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Patrick Guilfoile: The battle of the sexes continues

The mere presence of the opposite sex shortens the lifespan, at least for fruit flies.

Recent research by scientists at the University of Michigan and several other institutions showed all it takes to shave time off the lifespan of male or female flies is the odor of the opposite sex.

So how do you determine the connection between the smell of the opposite sex and a shortened lifespan?  Researchers created mutant fruit flies to test the idea. They modified male flies to produce the female scent, and modified female fruit flies to produce the typical male scent.

This allowed them to control other variables, so the only difference between test flies was their exposure to the odor from the opposite sex.

They found that male fruit flies exposed to males producing a female scent had a substantially shorter life span, as compared to genetically identical males that were not exposed to the scent.

The female fruit flies exposed to male scent also had a shorter life span, but the result was less dramatic compared to males exposed to the female scent.

Since the effect was most striking in males, researchers focused on the effects of female scent on males. Further research showed that males exposed to just the chemically isolated female scent also had a shortened life span.

Next, the scientists tried to determine how the fruit flies detected the female scent. (This is a little more complicated in fruit flies than people, as fruit flies have odor sensors on their legs, wings, and genitalia, in addition to their mouths.)  

They developed altered flies that lacked odor sensors in specific areas and narrowed it down to odor sensors on the legs and wings.

Next, they cut off the forelegs from a sample of hapless fruit flies, and found they no longer were affected by the scent of females, identifying the sensors on the legs as the key detector of the odor.

The researchers decided to probe further to gain an understanding of the brain circuits that responded to female scent. They used a genetic trick to block the function of neurons in different regions of the small fruit fly brain. Using this technique, they discovered a particular group of brain cells that responded to the female scent and likely played a role in sexual reward. Researchers also studied some of the metabolic changes that took place in males exposed to the female scent. They found changes that were typical of a stress response, and that may have contributed to the shortened life span of the males.

Based on these findings, the scientists tested whether mating could reduce the deleterious effects of the female scent. When placed together with an equal ratio of males and females, there was no improvement in life span of the males. However, when the males were housed in a ratio of five females to one male, their life span improved somewhat. This suggested to researchers that the female scent conveyed to males the possibility of mating, and if that expectation wasn’t realized, it created stress which ultimately reduced the lifespan of the frustrated male flies.

More information is available in the article by Christi Gendron and others “Drosophila life span and physiology are modulated by sexual perception and reward”  in Science 343: 544-548, January 30, 2014.

PATRICK GUILFOILE has a doctorate in bacteriology and is the associate vice president at Bemidji State University.