A transcriptomic approach of salinity response in the euryhaline teleost, Dicentrarchus labrax

Euryhaline teleosts possess the capacity to osmoregulate under various environmental conditions (freshwater to hypersaline water). This physiological capacity is generally monitored using enzyme activity assays (Na+/K+ -ATPase...), hormones quantification (prolactine, growth hormone) or their mRNAs expression. To date, few studies addressed the genetic correlates of adaptation to varying salinity at a molecular level in such fish. In the sea bass Dicentrarchus labrax, genetic differentiation was observed at specific allozyme loci between lagoon- and open-sea populations. In the present study, we investigated transcriptomic response of D. labrax to salt- and freshwater acclimation in two organs involved in osmoregulation, gill and intestine. By using suppression subtractive hybridisation, we characterised 586 partial cDNA sequences encoding proteins potentially involved in the metabolism of sea bass acclimated to salt- or freshwater under experimental conditions. Using these results, we first characterised complete genomic sequence of a carbonic anhydrase and then analysed mRNA expression of genes potentially involved in osmoregulation mechanisms (Na+/K+ -ATPase, carbonic anhydrase, angiotensin-converting enzyme and claudin-3), cell-cycle regulation (secretagogin) and immune system (nephrosin) in gill and intestine of wild fish from open sea and lagoons. Our analyses indicate a strong tissue- and environmental-dependant expression pattern for all the genes studied. A transcriptomic approach such as described in the present paper provides thus a first description of genes involved in metabolic or structural functions important for coping with environmental salinity variations in a euryhaline fish like the common sea bass D. labrax. It should be supplemented by proteomics to check the direct involvement of the gene products at the protein level, and by polymorphism analyses if one is to understand population or individual fluctuations in acclimation to salinity variation.