Molecular conservation of marsupial and eutherian placentation and lactation

Placenta: an old organ with new functions

The transition from oviparity to viviparity and the establishment of feto-maternal communications introduced the placenta as the major anatomical site to provide nutrients, gases, and hormones to the developing fetus. The placenta has endocrine functions, orchestrates maternal adaptations to pregnancy at different periods of pregnancy, and acts as a selective barrier to minimize exposure of developing fetus to xenobiotics, pathogens, and parasites. Despite the fact that this ancient organ is central for establishment of a normal pregnancy in eutherians, the placenta remains one of the least studied organs. The first step of pregnancy, embryo implantation, is finely regulated by the trophoectoderm, the precursor of all trophoblast cells. There is a bidirectional communication between placenta and endometrium leading to decidualization, a critical step for maintenance of pregnancy. There are three-direction interactions between the placenta, maternal immune cells, and the endometrium for adaptation of endometrial immune system to the allogeneic fetus. While 65% of all systemically expressed human proteins have been found in the placenta tissues, it expresses numerous placenta-specific proteins, whose expression are dramatically changed in gestational diseases and could serve as biomarkers for early detection of gestational diseases. Surprisingly, placentation and carcinogenesis exhibit numerous shared features in metabolism and cell behavior, proteins and molecular signatures, signaling pathways, and tissue microenvironment, which proposes the concept of "cancer as ectopic trophoblastic cells". By extensive researches in this novel field, a handful of cancer biomarkers has been discovered. This review paper, which has been inspired in part by our extensive experiences during the past couple of years, highlights new aspects of placental functions with emphasis on its immunomodulatory role in establishment of a successful pregnancy and on a potential link between placentation and carcinogenesis.

Quantifying maternal investment in mammals using allometry

Maternal investment influences the survival and reproduction of both mothers and their progeny and plays a crucial role in understanding individuals' life-history and population ecology. To reveal the complex mechanisms associated with reproduction and investment, it is necessary to examine variations in maternal investment across species. Comparisons across species call for a standardised method to quantify maternal investment, which remained to be developed. This paper addresses this limitation by introducing the maternal investment metric - MI - for mammalian species, established through the allometric scaling of the litter mass at weaning age by the adult mass and investment duration (i.e. gestation + lactation duration) of a species. Using a database encompassing hundreds of mammalian species, we show that the metric is not highly sensitive to the regression method used to fit the allometric relationship or to the proxy used for adult body mass. The comparison of the maternal investment metric between mammalian subclasses and orders reveals strong differences across taxa. For example, our metric confirms that Eutheria have a higher maternal investment than Metatheria. We discuss how further research could use the maternal investment metric as a valuable tool to understand variation in reproductive strategies.

ATG9B is a tissue-specific homotrimeric lipid scramblase that can compensate for ATG9A

Macroautophagy/autophagy is a fundamental aspect of eukaryotic biology, and the autophagy-related protein ATG9A is part of the core machinery facilitating this process. In addition to ATG9A vertebrates encode ATG9B, a poorly characterized paralog expressed in a subset of tissues. Herein, we characterize the structure of human ATG9B revealing the conserved homotrimeric quaternary structure and explore the conformational dynamics of the protein. Consistent with the experimental structure and computational chemistry, we establish that ATG9B is a functional lipid scramblase. We show that ATG9B can compensate for the absence of ATG9A in starvation-induced autophagy displaying similar subcellular trafficking and steady-state localization. Finally, we demonstrate that ATG9B can form a heteromeric complex with ATG2A. By establishing the molecular structure and function of ATG9B, our results inform the exploration of niche roles for autophagy machinery in more complex eukaryotes and reveal insights relevant across species.Abbreviation: ATG: autophagy related; CHS: cholesteryl hemisuccinate; cryo-EM: single-particle cryogenic electron microscopy; CTF: contrast transfer function: CTH: C- terminal α helix; FSC: fourier shell correlation; HDIR: HORMA domain interacting region; LMNG: lauryl maltose neopentyl glycol; MD: molecular dynamics simulations; MSA: multiple sequence alignment; NBD-PE: 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl ammonium salt); POPC: palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine; RBG: repeating beta groove domain; RMSD: root mean square deviation; SEC: size-exclusion chromatography; TMH: transmembrane helix.

