The physiological ecology of reptilian eggs and embryos, and the evolution of viviparity within the class reptilia

Convergent Evolution of Pregnancy in Vertebrates

Viviparity (live birth) represents a significant evolutionary innovation that has emerged in hundreds of lineages of invertebrate and vertebrate animals. The evolution of this trait from the ancestral state of egg laying has involved complex morphological, behavioral, physiological, and genetic changes, which enable internal development of embryos within the female reproductive tract. Comparable changes have also occurred in oviparous, brooding species that carry developing embryos in locations other than the female reproductive tract. This review explores the taxonomic distribution of vertebrate viviparity and brooding (collectively termed pregnancy), discusses the adaptations associated with internal incubation, and examines hypotheses surrounding the evolution of pregnancy in different lineages. Understanding the mechanisms that have led to the emergence of this trait can illuminate questions about the evolution of reproductive complexity and the processes that led to the emergence of evolutionary innovations that have shaped the remarkable diversity of Earth's fauna.

A synthetic review: natural history of amniote reproductive modes in light of comparative evolutionary genomics

There is a current lack of consensus on whether the ancestral parity mode was oviparity (egg-laying) or viviparity (live-birth) in amniotes and particularly in squamates (snakes, lizards, and amphisbaenids). How transitions between parity modes occur at the genomic level has primary importance for how science conceptualises the origin of amniotes, and highly variable parity modes in Squamata. Synthesising literature from medicine, poultry science, reproductive biology, and evolutionary biology, I review the genomics and physiology of five broad processes (here termed the 'Main Five') expected to change during transitions between parity modes: eggshell formation, embryonic retention, placentation, calcium transport, and maternal-fetal immune dynamics. Throughout, I offer alternative perspectives and testable hypotheses regarding proximate causes of parity mode evolution in amniotes and squamates. If viviparity did evolve early in the history of lepidosaurs, I offer the nucleation site hypothesis as a proximate explanation. The framework of this hypothesis can be extended to amniotes to infer their ancestral state. I also provide a mechanism and hypothesis on how squamates may transition from viviparity to oviparity and make predictions about the directionality of transitions in three species. After considering evidence for differing perspectives on amniote origins, I offer a framework that unifies (i) the extended embryonic retention model and (ii) the traditional model which describes the amniote egg as an adaptation to the terrestrial environment. Additionally, this review contextualises the origin of amniotes and parity mode evolution within Medawar's paradigm. Medawar posited that pregnancy could be supported by immunosuppression, inertness, evasion, or immunological barriers. I demonstrate that this does not support gestation or gravidity across most amniotes but may be an adequate paradigm to explain how the first amniote tolerated internal fertilization and delayed egg deposition. In this context, the eggshell can be thought of as an immunological barrier. If serving as a barrier underpins the origin of the amniote eggshell, there should be evidence that oviparous gravidity can be met with a lack of immunological responses in utero. Rare examples of two species that differentially express very few genes during gravidity, suggestive of an absent immunological reaction to oviparous gravidity, are two skinks Lampropholis guichenoti and Lerista bougainvillii. These species may serve as good models for the original amniote egg. Overall, this review grounds itself in the historical literature while offering a modern perspective on the origin of amniotes. I encourage the scientific community to utilise this review as a resource in evolutionary and comparative genomics studies, embrace the complexity of the system, and thoughtfully consider the frameworks proposed.

Extraembryonic Membranes and Placentation in the Mexican Snake Conopsis lineata

Extraembryonic membranes provide protection, oxygen, water, and nutrients to developing embryos, and their study generates information on the origin of the terrestrial egg and the evolution of viviparity. In this research, the morphology of the extraembryonic membranes and the types of placentation in the viviparous snake Conopsis lineata are described through optical microscopy during early and late gestation. When embryos develop inside the uterus, they become surrounded by a thin eggshell membrane. In early gestation, during stages 16 and 18, the embryo is already surrounded by the amnion and the chorion, and in a small region by the chorioallantois, which is product of the contact between the chorion and the growing allantois. A trilaminar omphalopleure covers the yolk sac from the embryonic hemisphere to the level of the equator where the sinus terminalis is located, and from there a bilaminar omphalopleure extends into the abembryonic hemisphere. Thus, according to the relationship of these membranes with the uterine wall, the chorioplacenta, the choriovitelline placenta, and the chorioallantoic placenta are structured at the embryonic pole, while the omphaloplacenta is formed at the abembryonic pole. During late gestation (stages 35, 36, and 37), the uterus and allantois are highly vascularized. The allantois occupies most of the extraembryonic coelom and at the abembryonic pole, it contacts the omphaloplacenta and form the omphalallantoic placenta. This is the first description of all known placenta types in Squamata for a snake species member of the subfamily Colubrinae; where an eggshell membrane with 2.9 μm in width present throughout development is also evident. The structure of extraembryonic membranes in C. lineata is similar to that of other oviparous and viviparous squamate species. The above indicates not only homology, but also that the functional characteristics have been maintained throughout the evolution of the reproductive type.

