Lipids of the eggs and neonates of oviparous and viviparous lizards

Maternal transmission of bacterial microbiota during embryonic development in a viviparous lizard

IMPORTANCE

We investigated the presence and diversity of bacteria in the embryos of the viviparous lizard Sceloporus grammicus and their amniotic environment. We compared this diversity to that found in the maternal intestine, mouth, and cloaca. We detected bacterial DNA in the embryos, albeit with a lower bacterial species diversity than found in maternal tissues. Most of the bacterial species detected in the embryos were also found in the mother, although not all of them. Interestingly, we detected a high similarity in the composition of bacterial species among embryos from different mothers. These findings suggest that there may be a mechanism controlling the transmission of bacteria from the mother to the embryo. Our results highlight the possibility that the interaction between maternal bacteria and the embryo may affect the development of the lizards.

Elongases of Long-Chain Fatty Acids ELO2 and ELO9 Are Involved in Cuticle Formation and Function in Fecundity in the Yellow Fever Mosquito, Aedes aegypti

Long-chain fatty acid elongases (ELOs) play important roles in the metabolism of fatty acids in insects. In this study, the genes for two elongases from Aedes aegypti were identified, AeELO2 and AeELO9. Quantitative real time PCR showed that AeELO2 and AeELO9 are expressed at all developmental stages and some body parts, but with different expression patterns. RNAi-mediated knockdown of AeELO2 and AeELO9 was performed to investigate their roles in the development, growth, osmotic balance, and cold tolerance of Ae. aegypti. Knockdown of AeELO2 slowed larval growth and development by causing molting abnormalities. Additionally, 33% ± 3.3% of adults died during oviposition, accompanied by an abnormal extension of cuticles in AeELO2-dsRNA knockdown mosquitos. Knockdown of AeEL09 resulted in abnormal balance of cuticular osmotic pressure and a reduction in egg production. The maximal mRNAs of AeELO2 and AeELO9 were detected in eggs at 72 h after oviposition. Moreover, AeELO2 knockdown reduced the egg hatching rates and AeELO9 knockdown larvae did not develop well. In summary, AeELO2 is involved in larval molting and growth, and its knockdown affects the flexibility and elasticity of adult mosquito cuticles. AeELO9 regulates cold tolerance, osmotic balance, and egg development in Ae. aegypti.

dLp/HDL-BGBP and MTP Cloning and Expression Profiles During Embryonic Development in the Mud Crab Scylla paramamosain

Lipids are the main energy source for embryonic development in oviparous animals. Prior to the utilization and catabolism, lipids are primarily transported from the yolk sac to embryonic tissues. In the present study, cDNA encoding a circulatory large lipid transfer protein (LLTP) superfamily member, the precursor of large discoidal lipoprotein (dLp) and high-density lipoprotein/β-1,3-glucan-binding protein (HDL-BGBP), named dLp/HDL-BGBP of 14,787 bp in length, was cloned from the mud crab Scylla paramamosain. dLp/HDL-BGBP was predicted to encode a 4,831 amino acids (aa) protein that was the precursor of dLp and HDL-BGBP, which were both detected in hemolymph by liquid chromatography-mass spectrometry (LC-MS/MS) analysis. For the intracellular LLTP, three microsomal triglyceride transfer protein (MTP) cDNAs of 2,905, 2,897, and 3,088 bp in length were cloned from the mud crab and were predicted to encode MTP-A of 881 aa, MTP-B of 889 aa, and MTP-C of 919 aa, respectively, which were different merely in the N-terminal region and shared an identical sequence of 866 aa. During embryonic development, the expression level of dLp/HDL-BGBP consecutively increased from the early appendage formation stage to the eye pigment-formation stage, which indicated that HDL-BGBP is probably the scaffolding protein for yolk lipid. For the MTP gene, MTP-C accounted for ~70% of MTP mRNA from the blastocyst stage to the nauplius stage, as well as the pre-hatching stage; MTP-C and MTP-A expression levels were comparable from the early appendage formation stage to the late eye pigment-formation stage; MTP-A was extremely low in blastocyst and gastrula stages; MTP-B was expressed at a relatively low-level throughout embryo development. The variations in the expression profiles among MTP transcripts suggested that MTP might play roles in the lipid droplet maturation and lipoprotein assembly during embryonic development.

