Arrested embryonic development: a review of strategies to delay hatching in egg-laying reptiles

Outbreeding reduces survival during metamorphosis in a headwater stream salamander

Assessing direct fitness effects of individual genetic diversity is challenging due to the intensive and long-term data needed to quantify survival and reproduction in the wild. But resolving these effects is necessary to determine how inbreeding and outbreeding influence eco-evolutionary processes. We used 8 years of capture-recapture data and single nucleotide polymorphism genotypes for 1906 individuals to test for effects of individual heterozygosity on stage-specific survival probabilities in the salamander Gyrinophilus porphyriticus. The life cycle of G. porphyriticus includes an aquatic larval stage followed by metamorphosis into a semi-aquatic adult stage. In our study populations, the larval stage lasts 6-10 years, metamorphosis takes several months, and lifespan can reach 20 years. Previous studies showed that metamorphosis is a sensitive life stage, leading us to predict that fitness effects of individual heterozygosity would occur during metamorphosis. Consistent with this prediction, monthly probability of survival during metamorphosis declined with multi-locus heterozygosity (MLH), from 0.38 at the lowest MLH (0.10) to 0.06 at the highest MLH (0.38), a reduction of 84%. Body condition of larvae also declined significantly with increasing MLH. These relationships were consistent in the three study streams. With evidence of localised inbreeding within streams, these results suggest that outbreeding disrupts adaptations in pre-metamorphic and metamorphic individuals to environmental gradients along streams, adding to evidence that headwater streams are hotspots of microgeographic adaptation. Our results also underscore the importance of incorporating life history in analyses of the fitness effects of individual genetic diversity and suggest that metamorphosis and similar discrete life stage transitions may be critical periods of viability selection.

The influence of oviposition status on measures of transmission potential in malaria-infected mosquitoes depends on sugar availability

BACKGROUND

Like other oviparous organisms, the gonotrophic cycle of mosquitoes is not complete until they have selected a suitable habitat to oviposit. In addition to the evolutionary constraints associated with selective oviposition behavior, the physiological demands relative to an organism's oviposition status also influence their nutrient requirement from the environment. Yet, studies that measure transmission potential (vectorial capacity or competence) of mosquito-borne parasites rarely consider whether the rates of parasite replication and development could be influenced by these constraints resulting from whether mosquitoes have completed their gonotrophic cycle.

METHODS

Anopheles stephensi mosquitoes were infected with Plasmodium berghei, the rodent analog of human malaria, and maintained on 1% or 10% dextrose and either provided oviposition sites ('oviposited' herein) to complete their gonotrophic cycle or forced to retain eggs ('non-oviposited'). Transmission potential in the four groups was measured up to 27 days post-infection as the rates of (i) sporozoite appearance in the salivary glands ('extrinsic incubation period' or EIP), (ii) vector survival and (iii) sporozoite densities.

RESULTS

In the two groups of oviposited mosquitoes, rates of sporozoite appearance and densities in the salivary glands were clearly dependent on sugar availability, with shorter EIP and higher sporozoite densities in mosquitoes fed 10% dextrose. In contrast, rates of appearance and densities in the salivary glands were independent of sugar concentrations in non-oviposited mosquitoes, although both measures were slightly lower than in oviposited mosquitoes fed 10% dextrose. Vector survival was higher in non-oviposited mosquitoes.

CONCLUSIONS

Costs to parasite fitness and vector survival were buffered against changes in nutritional availability from the environment in non-oviposited but not oviposited mosquitoes. Taken together, these results suggest vectorial capacity for malaria parasites may be dependent on nutrient availability and oviposition/gonotrophic status and, as such, argue for more careful consideration of this interaction when estimating transmission potential. More broadly, the complex patterns resulting from physiological (nutrition) and evolutionary (egg-retention) trade-offs described here, combined with the ubiquity of selective oviposition behavior, implies the fitness of vector-borne pathogens could be shaped by selection for these traits, with implications for disease transmission and management. For instance, while reducing availability of oviposition sites and environmental sources of nutrition are key components of integrated vector management strategies, their abundance and distribution are under strong selection pressure from the patterns associated with climate change.

Experimental evidence that competition strength scales with ecological similarity: a case study using Anolis lizards

Interspecific competition is widely considered a powerful process underlying species coexistence and ecological community structure. Although coexistence theory predicts stronger competition between more ecologically similar species, empirical support has largely relied on inferring competition from patterns of species co-occurrence. Coexistence theory also posits that species can only coexist when individuals compete more with conspecifics than with other species, however, few field studies-particularly in reptiles-have simultaneously estimated the strength of both intra- and interspecific competition among co-occurring species. Using an array of 12 experimental plots, we manipulated species presence and population size by plot of three native Anolis lizard species to empirically estimate the strength of competition on one anole species driven by two other species of varying ecological similarity. We observed that the strength of competition-as determined by relative growth rates and gravidity-was highly predictable and correlated to ecological similarity. Interspecific competition was strongest among species of highest ecological similarity, and intraspecific competition-induced by the addition or removal of conspecifics-was consistently the most intense. By employing direct experimental manipulations, our study provides an empirical investigation of the strength of competition as it relates to ecological similarity.

Increasing hypoxia progressively slows early embryonic development in an oviparous reptile, the green turtle, Chelonia mydas

Green turtle (Chelonia mydas) embryos are in an arrested state of development when the eggs are laid, but in the presence of oxygen, arrest is broken and development resumes within 12-16 h. However, the precise oxygen level at which embryos break arrest and continue development is not known. To better understand the impact of oxygen concentration on breaking of arrest and early embryonic development, we incubated freshly laid eggs of the green sea turtle for three days at each of six different oxygen concentrations (less than or equal to 1%, 3%, 5%, 7%, 9% and 21%) and monitored the appearance and growth of white spots on the shell, indicative of embryonic development. As reported previously, white spots did not develop on eggs incubated in anoxia (less than or equal to 1% oxygen). For all other treatments, mean time to white spot detection and white spot growth rate varied inversely with oxygen concentration. In nearly all cases the difference between eggs at different oxygen levels was statistically significant (p ≤ 0.05). This suggests that sea turtle embryonic development may respond to oxygen in a dose-dependent manner. Our results indicate that the development of green turtle embryos may be slowed if they are exposed to the most hypoxic conditions reported in mature natural nests.

