Geneticists solve a long-standing mystery at Finch's Beak

"I like good challenges and especially work on new issues!" said vonHoldt, an assistant professor of ecology and evolutionary biology at Princeton. "I've introduced a new problem into an entirely new system, which was an incredible opportunity to explore how different ecologies could promote different models of evolution."

The observer and biologist was Tom Smith, who has spent his entire career studying finches, especially black-bellied shrimps (Pyrenestes ostrinus) – in Cameroon and in his laboratory at the University of California at Los Angeles.

He and his colleagues spent years researching why some of these finches had a small beak, while others had a large beak. Much of their original work has identified differences in the hardness of the seeds they eat, a story quite similar to that of Darwin's finches. Smith, professor at UCLA and founding director of the Center for Tropical Research, has created a breeding colony of these finches to understand the legacy of the beak's height.

The result was a surprisingly simple and elegant: Mendelian genetics, better known to generations of high school students thanks to the Punnett squares. The biggest beak was the dominant trait, so two small-billed parents could only have a small-priced offspring, but if one of the parents had a big bill, their offspring would have a combination of large and small bills, perfectly matching the 3: 1 model predicted by Gregor Mendel centuries ago.

"You never have that!" vonHoldt laughed. "Features rarely present such a legacy, especially those who play a vital role in the fitness of a wild population."

This Mendelian pattern was the key, vonHoldt said. With new technologies to analyze the entire genome, combined with years of Smith 's ecological and information data, they had all the elements to find and understand the genes at their fingertips. origin of this mystery.

Smith had discovered the observation of the Mendelian pattern in these Cameroonian birds, but he had not been able to identify the gene that was responsible for it. But when vonHoldt compared the genes of big-eared birds with those of their smaller-beaked counterparts, she discovered a DNA segment – 300,000 base pairs, apparently inherited in block form – that still varied between finches. small and large. And right in the middle of this part of the chromosome was the IGF-1 gene, which vonHoldt knew about canine genetics.

"In dogs, it is a giant gene, literally and figuratively," she said. "It's a growth factor gene." In the dog, if you change its expression, some genetic modifications are enough to turn a normal sized dog into a dwarf dog the size of a cup of tea. "

The gene can affect a specific trait or an entire animal, depending on its location on the genome and its expression. "If this gene is more expressed, you expect a larger trait: a larger body, a larger foot, a larger ear, whatever control is exercised." It is then easy to imagine that 39 with a small change of this gene, the traits could very easily change in size or shape.We suspect that's the story here, with these beaks, "vonHoldt said.

Smith and his colleagues had previously determined that beak size affected the diet – whether a finch lived with large or small seeds – but this did not seem to have an impact on partner selection. "Women do not prefer men with a big beak, or vice versa," vonHoldt said.

In these birds, the bill was the only trait to change size; Black-beaked and beak-billed seedcatchers are otherwise identical. But Smith also discovered a third morph of these finches, which he called the "mega" variety, with an even larger beak and larger body size.

After examining the genes, vonHoldt discovered that the "mega" is genetically distinct from both small and large beak morphs. It bears not only two copies of the great allele, such as the big-billed finches, but it also presents other chromosomal modifications, which are apparently the product of an additional evolutionary stage.

VonHoldt said that she appreciated the opportunity to get out of her usual subjects. "Dogs and wolves have helped me to ask the questions that fascinate me, from domestication to conservation," she said, particularly as part of her work with wolves in the National Park. Yellowstone. "There is so much to explore beyond dogs, it was a great opportunity to make new collaborations, think about a different problem and use new methods to deal with an old issue. "

Explore further:
Evolution in action detected in Darwin's finches

More information:
Bridgett M. vonHoldt et al., The growth factor gene IGF1 is associated with beak size in the black-bellied seed cracker, Pyrenestes ostrinus, Nature Communications (2018). DOI: 10.1038 / s41467-018-07374-9