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Organisms respond to environmental stress by synthesizing a small number of highly conserved heat shock proteins. In organisms as diverse as bacteria, plants, invertebrates, and vertebrates, synthesis of these proteins is directly correlated with the acquisition of thermotolerance. While studying the freshwater coelenterate hydra, we observed that Hydra oligactis was extremely sensitive to thermal stress. In contrast, the related species Hydra attenuata survives short-term exposure to high temperatures. Furthermore, after incubation at an elevated but nonlethal temperature, H. oligactis did not become thermotolerant. H. attenuata, however, acquired thermotolerance after such a preincubation. In H. attenuata the major heat shock protein was found to be 60 kDa in size. H. oligactis did not synthesize detectable levels of this protein or any new species of proteins in response to stress. Several other species of hydra were found to behave like H. oligactis in response to stress. Thus, these findings provide direct support for the hypothesis that heat shock proteins are required for stress tolerance and that the major heat shock protein in hydra does not have any effects on normal growth or physiology. The findings also indicate that the presence of a heat shock response might be related to the natural environment in which an organism lives.

Thermotolerance and synthesis of heat shock proteins: these responses are present in Hydra attenuata but absent in Hydra oligactis.