Quantifying the effects of embryonic phenotypic plasticity on adult phenotypes in reptiles: A review of current knowledge and major gaps

Stressful Body Temperatures as a Maternal Effect on Lizard Reproduction

AbstractUnderstanding the relationship between the environment parents experience during reproduction and the environment embryos experience in the nest is essential for determining the intergenerational responses of populations to novel environmental conditions. Thermal stress has become commonplace for organisms inhabiting areas affected by rising temperatures. Exposure to body temperatures that approach, but do not exceed, upper thermal limits often induces adverse effects in organisms, but the propensity for these temperatures to have intergenerational consequences has not been explored in depth. Here, we quantified the effects of thermal stress on the reproductive physiology and development of brown anoles (Anolis sagrei) when thermal stress is experienced by mothers and by eggs during incubation. Mothers exposed to thermal stress produced smaller eggs and smaller offspring with reduced growth rates, while egg stress reduced developmental time and offspring mass. Hatchling survival and growth were negatively affected by thermal stress experienced by mothers but not by thermal stress experienced as eggs. We found mixed evidence for an additive effect of thermal stress on offspring; rather, thermal stress had specific (and most often negative) effects on different components of offspring phenotypes and fitness proxies when experienced either by mothers or by eggs. Stressful body temperatures therefore can function in a similar manner to other types of maternal effects in reptiles; however, this maternal effect has predominantly negative consequences on offspring.

Geographic variation in incubation temperatures promoting viable offspring production in broadly co-distributed turtles

Organisms whose early life stages are environmentally sensitive produce offspring within a relatively narrow range of suitable abiotic conditions. In reptiles, development rate and survival are often maximized if incubation temperatures remain under 31°C, though this upper bound may vary within and among species. We addressed this expectation by comparing responses to egg incubation at 30°C versus 33°C in congeneric turtle species pairs with broad syntopic geographic distributions. In the two softshell turtles (Apalone spp.), the greatest changes in development rate and phenotypic variance were observed in the northernmost population, which had a low survival rate (40%) at 33°C. The presumably suboptimal temperature (33°C) for northern populations otherwise yielded 76%-93% survival rates and fast swimming speeds in more southern populations. Still, in one species, northern hatchlings incubated at 33°C matched the elevated speeds of their southern counterparts, revealing a countergradient response. In northern populations of the two map turtles (Graptemys spp.), survival was also reduced (28%-60%) at 33°C and the development rate (relative to 30°C) increased by up to 75%. Our experiments on divergent taxa with similar nesting ecologies substantiate that the optimal thermal range for offspring production is variable. These findings encourage further work on how population- and species-level differences relate to local adaptation in widely distributed oviparous species.

Toward a universal measure of robustness across model organs and systems

The development of an individual must be capable of resisting the harmful effects of internal and external perturbations. This capacity, called robustness, can make the difference between normal variation and disease. Some systems and organs are more resilient in their capacity to correct the effects of internal disturbances such as mutations. Similarly, organs and organisms differ in their capacity to be resilient against external disturbances, such as changes in temperature. Furthermore, all developmental systems must be somewhat flexible to permit evolutionary change, and understanding robustness requires a comparative framework. Over the last decades, most research on developmental robustness has been focusing on specific model systems and organs. Hence, we lack tools that would allow cross-species and cross-organ comparisons. Here, we emphasize the need for a uniform framework to experimentally test and quantify robustness across study systems and suggest that the analysis of fluctuating asymmetry might be a powerful proxy to do so. Such a comparative framework will ultimately help to resolve why and how organs of the same and different species differ in their sensitivity to internal (e.g., mutations) and external (e.g., temperature) perturbations and at what level of biological organization buffering capacities exist and therefore create robustness of the developmental system.

