Over fifty years ago, scientists in Russia began to selectively breed silver foxes. They were interested in selecting the foxes that were friendly to people to see if they would be able to replicate the changes that led to the domestication of dogs. They also bred a population of foxes that were aggressive. Now, researchers at Cornell have looked at how gene expression is different in two regions of the brain in populations of tame and aggressive foxes. The findings have been reported in the Proceedings of the National Academy of Sciences.
Russian researchers have been breeding the fox population since 1959, at the Institute for Cytology and Genetics in Novosibirsk, Russia. After ten generations, a few of the foxes started to take on some of the characteristics of domesticated dogs when approached by humans. More and more of the dogs began to exhibit this friendliness over time.
"That such a radical change in temperament could be accomplished so quickly is truly remarkable," said senior co-author Andrew Clark, professor in the Department of Molecular Biology and Genetics at Cornell.
The Cornell researchers conducted their investigation on dogs created after 50 generations of such selective breeding. Genes that changed in the tame animals were seen in regions of the brain that are involved in learning and memory, in samples from the prefrontal cortex and basal forebrains of a dozen tame and one dozen aggressive foxes.
The researchers, led by Clark and first author Xu Wang, Ph.D, utilized two assays. In one, the RNA transcripts were sequenced, providing a snapshot of the active genes at the time of the sampling. The other test looked at the genetic background of the animals, specifically, at how alleles, or different versions of genes, changed over time.
Their work revealed the brain pathways that were altered by selecting foxes with different traits and promoting those traits through breeding, The team focused on areas of the brain known for social interaction and information processing. They wanted to see how chemicals in the brain, the neurotransmitters glutamine, dopamine, and serotonin were affected.
Clark hypothesized that because pleasure centers are stimulated by dopamine, they would be different in tame animals.
"Tame animals seem like they are blissed out all the time," he explained. "They're just so happy and adorable, so I thought certainly the dopaminergic [pathway would be affected]. But there was no signal."
Something else changed in the tame animals, however - genes that influence the function of two types of neurons, serotonergic and glutaminergic nerve cells. These neurons have roles in learning and memory.
Genes that are important in embryonic development and that influence a structure called the neural crest were implicated. These cells move around the developing embryo, and form many cells that are important in adult animals; that includes the formation of facial features. This may show how traits like fur spots, curly tails, floppy ears, or shorter noses show up in domesticated animals.
"Darwin, and many others since, observed that when people select for domestication, there is a tendency to see a reversion in these traits to a more juvenile form," explained Clark, who added that more research is necessary to understand how the neural crest influences domestication syndrome.