The evolution of highly variable immunity genes across a passerine bird radiation

To survive, individuals must be able to recognize and eliminate pathogens. The genes of the major histocompatibility complex (MHC) play an essential role in this process in vertebrates as their diversity affects the repertoire of pathogens that can be recognized by the immune system. Emerging evidence suggests that birds within the parvorder Passerida possess an exceptionally high number of MHC genes. However, this has yet to be directly investigated using a consistent framework, and the question of how this MHC diversity has evolved has not been addressed. We used next‐generation sequencing to investigate how MHC class I gene copy number and sequence diversity varies across the Passerida radiation using twelve species chosen to represent the phylogenetic range of this group. Additionally, we performed phylogenetic analyses on this data to identify, for the first time, the evolutionary model that best describes how MHC class I gene diversity has evolved within Passerida. We found evidence of multiple MHC class I genes in every family tested, with an extremely broad range in gene copy number across Passerida. There was a strong phylogenetic signal in MHC gene copy number and diversity, and these traits appear to have evolved through a process of Brownian motion in the species studied, that is following the pattern of genetic drift or fluctuating selection, as opposed to towards a single optimal value or through evolutionary ‘bursts’. By characterizing MHC class I gene diversity across Passerida in a systematic framework, this study provides a first step towards understanding this huge variation.     

Characterization of a divergent non-classical MHC class I gene in sharks

Sharks are the most ancient group of vertebrates known to possess members of the major histocompatibility complex (MHC) gene family. For this reason, sharks provide a unique opportunity to gain insight into the evolution of the vertebrate immune system through comparative analysis. Two genes encoding proteins related to the MHC class I gene family were isolated from splenic cDNA derived from spiny dogfish shark ( Squalus acanthias). The genes have been designated MhcSqac-UAA*01 and MhcSqac-UAA*NC1. Comparative analysis demonstrates that the Sqac-UAA*01 protein sequence clusters with classical MHC class I of several shark species and has structural elements common to most classical MHC class I molecules. In contrast, Sqac-UAA*NC1 is highly divergent from all vertebrate classical MHC class I proteins, including the Sqac-UAA *01 sequence and those of other shark species. Although Sqac-UAA*NC1 is clearly related to the MHC class I gene family, no orthologous genes from other species were identified due to the high degree of sequence divergence. In fact, the Sqac NC1 protein sequence is the most divergent MHC class-I-like protein identified thus far in any shark species. This high degree of divergence is similar in magnitude to some of the MHC class-I-related genes found in mammals, such as MICA or CD1. These data support the existence of a class of highly divergent non-classical MHC class I genes in the most primitive vertebrates known to possess homologues of the MHC and other components of the adaptive immune system.     

