Ig and T cell receptor (TCR) genes consist of separate genomic elements, which must undergo rearrangement and joining before a functional protein can be expressed. Considerable plasticity in the genomic arrangement of these elements has occurred during the evolution of the immune system. In tetrapods, all Ig and TCR chain elements are arranged as translocons. In teleosts, the Ig heavy and TCR chains are translocons, but light chain genes may occur as clusters. However, in chondrichthyes, all of the Ig light and heavy chain genes are arranged as clusters. These clusters vary in number from <10 to several hundred, depending on isotype and species. Here, we report that the germ-line gene for the TCR ␥ chain in a chondrichthyan, the sandbar shark (Carcharhinus plumbeus), is present as a single locus arranged in a classic translocon pattern. Thus, the shark utilizes 2 types of genomic arrangements, the unique cluster organization for Ig genes and the ''conventional'' translocon organization for TCR genes. The TCR ␥ translocon contains at least 5 V region genes, 3 J segment genes, and 1 C segment. As expected, the third hypervariable segment (CDR3), formed by the rearrangement of the V␥ and J␥ segments, contributed the major variability in the intact V region structure. Our data also suggest that diversity may be generated by mutation in the V regions.diversity ͉ translocon M odels proposed for the molecular evolution of the immune system are based on comparative studies of living species. Sharks are critical in this regard, because they belong to an ancient clade (chondrichthyans) that can be traced in the fossil record to the time of the earliest vertebrates. Approximately 450 million years ago, the gnathostomes diverged into 2 groups, the chondrichthyans and the line leading to modern teleosts and tetrapods. It can be concluded that the molecular components of the immune system are ancient and arose before this divergence. This conclusion follows from studies showing that all of the defining elements of the immune system, antibodies, T cell receptors (TCRs), MHC products, and recombination activator genes (RAG), are present in chondrichthyes (1, 2). Thus, continued studies of sharks and rays, the most distant living relatives of mammals with a vertebrate type (VDJ-C recombination) immune system, should provide insights into the molecular origins and evolution of the immune system.A surprising feature of the organization of shark Ig genes was that the V-J-C of light chains (3-5) and V-D-D-J-C of heavy chains (6-8) are arranged in individual clusters. This feature is in contrast to the translocons of higher vertebrates that consist of large arrays of multiple V segments distantly linked to several J and D segments, as well as C domains (9). Although the genes for the TCR chains ␣/ and ␥/␦ have been shown to be present in the skate (10) and horned shark (11, 12), the germ-line loci for these molecules have been only partially characterized. Analyses of cDNA sequences in the skate revealed multiple V region families, ...
