Evolution of the Monotremes: Phylogenetic Relationship to Marsupials and Eutherians, and Estimation of Divergence Dates Based on α-Lactalbumin Amino Acid Sequences

The amino acid sequences of the -lactalbumins of the echidna, Tachyglossus aculeatus, and the platypus, Ornithorhynchus anatinus, were compared with each other and with those of 13 eutherian and 3 marsupial species. Phylogenetic parsimony analyses, in which selected mammalian lysozymes were used as outgroups, yielded trees whose consensus indicated that the two monotremes are sister taxa to marsupials and eutherians and that the latter two clades are each other's closest relatives. The data do not support the notion of a Marsupionta (monotreme–marsupial) clade. Pairwise comparison between the -lactalbumins yielded maximum-likelihood distances from which divergence dates were estimated on the basis of three calibration points. The distance data support the view that the echidna and platypus lineages diverged from their last common ancestor at least 50 to 57 Ma (million years ago) and that monotremes diverged from marsupials and eutherian mammals about 163 to 186 Ma.     

Marsupial MHC class II beta genes are not orthologous to the eutherian beta gene families

The major histocompatibility complex (MHC) class II DRB, DQB, DPB, and DOB gene clusters are shared by different eutherian orders. Such an orthologous relationship is not seen between the beta genes of birds and eutherians. A high degree of uncertainty surrounds the evolutionary relationship of marsupial class II beta sequences with eutherian beta gene families. In particular, it has been suggested that marsupials utilize the DRB gene cluster. A cDNA encoding an MHC class II beta molecule was isolated from a brushtail possum mesenteric lymph node cDNA library. This clone is most similar to Macropus rufogriseus DBB. Our analysis suggests that all known marsupial beta-chain genes, excluding DMB, fall into two separate clades, which are distinct from the eutherian DRB, DQB, DPB, or DOB gene clusters. We recommend that the DAB and DBB nomenclature be reinstated. DAB and DBB orthologs are not present in eutherians. It appears that the marsupial and eutherian lineages have retained different gene clusters following gene duplication events early in mammalian evolution.     

Evolution and molecular characterization of a beta-globin gene from the Australian Echidna Tachyglossus aculeatus (Monotremata).

Coinciding with a period in evolution when monotremes, marsupials, and eutherians diverged from a common ancestor, a proto-beta-globin gene duplicated, producing the progenitors of mammalian embryonic and adult beta-like globin genes. To determine whether monotremes contain orthologues of these genes and to further investigate the evolutionary relationships of monotremes, marsupials, and eutherians, we have determined the complete DNA sequence of an echidna (Tachyglossus aculeatus) beta-like globin gene. Conceptual translation of the gene and sequence comparisons with eutherian and marsupial beta-like globin genes and echidna adult beta-globin indicate that the gene is adult expressed. Phylogenetic analyses do not clearly resolve the branching pattern of mammalian beta-like globin gene lineages and it is therefore uncertain whether monotremes have orthologues of the embryonic beta-like globin genes of marsupials and eutherians. Four models are proposed that provide a framework for interpreting further studies on the evolution of beta-like globin genes in the context of the evolution of monotremes, marsupials, and eutherians.     

Antibody response to sheep red blood cells in platypus and echidna

There is limited information regarding the kinetics of antibody responses exhibited by the platypus and the echidna in response to a T cell dependent antigen. In this preliminary study a platypus, an echidna and a rabbit were inoculated with sheep red blood cells to compare their antibody responses and kinetics. The antibody titres, produced by the platypus and echidna, were less than those elicited in the rabbit. Furthermore, the echidna and platypus exhibited a weak secondary response. This was most likely due to a failure of the platypus and echidna to undergo the characteristic IgM to IgG isotype switch following second antigen exposure. The conformational structure of these antibodies may differ from eutherian antibodies. This was further supported by a heat sensitivity experiment that indicated that these antibodies are more labile than rabbit immunoglobulins and therefore structurally less stable.     

