In 1959, Russian geneticist Dmitry Belyayev set up a selective breeding program intended to replicate the wolf-to-dog domestication process — this time, with foxes.

The experiment took place — and still continues — at the Institute of Cytology and Genetics in Novosibirsk, Russia, where researchers assigned a population of red foxes to different categories based on their behavior toward experimenters. Those considered the most friendly and tame were bred together, while the most aggressive were also bred together separately. Researchers observed behavioral and physiological changes in later generations of the former group, including seeking out human attention, sniffing and licking people, shorter legs, tail, snout, and upper jaw, and other dog-like traits.

Now, as part of a recent study published in G3: Genes | Genomes | Genetics, researchers believe they may have pinned down the biological mechanism responsible for such behavioral changes — namely, how these animals react to stress.

“Previous studies have found that ACTH [a stress response-driving hormone] levels in the anterior pituitary do not differ between tame and aggressive fox strains,” study author Anna Kukekova said. “This means that differential expression of the gene encoding ACTH may not cause the differences seen in blood levels of this hormone, and some other mechanism is reducing ACTH in the bloodstream of tame foxes.”

Using 12 foxes from the Institute of Cytology and Genetics’ breeding program, researchers compared the gene activity in their anterior pituitary glands. These canines belonged to an “elite” group — six of which were bred to be the most tame, and six which were bred to be the most aggressive.

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