Cytidine deaminase 2 is required for antibody gene assembly in lampreys.

The antibodies of jawless vertebrates consist of leucine-rich repeat arrays encoded by somatically assembled genes. It is unknown how the incomplete germline loci are converted into functional antibody genes during B lymphocyte development in lampreys. In larvae lacking the cytidine deaminase gene, assembly fails, whereas the T lineage-associated and antigen receptor gene assemblies occur normally. Thus, acts in a B cell lineage-specific fashion to support the somatic diversification of antibody genes. CDA2 is closely related to activation-induced cytidine deaminase (AID), which is essential for the elaboration of immunoglobulin gene repertoires in jawed vertebrates. Our results thus identify a convergent mechanism of antigen receptor gene assembly and diversification that independently evolved in the two sister branches of vertebrates.