The α-Globin Gene Family of an Australian Marsupial, Macropus eugenii: The Long Evolutionary History of the θ-Globin Gene and Its Functional Status in Mammals

Comparative evolutionary analyses of gene families among divergent lineages can provide information on the order and timing of major gene duplication events and evolution of gene function. Here we investigate the evolutionary history of the α-globin gene family in mammals by isolating and characterizing α-like globin genes from an Australian marsupial, the tammar wallaby, Macropus eugenii. Sequence and phylogenetic analyses indicate that the tammar α-globin family consists of at least four genes including a single adult-expressed gene (α), two embryonic/neonatally expressed genes (ζ and ζ′), and θ-globin, each orthologous to the respective α-, ζ-, and θ-globin genes of eutherian mammals. The results suggest that the θ-globin lineage arose by duplication of an ancestral adult α-globin gene and had already evolved an unusual promoter region, atypical of all known α-globin gene promoters, prior to the divergence of the marsupial and eutherian lineages. Evolutionary analyses, using a maximum likelihood approach, indicate that θ-globin, has evolved under strong selective constraints in both marsupials and the lineage leading to human θ-globin, suggesting a long-term functional status. Overall, our results indicate that at least a four-gene cluster consisting of three α-like and one β-like globin genes linked in the order 5′–ζ–α–θ–ω–3′ existed in the common ancestor of marsupials and eutherians. However, results are inconclusive as to whether the two tammar ζ-globin genes arose by duplication prior to the radiation of the marsupial and eutherian lineages, with maintenance of exon sequences by gene conversion, or more recently within marsupials.Reviewing Editor: Dr. John Oakeshott     

Isolation of monotreme T-cell receptor α and β chains

Monotremes are an ancient mammalian lineage that last shared a common ancestor with the marsupial and eutherian (placental) mammals about 170 million years ago. Characterization of their immune genes is allowing us to gain insights into the evolutionary processes that lead to the mammalian immune response. Here we describe the characterization of the first cDNA clones encoding T-cell receptors from a monotreme. Two TCR -chain cDNAs (TCRA) from the short-beaked echidna, Tachyglossus aculeatus, containing complete variable, joining and constant regions were isolated. The echidna TCRA constant region shares approximately 37% amino acid identity with other mammalian TCRA constant region sequences. The two variable regions belong to the TCRAV group C, which also contains V genes from humans, mice, cattle and chickens. One echidna TCR -chain cDNA (TCRB) containing the entire constant region was isolated and sequenced. It shares about 63% identity with other mammalian TCRB constant region sequences. The echidna TCRBV belongs to TCRBV group A, which also contains V genes from various eutherian species. Southern blot analysis indicates that, like in other mammalian species, there is only one TCRA constant region copy in the echidna genome, but at least two TCRB constant regions.     

The anatomy of the cerebral cortex of the echidna (Tachyglossus aculeatus)

The cerebral cortex of the echidna is notable for its extensive folding and the positioning of major functional areas towards its caudal extremity. The gyrification of the echidna cortex is comparable in magnitude to prosimians and cortical thickness and neuronal density are similar to that seen in rodents and carnivores. On the other hand, many pyramidal neurons in the cerebral cortex of the echidna are atypical with inverted somata and short or branching apical dendrites. All other broad classes of neurons noted in therian cortex are also present in the echidna, suggesting that the major classes of cortical neurons evolved prior to the divergence of proto- and eutherian lineages. Dendritic spine density on dendrites of echidna pyramidal neurons in somatosensory cortex and apical dendrites of motor cortex pyramidal neurons is lower than that found in eutheria. On the other hand, synaptic morphology, density and distribution in somatosensory cortex are similar to that in eutheria. In summary, although the echidna cerebral cortex displays some structural features, which may limit its functional capacities (e.g. lower spine density on pyramidal neurons), in most structural parameters (e.g. gyrification, cortical area and thickness, neuronal density and types, synaptic morphology and density), it is comparable to eutheria.     

A survey of type I interferons from a marsupial and monotreme: implications for the evolution of the type I interferon gene family in mammals

Sequence data for type I interferons (IFNs) have previously only been available for birds and eutherian ('placental') mammals, but not for the other two groups of extant mammals, the marsupials and monotremes. This has left a large gap in our knowledge of the evolutionary and functional relationships of what is a complex gene family in eutherians. In this study, a PCR-based survey of type I IFN genes from a marsupial, the tammar wallaby (Macropus eugenii), and a monotreme, the short-beaked echidna (Tachyglossus aculeatus), was conducted. Along with Southern blot and phylogenetic analysis, this revealed a large number of type I IFN genes for the wallaby, rivalling that of eutherians, but relatively few type I IFN genes in the echidna. The wallaby genes include both IFNA and IFNB orthologues, indicating that the gene duplication leading to these subtypes occurred prior to the divergence of marsupials and eutherians some 130 million years ago. Results from this study support the idea that the expansion of type I IFN gene complexity in mammals coincides with a concomitant expansion in the functionality of these molecules. For example, this expansion in complexity may have, at least partially, facilitated the evolution of viviparity in marsupials and eutherians. Other evolutionary aspects of these sequences are also discussed.     

