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.

Endocrine-disrupting chemical exposure and the American alligator: a review of the potential role of environmental estrogens on the immune system of a top trophic carnivore

Endocrine-disrupting chemicals (EDCs) alter cellular and organ system homeostasis by interfering with the body's normal physiologic processes. Numerous studies have identified environmental estrogens as modulators of EDC-related processes in crocodilians, notably in sex determination. Other broader studies have shown that environmental estrogens dysregulate normal immune function in mammals, birds, turtles, lizards, fish, and invertebrates; however, the effects of such estrogenic exposures on alligator immune function have not been elucidated. Alligators occupy a top trophic status, which may give them untapped utility as indicators of environmental quality. Environmental estrogens are also prevalent in the waters they occupy. Understanding the effects of these EDCs on alligator immunity is critical for managing and assessing changes in their health and is thus the focus of this review.

The mechanism of sex determination in vertebrates-are sex steroids the key-factor?

In many vertebrate species, sex is determined at fertilization of zygotes by sex chromosome composition, knows as genotypic sex determination (GSD). But in some species-fish, amphibians and reptiles-sex is determined by environmental factors; in particular by temperature-dependent sex determination (TSD). However, little is known about the mechanisms involved in TSD and GSD. How does TSD differ from GSD? As is well known, genes that activated downstream of sex-determining genes are conserved throughout all classes of vertebrates. What is the main factor that determines sex, then? Sex steroids can reverse sex of several species of vertebrate; estrogens induce the male-to-female sex-reversal, whereas androgens do the female-to-male sex-reversal. For such sex-reversal, a functioning sex-determining gene is not required. However, in R. rugosa CYP19 (P450 aromatase) is expressed at high levels in indifferent gonads before phenotypic sex determination, and the gene is also active in the bipotential gonad of females before sex determination. Thus, we may predict that an unknown factor, a common transcription factor locates on the X and/or W chromosome, intervenes directly or indirectly in the transcriptional up-regulation of the CYP19 gene for feminization in species of vertebrates with both TSD and GSD. Similarly, an unknown factor on the Z and/or Y chromosome probably intervenes directly or indirectly in the regulation of androgen biosynthesis for masculinization. In both cases, a sex-determining gene is not always necessary for sex determination. Taken together, sex steroids may be the key-factor for sex determination in some species of vertebrates.

Contaminant-induced feminization and demasculinization of nonmammalian vertebrate males in aquatic environments

Many chemicals introduced into the environment by humans adversely affect embryonic development and the functioning of the male reproductive system. It has been hypothesized that these developmental alterations are due to the endocrine-disruptive effects of various environmental contaminants. The endocrine system exhibits an organizational effect on the developing embryo. Thus, a disruption of the normal hormonal signals can permanently modify the organization and future function of the male reproductive system. A wide range of studies examining wildlife either in laboratories or in natural settings have documented alterations in the development of males. These studies have begun to provide the causal relationships between embryonic contaminant exposure and reproductive abnormalities that have been lacking in pure field studies of wild populations. An understanding of the developmental consequences of endocrine disruption in wildlife can lead to new indicators of exposure and a better understanding of the most sensitive life stages as well as the consequences of exposure during these periods.

Does alligator testis produce estradiol? A comparison of ovarian and testicular aromatase

Testicular secretion of estradiol is necessary for normal spermatogenesis and male reproductive physiology in humans and rodents. The role of estradiol in nonmammalian vertebrates remains unknown, but elevated circulating estradiol has been reported in male lizards, alligators, and various bird species. We have been unable to detect circulating estradiol in male alligators; therefore, we reexamined the question of testicular production of estradiol in alligators using more rigorous assay procedures. A large pool of plasma from a male alligator was extracted and run through an HPLC column. Immunoreactive estradiol-like material eluted coincident with authentic estradiol. By using an ultrasensitive RIA and processing large volumes of male plasma (1000 microl), we were able to measure estradiol. Estradiol in male alligators ranged from 0.23 to 3.14 pg/ml, whereas estradiol in immature female alligators ranged from 14 to 66 pg/ml. Aromatase activity in microsomes from adult alligator ovarian tissue was 36.2 +/- 1.6 pmol mg-1 h-1, whereas activity in testicular microsomes ranged between 0.92 and 2.38 pmol mg-1 h-1. Ovarian aromatase activity was inhibited in a concentration-dependent fashion by Fadrozole, but the essentially background activity of testicular aromatase was not inhibited at any concentration of Fadrozole. Likewise, a comparison of alligator testicular and ovarian aromatase mRNA expression gave a similar result: the ovarian expression was 600-fold higher and brain tissue was 10-fold higher than that of the testis. Circulating estradiol in male alligators is probably of extragonadal origin, and the testis produces little if any of this steroid.

