Eutherian-like reproductive specializations in a viviparous reptile

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.

Morphological research on amniote eggs and embryos: An introduction and historical retrospective

Evolution of the terrestrial egg of amniotes (reptiles, birds, and mammals) is often considered to be one of the most significant events in vertebrate history. Presence of an eggshell, fetal membranes, and a sizeable yolk allowed this egg to develop on land and hatch out well-developed, terrestrial offspring. For centuries, morphologically-based studies have provided valuable information about the eggs of amniotes and the embryos that develop from them. This review explores the history of such investigations, as a contribution to this special issue of Journal of Morphology, titled Developmental Morphology and Evolution of Amniote Eggs and Embryos. Anatomically-based investigations are surveyed from the ancient Greeks through the Scientific Revolution, followed by the 19th and early 20th centuries, with a focus on major findings of historical figures who have contributed significantly to our knowledge. Recent research on various aspects of amniote eggs is summarized, including gastrulation, egg shape and eggshell morphology, eggs of Mesozoic dinosaurs, sauropsid yolk sacs, squamate placentation, embryogenesis, and the phylotypic phase of embryonic development. As documented in this review, studies on amniote eggs and embryos have relied heavily on morphological approaches in order to answer functional and evolutionary questions.

Plasma estradiol and progesterone concentrations during the female reproductive cycle in a highly placentotrophic viviparous lizard, Mabuya sp

The neotropical genus Mabuya are obligate placentotrophic viviparous lizards, which have a short vitellogenesis that produces microlecithal oocytes and a prolonged time of gestation (9 to 10 months). The hormonal control of female reproductive activity during follicular growth and pregnancy has not been studied, although it is known that the corpus luteum can produce progesterone, but regresses early in pregnancy, being replaced in this function by the placenta. Through enzyme immunoassay (EIA) we measured the plasma concentrations of estradiol (E₂) and progesterone (P₄) in females of a population of Mabuya sp at different stages of their reproductive cycle. Previously, we confirmed the presence of P₄ in plasma by high-performance liquid chromatography methods with diode-array detector ultraviolet (HPLC-DAD-UV). The average concentration values of E₂ and P₄ were compared among reproductive stages and their dynamics were related to what is known in other oviparous and viviparous amniotes. The plasma E₂ concentrations of Mabuya sp. are below the levels found in other viviparous reptiles, probably related to the substantial reduction of its follicular growth phase. Its highest concentration was detected during vitellogenesis, related to its function in the growth and maturation of the ovarian follicles and oviduct preparation for pregnancy; lower levels were observed during pregnancy, but they increase at the end when a new vitellogenesis event begins and massive placental maternal-fetal nutrient transfer occurs. High concentrations of P₄ were found during pregnancy, related to its function in the maintenance of the developing embryos within the oviduct. The highest levels of P₄ were found at early gestation, then they descend from mid-gestation to the end of gestation. Although some characteristics of hormonal control related to the high level of placentotrophy were observed in this species, the changes in plasma sex steroid concentrations during the reproductive cycle in females of Mabuya sp. follow patterns seen in other viviparous amniotes.

Why do placentas evolve? Evidence for a morphological advantage during pregnancy in live-bearing fish

A live-bearing reproductive strategy can induce large morphological changes in the mother during pregnancy. The evolution of the placenta in swimming animals involves a shift in the timing of maternal provisioning from pre-fertilization (females supply their eggs with sufficient yolk reserves prior to fertilization) to post-fertilization (females provide all nutrients via a placenta during the pregnancy). It has been hypothesised that this shift, associated with the evolution of the placenta, should confer a morphological advantage to the females leading to a more slender body shape during the early stages of pregnancy. We tested this hypothesis by quantifying three-dimensional shape and volume changes during pregnancy and in full-grown virgin controls of two species within the live-bearing fish family Poeciliidae: Poeciliopsis gracilis (non-placental) and Poeciliopsis turneri (placental). We show that P. turneri is more slender than P. gracilis at the beginning of the interbrood interval and in virgins, and that these differences diminish towards the end of pregnancy. This study provides the first evidence for an adaptive morphological advantage of the placenta in live-bearing fish. A similar morphological benefit could drive the evolution of placentas in other live-bearing (swimming) animal lineages.

