Evolutionary Explosion of Wood Warblers Took Distinct Paths

According to a new study, the evolution of two color-related genes helps explain the striking diversity in colors and patterns among wood warblers, like the blackburnian warbler pictured here. (Credit: Darrell Cochran)

(CN) — Scientists have unlocked the evolutionary secrets behind the wood warbler in an effort to understand why there are so many different species of the songbird, according to new research revealed Monday.

A study published in the journal Current Biology detailed how a research team sequenced the genomes of 34 species of wood warblers to determine how they are interlinked.

“Wood warblers are an incredibly colorful and diverse group of birds, with more than a hundred species in total,” said Marcella Baiz, first author and researcher at Penn State, in a statement. “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.”

Baiz and the research team created a phylogenetic tree, a diagram that shows evolutionary relationships between various species. From that data, the team looked at nine related pairs of “sister species,” when two species diverge from the same base species.

“In most cases it is difficult to get at the genes underlying the diversification process because it can be hard to link specific genes to specific traits, like colors,” said David Toews, biology professor at Penn State and study team leader. “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. 

“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.”

The scientists looked at two genes in the divergent species: the Agouti-signaling protein (ASIP), which helps produce the pigment melanin responsible for black and brown plumage, and beta-carotene oxygenate 2 (BCO2), which produces carotenoid pigments that create brighter colors.

“We created an evolutionary tree based solely on the ASIP gene region, which more clearly shows how the gene has changed across the genus,” Baiz said. “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. However, the gene tree from the second gene, BCO2, showed a very different pattern that did not match up with the whole genome tree, which suggests different processes are at play.”

The team said introgression, the transfer of genes from one species to another, could explain why there were fewer genetic differences in the BCO2 gene than in the ASIP gene.

“Introgression can occur when two separate species hybridize, and the hybrid offspring go on to mate with one of the original species,” Baiz said. “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.”

The research team said this introgression, along with mutation, could help explain the origin of many variations of the wood warbler. 

“There’s a possibility that there may be introgression from another genus entirely,” Toews said. “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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