Paternal nutrient provisioning during male pregnancy in the seahorse Hippocampus abdominalis

Vertebrates that incubate embryos on or within the body cavity exhibit diverse strategies to provide nutrients to developing embryos, ranging from lecithotrophy (solely yolk-provided nutrition) to substantial matrotrophy (supplemental nutrients from the mother before birth). Syngnathid fishes (seahorses, pipefishes and sea dragons) are the only vertebrates to exhibit male pregnancy. Therefore, they provide a unique opportunity for comparative evolutionary research, in examining pregnancy independent of the female reproductive tract. Here, we tested the hypothesis that the most complex form of syngnathid pregnancy involves nutrient transport from father to offspring. We compared the dry masses of newly fertilised Hippocampus abdominalis eggs with those of fully developed neonates to derive a patrotrophy index. The patrotrophy index of H. abdominalis was 1, indicating paternal nutrient supplementation to embryos during gestation. We also measured the lipid content of newly fertilised eggs and neonates and found that there was no significant decrease in lipid mass during embryonic development. Since lipids are likely to be the main source of energy during embryonic development, our results suggest that lipid yolk reserves being depleted by embryonic metabolism are replaced by the brooding father. The results of our study support the hypothesis that nutrient transport occurs in the most advanced form of male pregnancy in vertebrates.

Mechanisms of reproductive allocation as drivers of developmental plasticity in reptiles

Developmental plasticity in offspring phenotype occurs as a result of the environmental conditions embryos experience during development. The nutritional environment provided to a fetus is an important source of developmental plasticity. Reptiles are a particularly interesting system to study this plasticity because of their varied routes of maternal nutrient allocation to reproduction. Most reptiles provide their offspring with all or most of the nutrients they require in egg yolk (lecithotrophy) while viviparous reptiles also provide their offspring with nutrients via a placenta (placentotrophy). We review the ways in which both lecithotrophy and placentotrophy can lead to differences in the nutrients embryonic reptiles receive, and discuss how these differences lead to developmental plasticity in offspring phenotype. We finish by reviewing the ecological and conservation consequences of nutritional-driven developmental plasticity in reptiles. If nutritional-driven developmental plasticity has fitness consequences, then understanding the basis of this plasticity has exciting potential to identify how reptile recruitment is affected by environmental changes in food supply. Such knowledge is critical to our ability to protect taxa threatened by environmental change.

Maternal transfer and embryonic assimilation of trace elements in freshwater turtles after remediation of a coal fly-ash spill

Oviparous vertebrates maternally transfer elements to their offspring during egg production. Maternal transfer occurs because elements mimic, or are incorporated into, nutrients allocated to eggs, but likely differs among species depending on the quantities of specific nutrients allocated to eggs. Developing embryos are often assumed to assimilate all of the elements allocated to eggs, but this assumption has rarely been tested. We tested the hypothesis that maternal transfer and embryonic assimilation of trace elements differed between two species of freshwater turtles exposed to a recently-remediated coal fly-ash spill. Sternotherus odoratus transferred As, Se, and Zn, while Trachemys scripta transferred As, Hg, Se, Sr, and Zn. Logarithmic non-linear relationships between hatchling and egg concentrations indicated that turtles partially assimilated elements present in eggs. In systems contaminated with multiple trace elements, our data show that maternal transfer and embryonic assimilation are element- and species-specific, and may be inconsistent even among closely-related species.

Metabolic fates of yolk lipid and individual fatty acids during embryonic development of the coot and moorhen

There is currently little information regarding the metabolic fates of yolk lipid and individual fatty acids during embryonic development of free-living avian species. Here we report the pattern of lipid utilization during embryonic development of the coot (Fulica atra) and the moorhen (Gallinula chloropus), two related species producing precocial offspring from eggs with a distinctive fatty acid composition and with an incubation period similar to that of the chicken. By the time of hatching, the proportions of the initial yolk lipid that had been transferred to the embryo were 88.2% and 79.8% for the coot and moorhen respectively. During the whole incubation period, 42.9% and 40.0% of the initial yolk lipid of the coot and moorhen respectively were lost from the system due to oxidation for energy, equating to 47.8% and 50.0% respectively of the actual amount of lipid transferred over this time. Thus, the lipid received by the embryos of both species is partitioned almost equally between the alternative fates of energy metabolism and incorporation into tissue lipids. In the coot, this 50:50 split between oxidation and tissue formation was maintained during the hatching process. The proportions of arachidonic (20:4n-6) and docosahexaenoic (22:6n-3) in the yolk lipids of these species were 2.5-3.5 times higher than in eggs of domestic poultry. In contrast to the situation in the chicken, there was no preferential uptake of 22:6n-3 from the yolk during coot and moorhen development. The fatty acid compositions of the whole body lipids of the coot and moorhen hatchlings were almost identical to those of the initial yolks indicating that, unlike the chicken, these species display relatively little overall biomagnification of 20:4n-6 and 22:6n-6 during development. It is suggested that the yolk fatty acid profiles of the coot and moorhen are particularly well matched to the requirements of the embryo, reducing the need for selective uptake of 22:6n-3 and for the overall biomagnification of 22:6n-3 and 20:4n-6.