An endogenous retroviral envelope syncytin and its cognate receptor identified in the viviparous placental Mabuya lizard

Syncytins are envelope genes from endogenous retroviruses that have been captured during evolution for a function in placentation. They have been found in all placental mammals in which they have been searched, including marsupials. Placental structures are not restricted to mammals but also emerged in some other vertebrates, most frequently in lizards, such as the viviparous Mabuya Scincidae. Here, we performed high-throughput RNA sequencing of a Mabuya placenta transcriptome and screened for the presence of retroviral env genes with a full-length ORF. We identified one such gene, which we named "syncytin-Mab1," that has all the characteristics expected for a syncytin gene. It encodes a membrane-bound envelope protein with fusogenic activity ex vivo, is expressed at the placental level as revealed by in situ hybridization and immunohistochemistry, and is conserved in all Mabuya species tested, spanning over 25 My of evolution. Its cognate receptor, required for its fusogenic activity, was searched for by a screening assay using the GeneBridge4 human/Chinese hamster radiation hybrid panel and found to be the MPZL1 gene, previously identified in mammals as a signal-transducing transmembrane protein involved in cell migration. Together, these results show that syncytin capture is not restricted to placental mammals, but can also take place in the rare nonmammalian vertebrates in which a viviparous placentotrophic mode of reproduction emerged. It suggests that similar molecular tools have been used for the convergent evolution of placentation in independently evolved and highly distant vertebrates.

Características estructurales del esqueleto en <i>Mabuya</i> sp. (Squamata: Scincidae): una comparación con escíncidos africanos

Para establecer las particularidades del plan corporal en Mabuya sp. se describen las principales características estructurales del esqueleto, se comparan con otros escíncidos africanos y se discuten en el contexto de la evolución de los escíncidos en América y del plan corporal en los Squamata. El taxón Mabuya sp. exhibe caracteres craneales diferentes a las especies africanas que podrían constituir sinapomorfias para el clado americano. Si bien, Mabuya sp. exhibe un cráneo con características similares a lagartos con un plan corporal lacertiforme, presenta transformaciones incipientes hacia un plan corporal serpentiforme, las cuales se evidencian en aumento del número de vértebras y en la reducción del tamaño de las extremidades. En Mabuya sp. el alargamiento corporal, representado en mayor número de vértebras, podría estar relacionado con la viviparidad, como también se observa en las especies vivíparas africanas, lo cual aumentaría el volumen en el abdomen para mantener los embriones en crecimiento. Por lo tanto, en su evolución a partir de formas africanas, esta característica del plan corporal probablemente subyace a la evolución del conjunto de especializaciones relacionadas con la viviparidad en el género Mabuya, las cuales son únicas dentro de reptiles y alcanzan su pináculo en este clado.

Proteomic Profile of Mabuya sp. (Squamata: Scincidae) Ovary and Placenta During Gestation

Reptiles are one of the most diverse groups of vertebrates, providing an integrated system for comparative studies on metabolic, animal physiology, and developmental biology. However, the molecular data available are limited and only recently have started to call attention in the "omics" sciences. Mabuya sp. is a viviparous placentrotrophic skink with particular reproductive features, including microlecithal eggs, early luteolysis, prolonged gestation, and development of a highly specialized placenta. This placenta is responsible for respiratory exchange and the transference of all nutrients necessary for embryonic development. Our aim was to identify differentially expressed proteins in the ovary and placenta of Mabuya sp. during early, mid, and late gestation; their possible metabolic pathways; and biological processes. We carried out a comparative proteomic analysis during gestation in both tissues by sodium dodecyl sulfate polyacrylamide gel electrophoresis, two-dimensional gel electrophoresis, and matrix-assisted laser desorption/ionization. Differential protein expression in both tissues (Student's t-test P < 0.05) was related to several processes such as cell structure, cell movement, and energy. Proteins found in ovary are mainly associated with follicular development and its regulation. In the placenta, particularly during mid and late gestation, protein expression is involved in nutrient metabolism, transport, protein synthesis, and embryonic development. This work provides new insights about the proteins expressed and their physiological mechanisms in Mabuya sp. placenta and ovary during gestation.

Matrotrophy and placentation in invertebrates: a new paradigm

Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy-placentotrophy-is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritive modes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent-offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and 'isolated' chambers with presumably more effective nutritional relationships, and (iii) internal secretory ('milk') glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels-positional, structural and physiological.