Sex reversal effects on Caiman latirostris exposed to environmentally relevant doses of the xenoestrogen bisphenol A

Exposure to environmental contaminants known as endocrine disruptors (EDs) alters the development and function of reproductive organs in several species. Bisphenol A (BPA) is an estrogenic chemical that leaches from dental materials and plastic food and beverage containers. BPA has been found in sewage, surface and drinking water, and therefore poses a potentially significant risk for human and wildlife. Prenatal exposure of rodents to environmentally relevant doses of BPA alters the development of the reproductive organs of male and female offspring. Species with temperature dependent sex determination (TSD) could act as sentinels of ecosystem health by providing sensitive biomarkers of endocrine disruptor's effects. We selected Caiman latirostris as an animal model to study endocrine disruption caused by BPA. The aim of this study was to determine whether exposure in ovum to BPA could cause estrogen-like effects on the reproductive system of C. latirostris. Sex determination and gonadal histoarchitecture were the endpoints evaluated after in ovum exposure to different doses of BPA and 17beta-estradiol (E(2)). We confirmed that C. latirostris is a species with TSD and additionally demonstrated that BPA causes estrogen-like developmental effects by reversing gonadal sex and altering gonadal histoarchitecture. Differences in responses to BPA and E(2) in our in vivo system were on the order of 100-fold. In contrast published in vitro studies have reported differences on the order of 10,000x or more. These results support the utility of C. latirostris, a species in which sex determination is temperature dependent, as a tool in assessing estrogenic activity in vivo and as a sentinel to monitor EDs in aquatic environment.

Effects of androgens on sex differentiation of the urodele Pleurodeles waltl

In nonmammalian vertebrates, steroids have been hypothesized to induce somatic sex differentiation, since manipulations of the steroidal environment of gonads have led to various degrees of sex reversal. Whereas the critical role of estrogens in ovarian differentiation is well documented, studies on androgens have produced a perplexing variety of results depending upon species variations and nature of androgens used. In this way, testosterone induces masculinization of females in some species but provokes paradoxical feminization of males in many other species such as the urodelan Pleurodeles waltl. In reptiles this phenomenon could be interpreted by conversion of exogenous testosterone to estradiol by aromatase. Treatments of Pleurodeles larvae with nonaromatizable androgens bring support to this hypothesis and suggest a role of androgens in sex differentiation. Dihydrotestosterone (DHT) could not induce the paradoxical feminization of ZZ larvae. In addition, DHT as well as 11beta-hydroxy-androstenedione could drive a functional male differentiation of ZW larvae. Moreover, other 5alpha reduced androgens also induced sex reversal of female larvae. Yet, the 5alpha reductase inhibitor CGP 53133 and antiandrogens such as flutamide or cyproterone acetate did not exert any effect on male sex differentiation of ZZ larvae. Though the precise role of androgens is still unknown, especially for 11-oxygenated androgens, our results suggest an implication in male sex differentiation. In this way, testosterone could play a pivotal role in being metabolized either into other androgens during testis differentiation or into estradiol during ovarian differentiation.

Expression of P450(arom) in Malaclemys terrapin and Chelydra serpentina: a tale of two sites

The formation of estrogens from androgens in all vertebrates is catalyzed by the "aromatase" complex, which consists of a membrane bound P(450) enzyme, P(450) aromatase (which binds the androgen substrate and inserts an oxygen into the molecule), and a flavoprotein (NADPH-cytochrome P450 reductase). Among vertebrates, the two major sites of aromatase expression are the brain and gonads. Given the importance of estrogen in reptile sex determination, we set out to examine whether P450arom was involved in the initiation and/or stabilization of sex determination in turtles. We examined the expression of aromatase activity in the brain and gonads of two turtle species exhibiting temperature dependent sex determination (TSD), the diamondback terrapin (Malaclemys terrapin), and the common snapping turtle (Chelydra serpentina). Estradiol when applied at stage 14 of the terrapin induces expression of aromatase in the gonad of embryos incubated at male temperatures (26.5 degrees C). The level of expression is similar to that of a normal embryonic ovary. When applied at stage 22, estradiol does not induce aromatase expression in the terrapin. The xenoestrogen, nonylphenol, sex reverses terrapin embryos at 26.5 degrees C. Letrazole, a nonsteroidal aromatase inhibitor, suppresses aromatase activity in the brain at either incubation temperature. Ovotestes are produced by letrazole administration in the terrapin when incubated at 30.5 degrees C. In the snapping turtle at stage 23, gonadal and brain aromatase activity in embryos incubated at female temperatures (30.5 degrees C) is nearly half that exhibited in terrapin embryos at the same temperature. Moreover, letrazole administration suppresses aromatase expression to nearly basal levels. At male incubation temperatures (26.5 degrees ), brain aromatase expression is nearly three times higher than at female temperatures, while gonadal expression levels are nearly one third lower. However, the gonadal expression levels at male temperatures in the snapping turtle are nearly 25 times higher than that found in the terrapin. Estradiol administration elevates this level nearly three fold. These data suggest that is not merely the expression of aromatase that is important for ovarian development, but that the level of expression may be more important.

