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Genes Play Role in Ant Social Status
  
By Nancy Touchette

Researchers studying ants have long believed that an ant’s standing in society is determined solely by the environment. But a new study of leaf-cutting ants challenges that notion, and suggests that genes also play a role.


Large worker leaf-cutting ants are at least twice as big as the small workers.

“We used to think something in the environment determined the division of labor in the nest,” says William O.H. Hughes of the University of Copenhagen. This could be something in the food or a pheromone emitted by the adult workers. “But instead, we found that the father’s genes also affect whether larvae become large or small workers.”

Leaf-cutting ants, or Acromyrmex echinatior, live in colonies containing up to 100,000 ants, almost all of which are sterile, female workers. Each colony normally has only a single queen, who lives for 10 years or more and is the only member of the colony that reproduces.

Females carry out all the work of the colony and can become one of two types of workers. Large workers go out and harvest vegetation to grow a fungus that nourishes larvae in the nest. Small workers stick close to the nest and tend to the fungus.

Large workers are two or three times the size of the small ones. The large ones cut a wider range of vegetation; small ones are better able to work within the densely packed structure of the fungus garden.

“Males don’t do a whole lot,” says Hughes. “They just sit in a corner and eat, then fly off, mate, and die.”

Leaf-cutting ant queens mate with up to 10 males at a time, which makes their offspring attractive candidates for genetic study. Every worker ant in a colony inherits her maternal genes from the same mother. The main differences in the genomes are due to the ants’ having different fathers.

Hughes and his colleagues, working with a research team led by Jacobus J. Boomsma, analyzed the DNA of a large sampling of small and large worker ants in five colonies collected from Panama.


Small leaf-cutting ants tend to the fungus.

“If the environment determines an ant’s working status, larvae should have an equal chance of becoming large or small, no matter who their fathers are,” says Hughes. “But we found that it wasn’t equal at all.”

In some lineages derived from the same father, there were 20 times as many small workers as large workers, but in other paternal lines, it was the large workers that outnumbered the small ones. This suggests that genetic factors make a significant contribution to an ant’s role in the colony.

Leaf-cutting ants are important pests of many crops. A single colony can completely defoliate a tree overnight. To control ant populations, it is important to understand how the colony functions.

“Leaf-cutting ants are excellent model systems for looking at social insect societies and figuring out how they work,” says Hughes. “The more you understand, the better your position for doing something about them.”

But the real reason for Hughes’ interest in ants?

“I study them because they’re cool,” he says.

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Hughes, W.O.H., et al. Worker caste polymorphism has genetic basis in Acromyrmex leaf-cutting ants. Proceedings of the National Academy of Sciences USA, July 23, 2003.
 

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