Evidence of maternal copper and cadmium transfer in two live-bearing fish species

We studied maternal transfer of an essential metal (copper) and a non-essential one (cadmium) in the live-bearing fishes Heterandria formosa and Gambusia affinis. The goals of this study were: (1) to determine whether metals are transferred from exposed females to their developing offspring; (2) to determine if this transfer differs between two fish species that differ in their degree of maternal provisioning during development; (3) to determine the duration of maternal metal transfer once females are no longer exposed; and (4) to determine whether copper and cadmium are transferred equivalently. We exposed gravid females to background levels (control) or 0.15 µM of metal for 10 days, and then transferred them to clean water. We allowed females to give birth to up to three broods, and then quantified metal levels in offspring born at least 3 days after the transfer. We detected maternal metal transfer for both metals and in both species. Offspring metal levels decreased as females spent more time in clean water. Similarly, metal levels were lower in later broods than in earlier ones. Maternal metal transfer was higher in H. formosa than in G. affinis. Our results constitute the first report of maternal metal transfer in live-bearing fishes, and show that developing embryos acquire both essential and non-essential metals from their mothers in both species. This shows that metal toxicity may be an issue for live-bearing fish in clean environments when the previous generation has encountered metal pollution.

The evolution of viviparity: molecular and genomic data from squamate reptiles advance understanding of live birth in amniotes

Squamate reptiles (lizards and snakes) are an ideal model system for testing hypotheses regarding the evolution of viviparity (live birth) in amniote vertebrates. Viviparity has evolved over 100 times in squamates, resulting in major changes in reproductive physiology. At a minimum, all viviparous squamates exhibit placentae formed by the appositions of maternal and embryonic tissues, which are homologous in origin with the tissues that form the placenta in therian mammals. These placentae facilitate adhesion of the conceptus to the uterus as well as exchange of oxygen, carbon dioxide, water, sodium, and calcium. However, most viviparous squamates continue to rely on yolk for nearly all of their organic nutrition. In contrast, some species, which rely on the placenta for at least a portion of organic nutrition, exhibit complex placental specializations associated with the transport of amino acids and fatty acids. Some viviparous squamates also exhibit reduced immunocompetence during pregnancy, which could be the result of immunosuppression to protect developing embryos. Recent molecular studies using both candidate-gene and next-generation sequencing approaches have suggested that at least some of the genes and gene families underlying these phenomena play similar roles in the uterus and placenta of viviparous mammals and squamates. Therefore, studies of the evolution of viviparity in squamates should inform hypotheses of the evolution of viviparity in all amniotes, including mammals.

The origin and evolution of genomic imprinting and viviparity in mammals

Genomic imprinting is widespread in eutherian mammals. Marsupial mammals also have genomic imprinting, but in fewer loci. It has long been thought that genomic imprinting is somehow related to placentation and/or viviparity in mammals, although neither is restricted to mammals. Most imprinted genes are expressed in the placenta. There is no evidence for genomic imprinting in the egg-laying monotreme mammals, despite their short-lived placenta that transfers nutrients from mother to embryo. Post natal genomic imprinting also occurs, especially in the brain. However, little attention has been paid to the primary source of nutrition in the neonate in all mammals, the mammary gland. Differentially methylated regions (DMRs) play an important role as imprinting control centres in each imprinted region which usually comprises both paternally and maternally expressed genes (PEGs and MEGs). The DMR is established in the male or female germline (the gDMR). Comprehensive comparative genome studies demonstrated that two imprinted regions, PEG10 and IGF2-H19, are conserved in both marsupials and eutherians and that PEG10 and H19 DMRs emerged in the therian ancestor at least 160 Ma, indicating the ancestral origin of genomic imprinting during therian mammal evolution. Importantly, these regions are known to be deeply involved in placental and embryonic growth. It appears that most maternal gDMRs are always associated with imprinting in eutherian mammals, but emerged at differing times during mammalian evolution. Thus, genomic imprinting could evolve from a defence mechanism against transposable elements that depended on DNA methylation established in germ cells.

