Is regulation of aromatase expression in reptiles the key to understanding temperature-dependent sex determination?

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.

Thermal Response of Circulating Estrogens in an Emydid Turtle, Chrysemys picta, and the Challenges of Climate Change

Maternal hormones such as estrogens deposited into the yolk of turtle eggs follow circulating levels in adult females, and they may alter the sexual fate of developing embryos in species with temperature-dependent sex determination (TSD). In temperate regions, this deposition occurs during the spring when estrogens increase in adult females as ambient temperatures rise, drop after the first clutch, and peak again (albeit less) in the fall. Global warming alters turtle nesting phenology (inducing earlier nesting), but whether it affects circulating hormones remains unknown, hindering our understanding of all potential challenges posed by climate change and the adaptive potential (or lack thereof) of turtle populations. Here, we addressed this question in painted turtles (Chrysemys picta) by quantifying estradiol, estrone, and testosterone via mass spectrometry in the blood of wild adult females exposed to 26 °C and 21 °C in captivity between mid-August and mid-October (15 females per treatment). Results from ANOVA and pairwise comparisons revealed no differences between treatments in circulating hormones measured at days 0, 2, 7, 14, 28, and 56 of the experiment. Further research is warranted (during the spring, using additional temperatures) before concluding that females are truly buffered against the indirect risk of climate change via maternal hormone allocation.

Sex determination mechanisms and sex control approaches in aquaculture animals

Aquaculture is one of the most efficient modes of animal protein production and plays an important role in global food security. Aquaculture animals exhibit extraordinarily diverse sexual phenotypes and underlying mechanisms, providing an ideal system to perform sex determination research, one of the important areas in life science. Moreover, sex is also one of the most valuable traits because sexual dimorphism in growth, size, and other economic characteristics commonly exist in aquaculture animals. Here, we synthesize current knowledge of sex determination mechanisms, sex chromosome evolution, reproduction strategies, and sexual dimorphism, and also review several approaches for sex control in aquaculture animals, including artificial gynogenesis, application of sex-specific or sex chromosome-linked markers, artificial sex reversal, as well as gene editing. We anticipate that better understanding of sex determination mechanisms and innovation of sex control approaches will facilitate sustainable development of aquaculture.

Sex-Biased miRNAs in the Gonads of Adult Chinese Alligator (Alligator sinensis) and Their Potential Roles in Sex Maintenance

MicroRNA (miRNA) is a category of single-stranded non-coding small RNA (sRNA) that regulates gene expression by targeting mRNA. It plays a key role in the temperature-dependent sex determination of Chinese alligator (Alligator sinensis), a reptile whose sex is determined solely by the temperature during the incubation period and remains stable thereafter. However, the potential function of miRNAs in the gonads of adult Chinese alligators is still unclear. Here, we prepared and sequenced sRNA libraries of adult female and male alligator gonads, from breeding (in summer) and hibernating (in winter) animals. We obtained 130 conserved miRNAs and 683 novel miRNAs, which were assessed for sex bias in summer and winter; a total of 65 miRNAs that maintained sex bias in both seasons were identified. A regulatory network of sex-biased miRNAs and genes was constructed. Sex-biased miRNAs targeted multiple genes in the meiosis pathway of adult Chinese alligator oocytes and the antagonistic gonadal function maintenance pathway, such as MOS, MYT1, DMRT1, and GDF9. Our study emphasizes the function of miRNA in the epigenetic mechanisms of sex maintenance in crocodilians.

The Aromatase-Estrogen System in the Testes of Non-Mammalian Vertebrates

Estrogens are important physiological regulators of testicular activity in vertebrates. Estrogen levels depend on the activity of P450 aromatase, the enzyme responsible for the irreversible conversion of testosterone into 17β-estradiol. Therefore, P450 aromatase is the key player in the aromatase-estrogen system. The present review offers a comparative overview of P450 aromatase activity in male gonads of amphibians, reptiles, and birds, with a particular emphasis on the functions of the aromatase-estrogen system in these organisms during their developmental and adult stages. The aromatase-estrogen system appears to be crucial for the sex differentiation of gonads in vertebrates. Administration of aromatase inhibitors prior to sexual differentiation of gonads results in the development of males rather than females. In adults, both aromatase and estrogen receptors are expressed in somatic cells, Leydig and Sertoli cells, as well as germ cells, with certain differences among different species. In seasonal breeding species, the aromatase-estrogen system serves as an "on/off" switch for spermatogenesis. In some amphibian and reptilian species, increased estrogen levels in post-reproductive testes are responsible for blocking spermatogenesis, whereas, in some species of birds, estrogens function synergistically with testosterone to promote spermatogenesis. Recent evidence indicates that the production of the aromatase enzyme in excessive amounts reduces the reproductive performance in avian species of commercial interest. The use of aromatase inhibitors to improve fertility has yielded suitable positive results. Therefore, it appears that the role of the aromatase-estrogen system in regulating the testicular activity differs not only among the different classes of vertebrates but also among different species within the same class.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Thermosensitive period for sex determination of the tropical freshwater turtle Malayemys macrocephala

Many egg-laying reptiles possess temperature-dependent sex determination (TSD) in which outcome of gonadogenesis is determined by incubation temperature during a temperature-sensitive period of development. Prior studies on Malayemys macrocephala showed that incubation temperatures influence gonadal development and suggested that M. macrocephala exhibits TSD. However, information on the temperature-sensitivity period in this species was unknown until the current study. Turtle eggs were collected from rice fields in central Thailand from December 2016 to February 2017. In the laboratory, eggs were incubated at male-biased temperature (26 °C) and shifted to female-biased temperature (32 °C), or vice versa. Single shift experiments were performed systematically during embryonic stages 13-21. After hatching, sex of individual turtles was determined by histological analysis. We found that the sex determination of M. macrocephala is affected by temperature up to stage 16 of embryonic development.