Univariate and multivariate plasticity in response to incubation temperature in an Australian lizard

Environments, particularly developmental environments, can generate a considerable amount of phenotypic variation through phenotypic plasticity. Plasticity in response to incubation temperature is well characterised in egg-laying reptiles. However, traits do not always vary independently of one another, and studies encompassing a broad range of traits spanning multiple categories are relatively rare but crucial to better understand whole-organism responses to environmental change, particularly if covariation among traits may constrain plasticity. In this study, we investigated multivariate plasticity in response to incubation across three temperatures in the delicate skink, Lampropholis delicata, and whether this was affected by covariation among traits. At approximately 1 month of age, a suite of growth, locomotor performance, thermal physiology and behavioural traits were measured. Plasticity in the multivariate phenotype of delicate skinks was distinct for different incubation temperatures. Cool temperatures drove shifts in growth, locomotor performance and thermal physiology, while hot temperatures primarily caused changes in locomotor performance and behaviour. These differences are likely due to variation in thermal reaction norms, as there was little evidence that covariation among traits or phenotypic integration influenced plasticity, and there was no effect of incubation temperature on the direction or strength of covariation. While there were broad themes in terms of which trait categories were affected by different incubation treatments, traits appeared to be affected independently by developmental temperature. Comparing reaction norms of a greater range of traits and temperatures will enable better insight into these patterns among trait categories, as well as the impacts of environmental change.

Genome-wide DNA methylation patterns harbour signatures of hatchling sex and past incubation temperature in a species with environmental sex determination

Conservation of thermally sensitive species depends on monitoring organismal and population-level responses to environmental change in real time. Epigenetic processes are increasingly recognized as key integrators of environmental conditions into developmentally plastic responses, and attendant epigenomic data sets hold potential for revealing cryptic phenotypes relevant to conservation efforts. Here, we demonstrate the utility of genome-wide DNA methylation (DNAm) patterns in the face of climate change for a group of especially vulnerable species, those with temperature-dependent sex determination (TSD). Due to their reliance on thermal cues during development to determine sexual fate, contemporary shifts in temperature are predicted to skew offspring sex ratios and ultimately destabilize sensitive populations. Using reduced-representation bisulphite sequencing, we profiled the DNA methylome in blood cells of hatchling American alligators (Alligator mississippiensis), a TSD species lacking reliable markers of sexual dimorphism in early life stages. We identified 120 sex-associated differentially methylated cytosines (DMCs; FDR < 0.1) in hatchlings incubated under a range of temperatures, as well as 707 unique temperature-associated DMCs. We further developed DNAm-based models capable of predicting hatchling sex with 100% accuracy (in 20 training samples and four test samples) and past incubation temperature with a mean absolute error of 1.2°C (in four test samples) based on the methylation status of 20 and 24 loci, respectively. Though largely independent of epigenomic patterning occurring in the embryonic gonad during TSD, DNAm patterns in blood cells may serve as nonlethal markers of hatchling sex and past incubation conditions in conservation applications. These findings also raise intriguing questions regarding tissue-specific epigenomic patterning in the context of developmental plasticity.

Nesting in Anolis Lizards: An Understudied Topic in a Well-Studied Clade

Maternal nesting behavior in oviparous species strongly influences the environmental conditions their embryos experience during development. In turn, these early-life conditions have consequences for offspring phenotypes and many fitness components across an individual’s lifespan. Thus, identifying the evolutionary and ecological causes and effects of nesting behavior is a key goal of behavioral ecology. Studies of reptiles have contributed greatly to our understanding of how nesting behavior shapes offspring phenotypes. While some taxonomic groups have been used extensively to provide insights into this important area of biology, many groups remain poorly studied. For example, the squamate genus Anolis has served as a model to study behavior, ecology, and evolution, but research focused on Anolis nesting behavior and developmental plasticity is comparatively scarce. This dearth of empirical research may be attributed to logistical challenges (e.g., difficulty locating nests), biological factors (e.g., their single-egg clutches may hinder some experimental designs), and a historical focus on males in Anolis research. Although there is a gap in the literature concerning Anolis nesting behavior, interest in nesting ecology and developmental plasticity in this group has grown in recent years. In this paper, we (1) review existing studies of anole nesting ecology and developmental plasticity; (2) highlight areas of anole nesting ecology that are currently understudied and discuss how research in these areas can contribute to broader topics (e.g., maternal effects and global change biology); and (3) provide guidelines for studying anole nesting in the field. Overall, this review provides a foundation for establishing anoles as models to study nesting ecology and developmental 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.

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.

Bridging the explanatory gaps: What can we learn from a biological agency perspective?

