A VpreB3 homologue in a marsupial, the gray short-tailed opossum, Monodelphis domestica

A VpreB surrogate light (SL) chain was identified for the first time in a marsupial, the opossum Monodelphis domestica. Comparing the opossum VpreB to homologues from eutherian (placental mammals) and avian species supported the marsupial gene being VpreB3. VpreB3 is a protein that is not known to traffic to the cell surface as part of the pre-B cell receptor. Rather, VpreB3 associates with nascent immunoglobulin chains in the endoplasmic reticulum. Homologues of other known SL chains VpreB1, VpreB2, and λ5, which are found in eutherian mammals, were not found in the opossum genome, nor have they been identified in the genomes of nonmammals. VpreB3 likely evolved from earlier gene duplication, independent of that which generated VpreB1 and VpreB2 in eutherians. The apparent absence of VpreB1, VpreB2, and λ5 in marsupials suggests that an extracellular pre-B cell receptor containing SL chains, as it has been defined in humans and mice, may be unique to eutherian mammals. In contrast, the conservation of VpreB3 in marsupials and its presence in nonmammals is consistent with previous hypotheses that it is playing a more primordial role in B cell development.

Comparative analysis of the chicken TCRα/δ locus

The access to whole genome sequences has provided the opportunity to study the evolution and organization of immunologically related genes on a large scale. The genes encoding the T cell receptor (TCR) α and δ chains are part of a complex locus that has shown remarkable conserved organization across different amniote lineages. In this study we have examined and annotated the TCRα/δ locus in chicken (Gallus gallus) and compared it to that of zebra finch (Taeniopygia guttata) and other avian species using the current available genome data. We also analyzed the expressed chicken TCRα/δ transcript repertoire and compared it with that previously described for zebra finch. The analyses conducted in this study show that the TCRα/δ locus in birds has undergone major rearrangements and expansion of the germ line repertoire in chicken, compared to zebra finch. A major expansion of the chicken variable gene repertoire appears to be driven by selection for genes from a limited number of subgroups.

A model for the evolution of the mammalian t-cell receptor α/δ and μ loci based on evidence from the duckbill Platypus

The specific recognition of antigen by T cells is critical to the generation of adaptive immune responses in vertebrates. T cells recognize antigen using a somatically diversified T-cell receptor (TCR). All jawed vertebrates use four TCR chains called α, β, γ, and δ, which are expressed as either a αβ or γδ heterodimer. Nonplacental mammals (monotremes and marsupials) are unusual in that their genomes encode a fifth TCR chain, called TCRµ, whose function is not known but is also somatically diversified like the conventional chains. The origins of TCRµ are also unclear, although it appears distantly related to TCRδ. Recent analysis of avian and amphibian genomes has provided insight into a model for understanding the evolution of the TCRδ genes in tetrapods that was not evident from humans, mice, or other commonly studied placental (eutherian) mammals. An analysis of the genes encoding the TCRδ chains in the duckbill platypus revealed the presence of a highly divergent variable (V) gene, indistinguishable from immunoglobulin heavy (IgH) chain V genes (VH) and related to V genes used in TCRµ. They are expressed as part of TCRδ repertoire (VHδ) and similar to what has been found in frogs and birds. This, however, is the first time a VHδ has been found in a mammal and provides a critical link in reconstructing the evolutionary history of TCRµ. The current structure of TCRδ and TCRµ genes in tetrapods suggests ancient and possibly recurring translocations of gene segments between the IgH and TCRδ genes, as well as translocations of TCRδ genes out of the TCRα/δ locus early in mammals, creating the TCRµ locus.