Topography and chemoarchitecture of the striatum and pallidum in a monotreme, the short-beaked echidna (Tachyglossus aculeatus)

The topography and chemoarchitecture of the striatum and pallidum in a monotreme, the short-beaked echidna (Tachyglossus aculeatus) have been studied using Nissl staining in conjunction with myelin staining, enzyme reactivity to acetylcholinesterase and NADPH diaphorase, and immunoreactivity to parvalbumin, calbindin, calretinin, tyrosine hydroxylase, neuropeptide Y, and neurofilament protein (SMI-32 antibody). All those components of the striatum and pallidum found in eutherian mammals could also be identified in the echidna's brain, with broad chemoarchitectural similarities to those regions in eutherian brains also apparent. There was a clear chemoarchitectural gradient visible with parvalbumin immunoreactivity of neurons and fibers, suggesting a subdivision of the echidna caudatoputamen into weakly reactive rostrodorsomedial and strongly reactive caudoventrolateral components. This may, in turn, relate to subdivision into associative versus sensorimotor CPu and reflect homology to the caudate and putamen of primates. Moreover, the chemoarchitecture of the echidna striatum suggested the presence of striosome-matrix architecture. The morphology of identified neuronal groups (i.e., parvalbumin, calbindin, and neuropeptide Y immunoreactive) in the echidna striatum and pallidum showed many similarities to those seen in eutherians, although the pattern of distribution of calbindin immunoreactive neurons was more uniform in the caudatoputamen of the echidna than in therians. These observations indicate that the same broad features of striatal and pallidal organization apply across all mammals and suggest that these common features may have arisen before the divergence of the monotreme and therian lineages.     

The major histocompatibility complex in monotremes: an analysis of the evolution of <Emphasis Type="Italic">Mhc</Emphasis> class I genes across all three mammalian subclasses

We report the isolation and characterization of cDNA clones of expressed, functional major histocompatibility complex class-I ( Mhc-I) genes from two species of monotremes: the duck-billed platypus and the short-beaked echidna. The cDNA clones were isolated from libraries constructed from spleen RNA, clearly establishing their expression in at least this one peripheral lymphoid organ. From the presence of conserved amino acid residues, it appears the expressed sequences encode molecules that likely function as classical Mhc-I. These clones were isolated using monotreme Mhc-I processed pseudogenes as probes. These processed pseudogenes were isolated from genomic DNA and, based on their structure, are likely independently derived in the platypus and echidna. When all the monotreme sequences were included in phylogenetic analyses, we found no apparent orthologous relationships between the platypus and echidna Mhc-I. Analyses that included a large number of Mhc-I sequences from other taxa support a separate monotreme Mhc-I clade, basal to a therian Mhc-I clade that is comprised of sequences from marsupial and placental mammals. The phylogenies also support the hypothesis that Mhc-I genes of placental mammals, marsupials, and monotremes are derived from three separate lineages of Mhc-I genes, best explained by two rounds of duplications and deletions. The first round would have occurred prior to the divergence of monotremes and therians, and the second prior to the divergence of marsupials and placental mammals. The sequences described here represent the first reported functional monotreme Mhc-I, as well as the first processed pseudogenes of any type from monotremes.     

Genes on the short arm of the human X chromosome are not shared with the marsupial X.

Eight genes located on the short arm of the human X chromosome (MAOA, SYN1, OAT, OTC, CYBB, DMD, ZFX, POLA) have been mapped in several marsupial species by cell hybrid analysis and/or in situ hybridization using probes derived from human cDNA. Seven appear to be autosomal in all marsupial species examined. The eighth, CYBB, detected a site on the X, as well as major autosomal sites. Although these genes are not conserved on the X chromosome in marsupials, at least some of them are arranged together in autosomal clusters. The autosomal location of human Xp genes in marsupials could mean that this region either was lost from a large ancestral X chromosome in the marsupial lineage or was acquired by a small ancestral X (and perhaps Y) in the eutherian lineage. Either explanation demands that the region was not subject to X chromosome inactivation in a common ancestor 120-150 MyrBP.     

