Evolution of immunoglobulin light chain genes: analysis of Xenopus IgL isotypes and their contribution to antibody diversity.

The amphibian Xenopus laevis expresses several types of immunoglobulin light chain (IgL). cDNA clones for two IgL isotypes, C sigma 1 and C sigma 2, were analysed. C sigma is expressed in spleen and mitogen-stimulated B cells, like another Xenopus IgL type, termed C rho. C sigma shares less than 33% residues with C rho or with CL regions of shark, chicken and mammals. This suggests that C sigma diverged from a common ancestor of CL regions before or at the emergence of amphibians. Two families of VL elements, V sigma 1 and V sigma 2 are associated with C sigma (but not with C rho). They rearrange to their own set of JL elements, J sigma 1 and J sigma 2, which are poorly related to other J elements of the Ig gene family. The Xenopus genome contains a few V sigma 2 and multiple V sigma 1 elements (comparable with mammalian V kappa), but only two C sigma genes. Thus, the organization and expression of Xenopus IgL loci are apparently similar to mammalian IgL loci but different from shark and chicken IgL loci. Only a few VL elements are expressed, since cDNA clones show extensive sharing of CDR1 and CDR2 sequences; some clones differ only in CDR3. Rearranging VL and JL elements increases CDR3 diversity in both V sigma families, but abortive rearrangements are frequent in V sigma 1 regions. The very poor heterogeneity of expressed VL elements therefore appears to limit antibody diversity in Xenopus.