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Dr. Ed Vargo examining fire ants

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New Light on Gender Determination in Ants

It may be the world's tiniest power struggle. Queens and workers in fire ant society are often at odds about the proportion of new male and female ants raised in their colonies.

Research into this struggle condcted at North Carolina State University has implications for how scientists understand the complicated structure of social insect societies, including those of ants, wasps, bees and termites, says Dr. Ed Vargo, NC State assistant professor of entomology. For decades, entomologists have thought that the workers in ant colonies - all female and all daughters of the queen - control how many males are allowed to live in the colony by killing many of their brothers while still larvae.

Scientists assumed the queen was simply an egg-laying machine. They were puzzled, though, that the ratio of females to males in some ant colonies was not quite as high as they would expect in a worker-dominated society.

The research by Vargo and his three European colleagues found an explanation. The queen can, in fact, alter the sex ratio of her colony by limiting the number of female eggs she lays.

"The queen and the workers are having a conflict, and in some circumstances the queens can exert more control, and in some circumstances the workers can exert more control," Vargo said.

To examine the determination of sex ratios in social insect societies, the scientists studied colonies of fire ants in Vargo's lab. Fire ants were the social insect of choice because they have only one queen per colony, and because their colonies tend to "specialize" in producing mostly male or mostly female offspring.

The scientists switched queens from about a dozen female-producing colonies with queens from about a dozen male-specialist colonies. Originally female-specializing colonies almost immediately began producing mostly males, while the originally male-specialist colonies produced females. Within five to six weeks, the sex ratio of the queen's new colony was nearly identical to her original colony.

"We found that the sex ratio of the queen in the original colony followed the queen to the new colony she was transferred to," Vargo said. "It completely reversed the sex ratio in that new colony, but stayed the same for the queen."

The researchers next examined the sex of the eggs produced by seven male- and 10 female-specialist colonies. They found that the queens from the female-producing colonies laid female eggs almost exclusively, and queens from male-producing colonies laid more than 50 percent male eggs. A queen is able to determine the sex of her offspring by allowing or not allowing stored sperm to fertilize the eggs. Fertilized eggs become females while unfertilized eggs become males.

The root of the power struggle between queens and workers results from the different interests they have in raising new members of the colony. Both - as a result of evolutionary pressures - are interested in ensuring the survival of their genes. The queen does this by producing new queens and male drones to mate with those queens, which will create new colonies. But males, which die after mating, are of no use to the female workers.

The queens, however, can force their daughters to raise male young by overwhelming them with male eggs. The workers may selectively eliminate some males, but won't kill many of them, Vargo speculates, because that would waste too much of their colony's limited resources.

That "tug-of-war" is what happens in a single colony. Among groups of fire ant colonies, Vargo says, an entirely different struggle is taking place, as queens compete among themselves to ensure that their genes survive to create the next generation of colonies.

For example, it's in the interest of the queen to produce mostly males in an area where most of the other queens are producing breeding females, and vice versa. It's still a mystery, though, whether queens can change the number of breeding males and females they produce over time, and what genetic or ecological factors cause queens to have a bias toward producing either sex.