EFFECTS OF EXPOSURE TO SHORT-TERM HEAT-STRESS ON FITNESS COMPONENTS IN DROSOPHILA-MELANOGASTER

Effects of thermal stress on survival and reproductive success in ten recently collected isofemale lines of Drosophila melanogaster were compared for flies treated as follows. always held at 25 degrees C, placed in an incubator set at 37 degrees C for 120 min, or exposed to 40 degrees C in an incubator for 90 min, with or without previous exposure to 37 degrees C. Short-term exposure to the higher temperature greatly reduced adult survival, the mating frequency of males and females, and female fecundity, which was measured as offspring produced over ten days. Male fertility, measured as the progeny produced by a female mated once, differed little among treatments. Previous exposure to a high, but non-lethal, temperature before exposure to the higher one, improved survival of males and females, and improved offspring production of females. Genetic variation was present among lines for offspring production, but genetic variation for survival was not significant, and genotype by environment interactions for fitness components of females were small. These results indicated low genetic variation in thermal resistance in the studied population, such that a threshold for temperature stress probably exists, above which local extinction is more likely than the evolution of resistance.