Antibody diversification in cartilaginous fishes: Mechanistic insights from the nurse shark and comparative perspectives across jawed vertebrates.
Antibody diversity in vertebrates arises through the coordinated actions of V(D)J recombination and somatic hypermutation (SHM). Cartilaginous fishes occupy a key phylogenetic position as the sister lineage to bony vertebrates and therefore provide important comparative insights into the evolution of adaptive immunity. This review focuses on the nurse shark (Ginglymostoma cirratum) as a representative model for examining antibody-diversification mechanisms in cartilaginous fishes. Shark immunoglobulin genes exhibit a multicluster organization, while immunoglobulin new antigen receptor (IgNAR), a heavy-chain-only isotype, contains a single variable domain with an extended complementarity-determining region 3 (CDR3) that can be stabilized by non-canonical disulfide bonds. These structural features, together with intracluster multi-D V(D)J recombination and distinctive SHM characterized by single and tandem substitutions and insertions/deletions, contribute to antibody diversification in sharks. By comparing cartilaginous fishes, ray-finned fishes, and mammals, this review highlights lineage-specific combinations of immunoglobulin gene organization, recombination, mutational processing, and affinity maturation. Within the heuristic framework proposed here, shark and mammalian systems are described as emphasizing "breadth-first" repertoire generation and "precision-first" affinity optimization, respectively. These terms indicate relative mechanistic emphases rather than mutually exclusive categories or sequential evolutionary stages, while ray-finned fishes exhibit a distinct combination of genomic organization and mutational features. Investigating antibody diversification in cartilaginous fishes not only advances our understanding of vertebrate immune evolution but also provides structural and mechanistic insights that may inform the development of engineered antibodies based on the IgNAR scaffold.