The Old German Owl (left), and Racing Homer(right) were two domestic pigeon breeds that researchers bred to conduct the study. Credit: Sydney Stringham
Charles Darwin was obsessed by domestic pigeons. Charles Darwin believed that their beaks held the secrets to selection. The 350+ domestic pigeon breeds are free from natural selection and have beaks that vary in size and shape within one species (Columba vicia). Beaks that are so short that parents sometimes have to stop their young from eating are the most striking. Interbreeding over centuries taught early pigeon enthusiasts that only a few genetic factors could regulate beak length. Modern geneticists have not been able to solve Darwin's puzzle by identifying the molecular machinery that controls short beaks.
Biologists from the University of Utah have discovered that beak size reductions in many domestic pigeon breeds can be linked to a mutation in ROR2 gene. Surprisingly mutations in ROR2 are also responsible for Robinow syndrome, a human disorder.
"Some of Robinow syndrome's most striking features are the facial characteristics, which include a prominent forehead, a narrow nose, and mouth. They are reminiscent of pigeons' short-beak pattern," stated Elena Boer, the lead author of this paper. Boer was a postdoctoral fellow at U and is now a clinical variation scientist at ARUP Laboratories. It makes sense developmentally, as we know that the ROR2 pathway plays an important part in vertebrate craniofacial growth.
The paper was published in Current Biology, Sept. 21, 2021.
Mapping genes and skulls
Researchers bred two pigeons, one with short beaks and one with medium beaks. The medium-beaked male was a Racing Homer, which is a bird that's bred to speed and has a beak similar to the ancestral rock. The small-beaked female was an Old German Owl. This fancy pigeon breed has a short, flat beak.
High resolution scans showing the grandchildren of the Racing Homer/German Owl cross. Animation showing the range of beak lengths, from the shortest to the longest. Credit: Elena Boer
"Breeders chose this beak for its aesthetics, and it would never have a chance in nature. Domestic pigeons have a great advantage in finding genes that cause size differences," stated Michael Shapiro (the James E. Talmage Presidential Endowed chair in Biology at the U) and the senior author of this paper. "One of Darwin’s major arguments was that artificial selection and natural selection are variations on the same process. The size of the pigeon's beak was crucial in understanding how this works.
A F1 brood was born from the parents of short- and medium-beaked children. They had beaks that were intermediate in length. The F 1 and F 2 children were mated by biologists. Their beaks ranged from large to small, with all sizes in between. Boer used micro-CT scans from the University of Utah Preclinical Imaging Core Facility to measure the beak shape and size in each of the F2 individuals.
Boer stated, "The cool part about this method is that we can look at the size and shape the entire skull. It turns out that not only beak length differs, but also the braincase changes shape at different times." These analyses showed that the beak variation found in F2 populations was caused by differences in beak length, not variations in skull size or overall body size.
The researchers then compared the genomes of the pigeons. They first used a technique known as quantitative trait loci mapping (QTL) to identify DNA sequence variants throughout the genome. Then, they looked to see if these mutations were present in the F2 grandkids' respective chromosomes.
Shapiro stated that the grandkids with small eyes had the same chromosome piece as their grandparents with small beaks. This meant that the piece of DNA was related to small beaks. "And it was on sex chromosome," Shapiro said. This is what classical genetic experiments had suggested. So we were excited.
The entire genome sequences of many different breeds of pigeons were then compared by the team. 56 pigeons came from 31 short-beaked breeds, while 121 came from 58 long- or medium-beaked breeds. All individuals with small beaks shared the same DNA sequence within the ROR2 gene area.
Representative images of individuals representing the short and medium beak pigeon breeds (left 4 birds, right 4 birds). Image credit to Thomas Hellmann. From left to right, short beak pigeons: English Short Face Tumbler (English Short Face Tumbler), African Owl, Oriental Frill and Budapest Tumbler. (B) Medium/long-beak pigeons. From left to right: West England, Cauchois and Scandaroon. All short-beak birds had the same ROR2 mutation. Credit: Adapted by Boer and Shapiro (2021). Current Biology
Boer stated, "The fact we received the same strong signal by two independent approaches was really thrilling and provided an additional level evidence that the ROR2 locus might be involved."
According to the authors, the short-beak mutation may cause the ROR2 protein's folding to change in a novel way. However, the team is planning to perform functional experiments to determine how the mutation affects craniofacial development.
Pigeon enthusiasts
Darwin was captivated by the charm of the domestic pigeon. The research team used blood from many members of the Utah Pigeon Club (NPA) to sequence their genomes. These groups are pigeon enthusiasts that breed and compete in competitions to showcase the striking differences among the breeds.
Shapiro stated, "Every paper that our lab published over the past 10 years relied in some way on their samples." "We couldn’t have done it without the pigeon-breeding community."
Explore more Variations of the same gene that drives diverse pigeon feather designs
More information: Craniofacial variation is associated with domestic pigeons' ROR2 coding variant, Current Biology (2021). www.cell.com/current-biology/f 0960-9822(21)01208-2 Journal information: Current Biology A ROR2 coding variant is associated with craniofacial variation in domestic pigeons,(2021). DOI: 10.1016/j.cub.2021.08.068