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.

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.

Immunolocalization of aromatase P450 in the epididymis of Podarcis sicula and Rattus rattus

The goal of this study was to evaluate P450 aromatase localization in the epididymis of two different vertebrates: the lizard Podarcis sicula, a seasonal breeder, and Rattus rattus, a continuous breeder. P450 aromatase is a key enzyme involved in the local control of spermatogenesis and steroidogenesis and we proved for the first time that this enzyme is represented in the epididymis of both P. sicula and R. rattus. In details, P450 aromatase was well represented in epithelial and myoid cells and in the connective tissue of P. sicula epididymis during the reproductive period; instead, during autumnal resumption this enzyme was absent in the connective tissue. During the non-reproductive period, P450 aromatase was localized only in myoid cells of P. sicula epididymis, whereas in R. rattus it was localized both in myoid cells and connective tissue. Our findings, the first on the epididymis aromatase localization in the vertebrates, suggest a possible role of P450 aromatase in the control of male genital tract function, particularly in sperm maturation.

Role for androgens in determination of ovarian fate in the common snapping turtle, Chelydra serpentina

Sex steroids are involved in sex determination in almost all vertebrates, including species with temperature-dependent sex determination (TSD). It is well established that aromatase and estrogens are involved in ovary determination in TSD species. In contrast, the role of non-aromatizable androgens in TSD is less clear. In this study, we used dihydrotestosterone (DHT) and an antagonist of the mammalian androgen receptor (flutamide) to examine the impact of androgens on sex determination in the snapping turtle. We incubated eggs at a male-producing temperature and treated embryos with drug delivery vehicle (5 L ethanol), DHT in vehicle, or flutamide in vehicle during the sex-determining period. We then measured expression of markers for ovarian and testicular development and genes involved in steroidogenesis. A subset of embryos and hatchlings were collected for histological analysis of gonad differentiation and sex determination. DHT and flutamide both induced ovarian development: 100% of vehicle-treated hatchlings had testes, while 60% of DHT-treated and 32% flutamide-treated hatchlings had ovaries. DHT and flutamide treatments also had feminizing effects on gene expression patterns and the structure of embryonic gonads. DHT treatment increased expression of FoxL2, androgen receptor, aromatase and several steroidogenic genes. Flutamide produced a similar, but weaker, pattern of gene expression. Genes involved in testis development (Sox9 and Amh) were influenced by flutamide treatment. Our findings support the hypothesis that androgens and the androgen receptor are involved in ovary determination in the common snapping turtle.

Mechanisms related to sexual determination by temperature in reptiles

A number of strategies have emerged that appear to relate to the evolution of mechanisms for sexual determination in vertebrates, among which are genetic sex determination caused by sex chromosomes and environmental sex determination, where environmental factors influence the phenotype of the sex of an individual. Within the reptile group, some orders such as: Chelonia, Crocodylia, Squamata and Rhynchocephalia, manifest one of the most intriguing and exciting environmental sexual determination mechanisms that exists, comprising temperature-dependent sex determination (TSD), where the temperature of incubation that the embryo experiences during its development is fundamental to establishing the sex of the individual. This makes them an excellent model for the study of sexual determination at the molecular, cellular and physiological level, as well as in terms of their implications at an evolutionary and ecological level. There are different hypotheses concerning how this process is triggered and this review aims to describe any new contributions to particular TSD hypotheses, analyzing them from the "eco-evo-devo" perspective.

Transcriptome Profiling Insights the Feature of Sex Reversal Induced by High Temperature in Tongue Sole Cynoglossus semilaevis

Sex reversal induced by temperature change is a common feature in fish. Usually, the sex ratio shift occurs when temperature deviates too much from normal during embryogenesis or sex differentiation stages. Despite decades of work, the mechanism of how temperature functions during early development and sex reversal remains mysterious. In this study, we used Chinese tongue sole as a model to identify features from gonad transcriptomic and epigenetic mechanisms involved in temperature induced masculinization. Some of genetic females reversed to pseudomales after high temperature treatment which caused the sex ratio imbalance. RNA-seq data showed that the expression profiles of females and males were significantly different, and set of genes showed sexually dimorphic expression. The general transcriptomic feature of pesudomales was similar with males, but the genes involved in spermatogenesis and energy metabolism were differentially expressed. In gonads, the methylation level of cyp19a1a promoter was higher in females than in males and pseudomales. Furthermore, high-temperature treatment increased the cyp19a1a promoter methylation levels of females. We observed a significant negative correlation between methylation levels and expression of cyp19ala. In vitro study showed that CpG within the cAMP response element (CRE) of the cyp19a1a promoter was hypermethylated, and DNA methylation decreased the basal and forskolin-induced activities of cyp19a1a promoter. These results suggested that epigenetic change, i.e., DNA methylation, which regulate the expression of cyp19a1a might be the mechanism for the temperature induced masculinization in tongue sole. It may be a common mechanism in teleost that can be induced sex reversal by temperature.

Influence of temperature variation on incubation period, hatching success, sex ratio, and phenotypes in Caiman latirostris

Temperature is crucial for reptiles, also during embryonic development, particularly for species with temperature-dependent sex determination. Under natural conditions, Broad-snouted caiman (Caiman latirostris) eggs are influenced by thermal changes in the interior of the nest related to the external environmental temperature. As nests are subject to variations in temperature and most lab studies on crocodilian incubation have been carried out at constant temperatures, we were interested in determining how temperature fluctuations may affect the development of caiman embryos. We investigated the effects of incubation at constant temperatures (31°C, 32°C, and 33°C) and fluctuating temperatures (31 ± 2, 32 ± 1, and 32 ± 2°C) on the following aspects: incubation period duration, hatching success, sex ratio, total length, and body mass of C. latirostris hatchlings. Eggs incubated at 31°C produced 100% females, those at 32°C produced 71.6% females (however, the sex ratio was nest related), and at 33°C produced 100% males. We found a masculinizing effect when incubation was at 31 ± 2°C compared with a constant 31°C; and temperature fluctuations at 32°C (32 ± 1 and 32 ± 2°C) had a negative effect on hatchlings size and mass, and hatching success compared with constant incubation temperatures of 32°C and 33°C. Finally, the effect of temperature variation during the incubation period on sex ratio, hatching success, and phenotype depends on the mean temperature, as the fluctuation around 31°C affected the sex ratios and incubation period, and the fluctuation around 32°C affected hatchling success and size.

