Aging Desexualizes Fly Brains

Researchers in Sweden have found that the brains of male and female fruit flies become desexualized as they age, although the male brain becomes feminized to a larger extent than the female brain becomes more masculine.

Research had already established that weaker individuals cannot afford to “invest” in sexual behaviors to the same extent as their healthier conspecifics, but whether or not the same could be said about the weakening that occurs due to aging is not clear.

To study this, the team from investigated how genes expressed to different degrees in young males and females change over time. The results showed that gene expression in male and female brains become more similar with age, and that both sexes contribute to this pattern. If the expression of a certain gene is higher in the brains of young females than in young males, the gene’s expressions is reduced in older females and increased in old males—and vice versa for genes with higher expression in young males.

“The results also show that the changes are larger in males than in females,” says coauthor Dr. Antonino Malacrinò, who worked at Sweden’s Linköping University at the time of the study but is now employed at Italy’s Università degli Studi Mediterranea di Reggio Calabria.

The fact that females change less with age than males is presumably the result of that the connection between investment in sexual traits and reproductive success is not as strong in females as in males. While a male fruit fly, in order to reproduce, must out-compete other males in quickly finding females and encouraging them to mate through an intricate and taxing dance performance, a female only needs to decide how much energy she has available for reproduction. Aging implies that reduced amounts of resources are available to invest in reproduction and other activities for both sexes, but stiff competition between males means that the cost of increased investment is greater for them than for females.

“If you keep investing as much as before in reproduction when you get old, you don’t have energy left for survival,” says Urban Friberg, who has led the study and is senior associate professor at the Department of Physics, Chemistry and Biology (IFM) at Linköping University.

Studies on other animals, including humans, which have mainly focused on age-related changes in gene expressions in one sex, have given results pointing in the same direction. This indicates that the results shown in fruit flies may also apply to many other animals. “We get the same general results in both the populations of fruit flies that we have examined, even if they differ quite a bit with respect to which genes are involved,” adds Friberg.

Previous work by Friberg’s lab compared sex differences in gene expression between male and female flies of high and low genetic quality. Similarly to how aging reduces sex differences, the earlier study show that reduced genetic quality makes gene expression more similar in male and female flies, and again it was males that changed their gene expression more than females.

The most recent study—detailed in a recent issue of the Proceedings of the Royal Society of London B—provides no answer to what molecular signal associated with aging lies behind the reduced sex differences in the brain. Further exploration on the topic could include determining if the signaling molecule is shared with other species.