Complexity in the timing of the first postnatal ecdysis in snakes

Lepidosaurian reptiles, particularly snakes, periodically shed the outer epidermal layers of their skin (ecdysis) to restore or enhance vital functions such as regulating water and gaseous exchange, growth, and protection against insult, infection or physical injury. Although many studies have focused on the nature and mechanisms of skin shedding, little attention has been paid to the timing of the first ecdysis in neonates following birth or hatching. A recent study investigated patterns of the time to first postnatal ecdysis in snakes based on a large dataset taken from the literature. The analysis demonstrated patterns in the time to first postnatal ecdysis related to phylogeny as well as several life history traits. While this assessment provides important advances in our knowledge of this topic, data on known biophysical drivers of ecdysis - temperature and humidity - were largely unavailable and were not evaluated. The first postnatal ecdysis of neonatal snakes can be viewed as an adaptive adjustment to the transition from the aqueous environment of the embryo to the aerial environment of the newborn. Hence, the timing of the first postnatal ecdysis is logically influenced by the aerial environment into which a newborn snake or hatchling finds itself. Therefore, in this Commentary, we first emphasize the putative plasticity of ecdysis with respect to epidermal lipids that structure the water permeability barrier and are established or renewed during ecdysis to reduce transepidermal evaporative water loss. We then discuss the likely importance of biophysical variables as influential covariates that need future investigation as potential co-determinants of the timing of first postnatal ecdysis.

The impact of climate change on sea turtles: Current knowledge, scientometrics, and mitigation strategies

Sea turtles are one of the most significant groups of marine species, playing a key role in the sustainability and conservation of marine ecosystems and the food chain. These emblematic species are threatened by several natural and anthropogenic pressures, and climate change is increasingly reported as one of the most important threats to sea turtles, affecting sea turtles at all stages of their life cycle and at both their marine and coastal habitats. The effect of climate change is expressed as global warming, sea-level rise, extreme storms, and alterations in predation and diseases' patterns, posing a potentially negative impact on sea turtles. In this systematic review, the author presented the current knowledge and research outcomes on the impact of climate change on sea turtles. Moreover, this study determined trends and hotspots in keywords, country collaborations, authors, and publications in the field through a scientometric analysis. Finally, this article reviewed proposed mitigation strategies by researchers, marine protected area (MPA) managers, and non-governmental organizations (NGOs) to reduce the impact of climate change on the conservation of sea turtles.

Embryonic and juvenile snakes (Natrix maura, Linnaeus 1758) compensate for high elevation hypoxia via shifts in cardiovascular physiology and metabolism

The colonization of novel environments requires a favorable response to conditions never, or rarely, encountered in recent evolutionary history. For example, populations colonizing upslope habitats must cope with lower atmospheric pressure at elevation, and thus reduced oxygen availability. The embryo stage in oviparous organisms is particularly susceptible, given its lack of mobility and limited gas exchange via diffusion through the eggshell and membranes. Especially little is known about responses of Lepidosaurian reptiles to reduced oxygen availability. To test the role of physiological plasticity during early development in response to high elevation hypoxia, we performed a transplant experiment with the viperine snake (Natrix maura, Linnaeus 1758). We maintained gravid females originating from low elevation populations (432 m above sea level [ASL]-normoxia) at both the elevation of origin and high elevation (2877 m ASL-extreme high elevation hypoxia; approximately 72% oxygen availability relative to sea level), then incubated egg clutches at both low and high elevation. Regardless of maternal exposure to hypoxia during gestation, embryos incubated at extreme high elevation exhibited altered developmental trajectories of cardiovascular function and metabolism across the incubation period, including a reduction in late-development egg mass. This physiological response may have contributed to the maintenance of similar incubation duration, hatching success, and hatchling body size compared to embryos incubated at low elevation. Nevertheless, after being maintained in hypoxia, juveniles exhibit reduced carbon dioxide production relative to oxygen consumption, suggesting altered energy pathways compared to juveniles maintained in normoxia. These findings highlight the role of physiological plasticity in maintaining rates of survival and fitness-relevant phenotypes in novel environments.

