Life history consequences of temperature transients in Drosophila melanogaster

The physiological and life history consequences of chronic temperatures are well studied in ectotherms. However, little is known about the consequences of short-term exposure to unusually high or low temperatures, as would occur during a weather front. What are the immediate life-history effects of such thermal transients? Can ectotherms recover quickly or do they suffer carry-over effects that persist after weather returns to normal? We measured the impact of thermal transients on egg and progeny production of Drosophila melanogaster Meigen from Washington State. We reared flies at 25 degrees C and then transferred 3- to 5-day old adults to one of three transient treatments (1 or 3 days at 18 degrees C, 1 day at 29 degrees C) before returning them to 25 degrees C. We monitored daily egg production and egg-to-adult viability before (as a control), during, and after the transient as well as fecundity and viability of flies held at constant 18 degrees , 25 degrees and 29 degrees C. This population appears particularly heat tolerant as neither constant nor transient exposure to 29 degrees C (usually a stressful temperature for this species) affected female fecundity or the viability of her progeny. However, a 1- or 3-day exposure to 18 degrees C reduced female fecundity by 75-90% relative to controls, and eggs laid during the 3-day exposure had greatly reduced viability. When returned to 25 degrees C after transient exposure to 18 degrees C, females immediately matched the fecundity and viability of females maintained constantly at 25 degrees C. Therefore, these flies do not suffer negative carry-over effects from these moderate thermal transients. Surprisingly, fitness (intrinsic rate of population growth) was not depressed by transient temperature exposure. However, the severity and especially the timing of the transient will probably determine the likelihood of carry-over effects as well as its effect on fitness.