Transcriptomic analysis of pre-ovipositional embryonic arrest in a non-squamate reptile (Chelonia mydas)

After gastrulation, oviductal hypoxia maintains turtle embryos in an arrested state prior to oviposition. Subsequent exposure to atmospheric oxygen upon oviposition initiates recommencement of embryonic development. Arrest can be artificially extended for several days after oviposition by incubation of the egg under hypoxic conditions, with development recommencing in an apparently normal fashion after subsequent exposure to normoxia. To examine the transcriptomic events associated with embryonic arrest in green sea turtles (Chelonia mydas), RNA‐sequencing analysis was performed on embryos from freshly laid eggs and eggs incubated in either normoxia (oxygen tension ~159 mmHg) or hypoxia (<8 mmHg) for 36 h after oviposition (n = 5 per group). The patterns of gene expression differed markedly among the three experimental groups. Normal embryonic development in normoxia was associated with upregulation of genes involved in DNA replication, the cell cycle, and mitosis, but these genes were commonly downregulated after incubation in hypoxia. Many target genes of hypoxia inducible factors, including the gene encoding insulin‐like growth factor binding protein 1 (igfbp1), were downregulated by normoxic incubation but upregulated by incubation in hypoxia. Notably, some of the transcriptomic effects of hypoxia in green turtle embryos resembled those reported to be associated with hypoxia‐induced embryonic arrest in diverse taxa, including the nematode Caenorhabditis elegans and zebrafish (Danio rerio). Hypoxia‐induced preovipositional embryonic arrest appears to be a unique adaptation of turtles. However, our findings accord with the proposition that the mechanisms underlying hypoxia‐induced embryonic arrest per se are highly conserved across diverse taxa.

HREM, RNAseq and Cell Cycle Analyses Reveal the Role of the G2/M-Regulatory Protein, WEE1, on the Survivability of Chicken Embryos during Diapause

Avian blastoderm can enter into diapause when kept at low temperatures and successfully resume development (SRD) when re-incubated in body temperature. These abilities, which are largely affected by the temperature and duration of the diapause, are poorly understood at the cellular and molecular level. To determine how temperature affects embryonic morphology during diapause, high-resolution episcopic microscopy (HREM) analysis was utilized. While blastoderms diapausing at 12 °C for 28 days presented typical cytoarchitecture, similar to non-diapaused embryos, at 18 °C, much thicker blastoderms with higher cell number were observed. RNAseq was conducted to discover the genes underlying these phenotypes, revealing differentially expressed cell cycle regulatory genes. Among them, WEE1, a negative regulator of G2/M transition, was highly expressed at 12 °C compared to 18 °C. This finding suggested that cells at 12 °C are arrested at the G2/M phase, as supported by bromodeoxyuridine incorporation (BrdU) assay and phospho-histone H3 (pH 3) immunostaining. Inhibition of WEE1 during diapause at 12 °C resulted in cell cycle progression beyond the G2/M and augmented tissue volume, resembling the morphology of 18 °C-diapaused embryos. These findings suggest that diapause at low temperatures leads to WEE1 upregulation, which arrests the cell cycle at the G2/M phase, promoting the perseverance of embryonic cytoarchitecture and future SRD. In contrast, WEE1 is not upregulated during diapause at higher temperature, leading to continuous proliferation and maladaptive morphology associated with poor survivability. Combining HREM-based analysis with RNAseq and molecular manipulations, we present a novel mechanism that regulates the ability of diapaused avian embryos to maintain their cytoarchitecture via cell cycle arrest, which enables their SRD.

The behavioural and physiological ecology of embryos: responding to the challenges of life inside an egg

Adaptations of post-hatching animals have attracted far more study than have embryonic responses to environmental challenges, but recent research suggests that we have underestimated the complexity and flexibility of embryos. We advocate a dynamic view of embryos as organisms capable of responding - on both ecological and evolutionary timescales - to their developmental environments. By viewing embryos in this way, rather than assuming an inability of pre-hatching stages to adapt and respond, we can broaden the ontogenetic breadth of evolutionary and ecological research. Both biotic and abiotic factors affect embryogenesis, and embryos exhibit a broad range of behavioural and physiological responses that enable them to deal with changes in their developmental environments in the course of interactions with their parents, with other embryos, with predators, and with the physical environment. Such plasticity may profoundly affect offspring phenotypes and fitness, and in turn influence the temporal and spatial dynamics of populations and communities. Future research in this field could benefit from an integrated framework that combines multiple approaches (field investigations, manipulative experiments, ecological modelling) to clarify the mechanisms and consequences of embryonic adaptations and plasticity.

A review of the effects of incubation conditions on hatchling phenotypes in non-squamate reptiles

Developing embryos of oviparous reptiles show substantial plasticity in their responses to environmental conditions during incubation, which can include altered sex ratios, morphology, locomotor performance and hatching success. While recent research and reviews have focused on temperature during incubation, emerging evidence suggests other environmental variables are also important in determining hatchling phenotypes. Understanding how the external environment influences development is important for species management and requires identifying how environmental variables exert their effects individually, and how they interact to affect developing embryos. To address this knowledge gap, we review the literature on phenotypic responses in oviparous non-squamate (i.e., turtles, crocodilians and tuataras) reptile hatchlings to temperature, moisture, oxygen concentration and salinity. We examine how these variables influence one another and consider how changes in each variable alters incubation conditions and thus, hatchling phenotypes. We explore how incubation conditions drive variation in hatchling phenotypes and influence adult populations. Finally, we highlight knowledge gaps and suggest future research directions.

