First de novo transcriptome assembly of Iwagaki oyster, Crassostrea nippona, and comparative evolutionary analysis of salinity-stress response genes in Crassostrea oysters.

Crassostrea nippona is a commercially important oyster species in East Asia for it is edible during the summer when the other oyster species are unavailable. Salinity is one of the important limiting factors to the survival and distribution of this stenohaline species. In this study, 535 million reads (74G data) from C. nippona were produced and assembled into 66,742 transcripts. The number of 19,253 differentially expressed genes (DEGs) under salinity stress were identified as salinity stress-response genes. Through comparative evolutionary analysis in five Crassostrea species from East Asia, salinity stress-response genes were noticed to have higher adaptive evolution rates than other genes. This study presents the first de novo transcriptome of C. nippona. Furthermore, comparative evolutionary analysis implies that salinity plays an important role in speciation of Crassostrea species.