In the area of behavior, Penn investigators have found that epigenetic changes determine whether an ant becomes a soldier or a nurse. Whereas, in mice, epigenetic changes appear to reduce how susceptible male offspring are to cocaine addiction if their father was exposed to the drug.

In 2010, Shelley Berger's group, working with a large collaborative team, reported on the genomes of two common ant species, the Florida carpenter ant (Camponotus floridanus) and Jerdon’s jumping ant (Harpegnathos saltator). They found that although there are large differences in behavior between female queens and female workers, there are few genetic differences. Rather, epigenetic modifications to the chromatin appear to control their fate.

Similarly, the group later reported that epigenetic differences distinguish between two very different types of worker ants in Florida carpenter ant colonies. "Majors" are soldier ants that have enlarged heads and strong jaws. They are primarily responsible for protecting the colony. By contrast, "minors" are smaller and play the role of caretakers, foraging for food and nursing the young.

To learn how the differences come about, Berger's team conducted a genome-wide survey looking at how the DNA winds around histone proteins. Depending on the epigenetic marks on histones, the DNA winds more or less tightly around them. Tightly wound DNA is essentially turned off because it is inaccessible to the cellular machinery that would copy it into RNA and then protein. More loosely wound DNA, by contrast, is available for transcription and the gene can be turned on.

In examining several different histone modifications, the team found a number of distinct differences between the major and minor castes – and the differences correlate nicely with the different roles of majors and minors. For example, muscle-related genes are more highly expressed in majors, whereas brain- and neuron-related genes, which might help with complex tasks such as foraging, are more active in minors.

But epigenetics influence on behavior isn't limited to bugs. Last year, R. Christopher Pierce, PhD, associate professor of Neuroscience in Psychiatry at Penn, and colleagues at Massachusetts General Hospital in Boston, reported that cocaine-induced changes in physiology are passed down from father to son in rats.

The investigators allowed male rats to self-administer cocaine for 60 days, and then mated those animals with females that had no cocaine exposure. When the researchers offered the offspring the opportunity to self-administer cocaine, the males were slower to administer the drug than either the females or male offspring from control rats that had no drug exposure.

Further examination showed that the male offspring from cocaine-addicted fathers expressed more brain-derived neurotrophic factor in their brains than male offspring from non-exposed fathers. The researchers hypothesize that the difference in expression is due to cocaine-induced changes in the chromatin in sperm and that that change is passed on to male offspring, providing them with some protection from addiction.

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