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Queens Rule Ant Nests
16 August 2001 7:00 pm
Insect queens are well-known as matriarchs that lay all the eggs in their colonies. But surprisingly, they can also influence its sex ratio, according to a study published in the 17 August Science. Most researchers thought that the workers, the queen's daughters who actually raise the colony's young, determine the ratio of males to females, as they can kill or starve unwanted eggs.
There's a good reason why female insects would want to kill their brothers. Male social insects develop from unfertilized eggs and carry just one copy of every chromosome, while the females arise from fertilized eggs and have two copies, one from each parent. As a result, sisters are more closely related to one another than to their brothers. Thus, while queens benefit equally by producing sons or daughters, worker females should prefer to raise sisters so they can pass on more of their genes. Indeed, researchers have found that ant colonies usually have about three females to every male.
It was the exceptions to this rule that intrigued evolutionary biologist Laurent Keller of the University of Lausanne, Switzerland, and his colleagues. They collected 24 fire ant colonies, reared them in the lab for at least a week, then counted the number of males and females among 100 individuals plucked from each colony. Eleven samples turned out to have mostly males, and 13 consisted almost entirely of females.
But when the researchers switched queens from these two kinds of colonies, sex ratios reversed. Five weeks later, the formerly male-dominated colonies had become biased toward females, and vice versa. An examination of the DNA in the eggs revealed the reason: More than half the eggs laid by queens from male-dominated colonies were haploid--or male. The queens--not their worker daughters--were influencing the sex ratio, by laying either more male or more female eggs. Most likely, they controlled the number of eggs fertilized by their internal stores of sperm. The researchers still don't know why some colonies are nonetheless biased toward females.
"This is really the first experimental test of who determines [sex] ratios," says evolutionary biologist Lotta Sundström of the University of Helsinki, Finland. The results are "compelling," adds Jon Seger, an evolutionary biologist at the University of Utah in Salt Lake City.