COSTS AND BENEFITS OF ACTIVATION OF THE HEAT-SHOCK RESPONSE IN DROSOPHILA-MELANOGASTER

1. The costs of conditioning adult Drosophila melanogaster with a mild thermal stress that activates the genes for heat-shock proteins were examined by comparing the number of offspring produced by females main tained continuously at 25 degrees C with females exposed to a non-leth al stress, 36 degrees C for 75 min, once, twice or three times. The co mparison was done under two nutritional treatments, with or without ye ast added to the medium. 2. Benefits of conditioning adult D. melanoga ster to thermal stress were examined by comparing survival after a sev ere stress (39 degrees C for 100 min) among flies that were not condit ioned with those conditioned once, twice or three times by exposure to 36 degrees C for 75 min. Additional comparisons were made varying eit her the duration of conditioning or the time elapsed between condition ing and exposure to the severe stress. 3. Fewer offspring were produce d by females receiving the conditioning treatment than for those not r eceiving it and the decrease in fecundity was greater for females cond itioned more often. Proportional differences in fecundity were larger for the no-yeast treatment than for females held with yeast. 4. Surviv al after the severe stress was much greater for flies that: were condi tioned than for those not conditioned and survival increased with incr easing number of conditioning bouts. Survival also was greater for fli es conditioned closer to the exposure to severe stress, with the excep tion that survival for flies conditioned only 2 or 4 h before exposure to severe stress was less than that for those conditioned 8 h before.