Heat shock protein gene expression is higher and more variable with thermal stress and mutation accumulation in Daphnia

Understanding the genetic architecture of the stress response and its ability to evolve in response to different stressors requires an integrative approach. Here we quantify gene expression changes in response to two stressors associated with global climate change and habitat loss--heat shock and mutation accumulation. We measure expression levels for two Heat Shock Proteins (HSP90 and HSP60)--members of an important family of conserved molecular chaperones that have been shown to play numerous roles in the cell. While HSP90 assists with protein folding, stabilization, and degradation throughout the cell, HSP60 primarily localizes to the mitochondria and mediates de novo folding and stress-induced refolding of proteins. We perform these assays in Daphnia magna originally collected from multiple genotypes and populations along a latitudinal gradient, which differ in their annual mean, maximum, and range of temperatures. We find significant differences in overall expression between loci (10-fold), in response to thermal stress (~6x increase) and with mutation accumulation (~4x increase). Importantly, stressors interact synergistically to increase gene expression levels when more than one is applied (increasing, on average, >20x). While there is no evidence for differences among the three populations assayed, individual genotypes vary considerably in HSP90 expression. Overall, our results support previous proposals that HSP90 may act as an important buffer against not only heat, but also mutation, and expands this hypothesis to include another member of the gene family acting in a different domain of the cell.