VANCOUVER--In the battle between the sexes, centuries of dueling over female fruit flies has produced in males a nasty chemical weapon: toxic semen that helps kill off their mates sooner than if the females were able to keep their virginity. But in a neat experiment that forced fruit flies to be monogamous, researchers have allowed evolution to disarm the sperm. And, according to findings presented here yesterday at the annual meeting of the Society for the Study of Evolution, there was a substantial peace dividend: The monogamous population produced more offspring overall than control populations did.
For a male, the best reproductive strategy is to ensure that his sperm fertilize the maximum number of eggs. To gain an edge over his mate's other suitors, a male fly laces his seminal fluid with proteins that are toxic to other flies' sperm. Unfortunately, the female gets caught in the competitive crossfire; the seminal fluid is also mildly toxic to her. Two years ago, evolutionary biologist William Rice of the University of California, Santa Cruz, showed that when he prevented females from evolving defenses to the males' power plays, "super males" evolved with very toxic sperm and aggressive mating habits.
Now evolutionary biologist Brett Holland, a graduate student working with Rice, has shown that sensitive nice-guy flies can evolve, too--albeit in a situation far removed from real life. The crucial step, they reasoned, would be to remove competition among males. So Holland imposed monogamy on the normally promiscuous flies by pairing the flies off and putting just one male and female into each vial. After 30 generations, the males were docile. Compared with male progeny of control flies that had to compete for a single female, descendants of monogamous males had less toxic sperm and harassed females less during mating. Females, in turn, were less resistant to the males' sperm. The cooperation paid off. The monogamous flies, on average, produced about 28% more viable offspring than controls.
The experiment is a clear, and clever, demonstration of the costs of competition in evolution, says evolutionary biologist Locke Rowe of the University of Toronto in Canada: "It's similar to a real arms race where competition drags the whole economy down."