Web edition: September 18, 2012
Print edition: October 20, 2012; Vol.182 #8 (p. 10)
Reversible changes that dictate how genes function may determine what jobs honeybees do in the hive.
Worker honeybees are literally born to be nurses that take care of larvae. After two to three weeks, the workers become foragers. That career change is accompanied by the addition of chemical tags called methyl groups to some of their genes, says a team of researchers led by epigeneticist Andrew Feinberg of Johns Hopkins University and biologist Gro Amdam of Arizona State University in Tempe and the Norwegian University of Life Sciences in Aas.
The addition of chemical tags, tweaks known as epigenetic modifications, changes genes’ activity but doesn’t alter the genes themselves. In honeybees, these modifications are also reversible: Forager bees that go back to being nurses also revert to the epigenetic pattern seen in nurses, the researchers report online September 16 in Nature Neuroscience.
Although other researchers have previously detected different patterns of chemical tags between nurses and foragers, this is the first time behavior changes have been linked to reversal of those patterns.
“This is an exciting paper because it implicates epigenetics in the establishment and stability of distinct behavioral predispositions,” says Gene Robinson who studies bees, genes and behavior at the University of Illinois at Urbana-Champaign. The new study does not show that methylation changes cause bees to behave differently, but it does show that behavior and epigenetic modifications are both reversible and associated with each other.
For the new study, Amdam and her colleagues set up bee colonies in which all the workers were the same age. The researchers continually removed larvae just before hatching so no relief workers could be added to the workforce. Dots of paint distinguished foragers from nurses. An analysis of the chemical tags in bee brains showed that the nurse-to-forager career transition was associated with dropped or added epigenetic tags on 155 different genes.
When all the foragers were out of the colony, Amdam and her colleagues snatched the remaining nurses. After a few days of confusion, some foragers returned to their old nursing jobs. That reversion was accompanied by DNA methylation changes in 107 genes, including 57 involved in the original nurse-to-forager change, the team found.
Epigeneticists would have predicted a result similar to these findings, says Wolf Reik, an epigeneticist who studies wasps at the Babraham Institute in Cambridge, England. “This is not surprising, but it is actually quite exciting to see it,” Reik says.
Researchers in Australia had previously described epigenetic differences between queens and workers, a difference Feinberg and Amdam’s analysis failed to uncover. Reik and Solenn Patalano, also of the Babraham Institute, attribute the different findings to the age of the bees in the studies. The Australian group studied established workers and queens, while the new study examined newly emerged bees. Epigenetic patterns are shaped over time by experience, age and interactions with the environment. So the differences noted in the earlier study might reflect the different experiences queens and workers encounter in their lifetimes.
Similarly, Ryszard Maleszka, an epigeneticist at the Australian National University in Canberra who led the earlier studies, isn’t convinced that reversing epigenetic tags causes bees to revert to old behaviors. Rather, flexible tags may reflect the reverted nurses’ switch from flying alone in a visually exciting world to returning to a dark, crowded nest alive with smells.
B. R. Herb et al. Reversible switching between epigenetic states in honeybee behavioral subcastes. Nature Neuroscience. doi:10.1038/nn.3218
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