The structure of the eggshell and eggshell membranes of Crocodylus niloticus

The macro- and microstructure, elemental composition, and crystallographic characteristics of the eggshell and eggshell membranes of the Crocodylus niloticus egg was investigated using optical and electron microscopy, energy-dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD) and computerised tomography. The translucent ellipsoid egg is composed of two basic layers, the outer calcified layer referred to as the shell and an inner organic fibre layer, referred to as the shell membrane. The outer inorganic calcite shell is further divided into an external, palisade and mammillary layers with pore channels traversing the shell. The external layer is a thin layer of amorphous calcium and phosphorus, the underlying palisade layer consist of irregular wedge-shaped crystals composed calcite with traces of magnesium, sodium, sulphur and phosphorus. The crystals are mostly elongated, orientated perpendicular to the shell surface ending in cone-shaped knobs, which forms the inner mammillary layer. The elemental composition of the mammillae is like that of the palisade layer, but the crystal structure is much smaller and orientated randomly. The highest number of mammillae and shell pores are found at the equator of the egg, becoming fewer towards the egg poles. The shell thickness follows the same pattern, with the thickest area located at the equator. The eggshell membrane located right beneath and embedded in the mammillary layer of the shell; it is made up of unorganised fibre sheets roughly orientated at right angles to one another. Individual fibres consist of numerous smaller fibrils forming open channels that run longitudinally through the fibre.

Comparative transcriptomic analysis and genome-wide characterization of the Semaphorin family reveal the potential mechanism of angiogenesis around embryo in ovoviviparous black rockfish (Sebastes schlegelii)

To guarantee the quality and survival rate of their offspring, ovoviviparous teleost evolved special characteristics of in vivo fertilization and embryo development. Maternal black rockfish, having over 50 thousand embryos developing within the ovary simultaneously, provided around 40% nutrition throughout oocyte development, while the capillaries around each embryo contributed the rest 60% during pregnancy. Since fertilization, capillaries started to proliferate and developed into a placenta-like structure that covered over half of each embryo. Aimed to characterize the potential mechanism behind, comparative transcriptome analysis of samples collected according to the process of pregnancy. Three important time point in the process, including mature oocyte stage, fertilization and sarcomere period, were chosen for the transcriptome sequencing. Our study identified key pathways and genes involved in the cell cycle as well as DNA replication and repair, cell migration and adhesion, immune, and metabolic functions. Notably, several of the semaphoring gene family members were differently expressed. To confirm the accuracy of these genes, total of 32 sema genes were identified from the whole genome and distinct expression pattern of sema genes was observed in different pregnant stages. Our results revealed a novel insight for further investigating the functions of sema genes in reproduction physiology and embryo processes in ovoviviparous teleost.

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

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

No evidence for cold-adapted life-history traits in cool-climate populations of invasive cane toads (Rhinella marina)

As an invasive organism spreads into a novel environment, it may encounter strong selective pressures to adapt to abiotic and biotic challenges. We examined the effect of water temperature during larval life on rates of survival and growth of the early life-history stages of cane toads (Rhinella marina) from two geographic regions (tropical vs. temperate) in the species’ invaded range in eastern Australia. If local adaptation at the southern (cool-climate) invasion front has extended the cold-tolerance of early life-stages, we would expect to see higher viability of southern-population toads under cooler conditions. Our comparisons revealed no such divergence: the effects of water temperature on rates of larval survival and growth, time to metamorphosis, size at metamorphosis and locomotor performance of metamorphs were similar in both sets of populations. In two cases where tropical and temperate-zone populations diverged in responses to temperature, the tropical animals performed better at low to medium temperatures than did conspecifics from cooler regions. Adaptation to low temperatures in the south might be constrained by behavioural shifts (e.g., in reproductive seasonality, spawning-site selection) that allow toads to breed in warmer water even in cool climates, by gene flow from warmer-climate populations, or by phylogenetic conservatism in these traits.

