Between night and day: The power of flies to adapt

Most plants and animals are exposed to a wide range of environmental variations. A study published in the journal Nature and conducted by the team of Richard Benton, professor at the Center for Integrative Genomics in the Faculty of Biology and Medicine at the University of Lausanne, looks at the ability of drosophila to adapt to fluctuations in day length.

Species with a wide geographical distribution, such as humans, are confronted with multiple environmental variations, which they manage thanks to the flexibility, or ‘plasticity’, of their behaviour. This ability to adapt to the world around them is crucial to their survival. However, the molecular mechanisms underlying it remain poorly understood. It is therefore important to decipher how behavioural plasticity is regulated by genes and the nervous system, in order to understand how widespread species have evolved to cope with environmental changes and how they will adapt to a changing climate.

At the heart of the circadian cycle

Day length is a factor that fluctuates according to season and latitude. Many species, such as certain flies, adjust their circadian rhythm (daily activity cycle) to adapt to these variations in day length.

In a study published on 16 October 2024 in the journal Nature, Michael Shahandeh, a former post-doctoral fellow in Prof. Richard Benton ‘s group at the Center for Integrative Genomics in the Faculty of Biology and Medicine at UNIL, compares two species of Drosophila to examine differences in behavioural flexibility. Drosophila melanogaster, also known as the ‘vinegar fly’, is found all over the world, so it undergoes major changes in the length of the day and demonstrates strong circadian plasticity. In contrast, Drosophila sechellia, endemic to the Seychelles, a region close to the equator, undergoes much smaller fluctuations in day length and displays less plasticity.

To compare the circadian plasticity of these two species, the scientists subjected them to an imposed circadian cycle of a long day: 16 hours of light. This constraint had harmful consequences for the fitness (understood here as the ability to survive and reproduce) of D. sechellia, which is used to a constant day length of 12 hours. “This species has lost its ability to delay its evening peak activity in the event of a longer photoperiod; as a result, long days are stressful for it and its reproductive rate has halved, whereas D. melanogaster has remained perfectly fertile”, comments Richard Benton.

Defining the critical genetic elements

Using a genetic screen, the biologists then managed to discover a key role for the Pdf (Pigment-dispersing factor) gene in this divergence between species. “This gene is responsible for the expression of the Pdf neuropeptide, which is critical for circadian activity. As expected, replacing the D. melanogaster gene with that of D. sechellia reduced the ability of D. melanogaster to delay its peak activity under long daylight conditions", reports the Lausanne-based professor. "D. melanogaster is like a ‘genetic test tube’ for us. This experiment revealed that the differences between the Pdf genes of D. sechellia and D. melanogaster contribute to the behavioural differences between these two species.” The specific features of the D. melanogaster Pdf gene partly explain why this species has spread widely throughout the world, whereas D. sechellia has specialised in a single niche.

Exploring other forms of behavioural plasticity

Richard Benton also mentions previous research suggesting the importance of the Pdf neuropeptide in Drosophila species living at higher latitudes, exhibiting even greater flexibility in circadian activity than D. melanogaster. “These observations suggest that this neuropeptide is a key evolutionary factor in the development of circadian plasticity in Drosophila. Given that Pdf is also present in many other arthropods, such as mosquitoes, which are widely distributed throughout the world, it could play a similar role in the latter. More generally, our work could inspire the exploration of other forms of behavioural plasticity involving other cellular and molecular mechanisms in different animal species. For example, songbirds change their vocalisation frequencies in response to noise caused by human activity, and lizards change their resting behaviour in response to altitude, but we know nothing about the mechanisms underlying these phenomena.”