ANALYSIS OF POLYMORPHISMS IN THE MAJOR EXPRESSED CLASS-I LOCUS (B-FIV) OF THE CHICKEN

We analyzed the polymorphic nature of eleven alleles expressed by the major class I locus (B-FIV) in chickens. Similar to mammalian class I loci, the nucleotide substitutions with high variability occur in exon s 2 and 3 encoding the alpha 1 and alpha 2 domains. However, the nonsy non ymous to synonymous ratio of nucleotide substitutions in exon 3 en coding the alpha helix and beta sheets is reversed compared with HLA. The region of exon 3 encoding the alpha 2 helix demonstrates a much lo wer nonsynonymous to synonymous ratio, suggesting evolutionary selecti on of a more conserved alpha 2 helix in B-FIV compared with HLA. Amino acid residues with high Wu-Kabat variability are typically located in positions predicted to impact antigen presentation. B-FIV amino acid residues predicted to interact with the CDR1 alpha region of the T-cel l receptor (Tcr) demonstrate less variability than in mouse and human class I alleles. The combination of a reduced nonsynonymous to synonym ous ratio in exon 3 encoding the alpha 2 helix and the limited variabi lity in CDR1 alpha contact residues is discussed with regard to concer ted evolution between a minimal major histocompatibility complex and c ompaction of Tcr variable gene segments in the chicken.