The Major Histocompatibility Complex in Old World Camelids and Low Polymorphism of its Class II Genes

This study brings first information on the major histocompatibility complex (MHC) of three species of Old World camelids, Camelus bactrianus, Camelus dromedarius and Camelus ferus. Physical mapping located the MHC region to the chromosome 20 in Camelus bactrianus. Cytogenetic and comparative analyses of whole genome sequences showed that the order of the three major sub-regions on this chromosome is “Centromere - Class II – Class III – Class I”. The overall genomic organization is highly similar to the cattle MHC region, including the presence of DYA exon 2 genomic sequences, identified in the three camel species. However, full-length DYA gene sequences could not be retrieved from the sequences available. The structure and functional importance of this region for camels thus remain unknown. All Bactrian camels analyzed had a stop codon in the DYA exon 2 reading frame, while in the dromedaries, a single nucleotide polymorphism with one potentially functional allele was observed in this nucleotide position.

DRA, DRB, DQA, DQB and DYA exon 2 sequences encoding the antigen binding site of the corresponding class II antigen presenting molecules showed high degree of sequence similarity and extensive allele sharing across the three species. Unexpectedly low extent of polymorphism with low numbers of alleles and haplotypes was observed in all species, despite different geographic origins of the camels analyzed. Interestingly, the DRA locus was found to be polymorphic, with three alleles shared by all three species. DRA and DQA sequences retrieved from ancient DNA samples of Camelus dromedarius suggested that additional polymorphism might exist.