Evolution of placental specializations in viviparous African and South American lizards

Phylogenetic information offers an important resource in analyses of reproductive diversity, including interpretations of fetal membrane evolution. In this paper, we draw upon ongoing studies of South American and African lizards to consider the value of combining phylogenetic and reproductive evidence in the construction of evolutionary interpretations. South American lizards of the genus Mabuya exhibit several reproductive specializations that are convergent on those of eutherian mammals, including viviparity, long gestation periods, ovulation of tiny eggs, and placental supply of the nutrients for development. Studies of placental morphology and development indicate that New World Mabuya share several other derived features, including chorionic areolae and a "Type IV" epitheliochorial placenta with a villous, mesometrial placentome. Some characteristics of these lizards are shared by two African skinks, M. ivensii and Eumecia anchietae, including minuscule eggs, placentotrophy, an absorptive chorioallantois, and features of the yolk sac. Available evidence is consistent with two explanations: (1) placentotrophy originated in Africa, predating a trans-Atlantic colonization by Mabuya of the New World; and (2) placentotrophy arose two or three separate times among these closely related skinks. As illustrated by analysis of these animals, not only can data on fetal membrane morphology yield phylogenetic information, but phylogenetic evidence in turn provides a valuable way to reconstruct the evolution of fetal membranes in a biogeographic context. When appropriately interpreted, morphological and phylogenetic evidence can be combined to yield robust evolutionary conclusions that avoid the pitfalls of circular reasoning.     

Utilisation of nutrients by embryos of the enigmatic Australian viviparous skink Niveoscincus coventryi

The Eugongylus species group of Australian lygosomine skinks provides an unparalleled opportunity to study the evolution of placentotrophy. Viviparity and placentotrophy have evolved in two lineages, currently recognised as the genera Pseudemoia and Niveoscincus. The genus Niveoscincus is important because it is the only lineage of squamates in which variation in placental morphology and in the pattern of embryonic nutrition is known. Niveoscincus coventryi has the least complex placental morphology among species currently assigned to the genus. We quantified the net uptake of nutrients across the placenta of N. coventryi for comparison with other species in the genus and with other viviparous and oviparous lizards. The pattern of embryonic nutrition of N. coventryi is similar to other viviparous lizards with simple placentae in that there is no net uptake of dry matter during development but there is a net uptake of water, calcium, potassium, and sodium. There is no net uptake of lipid, nitrogen (an index of protein), or magnesium. We conclude that N. coventryi is predominantly lecithotrophic. Further, if N. coventryi is the sister taxon to Tasmanian Niveoscincus, then the distribution of patterns of embryonic nutrition among members of this clade suggests that the evolution of placentotrophy occurred during radiation of this lineage in Tasmania.     

Allantoplacental ultrastructure of an Andean population of Mabuya (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.     

Morphology, development, and evolution of fetal membranes and placentation in squamate reptiles

Current studies on fetal membranes of reptiles are providing insight into three major historical transformations: evolution of the amniote egg, evolution of viviparity, and evolution of placentotrophy. Squamates (lizards and snakes) are ideal for such studies because their fetal membranes sustain embryos in oviparous species and contribute to placentas in viviparous species. Ultrastructure of the fetal membranes in oviparous corn snakes (Pituophis guttatus) shows that the chorioallantois is specialized for gas exchange and the omphalopleure, for water absorption. Transmission and scanning electron microscopic studies of viviparous thamnophine snakes (Thamnophis, Storeria) have revealed morphological specializations for gas exchange and absorption in the intra-uterine environment that represent modifications of features found in oviparous species. Thus, fetal membranes in oviparous species show morphological differentiation for distinct functions that have been recruited and enhanced under viviparous conditions. The ultimate in specialization of fetal membranes is found in viviparous skinks of South America (Mabuya) and Africa (Trachylepis, Eumecia), in which placentotrophy accounts for nearly all of the nutrients for development. Ongoing research on these lizards has revealed morphological specializations of the chorioallantoic placenta through which nutrient transfer is accomplished. In addition, African Trachylepis show an invasive form of implantation, in which uterine epithelium is replaced by invading chorionic cells. Ongoing analysis of these lizards shows how integration of multiple lines of evidence can provide insight into the evolution of developmental and reproductive specializations once thought to be confined to eutherian mammals.     

Nutrient uptake by embryos of the Australian viviparous lizard Eulamprus tympanum.

