Characterization of immunoglobulin light chain genes in the sandbar shark, Carcharhinus plumbeus.

Abstract

In order to understand the evolution of the rearranging immunoglobulin system, it is necessary to examine living representatives of the most primitive vertebrate phyla. Immunoglobulins are the major recognition and defense molecules of the humoral immune response and are found in all vertebrates. While early studies demonstrated that the general structure of immunoglobulins has remained relatively unchanged throughout evolution, the organization of their encoding genes differs dramatically. Elasmobranchs, which include the sharks, skates, and rays, are the most ancient phylogenetic class of vertebrates from which immunoglobulin DNA sequences have been obtained. The Carcharhinoid sharks are of considerable interest for evolutionary studies because they are an old order whose ancestors date back to the Jurassic period. Immunoglobulin light chain genes of the sandbar shark (Carcharhinus plumbeus) were characterized as to their DNA sequence as well as their number and arrangement within the genome. Sequence of a cDNA clone encoding sandbar shark light chain demonstrates that their Ig light chains are homologous to mammalian λ chains with shark sequences sharing ∼40-50% identity with human λ chains. Analysis of sandbar shark genomic light chain clones by mapping and DNA sequencing demonstrates that sharks have a unique Ig gene arrangement system in which the genes are organized into clusters or cassettes spanning 4.3 to 6 kilobases and contain a single variable (V), joining (J), and constant (C) gene. The light chain clusters can be divided into two patterns based upon spacing differences between the J and C genes. A unique finding of this study is that the V and J genes are fused within the germline. PCR analysis of genomic DNA extends this finding, demonstrating that VJ fusion is the predominant organizational feature of sandbar shark immunoglobulin light chain genes. This finding raises questions concerning the necessity of recombination to produce an antibody repertoire capable of reacting against a diverse array of antigens. While such fusion may initially suggest a lack of light chain diversity in these animals, the results of this study strongly supports the hypothesis that sandbar sharks can potentially express a highly diverse light chain repertoire.