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

Could Exposure to Glyphosate Pose a Risk to the Survival of Wild Animals? A Case Study on the Field Lizard Podarcis siculus

Soil contaminants (herbicides, pesticides, and heavy metals) are among the main causes of change in terrestrial ecosystems. These substances lead to a general loss of biodiversity, both of flora and fauna and being able to biomagnify and pass through the food chain, they can endanger the survival of terrestrial vertebrates at the top of this chain. This review analyzes the risks associated with exposure to glyphosate, the active principle of many herbicide products, for the reproductive health of the field lizard (Podarcis siculus) potentially exposed to the substance in its natural habitat; therefore, introducing it as a possible model organism. Data demonstrate that glyphosate is toxic for this animal, affecting the health of the reproductive organs, both in males and females, and of the liver, the main detoxifying organ and closely involved in the female reproductive process. Sharing structural and functional characteristics of these organs with many other vertebrates, the information obtained with this reptile represents a wake-up call to consider when analyzing the cost/benefit ratio of glyphosate-based substances. The data clearly demonstrate that the P. siculus lizard can be considered a good target organism to study the reproductive risk assessment and hazards of exposure to soil contaminants on wild terrestrial vertebrates.

Systematic review of reptile reproductive toxicology to inform future research directions on endangered or threatened species, such as sea turtles

Threatened or endangered reptiles, such as sea turtles, are generally understudied within the field of wildlife toxicology, with even fewer studies on how contaminants affect threatened species reproduction. This paper aimed to better inform threatened species conservation by systematically and quantitatively reviewing available research on the reproductive toxicology of all reptiles, threatened and non-threatened. This review found 178 studies that matched our search criteria. These papers were categorized into location conducted, taxa studied, species studied, effects found, and chemicals investigated. The most studied taxa were turtles (n = 87 studies, 49%), alligators/crocodiles (n = 54, 30%), and lizards (n = 37, 21%). Maternal transfer, sex steroid alterations, sex reversal, altered sexual development, developmental abnormalities, and egg contamination were the most common effects found across all reptile taxa, providing guidance for avenues of research into threatened species. Maternal transfer of contaminants was found across all taxa, and taking into account the foraging behavior of sea turtles, could help elucidate differences in maternal transfer seen at nesting beaches. Sex steroid alterations were a common effect found with contaminant exposure, indicating the potential to use sex steroids as biomarkers along with traditional biomarkers such as vitellogenin. Sex reversal through chemical exposure was commonly found among species that exhibit temperature dependent sex determination, indicating the potential for both environmental pollution and climate change to disrupt population dynamics of many reptile species, including sea turtles. Few studies used in vitro, DNA, or molecular methodologies, indicating the need for more research using high-throughput, non-invasive, and cost-effective tools for threatened species research. The prevalence of developmental abnormalities and altered sexual development and function indicates the need to further study how anthropogenic pollutants affect reproductive output in threatened reptiles.

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.

Estrogens in Male Physiology

Estrogens have historically been associated with female reproduction, but work over the last two decades established that estrogens and their main nuclear receptors (ESR1 and ESR2) and G protein-coupled estrogen receptor (GPER) also regulate male reproductive and nonreproductive organs. 17β-Estradiol (E2) is measureable in blood of men and males of other species, but in rete testis fluids, E2 reaches concentrations normally found only in females and in some species nanomolar concentrations of estrone sulfate are found in semen. Aromatase, which converts androgens to estrogens, is expressed in Leydig cells, seminiferous epithelium, and other male organs. Early studies showed E2 binding in numerous male tissues, and ESR1 and ESR2 each show unique distributions and actions in males. Exogenous estrogen treatment produced male reproductive pathologies in laboratory animals and men, especially during development, and studies with transgenic mice with compromised estrogen signaling demonstrated an E2 role in normal male physiology. Efferent ductules and epididymal functions are dependent on estrogen signaling through ESR1, whose loss impaired ion transport and water reabsorption, resulting in abnormal sperm. Loss of ESR1 or aromatase also produces effects on nonreproductive targets such as brain, adipose, skeletal muscle, bone, cardiovascular, and immune tissues. Expression of GPER is extensive in male tracts, suggesting a possible role for E2 signaling through this receptor in male reproduction. Recent evidence also indicates that membrane ESR1 has critical roles in male reproduction. Thus estrogens are important physiological regulators in males, and future studies may reveal additional roles for estrogen signaling in various target tissues.

