Distinct Patterns of Gene-Specific Methylation in Mammalian Placentas: Implications for Placental Evolution and Function

Epigenetic inheritance of phenotypes associated with parental exposure to cocaine

Parental exposure to drugs of abuse induces changes in the germline that can be transmitted across subsequent generations, resulting in enduring effects on gene expression and behavior. This transgenerational inheritance involves a dynamic interplay of environmental, genetic, and epigenetic factors that impact an individual's vulnerability to neuropsychiatric disorders. This chapter aims to summarize recent research into the mechanisms underlying the inheritance of gene expression and phenotypic patterns associated with exposure to drugs of abuse, with an emphasis on cocaine. We will first define the epigenetic modifications such as DNA methylation, histone post-translational modifications, and expression of non-coding RNAs that are impacted by parental cocaine use. We will then explore how parental cocaine use induces heritable epigenetic changes that are linked to alterations in neural circuitry and synaptic plasticity within reward-related circuits, ultimately giving rise to potential behavioral vulnerabilities. This discussion will consider phenotypic differences associated with gestational as well as both maternal and paternal preconception drug exposure and will emphasize differences based on offspring sex. In this context, we explore the complex interactions between genetics, epigenetics, environment, and biological sex. Overall, this chapter consolidates the latest developments in the multigenerational effects and long-term consequences of parental substance abuse.

The transposable element-derived transcript of LIN28B has a placental origin and is not specific to tumours

Transposable elements (TEs) are genetic elements that have evolved as crucial regulators of human development and cancer, functioning as both genes and regulatory elements. When TEs become dysregulated in cancer cells, they can serve as alternate promoters to activate oncogenes, a process known as onco-exaptation. This study aimed to explore the expression and epigenetic regulation of onco-exaptation events in early human developmental tissues. We discovered co-expression of some TEs and oncogenes in human embryonic stem cells and first trimester and term placental tissues. Previous studies identified onco-exaptation events in various cancer types, including an AluJb SINE element-LIN28B interaction in lung cancer cells, and showed that the TE-derived LIN28B transcript is associated with poor patient prognosis in hepatocellular carcinoma. This study further characterized the AluJb-LIN28B transcript and confirmed that its expression is restricted to the placenta. Targeted DNA methylation analysis revealed differential methylation of the two LIN28B promoters between placenta and healthy somatic tissues, indicating that some TE-oncogene interactions are not cancer-specific but arise from the epigenetic reactivation of developmental TE-derived regulatory events. In conclusion, our findings provide evidence that some TE-oncogene interactions are not limited to cancer and may originate from the epigenetic reactivation of TE-derived regulatory events that are involved in early development. These insights broaden our understanding of the role of TEs in gene regulation and suggest the potential importance of targeting TEs in cancer therapy beyond their conventional use as cancer-specific markers.

Dynamics of the Equine Placental DNA Methylome and Transcriptome from Mid- to Late Gestation

The placenta is a temporary organ that is essential for the survival of the fetus, with a lifelong effect on the health of both the offspring and the dam. The functions of the placenta are controlled by its dynamic gene expression during gestation. In this study, we aimed to investigate the equine placental DNA methylome as one of the fundamental mechanisms that controls the gene expression dynamic. Chorioallantois samples from four (4M), six (6M), and ten (10M) months of gestation were used to map the methylation pattern of the placenta. Globally, methylation levels increased toward the end of gestation. We identified 921 differentially methylated regions (DMRs) between 4M and 6M, 1225 DMRs between 4M and 10M, and 1026 DMRs between 6M and 10M. A total of 817 genes carried DMRs comparing 4M and 6M, 978 comparing 4M and 10M, and 804 comparing 6M and 10M. We compared the transcriptomes between the samples and found 1381 differentially expressed genes (DEGs) when comparing 4M and 6M, 1428 DEGs between 4M and 10M, and 741 DEGs between 6M and 10M. Finally, we overlapped the DEGs and genes carrying DMRs (DMRs-DEGs). Genes exhibiting (a) higher expression, low methylation and (b) low expression, high methylation at different time points were identified. The majority of these DMRs-DEGs were located in introns (48.4%), promoters (25.8%), and exons (17.7%) and were involved in changes in the extracellular matrix; regulation of epithelial cell migration; vascularization; and regulation of minerals, glucose, and metabolites, among other factors. Overall, this is the first report highlighting the dynamics in the equine placenta methylome during normal pregnancy. The findings presented serve as a foundation for future studies on the impact of abnormal methylation on the outcomes of equine pregnancies.