Macro- and Microhabitat Predictors of Nest Success and Hatchling Survival in Eastern Box Turtles (Terrapene carolina carolina) and Spotted Turtles (Clemmys guttata) in Oak Savanna Landscapes

Differing selection pressures on stationary nest contents compared to mobile offspring mean that the nest-site characteristics resulting in the highest nest success may not be the same characteristics that result in the highest survival of juveniles from those nests. In such cases, maternal nest-site choice may optimize productivity overall by selecting nest sites that balance opposing pressures on nest success and juvenile survival, rather than maximizing survival of either the egg or the juvenile stage. Determining which macro- and microhabitat characteristics best predict overall productivity is critical for ensuring that land management activities increase overall recruitment into a population of interest, rather than benefiting one life stage at the inadvertent expense of another. We characterized nest-site choice at the macro- and microhabitat scale, and then quantified nest success and juvenile survival to overwintering in two declining turtle species, eastern box turtles and spotted turtles, that co-occur in oak savanna landscapes of northwestern Ohio and southern Michigan. Nest success in box turtles was higher in nests farther from macrohabitat edges, constructed later in the year, and at greater total depths. In contrast, survival of juvenile box turtles to overwintering was greater from nests under less shade cover and at shallower total depths. Spotted turtle nest success and juvenile survival were so high that we were unable to detect relationships between nest-site characteristics and the small amount of variation in survival. Our results demonstrate, at least for eastern box turtles, a tradeoff in nest depth between favoring nest success vs. juvenile survival to overwintering. We suggest that heterogeneity in microhabitat structure within nesting areas is important for allowing female turtles to both exercise flexibility in nest-site choice to match nest-site characteristics to prevailing weather conditions, and to place nests in close proximity to habitat that will subsequently be used by hatchlings for overwintering.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

DISCUSSION

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

Developmental programming of DNA methylation and gene expression patterns is associated with extreme cardiovascular tolerance to anoxia in the common snapping turtle

Background

Environmental fluctuation during embryonic and fetal development can permanently alter an organism’s morphology, physiology, and behaviour. This phenomenon, known as developmental plasticity, is particularly relevant to reptiles that develop in subterranean nests with variable oxygen tensions. Previous work has shown hypoxia permanently alters the cardiovascular system of snapping turtles and may improve cardiac anoxia tolerance later in life. The mechanisms driving this process are unknown but may involve epigenetic regulation of gene expression via DNA methylation. To test this hypothesis, we assessed in situ cardiac performance during 2 h of acute anoxia in juvenile turtles previously exposed to normoxia (21% oxygen) or hypoxia (10% oxygen) during embryogenesis. Next, we analysed DNA methylation and gene expression patterns in turtles from the same cohorts using whole genome bisulfite sequencing, which represents the first high-resolution investigation of DNA methylation patterns in any reptilian species.

Results

Genome-wide correlations between CpG and CpG island methylation and gene expression patterns in the snapping turtle were consistent with patterns observed in mammals. As hypothesized, developmental hypoxia increased juvenile turtle cardiac anoxia tolerance and programmed DNA methylation and gene expression patterns. Programmed differences in expression of genes such as SCN5A may account for differences in heart rate, while genes such as TNNT2 and TPM3 may underlie differences in calcium sensitivity and contractility of cardiomyocytes and cardiac inotropy. Finally, we identified putative transcription factor-binding sites in promoters and in differentially methylated CpG islands that suggest a model linking programming of DNA methylation during embryogenesis to differential gene expression and cardiovascular physiology later in life. Binding sites for hypoxia inducible factors (HIF1A, ARNT, and EPAS1) and key transcription factors activated by MAPK and BMP signaling (RREB1 and SMAD4) are implicated.

Conclusions

Our data strongly suggests that DNA methylation plays a conserved role in the regulation of gene expression in reptiles. We also show that embryonic hypoxia programs DNA methylation and gene expression patterns and that these changes are associated with enhanced cardiac anoxia tolerance later in life. Programming of cardiac anoxia tolerance has major ecological implications for snapping turtles, because these animals regularly exploit anoxic environments throughout their lifespan.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-021-00414-7.

Elevation, oxygen, and the origins of viviparity

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

Phylogeny and evolutionary history of the amniote egg

We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell, and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition, and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order, and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid-filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates-the amniote egg presenting one "solution" that enabled the conquest of land for reproduction.

