Temperature-dependent Sex Determination Research Articles

Temperature regulates sex determination and growth in the paralichthid flatfishCalifornia halibut.

California halibut (Paralichthys californicus) is a candidate species for aquaculture and stock enhancement. These applications rely on sex control, either to maximize the production of faster growing females or to match sex ratios in the wild. Other paralichthids exhibit temperature-dependent sex determination (TSD), but the presence and pattern of TSD is not well defined in California halibut. Juvenile California halibut were cultured at three distinct temperatures (15°C, 19°C, and 23°C) through the developmental period presumed to be thermosensitive based on findings from congeners. Sex ratios were quantified in each treatment using phenotypic sex identification techniques applied early (molecular biomarkers; 51-100 mm total length [TL]) and late (visual examination of the gonads; ≥100 mm TL) in the juvenile phase. Both techniques indicated similar sex determination trends at each temperature, with overall sex ratios assessed as 49.9% maleat 15°C, 74.5% maleat 19°C, and 98.2% maleat 23°C. Growth rates were highest at 23°C and lowest at 15°C, with intrinsically fast- and slow-growing individuals at all temperatures. At 15°C and 19°C, females comprised a higher proportion among the fast growers than they did among the slow growers. These data show that California halibut exhibit TSD, with temperatures of 19°C and 23°C masculinizing fish while 15°C appears to produce a 1:1 sex ratio. This study will help optimize sex ratios and growth in hatcheries through thermal manipulation. Furthermore, the developed biomolecular tools and identified temperature thresholds will be important in future work to understand the influence of global warming on wild population demographics.

Screening of temperature-responsive signalling molecules during sex differentiation in Asian yellow pond turtle (Mauremys mutica)

BackgroundThe Asian yellow pond turtle (Mauremys mutica) is an important commercial freshwater aquaculture species in China. This species is a highly sexually dimorphic species, with males growing at a faster rate than females and exhibits temperature-dependent sex determination (TSD), in which the incubation temperature during embryonic development determines the sexual fate. However, the mechanisms of the sex determination or sex differentiation in the Asian yellow pond turtle are remain a mystery.ResultsTemperature-specific gonadal transcriptomics of the Asian yellow pond turtle were performed during the thermosensitive period (stage 15) using RNA-seq technology to identify candidate genes that initiate gonadal differentiation. We uncovered candidates that were the first to respond to temperature. These candidates were sexually dimorphic in expression, reflecting differences in gonadal (Cirbp, Runx1) and germline differentiation (Vasa, Nanos1, Piwil2), gametogenesis (Hmgb3, Zar1, Ovoinhibitor-like, Kif4), steroid hormone biosynthesis (Hsd17b5, Hsd17b6), heat shock (Dnajb6, Hsp90b1, Hsp90aa1) and transient receptor potential channel genes (Trpm1, Trpm4, Trpm6, Trpv1).ConclusionsOur work will provide important genetic information to elucidate the mechanisms of sex control in the Asian yellow pond turtles, and will contribute important genetic resources for further studies of temperature-dependent sex determination in turtles.

Environmental (and random?) sex determination in endangered and invasive Phelsuma geckos.

Sex is a fundamental characteristic of an individual. It is therefore puzzling why in some systems sex is precisely determined by a genotype, while in others it is influenced by the environment or even subtle, not to say random, factors. Some stochasticity in sex determination would be expected if environmental conditions did not have a large sex-specific effect on fitness. Although data are only available for a small fraction of species, geckos show exceptional variability in sex determination. We tested the effects of three incubation temperatures on sex ratio and adult body size in the invasive gecko Phelsuma laticauda and the vulnerable gecko Phelsuma nigristriata. We document temperature-dependent sex determination (TSD) in both species. Only females hatched at a low temperature (24 °C), whereas male production peaked at an intermediate temperature (29 °C) and declined, at least in P. laticauda, again at the highest temperature (31 °C). Interestingly, full siblings hatched from eggs glued together during the incubation at temperatures producing both sexes are often of the opposite sex. We found no significant effect of incubation temperature on adult body length. Documentation of TSD in the day geckos has implications for conservation practice in environmental management of endangered species or eradication of invasive species. However, it appears that a very subtle (random?) factor may also be involved in their sex determination. In line with this, we found no significant effect of incubation temperature on adult body length, suggesting that, at least in this trait, there is no strong selection against producing females at "male" temperatures.

