Association of MHC class II β domain polymorphism in channel catfish (Ictalurus punctatus) with natural infection by Ligictaluridus floridanus

The major histocompatibility complex (MHC) class II β genes are highly polymorphic and trigger the adaptive immune response in fish against parasitic infections. In channel catfish, exon 2 of the HLA (human leukocyte antigens) and H-2 genes encodes the MHC class II β domain. HLA and H-2 have not been documented in channel catfish populations or associated with parasite infections. We used Ligictaluridus floridanus, a common monogenean ectoparasite, to uncover SNPs in channel catfish. This study uses massive sequencing technology to find single nucleotide polymorphisms (SNPs) in a portion of the MHC class II β domain of the HLA and H-2 genes. Based on an adjusted parasite load, we ranked fish as resistant (RR) or susceptible (SS) to L. floridanus natural infection. We recovered SNPs from 58 fish, 32 classified as RR and 26 as SS. We found 13 SNPs, resulting in 12 non-synonymous changes (4 in HLA and 8 in H-2) and one synonymous change in the H-2 gene. The GG homozygote genotype for the alternative SNP HLA:1849444(C/G), which changes an aspartate (D) to glutamate (E), was significantly associated with susceptibility to L. floridanus. Our study identified 11 haplotypes in the HLA gene, with only one identical to the previously reported allele Icpu-DAB*03 (FKDN). We identified four non-synonymous SNPs in the HLA gene, with a significant association with susceptibility to L. floridanus observed in the genotype GG of SNP HLA:1849444(C/G). In the H-2 gene, the nine SNPs identified were non-significant in any test conducted.