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 com pared for flies treated as follows. always held at 25 degrees C, place d in an incubator set at 37 degrees C for 120 min, or exposed to 40 de grees 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, an d female fecundity, which was measured as offspring produced over ten days. Male fertility, measured as the progeny produced by a female mat ed once, differed little among treatments. Previous exposure to a high , but non-lethal, temperature before exposure to the higher one, impro ved survival of males and females, and improved offspring production o f females. Genetic variation was present among lines for offspring pro duction, but genetic variation for survival was not significant, and g enotype by environment interactions for fitness components of females were small. These results indicated low genetic variation in thermal r esistance in the studied population, such that a threshold for tempera ture stress probably exists, above which local extinction is more like ly than the evolution of resistance.