One of biology’s enduring mysteries is how some animals—from humans to honey bees—became so social. Now, a study suggests that, in the inconspicuous sweat bee, changes to the expression of a single gene could determine which bees are solitary and which are social. The gene, which has previously been linked to autism in humans, has also been connected to social behavior in animals like mice and locusts. The new discovery puts scientists one step closer toward demonstrating a common evolutionary basis for social behavior.
“People have been taking about the genetics of sociality for years,” says Bernard Crespi, an evolutionary biologist at Simon Fraser University in Vancouver, Canada, who was not involved with the work. “Finding this gene is a real watershed for the field.”
Sweat bees don’t have the same massive colonies as honey bees, whose hundreds of workers care for and protect a single egg-laying queen. But the tiny, gentle bees have some interesting social arrangements: In some groups and species, workers help a reproducing queen, as honey bees do; in other groups, sweat bee females tend their own broods. This difference has led scientists to think sweat bees may hold the key to understanding how more complex insect societies began to evolve.
In the 1950s, French biologist Cécile Plateaux-Quénu documented two distinct behaviors in a single species of sweat bee, Lasioglossum albipes. Females in cooler parts of France didn’t generally have helpers, whereas those in warmer parts did—there, female bees would lay two sets of eggs, and hatchlings from the first set would tend to the second set of eggs. Plateaux-Quénu’s studies showed, too, that this difference was inherited.
Two decades later, Sarah Kocher, an evolutionary geneticist now at Princeton University, decided to follow up on Plateaux-Quénu’s pioneering studies. She collected 150 bees from three cool and three warm regions of France. While a postdoctoral fellow at Harvard University, she and colleagues analyzed the bees’ DNA to find genetic differences that might explain the two behaviors.
After sequencing and comparing genomes from the six groups, the researchers found 200 differences centered around 62 genes. One gene, called syntaxin 1a, stood out. It is responsible for creating syntaxin, a protein important in the transmission of signals between nerve cells. The gene, linked to social behavior in a number of animals, was the one that best differentiated social from solitary sweat bees, Kocher says. “It seemed a good place to start.”
So Kocher measured how active the gene was in the social and the solitary bees. The social bees’ gene was about 15 times as active as that of the solitary bees, her team reports today in Nature Communications, making it a strong candidate for the switch to sociality. Next, her team evaluated seven differences to DNA on or near the syntaxin gene to see which were the most powerful in controlling its activity. The answer? A DNA sequence in part of the syntaxin gene that does not code for a protein.
These findings are in line with syntaxin function in other animals, Kocher says. For example, locusts that feed on their own but come together to migrate have increased syntaxin activity during migration. Mice lacking syntaxin have altered levels of hormones that influence social behavior. And several studies in humans have implicated syntaxin in autism and other disorders that lead to hyper- or hyposocial behavior.
This is not the first gene linked to both insect social behavior and autism. Last year, researchers showed that honey bees that react less to danger than most other workers have altered levels of activity in other genes implicated in autism. These genes code for proteins with functions similar to syntaxin’s.
The next step, says Gene Robinson, a behavioral genomicist and director of the University of Illinois Carl R. Woese Institute for Genomic Biology in Urbana, who did the honey bee work, is to figure out how the genes in his study—and syntaxin—affect brain development and function. And Crespi urges researchers to take a closer look at syntaxin’s activity in people with autism: “If it’s involved with cooperation and social behaviors, then it could be a target for a new therapeutic agent.”