Unusual angiogenic factor plays a role in lizard pregnancy but is not unique to viviparity

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.

Understanding the evolution of viviparity using intraspecific variation in reproductive mode and transitional forms of pregnancy

How innovations such as vision, flight and pregnancy evolve is a central question in evolutionary biology. Examination of transitional (intermediate) forms of these traits can help address this question, but these intermediate phenotypes are very rare in extant species. Here we explore the biology and evolution of transitional forms of pregnancy that are midway between the ancestral state of oviparity (egg-laying) and the derived state, viviparity (live birth). Transitional forms of pregnancy occur in only three vertebrates, all of which are lizard species that also display intraspecific variation in reproductive phenotype. In these lizards (Lerista bougainvillii, Saiphos equalis, and Zootoca vivipara), geographic variation of three reproductive forms occurs within a single species: oviparity, viviparity, and a transitional form of pregnancy. This phenomenon offers the valuable prospect of watching 'evolution in action'. In these species, it is possible to conduct comparative research using different reproductive forms that are not confounded by speciation, and are of relatively recent origin. We identify major proximate and ultimate questions that can be addressed in these species, and the genetic and genomic tools that can help us understand how transitional forms of pregnancy are produced, despite predicted fitness costs. We argue that these taxa represent an excellent prospect for understanding the major evolutionary shift between egg-laying and live birth, which is a fundamental innovation in the history of animals.

Different Genes are Recruited During Convergent Evolution of Pregnancy and the Placenta

The repeated evolution of the same traits in distantly related groups (convergent evolution) raises a key question in evolutionary biology: do the same genes underpin convergent phenotypes? Here, we explore one such trait, viviparity (live birth), which, qualitative studies suggest, may indeed have evolved via genetic convergence. There are >150 independent origins of live birth in vertebrates, providing a uniquely powerful system to test the mechanisms underpinning convergence in morphology, physiology, and/or gene recruitment during pregnancy. We compared transcriptomic data from eight vertebrates (lizards, mammals, sharks) that gestate embryos within the uterus. Since many previous studies detected qualitative similarities in gene use during independent origins of pregnancy, we expected to find significant overlap in gene use in viviparous taxa. However, we found no more overlap in uterine gene expression associated with viviparity than we would expect by chance alone. Each viviparous lineage exhibits the same core set of uterine physiological functions. Yet, contrary to prevailing assumptions about this trait, we find that none of the same genes are differentially expressed in all viviparous lineages, or even in all viviparous amniote lineages. Therefore, across distantly related vertebrates, different genes have been recruited to support the morphological and physiological changes required for successful pregnancy. We conclude that redundancies in gene function have enabled the repeated evolution of viviparity through recruitment of different genes from genomic "toolboxes", which are uniquely constrained by the ancestries of each lineage.

Structural changes to the brood pouch of male pregnant seahorses (Hippocampus abdominalis) facilitate exchange between father and embryos

INTRODUCTION

Embryonic growth and development require efficient respiratory gas exchange. Internal incubation of developing young thus presents a significant physiological challenge, because respiratory gas diffusion to embryos is impeded by the additional barrier of parental tissue between the embryo and the environment. Therefore, live-bearing species exhibit a variety of adaptations facilitating respiratory gas exchange between the parent (usually the mother) and embryos. Syngnathid fishes are the only vertebrates to exhibit male pregnancy, allowing comparative studies of the biology and evolution of internal incubation of embryos, independent of the female reproductive tract. Here, we examine the fleshy, sealed, seahorse brood pouch, and provide the first quantification of structural changes to this gestational organ across pregnancy.

METHODS

We used histological analysis and morphometrics to quantify the surface area for exchange across the brood pouch epithelium, and the structure of the vascular bed of the brood pouch.

RESULTS

We show dramatic remodelling of gestational tissues as pregnancy progresses, including an increase in tortuosity of the gestational epithelium, an increase in capillary density, and a decrease in diffusion distance between capillaries and the pouch lumen.

DISCUSSION

These changes produce an increased surface area and expansion of the vascular bed of the placenta that likely facilitates respiratory gas exchange. These changes mirror the remodelling of gestational tissue in viviparous amniotes and elasmobranchs, and provide further evidence of the convergence of adaptations to support pregnancy in live-bearing animals.