Too Hot to Nest? In a Hot Summer the Tortoise Chersina angulata Can Switch From Nesting to Facultative Viviparity

In a captive colony of Chersina angulata in Cape Town, South Africa, we observed in 2015/16 retention of the last egg clutch inside the female until the hatching stage was reached, conforming to the generally accepted definition of viviparity. Retrospective climatic analysis indicates egg retention until the hatching stage co-occurred with unusually hot summer weather: the average air temperatures in December 2015 and January and February 2016 were higher than during the preceding five and the following 5 years when facultative viviparity could not be observed. Late December and January appears to be the critical period for females to either deposit their last clutch of the nesting season into a nest, or to retain the last clutch for embryonic development inside the female. Over the 28 December to 24 January period the minimum, average and maximum air temperatures in 2015–16 were about 3°C higher than in the five following years. This association of facultative viviparity with unusual summer heat suggests that hot ambient temperatures at the end of the nesting season may cue females to switch from oviposition to facultative viviparity. Compared to incubation in a nest this phenotypic plasticity of the reproductive mode—to retain during hot summers the season’s last clutch inside the female—may buffer the developing embryos from excessive heat exposure: females can thermo-regulate by moving among microhabitats whereas sun exposed shallow nests cannot escape high ground temperatures. This novel reproductive strategy has the potential to enhance the resilience of species to global warming.

Effects of moisture during incubation on green sea turtle (Chelonia mydas) development, morphology and performance

While the effect of temperature on embryonic development in sea turtles has been well studied over recent years, our understanding of the effect of substrate moisture, another important environmental variable, is limited. High sand moisture decreases nest temperature through evaporative and direct cooling during rainfall, but its direct effect on hatchling development, morphology and performance is unclear. To address this knowledge gap, we incubated 40 green sea turtle Chelonia mydas clutches in a beach hatchery under either high (~8% v/v) or low (~5% v/v) sand moisture concentrations for the duration of embryonic development. In half of the clutches, temperature sensors were deployed to measure any effect of sand moisture on nest temperature. As hatchlings emerged, we measured body size and locomotory performance during the first 24 h, an important period of frenzied activity for sea turtles. We excavated clutches post-emergence to determine hatching success, emergence success and to determine the stage of embryonic death for unsuccessful eggs. High moisture concentrations increased incubation duration, decreased nest temperature and had marginal effects on hatchling morphology, but no effect on hatching success, stage of embryonic death, crawling speed or initial swimming performance. However, after 24 h of swimming, hatchlings from high-moisture clutches produced less mean swim thrust and spent less time powerstroking than hatchlings from low-moisture clutches, suggesting reduced swimming endurance and potentially impacting the ability of hatchlings to successfully disperse. The effect of moisture on nest temperature and hatchling endurance highlights the importance of considering rainfall patterns when predicting future impacts of climate change on sea turtle populations.

Sperm storage in the oviduct of the Chinese pond turtle Mauremys reevesii depends on oestrogen-based suppression of the TLR2/4 immune pathway

The long-term storage of spermatozoa in the female reproductive tract is limited by the innate immune system. Oestrogen plays a role in regulating the innate immune system. Thus, exploring the expression of genes in the Toll-like receptor (TLR) 2/4 pathway and oestrogen receptors in the oviduct of Mauremys reevesii could contribute to our understanding of the mechanism of sperm storage. In this study, three parts of the oviduct (isthmus, uterus and vagina) in three mated and unmated female turtles were used to perform immunohistochemistry and real-time quantitative polymerase chain reaction (qPCR). Immunohistochemistry revealed that the TLR2/4 protein was mainly distributed in epithelial tissues and glandular cell membranes, and that TLR2/4 levels in the oviduct were significantly decreased in mated compared with unmated turtles. Real-time qPCR indicated that TLR2/4, myeloid differentiation factor 88 (MyD88), interleukin 1 receptor associated kinase 4 (IRAK4), TNF receptor associated factor 6 (TRAF6), interferon regulatory factor 3 (IRF3) and interleukin 6 (IL6) mRNA expression was significantly higher in the oviduct of unmated than mated turtles, whereas the opposite was true for the expression of oestrogen receptor 1 (ESR1) and progesterone receptor (PGR). These results indicate that when spermatozoa are stored in the oviduct, an increase in oestrogen suppresses the immune response induced by the TLR2/4 pathway so that spermatozoa are not removed as a foreign substance, but stored until fertilisation. The findings of this study are relevant to our understanding of the relationship between sperm storage and the innate immune system in the oviduct of reptiles.

Finding of a two-headed green turtle embryo during nest monitoring in Baa Atoll, Maldives

Green sea turtles are one of the two species of marine turtles known to nest in the Maldives. The prevalent time of nesting seems to be inconsistent throughout the island nation. In this study, sea turtle nesting activity was monitored on the island of Coco Palm Dhuni Kolhu in Baa Atoll over a period of 12 months. A total of 13 nests were confirmed with a median hatching success rate of 89.58% as ascertained by nest excavation. In one of the nests, a severely deformed hatchling with polycephaly, an opening in the neck area and a lordotic spine was found, and we investigated in detail with radiographic images and a necropsy. Our findings support the importance of consistent nesting activity and nest monitoring efforts in the country as a basis for conservation efforts.

Molecular Drivers of Developmental Arrest in the Human Preimplantation Embryo: A Systematic Review and Critical Analysis Leading to Mapping Future Research

Developmental arrest of the preimplantation embryo is a multifactorial condition, characterized by lack of cellular division for at least 24 hours, hindering the in vitro fertilization cycle outcome. This systematic review aims to present the molecular drivers of developmental arrest, focusing on embryonic and parental factors. A systematic search in PubMed/Medline, Embase and Cochrane-Central-Database was performed in January 2021. A total of 76 studies were included. The identified embryonic factors associated with arrest included gene variations, mitochondrial DNA copy number, methylation patterns, chromosomal abnormalities, metabolic profile and morphological features. Parental factors included, gene variation, protein expression levels and infertility etiology. A valuable conclusion emerging through critical analysis indicated that genetic origins of developmental arrest analyzed from the perspective of parental infertility etiology and the embryo itself, share common ground. This is a unique and long-overdue contribution to literature that for the first time presents an all-inclusive methodological report on the molecular drivers leading to preimplantation embryos’ arrested development. The variety and heterogeneity of developmental arrest drivers, along with their inevitable intertwining relationships does not allow for prioritization on the factors playing a more definitive role in arrested development. This systematic review provides the basis for further research in the field.

