Microgeographic variation in gene expression plasticity under osmotic stress in wild eastern oyster juveniles (Crassostrea virginica)

Gene expression plasticity is an essential mechanism of physiological acclimation. In adult eastern oysters, Crassostrea virginica (Gmelin, 1791), transcriptomic responses to experimental osmotic stress differ between oyster bed sources within the estuarine salinity gradient. Here RNA-seq was used to characterize spatial variation in gene expression plasticity in wild oyster spat (juveniles), collected from upper and lower Delaware Bay oyster beds, and reciprocally exposed to moderate and low salinity experimental treatments. Low salinity conditions contribute significantly to mortality in the species and stressful upper bay episodes of extended hypo-salinity are expected to increase in frequency under future climate conditions. The spat collected at moderate salinity (lower bay) sites show more than twice as many differentially expressed transcripts between treatments, relative to their upper bay counterparts – a pattern of spatial variation in gene expression plasticity consistent with previous findings in adults. However, an alternate measure of gene expression plasticity, the log-fold degree of expression difference between treatments, was greater in oysters collected at low salinity (upper bay). Observed spatial differences in gene expression plasticity indicate that early pre- or post-settlement environmental conditions likely generated distinct transcriptome-wide osmotic stress responses between upper and lower bay populations of a single estuary.