EXPERIMENTAL EVOLUTION OF HSP70 EXPRESSION AND THERMOTOLERANCE IN DROSOPHILA MELANOGASTER

To examine whether recent evolutionary history affects the expression of Hsp70, the major heat-induced-heat shock protein in Drosophila melanogaster, we measured Hsp70 expression, thermotolerance, and hsp70 gene number in replicate populations undergoing laboratory evolution at different temperatures. Despite Hsp70's ancient and highly conserved nature, experimental evolution effectively and replicably modified its expression and phenotype (thermotolerance). Among five D. melanogaster populations founded from a common ancestral population and raised at three different temperatures (one at 18°C, two each at 25°C and 28°C) for twenty years, Hsp70 expression varies in a consistent pattern: the replicate 28°C lines expressed 30-50% less Hsp70 than the other lines at a range of inducing temperatures. This modification was refractory to acclimation, and correlated with thermotolerance: the 28°C lines had significantly lower inducible tolerance of 38.5°C and 39°C. We verified the presence of five hsp70 genes in the genome of each line, excluding copy number variation as a candidate molecular basis of the evolved difference in expression. These findings support the ability of Hsp70 levels in D. melanogaster populations to change over microevolutionary time scales and implicate constancy of environmental temperature as a potentially important selective agent.