Reproductive tradeoffs and yolk steroids in female leopard geckos, Eublepharis macularius

Life history theory predicts tradeoffs among reproductive traits, but the physiological mechanisms underlying such tradeoffs remain unclear. Here we examine reproductive tradeoffs and their association with yolk steroids in an oviparous lizard. Female leopard geckos lay two eggs in a clutch, produce multiple clutches in a breeding season, and reproduce for several years. We detected a significant tradeoff between egg size and the number of clutches laid by females during their first two breeding seasons. Total reproductive effort was strongly condition-dependent in the first season, but much less so in the second season. Although these and other tradeoffs were unmistakable, they were not associated with levels of androstenedione, oestradiol, or testosterone in egg yolk. Female condition and egg size, however, were inversely related to dihydrotestosterone (DHT) levels in egg yolk. Finally, steroid levels in egg yolk were not directly related to steroid levels in the maternal circulation when follicles were developing, indicating that steroid transfer to eggs is regulated. These findings suggest that maternal allocation of DHT could mitigate tradeoffs that lead to poor offspring quality (i.e. poor female condition) and small offspring size (i.e. small egg size).

Anopheles gambiae lipophorin: characterization and role in lipid transport to developing oocyte

Lipid transport in arthropods is achieved by highly specialized lipoproteins, which resemble those described in vertebrate blood. Here, we describe purification and characterization of the lipid-apolipoprotein complex, lipophorin (Lp), in the malaria vector mosquito Anopheles gambiae. We also describe the Lp-mediated lipid transfer to developing eggs and the distribution of the imported lipid in developing embryos. The density of the Lp complex was 1.135 g/ml with an apparent molecular weight of 630 kDa. It is composed of two major polypeptides, apoLp I (260 kDa) and apoLp II (74 kDa) and composed of 50% protein, 48% lipid and 2% carbohydrate (w/w). Hydrocarbon, cholesterol, phosphatidyl choline, phosphatidyl ethanolamine, cholesteryl ester and diacylglyceride were the major Lp-associated lipids. Using fluorescently tagged lipids, we observed patterns that suggest that in live developing oocytes, the Lp was taken up by a receptor-mediated endocytic process. Such process was blocked at low temperature and in the presence of excess unlabeled Lp, but not by bovine serum albumin. Imported Lp was segregated in the spherical yolk bodies (mean size 1.8 microm) and distributed evenly in the cortex of the oocyte. In embryonic larvae, before hatching, a portion of the fatty acid in vesicles was found evenly distributed along the body, whereas portion of phospholipids was accumulated in the intestine.

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.

Effects of ambient temperature on lipid and fatty acid composition in the oviparous lizards, Phrynocephalus przewalskii

This study was designed to assess the effect of ambient temperature on lipid content, lipid classes and fatty acid compositions of heart, liver, muscle and brain in oviparous lizards, Phrynocephalus przewalskii, caught in the desert area of China. Significant differences could be observed in the contents of the total lipid and fatty acid compositions among different temperatures (4, 25 and 38 degrees C). The study showed that liver and muscle were principal sites of lipid storage. Triacylglycerol (TAG) mainly deposited in the liver, while phospholipids (PL) was identified as the predominant lipid class in the muscle and brain. Palmitic and stearic acid generally occupied the higher proportion in saturated fatty acids (SFA), while monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) consisted mainly of 16:1n-7, 18:1n-9, 18:2n-6, 18:3n-3, 20:4n-6 and 22:6n-3 regardless of tissue and temperature. These predominant fatty acids proportion fluctuations caused by temperature affected directly the ratio of unsaturated to saturated fatty acids. There was a tendency to increase the degree of unsaturation in the fatty acids of TAG and PL as environmental temperature dropped from 38 to 4 degrees C, although the different extent in different tissues. These results suggested that lipid characteristics of P. przewalskii tissues examined were influenced by ambient temperature.

