Structure and organization of pig MHC class II DRB genes: evidence for genetic exchange between loci.

The pig major histocompatibility complex DRB genes were studied by polymerase chain reaction (PCR) amplification of exon 2 from eight domestic pigs and two European wild boars. Sequence comparisons together with a phylogenetic analysis showed the existence of at least three DRB genes of which only one appears to be expressed. The two putative DRB pseudogenes contained deletions in exon 2, making it possible to confirm the presence of three non-allelic DRB genes by analyzing the length polymorphism of the amplified PCR products. The expressed gene shows allelic polymorphism at the same positions as in the human DRB1 gene. In addition, this pig gene shows extensive allelic polymorphism at positions 84-88, whereas, e.g., human DRB genes do not. Surprisingly, the two putative DRB pseudogenes also display a considerable amount of allelic polymorphism, albeit of a different character as compared with the expressed DRB gene. Short stretches of sequences are shared between individual alleles at different loci. These sequence similarities cannot be due to natural selection, since two of the three DRB genes involved are polymorphic pseudogenes constituting allelic series that have diverged after the inactivation event. Instead, the results indicate that the sequences have been exchanged between the DRB genes by intergenic recombination.