Ant genomes offer new ways to explore social behavior

The complicated social lives of ants, more than 100 million years in the making, are getting a fresh look by researchers who have sequenced the complete genomes for seven ant species.

The data offer a rich and detailed look at how social behavior is reflected in the genes of different ants, said Assistant Professor of Biology Christopher Smith, who hopes to use the ant genomes to explore behavior in other social animals -- humans.

"The bottom line is that ants cooperate like few other species and I am very hopeful we can use that as a way to gain insight into human social interactions and diseases," Smith said. "This is why I went into working with ants."

Photo of brown ants tending to a larger queen ant, while one ant tends to a white ant larvae. The background is the genetic code that makes up the genome sequence for fire ants.

Worker ants from the fire ant species (the smaller ants) tend to their queen (larger ant) and brood (white ant larvae) against a backdrop of a portion of the genome sequence for this species. Photo courtesy of Romain Libbrecht and Yannick Wurm.

The seven genomes, discussed in a review article by Smith and colleagues in the journal Trends in Genetics, are full of other intriguing bits of information. Inside the genomes, scientists hope to find clues to how ants develop their caste system of queen and worker, why some ants live so much longer than others and even how genes changed in two species of ant that became fungus farmers. Smith helped sequenced the genomes of three of the ant species, details of which were published earlier this year.

 

Social genes

Ants are eusocial animals. They live in groups, care cooperatively for their young, overlap in generations -- and not every individual gets to reproduce.

Since honeybees are also eusocial, researchers have been eager to compare ant and bee genomes. So far, the genomes of the two insects are surprisingly different, the study authors suggest, containing different stretches of repetitive DNA and different sets of genes used to sense chemicals.

Ants and honeybees may also use DNA methylation -- a chemical method of modifying gene expression -- in different ways. Methylation is critical to guiding separate development in bee workers and queens, but it's less clear how the process affects ant social organization.

"What I think is cool is the fact that they have similar social organizations despite all these differences," Smith said. "It tells us that what we thought was important -- the genes and genome -- is less important than we thought and there is something else that explains how different organisms happen upon highly complex social organization."

The researchers note that there are more than 14,000 described species of ants (this diversity is catalogued in large part at Antweb.org, based at the California Academy of Sciences), and that species vary considerably when it comes to their social organization. One goal of the project is to compare several ant genomes to see whether different species of ants have evolved different "sociogenomes" over the past 100 million years.

Could ant genes have anything to say about human behavior? Smith is using data from the ant genomes to look at Williams-Beuren Syndrome, a rare disorder that causes those affected to be hyper-gregarious. People with the disease are missing one copy of about 30 different genes, but "we have little or no idea how having a half dose of these 30 genes leads to this complex social phenotype," Smith said.

Nearly 70 percent of these genes can be found in ants, so Smith and his students have developed a system to alter the expression of these genes and see if the changes have any effects on ant social behavior.

"The problem with human studies, especially genome-wide-association-studies, is that they come up with long lists of genes that are associated with a condition, but this correlation is not causation," Smith explained. "In order to prove direct links, you need to manipulate the gene of interest to see a direct effect."

"This is expensive and unethical in social vertebrates and current model animal systems don't exhibit the level of social complexity we want to model for humans," Smith said. "But social insects could be a way to explore these kinds of questions."

 

The secrets of aging, antibiotics -- and pest control

Ants are unusual in other ways that make their genomes especially interesting. For instance, researchers who study aging would like to know more about how and why certain ant castes grow older faster.

"Ant queens and workers are all female. Ant queens live 10 to 100- fold longer than workers, and queens are fertile while workers are sterile. But they have the same genome," Smith said. "That one genome is read in two different ways. If we understood just how queens and workers used the same genes in different ways I think we'd get a pretty big window on how aging and fertility are controlled. This could also apply to similar processes in humans."

Photo of brown colored fire ants fighting off an ant from a larger species (the rough harvester ant), set against a backdrop of the letters that make up the genetic code of the fire ant genome.

Fire ant workers (smaller ants) defend themselves against a worker ant from the rough harvester ant species (larger ant). The backdrop is the genetic code that makes up the sequenced fire ant genome. Photo courtesy of Romain Libbrecht and Yannick Wurm.

Ants also produce a variety of antibiotic substances and their poison could contain elements useful in making new medicines. Details from the ant genomes may also point the way to new pest controls, the researchers suggest. For instance, new information about the genetics of how ants signal to each other could be used to subvert the signals and encourage a colony to turn on itself.

"The most effective pest control methods rely on behaviors we exploit," Smith said. "The potential is for using their natural chemical language against them to direct them to self-destruct, instead of saturating them with poisons that contaminate our food and watersheds."

The ant genomes have yielded a number of other fascinating features:

• Ants have an expanded number of genes related to the chemicals given off by their exoskeletons, which help the ants recognize their colonies and signal their queens. These genes suggest that chemical communication may play a more important role in ant societies than in honeybees, the researchers say.

• Social insects such as ants and honeybees also have fewer genes related to innate immunity than solitary-living insects. Social insects are scrupulous about grooming each other and removing dead or sick insects from their colonies, and they may need fewer immune defenses as a result of such clean living.

• Some ants even bear the mark of agriculture in their genes, the studies show. Leaf-cutter ants lack genes to synthesize the amino acid arginine, presumably because they get the arginine they need from the fungus they farm.

 

Smith praised the international efforts of the ant genome teams, many of whom had no formal funding sources. People "came together in a very organic way to cooperate, share data and even coordinate publications," he said.

Smith said social networking software called MAKER, developed by Mark Yandell at the University of Utah, was a critical part of the effort, allowing researchers to work simultaneously with colleagues around the globe on the same genome.

"Let’s just say if you put these seven ant species in a box, they would kill one another quickly," Smith joked. "Fortunately the human researchers applied their understanding of the power of ant cooperation to get some serious work done."

-- University Communications