Yuko Ohta

Evolutionary Origins of Immunoglobulin Gene Diversity

During the past several years, our laboratory has identified immunoglobulin genes in species that represent major phylogenetic groups of vertebrates (Litman et al. 1989). Taken together, the descriptions of immunoglobulin genes in chondrichthyes (Litman et al. 1985a; Hinds and Litman 1986; Kokubu et al 1988a), bony fishes (Amemiya and Litman 1990, 1991), amphibians (Schwager et al. 1988b, 1989; Haire et al. 1990, 1991), avians (Reynaud et al. 1987, 1989) and mammals (Blackwell and Alt 1988), as well as ongoing studies in our laboratory involving other phylogenetically important species are revealing, the overall patterns in the evolution of immunoglobulin gene structure and diversity. In all of these species, segmental organization and DNA sequence-mediated selective rearrangement in somatic tissues, the most distinctive features of the immunoglobulin gene system, are conserved. The nucleotide and predicted peptide sequences of individual VH, VL, DH, JH, JL and some CH exons, in most instances, are highly conserved as are the recombination signal sequences (RSSs) flanking the VH, VL, DH, JH and JL segmental elements (Litman et al. 1985a; Hinds and Litman 1986; Kokubu et al. 1988a). In addition, the exon-intron organization of the CH exons in Heterodontus (horned shark), a phylogenetically primitive vertebrate, as well as other chondrichthyes (see below), is equivalent to that described in mammals (Kokubu et al. 1988b). To a certain degree, some of these findings are not unexpected since in many cases the genes in lower vertebrates were detected by cross-hybridization with a mammalian immunoglobulin heavy chain variable region gene-specific probe (Litman et al. 1983, 1985a). Typically, any gene detected using this procedure would have to be at least 60% related at the nucleotide sequence level. Although it is informative to compare the sequences of these genes to one another in an overall sense, it must be emphasized that the VH genes are members of extensively diversified multigene families and few guidelines can be applied to determine whether similarities and/or differences in gene sequence reflect orthologous evolutionary relationships (Litman et al. 1985b).

Evolutionary Development of Immunoglobulin Gene Diversity

In all jawed vertebrate species, antibody diversity is specified by an immunoglobulin monomeric structure consisting of two heavy chains and two light chains. Different immuno-globulin classes are associated with various polymeric configurations of the basic monomer. Immunoglobulin-like heterodimers also occur in the jawless vetebrates (cyclostomes) of which the only extant representatives are the lampreys and hagfishes (Varner and Litman, unpub-lished observations);1-2 primary structure data that would firmly establish these as antibodies, however, are not available presently. During the past several years, our labora-tory has identified immunoglobulin genes in species that are considered to represent signifi-cant departure points in vertebrate phylogeny.3 Taken together with the descriptions of im-munoglobulin genes in avians4,5 and mammals,6 a reasonably complete picture of the over-all evolution of immunoglobulin gene structure and diversity is emerging. In all of these species, the most distinctive features of the immunoglobulin gene system, segmental organiza-tion and selective rearrangement in somatic tissues, are preserved. The structures of indivi-dual heavy chain variable (VH), diversity (DR), joining (JH) and certain constant region (CR) segmental elements are also remarkably similar. Two additional features of higher, vertebrate immunoglobulin genomic organization, the split leader and recombination signal sequences (RSSs), also are found in lower vertebrates. Furthermore, the exon-intron organization of the constant region found in Heterodontus (horned shark), the most phylogenetically primitive vertebrate studied thus far, is equivalent to that described in mammals.7 To a certain degree, some of these findings are not unexpected since in many cases the genes in lower vetebrates were detected by cross-hybridization with a particular mammalian immunoglobulin heavy chain gene probe.8,9