The ancestry and cumulative evolution of immune reactions

The last two decades of study enriched greatly our knowledge of how the immune system originated and the sophisticated immune mechanisms of today's vertebrates and invertebrates developed. Even unicellular organisms possess mechanisms for pathogen destruction and self recognition. The ability to distinguish self from non-self is a prerequisite for recognition of sexual compatibility and ensuring survival. Molecules involved in these processes resemble those found in the phagocytic cells of higher organisms. Recognition of bacteria by scavenger receptors induces phagocytosis or endocytosis. The phagocytic mechanisms characterizing the amoeboid protozoans developed further during the evolution towards innate immunity. The scavenger receptor cysteine-rich domain SRCR is encoded in the genomes from the most primitive sponges to mammals. The immune system of sponges comprises signal transduction molecules which occur in higher metazoans as well. Sponges already possess recognition systems for pathogenic bacteria and fungi, based on membrane receptors (a lipopolysaccharide-interacting protein, a cell surface receptor recognizing β(1 → 3)-d-glucans of fungi). Perforin-like molecules and lysozymes are involved, among others, in defense in sponges. Reactive oxygen and nitrogen species function in the immunity of early metazoan. Genes encoding the family of reactive oxygen-generating NADPH oxidases (Noxes) are found in a variety of protists and plants. The NO synthases of cnidarians, mollusks, and chordates are conserved with respect to the mammalian NOS. The antimicrobial peptides of protozoans, amoebapores, are structural and functional analogs of the natural killer cell peptide, NK-lysin, of vertebrates. An ancestral S-type lectin has been found in sponges. Opsonizing properties of lectins and the ability to agglutinate cells justify their classification as primitive recognition molecules. Invertebrate cytokines are not homologous to those of vertebrate, and their functional convergence was presumably enabled by the general similarity of the lectin-like recognition domain three-dimensional structure. Sponges contain molecules with SCR/CCP domains that show high homology to the mammalian regulators of complement activation (RCA family). A multi-component complement system comprising at least the central molecule of the complement system, C3, Factor B, and MASP developed in the cnidarians and evolved into the multilevel cascade engaged in innate and acquired immunity of vertebrates. The adaptive immune system of mammals is also deeply rooted in the metazoan evolution. Some its precursors have been traced as deep as in sponges, namely, two classes of receptors that comprise Ig-like domains, the receptor tyrosine kinases (RTK), and the non-enzymic sponge adhesion molecules (SAM). The antibody-based immune system defined by the presence of the major histocompatibility complex (MHC), T-cell receptor (TCR), B-cell receptor (BCR) or recombination activating genes (RAGs) is known beginning from jawed fishes. However, genes closely resembling RAG1 and RAG2 have been uncovered in the genome of a see urchin. The ancestry of MHC gene remains unknown. Similarly, no homologue of the protein binding domain (PBD) in MHC molecules has been found in invertebrates. The pathway by which endogenous peptides are degraded for presentation with class I MHC molecules utilizes mechanisms similar to those involved in the normal turnover of intracellular proteins, apparently recruited to work also for the immune system. Several cDNAs coding for lysosomal enzymes, e.g., cathepsin, have been isolated from sponges. All chromosomal duplication events in the MHC region occurred after the origin of the agnathans but before the gnathostomes split from them. The V-domains of the subtype found in the receptors of T and B-cells are known from both agnathans and cephalochordates, although they do not rearrange. The rearrangement mechanism of the lymphocyte V-domains suggests its origin from a common ancestral domain existing before the divergence of the extant gnathostome classes. Activation-induced deaminase (AID) - homologous proteins have been found only in the gnathostomes. It appears thus that the adaptive immunity of vertebrates is a result of stepwise accumulation of small changes in molecules, cells and organs over almost half a billion years.     

Retained orthologous relationships of the MHC Class I genes during euteleost evolution

Major histocompatibility complex (MHC) class I molecules play a pivotal role in immune defense system, presenting the antigen peptides to cytotoxic CD8+ T lymphocytes. Most vertebrates possess multiple MHC class I loci, but the analysis of their evolutionary relationships between distantly related species has difficulties because genetic events such as gene duplication, deletion, recombination, and/or conversion have occurred frequently in these genes. Human MHC class I genes have been conserved only within the primates for up to 46-66 My. Here, we performed comprehensive analysis of the MHC class I genes of the medaka fish, Oryzias latipes, and found that they could be classified into four groups of ancient origin. In phylogenetic analysis using these genes and the classical and nonclassical class I genes of other teleost fishes, three extracellular domains of the class I genes showed quite different evolutionary histories. The α1 domains generated four deeply diverged lineages corresponding to four medaka class I groups with high bootstrap values. These lineages were shared with salmonid and/or other acanthopterygian class I genes, unveiling the orthologous relationships between the classical MHC class I genes of medaka and salmonids, which diverged approximately 260 Ma. This suggested that the lineages must have diverged in the early days of the euteleost evolution and have been maintained for a long time in their genome. In contrast, the α3 domains clustered by species or fish groups, regardless of classical or nonclassical gene types, suggesting that this domain was homogenized in each species during prolonged evolution, possibly retaining the potential for CD8 binding even in the nonclassical genes. On the other hand, the α2 domains formed no apparent clusters with the α1 lineages or with species, suggesting that they were diversified partly by interlocus gene conversion, and that the α1 and α2 domains evolved separately. Such evolutionary mode is characteristic to the teleost MHC class I genes and might have contributed to the long-term conservation of the α1 domain.     

