Genomic organisation and allelic diversity within coding and non-coding regions of the Ovar-DRB1 locus

In all but the most primitive vertebrates, multiple polymorphic genes associated with lymphocyte-mediated immunological surveillance are linked together within genomic regions termed the major histocompatibility complex (MHC). The extensive diversity at many MHC loci provides a valuable source of genetic markers for examining the complex relationships between host genotype and disease resistance or susceptibility. Such studies in domestic sheep (Ovis aries) have generally focused on exon 2 of the polymorphic class II MHC DRB1 gene and its adjacent sequences. We have determined the complete genomic sequences of two Ovar-DRB1 alleles, which has allowed an analysis of diversity at coding, non-coding and promoter regions of this gene. On the basis of these sequences, oligonucleotide primers were designed for amplification of full-length Ovar-DRB1 transcripts and used to evaluate diversity within an MHC-defined resource flock maintained at the Moredun Institute. We describe nine novel full-length Ovar-DRB1 sequences along with an improved direct-sequencing method for analysis of the entire exon 2 region of the Ovar-DRB1 gene based on previously unknown intronic sequences. We discuss how these data provide evidence on the evolution of MHC DRB diversity in domestic sheep.