Alligator aromatase cDNA sequence and its expression in embryos at male and female incubation temperatures

In all species of crocodilians, sex is determined not by genetic mechanisms, but by the temperature at which the egg is incubated. In the American alligator (Alligator mississippiensis) the thermosensitive period (TSP) for sex determination is a 7- to 10-day window within stages 21-24 of development, around the middle third of the incubation period. Treating embryos with estrogen during the TSP produces female offspring, even at male incubation temperatures. Conversely, blocking embryonic estrogen synthesis at female-inducing temperature prevents development of the female phenotype. Therefore, it has been suggested that estrogen plays a role in determination of sex in the alligator. Estrogen is produced from an androgen substrate by cytochrome P450 aromatase (CYP19). If estrogen plays a critical role in sex determination, there should be differences in aromatase expression between embryos at male- and female-producing temperatures during the TSP. Therefore, to address this question, we cloned and characterized the alligator CYP19 cDNA. Based on the sequence information, a quantitative kinetic reverse transcriptase-polymerase chain reaction (TaqMan) assay was designed to measure expression of the alligator aromatase gene in RNA extracted from the gonadal and brain regions of alligator embryos incubated at male- or female-producing temperatures from prior to the TSP through hatching. Aromatase expression was detected in the brain region from the earliest stage tested (stage 20) through hatching. The hypothalamus had significantly higher expression than the forebrain or hindbrain in both male and female embryos. Expression was not significantly different in the gonadal region between embryos at male and female temperatures until after the TSP, when there was a dramatic increase in expression at female temperature. These data indicate that aromatase expression and, thus, estrogen production, are not the initial trigger for sex determination but play an essential role in ovarian differentiation in the alligator. J. Exp. Zool. 290:439-448, 2001.

Temperature-dependent sex determination and gonadal differentiation in reptiles

In many reptile species, sexual differentiation of gonads is sensitive to temperature (temperature-dependent sex determination, TSD) during a critical period of embryonic development (thermosensitive period, TSP). Experiments carried out with different models including turtles, crocodilians and lizards have demonstrated the implication of estrogens and the key role played by aromatase (the enzyme complex that converts androgens to estrogens) in ovary differentiation during TSP and in maintenance of the ovarian structure after TSP. In some of these experiments, the occurrence of various degrees of gonadal intersexuality is related to weak differences in aromatase activity, suggesting subtle regulations of the aromatase gene at the transcription level. Temperature could intervene in these regulations. Studies presently under way deal with cloning (cDNAs) and expression (mRNAs) of genes that have been shown, or are expected, to be involved in gonadal formation and/or differentiation in mammals. Preliminary results show that homologues of the WT1, SF1, SOX9, DAX1 and AMH genes exist in TSD reptiles. However, the expression patterns of these genes during gonadal differentiation may be different between mammals and TSD reptiles and also between different reptile species. How these genes could interact with aromatase is being examined.

Temperature-dependent sex determination in the American alligator: expression of SF1, WT1 and DAX1 during gonadogenesis

Sex determination in mammals and birds is chromosomal, while in many reptiles sex determination is temperature dependent. Morphological development of the gonads in these systems is conserved, suggesting that many of the genes involved in gonad development are also conserved. The genes SF1, WT1 and DAX1 play various roles in the mammalian testis-determining pathway. SF1 and WT1 are thought to interact to cause male-specific gene expression during testis development, while DAX1 is believed to inhibit this male-specific gene expression. We have cloned SF1 and DAX1 from the American alligator, a species with temperature-dependent sex determination (TSD). SF1, DAX1 and WT1 are expressed in the urogenital system/gonad throughout the period of alligator gonadogenesis which is temperature sensitive. SF1 appears to be expressed at a higher level in females than in males. This SF1 expression pattern is concordant with the observed pattern during chicken gonadogenesis, but opposite to that observed during mouse gonadogenesis. Although the observed sexual dimorphism of gonadal SF1 expression in alligators and chickens is opposite that observed in the mouse, it is probable that SF1 is involved in control of gonadal steroidogenesis in all these vertebrates. DAX1 and WT1 are both expressed during stages 22-25 of both males and females. However, there appear to be no sex differences in the expression patterns of these genes. We conclude that DAX1, WT1 and SF1 may be involved in gonadal development of the alligator. These genes may form part of a gonadal-development pathway which has been conserved through vertebrate evolution.

