Reversible ‘tags’ on genes tell bees their jobs
JOHNS HOPKINS (US) — Genetically identical worker bees take on very different roles in the hive—a division of labor that now appears to be linked to chemical tags on those identical genes.
A new study of worker bees in nursing and foraging roles provides the first evidence that the tagging, known as DNA methylation, impacts complex behavioral patterns of an entire organism. It also suggests that both the tagging and the behavior—in the bees, and presumably other animals—can be reversed.
“Genes themselves weren’t going to tell us what is responsible” for making one bee a nurse and another a forager, lead researcher Andy Feinberg says. “But epigenetics—and how it controls genes—could.”
Nursing bees generally remain in the hive to feed and take care of the queen and her larvae. Genetically, they are indistinguishable from those sent out to bring back pollen and nectar. But scientists have found differences in the epigenetic tags on the genes. (Credit: Christofer Bang)
Feinberg and his team say that the finding, described online September 16 in Nature Neuroscience, could have important implications for understanding behavior-related aspects of human health.
DNA methylation has long been shown to play an important role in regulating gene activity, like fate determination in stem cells or the creation of cancer cells, says Feinberg, professor of molecular medicine and director of the Center for Epigenetics at Johns Hopkins University’s Institute for Basic Biomedical Sciences,
Curious about how epigenetics might contribute to behavior, the team turned to bees, a tried-and-true model of animal behavior.
All worker bees are female and, within a given hive, all are genetically identical sisters. They don’t, however, all do the same thing; some nurse the queen bee and her larvae and some leave the hive in search of pollen and other supplies.
Working with bee expert Gro Amdam, associate professor of life sciences at Arizona State University and the Norwegian University of Life Sciences, Feinberg’s epigenetics team found significant differences in DNA methylation patterns in bees with these vastly different behavioral patterns.
Feinberg and Amdam started their experiment with new hives populated by bees of the same age. That removed the possibility that any differences they might find could be attributed to differences in age.
“When young, age-matched bees enter a new hive, they divvy up their tasks so that the right proportion becomes nurses and foragers,” Amdam says.
Analyzing the patterns of DNA methylation in the brains of 21 nurses and 21 foragers, the team found 155 regions of DNA that had different tag patterns in the two types of bees. The genes associated with the methylation differences were mostly regulatory genes known to affect the status of other genes.
“Gene sequences without these tags are like roads without stop lights—gridlock,” says Feinberg.
Once they knew differences existed, they could take the next step to determine if they were permanent.
“When there are too few nurses, the foragers can step in and take their places, reverting to their former practices,” Amdam says. The researchers removed all of the nurses from their hives and waited several weeks for the hive to restore balance between the two professions.
That done, the team again looked for differences in DNA methylation patterns, this time between foragers that remained foragers and those that became nurses. One hundred and seven DNA regions showed different tags between the foragers and the reverted nurses, suggesting that the epigenetic marks were not permanent but reversible and connected to the bees’ behavior and the facts of life in the hive.
More than half of those regions had already been identified among the 155 regions that change when nurses mature into foragers. These 57 regions are likely at the heart of the different behaviors exhibited by nurses and foragers, says Amdam.
“It’s like one of those pictures that portray two different images depending on your angle of view,” she says. “The bee genome contains images of both nurses and foragers. The tags on the DNA give the brain its coordinates so that it knows what kind of behavior to project.”
The researchers say they hope their results begin to shed light on complex behavioral issues in humans, such as learning, memory, stress response, and mood disorders, which all involve interactions between genetic and epigenetic components similar to those in the study.
A person’s underlying genetic sequence is acted upon by epigenetic tags, which may be affected by external cues to change in ways that create stable—but reversible—behavioral patterns.
The National Institute of Environmental Health Sciences, the Research Council of Norway, and the Pew Charitable Trust funded the study.
Source: Johns Hopkins University