Comparative analysis reveals epigenomic evolution related to species traits and genomic imprinting in mammals

DNA methylation is an epigenetic modification that plays a crucial role in various regulatory processes, including gene expression regulation, transposable element repression, and genomic imprinting. However, most studies on DNA methylation have been conducted in humans and other model species, whereas the dynamics of DNA methylation across mammals remain poorly explored, limiting our understanding of epigenomic evolution in mammals and the evolutionary impacts of conserved and lineage-specific DNA methylation. Here, we generated and gathered comparative epigenomic data from 13 mammalian species, including two marsupial species, to demonstrate that DNA methylation plays critical roles in several aspects of gene evolution and species trait evolution. We found that the species-specific DNA methylation of promoters and noncoding elements correlates with species-specific traits such as body patterning, indicating that DNA methylation might help establish or maintain interspecies differences in gene regulation that shape phenotypes. For a broader view, we investigated the evolutionary histories of 88 known imprinting control regions across mammals to identify their evolutionary origins. By analyzing the features of known and newly identified potential imprints in all studied mammals, we found that genomic imprinting may function in embryonic development through the binding of specific transcription factors. Our findings show that DNA methylation and the complex interaction between the genome and epigenome have a significant impact on mammalian evolution, suggesting that evolutionary epigenomics should be incorporated to develop a unified evolutionary theory.

Embryonic specializations for vertebrate placentation

The vertebrate placenta, a close association of fetal and parental tissue for physiological exchange, has evolved independently in sharks, teleost fishes, coelacanths, amphibians, squamate reptiles and mammals. This transient organ forms during pregnancy and is an important contributor to embryonic development in both viviparous and oviparous, brooding species. Placentae may be involved in transport of respiratory gases, wastes, immune molecules, hormones and nutrients. Depending on the taxon, the embryonic portion of the placenta is comprised of either extraembryonic membranes (yolk sac or chorioallantois) or temporary embryonic tissues derived via hypertrophy of pericardium, gill epithelium, gut, tails or fins. These membranes and tissues have been recruited convergently into placentae in several lineages. Here, we highlight the diversity and common features of embryonic tissues involved in vertebrate placentation and suggest future studies that will provide new knowledge about the evolution of pregnancy. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Socially transferred materials: why and how to study them

When biological material is transferred from one individual's body to another, as in ejaculate, eggs, and milk, secondary donor-produced molecules are often transferred along with the main cargo, and influence the physiology and fitness of the receiver. Both social and solitary animals exhibit such social transfers at certain life stages. The secondary, bioactive, and transfer-supporting components in socially transferred materials have evolved convergently to the point where they are used in applications across taxa and type of transfer. The composition of these materials is typically highly dynamic and context dependent, and their components drive the physiological and behavioral evolution of many taxa. Our establishment of the concept of socially transferred materials unifies this multidisciplinary topic and will benefit both theory and applications.

Placental imprinting of SLC22A3 in the IGF2R imprinted domain is conserved in therian mammals

Background

The eutherian IGF2R imprinted domain is regulated by an antisense long non-coding RNA, Airn, which is expressed from a differentially methylated region (DMR) in mice. Airn silences two neighbouring genes, Solute carrier family 22 member 2 (Slc22a2) and Slc22a3, to establish the Igf2r imprinted domain in the mouse placenta. Marsupials also have an antisense non-coding RNA, ALID, expressed from a DMR, although the exact function of ALID is currently unknown. The eutherian IGF2R DMR is located in intron 2, while the marsupial IGF2R DMR is located in intron 12, but it is not yet known whether the adjacent genes SLC22A2 and/or SLC22A3 are also imprinted in the marsupial lineage. In this study, the imprinting status of marsupial SLC22A2 and SLC22A3 in the IGF2R imprinted domain in the chorio-vitelline placenta was examined in a marsupial, the tammar wallaby.

