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TetrapodsEutherian Mammals and Nonmammalian the Evolution of Immunoglobulin Genes in (Platypus) Links Ornithorhynchus anatinus

The evolutionary origins of mammalian immunoglobulin H chain isotypes (IgM, IgD, IgG, IgE, and IgA) are still incompletely understood as these isotypes differ considerably in structure and number from their counterparts in nonmammalian tetrapods. We report in this study that the platypus ( Ornithorhynchus anatinus ) Ig H chain constant region gene locus contains eight Ig encoding genes, which are arranged in an (cid:1) - (cid:2) - (cid:3) - (cid:4) 2- (cid:4) 1- (cid:5) 1- (cid:6) - (cid:5) 2 order, spanning a total of (cid:1) 200 kb DNA, encoding six distinct isotypes. The (cid:3) ( (cid:3) for Ornithorhynchus ) gene encodes a novel Ig H chain isotype that consists of four constant region domains and a hinge, and is structurally different from any of the ﬁve known mammalian Ig classes. This gene is phylogenetically related to (cid:7) ( (cid:6) ) and (cid:4) , and thus appears to be a structural intermediate between these two genes. The platypus (cid:2) gene encodes ten heavy chain constant region domains, lacks a hinge region and is similar to IgD in amphibians and ﬁsh, but strikingly different from that in eutherian mammals. The platypus Ig H chain isotype repertoire thus shows a unique combination of genes that share similarity both to those of nonmammalian tetrapods