Evolution of Duplicated IgH Loci in Atlantic salmon, Salmo salar

Background: The Atlantic salmon (Salmo salar) immunoglobulin heavy chain (IgH) locus possesses two parallel IgHisoloci (IGH-A and IGH-B), that are related to the genomic duplication event in the family Salmonidae. Theseduplicated IgH loci in Atlantic salmon provide a unique opportunity to examine the mechanisms of genomediversity and genome evolution of the IgH loci in vertebrates. In this study, we defined the structure of these lociin Atlantic salmon, and sequenced 24 bacterial artificial chromosome (BAC) clones that were assembled into theIGH-A (1.1 Mb) and IGH-B (0.9 Mb) loci. In addition, over 7,000 cDNA clones from the IgH variable (VH) region havebeen sequenced and analyzed.Results: The present study shows that the genomic organization of the duplicated IgH loci in Atlantic salmondiffers from that in other teleosts and other vertebrates. The loci possess multiple Cτ genes upstream of the Cμregion, with three of the Cτ genes being functional. Moreover, the duplicated loci possess over 300 VH segmentswhich could be classified into 18 families. This is the largest number of VH families currently defined in anyvertebrate. There were significant structural differences between the two loci, indicating that both IGH-A and -B locihave evolved independently in the short time after the recent genome duplication approximately 60 mya.Conclusions: Our results indicate that the duplication of the IgH loci in Atlantic salmon significantly contributes tothe increased diversity of the antibody repertoire, as compared with the single IgH locus in other vertebrates.