The Adaptive Sex in Stressful Environments

The impact of early stress on juvenile development has intrigued scientists for decades, but the adaptive significance of such effects remains an ongoing debate. This debate has largely ignored some characteristics of the offspring, such as their sex, despite strong evolutionary and demographic implications of sex-ratio variation. We review recent studies that examine associations between glucocorticoids (GCs), the main class of stress hormones, and offspring sex. Whereas exposure to GCs at around the time of sex determination in fish consistently produces males, the extent and direction of sex-ratio bias in response to stress vary in reptiles, birds, and mammals. We propose proximate and ultimate explanations for most of these trends.

Dynamic epimarks in sex-related genes predict gonad phenotype in the European sea bass, a fish with mixed genetic and environmental sex determination

The integration of genomic and environmental influences into methylation patterns to bring about a phenotype is of central interest in developmental epigenetics, but many details are still unclear. The sex ratios of the species used here, the European sea bass, are determined by genetic and temperature influences. We created four families from parents known to produce offspring with different sex ratios, exposed larvae to masculinizing temperatures and examined, in juvenile gonads, the DNA methylation of seven genes related to sexual development by a targeted sequencing approach. The genes most affected by both genetics and environment were cyp19a1a and dmrt1, with contrasting sex-specific methylation and temperature responses. The relationship between cyp19a1a methylation and expression is relevant to the epigenetic regulation of vertebrate sex, and we report the evidence of such relationship only below a methylation threshold, ~ 80%, and that it was sex-specific: negatively correlated in females but positively correlated in males. From parents to offspring, the methylation in gonads was midway between oocytes and sperm, with bias towards oocytes for amh-r2, er-β2, fsh-r and cyp19a1a. In contrast, dmrt1 levels resembled those of sperm. The methylation of individual CpGs from foxl2, er-β2 and nr3c1 were conserved from parents to offspring, whereas those of cyp19a1a, dmrt1 and amh-r2 were affected by temperature. Utilizing a machine-learning procedure based on the methylation levels of a selected set of CpGs, we present the first, to our knowledge, system based on epigenetic marks capable of predicting sex in an animal with ~ 90% accuracy and discuss possible applications.

Conservation of Ovary-Specific Genes, Foxl2, Aromatase, and Rspo1, in the Common Indian Garden Lizard, Calotes versicolor, That Lacks Chromosomal or Temperature-Dependent Sex Determination

Foxl2,Rspo1, and Aromatase are genes important in the ovary developmental pathway in mammals and birds. Here, we show their presence in the lizard, Calotes versicolor, which is known to lack a chromosomal as well as a temperature-dependent mode of sex determination and has an indeterminate, bipotential gonad throughout embryonic development. The expression of the 3 genes, as well as that of CvSox9 and Wnt4 - the known testis and ovary pathway genes - was studied by RT-PCR and whole tissue RNA in situ hybridization (WRISH) on the developing mesonephros gonadal complex (MGC). The expression of all 3 genes was initiated in the gonad shortly after its evagination from the mesonephros (day 5 onwards). CvFoxl2 generally was expressed in those MGCs in which CvSox9 was either not expressed or lowly expressed and vice versa. On the other hand, CvArom was expressed rather sporadically and randomly, showing no association with CvFoxl2, CvRspo1, or CvSox9, though in later stages WRISH preparations showed its coincidence with CvWnt4. CvRspo1 was expressed in almost all embryos right from day 5. Immunofluorescence localization of Rspo1 and Foxl2 proteins showed their presence in the gonads from day 10 onwards, and by day 25 it was primarily confined to the cortex but away from the coelomic epithelium of the gonadal cortex. Apparently both proteins were localized in the pregranulosa cells, Rspo1 in the cytoplasm and Foxl2 in the nucleus. Thus, it is clear that both CvFoxl2 and CvRspo1 are active in ovary formation, but whether they are expressed in the same or different cells is unknown. Though the transcription pattern of CvArom remains circumspect for its role in differentiation of the ovary, earlier evidence on aromatase inhibitor-induced reversal to the male sex indicates its importance in ovary function.

Temperature fluctuations and maternal estrogens as critical factors for understanding temperature-dependent sex determination in nature

Vertebrates with temperature-dependent sex determination (TSD) have justifiably received a lot of attention when it comes to the potential effects of climate change. Freshwater turtles have long been used to characterize the physiological and genetic mechanisms underlying TSD and provide a great system to investigate how changing climatic conditions will affect vertebrates with TSD. Unfortunately, most of what we know about the mechanisms underlying TSD comes from laboratory conditions that do not accurately mimic natural conditions (i.e., constant incubation temperatures and supraphysiological steroid manipulations). In this paper, we review recent advances in our understanding of how TSD operates in nature that arose from studies using more natural fluctuating incubation temperatures and natural variation in maternal estrogens within the yolk. By incorporating more natural conditions into laboratory studies, we are better able to use these studies to predict how changing climatic conditions will affect species with TSD.

