Y-chromosome DNA is present in the blood of female dogs suggesting the presence of fetal microchimerism

A synthetic review: natural history of amniote reproductive modes in light of comparative evolutionary genomics

There is a current lack of consensus on whether the ancestral parity mode was oviparity (egg-laying) or viviparity (live-birth) in amniotes and particularly in squamates (snakes, lizards, and amphisbaenids). How transitions between parity modes occur at the genomic level has primary importance for how science conceptualises the origin of amniotes, and highly variable parity modes in Squamata. Synthesising literature from medicine, poultry science, reproductive biology, and evolutionary biology, I review the genomics and physiology of five broad processes (here termed the 'Main Five') expected to change during transitions between parity modes: eggshell formation, embryonic retention, placentation, calcium transport, and maternal-fetal immune dynamics. Throughout, I offer alternative perspectives and testable hypotheses regarding proximate causes of parity mode evolution in amniotes and squamates. If viviparity did evolve early in the history of lepidosaurs, I offer the nucleation site hypothesis as a proximate explanation. The framework of this hypothesis can be extended to amniotes to infer their ancestral state. I also provide a mechanism and hypothesis on how squamates may transition from viviparity to oviparity and make predictions about the directionality of transitions in three species. After considering evidence for differing perspectives on amniote origins, I offer a framework that unifies (i) the extended embryonic retention model and (ii) the traditional model which describes the amniote egg as an adaptation to the terrestrial environment. Additionally, this review contextualises the origin of amniotes and parity mode evolution within Medawar's paradigm. Medawar posited that pregnancy could be supported by immunosuppression, inertness, evasion, or immunological barriers. I demonstrate that this does not support gestation or gravidity across most amniotes but may be an adequate paradigm to explain how the first amniote tolerated internal fertilization and delayed egg deposition. In this context, the eggshell can be thought of as an immunological barrier. If serving as a barrier underpins the origin of the amniote eggshell, there should be evidence that oviparous gravidity can be met with a lack of immunological responses in utero. Rare examples of two species that differentially express very few genes during gravidity, suggestive of an absent immunological reaction to oviparous gravidity, are two skinks Lampropholis guichenoti and Lerista bougainvillii. These species may serve as good models for the original amniote egg. Overall, this review grounds itself in the historical literature while offering a modern perspective on the origin of amniotes. I encourage the scientific community to utilise this review as a resource in evolutionary and comparative genomics studies, embrace the complexity of the system, and thoughtfully consider the frameworks proposed.

In utero position matters for littermate cell transfer in mice: an additional and confounding source with maternal microchimerism

Introduction

Feto-maternal cell transfer during pregnancy is called microchimerism (Mc). Its persistence in respective hosts is increasingly studied as to its potential role in immune tolerance, autoimmunity, cancer, and degenerative diseases. Murine models with transgenic reporter genes, heterozygously carried by the mother, allow maternal Mc tracking in wild-type (WT) offspring. However, as gestation in mice is multi-embryonic, an exchange of cells between fetuses carrying the same reporter gene as their mother and negative WT littermate, named littermate Mc (LMc), can occur and be confounded with the maternal source. We propose here to evaluate LMc contribution in mice.

Methods

To avoid the maternal confounding source of Mc, transgenic males, heterozygous for a reporter gene, here, the human leukocyte antigen DRB1*04 (DR4+/-), were crossed with WT females (DR4-/-). DR4+/- LMc was specifically quantified by HLA-DR4 quantitative PCR, i) in utero in main organs from 15 DR4-/- fetuses from three litters of 11, nine, and five; and ii) after birth in two litters of eight pups: in two DR4-/- stillborns and four DR4-/- adult mice.

Results

At embryonic stages, DR4-/- fetuses having one or two nearby DR4+/- littermates in the same uterine horn were almost seven times more frequently positive for DR4- microchimerism in their organs (p = 0.01) and had quantitatively more LMc (p = 0.009) than those without nearby DR4+/- littermates. Furthermore, LMc persists at birth and into adulthood with interindividual heterogeneity.

