Ant Study Shows Link Between Single Gene, Colony Formation

By John Roach
for National Geographic News

January 24, 2002

The complex group behavior of social insects such as ants and bees has long intrigued scientists and other observers. This activity is thought to be shaped by a combination of factors, including genetics, learning, and the environment. But a new study shows that when it comes to fire ants, a single gene plays a major role.

The finding may offer important insight to researchers who are working to determine what genes influence social behavior in people.

Tracing the roots of human behavior is difficult because a complicated web of neural processes governs how people behave and interact, and these workings in the brain are controlled by a broad and diverse range of genes.

The new research, said Gene Robinson, a professor of entomology and neuroscience at the University of Illinois at Urbana–Champaign, raises the possibility that "it will be possible to trace the effects of genes through the long and winding roads that lead through effects on brain function, to elucidate effects on behavior."

Distinctive Colonies

The study focused on the social organization of South American fire ants, an aggressive species that invaded the United States in the 1920s.

The ants form two kinds of colonies. Some of the ants live in robust, sprawling compounds of interconnected nests that are inhabited by many queens. Others form tight settlements and are fiercely loyal to a single fat queen.

The research results showed that whether a colony has one queen or many is a matter of what variant of a particular gene the ants possess.

The gene, named Gp-9, is one of about 15,000 genes in the genome of fire ants. The gene encodes a protein involved in the perception of an odorant that appears to be crucial in determining how ants perceive the world.

Entomologists Michael Krieger and Kenneth Ross of the University of Georgia in Athens conducted the study, which was published last November in the journal Science.

Explaining the significance of the results, Krieger said the finding "demonstrates that single genes of major effect can underlie the expression of complex behaviors important in social evolution."

"However," he added, "it is possible that variation in particular genes can cause striking variation in social behavior."

Influential Queens

The scientists found that some of the South American fire ants they studied had a mutation in an odorant binding protein, which is responsible for chemo-reception, or the ability to sense and respond to chemical stimuli.

"Chemo-reception is a very important sense of these critters, like eyesight in humans," said Krieger. "Having this mutation lets the ants perceive the world differently."

Several years ago Ross discovered a major genetic difference among fire ant queens. Those that ruled their own domains had one variant of the gene Gp-9, whereas queens who make their home in a colony of interconnected nests have a different variant of the gene.

A so-called monogyne queen establishes her own independent colony after a mating flight. She nourishes her eggs with her own fat reserves, without the assistance of any workers, until the eggs hatch and become workers themselves.

Polygyne queens, on the other hand, do not have enough energy reserves to found a nest independently. "The only option they have left is to seek adoption into an already established polygyne nest," said Krieger, who identified the protein in Gp-9 that appears to underlie the social structure of fire ant colonies.

Faulty Perception

Genes have two or more alternative forms, or alleles, that occur at a particular location on a chromosome. In most organisms alleles work in pairs—one inherited from the mother and one from the father—to determine various traits.

The particular combination of these two alleles produces variations in inherited traits. The researchers found that which variant of the Gp-9 gene the fire ants carried influenced their behavior in colonies.

The workers in monogyne colonies are loyal to their resident queen and kill off any would-be queen that tries to infiltrate their colony. The study revealed that the ants in monogyne colonies have two copies of the dominant allele of the Gp-9 gene—that is, the form of the gene that exerts greater control over a trait than another form.

In contrast, the workers in polygyne fire ant colonies are more accepting, according to the researchers. All of the queens and some of the workers in the polygyne communities were found to carry a mutant allele, which produces a variation of the trait controlled by Gp-9.

In polygyne colonies, the workers kill off all queens that possess the two copies of the dominant gene. But the workers that carry the mutant allele somehow persuade the rest of the community to accept the queens that also carry the mutant allele, which enables the polygyne colony to grow like a city and its suburbs.

Krieger said the surviving queens appear to escape attack because the mutant allele seems to encode a faulty protein that makes the polygyne ants less effective than the monogyne ants at detecting pheromones—chemical substances that animals produce to stimulate interest and behavioral responses by other members of the species.

"Since the presence of the [mutant] allele allows several queens to stay in a colony, we think that the mutation causes them to be less effective in recognizing queens," Krieger said.