Evolution of MHC class I in the Order Crocodylia

The major histocompatibility complex (MHC) is a dynamic genomic region with an essential role in the adaptive immunity of jawed vertebrates. The evolution of the MHC has been dominated by gene duplication and gene loss, commonly known as the birth-and-death process. Evolutionary studies of the MHC have mostly focused on model species. However, the investigation of this region in non-avian reptiles is still in its infancy. To provide insights into the evolutionary mechanisms that have shaped the diversity of this region in the Order Crocodylia, we investigated MHC class I exon 3, intron 3, and exon 4 across 20 species of the families Alligatoridae and Crocodilidae. We generated 124 DNA sequences and identified 31 putative functional variants as well as 14 null variants. Phylogenetic analyses revealed three gene groups, all of which were present in Crocodilidae but only one in Alligatoridae. Within these groups, variants generally appear to cluster at the genus or family level rather than in species-specific groups. In addition, we found variation in gene copy number and some indication of interlocus recombination. These results suggest that MHC class I in Crocodylia underwent independent events of gene duplication, particularly in Crocodilidae. These findings enhance our understanding of MHC class I evolution and provide a preliminary framework for comparative studies of other non-avian reptiles as well as diversity assessment within Crocodylia.     

Evolution of the proteasome components

 A phylogenetic analysis of proteasome subunits revealed two major families (α and β) which originated by an ancient gene duplication prior to the divergence of archaebacteria and eukaryotes. Numerous gene duplications have subsequently occurred in eukaryotes; at least nine of these duplications were shown to have occurred prior to the divergence of animals and fungi. In mammals, two genes encoding proteasome subunits (LMP2 and LMP7) are located in the major histocompatibility complex (MHC) region and play a specific role in generation of peptides for presentation by class I MHC molecules. Phylogenetic analysis of LMP7 and related sequences from mammals and lower vertebrates indicated that this locus arose by gene duplication prior to the divergence of jawed and jawless vertebrates; the time of this duplication was estimated to have been about 600 million years ago. The evolutionary history of the proteasome subunits provides support for a model of the evolution of new gene function postulating that, after gene duplication, the proteins encoded by daughter loci can adapt to specialized functions previously performed by the product of a single generalized ancestral locus. Received: 19 August 1996 / Revised: 24 December 1996     

Contrasting patterns of selection acting on MHC class I and class II DRB genes in the Alpine marmot (Marmota marmota)

The major histocompatibility complex (MHC) genes code for proteins that play a critical role in the immune system response. The MHC genes are among the most polymorphic genes in vertebrates, presumably due to balancing selection. The two MHC classes appear to differ in the rate of evolution, but the reasons for this variation are not well understood. Here, we investigate the level of polymorphism and the evolution of sequences that code for the peptide-binding regions of MHC class I and class II DRB genes in the Alpine marmot (Marmota marmota). We found evidence for four expressed MHC class I loci and two expressed MHC class II loci. MHC genes in marmots were characterized by low polymorphism, as one to eight alleles per putative locus were detected in 38 individuals from three French Alps populations. The generally limited degree of polymorphism, which was more pronounced in class I genes, is likely due to bottleneck the populations undergone. Additionally, gene duplication within each class might have compensated for the loss of polymorphism at particular loci. The two gene classes showed different patterns of evolution. The most polymorphic of the putative loci, Mama-DRB1, showed clear evidence of historical positive selection for amino acid replacements. However, no signal of positive selection was evident in the MHC class I genes. These contrasting patterns of sequence evolution may reflect differences in selection pressures acting on class I and class II genes.     

Molecular cloning of C4 gene and identification of the class III complement region in the shark MHC

To clarify the evolutionary origin of the linkage of the MHC class III complement genes with the MHC class I and II genes, we isolated C4 cDNA from the banded hound shark (Triakis scyllium). Upon phylogenetic tree analysis, shark C4 formed a well-supported cluster with C4 of higher vertebrates, indicating that the C3/C4 gene duplication predated the divergence of cartilaginous fish from the main line of vertebrate evolution. The deduced amino acid sequence predicted the typical C4 three-subunits chain structure, but without the histidine residue catalytic for the thioester bond, suggesting the human C4A-like specificity. The linkage analysis of the complement genes, one C4 and two factor B (Bf) genes, to the shark MHC was performed using 56 siblings from two typing panels of T. scyllium and Ginglymostoma cirratum. The C4 and one of two Bf genes showed a perfect cosegregation with the class I and II genes, whereas two recombinants were identified for the other Bf gene. These results indicate that the linkage between the complement C4 and Bf genes, as well as the linkage between these complement genes and the MHC class I and II genes were established before the emergence of cartilaginous fish >460 million years ago.     

