Life-time protection against severe heat stress by exposing young Drosophila melanogaster flies to a mild cold stress.

Previous studies in the laboratory of the author have shown that subjecting flies to a mild stress (e.g. a cold stress) during the first 2 weeks of adult life can increase lifespan and resistance to severe stresses (e.g. heat and fungal infection) at 6 weeks of age (ca the mean lifespan at 25 °C). This result could either show that a mild stress protects flies against severe stress for the entire life or for a duration of 4 weeks. To clarify the issue, young flies living at 25 °C were pretreated with a cold stress and thereafter transferred at 19 or 22 °C, which increases lifespan. The mild cold stress protected these flies from heat at ages when flies kept at 25 °C are dead, i.e. at 10 weeks of age or 8 weeks after the end of cold stress. Thus, a mild stress protects flies for life, even if the duration of life is increased. Because temperature can strongly vary from day to day in the wild, and lifespan of flies too, it would be a selective advantage if the ability to survive a strong stress after having been subjected to a mild stress would be maintained not only for a few days but for life, whatever its duration could be. If flies would be subjected to a mild stress when living at 25 °C, a temperature change from e.g. 25 to 22 °C would increase their lifespan and they could survive a strong stress at an age when flies kept at 25 °C are dead.