Conclusions

This study identifies heterogeneity for LMc acquisition according to in utero position and different interpretation of previously published results on maternal Mc in mice.

Is chimerism associated with cancer across the tree of life?

Chimerism is a widespread phenomenon across the tree of life. It is defined as a multicellular organism composed of cells from other genetically distinct entities. This ability to 'tolerate' non-self cells may be linked to susceptibility to diseases like cancer. Here we test whether chimerism is associated with cancers across obligately multicellular organisms in the tree of life. We classified 12 obligately multicellular taxa from lowest to highest chimerism levels based on the existing literature on the presence of chimerism in these species. We then tested for associations of chimerism with tumour invasiveness, neoplasia (benign or malignant) prevalence and malignancy prevalence in 11 terrestrial mammalian species. We found that taxa with higher levels of chimerism have higher tumour invasiveness, though there was no association between malignancy or neoplasia and chimerism among mammals. This suggests that there may be an important biological relationship between chimerism and susceptibility to tissue invasion by cancerous cells. Studying chimerism might help us identify mechanisms underlying invasive cancers and also could provide insights into the detection and management of emerging transmissible cancers.

Chimerism in health and potential implications on behavior: A systematic review

In this review, we focus on the phenomenon of chimerism and especially microchimerism as one of the currently underexplored explanations for differences in health and behavior. Chimerism is an amalgamation of cells from two or more unique zygotes within a single organism, with microchimerism defined by a minor cell population of <1%. This article first presents an overview of the primary techniques employed to detect and quantify the presence of microchimerism and then reviews empirical studies of chimerism in mammals including primates and humans. In women, male microchimerism, a condition suggested to be the result of fetomaternal exchange in utero, is relatively easily detected by polymerase chain reaction molecular techniques targeting Y-chromosomal markers. Consequently, studies of chimerism in human diseases have largely focused on diseases with a predilection for females including autoimmune diseases, and female cancers. We detail studies of chimerism in human diseases and also discuss some potential implications in behavior. Understanding the prevalence of chimerism and the associated health outcomes will provide invaluable knowledge of human biology and guide novel approaches for treating diseases.

Immunological implications of pregnancy-induced microchimerism

Immunological identity is traditionally defined by genetically encoded antigens, with equal maternal and paternal contributions as a result of Mendelian inheritance. However, vertically transferred maternal cells also persist in individuals at very low levels throughout postnatal development. Reciprocally, mothers are seeded during pregnancy with genetically foreign fetal cells that persist long after parturition. Recent findings suggest that these microchimeric cells expressing non-inherited, familially relevant antigenic traits are not accidental 'souvenirs' of pregnancy, but are purposefully retained within mothers and their offspring to promote genetic fitness by improving the outcome of future pregnancies. In this Review, we discuss the immunological implications, benefits and potential consequences of individuals being constitutively chimeric with a biologically active 'microchiome' of genetically foreign cells.

Does microchimerism mediate kin conflicts?

Fetal microchimerism (FMc) is predicted to promote the fitness of the fetus and maternal microchimerism (MMc) to promote the fitness of the mother. Offspring and mothers benefit from each other's health. Therefore, microchimeric cells should usually not be harmful to their host. However, the evolutionary interests of mothers and offspring diverge when there is competition among siblings for maternal investment. Fetal cells in mothers' bodies could benefit their own offspring at the expense of its sibs by promoting lactogenesis or by extending the interbirth interval. Maternal cells in fetal bodies could benefit from the suppression of sibling rivalry. Non-inherited haplotypes in MMc or sibling microchimerism (SMc) gain no direct benefit from their hosts' health and could be associated with substantial detrimental effects.

