Haplotype variation, recombination, and gene conversion within the turkey MHC-B locus

The major histocompatibility complex (MHC) is a gene dense region with profound effects on the disease phenotype. In many species, characterizations of MHC polymorphisms have focused on identifying allelic haplotypes of the highly polymorphic class I and class II loci through direct immunological approaches such as monoclonal antibodies specific for the major antigens or indirectly through DNA sequence-based approaches. Invariably, these studies fail to assess the broader range of variation at the other loci within the MHC. This study examines variation in the turkey MHC by resequencing 15 interspersed amplicons (∼14 kb) spaced across the MHC-B locus in a representative sampling of 52 commercial birds. Over 200 single nucleotide polymorphisms (SNPs) were identified with high levels of polymorphism (1 SNP/70 bp) and heterozygosity (average minor allele frequency of 0.15). SNP genotypes were used to identify the major haplotypes segregating in the commercial lines. Sequencing of the peptide binding region (PBR, exon 2) of the class IIB loci of select individuals identified 10 PBR alleles/isotypes among the major MHC haplotypes. Examination of pedigreed families provides direct evidence of gene conversion and recombination within the B locus. Results of this study demonstrate the MHC diversity available in commercial flocks and provide genomic resources for studying the effect of this diversity (alleles and/or haplotypes) on disease susceptibility and resistance.