Bees forage with their guts

When faced with a choice between carb loading and a protein-rich, Atkins-style diet, honeybees let their guts decide.

Insulin signals from fat cells in the bees’ abdomens help determine whether they forage for high-protein pollen or sugar-filled nectar, a new study shows. The study, published April 1 in PLoS Genetics, is the first to manipulate insulin signals in honeybees and to show how changes in the signals influence behavior.

Reducing the activity of the insulin receptor substrate, or IRS, gene caused bees to forage more for pollen than for nectar, report researchers led by Gro Amdam, a biologist at Arizona State University in Tempe and the Norwegian University of Life Sciences in Aas. The researchers showed that the gene, which is involved in sugar uptake by cells, regulates not just how nutrients are turned into energy but also the bees’ preferences for which foods to consume in the first place.

Reducing the gene’s activity in fat cells affects the bees’ behavior even if the gene is functioning normally in the brain, Amdam’s group discovered. That suggests the gene causes fat cells to generate a chemical signal that tells the brain what kind of food to look for.

“That’s something that I find quite remarkable,” says Thomas Flatt, a geneticist at the University of Veterinary Medicine in Vienna, Austria. “I don’t think many people have considered how insulin is affecting food choices, not just what happens after food has entered the body. The behavior dimension is new and interesting.”

Insulin is an important hormone that regulates sugar intake by cells. In conditions such as diabetes, where insulin is missing or cells are insensitive to its effects, cells don’t take in the sugar they need to burn for fuel and essentially starve to death.

Bees usually collect both nectar, which is a rich sugar source that is also the starting material for honey, and pollen, a high-protein food needed to feed bee larvae.

In the new study, Amdam and her colleagues show that the IRS gene is partially responsible for influencing the bees’ foraging behavior.

Using bee strains that have been bred in the laboratory to show a high or low preference for pollen, Amdam’s group showed that pollen-hoarding bees have lower IRS activity.

The researchers also demonstrated the importance of IRS by reducing its activity in normal bees using a technique called RNA interference. Researchers injected RNA into the bees’ abdomens, blocking genes that are normally turned on in the thin layer of fat cells lining the abdominal cavity.

Then the researchers compared the foraging behavior of bees with reduced IRS activity in their fat cells to that of a control group that got sham injections with no effect on the gene’s activity. As a group, bees that got the IRS-inhibiting injections collected more pollen than the control group did.

Previous work by the group had shown that changing bees’ sensitivity to sugar could affect nectar collection. The new study demonstrates that bees with reduced insulin signaling like sugar just as much as bees with normal insulin signaling. That result, together with the fact that IRS levels remain normal in the brain, means that some other message must travel from fat cells in the abdomen to the brain to induce bees’ pollen craving.

“This taps in very nicely to well-known connections” between fat tissue and the brain in vertebrates, says Gene E. Robinson, a geneticist, neuroscientist and entomologist at the University of Illinois at Urbana-Champaign. For instance, leptin, another hormone produced by fat cells, is known to regulate hunger in mammals, including people. “We want to know what the fat cells are telling the brain about what food to choose,” Amdam says. For now, the content of that message is a mystery.