We begin this article by delineating the explanatory gaps left by prevailing gene-focused approaches in our understanding of phenotype determination, inheritance, and the origin of novel traits. We aim not to diminish the value of these approaches but to highlight where their implementation, despite best efforts, has encountered persistent limitations. We then discuss how each of these explanatory gaps can be addressed by expanding research foci to take into account biological agency-the capacity of living systems at various levels to participate in their own development, maintenance, and function by regulating their structures and activities in response to conditions they encounter. Here we aim to define formally what agency and agents are and-just as importantly-what they are not, emphasizing that agency is an empirical property connoting neither intention nor consciousness. Lastly, we discuss how incorporating agency helps to bridge explanatory gaps left by conventional approaches, highlight scientific fields in which implicit agency approaches are already proving valuable, and assess the opportunities and challenges of more systematically incorporating biological agency into research programs.

A guide to incubate eggs of Tropidurus lizards under laboratory conditions

Studies in Evo-Devo benefit from the use of a variety of organisms, as comparative approaches provide a better understanding of Biodiversity and Evolution. Standardized protocols to incubate eggs and manipulate embryo development enable postulation of additional species as suitable biological systems for research in the field. In the past decades, vertebrate lineages such as Squamata (lizards, snakes, and amphisbaenians) emerged as crucial study systems for addressing topics as diverse as phenotypic evolution and climate change. However, protocols for maintaining gravid females and incubating eggs in the lab under experimental conditions are available to only a few squamate species. This resource article presents a simple incubation guide that standardizes conditions to maintain embryos of Tropidurus catalanensis (Squamata: Tropiduridae) under different experimental conditions, manipulating relevant environmental factors like temperature and humidity. We identified associated effects relating the egg incubation condition to developmental stage, incubation time, hatching success, and resulting morphotypes. Temperature and humidity play a key role in development and require attention when establishing the experimental design. Current literature comprises information for Tropidurus lizards that ponders how general in Squamata are the ecomorphs originally described for Anolis. Studies evaluating phenotypic effects of developmental environments suggest plasticity in some of the traits that characterize the ecomorphological associations described for this family. We expect that this incubation guide encourages future studies using Tropidurus lizards to address Evo-Devo questions.

Good alimentation can overcome the negative effects of climate change on growth in reptiles

Climate change may lead to higher nest temperatures, which may increase embryo development rate but reduce hatchling size and growth. Larger body size permits better performance, making growth an important fitness trait. In ectotherms, growth is affected by temperature and food quality. To segregate the effects of incubation temperature vs. alimentation on the growth of the Mexican black spiny-tailed iguana, Ctenosaura pectinata, we incubated eggs at 29 or 32 °C, and hatchlings were kept at 30 °C and fed either high- or low-quality food for 1 year, with body size and mass being recorded every 2 weeks. Iguanas incubated at 29 °C grew faster than those incubated at 32 °C. However, food quality had a larger effect on growth than incubation temperature; iguanas fed with high-quality food reached larger body sizes. Growth models suggested that differences in growth between incubation temperatures and food types remain throughout their lives. We found that incubation temperature had long-lasting effects on an ectotherm, and higher incubation temperatures might lead to reduced growth and maturation at a later age. However, food might transcend the effect of increased incubation temperature; therefore, good alimentation might mitigate effects of climate change on growth.

High temperatures limit developmental resilience to high-elevation hypoxia in the snake Natrix maura (Squamata: Colubridae)

Climate change is generating range shifts in many organisms, notably along the altitudinal gradient. However, moving up in altitude exposes organisms to lower oxygen availability, which may negatively affect development and fitness, especially at high temperatures. To test this possibility in a potentially upward-colonizing species, we artificially incubated developing embryos of the viperine snake Natrix maura Linnaeus 1758, using a split-clutch design, in conditions of extreme high elevation or low elevation at two ecologically-relevant incubation temperatures (24 and 32 °C). Embryos at low and extreme high elevations incubated at cool temperatures did not differ in development time, hatchling phenotype or locomotor performance. However, at the warmer incubation temperature and at extreme high elevation, hatching success was reduced. Further, embryonic heart rates were lower, incubation duration longer and juveniles born smaller. Nonetheless, snakes in this treatment were faster swimmers than siblings in other treatment groups, suggesting a developmental trade-off between size and performance. Constraints on development may be offset by the maintenance of important performance metrics, thus suggesting that early life-history stages will not prevent the successful colonization of high-elevation habitat even under the dual limitations of reduced oxygen and increased temperature.