Climate variability and parent nesting strategies influence gas exchange across avian eggshells

Embryo survival in birds depends on a controlled transfer of water vapour and respiratory gases through the eggshell, and this exchange is critically sensitive to the surrounding physical environment. As birds breed in most terrestrial habitats worldwide, we proposed that variation in eggshell conductance has evolved to optimize embryonic development under different breeding conditions. This is the first study to take a broad-scale macro-ecological view of avian eggshell conductance, encompassing all key avian taxonomic groups, to assess how life history and climate influence the evolution of this trait. Using whole eggs spanning a wide phylogenetic diversity of birds, we determine that body mass, temperature seasonality and whether both parents attend the nest are the main determinants of eggshell conductance. Birds breeding at high latitudes, where seasonal temperature fluctuations are greatest, will benefit from lower eggshell conductance to combat temporary periods of suspended embryo growth and prevent dehydration during prolonged incubation. The nest microclimate is more consistent in species where parents take turns incubating their clutch, resulting in lower eggshell conductance. This study highlights the remarkable functional qualities of eggshells and their importance for embryo survival in extreme climates.

Disentangling the correlated evolution of body size, life history, and ontogeny in miniaturized chelydroid turtles

Organismal miniaturization is defined by a reduction in body size relative to a large ancestor. In vertebrate animals, miniaturization is achieved by suppressing the energetics of growth. However, this might interfere with reproductive strategies in egg-laying species with limited energy budgets for embryo growth and differentiation. In general, the extent to which miniaturization coincides with alterations in animal development remains obscure. To address the interplay among body size, life history, and ontogeny, miniaturization in chelydroid turtles was examined. The analyses corroborated that miniaturization in the Chelydroidea clade is underlain by a dampening of the ancestral growth trajectory. There were no associated shifts in the early sequence of developmental transformations, though the relative duration of organogenesis was shortened in miniaturized embryos. The size of eggs, hatchlings, and adults was positively correlated within Chelydroidea. A phylogenetically broader exploration revealed an alternative miniaturization mode wherein exceptionally large hatchlings grow minimally and thus attain diminutive adult sizes. Lastly, it is shown that miniaturized Chelydroidea turtles undergo accelerated ossification coupled with a ~10% reduction in shell bones. As in other vertebrates, the effects of miniaturization were not systemic, possibly owing to opposing functional demands and tissue geometric constraints. This underscores the integrated and hierarchical nature of developmental systems.

Integrative developmental biology in the age of anthropogenic change

Humans are changing and challenging nature in many ways. Conservation Biology seeks to limit human impacts on nature and preserve biological diversity. Traditionally, Developmental Biology and Conservation Biology have had nonoverlapping objectives, operating in distinct spheres of biological science. However, this chasm can and should be filled to help combat the emerging challenges of the 21st century. The means by which to accomplish this goal were already established within the conceptual framework of evo- and eco-devo and can be further expanded to address the ways that anthropogenic disturbance affect embryonic development. Herein, I describe ways that these approaches can be used to advance the study of reptilian embryos. More specifically, I explore the ways that a developmental perspective can advance ongoing studies of embryonic physiology in the context of global warming and chemical pollution, both of which are known stressors of reptilian embryos. I emphasize ways that these developmental perspectives can inform conservation biologists trying to develop management practices that will address the complexity of challenges facing reptilian embryos.

Static magnetic field halves cryoinjuries of vitrified mouse COCs, improves their functions and modulates pluripotency of derived blastocysts

This study was performed with the aim of evaluating the influence of static magnetic field (SMF) of 60 mT on mouse Cumulus Oocytes Complexes (COCs) vitrification. The COCs were vitrified in the presence (Vit_SMF⁺) and absence of SMF (Vit_SMF^-). Along with these groups, non-vitrified or fresh COCS, which exposed (nVit_SMF⁺) and non-exposed (nVit_SMF^-) to magnetic field, were also considered. Survival and viability rates and mitochondrial activity as well as ultrastructure of oocytes were examined by trypan blue Staining (TBS), Annexin-PI Staining, JC1 staining and transition electron microscopy, respectively. Following in vitro fertilization (IVF) and embryo development, gene expression was carried out through qRT-PCR at blastocyst (BL) stage. The survival rate in Vit_SMF⁺ and Vit_SMF^- decreased meaningfully in comparison with nVit_SMF^- (P < 0.05), but there was no significant difference between SMF⁺ and SMF^- groups. The mitochondrial activity in Vit_SMF^- was significantly reduced compared to the nVit_SMF^- group (P < 0.05), however its value in Vit_SMF⁺ returned to the control level. Ultrastructural study demonstrated that SMF could protect the COCs from cryoinjuries and reduced damaged features in ooplasm of the vitrified oocytes. There was no significant difference in fertilization rate. Although, BL formation was the highest rate in the Vit_SMF⁺ group, it was just substantially higher than the non-vitrified groups (P < 0.05). The significant changes of Oct4, Cdx2 and Nanog genes expression due to vitrification (Vit_SMF^-) or SMF (nVit_SMF⁺) treatments (P < 0.05) as compared to control (nVit_SMF^-), returned to the natural level after using SMF in vitrified derived blastocysts (Vit_SMF⁺). Totally based on the results, it is clear that static magnetic field improves mitochondrial potential activity and ultrastructure of mouse vitrified COCs. In addition, SMF enhances the embryo cleavage rate to blastocyst stage and modulates pluripotency in blastocyst embryos derived from vitrified COCs.

