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.

本研究调查了阿拉伯骆驼 ( Camelus dromedarius ) 的MHC DRB基因。结果表明，20 号染色体上至少存在两个转录的 DRB 样基因，分别命名为MhcCadr-DRB1和MhcCadr-DRB2。这些基因相距 155 Kb，具有相似的基因结构，并且转录方向相反。与DRB1相比，DRB2基因座在第二外显子(270bp)中包含12个核苷酸的缺失，表现出较低的转录物丰度，并且表达为因外显子2跳跃而不同的两个剪接变体。该基因似乎与单峰骆驼的功能相关性较小。相反，DRB1被认为是该物种中表现出较高转录丰度和多态性水平的主要基因。在突尼斯单峰骆驼中鉴定出总共 7 个DRB1外显子 2 等位基因，这些等位基因由 18 个氨基酸取代产生。在 mRNA 水平上表征了六个全长等位基因。尽管没有明确的证据证明平衡选择（即杂合子优势），但检测到作用于DRB1基因的弱历史正选择信号，如被正选择的位点数量有限所示。这种趋势可能与病原体接触率低和该物种的人口统计历史有关。双峰驼和野骆驼基因组的比较分析表明，骆驼属中存在跨物种多态性（TSP）。该结果为该属的 MHC DRB1遗传多样性分析奠定了基础，因为开发的基因分型方案完全适用于三个骆驼物种。