Allelic polymorphism of Immunoglobulin heavy chain genes in the Antarctic teleost Trematomus bernacchii

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.