Can Immune Suppression and Epigenome Regulation in Placenta Offer Novel Insights into Cancer Immune Evasion and Immunotherapy Resistance?

Cancer is the second leading cause of mortality and morbidity in the developed world. Cancer progression involves genetic and epigenetic alterations, accompanied by aggressive changes, such as increased immune evasion, onset of metastasis, and drug resistance. Similar to cancer, DNA hypomethylation, immune suppression, and invasive cell behaviours are also observed in the human placenta. Mechanisms that lead to the acquisition of invasive behaviour, immune evasion, and drug and immunotherapy resistance are presently under intense investigations to improve patient outcomes. Here, we review current knowledge regarding the similarities between immune suppression and epigenome regulation, including the expression of repetitive elements (REs), endogenous retroviruses (ERVs) and transposable elements (TEs) in cells of the placenta and in cancer, which are associated with changes in immune regulation and invasiveness. We explore whether immune suppression and epigenome regulation in placenta offers novel insights into immunotherapy resistance in cancer, and we also discuss the implications and the knowledge gaps relevant to these findings, which are rapidly being accrued in these quite disparate research fields. Finally, we discuss potential linkages between TE, ERV and RE activation and expression, regarding mechanisms of immune regulation in placenta and cancer. A greater understanding of the role of immune suppression and associated epigenome regulation in placenta could help to elucidate some comparable mechanisms operating in cancer, and identify potential new therapeutic targets for treating cancer.

Reawakening the Developmental Origins of Cancer Through Transposable Elements

Transposable elements (TEs) have an established role as important regulators of early human development, functioning as tissue-specific genes and regulatory elements. Functional TEs are highly active during early development, and interact with important developmental genes, some of which also function as oncogenes. Dedifferentiation is a hallmark of cancer, and is characterized by genetic and epigenetic changes that enable proliferation, self-renewal and a metabolism reminiscent of embryonic stem cells. There is also compelling evidence suggesting that the path to dedifferentiation in cancer can contribute to invasion and metastasis. TEs are frequently expressed in cancer, and recent work has identified a newly proposed mechanism involving extensive recruitment of TE-derived promoters to drive expression of oncogenes and subsequently promote oncogenesis—a process termed onco-exaptation. However, the mechanism by which this phenomenon occurs, and the extent to which it contributes to oncogenesis remains unknown. Initial hypotheses have proposed that onco-exaptation events are cancer-specific and arise randomly due to the dysregulated and hypomethylated state of cancer cells and abundance of TEs across the genome. However, we suspect that exaptation-like events may not just arise due to chance activation of novel regulatory relationships as proposed previously, but as a result of the reestablishment of early developmental regulatory relationships. Dedifferentiation in cancer is well-documented, along with expression of TEs. The known interactions between TEs and pluripotency factors such as NANOG and OCTt4 during early development, along with the expression of some placental-specific TE-derived transcripts in cancer support a possible link between TEs and dedifferentiation of tumor cells. Thus, we hypothesize that onco-exaptation events can be associated with the epigenetic reawakening of early developmental TEs to regulate expression of oncogenes and promote oncogenesis. We also suspect that activation of these early developmental regulatory TEs may promote dedifferentiation, although at this stage it is hard to predict whether TE activation is one of the initial drivers of dedifferentiation. We expect that developmental TE activation occurs as a result of the establishment of an epigenetic landscape in cancer that resembles that of early development and that developmental TE activation may also enable cancers to exploit early developmental pathways, repurposing them to promote malignancy.

