The origin of eutherian mammals

Palaeontologically recognizable eutherians originated no later than the Early Cretaceous in warm, probably moderately seasonal climates. Immediate ancestors were small, sharing many anatomical, physiological and reproductive features with small modern marsupials. Development of characteristically eutherian features involved interactions of body size, rates of metabolism, energetic costs of reproduction, anatomical/physiological processes of development and effects of each upon rates of population growth. In contrast to eutherians, marsupials have a narrow range of basal metabolic rates (lacking high rates), and show no direct links between rate of energy expenditure and gestation period, postnatal growth rate, fecundity or reproductive potential. Biological implications of this contrast are most pronounced at small body sizes. When resources are abundant, the relatively higher growth rates and earlier maturation of small eutherians (particularly those with high rates of metabolism) can lead to rapid population growth; among most marsupials, however, both pre- and postnatal constraints apparently preclude attainment of such high rates of reproduction. Also, only eutherians among the amniotes combine intimacy of placentation with prolonged active intra-uterine morphogenesis. Once established, that combination permitted (and even favoured) increases in diversity of adaptation in such disparate aspects as elevated metabolic rate, increased pre- and postnatal growth rates, increased encephalization, greater longevity, increased gregariousness, greater karyotypic flexibility, and augmented variability in adult morphology. However, all such boosts in diversity were probably secondary and dependent upon prior innovation of trophoblastic/uterine wall immunological protection of foetal tissues during prolonged intra-uterine development. Increased metabolic rates followed thereafter, with synergisms that may have speeded evolution among early eutherians. Eutherian-style trophoblast probably originated in the Mesozoic. Dependent adaptations, variably expressed, evolved later in sundry descendant lineages. Reproductive differences between marsupials and eutherians are not biologically trivial; to the contrary, breakthroughs among eutherians assured their dominance: (1) in high intensity food habits; (2) at small body masses; and (3) in very cold climates.     

Cardiovascular characteristics of two resting marsupials: An insight into the cardio-respiratory allometry of marsupials

Summary Minimum resting values for several cardiovascular and respiratory characteristics were established for two marsupial species,Trichosurus vulpecula andMacropus eugenii. Certain characteristics including heart rate, stroke volume and cardiac output varied significantly with body mass and allometric equations of the formy=aM ^b were derived to describe the relationships. The exponents of body mass,b, were generally similar to those for eutherian mammals, but in the marsupials they intercept,a, differed significantly from reported eutherian values.Although resting cardiac output in the marsupials appeared reduced in proportion to their lower resting oxygen consumption this pattern was not repeated for other variables. The stroke volume of the marsupials was 156% of eutherian levels while heart rate was less than 50% of the eutherian values.Initial data for respiratory variables also indicated comparable differences in this aspect of oxygen transport between marsupials and eutherians. Minimum respiratory rates of the marsupials were much lower than those of eutherians and tidal volumes appear larger in marsupials. The results are interpreted as suggesting that marsupials may have a large aerobic capacity.     

Evolution of the shell coat and yolk in amniotes: a marsupial perspective

Two characters distinguish oogenesis and early development in marsupials and monotremes: (1) the shell coat that persists from the zygote to somite stages in marsupials or until hatching in monotremes; and (2) the numerous, apparently almost empty vesicles that appear in primary oocytes, increase during oogenesis in marsupials and monotremes before being shed into the cleavage cavity and are preferentially distributed to the trophoblast lineage in marsupials, but comprise the latebra in monotremes. Analysis of these unusual characters used Southern analysis of genomic DNA dot blots and histology and electron microscopy. The evidence suggests that the marsupial shell coat protein, CP4, was probably characteristic of the egg of the mammalian ancestor. Further, the vesicles, present in marsupials during oogensis and cleavage and in eutherian mammals during blastocyst formation are the residual elements of white yolk present in the larger yolky eggs of monotemes and sauropsids. By comparison with the function of the vesicle components in marsupials, it is suggested that one role for the white yolk in monotremes and the sauropsids is to provide extracellular matrix (ECM), especially hyaluronan containing stabilizing proteins, for epithelial construction. Thus, as oviparity was replaced by viviparity, egg size was reduced, the germinal cytoplasm was retained, and yellow yolk was markedly reduced or lost in marsupials and eutherians. The white yolk was retained in monotremes and marsupials where blastocyst epithelial construction requires ECM support, and its appearance is heterochronously shifted to after compaction, when blastocyst formation and expansion occurs, in eutherian mammals.     

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.     