Influence of size, sex, and reproductive status on the thermal biology of endemic Florida scrub lizards

Climate change is impacting species globally, with many populations declining at an accelerated rate toward extinction. Ectothermic species are particularly vulnerable given their reproductive success is linked to environmental temperatures. Studies of the effect of temperature on reproductive success in oviparous squamates have focused mostly on nest temperatures, after eggs are deposited. However, in some species gravid females are known to thermoregulate differently than other adults to increase reproductive success. It is essential to understand what influences the thermal biology of breeding adults in a population to implement targeted conservation strategies. The Florida scrub lizard Sceloporus woodi is an endemic species listed as near-threatened due to decreasing populations. This study is the first to document the thermal biology of these breeding adults in relation to size, sex, and reproductive status. A t test was used to determine whether sexual dimorphism was present in the sampled S. woodi. Full linear mixed-effects models were used to test the influence of size, sex, and reproductive status on the thermal biology of S. woodi. Despite female-biased sexual size dimorphism, there were no sex-based differences in body temperature in the studied population. Interestingly, reproductive status influenced thermal biology of females during the breeding season, with gravid females maintaining lower body temperatures compared to nongravid females. However, gravid females did not regulate their body temperatures more precisely compared to nongravid females. These results indicate the population viability of this endemic species is potentially linked to the different thermoregulatory requirements of gravid females as compared to other adults. Lower body temperatures of gravid females, exacerbated by their lack of thermal precision, have disconcerting conservation implications in the face of climate warming. Future studies focusing on gravid females are warranted to attain effective biodiversity conservation strategies mitigating the impacts of climate warming.

A method for the collection of early-stage sea turtle embryos

Early-stage turtle embryos, immediately after oviposition, are very small (<5 mm diameter), hindering research on the initial period of embryonic development. For example, assessing whether turtle eggs had been fertilized and contained a viable embryo at oviposition, especially under field conditions, is complicated by the microscopic size of embryos that may have died at an early stage of development. Further, little is known about the molecular pathways that promote and regulate early developmental processes in turtles, such as pre-ovipositional embryonic arrest. To enable further investigation of the processes critical to early embryonic development in turtle species, a reliable method is required for extraction of early-stage embryos from the egg. Therefore, our aim was to develop a novel and reproducible method for extracting early-stage sea turtle embryos. Herein, we describe the technique for extracting Chelonia mydas embryos before and after white spot formation. Once the embryos were collected, the total RNA of 10 embryos was extracted to validate the method. The total RNA concentration was above 5 ng µl-1 and the RNA integrity number varied between 7.0 and 10.0, which is considered acceptable for further RNA-sequencing analyses. This extraction technique could be employed when investigating fertilization rates of turtle nests and for further investigation of the molecular biology of embryonic development in turtles. Furthermore, the technique should be adaptable to other turtle species or any oviparous species with similar eggs.

Abnormal persistence of the chorioallantoic membrane is associated with severe developmental abnormalities in freshwater turtles

Development in oviparous reptiles requires the correct formation and function of extra-embryonic membranes in the egg. In 2017, we incubated 2583 eggs from five species of freshwater turtle during a long-term ecological study and opened eggs that failed to hatch. We described a previously unreported developmental anomaly: the retention of an extra-embryonic membrane around 7 turtles (1 Spiny Softshell Turtle (Apalone spinifera (Le Sueur, 1827)), 1 Snapping Turtle (Chelydra serpentina (Linnaeus, 1758)), and 5 Northern Map Turtles (Graptemys geographica (Le Sueur, 1817))) that were alive but unhatched >14 days after their clutch mates had emerged. We investigated the association between retention of this membrane and the exhibition of other developmental deformities of varying severity, and we tested whether this novel abnormality was associated with reduced fertility or hatching success in affected clutches. Consultation of ∼150 years of literature suggests that we observed persistence of the chorioallantoic membrane (CAM; also called the chorioallantois). Our data suggest that clutches where at least one turtle exhibits a persistent CAM may also exhibit slightly reduced fertility or hatch success in the rest of the clutch compared with conspecific clutches that do not contain this anomaly. Future research should investigate the factors predicting CAM retention and other developmental abnormalities.

Investigating the gene expression profiles of cells in seven embryonic stages with machine learning algorithms

The development of embryonic cells involves several continuous stages, and some genes are related to embryogenesis. To date, few studies have systematically investigated changes in gene expression profiles during mammalian embryogenesis. In this study, a computational analysis using machine learning algorithms was performed on the gene expression profiles of mouse embryonic cells at seven stages. First, the profiles were analyzed through a powerful Monte Carlo feature selection method for the generation of a feature list. Second, increment feature selection was applied on the list by incorporating two classification algorithms: support vector machine (SVM) and repeated incremental pruning to produce error reduction (RIPPER). Through SVM, we extracted several latent gene biomarkers, indicating the stages of embryonic cells, and constructed an optimal SVM classifier that produced a nearly perfect classification of embryonic cells. Furthermore, some interesting rules were accessed by the RIPPER algorithm, suggesting different expression patterns for different stages.

Synchronised nesting aggregations are associated with enhanced capacity for extended embryonic arrest in olive ridley sea turtles

Sea turtle species in the genus Lepidochelys exhibit an unusual behavioural polymorphism, nesting in both aggregations and solitarily. Aggregated nesting events, termed ‘arribadas’, involve hundreds of thousands of females congregating at a single nesting beach over a few days to oviposit their eggs. Aggregate and solitary nesting behaviours are associated with distinct inter-nesting intervals, three and four weeks for non-arribada and arribada nesters respectively. Consequently, embryos are maintained in pre-ovipositional embryonic arrest in the hypoxic oviduct for different lengths of time depending on the mother’s reproductive behaviour. However, sea turtle embryos are limited in their capacity to remain in arrest and will subsequently die if held in hypoxia too long. Here, we tested whether embryos oviposited during arribada or non-arribada nesting differ in their capacity to be maintained in pre-ovipositional arrest. Olive ridley turtle (Lepidochelys olivacea) eggs from eight clutches (four from each nesting tactic) were divided among seven treatments after oviposition; normoxia (control; 21% O₂), or hypoxia (1% O₂) for 3, 3.5, 4, 8, 15 or 30 days, before being returned to normoxia. Arribada eggs were capable of extending pre-ovipositional arrest for longer, with some eggs from the 8- and 15-day hypoxia treatment still hatching while no non-arribada eggs hatched after more than four days in hypoxia. This difference in embryonic capacity to survive extended periods of arrest may be an important mechanism facilitating arribada behaviour by allowing longer inter-nesting intervals. Our finding provides an intriguing insight into the physiological mechanisms that are integral to this unique mass-nesting behaviour.

