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 (Ictalurus 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 mu exon is spliced into the delta transcripts. In agreement with the tetraploid 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 delta1-(delta2-delta3-delta4)*-(delta2- delta3-delta4)-delta5-delta6-d elta7. Sequence analysis indicates that delta1-delta7 arose from two duplication events. Accordingly, salmon delta can be reduced to a unit 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 delta1-delta5-delta6. Phylograms indicate a relationship between teleost and mammalian IgD mainly because of the similarity between the teleost delta5 and human delta2. The corresponding domain in mouse IgD has been deleted during evolution. The teleost delta1 and delta6 sequences are most similar to domains of other non-IgM isotypes, including those in cartilaginous fishes.