Why are some people shy while others are outgoing? A study in the current issue of Science demonstrates for the first time that social behavior may be shaped by differences in the length of seemingly non-functional DNA, sometimes referred to as junk DNA.
The finding by researchers at the Yerkes National Primate Research Center of Emory University and the Atlanta-based Center for Behavioral Neuroscience (CBN) has implications for understanding human social behavior and disorders, such as autism.
In the study, Yerkes and former CBN graduate student Elizabeth A.D. Hammock, PhD, and Yerkes and CBN researcher Larry J. Young, PhD, also of the Department of Psychiatry and Behavioral Sciences at Emory University's School of Medicine, examined whether the junk DNA, more formally known as microsatellite DNA, associated with the vasopressin receptor gene affects social behavior in male prairie voles, a rodent species. Previous studies, including Dr. Young's gene-manipulation study reported in Nature's June 17, 2004, issue, have shown the vasopressin receptor gene regulates social behaviors in many species.
The researchers bred two groups of prairie voles with short and long versions of the junk DNA. By comparing the behavior of male offspring after they matured, they discovered microsatellite length affects gene expression patterns in the brain. In the prairie voles, males with long microsatellites had higher levels of vasopressin receptors in brain areas involved in social behavior and parental care, particularly the olfactory bulb and lateral septum. These males spent more time investigating social odors and approached strangers more quickly. They also were more likely to form bonds with mates, and they spent more time nurturing their offspring.
"This is the first study to demonstrate a link between microsatellite length, gene expression patterns in the brain and social behavior across several species," said Young. "Because a significant portion of the human genome consists of junk DNA and due to the way microsatellite DNA expands and contracts over time, microsatellites may represent a previously unknown factor in social diversity."
Hammock and Young's finding extends beyond social diversity in rodents to that in apes and humans. Chimpanzees and bonobos, humans' closest relatives, have the vasopressin receptor gene, yet only the bonobo, which has been called the most empathetic ape, has a microsatellite similar to that of humans. According to Yerkes researcher Frans de Waal, PhD, "That this specific microsatellite is missing from the chimpanzee's DNA may mean the last common ancestor of humans and apes was socially more like the bonobo and less like the relatively aggressive and dominance-oriented chimpanzee."
The researchers' finding also has set a clear course for the next step. They want to build upon previous studies that identified a microsatellite sequence in the human vasopressin receptor that varies in length. "The variability in the microsatellite could account for some of the diversity in human social personality traits," explains Hammock. "For example, it may help explain why some people are naturally gregarious while others are shy." In particular, Young wants his research team to expound upon studies that have identified a link with autism.