Twins at Conspicuously Different Developmental Stages in a Turtle Egg

One-egg twins, in general, initiate embryonic development at the same time, and their developmental stages proceed in parallel. Here we report a rare case of the embryonic development of the red-eared slider turtle, Trachemys scripta, in which twins at conspicuously different developmental stages developed on a single yolk. One of the twins appeared to have developed at the normal developmental rate, whereas the development of the other was markedly delayed, despite the absence of any overt anomalies. This observation suggests uncoupled or fully independent differential regulation of embryonic development from either a single or, more likely, two distinct pluripotent blastoderms sharing the same yolk and same environmental conditions.

Evidence for atypical nest overwintering by hatchling lizards, Heloderma suspectum

The timing of reproductive events (e.g. oviposition and hatching) to coincide with favourable seasonal conditions is critical for successful reproduction. However, developmental time may not match the duration between the optimal time for oviposition and the optimal time for hatchling survival. Thus, strategies that alter the time between oviposition and hatchling emergence can be highly advantageous. Arrested development and the resulting extension of the duration between oviposition and hatching has been widely documented across oviparous amniotes, but nest overwintering by hatchlings has only been documented in aquatic chelonians that live where winters are quite cold. Herein, we present a compilation of evidence regarding reproductive phenology by hatchlings of the Gila monster (Heloderma suspectum), a lizard inhabiting the Sonoran Desert of North America. Our data demonstrate that (i) Gila monster hatchlings from eggs oviposited in July do not emerge from their nests until late spring or summer of the following year, yet (ii) Gila monster eggs artificially incubated at field-relevant temperatures hatch in 4-5 months. Furthermore, we describe a fortuitous excavation of a hatching Gila monster nest in late October, which coincides with the artificial incubation results. Together, these results provide strong support for the existence of overwintering in the nest by a lizard, and suggest that this reproductive strategy should be explored in a broader array of taxa.

Skull Development, Ossification Pattern, and Adult Shape in the Emerging Lizard Model Organism Pogona vitticeps: A Comparative Analysis With Other Squamates

The rise of the Evo-Devo field and the development of multidisciplinary research tools at various levels of biological organization have led to a growing interest in researching for new non-model organisms. Squamates (lizards and snakes) are particularly important for understanding fundamental questions about the evolution of vertebrates because of their high diversity and evolutionary innovations and adaptations that portrait a striking body plan change that reached its extreme in snakes. Yet, little is known about the intricate connection between phenotype and genotype in squamates, partly due to limited developmental knowledge and incomplete characterization of embryonic development. Surprisingly, squamate models have received limited attention in comparative developmental studies, and only a few species examined so far can be considered as representative and appropriate model organism for mechanistic Evo-Devo studies. Fortunately, the agamid lizard Pogona vitticeps (central bearded dragon) is one of the most popular, domesticated reptile species with both a well-established history in captivity and key advantages for research, thus forming an ideal laboratory model system and justifying his recent use in reptile biology research. We first report here the complete post-oviposition embryonic development for P. vitticeps based on standardized staging systems and external morphological characters previously defined for squamates. Whereas the overall morphological development follows the general trends observed in other squamates, our comparisons indicate major differences in the developmental sequence of several tissues, including early craniofacial characters. Detailed analysis of both embryonic skull development and adult skull shape, using a comparative approach integrating CT-scans and gene expression studies in P. vitticeps as well as comparative embryology and 3D geometric morphometrics in a large dataset of lizards and snakes, highlights the extreme adult skull shape of P. vitticeps and further indicates that heterochrony has played a key role in the early development and ossification of squamate skull bones. Such detailed studies of embryonic character development, craniofacial patterning, and bone formation are essential for the establishment of well-selected squamate species as Evo-Devo model organisms. We expect that P. vitticeps will continue to emerge as a new attractive model organism for understanding developmental and molecular processes underlying tissue formation, morphology, and evolution.

Ecological and evolutionary significance of a lack of capacity for extended developmental arrest in crocodilian eggs

Hypoxia within the oviducts maintains embryonic arrest in turtles at the pre-ovipositional stage, which expands the timeframe over which nesting can occur without compromising embryo survival. The arrest can be extended post-oviposition through incubation of eggs in hypoxia. We determined whether crocodilian embryos have this same capacity. We also tested whether increased oxygen availability during incubation alters hatching success. We incubated freshly laid saltwater crocodile (Crocodylus porosus) eggs (N = 83) at 32°C in one of five treatments; control (normoxia; 21% O₂), 3-day and 6-day hypoxia (1% O₂), or 3-day and 6-day hyperoxia (42% O₂). Incubation (approx. 82 days) was then completed in normoxia. There was a significant effect of treatment on survival of embryos through to hatching (p < 0.001). The hypoxic treatments resulted in almost no hatching (6.7% and 0% survival for the 3- and 6-day treatments, respectively), while the hyperoxic and control treatments resulted in normal to high hatching success (86.6%, 100% and 64.2% for the control, 3- and 6-day hyperoxic treatments, respectively). Unlike turtles, hypoxic incubation of crocodile eggs failed to delay development. Our results provide the first experimental evidence that, unlike turtles, crocodiles do not exhibit embryonic arrest when incubated under hypoxic conditions immediately following oviposition. An absence of embryonic arrest is of ecological and evolutionary significance, as it implies that crocodilians lack an ability to avoid adverse environmental conditions through delayed nesting and that, unlike turtles, embryonic arrest may not be a potential explanation for the lack of viviparity in the order Crocodylia.

