The biology of environmental stress: adaptive responses in Sydney rock oysters (Saccostrea glomerata) from an urbanized estuary

This thesis investigates the impacts of chronic exposure to environmenta ldisturbance on wild populations of Sydney rock oysters (Saccostrea glomerata) in the Sydney Harbour estuary. It examines the hypothesis that intergenerational exposure to chemical contaminants and altered hydrological conditions leads to localized adaptation of oyster populations at the subcellular level. Chapters 2 and 3 focus on Sydney rock oysters from four bays that exhibit strong gradients of disturbance allowing the selection of high- and low-impact sites within each embayment. Tissue chemistry and hydrological measurements illustrated a unique mixture of stressors that differed significantly between these sites. Proteomic analysis showed clear differences in oyster proteomes among sites, and these differences were consistent across the four bays tested (Chapter 2). In contrast, population genetic analysis found no substantial differentiation of neutral genetic markers (microsatellites) in oysters throughout thestudy area (Chapter 3). This suggests that the proteomic differences observed were not due to neutral genetic variation. Instead, it was concluded that proteomic differentiation between populations reflected either transient changes in gene expression and protein turnover, or longer-term alterations that may involve genome-level adaptation. Chapters 4 and 5 describe results of a recovery experiment that was conducted to differentiate between these two explanations for the differences observed between populations. Oysters were collected from high- and low-impact sites within a single bay and transferred to the laboratory where they were acclimated in clean seawater for 60 days. Proteomic analysis revealed that the concentrations of most proteins in oysters from the high-impact site rapidly returned to levels equivalent to the ambient state, as reflected by the proteomes of oysters from the low-impact site (Chapter 4). Thissuggests that responses to environmental disturbance were transient at the level of the proteome and that oysters rapidly returned to cellular homeostasis when the disturbance was removed. However, the transcriptional analysis found that the expression of numerous genes remained significantly altered in oysters from the high-impact site throughout the course of the 60-day recovery period in clean seawater (Chapter 5). It was concluded that oysters from populations that suffer chronic environmental disturbance can rapidly return to functional homeostasis once the disturbance is removed, whilst their transcriptome remains substantially altered. The data suggest transcriptional modification may be a mechanism for localized responses to stress.They are also consistent with long-term adaptation to chronic environmental stress over small spatial scales.