Darwin’s Short Beak Riddle Solved | @theU

Charles Darwin was obsessed with domestic pigeons. He thought they held the secrets of breeding in their beaks. Freed from the bonds of natural selection, the more than 350 breeds of domestic pigeons have beaks of all shapes and sizes within a single species (Columba livia). Most striking are the beaks so short that they sometimes prevent parents from feeding their own young. Centuries of interbreeding taught the first pigeon fanciers that the length of the beak was probably regulated by a few hereditary factors. Yet modern geneticists have failed to solve Darwin’s mystery by identifying the molecular machinery controlling shortbeaks – until now.

In a new study, biologists at the University of Utah have found that a mutation in the ROR2 gene is linked to reduced beak size in many breeds of domestic pigeons. Surprisingly, mutations in ROR2 also underlie a human disorder called Robinow syndrome.

“Some of the most striking features 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-beaked phenotype in pigeons,” said Elena Boer. , lead author of the paper who completed the research as a postdoctoral fellow at U and is now a clinical variant scientist at ARUP Laboratories. “This makes sense from a developmental perspective because we know that the ROR2 signaling pathway plays an important role in craniofacial development in vertebrates.”

The paper published in the journal Current Biology on September 21, 2021.

Mapping genes and skulls

PHOTO CREDIT: Sydney stringham

Old German Owl (left) and Racing Homer (right) breeds of domestic pigeons.

The researchers bred two short and medium-billed pigeons – the medium-billed male was a Racing Homer, a speed-bred bird with a beak length similar to that of the ancestral rock pigeon. The small-billed female was an old German owl, a fancy breed of pigeon that has a small, stocky bill.

“Breeders selected this beak purely for aesthetics to the point that it is damaging – it would never appear in the wild. So domestic pigeons are a huge advantage in finding the genes responsible for height differences, ”said Michael Shapiro, James E. Talmage Presidential Chair in Biology at U and lead author of the article. “One of Darwin’s great arguments was that natural selection and artificial selection are variants of the same process. The size of the pigeon’s beak was instrumental in understanding how it works.

Short- and medium-billed parents produced a first F1 brood of children with beaks of medium length. When biologists mated the F1 birds to each other, the result F2 the grandchildren had beaks ranging from large to small, and all sizes in between. To quantify the variation, Boer measured the size and shape of the beak in the 145 F2 people using micro-scans generated at the central pre-clinical imaging facility at the University of Utah.

“The good thing about this method is that it allows us to look at the size and shape of the entire skull, and it turns out that it’s not just the length of the beak that differs.” : the skull changes shape at the same time, ”said Boer. “These analyzes demonstrated that the variation of the beak within the F2 population was due to actual differences in beak length and not to variation in overall skull or body size.

An animation of bird skulls showing the variety of beak lengths from short to long.


High resolution scans of the Racing Homer and German Owl grandchildren. Animation shows variety of beak lengths, from shortest to longest.

Next, the researchers compared the genomes of the pigeons. First, using a technique called quantitative loci mapping (QTL), they identified variants of DNA sequences scattered throughout the genome, and then looked to see if these mutations appeared in the F2 chromosomes of grandchildren.

“The grandchildren with the small beaks had the same piece of chromosome as their grandparent with the small beak, which told us that this piece of chromosome has something to do with the small beaks,” Shapiro said. “And it was on the sex chromosome, which classic genetic experiments had suggested, so we got excited.”

The team then compared the complete genome sequences of many different breeds of pigeons; 56 pigeons of 31 short-billed breeds and 121 pigeons of 58 medium or long-billed breeds. Analysis showed that all individuals with small beaks had the same DNA sequence in an area of ​​the genome that contains the ROR2 gene.

“The fact that we got the same strong signal from two independent approaches was really exciting and provided a further level of evidence that the ROR2 locus is involved,” said Boer.

The authors speculate that the short-nosed mutation causes the ROR2 protein to fold in a new way, but the team plans to do functional experiments to understand how the mutation affects craniofacial development.

Heads of domestic pigeon breeds.  The four on the left have a short bill, the four on the right have a medium or long bill.

PHOTO CREDIT: Thomas Hellmann, adapted from Boer et al. (2021) Current biology

Representative images of individuals representing short-billed (four birds on the left) and medium or long-billed (four birds on the right) breeds of pigeons (image credit: Thomas Hellmann). Short-billed pigeons, from left to right: English Short-faced Tumbler, African Owl, Eastern Frill, Budapest Tumbler. (B) Medium / long-billed pigeons, from left to right: West of England, Cauchois, Scandaroon, Show King. Short-billed birds all had the same ROR2 mutation.

Pigeon lovers

The attraction of the domestic pigeon which hypnotized Darwin still captivates the curious today. Many of the blood samples the research team used for genome sequencing were donated by members of the Utah Pigeon Club and the National Pigeon Association, groups of pigeon enthusiasts who continue to breed pigeons and to participate in competitions to show the striking variation between races.

“Every paper our lab has published over the past 10 years has relied on their samples in one way or another,” Shapiro said. “We couldn’t have done this without the pigeon breeding community.”

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