17β-Estradiol modulates cell proliferation of medullary cords during ovarian differentiation of the Lepidochelys olivacea sea turtle

In turtles undergoing temperature sex determination (TSD), bipotential gonads express Sox9 in medullary cords at both female- (FPT) and male-producing temperatures (MPT). Subsequently, when the sex fate of medullary cords becomes dimorphic, at FPT, Sox9 is downregulated, whereas at MPT, its expression is maintained. Medullary cords in the ovary turn into ovarian lacuna, whereas in the testis they differentiate as seminiferous cords. When embryos of Lepidochelys olivacea sea turtle are incubated at MPT and treated with estradiol, Sox9 expression persists in the medullary cords in the form of tiny ovotestis-like formations. The perturbed development of the treated gonads is due to a significant decrease in the number of proliferating cells. This suggests that the disturbed effect caused by exogenous estradiol may be due to a conflict between the gene networks regulated by temperature and the increased level of endogenous estrogens, induced by the treatment. Here, we decided to use fadrozole and fulvestrant, an aromatase inhibitor and an estrogen-receptor antagonist, respectively, to provide insights into the role played by endogenous estrogens in regulating the cell proliferation of the two main gonadal compartments: the medullary cords and the cortex. Comparing cell proliferation patterns, our current results suggest that the endogenous estrogens are involved in determining the sex fate of medullary cords, by repressing proliferation. Interestingly, our results showed that endogenous estradiol levels are unnecessary for the thickening of the ovarian cortex.

Improved genome assembly of American alligator genome reveals conserved architecture of estrogen signaling

The American alligator, Alligator mississippiensis, like all crocodilians, has temperature-dependent sex determination, in which the sex of an embryo is determined by the incubation temperature of the egg during a critical period of development. The lack of genetic differences between male and female alligators leaves open the question of how the genes responsible for sex determination and differentiation are regulated. Insight into this question comes from the fact that exposing an embryo incubated at male-producing temperature to estrogen causes it to develop ovaries. Because estrogen response elements are known to regulate genes over long distances, a contiguous genome assembly is crucial for predicting and understanding their impact. We present an improved assembly of the American alligator genome, scaffolded with in vitro proximity ligation (Chicago) data. We use this assembly to scaffold two other crocodilian genomes based on synteny. We perform RNA sequencing of tissues from American alligator embryos to find genes that are differentially expressed between embryos incubated at male- versus female-producing temperature. Finally, we use the improved contiguity of our assembly along with the current model of CTCF-mediated chromatin looping to predict regions of the genome likely to contain estrogen-responsive genes. We find that these regions are significantly enriched for genes with female-biased expression in developing gonads after the critical period during which sex is determined by incubation temperature. We thus conclude that estrogen signaling is a major driver of female-biased gene expression in the post-temperature sensitive period gonads.

A timecourse analysis of systemic and gonadal effects of temperature on sexual development of the red-eared slider turtle Trachemys scripta elegans

Temperature dependent sex determination (TSD) is the process by which the environmental temperature experienced during embryogenesis influences the sex of an organism, as in the red-eared slider turtle Trachemys scripta elegans. In accord with current paradigms of vertebrate sex determination, temperature is believed to exert its effects on sexual development in T. scripta entirely within the middle third of development, when the gonad is forming. However, whether temperature regulates the transcriptome in T. scripta early embryos in a manner that could influence secondary sex characteristics or establish a pro-male or pro-female environment has not been investigated. In addition, apart from a handful of candidate genes, very little is known about potential similarities between the expression cascade during TSD and the genetic cascade that drives mammalian sex determination. Here, we conducted an unbiased transcriptome-wide analysis of the effects of male- and female-promoting temperatures on the turtle embryo prior to gonad formation, and on the gonad during the temperature sensitive period. We found sexually dimorphic expression reflecting differences in steroidogenic enzymes and brain development prior to gonad formation. Within the gonad, we mapped a cascade of differential expression similar to the genetic cascade established in mammals. Using a Hidden Markov Model based clustering approach, we identified groups of genes that show heterochronic shifts between M. musculus and T. scripta. We propose a model in which multiple factors influenced by temperature accumulate during early gonadogenesis, and converge on the antagonistic regulation of aromatase to canalize sex determination near the end of the temperature sensitive window of development.

P450 aromatase: a key enzyme in the spermatogenesis of the Italian wall lizard, Podarcis sicula

P450 aromatase is a key enzyme in steroidogenesis involved in the conversion of testosterone into 17β-estradiol. We investigated the localization and the expression of P450 aromatase in Podarcis sicula testes during the different phases of the reproductive cycle: summer stasis (July–August), early autumnal resumption (September), middle autumnal resumption (October–November), winter stasis (December–February), spring resumption (March–April) and the reproductive period (May–June). Using immunohistochemistry, we demonstrated that the P450 aromatase is always present in somatic and germ cells of P. sicula testis, particularly in spermatids and spermatozoa, except in early autumnal resumption, when P450 aromatase is evident only within Leydig cells. Using real-time PCR and semi-quantitative blot investigations, we also demonstrated that both mRNA and protein were expressed in all phases, with two peaks of expression occurring in summer and in winter stasis. These highest levels of P450 aromatase are in line with the increase of 17β-estradiol, responsible for the spermatogenesis block typical of this species. Differently, in autumnal resumption, the level of P450 aromatase dramatically decreased, along with 17β-estradiol levels, and testosterone titres increased, responsible for the subsequent renewal of spermatogenesis not followed by spermiation. In spring resumption and in the reproductive period we found intermediate P450 aromatase amounts, low levels of 17β-estradiol and the highest testosterone levels determining the resumption of spermatogenesis needed for reproduction. Our results, the first collected in a non-mammalian vertebrate, indicate a role of P450 aromatase in the control of steroidogenesis and spermatogenesis, particularly in spermiogenesis.