Elevation, oxygen, and the origins of viviparity

Research focused on understanding the evolutionary factors that shape parity mode evolution among vertebrates have long focused on squamate reptiles (snakes and lizards), which contain all but one of the evolutionary transitions from oviparity to viviparity among extant amniotes. While most hypotheses have focused on the role of cool temperatures in favoring viviparity in thermoregulating snakes and lizards, there is a growing appreciation in the biogeographic literature for the importance of lower oxygen concentrations at high elevations for the evolution of parity mode. However, the physiological mechanisms underlying how hypoxia might reduce fitness, and how viviparity can alleviate this fitness decrement, has not been systematically evaluated. We qualitatively evaluated previous research on reproductive and developmental physiology, and found that (1) hypoxia can negatively affect fitness of squamate embryos, (2) oxygen availability in the circulatory system of adult lizards can be similar or greater than an egg, and (3) gravid females can possess adaptive phenotypic plasticity in response to hypoxia. These findings suggest that the impact of hypoxia on the development and physiology of oviparous and viviparous squamates would be a fruitful area of research for understanding the evolution of viviparity. To that end, we propose an integrative research program for studying hypoxia and the evolution of viviparity in squamates.

The evolution and physiology of male pregnancy in syngnathid fishes

The seahorses, pipefishes and seadragons (Syngnathidae) are among the few vertebrates in which pregnant males incubate developing embryos. Syngnathids are popular in studies of sexual selection, sex-role reversal, and reproductive trade-offs, and are now emerging as valuable comparative models for the study of the biology and evolution of reproductive complexity. These fish offer the opportunity to examine the physiology, behavioural implications, and evolutionary origins of embryo incubation, independent of the female reproductive tract and female hormonal milieu. Such studies allow us to examine flexibility in regulatory systems, by determining whether the pathways underpinning female pregnancy are also co-opted in incubating males, or whether novel pathways have evolved in response to the common challenges imposed by incubating developing embryos and releasing live young. The Syngnathidae are also ideal for studies of the evolution of reproductive complexity, because they exhibit multiple parallel origins of complex reproductive phenotypes. Here we assay the taxonomic distribution of syngnathid parity mode, examine the selective pressures that may have led to the emergence of male pregnancy, describe the biology of syngnathid reproduction, and highlight pressing areas for future research. Experimental tests of a range of hypotheses, including many generated with genomic tools, are required to inform overarching theories about the fitness implications of pregnancy and the evolution of male pregnancy. Such information will be widely applicable to our understanding of fundamental reproductive and evolutionary processes in animals.

Australian lizards are outstanding models for reproductive biology research

Australian lizards are a diverse group distributed across the continent and inhabiting a wide range of environments. Together, they exhibit a remarkable diversity of reproductive morphologies, physiologies, and behaviours that is broadly representative of vertebrates in general. Many reproductive traits exhibited by Australian lizards have evolved independently in multiple lizard lineages, including sociality, complex signalling and mating systems, viviparity, and temperature-dependent sex determination. Australian lizards are thus outstanding model organisms for testing hypotheses about how reproductive traits function and evolve, and they provide an important basis of comparison with other animals that exhibit similar traits. We review how research on Australian lizard reproduction has contributed to answering broader evolutionary and ecological questions that apply to animals in general. We focus on reproductive traits, processes, and strategies that are important areas of current research, including behaviours and signalling involved in courtship; mechanisms involved in mating, egg production, and sperm competition; nesting and gestation; sex determination; and finally, birth in viviparous species. We use our review to identify important questions that emerge from an understanding of this body of research when considered holistically. Finally, we identify additional research questions within each topic that Australian lizards are well suited for reproductive biologists to address.

Facultative oviparity in a viviparous skink ( Saiphos equalis)

Facultative changes in parity mode (oviparity to viviparity and vice versa) are rare in vertebrates, yet offer fascinating opportunities to investigate the role of reproductive lability in parity mode evolution. Here, we report apparent facultative oviparity by a viviparous female of the bimodally reproductive skink Saiphos equalis-the first report of different parity modes within a vertebrate clutch. Eggs oviposited facultatively possess shell characteristics of both viviparous and oviparous S. equalis, demonstrating that egg coverings for viviparous embryos are produced by the same machinery as those for oviparous individuals. Since selection may act in either direction when viviparity has evolved recently, squamate reproductive lability may confer a selective advantage. We suggest that facultative oviparity is a viable reproductive strategy for S. equalis and that squamate reproductive lability is more evolutionarily significant than previously acknowledged.