Reconstructed and projected beach temperatures reveal where flatback turtles are most at risk from climate change

Nest temperature is the predominant driver of emergence success and primary sex ratios in sea turtles, with female offspring produced at higher temperatures due to temperature-dependent sex determination. However, emergence success and primary sex ratios are unfeasible to measure at scale, making methods to estimate these life-history traits from predicted sand temperatures highly desirable for long-term conservation planning in the context of climate change. To address this, we used a mechanistic microclimate model to predict hourly sand temperatures, over 32 nesting seasons since 1986, at 402 West Australian beaches supporting nesting by flatback turtles (Natator depressus). Predicted sand temperatures indicated that ∼70% of these beaches carried a 'very low' to 'intermediate' risk of subjecting embryos to thermal stress. By combining these temperature predictions with a physiological model, current and future emergence success and sex ratios were projected for ten different beaches spanning a range of thermal microclimates, under various climate change scenarios. Under recent climate conditions, emergence success averaged 76%, but declined to 63% and 37% with a 2°C and 4°C increase in air temperature, respectively. The sex ratios of hatchlings varied by location, but extremely skewed sex ratios were anticipated in a 4°C warming scenario. Our projections reveal that 'high risk' nesting beaches will regularly experience clutch failure as climate change progresses, while cooler beaches offer long-term nesting potential and require protection from additional anthropogenic impacts. These projections, covering an entire genetic stock, supply demographic data for assessing extinction risks and this method can be applied to sea turtle populations worldwide.

Assessment of Acetylcholinesterase Enzyme (AChE) Activity in the Developing Brain of TSD Lizard, Calotes Versicolor (Daud.)

Acetylcholinesterase (AChE) is a critical enzyme in the neuronal cholinergic system of vertebrates. It is one of the efficient cholinesterases that is involved in the termination of acetylcholine-mediated neurotransmission, rapidly hydrolyzing acetylcholine into acetate and choline. Barring a few reports on the distribution/presence of AChE enzyme activity in the adult reptilian species there are nil reports on the presence of this enzyme activity in the developing brain. Hence, the present investigation aims to evaluate AChE enzyme activity in the developing brain of an oviparous lizard, Calotes versicolor which exhibits a novel Female-Male-Female-Male (FMFM) pattern of temperature-dependent sex determination (TSD). The eggs were collected during breeding season, padded with moist cotton, and incubated in an incubator at 30o C. The whole brain from the embryos was collected during different developmental stages from oviposition (Stage 27) to hatching (stage 42). The AChE enzyme activity was quantified in the brain, according to Ellman’s protocol using a spectrophotometer. The findings reveal that the onset of AChE enzyme activity is observed as early as at oviposition, which reflects the early action of AChE activity in the developing brain. The observed results albeit indirectly suggest the involvement of AChE enzyme activity in the morphogenetic process. Further, an exponential increase in AChE activity during the post-gonadal differentiation phase indicates age-related elevation in AChE enzyme activity which in turn unveils its involvement in neuronal transmission of the embryonic brain. Besides, this specific AChE enzyme activity in the developing brain of Calotes versicolor represents a conserved pattern for the cholinergic system in vertebrates.

Mathematical modelling to assess the impact of stress on temperature-dependent sex determination in teleost fish

Mathematical modelling to assess the impact of stress on temperature-dependent sex determination in teleost fish