Eulamprus tympanum is a high-altitude viviparous lizard that was probably used to help define a Type I chorioallantoic placenta. In this article, we (1) describe the net transport of nutrients across the placenta of E. tympanum, and (2) compare placental uptake in E. tympanum with a previous study of Eulamprus quoyii, which occurs in warmer environments, to assess the potential importance of thermal regime on placentotrophy. Freshly ovulated eggs are 387.3+/-19.7 mg. There is a significant net uptake of water and a net loss of dry matter during development, so the dry neonate is only 84% the size of the dry egg. There is no significant change in the total ash or nitrogen in eggs during embryonic development, with the entire loss of dry matter being lipid. Almost the entire loss of lipid occurs in the triacylglycerol fraction, with no net change in phospholipids. A net increase in total cholesterol suggests that cholesterol is synthesised by the developing embryo. The lipid profile of eggs of E. tympanum reflects that of other species with simple placentae in having a relatively high proportion of triacylglycerol and little cholesterol. The fatty acid composition of eggs reflects that expected in the diet of E. tympanum. There is a preservation and some synthesis of arachidonic (20:4n-6) and docosahexaenoic (22:6n-3) acids in the phospholipid fraction during embryonic development. Despite there being no net uptake of ash, there is a net increase in calcium, potassium, sodium, and magnesium in the neonate compared with the egg. We conclude that E. tympanum, like E. quoyii, is predominantly lecithotrophic with little, if any, uptake of organic molecules but with significant uptake of some inorganic ions and water. In addition, there is no difference in placentotrophy correlated with differences in the environments inhabited by each species.     

Evolution of viviparity: what can Australian lizards tell us?

Historically, Australia has been important in the study of, and the development of hypotheses aimed at understanding, the evolution of viviparity in amniote vertebrates. Part of the importance of Australia in the field results from a rich fauna of skinks, including one of the broadest ranges of diversity of placental structures within one geographic region. During the last decade, we have focussed our studies on one lineage, the Eugongylus group of skinks of the subfamily Lygosominae because it contains oviparous species and some that exhibit complex placentae. Our specific objective has been to attempt to understand the fundamental steps required when viviparity, and ultimately complex placentae, evolve from oviparous ancestors. We have taken a three-prong approach: (1) detailed study of the morphology and ontogeny of the placentae of key species at the light microscope level; (2) study of changes in the uterus associated with pregnancy, or the plasma membrane transformation; and (3) measures of the net exchange of nutrients across the placenta or eggshell of key species. In turn, we have found that: (1) details of the morphology and ontogeny of placentae are more complex that originally envisaged, and that the early conclusions about a sequence in the evolution of complex placentae was naïve; (2) a plasma membrane transformation occurs in viviparous, but not oviparous lizards, and thus may be a fundamental feature of the evolution of viviparity in amniotes; and (3) species with more complex chorioallantoic placentae tend to transport more nutrients across the placenta during pregnancy than those with simpler chorioallantoic placentae but, because the correlation is not tight, the importance of the omphaloplacenta in transporting nutrients may have been overlooked. Also, the composition of yolk of highly matrotrophic species is broadly similar, but not identical, to the yolk of oviparous species. Some of the interpretation of our data within the context of our specific objective is not yet possible, pending the publication of a robust phylogeny of Eugongylus group skinks. Once such a phylogeny is available, we are in a position to propose specific hypotheses about the evolution of viviparity that can be tested using another lineage of amniotes, possibly Mabuya group skinks.     

Evidence for placental transfer of lipids during gestation in the viviparous lizard, Pseudemoia entrecasteauxii

During gestation in the viviparous lizard Pseudemoia entrecasteauxii, the fetus obtains nutrients from two sources: uptake of yolk components from the retained egg (lecithotrophy) and transfer of nutrients from the maternal circulation via the placenta (placentotrophy). Although net placentotrophy in this species is indicated by the observation that the neonate contains 1.7 times more dry matter than the egg, the placental transfer of lipid has not been previously demonstrated. Lipid analysis was performed on newly ovulated eggs and on neonates. The weight of total lipid per neonate (8.2+/-0.5 mg) is significantly (P=0.049) greater than that in the egg (6.8+/-0.4 mg), indicating that the placenta must contribute some lipid to the fetus. On the assumption that 50% of the lipid delivered to the fetus from either source is oxidized for energy, it is calculated that the placenta accounts for 58.5% of the fetal lipid requirements, with the remaining 41.5% being derived from the egg. The fatty acid compositions of the triacylglycerol and phospholipid recovered in the neonatal tissue differ substantially from those of the egg. In particular, the proportions of 18:2n-6 and 18:3n-3 are far lower in the neonatal lipids compared with the egg lipids. On the other hand, the proportion of 22:6n-3 in the phospholipid of the neonate is six times higher than in the phospholipid of the egg. The absolute amount (mg) of 22:6n-3 recovered in the total lipid of the neonate is 3.8 times greater than the amount initially present in the egg. By comparison, the amount of total fatty acid in neonatal lipid is 1.2 times greater than the amount in the egg. Thus, there is a preferential use of 22:6n-3 for tissue phospholipid synthesis during development. We conclude that there is net transfer of fatty acids across the placenta to the fetus of P. entrecasteauxii and a high degree of selectivity in the use of the various fatty acids for fetal tissue lipid synthesis.     