A second TCRδ locus in Galliformes uses antibody-like V domains: insight into the evolution of TCRδ and TCRμ genes in tetrapods

Analyses of the available avian genomes revealed the presence of a second TCRδ locus in the Galliformes. This second TCRδ locus is nonsyntenic to the conventional TCRα/δ and is unusual in that the V genes are more related to IgH V genes (VH) than to TCR V genes. The second TCRδ is not found in another avian lineage, the passerine zebra finch. Rather the finch's conventional TCRα/δ locus contains VH genes that are expressed with the conventional Cδ gene, similar to what has been found in amphibians. A comparison between Galliformes and Passeriformes genomic organization suggests an origin of the second TCRδ in the former lineage involving gene duplication. Expression of these atypical TCRδ transcripts with a VH domain paired with Cδ was found in lymphoid tissues of both avian lineages. The configuration of the second TCRδ in chicken and turkey is reminiscent of the TCRδ duplication that is present in nonplacental mammals and provides insight into the origin of the uniquely mammalian TCRμ locus.

Platypus TCRμ provides insight into the origins and evolution of a uniquely mammalian TCR locus

TCRμ is an unconventional TCR that was first discovered in marsupials and appears to be absent from placental mammals and nonmammals. In this study, we show that TCRμ is also present in the duckbill platypus, an egg-laying monotreme, consistent with TCRμ being ancient and present in the last common ancestor of all extant mammals. As in marsupials, platypus TCRμ is expressed in a form containing double V domains. These V domains more closely resemble Ab V than that of conventional TCR. Platypus TCRμ differs from its marsupial homolog by requiring two rounds of somatic DNA recombination to assemble both V exons and has a genomic organization resembling the likely ancestral form of the receptor genes. These results demonstrate that the ancestors of placental mammals would have had TCRμ but it has been lost from this lineage.

The dynamic TCRδ: TCRδ chains in the amphibian Xenopus tropicalis utilize antibody-like V genes

The content and organization of the Xenopus tropicalis TCRα/δ locus was determined. This locus is highly conserved among tetrapods, with the genes encoding the TCRδ chains embedded with those encoding TCRα. However, the frog TCRα/δ is unusual in that it contains V genes that appear indistinguishable from those in the IgH locus (VH). These V genes, termed VHδ, make up 70% of the V genes at the TCRδ locus and are expressed exclusively in TCRδ chains. Finding TCRδ chains that use antibody-like V domains in frogs is similar to the situation in shark TCRδ variants and TCRμ in marsupials. These results suggest that such unconventional TCR may be more widespread across vertebrate lineages than originally thought and raise the possibility of previously unrealized subsets of T cells. We also revealed close linkage of TCRα/δ, IgH, and Igλ in Xenopus which, in combination with linkage analyses in other species, is consistent with the previous models for the emergence of these antigen receptor loci.

TCR mu recombination and transcription relative to the conventional TCR during postnatal development in opossums

Marsupials are a distinct lineage of mammals notable for giving birth to highly altricial (relatively less developed) young. The recent discovery of a unique TCR chain in marsupials, TCRmu, raises questions about its possible role in early development. Here we compare the timing of V(D)J recombination and appearance of TCRmu transcripts relative to the conventional TCRalpha, beta, gamma, and delta mRNA during postnatal development in the opossum. There are two TCRmu transcript isoforms, TCRmu1.0 and TCRmu2.0. TCRmu1.0, which uses prejoined V(D)J segments, is detectable as early as day 1, when the thymus is primarily undifferentiated epithelium. The other isoform, TCRmu2.0, which requires V(D)J recombination and contains an unusual double V configuration, is not detectable until day 13 when the thymus is histologically mature. Surprisingly, we were able to detect TCRalpha, beta, and delta mRNA transcribed from loci that had completed V(D)J recombination as early as day 1 as well. At this early age there is apparent evidence for preference in the V segments used in the TCRalpha and beta genes. In the case of Valpha this preference appears to be associated with position in the TCRalpha/delta locus. In Vbeta, however, preference may be due to the use of microhomology in the V, D, and J segments. Mature TCRgamma transcripts were not detected until day 8, suggesting that, in contrast to eutherian mammals, in the opossum alphabeta T cell development precedes gammadelta T cell development. The results support that there may be differences in T cell subset development between marsupials and placental mammals.

