions related to plumage coloration within a particularly colorful subset of warblers," said Baiz, a postdoctoral researcher at Penn State University.

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Identifying connections between genes and animal traits remains quite difficult, complicating the task of linking genes with the evolutionary processes underlying speciation.

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"But many species of warblers readily interbreed, producing hybrid offspring with a mix of the parent species' traits, so we were previously able to link certain color patterns with their underlying genetic regions," said lead researcher David Toews.

"In this study, we focused on two coloration genes, but were able to study them across all the species in this large genus, to give us a window into the rest of the radiation," said Toews, an assistant professor of biology at Penn State.

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For the study, scientists sequenced and analyzed the genomes of 34 species within the Setophaga genus of wood warblers, which allowed them to construct a detailed phylogenetic tree of the New World warblers.

By untangling the evolutionary relationships among the group's many species, researchers were able to identify a variety of sister species -- pairs of species that diverged from a common ancestor.

Through their analysis, Toews and company determined that brown and black plumage among wood warbler species is controlled by the Agouti-signaling protein gene, or ASIP gene.

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When researchers compared sister species with significant differences in the amount or positioning of their black and brown feathers, they found discernible genetic differences near the ASIP gene.

"We created an evolutionary tree based solely on the ASIP gene region, which more clearly shows how the gene has changed across the genus," said Baiz.

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"The patterns in this gene tree mirror patterns in the phylogenetic tree based on what we see across the whole genome. This implies that the differences we see in ASIP resulted from mutations that arose independently in different species," Baiz said.

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However, when researchers created an evolutionary tree based on the BCO2 gene region, or beta-carotene oxygenate 2, the patterns failed to match up with the genome tree.

The discrepancy suggests BCO2 -- which codes for bright yellow, red, and orange plumage in warblers -- evolved through a different process.

Researchers suspect the distribution of genetic changes in BCO2 came about through introgression, the exchange of genes between species that evolved separately.

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"Introgression can occur when two separate species hybridize, and the hybrid offspring go on to mate with one of the original species," said Baiz. "After several generations, genetic material from one species can be incorporated into the other. The signal of this kind of ancient introgression can be maintained in the genomes of living individuals -- like how ancestry tests can reveal how many Neanderthal genes you have."

"In this instance, we unexpectedly found evidence for ancient introgression at BCO2 in two otherwise distantly related warblers in this genus," Baiz said.

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Researchers identified evidence of introgression between the yellow warbler and magnolia warbler, as well as between the prairie warbler and vitelline warbler. However, scientists were unable to determine the direction of the gene transfer.

"One possibility is that the magnolia warbler version of BCO2 introgressed into the yellow warbler, and this 'new to them' version produced a broader deposition of carotenoids in the feathers of the yellow warbler," said Toews. "It is fun to think that ancient introgression is what made the yellow warbler so yellow!"

The new research suggests both inheritable mutations and introgression combined to produce the tremendous diversity of colors and plumage patterns displayed by warbler species. Scientists suspect these two modes of evolutionary development could have also fueled the rapid radiation of warbler species.

In followup studies, scientists hope to precisely pinpoint the specific genetic mutations and gene transfers that led to changes in pigment production.

"There's a possibility that there may be introgression from another genus entirely," said Toews. "Expanding to other warblers would allow us to explore this possibility, and to more broadly understand the radiation of these fascinating birds."

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