’At five degrees more, all the males were sterile’

In her doctoral thesis, Dr Berta Canal investigated the effects of heat on the reproductive capacity of fruit flies

Berta Canal currently works as a postdoc at the University of Lausanne in Switze
Berta Canal currently works as a postdoc at the University of Lausanne in Switzerland. © private
Dr Berta Canal recently completed her doctoral thesis at the Institute for Evolution and Biodiversity (IEB) at the University of Münster that examined the effects of heat stress on the reproductive capacity of fruit flies (Drosophila). The results can provide insights into the possible consequences of climate change for the survival of insect populations and help us understand heat-related reproductive issues in other animal species. In an interview with Christina Hoppenbrock, the biologist offers some insights into her work.

You’ve investigated fruit flies. Most people think that flies always have enough offspring. Are small increases in temperature really such a problem for these animals?

Yes, a small increase in temperature can have serious consequences for them, as the males can no longer reproduce if the temperature is not lowered again. I have observed that about half of the male flies become sterile if the temperature during their development is only four degrees above the optimum temperature of 25 degrees Celsius. At five degrees above that, all the males were sterile. Even if the females can still reproduce at these temperatures, reproduction is no longer possible without fertile males. This jeopardises the survival of the population.

What happens in the flies’ bodies when the ambient temperature is higher?

Reproductive organs such as the testes and accessory glands, the two most important tissues involved in the production of sperm and the seminal proteins necessary for reproduction, are damaged by heat stress. I also observed that the functionality of the reproductive organs does not fully regenerate during recovery in the optimal temperature range. My measurements are consistent with previous observations of fertility losses, as it is possible to restore fertility if the temperature is lowered, but not completely. Even after several days of recovery, the flies were less fertile and produced fewer offspring.

You used the method of experimental evolution in your work. What is that?

Experimental evolution makes it possible to measure evolutionary responses to changing environmental conditions in real time in the laboratory. I kept the flies at warmer temperatures for several generations and analysed their performance under heat stress. I looked at different traits, with a focus on reproduction to determine whether long-term exposure to warm temperatures leads to adaptation.

What consequences will climate change have for wild fruit flies?

It’s hard to say - like many other species, fruit flies could react differently to the expected rise in temperature and increasing frequency of heatwaves. If the flies are able to adapt to warmer temperatures, they would be able to survive in the new environment. In the worst-case scenario, if they’re unable to adapt or spread to another suitable habitat, some populations could experience significant declines and eventually become extinct.

And what conclusions do you draw about other animal species from your work?

These results are useful for inferring heat stress damage in other ’cold-blooded’ species. The temperatures tested in my study are easily reached in summer in many places around the world. My work contributes to a better understanding of organisms’ responses to climate change. In addition, the results help to improve predictions about the impact of climate change on species’ geographic ranges that may shift due to temperature changes and on the potential of species to survive under heat stress. The findings, particularly from the investigations at the cellular level, could also help us understand heat-related reproductive problems in mammals.

What new insight from your work do you find particularly exciting?

The results of the experimental evolution study, as they highlight the complexity of evolutionary processes; the evolution of traits involved in reproduction is influenced by several factors and their interactions. In the context of thermal adaptation, for example, evolutionary processes not only depend on increases in temperature, but also on the thermal fluctuations to which an individual or a population is exposed over time.

The environmental conditions in temperate areas with large temperature fluctuations differ from those in tropical habitats. I would expect different reproductive outcomes and evolutionary responses accordingly. Species from areas with strong seasonal fluctuations might adapt better to temperature changes than species living under more constant and stable conditions, such as in the tropics.

Berta Canal’s dissertation was supervised by Dr Claudia Fricke (formerly of the IEB, now professor of animal ecology at the University of Halle), Stefan Schlatt (Centre for Reproductive Medicine and Andrology, University of Münster) and Klaus Reinhardt (applied zoology, Dresden University of Technology). Berta Canal is now a postdoctoral researcher at the University of Lausanne in Switzerland.