Comparative genomic analysis and evolution of the T cell receptor loci in the opossum Monodelphis domestica

BACKGROUND

All jawed-vertebrates have four T cell receptor (TCR) chains: alpha (TRA), beta (TRB), gamma (TRG) and delta (TRD). Marsupials appear unique by having an additional TCR: mu (TRM). The evolutionary origin of TRM and its relationship to other TCR remain obscure, and is confounded by previous results that support TRM being a hybrid between a TCR and immunoglobulin locus. The availability of the first marsupial genome sequence allows investigation of these evolutionary relationships.

RESULTS

The organization of the conventional TCR loci, encoding the TRA, TRB, TRG and TRD chains, in the opossum Monodelphis domestica are highly conserved with and of similar complexity to that of eutherians (placental mammals). There is a high degree of conserved synteny in the genomic regions encoding the conventional TCR across mammals and birds. In contrast the chromosomal region containing TRM is not well conserved across mammals. None of the conventional TCR loci contain variable region gene segments with homology to those found in TRM; rather TRM variable genes are most similar to that of immunoglobulin heavy chain genes.

CONCLUSION

Complete genomic analyses of the opossum TCR loci continue to support an origin of TRM as a hybrid between a TCR and immunoglobulin locus. None of the conventional TCR loci contain evidence that such a recombination event occurred, rather they demonstrate a high degree of stability across distantly related mammals. TRM, therefore, appears to be derived from receptor genes no longer extant in placental mammals. These analyses provide the first genomic scale structural detail of marsupial TCR genes, a lineage of mammals used as models of early development and human disease.

A unique T cell receptor discovered in marsupials

T cells recognize antigens by using T cell receptors (TCRs) encoded by gene segments, called variable (V), diversity (D), and joining (J), that undergo somatic recombination to create diverse binding specificities. Four TCR chains (alpha, beta, gamma, and delta) have been identified to date, and, as T cells develop in the thymus, they express exclusively either an alphabetaTCR or a gammadeltaTCR heterodimer. Here, we show that marsupials have an additional TCR (TCRmu) that has V, D, and J that are either somatically recombined, as in conventional TCRs, or are already prejoined in the germ-line DNA in a manner consistent with their creation by retrotransposition. TCRmu does not have a known homolog in eutherian mammals but has features analogous to a recently described TCRdelta isoform in sharks. TCRmu is expressed in at least two mRNA isoforms that appear capable of encoding a full-length protein, both of which are transcribed in the thymus and spleen. One contains two variable domains: a somatically recombined V and a prejoined V. This appears to be the dominant isoform in peripheral lymphoid tissue. The other isoform contains only the prejoined V and is structurally more similar to conventional TCR chains, however invariant. Unlike other TCRs, TCRmu uses prejoined gene segments and is likely present in all marsupials. Its similarity to a TCR isoform in sharks suggests that it, or something similar, may be present in other vertebrate lineages and, therefore, may represent an ancient receptor system.

A unique T cell receptor discovered in marsupials

T cells recognize antigens by using T cell receptors (TCRs) encoded by gene segments, called variable (V), diversity (D), and joining (J), that undergo somatic recombination to create diverse binding specificities. Four TCR chains (alpha, beta, gamma, and delta) have been identified to date, and, as T cells develop in the thymus, they express exclusively either an alphabetaTCR or a gammadeltaTCR heterodimer. Here, we show that marsupials have an additional TCR (TCRmu) that has V, D, and J that are either somatically recombined, as in conventional TCRs, or are already prejoined in the germ-line DNA in a manner consistent with their creation by retrotransposition. TCRmu does not have a known homolog in eutherian mammals but has features analogous to a recently described TCRdelta isoform in sharks. TCRmu is expressed in at least two mRNA isoforms that appear capable of encoding a full-length protein, both of which are transcribed in the thymus and spleen. One contains two variable domains: a somatically recombined V and a prejoined V. This appears to be the dominant isoform in peripheral lymphoid tissue. The other isoform contains only the prejoined V and is structurally more similar to conventional TCR chains, however invariant. Unlike other TCRs, TCRmu uses prejoined gene segments and is likely present in all marsupials. Its similarity to a TCR isoform in sharks suggests that it, or something similar, may be present in other vertebrate lineages and, therefore, may represent an ancient receptor system.

