Some ants live longer than others—way longer. And the mapping of the first full genome sequences of ants helps to reveal how two ants from the same colony, and with much the same genetic material, can have such different life histories. The work may also provide insights into longevity in another social species with which ants share about one third of their genes: humans.
Researchers sequenced the genomes of two ant species: Jerdon's jumping ant (Harpegnathos saltator) and the Florida carpenter ant (Camponotus floridanus), which have quite different levels of social—and hence, biological—mobility. Carpenter ants live in large colonies that revolve around a queen that lays all of the fertilized eggs. Once the queen dies, the colony perishes as well. Jerdon's jumping ants, on the other hand, have smaller colonies in which worker ants can replace the queen after she dies. These so-called gamergate queens change physically and behaviorally as they take on the queen's duties.
All of these ant castes seem to start with the same basic genetic blueprint, yet end up looking—and behaving—very differently. Scientists point to epigenetics, the change in gene expression (rather than direct alterations in the DNA code), as a likely explanation. "It's not changes in the genome," says Shelley Berger, a professor of cell and developmental biology at the University of Pennsylvania School of Medicine and co-author of the new paper. "It seems to me this was an epigenetic question of how they took on these different forms," she says of the ants.
The genome sequencing, published in the August 27 issue of Science, reveals that the carpenter ant has about 240 million base pairs and 17,064 genes, whereas the jumping ant has some 330 million base pairs and 18,564 genes (compared with about three billion base pairs and 23,000 genes in humans). Berger notes that the group could only sequence the genomes of male ants, but based on that data they had little reason to think the females would be radically different.
The first two ant genomes "are going to be incredibly useful to compare against other genomes," says Christopher Smith, a cell and molecular biologist at San Francisco State University, who was not involved in the recent research. He calls the new work "a really forward-looking study."
Long live the queen—and her replacements
The sequenes provide clues that explain why queen ants can live as much as 10 times as long as female worker ants, and researchers are keen to figure out what factors go into determining this extreme longevity.
When a queen from an H. saltator colony dies, female worker ants fight to decide who will take over. Once a new queen is selected, her form and function change. She shifts from workaday laborer to fertile egg layer, adjusting body and life history in the process.
The researchers found that in the ants expression of telomerase, enzymes that help to protect the genetic information at the end of chromosomes, changed as gamergate queens transformed from worker to egg-layer. "That gamergate queen, she starts expressing higher level of telomerase," and her life span increases from that of an average worker ant, Berger explains. She and her colleagues are interested in finding the "aspects of longevity that correlate with this genetic switch."
Ants provide "a natural system where there's a life span difference," Smith explains. "We can see where nature has leveraged these [epigenetic] pathways" to extend life.
In addition to questions of longevity, the genetic and epigenetic profiles of ants can provide interesting insights about metabolism. "Queens and workers have very different energy usage profiles," Smith notes. Ants' fat reserves seem to help determine behavior, he explains. "Worker ants don't have much to run on, so they run off to find more food." And because ants have insulin signaling pathways similar to those of humans, researchers might also be able to study crucial health issues such as metabolic syndrome and calorie use.
On a broader scale, drilling down into the ant genome could uncover some of the mechanisms of epigenetic shifts themselves. Environmental signals, such as the quality of food or ambient temperature, can influence how ants express different genes. "Those environmental signals get translated into higher or lower expression of a gene—and there's a big black box in the middle" that researchers are trying to crack open, Smith says.
Other ant genomes are slated for publication soon. And after working on a few of the forthcoming genomes, Smith thinks ants could be promising model organisms. Unlike genes of fruit flies, which have been common for genetic studies for decades, those of ants undergo DNA methylation, which is a key process by which many higher organisms, including mammals, regulate gene expression.
"Ants' true strength is an epigenetic system," Smith says. Mice also have a complex epigenetic profile, but they can be more troublesome for bigger studies because, as Smith notes of higher order experimental animals, "once you have a backbone, you have a lot of paperwork and expense."
In addition to cutting down on potential administrative headaches, ants draw particular interest because they are highly social organisms. "What we have been lacking," Smith notes "is a good invertebrate social model system" Although the honeybee genome was published in 2006, ants, he explains, "are a lot easier to keep in a lab."
But ants are by no means ideal. In fact, most species cannot be persuaded to reproduce in a laboratory setting. And Smith notes that he does not think the new paper with the two genome sequences "brings us any closer" to having a viable genetic ant model. This is in large part because H . saltator is "one of the very few species of ants that can stay alive and mate in the lab" and are currently only available for study in India, he notes.
Nevertheless, the new genomes show ants to be "a pretty great system to address epigenetic regulation," Berger says. And with a genetically modifiable ant, for instance, researchers could try knocking out a gene that regulates a life-extending enzyme (sirtuin) in female worker ants (a change noted in transition from worker to gamergate queen) to see if it might extend life span, she explains.
"It's going to take some time to fully exploit it as a model," Berger notes. She hopes, however, that someday soon, the ant model will provide a way to investigate the role of epigenetic change in development, longevity and behavior—a batch of attributes she calls the "triple whammy"—in insects as well as humans.