Estrogenic environmental contaminants alter the mRNA abundance profiles of genes involved in gonadal differentiation of the American bullfrog

Wildlife and human populations are exposed to anthropogenic mixtures of chemicals in the environment that may adversely influence normal reproductive function and development. We determined the effects of exposure to estrogenic chemicals and wastewater effluent (WWE) on developing gonads of the American bullfrog, Rana (Lithobates) catesbeiana, a species whose widespread distribution make it an ideal model for environmental monitoring of endocrine effects of chemical contaminants. Premetamorphic bullfrog tadpoles were exposed to treatment vehicle, 17β-estradiol (E2; 10(-9)M) or 4-tert-octylphenol (OP; 10(-9)M, 10(-8)M, and 10(-7)M). Additionally, gonadal differentiation was evaluated in bullfrog tadpoles from a WWE-containing site versus those from a reference location receiving no WWE. In both studies, phenotypic sex, steroidogenic factor-1 (nr5a1), and aromatase (cyp19a1) mRNA levels using quantitative real-time PCR were determined. Exposure to E2 or OP did not alter sex ratios. In controls, both nr5a1 and cyp19a1 transcript levels exhibited sexual dimorphism, with males demonstrating higher levels of nr5a1 and females greater abundance of cyp19a1. However, E2 exposure increased cyp19a1 mRNA abundance in testes and decreased levels in ovaries, eliminating the sexual dimorphism observed in controls. E2-exposed males exhibited increased nr5a1 transcript levels in the testes compared to controls, while females demonstrated no E2 effect. OP treatment had no effect on female cyp19a1 mRNA abundance, but exposure to 10(-7)M OP increased testicular transcript levels. Treatment with 10(-9) and 10(-8)M OP, but not 10(-7)M, resulted in decreased abundance of nr5a1 transcript in both ovaries and testes. Animals from the field had sexually dimorphic gonadal levels of cyp19a1, but both sexes from the WWE site exhibited elevated cyp19a1 transcript abundance compared to the reference location. Individual chemical compounds and anthropogenic wastewater effluent dispersed within the environment influence the levels of gonadal mRNA encoding key proteins involved in gonadal differentiation.

Placental and embryonic tissues exhibit aromatase activity in the viviparous lizard Niveoscincus metallicus

Aromatase is a key regulator of circulating testosterone (T) and 17-β-oestradiol (E2), two steroids which are critical to the development, maintenance and function of reproductive tissues. The role of aromatase in sexual differentiation in oviparous (egg-laying) reptiles is well understood, yet has never been explored in viviparous (live-bearing) reptiles. As a first step towards understanding the functions of aromatase during gestation in viviparous reptiles, we measured aromatase activity in maternal and embryonic tissues at three stages of gestation in the viviparous skink, Niveoscincus metallicus. Maternal ovaries and adrenals maintained high aromatase activity throughout gestation. During the early phases of embryonic development, placental aromatase activity was comparable to that in maternal ovaries, but declined significantly at progressive stages of gestation. Aromatase activity in the developing brains and gonads of embryos was comparable with measurements in oviparous reptiles. Aromatase activity in the developing brains peaked mid development, and declined to low levels in late stage embryos. Aromatase activity in the embryonic gonads was low at embryonic stage 29-34, but increased significantly at mid-development and then remained high in late stage embryos. We conclude that ovarian estrogen synthesis is supplemented by placental aromatase activity and that maternal adrenals provide an auxiliary source of sex steroid. The pattern of change in aromatase activity in embryonic brains and gonads suggests that brain aromatase is important during sexual differentiation, and that embryonic gonads are increasingly steroidogenic as development progresses. Our data indicate vital roles of aromatase in gestation and development in viviparous lizards.