Male hatchling production in sea turtles from one of the world's largest marine protected areas, the Chagos Archipelago

Sand temperatures at nest depths and implications for hatchling sex ratios of hawksbill turtles (Eretmochelys imbricata) and green turtles (Chelonia mydas) nesting in the Chagos Archipelago, Indian Ocean are reported and compared to similar measurements at rookeries in the Atlantic and Caribbean. During 2012–2014, temperature loggers were buried at depths and in beach zones representative of turtle nesting sites. Data collected for 12,546 days revealed seasonal and spatial patterns of sand temperature. Depth effects were minimal, perhaps modulated by shade from vegetation. Coolest and warmest temperatures were recorded in the sites heavily shaded in vegetation during the austral winter and in sites partially shaded in vegetation during summer respectively. Overall, sand temperatures were relatively cool during the nesting seasons of both species which would likely produce fairly balanced hatchling sex ratios of 53% and 63% male hatchlings, respectively, for hawksbill and green turtles. This result contrasts with the predominantly high female skew reported for offspring at most rookeries around the globe and highlights how local beach characteristics can drive incubation temperatures. Our evidence suggests that sites characterized by heavy shade associated with intact natural vegetation are likely to provide conditions suitable for male hatchling production in a warming world.

RNA-seq analysis of the gonadal transcriptome during Alligator mississippiensis temperature-dependent sex determination and differentiation

Background

The American alligator (Alligator mississippiensis) displays temperature-dependent sex determination (TSD), in which incubation temperature during embryonic development determines the sexual fate of the individual. However, the molecular mechanisms governing this process remain a mystery, including the influence of initial environmental temperature on the comprehensive gonadal gene expression patterns occurring during TSD.

Results

Our characterization of transcriptomes during alligator TSD allowed us to identify novel candidate genes involved in TSD initiation. High-throughput RNA sequencing (RNA-seq) was performed on gonads collected from A. mississippiensis embryos incubated at both a male and a female producing temperature (33.5 °C and 30 °C, respectively) in a time series during sexual development. RNA-seq yielded 375.2 million paired-end reads, which were mapped and assembled, and used to characterize differential gene expression. Changes in the transcriptome occurring as a function of both development and sexual differentiation were extensively profiled. Forty-one differentially expressed genes were detected in response to incubation at male producing temperature, and included genes such as Wnt signaling factor WNT11, histone demethylase KDM6B, and transcription factor C/EBPA. Furthermore, comparative analysis of development- and sex-dependent differential gene expression revealed 230 candidate genes involved in alligator sex determination and differentiation, and early details of the suspected male-fate commitment were profiled. We also discovered sexually dimorphic expression of uncharacterized ncRNAs and other novel elements, such as unique expression patterns of HEMGN and ARX. Twenty-five of the differentially expressed genes identified in our analysis were putative transcriptional regulators, among which were MYBL2, MYCL, and HOXC10, in addition to conventional sex differentiation genes such as SOX9, and FOXL2. Inferred gene regulatory network was constructed, and the gene-gene and temperature-gene interactions were predicted.

Conclusions

Gonadal global gene expression kinetics during sex determination has been extensively profiled for the first time in a TSD species. These findings provide insights into the genetic framework underlying TSD, and expand our current understanding of the developmental fate pathways during vertebrate sex determination.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2396-9) contains supplementary material, which is available to authorized users.

TRPV4 associates environmental temperature and sex determination in the American alligator

Temperature-dependent sex determination (TSD), commonly found among reptiles, is a sex determination mode in which the incubation temperature during a critical temperature sensitive period (TSP) determines sexual fate of the individual rather than the individual’s genotypic background. In the American alligator (Alligator mississippiensis), eggs incubated during the TSP at 33 °C (male producing temperature: MPT) yields male offspring, whereas incubation temperatures below 30 °C (female producing temperature: FPT) lead to female offspring. However, many of the details of the underlying molecular mechanism remains elusive, and the molecular link between environmental temperature and sex determination pathway is yet to be elucidated. Here we show the alligator TRPV4 ortholog (AmTRPV4) to be activated at temperatures proximate to the TSD-related temperature in alligators, and using pharmacological exposure, we show that AmTRPV4 channel activity affects gene expression patterns associated with male differentiation. This is the first experimental demonstration of a link between a well-described thermo-sensory mechanism, TRPV4 channel, and its potential role in regulation of TSD in vertebrates, shedding unique new light on the elusive TSD molecular mechanism.

Epigenetics of sex determination in mammals

Epigenetics is the study of changes in gene function that cannot be explained by changes in DNA sequence. A mammalian body contains more than two hundred types of cells. Since all of them are derived from a single fertilized egg, their genotypes are identical. However, the gene expression patterns are different between the cell types, indicating that each cell type has unique own "epigenotype". Epigenetic gene regulation mechanisms essentially contribute to various processes of mammalian development. The essence of epigenetic regulation is the structural change of chromatin to modulate gene activity in a spatiotemporal manner. DNA methylation and histone modifications are the major epigenetic mechanisms. Sex determination is the process for gender establishment. There are two types of sex-determining mechanisms in animals, environmental sex determination (ESD) and genotypic sex determination (GSD). Recent studies have provided some evidence that epigenetic mechanisms play indispensable roles in ESD and GSD. Some fishes undergo ESD, in which DNA methylation is essentially involved. GSD is employed in therian mammals, where Sry (sex-determining region on the Y chromosome) triggers testis differentiation from undifferentiated gonads. Sry expression is tightly regulated in a spatiotemporal manner. A recent study demonstrated that histone modification is involved in Sry regulation. In this review, we discuss the role of epigenetic mechanisms for sex determination in mammals and other vertebrates.

Differential expression of estrogen receptor alpha in the embryonic adrenal-kidney-gonadal complex of the oviparous lizard, Calotes versicolor (Daud.)