Human placental methylome in the interplay of adverse placental health, environmental exposure, and pregnancy outcome

The placenta is the interface between maternal and fetal circulations, integrating maternal and fetal signals to selectively regulate nutrient, gas, and waste exchange, as well as secrete hormones. In turn, the placenta helps create the in utero environment and control fetal growth and development. The unique epigenetic profile of the human placenta likely reflects its early developmental separation from the fetus proper and its role in mediating maternal–fetal exchange that leaves it open to a range of exogenous exposures in the maternal circulation. In this review, we cover recent advances in DNA methylation in the context of placental function and development, as well as the interaction between the pregnancy and the environment.

Vitamin D status and metabolism in an ovine pregnancy model: effect of long-term, high-altitude hypoxia

Vitamin D status increases during healthy mammalian pregnancy, but the molecular determinants remain uncharacterized. The first objective of this study was to determine the effects of pregnancy, and the second objective was to examine the role of chronic hypoxia on vitamin D status and metabolism in an ovine model. We analyzed the plasma levels of cholecalciferol, 25-OH-D, and 1α,25-(OH)2D in nonpregnant ewes, near-term pregnant ewes, and their fetuses exposed to normoxia (low altitude) or hypoxia (high-altitude) for 100 days. Hypoxic sheep had increased circulating levels of 25-OH-D and 1α,25-(OH)2D compared with normoxic sheep. Hypoxia increases in 25-OH-D were associated with increased expression of renal 25-hydroxylases CYP2R1 and CYP2J. Pregnancy did not increase further the plasma levels of 25-OH-D, but it significantly increased those of the active metabolite, 1α,25-(OH)2D, in both normoxic and hypoxic ewes. Increased bioactivation of vitamin D correlated with increased expression of the vitamin D-activating enzyme CYP27b1 and decreased expression of the inactivating enzyme CYP24a1 in maternal kidneys and placentas. Hypoxia increased parathyroid hormone levels and further increased renal CYP27b1. Pregnancy and hypoxia decreased the expression of vitamin D receptor (VDR) in maternal kidney and lung, with opposite effects on placental VDR. We conclude that ovine pregnancy is a model of increased vitamin D status, and long-term hypoxia further improves vitamin D status due to pregnancy- and hypoxia-specific regulation of VDR and metabolic enzymes.

Early Developmental and Evolutionary Origins of Gene Body DNA Methylation Patterns in Mammalian Placentas

Over the last 20-80 million years the mammalian placenta has taken on a variety of morphologies through both divergent and convergent evolution. Recently we have shown that the human placenta genome has a unique epigenetic pattern of large partially methylated domains (PMDs) and highly methylated domains (HMDs) with gene body DNA methylation positively correlating with level of gene expression. In order to determine the evolutionary conservation of DNA methylation patterns and transcriptional regulatory programs in the placenta, we performed a genome-wide methylome (MethylC-seq) analysis of human, rhesus macaque, squirrel monkey, mouse, dog, horse, and cow placentas as well as opossum extraembryonic membrane. We found that, similar to human placenta, mammalian placentas and opossum extraembryonic membrane have globally lower levels of methylation compared to somatic tissues. Higher relative gene body methylation was the conserved feature across all mammalian placentas, despite differences in PMD/HMDs and absolute methylation levels. Specifically, higher methylation over the bodies of genes involved in mitosis, vesicle-mediated transport, protein phosphorylation, and chromatin modification was observed compared with the rest of the genome. As in human placenta, higher methylation is associated with higher gene expression and is predictive of genic location across species. Analysis of DNA methylation in oocytes and preimplantation embryos shows a conserved pattern of gene body methylation similar to the placenta. Intriguingly, mouse and cow oocytes and mouse early embryos have PMD/HMDs but their placentas do not, suggesting that PMD/HMDs are a feature of early preimplantation methylation patterns that become lost during placental development in some species and following implantation of the embryo.