Natural nest substrates influence squamate embryo physiology but have little effect on hatchling phenotypes

Vertebrate embryos require access to water; however, many species nest in terrestrial habitats that vary considerably in moisture content. Oviparous, non-avian reptiles have served as models to understand how environmental factors, like moisture availability, influence development because eggs are often exposed to prevailing environments in the absence of parental care. Though much research demonstrates the importance of water absorption by eggs, many ecological factors that influence moisture availability in natural nests have received little attention. For example, the type of substrate in which nests are constructed is understudied. We experimentally incubated eggs of the brown anole lizard (Anolis sagrei) in 2 naturally occurring nest substrates that were treated with varying amounts of water to determine how natural substrates influence development at different moisture concentrations. One substrate consisted of sand and crushed seashells and the other was mostly organic material (i.e. decayed plant material). Both are common nesting substrates at our field site. When controlling for water uptake by eggs, we found that egg survival and hatchling phenotypes were similar between substrates; however, embryos developed more quickly in the sand/shell substrate than the organic substrate, indicating substrate-specific effects on embryo physiology. These results demonstrate that different natural substrates can result in similar developmental outcomes if the water available to eggs is comparable; however, some aspects of development, like developmental rate, are affected by the type of substrate, independent of water availability. Further study is required to determine how natural substrates influence embryo physiology independent of water content.

Climate change and marine turtles: recent advances and future directions

Climate change is a threat to marine turtles that is expected to affect all of their life stages. To guide future research, we conducted a review of the most recent literature on this topic, highlighting knowledge gains and research gaps since a similar previous review in 2009. Most research has been focussed on the terrestrial life history phase, where expected impacts will range from habitat loss and decreased reproductive success to feminization of populations, but changes in reproductive periodicity, shifts in latitudinal ranges, and changes in foraging success are all expected in the marine life history phase. Models have been proposed to improve estimates of primary sex ratios, while technological advances promise a better understanding of how climate can influence different life stages and habitats. We suggest a number of research priorities for an improved understanding of how climate change may impact marine turtles, including: improved estimates of primary sex ratios, assessments of the implications of female-biased sex ratios and reduced male production, assessments of the variability in upper thermal limits of clutches, models of beach sediment movement under sea level rise, and assessments of impacts on foraging grounds. Lastly, we suggest that it is not yet possible to recommend manipulating aspects of turtle nesting ecology, as the evidence base with which to understand the results of such interventions is not robust enough, but that strategies for mitigation of stressors should be helpful, providing they consider the synergistic effects of climate change and other anthropogenic-induced threats to marine turtles, and focus on increasing resilience.

Congenital Malformations in Sea Turtles: Puzzling Interplay between Genes and Environment

Simple Summary

Congenital malformations can lead to embryonic mortality in many species, and sea turtles are no exception. Genetic and/or environmental alterations occur during early development in the embryo, and may produce aberrant phenotypes, many of which are incompatible with life. Causes of malformations are multifactorial; genetic factors may include mutations, chromosomal aberrations, and inbreeding effects, whereas non-genetic factors may include nutrition, hyperthermia, low moisture, radiation, and contamination. It is possible to monitor and control some of these factors (such as temperature and humidity) in nesting beaches, and toxic compounds in feeding areas, which can be transferred to the embryo through their lipophilic properties. In this review, we describe possible causes of different types of malformations observed in sea turtle embryos, as well as some actions that may help reduce embryonic mortality.

