Identification, evolutionary and characteristic analysis of apoptosis - related caspase gene family in Tachysurus fulvidraco and their expression profiles under bacterial, parasitic, and viral infections.

The caspase family, conserved cysteine proteases, play pivotal roles in host anti-infectious immunity and apoptotic regulation. However, systematic investigations into this family in Siluriformes are scarce. Here, we systematically identified 9 apoptosis-related caspase genes in the genome of yellow catfish (Tachysurus fulvidraco) and analyzed their molecular characteristics, evolutionary relationships, and expression dynamics during pathogen infections. Results showed that T. fulvidraco encodes caspase2, 3, 3a, 3b, 6, 7, 8, 9, and 10, but lacks the canonical caspase1. Apoptosis-related caspases were classified into initiators (2/8/9/10) and effectors (3/6/7), with caspase3 existing in three isoforms (3, 3a, 3b). Phylogenetic and synteny analyses indicated strong evolutionary conservation, with close correlations between their conservation and species phylogeny. Tissue-specific expression profiles showed that all caspase genes were ubiquitously expressed across tested tissues, with relatively high levels in the hindgut, gill, and liver. Pathogen challenge assays showed: (1) During Vibrio mimicus infection, most caspase genes were upregulated in the head kidney, skin, and gill at the early phase, whereas in the spleen, their expression was predominantly elevated during the middle to late stages; (2) Following long-term Ichthyophthirius multifiliis infection (20 days post-infection, dpi), all caspase genes in the head kidney were activated, while most in the skin were activated during the dying state, with differential activation patterns between the gill and skin; (3) After yellow catfish calicivirus (YCCV) infection, the majority of caspases were activated in the skin, gill, and head kidney, with caspase3b exhibiting relatively high fold changes (63.81-104.81) across four tissues. In conclusion, the caspase family in T. fulvidraco exhibits differential expression during bacterial, viral, and parasitic infections through evolutionary conservation and specific divergence. These findings provide a foundation for deciphering apoptotic regulation and disease resistance mechanisms in Siluriformes fish.