Gill Transcriptome of Freshwater Fish Tor putitora
Climate induced water temperature rise and fluctuation pose serious challenges to aquatic species, especially the coldwater and hillstream fishes such as Tor putitora. The present study investigated the physiological, histological, and transcriptomic responses of Tor putitora to acute constant heat stress and diurnal temperature fluctuations to elucidate molecular mechanisms underlying thermal sensitivity and adaptation. Fish were exposed to elevated temperatures 30 and 32 degree Celsius and diurnal thermal cycling 24 to 32 degree C for 96 h, with 24 degree C serving as the control. Heat stress resulted in a significant increase in lipid peroxidation, as evidenced by elevated malondialdehyde levels in gill, liver, and kidney tissues, indicating enhanced oxidative damage. In contrast to some warm-water species, the activities of key antioxidant enzymes like catalase, superoxide dismutase, and glutathione reductase, were significantly reduced with increasing temperature and exposure duration, reflecting a compromised antioxidant defence system. Histological examination revealed severe tissue-specific alterations under heat stress, including fusion and degeneration of secondary gill lamellae, telangiectasis, hepatic vacuolization and sinusoidal congestion, and degeneration of nephrotic tubules with macrophage infiltration. Gill transcriptome profiling using RNA Seq demonstrated extensive transcriptional reprogramming under both constant and fluctuating thermal regimes. Differential expression analysis identified widespread downregulation of genes associated with immune response, apoptosis, ferroptosis, cytokine signalling, and cellular maintenance, suggesting systemic immunosuppression and energy conservation in response to thermal stress.