Abstract

The completion of embryonic development depends, in part, on the interplay between genetic factors and environmental conditions, and any alteration during development may affect embryonic genetic and epigenetic regulatory pathways leading to congenital malformations, which are mostly incompatible with life. Oviparous reptiles, such as sea turtles, that produce numerous eggs in a clutch that is buried on the beach provide an opportunity to study embryonic mortality associated with malformations that occur at different times during development, or that prevent the hatchling from emerging from the nest. In sea turtles, the presence of congenital malformations frequently leads to mortality. A few years ago, a detailed study was performed on external congenital malformations in three species of sea turtles from the Mexican Pacific and Caribbean coasts, the hawksbill turtle, Eretmochelys imbricata (n = 23,559 eggs), the green turtle, Chelonia mydas (n = 17,690 eggs), and the olive ridley, Lepidochelys olivacea (n = 20,257 eggs), finding 63 types of congenital malformations, of which 38 were new reports. Of the three species, the olive ridley showed a higher incidence of severe anomalies in the craniofacial region (49%), indicating alterations of early developmental pathways; however, several malformations were also observed in the body, including defects in the carapace (45%) and limbs (33%), as well as pigmentation disorders (20%), indicating that deviations occurred during the middle and later stages of development. Although intrinsic factors (i.e., genetic mutations or epigenetic modifications) are difficult to monitor in the field, some environmental factors (such as the incubation temperature, humidity, and probably the status of feeding areas) are, to some extent, less difficult to monitor and/or control. In this review, we describe the aetiology of different malformations observed in sea turtle embryos, and provide some actions that can reduce embryonic mortality.

Different solutions lead to similar life history traits across the great divides of the amniote tree of life

Amniote vertebrates share a suite of extra-embryonic membranes that distinguish them from anamniotes. Other than that, however, their reproductive characteristics could not be more different. They differ in basic ectothermic vs endothermic physiology, in that two clades evolved powered flight, and one clade evolved a protective shell. In terms of reproductive strategies, some produce eggs and others give birth to live young, at various degrees of development. Crucially, endotherms provide lengthy parental care, including thermal and food provisioning—whereas ectotherms seldom do. These differences could be expected to manifest themselves in major differences between clades in quantitative reproductive traits. We review the reproductive characteristics, and the distributions of brood sizes, breeding frequencies, offspring sizes and their derivatives (yearly fecundity and biomass production rates) of the four major amniote clades (mammals, birds, turtles and squamates), and several major subclades (birds: Palaeognathae, Galloanserae, Neoaves; mammals: Metatheria and Eutheria). While there are differences between these clades in some of these traits, they generally show similar ranges, distribution shapes and central tendencies across birds, placental mammals and squamates. Marsupials and turtles, however, differ in having smaller offspring, a strategy which subsequently influences other traits.

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

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

Additive effects of temperature and water availability on pregnancy in a viviparous lizard

One of the greatest current threats to biodiversity is climate change. However, understanding of organismal responses to fluctuations in temperature and water availability is currently lacking, especially during fundamental life-history stages such as reproduction. To further explore how temperature and water availability impact maternal physiology and reproductive output, we used the viviparous form of the European common lizard (Zootoca vivipara) in a two-by-two factorial design manipulating both hydric and thermal conditions, for the first time. We collected blood samples and morphological measurements during early pregnancy and post-parturition to investigate how water availability, temperature and a combination of the two influence maternal phenology, morphology, physiology and reproductive output. We observed that dehydration during gestation negatively affects maternal physiological condition (lower mass gain, higher tail reserve mobilization) but has little effect on reproductive output. These effects are mainly additive to temperature regimes, with a proportional increase in maternal costs in warmer environments. Our study demonstrates the importance of considering combined effects of water and temperature when investigating organismal responses to climate changes, especially during periods crucial for species survival such as reproduction.

Effect of early thermal environment on the morphology and performance of a lizard species with bimodal reproduction

Early developmental environments influence life-history traits and survival of reptiles. In fact, rising global temperatures have already caused widespread extinctions among lizards. Viviparous species might be more susceptible to increasing temperatures because of their inability to meet their energetic demands following rapid environmental changes. Nevertheless, we do not yet fully understand how lizards with different reproductive modes can respond to climate change. We investigated the effect of both maternal thermal environment during pregnancy and incubation temperature on hatchling morphology and physiological performance of two populations of the lizard Saiphos equalis differing in their mode of reproduction, to test whether reproductive mode affects the ability to buffer against rising temperatures. Gravid females from both populations were subjected to current or projected end-of-century (future) thermal environments, to evaluate differences in the body size, growth rate, thermal preference, and locomotor performance of their offspring. Our results show that independently of the mode of reproduction, high temperatures accelerated gestation periods. Thermal environments did not affect hatchling thermal preference, but viviparous hatchlings consistently preferred lower temperatures. Unlike viviparous lizards, oviparous hatchlings incubated under future temperatures were smaller and had a lower growth rate compared to current-incubated hatchlings. Finally, thermal environments did not affect hatchling endurance and speed when controlling for body size. Our results show that global warming is likely to have a negative impact on S. equalis, but suggest that some of its effects may be ameliorated by maternal responses during pregnancy, particularly in viviparous populations.