Expression of vascular endothelial growth factor A isoforms is dysregulated in women with endometriosis

Angiogenesis is a critical step in the development of ectopic lesions during endometriosis. Although total vascular endothelial growth factor (VEGF) A is elevated in the peritoneal fluid of women with endometriosis, there are contradictory reports on how levels of total endometrial VEGFA are altered in this disease. Furthermore, limited research is available on different VEGFA isoforms in women with endometriosis. Thus, the aim of the present study was to analyse levels of various VEGFA isoforms in women with and without endometriosis at different stages of the menstrual cycle. Quantitative polymerase chain reaction analysis showed that total VEGFA was highest during menstruation in endometriosis compared with controls (P=0.0373). VEGF121 and VEGF189 were similarly highest during menstruation in endometriosis compared with controls (P=0.0165 and 0.0154 respectively). The present study is also the first to identify the natural expression of VEGF111 in human tissue, which is also highest during menstruation in endometriosis (P=0.0464). This discovery of the natural production of VEGF111 in human endometrium, as well as the upregulation of VEGFA isoforms during menstruation in endometriosis, may shed further light on the development and progression of the disease, and improve our understanding of the regulation of endometrial angiogenesis.

Ovarian Hyperstimulation Reduces Vascular Endothelial Growth Factor-A During Uterine Receptivity

The angiogenic factor vascular endothelial growth factor-A (VEGFA) plays a critical role during early pregnancy in many species including the rat, and any alterations in VEGFA levels can severely impact blastocyst implantation rates. The rat ovarian hyperstimulation (OH) model is useful in studying how the induction of superovulation affects VEGFA levels and endometrial receptivity to blastocyst implantation. The present study shows that the major isoform in the rat uterus, Vegf₁₈₈, is reduced at the time of receptivity in OH compared to normal pregnancy, whereas there is no change in Vegf₁₆₄ and Vegf₁₂₀ messenger RNA (mRNA). The VEGFA receptor 2 (VEGFR2) protein levels are also reduced at the time of receptivity in OH. Our ovariectomy studies show that Vegf₁₆₄, Vegf₁₈₈, and Vegf₁₂₀ are significantly decreased by estrogen, and, to a lesser extent progesterone, when compared to control animals. Although no change in the percentage of endometrial blood vessels was seen across all stages of pregnancy, at the time of receptivity in OH pregnancies, blood vessels were typically larger compared to other stages. The altered progesterone-estrogen ratio seen in OH, taken together with our ovariectomy studies, explains the changes to Vegfa mRNA in OH at the time of receptivity. Since VEGFA is important during implantation, the changes to Vegfa and VEGFR2 levels in the endometrium may help explain the observed lower endometrial receptivity following OH. This study aimed to analyse how ovarian hyperstimulation alters the levels of vascular endothleial growth factor and its major receptor, VEGFR2 in the uterus in a rat model.

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.

Comparative genomics of hormonal signaling in the chorioallantoic membrane of oviparous and viviparous amniotes

In oviparous amniotes (reptiles, birds, and mammals) the chorioallantoic membrane (CAM) lines the inside of the egg and acts as the living point of contact between the embryo and the outside world. In livebearing (viviparous) amniotes, communication during embryonic development occurs across placental tissues, which form between the uterine tissue of the mother and the CAM of the embryo. In both oviparous and viviparous taxa, the CAM is at the interface of the embryo and the external environment and can transfer signals from there to the embryo proper. To understand the evolution of placental hormone production in amniotes, we examined the expression of genes involved in hormone synthesis, metabolism, and hormone receptivity in the CAM of species across the amniote phylogeny. We collected transcriptome data for the chorioallantoic membranes of the chicken (oviparous), the lizards Lerista bougainvillii (both oviparous and viviparous populations) and Pseudemoia entrecasteauxii (viviparous), and the horse Equus caballus (viviparous). The viviparous taxa differ in their mechanisms of nutrient provisioning: L. bougainvillii is lecithotrophic (embryonic nourishment is provided via the yolk only), but P. entrecasteauxii and the horse are placentotrophic (embryos are nourished via placental transport). Of the 423 hormone-related genes that we examined, 91 genes are expressed in all studied species, suggesting that the chorioallantoic membrane ancestrally had an endocrine function. Therefore, the chorioallantoic membrane appears to be a highly hormonally active organ in all amniotes. No genes are expressed only in viviparous species, suggesting that the evolution of viviparity has not required the recruitment of any specific hormone-related genes. Our data suggest that the endocrine function of the CAM as a placental tissue evolved in part through co-option of ancestral gene expression patterns.