Contrasting evolutionary histories among tightly linked HLA loci

Genes comprising the major histocompatibility complex (MHC) play a central role in governing the immune response of vertebrates. A great deal of information has been revealed on the molecular biology and physiology of these loci, but three features-the high polymorphism, tight linkage among the loci, and the nonrandom association of alleles-make the system of particular interest from the perspective of population genetics. Information on the dynamic evolutionary forces that have acted on a locus can be inferred from the number and distribution of alleles that it carries. Ten loci from the HLA region of the human MHC, each sampled from several different populations, have been examined for departures from the expected value of homozygosity under the condition of selective neutrality. The homozygosities of five class I and II loci that code for membrane glycoproteins, HLA-A, -B, -C, -DR, and -DQ, and of glyoxylase I (GLO) were significantly less than the neutrality expectations. This suggests the presence of some form of balancing selection. In spite of being closely linked, in fact, located between the class I and class II histocompatibility loci, the homozygosities of the four class III or complement loci C2, Bf, C4A, and C4B, which are detected by electrophoresis, were indistinguishable from, or exceeded, that expected under neutrality. Although this conforms to the suggestion that, in general, electrophoretic variants are neutral, because of the tight linkage to loci demonstrating a history of selection, it is possible that the mechanism for generating variation in the class III loci may be different from that of the class I and class II loci.     

Birth-and-death evolution in primate MHC class I genes: divergence time estimates

The major histocompatibility complex (MHC) is a multigene family that mediates the host immune response by helping T lymphocytes to recognize and respond to foreign antigens. The high degree of polymorphism and a quick turnover of the genetic loci make the evolution of MHC genes an intriguing subject of study. To understand the evolutionary pattern of this multigene family, we studied the phylogeny and divergence times of six functional MHC class I loci from primate species. On the phylogenetic trees, locus F occupies the most basal position among these loci. Our results suggest that the F locus diverged from the other MHC class I loci about 46-66 MYA. The major diversification of the other class I loci was estimated to have occurred at about 35-49 MYA, which is before the time of separation of Old World-New World monkeys. The gene duplication leading to the classical C locus in great apes appears to have occurred about 21-28 MYA. At approximately the same time the duplication of the B locus occurred in macaques. The oldest allelic lineages of A, B, and C loci in humans seem to have appeared at least 14-19, 10-15, and 13-17 MYA, respectively. Our phylogenetic analysis supports the hypothesis that the nonclassical locus F has diverged from the rest of class I loci very early in primate evolution. The overall phylogenetic pattern observed among class I genes is consistent with the model of birth-and-death evolution.     

Molecular cloning and linkage analysis of complement C3 and C4 genes of the Japanese medaka fish

 The thioester-containing complement components, C3 and C4, are believed to have arisen by gene duplication from a common ancestor, and the mammalian C4 gene resides in the vicinity of the C2 and B genes within the major histocompatibility complex (MHC) class III region. To analyze the evolution of both the complement system and the MHC, we determined the complete primary structures of two C3 genes, termed Orla C3-1 and Orla C3-2, and one C4 gene, termed Orla C4, of a teleost, Japanese medaka fish (Oryzias latipes), by analyzing cDNA clones isolated from a liver library constructed using the inbred AA2 strain. The deduced basic structures of Orla C3-1, C3-2, and C4, such as the subunit chain structure, the thioester site, and the proteolytic activation site, are similar to their mammalian counterparts. However, the catalytic His residue which greatly increases the rate of thioester reaction, is replaced by Ala in Orla C3-2, implying functional differentiation between two C3 molecules. Mapping analysis revealed a close linkage between the C3-1 and C3-2 genes, indicating that they arose by a local duplication rather than by a genome-wide tetraploidization. The C4 gene belongs to a different linkage group, and no linkage was observed among the C3, C4, Bf/C2, MHC class I, and MHC class II loci. These results suggest that the MHC class III complement region was established in the tetrapod lineage, or lost in the teleost lineage. Received: 15 July 1999 / Revised: 3 September 1999     

Feline herpesvirus‐1 down‐regulates MHC class I expression in an homologous cell system

