Phylogenetic diversification of immunoglobulin genes and the antibody repertoire

Immunoglobulins are encoded by a large multigene system that undergoessomatic rearrangement and additional genetic change during the developmentof immunoglobulin-producing cells. Inducible antibody and antibody-likeresponses are found in all vertebrates. However, immunoglobulin possessingdisulfide-bonded heavy and light chains and domain-type organization hasbeen described only in representatives of the jawed vertebrates. Highdegrees of nucleotide and predicted amino acid sequence identity areevident when the segmental elements that constitute the immunoglobulin geneloci in phylogenetically divergent vertebrates are compared. However, theorganization of gene loci and the manner in which the independent elementsrecombine (and diversify) vary markedly among different taxa. One strikingpattern of gene organization is the "cluster type" that appears to berestricted to the chondrichthyes (cartilaginous fishes) and limitssegmental rearrangement to closely linked elements. This type of geneorganization is associated with both heavy- and light-chain gene loci. Insome cases, the clusters are "joined" or "partially joined" in the germline, in effect predetermining or partially predetermining, respectively,the encoded specificities (the assumption being that these are expressed)of the individual loci. By relating the sequences of transcribed geneproducts to their respective germ-line genes, it is evident that, in somecases, joined-type genes are expressed. This raises a question about theexistence and/or nature of allelic exclusion in these species. Theextensive variation in gene organization found throughout the vertebratespecies may relate directly to the role of intersegmental(V<==>D<==>J) distances in the commitment of the individualantibody-producing cell to a particular genetic specificity. Thus, theevolution of this locus, perhaps more so than that of others, may reflectthe interrelationships between genetic organization and function.