The central importance of the heat shock response to survival of heat stress has become an almost universally accepted concept in physiology. Heat shock proteins of the 70 kDa class (Hsp70) are some of the best studied proteins involved in the stress response and levels of this protein are often used as an indicator of resistance to heat stress. For many years it has been recognized that individuals within a population of organisms express a wide range of Hsp70 levels. Louise Jensen and her colleagues in Denmark and Australia explored the role that this genetically based individual variation in Hsp70 levels plays in determining the heat tolerance of adult fruit flies Drosophila melanogaster.
The team conducted three separate experiments to test the importance of Hsp70 expression levels to heat tolerance. Each experiment was conducted on 20–50 inbred lines of flies. Inbred lines of flies have reduced genetic variation making it easier to identify genetic differences in gene expression. To create the inbred lines the team collected wild female flies from locations in Australia and bred their progeny in the lab for at least 3 generations, and up to 45 generations for some experiments.
The team then measured the levels of Hsp70 expression induced by exposing the flies to temperatures of 35°C followed by 1 h of recovery at 25°C. Then they determined each line's heat tolerance using two different methods: in the first they recorded how many of the insects survived a severe 1 h heat shock at 38.3–38.8°C; and in the second they measured the time required for the flies to lose activity at 38.5–39°C. Finally, Jensen and her colleagues evaluated the long-term effects of heat shock protein expression on the life history of female flies from the various inbred lines by measuring the number of progeny produced per female, the proportion of eggs that successfully developed to become adults, and the total development time for eggs produced by each female.
The team found that Hsp70 expression levels in response to their heat shock regime were different in all the inbred lines produced for all of their experiments. However, these differences in Hsp70 expression level did not correlate significantly with the insects' heat tolerances. Thus, the authors concluded that variation in Hsp70 expression is relatively unimportant in determining the heat tolerance of adult flies.
This result is in opposition to studies conducted on larval flies and suggests that Hsp70 may play different roles in heat tolerance in different life history stages of an organism. In addition, there were no strong correlations with any of the life history characters that were measured in this study, which suggests there is no ‘cost’ of increased Hsp70 expression as has been suggested by previous studies.
The results of this study suggest that we should seriously rethink the central importance of Hsp70 in determining the heat tolerance of all organisms. The study also provides evidence that measuring levels of Hsp70 in wild organisms may not be a good indicator of thermal tolerance. It is also a warning that we should not make assumptions about the importance of Hsp70 expression levels, or any other protein for that matter, no matter how much we know about that protein. Future studies should confirm a role for this protein in heat tolerance for individual species, or even a different life history stage within a single species, before we draw conclusions about the heat shock response. Finally, the study confirms what many have observed in recent years: there is much more to the heat shock response than Hsp70 and it is time to explore other options.
