Functional Genetics Of Eastern Oysters (Crassostrea Virginica) Across Salinity Variation In A Single Estuary

Abstract

Understanding the interaction between selection and phenotypic plasticity is important for predicting species' persistence in rapidly changing environments. For sessile organisms, phenotypic plasticity is a typical mechanism for responding to environmental variation. However, the additional characteristics of high fecundity and widely dispersing offspring present an opportunity for selection to reshape the functional genetic composition of populations across habitat heterogeneities every generation. I tested for this pattern of recurrent viability selection in eastern oysters, Crassostrea virginica, using two experimental approaches. First, I made withinreef pair crosses from low, intermediate and high salinity source oysters and analyzed larval survival after a ten day exposure to 10 and 30 salinity treatments. The parental reef source-bylarval salinity treatment interaction term was a significant predictor of larval survival. Second, I sequenced and assembled the oyster transcriptome de novo in order to conduct RNA-seq to identify differential gene expression patterns in response to salinity treatment and oyster reef source. The samples for RNA-seq were twenty-four adult oysters collected from high and low salinity source reefs and acclimated for 9 weeks in 10 and 30 salinity common gardens. A total of 9,921 reference transcriptome contigs (reftigs; 23.6%) were significantly differentially expressed (DE), with 0.6% of all reftigs DE for the reef source, 18.9% DE for treatment salinity, and 13.9% DE for the reef-by-treatment factor. The reftigs responding to treatment and the reef- by-treatment factors demonstrate a genomically pervasive pattern of plastic gene expression in response to salinity. Additionally, the abundant genotype-by-environment patterns suggest that the history of selection at each reef is generating different plastic responses after acclimation to the same osmotic condition. Overall, the reef-specific patterns of gene expression and larval survival indicate that oyster responses to habitat heterogeneity are shaped by both phenotypic plasticity and recurrent viability selection. Furthermore, the larval results suggest that the functional plasticity differences observed in the adults were heritable. Studies on the interaction between plasticity and evolutionary responses typically classify plasticity as acting within generations and selection as acting between generations. I suggest that for high dispersal species with type III survivorship, intra-generational selection can shape patterns of plasticity across habitats.