Results

In the tammar placenta, SLC22A3 but not SLC22A2 was imprinted. Tammar SLC22A3 imprinting was evident in placental tissues but not in the other tissues examined in this study. A putative promoter of SLC22A3 lacked DNA methylation, suggesting that this gene is not directly silenced by a DMR on its promoter as seen in the mouse. Based on immunofluorescence, we confirmed that the tammar SLC22A3 is localised in the endodermal cell layer of the tammar placenta where nutrient trafficking occurs.

Conclusions

Since SLC22A3 is imprinted in the tammar placenta, we conclude that this placental imprinting of SLC22A3 has been positively selected after the marsupial and eutherian split because of the differences in the DMR location. Since SLC22A3 is known to act as a transporter molecule for nutrient transfer in the eutherian placenta, we suggest it was strongly selected to control the balance between supply and demand of nutrients in marsupial as it does in eutherian placentas.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-022-00465-4.

Origination of LTR retroelement-derived NYNRIN coincides with therian placental emergence

The emergence of the placenta is a revolutionary event in the evolution of therian mammals, to which some LTR retroelement–derived genes, such as PEG10, RTL1, and syncytin, are known to contribute. However, therian genomes contain many more LTR retroelement–derived genes that may also have contributed to placental evolution. We conducted large-scale evolutionary genomic and transcriptomic analyses to comprehensively search for LTR retroelement–derived genes whose origination coincided with therian placental emergence and that became consistently expressed in therian placentae. We identified NYNRIN as another Ty3/Gypsy LTR retroelement–derived gene likely to contribute to placental emergence in the therian stem lineage. NYNRIN knockdown inhibited the invasion of HTR8/SVneo invasive-type trophoblasts, whereas the knockdown of its nonretroelement-derived homolog KHNYN did not. Functional enrichment analyses suggested that NYNRIN modulates trophoblast invasion by regulating epithelial-mesenchymal transition and extracellular matrix remodeling and that the ubiquitin-proteasome system is responsible for the functional differences between NYNRIN and KHNYN. These findings extend our knowledge of the roles of LTR retroelement–derived genes in the evolution of therian mammals.

Progesterone Receptor Signaling in the Uterus Is Essential for Pregnancy Success

The uterus plays an essential role in the reproductive health of women and controls critical processes such as embryo implantation, placental development, parturition, and menstruation. Progesterone receptor (PR) regulates key aspects of the reproductive function of several mammalian species by directing the transcriptional program in response to progesterone (P4). P4/PR signaling controls endometrial receptivity and decidualization during early pregnancy and is critical for the establishment and outcome of a successful pregnancy. PR is also essential throughout gestation and during labor, and it exerts critical roles in the myometrium, mainly by the specialized function of its two isoforms, progesterone receptor A (PR-A) and progesterone receptor B (PR-B), which display distinct and separate roles as regulators of transcription. This review summarizes recent studies related to the roles of PR function in the decidua and myometrial tissues. We discuss how PR acquired key features in placental mammals that resulted in a highly specialized and dynamic role in the decidua. We also summarize recent literature that evaluates the myometrial PR-A/PR-B ratio at parturition and discuss the efficacy of current treatment options for preterm birth.