Recommendations for Accurate Resolution of Gene and Isoform Allele-Specific Expression in RNA-Seq Data

Genetic variation modulates gene expression transcriptionally or post-transcriptionally, and can profoundly alter an individual’s phenotype. Measuring allelic differential expression at heterozygous loci within an individual, a phenomenon called allele-specific expression (ASE), can assist in identifying such factors. Massively parallel DNA and RNA sequencing and advances in bioinformatic methodologies provide an outstanding opportunity to measure ASE genome-wide. In this study, matched DNA and RNA sequencing, genotyping arrays and computationally phased haplotypes were integrated to comprehensively and conservatively quantify ASE in a single human brain and liver tissue sample. We describe a methodological evaluation and assessment of common bioinformatic steps for ASE quantification, and recommend a robust approach to accurately measure SNP, gene and isoform ASE through the use of personalized haplotype genome alignment, strict alignment quality control and intragenic SNP aggregation. Our results indicate that accurate ASE quantification requires careful bioinformatic analyses and is adversely affected by sample specific alignment confounders and random sampling even at moderate sequence depths. We identified multiple known and several novel ASE genes in liver, including WDR72, DSP and UBD, as well as genes that contained ASE SNPs with imbalance direction discordant with haplotype phase, explainable by annotated transcript structure, suggesting isoform derived ASE. The methods evaluated in this study will be of use to researchers performing highly conservative quantification of ASE, and the genes and isoforms identified as ASE of interest to researchers studying those loci.

Early origins of adult disease: approaches for investigating the programmable epigenome in humans, nonhuman primates, and rodents

According to the developmental origins of health and disease hypothesis, in utero experiences reprogram an individual for immediate adaptation to gestational perturbations, with the sequelae of later-in-life risk of metabolic disease. An altered gestational milieu with resultant adult metabolic disease has been observed in instances of both in utero constraint (e.g., from famine or uteroplacental insufficiency) and overt caloric abundance (e.g., from a maternal high-fat, caloric-dense diet). The commonality of the adult metabolic phenotype begs the question of how diverse in utero experiences (i.e., reprogramming events) converge on common metabolic pathways and how the memory of these events is maintained across the lifespan. We and others have investigated the molecular mechanisms underlying fetal programming and observed that epigenetic modifications to the fetal and placental epigenome accompany these reprogramming events. Based on several lines of emerging data in human and nonhuman primates, it is now felt that modified epigenetic signature--and the histone code in particular--underlies alterations in postnatal gene expression and metabolic pathways central to accurate functioning and maintenance of health. Because of the tissue lineage specificity of many of these modifications, nonhuman primates serve as an apt model system for the capacity to recapitulate human gene expression and regulation during development. This review summarizes recent epigenetic advances using rodent and primate (both human and nonhuman) models during in utero development and contributing to adult diseases later in life.

Placental pseudo-malignancy from a DNA methylation perspective: unanswered questions and future directions

The growing fetus is dependent on adequate placental function for delivery of essential nutrients and oxygen, and for waste removal. The placenta also plays an important protective role; shielding the developing baby from the maternal immune system and adverse environmental exposures. Fundamental to these processes is correct invasion of the decidua and remodeling of maternal vasculature, each of which show remarkable parallels to tumorogenesis, with the obvious exception that the former is usually a tightly controlled process. It is not surprising that these physiological similarities are mirrored in gene expression and epigenetic parallels, many not found in any other aspect of human development. In this perspective, we summarize known DNA methylation similarities between placenta and human tumors, and discuss the implications and knowledge gaps associated with these findings. We also speculate on the potential origin of common DNA methylation features in these two disparate aspects of human physiology.

