RNA sequencing, de novo assembly and differential analysis of the gill transcriptome of freshwater climbing perch Anabas testudineus after 6 days of seawater exposure

To obtain transcriptomic insights into branchial responses to salinity challenge in Anabas testudineus, this study employed RNA sequencing (RNA-Seq) to analyse the gill transcriptome of A. testudineus exposed to seawater (SW) for 6 days compared with the freshwater (FW) control group. A combined FW and SW gill transcriptome was de novo assembled from 169.9 million 101 bp paired-end reads. In silico validation employing 17 A. testudineus Sanger full-length coding sequences showed that 15/17 of them had greater than 80% of their sequences aligned to the de novo assembled contigs where 5/17 had their full-length (100%) aligned and 9/17 had greater than 90% of their sequences aligned. The combined FW and SW gill transcriptome was mapped to 13,780 unique human identifiers at E-value <= 1.0E-20 while 952 and 886 identifiers were determined as up and down-regulated by 1.5 fold, respectively, in the gills of A. testudineus in SW when compared with FW. These genes were found to be associated with at least 23 biological processes. A larger proportion of genes encoding enzymes and transporters associated with molecular transport, energy production, metabolisms were up-regulated, while a larger proportion of genes encoding transmembrane receptors, G-protein coupled receptors, kinases and transcription regulators associated with cell cycle, growth, development, signalling, morphology and gene expression were relatively lower in the gills of A. testudineus in SW when compared with FW. High correlation (R=0.99) was observed between RNA-Seq data and real-time quantitative PCR validation for 13 selected genes. The transcriptomic sequence information will facilitate development of molecular resources and tools while the findings will provide insights for future studies into branchial iono-osmoregulation and related cellular processes in A. testudineus.