Estrogen signalling is critical for ovarian differentiation in reptiles with temperature-dependent sex determination (TSD). To elucidate the involvement of estrogen in this process, adrenal-kidney-gonadal (AKG) expression of estrogen receptor (ERα) was studied at female-producing temperature (FPT) in the developing embryos of the lizard, Calotes versicolor which exhibits a distinct pattern of TSD. The eggs of this lizard were incubated at 31.5±0.5°C (100% FPT). The torso of embryos containing adrenal-kidney-gonadal complex (AKG) was collected during different stages of development and subjected to Western blotting and immunohistochemistry analysis. The ERα antibody recognized two protein bands with apparent molecular weight ∼55 and ∼45kDa in the total protein extracts of embryonic AKG complex of C. versicolor. The observed results suggest the occurrence of isoforms of ERα. The differential expression of two different protein isoforms may reveal their distinct role in cell proliferation during gonadal differentiation. This is the first report to reveal two isoforms of the ERα in a reptile during development. Immunohistochemical studies reveal a weak, but specific, cytoplasmic ERα immunostaining exclusively in the AKG during late thermo-sensitive period suggesting the responsiveness of AKG to estrogens before gonadal differentiation at FPT. Further, cytoplasmic as well as nuclear expression of ERα in the medulla and in oogonia of the cortex (faint activity) at gonadal differentiation stage suggests that the onset of gonadal estrogen activity coincides with sexual differentiation of gonad. Intensity and pattern of the immunoreactions of ERα in the medullary region at FPT suggest endogenous production of estrogen which may act in a paracrine fashion to induce neighboring cells into ovarian differentiation pathway.

Chronology, magnitude and duration of expression of putative sex-determining/differentiation genes in a turtle with temperature-dependent sex determination

The red-eared slider turtle (Trachemys scripta) possesses temperature-dependent sex determination (TSD) in which the incubation temperature determines gonadal sex. Although a number of mammalian gene homologues have been identified in reptiles with TSD, the exact sex-determining trigger(s) is not known. To date, the current study represents the most comprehensive simultaneous evaluation of the chronology of mRNA expression profiles of putative sex-determining/differentiation genes (Dmrt1, Sox9, Amh, Lhx9, and Foxl2) from gonads incubated at male- and female-producing temperatures in T. scripta. Additionally, sex-reversing treatments with 17β-estradiol and letrozole were examined. At a male-producing temperature, Dmrt1 expression was sexually dimorphic by stage 17, Sox9 by 19 and Amh by 21. In contrast, Foxl2 did not significantly increase until after the thermosensitive period at a female-producing temperature. Treatment with 17β-estradiol resulted in reduced gonad size and/or inhibited gonadal development and differentiation. Gene expression was subsequently low in this group. Sex reversal utilizing letrozole failed to produce testes at a female-producing temperature and as such, gene expression was comparable to ovary. These results indicate that Dmrt1 and Sox9 are potential triggers for testis differentiation and Amh, Lhx9 and Foxl2 represent a conserved core set of genes in the sex-determining/differentiation pathway of TSD species.

Reproductive science as an essential component of conservation biology

In this chapter we argue that reproductive science in its broadest sense has never been more important in terms of its value to conservation biology, which itself is a synthetic and multidisciplinary topic. Over recent years the place of reproductive science in wildlife conservation has developed massively across a wide and integrated range of cutting edge topics. We now have unprecedented insight into the way that environmental change affects basic reproductive functions such as ovulation, sperm production, pregnancy and embryo development through previously unsuspected influences such as epigenetic modulation of the genome. Environmental change in its broadest sense alters the quality of foodstuffs that all animals need for reproductive success, changes the synchrony between breeding seasons and reproductive events, perturbs gonadal and embryo development through the presence of pollutants in the environment and drives species to adapt their behaviour and phenotype. In this book we explore many aspects of reproductive science and present wide ranging and up to date accounts of the scientific and technological advances that are currently enabling reproductive science to support conservation biology.

The first case of intersexuality in an African dwarf crocodile (Osteolaemus tetraspis)

To the authors knowledge this is the first case of intersexuality in an African dwarf crocodile (Osteolaemus tetraspis). An adult African dwarf crocodile with a male-typical phenotype lived at Zoo Duisburg in Germany for 10 years. It died in October 2012 despite intensive treatment as a result of terminal septicemia. After a detailed pathological examination the gonads were histologically confirmed as ovotestes. Half of the 22 extant species of crocodilians have been examined for occurrence of temperature dependent sex determination (TSD). In TSD reptiles, masculinizing temperatures yield 100% or a majority of males, whereas feminizing temperatures yield 100% or a majority of females. In the transition range of temperature (TRT), a mix of males, females and sometimes intersexes are obtained. However, the molecular mechanisms behind TSD and an explanation for the occurrence of intersexuality remain elusive.

Mechanisms of crosstalk between endocrine systems: regulation of sex steroid hormone synthesis and action by thyroid hormones

Thyroid hormones (THs) are well-known regulators of development and metabolism in vertebrates. There is increasing evidence that THs are also involved in gonadal differentiation and reproductive function. Changes in TH status affect sex ratios in developing fish and frogs and reproduction (e.g., fertility), hormone levels, and gonad morphology in adults of species of different vertebrates. In this review, we have summarized and compared the evidence for cross-talk between the steroid hormone and thyroid axes and present a comparative model. We gave special attention to TH regulation of sex steroid synthesis and action in both the brain and gonad, since these are important for gonad development and brain sexual differentiation and have been studied in many species. We also reviewed research showing that there is a TH system, including receptors and enzymes, in the brains and gonads in developing and adult vertebrates. Our analysis shows that THs influences sex steroid hormone synthesis in vertebrates, ranging from fish to pigs. This concept of crosstalk and conserved hormone interaction has implications for our understanding of the role of THs in reproduction, and how these processes may be dysregulated by environmental endocrine disruptors.

