Variation in thermal traits describing sex determination and development in Western Australian sea turtle populations

Understanding the role of environmental temperature on sex determination through comparative studies in reptiles and amphibians

In vertebrates, species exhibit phenotypic plasticity of sex determination that the sex can plastically be determined by the external environmental temperature through a mechanism, temperature-dependent sex determination (TSD). Temperature exerts influence over the direction of sexual differentiation pathways, resulting in distinct primary sex ratios in a temperature-dependent manner. This review provides a summary of the thermal sensitivities associated with sex determination in reptiles and amphibians, with a focus on the pattern of TSD, gonadal differentiation, temperature sensing, and the molecular basis underlying thermal sensitivity in sex determination. Comparative studies across diverse lineages offer valuable insights into comprehending the evolution of sex determination as a phenotypic plasticity. While evidence of molecular mechanisms governing sexual differentiation pathways continues to accumulate, the intracellular signaling linking temperature sensing and sexual differentiation pathways remains elusive. We emphasize that uncovering these links is a key for understanding species-specific thermal sensitivities in TSD and will contribute to a more comprehensive understanding of ecosystem and biodiversity conservations.

Can a present-day thermal niche be preserved in a warming climate by a shift in phenology? A case study with sea turtles

How species respond to climate change may impact their extinction probability. Here we link climatology and ecology to tackle a globally important conservation question. For sea turtles, there are concerns that climate warming will cause both the feminization of populations as well as reduced hatchling survival. For 58 nesting sites across the world spanning all seven sea turtle species, we investigated whether warming might be avoided by shifts in nesting phenology to a cooler part of the year. We show that even with the most extreme phenological shift that has been reported to date-an 18-day advance in nesting per °C increase in sea surface temperature (SST)-temperatures will continue to increase at nesting sites with climate warming. We estimate that SST at nesting sites will rise by an average of 0.6°C (standard deviation = 0.9°C, n = 58) when we model a 1.5°C rise in SST combined with a best-case-scenario shift in nesting. Since sea turtles exhibit temperature-dependent sex determination, these temperature rises could lead to increasingly female-biased sex ratios as well as reduced hatchling production at sites across the world. These findings underscore concerns for the long-term survival of this iconic group.

How well do embryo development rate models derived from laboratory data predict embryo development in sea turtle nests?

Development rate of ectothermic animals varies with temperature. Here we use data derived from laboratory constant temperature incubation experiments to formulate development rate models that can be used to model embryonic development rate in sea turtle nests. We then use a novel method for detecting the time of hatching to measure the in situ incubation period of sea turtle clutches to test the accuracy of our models in predicting the incubation period from nest temperature traces. We found that all our models overestimated the incubation period. We hypothesize three possible explanations which are not mutually exclusive for the mismatch between our modeling and empirically measured in situ incubation period: (1) a difference in the way the incubation period is calculated in laboratory data and in our field nests, (2) inaccuracies in the assumptions made by our models at high incubation temperatures where there is no empirical laboratory data, and (3) a tendency for development rate in laboratory experiments to be progressively slower as temperature decreases compared with in situ incubation.

We determined the hatching time in sea turtle nests and compared those with hatching times predicted from nest temperature traces. We found that nest temperature traces overestimated hatching time.

Latitudinal Variation in the Pattern of Temperature-Dependent Sex Determination in the Japanese Gecko, Gekko japonicus

Simple Summary

In egg-laying lizards, sex is determined by genetic factors in species with sex chromosomes or egg incubation temperatures in species without sex chromosomes, i.e., temperature-dependent sex determination (TSD). Surprisingly, recent studies find sex chromosomes and TSD co-occur in the same species in some lizards. The Japanese gecko from Japan may be this case. However, Japanese gecko with TSD from a Chinese population does not have sex chromosomes, suggesting that the pattern of TSD in this gecko may vary among populations. We incubated gecko eggs from three populations in China at constant temperatures of 24, 26, 28, 30, and 32 °C to quantify the sex determination pattern. We found that the temperature yielding an equal number of sons and daughters of the low-latitude population was lower than that of the two high-latitude populations. Moreover, the low-latitude population had a narrower temperature range producing mixed sex offspring at lower temperatures, but a wider range at higher temperatures. Sex ratio was almost 1:1 for the low-latitude population when incubated from 26 to 30 °C. Conversely, more male offspring were produced at 28 or 30 °C in the two high-latitude populations. Our study may provide an interesting system to explore the evolution of sex determination mechanisms in animals.