Temperature-dependent sex determination modulates cardiovascular maturation in embryonic snapping turtles Chelydra serpentina

We investigated sex differences in cardiovascular maturation in embryos of the snapping turtle Chelydra serpentina, a species with temperature-dependent sex determination. One group of eggs was incubated at 26.5°C to produce males. Another group of eggs was incubated at 26.5°C until embryos reached stage 17; eggs were then shifted to 31°C for 6 days to produce females, and returned to 26.5°C for the rest of embryogenesis. Thus, males and females were at the same temperature when autonomic tone was determined and for most of development. Cholinergic blockade increased resting blood pressure (P(m)) and heart rate (f(H)) in both sexes at 75% and 90% of incubation. However, the magnitude of the f(H) response was enhanced in males compared with females at 90% of incubation. β-adrenergic blockade increased P(m) at 75% of incubation in both sexes but had no effect at 90% of incubation. β-adrenergic blockade reduced f(H) at both time points but produced a stronger response at 90% versus 75% of incubation. We found that α-adrenergic blockade decreased P(m) in both sexes at 75% and 90% of incubation and decreased f(H) at 75% of incubation in both sexes. At 90% of incubation, f(H) decreased in females but not males. Although these data clearly demonstrate sexual dimorphism in the autonomic regulation of cardiovascular physiology in embryos, further studies are needed to test whether differences are caused by endocrine signals from gonads or by a hormone-independent temperature effect.

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.

Fish with thermolabile sex determination (TSD) as models to study brain sex differentiation

As fish are ectothermic animals, water temperature can affect their basic biological processes such as larval development, growth and reproduction. Similar to reptiles, the incubation temperature during early phases of development is capable to modify sex ratios in a large number of fish species. This phenomenon, known as thermolabile sex determination (TSD) was first reported in Menidia menidia, a species belonging to the family Atherinopsidae. Since then, an increasing number of fish have also been found to exhibit TSD. Traditionally, likewise in reptiles, several TSD patterns have been described in fish, however it has been recently postulated that only one, females at low temperatures and males at high temperatures, may represent the "real" or "true" TSD. Many studies regarding the influence of temperature on the final sex ratios have been focused on the expression and activity of gonadal aromatase, the enzyme involved in the conversion of androgens into estrogens and encoded by the cyp19a1a gene. In this regard, teleost fish, may be due to a whole genome duplication event, produce another aromatase enzyme, commonly named brain aromatase, encoded by the cyp19a1b gene. Contrary to what has been described in other vertebrates, fish exhibit very high levels of aromatase activity in the brain and therefore they synthesize high amounts of neuroestrogens. However, its biological significance is still not understood. In addition, the mechanism whereby temperature can induce the development of a testis or an ovary still remains elusive. In this context the present review is aimed to discuss several theories about the possible role of brain aromatase using fish as models. The relevance of brain aromatase and therefore of neuroestrogens as the possible cue for gonadal differentiation is raised. In addition, the possible role of brain aromatase as the way to keep the high levels of neurogenesis in fish is also considered. Several key examples of how teleosts and aromatase regulation can offer more insight into basic mechanisms of TSD are also reviewed.

Molecular mechanisms of sex determination in reptiles

Charles Darwin first provided a lucid explanation of how gender differences evolve nearly 140 years ago. Yet, a disconnect remains between his theory of sexual selection and the mechanisms that underlie the development of males and females. In particular, comparisons between representatives of different phyla (i.e., flies and mice) reveal distinct genetic mechanisms for sexual differentiation. Such differences are hard to comprehend unless we study organisms that bridge the phylogenetic gap. Analysis of variation within monophyletic groups (i.e., amniotes) is just as important if we hope to elucidate the evolution of mechanisms underlying sexual differentiation. Here we review the molecular, cellular, morphological, and physiological changes associated with sex determination in reptiles. Most research on the molecular biology of sex determination in reptiles describes expression patterns for orthologs of mammalian sex-determining genes. Many of these genes have evolutionarily conserved expression profiles (i.e., DMRT1 and SOX9 are expressed at a higher level in developing testes vs. developing ovaries in all species), which suggests functional conservation. However, expression profiling alone does not test gene function and will not identify novel sex-determining genes or gene interactions. For that reason, we provide a prospectus on various techniques that promise to reveal new sex-determining genes and regulatory interactions among these genes. We offer specific examples of novel candidate genes and a new signaling pathway in support of these techniques.

