Coscia Maria Rosaria, Giacomelli Stefano, Oreste Umberto
Institute of Protein Biochemistry, CNR Via P. Castellino, 11180131 Naples, Italy.
Mar Genomics. 2012 Dec;8:43-8. doi: 10.1016/j.margen.2012.04.002. Epub 2012 May 9.
IgM represents the main immunoglobulin isotype shared by all teleost fish. However, Antarctic fish IgM possess a peculiar hinge region, which connects the CH2 and CH3 domains, not seen in any other teleost species. In the present study allelic polymorphism of IgM gene of the Antarctic teleost Trematomus benacchii was investigated. By nucleotide sequencing the entire Immunoglobulin heavy chain constant region from ten T. bernacchii individuals, 47 positions were found to be polymorphic. The largest number of polymorphic positions, accounting for 51% of the total, was found to fall within the hinge region. This region not only displayed extensive nucleotide variation, but also length diversity; in fact several sequences were one amino acid shorter as resulting from the usage of a different splice acceptor site of the CH3 exon, as demonstrated by genomic DNA analysis. The Ka/Ks ratios of the polymorphic positions showed typical values higher than 1, indicative of positive selection acting to polymorphic codons to favor amino acid replacements and maintain allelic variants.
IgM是所有硬骨鱼共有的主要免疫球蛋白同种型。然而，南极鱼类的IgM具有一个特殊的铰链区，该区域连接CH2和CH3结构域，这在其他硬骨鱼物种中未见。在本研究中，对南极硬骨鱼伯氏南极鱼IgM基因的等位基因多态性进行了研究。通过对10个伯氏南极鱼个体的整个免疫球蛋白重链恒定区进行核苷酸测序，发现47个位置存在多态性。发现多态性位置数量最多，占总数的51%，位于铰链区内。该区域不仅显示出广泛的核苷酸变异，还存在长度多样性；事实上，如基因组DNA分析所示，由于CH3外显子使用了不同的剪接受体位点，一些序列短了一个氨基酸。多态性位置的Ka/Ks比值显示出典型值高于1，表明正选择作用于多态密码子以促进氨基酸替换并维持等位基因变体。