Genome of the marsupial Monodelphis domestica reveals innovation in non-coding sequences

We report a high-quality draft of the genome sequence of the grey, short-tailed opossum (Monodelphis domestica). As the first metatherian ('marsupial') species to be sequenced, the opossum provides a unique perspective on the organization and evolution of mammalian genomes. Distinctive features of the opossum chromosomes provide support for recent theories about genome evolution and function, including a strong influence of biased gene conversion on nucleotide sequence composition, and a relationship between chromosomal characteristics and X chromosome inactivation. Comparison of opossum and eutherian genomes also reveals a sharp difference in evolutionary innovation between protein-coding and non-coding functional elements. True innovation in protein-coding genes seems to be relatively rare, with lineage-specific differences being largely due to diversification and rapid turnover in gene families involved in environmental interactions. In contrast, about 20% of eutherian conserved non-coding elements (CNEs) are recent inventions that postdate the divergence of Eutheria and Metatheria. A substantial proportion of these eutherian-specific CNEs arose from sequence inserted by transposable elements, pointing to transposons as a major creative force in the evolution of mammalian gene regulation.

Modo-UG, a marsupial nonclassical MHC class I locus

Modo-UG is a class I gene located in the MHC of the marsupial Monodelphis domestica, the gray, short-tailed opossum. Modo-UG is expressed as three alternatively spliced mRNA forms, all of which encode a transmembrane form with a short cytoplasmic tail that lacks phosphorylation sites typically found in classical class I molecules. The three alternative mRNAs would encode a full-length form, an isoform lacking the alpha2 domain, and one lacking both alpha2 and alpha3 domains. Genotyping both captive-bred and wild M. domestica from different geographic regions revealed no variation in the residues that make up Modo-UG's peptide-binding groove. Modo-UG's low polymorphism is contrasting to that of a nearby class I locus, Modo-UA1, which has a highly polymorphic peptide-binding region. Absence of functional polymorphism in Modo-UG is therefore not a general feature of opossum class I genes but the result of negative selection. Modo-UG is the first MHC linked marsupial class I to be described that appears to clearly have nonclassical features.

TCR gamma chain diversity in the spleen of the duckbill platypus (Ornithorhynchus anatinus)

TCR gamma (TRG) chain diversity in splenic gammadelta T cells was determined for an egg-laying mammal (or monotreme), the duckbill platypus. Three distinct V subgroups were found in the expressed TRG chains and these three subgroups are members of a clade not found so far in eutherian mammals or birds. Each subgroup contains approximately five V gene segments, and their overall divergence is much less than is found in eutherians and birds, consistent with their recent evolution from an ancestral V gene segment. The platypus TRG locus also contains three C region genes and many of the residues involved in TCR function, such as interactions with CD3, were conserved in the monotreme C regions. All non-eutherian mammals (monotremes and marsupials) lacked the second cysteine residue necessary to form the intradomain disulfide bond in the C region, a loss apparently due to independent mutations in marsupials and monotremes. Monotreme TRGC regions also had among the most variation in the length of the connecting peptide region described for any species due to repeated motifs.

The complexity of expressed kappa light chains in egg-laying mammals

Complementary DNAs encoding immunoglobulin light chains were isolated from two monotreme species, Ornithorhynchus anatinus (duckbill platypus) and Tachyglossus aculeatus (echidna). The sequences of both the variable and constant regions of these clones had greater similarity to IGK than to other light chain classes and phylogenetic analyses place them squarely within the mammalian IGK group, establishing them as monotreme IGK homologues. The constant region sequences of all clones were essentially identical within each species and, along with Southern blot results, the data are consistent with a single IGKC in each species. The expressed IGKV repertoires from both platypus and echidna were randomly sampled and there appear to be at least four platypus and at least nine echidna IGKV subgroups. The IGKV subgroups are highly divergent within species, in some cases sharing as little as 57% nucleotide identity. Two of the IGKV subgroups are present in both species, so there is some degree of overlap in the germline repertoires of these two monotremes. Overall the complexity seen in platypus and echidna IGK light chains is comparable with that of other mammals considered to have high levels of germline diversity and is in contrast to what has been found so far for monotreme IGL.