A novel pattern of placental leucine transfer during mid to late gestation in a highly placentotrophic viviparous lizard

Placentotrophy is the nourishment of embryos by resources provided via the placenta during gestation. The magnitude and timing of placental nutrient support during pregnancy are important for embryonic growth, especially in highly placentotrophic animals such as mammals. However, no study has yet investigated how placental organic nutrient support may change during pregnancy in highly placentotrophic viviparous reptiles. Amino acids are essential nutrients for embryonic growth and leucine is a common amino acid. The magnitude and timing of placental leucine transfer may affect embryonic growth and mass and, therefore, offspring phenotype. In this study, female Pseudemoia entrecasteauxii, a highly placentotrophic viviparous skink, were collected throughout gestation. We injected (3)H-leucine into these gravid females and assessed the transfer of (3)H-leucine into maternal compartments (i.e., the blood and the liver), and into embryonic compartments (i.e., the embryo, the yolk, and the amniotic fluid). At either 60 or 120 min post-injection, the radioactivity in each sample was extracted and then counted, and the transfer ratio was calculated. Our results provide direct evidence that circulating maternal leucine passes through the placenta into the embryos in this species. The relative rate of placental leucine transfer did not alter during mid to late gestation. This suggests the steady somatic growth of the embryos during mid-late pregnancy is dependent upon the placental transfer of nutrients rather than yolk stores. This pattern of placental nutrient support may determine offspring body size at birth and, therefore, offspring fitness in P. entrecasteauxii.

Stable isotope tracer reveals that viviparous snakes transport amino acids to offspring during gestation

Viviparity and placentation have evolved from oviparity over 100 times in squamate reptiles (lizards and snakes). The independent origins of placentation have resulted in a variety of placental morphologies in different taxa, ranging from simple apposition of fetal and maternal tissues to endotheliochorial implantation that is homoplasious with mammalian placentation. Because the eggs of oviparous squamates transport gases and water from the environment and calcium from the eggshell, the placentae of viviparous squamates are thought to have initially evolved to accomplish these functions from within the maternal oviduct. Species with complex placentae have also been shown to rely substantially, or even primarily, on placental transport of organic nutrients for embryonic nutrition. However, it is unclear whether species with only simple placentae are also capable of transporting organic nutrients to offspring. Among viviparous squamates, all of the snakes that have been studied thus far have been shown to have simple placentae. However, most studies of snake placentation are limited to a single lineage, the North American Natricinae. We tested the abilities of four species of viviparous snakes – Agkistrodon contortrix (Viperidae), Boa constrictor (Boidae), Nerodia sipedon (Colubridae: Natricinae) and Thamnophis sirtalis (Colubridae: Natricinae) – to transport diet-derived amino acids to offspring during gestation. We fed [15N]leucine to pregnant snakes, and compared offspring 15N content with that of unlabeled controls. Labeled females allocated significantly more 15N to offspring than did controls, but 15N allocation did not differ among species. Our results indicate that viviparous snakes are capable of transporting diet-derived amino acids to their offspring during gestation, possibly via placentation.

Uterine gene expression in the live-bearing lizard, Chalcides ocellatus, reveals convergence of squamate reptile and mammalian pregnancy mechanisms

Although the morphological and physiological changes involved in pregnancy in live-bearing reptiles are well studied, the genetic mechanisms that underlie these changes are not known. We used the viviparous African Ocellated Skink, Chalcides ocellatus, as a model to identify a near complete gene expression profile associated with pregnancy using RNA-Seq analyses of uterine transcriptomes. Pregnancy in C. ocellatus is associated with upregulation of uterine genes involved with metabolism, cell proliferation and death, and cellular transport. Moreover, there are clear parallels between the genetic processes associated with pregnancy in mammals and Chalcides in expression of genes related to tissue remodeling, angiogenesis, immune system regulation, and nutrient provisioning to the embryo. In particular, the pregnant uterine transcriptome is dominated by expression of proteolytic enzymes that we speculate are involved both with remodeling the chorioallantoic placenta and histotrophy in the omphaloplacenta. Elements of the maternal innate immune system are downregulated in the pregnant uterus, indicating a potential mechanism to avoid rejection of the embryo. We found a downregulation of major histocompatability complex loci and estrogen and progesterone receptors in the pregnant uterus. This pattern is similar to mammals but cannot be explained by the mammalian model. The latter finding provides evidence that pregnancy is controlled by different endocrinological mechanisms in mammals and reptiles. Finally, 88% of the identified genes are expressed in both the pregnant and the nonpregnant uterus, and thus, morphological and physiological changes associated with C. ocellatus pregnancy are likely a result of regulation of genes continually expressed in the uterus rather than the initiation of expression of unique genes.

