[Cloning of Xenopus laevis major histocompatibility complex class II beta-chain genes]

The amphibian Xenopus laevis is the most primitive vertebrate in which the major histocompatibility complex (MHC) has been identified both at the biochemical and functional level. In the present study, cDNA clones for MHC class II beta-chain genes of Xenopus laevis were isolated using the polymerase chain reaction. In partially inbred animals with the MHC f haplotype, two distinct cDNA clones assumed to encode isotypes were identified. The deduced MHC class II beta-chain of Xenopus laevis had structures fundamentally similar to those of their mammalian counterparts. The overall amino acid sequence identity between Xenopus laevis and representative mammalian MHC class II beta-chains was approximately 41-50%. Sequence comparison of cDNA clones isolated from outbred individuals and the partially inbred frog with the MHC f haplotype showed that nucleotide substitutions are located primarily in the beta 1 domain and that non-synonymous substitutions occur more frequently than synonymous substitutions in the putative antigen binding site. This result indicates that, as in mammalian MHC genes, the MHC class II beta-chain genes of Xenopus laevis are under positive Darwinian selection. Despite the fact that Xenopus laevis is a tetraploid species, its MHC, as defined by serology, immunochemistry, and mixed lymphocyte reaction, is diploidized. Family studies showed that all MHC class II beta-chain genes detectable in the genome of Xenopus laevis segregate with serologically defined MHC haplotypes, suggesting that the second, inactivated set of MHC class II beta-chain genes was either deleted or that their sequences diverged too far to be detected by DNA hybridization. Furthermore, a nearly constant number of MHC class II beta-chain genes was detectable in most, if not all, Xenopus species with various degrees of ploidy. Thus, various polyploid species of Xenopus appear to have used a similar strategy to diploidize their MHC genes.