HERITABILITY OF EXPRESSION OF THE 70KD HEAT-SHOCK PROTEIN IN DROSOPHILA MELANOGASTER AND ITS RELEVANCE TO THE EVOLUTION OF THERMOTOLERANCE.

The principle inducible heat-shock protein of Drosophila melanogaster, Hsp70, contributes to thermotolerance throughout the entire life cycle of the species but may also reduce fitness in some life stages. In principle, selection might maximize the benefits of Hsp70 expression relative to its costs by adjusting the magnitude of Hsp70 expression for each life-cycle stage independently. Therefore we examined whether the magnitude of Hsp70 expression varied during the life cycle and the relationship of this variation to several life-history traits. For 28 isofemale lines derived from a single natural population, estimates of heritable variation in Hsp70 expression ranged between 0.25 and 0.49, and the association among variation in first- and third-instar larvae and in adults correlated highly. Thus, Hsp70 expression is genetically coupled at these developmental stages. A line engineered with extra copies of the hsp70 gene produced more Hsp70 and survived heat shock much better than did a control strain. Among natural lines, Hsp70 expression was only weakly related to tolerance of heat shock and to larva-to-adult survival and developmental time at permissive temperatures. Additionally, lines with high adult survival developed slowly as larvae, which is a possible trade-off. These and other findings suggest that trade-offs may maintain quantitative variation both in heat-shock protein expression and in life-history traits that associate with thermotolerance.