Population-specific responses in eastern oysters exposed to low salinity in the northern Gulf of Mexico.

Eastern oysters, Crassostrea virginica, are facing rapid environmental changes in the northern Gulf of Mexico and can respond to these changes via plasticity or evolution. Plastic responses can immediately buffer against environmental changes, although this buffering may impact the organism's ability to evolve in subsequent generations. While plasticity and evolution are not mutually exclusive, the relative contribution and interaction between them remain unclear. In this study, we investigate the roles of plastic and evolved responses of C. virginica acclimated to low salinity using a common garden experiment with four populations exposed to two salinities. We use three transcriptomic analyses (edgeR, PERMANOVA, and WGCNA) combined with physiology data to identify the effect of genotype (population), environment (salinity), and genotype-by-environment interaction on both whole organism and molecular phenotypes. We demonstrate that variation in gene expression is mainly driven by population, with relatively small changes in response to salinity. In contrast, the morphology and physiology data reveal that salinity has a larger influence on oyster performance than the population of origin. All analyses lacked signatures of genotype-by-environment interaction, and in contrast to previous studies, we find no evidence for population-specific responses to low salinity. However, individuals from the highest salinity estuary displayed highly divergent gene expression from other populations, which could potentially drive population-specific responses to other stressors. Our findings suggest that C. virginica largely rely on plasticity in physiology to buffer the effects of low salinity, but that these changes in physiology do not rely on large persistent changes in gene expression.