Both cell-autonomous mechanisms and hormones contribute to sexual development in vertebrates and insects

The differentiation of male and female characteristics in vertebrates and insects has long been thought to proceed via different mechanisms. Traditionally, vertebrate sexual development was thought to occur in two phases: a primary and a secondary phase, the primary phase involving the differentiation of the gonads, and the secondary phase involving the differentiation of other sexual traits via the influence of sex hormones secreted by the gonads. In contrast, insect sexual development was thought to depend exclusively on cell-autonomous expression of sex-specific genes. Recently, however, new evidence indicates that both vertebrates and insects rely on sex hormones as well as cell-autonomous mechanisms to develop sexual traits. Collectively, these new data challenge the traditional vertebrate definitions of primary and secondary sexual development, call for a redefinition of these terms, and indicate the need for research aimed at explaining the relative dependence on cell-autonomous versus hormonally guided sexual development in animals.

RNAi-Mediated Gene Silencing in a Gonad Organ Culture to Study Sex Determination Mechanisms in Sea Turtle

The autosomal Sry-related gene, Sox9, encodes a transcription factor, which performs an important role in testis differentiation in mammals. In several reptiles, Sox9 is differentially expressed in gonads, showing a significant upregulation during the thermo-sensitive period (TSP) at the male-promoting temperature, consistent with the idea that SOX9 plays a central role in the male pathway. However, in spite of numerous studies, it remains unclear how SOX9 functions during this event. In the present work, we developed an RNAi-based method for silencing Sox9 in an in vitro gonad culture system for the sea turtle, Lepidochelys olivacea. Gonads were dissected as soon as the embryos entered the TSP and were maintained in organ culture. Transfection of siRNA resulted in the decrease of both Sox9 mRNA and protein. Furthermore, we found coordinated expression patterns for Sox9 and the anti-Müllerian hormone gene, Amh, suggesting that SOX9 could directly or indirectly regulate Amh expression, as it occurs in mammals. These results demonstrate an in vitro method to knockdown endogenous genes in gonads from a sea turtle, which represents a novel approach to investigate the roles of important genes involved in sex determination or differentiation pathways in species with temperature-dependent sex determination.

Epigenetics of sex determination and gonadogenesis

Epigenetics is commonly defined as the study of heritable changes in gene function that cannot be explained by changes in DNA sequence. The three major epigenetic mechanisms for gene expression regulation include DNA methylation, histone modifications, and non-coding RNAs. Epigenetic mechanisms provide organisms with the ability to integrate genomic and environmental information to modify the activity of their genes for generating a particular phenotype. During development, cells differentiate, acquire, and maintain identity through changes in gene expression. This is crucial for sex determination and differentiation, which are among the most important developmental processes for the proper functioning and perpetuation of species. This review summarizes studies showing how epigenetic regulatory mechanisms contribute to sex determination and reproductive organ formation in plants, invertebrates, and vertebrates. Further progress will be made by integrating several approaches, including genomics and Next Generation Sequencing to create epigenetic maps related to different aspects of sex determination and gonadogenesis. Epigenetics will also contribute to understand the etiology of several disorders of sexual development. It also might play a significant role in the control of reproduction in animal farm production and will aid in recognizing the environmental versus genetic influences on sex determination of sensitive species in a global change scenario.

Genomic approaches to study genetic and environmental influences on fish sex determination and differentiation

The embryonic gonad is the only organ that takes two mutually exclusive differentiating pathways and hence gives rise to two different adult organs: testes or ovaries. The recent application of genomic tools including microarrays, next-generation sequencing approaches, and epigenetics can significantly contribute to decipher the molecular mechanisms involved in the processes of sex determination and sex differentiation. However, in fish, these studies are complicated by the fact that these processes depend, perhaps to a larger extent when compared to other vertebrates, on the interplay of genetic and environmental influences. Here, we review the advances made so far, taking into account different experimental approaches, and illustrate some technical complications deriving from the fact that as development progresses it becomes more and more difficult to distinguish whether changes in gene expression or DNA methylation patterns are the cause or the consequence of such developmental events. Finally, we suggest some avenues for further research in both model fish species and fish species facing specific problems within an aquaculture context.

Expression of aromatase and two isozymes of 5α-reductase in the developing green anole forebrain

Neural steroids, as well as the enzymes that produce these hormones, are important for sexual differentiation of the brain during development. Aromatase converts testosterone into oestradiol. 5α-reductase converts testosterone to 5α-dihydrotestosterone and occurs in two isozymes: type 1 (5αR1) and type 2 (5αR2). Each of these enzymes is present in the developing brain in many species, although no work has been carried out examining the expression of all three enzymes in non-avian reptiles with genetic sex determination. In the present study, we evaluated mRNA expression of neural aromatase, 5αR1 and 5αR2, on the day of hatching and at day 50 in one such lizard, the green anole. We describe the distribution of these enzymes throughout the brain and the quantification of mRNA expression in three regions that control adult sexual behaviours: the preoptic area (POA) and ventromedial amygdala (AMY), which are involved in male displays, as well as the ventromedial hypothalamus, which regulates female receptivity. Younger animals had a greater number (POA) and density (AMY) of 5αR1 mRNA expressing cells. We detected no effects of sex or age on aromatase or 5αR2. In comparison with data from adults, the present results support the idea that the green anole forebrain has not completely differentiated by 50 days after hatching and that 5αR1 may play a role in the early development of regions important for masculine function.

