The major histocompatibility class I locus in Atlantic salmon (Salmo salar L.): polymorphism, linkage analysis and protein modelling

Abstract.

A cDNA library screening using the conserved exon 4 of Atlantic salmon Mhc class I as probe provided the basis for a study on Mhc class I polymorphism in a breeding population. Twelve different alleles were identified in the 82 dams and sires studied. No individual expressed more than two alleles, which corresponded to the diploid segregation patterns of the polymorphic marker residing within the 3′-untranslated tail. Close linkage between the Sasa-UBA and Sasa-TAP2B loci strengthens the claim that Sasa-UBA is the major Mhc class I locus in Atlantic salmon. We found no evidence for a second expressed classical or non-classical Mhc class I locus in Atlantic salmon. A phylogenetic analysis of salmonid Mhc class I sequences showed domains conserved between rainbow trout, brown trout and Atlantic salmon. Evidence for shuffling of the α₁ domain was identified and lineages of the remaining α₂ through the cytoplasmic tail gene segment can be defined. The coding sequence of one allele was found associated with two different markers, suggesting recombination within the 3′-tail dinucleotide repeat itself. Protein modelling of several Sasa-UBA alleles shows distinct differences in their peptide binding domains and enables a further understanding of the functionality of the high polymorphism.