Oogenesis and egg development in triatomines: a biochemical approach

In triatomines, as well as in other insects, accumulation of yolk is a process in which an extra-ovarian tissue, the fat body, produces yolk proteins that are packed in the egg. The main protein, synthesized by the fat body, which is accumulated inside the oocyte, is vitellogenin. This process is also known as vitellogenesis. There are growing evidences in triatomines that besides fat body the ovary also produces yolk proteins. The way these yolk proteins enter the oocyte will be discussed. Yolk is a complex material composed of proteins, lipids, carbohydrates and other minor components which are packed inside the oocyte in an organized manner. Fertilization triggers embryogenesis, a process where an embryo will develop. During embryogenesis the yolk will be used for the construction of a new individual, the first instar nymph. The challenge for the next decade is to understand how and where these egg proteins are used up together with their non-protein components, in pace with the genetic program of the embryo, which enables cell differentiation (early phase of embryogenesis) and embryo differentiation (late phase) inside the egg.

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.

Ovarian histology of the placentotrophicMabuya mabouya(Squamata, Scincidae)

Ovarian structure and folliculogenesis of females at different reproductive stages are described for the viviparous placentotrophic lizard Mabuya mabouya. The small ovaries have a thin wall formed by the ovarian epithelium and a thin tunica albuginea. One to two germinal beds that contain numerous oogonia and developing primordial follicles are derived from the ovarian epithelium and are next to the ovarian hilum. The ovarian cortex contains follicles at different stages of development, corpora lutea, and atretic follicles. The yolk nucleus and Balbiani complex were not evident in the ooplasm of previtellogenic follicles. The follicular epithelium of these follicles is polymorphic, as in other species of Squamata, but the larger cells are spherical and monolayered rather than pyriform. The zona radiata of the preovulatory follicles is less developed than in lecithotrophic species. These features suggest a decrease in metabolic and absorptive processes during follicular growth. In preovulatory follicles (1.5-1.8 mm diameter), primordial yolk vacuoles and small cortical granules are deposited in the ooplasm instead of fatty yolk platelets, so that only one stage of vitellogenesis is observed. Polyovular atretic follicles occur in some females. Follicular atresia is minimal for preovulatory follicles, but is more frequent in follicles with polymorphic epithelia. In the corpus luteum, the luteal tissue is formed from granulosa cells and luteolysis occurs during the late gastrula -- late neurula embryonic stages. Thus, the maintenance of gestation from the pharyngula to preparturition stages seems to be related to secretion of extraluteal progesterone, possibly of placental origin. These observed ovarian features are related to the high degree of placental complexity of this species and show that the evolution of advanced placentotrophy in species of this lineage has been accompanied by concomitant changes in ovarian function.

Metabolic fate of yolk fatty acids in the developing king penguin embryo

This study examines the metabolic fate of total and individual yolk fatty acids (FA) during the embryonic development of the king penguin, a seabird characterized by prolonged incubation (53 days) and hatching (3 days) periods, and a high n-3/n-6 polyunsaturated FA ratio in the egg. Of the approximately 15 g of total FA initially present in the egg lipid, 87% was transferred to the embryo by the time of hatching, the remaining 13% being present in the internalized yolk sac of the chick. During the whole incubation, 83% of the transferred FA was oxidized for energy, with only 17% incorporated into embryo lipids. Prehatching (days 0-49), the fat stores (triacylglycerol) accounted for 58% of the total FA incorporated into embryo lipid. During hatching (days 49-53), 40% of the FA of the fat stores was mobilized, the mobilization of individual FA being nonselective. At hatch, 53% of the arachidonic acid (20:4n-6) of the initial yolk had been incorporated into embryo lipid compared with only 15% of the total FA and 17-24% of the various n-3 polyunsaturated FA. Similarly, only 32% of the yolk's initial content of 20:4n-6 was oxidized for energy during development compared with 72% of the total FA and 58-66% of the n-3 polyunsaturated FA. The high partitioning of yolk FA toward oxidization and the intense mobilization of fat store FA during hatching most likely reflect the high energy cost of the long incubation and hatching periods of the king penguin. The preferential partitioning of 20:4n-6 into the structural lipid of the embryo in the face of its low content in the yolk may reflect the important roles of this FA in tissue function.

