Bee role

Wednesday, February 2, 2011
by Diane Boudreau

Social bees Photo by Bente Smedal.

All complex societies require a division of labor. Human civilization has advanced to modern levels because people can specialize and become experts. Some produce food. Others design buildings. Still others care for the sick.

When it comes to division of labor, however, humans have got nothing on the social insects. These animals, such as honeybees, take on very rigid roles. The system is very efficient, and it has helped social insects become one of the greatest success stories of evolution.

A female bee can become a worker or a queen. Queen bees are larger and live longer than workers. Queen bees can also produce offspring. A queen lays eggs and keeps society together with pheromones that attract the workers. Workers tend the hive, care for the queen and larvae, and forage for food.

Male bees (drones) only come along when it’s time for the queen to mate.

“The queen will fly out after she’s about one week old, and she will meet with many drones in a congregation area. She will fly through the area and mate with the drones in flight. And the drones, they just die directly after they mate with the queen. That is really their sole purpose. While they are in the colony waiting for their flights, they basically sit around and get fed by the workers,” says Florian Wolschin, a biologist at ASU.

The drones don’t fertilize the queen’s eggs right away. Instead, she holds onto their sperm and fertilizes her eggs as needed. To produce male bees, she lays unfertilized eggs.

Worker bees control the fate of the female larvae. The amount and type of food the larvae receive determine whether they become workers or queens. Among workers, tasks are further divided. Some take care of the queen and larvae. Others forage for pollen or nectar. Some workers collect mostly pollen, while others prefer nectar.

The bees’ genes
Bee social behavior is so structured that many people assumed it was hard-coded in their genes. However, when scientists sequenced the honeybee genome, they didn’t find any new genes that had evolved specifically to control social life.

“There wasn’t much evidence in the genome for social genes or new gene products that had evolved to specifically contribute to controlling social life,” says ASU biologist Gro Amdam, who helped sequence the bee genome.

“The incredible thing is that both of these types of female honeybees emerge from the same genome,” adds Wolschin. “So how does that happen?”

Nurse bees with larvae Nurse bees take care of the brood. Photo by Bente Smedal.

The fact that bees can develop so differently from the same genome makes them useful model animals for exploring our own development. Model organisms have contributed greatly to our understanding of biology. For instance, most of what we know about classical genetics comes from studies of fruit flies. And most of what we know about molecular genetics comes from looking at E. coli, a type of bacteria.

Social insects can help us advance a field called “epigenetics.” Epigenetics explores how the function of genes can change without changing the sequence of the genetic code, or DNA.

Take stem cells, for example. Stem cells are like “wild cards.” They can develop into any type of cell in the body. But how does one stem cell “know” to develop into a liver cell, while another stem cell—carrying the exact same genes—becomes a neuron?

Epigenetics explores why certain genes are expressed at some times and not others. Why do some people with a genetic predisposition for breast cancer get the disease, while others, who carry the same mutation, don’t?

Epigenetics combines what’s inside our DNA with the environment around it—nature and nurture put together. Environment is tough to study in humans, particularly when you are studying behavior. And we cannot ethically tinker with peoples’ genes to see what will happen if we change them.

Enter the insects.

“Bees have been used from early on as a model to study the genetics of complicated behavior, including social behavior,” says Amdam.

A woman’s role
Ancient bees were solitary creatures. Unlike the social bees we see today, these ancestors were do-it-yourselfers. Every female reproduced, foraged and nurtured larvae. Over time, social bees such as honeybees evolved and divided the workload.

We know that social bees don’t have new genes to make them behave this way. So how did they change their behavior? Amdam and ASU biologist Robert Page proposed that the ability to be social was in them from the start.

“I think it’s really easy to relate to that by thinking about dogs,” says Amdam. Humans have bred dogs to have a wide variety of traits, from the tiny Chihuahua to the massive Great Dane. “We have retrievers, we have pointers, we have dogs that fight, we have dogs that run,” she says.

All of these behaviors are present in wolves, the ancestors of our pet dogs. In dogs, however, certain behaviors are emphasized. For example, a retriever over-emphasizes fetching, while a greyhound over-emphasizes speed.