Calcium-binding and structural stability of echidna and canine milk lysozymes.

For echidna and canine milk lysozymes, which were presumed to be the calcium-binding lysozymes by their amino acid sequences, we have quantitated their calcium-binding strength and examined their guanidine unfolding profiles. The calcium-binding constants of echidna and canine lysozymes were determined to be 8.6 x 10(6) M(-1) and 8.9 x 10(6) M(-1) in 0.1 M KCl at pH 7.1 and 20 C, respectively. The unfolding of decalcified canine lysozyme proceeds in the same manner as that of alpha-lactalbumin, through a stable molten globule intermediate. However, neither calcium-bound nor decalcified echidna lysozyme shows a stable molten globule intermediate. This unfolding profile of echidna lysozyme is identical to that of conventional lysozymes and pigeon egg-white lysozyme, avian calcium-binding lysozyme. This result supports the suggestion of Prager and Jolles (Prager EM, Jolles P. 1996. Animal lysozymes c and g: An overview. In: Jolles P, ed. Lysozymes: Model enzymes in biochemistry and biology. Basel-Boston-Berlin: Birkhauzer Verlag. pp 9-31) that the lineage of avian and echidna calcium-binding lysozymes and that of eutherian calcium-binding lysozymes diverged separately from that of conventional lysozymes.     

Immunoglobulin genetics of Ornithorhynchus anatinus (platypus) and Tachyglossus aculeatus (short-beaked echidna)

In this paper, we review data on the monotreme immune system focusing on the characterisation of lymphoid tissue and of antibody responses, as well the recent cloning of immunoglobulin genes. It is now known that monotremes utilise immunoglobulin isotypes that are structurally identical to those found in marsupials and eutherians, but which differ to those found in birds and reptiles. Monotremes utilise IgM, IgG, IgA and IgE. They do not use IgY. Their IgG and IgA constant regions contain three domains plus a hinge region. Preliminary analysis of monotreme heavy chain variable region diversity suggests that the platypus primarily uses a single VH clan, while the short-beaked echidna utilises at least 4 distinct VH families which segregate into all three mammalian VH clans. Phylogenetic analysis of the immunoglobulin heavy chain constant region gene sequences provides strong support for the Theria hypothesis. The constant region of IgM has proven to be a useful marker for estimating the time of divergence of mammalian lineages.     

Identification of tammar wallaby SIRH12, derived from a marsupial-specific retrotransposition event

In humans and mice, there are 11 genes derived from sushi-ichi related retrotransposons, some of which are known to play essential roles in placental development. Interestingly, this family of retrotransposons was thought to exist only in eutherian mammals, indicating their significant contributions to the eutherian evolution, but at least one, PEG10, is conserved between marsupials and eutherians. Here we report a novel sushi-ichi retrotransposon-derived gene, SIRH12, in the tammar wallaby, an Australian marsupial species of the kangaroo family. SIRH12 encodes a protein highly homologous to the sushi-ichi retrotransposon Gag protein in the tammar wallaby, while SIRH12 in the South American short-tailed grey opossum is a pseudogene degenerated by accumulation of multiple nonsense mutations. This suggests that SIRH12 retrotransposition occurred only in the marsupial lineage but acquired and retained some as yet unidentified novel function, at least in the lineage of the tammar wallaby.     

Trophoblast and hypoblast in the monotreme, marsupial and eutherian mammal: evolution and origins

The pregastrula stage mammalian conceptus consists of both embryonic and non-embryonic components. The latter forms the bulk of the tissues, provides nutrition for the developing embryo and also contributes developmental signals that influence events within the embryo itself. Understanding the origins and relationships between the embryonic and extraembryonic cell lineages is thus central to understanding development in mammals. Despite the apparent gross differences in early developmental strategy and form, the conceptuses of eutherian, marsupial and monotreme mammals show some remarkable similarities in the lineage allocation to trophoblast and hypoblast and in the emergent properties of the two cell types. We suggest that the gross differences can be explained by two relatively small evolutionary timing changes affecting cell adhesion patterns and the polarisation of developmentally significant information. These changes result in the conversion of a unilaminar blastocyst to a morula form composed of blastomeres with increased regulatory capacity.     

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.