A 5′ Promoter Region SNP in CTSC Leads to Increased Hypoxia Tolerance in Changfeng Silver Carp (Hypophthalmichthys molitrix)

Silver carp is a critically significant species in freshwater aquaculture in China, characterized by its limited tolerance to hypoxia. In this study, a significant SNP locus at Chr8: 29647765 (T/C) associated with hypoxia tolerance traits was identified in Changfeng silver carp, and the homozygotic CC genotype exhibited higher hypoxic tolerance than the homozygotic TT and heterozygotic TC genotypes. Under hypoxic conditions, the hemoglobin concentration increased, with the CC genotype demonstrating a significantly higher level compared with the TT genotype; the activities of antioxidant enzymes including catalase and superoxide dismutase were significantly higher in the CC genotype than in the other genotypes; the area of the gill lamellae was significantly smaller in the CC genotype than in the TT and TC genotypes; and the number of apoptotic cells in the brain was significantly lower in the CC genotype than in the TT and TC genotypes. Sequence analysis showed that this SNP was located in the promoter region of the cathepsin C (CTSC) gene. The expression levels of the CTSC gene were analyzed across the three genotypes, revealing that the CC genotype exhibited significantly lower expression compared with the TT and TC genotypes under hypoxia. This finding suggests that the SNP associated with the CC genotype leads to reduced CTSC expression, which may facilitate better physiological adaptation to hypoxia. Analysis of the promoter region of CTSC found a unique predicted hypoxia-inducible factor 1-alpha (HIF-1 alpha) binding site (CGTG) in the T genotype, implying that the differential expression of CTSC among the three genotypes under hypoxic stress may be regulated by HIF-1 alpha, a transcription factor integral to hypoxia adaptation, thereby affecting hypoxia tolerance, which further affects the immune response of the Changfeng silver carp in response to the hypoxic environment. Although SNPs represent significant genetic determinants, their phenotypic effects are predominantly mediated through complex interactions within gene regulatory networks and environmental influences. This study identified an effective SNP site in Changfeng silver carp, providing valuable guidance for future selective breeding and the development of new hypoxia-tolerant varieties.