Temperature-dependent sex determination in the red-eared slider turtle, Trachemys scripta

Temperature-dependent sex determination (TSD) in the red-eared slider turtle, Trachemys scripta, has been the subject of a variety of past studies. Incubation temperature appears to affect sex determination in a dose-dependent fashion. This suggests that temperature could be affecting a dosage-sensitive element in the sex-determination cascade. Sex determination in T. scripta is sensitive to estrogen, and data from many studies support the hypothesis that endogenous estrogen production may be involved in female sex determination. However, this hypothesis has not yet been evaluated through aromatase expression studies in this species. Several recent studies have cloned cDNAs for genes that could be involved in sex determination and/or sex differentiation. The cDNAs for SF-1 and MIS have been cloned in T. scripta, indicating that these may represent conserved elements in the sex-determination/sex-differentiation cascade of reptiles. The SOX9 cDNA also has been cloned in T. scripta (Spotila et al., '98), and it shows a sex-specific expression pattern. Future studies targeted at aromatase expression as well as the expression of factors such as SOX9, SF-1, and MIS will begin to provide a more comprehensive picture of the events involved in TSD in T. scripta. Further, such studies could help pinpoint the temperature-sensitive element(s).

Response of diencephalon but not the gonad to female-promoting temperature with elevated estradiol levels in the sea turtle Lepidochelys olivacea

Although temperature sex determination is well known in several reptile species, the physiological mechanism underlying this process remains to be elucidated. In the current work, we analyzed the levels of testosterone (T) and estradiol (E2) in the gonads; two brain regions--telencephalon (Te) and diencephalon/mesencephalon (Di)--and the serum of developing embryos of the olive ridley Lepidochelys olivacea incubated at male- or female-promoting temperatures. Conversion of pregnenolone (P5) to T and T to E2 were studied in the gonads and brain. The analyses were performed during three periods: the thermosensitive period (TSP), histologically undifferentiated gonads (UDG), and differentiated gonads (DG). In the gonads, serum, and brain, T concentrations were higher at the female-promoting temperature during the three periods, whereas in the gonads and serum, E2 levels were similar at the female and male-promoting temperature. In Di, the concentration of E2 was significantly higher at the female-promoting temperature. Biotransformation of P5 to T in gonadal tissues were slightly higher at the female-promoting temperature in TSP and increased during UDG and DG. Conversion of T to E2, however, was similar at the two temperatures during the three periods. In the brain, the Di showed a higher efficiency for transforming T to E2 at the female-promoting temperature. Our present results do not allow us to decide whether the diencephalon is the cause or the effect, but they conclusively demonstrate that, in L. olivacea, this region of the brain senses temperature during sex determination.

Yolk steroids decline during sexual differentiation in the alligator

The leading explanation of temperature-dependent sex determination (TSD) in reptiles postulates that (1) ovarian differentiation is directed by estrogen and that (2) estrogen is synthesized in the developing gonad following induction of aromatase expression. However, the source of steroid substrate for aromatization has not yet been identified. In addition, sex ratios vary as a function of clutch, but such biases are as yet unexplained. To address these issues, we measured estradiol, testosterone, and androstenedione in yolks of the American alligator (Alligator mississippiensis) before, during, and after the period of gonadal differentiation in this TSD species. Eggs were collected from a wild population in Louisiana and were incubated at male- and female-determining constant temperatures in the lab, as well as at intermediate temperatures that produced both sexes. Steroids were assayed in yolk extracts after celite column chromatography. All three steroids were found to be in the range of nanograms/gram of yolk at stage 16. Androstenedione was the predominant steroid, 2- to 3-fold higher in concentration than estradiol and 15- to 20-fold higher than testosterone. The levels of these steroids declined (5- to 30-fold) between stages 16 and 25, most markedly between stages 21 and 23, regardless of incubation temperature. The chronology of this sharp decline in steroid levels in our study coincides with the timing of gonadal differentiation in this species, between stages 21 to 23 based on previous reports. Estradiol levels in yolks differed by 3-fold in some clutches relative to others, whereas, no clutch differences were apparent for either androstenedione or testosterone. These data demonstrate that alligator yolk contains high concentrations of two steroid substrates utilized for estrogen synthesis, as well as significant quantities of estradiol itself. We hypothesize that estradiol levels in yolk provide a steroid background, variable among and within clutches, on which gonadal development is initiated and proceeds. As a consequence, we suggest that yolk provides an epigenetic maternal contribution that modulates the effect of incubation temperature on hatchling sex.