Identification of a novel immunoglobulin delta transcript and comparative analysis of the genes encoding IgD in Atlantic salmon and Atlantic halibut.

Atlantic salmon possesses two parallel Ig heavy chain gene complexes, A and B, most probably as a result of ancestral tetraploidy. Consequently, there are two distinct IgD heavy chain (delta) subvariants in this species. The Igdelta(B) gene was characterised in a previous study. In the present work the Igdelta(A) gene was amplified by PCR and sequenced. Both Igdelta genes in salmon have a structure like delta1-(delta2-delta3-delta4)(2)-delta5-delta6-delta7-TM1-TM2 and show a high degree of sequence identity (approximately 95%). 3'RACE and RT-PCR analyses performed in the present study indicate that Igdelta transcripts of membrane type are dominating in Atlantic salmon and Atlantic halibut. However, a different transcript, originating from the Igdelta(B) gene in salmon, was identified by PCR. This RNA fragment is spliced between the regular donor/acceptor sites in delta6 and TM2. Cloning and characterisation of cDNA encoding the membrane form of halibut IgD revealed an overall Ig domain structure equivalent to that in salmon. Corresponding duplications of delta2-delta3-delta4 have now been found in three teleost fishes: salmon, halibut and catfish. The tandem duplicated fragments are highly similar within each species, while not being especially conserved between the species. Thus, the duplicated gene fragments have either arisen independently in each species or are subjected to homogenisation by some means.