Reptile Pregnancy Is Underpinned by Complex Changes in Uterine Gene Expression: A Comparative Analysis of the Uterine Transcriptome in Viviparous and Oviparous Lizards

The evolution of new organs is difficult to study because most vertebrate organs evolved only once, more than 500 million years ago. An ideal model for understanding complex organ evolution is the placenta, a structure that is present in live bearing reptiles and mammals (amniotes), which has evolved independently more than 115 times. Using transcriptomics, we characterized the uterine gene expression patterns through the reproductive cycle of a viviparous skink lizard, Pseudemoia entrecasteauxii. Then we compare these patterns with the patterns of gene expression from two oviparous skinks Lampropholis guichenoti and Lerista bougainvillii. While thousands of genes are differentially expressed between pregnant and non-pregnant uterine tissue in the viviparous skink, few differentially expressed genes were identified between gravid and non-gravid oviparous skinks. This finding suggests that in P. entrecasteauxii, a pregnant-specific gene expression profile has evolved, allowing for the evolution of pregnancy-specific innovations in the uterus. We find substantial gene expression differences between the uterus of the chorioallantoic and the yolk sac placenta in P. entrecasteauxii, suggesting these placental regions are specialized for different placental functions. In particular, the chorioallantoic placenta is likely a major site of nutrient transport by membrane-bound transport proteins, while the yolk sac placenta also likely transports nutrients but via apocrine secretions. We discuss how the evolution of transcription factor networks is likely to underpin the evolution of the new transcriptional states in the uterine tissue of viviparous reptiles.

Expression of VEGF 111 and other VEGF-A variants in the rat uterus is correlated with stage of pregnancy

Vascular endothelial growth factor A is a major mediator of angiogenesis, a critically important process in vertebrate growth and development as well as pregnancy. Here we report for the first time the expression of a rare and unusually potent splice variant, VEGF ₁₁₁ , in vivo in mammals. This variant has previously only been found in mammals in cultured human cells exposed to genotoxic agents. Our discovery of VEGF ₁₁₁ in the uterus of both a eutherian (rat) and a marsupial (fat-tailed dunnart) suggests that the splice variant may be common to all mammals. As VEGF ₁₁₁ is also expressed in the uterus of at least one lineage of lizards, the expression of this splice variant may be a widespread amniote phenomenon. We measured expression of VEGF ₁₁₁ and two major VEGF-A splice variants in the uterus of pregnant rats, showing that the three variants show different expression patterns across pregnancy. Our results suggest that viviparous mammals possess a precisely regulated milieu of VEGF isoforms producing the angiogenesis required for successful pregnancy. The discovery of VEGF ₁₁₁ in rat uterus paves the way for the development of in vivo models of VEGF ₁₁₁ activity in a highly tractable laboratory animal, and is particularly significant in the context of early pregnancy loss and cancer research.

Seahorse Brood Pouch Transcriptome Reveals Common Genes Associated with Vertebrate Pregnancy

Viviparity (live birth) has evolved more than 150 times in vertebrates, and represents an excellent model system for studying the evolution of complex traits. There are at least 23 independent origins of viviparity in fishes, with syngnathid fishes (seahorses and pipefish) unique in exhibiting male pregnancy. Male seahorses and pipefish have evolved specialized brooding pouches that provide protection, gas exchange, osmoregulation, and limited nutrient provisioning to developing embryos. Pouch structures differ widely across the Syngnathidae, offering an ideal opportunity to study the evolution of reproductive complexity. However, the physiological and genetic changes facilitating male pregnancy are largely unknown. We used transcriptome profiling to examine pouch gene expression at successive gestational stages in a syngnathid with the most complex brood pouch morphology, the seahorse Hippocampus abdominalis. Using a unique time-calibrated RNA-seq data set including brood pouch at key stages of embryonic development, we identified transcriptional changes associated with brood pouch remodeling, nutrient and waste transport, gas exchange, osmoregulation, and immunological protection of developing embryos at conception, development and parturition. Key seahorse transcripts share homology with genes of reproductive function in pregnant mammals, reptiles, and other live-bearing fish, suggesting a common toolkit of genes regulating pregnancy in divergent evolutionary lineages.