Sex Ratio Estimation of Green Turtle Hatchlings in Kazanlı Beach, Türkiye

Sea turtles are distributed worldwide, mostly in the tropical and subtropical waters. All seven species of sea turtles in the world have temperature-dependent sex determination. The pivotal temperature for hatchlings with a female:male ratio of 1:1 is usually around 29°C. Female hatchlings are observed at values higher than the pivotal temperature and male hatchlings at lower values. It is expected that wildlife threatened by global warming will be more severely affected, especially species that sex is determined by temperature. This study was carried out by measuring air, sand and nest temperatures during two nesting seasons at Kazanlı Beach, one of the largest nesting sites of Chelonia mydas (Green turtle) in the Mediterranean. Sand grain size was also measured, and gonads of dead hatchlings were examined histologically. Both sand (n=4) and nest (n=6) temperatures were above the pivotal temperature (between 29.1-31.6°C) in the study area. According to the estimation based on temperatures, hatchlings were female (♀>50%) biased. Of the hatchling samples, 75 were examined histologically. According to histological examination, the female:male ratio was 2:1. According to the direct examination and indirect estimation; the sex ratio of green turtle hatchlings in Kazanlı Beach, one of the highest hatchling producing nesting sites of the Mediterranean, was reported for the first time in this study. There is a need to fill the data gaps regarding sex ratios, especially for major beaches with high hatchling production. Knowing the temperature profiles in and around the nest and sex ratios of hatchlings is critical for the development of conservation measures for endangered sea turtles.

Feminization of a mixed-stock foraging aggregation of immature green turtles (Chelonia mydas), 1975–2018

Marine turtles exhibit temperature-dependent sex determination (TSD), and there is widespread concern that global warming is raising nest incubation temperatures, resulting in increasingly female-skewed sex ratios in “feminized” populations. We assessed the sex ratio of a mixed-stock aggregation of immature green turtles (Chelonia mydas) at a midocean developmental foraging ground in the Northwest Atlantic from 1975 to 2018. We used plasma testosterone concentrations, measured by radioimmunoassay (RIA) and calibrated by laparoscopy, to determine the sex of 2,724 green turtles captured 3,940 times in Bermuda (32°18′ N, 64°46′ W) waters. A logistic regression model correctly predicted the sex of 99.5% of turtles (189/190) with associated testosterone concentrations whose sex had been verified via laparoscopy. Empirical evaluation of the trend in sex ratio using four related data sets showed a significant increase (2.8–4.0% yr−1) in the sex ratio (F/M) over the course of the study. Using growth rates to predict the year of arrival of turtles in Bermuda, we estimated the sex ratio of recruiting cohorts over 4 decades. Mixed-stock analysis of mtDNA sequences of 602 turtles that recruited to Bermuda between 1970 and 2018 suggested that multiple, geographically dispersed rookeries contributed to the Bermuda aggregation making it regionally representative. Changes in rookery contributions and strong population increases at certain rookeries may partly explain the increasing trend in the sex ratio. But the steady rate of increase over decades and the increasing female percentage of arriving cohorts are consistent with impacts of global warming at source rookeries.

Exploring contaminants as a disruptor of temperature-dependent sex determination in sea turtle hatchlings

Sea turtle nesting beaches are experiencing increased sand temperatures as climate change progresses. In one major green turtle (Chelonia mydas) nesting beach in the northern Great Barrier Reef, over 99 percent of hatchlings are female. The effects of contaminants on sea turtle hatchling sex determination are not often explored. Liver samples were collected from green turtle hatchlings that were sacrificed for histological sex determination in a parallel study on the effects of sand cooling on sex ratios, and analysed for trace elements via acid digestion and organic contaminants via in vitro cytotoxicity bioassays. Chromium, antimony, barium, and cadmium have previously been demonstrated to be estrogenic, and concentrations of these elements were used to calculate three estrogenic indexes for each clutch: predicted relative estrogenic potency (PEEQA), the sum of percent trace elements above the median of all samples (TEOM), and the sum of percent estrogenic elements above the median of all samples (EstroEOM). Excluding an outlier clutch, cadmium, antimony, and EstroEOM had significant positive relationships with sex ratio deviation. Mean clutch cobalt, lead, antimony and barium, also had a significant positive relationship with clutch sex ratio. There was no relationship between in vitro cytotoxicity of liver extracts and sex ratio, however, 9% of hatchlings had organic contaminants high enough to suggest potential cellular damage. Contaminant effects on sex determination are likely to be caused by a mixture of contaminant interactions as well as temperature. Many trace elements detected in this study have also been linked to negative health effects on hatchlings in previous studies. Considering the risks of feminization due to climate change and potential contaminant effects on hatchling health and sex determination, future studies exploring contaminant effects on sea turtle hatchling sex determination are recommended.