The Evolution of Viviparity in Vertebrates

In the vertebrate tree of life, viviparity or live birth has independently evolved many times, resulting in a rich diversity of reproductive strategies. Viviparity is believed to be a mode of reproduction that evolved from the ancestral condition of oviparity or egg laying, where most of the fetal development occurs outside the body. Today, there is not a simple model of parity transition to explain this species-specific divergence in modes of reproduction. Most evidence points to a gradual series of evolutionary adaptations that account for this phenomenon of reproduction, elegantly displayed by various viviparous squamates that exhibit placentae formed by the appositions of maternal and embryonic tissues, which share significant homology with the tissues that form the placenta in therian mammals. In an era where the genomes of many vertebrate species are becoming available, studies are now exploring the molecular basis of this transition from oviparity to viviparity, and in some rare instances its possible reversibility, such as the Australian three-toed skink (Saiphos equalis). In contrast to the parity diversity in squamates, mammals are viviparous with the notable exception of the egg-laying monotremes. Advancing computational tools coupled with increasing genome availability across species that utilize different reproductive strategies promise to reveal the molecular underpinnings of the ancestral transition of oviparity to viviparity. As a result, the dramatic changes in reproductive physiology and anatomy that accompany these parity changes can be reinterpreted. This chapter will briefly explore the vertebrate modes of reproduction using a phylogenetic framework and where possible highlight the role of potential candidate genes that may help explain the polygenic origins of live birth.

Placentation in Marsupials

It is sometimes implied that marsupials are "aplacental," on the presumption that the only mammals that have a placenta are the eponymous "placental" mammals. This misconception has persisted despite the interest in and descriptions of the marsupial placenta, even in Amoroso's definitive chapter. It was also said that marsupials had no maternal recognition of pregnancy and no placental hormone production. In addition, it was thought that genomic imprinting could not exist in marsupials because pregnancy was so short. We now know that none of these ideas have held true with extensive studies over the last four decades definitively showing that they are indeed mammals with a fully functional placenta, and with their own specializations.

Mammalian tumor-like organs. 1. The role of tumor-like normal organs and atypical tumor organs in the evolution of development (carcino-evo-devo)

Background

Earlier I hypothesized that hereditary tumors might participate in the evolution of multicellular organisms. I formulated the hypothesis of evolution by tumor neofunctionalization, which suggested that the evolutionary role of hereditary tumors might consist in supplying evolving multicellular organisms with extra cell masses for the expression of evolutionarily novel genes and the origin of new cell types, tissues, and organs. A new theory—the carcino-evo-devo theory—has been developed based on this hypothesis.

Main text

My lab has confirmed several non-trivial predictions of this theory. Another non-trivial prediction is that evolutionarily new organs if they originated from hereditary tumors or tumor-like structures, should recapitulate some tumor features in their development. This paper reviews the tumor-like features of evolutionarily novel organs. It turns out that evolutionarily new organs such as the eutherian placenta, mammary gland, prostate, the infantile human brain, and hoods of goldfishes indeed have many features of tumors. I suggested calling normal organs, which have many tumor features, the tumor-like organs.

Conclusion

Tumor-like organs might originate from hereditary atypical tumor organs and represent the part of carcino-evo-devo relationships, i.e., coevolution of normal and neoplastic development. During subsequent evolution, tumor-like organs may lose the features of tumors and the high incidence of cancer and become normal organs without (or with almost no) tumor features.

Molecular Signatures of Placentation and Secretion Uncovered in Poeciliopsis Maternal Follicles

Placentation evolved many times independently in vertebrates. Although the core functions of all placentas are similar, we know less about how this similarity extends to the molecular level. Here, we study Poeciliopsis, a unique genus of live-bearing fish that have independently evolved complex placental structures at least three times. The maternal follicle is a key component of these structures. It envelops yolk-rich eggs and is morphologically simple in lecithotrophic species but has elaborate villous structures in matrotrophic species. Through sequencing, the follicle transcriptome of a matrotrophic, Poeciliopsis retropinna, and lecithotrophic, P. turrubarensis, species we found genes known to be critical for placenta function expressed in both species despite their difference in complexity. Additionally, when we compare the transcriptome of different river populations of P. retropinna, known to vary in maternal provisioning, we find differential expression of secretory genes expressed specifically in the top layer of villi cells in the maternal follicle. This provides some of the first evidence that the placental structures of Poeciliopsis function using a secretory mechanism rather than direct contact with maternal circulation. Finally, when we look at the expression of placenta proteins at the maternal–fetal interface of a larger sampling of Poeciliopsis species, we find expression of key maternal and fetal placenta proteins in their cognate tissue types of all species, but follicle expression of prolactin is restricted to only matrotrophic species. Taken together, we suggest that all Poeciliopsis follicles are poised for placenta function but require expression of key genes to form secretory villi.

