Quantitative variation in stress-protein production with length of exposure and age in Daphnia magna

The rapid production of stress proteins by organisms in response to heat or chemical stressors has been correlated with increased resistance and survival. Studies in several species have shown that the ability to produce stress proteins is acquired at the same time as heat resistance occurs in development. The relationship between stress-protein synthesis in Daphnia magna and length of stressor exposure was examined at several ages. Protein patterns were determined by immunoblot probing of HSP sub(70) and by silver-staining visualization after separation of proteins by electrophoresis. Test organisms were exposed to a continuous heat shock or a 2-h heat shock followed by recovery at ambient temperature. In continuously exposed animals, protein levels were generally lower in heat shock than in controls over the span of the experiment. At a younger age (5 days), D. magna produced stress proteins in response to heat shock. They produced fewer stress proteins around the age at first brood (8-11 days), regaining their sensitivity to heat shock again at 12 days. Allowing the animals to recover after an initial shock of 2 h resulted in a different pattern of stress-protein production. Protein production is apparently decreased under constant stress and is resumed after removal from the stress.