Abstract

Identifying latitudinal variation in the pattern of temperature-dependent sex determination (TSD) may provide insight into the evolution of sex determining system in vertebrates, but such studies remain limited. Here, we quantified TSD patterns of three geographically separated populations of the Japanese gecko (Gekko japonicus) along the latitudinal cline of China. We incubated gecko eggs from the three populations at constant temperatures of 24, 26, 28, 30, and 32 °C to quantify the TSD pattern. Our study demonstrated that G. japonicus exhibited a FMF pattern of TSD, with the low and high incubation temperatures yielding significantly female-biased hatchlings, and the medium temperatures producing male-biased hatchlings. More interestingly, we found latitudinal variations in the TSD pattern in terms of pivotal temperatures (T_pivs), transitional range of temperatures (TRT), and the sex ratios at the medium temperatures. The T_pivs for the low-latitude population were lower than those for the two high-latitude populations. The low-latitude population has a narrower FM TRT, but a wider MF TRT. The sex ratio is almost 50:50 for the low-latitude population when eggs were incubated from 26 to 30 °C. Conversely, the sex ratio is male-biased for the two high-latitude populations at 28 or 30 °C. Therefore, G. japonicus may provide an interesting system to explore the evolution of TSD in reptiles given the diversity of TSD patterns among populations.

Reconstructing past thermal conditions in beach microclimates

Reconstruction of past conditions provides important information on how ecosystems have been impacted by climate change, but generally for microhabitats worldwide there are no long-term empirical measurements. In these cases, there has been protracted debate about how various large-scale environmental proxies can best be used to reconstruct local temperatures. Here we help resolve this debate by examining how well environmental proxies hindcast sand temperatures at nest depths for five sea turtle nesting sites across the world. We link instrumental air temperature and sea surface temperature records with empirical sand temperature observations in the Atlantic (Ascension Island and Cape Verde), the Indian Ocean (Chagos Archipelago), the Caribbean (St Eustatius) and the Pacific (French Polynesia). We found strong correlations between sea surface temperatures, air temperatures and sand temperatures at all our study sites. Furthermore, Granger causality testing shows variations in sea surface temperature and air temperature precede variations in sand temperatures. We found that different proxies (air or sea temperature or a combination of both) predicted mean monthly sand temperatures within <0.5°C of empirical observations. Reconstructions of sand temperatures over the last 170 years reveal a slight warming of temperatures (maximum 0.5°C per century). An analysis of 36 published datasets revealed that the gradient of the relationship between sand temperature and air temperature is relatively constant, suggesting long-term changes in sand temperature could be extended around the world to include nesting sites where there are no empirical measurements of sand temperature. Our approaches are likely to have utility for a range of microhabitats where there is an interest in long-term changes in temperature.

Effects of global warming on species with temperature-dependent sex determination: Bridging the gap between empirical research and management

Global warming could threaten over 400 species with temperature-dependent sex determination (TSD) worldwide, including all species of sea turtle. During embryonic development, rising temperatures might lead to the overproduction of one sex and, in turn, could bias populations' sex ratios to an extent that threatens their persistence. If climate change predictions are correct, and biased sex ratios reduce population viability, species with TSD may go rapidly extinct unless adaptive mechanisms, whether behavioural, physiological or molecular, exist to buffer these temperature-driven effects. Here, we summarize the discovery of the TSD phenomenon and its still elusive evolutionary significance. We then review the molecular pathways underpinning TSD in model species, along with the hormonal mechanisms that interact with temperatures to determine an individual's sex. To illustrate evolutionary mechanisms that can affect sex determination, we focus on sea turtle biology, discussing both the adaptive potential of this threatened TSD taxon, and the risks associated with conservation mismanagement.

Metabolic Rates and Thermal Thresholds of Embryonic Flatback Turtles (Natator depressus) from the North West Shelf of Australia

AbstractNest microclimates influence embryonic development and survival in many lineages, including reptiles with temperature-dependent sex determination. These microclimates are dependent on physical drivers and biological processes, such as embryonic metabolism, that generate heat. The flatback turtle (Natator depressus) has among the largest hatchlings of the seven extant sea turtle species, making it an excellent candidate for quantifying the contribution of embryonic metabolism to the nest microclimate. Consequently, we measured embryonic metabolic rates, development rates, and the relationship between temperature and sex determination for a N. depressus population nesting at Cemetery Beach in Western Australia, a mainland beach characterized by high sand temperatures. Total oxygen consumed at 29.5°C during an average 52-d incubation period was 2,622 mL, total carbon dioxide produced was 1,886 mL, and estimated embryonic heat production reached 38 mW at 90% of development. Adjustment of metabolic rates to 32°C and 34°C increased peak heat production by 18% and 27%, respectively. The pivotal temperature (T_PIV) producing an equal sex ratio was 30.3°C, mixed sexes were produced between 29.3°C and 31.2°C, and only females were produced above 31.2°C. The T_PIV was similar (within 0.2°C) to that of an island rookery within the same genetic stock (North West Shelf), but the peak development rate (2.5% d^-1) was estimated to be achieved at a temperature ~2.5°C higher (34.7°C) than the island rookery. Our results add to a growing consensus that thermal thresholds vary among sea turtle populations, even within the same genetic stock. Furthermore, we show that metabolic heat will have an appreciable impact on the nest microclimate, which has implications for embryonic survival and fitness under a future climate with warmer sand temperatures.