Testing hypotheses of marsupial brain size variation using phylogenetic multiple imputations and a Bayesian comparative framework

Considerable controversy exists about which hypotheses and variables best explain mammalian brain size variation. We use a new, high-coverage dataset of marsupial brain and body sizes, and the first phylogenetically imputed full datasets of 16 predictor variables, to model the prevalent hypotheses explaining brain size evolution using phylogenetically corrected Bayesian generalized linear mixed-effects modelling. Despite this comprehensive analysis, litter size emerges as the only significant predictor. Marsupials differ from the more frequently studied placentals in displaying a much lower diversity of reproductive traits, which are known to interact extensively with many behavioural and ecological predictors of brain size. Our results therefore suggest that studies of relative brain size evolution in placental mammals may require targeted co-analysis or adjustment of reproductive parameters like litter size, weaning age or gestation length. This supports suggestions that significant associations between behavioural or ecological variables with relative brain size may be due to a confounding influence of the extensive reproductive diversity of placental mammals.

Baby Genomics: Tracing the Evolutionary Changes That Gave Rise to Placentation

It has long been challenging to uncover the molecular mechanisms behind striking morphological innovations such as mammalian pregnancy. We studied the power of a robust comparative orthology pipeline based on gene synteny to address such problems. We inferred orthology relations between human genes and genes from each of 43 other vertebrate genomes, resulting in ∼18,000 orthologous pairs for each genome comparison. By identifying genes that first appear coincident with origin of the placental mammals, we hypothesized that we would define a subset of the genome enriched for genes that played a role in placental evolution. We thus pinpointed orthologs that appeared before and after the divergence of eutherian mammals from marsupials. Reinforcing previous work, we found instead that much of the genetic toolkit of mammalian pregnancy evolved through the repurposing of preexisting genes to new roles. These genes acquired regulatory controls for their novel roles from a group of regulatory genes, many of which did in fact originate at the appearance of the eutherians. Thus, orthologs appearing at the origin of the eutherians are enriched in functions such as transcriptional regulation by Krüppel-associated box-zinc-finger proteins, innate immune responses, keratinization, and the melanoma-associated antigen protein class. Because the cellular mechanisms of invasive placentae are similar to those of metastatic cancers, we then used our orthology inferences to explore the association between placenta invasion and cancer metastasis. Again echoing previous work, we find that genes that are phylogenetically older are more likely to be implicated in cancer development.

The influence of evolutionary history on human health and disease

Nearly all genetic variants that influence disease risk have human-specific origins; however, the systems they influence have ancient roots that often trace back to evolutionary events long before the origin of humans. Here, we review how advances in our understanding of the genetic architectures of diseases, recent human evolution and deep evolutionary history can help explain how and why humans in modern environments become ill. Human populations exhibit differences in the prevalence of many common and rare genetic diseases. These differences are largely the result of the diverse environmental, cultural, demographic and genetic histories of modern human populations. Synthesizing our growing knowledge of evolutionary history with genetic medicine, while accounting for environmental and social factors, will help to achieve the promise of personalized genomics and realize the potential hidden in an individual's DNA sequence to guide clinical decisions. In short, precision medicine is fundamentally evolutionary medicine, and integration of evolutionary perspectives into the clinic will support the realization of its full potential.

