Natural thermal stress and heat‐shock protein expression in Drosophila larvae and pupae

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Abstract
1. Whether Drosophila larvae and pupae naturally experience temperatures that can cause heat damage or death is poorly understood, but bears directly on numerous investigations of the thermal biology and heat‐shock response in Drosophila. Accordingly, the temperatures of necrotic fruit, which Drosophila larvae and pupae inhabit, the temperatures of larvae and pupae outside the laboratory, and the levels of the heat‐shock protein hsp 70 expressed by larvae in nature were examined. 2. When necrotic fruit was sunlit, internal temperatures rose to levels that can harm indwelling insects. Fruit size and evaporative water loss affected these temperatures. Temperatures of larvae and pupae in the field commonly exceeded 35 °C, with living larvae recorded at >44°C and pupae at >41°C. Natural mortality was evident, presumably because of heat. 3. In the laboratory, these temperatures kill larvae rapidly, with LT50s (time taken for half the sample to be killed) of 30 min at 39 °C, 15 min at 40 °C and 8·5 min at 41 °C. Gradual transfer from 25°C to these temperatures resulted in no lesser mortality than did direct transfer. 4. Hsp 70 levels in lysates of whole larvae were measured by ELISA (enzyme‐link immunosorbent assay) with an hsp 70‐specific antibody. For larvae within necrotic apples experimentally transferred from shade to sun and within necrotic fruit in situ, hsp 70 levels equalled or exceeded levels detected in parallel laboratory studies of whole larvae or cells in culture. 5. These data provide an ecological context for studies of thermal stress and the heat‐shock response in Drosophila that has heretofore been lacking.

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