What does it mean to be an ant?
Genome sequences offer clues on how to be a queen and live a long life.
Some are nomadic warriors, others the world's oldest farmers, and still others power their societies with slavery or child labour — such is the diversity of ant communities. A paper published in Science this week1 sheds light on the molecular forces that drive such differences.
The paper presents the genome sequences of two ant species — one (Harpegnathos saltator) with a primitive social structure, and the other, the carpenter ant (Camponotus floridanus), which has a more complex social structure.
Ants of the same species but in different social castes have the same DNA sequence but assume radically different characteristics as a result of 'epigenetic changes' — DNA modifications that affect the expression of genes rather than the genes themselves. By examining the sequences from the two ant species, the researchers were able to identify how these epigenetic changes affect the ants' characteristics.
"We were looking for a system where we could really look at epigenetics in a living organism," says Danny Reinberg, a biochemist at the New York University Medical School and one of four lead authors on the study.
Harpegnathos saltator communities generally consist of only about 60 individuals; there is little difference in the physiological make-up between queens and workers, and workers can become queens if her majesty dies. By contrast, carpenter ant societies include thousands. Workers belong to one of two castes, each distinguished by specific physiological and behavioural features, and only the queen can lay fertilized eggs — when she dies, then the entire colony dies with her.
The researchers analysed the genomes of individuals with different positions in each species' social hierarchy, as well as of individuals with the same roles in the two species. They also compared differences in gene expression and examined key epigenetic markers such as DNA methylation, a process thought to silence genes.
Force of numbers
With information from both genomes, says Reinberg, "we can now look at the molecular events that happen when a queen lives longer - and do so without manipulating the system".
The results provide a tantalizing link between ageing mechanisms in ants and other organisms. By removing the queen in H. saltator communities, the researchers induced other females in the colony to shed their role as workers and acquire characteristics of the queen — including an increased lifespan. Those organisms showed a boost in the expression levels of two genes — one encoding the enzyme telomerase and the other encoding the protein SIRT1 — both of which have been associated with longevity in humans.
They also found that epigenetic markings such as DNA methylation were present on the ants' genomes. "We think that's pretty exciting," says Shelly Berger, a geneticist at the University of Pennsylvania Medical School in Philadelphia and another lead author on the study. "It means that we are going to ber able to understand how these epigenetic systems are involved in these behavioural transitions."
In carpenter ants, the two worker castes have different behavioural traits — major workers protect the colony, whereas minor workers scavenge for food. Reinberg and his colleagues' analysis showed that there were differences in the expression levels of genes that function in the brain between these two worker groups, for example, in genes associated with the perception of smell.
Ants are not the only eusocial insects, in which some individuals in the group give up reproduction to work and care for others in the group. Their cousins the honeybees also live in similarly organized societies. Gene Robinson, an entomologist at the University of Illinois at Urbana-Champaign, spearheaded the effort to sequence the honeybee genome, which was published in 2006 and provided intriguing insights in its comparisons with the Drosophila and mosquito genomes2 — but that was just the beginning. "It is very difficult to draw conclusions from just a couple of species," he says. "What is really needed is the ability to compare more similar species with similar attributes."
The publication of these two ant genome sequences is "a harbinger of the future" — of a time when sequencing the genomes of related species will become routine, says Robinson. Ant genomes are about one-tenth of the size of human genomes. "When we get to the US$1000 genome, then these insects, just for sequencing costs, are going to be $99," he notes. "For groups like social insects, we really will be able to have hundreds of them."