The evolutionary significance of placental interdigitation in mammalian reproduction: contributions from comparative studies

The placenta is fundamental to mammalian reproduction and is surprisingly diverse in gross morphology among species. Whether and how this diversity affects maternal investment and fetal growth is still poorly understood. Contrary to suggestions that highly invasive hemochorial placentation is beneficial to fetal development, recent comparative studies have revealed that interdigitation - the degree of contact between maternal and fetal tissues at the area of exchange - strongly influences fetal growth rates. Species with labyrinthine placentae give birth to neonates of similar size to those of species with villous or trabecular placentae but in less than half the time. These findings suggest that there might be tradeoffs between fetal growth rates (higher with greater interdigitation) and gestation time (shorter with greater interdigitation), in association with type of interdigitation. Such tradeoffs might be the results of maternal-offspring conflict over the allocation of maternal resources, with paternal genes favouring greater interdigitation and so higher fetal growth, and maternal genes responding by reducing gestation time. These results emphasize the role of interdigitation as a means to increase the surface area for exchange, and are consistent with within species studies demonstrating that a higher surface area for exchange is associated with heavier neonates. Further studies could investigate the role of other traits in the evolution of placental diversity and their impact on fetal development.

Cigarette smoke-induced transgenerational alterations in genome stability in cord blood of human F1 offspring

The relevance of preconceptional and prenatal toxicant exposures for genomic stability in offspring is difficult to analyze in human populations, because gestational exposures usually cannot be separated from preconceptional exposures. To analyze the roles of exposures during gestation and conception on genomic stability in the offspring, stability was assessed via the Comet assay and highly sensitive, semiautomated confocal laser scans of γH2AX foci in cord, maternal, and paternal blood as well as spermatozoa from 39 families in Crete, Greece, and the United Kingdom. With use of multivariate linear regression analysis with backward selection, preconceptional paternal smoking (% tail DNA: P>0.032; γH2AX foci: P>0.018) and gestational maternal (% tail DNA: P>0.033) smoking were found to statistically significantly predict DNA damage in the cord blood of F1 offspring. Maternal passive smoke exposure was not identified as a predictor of DNA damage in cord blood, indicating that the effect of paternal smoking may be transmitted via the spermatozoal genome. Taken together, these studies reveal a role for cigarette smoke in the induction of DNA alterations in human F1 offspring via exposures of the fetus in utero or the paternal germline. Moreover, the identification of transgenerational DNA alterations in the unexposed F1 offspring of smoking-exposed fathers supports the claim that cigarette smoke is a human germ cell mutagen.

DNA methylation profiling highlights the unique nature of the human placental epigenome

As the 'gateway' to the fetus, the placenta is subject to a myriad of environmental factors, each with the potential to alter placental epigenetic and gene expression profile. This can have direct consequences for the developing fetus and potentially even long-term health implications. As a result, interest in placental epigenetics generally, and changes occurring in placenta-associated disease, has intensified over recent years. This article will discuss the general features of placental DNA methylation and will describe current technologies for profiling genome-wide DNA methylation patterns in this tissue, the approaches to data analysis and some of the major findings from recent studies.

Retroelement demethylation associated with abnormal placentation in Mus musculus x Mus caroli hybrids

The proper functioning of the placenta requires specific patterns of methylation and the appropriate regulation of retroelements, some of which have been co-opted by the genome for placental-specific gene expression. Our inquiry was initiated to determine the causes of the placental defects observed in crosses between two species of mouse, Mus musculus and Mus caroli. M. musculus × M. caroli fetuses are rarely carried to term, possibly as a result of genomic incompatibility in the placenta. Taking into account that placental dysplasia is observed in Peromyscus and other Mus hybrids, and that endogenous retroviruses are expressed in the placental transcriptome, we hypothesized that these placental defects could result, in part, from failure of the genome defense mechanism, DNA methylation, to regulate the expression of retroelements. Hybrid M. musculus × M. caroli embryos were produced by artificial insemination, and dysplastic placentas were subjected to microarray and methylation screens. Aberrant overexpression of an X-linked Mus retroelement in these hybrid placentas is consistent with local demethylation of this retroelement, concomitant with genome instability, disruption of gene regulatory pathways, and dysgenesis. We propose that the placenta is a specific site of control that is disrupted by demethylation and retroelement activation in interspecific hybridization that occur as a result of species incompatibility of methylation machinery. To our knowledge, the present data provide the first report of retroelement activation linked to decreased methylation in a eutherian hybrid system.

