Expression of IgM, IgD, and IgY in a Reptile, Anolis carolinensis

The reptiles are the last major group of jawed vertebrates in which the organization of the IGH locus and its encoded Ig H chain isotypes have not been well characterized. In this study, we show that the green anole lizard (Anolis carolinensis) expresses three Ig H chain isotypes (IgM, IgD, and IgY) but no IgA. The presence of the 5 gene in the lizard demonstrates an evolutionary continuity of IgD from fishes to mammals. Although the germline 8 gene contains 11 C-H exons, only the first 4 are used in the expressed IgD membrane-bound form. The mu chain lacks the cysteine in C(H)1 that forms a disulfide bond between H and L chains, suggesting that (as in IgM of some amphibians) the H and L polypeptide chains are not covalently associated. Although conventional IgM transcripts (four C-H domains) encoding both secreted and membrane-bound forms were detected, alternatively spliced transcripts encoding a short membrane-bound form were also observed and shown to lack the first two C-H domains (VDJ-C(H)3-C(H)4-transmembrane region). Similar to duck IgY, lizard IgY H chain (nu) transcripts encoding both full-length and truncated (IgY Delta Fc) forms (with two C-H domains) were observed. The absence of an IgA-encoding gene in the lizard IGH locus suggests a complex evolutionary history for IgA in the saurian lineage leading to modern birds, lizards, and their relatives. The Journal of Immunology, 2009, 183: 3858-3864.