Facultative placentotrophy: half-way house or strategic solution?

While yolk is generally the primary source of embryo nutrients in squamates, numerous species supplement this with facultative placentotrophy. We argue that facultative placentotrophy should have selective importance relevant to offspring fitness. In the skink Niveoscincus metallicus, the size of ovulated eggs is unrelated to maternal size but large females produce offspring that are larger than is necessary for survival, providing evidence for facultative placentotrophy. We discuss the circumstances in which facultative placentotrophy might be used to supplement the nutritional support provided by yolk and obligate placentotrophy in this species, and present summary data from experiments designed to investigate these circumstances. Clutch reduction by oviduct removal had no effect on neonate mass or snout-vent length, indicating that the number of embryos does not influence allocation of maternal resources once gestation has commenced. Manipulation of maternal basking opportunity in combination with food intake during pregnancy suggested that an important role of facultative placentotrophy is the optimization of embryonic fat reserves. This hypothesis was supported by the observation that larger neonates have larger abdominal fat bodies. These reserves presumably facilitate survival in the relatively short pre-hibernatory period available to newborn animals. Our data indicate that they also play a vital role in maintaining pre-natal condition if birth is delayed by adverse weather, a common circumstance in this species. In such circumstances the yolk has been used up and the placental membranes have degenerated. Experimental induction of premature ovulation of eggs with reduced yolk, achieved by injecting females with FSH, was followed by fertilization using stored sperm. Gestation length was greatly reduced and the resulting neonates were all < or =75% normal birth mass, with two of the six births being stillborn. Thus facultative placentotrophy does not appear to be a means of compensating for a poor yolk supply. We suggest that facultative placentotrophy in N. metallicus is not a transitional stage en route to greater reliance on obligate placentotrophy, but a uniquely squamate adaptation that provides flexibility in embryonic nutrition, and optimizes offspring fitness in an unpredictable temperate climate.     

Morphogenesis of extraembryonic membranes and placentation in Mabuya mabouya (Squamata, Scincidae)

Topological and histological analyses of Mabuya mabouya embryos at different developmental stages showed an extraembryonic membrane sequence as follows: a bilaminar omphalopleure and progressive mesodermal expansion around the whole yolk sac at gastrula stages; mesodermal split and formation of an exocoelom in the entire embryonic chamber at neurula stages; beginning of the expansion of the allantois into the exocoelom to form a chorioallantoic membrane at pharyngula stages; complete extension of the allantois into the exocoelom between limb-bud to preparturition stages. Thus, a placental sequence could be enumerated: bilaminar yolk sac placenta; chorioplacenta; allantoplacenta. All placentas are highly specialized for nutrient absorption from early developmental stages. The bistratified extraembryonic ectoderm possesses an external layer with cuboidal cells and a microvillar surface around the whole yolk sac, which absorbs uterine secretions during development of the bilaminar yolk sac placenta and chorioplacenta. During gastrulation, with mesodermal expansion a dorsal absorptive plaque forms above the embryo and several smaller absorptive plaques develop antimesometrially. Both structures are similar histologically and are active in histotrophic transfer from gastrula stages until the end of development. The dorsal absorptive plaque will constitute the placentome and paraplacentome during allantoplacental development. At late gastrula-early neurula stages some absorptive plaques form chorionic concavities or chorionic bags that are penetrated by a long uterine fold and seem to have a specialized histotrophic and/or metabolic role. The extraembryonic mesoderm does not ingress into the yolk sac and neither an isolated yolk mass nor a yolk cleft are formed. This derived pattern of development may be related to the drastic reduction of the egg size and obligatory placentotrophy from early developmental stages. Our results show new specialized placentotrophic structures and a novel arrangement of extraembryonic membrane morphogenesis for Squamata.     

Changes to the uterine epithelium during the reproductive cycle of two viviparous lizard species (Niveoscincus spp.)