Short-term resilience to climate-induced temperature increases for equatorial sea turtle populations.

Projection models are being increasingly used to manage threatened taxa by estimating their responses to climate change. Sea turtles are particularly susceptible to climate change as they have temperature-dependent sex determination and increased sand temperatures on nesting beaches could result in the 'feminisation' of hatchling sex ratios for some populations. This study modelled likely long-term trends in sand temperatures and hatchling sex ratios at an equatorial nesting site for endangered green turtles (Chelonia mydas) and critically endangered hawksbill turtles (Eretmochelys imbricata). A total of 1078 days of sand temperature data were collected from 28 logger deployments at nest depth between 2018 and 2022 in Papua New Guinea (PNG). Long-term trends in sand temperature were generated from a model using air temperature as an environmental proxy. The influence of rainfall and seasonal variation on sand temperature was also investigated. Between 1960 and 2019, we estimated that sand temperature increased by ~0.6°C and the average hatchling sex ratio was relatively balanced (46.2% female, SD = 10.7). No trends were observed in historical rainfall anomaliesand projections indicated no further changes to rainfall until 2100. Therefore, the sex ratio models were unlikely to be influenced by changing rainfall patterns. A relatively balanced sex ratio such as this is starkly different to the extremely female-skewed hatchling sex ratio (>99% female) reported for another Coral Sea nesting site, Raine Island (~850 km West). This PNG nesting site is likely rare in the global context, as it is less threatened by climate-induced feminisation. Although there is no current need for 'cooling' interventions, the mean projected sex ratios for 2020-2100 were estimated 76%-87% female, so future interventions may be required to increase male production. Our use of long-term sand temperature and rainfall trends has advanced our understanding of climate change impacts on sea turtles.

Olive Ridley Sea Turtle Incubation in Natural Conditions Is Possible on Guatemalan Beaches

The Guatemalan strategy for sea turtle conservation was defined by the National Council of Protected Areas (CONAP) in 1989. Hatcheries lie at the core of this strategy: egg collectors are allowed to deliver 20% of a nest to a hatchery in exchange for selling or eating the remaining eggs. Consequently, nearly 100% of nests are collected, with no nests being left on the beaches. Hatchery design promotes shading using roofs made from vegetation. The logic behind this recommendation is that the natural incubation of eggs is supposedly impossible due to the overly high temperatures on the beach. However, changing the incubation temperature of sea turtle eggs can profoundly alter the sex ratio in sea turtles with temperature-dependent sex determination. It can also modify the physiology or behavior of juvenile turtles. Here, we test whether incubation in natural conditions is possible on Guatemalan beaches, and for the first time, we determine the thermal reaction norm of embryo growth to ensure hatching success in sea turtles. We show that incubation in natural conditions is possible since three out of the four monitored nests produced hatchlings. We urge the Guatemala National Council of Protected Areas to reevaluate its strategy for sea turtle conservation in Guatemala in light of these results.

Differential early‐life survival underlies the adaptive significance of temperature‐dependent sex determination in a long‐lived reptile

