Models proposed for the molecular evolution of the immune system are based on comparative studies of living species. Our data also suggest that diversity may be generated by mutation in the V regions. As expected, the third hypervariable segment (CDR3), formed by the rearrangement of the Vγ and Jγ segments, contributed the major variability in the intact V region structure.
The TCR γ translocon contains at least 5 V region genes, 3 J segment genes, and 1 C segment. Thus, the shark utilizes 2 types of genomic arrangements, the unique cluster organization for Ig genes and the “conventional” translocon organization for TCR genes. Here, we report that the germ-line gene for the TCR γ chain in a chondrichthyan, the sandbar shark ( Carcharhinus plumbeus), is present as a single locus arranged in a classic translocon pattern. These clusters vary in number from <10 to several hundred, depending on isotype and species. However, in chondrichthyes, all of the Ig light and heavy chain genes are arranged as clusters. In teleosts, the Ig heavy and TCR chains are translocons, but light chain genes may occur as clusters. In tetrapods, all Ig and TCR chain elements are arranged as translocons. Considerable plasticity in the genomic arrangement of these elements has occurred during the evolution of the immune system. 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.
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