We investigated morphological differences in uterine epithelia of the reproductive cycle between two closely related viviparous skinks, Niveoscincus metallicus (lecithotrophic) and Niveoscincus ocellatus (placentotrophic), which have similar placental complexity but different degrees of placentotrophy. Scanning (SEM) and transmission electron microscopy (TEM) revealed that the uterine surface of non‐reproductive females of both species is mainly covered by ciliated cells. As vitellogenesis begins, the uterine epithelium consists of ciliated and non‐ciliated cells under a thin glycocalyx. Microvilli are greatly reduced at mid‐pregnancy, and the uterus differentiates into two structurally distinct regions: the chorioallantoic and the omphaloplacenta. At late stages of pregnancy, the uterine epithelium of chorioallantoic placenta in both species is further ridged, forming a knobbly uterine surface. The ultrastructural evidence between N. metallicus and N. ocellatus cannot strictly account for the distinct differences in their placentotrophy; as yet unexplored molecular nutrient transport mechanisms that are not reflected in uterine ultrastructure must play significant roles in nutrient transportation. Characteristics consistent with a plasma membrane transformation were confirmed in both species.     

Energy and nutrient utilisation by embryonic reptiles

Most reptiles are oviparous, with the developing embryos relying on the contents of the yolk to sustain development until hatching (lecithotrophy). The yolk is composed primarily of lipid and protein, which act as an energy source and the essential components to build embryonic tissue. Nevertheless, yolk and the resulting embryos contain many other nutrients, including inorganic ions, vitamins, carotenoids, water and hormones. Apart from water and oxygen, which may be taken up by eggs, and some inorganic ions that can come from the eggshell or even from outside the egg, everything required by the embryo must be in the egg when it is laid. Approximately 20% of squamate reptiles are viviparous, exhibiting a variety of placental complexities. Species with complex placentae have reduced yolk volumes, with the mother augmenting embryonic nutrition by provision across the placenta (placentotrophy). Despite assumed advantages of placentotrophy, only 5 out of approximately 100 lineages of viviparous squamates exhibit substantial placentotrophy. This paper reviews available and recent information on the yolk contents of a variety of squamate reptiles to ask the question, how are nutrients transported from the yolk to the embryo or across the placenta? Although, current available data suggest that, in broad terms, yolk is taken up by embryos without discrimination of the nutrients, there are some apparent exceptions, including the very long chain polyunsaturated fatty acids. In addition, fundamental differences in the patterns of energy utilisation in lizards and snakes suggest fundamental differences in lipid profiles in these taxa, which appear to reflect the differences between placentotrophic and lecithotrophic viviparous lizards.     

Dynamic changes to claudins in the uterine epithelial cells of the marsupial Sminthopsis crassicaudata (Dasyuridae) during pregnancy

The fluid that surrounds the embryo in the uterus contains important nourishing factors and secretions. To maintain the distinct microenvironment in the uterine lumen, the tight junctions between uterine epithelial cells are remodeled to decrease paracellular movement of molecules and solutes. Modifications to tight junctions between uterine epithelial cells is a common feature of pregnancy in eutherian mammals, regardless of placental type. Here we used immunofluorescence microscopy and western blot analysis to describe distributional changes to tight junctional proteins, claudin‐1, ‐3, ‐4, and ‐5, in the uterine epithelial cells of a marsupial species, Sminthopsis crassicaudata. Immunofluorescence microscopy revealed claudin‐1, ‐3, and ‐5 in the tight junctions of the uterine epithelium of S. crassicaudata during pregnancy. These specific claudins are associated with restricting passive movement of fluid between epithelial cells in eutherians. Hence, their function during pregnancy in S. crassicaudata may be to maintain the uterine luminal content surrounding developing embryos. Claudin‐4 disappears from all uterine regions of S. crassicaudata at the time of implantation, in contrast with the distribution of this claudin in some eutherian mammals. We conclude that like eutherian mammals, distributional changes to claudins in the uterine epithelial cells of S. crassicaudata are necessary to support pregnancy. However, the combination of individual claudin isoforms in the tight junctions of the uterine epithelium of S. crassicaudata differs from that of eutherian mammals. Our findings suggest that the precise permeability of the paracellular pathway of the uterine epithelium is species‐specific.     