Abstract Many ectotherms rely on temperature cues experienced during development to determine offspring sex. The first descriptions of temperature‐dependent sex determination (TSD) were made over 50 years ago, yet an understanding of its adaptive significance remains elusive, especially in long‐lived taxa. One novel hypothesis predicts that TSD should be evolutionarily favoured when two criteria are met—(a) incubation temperature influences annual juvenile survival and (b) sexes mature at different ages. Under these conditions, a sex‐dependent effect of incubation temperature on offspring fitness arises through differences in age at sexual maturity, with the sex that matures later benefiting disproportionately from temperatures that promote juvenile survival. The American alligator (Alligator mississippiensis) serves as an insightful model in which to test this hypothesis, as males begin reproducing nearly a decade after females. Here, through a combination of artificial incubation experiments and mark‐recapture approaches, we test the specific predictions of the survival‐to‐maturity hypothesis for the adaptive value of TSD by disentangling the effects of incubation temperature and sex on annual survival of alligator hatchlings across two geographically distinct sites. Hatchlings incubated at male‐promoting temperatures (MPTs) consistently exhibited higher survival compared to those incubated at female‐promoting temperatures. This pattern appears independent of hatchling sex, as females produced from hormone manipulation at MPT exhibit similar survival to their male counterparts. Additional experiments show that incubation temperature may affect early‐life survival primarily by affecting the efficiency with which maternally transferred energy resources are used during development. Results from this study provide the first explicit empirical support for the adaptive value of TSD in a crocodilian and point to developmental energetics as a potential unifying mechanism underlying persistent survival consequences of incubation temperature. Read the free Plain Language Summary for this article on the Journal blog.

Experiencing short heat waves early in development changes thermal responsiveness of turtle embryos to later heat waves.

Although physiological responses to the thermal environment are most frequently investigated using constant temperatures, the incorporation of thermal variability can allow for a more accurate prediction of how thermally sensitive species respond to a rapidly changing climate. In species with temperature-dependent sex determination (TSD), developmental responses to incubation temperature are mediated by several genes involved in gonadal differentiation. Kdm6b and Dmrt1 respond to cool incubation temperatures and are associated with testis development, while Foxl2 and Cyp19A1 respond to warm incubation temperatures and are associated with ovary development. Using fluctuating incubation temperatures, we designed two studies, one investigating how conflicting thermal cues affect the timing of commitment to gonadal development, and another investigating the rapid molecular responses to conflicting thermal cues in the red-eared slider turtle (Trachemys scripta). Using gene expression as a proxy of timing of commitment to gonadal fate, results from the first study show that exposure to high amounts of conflicting thermal cues during development delays commitment to gonadal fate. Results from the second study show that Kdm6b splice variants exhibit differential responses to early heat wave exposure, but rapidly (within 2 days) recover to pre-exposure levels after the heat wave. Despite changes in the expression of Kdm6b splice variants, there was no effect on Dmrt1 expression. Collectively, these findings demonstrate how short exposures to heat early in development can change how embryos respond to heat later in development.

Testosterone identifies hatchling sex for Mojave desert tortoises (Gopherus agassizii)

The threatened Mojave desert tortoise (Gopherus agassizii) exhibits temperature-dependent sex determination, and individuals appear externally sexually monomorphic until sexual maturity. A non-surgical sex identification method that is suitable for a single in situ encounter with hatchlings is essential for minimizing handling of wild animals. We tested (1) whether plasma testosterone quantified by enzyme-linked immunosorbent assay differentiated males from females in 0–3 month old captive hatchlings, and (2) whether an injection of follicle-stimulating hormone (FSH) differentially elevates testosterone in male hatchlings to aid in identifying sex. We validated sex by ceolioscopic (laparoscopic) surgery. We then fit the testosterone concentrations to lognormal distributions and identified the concentration below which individuals are more likely female, and above which individuals are more likely male. Using a parametric bootstrapping procedure, we estimated a 0.01–0.04% misidentification rate for naïve testosterone samples, and a 1.26–1.39% misidentification rate for challenged (post-FSH injection) testosterone samples. Quantification of plasma testosterone concentration from small volume (0.1 mL) blood samples appears to be a viable, highly accurate method to identify sex of 0–3 month old hatchlings and could be a valuable tool for conservation measures and investigation of trends and variation in sex ratios for in situ wild nests.

