© 1999 by The Society for Integrative and Comparative Biology
Evolution of Thermotolerance and Variation in the Heat Shock Protein, Hsp701
*Department of BGES, Cleveland State Univ. 2399 Euclid Ave, Cleveland Ohio 44115
Department of Organismal Biology and Anatomy, The University of Chicago 1027 East 57th St., Chicago, Illinois 60637
Correspondence: 2 E-mail: r.krebs{at}popmail.csuohio.edu
Low to moderate levels of stress induce a class of molecular chaperones called heat shock proteins (Hsps), which protect cells, tissues and whole organisms from more severe stress. In higher Eukaryotes, Hsp70 is one of the principle heat-induced chaperones. This response is general, and how much Hsp70 an animal produces correlates with the level of stress to which it is exposed. Nonetheless, definitively linking high Hsp70 expression as an adaptation to stress tolerance is problematic, because organisms and cells respond to stress in many ways. By molecular manipulation of Hsp70 in one animal group, Drosophila, differences in hsp70 copy number are shown to directly influence heat-induced expression of Hsp70 and tolerance of heat. However, too high an expression level of Hsp70 can harm individuals during periods of rapid growth. This strong physiological relationship between Hsp70 concentration and thermotolerance, along with Hsp70's remarkable degree of interspecific coding sequence conservation, suggest that hsp70 regulatory elements may evolve as an adaptation in diverse species to their thermal environments. To examine this possibility, correlative studies within species and research on phylogenetic covariation between these traits is reviewed with a focus on Drosophila species. However, the techniques and results discussed should broadly apply to other animal groups where evolutionary approaches can be used to test whether genetic variation in both thermotolerance and Hsp expression within and among species select locally on either hsp70 sequence and/or expression.