Food habits, energetics, and the reproduction of marsupials

Basal rate of metabolism in marsupials and in eutherian mammals is principally correlated with body mass, food habits and activity. Feeding on fruit, the leaves of woody plants, or invertebrates is associated with low basal rates, especially at large masses, in both groups of mammals. These foods lead to low basal rates because they are seasonally unavailable, are indigestible, or need to be detoxified. The depression in basal rate associated with frugivory and folivory is increased when coupled with sedentary, arboreal habits in both marsupials and eutherians. In contrast, eutherians that feed on vertebrates or herbs generally have high basal rates, while marsupials that eat these foods do not have high basal rates. These foods permit high basal rates, which are exploited by eutherians because high basal rates in these mammals lead to high rates of reproduction. Marsupials have, at best, a limited correlation of reproduction with rate of metabolism, so that feeding on vertebrates or herbs does not lead to high basal rates in these mammals. This difference between marsupials and eutherians in the coupling of reproduction to energetics has at least two ecological consequences. 1) Marsupials generally do not tolerate cold-temperate environments because they do not accelerate growth and development to complete reproduction within a short spring and summer. 2) Marsupials coexist with ecologically similar eutherians as long as marsupials have food habits that are correlated with low rates of metabolism in eutherians (i.e. they feed on fruit, the leaves of woody plants, or invertebrates), but they tend to be displaced by eutherians when marsupials have food habits that are associated with high rates of metabolism in eutherians (i.e. when they feed on vertebrates and, probably, herbs).     

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.     

The evolution of the thyroid hormone distributor protein transthyretin in the order insectivora, class mammalia

Thyroid hormones are involved in the regulation of growth and metabolism in all vertebrates. Transthyretin is one of the extracellular proteins with high affinity for thyroid hormones which determine the partitioning of these hormones between extracellular compartments and intracellular lipids. During vertebrate evolution, both the tissue pattern of expression and the structure of the gene for transthyretin underwent characteristic changes. The purpose of this study was to characterize the position of Insectivora in the evolution of transthyretin in eutherians, a subclass of Mammalia. Transthyretin was identified by thyroxine binding and Western analysis in the blood of adult shrews, hedgehogs, and moles. Transthyretin is synthesized in the liver and secreted into the bloodstream, similar to the situation for other adult eutherians, birds, and diprotodont marsupials, but different from that for adult fish, amphibians, reptiles, monotremes, and Australian polyprotodont marsupials. For the characterization of the structure of the gene and the processing of mRNA for transthyretin, cDNA libraries were prepared from RNA from hedgehog and shrew livers, and full-length cDNA clones were isolated and sequenced. Sections of genomic DNA in the regions coding for the splice sites between exons 1 and 2 were synthesized by polymerase chain reaction and sequenced. The location of splicing was deduced from comparison of genomic with cDNA nucleotide sequences. Changes in the nucleotide sequence of the transthyretin gene during evolution are most pronounced in the region coding for the N-terminal region of the protein. Both the derived overall amino sequences and the N-terminal regions of the transthyretins in Insectivora were found to be very similar to those in other eutherians but differed from those found in marsupials, birds, reptiles, amphibians, and fish. Also, the pattern of transthyretin precursor mRNA splicing in Insectivora was more similar to that in other eutherians than to that in marsupials, reptiles, and birds. Thus, in contrast to the marsupials, with a different pattern of transthyretin gene expression in the evolutionarily "older" polyprotodonts compared with the evolutionarily "younger" diprotodonts, no separate lineages of transthyretin evolution could be identified in eutherians. We conclude that transthyretin gene expression in the liver of adult eutherians probably appeared before the branching of the lineages leading to modern eutherian species.     

Water usage and diet preferences of free ranging kangaroos, sheep and feral goats in the Australian arid zone during summer

In much of the arid rangelands of Australia the common large mammalian herbivores are the native marsupials, the Red kangaroo ( Megalein rufa ) and the Euro ( Mucropus robustus ), and the introduced eutherian species, the domestic sheep ( Ovis aries ) and the feral goat ( Capra hircus ). In the north west of New South Wales a study was carried out to examine the patterns of water usage during summer of these species when they were free ranging in the same area. Drinking frequencies, water turnovers, electrolyte concentrations and diet preferences were determined and information obtained on movements and dispersal from watering points.
The marsupials had a much lower water usage than either of the eutherians. The tritiated water turnover of the goats was three times and that of the sheep four times the value obtained for kangaroos. While much of the difference between the marsupials and eutherians appeared to be due to fundamental physiological differences the high water usage of sheep was in part related to the high intake of halophytic plants in their diet.
The water turnovers of the Red kangaroos and Euros were not found to be different. Drinking studies and urine osmolalities suggested, however, that the open plains dwelling Red kangaroos had a higher water requirement than the hill inhabiting Euro.
The principal components of the diets of the Red kangaroo were grasses; of the Euro, grasses and shrubs; of goats, trees and shrubs; of sheep, halophytic shrubs.     

