Cloning and sequencing of expressed DRB genes of the red deer (Cervus elaphus) Mhc.

The expressed major histocompatibility complex (Mhc) class II DRB genes of 50 unrelated deer were examined by reverse transcription polymerase chain reaction, cloning, and sequencing of DRB exon 2. Deer, like other mammals, have at least one highly polymorphic Mhc class II DRB gene. Thirty-four different sequences were identified. Most of the variation in amino acid composition occurred at positions that have been shown to form the peptide binding site (PBS). Eighteen deer-specific substitutions were found, 11 of these occurred in the PBS. Significantly higher rates of replacement substitutions than silent substitutions were found in the deer sequences, indicating strong positive selection pressure for diversity in DRB sequences. Between one and four DRB sequences were found per deer. Inheritance of these sequences in pedigrees showed Mendelian segregation with up to two expressed DRB genes per haplotype. Sheep are the only other ruminant in which the presence of more than one expressed DRB gene has been demonstrated. Phylogenetic trees were constructed in an attempt to assign the deer DRB sequences to specific loci, but no clear segregation of the DRB sequences for different loci was found. It would seem likely that sequence exchange between the loci has occurred. As has been shown in other species, the alpha-helix and beta-sheet regions of exon 2 appeared to have different evolutionary histories.