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October 16, 2021

U. researchers say they’ve cracked Darwin’s pigeon ‘enigma’

An image of an Old German Owl, left, and Racing Homer, right. The two domestic pigeon species were the the grandparents of over 100 pigeons researched in a study regarding why domestic pigeon beaks widely range in size. (Sydney Stringham via University of Utah)

SALT LAKE CITY — There are many animals that piqued Charles Darwin’s interest during his legendary studies in the 19th century.

He may be mostly associated with tortoises and finches, but he also dwelled often on the domestic pigeon. That’s because the species helped form his theory of natural selection because he noted domestic pigeons were artificially selected, Michael Wheelock wrote in a piece for Rockefeller University’s “The Incubator” back in 2013.

But one facet of pigeons he wondered about was why, exactly, did the more than 300 various breeds of pigeons have beaks of different shapes and sizes, including beaks short enough to make it difficult for parents to feed their young?

Well over a century later, University of Utah researchers say they now have an answer to what they called “Darwin’s short-beak enigma.” They say short beaks in pigeons are the result of a genetic mutation, the same genetic mutation that causes Robinow syndrome in humans. Their findings were published Tuesday in the journal “Current Biology.”

To get to their finding, a team of researchers bred two pigeons with different beaks. Michael Shapiro, who is the James E. Talmage Presidential Endowed Chair in Biology at the University of Utah and senior author of the study, explained that domestic pigeon breeders selected beaks based on aesthetics and not anything that would benefit the species in nature. Because of this, the researchers knew they could find genes responsible for different beak sizes.

“One of Darwin’s big arguments was that natural selection and artificial selection are variations of the same process,” Shapiro said in a statement Tuesday. “Pigeon beak sizes were instrumental in figuring out how that works.”

The team started by breeding a Racing Homer with a medium-sized beak similar to the ancestral rock pigeon with an Old German Owl, which despite the name is a fancy pigeon breed with a small beak. Their brood featured intermediate-length beaks; when those birds mated with another, their offspring featured various beak sizes and shapes.

Elena Boer — a clinical variant scientist at ARUP Laboratories, former postdoctoral research fellow at the University of Utah and the study’s lead author — then used micro-CT scans to measure the beaks of the over 100 birds produced as grandchildren of the original pigeon couple. She found that not only did the beaks differ but so did the shape of the birds’ braincases.

“These analyses demonstrated that beak variation within the (grandchild) population was due to actual differences in beak length and not variation in overall skull or body size,” she said in a statement.

But the largest finding of the paper is that the short beaks are the result of alterations to the ROR2 gene. This was discovered through two steps.

They first used a process called quantitative trait loci mapping, which helped them identify DNA sequence variants and also the ability to search for mutations in the chromosomes of the grandchildren. The results confirmed what the researchers expected based on previous classical genetic experiments, according to Shapiro. He said they found that the grandchildren with small beaks had “the same piece of chromosome” as the grandparent with a small beak.”

They then analyzed all of the genome sequences from the different pigeon breeds. This research showed that all the birds with small beaks had the same DNA sequence in the genome that contains the ROR2 gene. Boer said finding the same results in two different approaches was “really exciting” because it strongly indicates ROR2 gene is the leading factor for the beak size.

She added that ROR2 gene mutations also result in Robinow syndrome within humans.

“Some of the most striking characteristics of Robinow syndrome are the facial features, which include a broad, prominent forehead and a short, wide nose and mouth, and are reminiscent of the short-beak phenotype in pigeons,” she explained. “It makes sense from a developmental standpoint because we know that the ROR2 signaling pathway plays an important role in vertebrate craniofacial development.”

And one of Darwin’s many quandaries regarding animal mutations is now solved.

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