A single base pair change that turned a colorful bird entirely black probably guided the formation of a new species, researchers report in the August issue of The American Naturalist.

Melanic (above) and chestnut- bellied (below) Monarch flycatchers Image: J. Albert Uy

"It looks like we have a single mutation that's driving speciation in these birds," J. Albert Uy, an evolutionary biologist at Syracuse University in New York, who led the study, told The Scientist. "It's one of the first if not only examples of this kind of thing in vertebrates."

Eighty years ago, the late Harvard zoologist Ernst Mayr visited the Solomon Islands in the South Pacific and marveled at the variation in plumage color of the Monarch flycatcher (Monarcha castaneiventris), a small, insect-eating songbird with a long, fanned tail. In particular, two flycatcher populations that lived on islands eight kilometers apart caught Mayr's eye. One had a chestnut-colored belly with an iridescent bluish-black backside, and the other was all black, or "melanic." Mayr discussed these birds as an archetypal example of speciation in action in his 1942 opus Systematics and the Origin of Species, but he didn't know what was driving the two birds apart. Now, Uy thinks he has the answer -- a single nucleotide substitution in a gene underlying plumage color.

Uy and his colleagues sequenced the coding region of the melanocortin 1 receptor (MC1R) gene, a G protein-coupled receptor that regulates melanin production, in the chestnut-bellied and melanic birds as well as three other subspecies of Monarch flycathers. They discovered three nucleotide differences, but only one affected the amino acid sequence. Strikingly, all the melanic birds had the derived amino acid change, whereas the chestnut-bellied birds and the three other subspecies all had the ancestral sequence. The researchers also used stuffed taxidermic mounts to test the birds' ability to recognize their own subspecies and found that the two groups of flycatchers consistently preferred their own kind. Together, these results indicate that the single genetic swap probably set speciation in motion, Uy said.

"I think we do have a speciation gene because this substitution creates a different physical appearance for the bird, which in turn causes the birds to recognize members" of its own subspecies, Uy said. "We're catching them right now in the act of becoming new species."

Daven Presgraves, an evolutionary biologist at the University of Rochester, New York, who was not involved in the study, agrees that MC1R probably counts as a speciation gene, but pointed out that the ultimate test would be to show that the gene directly influences mate choice. Nonetheless, the "study is as strong as we can reasonably expect," he wrote in an email, because mate choice trials are "nearly impossible for monogamous birds on remote islands." Uy, who is now building aviaries and applying for licenses to keep the tropical, endangered birds in captivity, plans to carry out mating trials, but conceded that it "may take five years" or more.

Uy is also investigating the genetic basis of another all-black subspecies that lives on a separate island some 150 kilometers away from the other melanic population but which does not have the same mutation in the MC1R gene. He intends to sequence the "next group of candidates" -- genes that encode proteins that interact with the MC1R to control pigmentation. Slowly but surely, "we are now getting at the underlying basis of species divergence" in these birds, Uy said.


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