Cytotoxic T lymphocytes (CTLs) are an essential component of the immune defense against many virus infections. CTLs recognize viral peptides in the context of the major histocompatibility complex (MHC) class I molecules on the surface of infected cells. Many viruses have evolved mechanisms to interfere with MHC class I expression as a means of evading the host immune response. In the present research we have studied the effect of in vitro Feline Herpesvirus 1 (FeHV‐1) infection on MHC class I expression. The results of this study demonstrate that FeHV‐1 down regulates surface expression of MHC class I molecules on infected cells, presumably to evade cytotoxic T‐cell recognition and, perhaps, attenuate induction of immunity. Sensitivity to UV irradiation and insensitivity to a viral DNA synthesis inhibitor, like phosphonacetic acid, revealed that immediate early or early viral gene(s) are responsible. Use of the protein translation inhibitor cycloheximide confirmed that an early gene is primarily responsible. J. Cell. Biochem. 106: 179–185, 2009. © 2008 Wiley‐Liss, Inc.     

Sequence and evolution of cattle MHC class I cDNAs: concerted evolution has not taken place in cattle

To explore genetic mechanisms responsible for major histocompatibility complex (MHC) class I evolution in the artiodactyls, we cloned and sequenced MHC class I cDNAs from a Bos taurus bull heterozygous for cattle MHC (BoLA) class I serological specificities w2 and w30. Four unique cDNAs were found, indicating the presence of at least two MHC class I loci. Analysis of these four cDNAs and all previously published BoLA cDNA sequences suggested that there may be three cattle MHC class I loci. Additionally, comparison of all of the BoLA class I cDNAs to MHC class I cDNAs of other artiodactyls showed that some of the BoLA class I cDNAs were more similar to certain sheep cDNAs than they were to other cattle cDNAs. These data indicate that each BoLA class I locus has evolved independently after an ancestral gene duplication event and that inter-locus segmental exchange o or concerted evolution has not occurred rapidly enough to cause extensive divergence between the orthologous MHC class I loci of sheep and cattle.The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession numbers L02832–L02835. Correspondence to: T. L. Garber at the present address.     

Zwitterionic capsular polysaccharides: the new MHCII-dependent antigens

The immune system has evolved the ability for T cells to recognize nearly any biological polymer, including peptides, protein superantigens, and glycolipids through presentation by the major histocompatibility complex (MHC) proteins such as MHC class I (MHCI), MHC class II (MHCII), and CD1. A recent and unexpected addition to this list is the zwitterionic capsular polysaccharide (ZPS). These bacterial molecules utilize MHCII presentation to activate T cells via recognition by alphabeta T cell receptor (alphabetaTCR) proteins. In this review, we explore what is currently known about ZPS processing and presentation within antigen-presenting cells (APCs) and the immune response that follows.     

The oxidoreductase ERp57 efficiently reduces partially folded in preference to fully folded MHC class I molecules

The oxidoreductase ERp57 is an integral component of the peptide loading complex of major histocompatibility complex (MHC) class I molecules, formed during their chaperone-assisted assembly in the endoplasmic reticulum. Misfolded MHC class I molecules or those denied suitable peptides are retrotranslocated and degraded in the cytosol. The presence of ERp57 during class I assembly suggests it may be involved in the reduction of intrachain disulfides prior to retrotranslocation. We have studied the ability of ERp57 to reduce MHC class I molecules in vitro. Recombinant ERp57 specifically reduced partially folded MHC class I molecules, whereas it had little or no effect on folded and peptide-loaded MHC class I molecules. Reductase activity was associated with cysteines at positions 56 and 405 of ERp57, the N-terminal residues of the active CXXC motifs. Our data suggest that the reductase activity of ERp57 may be involved during the unfolding of MHC class I molecules, leading to targeting for degradation.     

Gene duplication and evidence for balancing selection acting on MHC class II DAA gene of the half-smooth tongue sole (Cynoglossus semilaevis)

Allelic polymorphism and evolution mechanism of major histocompatibility complex (MHC) genes has been investigated in many mammals, however, much less is known in teleost. In order to investigate the mechanisms creating and maintaining variability at the MHC class II DAA locus, we examined the polymorphism, gene duplication and balancing selection of MHC class II DAA gene of the half-smooth tongue sole (Cynoglossus semilaevis). We described 33 alleles in the C. semilaevis, recombination and gene duplication seems to play more important roles in the origin of new alleles. The rate of non-synonymous substitutions (d(N)) occurred at a significantly higher frequency than that of synonymous substitutions (d(S)) in peptide-binding region (PBR) and non-PBR, suggesting balancing selection for maintaining polymorphisms at the MHC II DAA locus. Many positive selection sites were found to act very intensively on antigen-binding sites. Our founding suggests a snapshot in an evolutionary process of MHC-DAA gene evolution of the C. semilaevis.     