Distribution of lipids from the yolk to the tissues during development of the water python (Liasis fuscus)

Energy metabolism during embryonic development of snakes differs in several respects from the patterns displayed by other reptiles. There are, however, no previous reports describing the main energy source for development, the yolk lipids, in snake eggs. There is also no information on the distribution of yolk fatty acids to the tissues during snake development. In eggs of the water python ( Liasis fuscus), we report that triacylglycerol, phospholipid, cholesteryl ester and free cholesterol, respectively, form 70.3%, 14.1%, 5.7% and 2.1% of the total lipid. The main polyunsaturate of the yolk lipid classes is 18:2n-6. The yolk phospholipid contains 20:4n-6 and 22:6n-3 at 13.0% and 3.6% (w/w), respectively. Approximately 10% and 30% of the initial egg lipids are respectively recovered in the residual yolk and the fat body of the hatchling. A major function of yolk lipid is, therefore, to provision the neonate with large energy reserves. The proportion of 22:6n-3 in brain phospholipid of the hatchling is 11.1% (w/w): this represents only 0.24% of the amount of 22:6n-3 originally present in the egg. This also contrasts with values for free-living avian species where the proportion of DHA in neonatal brain phospholipid is 16-19%. In the liver of the newly hatched python, triacylglycerol, phospholipid and cholesteryl ester, respectively, form 68.2%, 7.7% and 14.3% of total lipid. This contrasts with embryos of birds where cholesteryl ester forms up to 80% of total liver lipid and suggests that the mechanism of lipid transfer in the water python embryo differs in some respects from the avian situation.

Morphogenesis of extraembryonic membranes and placentation inMabuya 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.

The fatty acid composition of oophagous tadpoles (Chirixalus eiffingeri) fed conspecific or chicken egg yolk

We compared the lipid content and fatty acid composition of (1) the egg yolk of three anuran species (Chirixalus eiffingeri, Rhacophorus moltrechti and Buergeria robustus) and chicken eggs; and (2) C. eiffingeri tadpoles fed conspecific eggs or chicken egg yolk. Anuran and chicken egg yolk contained more non-polar than polar lipids but the proportions varied among species. Chicken egg yolk contained low amounts of 22:5n-3 in the polar lipid fraction, and B. robustus eggs did not contain any n-3 or n-6 non-polar lipids. The specific variation of lipid contents and fatty acid composition may relate to the maternal diet and/or breeding biology. In C. eiffingeri tadpoles that fed chicken yolk or frog egg yolk, the dominant components of polar and non-polar lipids were 16:0, 18:0, 18:1, and 18:2n-6, or 20:4n-6 fatty acids. C. eiffingeri eggs contained more n-3 fatty acids (e.g. 18:3n-3 and 20:5n-3) than chicken egg yolk, and tadpoles fed conspecific eggs contained more of these fatty acids than tadpoles fed chicken egg yolk. The compositional differences in the fatty acids between C. eiffingeri tadpoles that fed frog egg or chicken egg yolk are the reflection of the variation in the dietary sources. Our results suggest a direct incorporation of fatty acids into the body without or minimal modification, which provide an important insight into the physiological aspects of cannibalism.

Differential partitioning of maternal fatty acid and phospholipid in neonate mosquito larvae

In animals, lipids are a source of energy, cell membrane components,signaling pathway modulators and emulsifying agents. In egg-laying animals,maternal yolk lipids, imported into the egg before laying, are metabolized or distributed in the developing embryo to serve these functions. Studies with birds, reptiles and insects have described lipid metabolism in adults and in eggs, but no studies have addressed how lipids are distributed in developing organs in the embryo. Here we show that maternal fatty acid and phospholipids segregate differently in tissues of newly hatched mosquito larvae. In the mother, both lipids are colocalized in yolk granules of developing oocytes and distributed evenly. In neonate larvae, however, the maternal fatty acid is stored along the side of the body, especially at the base of the body hair,and in the thorax, where the muscles are located, probably to provide energy for the rapid movements needed to find food immediately after birth. Most maternal phospholipids, however, are concentrated in the motile intestinal gastric caeca, from which they are released into the gut lumen where they may act as emulsifiers, probably to facilitate assimilation of the food the neonate ingests. Similar phenomena were observed in both Anopheles gambiae and Aedes aegypti mosquitoes, suggesting that such differential segregation of lipids is common to both insects. This study may lead to improved delivery of larvicidal agents and to efficient killing of newly hatched mosquito larvae as a control strategy for mosquito-borne diseases.