Angiogenesis of the uterus and chorioallantois in the eastern water skink Eulamprus quoyii

We have discovered a modification of the uterus that appears to facilitate maternal-fetal communication during pregnancy in the scincid lizard Eulamprus quoyii. A vessel-dense elliptical area (VDE) on the mesometrial side of the uterus expands as the embryo grows, providing a large vascular area for physiological exchange between mother and embryo. The VDE is already developed in females with newly ovulated eggs, and is situated directly adjacent to the chorioallantois of the embryo when it develops. It is likely that signals from the early developing embryo determine the position of the VDE, as the VDE is off-centre in cases where the embryo sits obliquely in the uterus. The VDE is not a modification of the uterus over the entire chorioallantoic placenta, as the VDE is smaller than the chorioallantois after embryonic stage 33, but expansion of the VDE and growth of the chorioallantois during pregnancy are strongly correlated. The expansion of the VDE is also strongly correlated with embryonic growth and increasing embryonic oxygen demand (). We propose that angiogenic stimuli are exchanged between the VDE and the chorioallantois in E. quoyii, allowing the simultaneous growth of both tissues.

Gene expression during the oocyte-to-embryo transition in mammals

The seminal question in modern developmental biology is the origins of new life arising from the unification of sperm and egg. The roots of this question begin from 19th to 20th century embryologists studying fertilization and embryogenesis. Although the revolution of molecular biology has yielded significant insight into the complexity of this process, the overall orchestration of genes, molecules, and cells is still not fully formed. Early mammalian development, specifically the oocyte-to-embryo transition, is essentially under "maternal command" from factors deposited in the cytoplasm during oocyte growth, independent of de novo transcription from the nascent embryo. Many of the advances in understanding this developmental period occurred in tandem with application of new methods and techniques from molecular biology, from protein electrophoresis to sequencing and assemblies of whole genomes. From this bed of knowledge, it appears that precise control of mRNA translation is a key regulator coordinating the molecular and cellular events occurring during oocyte-to-embryo transition. Notably, oocyte transcriptomes share, yet retain some uniqueness, common genetic motifs among all chordates. The common genetic motifs typically define fundamental processes critical for cellular maintenance, whereas the unique genetic features may be a source of variation and a substrate for sexual selection, genetic drift, or gene flow. One purpose for this complex interplay among genes, proteins, and cells may allow for evolution to transform and act upon the underlying processes, at molecular, structural and organismal levels, to increase diversity, which is the ultimate goal of sexual reproduction.

Genotypic sex determination enabled adaptive radiations of extinct marine reptiles

Adaptive radiations often follow the evolution of key traits, such as the origin of the amniotic egg and the subsequent radiation of terrestrial vertebrates. The mechanism by which a species determines the sex of its offspring has been linked to critical ecological and life-history traits but not to major adaptive radiations, in part because sex-determining mechanisms do not fossilize. Here we establish a previously unknown coevolutionary relationship in 94 amniote species between sex-determining mechanism and whether a species bears live young or lays eggs. We use that relationship to predict the sex-determining mechanism in three independent lineages of extinct Mesozoic marine reptiles (mosasaurs, sauropterygians and ichthyosaurs), each of which is known from fossils to have evolved live birth. Our results indicate that each lineage evolved genotypic sex determination before acquiring live birth. This enabled their pelagic radiations, where the relatively stable temperatures of the open ocean constrain temperature-dependent sex determination in amniote species. Freed from the need to move and nest on land, extreme physical adaptations to a pelagic lifestyle evolved in each group, such as the fluked tails, dorsal fins and wing-shaped limbs of ichthyosaurs. With the inclusion of ichthyosaurs, mosasaurs and sauropterygians, genotypic sex determination is present in all known fully pelagic amniote groups (sea snakes, sirenians and cetaceans), suggesting that this mode of sex determination and the subsequent evolution of live birth are key traits required for marine adaptive radiations in amniote lineages.

