Characterization and genetic diversity of MHC class II DRB genes in the Arabian camel (Camelus dromedarius)

This study investigated the MHC DRB genes in the Arabian camel (Camelus dromedarius). The results revealed the presence of - at least - two transcribed DRB-like genes in chromosome 20, designated MhcCadr-DRB1 and MhcCadr-DRB2. These genes are 155 Kb apart, have similar gene structure, and are transcribed in opposite directions. Compared to DRB1, the DRB2 locus contains a deletion of 12 nucleotides in the second exon (270 bp), exhibits lower transcript abundance, and is expressed as two splice variants differing by exon 2 skipping. This gene seems to be of minor functional relevance in the dromedary camel. Conversely, the DRB1 is thought to be the main gene in this species showing higher transcript abundance and polymorphism levels. A total of seven DRB1 exon 2 alleles were identified in the Tunisian dromedary camel resulting from 18 amino acid substitutions. Six full length alleles were characterized at the mRNA level. Although there is no clear evidence for balancing selection (i.e., heterozygote advantage), signals of weak historical positive selection acting on the DRB1 gene were detected, as indicated by the limited number of the sites being positively selected. This trend might be related to the low exposure to pathogens and to the demographic history of the species. Comparative analysis with Bactrian and wild camel genomes suggested occurrence of trans species polymorphism (TSP) in the Camelus genus. The results lay the foundation for the MHC DRB1 genetic diversity analysis in this genus since the developed genotyping protocols are fully applicable in the three Camelus species.