Immunoglobulin and T-cell receptor (TCR) molecules are central to the adaptive immune system. Sequence conservation, similarities in domain structure, and usage of similar recombination signal sequences and recombination machinery indicate that there was probably a time during evolution when an ancestral receptor diverged to the modern-day immunoglobulin and TCR. Other molecules that undergo rearrangement have not been described in vertebrates, nor have intermediates been identified that have features of both these gene families. We report here the isolation of a new member of the immunoglobulin superfamily from the nurse shark, Ginglymostoma cirratum, which contains one variable and five constant domains and is found as a dimer in serum.

Unlike all other vertebrates examined to date, there is only one detectable class I locus in the Xenopus MHC. On the bases of a nearly ubiquitous and high tissue expression, extensive polymorphism, and MHC linkage, this gene is of the classical or class Ia type. Sequencing analysis of class Ia cDNAs encoded by eight defined MHC haplotypes reveals two very old allelic lineages that perhaps emerged when humans and mice diverged from a common ancestor up to 100 million years ago. The unprecedented age of these lineages suggests that different class Ia genes from ancestors of the laboratory model Xenopus laevis are now expressed as alleles in this species. The lineages are best defined by their cytoplasmic and α2 peptide-binding domains, and there are highly diverse alleles (defined by the α1 peptide-binding domain) in each lineage. Surprisingly, the α3 domains are homogenized in both lineages, suggesting that interallelic gene conversion/recombination maintains the high sequence similarity.