Solitary bees are like wolves. They do many tasks equally well. Honeybees are more like dogs. Each bee does one task extremely well, to the detriment of other skills. How does this happen?

Solitary bees go through phases in their behavior. These phases are related to the state of the bee’s ovary—the organ that produces eggs. A solitary bee will collect and hoard pollen when her ovary is large and she is getting ready to lay eggs. She does this because the larvae will need pollen to eat.

Among social bees, the workers still have ovaries, even though they don’t lay eggs. But the ovaries do play a role in the bees’ division of labor. Honeybees with large ovaries will collect lots of pollen, just like the solitary bees preparing to lay eggs. Bees with smaller ovaries prefer to collect nectar.

Bee collecting pollen A bee collects pollen and carries it in sacs on her back legs. Photo by Osman Kaftanoglu

“We suggested that the female reproductive cycle had been adopted by social evolution. So instead of getting rid of the ovary, it was actually a resource for social life. The colony could, by creating variation in ovary size, create variation in the propensity of different bees to collect different kinds of food,” says Amdam.

The hive benefits from having specialists, and so bees have used the link between the ovary and behavior to create variation in what workers do. This helps explain why the genome doesn’t change much from solitary to social bees, says Amdam. The same genes are involved in both solitary and social behaviors.

Unlocking the secrets of youth
Why should we care about how bees develop their different roles? For one thing, bees may be able to help us understand the aging process in humans.

For centuries, people have used bee products like honey and royal jelly to make cosmetics and supplements that claim to prolong youth. But the ASU scientists are not interested in pills and lotions. They are looking at the bees’ genes to unlock the mysteries of aging.

Think about it. All bees develop from the same genome. All female larvae have the potential to become either workers or queens. But the bees that do become queens will live 10 or even 20 times as long as the bees that become workers.

Bees have another amazing ability related to aging. “In most organisms, aging cannot be reversed. In bees, some signs of aging can be reversed by manipulating the colony,” says Wolschin.

When worker bees are young, they stay in the nest, tending the brood and caring for the queen. At two to three weeks of age, they leave the nest and begin foraging for pollen, nectar and water.

“What you can do experimentally is take away all the young bees from the colony, but leave the queen and brood in there. The foragers come back from their flights and there’s no one to take care of the brood,” says Wolschin.

Some of the foragers will rise to the occasion, returning to their youthful tasks of tending to the queen and cleaning the nest. Studies in Amdam’s laboratory show that they return to a physically younger state, as well. For instance, certain proteins that decline in older bees return to their youthful levels.

As if that isn’t odd enough, the bees’ get “younger” performance-wise, as well. As bees age they lose some of their learning ability. Nicholas Baker, a research technician at ASU, showed that older foragers begin performing better on learning tests when they are forced back into their younger roles.

How do you measure a bee’s ability to learn? Scientists test this by exposing the bee to a specific scent and then rewarding it with sugar. Bees respond to sugar by extending their tongues. Scientists measure how many times the bee must be exposed to the scent/reward pair before she will extend her tongue in response to the scent alone.

Bee sticks out its tongue during a learning test A bee extends her tongue during a learning trial. Photo by Bente Smedal.

“There are signs on several levels that show us that parts of the bees’ overall biology can be reverted. That’s pretty unique. You don’t have other model organisms in aging research that can do that,” says Wolschin.

The scientists hope that learning how bees develop can help us understand our own development and aging. Perhaps someday we can use this knowledge to retain our mental abilities with age or to improve recovery after illnesses and injuries.

“We don’t work with people or furry little animals that are probably closer to people than insects are,” cautions Amdam. Yet, she says, insect research can inspire other scientists to explore areas they may not have considered before.

She continues: “We unravel, we bring to the surface, a new insight. And then a researcher that works with animals close to people, or with people in a clinical setting, can pick up on that and try it. That’s one of the most important contributions of model organism science—the inspiration.”

Want to know more about bees and the people who study them? Try these links:

Bee movie maker
ASU biologist Brian Smith captures the high-speed communication of bees on film.

Podcast with Gro Amdam
The ASU scientist talks about Africanized bees, how bees communicate, bee aging and why she considers herself a control freak.