Selection on knockdown performance in Drosophila melanogaster impacts thermotolerance and heat-shock response differently in females and males

We studied adaptive thermotolerance in replicate populations of Drosophila melanogaster artificially selected for high and low knockdown temperature (T-KD), the upper temperature at which flies can no longer remain upright or locomote effectively. Responses to selection have generated High T-KD populations capable of maintaining locomotor function at 40 degrees C, and Low T-KD populations with T-KD of similar to 35 degrees C. We examined inducible knockdown thermotolerance, as well as inducible thermal survivorship, following a pretreatment heat-shock (known to induce heat-shock proteins) for males and females from the T-KD selected lines. Both selection for knockdown and sex influenced inducible knockdown thermotolerance, whereas inducible thermal survivorship was influenced only by sex, and not by selection. Overall, our findings suggest that the relationships between basal and inducible thermotolerance are contingent upon the methods used to gauge thermotolerance, as well as the sex of the flies. Finally, we compared temporal profiles of the combined expression of two major heat-shock proteins, HSC70 and HSP70, during heat stress among the females and males from the selected T-KD lines. The temporal profiles of the proteins differed between High and Low T-KD females, suggesting divergence of the heat-shock response. We discuss a possible mechanism that may lead to the heat-shock protein patterns observed in the selected females.