Changes in transcriptomic response to salinity stress induce the brackish water adaptation in a freshwater snail

Abstract

Studying the mechanisms of the establishment of a population in a novel environment allows us to examine the process of local adaptations and subsequent range expansion. In a river system, detecting genetic or phenotypic differences between a freshwater and brackish water population could contribute to our understanding of the initial process of brackish water adaptation. Here, we investigated behavioural and gene expression responses to salt water in a freshwater and brackish water population of the freshwater snail, Semisulcospira reiniana. Although the individuals in brackish water exhibited significantly higher activity in saltwater than freshwater individuals just after sampling, the activity of freshwater individuals had increased in the second observation after rearing, suggesting that their salinity tolerance was plastic rather than genetic. We found 476 and 1002 differentially expressed genes across salinity conditions in the freshwater and brackish water populations, respectively. The major biological process involved in the salinity response of the freshwater population was the biosynthesis and metabolic processing of nitrogen-containing compounds, but that of the brackish water population was influenced by the chitin metabolic process. These results suggest that phenotypic plasticity induces adaptation to brackish water in the freshwater snail by modifying its physiological response to salinity.

Figure 1

Changes in the locomotive distance of individuals collected in freshwater and brackish water area with observation in 0% (a) and 0.4% saltwater (b). The first observation was conducted just after sampling and the second was conducted after rearing in freshwater. Error bars are SEM. **P < 0.01 in post hoc comparison (α = 0.0125).

Figure 2

The number of differentially expressed genes (DEGs) and their expression patterns. (a) The number of DEGs of the freshwater and brackish water populations. (b,c) The expression patterns of DEGs shared between the two populations (b) and all DEGs in each population (c). Colour scale represents the log scaled value of mean FPKM of three individuals in each salt concentration.