DNA methylation of the gonadal aromatase (cyp19a) promoter is involved in temperature-dependent sex ratio shifts in the European sea bass

Sex ratio shifts in response to temperature are common in fish and reptiles. However, the mechanism linking temperature during early development and sex ratios has remained elusive. We show in the European sea bass (sb), a fish in which temperature effects on sex ratios are maximal before the gonads form, that juvenile males have double the DNA methylation levels of females in the promoter of gonadal aromatase (cyp19a), the enzyme that converts androgens into estrogens. Exposure to high temperature increased the cyp19a promoter methylation levels of females, indicating that induced-masculinization involves DNA methylation-mediated control of aromatase gene expression, with an observed inverse relationship between methylation levels and expression. Although different CpGs within the sb cyp19a promoter exhibited different sensitivity to temperature, we show that the increased methylation of the sb cyp19a promoter, which occurs in the gonads but not in the brain, is not a generalized effect of temperature. Importantly, these effects were also observed in sexually undifferentiated fish and were not altered by estrogen treatment. Thus, methylation of the sb cyp19a promoter is the cause of the lower expression of cyp19a in temperature-masculinized fish. In vitro, induced methylation of the sb cyp19a promoter suppressed the ability of SF-1 and Foxl2 to stimulate transcription. Finally, a CpG differentially methylated by temperature and adjacent to a Sox transcription factor binding site is conserved across species. Thus, DNA methylation of the aromatase promoter may be an essential component of the long-sought-after mechanism connecting environmental temperature and sex ratios in vertebrate species with temperature-dependent sex determination.

From the origin of sex-determining factors to the evolution of sex-determining systems

Sex determination is typically classified as either genotypic or environmental. However, this dichotomy obscures the developmental origin and evolutionary modification of determinants of sex, and therefore hinders an understanding of the processes that generates diversity in sex-determining systems. Recent research on reptiles and fish emphasizes that sex determination is a multifactorial regulatory process that is best understood as a threshold dichotomy rather than as the result of genetically inherited triggers of development. Here we critically assess the relationship between the developmental origin of sex-determining factors and evolutionary transitions in sex-determining systems. Our perspective emphasizes the importance of both genetic and nongenetic causes in evolution of sex determination and may help to generate predictions with respect to the evolutionary patterns of sex-determining systems and the underlying diversity of developmental and genetic regulatory networks.

Effect of the N-terminal sequence on the binding affinity of transthyretin for human retinol-binding protein

During vertebrate evolution, the N-terminal region of transthyretin (TTR) subunit has undergone a change in both length and hydropathy. This was previously shown to change the binding affinity for thyroid hormones (THs). However, it was not known whether this change affects other functions of TTR. In the present study, the effect of these changes on the binding of TTR to retinol-binding protein (RBP) was determined. Two wild-type TTRs from human and Crocodylus porosus, and three chimeric TTRs, including a human chimeric TTR in which its N-terminal sequence was changed to that of C. porosus TTR (croc/huTTR) and two C. porosus chimeric TTRs (hu/crocTTR in which its N-terminal sequence was changed to that of human TTR and xeno/crocTTR in which its N-terminal sequence was changed to that of Xenopus laevis TTR), were analyzed for their binding to human RBP by native-PAGE followed by immunoblotting and a chemilluminescence assay. The K(d) of human TTR was 30.41 ± 2.03 μm, and was similar to that reported for the second binding site, whereas that of crocodile TTR was 2.19 ± 0.24 μm. The binding affinities increased in croc/huTTR (K(d) = 23.57 ± 3.54 μm) and xeno/crocTTR (K(d) = 0.61 ± 0.16 μm) in which their N-termini were longer and more hydrophobic, but decreased in hu/crocTTR (K(d) = 5.03 ± 0.68 μm) in which its N-terminal region was shorter and less hydrophobic. These results suggest an influence of the N-terminal primary structure of TTR on its function as a co-carrier for retinol with RBP.

Segregating variation for temperature-dependent sex determination in a lizard

Temperature-dependent sex determination (TSD) was first reported in 1966 in an African lizard. It has since been shown that TSD occurs in some fish, several lizards, tuataras, numerous turtles and all crocodilians. Extreme temperatures can also cause sex reversal in several amphibians and lizards with genotypic sex determination. Research in TSD species indicates that estrogen signaling is important for ovary development and that orthologs of mammalian genes have a function in gonad differentiation. Nevertheless, the mechanism that actually transduces temperature into a biological signal for ovary versus testis development is not known in any species. Classical genetics could be used to identify the loci underlying TSD, but only if there is segregating variation for TSD. Here, we use the 'animal model' to analyze inheritance of sexual phenotype in a 13-generation pedigree of captive leopard geckos, Eublepharis macularius, a TSD reptile. We directly show genetic variance and genotype-by-temperature interactions for sex determination. Additive genetic variation was significant at a temperature that produces a female-biased sex ratio (30°C), but not at a temperature that produces a male-biased sex ratio (32.5°C). Conversely, dominance variance was significant at the male-biased temperature (32.5°C), but not at the female-biased temperature (30°C). Non-genetic maternal effects on sex determination were negligible in comparison with additive genetic variance, dominance variance and the primary effect of temperature. These data show for the first time that there is segregating variation for TSD in a reptile and consequently that a quantitative trait locus analysis would be practicable for identifying the genes underlying TSD.

Progesterone metabolites, "xenobiotic-sensing" nuclear receptors, and the metabolism of maternal steroids

During development, embryos utilize steroid signals to direct sexual differentiation of tissues necessary for reproduction. Disruption of these signals by exogenous substances (both natural and synthetic) frequently produce phenotypic effects that can persist into adulthood and influence reproduction. This paper reviews the evidence that during embryonic development, progesterone metabolites and xenobiotic-sensing nuclear receptors may interact to increase the expression of numerous enzymes responsible for steroid metabolism in oviparous and placental amniotes. In these groups, embryonic development is characterized by (1) elevated progesterone concentrations, (2) 5 beta reduction being the primary metabolic pathway of progesterone, (3) the presence of xenobiotic-sensing nuclear receptors that can bind 5 beta metabolites of progesterone, and (4) increased expression of a suite of enzymes responsible for the metabolism of multiple steroids. We propose that xenobiotic-sensing nuclear receptors initially evolved to buffer the developing embryo from the potentially adverse effects of various maternal steroids on sexual differentiation.