Evidence of Reproductive Stress in Titanosaurian Sauropods Triggered by an Increase in Ecological Competition

The occurrence of dinosaur pathologic eggs in the Late Cretaceous of Europe is well known, but their origin remains unclear. Here we expose the results of a detailed sampling of the conspicuous fossil record of Late Cretaceous titanosaurian eggs (oogenus Megaloolithius) from several southwestern Europe basins. After examining more than 450 samples, we observed a remarkable and statistically supported occurrence of multiple pathologic eggs in a relatively short stratigraphic range at the end of the early Maastrichtian, circa 71-70 Ma. All pathologic specimens exhibit multi-layered eggshell condition, a characteristic related to dystocia, or egg retention within the female uterus for an abnormal prolonged period of time. After exploring various scenarios, the occurrence of pathologic eggs is strongly correlated with an intense dinosaur faunal replacement that occurred during the early Maastrichtian in the Ibero-Armorican Island. Given that inter-species competiveness is proved to produce major affects in ecological communities, our results suggest that pathologies in the eggs of European titanosaurians could be a consequence of an increase in reproductive stress triggered by direct ecological competition between different dinosaurs. Thus, the present study provides a new perspective of how dinosaurs might have been affected by ecological/environmental disturbance.

A 20-year investigation of declining leatherback hatching success: implications of climate variation

Unprecedented increases in air temperature and erratic precipitation patterns are predicted throughout the twenty-first century as a result of climate change. A recent global analysis of leatherback turtle hatchling output predicts that the nesting site at Sandy Point National Wildlife Refuge (SPNWR) will experience the most significant regional climate alterations. We aimed to identify how local air temperatures and precipitation patterns influenced within-nest mortality and overall hatchling output at this site between 1990 and 2010. We show that while the greatest mortality occurred during the latest stages of development (stage three), the rate of embryo mortality was highest during the initial stages (stage zero) of development (approx. 3.8 embryos per day per clutch). Increased mortality at stage three was associated with decreased precipitation and increased temperature during this developmental period, whereas precipitation prior to, and during stage zero had the greatest influence on early mortality. There was a significant decline in overall hatching success (falling from 74% to 55%) and emergence rate (calculated from the number of hatchlings that emerged from the nest as a percentage of hatched eggs) which fell from 96% to 91%. However, there was no trend observed in local temperature or precipitation during this timeframe, and neither variable was related to hatching success or emergence rate. In conclusion, our findings suggest that despite influencing within-nest mortality, climatic variability does not account for the overall decline in hatchling output at SPNWR from 1990 to 2010. Further research is therefore needed to elicit the reasons for this decline.

When Is Embryonic Arrest Broken in Turtle Eggs?

Turtle embryos enter a state of arrested development in the oviduct, allowing the mother greater flexibility in her reproductive schedule. Development recommences once eggs transition from the hypoxic oviduct to the normoxic nest. Significant mortality can occur if turtle eggs are moved between 12 h and 20 d after oviposition, and this is linked to the recommencement of embryonic development. To better understand the timing of developmental arrest and to determine how movement-induced mortality might be avoided, we determined the latency (i.e., time elapsed since oviposition) to recommencement of development following oviposition by exposing the eggs of green turtles (Chelonia mydas) to hypoxia (oxygen tension <8 mmHg) for 3 d, commencing 30 min to 48 h after oviposition. Embryonic development-including development of the characteristic opaque white spot on the eggshell-was halted by hypoxic incubation. When the delay before hypoxic incubation was 12 h or less, hatching success did not differ from a control group. If the hypoxic treatment began after 16 h or more in normoxia, then all embryos died. Thus, by returning eggs to a hypoxic environment before they have broken from arrest (i.e., within 12 h of oviposition), it is possible to extend embryonic arrest for at least 3 d, with no apparent detriment to hatching success. Therefore, hypoxic incubation may provide a new approach for avoidance of movement-induced mortality when conservation or research efforts require the relocation of eggs. Our findings also suggest that movement-induced mortality may have constrained the evolution of viviparity in turtles.

The enigma of embryonic diapause

Embryonic diapause – a period of embryonic suspension at the blastocyst stage – is a fascinating phenomenon that occurs in over 130 species of mammals, ranging from bears and badgers to mice and marsupials. It might even occur in humans. During diapause, there is minimal cell division and greatly reduced metabolism, and development is put on hold. Yet there are no ill effects for the pregnancy when it eventually continues. Multiple factors can induce diapause, including seasonal supplies of food, temperature, photoperiod and lactation. The successful reactivation and continuation of pregnancy then requires a viable embryo, a receptive uterus and effective molecular communication between the two. But how do the blastocysts survive and remain viable during this period of time, which can be up to a year in some cases? And what are the signals that bring it out of suspended animation? Here, we provide an overview of the process of diapause and address these questions, focussing on recent molecular data.

An evolutionary solution of terrestrial isopods to cope with low atmospheric oxygen levels

The evolution of current terrestrial life was founded by major waves of land invasion coinciding with high atmospheric oxygen content. These waves were followed by periods with substantially reduced oxygen concentration and accompanied by evolution of novel traits. Reproduction and development are limiting factors for evolutionary water-land transitions, and brood care has likely facilitated land invasion. Peracarid crustaceans provide parental care for their offspring by brooding the early stages within the motherly pouch, marsupium. Terrestrial isopod progeny begins ontogenetic development within the marsupium in water, but conclude development within the marsupium in air. Our results for progeny growth until hatching from the marsupium provide evidence for the limiting effects of oxygen concentration and for a potentially adaptive solution. Inclusion of air within the marsupium compensates for initially constrained growth in water through catch-up growth, and it may explain how terrestrial isopods adapted to short- and long-term changes in oxygen concentration.

