Transcriptome analysis reveals novel insights in air-breathing magur catfish (Clarias magur) in response to high environmental ammonia

The facultative air-breathing magur catfish (Clarias magur) frequently face different environmental challenges, such as hyper-ammonia, and desiccation stresses in their natural habitats. All these stresses lead to higher accumulation of body ammonia, thereby causing various harmful effects to the fish due to its toxicity. Nonetheless, the mechanisms underlying ammonia-induced toxicity is yet not clear. In the present study, we used RNA sequencing and utilized a modified method for de novo assembly of the transcriptome to provide an exhaustive study on the transcriptomic alterations of magur catfish in response to high environmental ammonia (HEA; 25 mM NH4Cl). The final contig assembly produced a total of 311,076 unique transcripts (termed as unigenes) with a GC content of 48.3% and the average length of 599 bp. A considerable number of SSR marker associated with these unigenes were also detected. A total of 279,156 transcripts were successfully annotated by using various databases. Comparative transcriptomic analysis revealed a total of 3453 and 19,455 genes were differentially expressed in the liver and brain tissues, respectively, in ammonia-treated fish compared to the control. Enrichment analysis of the differentially expressed genes (DEGs) showed that several GO and KEGG pathway terms were significantly over-represented. Functional analysis of significantly elevated DEGs demonstrated that ammonia stress tolerance of the magur catfish was associated with quite a few pathways related to immune response, oxidative stress, and apoptosis, as well as few transporter proteins involved with ammonia and urea transport. Both liver and brain tissues showed HEA-mediated oxidative damage with consequent activation of antioxidant machinery. However, elevated ROS levels led to an activation of inflammatory cytokines and thus innate immune response in the liver. Conversely, in the brain ROS-mediated irreversible cell damages activated apoptosis via both p53-Bax-Bcl2 and caspase-mediated pathways. The present study provides a novel understanding of the molecular responses of this air-breathing catfish against the ammonia-induced stressors, which could elucidate the underlying mechanisms of adaptation of this facultative air-breather living under various environmental constraints.