Scientists have been 'abuzz' with the results of a recent study published in Nature Neuroscience in which it was reported that differences in the behavior of honeybees correlated to variations in DNA methylation patterns. These changes, moreover, could be reversed, allowing bees to transition between two distinct roles and alter their behavior accordingly.
Within most honeybee colonies there is typically a single queen bee whose sole responsibility is to propagate the colony. Out of the tens of thousands of bees within the colony the queen alone retains the ability to reproduce, laying up to 2,000 eggs per day. The rest of the colony is made up of sterile worker bees which are unable to reproduce, and so their lives are devoted to maintaining the colony and ensuring the safety of the queen. Most worker bees are nurses for the first 2-3 weeks of their adult lives and tend to the queen and larvae before becoming foragers, collecting pollen, nectar, and water for the colony. These two behavioral programs are fairly flexible, however, with foragers retaining the ability to revert back to nurses as needed.
Although all of the honeybees within a colony share the same genome, these three very distinct behaviors and castes emerge despite their genetic similarity. It has previously been demonstrated that nurses and foragers differ in gene expression within the brain, and that the difference in the expression profile of nurses and foragers correlate with the observable differences in behavior. What effects these changes is still unknown, but these observations suggested that epigenetics may play a role in determining which bees within the colony would adopt the role of queen, nurses, and foragers.
Epigenetics refers to the phenomenon in which DNA containing the same sequence of ‘letters’ can produce two very different ‘phenotypes’, or physical characteristics. Epigenetic modifications are the ways in which DNA can be altered without changing the sequence of the ‘letters’. One such modification is the addition of small molecules known as methyl groups to the letters within a DNA sequence. Addition of these methyl groups to a length of DNA shuts off any active genes in the affected region. If you were to compare the DNA of one of your skin cells to that of a muscle cell, the letter sequence will be (more or less) the same, but chances are you will find variations in DNA methylation and subsequently differences in which genes are turned on or off.
In a recent study published in Nature Neuroscience researchers analyzed differences in the methylation patterns of DNA in the brains of queen bees and worker bees. The brain was the logical starting point for the study, as the brain is where the behavior of honeybees originates and is roughly the same size in both queen and worker bees. Researchers found no significant difference between the DNA methylation patterns of queen bees and worker bees shortly after they emerged as adults, however, comparison of the DNA methylation patterns of nurse and forager worker bees revealed extensive differences between the two. To confirm whether each set of methylation patterns was specific to the role of the worker bee, or simply the result of having undergone a transition from nurse to forager, foragers were switched into a hive containing only a queen and larvae. The lack of nurses in the hive compelled a subset of the forager bees to revert back into nurses to compensate. The methylation patterns of the revertant nurses matched the methylation patterns of the nurses in the original experiment, indicating that the forager and nurse bees have unique and consistent methylation patterns. This led researchers to speculate that the difference in DNA methylation between nurse and forager bees may account for the difference in their respective behaviors.
Epigenetics has become one of the most ubiquitous 'buzz words' in current scientific literature, and rightfully so. Although it is not clear whether the differences described above are cause or consequence of the nurse-to-forager transition, it appears that the role of epigenetics in natural phenomena cannot be understated.Herb, B. R., Wolschin, F., Hansen, K. D., Aryee, M. J., Langmead, B., Irizarry, R., . . . Feinberg, A. P. (2012). Reversible switching between epigenetic states in honeybee behavioral subcastes. Nature Neuroscience, 15(10), 1371-1373.