Sato K, Katagiri M
Second Department of Pathology, Ashikawa Medical College, Japan.
Hokkaido Igaku Zasshi. 1994 Mar;69(2):202-16.
The amphibian Xenopus laevis is the most primitive vertebrate in which the major histocompatibility complex (MHC) has been identified both at the biochemical and functional level. In the present study, cDNA clones for MHC class II beta-chain genes of Xenopus laevis were isolated using the polymerase chain reaction. In partially inbred animals with the MHC f haplotype, two distinct cDNA clones assumed to encode isotypes were identified. The deduced MHC class II beta-chain of Xenopus laevis had structures fundamentally similar to those of their mammalian counterparts. The overall amino acid sequence identity between Xenopus laevis and representative mammalian MHC class II beta-chains was approximately 41-50%. Sequence comparison of cDNA clones isolated from outbred individuals and the partially inbred frog with the MHC f haplotype showed that nucleotide substitutions are located primarily in the beta 1 domain and that non-synonymous substitutions occur more frequently than synonymous substitutions in the putative antigen binding site. This result indicates that, as in mammalian MHC genes, the MHC class II beta-chain genes of Xenopus laevis are under positive Darwinian selection. Despite the fact that Xenopus laevis is a tetraploid species, its MHC, as defined by serology, immunochemistry, and mixed lymphocyte reaction, is diploidized. Family studies showed that all MHC class II beta-chain genes detectable in the genome of Xenopus laevis segregate with serologically defined MHC haplotypes, suggesting that the second, inactivated set of MHC class II beta-chain genes was either deleted or that their sequences diverged too far to be detected by DNA hybridization. Furthermore, a nearly constant number of MHC class II beta-chain genes was detectable in most, if not all, Xenopus species with various degrees of ploidy. Thus, various polyploid species of Xenopus appear to have used a similar strategy to diploidize their MHC genes.
两栖动物非洲爪蟾是最原始的脊椎动物，在其体内已在生化和功能水平上鉴定出主要组织相容性复合体（MHC）。在本研究中，利用聚合酶链反应分离出了非洲爪蟾MHC II类β链基因的cDNA克隆。在具有MHC f单倍型的部分近交动物中，鉴定出了两个假定编码同种型的不同cDNA克隆。非洲爪蟾推导的MHC II类β链结构与哺乳动物对应物的结构基本相似。非洲爪蟾与代表性哺乳动物MHC II类β链之间的总体氨基酸序列同一性约为41%-50%。从远交个体和具有MHC f单倍型的部分近交蛙中分离出的cDNA克隆的序列比较表明，核苷酸取代主要位于β1结构域，并且在假定的抗原结合位点非同义取代比同义取代更频繁地发生。这一结果表明，与哺乳动物MHC基因一样，非洲爪蟾的MHC II类β链基因处于正达尔文选择之下。尽管非洲爪蟾是四倍体物种，但其通过血清学、免疫化学和混合淋巴细胞反应定义的MHC是二倍体化的。家系研究表明，在非洲爪蟾基因组中可检测到的所有MHC II类β链基因都与血清学定义的MHC单倍型分离，这表明第二套失活的MHC II类β链基因要么被删除，要么其序列差异太大以至于无法通过DNA杂交检测到。此外，在大多数（如果不是全部）具有不同倍性程度的非洲爪蟾物种中，可检测到数量几乎恒定的MHC II类β链基因。因此，非洲爪蟾的各种多倍体物种似乎采用了类似的策略来使其MHC基因二倍体化。
