Sex chromosome genes influence aggression

It has been well documented that, across human cultures and in most mammals, males are usually more aggressive and less nurturing than females. It's simple to blame male hormones, like testosterone, for male behavior such as aggression. But maybe it's in our genes, too.

Indeed such social behavior also has a genetic basis, according to new research on mice by neuroscientists at the University of Virginia Health System. "The differences in sex chromosomes, XX versus XY, are also responsible for differences in adult behavior," explained Emilie Rissman, PhD, a professor of biochemistry and molecular genetics at UVa, who studied aggression and maternal behavior in genetically engineered mice. "Sex chromosome genes may not be the whole story that determines how aggressive or motherly we are, but they are a part of it."

Rissman's work is published in the The Journal of Neuroscience. Co-authors on the paper are scientists at the University of California Los Angeles and the National Institute for Medical Research in London, England.

Using mouse models, Rissman and the research team uncoupled the testis-determining gene Sry on the male Y chromosome from other sex chromosome genes. The presence of Sry leads to the development of the testes and high levels of androgens in males, which is partly responsible for aggression. Sry was deleted from the Y chromosome and replaced by a transgenic copy.

In their experiments, the researchers compared mice with or without the Sry gene (either males with testes or ovary-bearing females) to mice with the XX versus XY sex chromosomes. They tested how long it took for mice to become aggressive when another mouse was placed in their home territory. The researchers also clocked the time it took to exhibit spontaneous parental behavior by retrieving a pup.

The researchers found that the sex chromosome complement, as well as gonadal sex differences, influences behavior. Gonadal male mice (those with Sry with either the XX or XY sex complement) behaved most aggressively. But females with the Y chromosome were aggressive as well. Females with the XX complement were not aggressive, but in contrast they exhibited high levels of maternal behavior by picking up pups and building nests.

"We know that hormones are important for development of sex differences in brain and behavior," Rissman said. "But this is another component of the sex difference that needs to be examined." In all probability, Rissman said, genes and hormones interact with each other, especially in males, because they have testosterone and the Y chromosome. "It is our hope that these data could lead to the discovery of new genetic bases for aggression and parental behavior in other animals, including humans."