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Open Access Research article

Comprehensive analysis of MHC class I genes from the U-, S-, and Z-lineages in Atlantic salmon

Morten F Lukacs1, Håvard Harstad1, Hege G Bakke2, Marianne Beetz-Sargent3, Linda McKinnel3, Krzysztof P Lubieniecki4, Ben F Koop3 and Unni Grimholt2*

Author affiliations

1 Department of Basic Science and Aquatic Medicine, Norwegian School of Veterinary Science, Oslo, Norway

2 Centre for Ecology and Evolutionary Synthesis, Dept of Biology, University of Oslo, Norway

3 Department of Biology, University of Victoria, Victoria BC V8W 2Y2, Canada

4 Department of Molecular Biology & Biochemistry, Simon Fraser University, Burnaby BC, Canada

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Citation and License

BMC Genomics 2010, 11:154  doi:10.1186/1471-2164-11-154

Published: 5 March 2010

Abstract

Background

We have previously sequenced more than 500 kb of the duplicated MHC class I regions in Atlantic salmon. In the IA region we identified the loci for the MHC class I gene Sasa-UBA in addition to a soluble MHC class I molecule, Sasa-ULA. A pseudolocus for Sasa-UCA was identified in the nonclassical IB region. Both regions contained genes for antigen presentation, as wells as orthologues to other genes residing in the human MHC region.

Results

The genomic localisation of two MHC class I lineages (Z and S) has been resolved. 7 BACs were sequenced using a combination of standard Sanger and 454 sequencing. The new sequence data extended the IA region with 150 kb identifying the location of one Z-lineage locus, ZAA. The IB region was extended with 350 kb including three new Z-lineage loci, ZBA, ZCA and ZDA in addition to a UGA locus. An allelic version of the IB region contained a functional UDA locus in addition to the UCA pseudolocus. Additionally a BAC harbouring two MHC class I genes (UHA) was placed on linkage group 14, while a BAC containing the S-lineage locus SAA (previously known as UAA) was placed on LG10. Gene expression studies showed limited expression range for all class I genes with exception of UBA being dominantly expressed in gut, spleen and gills, and ZAA with high expression in blood.

Conclusion

Here we describe the genomic organization of MHC class I loci from the U-, Z-, and S-lineages in Atlantic salmon. Nine of the described class I genes are located in the extension of the duplicated IA and IB regions, while three class I genes are found on two separate linkage groups. The gene organization of the two regions indicates that the IB region is evolving at a different pace than the IA region. Expression profiling, polymorphic content, peptide binding properties and phylogenetic relationship show that Atlantic salmon has only one MHC class Ia gene (UBA), in addition to a multitude of nonclassical MHC class I genes from the U-, S- and Z-lineages.