Stress-induced plastic responses in Drosophila simulans following exposure to combinations of temperature and humidity levels.

Plastic responses to heat and desiccation stress in insects have been studied in many laboratory experiments on Drosophila. However, in these studies the possible interaction between the corresponding stress factors in natural environments has not been taken into consideration. We investigated changes in heat and desiccation resistance of adult Drosophila simulans after short-term exposures to different temperatures (35, 31 and 18°C) in combination with high and low relative humidity (ca. 90 and 20%, respectively). Hardening under extreme conditions (35 or 31°C and low relative humidity) commonly resulted in higher resistance to heat and desiccation as compared with other less stressful combinations of temperature and humidity levels. The concentration of the heat-shock protein Hsp70 in the experimental flies increased following almost all applied treatments. Life span of the hardened flies under non-stressful conditions was reduced irrespective of the stress dose, indicating a fitness cost for the plastic responses. The results of the study show that hardening using combined heat and desiccation stress can be very efficient with regard to induction of plastic responses improving tolerance to both types of stress. This may favour adaptation to hot and dry climatic conditions, though the negative effects on fitness are likely to constrain evolution of such plastic responses.