Intraspecific differences in the transcriptional stress response of two populations of Sydney rock oyster increase with rising temperatures

The vulnerability of sessile organisms to warming temperatures may depend on their capacity to adaptively alter their expression of genes associated with stress mitigation. We compared the transcriptional profile of 2 populations of Sydney rock oysters Saccostrea glomerata (one that had been selectively bred over 7 generations for fast growth and disease resistance and one wild type that had not been subjected to selection) following exposure to an artificial temperature gradient in the field. Oysters were attached to white, grey or black stone pavers that experienced mean maximum substrate temperatures of approximately 34, 37 and 40 degrees C, respectively, at low tide. Across all pavers, selectively bred oysters suffered 12% higher mortality than wild-type oysters, although this difference was not significant. Expression profiles did not differ between oyster populations on the coolest (white) pavers. However, divergent transcriptional profiles of genes associated with the key intracellular stress mechanisms of antioxidant defence, heat shock re sponse, energy metabolism and the cytoskeleton were detected in oysters on the hottest (black) pavers. Expression of these genes was upregulated at high temperatures by the selectively bred oysters but displayed little change, or was suppressed at high temperature among the non-selected wild-type oysters. One potential explanation is that the selectively bred oysters have traded off rapid growth for a lower thermal maximum. Complementary physiological and ecological studies are needed to confirm this hypothesis.