Ecologically relevant thermal fluctuations enhance offspring fitness: biological and methodological implications for studies of thermal developmental plasticity

Natural thermal environments are notably complex and challenging to mimic in controlled studies. Consequently, our understanding of the ecological relevance and underlying mechanisms of organismal responses to thermal environments is often limited. For example, studies of thermal developmental plasticity have provided key insights into the ecological consequences of temperature variation, but most laboratory studies use treatments that do not reflect natural thermal regimes. While controlling other important factors, we compared the effects of naturally fluctuating temperatures with those of commonly used laboratory regimes on development of lizard embryos and offspring phenotypes and survival. We incubated eggs in four treatments: three that followed procedures commonly used in the literature, and one that precisely mimicked naturally fluctuating nest temperatures. To explore context-dependent effects, we replicated these treatments across two seasonal regimes: relatively cool temperatures from nests constructed early in the season and warm temperatures from late-season nests. We show that natural thermal fluctuations have a relatively small effect on developmental variables but enhance hatchling performance and survival at cooler temperatures. Thus, natural thermal fluctuations are important for successful development and simpler approximations (e.g. repeated sine waves, constant temperatures) may poorly reflect natural systems under some conditions. Thus, the benefits of precisely replicating real-world temperatures in controlled studies may outweigh logistical costs. Although patterns might vary according to study system and research goals, our methodological approach demonstrates the importance of incorporating natural variation into controlled studies and provides biologists interested in thermal ecology with a framework for validating the effectiveness of commonly used methods.

Sex-specific effects of developmental temperature on morphology, growth and survival of offspring in a lizard with temperature-dependent sex determination

The developmental environment plays a pivotal role in shaping fitness-relevant phenotypes of all organisms. Phenotypes are highly labile during embryogenesis, and environmental factors experienced early in development can have profound effects on fitness-relevant traits throughout life. Many reptiles exhibit temperature-dependent sex determination (TSD), whereby temperature during embryonic development permanently determines offspring sex. The leading hypothesis for the adaptive significance of TSD posits that egg incubation temperature differentially affects the fitness of males vs. females so that each sex is produced at its optimal temperature. The goal of this research is to address this hypothesis by quantifying the sex-specific effects of incubation temperature on phenotypes and survival in a lizard (Agama picticauda) with TSD. By incubating eggs under constant and fluctuating temperatures, we demonstrated that incubation temperature affects fitness-relevant phenotypes in A. picticauda; but males and females had similar reaction norms. However, females produced from female-biased incubation temperatures had greater survival than those from male-biased temperatures, and male survival was lowest for individuals produced from a female-biased temperature. In addition, eggs incubated at male-biased temperatures hatched earlier than those incubated at female-biased temperatures, which may have sex-specific consequences later in life as predicted by models for the adaptive significance of TSD.

Embryonic Temperature Programs Phenotype in Reptiles

Reptiles are critically affected by temperature throughout their lifespan, but especially so during early development. Temperature-induced changes in phenotype are a specific example of a broader phenomenon called phenotypic plasticity in which a single individual is able to develop different phenotypes when exposed to different environments. With climate change occurring at an unprecedented rate, it is important to study temperature effects on reptiles. For example, the potential impact of global warming is especially pronounced in species with temperature-dependent sex determination (TSD) because temperature has a direct effect on a key phenotypic (sex) and demographic (population sex ratios) trait. Reptiles with TSD also serve as models for studying temperature effects on the development of other traits that display continuous variation. Temperature directly influences metabolic and developmental rate of embryos and can have permanent effects on phenotype that last beyond the embryonic period. For instance, incubation temperature programs post-hatching hormone production and growth physiology, which can profoundly influence fitness. Here, we review current knowledge of temperature effects on phenotypic and developmental plasticity in reptiles. First, we examine the direct effect of temperature on biophysical processes, the concept of thermal performance curves, and the process of thermal acclimation. After discussing these reversible temperature effects, we focus the bulk of the review on developmental programming of phenotype by temperature during embryogenesis (i.e., permanent developmental effects). We focus on oviparous species because eggs are especially susceptible to changes in ambient temperature. We then discuss recent work probing the role of epigenetic mechanisms in mediating temperature effects on phenotype. Based on phenotypic effects of temperature, we return to the potential impact of global warming on reptiles. Finally, we highlight key areas for future research, including the identification of temperature sensors and assessment of genetic variation for thermosensitivity.