ZBED6 binding motifs correlate with endogenous retroviruses and Syncytin genes

Retroviruses have infiltrated vertebrate germlines for millions of years as inherited endogenous retroviruses (ERVs). Mammalian genomes host large numbers of ERVs and transposable elements (TEs), including retrotransposons and DNA transposons, that contribute to genomic innovation and evolution as coopted genes and regulators of diverse functions. To explore features distinguishing coopted ERVs and TEs from other integrations, we focus on the potential role of ZBED6 and repeated ERV domestication as repurposed Syncytin genes. The placental mammal-specific ZBED6 is a DNA transposon-derived transcription regulator and we demonstrate that its binding motifs are associated with distinct Syncytins and that ZBED6 binding motifs are 2- to 3-fold more frequent in ERVs than in flanking DNA. Our observations suggest that ZBED6 could contribute an extended regulatory role of genomic expression, utilizing ERVs as platforms for genomic innovation and evolution.

Regulation of maternal-fetal metabolic communication

Pregnancy may be the most nutritionally sensitive stage in the life cycle, and improved metabolic health during gestation and early postnatal life can reduce the risk of chronic disease in adulthood. Successful pregnancy requires coordinated metabolic, hormonal, and immunological communication. In this review, maternal-fetal metabolic communication is defined as the bidirectional communication of nutritional status and metabolic demand by various modes including circulating metabolites, endocrine molecules, and other secreted factors. Emphasis is placed on metabolites as a means of maternal-fetal communication by synthesizing findings from studies in humans, non-human primates, domestic animals, rabbits, and rodents. In this review, fetal, placental, and maternal metabolic adaptations are discussed in turn. (1) Fetal macronutrient needs are summarized in terms of the physiological adaptations in place to ensure their proper allocation. (2) Placental metabolite transport and maternal physiological adaptations during gestation, including changes in energy budget, are also discussed. (3) Maternal nutrient limitation and metabolic disorders of pregnancy serve as case studies of the dynamic nature of maternal-fetal metabolic communication. The review concludes with a summary of recent research efforts to identify metabolites, endocrine molecules, and other secreted factors that mediate this communication, with particular emphasis on serum/plasma metabolomics in humans, non-human primates, and rodents. A better understanding of maternal-fetal metabolic communication in health and disease may reveal novel biomarkers and therapeutic targets for metabolic disorders of pregnancy.

The origins of genomic imprinting in mammals

Genomic imprinting is a process that causes genes to be expressed according to their parental origin. Imprinting appears to have evolved gradually in two of the three mammalian subclasses, with no imprinted genes yet identified in prototheria and only six found to be imprinted in marsupials to date. By interrogating the genomes of eutherian suborders, we determine that imprinting evolved at the majority of eutherian specific genes before the eutherian radiation. Theories considering the evolution of imprinting often relate to resource allocation and recently consider maternal-offspring interactions more generally, which, in marsupials, places a greater emphasis on lactation. In eutherians, the imprint memory is retained at least in part by zinc finger protein 57 (ZFP57), a Kruppel associated box (KRAB) zinc finger protein that binds specifically to methylated imprinting control regions. Some imprints are less dependent on ZFP57invivo and it may be no coincidence that these are the imprints that are found in marsupials. Because marsupials lack ZFP57, this suggests another more ancestral protein evolved to regulate imprints in non-eutherian subclasses, and contributes to imprinting control in eutherians. Hence, understanding the mechanisms acting at imprinting control regions across mammals has the potential to provide valuable insights into our understanding of the origins and evolution of genomic imprinting.

