RNA sequencing analysis of transcriptional change in the freshwater mussel Elliptio complanata after environmentally relevant sodium chloride exposure

Laura S. Robertson, Heather S. Galbraith, Deborah Iwanowicz, Carrie J. Blakeslee, R. Scott Cornman

To identify potential biomarkers of salt stress in a freshwater sentinel species, we examined transcriptional responses of the common mussel Elliptio complanata to controlled NaCl exposures. RNA-Seq of mantle tissue identified 481 transcripts differentially expressed in adult mussels exposed to 2 ppt NaCl (1.2 ppt chloride) for seven days, of which 290 had non-overlapping intervals. Differentially expressed gene categories included ion and transmembrane transport, oxidoreductase activity, maintenance of protein folding, and amino acid metabolism. The rate-limiting enzyme for synthesis of taurine, an amino acid frequently linked to osmotic stress in aquatic species, was up-regulated, as was the transmembrane ion pump sodium/potassium ATPase. These patterns confirm a primary transcriptional response to the experimental dose, albeit likely overlapping with non-specific secondary stress responses. Substantial involvement of the protein chaperone family Hsp70 and the water-transporting aquaporin family was not detected, however, in contrast to some studies in other bivalves. A subset of the most significantly regulated genes was confirmed by qPCR in an independent sample. Cluster analysis showed separation of mussels exposed to 2 ppt NaCl from control mussels in multivariate space, but mussels exposed to 1 ppt NaCl were largely indistinguishable from controls. Transcriptome-scale analysis of salt exposure under laboratory conditions efficiently identified candidate biomarkers for further functional analysis and field validation. This article is protected by copyright. All rights reserved.