Identification of IgF, a hinge-region-containing Ig class, and IgD inXenopus tropicalis

Abstract

Only three Ig isotypes, IgM, IgX, and IgY, were previously known in amphibians. Here, we describe a heavy-chain isotype in Xenopus tropicalis, IgF (encoded by C ), with only two constant region domains. IgF is similar to amphibian IgY in sequence, but the gene contains a hinge exon, making it the earliest example, in evolution, of an Ig isotype with a separately encoded genetic hinge. We also characterized a gene for the heavy chain of IgD, located immediately 3 of C , that shares features with the C␦ gene in fish and mammals. The latter gene contains eight constant-region-encoding exons and, unlike the chimeric splicing of CH1 onto the IgD heavy chain in teleost fish, it is expressed as a unique IgD heavy chain. The IgH locus of X. tropicalis shows a 5 V H-DH-JH-C -C␦-C -C -C 3 organization, suggesting that the mammalian and amphibian Ig heavy-chain loci share a common ancestor.amphibians ͉ antibody evolution

IgM, IgD and IgY and their expression pattern in the Chinese soft-shelled turtle Pelodiscus sinensis

“…The Xenopous ␦ gene (IgD, possess eight Cdomains) locates in the same gene cluster, following immediately 3 end of the IgM gene, as in mammals (Ohta and Flajnik, 2006). IgF heavy chain with only two constant domains, is similar to amphibian IgY in sequence, but its gene contains a hinge exon making it the earliest example in evolution of an Ig isotype with a separately encoded genetic hinge (Zhao et al, 2006).…”

Extensive diversification of IgH subclass-encoding genes and IgM subclass switching in crocodilians

“…7). As IgM and IgD(W) are now commonly accepted to be the most primordial IgH classes in evolution 17,21,22 , it is reasonable to conclude that all other IgH classes evolved from them through various molecular mechanisms such as gene duplication, conversion or recombination. These processes gave rise to IgNAR and IgZ(T) after the divergence of cartilaginous fish and bony fish, respectively [12][13][14] .…”

“…During tetrapod evolution, lineage-specific IgH class addition or deletion was adopted to shape the IgH locus of the modern living tetrapods. In Xenopus, the new isotype IgF, apparently derived from IgY gene duplication, was identified in addition to IgM, IgD, IgY and IgX 17 (Fig. 7).…”

“…Both V(D)J recombination and SHM are utilized by all of the species that have been examined, whereas gene conversion has a major role only in birds 1,8,9 . Additionally, more than 10 genes encoding different Ig classes, such as IgM, IgD (IgW), IgNAR, IgZ (IgT), IgA (IgX), IgY, IgF, IgO, IgG and IgE, have been identified in various species 3,[10][11][12][13][14][15][16][17][18][19][20] , and IgG and IgA are further diversified into a variable number of subclasses in mammalian species. Differential diversification of the Ig classes or subclasses may have arisen from a long evolutionary period of environmental selection pressure, thus conferring survival advantages.…”

“…Differential diversification of the Ig classes or subclasses may have arisen from a long evolutionary period of environmental selection pressure, thus conferring survival advantages. IgM and IgD are thought to be the most primitive Ig classes 17,21 , as IgM has been identified in all species examined to date and IgD is found in most species except for birds and certain mammals 22 . Logically, other IgH (H: heavy-chain genes) classes should be evolutionarily derived from IgM and IgD, likely through various mechanisms such as gene duplication, gene conversion or recombination.…”