Comparative analysis of gonadal transcriptomes between turtle and alligator identifies common molecular cues activated during the temperature-sensitive period for sex determination

The mode of sex determination in vertebrates can be categorized as genotypic or environmental. In the case of genotypic sex determination (GSD), the sexual fate of an organism is determined by the chromosome composition with some having dominant genes, named sex-determining genes, that drive the sex phenotypes. By contrast, many reptiles exhibit environmental sex determination (ESD), whereby environmental stimuli drive sex determination, and most notably temperature. To date, temperature-dependent sex determination (TSD) has been found in most turtles, some lizards, and all crocodylians, but commonalities in the controlling processes are not well established. Recent innovative sequencing technology has enabled investigations into gonadal transcriptomic profiles during temperature-sensitive periods (TSP) in various TSD species which can help elucidate the controlling mechanisms. In this study, we conducted a time-course analysis of the gonadal transcriptome during the male-producing temperature (26℃) of the Reeve’s turtle (Chinese three-keeled pond turtle) Mauremys reevesii. We then compared the transcriptome profiles for this turtle species during the TSP with that for the American alligator Alligator mississippiensis to identify conserved reptilian TSD-related genes. Our transcriptome-based findings provide an opportunity to retrieve the candidate molecular cues that are activated during TSP and compare these target responses between TSD and GSD turtle species, and between TSD species.

Sexually dimorphic expression of AMH facilitates testis differentiation pathway in the tropical lizard, Calotes versicolor (Daud.)

The Anti-mullerian hormone (AMH), also known as Mullerian inhibiting substance (MIS), is a glycoprotein that belongs to transforming growth factor β superfamily. The significance of AMH during gonadal differentiation is not clearly deciphered in reptiles. Hence, current study aims to know the onset of AMH secretion and its functional role in Mullerian duct regression gonadal differentiation in tropical lizard, Calotes versicolor which exhibits a novel Female-Male-Female-Male (FMFM) pattern of temperature-dependent sex determination (TSD). The Immunohistochemistry and qRT-PCR techniques were employed to analyze the gonadal expression profile of AMH during different stages of embryonic development. The eggs of the lizard were incubated at both male-producing temperature (MPT: 25.5 ± 0.5 °C) and female-producing temperatures (FPT: 31.5 ± 0.5 °C). The results reveal that the onset of AMH gene expression was observed as early as oviposition prior to the immunolocalization of AMH protein at early-TSP (Temperature-sensitive period). The substantial rise in the intensity of the immunoreaction of AMH protein in the cytoplasm confining to Sertoli cells of seminiferous cords at MPT with low level of expression at FPT during gonadal sex differentiation, specify sexually dimorphic expression of AMH protein. Further, with the onset of sexual differentiation, the developing testis immensely expresses AMH gene which is 7-fold greater than that of transcripts levels in female embryos; signifies its conserved role in Mullerian duct regression thereby promoting testis differentiation. The robust immunnoexpression of AMH protein during post-gonadal differentiation coincides with the onset of the regression of Mullerian duct point out a positive correlation between testis differentiation and Mullerian duct regression, thus facilitating testis differentiation pathway. Based on the immunoexpression pattern of AMH protein and transcript levels of AMH gene, it is inferred that AMH plays a significant role in Mullerian duct regression, favoring testis differentiation.

Sex-Linked Loci on the W Chromosome in the Multi-Ocellated Racerunner (Eremias multiocellata) Confirm Genetic Sex-Determination Stability in Lacertid Lizards.

The multi-ocellated racerunner, Eremias multiocellata, was considered to have temperature-dependent sex determination (TSD), as its sex ratio can be influenced at different temperatures. However, such an observation contrasts with recent findings that suggest TSD is less common than previously thought. Here, a genotyping-by-sequencing (GBS) approach was employed to identify sex-linked markers in the E. multiocellata, for which the mechanism choice of TSD or GSD is still controversial. We preliminarily identified 119 sex-linked markers based on sex-associated sex-specific sequences, 97% of which indicated female heterogamety. After eliminating the false positives, 38 sex-linked markers were recognized, all of which showed the ZW/ZZ system. Then, eight of the novel markers were verified by PCR amplification from 15 populations of E. multiocellata, which support the GSD in E. multiocellata without geographic variation. To test the conservation of sex chromosome in Eremias, the eight markers were further cross-tested by PCR amplification in 10 individuals of the Mongolian racerunner (Eremias argus), two of which exhibited cross-utility. The novel sex-linked markers could be mapped on the W chromosome of the sand lizard (Lacerta agilis). Our finding that the sex-linked markers are shared in closely related species, along with a conserved synteny of the W chromosome, further supports the homology and conservation of sex chromosomes in the lacertid lizards.

