What fruit flies can tell us about being social

Professor of neurogenetics Jean-Christophe Billeter is convinced that all life is in essence social. Even fruit flies, which are typically characterized as a solitary species, are social. In Billeter’s lab at the University of Groningen, PhD student Tiphaine Bailly recently found that simply seeing other fruit flies directly affects their hormone levels and, ultimately, the speed and timing of laying eggs.

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Billeter is used to the fact that people do not always recognize the relevance of studying fruit flies. ‘But,’ he explains, ‘many of the fundamental findings in genetics over the past 100 years have been based on fruit flies.’ Fruit flies have been used for research on biological phenomena as diverse as sleep, cancer, Alzheimer’s disease, and Parkinson’s disease. We have learned things about immunity thanks to fruit flies and we know that our sex is determined by chromosomes thanks to fruit flies.

And now, fruit flies show us how their bodies are fundamentally affected by their social environment. Billeter: ‘If you think about it, how we (humans) experience the world is also affected by who we are with. There have been experiments to show that our perception of pain is lowered when we are in the presence of others.’ By studying the effect of social context in fruit flies, Billeter hopes to understand more about the underlying mechanism.

The effect of the group on the individual’s body

We tend to expect that what happens in our bodies depends only on us as individuals. ‘But,’ says Billeter, ‘it’s also about who we are with. You only need a male and a female to reproduce, but the group of individuals around them also influences the success of reproduction.’

When fruit flies are alone, they will lay their eggs only under cover of night, probably to be safe from predators. But when they are in a group, they tend to synchronize their egg production and lay them during the day. Billeter: ‘They have to synchronize because if their young are of different ages, they will end up fighting and none of them will survive. But if they grow up together, they can fight off other threats together. So, this social behaviour functions to ward of competition within the group.’

These results are the work of PhD student Tiphaine Bailly, who also studied in more detail the mechanisms underlying this synchronization of reproduction. The hormone that normally regulates egg production, the juvenile hormone, is influenced by the social environment. When there are other fruit flies around—no matter their age, or race, or reproductive stage—it causes the reproductive cycle to speed up.

Billeter: ‘As a result, the effect of light on egg laying, which would normally cause flies to lay their eggs only during the night, is no longer there when in the presence of others.’ And to know whether others are present, all they do is look. ‘Which is somewhat surprising because in another study we found that they use smell to determine whether or not they want to join a group of fruit flies.’

What fruit flies can tell us about humans

These results on fruit flies are reminiscent of the urban legend that women’s menstrual cycles tend to synchronize when they live in the same house. But Billeter emphasizes: ‘There are as many papers to support this idea as there are papers to refute it; so, the jury is still out on this phenomenon.’ Knowing how fruit fly hormones are affected by the social environment does not yet tell us anything definitive about how it might work in humans, though the function of the juvenile hormone—an insect hormone—is rather similar to that of human hormones such as oestrogen.

‘In any case,’ says Billeter, ‘we know that in humans, the feeling of being alone can be as damaging as smoking or obesity, illustrating how the presence of others fundamentally affects us. It can take several years off a person’s life. We can study the effect of the social environment on the body much easier in fruit flies. And if we can understand the mechanism, perhaps it will help us to unravel similar mechanisms in other species. But this kind of thing takes decades.’

Reference: Tiphaine P.M. Bailly, Philip Kohlmeier, Rampal S. Etienne, Bregje Wertheim, Jean-Christophe Billeter, Social modulation of oogenesis and egg laying in Drosophila melanogaster, Current Biology, 2023.