Maximization of evolutionary trends for placental viviparity in the spadenose shark,Scoliodon taticaudus

Synopsis Placental viviparity has evolved inScoliodon taticaudus to a degree that rivals some eutherian mammals. Its eggs are the smallest known of any shark. They have a diameter of 1 mm, a dry weight of 0.0654 ± 0.0100 mg and are nearly yolk-free. Implantation takes place at an early (3 mm) stage of development, and gestation is short (5–6 months). Comparison of the dry weight of the egg (0.065 mg) with the estimated dry weights of a mid-late term 90 mm embryo (910 mg) and a 152 mm neonate (3815.4 mg) reveals weight changes of 14219 × and 58338 ×, respectively. Its normalized brood weight, a measure of maternal nutrient investment, is 49.5 g · kg^–1 female body weight for a six-month gestation. Comparisons with other species of placental and nonplacental sharks show thatS. laticaudus has a highly advanced form of matrotrophy. Maternal nutrients appear to be acquired by placental transport and by imbibition of uterine fluid. Hemotrophic placental nutrient transfer occurs across a unique uterine implantation site, termed the trophonematous cup, in which maternal blood appears to bathe the outer epithelium of the embryonic yolksac placenta. The latter is solid and filled with a three-dimensional network of capillaries and many free interstitial cells. The umbilical stalk contains the vitelline vessels but lacks a yolk duct. Its surface is amplified by many long, villous appendiculae, which consist of a vascular core that ramifies into a massive surface capillary network invested by a simple squamous epithelium. The appendiculae ofS. laticaudus most likely are sites of gas exchange and possibly the uptake of small molecules. They are unlike the appendiculae described in any other placental shark and exhibit design principles similar to those of the uterine trophonemata of matrotrophic rays.     

Developmental genes during placentation: insights from mouse mutants

Placenta,a temporary organ first formed during the development of a new life is essential for the survival and growth of the fetus in eutherian mammals.It serves as an interface for the exchange of nutrients,gases and wastes between the maternal and fetal compartments.During the past decades,studies employing gene-engineered mouse mutants have revealed a wide range of signaling molecules governing the trophoblast development and function during placentation under various pathophysiological conditions.Here,we summarize the recent progress with particular respect to the involvement of developmental genes during placentation.     

Morphogenesis of placental membranes in the viviparous,placentotrophic lizard Chalcides chalcides (Squamata: Scincidae)

In the scincid lizard Chalcides chalcides, females ovulate small ova and supply most of the nutrients for development by placental means. The yolk is enveloped precocially by extraembryonic ectoderm and endoderm during the gastrula stage, establishing a simple bilaminar yolk sac placenta. The shell membrane begins to degenerate at this time, resulting in apposition of extraembryonic and maternal tissues. A true chorioplacenta has developed by the early pharyngula stage, as has a choriovitelline placenta and the first stages of an omphaloplacenta. Although the choriovitelline membrane disappears rapidly, the omphaloplacenta spreads to occupy the entire abembryonic pole. The yolk cleft is not confluent with the exocoelom, and no omphalallantoic placenta develops. By the limb-bud stage, an allantoplacenta has been established, with a mesometrial placentome composed of interdigitating ridges of chorioallantois and uterine mucosa. The discovery of five distinct placental arrangements in this species, three of which are transitory and two of which have not previously been recorded in reptiles, emphasizes the need for accounts that specify ontogenetic stages and the precise identity and composition of squamate placental membranes. Contrary to previous interpretations, the pattern of extraembryonic membrane development in C. chalcides is evolutionarily conservative, despite the presence of a reduced yolk mass and cytological specializations for nutrient transfer. Our observations indicate that substantial placentotrophy can evolve in squamates without major modifications of morphogenetic patterns. J Morphol 232:35–55, 1997. © 1997 Wiley-Liss, Inc.     

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.     

Placental nutrition in the Tasmanian skink, Niveoscincus ocellatus

Niveoscincus ocellatus is an important species in historical analyses of the evolution of viviparity because it is the species upon which the type II chorioallantoic placenta was based. Here we describe the net nutrient uptake across the placenta of N. ocellatus for comparison with other species of skinks with complex placentae. N. ocellatus is highly placentotrophic, with neonates being 1.68-times larger in dry matter than the fresh eggs. There is an increase of nitrogen from 6.3 +/- 0.2 mg to 9.2 +/- 0.6 mg, and ash from 3.8 +/- 0.3 mg to 6.7 +/- 0.6 mg. The increase in ash is made up by a more than two-fold increase in the amounts of calcium, potassium and sodium. There is no significant difference in lipids in the neonates compared to fresh eggs, so considerable lipid must have crossed the placenta to provide energy for embryonic development. N. ocellatus is significantly more placentotrophic than Niveoscincus metallicus, which also has a complex chorioallantoic placenta. Discovery of substantial placentotrophy in this genus confirms that two lineages of Australian lygosomine skinks (represented by the genera Pseudemoia and Niveoscincus) have evolved this pattern of embryonic nutrition and supports the hypothesis that the evolution of reptilian placentotrophy involves specialisations in addition to structural modifications of the chorioallantoic placenta.     

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.