Contact: Robert Irion (408) 459-2495; irion@ua.ucsc.edu

FEMALES AND MALES MAY CONTINUALLY COEVOLVE IN AN ANTAGONISTIC DANCE, ACCORDING TO STUDY ON FRUIT FLIES

* This news release is EMBARGOED until 4 p.m. EDT Wednesday, May 15, 1996. The author will publish his study in the May 16 issue of the journal Nature.

SANTA CRUZ, CA--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 the University of California, Santa Cruz, 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 appears in the May 16 issue of the journal Nature. His paper is 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.

Each generation began with about 800 male flies. Rice mated these with about 600 females, making detailed observations of their mating behaviors and how long they survived after mating. From the thousands of offspring, he chose 800 sons with the desired genetic characteristics and began the process anew. The experiment lasted more than eighteen months.

After 30 generations, Rice saw strong evidence that his "supermales" had emerged. Their fitness as fathers--defined by Rice as their ability to leave more male descendants for the next generation--went up by 24 percent over the first generation of males. So, for every four adult male sons produced by fathers in the first batch of flies, the supermales spawned five sons. (Supermales also sired more daughters, but only the sons carried the full genetic complement of interest to Rice.)

By every other measure, Rice says, the supermales were superior. For instance, when Rice introduced competitor males into the fray to mate the females, the supermale flies remated the females more frequently. Further, their sperm continued to fertilize the eggs of the females at a higher rate, even after Rice let the females mate with competitor males.

The success of the supermales took its toll on their female partners. Many more females died prematurely as the generations wore on. "Seminal fluid is toxic to female flies," Rice says. "The more they mate, the faster they die." Indeed, evolutionary adaptations in the supermales seemed to boost the toxicity of their seminal fluid--a genetically based change that led to more supermales in the next generation, with a deadly cost to females. The fact that more females died after bearing their offspring was of no consequence to the supermales, whose sole goals in life already had been realized.

All of these measurements, Rice says, painted a convincing picture of sexually antagonistic coevolution among his winged charges. In nature, he suspects, female flies would counteract any genetic movement toward supermales by evolving some way of making the seminal fluid less toxic to them. However, researchers don't yet know how that might occur.

"These molecular interactions can go on indefinitely," Rice says. "Because they occur at the level of simple gene products, there's no physical constraint to prevent the point-counterpoint adaptations from going on forever." In that light, he concludes, biologists should consider sexually antagonistic coevolution as one of the major driving forces behind the origin of new species--a field of study in which new theories often fly as fast and furious as one of Rice's supermales.

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

Editor's note: You may reach Rice at (408) 459-3176 or rice@biology.ucsc.edu. For a copy of his study, contact Robert Irion at (408) 459-2495 or irion@ua.ucsc.edu.

This release is also available on the World Wide Web at UCSC's "Services for Journalists" site (http://www.ucsc.edu/news/journalist.html) or via modem from UC NewsWire (209/244-6971).

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