Reusch Thorsten B H, Schaschl Helmut, Wegner K Mathias
Max-Planck-Institut für Limnologie, August-Thienemann-Strasse 2, 24306 Plön, Germany.
Immunogenetics. 2004 Sep;56(6):427-37. doi: 10.1007/s00251-004-0704-z. Epub 2004 Aug 21.
Using a bacterial artificial chromosome (BAC) library, we analysed a 99.5 kb genomic segment containing the major histocompatibility class II genes of a teleost, the three-spined stickleback Gasterosteus aculeatus. Experiments with G. aculeatus have provided direct evidence for balancing selection by pathogens and mate choice driving MH class II beta polymorphism. Two sets of paralogous class II alpha genes and beta genes in a tandem arrangement were identified, designated Gaac-DAA/DAB and Gaac-DBA/DBB. Expression analysis of the beta genes using single-strand conformation polymorphism revealed that both gene copies are expressed. Based on an analysis of pairwise nucleotide polymorphisms, we estimate that the duplication into two paralogous class II loci occurred only 1.2-2.4 million years ago, 1-2 orders of magnitude more recently than in other fish, bird or mammalian species. At the 3'-direction of the classical MH loci, we identified another seven genes or gene fragments, two of which (small inducible cytokine, complement regulatory factor) are related to immune function in other vertebrates. None of these genes were associated with MH class II genes in zebrafish, suggesting a markedly different organisation of the MH class II region in sticklebacks, and presumably, across bony fishes. When the nucleotide substitution pattern of the novel class II beta genes was analysed together with a representative sequence sample isolated from fish in northern Germany (n=27), we found that the peptide binding region of the Gaac-DAB and Gaac-DBB loci had undergone an inter-locus gene conversion (P=0.007). In accordance, we found a 10- to 20-fold higher frequency of CpG-islands on the MH class II segment compared to other species, a feature that may be conducive for inter-locus recombination.
利用细菌人工染色体（BAC）文库，我们分析了一段99.5 kb的基因组片段，该片段包含硬骨鱼三刺鱼（Gasterosteus aculeatus）的主要组织相容性复合体II类基因。对三刺鱼的实验为病原体驱动的平衡选择和配偶选择导致MH II类β多态性提供了直接证据。我们鉴定出两组串联排列的II类α基因和β基因旁系同源物，分别命名为Gaac-DAA/DAB和Gaac-DBA/DBB。利用单链构象多态性对β基因进行表达分析，结果显示两个基因拷贝均有表达。基于对成对核苷酸多态性的分析，我们估计两个II类旁系同源基因座的复制发生在仅120万至240万年前，比其他鱼类、鸟类或哺乳动物物种的复制时间晚1至2个数量级。在经典MH基因座的3'方向，我们鉴定出另外7个基因或基因片段，其中两个（小诱导细胞因子、补体调节因子）与其他脊椎动物的免疫功能相关。这些基因在斑马鱼中均与MH II类基因无关，这表明三刺鱼中MH II类区域的组织方式明显不同，可能在硬骨鱼类中也是如此。当将新的II类β基因的核苷酸替换模式与从德国北部鱼类中分离出的代表性序列样本（n = 27）一起分析时，我们发现Gaac-DAB和Gaac-DBB基因座的肽结合区域发生了基因座间的基因转换（P = 0.007）。相应地，我们发现MH II类片段上的CpG岛频率比其他物种高10至20倍，这一特征可能有利于基因座间的重组。
