Immunoglobulin D (IgD) of Atlantic cod has a unique structure

A new immunoglobulin heavy-chain gene with some homology to mammalian IgD was recently cloned from the channel catfish and Atlantic salmon, two species of teleost fish. We have cloned and sequenced a new H-chain gene from Atlantic cod (Gadus morhua L.) which has clear similarities to these genes, but which also differs in several ways. The similarities of catfish, salmon, and cod delta to the mammalian delta genes are sequence homology, location immediately downstream of IgM (mu), and expression by alternative splicing rather than class switching. A unique feature of catfish, salmon, and cod delta is the chimeric nature of the gene product, as the mu 1 exon is spliced to the delta 1 exon. Several unique features of cod IgD were found: (1) a deletion of the delta 3, delta 4, delta 5, and delta 6 domains described in catfish and salmon IgD, (2) a tandem duplication of a part of the delta locus including the delta 1 and delta 2 domains. (3) the presence of a truncated delta 7 domain downstream of the STM exons, and (4) the separation of the duplicated domains by a short exon (delta y) which has homology to a conserved part of the transmembrane exon 1 (TM1) of some H-chain isotypes. This unique organization of the delta locus of cod probably developed after the evolutionary split from the catfish and salmon branches.