Fetal microchimerism and maternal health: a review and evolutionary analysis of cooperation and conflict beyond the womb

The presence of fetal cells has been associated with both positive and negative effects on maternal health. These paradoxical effects may be due to the fact that maternal and offspring fitness interests are aligned in certain domains and conflicting in others, which may have led to the evolution of fetal microchimeric phenotypes that can manipulate maternal tissues. We use cooperation and conflict theory to generate testable predictions about domains in which fetal microchimerism may enhance maternal health and those in which it may be detrimental. This framework suggests that fetal cells may function both to contribute to maternal somatic maintenance (e.g. wound healing) and to manipulate maternal physiology to enhance resource transmission to offspring (e.g. enhancing milk production). In this review, we use an evolutionary framework to make testable predictions about the role of fetal microchimerism in lactation, thyroid function, autoimmune disease, cancer and maternal emotional, and psychological health.

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Fetal Microchimerism in Cancer Protection and Promotion: Current Understanding in Dogs and the Implications for Human Health

Fetal microchimerism is the co-existence of small numbers of cells from genetically distinct individuals living within a mother's body following pregnancy. During pregnancy, bi-directional exchange of cells occurs resulting in maternal microchimerism and even sibling microchimerism in offspring. The presence of fetal microchimerism has been identified with lower frequency in patients with cancers such as breast and lymphoma and with higher frequency in patients with colon cancer and autoimmune diseases. Microchimeric cells have been identified in healing and healed tissues as well as normal and tumor tissues. This has led to the hypothesis that fetal microchimerism may play a protective role in some cancers and may provoke other cancers or autoimmune disease. The long periods of risk for these diseases make it a challenge to prospectively study this phenomenon in human populations. Dogs get similar cancers as humans, share our homes and environmental exposures, and live compressed life-spans, allowing easier prospective study of disease development. This review describes the current state of understanding of fetal microchimerism in humans and dogs and highlights the similarities of the common cancers mammary carcinoma, lymphoma, and colon cancer between the two species. Study of fetal microchimerism in dogs might hold the key to characterization of the type and function of microchimeric cells and their role in health and disease. Such an understanding could then be applied to preventing and treating disease in humans.

Assessment of fetal cell chimerism in transgenic pig lines generated by Sleeping beauty transposition

Human cells migrate between mother and fetus during pregnancy and persist in the respective host for long-term after birth. Fetal microchimerism occurs also in twins sharing a common placenta or chorion. Whether microchimerism occurs in multiparous mammals such as the domestic pig, where fetuses have separate placentas and chorions, is not well understood. Here, we assessed cell chimerism in litters of wild-type sows inseminated with semen of transposon transgenic boars. Segregation of three independent monomeric transposons ensured an excess of transgenic over non-transgenic offspring in every litter. Transgenic siblings (n = 35) showed robust ubiquitous expression of the reporter transposon encoding a fluorescent protein, and provided an unique resource to assess a potential cell trafficking to non-transgenic littermates (n = 7) or mothers (n = 4). Sensitive flow cytometry, fluorescence microscopy, and real-time PCR provided no evidence for microchimerism in porcine littermates, or piglets and their mothers in both blood and solid organs. These data indicate that the epitheliochorial structure of the porcine placenta effectively prevents cellular exchange during gestation.

The health effects of fetal microchimerism can be modeled in companion dogs

Fetal microchimerism (FMC) has been described to have a range of effects on health and disease. Y-chromosomal DNA has been detected in Golden Retrievers suggesting persistent FMC. In that report, nine dogs had evidence of microchimerism without prior pregnancy. To further understand this finding, a dam with prior male live births giving birth to her fourth litter of puppies, all females, was evaluated for FMC along with two of her daughters. All three female dogs had evidence of Y-chromosomal DNA in their blood. This suggests that male cells carried by the dam from previous pregnancy trafficked to her daughters to establish microchimerism in younger siblings. Companion dogs share many of the same cancers as humans, have out-bred genetics, and share the human environment, making them optimal models of human disease. Understanding the impact of FMC on health and disease of dogs could elucidate mechanisms useful for clinical interventions in humans.