Higher temperatures directly increase germ cell number, promoting feminization of red-eared slider turtles.

In many reptile species, gonadal sex is affected by environmental temperature during a critical period of embryonic development-a process known as temperature-dependent sex determination (TSD).1 The oviparous red-eared slider turtle, Trachemys scripta, has a warm-female/cool-male TSD system and is among the best-studied members of this group.2 When incubated at low temperatures, the somatic cells of the bipotential gonad differentiate into Sertoli cells, the support cells of the testis, whereas at high temperatures, they differentiate into granulosa cells, the support cells of the ovary.3 Here, we report the unexpected finding that temperature independently affects the number of primordial germ cells (GCs) in the embryonic gonad at a time before somatic cell differentiation has initiated. Specifically, embryos incubated at higher, female-inducing temperatures have more GCs than those incubated at the male-inducing temperature. Furthermore, elimination of GCs in embryos incubating at intermediate temperatures results in a strong shift toward male-biased sex ratios. This is the first evidence that temperature affects GC number and the first evidence that GC number influences sex determination in amniotes. This observation has two important implications. First, it supports a new model in which temperature can impact sex determination in incremental ways through multiple cell types. Second, the findings have important implications for a major unresolved question in the fields of ecology and evolutionary biology-the adaptive significance of TSD. We suggest that linking high GC number with female development improves female reproductive potential and provides an adaptive advantage for TSD.

Metabolic consequences of sex reversal in two lizard species: a test of the like-genotype and like-phenotype hypotheses.

Vertebrate sex is typically determined genetically, but in many ectotherms sex can be determined by genes (genetic sex determination, GSD), temperature (temperature-dependent sex determination, TSD), or interactions between genes and temperature during development. TSD may involve GSD systems with either male or female heterogamety (XX/XY or ZZ/ZW) where temperature overrides chromosomal sex determination to cause a mismatch between genetic sex and phenotypic sex (sex reversal). In these temperature-sensitive lineages, phylogenetic investigations point to recurrent evolutionary shifts between genotypic and temperature-dependent sex determination. These evolutionary transitions in sex determination can occur rapidly if selection favours the reversed sex over the concordant phenotypic sex. To investigate the consequences of sex reversal on offspring phenotypes, we measured two energy-driven traits (metabolism and growth) and 6month survival in two species of reptile with different patterns of temperature-induced sex reversal. Male sex reversal occurs in Bassiana duperreyi when chromosomal females (female XX) develop male phenotypes (maleSR XX), while female sex reversal occurs in Pogona vitticeps when chromosomal males (male ZZ) develop female phenotypes (femaleSR ZZ). We show metabolism in maleSR XX was like that of male XY; that is, reflective of phenotypic sex and lower than genotypic sex. In contrast, for Pogona vitticeps, femaleSR ZZ metabolism was intermediate between male ZZ and female ZW metabolic rate. For both species, our data indicate that differences in metabolism become more apparent as individuals become larger. Our findings provide some evidence for an energetic advantage from sex reversal in both species but do not exclude energetic processes as a constraint on the distribution of sex reversal in nature.

HerpSexDet: the herpetological database of sex determination and sex reversal

Wildlife exhibits various sex-determination systems where sex chromosomes and environmental temperatures may both contribute to individual sexual development. The causes and consequences of this variability are important questions for evolutionary ecology, especially in light of ongoing environmental change. Amphibians and reptiles are emerging as a key group for studying these questions, with new data accumulating acceleratingly. We collected empirical data from earlier databases, reviews and primary literature to create the most up-to-date database on herpetological sex determination. We named our database HerpSexDet, which currently features data on genetic and temperature-dependent sex determination as well as reports on sex reversal for a total of 192 amphibian and 697 reptile species. This dataset, which we will regularly update in the future, facilitates interspecific comparative studies on the evolution of sex determination and its consequences for species-specific traits such as life history and conservation status, and may also help guiding future research by identifying species or higher taxa that are potentially most enlightening for the study of environmentally driven sex reversal.