NATURAL VARIATION IN THE EXPRESSION OF THE HEAT-SHOCK PROTEIN HSP70 IN A POPULATION OF DROSOPHILA-MELANOGASTER AND ITS CORRELATION WITH TOLERANCE OF ECOLOGICALLY RELEVANT THERMAL-STRESS

Although Hsp70, the principal inducible heat-shock protein of Drosophi la melanogaster, has received intense scrutiny in laboratory strains, its variation within natural populations and the consequences of such variation for thermotolerance are unknown. We have characterized varia tion in first-instar larvae of 20 isofemale lines isolated from a sing le natural population of D. melanogaster, in which larvae are prone to thermal stress in nature. Hsp70 expression varied more than twofold a mong lines after induction by exposure to 36 degrees C for one hour, w ith an estimated proportion of the variation due to genetic difference s of 0.24 +/- 0.08. Thermotolerance with and without a Hsp70-inducing pretreatment, survival at 25 degrees C, and developmental time also va ried significantly. As expected, expression of Hsp70 correlated positi vely with larval thermotolerance. By contrast, lines in which larval s urvival was high in the absence of heat stress showed lower than avera ge Hsp70 expression and lower than average inducible thermotolerance. This conditional performance suggests an evolutionary trade-off betwee n thermotolerance and the ability to produce higher concentrations of Hsp70, and survival in a benign environment.