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The Secret of the Pigeon's Crest
31 January 2013 2:00 pm
Domesticated pigeons are a bit like dogs: For centuries, fanciers have cultivated a startling variety of colors, feather arrangements, and behaviors, creating more than 350 breeds used in shows for looks and competitions for how fast they fly, how much they tumble in the sky, or how long they can remain airborne. After sequencing the rock pigeon genome, researchers have now tapped into that diversity to track down the genetic basis of one of the pigeon's more ostentatious traits: the head crest. The gene could be the same one used in wild birds for head crest features, and its discovery paves the way for uncovering the genetic basis of other important avian traits.
Science's interest in pigeon diversity dates back to at least the mid-19th century. The father of evolution, Charles Darwin, was fascinated by all the different kinds and bred these birds at his home. He wrote about them extensively in his books on natural selection and evolution and speculated, along with others, that the 150 breeds known at the time all descended from the rock dove, also known as rock pigeon. "The domesticated pigeon was just as, if not more, important [as Darwin's finches] to the evolution of his thinking about how natural selection worked," says Nathan Young, an evolutionary biologist at the University of California, San Francisco.
At the turn of the century, classical geneticists were also intrigued by pigeons, and in 1911, breeding studies by T. H. Morgan suggested that head crests were a simple trait. "But this system has remained remarkably unknown among modern evolutionary biologists and geneticists," says Hans Ellegren, an evolutionary biologist at Uppsala University in Sweden.
Michael Shapiro, an evolutionary biologist at the University of Utah in Salt Lake City, wanted to bring pigeons into the scientific limelight again. "We know much more about genetic variation in fish and in mammals than we know about birds," he points out. Until he came to Utah in 2006, Shapiro had focused on stickleback fish. But then someone pointed out that the range of traits in those fish was meager compared with pigeon traits and proved the point by showing Shapiro the Encyclopedia of Pigeon Breeds. "I was just blown away," Shapiro recalls.
Utah proved to be a good place to start studying pigeons, as it has a very active pigeon breeders group. In 2008, Shapiro's team began collecting blood samples and feathers for DNA analyses and last year developed a genealogy of pigeon breeds. Head crests jumped out as a potentially interesting trait to study. Different breeds boast different designs, from a simple peak to a hood of plumage that buries the head. And breeds with crests were distributed throughout the pigeon family tree. Furthermore, wild birds such as woodpeckers and doves have head crests thought to be important in mate selection and communication.
So Shapiro teamed up with by BGI-Shenzhen in China, which sequenced the pigeon genome. The group also sequenced 40 other pigeon genomes, including breeds with and without head crests as well as two free-living pigeon populations. Those data helped establish the range of normal variation among these genomes. With a computer program originally designed to find DNA variants important in human disease, they homed in on places in the genome where DNA's letters, called bases, were consistently different between those with and without head crests. That search led them to a gene called EphB2 that was responsible for the presence or absence of a head crest. A survey of an additional 69 uncrested birds from 57 breeds and 61 crested birds from 22 breeds confirmed that uncrested birds had one version of this gene and all the crested birds had another version, they report online today in Science. The single base change involved causes the protein to have a different amino acid at a crucial spot, one that likely renders the protein ineffective.
Researchers already know that pigeon head crests form because feathers there grow up toward the head rather than down along the body. The researchers didn't find any gene activity differences between crested and uncrested birds where and when this feather polarity is established in the embryo. So they think the decision to grow a head crest must be made earlier in development, Shapiro says.
"It's an interesting example of how a single gene change can have a profound effect," says Cliff Tabin, a developmental geneticist at Harvard University.
Now Shapiro's team is looking at other traits. For example, there are birds bred for their tendency to tumble in midair and he'd like to understand the genetics behind these "rollers" or "tumblers" as they are called. Beak shape, size, and color also vary greatly, and their genetics might be simple to work out.
Other researchers may follow suit, Ellegren says. "The work now puts the spot on pigeons … [and] may attract other scientists to start using pigeons as models."