Molecular cloning and phylogenetic analysis of the Atlantic salmon immunoglobulin D gene

A gene homologous to the IgD heavy chain (delta) gene in channel catfish (I ctalurus punctatus) was found 0.9 kb downstream of the IgM heavy chain (mu) gene in Atlantic salmon (Salmo salar). As in catfish, the first constant m u exon is spliced into the delta transcripts. In agreement with the tetrapl oid ancestry of the salmonid fish family there are two highly similar delta genes in Atlantic salmon. Characterization of these genes showed that they encode seven 'unique' Ig domains, three of which are tandem duplicated, i. e. like delta 1-(delta 2-delta 3-delta 4)*-(delta 2-delta 3-delta 4)-delta 5-delta 6-delta 7. Sequence analysis indicates that delta 1-delta 7 arose f rom two duplication events. Accordingly, salmon delta can be reduced to a u nit of three Ig domains corresponding to the three C-terminal domains of a prototypic Ig molecule. The ancestral three-domain unit is apparently best conserved in delta 1-delta 5-delta 6. Phylograms indicate a relationship be tween teleost and mammalian IgD mainly because of the similarity between th e teleost delta 5 and human delta 2. The corresponding domain in mouse IgD has been deleted during evolution. The teleost delta 1 and delta 6 sequence s are most similar to domains of other non-IgM isotypes, including those in cartilaginous fishes.