Phylogenetic evidence for early hemochorial placentation in eutheria

The eutherian placenta is remarkable for its structural and functional variability. In order to construct and test comparative hypotheses relating ecological, behavioral and physiological traits to placental characteristics it is first necessary to reconstruct the historical course of placental evolution. Previous attempts to do so have yielded inconsistent results, particularly with respect to the early evolution of structural relationships between fetal and maternal circulatory systems. Here, we bring a battery of phylogenetic methods - including parsimony, likelihood and Bayesian approaches - to bear on the question of placental evolution. All of these approaches are consistent in indicating that highly invasive hemochorial placentation, as found in human beings and numerous other taxa, was an early evolutionary innovation present in the most ancient ancestors of the living placental mammals.

Heliotherms in tropical rain forest: the ecology of Kentropyx calcarata (Teiidae) and Mabuya nigropunctata (Scincidae) in the Curuá-Una of Brazil

Kentropyx calcarata (Teiidae) and Mabuya nigropunctata (Scincidae) occur together in lowland tropical forest of the Amazon near the Rio Curuá-Una of Brazil. During the wet season of 1995 these lizards were common at forest edge along narrow roads that transect forest, in treefalls and along streams where sun reaches the ground. Both species are heliothermic, basking to gain heat. Their association with open patches results from high activity temperature requirements in an environment where sun availability is low. Null temperature distributions from forest and treefalls showed that forest does not offer opportunities for heat gain similar to treefalls. Moreover, the large proportion of time spent basking by both species indicates the importance of these patches for thermoregulation. K. calcarata is slightly larger in body length and heavier at a given body length than M. nigropunctata. Both species are active foragers that seek out prey while moving through the habitat, feeding on orthopterans, roaches and spiders. M. nigropunctata also eat significant numbers of insects that occur on vegetation, such as hemipterans. Prey size is larger in K. calcarata and associated with lizard body size. Prey size does not vary with body size in M. nigropunctata and prey are typically relatively small.

Many of the ecological differences between these two lowland forest species appear to be historical: the ecology of K. calcarata is very similar to that of other species of Kentropyx and teiids in general and the ecology of M. nigropunctata is most similar to that of other studied species of south American Mabuya.

Comparison of the respiratory transition at birth or hatching in viviparous and oviparous amniote vertebrates

Regardless of the mode of reproduction, three things must occur at birth or hatching in amniote vertebrates: initiation of breathing, pulmonary fluid elimination and reabsorption, and adequate perfusion of pulmonary circulation. Although data on these events are few, there appears to be no fundamental difference in them that can be associated with the oviparity to viviparity transition. There are, however, differences in the timing of these events in oviparous and viviparous amniotes. The transition to neonatal respiration tends to be very quick in viviparous species because the vascular support for oxygen uptake provided by the mother is rapidly disassociated from the mechanism for uptake by the embryo. By contrast, hatching often is a slow process, taking 24 h or more in some species, as chorioallantoic blood flow slowly gives way to clearing of the lungs and pulmonary gas exchange. Little is known of the mechanisms of pulmonary fluid elimination and reabsorption or lung inflation in reptiles, but the cellular structures and surfactant systems are similar in all amniote vertebrates. Nevertheless, there are differences, particularly of timing and maturation of various systems, but there has been no exploration of the functional (or phylogenetic) bases of these differences. Perfusion of the neonatal pulmonary system to support respiration in reptiles remains to be investigated. In mammals and birds, closure of the ductus arteriosus is important, but the role played by the ductus arterioisus in reptilian hatching or birth is not known.

Interpopulation variation in life-history traits ofPoeciliopsis prolifica: implications for the study of placental evolution

Placental reproduction is widespread across vertebrate taxa, but little is known about its life-history correlates and putative adaptive value. We studied variation in life-history traits in two populations of the placental poeciliid fish Poeciliopsis prolifica to determine whether differences in post-fertilization maternal provisioning to embryos have a genetic basis and how food availability affects reproduction. Life histories were characterized for wild-caught females and for second-generation lab-born females raised under two levels of food availability. We found that the two populations did not differ significantly in the wild for any life-history traits except for the lipid dry weight in females and in embryos at an advanced stage of development. When environmental effects were experimentally controlled, however, populations exhibited significant differences in several traits, including the degree of maternal provisioning to embryos. Food availability significantly affected female size at first parturition, brood size and offspring dry weight at birth. Altogether, these results demonstrate that population differences in maternal provisioning and other life-history traits have a genetic basis and show a plastic response to food availability. We infer that phenotypic plasticity may mask population differences in the field. In addition, when comparing life-history patterns in these two populations with known patterns in placental and non-placental poeciliids, our results support the hypotheses that placentation is an adaptive reproductive strategy under high-resource conditions but that it may represent a cost under low-food conditions. Finally, our results highlight that age at maturity and reproductive allotment may be key life-history traits accompanying placental evolution.

