Structure and organization of the immunoglobulin M heavy chain genes in Atlantic salmon, Salmo salar.

To determine the structure and organization of the germline immunoglobulin M heavy chain (IgH) genes in Atlantic salmon, Salmo salar, relevant clones from a genomic library (of one individual fish) have been characterized. Two closely related IgH constant region genes, CHA and CHB, have been sequenced completely. In addition, an allotypic variant of CHA was identified and partially sequenced. Five joining (JH) elements were found in a distance of 0.5-1.6 kb upstream of the first constant exon (CH1), in both CHA and CHB, substantiating the hypothesis that the entire gene complex is duplicated; possibly a remnant of a tetraploid event in the salmonid ancestor. An octamer motif (ATGTATTT, and its reverse complementary sequence) was found to be dispersed in the JH-CH1 region, but not elsewhere, signifying a role in these loci. Four closely related variable (VH) genes which were subcloned from three distinct lambda clones showed the classical structure of a two exon unit split by a 100 bp intron. The split-intron and a few hundred base pairs of the flanking sequences of the genes were highly similar. Three of the four genes were interrupted by stop codons and/or frame shifts, indicating a high proportion of VH-pseudogenes in this species. Based on the present results, and comparison with sequences of rainbow trout, Oncorhynchus mykiss, it is likely that the IgH loci have remained tetrasomicly inherited throughout the radiation of the genus Salmo and Oncorhynchus, and that the duplicated loci have gone into a disomic inheritance pattern in the comparatively recent past.