Sexual development in marsupials: genetic characterization of bandicoot siblings with scrotal and testicular maldevelopment

In marsupials testis determination requires the presence of a Y chromosome. The sex determining region on the Y gene (SRY) is necessary for testicular development in eutherians and it is assumed to play a similar role in marsupials. Relatively few studies have investigated the genetic basis of sexual development, and as yet there is no direct evidence that SRY is required for testis development in marsupials. Studies on intersexual marsupials have revealed a fundamental difference between marsupial and eutherian sex determination. The scrotum of marsupials is analogous, not homologous, to the eutherian scrotum and is under the control of X-linked genes not androgens. The current study describes two bandicoot (Isoodon macrourus) siblings. Both siblings had underdeveloped male reproductive tracts and testicular dysgenesis, one was ascrotal and the other had a diminutive scrotum. Their karyotypes were normal for this species which eliminates the Y chromosome from some somatic tissues. SRY was detected by Southern blotting. SRY, ubiquitin activating enzyme-1 on the Y (UBE1Y) and glucose 6-phosphate dehydrogenase (G6PD) gene expression were examined. UBE1Y was widely expressed in many tissues. SRY gene expression was much lower than normal in the abnormal siblings and may be responsible for their failure of testicular and epididymal development. The cause of their scrotal abnormalities is unknown. It is possible that the separate defects of scrotal and testis development in the two siblings, which had normal relatives, were due to a mutation in a gene common to both developmental pathways.     

C-->U editing of apolipoprotein B mRNA in marsupials: identification and characterisation of APOBEC-1 from the American opossum Monodelphus domestica.

The C->U editing of RNA is widely found in plant and animal species. In mammals it is a discrete process confined to the editing of apolipoprotein B (apoB) mRNA in eutherians and the editing of the mitochondrial tRNA for glycine in marsupials. Here we have identified and characterised apoB mRNA editing in the American opossum Monodelphus domestica. The apoB mRNA editing site is highly conserved in the opossum and undergoes complete editing in the small intestine, but not in the liver or other tissues. Opossum APOBEC-1 cDNA was cloned, sequenced and expressed. The encoded protein is similar to APOBEC-1 of eutherians. Motifs previously identified as involved in zinc binding, RNA binding and catalysis, nuclear localisation and a C-terminal leucine-rich domain are all conserved. Opossum APOBEC-1 contains a seven amino acid C-terminal extension also found in humans and rabbits, but not present in rodents. The opossum APOBEC-1 gene has the same intron/exon organisation in the coding sequence as the eutherian gene. Northern blot and RT-PCR analyses and an editing assay indicate that no APOBEC-1 was expressed in the liver. Thus the far upstream promoter responsible for hepatic expression in rodents does not operate in the opossum. An APOBEC-1-like enzyme such as might be involved in C->U RNA editing of tRNA in marsupial mitochondria was not demonstrated. The activity of opossum APOBEC-1 in the presence of both chicken and rodent auxiliary editing proteins was comparable to that of other mammals. These studies extend the origins of APOBEC-1 back 170 000 000 years to marsupials and help bridge the gap in the origins of this RNA editing process between birds and eutherian mammals.     

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.     

CSF2RA, ANT3, and STS are autosomal in marsupials: implications for the origin of the pseudoautosomal region of mammalian sex chromosomes

The X and Y Chromosomes (Chrs) of eutherian (``placental') mammals share a pseudo-autosomal region (PAR) that pairs and recombines at meiosis. In humans and other eutherians, the PAR contains several active genes and has also been thought to be critical for pairing and fertility. In order to explore the origin of the PAR, we cloned and mapped three human or mouse pseudoautosomal genes in marsupials, a group of mammals that diverged from eutherians about 130 (MYrBP). All three genes were autosomal in marsupials, and two co-localized with other human Xp genes on an autosome. This implies that the human PAR, like most of human Xp, represents a relic of an autosomal region added to both X and Y Chrs between 80 and 150 MYrBP. Received: 19 September 1997 / Accepted: 20 January 1998     

Meiotic sex chromosome inactivation in the marsupial Monodelphis domestica

In eutherian mammals, the X and Y chromosomes undergo meiotic sex chromosome inactivation (MSCI) during spermatogenesis in males. However, following fertilization, both the paternally (Xp) and maternally (Xm) inherited X chromosomes are active in the inner cell mass of the female blastocyst, and then random inactivation of one X chromosome occurs in each cell, leading to a mosaic pattern of X-chromosome activity in adult female tissues. In contrast, marsupial females show a nonrandom pattern of X chromosome activity, with repression of the Xp in all somatic tissues. Here, we show that MSCI also occurs during spermatogenesis in marsupials in a manner similar to, but more stable than that in eutherians. These findings support the suggestion that MSCI may have provided the basis for an early dosage compensation mechanism in mammals based solely on gametogenic events, and that random X-chromosome inactivation during embryogenesis may have evolved subsequently in eutherian mammals.     

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.