Uptake of dextran-FITC by epithelial cells of the chorioallantoic placentome and the omphalopleure of the placentotrophic lizard,Pseudemoia entrecasteauxii

Placental nutrient provision has evolved in multiple lineages of squamate reptiles and although possible structural specializations for placentotrophy have been described in a variety of species, neither the pathways nor the mechanisms of placental transfer are known. Lizards of the Australian genus Pseudemoia are placentotrophic and have elaborate placental structures that are thought to enhance nutrient transfer. The chorioallantoic placenta, which occupies the embryonic hemisphere of the egg, is regionally diversified into a large area with low epithelial height and a smaller placentome with cuboidal or columnar epithelia. Both regions are underlain by an extensive vascular bed. The abembryonic hemisphere of the egg is covered by an omphaloplacenta, which is similar to the placentome in having cuboidal or columnar epithelia but with a different embryonic vascular supply. We tested the hypothesis that embryonic epithelial cells of the placentome and the omphaloplacenta of Pseudemoia entrecasteauxii are each capable of endocytosis. Embryos (stages 33-39) with intact extraembryonic membranes were surgically removed from the uterus and incubated in a solution containing fluorescein isothiocyanate-dextran (77,000 MW). The fluorescent label was detected in the cytoplasm of scattered populations of epithelial cells in both placental regions of all embryonic stages. We conclude that both the placentome and the omphaloplacenta of P. entrecasteauxii are sites of histotrophic nutrient transport. However, there are histological and cytological differences in the embryonic epithelia of these two placental regions. The histological differences reflect differences in the evolutionary precursors of each tissue. The cytological differences likely portray different functional specializations.

Interembryonic regions of the uterus of the viviparous lizardMabuya brachypoda (Squamata: Scincidae)

Analysis of the structure and physiology of the uterine incubation chambers of viviparous squamates has provided insight concerning adaptations for gestation. However, the literature addressing the biology of the interembryonic regions of the uterus is very limited, presumably because it has been assumed that this area has little role in the development and support of embryos in viviparous squamates. This study was undertaken to examine the histology of the interembryonic regions of Mabuya brachypoda, a viviparous lizard with microlecithal ova and consequently substantial matrotrophic activity. The incubation chambers are oval, distended zones of the uterus, adjacent to the interembryonic regions. The wall of the interembryonic regions includes: mucosa, formed by a cuboidal or columnar epithelium with ciliated and nonciliated cells, and a lamina propria of vascularized connective tissue containing abundant acinar glands; myometrial smooth muscle consisting of inner circular and outer longitudinal layers; and serosa. The segment of the interembryonic region adjacent to the incubation chamber forms a transitional segment that displays folds of the mucosa that protrude into the uterine lumen. The limit of the incubation chamber is well defined by the long mucosal folds of the transitional segment. Long and thin extensions of extraembryonic membranes are present in the lumen of the transitional segment, outside of the incubation chamber region. The presence of abundant uterine glands and extraembryonic membranes in the interembryonic regions during gestation suggests uterine secretory activity and histotrophic transfer of nutrients to embryos in these regions.

Allantoplacental ultrastructure of an Andean population ofMabuya (Squamata, Scincidae)