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.

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.

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.

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.

Utilisation of lipids, protein, ions and energy during embryonic development of Australian oviparous skinks in the genus Lampropholis

The contents of eggs and neonates of the Australian skinks, Lampropholis guichenoti and L. delicata, are described and compared to allow interpretation of nutrient utilisation by the developing embryo. Even though the females are the same size, L. guichenoti lay smaller clutches of larger eggs (egg contents=41.6+/-1.2 mg dry mass) than L. delicata (26.6+/-2.8 mg). The energy density is the same for eggs (30.5+/-0.9 J/g ash-free dry mass for L. guichenoti and 29.9+/-1.1 J/mg for L. delicata) and neonates (22.5+/-1.3 J/mg for L. guichenoti and 23.5+/-0.4 J/mg for L. delicata) between species. The amount of nitrogen (protein) in neonates is only slightly lower than that in eggs, whereas there is a large and significant decline in total lipids. Thus, like some other skinks, protein is a source of metabolic energy during embryogenesis, although not as important as lipid. Triacylglycerol is the major lipid component of the eggs (80% of total lipid), with phospholipid forming only approximately 10% of the total lipid. The fatty acid profile of the phospholipid is distinguished by a high proportion of arachidonic acid (8%), a significant proportion of eicosapentaenoic acid (2-4%) and a relatively low proportion of docosahexaenoic acid (2-3%) compared to chickens. Eggs of both species have remarkably low concentrations of free cholesterol compared to other amniote eggs (0.7% for L. guichenoti and 1.3% for L. delicata). The loss of lipid during embryonic development is almost entirely due to the selective utilisation of yolk triacylglycerol, presumably for energy. By contrast, the amount of phospholipid recovered from the neonates was the same as that originally in the eggs. Moreover, significantly more total cholesterol was present in the neonates than in the eggs, suggesting that biosynthesis of additional cholesterol occurred during development. The phospholipids of the neonates contain higher proportions of arachidonic (11-12%) and docosahexaenoic (8%) acids than the phospholipids of the eggs. Eicosapentaenoic acid is less prevalent in phospholipids in neonates than in eggs. Neonates of both species contain significantly more calcium than the fresh egg contents (L. guichenoti, eggs 0.303+/-0.051 mg, neonates 0.641+/-0.047 mg; L. delicata, eggs 0.187+/-0.013 mg, neonates 0.435+/-0.033 mg), presumably as a result of resorption of calcium from the eggshell. Interestingly, there is also significantly more sodium in neonates than in the contents of fresh eggs (L. guichenoti, eggs 0.094+/-0.010 mg, neonates 0.184+/-0.011 mg; L. delicata, eggs 0.084+/-0.011 mg, neonates 0.151+/-0.010 mg). There is no significant difference in the content of potassium and magnesium in eggs and neonates of either species. Although the fresh eggs of L. delicata have a significantly higher sodium concentration than L. guichenoti, there is no difference in the concentrations of calcium, magnesium, potassium or sodium in the neonates of the two species.

Comparison of nutrient transport across the placenta of lizards differing in placental complexity

We have reviewed published and new quantitative data on the net uptake of nutrients by embryos of oviparous and viviparous lizards that vary in chorioallantoic placental complexity to better understand the evolution of complex placentae. We assessed net nutrient uptake during embryonic development by measuring the total dry mass, or the mass of separate nutrients, in the egg at about the time of ovulation and in the neonate. There is no significant difference in the fresh egg to neonate dry mass ratio of oviparous and viviparous species that have simple placentae, indicating that there is little, if any, net uptake of nutrients by viviparous species with simple chorioallantoic placentae. In contrast, there is significant uptake of dry matter and individual nutrients across the placenta of species with complex chorioallantoic placentae. Species of the genus Niveoscincus have a range of placentae and nutrient uptakes, even among populations of one species, suggesting that further studies among populations of single species are required. Data are available for relatively few clades, and all the data for the three most complex chorioallantoic placental types are derived from a single genus. Thus, further research on new genera of lizards is required to overcome the potentially confounding effects of phylogeny in our analyses.