Population Viability of Sea Turtles in the Context of Global Warming

Sea turtles present a model for the potential impacts of climate change on imperiled species, with projected warming generating concern about their persistence. Various sea turtle life-history traits are affected by temperature; most strikingly, warmer egg incubation temperatures cause female-biased sex ratios and higher embryo mortality. Predictions of sea turtle resilience to climate change are often focused on how resulting male limitation or reduced offspring production may affect populations. In the present article, by reviewing research on sea turtles, we provide an overview of how temperature impacts on incubating eggs may cascade through life history to ultimately affect population viability. We explore how sex-specific patterns in survival and breeding periodicity determine the differences among offspring, adult, and operational sex ratios. We then discuss the implications of skewed sex ratios for male-limited reproduction, consider the negative correlation between sex ratio skew and genetic diversity, and examine consequences for adaptive potential. Our synthesis underscores the importance of considering the effects of climate throughout the life history of any species. Lethal effects (e.g., embryo mortality) are relatively direct impacts, but sublethal effects at immature life-history stages may not alter population growth rates until cohorts reach reproductive maturity. This leaves a lag during which some species transition through several stages subject to distinct biological circumstances and climate impacts. These perspectives will help managers conceptualize the drivers of emergent population dynamics and identify existing knowledge gaps under different scenarios of predicted environmental change.

Feminization of hawksbill turtle hatchlings in the twenty-first century at an important regional nesting aggregation

Projected climate change is forecasted to have significant effects on biological systems worldwide. Marine turtles in particular may be vulnerable, as the sex of their offspring is determined by their incubating temperature, termed temperature-dependent sex determination. This study aimed to estimate historical, and forecast future, primary sex ratios of hawksbill turtle Eretmochelys imbricata hatchlings at an important nesting ground in northeastern Qatar. Incubation temperatures from the Arabian/Persian Gulf were measured over 2 nesting seasons. Climate data from same period were regressed with nest temperatures to estimate incubation temperatures and hatchling sex ratios for the site from 1993 to 2100. Future hatchling sex ratios were estimated for 2 climate forecasts, one mid-range (SSP245) and one extreme (SSP585). Historical climate data showed female-biased sex ratios of 73.2 ± 12.1% from 1993 to 2017. Female biases from 2018 to 2100 averaged 85.7% ± 6.7% under the mid-range scenario and 87.9% ± 5.4% under the high-range scenario. In addition, predicted female hatchling production was >90% from 2054 and 2052 for SSP245 and SSP585, respectively. These results show that hawksbill primary sex ratios in Qatar are at risk of significant feminization by the year 2100 and that hawksbill turtle incubation temperatures in an extreme, understudied environment are already comparable to those predicted in tropical rookeries during the latter half of the 21st century. These results can help conservationists predict primary sex ratios for hawksbill turtles in the region in the face of 21st-century climate change.

Predicting the effects of climate change on incubation in reptiles: methodological advances and new directions

The unprecedented advancement of global climate change is affecting thermal conditions across spatial and temporal scales. Reptiles with temperature-dependent sex determination (TSD) are uniquely vulnerable to even fine-scale variation in incubation conditions and are a model system for investigating the impacts of shifting temperatures on key physiological and life-history traits. The ways in which current and predicted future climatic conditions translate from macro- to ultra-fine scale temperature traces in subterranean nests is insufficiently understood. Reliably predicting the ways in which fine-scale, daily and seasonally fluctuating nest temperatures influence embryonic development and offspring phenotypes is a goal that remains constrained by many of the same logistical challenges that have persisted throughout more than four decades of research on TSD. However, recent advances in microclimate and developmental modeling should allow us to move farther away from relatively coarse metrics with limited predictive capacity and towards a fully mechanistic model of TSD that can predict incubation conditions and phenotypic outcomes for a variety of reptile species across space and time and for any climate scenario.