A concise review on epigenetic regulation: insight into molecular mechanisms

Epigenetic mechanisms are responsible for the regulation of transcription of imprinted genes and those that induce a totipotent state. Starting just after fertilization, DNA methylation pattern undergoes establishment, reestablishment and maintenance. These modifications are important for normal embryo and placental developments. Throughout life and passing to the next generation, epigenetic events establish, maintain, erase and reestablish. In the context of differentiated cell reprogramming, demethylation and activation of genes whose expressions contribute to the pluripotent state is the crux of the matter. In this review, firstly, regulatory epigenetic mechanisms related to somatic cell nuclear transfer (SCNT) reprogramming are discussed, followed by embryonic development, and placental epigenetic issues.

Impact of DNA methylation on trophoblast function

The influence of epigenetics is evident in many fields of medicine today. This is also true in placentology, where versatile epigenetic mechanisms that regulate expression of genes have shown to have important influence on trophoblast implantation and placentation. Such gene regulation can be established in different ways and on different molecular levels, the most common being the DNA methylation. DNA methylation has been shown today as an important predictive component in assessing clinical prognosis of certain malignant tumors; in addition, it opens up new possibilities for non-invasive prenatal diagnosis utilizing cell-free fetal DNA methods. By using a well known demethylating agent 5-azacytidine in pregnant rat model, we have been able to change gene expression and, consequently, the processes of trophoblast differentiation and placental development. In this review, we describe how changes in gene methylation effect trophoblast development and placentation and offer our perspective on use of trophoblast epigenetic research for better understanding of not only placenta development but cancer cell growth and invasion as well.

Genome-wide mapping of imprinted differentially methylated regions by DNA methylation profiling of human placentas from triploidies

Background

Genomic imprinting is an important epigenetic process involved in regulating placental and foetal growth. Imprinted genes are typically associated with differentially methylated regions (DMRs) whereby one of the two alleles is DNA methylated depending on the parent of origin. Identifying imprinted DMRs in humans is complicated by species- and tissue-specific differences in imprinting status and the presence of multiple regulatory regions associated with a particular gene, only some of which may be imprinted. In this study, we have taken advantage of the unbalanced parental genomic constitutions in triploidies to further characterize human DMRs associated with known imprinted genes and identify novel imprinted DMRs.

Results

By comparing the promoter methylation status of over 14,000 genes in human placentas from ten diandries (extra paternal haploid set) and ten digynies (extra maternal haploid set) and using 6 complete hydatidiform moles (paternal origin) and ten chromosomally normal placentas for comparison, we identified 62 genes with apparently imprinted DMRs (false discovery rate <0.1%). Of these 62 genes, 11 have been reported previously as DMRs that act as imprinting control regions, and the observed parental methylation patterns were concordant with those previously reported. We demonstrated that novel imprinted genes, such as FAM50B, as well as novel imprinted DMRs associated with known imprinted genes (for example, CDKN1C and RASGRF1) can be identified by using this approach. Furthermore, we have demonstrated how comparison of DNA methylation for known imprinted genes (for example, GNAS and CDKN1C) between placentas of different gestations and other somatic tissues (brain, kidney, muscle and blood) provides a detailed analysis of specific CpG sites associated with tissue-specific imprinting and gestational age-specific methylation.

Conclusions

DNA methylation profiling of triploidies in different tissues and developmental ages can be a powerful and effective way to map and characterize imprinted regions in the genome.