The role of mammalian foetal membranes in early embryogenesis: Lessons from marsupials

Across mammals, early embryonic development is supported by uterine secretions taken up through the yolk sac and other foetal membranes (histotrophic nutrition). The marsupial conceptus is enclosed in a shell coat for the first two-thirds of gestation and nutrients pass to the embryo through the shell and the avascular bilaminar yolk sac. At around the time of shell rupture, part of the yolk sac is trilaminar and supplied with blood vessels. It attaches to the uterus and forms a choriovitelline placenta. Rapid growth of the embryo ensues, still supported by histotrophe as well as exchange of oxygen and nutrients between maternal and foetal blood vessels (haemotrophic nutrition). Few marsupials have a chorioallantoic placenta and the highly altricial newborn is delivered after a short gestation. Eutherian embryos pass through a similar sequence before there is a fully functional chorioallantoic placenta. In most orders, there is transient yolk sac placentation, but even before this, nutrients are transferred through an avascular yolk sac. Yolk sac placentation does not occur in rodents or catarrhine primates. Early embryonic development in the mouse is nonetheless dependent on histotrophic nutrition. In the first trimester of human pregnancy, uterine glands open to the intervillous space and secretion products are taken up by the trophoblast. Transfer of nutrients to the early human embryo also involves the yolk sac, which floats free in the exocoelom. Marsupials can therefore inform us about the role of foetal membranes and histotrophic nutrition in early embryogenesis, knowledge that can translate to eutherians.

Didelphis albiventris: an overview of unprecedented transcriptome sequencing of the white-eared opossum

BACKGROUND

The white-eared opossum (Didelphis albiventris) is widely distributed throughout Brazil and South America. It has been used as an animal model for studying different scientific questions ranging from the restoration of degraded green areas to medical aspects of Chagas disease, leishmaniasis and resistance against snake venom. As a marsupial, D. albiventris can also contribute to the understanding of the molecular mechanisms that govern the different stages of organogenesis. Opossum joeys are born after only 13 days, and the final stages of organogenesis occur when the neonates are inside the pouch, depending on lactation. As neither the genome of this opossum species nor its transcriptome has been completely sequenced, the use of D. albiventris as an animal model is limited. In this work, we sequenced the D. albiventris transcriptome by RNA-seq to obtain the first catalogue of differentially expressed (DE) genes and gene ontology (GO) annotations during the neonatal stages of marsupial development.

RESULTS

The D. albiventris transcriptome was obtained from whole neonates harvested at birth (P0), at 5 days of age (P5) and at 10 days of age (P10). The de novo assembly of these transcripts generated 85,338 transcripts. Approximately 30% of these transcripts could be mapped against the amino acid sequences of M. domestica, the evolutionarily closest relative of D. albiventris to be sequenced thus far. Among the expressed transcripts, 2077 were found to be DE between P0 and P5, 13,780 between P0 and P10, and 1453 between P5 and P10. The enriched GO terms were mainly related to the immune system, blood tissue development and differentiation, vision, hearing, digestion, the CNS and limb development.

CONCLUSIONS

The elucidation of opossum transcriptomes provides an out-group for better understanding the distinct characteristics associated with the evolution of mammalian species. This study provides the first transcriptome sequences and catalogue of genes for a marsupial species at different neonatal stages, allowing the study of the mechanisms involved in organogenesis.

Evolution and Development of the Atrial Septum

The complete division of the atrial cavity by a septum, resulting in a left and right atrium, is found in many amphibians and all amniotes (reptiles, birds, and mammals). Surprisingly, it is only in eutherian, or placental, mammals that full atrial septation necessitates addition from a second septum. The high incidence of incomplete closure of the atrial septum in human, so-called probe patency, suggests this manner of closure is inefficient. We review the evolution and development of the atrial septum to understand the peculiar means of forming the atrial septum in eutherian mammals. The most primitive atrial septum is found in lungfishes and comprises a myocardial component with a mesenchymal cap on its leading edge, reminiscent to the primary atrial septum of embryonic mammals before closure of the primary foramen. In reptiles, birds, and mammals, the primary foramen is closed by the mesenchymal tissues of the atrioventricular cushions, the dorsal mesenchymal protrusion, and the mesenchymal cap. These tissues are also found in lungfishes. The closure of the primary foramen is preceded by the development of secondary perforations in the septal myocardium. In all amniotes, with the exception of eutherian mammals, the secondary perforations do not coalesce to a secondary foramen. Instead, the secondary perforations persist and are sealed by myocardial and endocardial growth after birth or hatching. We suggest that the error-prone secondary foramen allows large volumes of oxygen-rich blood to reach the cardiac left side, needed to sustain the growth of the extraordinary large offspring that characterizes eutherian mammals. Anat Rec, 302:32-48, 2019. © 2018 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

