Bioinformatic Analysis and Tissue-Specific Expression of Clock Genes and Opn5 in the Chinese Giant Salamander (Andrias davidianus) Under Different Light Conditions

Circadian rhythms are essential for physiological regulation and environmental adaptation of animals. Light, as a primary exogenous synchronizing signal, significantly influences the maintenance and remodeling of the circadian system. In vertebrates, circadian rhythms depend on the synergistic action of central and peripheral clocks; however, systematic studies on the molecular responses to light across different tissues in amphibians are still lacking. In this study, 64 juvenile Chinese giant salamanders (Andrias davidianus) were randomly assigned to four groups: a control group (approximately 12L:12D, 5–12 lx, C), a continuous intermediate-light group (24L:0D, 80 ± 5 lx, I), a continuous high-light group (24L:0D, 220 ± 10 lx, H), and a continuous dark group (0L:24D, 0 lx, B). The experiment spanned 13 days. We first identified candidate Clock and Opn5 homologues from transcriptome-derived sequences using homology searches, phylogenetic analysis and conserved-domain analysis. We then used RT-qPCR to measure expression changes across sampling days of Clock, Bmal1, Per2, Cry1 and Opn5 in the brain, eye and skin. The results indicated that under control and continuous intermediate-light conditions, the expression changes in core clock genes in brain and eye tissues were generally consistent, while the skin exhibited a relatively independent response pattern. Under continuous high-light conditions, the brain and eye exhibited distinct temporal expression patterns, whereas constant darkness was associated with a later-stage transcriptional response in the skin. In summary, prolonged exposure to bright light and continuous darkness were associated with distinct tissue-specific and time-dependent transcriptional responses of Opn5 and core clock genes in the brain, eye, and skin of juvenile Chinese giant salamanders. This study provides foundational data on the molecular responses of the clock genes and non-visual opsin in amphibians to abnormal lighting conditions.