Haplotype analysis of the DQA genes in sheep: evidence supporting recombination between the loci.

The ovine class II major histocompatibility complex mediates specific immune responses to exogenous antigens in sheep. A number of ovine class II loci have been identified, and most of them appear to be polymorphic. In this study we investigated the DQA1 locus of 520 sheep and the DQA2 locus of over 40,000 sheep, finding 12 sequences and 22 sequences, respectively, using DQA1- and DQA2-specific PCR primers. Among the DQA2 sequences, 2 groups of sequences can be found: those that share homology with the DQA2 sequences from closely related species and those that cluster with bovine DQA3 and DQA4 sequences and have been called DQA2-like in sheep. The occurrence of these DQA2-like sequences was once again confirmed to correspond with the absence of detectable DQA1 sequences, suggesting that they are found at the same location as DQA1. Within the sheep studied, 37 haplotypes could be detected, 23 being haplotypes of DQA1 and DQA2 sequences and with frequencies ranging from 0.38 to 9.27%, and 14 being haplotypes of DQA2 and DQA2-like sequences and with frequencies ranging from 0.03 to 14.53%. We discovered 12 DQA1-DQA2 combinations that were derived from 5 DQA1 alleles and 4 DQA2 alleles, and 8 DQA2-DQA2-like combinations from 5 DQA2 alleles and 2 DQA2-like sequences. The frequency of occurrence of recombined DQA1-DQA2 sequences and recombined DQA2-DQA2-like sequences is similar, once again suggesting the DQA2-like sequences are found at the DQA1 locus.