A missing piece: the spiny mouse and the puzzle of menstruating species

We recently discovered the first known menstruating rodent. With the exception of four bats and the elephant shrew, the common spiny mouse (Acomys cahirinus) is the only species outside the primate order to exhibit menses. There are few widely accepted theories on why menstruation developed as the preferred reproductive strategy of these select mammals, all of which reference the evolution of spontaneous decidualisation prior to menstrual shedding. Though menstruating species share several reproductive traits, there has been no identifiable feature unique to menstruating species. Such a feature might suggest why spontaneous decidualisation, and thus menstruation, evolved in these species. We propose that a ≥3-fold increase in progesterone during the luteal phase of the reproductive cycle is a unique characteristic linking menstruating species. We discuss spontaneous decidualisation as a consequence of high progesterone, and the potential role of prolactin in screening for defective embryos in these species to aid in minimising implantation of abnormal embryos. We further explore the possible impact of nutrition in selecting species to undergo spontaneous decidualisation and subsequent menstruation. We summarise the current knowledge of menstruation, discuss current pre-clinical models of menstruation and how the spiny mouse may benefit advancing our understanding of this rare biological phenomenon.

Transcriptomic changes in the pre-implantation uterus highlight histotrophic nutrition of the developing marsupial embryo

Early pregnancy is a critical time for successful reproduction; up to half of human pregnancies fail before the development of the definitive chorioallantoic placenta. Unlike the situation in eutherian mammals, marsupial pregnancy is characterised by a long pre-implantation period prior to the development of the short-lived placenta, making them ideal models for study of the uterine environment promoting embryonic survival pre-implantation. Here we present a transcriptomic study of pre-implantation marsupial pregnancy, and identify differentially expressed genes in the Sminthopsis crassicaudata uterus involved in metabolism and biosynthesis, transport, immunity, tissue remodelling, and uterine receptivity. Interestingly, almost one quarter of the top 50 genes that are differentially upregulated in early pregnancy are putatively involved in histotrophy, highlighting the importance of nutrient transport to the conceptus prior to the development of the placenta. This work furthers our understanding of the mechanisms underlying survival of pre-implantation embryos in the earliest live bearing ancestors of mammals.

Recent advances in understanding evolution of the placenta: insights from transcriptomics

The mammalian placenta shows an extraordinary degree of variation in gross and fine structure, but this has been difficult to interpret in physiological terms. Transcriptomics offers a path to understanding how structure relates to function. This essay examines how studies of gene transcription can inform us about placental evolution in eutherian and marsupial mammals and more broadly about convergent evolution of viviparity and placentation in vertebrates. Thus far, the focus has been on the chorioallantoic placenta of eutherians at term, the reproductive strategies of eutherians and marsupials, and the decidual response of the uterus at implantation. Future work should address gene expression during early stages of placental development and endeavor to cover all major groups of mammals. Comparative studies across oviparous and viviparous vertebrates have centered on the chorioallantoic membrane and yolk sac. They point to the possibility of defining a set of genes that can be recruited to support commonalities in reproductive strategies. Further advances can be anticipated from single-cell transcriptomics if those techniques are applied to a range of placental structures and in species other than humans and mice.

What is a placental mammal anyway?

Many developmental functions in marsupials and eutherian mammals are accomplished by different tissues, but similar genes.