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

Abstract

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 δ gene in the lizard demonstrates an evolutionary continuity of IgD from fishes to mammals. Although the germline δ gene contains 11 CH exons, only the first 4 are used in the expressed IgD membrane-bound form. The μ chain lacks the cysteine in CH1 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 CH 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 CH domains (VDJ-CH3-CH4-transmembrane region). Similar to duck IgY, lizard IgY H chain (υ) transcripts encoding both full-length and truncated (IgYΔFc) forms (with two CH 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.

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

“…7). Interestingly, IgA was found to be lost in a number of reptilian species, including lizards and turtles 25,41,43 , whereas IgD is absent in all birds investigated thus far 22,27,[44][45][46] (Fig. 7).…”

“…Compared with birds and mammals, few studies have been conducted on reptilian Ig genes 25,28,[36][37][38][39][40][41] . Crocodilians are a group of reptiles that phylogenetically link reptiles and birds, and they are thought to have a strong immune system [29][30][31] .…”

“…This finding suggests that the amphibian IgX is orthologous to IgA 51 and that the a gene emerged before the divergence of tetrapods (including amphibians, reptiles, birds and mammals). Considering the early emergence of the tetrapod a gene, it is remarkable that this antibody class, with a major importance in mucosal immunity, is missing in some reptiles 25 . Second, similar to birds, the crocodilian a genes were all inverted, suggesting that the a gene inversion also occurred before the divergence of crocodilians and birds.…”

“…For example, IgD is absent in all birds examined to date 27 , although it has been identified in all other groups of jawed vertebrates, including reptiles 22,25,28 . Furthermore, although IgA (or IgX) is expressed in mammals, birds and amphibians 1 , this Ig class, which is involved in mucosal immunity, is missing in several reptiles 25,28 . This absence is surprising considering that even bony fish express a special Ig class, IgT, that is dedicated to mucosal immunity 15 .…”

“…These processes led to the origin of IgNAR in cartilaginous fish and IgZ(T) in bony fish 13,14,23 . Although both IgY and IgA(X) appeared only after the emergence of tetrapods, the former likely preceded the latter in evolution based on the evolutionary evidence that IgA (X) resulted from a genetic recombination between IgM and IgY 24,25 . From lower tetrapods to mammals, IgY has been functionally diversified into IgG and IgE 26 .…”