Livestock and Wildlife Laboratory LR16IRA04, Arid Lands Institute, Medenine, Tunisia.
Department of Life Sciences, Faculty of Sciences, University of Gabès, Gabès, Tunisia.
Immunogenetics. 2023 Aug;75(4):355-368. doi: 10.1007/s00251-023-01303-x. Epub 2023 Apr 4.
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单峰驼（Camelus dromedarius）的 MHC DRB 基因。结果表明，在 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。这两个基因相距 155kb，具有相似的基因结构，且转录方向相反。与 DRB1 相比，DRB2 基因座在第二外显子中缺失了 12 个核苷酸（270bp），转录丰度较低，并且作为两个通过外显子 2 跳跃差异表达的剪接变体。该基因在单峰驼中似乎具有较小的功能相关性。相反，DRB1 被认为是该物种的主要基因，具有更高的转录丰度和多态性水平。在突尼斯单峰驼中鉴定出了七个 DRB1 外显子 2 等位基因，导致 18 个氨基酸取代。在 mRNA 水平上对六个全长等位基因进行了特征描述。尽管没有明确的平衡选择（即杂合优势）的证据，但对 DRB1 基因的弱历史正选择的信号被检测到，这表明正选择的位点数量有限。这种趋势可能与该物种较低的病原体暴露和种群历史有关。与双峰驼和野生骆驼基因组的比较分析表明，在骆驼属中存在跨物种多态性（TSP）。由于开发的基因分型方案完全适用于这三个骆驼物种，因此本研究为该属 MHC DRB1 遗传多样性分析奠定了基础。