Genomics of Developmental Plasticity in Animals

Developmental plasticity refers to the property by which the same genotype produces distinct phenotypes depending on the environmental conditions under which development takes place. By allowing organisms to produce phenotypes adjusted to the conditions that adults will experience, developmental plasticity can provide the means to cope with environmental heterogeneity. Developmental plasticity can be adaptive and its evolution can be shaped by natural selection. It has also been suggested that developmental plasticity can facilitate adaptation and promote diversification. Here, we summarize current knowledge on the evolution of plasticity and on the impact of plasticity on adaptive evolution, and we identify recent advances and important open questions about the genomics of developmental plasticity in animals. We give special attention to studies using transcriptomics to identify genes whose expression changes across developmental environments and studies using genetic mapping to identify loci that contribute to variation in plasticity and can fuel its evolution.

Egg incubation temperature influences the growth and foraging behaviour of juvenile lizards

After laying their eggs, oviparous reptiles are reliant on the external environment to provide the required incubation conditions for successful embryonic development. Egg incubation temperature can impact the behaviour of various species of reptiles, but previous experiments have focused on the impact of incubation environment on hatchlings, with only a limited number of studies focussing on the longer-term behavioural consequences of incubation environment. This study investigated the effects of developmental environment on bearded dragon lizards (Pogona vitticeps) that were incubated at different temperatures within the natural range; half of them were incubated at a 'hot' temperature (30 ± 3 °C) and half at a 'cold' temperature (27 ± 3 °C). The growth and foraging behaviour of the lizards was then compared over 18 weeks of development. Although the lizards incubated at a cool temperatures grew more quickly, those incubated at the hotter temperature completed the foraging task more often and had significantly faster running speeds. These results show that egg incubation temperature impacts the foraging behaviour of juvenile lizards and suggest a potential trade-off between growth and foraging speed, which could influence an animal's life history trajectory.

Lizard nest environments differ between suburban and forest habitats

Nesting success is critical for oviparous species to maintain viable populations. Many species often do not provide parental care (e.g. oviparous reptiles), so embryos are left to develop in the prevailing conditions of the nest. For species that occupy diverse habitats, embryos must be able to complete development across a broad range of environmental conditions. Although much research has investigated how environmental conditions influence embryo development, we know little about how nest conditions differ between diverse habitats. Anolis lizards are commonly found in various habitats including those heavily modified by humans (e.g. cities). We describe nest sites of anoles in two different habitat types: a suburban area and a nearby forest. The suburban area had less total nesting habitat but a greater variety of microenvironment conditions for females to use for nesting, compared to the forest. Suburban nests were warmer and drier with greater thermal variance compared to forest nests. Finally, we use data from the literature to predict how nest conditions may influence development. Our study provides the first quantitative assessment of anole nest sites in human-modified environments and shows how suburban habitats may generate variation in developmental rate.

Transplanting gravid lizards to high elevation alters maternal and embryonic oxygen physiology, but not reproductive success or hatchling phenotype

Increased global temperatures have opened previously inhospitable habitats, such as at higher elevations. However, the reduction of oxygen partial pressure with increase in elevation represents an important physiological constraint that may limit colonization of such habitats, even if the thermal niche is appropriate. To test the mechanisms underlying the response to ecologically-relevant levels of hypoxia, we performed a translocation experiment with the common wall lizard (Podarcis muralis), a widespread European lizard amenable to establishing populations outside its natural range. We investigated the impacts of hypoxia on the oxygen physiology and reproductive output of gravid common wall lizards and the subsequent development and morphology of their offspring. Lowland females transplanted to high elevations increased their haematocrit and haemoglobin concentration within days and maintained routine metabolism compared to lizards kept at native elevations. However, transplanted lizards suffered from increased reactive oxygen metabolite production near the oviposition date, suggesting a cost of reproduction at high elevation. Transplanted females and females native to different elevations did not differ in reproductive output (clutch size, egg mass, relative clutch mass, or embryonic stage at oviposition) or in post-oviposition body condition. Developing embryos reduced heart rates and prolonged incubation times at high elevations within the native range and at extreme high elevations beyond the current range, but this reduced oxygen availability did not affect metabolic rate, hatching success, or hatchling size. These results suggest that this opportunistic colonizer is capable of successfully responding to novel environmental constraints in these important life-history stages.

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 .