Haplotype exclusion: the unique case presented by multiple immunoglobulin gene loci in cartilaginous fish

Cartilaginous fish represent the most phylogenetically distant species from man in which immunoglobulin and T cell antigen receptor genes have been identified. Immunoglobulin genes in cartilaginous fish are organized in hundreds of clusters, located on different chromosomes and presumably are under independent regulation; large numbers of immunoglobulin gene clusters are germline-joined and thus their expression is not directly dependent on somatic rearrangement. Despite the unusual nature of immunoglobulin gene genetics in these species, preliminary characterization of the transcription products of immunoglobulin loci in single cell isolates is consistent with haplotype exclusion. Certain features of immunoglobulin gene organization and expression in cartilaginous fish are remarkably similar to that of odorant receptors and suggest that at the level of transcriptional regulation, at least two different mechanisms could exist that relate to haplotype exclusion.