Evolution of thermotolerance and variation in the heat shock protein, Hsp70

LOW to moderate levels of stress induce a class of molecular chaperones cal led heat shock proteins (Hsps), which protect cells, tissues and whole orga nisms from more severe stress. In higher Eukaryotes, Hsp70 is one of the pr inciple heat-induced chaperones. This response is general, and how much Hsp 70 an animal produces correlates with the level of stress to which it is ex posed. Nonetheless, definitively linking high Hsp70 expression as an adapta tion to stress tolerance is problematic, because organisms and cells respon d to stress in many ways. By molecular manipulation of Hsp70 in one animal group, Drosophila, differences in hsp70 copy number are shown to directly i nfluence heat-induced expression of Hsp70 and tolerance of heat. However, t oo high an expression level of Hsp70 can harm individuals during periods of rapid growth. This strong physiological relationship between Hsp70 concent ration and thermotolerance, along with Hsp70's remarkable degree of intersp ecific coding sequence conservation, suggest that hsp70 regulatory elements may evolve as an adaptation in diverse species to their thermal environmen ts, To examine this possibility, correlative studies within species and res earch on phylogenetic covariation between these traits is reviewed with a f ocus on Drosophila species. However, the techniques and results discussed s hould broadly apply to other animal groups where evolutionary approaches ca n be used to test whether genetic variation in both thermotolerance and Hsp expression within and among species select locally on either hsp70 sequenc e and/or expression.