Nature and Nurture and Something Else (Plus Bees)Posted: October 19, 2012
What makes you, you? The nature vs. nurture debate has been going on for more than a century, and recent work with honeybees has managed to make it even more complex. Researchers focused not on the nature part, the bees’ DNA, nor on the nurture part, how the bees grew up and lived, but on a fuzzy gray area in between.
All of the worker bees in a hive are sisters, descended from the same queen. All of these bees grow up together sharing the same environment. Worker bees divide into two groups: nurse bees, who take care of the eggs and larvae, and forager bees, who fly around collecting pollen and nectar. Nurses’ and foragers’ missions are different, but the DNA of these sister bees is quite similar and their nurturing seems to have been the same. How do they end up with different purposes?
Enter epigenetics, a relatively new field that studies how the environment affects the expression of genes. Genes were once thought to be instructions written in stone, unchangable directives for the cell. The nurture effects were thought to go on after the instructions were read, a result of environmental factors, i.e. where a toxic chemical causes cancer or someone overcomes a natural stutter. In reality, the genes themselves get buffeted by the winds of chance and circumstance from the outside world. Epigenetics has found “tags” sitting on top of sections of DNA. These tags control whether the cell will “read” a gene or if it will remain silent. The DNA itself is not actually changed, but its accessibility is. The whole set of DNA, called the genome, is overlaid with a pattern of these tags, called the epigenome.
This epigenome develops throughout life, starting with very few tags at birth. Tags are added or removed due to environmental factors such as nutrition, stress, and disease, allowing cells access to some genes and not others. Not only do these tags accumulate throughout one lifetime, some of them can be passed down to offspring. Which means that parents, grandparents, and various distant relatives all gave some of their epigenome to an individual, contributing to how their genes are expressed. They also contributed DNA, too, but unlike DNA, the epigenome is influenced by lifestyle choices. Those grandparents’ actions and experiences, not just their genes, influence who you are.
After finding no difference between the genome and the epigenomes of the queen bee and workers right after birth, Dr. Andrew Feinberg and his colleagues at Johns Hopkins University examined the differences between the two castes of worker bees: nurses and foragers. These workers perform very different roles in the hive. Usually, newly born bees start out as a nurses, and as older foragers die in the risky outdoors, some of them start foraging. Researchers took care to compare the epigenomes of workers that were the same age, each nurse and forager getting the same amount of time to accumulate epigenetic tags.
In their experimental design, the researchers were sneaky. They took advantage of the ability of the workers’ ability to change jobs: switching back to nurse from forager if the need arises. This doesn’t happen very often, but researchers created the need. If we manipulate their hive a bit, and get some of the forager bees to change back to being nurses, they asked, what will their genome look like then?
While the forager bees were out foraging, the researchers moved the hives, so that the bees came back to another hive, empty of bees but not of honeycomb full of larvae that needed tending. With a distinct need for nurse bees, half of the workers went back to their old jobs. Researchers looked to see if foragers and nurses have different epigenomes, and what type of epigenome the foragers-turned-back-to-nurses had.
It turned out that not only did nurses and foragers have distinct epigenomes, but it seems the epigenome changed with the job. When foraging worker bees were steered back to being nurses, their epigenome, and the genes it allowed to be expressed, reverted back to look like it did when it was a young nurse bee. It was like flipping a switch attached to about 100 genes at the same time, turning them on or off if the worker was fulfilling a nurse role or a forager role. These worker bees acted very differently, and the specific epigenome patterns seem to be the key to why.
This is, as the researchers note in very understated tones, “the first evidence in any organism of reversible epigenetic changes associated with behavior.” Does our epigenome change our behavior or does our behavior change our epigenome? No one knows, but this is evidence of the large role epigenetics plays in each individual. And our epigenome is greatly affected by every facet of the environment we live in. So how did you become you? A murky causal soup of your environment, combined with your genes, combined with the gray area of environmentally-affected gene expression. Epigenetics, and bees, have just made the nature-nurture debate much more interesting.