Genetic variation in thermal tolerance among natural populations of Drosophila buzzatii: down regulation of Hsp70 expression and variation in heat stress resistance traits

1. Thermal adaptation was investigated in the fruitfly Drosophila buzzatii Patterson and Wheeler. Two natural populations originating from a high- and a low-temperature environment, respectively, were compared with respect to Hsp70 (heat shock protein) expression, knock-down resistance and heat shoc k resistance. 2. Three main hypotheses were tested: (i) The expression level of Hsp70 in flies from the high-temperature habitat should be down-regulated relative t o flies from the colder habitat. (ii) Flies having higher Hsp70 expression levels should be weakened most by a hardening treatment and go faster into coma, as Hsp70 level reflects stress intensity, and therefore display reduc ed heat knock-down resistance. (iii) Heat shock resistance should be increa sed in the population with highest Hsp70 expression because the level of Hs p70 is positively associated with this trait. 3. The results generally matched the hypotheses. Hsp70 expression was reduc ed in the high-temperature population. Knock-down resistance was higher in the high-temperature population and survival after heat shock was lower in the high-temperature population. 4. This study showed genetic differences in thermal tolerance between popul ations, indicating that high temperature in nature may be an important sele ctive factor. Moreover, knock-down resistance in this study seems to be a m ore relevant trait than standard heat shock resistance for identifying ther mal adaptation in natural populations.