Love hurts -- especially for the female fruit fly. A new study published online in the Journal of Evolutionary Biology shows that after fruit flies mate, females ramp up their immune systems in roughly the same fashion as they do when fighting bacterial and fungal infections.

"Of course the immune system is there to fight pathogens, but it might be there to protect you against members of your own species, too," Ted Morrow, an evolutionary biologist at Uppsala University in Sweden who led the study, told The Scientist.

"It's an exciting and very clever study," said Andrew Stewart, a University of California, Santa Barbara, evolutionary biologist, who was not involved in the work. "It gets at some of the areas in sexual conflict where we don't really know what's going on."

Image: T. Chapman/PLoS Biology

In the fruit fly Drosophila melanogaster, sex is a fairly dangerous game. Females that mate more than once are known to have shortened lifespans and fewer offspring. And yet they do it anyway -- studies of wild flies show that around 80% of females contain sperm from multiple males. Thus, researchers have long wondered what drives this so-called "cost of mating."

Several researchers have tried to decipher the molecular cause of the procreation price tag by comparing genome-wide gene activation levels before and after mating, but these microarray studies have always stopped at one round of mating. After comparing two-timers' microarray profiles with those of virgins and singly-mated females, Morrow and his grad student Paolo Innocenti showed that female fruit fly libido first stimulated a large number of physiological processes related to egg production, but once activated, additional dalliances had little effect on these genes. Instead, extra mating aroused an immune response, which suggests that the cost of multiple mating may be largely driven by this system's activation.

"Mounting a large scale immune response could be costly to the organism and that could cause a female to die sooner as a function of having that second mating," said Lawrence Harshman, an evolutionary geneticist at the University of Nebraska-Lincoln, who was not involved in the research.

Harshman noted, however, that because "microarray studies are typically correlational, [the authors] don't provide strong inference about mechanism." Morrow suggested that males might introduce proteins that hijack the female's signaling system used to control her sexual appetite, so "maybe the immune system of females is being used to mop up the [male] signals," he said.

"It makes sense," Stewart said. "Imagine you have these proteins that get into the female and manipulate her. If she was able to combat them she would live longer and have more offspring." But it's also possible that males introduce more pathogens in their ejaculate into previously mated females, perhaps because of sperm competition, Harshman noted.

The study provides "one of the first pieces of evidence about where some of the battle field is [between the sexes], and it's being waged via the immune system," said Stewart. "But the critical link that's missing is the direct evidence that products in the immune system can recognize or breakdown the [male] seminal proteins," which are known to shorten lifespan.


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