Hsp70 and larval thermotolerance in Drosophila melanogaster: how much is enough and when is more too much?

Heat shock proteins (Hsps) and other molecular chaperones perform diverse c ellular roles (e.g., inducible thermotolerance) whose functional consequenc es are concentration dependent. We manipulated Hsp70 concentration quantita tively in intact larvae of Drosophila melanogaster to examine its effect on survival, developmental time and tissue damage after heat shock. Larvae of an extracopy strain, which has 22 hsp70 copies, produced Hsp70 more rapidl y and to higher concentrations than larvae of a control strain. which has t he wild-type 10 copies of the gene. Increasing the magnitude and duration o f pretreatment increased Hsp70 concentrations, improved tolerance of more s evere stress, and reduced delays in development. Pretreatment, however, did not protect against acute tissue damage. For larvae provided a brief or mi ld intensity pretreatment, faster expression of Hsp70 in the extracopy stra in improved survival to adult and reduced tissue damage 21 h after heat sho ck. Negative effects on survival ensued in extra-copy larvae pretreated mos t intensely, but their overexpression of Hsp70 did not increase tissue dama ge. Because rapid expression to yield a low Hsp70 concentration benefits la rvae but overexpression harms them, natural selection may balance benefits and costs of high and low expression levels in natural populations. (C) 199 8 Elsevier Science Ltd. All rights reserved.