Mabuya species are highly matrotrophic viviparous lizards with Type IV epitheliochorial allantoplacenta. The allantoplacenta of an Andean population of this genus, currently assigned to Mabuya sp., possesses specializations related to histotrophic nutrition at the embryonic hemisphere (placentome, paraplacentome, and chorionic areolas), while at the abembryonic hemisphere it has a mixed function: histotrophic transfer (absorptive plaques) and hemotrophic nutrition (gas exchange in respiratory segments). These placental specializations were studied using high-resolution light microscopy and transmission electron microscopy, and were compared with those found in other squamate reptiles and eutherian mammals. Cytological features of the placentome suggest that this is an important region for nutritional provision; the paraplacentome also shows characteristics for nutrient transfer, especially lipids. Chorionic areolas allow the absorption of glandular products, as well as uterine and chorionic cellular debris produced by lysis of some cells of both epithelia during areola formation. In the absorptive plaques both uterine and chorionic epithelia are firmly attached and their cellular apices exhibit electron-dense granules that could be related to autocrine and paracrine functions. The short interhemal distance found in the respiratory segments confirms their role in gas exchange. A common feature of all regional specializations in the Mabuya sp. allantoplacenta is the presence of lipids in the interacting chorionic and uterine epithelia, suggesting that lipids are transferred throughout the entire embryonic chamber; placental transfer of lipids may be the principal fetal energy and lipid source in this species. In spite of this feature, each one of the specialized areas of the allantoplacenta has different features suggesting particular functions in the transfer of nutrients (as ions, lipids, proteins, amino acids, sugar, water, and gases), and in the possible synthesis of hormones and proteins. The placental complexity observed in this species of Mabuya is greater than in any other reptile, and resembles that of eutherian mammals: Each one of these specializations of the placental membranes in Mabuya sp. is similar to those found among different eutherian mammals, indicating a very impressive evolutionary convergence at the histological and cytological levels between both clades. However, no eutherian mammal species simultaneously displays all of these specializations in the embryonic chamber as does Mabuya sp.

A review of the evolution of viviparity in lizards: structure, function and physiology of the placenta

The aim of this review is to collate data relevant to understanding the evolution of viviparity in general, and complex placentae in particular. The wide range of reproductive modes exhibited by lizards provides a solid model system for investigating the evolution of viviparity. Within the lizards are oviparous species, viviparous species that have a very simple placenta and little nutrient uptake from the mother during pregnancy (lecithotrophic viviparity), through a range of species that have intermediate placental complexities and placental nutrient provision, to species that lay microlecithal eggs and most nutrients are provided across the placenta during development (obligate placentotrophy). In its commonest form, lecithotrophic viviparity, some uptake of water, inorganic ions and oxygen occurs from the mother to the embryo during pregnancy. In contrast, the evolution of complex placentae is rare, but has evolved at least five times. Where there is still predominantly a reliance on egg yolk, the omphaloplacenta seems to be paramount in the provision of nutrition to the embryo via histotrophy, whereas the chorioallantoic placenta is more likely involved in gas exchange. Reliance on provision of substantial organic nutrient is correlated with the regional specialisation of the chorioallantoic placenta to form a placentome for nutrient uptake, particularly lipids, and the further development of the gas exchange capabilities of the other parts of the chorioallantois.

Cyto-epitheliochorial placenta of the viviparous lizardPseudemoia entrecasteauxii: A new placental morphotype

The structural features of the uterine epithelium of the chorioallantoic placenta and omphalloplacenta in the viviparous Australian skink, Pseudemoia entrecasteauxii, were investigated using SEM and TEM techniques. In particular, the structural characteristics that would allow interpretation of function were analyzed, particularly those of gas exchange in the chorioallantoic placenta and histotrophy in the omphaloplacenta. Pseudemoia entrecasteauxii has a complex placenta consisting of a placentome, paraplacentome, and omphaloplacenta. The paraplacentome has a well-vascularized lamina propria in which projecting uterine capillaries displace the overlying uterine epithelial cells, reducing them to attenuated cytoplasmic extensions. Associated cell nuclei and organelles are lost from this region, to provide a capillary lumen to uterine lumen barrier of 0.5-1.0 microm. Hence, the paraplacentome is likely a prominent site for gaseous exchange via simple diffusion. The omphaloplacenta has a similar cytology to that of the placentome, but the uterine epithelial cells are hypertrophied and the apical plasma membrane actively secretes vesicles into the uterine lumen. The omphaloplacenta shows features that are associated with histotrophic transport of nutrients via vesicle secretion, very similar to that of lipid apocrine secretion. The placentome consists of cuboidal cells in the uterine epithelium, with large centrally located nuclei overlying the well-vascularized lamina propria. Although the placentome has a similar cytological structure to that of the omphaloplacenta, granules or active vesicle secretion were not observed. Thus, the placentome may be associated with histotrophy, but not via apocrine secretion. Squamate placentation is epitheliochorial; however, we propose a new term be used to describe the type of placentation in P. entrecasteauxii: "cyto-epitheliochorial," because of the extreme attenuation of uterine epithelial cells of the paraplacentome.