Lineage-Restricted Retention of a Primitive Immunoglobulin Heavy Chain Isotype within the Dipnoi Reveals an Evolutionary Paradox

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 100; no. 5; pp. 2501 - 2506
• Main Authors: Ota, Tatsuya, Rast, Jonathan P., Litman, Gary W., Amemiya, Chris T.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 04.03.2003; National Acad Sciences; The National Academy of Sciences
• Subjects: Amino Acid Sequence; Amino acids; Animals; Biological Sciences; Blotting, Northern; Cloning, Molecular; Dipnoi; DNA, Complementary - metabolism; Evolution; Evolution, Molecular; Fish; Fishes; Gene Library; Genetic loci; Immunoglobulin heavy chains; Immunoglobulin Heavy Chains - chemistry; Immunoglobulin Heavy Chains - genetics; Immunoglobulin M - metabolism; Immunology; Isotypes; Molecular Sequence Data; Phylogenetics; Phylogeny; Protein Structure, Tertiary; Protopterus aethiopicus; RNA; RNA - metabolism; Sequence Homology, Amino Acid; Sharks; Vertebrates
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0538029100

The lineage leading to lungfishes is one of the few major jawed vertebrate groups in which Ig heavy chain isotype structure has not been investigated at the genetic level. In this study, we have characterized three different Ig heavy chain isotypes of the African lungfish, Protopterus aethiopicus, including an IgM-type heavy chain and short and long forms of non-IgM heavy chains. Northern blot analysis as well as patterns of VH utilization suggest that the IgM and non-IgM isotypes are likely encoded in separate loci. The two non-IgM isotypes identified in Protopterus share structural features with the short and long forms of IgX/W/NARC (referred to hereafter as IgW), which were previously considered to be restricted to the cartilaginous fish. It seems that the IgW isotype has a far broader phylogenetic distribution than considered originally and raises questions with regard to the origin and evolutionary divergence of IgM and IgW. Moreover, its absence in other gnathostome lineages implies paradoxically that the IgW-type genes were lost from teleost and tetrapod lineages.