Climate Change and Green Sea Turtle Sex Ratio-Preventing Possible Extinction

Climate change poses a threat to species with temperature-dependent sex determination (TSD). A recent study on green sea turtles (Chelonia mydas) at the northern Great Barrier Reef (GBR) showed a highly female-skewed sex ratio with almost all juvenile turtles being female. This shortage of males might eventually cause population extinction, unless rapid evolutionary rescue, migration, range shifts, or conservation efforts ensure a sufficient number of males. We built a stochastic individual-based model inspired by C. mydas but potentially transferrable to other species with TSD. Pivotal temperature, nest depth, and shading were evolvable traits. Additionally, we considered the effect of crossbreeding between northern and southern GBR, nest site philopatry, and conservation efforts. Among the evolvable traits, nest depth was the most likely to rescue the population, but even here the warmer climate change scenarios led to extinction. We expected turtles to choose colder beaches under rising temperatures, but surprisingly, nest site philopatry did not improve persistence. Conservation efforts promoted population survival and did not preclude trait evolution. Although extra information is needed to make reliable predictions for the fate of green sea turtles, our results illustrate how evolution can shape the fate of long lived, vulnerable species in the face of climate change.

Incubation environment and parental identity affect sea turtle development and hatchling phenotype

For reptiles, the incubation environment experienced by embryos during development plays a major role in many biological processes. The unprecedented rate of climate change makes it critical to understand the effects that the incubation environment has on developing embryos, particularly in imperiled species such as chelonians. Consequently, a number of studies have focused on the effects of different environmental conditions on several developmental processes and hatchling phenotypic traits. In addition to the incubation environment, it is also essential to understand how parental contributions can influence hatchling quality. This is the first study that investigates the effects of parental origin and incubation conditions on sea turtle embryonic development and hatchling phenotype in nests incubating in the field (rather than under controlled laboratory conditions). Here, we used the loggerhead sea turtle (Caretta caretta) to investigate the effects of parental origin (clutch), incubation temperature, and the nest hydric environment on embryonic growth, incubation durations, hatching success, and hatchling phenotype. Our results show that nest moisture and temperature affect embryo mass towards the last third of development, with hatchling size positively correlated with nest moisture content, and maternal origin had a strong impact on hatching success and hatchling size regardless of the incubation conditions. The results from this experiment identify multiple factors that affect turtle embryonic development under field incubation conditions, a fundamental consideration when interpreting the potential impacts of climate change on reptilian development.

Genomic and transcriptomic investigations of the evolutionary transition from oviparity to viviparity

Viviparous (live-bearing) vertebrates have evolved repeatedly within otherwise oviparous (egg-laying) clades. Over two-thirds of these changes in vertebrate reproductive parity mode happened in squamate reptiles, where the transition has happened between 98 and 129 times. The transition from oviparity to viviparity requires numerous physiological, morphological, and immunological changes to the female reproductive tract, including eggshell reduction, delayed oviposition, placental development for supply of water and nutrition to the embryo by the mother, enhanced gas exchange, and suppression of maternal immune rejection of the embryo. We performed genomic and transcriptomic analyses of a closely related oviparous-viviparous pair of lizards (Phrynocephalus przewalskii and Phrynocephalus vlangalii) to examine these transitions. Expression patterns of maternal oviduct through reproductive development of the egg and embryo differ markedly between the two species. We found changes in expression patterns of appropriate genes that account for each of the major aspects of the oviparity to viviparity transition. In addition, we compared the gene sequences in transcriptomes of four oviparous-viviparous pairs of lizards in different genera (Phrynocephalus, Eremias, Scincella, and Sphenomorphus) to look for possible gene convergence at the sequence level. We discovered low levels of convergence in both amino acid replacement and evolutionary rate shift. This suggests that most of the changes that produce the oviparity-viviparity transition are changes in gene expression, so occasional reversals to oviparity from viviparity may not be as difficult to achieve as has been previously suggested.

