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

Abstract

Crocodilians are a group of reptiles that are closely related to birds and are thought to possess a strong immune system. Here we report that the IgH locus in the Siamese crocodile and the Chinese alligator contains multiple m genes, in contrast to other tetrapods. Both the m2 and m3 genes are expressed through class-switch recombination involving the switch region and germline transcription. Both IgM1 and IgM2 are present in the serum as polymers, which implies that IgM class switching may have significant roles in humoural immunity. The crocodilian a genes are the first IgA-encoding genes identified in reptiles, and these genes show an inverted transcriptional orientation similar to that of birds. The identification of both a and d genes in crocodilians suggests that the IgH loci of modern living mammals, reptiles and birds share a common ancestral organization.

Multiple IgH Isotypes Including IgD, Subclasses of IgM, and IgY Are Expressed in the Common Ancestors of Modern Birds

“…However, despite this deep evolutionary conservation, IgD, compared with IgM, does show a great deal of variation with respect to its structure and expression patterns among different jawed vertebrates. Whereas IgD H chains possess no more than three CH domains in placental mammals, the number of CH domains varies from 2 to 11 in the expressed IgD H chains of the platypus and other tetrapods, including birds, modern reptiles, and amphibians (5,8,12,(33)(34)(35)37). This variation is even greater in fish, with the rainbow trout having the shortest IgD H chains, with four CH domains (22205025) (42), and the Siberian sturgeon having the longest, with 19 CH domains (24).…”

“…For example, in platypus and lizards, only membrane-bound IgD-encoding transcripts can be detected (5,33). In other species, such as snakes, turtles, crocodiles, and Xenopus, the IgD expression pattern is more like that in placental mammals (8,12,34,35,37). In all fish examined to date, membrane-bound IgD-encoding transcripts have been detected, and secreted IgD has been clearly detected at the protein level in selected species, including catfish and rainbow trout (46,47).…”

“…In amphibians, such as X. tropicalis, the IgH gene locus essentially maintains this ancestral organization, with the addition of an IgF-encoding gene originally duplicated from the y gene (8). In the common direct ancestor of birds and modern reptiles, the basic genomic organization of m-d-a(x)-y was maintained, although the a gene was inverted due to an unknown genetic rearrangement; this backbone structure can be found in both ostriches and crocodilians (37). Based on this genomic backbone structure, the a gene was then lost in specific reptiles, such as lizards (33), whereas the d gene was lost in specific birds, such as chickens and ducks (30,31) (Fig.…”

“…However, it was recently reported that there are three functional m genes in the IgH locus of crocodilians (37). As crocodilians are phylogenetically close to the ostrich, we performed a separate phylogenetic analysis using only the m sequences derived from various tetrapods (Figs.…”

“…As nonavian reptiles are close relatives of birds, these data caused us to speculate that a genetic rearrangement event in the IgH locus may have occurred in the common direct ancestor of modern reptiles, which led to either loss of the d gene and the inversion of the a gene in birds, or the loss of the a gene in nonavian reptiles (36). However, this hypothesis was strongly challenged by one of our recent studies on crocodilians, which showed that both d and the inverted a genes are present in the IgH locus of this group (37). That study suggested that the loss and inversion of the a or d genes were independent genetic events that occurred after the divergence of birds and nonavian reptiles, suggesting that the d gene may be present in some birds.…”