Identification of molecular and physiological responses to chronic environmental challenge in an invasive species: the Pacific oyster, Crassostrea gigas

Abstract

Understanding the environmental responses of an invasive species is critical in predicting how ecosystem composition may be transformed in the future, especially under climate change. In this study, Crassostrea gigas, a species well adapted to the highly variable intertidal environment, was exposed to the chronic environmental challenges of temperature (19 and 24°C) and pH (ambient seawater and a reduction of 0.4 pH units) in an extended 3-month laboratory-based study. Physiological parameters were measured (condition index, shell growth, respiration, excretion rates, O:N ratios, and ability to repair shell damage) alongside molecular analyses. Temperature was by far the most important stressor, as demonstrated by reduced condition indexes and shell growth at 24°C, with relatively little effect detected for pH. Transcriptional profiling using candidate genes and SOLiD sequencing of mantle tissue revealed that classical "stress" genes, previously reported to be upregulated under acute temperature challenges, were not significantly expressed in any of the treatments, emphasizing the different response between acute and longer term chronic stress. The transcriptional profiling also elaborated on the cellular responses underpinning the physiological results, including the identification of the PI3K/AKT/mTOR pathway as a potentially novel marker for chronic environmental challenge. This study represents a first attempt to understand the energetic consequences of cumulative thermal stress on the intertidal C. gigas which could significantly impact on coastal ecosystem biodiversity and function in the future.

Keywords: Candidate genes; condition index; energetic trade-offs; mTOR pathway; transcriptional profiling.

Figure 1

Picture of Crassostrea gigas.

Figure 2

95% confidence intervals for mean values based on pooled standard deviation of (A) condition index; (B) shell dry weight (g); (C) oxygen consumption rates (mL/O₂/h, corrected to a standardized weight of an individual); and (D) excretion rates (NH₄ μmol/L/dry weights [g]/individual/h, corrected to a standardized weight of an individual). X-axis symbols: NH: 24°C, normal pH; PHH: 24°C low pH; PHL: 19°C low pH; NL: 19°C normal pH.

Figure 3

Mean mRNA levels (n = 6) of ATPase (P-type sodium–potassium ATPase), Hsp70, Hsc72, bone-specific (similar to bone-specific cmf608), calreticulin, protein kinase (calcium-dependant protein kinase), ferritin, calnexin, and GRP78 in the NL, NH, PHL, and PHH treatments. The error bars represent standard errors. The Y-axis represents relative gene expression levels compared with the reference gene.

Figure 4

Diagrammatic representation of molecular results, which are fully described in Tables S3–S6.