UCSC Review Summer 1996

Fruit flies document natural antagonism between the sexes

Longtime viewers of 60 Minutes may recall the weekly segment known as "Point Counterpoint," in which a female and a male commentator put up their intellectual dukes over issues of the day. Even those wise pundits could not have realized how closely their verbal sparring resembled a genetic joust of the sexes that, says biologist William Rice, happens all around us and has helped drive the evolution of every species on earth.

Rice, a professor at UCSC, found evidence of surprising sexual one-upsmanship during a clever lab experiment using tens of thousands of fruit flies. His experimental setup and a bag of genetic tricks let Rice freeze the evolution of female flies as the males evolved through 41 generations. With time, the males became "supermales," prodigiously successful at passing on their improved genes to their sons--but at the expense of their female mates, who succumbed to increasingly toxic seminal fluid.

Females and males within all species, including humans, may continually evolve new genetic strategies to gain a leg up on each other, says Rice. However, this "sexually antagonistic coevolution" would happen slowly in nature and would be hard to observe. Each sex would adapt to changes in the other, preserving an uneasy balance--much as each member of a waltzing pair holds still, relative to the other person, even though they both sweep across the dance floor.

By pinning half of each fruit-fly pair in place evolutionarily, Rice exposed the dance of the males. "The two sexes are an important part of each other's environment," he says. "They evolve substantially in response to one another, not just in response to the physical environment or to predators and competitors. This point- counterpoint kind of evolution can happen perpetually, and it may play a critical role in splitting one species off from another."

Rice's study appeared in the May 16 issue of the journal Nature. His paper was accompanied by a "News and Views" commentary from Linda Partridge and Tracey Chapman of University College London, leading researchers in the field.

Rice used fruit flies (Drosophila melanogaster) for his work because they breed quickly--each generation lasts just two weeks-- and because scientists have devised useful techniques to manipulate their genes. Several of these tools let Rice build a population of male flies that evolved from generation to generation while mating with females genetically identical to those in the first batch.

Rice's research was funded by a grant from the National Science Foundation.