Experimental evolution of Hsp70 expression and thermotolerance in Drosophila melanogaster.

To examine whether recent evolutionary history affects the expression of Hs p70, the major heat-induced-heat shock protein in Drosophila melanogaster, we measured Hsp70 expression, thermotolerance, and hsp70 gene number in rep licate populations undergoing laboratory evolution at different temperature s. Despite Hsp70's ancient and highly conserved nature, experimental evolut ion effectively and replicably modified its expression and phenotype (therm otolerance). Among five D, melanogaster populations founded from a common a ncestral population and raised at three different temperatures tone at 18 d egrees C, two each at 25 degrees C and 28 degrees C for twenty years, Hsp70 expression varies in a consistent pattern: the replicate 28 degrees C line s expressed 30-50% less Hsp70 than the other Lines at a range of inducing t emperatures. This modification was refractory to acclimation, and correlate d with thermotolerance: the 28 degrees C lines had significantly lower indu cible tolerance of 38.5 degrees C and 39 degrees C. We verified the presenc e of five hsp70 genes in the genome of each line, excluding copy number var iation as a candidate molecular basis of the evolved difference in expressi on. These findings support the ability of Hsp70 levels in D. melanogaster p opulations to change over microevolutionary time scales and implicate const ancy of environmental temperature as a potentially important selective agen t.