The impact of extended preovipositional arrest on embryonic development and hatchling fitness in the flatback sea turtle

Turtle embryos pause development before oviposition in a process known as preovipositional arrest. Embryonic development arrests due to hypoxia (low oxygen) in the maternal oviducts and resumes only after exposure to normoxia when eggs are laid. Recently, several studies have hypothesized that the prolonged periods of preovipositional arrest may have a detrimental effect on embryo survival and development after eggs are laid. We tested this hypothesis by comparing embryo survival (determined by white spot formation and hatching success) and hatchling fitness (measured by self-righting, crawling, and swimming ability) of flatback sea turtle (Natator depressus) eggs following incubation in hypoxic (∼ 1%) and normoxic (∼ 21%) treatments for 5 d immediately following oviposition. We also measured embryo survival and hatchling fitness when eggs were incubated in hyperoxic conditions (42% oxygen), to determine whether hyperoxia could improve developmental outcome or whether some consequence of oxidative stress might manifest. Eggs incubated in hypoxia remained arrested during the 5-d treatment, and 97.5% of the eggs successfully recommenced development after exposure to normoxia when the treatment finished. At treatment commencement, 100% and 97.5% of eggs in the hyperoxic and normoxic treatments, respectively, began developing. Although hatching success was significantly lower following hypoxia (15%) compared to normoxia (80%) and hyperoxia (85%), hatchings from the hypoxic treatment were larger (carapace length and width and plastron length) than normoxic hatchlings. Similarly, hypoxic hatchings also swam significantly faster than hyperoxic hatchlings. Considering larger hatchlings may have a greater chance of survival, the production of larger hatchings may offset the high cost (lower hatching success) when preovipositional arrest is prolonged. Hyperoxia does not appear to have deleterious consequences for development.

A model to predict the thermal reaction norm for the embryo growth rate from field data

The incubation of eggs is strongly influenced by temperature as observed in all species studied to date. For example, incubation duration, sexual phenotype, growth, and performances in many vertebrate hatchlings are affected by incubation temperature. Yet it is very difficult to predict temperature effect based on the temperature within a field nest, as temperature varies throughout incubation. Previous works used egg incubation at constant temperatures in the laboratory to evaluate the dependency of growtProd. Type: FTPh rate on temperature. However, generating such data is time consuming and not always feasible due to logistical and legislative constraints. This paper therefore presents a methodology to extract the thermal reaction norm for the embryo growth rate directly from a time series of incubation temperatures recorded within natural nests. This methodology was successfully applied to the nests of the marine turtle Caretta caretta incubated on Dalyan Beach in Turkey, although it can also be used for any egg-laying species, with some of its limitations being discussed in the paper. Knowledge about embryo growth patterns is also important when determining the thermosensitive period for species with temperature-dependent sex determination. Indeed, in this case, sexual phenotype is sensitive to temperature only during this window of embryonic development.

Potential sources of intra-population variation in painted turtle (Chrysemys picta) hatchling overwintering strategy

Many temperate animals spend half their lives in a non-active, overwintering state, and multiple adaptations have evolved to enable winter survival. One notable vertebrate model is Chrysemys picta whose hatchlings display dichotomous overwintering strategies: some hatchlings spend their first winter aquatically after nest emergence in fall, while others overwinter terrestrially within their natal nest with subsequent spring emergence. Occurrence of these strategies varies among populations and temporally within populations; however, factors that determine the strategy employed by a nest in nature are unknown. We examined potential factors that influence intra-population variation in C. picta hatchling overwintering strategy over two winters in Algonquin Park, Ontario. We found that environmental factors may be a trigger for hatchling overwintering strategy: fall-emerging nests were sloped towards the water and were surrounded by a relatively higher percentage of bare ground compared to spring-emerging nests. Fall-emerging hatchlings were also relatively smaller. Overwintering strategy was not associated with clutch oviposition sequence, or mammalian or avian predation attempts. Instead, fall emergence from the nest was associated with the direct mortality threat of predation by Sarcophagid fly larvae. Body condition and righting response, measured as proxies of hatchling fitness, did not differ between overwintering strategies. Costs and benefits of overwintering aquatically versus terrestrially in hatchling C. picta are largely unknown, and have the potential to affect population dynamics. Understanding winter survival has great implications for turtle ecology, thus we emphasize future research areas on dichotomous overwintering strategies in temperate hatchling turtles.

Evaluation of predator-exclusion cages used in turtle conservation: cost analysis and effects on nest environment and proxies of hatchling fitness

Context A main goal of conservation is to mitigate anthropogenic impacts on natural ecosystems, thus conservation tools themselves should not negatively affect target species. Predator-exclusion cages are effectively used to reduce predation of turtle nests; however, their effects on nest environment and developing hatchlings have not been examined. Aims Our study had the following four goals: (1) to examine effects of cages on the nest environment, (2) determine whether nest caging affects proxies for hatchling fitness, (3) evaluate whether nest predators preferentially interact with certain cage types, and (4) assess the cost-effectiveness of different nest caging designs. Methods In 2010 and 2011 in Algonquin Provincial Park, Ontario, painted turtle (Chrysemys picta; n = 93) and snapping turtle (Chelydra serpentina; n = 91) nests were assigned to one of three treatments (wooden-sided cages, above- and below-ground wire cages) or a control (no nest cage) and outfitted with a data logger to record incubation temperature. After emergence, hatching success and proxies of hatchling fitness were measured. Key results Nest temperature, hatching success, frequency of hatchling deformities and locomotor performance did not differ among cage treatments. However, hatchling body condition differed among treatments; wooden-sided and below-ground cages had the most positive influence on body condition in painted and snapping turtles, respectively. Predator interactions did not differ among treatments, and wooden-sided cages were the most inexpensive to construct. Conclusions Nest cages did not alter the nest environment from natural conditions but did alter hatchling body condition, and nest caging affected species differently. Implications Nest cages are known to reduce nest depredation, and our data indicated that, in general, nest cages also do not affect the nest environment or proxies for hatchling fitness. Thus, our findings indicated that cages are effective conservation tools that do not present secondary deleterious effects on potential recruitment.