Genome-Wide Identification and Expression Analysis of Duplicated MRC1 Gene in Cyprinus carpio

Mannose receptor C-type 1 (MRC1) is a member of the C-type lectin superfamily that encodes the mannose receptor. This pattern recognition receptor plays a key role in the innate immune response. The role of the MRC1 gene in the immune response of mammals has been extensively investigated but less so in fish. The emergence of high-density intensive culture in recent years has increased the frequency with which diseases caused by pathogenic microorganisms have occurred. Aeromonas hydrophila is one of the most prevalent pathogenic bacteria. In this study, 11 copies of the MRC1 gene were identified in Cyprinus carpio for the first time. Domain prediction, collinearity analysis, multiple sequence alignment, and phylogenetic analysis were conducted. The results showed that the MRC1 gene has been highly conserved during evolution. We found that the MRC1 gene exhibited varying degrees of the multicopy phenomenon in different species, ranging from 2 copies in most fish to 11 copies in C. carpio. Simultaneously, the expression level of each copy in the brain, muscle, liver, and spleen tissues of healthy carp was examined and compared. It was discovered that immune-related tissue expression was relatively higher in the spleen than that in other tissues. Further comparative analysis of expression in the spleen at 4 h, 12 h, and 24 h after infection with A. hydrophila revealed that the expression characteristics of different copies were distinct, with the expression of HHLG13g0734 being significantly up-regulated after 4 h of infection, HHLG13g0734 being significantly up-regulated after 24 h of infection, and HHLG3g0497 being significantly down-regulated throughout the infection. It indicated that only a portion of the MRC1 gene of C. carpio retained immune-related functions and participated in the immune response. Our findings contribute to a better understanding of the immune function of the MRC1 gene during the process of defending against A. hydrophila infection and serve as a fundamental data reference for the molecular-assisted breeding of new disease-resistant strains.