Facultative mobilization of eggshell calcium promotes embryonic growth in an oviparous snake

The mineralized eggshell of Reptilia was a major innovation in the evolution of the amniotic egg. Inorganic components strengthen the eggshell and are a potential source of nutrients to developing embryos. Embryos of oviparous reptiles do extract calcium from eggshells but vary interspecifically in exploitation of this resource. The pattern of embryonic calcium nutrition of the corn snake, Pantherophis guttatus, is similar to a diversity of squamate species; embryos obtain most calcium from yolk, yet also mobilize calcium from the eggshell. We tested the hypothesis that embryonic development is not dependent on eggshell calcium by manipulating calcium availability. We peeled away the outer calcareous layer of the eggshell of recently oviposited eggs; control eggs were left intact. Eggs were sampled periodically and calcium content of egg compartments (embryo, yolk, eggshell) was measured. We also analyzed skeletal development and size of hatchlings. There was no difference in survivorship or length of incubation between treatments. However, hatchlings from intact eggs contained more calcium and were larger in mass and length than siblings from peeled eggs. There were no observable differences in ossification but hatchlings from intact eggs had larger skeletal elements (skull, vertebrae). Our results indicate that mobilization of eggshell calcium is not a requirement for embryonic development of P. guttatus and that embryos augment yolk calcium by extracting calcium from the eggshell. This pattern of embryonic calcium nutrition would favor embryos with a greater capacity to mobilize calcium from the eggshell by promoting growth and thereby potentially enhance hatchling fitness.

Introduction to the special issue-Developmental plasticity in reptiles: Physiological mechanisms and ecological consequences

Scientific interest in developmental plasticity spans many disciplines, and research on reptiles has provided many insights into this field. We highlight these contributions, review the field's history, and introduce the special issue on this topic .

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.

Use of field-portable ultrasonography reveals differences in developmental phenology and maternal egg provisioning in two sympatric viviparous snakes

A thorough understanding of the life cycles underlying the demography of wild species is limited by the difficulty of observing hidden life‐history traits, such as embryonic development. Major aspects of embryonic development, such as the rate and timing of development, and maternal–fetal interactions can be critical features of early‐life fitness and may impact population trends via effects on individual survival. While information on development in wild snakes and lizards is particularly limited, the repeated evolution of viviparity and diversity of reproductive mode in this clade make it a valuable subject of study. We used field‐portable ultrasonography to investigate embryonic development in two sympatric garter snake species, Thamnophis sirtalis and Thamnophis elegans in the Sierra Nevada mountains of California. This approach allowed us to examine previously hidden reproductive traits including the timing and annual variation in development and differences in parental investment in young. Both species are viviparous, occupy similar ecological niches, and experience the same annual environmental conditions. We found that T. sirtalis embryos were more developmentally advanced than T. elegans embryos during June of three consecutive years. We also found that eggs increased in volume more substantially across developmental stages in T. elegans than in T. sirtalis, indicating differences in maternal provisioning of embryos via placental transfer of water. These findings shed light on interspecific differences in parental investment and timing of development within the same environmental context and demonstrate the value of field ultrasonography for pursuing questions relating to the evolution of reproductive modes, and the ecology of development.

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.

Crocodilian perivitelline membrane-bound sperm detection

Advanced reproductive technologies (ART's) are often employed with various taxa to enhance captive breeding programs and maintain genetic diversity. Perivitelline membrane-bound (PVM-bound) sperm detection has previously been demonstrated in avian and chelonian species as a useful technique for breeding management. In the absence of embryotic development within an egg, this technique can detect the presence of sperm trapped on the oocyte membrane confirming breeding, male reproductive status, and pair compatibility. PVM-bound sperm were successfully detected in three clutches of Cuban crocodile (Crocodylus rhombifer) eggs at the Smithsonian's National Zoological Park (NZP) for the first time in any crocodilian species. PVM-bound sperm were detected in fresh and incubated C. rhombifer eggs, as well as eggs that were developing (banded) and those that were not (not banded). The results of this study showed significant differences in average sperm densities per egg between clutches (p = 0.001). Additionally, there was not a significant difference within clutches between eggs that banded and those that did not band (Clutch A, p = 0.505; Clutch B, p = 0.665; Clutch C, p = 0.266). The results of this study demonstrate the necessity to microscopically examine eggs that do not develop (do not band), to determine if sperm is present, which can help animal managers problem solve reproductive shortcomings. PVM-bound sperm detection could be a useful technique in assessing crocodilian breeding programs, as well as have potential uses in studies assessing sperm storage, artificial insemination, and artificial incubation.