Michelle M. Elekonich and Stephen P. Roberts. Biological Sciences, University of Nevada Las Vegas, 4505 Maryland Parkway, Biology 454004, Las Vegas, NV 89154-4004
Honey bees spend the first 2-3 weeks of their adult lives working inside the constant environment of the hive which they maintain at 33-35 °C through multiple behavioral and physiological thermoregulatory mechanisms. At about 3 weeks of age workers frequently leave the hive as foragers to gather pollen and nectar for the colony. As foragers they undergo periodic episodes of potentially stressful extreme body temperature (> 40 °C) resulting from both environmental and metabolic heat loads. Heat shock proteins (Hsps) or stress proteins are expressed in response to heat, and various environmental stressors. Hsps function as molecular chaperones to bind damaged proteins and refold them or tag them for degradation. We measured mortality and expression of Hsp70 family proteins in heads and flight muscles of bees surviving heat treatments varying from 33 to 50 °C and from 0.5 to 4 hours. Levels of a subset of encoding genes, hsp70 and hsc70-4, were also measured following 1-hour heat treatments across the same temperature range. There was little or no mortality in bees held at 33 and 42 °C. The time prior to 50% mortality at 46, 48 and 50 °C was 3.25, 1.75 and 1.25 hours, respectively. The threshold induction temperature for Hsp70 expression and elevated hsp70/hsc70-4 activity in heads was 46 °C, although heat-induction of Hsp70 and elevated hsp70/hsc70-4 in flight muscles were undetectable. Furthermore, the maximum induction of brain Hsp70 expression and hsp70/hsc70-4 activity occurred only after longer exposures, yet was very modest at levels only 2-3 times baseline values (vs. approximately 1000-fold for ectothermic insects such as Drosophila). These data indicate honey bee tissues, especially flight muscles, are extremely thermotolerant and suggest that the thermal kinetics of Hsp70 induction and hsp70 transcription have co-evolved with endothermy.
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