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.
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