ABSTRACT
Abstract ................................................................................................... 19 2.1 Introduction .................................................................................... 19 2.2 Amphibians .................................................................................... 21 2.2.1 IgD(δ) Genes in Amphibians ............................................. 23 2.2.2 IgX, An Ortholog of IgA, Originated From
Recombination Between IgM and IgY .............................. 24 2.2.3 IgF: The Earliest IgH Isotype with a Separately Encoded
Genetic Hinge .................................................................... 25 2.2.4 κ, λ, and σ Light Chains Are Expressed in Xenopus ......... 25 2.2.5 Germline Diversity of the V, D, and J Gene Segments in
IgH and IgL Loci of Xenopus ............................................ 26 2.2.6 V Segments Were Rearranged in a Stepwise Manner
During the Ontogeny of Xenopus ...................................... 27 2.2.7 Diversity of Heavy Chain Complementarity Determining
Region 3 (CDR-H3) in larvae and Adults of Xenopus ...... 28 2.3 Reptiles .......................................................................................... 28 2.3.1 Multiple Copies of IgM(μ) Genes Can Be Expressed
Through CSR in Crocodiles ............................................... 30 2.3.2 Multiple Copies of the IgD(δ) and IgD2(d2) Genes
are Present in the Turtle Genome ....................................... 30
2.3.3 Diversification of IgY(υ) Genes in Turtles, Snakes, and Crocodiles .......................................................................... 31
2.3.4 Crocodilian IgA: A Missing Link Between Amphibians and Aves ............................................................................. 32
2.3.5 Both κ and λ Light Chains Are Expressed in Some Reptiles .................................................................... 33
2.4 Aves................................................................................................ 33 2.4.1 Avian µ Genes .................................................................... 34 2.4.2 Avian a Genes .................................................................... 35 2.4.3 Avian υ Genes .................................................................... 35 2.4.4 Genomic Organization of the λ Loci in Turkey
and Zebra Finch ................................................................. 36 2.5 Mammals ....................................................................................... 37 2.5.1 Monotremes ....................................................................... 37 2.5.2 Marsupials .......................................................................... 41 2.5.3 Eutherians .......................................................................... 41 Keywords ................................................................................................ 43 References ............................................................................................... 43
ABSTRACT
Over the past several decades, comparative examinations of the immunoglobulin (Ig) genes of various tetrapod species have provided interesting knowledge that is unavailable from conventional mouse and human models. This chapter summarizes the progress of research on Ig gene organization and repertoire diversification in tetrapods, highlighting the evolutionary relationships among different Ig isotypes. A number of distinct Ig heavy chain (H) isotypes, such as IgM, IgD, IgG, IgA, IgE, IgY, IgX, IgF, and IgO, have been discovered in different tetrapod species. IgM and IgD are two primordial Ig isotypes that are expressed in almost all vertebrate species. Unlike IgM, IgD shows a high degree of variation in the number of δCH exons and alternative RNA splicing patterns. IgY is the major low-molecular-weight serum Ig encoded by the υ gene in non-mammalian tetrapods and is thought to be the evolutionary precursor of mammalian IgG and IgE. IgA and its analog IgX, which play important roles in mucosal immunity, are also present in the majority of tetrapods (except for some reptiles) and are generated via the genetic combination of the IgM and IgY genes. Some species possess unique Ig isotypes such as IgO in platypus, IgF in Xenopus, and IgY (∆Fc) in duck and turtle. Regarding the light (L) chains, both Igκ and Igλ are expressed in tetrapods, but the frequency of expression of these two light chains differs greatly among species. A third IgL isotype, Igσ, is expressed only in amphibians and lower vertebrates (teleosts and cartilaginous fishes). Due to the translocon configuration of the IgH and IgL loci in tetrapods, V(D)J recombination is the major mechanism for generation of Ig diversity in most tetrapod species. However, two post-recombinatorial mechanisms, that is, somatic gene conversion (sGC) and somatic hypermutation (SHM), are used by some species to further expand their Ig repertoires when V(D) J-recombinational diversity alone is insufficient.