Allelic polymorphism, gene duplication and balancing selection of the MHC class II DAB gene of Cynoglossus semilaevis (Cynoglossidae)

Major histocompatibility complex (MHC) genes play an important role in the immune response of vertebrates. Allelic polymorphism and evolutionary mechanism of MHC genes have been investigated in many mammals, but much less is known in teleosts. We examined the polymorphism, gene duplication and balancing selection of the MHC class II DAB gene of the half-smooth tongue sole (Cynoglossus semilaevis); 23 alleles were found in this species. Gene duplication manifested as three to six distinct sequences at each domain in the same individuals. Non-synonymous substitutions occurred at a significantly higher frequency than synonymous substitutions in the PBR domain, suggesting balancing selection for maintaining polymorphisms at the MHC II DAB locus. Many positive selection sites were found to act very intensely on antigen-binding sites of MHC class II DAB gene.     

Downregulation of major histocompatibility complex class I molecules by Kaposi's sarcoma-associated herpesvirus K3 and K5 proteins

The T-cell-mediated immune response plays a central role in the defense against intracellular pathogens. To avoid this immune response, viruses have evolved elaborate mechanisms that target and modulate many different aspects of the host's immune system. A target common to many of these viruses is the major histocompatibility complex (MHC) class I molecules. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes K3 and K5 zinc finger membrane proteins which remove MHC class I molecules from the cell surface. K3 and K5 exhibit 40% amino acid identity to each other and localize primarily near the plasma membrane. While K3 and K5 dramatically downregulated class I molecules, they displayed different specificities in downregulation of HLA allotypes. K5 significantly downregulated HLA-A and -B and downregulated HLA-C only weakly, but not HLA-E, whereas K3 downregulated all four HLA allotypes. This selective downregulation of HLA allotypes by K5 was partly due to differences in amino acid sequences in their transmembrane regions. Biochemical analyses demonstrated that while K3 and K5 did not affect expression and intracellular transport of class I molecules, their expression induced rapid endocytosis of the molecules. These results demonstrate that KSHV has evolved a novel immune evasion mechanism by harboring similar but distinct genes, K3 and K5, which target MHC class I molecules in different ways.     

Cross-presentation in viral immunity and self-tolerance

T lymphocytes recognize peptide antigens presented by class I and class II molecules encoded by the major histocompatibility complex (MHC). Classical antigen-presentation studies showed that MHC class I molecules present peptides derived from proteins synthesized within the cell, whereas MHC class II molecules present exogenous proteins captured from the environment. Emerging evidence indicates, however, that dendritic cells have a specialized capacity to process exogenous antigens into the MHC class I pathway. This function, known as cross-presentation, provides the immune system with an important mechanism for generating immunity to viruses and tolerance to self.     

Isolation of major histocompatibility complex Class I genes from the tammar wallaby (Macropus eugenii)

The major histocompatibility complex (MHC) plays an essential role in the adaptive immune system of vertebrates through antigen recognition. Although MHC genes are found in all vertebrates, the MHC region is dynamic and has changed throughout vertebrate evolution, making it an important tool for comparative genomics. Marsupials occupy an important position in mammalian phylogeny, yet the MHC of few marsupials has been studied in detail. We report the isolation and analysis of expressed MHC Class I genes from the tammar wallaby, a model marsupial used extensively for the study of mammalian reproduction, genetics, and immunology. We determined that there are at least 11 Class I loci in the tammar genome and isolated six expressed Class I sequences from spleen and testes cDNA libraries, representing at least four loci. Two of the Class I sequences contain substitutions at sites known to be important for antigen binding, perhaps impacting their ability to bind peptides, or the types of peptide to which they bind. Phylogenetic analysis of tammar wallaby Class I sequences and other mammalian Class I sequences suggests that some tammar wallaby and red-necked wallaby loci evolved from common ancestral genes.