Imprinted genes and the epigenetic regulation of placental phenotype

Imprinted genes are expressed in a parent-of-origin manner by epigenetic modifications that silence either the paternal or maternal allele. They are widely expressed in fetal and placental tissues and are essential for normal placental development. In general, paternally expressed genes enhance feto-placental growth while maternally expressed genes limit conceptus growth, consistent with the hypothesis that imprinting evolved in response to the conflict between parental genomes in the allocation of maternal resources to fetal growth. Using targeted deletion, uniparental duplication, loss of imprinting and transgenic approaches, imprinted genes have been shown to determine the transport capacity of the definitive mouse placenta by regulating its growth, morphology and transporter abundance. Imprinted genes in the placenta are also responsive to environmental challenges and adapt placental phenotype to the prevailing nutritional conditions, in part, by varying their epigenetic status. In addition, interplay between placental and fetal imprinted genes is important in regulating resource partitioning via the placenta both developmentally and in response to environmental factors. By balancing the opposing parental drives on resource allocation with the environmental signals of nutrient availability, imprinted genes, like the Igf2-H19 locus, may act as nutrient sensors and optimise the fetal acquisition of nutrients for growth. These genes, therefore, have a major role in the epigenetic regulation of placental phenotype with long term consequences for the developmental programming of adult health and disease.

Mitochondrial inhibition during preimplantation embryogenesis shifts the transcriptional profile of fetal mouse brain

Environmental stress results in perturbations to mitochondrial function in the preimplantation embryo and hinders subsequent embryo and possibly offspring development. Global gene expression in fetal mouse brain was investigated following targeted mitochondrial inhibition by amino-oxyacetate (AOA) from the 2-cell to the blastocyst stage. Blastocysts were transferred to pseudopregnant recipients and RNA extracted from Day 18 fetal brains for microarray interrogation. Exposure to 5 μM AOA during preimplantation embryo development induced differential expression of 166 genes (>1.25 fold) in the fetal brain, relative to control medium-cultured embryos. Altered expression pathways included carbohydrate metabolism, neurological development, cellular proliferation and death, DNA replication, recombination and repair. Of 28 genes exhibiting the greatest change in expression, qPCR confirmed that 16 were significantly altered. Targeted qPCR assessment of a further 20 genes associated with methylation, acetylation and mitochondrial dysfunction revealed that three were significantly altered (Immp1l, Nars2, Sat2) and Dmap1 exhibited a sex-specific response to AOA exposure. Only 2/48 genes had significantly altered expression by qPCR (Nola3, Timm8b) in fetal brains exposed to 50 μM AOA embryo culture, excluding an AOA dose-dependent response. It was concluded that perturbation of mitochondrial function induced by 5 μM AOA during preimplantation embryo development alters gene expression in the neonatal brain in a manner that suggests that proper brain development may be compromised.

Placentation and maternal investment in mammals

The mammalian placenta exhibits striking interspecific morphological variation, yet the implications of such diversity for reproductive strategies and fetal development remain obscure. More invasive hemochorial placentas, in which fetal tissues directly contact the maternal blood supply, are believed to facilitate nutrient transfer, resulting in higher fetal growth rates, and to be a state of relative fetal advantage in the evolution of maternal-offspring conflict. The extent of interdigitation between maternal and fetal tissues has received less attention than invasiveness but is also potentially important because it influences the surface area for exchange. We show that although increased placental invasiveness and interdigitation are both associated with shorter gestations, interdigitation is the key variable. Gestation times associated with highly interdigitated labyrinthine placentas are 44% of those associated with less interdigitated villous and trabecular placentas. There is, however, no relationship between placental traits and neonatal body and brain size. Hence, species with more interdigitated placentas produce neonates of similar body and brain size but in less than half the time. We suggest that the effects of placental interdigitation on growth rates and the way that these are traded off against gestation length may be promising avenues for understanding the evolutionary dynamics of parent-offspring conflict.