Picture courtesy of David Toews

A yellow-rumped warbler sits in hybridization region in western Canada

September 26, 2016

Genomic Testing Suggests ‘Butterbutt’ Warbler is Three Distinct Species

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Warblers are small, perching, singing birds that may seem similar to one another to the untrained eye and ear. But for David Toews, a postdoctoral researcher at the Fuller Evolutionary Biology Program of Cornell’s Lab of Ornithology, these colorful woodland birds are anything but similar.

In particular, one specific species of warbler can actually be differentiated into three separate species — a breakthrough that spells out a slew of new knowledge and questions in our understanding of genomics and conservation.

In a study entitled “Genomic variation across the Yellow-rumped Warbler species complex” published in The Auk: Ornithological Advances, the Yellow-rumped warbler, affectionately called the “butterbutt” warbler, has been subject to new genomic analysis methods that have confirmed the species to be three closely related species.

These grey, yellow streaked warblers are migratory, insect-eating birds that spend their summers in the boreal forests of North America and winters in the southern U.S. and Central America. They have four distinct forms — the myrtle warbler of eastern North America, Audubon’s warbler of western North America and two isolated populations, the black-fronted and Goldman’s warblers, of Mexico and Central America, respectively.

During much of the 20th century, the myrtle and Audubon’s warbler were considered distinct species. This changed once ornithologists discovered a region of species hybridization, a zone in western Canada where myrtle and Audubon’s warblers can interbreed and yield offspring, thus reclassifying these warblers into a single species. Toews’ new study appears to revert this status quo, but also generates new implications for our genetic understanding of birds.

Audubon's warbler is a separate species

Picture courtesy of David Toews

Audubon’s warbler is a separate species

These birds have been the primary interest of Toews’ research since he was working on his doctorate at the University of British Columbia.

“I started studying them for my Ph.D,” Toews said. “I was interested in patterns of hybridization and introgression, that is, changes in genes, between Audubon’s and myrtle warblers. As I was doing that, this new type of technology became available called reduced representation genome sequencing.”

Also called genotype by sequencing, this new method quantitatively evaluates the differences between regions of interest in the complete set of genetic material, or genome, of the birds.

Looking at the warbler through a genomic lens has been key to differentiating the four forms of the bird.

“We look at a whole bunch of genomic markers, scattered throughout the genome, in a large number of individuals,” Toews said. “The important thing about this, compared to previous studies, is that we have some idea of where the genetic markers are and we can look for small clusters of highly different markers.”

Toews describes how this process can trace the distinctions between the myrtle and Audubon’s warbler.

“One has a white throat. One has a yellow throat. One of those many regions probably has a gene that gives us this yellow or white throat,” Toews said.

From the analysis of over 30,000 genetic markers in the genomes of the different forms of Yellow-rumped warbler, Toews and the team were able to find several dozen small regions in the genetic material that distinguish the myrtle, Audubon’s and Goldman’s warblers as three distinct species.

This analysis, coupled with an observation of the characteristics of the hybrid warblers in western Canada, allowed Toews and the research team to conclude separate species status for three of the four forms.

“For a while, it was that if two things hybridized at all, they were considered the same species. The definition was super rigid,” Toews said. “But then further research showed that the hybrid zones are really narrow for the myrtle and Audubon’s warbler and it looks like the hybrids are not very fit.”

This finding has contributed to not only genetic knowledge of Yellow-rumped warblers but of genetic variation and its drivers between all types of species.

“We’re starting to get a picture by looking at multiple different groups, on how genomes evolve and what are the drivers of that change and in many cases it’s natural selection,” Toews said. “We’re starting to pick up on some common themes and common patterns when comparing closely related species at a genomic level.”

Toews attributes the discovery to advancements in genetic sequencing.

“The field itself, the ability to look at so many genetic markers is new, the methods are being refined and it is very much in a discovery phase,” Toews said. “So it’s awesome that we can sequence thousands of regions of the genome when we could only do a couple hundred before.”

In addition, the finding provides critical information regarding the conservation of the warbler.

“In Guatemala, Goldman’s warbler, is close to one of their only endemic bird species, but there has been pretty big habitat degradation where these warblers live. I think this finding will have a pretty big conservation implication globally,” Toews said.

Conservationists will have to consider the separate species status of Goldman’s warbler in their habitat management plans.

Regarding further research questions to pursue, Toews points to the narrow hybrid zone in the range of the two warblers.

“Hybrids are continually being produced and a lot of things we thought that made these hybrids unfit; their song, their migratory patterns, they don’t seem to be the case,” Toews said. “Right now, we’re interested in looking at nesting and hatching success of the hybrids – it’s the 65 million dollar question.”

The other researchers on this paper include Alan Brelsford Ph.D. University of California, Riverside; Christine Grossen Ph.D. University of Zürich; Borja Milá Ph.D. Spanish National Research Council and Prof. Darren Irwin, the University of British Columbia.