Antimicrobial peptides (AMPs), also known as host defence peptides, are evolutionarily ancient defensive weapons that can combat infections caused by pathogens. Fish‐derived AMPs have shown microbicidal properties, host immunomodulatory responses, and are induced by a variety of factors, especially pathogenic infection and abiotic stress, which may activate downstream signalling pathways to initiate the expression of antimicrobial peptide genes. Large‐scale genome sequencing has been applied in many aquatic animals, providing databases for the systematic identification of putative AMPs in these species. Recently, the whole genome and the proteome of channel catfish (Ictalurus punctatus) were released; by taking advantage of the antimicrobial peptide database and combining the proteome of catfish, we established a bioinformatic analysis pipeline to identify catfish AMP repertoire. This review introduces an effective method for the identification and development of putative AMPs based on protein datasets, and summarizes the structural properties, immunomodulation and molecular responses in vivo of catfish AMPs. In addition, the evaluation of the antibacterial activity of synthetic or isolated AMPs in vitro, and the applications of AMPs in fish via genetic engineering and clustered regularly interspaced short palindromic repeats associated proteins system (CRISPR/Cas9) were presented, which supports antimicrobial research and aquaculture therapy. The summary and comparison of these AMPs will enhance our understanding and their cross‐species applications. Combined with current genetic engineering and genome editing techniques, it is possible to promote the development of immune processes to protect valuable aquatic animals from infectious diseases.

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The X and Y chromosomes of channel catfish have the same gene contents. Here, we report allelic hypermethylation of the X chromosome within the sex determination region (SDR). Accordingly, the X-borne hydin-1 gene was silenced, whereas the Y-borne hydin-1 gene was expressed, making monoallelic expression of hydin-1 responsible for sex determination, much like genomic imprinting. Treatment with a methylation inhibitor, 5-aza-dC, erased the epigenetic marks within the SDR and caused sex reversal of genetic females into phenotypic males. After the treatment, hydin-1 and six other genes related to cell cycle control and proliferative growth were up-regulated, while three genes related to female sex differentiation were down-regulated in genetic females, providing additional support for epigenetic sex determination in catfish. This mechanism of sex determination provides insights into the plasticity of genetic sex determination in lower vertebrates and its connection with temperature sex determination where DNA methylation is broadly involved.

Previous studies have demonstrated that CRISPR/Cas9‐mediated genome editing and transgenesis by integrating vector‐engineered antimicrobial peptide genes (AMGs) are effective to modulate the fish's innate immune system. To generalise the knowledge of AMG application in aquaculture, we recruited 544 data entries from a pool of empirical studies, which included 23 studies (two unpublished from our team) spanning 12 diseases. We systematically re‐processed and re‐analysed these data by harnessing a cross‐disease meta‐analysis. On aggregate, AMG‐genetic engineering aimed at enhancing disease resistance was shown to decrease the number of colony‐forming units of bacteria from fish tissues, increase post‐infection survival rates, and alter the expression of AMGs and immune‐related genes. Further, AMG pathogen‐combating activity was triggered within 2 h after infection and lasted 48 h. The overexpression of AMGs was highest in the spleen and skin, followed by the kidney and liver during this period. Typically, regardless of the type of AMGs, the synergistic expression of AMGs with IL‐10, IKβ, TGFβ, C3b and TLR genes in AMG‐integrated fish contributed to activating inflammatory/immune responses against pathogens. Our findings revealed that the efficiency of transgenic AMGs against pathogens was fish‐, pathogen‐ and AMG‐specific. Compared to bacteria, transgenic AMGs were less inhibitory to viruses and parasites. In addition, innovative CRISPR/Cas9‐based transgenesis enabling the site‐directed knock‐in of foreign genes at multiple loci was assessed for the enhancement of disease resistance in combination with other favourable fish‐production traits, including fast growth, sterility and enriched fatty‐acid content. Altogether, our findings indicated that AMGs as transgenes have substantial promise to modulate the fish's innate immune system and enhance disease resistance.