Genetic analysis of the ovine major histocompatibility complex and association with footrot

An investigation of the ovine major histocompatibility complex (MhcOvar) was conducted using human-derived MHC class II D-region gene sequences. A human HLA-DQA probe (pII-α-4, exons 1 - 5) detected a single Ovar-DQA locus containing three alleles. The HLA-DQA probe (pII-a-5, exons 2 - 5) detected the more variable class II A gene region; Ovar-DQA. Detection of the Ovar-DRB and Ovar-DRB gene regions using the human derived probes HLA-DRB (pII-β-1, exons 1 - 6) and HLA-DRB (pII-β-3, exons 1 - 6) respectively, was partially obscured by crossreactivity, due to high sequence homology between the D region B genes. A necessity of any segregation study is the verification of pedigree data. An investigation of ovine 'DNA fingerprints' generated using minisatellite probes 33.6 (Jeffreys et al., 1985b) and pV47-2 (Longmire et al., 1990) revealed the latter to detect the more polymorphic minisatellite population within the ovine genome, with a lower bandsharing probability between unrelated individuals (x = 0.46), and consequently a higher exclusion probability (5.1 x 10⁻³) that any two unrelated individuals will share the same fingerprint. This made pV47-2 a more suitable multilocus probe for ovine pedigree verification than 33.6. A putative Coopworth family's relatedness was investigated. Analysis based on DNA fingerprints generated using 33.6 and pV47-2, indicated that both offspring appeared to be siblings, although non-identical, with initial assignment of these offspring 641/0 and 642/0 to the parents 722/8 and 696/7 appearing correct. However there exists a possibility (between 2.2 - 5.2%) that purely by chance the offspring's fingerprints fit with those of the assigned parents. Using this Coopworth family, Mendelian inheritance was observed for all RFLP' s recognised across the four detectable MhcOvar class II gene regions, consequently detection of these restriction fragment length polymorphisms reflect stable heritable sequences (alleles) within which lie MhcOvar loci. Genetic marker analysis was performed with the MHC class II gene sequences: HLA-DQA (pII-α-4, exons 1 - 5), Ovar-DQA1 and Ovar-DQA2 (exon 2) by hybridisation to restricted ovine genomic DNA from three populations containing unrelated Romneys, Corriedales and Merinos. The Merinos were subdivided into those that had succumbed to Dichelobader nodosus (footrot) infection following field challenge. The Ovar-DRA locus probed with HLA-DRA detected one allele (B) at high population frequency, while another (allele C) appeared to be variable in frequency between breeds, with Romneys containing a significantly higher proportion (P < 0.01). No association was observed between the Ovar-DRA locus and an individuals footrot status. Six alleles were detected at each of the Ovar-DQA1 and Ovar-DQA2 loci. No significant difference in allele frequencies across the three breeds was observed. A significant proportion of individuals without a history of D.nodosus infection contain a genotype with allele C (1.8kb) at the Ovar-DQA1 locus (P < 0.05). Conversely, no association was observed between alleles at the Ovar-DQA2 locus and an individuals footrot status