After copulation, Drosophila melanogaster females are able to create long-term memories of unpleasant events—electric shocks—that virgin females cannot, according to a study published today in Science Advances (November 20). The authors suspect the memory boost may improve the chance of survival of the female during the subsequent egg-laying period as well as guide her choice of laying sites. Whatever the reason, the enhanced memory joins a list of physiological and behavioral effects on female flies that result from sex.
“It’s quite impressive and convincing [data],” says entomologist Elwyn Isaac of the University of Leeds who was not involved in the research. “They propose that the sex peptide gets into the [female’s] circulation and somehow gets across the blood brain barrier [to activate memory].”
It’s “very interesting,” Isaac continues, because until now, sex peptide—a protein produced in the male reproductive system and found in ejaculate—was thought to act on sensory neurons in the female’s uterus. These neurons produce a receptor protein to which sex peptide binds and are thought to be necessary for the peptide’s many effects on females, which include ramping up ovulation, increasing egg-laying behavior, changing food preference to a high-protein diet, and causing the female to reject other males. But, the authors of the new study, “show definitely that those neurons are not required for this [long-term memory] effect,” Isaac says. Indeed, deletion of the receptor in these neurons made no difference to the flies’ long-term memory formation after sex.
Instead, a single pair of neurons in the female fly’s brain mediates the sex-induced memory effect, according to study author Thomas Preat, a neuroscientist at the Industrial Physics and Chemistry Higher Educational Institution (ESPCI) in Paris. Preat’s team had previously shown that the pair of neurons was required for long-term memory in adult flies. Then, “by chance” Preat says, he was attending a talk at a fly conference when the presenter revealed incidental data showing those neurons produced the sex peptide receptor.
Back in the lab, Preat confirmed this finding, which begged the question, was long-term memory induced in females by the sex peptide? Sure enough, the team showed that while mated females that had learned how to avoid an electric shock in response to a particular odor consistently avoided shocks when presented with the odor a day after their training, virgin females had a far less robust recall and received far more shocks as a result.
Furthermore, females that mated with sex peptide–lacking males had a similar memory recall to that of virgins, while virgins injected with sex peptide had memory recall equivalent to their mated counterparts.
The requirement for sex peptide is “very clearly shown . . . with good controls,” says neurogeneticist Jean-Maurice Dura of the Institute of Human Genetics in Montpellier, France, who also did not participate in the research.
“It’s not that virgin females have no memory at all,” Dura says. It’s just short-term. They could learn to avoid shocks during the training, but they couldn’t retain that information the next day. Unlike short-term memory, long-term memory in fruit flies requires protein synthesis and is therefore “energetically costly,” Dura explains. Perhaps flies have evolved to avoid such a costly process until it’s crucial—when the survival of offspring is at stake—he suggests.
“Changes in memory performance could allow mated females to increase their chances of survival, to find food resources and identify suitable places to lay their eggs,” writes Carolina Rezaval in an email to The Scientist. “In this regard, it would be informative to know if mated females also show enhanced appetitive memory”—the ability to remember cues associated with rewards rather than pain—she adds. Rezaval is a behavioral neurobiologist at the University of Birmingham and was not involved in the research.
Other questions arising from the work include those relating to a protein called MIP, which also binds to the sex peptide receptor and is produced in mated and unmated female and male flies alike. The team showed that females lacking MIP were unable to generate long-term memories as effectively after mating as their wildtype counterparts. Why MIP does not enable long-term memory formation in the absence of copulation and whether males, which produce both MIP and the receptor, also experience a sex-induced memory boost are ongoing research investigations, says Preat.
Perhaps the most intriguing question is how these results relate to mammals and humans. “Several studies in rodents show that females experience remarkable plasticity within the brain during pregnancy,” writes Rezaval. And this study now “opens the door to investigate if sperm compounds identified in mammals, including humans, could underlie [such] mating-induced plasticity.”
L. Scheunemann et al., “A sperm peptide enhances long-term memory in female Drosophila,” Sci Adv, 5eaax3432, 2019.
Ruth Williams is a freelance journalist based in Connecticut. Email her at email@example.com or find her on Twitter @rooph.
Clarification (November 22): The original image showed one female fruit fly on top of another, giving the mistaken impression they were mating. The updated photo shows a male fruit fly.