Sammut Bénédicte, Marcuz Anne, Pasquier Louis Du
Basel Institute for Immunology, Switzerland.
Eur J Immunol. 2002 Sep;32(9):2698-709. doi: 10.1002/1521-4141(200209)32:9<2698::AID-IMMU2698>3.0.CO;2-U.
The dodecaploid anuran amphibian Xenopus ruwenzoriensis represents the only polyploid species of Xenopus in which the full silencing of the extra copies of the major histocompatibility complex (MHC) has not occurred. Xenopus ruwenzoriensis is a recent polyploid that has evolved within one of the two tetraploid groups of Xenopus through allopolyploidization. Family studies of its MHC haplotype suggested a polysomic inheritance of the MHC class I and II genes. Four class Ia bands can be detected per individual in Southern blot analysis and, similarly, four different cDNA sequences are expressed per individual. The Xenopus class Ia sequences we analyzed belong to only one of the old class I lineages and show a homogenization of their alpha3 domain sequences. This homogenization occurred after speciation within the Xenopus ruwenzoriensis species, either due to gene conversion or inter-alleles/loci recombination.A re-evaluation of the polymorphism of class Ia in Xenopus, by looking at the rate of non-synonymous versus synonymous substitutions, suggests that Xenopus MHC class Ia genes are not under strong overdominant selection. This is a rare situation among vertebrates. The observed polymorphism is most likely due to the interlocus genetic exchanges related to the peculiar mode of speciation of the genus.
十二倍体无尾两栖动物鲁文佐里山爪蟾是爪蟾属中唯一主要组织相容性复合体（MHC）额外拷贝未完全沉默的多倍体物种。鲁文佐里山爪蟾是一种近期形成的多倍体，通过异源多倍体化在爪蟾属的两个四倍体组之一中进化而来。对其MHC单倍型的家系研究表明，MHC I类和II类基因存在多体遗传。在Southern印迹分析中，每个个体可检测到四条Ia类条带，同样，每个个体表达四种不同的cDNA序列。我们分析的爪蟾Ia类序列仅属于一个古老的I类谱系，并且其α3结构域序列呈现出同质化。这种同质化发生在鲁文佐里山爪蟾物种形成之后，可能是由于基因转换或等位基因/基因座间的重组。通过观察非同义替换与同义替换的比率对爪蟾Ia类多态性进行重新评估，结果表明爪蟾MHC Ia类基因并非处于强烈的超显性选择之下。这在脊椎动物中是一种罕见的情况。观察到的多态性很可能是由于与该属特殊物种形成模式相关的基因座间遗传交换所致。
