SUMMARY Drosophila melanogaster collected in sub-equatorial Africa in the 1970s are remarkably tolerant of sustained laboratory culture above 30°C and of acute exposure to much warmer temperatures. Inducible thermotolerance of high temperatures, which in Drosophila melanogaster is due in part to the inducible molecular chaperone Hsp70, is only modest in this strain. Expression of Hsp70 protein and hsp70 mRNA is likewise reduced and has slower kinetics in this strain (T) than in a standard wild-type strain (Oregon R). These strains also differed in constitutive and heat-inducible levels of other molecular chaperones. The lower Hsp70 expression in the T strain apparently has no basis in the activation of the heat-shock transcription factor HSF, which is similar in T and Oregon R flies. Rather, the reduced expression may stem from insertion of two transposable elements, H.M.S. Beagle in the intergenic region of the 87A7 hsp70 gene cluster and Jockey in the hsp70Ba gene promoter. We hypothesize that the reduced Hsp70 expression in a Drosophila melanogaster strain living chronically at intermediate temperatures may represent an evolved suppression of the deleterious phenotypes of Hsp70.

We demonstrated that in the desert lizard Phrynocephalus interscapularis constitutive synthesis of both members of the hsp70 family of heat shock proteins takes place in summer. Elevation of environmental and body temperatures during the day was accompanied by a pronounced (2.0-2.5 times) increase in hsp70 synthesis. Synthesis of hsps continues during the day and drops to a basal level at night. Northern hybridization analysis demonstrated a higher content of hsp-coding RNA in the cells of P. interscapularis under normal physiological conditions in comparison with the lizard Lacerta vivipara, which inhabits regions with moderate climates.

We compare the properties and activation of heat-shock transcription factor (HSF1) and the synthesis of a major family of heat-shock proteins (HSP70) in lizard species inhabiting ecological niches with strikingly different thermal parameters. Under normal non-heat-shock conditions, all desert-dwelling lizard species studied so far differ from a northern, non-desert species (Lacerta vivipara) in the electrophoretic mobility and content of proteins constitutively bound to the regulatory heat-shock elements in the heat-shock gene promoter. Under these conditions, levels of activated HSF1 and of both HSP70 mRNA and protein are higher in the desert species than in the non-desert species. Upon heat shock, HSF1 aggregates in all species studied, although in desert species HSF1 subsequently disaggregates more rapidly. Cells of the northern species have a lower thermal threshold for HSP expression than those of the desert species, which correlates with the relatively low constitutive level of HSPs and high basal content of HSF1 in their cells.

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