Cellular mechanisms of sex determination in the red-eared slider turtle, Trachemys scripta

In all vertebrates sex determination is the step at which development of a testis or ovary is initiated in the bipotential gonad. Although Mus musculus and the red-eared slider turtle, Trachemys scripta, use different mechanisms to initiate organogenesis of the testis (the Y-linked gene, Sry, in the mouse vs. the incubation temperature of the egg in the turtle), the structure of the adult testis is strikingly similar. We have identified several cellular mechanisms involved in testis organogenesis in mouse. Here we investigated whether these cellular mechanisms are conserved in T. scripta downstream of the temperature-dependent switch. Cell tracing experiments indicated that the coelomic epithelium in T. scripta contributes precursors for Sertoli cells and interstitial cells as in mouse. However, we detect no male-specific mesonephric cell migration, a process required for the de novo testis cord-forming process in mouse. In contrast to mouse gonads, where no cord structure is discernible until after the divergence of testis development, we find that primitive sex cords continuous with the coelomic epithelium exist in all T. scripta gonads from the earliest bipotential stages examined. We conclude that typical testis architecture results from the maintenance and elaboration of primitive sex cords in T. scripta rather than the assembly of de novo structures as in mouse.

Yolk steroid hormones and sex determination in reptiles with TSD

In reptiles with temperature-dependent sex determination (TSD), the temperature at which the eggs are incubated determines the sex of the offspring. The molecular switch responsible for determining sex in these species has not yet been elucidated. We have examined the dynamics of yolk steroid hormones during embryonic development in the snapping turtle, Chelydra serpentina, and the alligator, Alligator mississippiensis, and have found that yolk estradiol (E(2)) responds differentially to incubation temperature in both of these reptiles. Based upon recently reported roles for E(2) in modulation of steroidogenic factor 1, a transcription factor known to be significant in the sex differentiation process, we hypothesize that yolk E(2) is a link between temperature and the gene expression pathway responsible for sex determination and differentiation in at least some of these species. Here we review the evidence that supports our hypothesis.

Environmental endocrine disrupters dysregulate estrogen metabolism and Ca2+ homeostasis in fish and mammals via receptor-independent mechanisms

Xenoestrogen endocrine disrupters (EDs) in the environment are thought to be responsible for a number of examples of sexual dysfunction that have recently been reported in several species. There is growing concern that these compounds may also cause abnormalities of the male reproductive tract and reduced spermatogenesis in man. Whilst some effects of EDs may be receptor-mediated, there is growing evidence that these compounds can exert potent effects in vivo by directly interacting with cellular enzyme targets. Here we report on, and review, the effects of alkylphenols and other EDs on two such enzymes: (1) sulfotransferases, which convert active estrogenic steroids to inactive steroid sulfates; and (2) Ca(2+)-ATPases, which are responsible for maintaining low, physiological, intracellular Ca(2+) concentrations. These enzymes are potently inhibited by EDs in both fish and mammalian species. The increased concentrations of active estrogens and the likely cytotoxic effects of elevated concentrations of intracellular Ca(2+) arising from these effects may underlie some of the endocrine disrupting potential of these widespread industrial pollutants.

Cloning and expression of aromatase in a turtle with temperature-dependent sex determination

It has been hypothesized that estrogen production may play a pivotal role in the sex determination of reptiles with temperature-dependent sex determination (TSD). This hypothesis has been furthered by studies that have shown higher aromatase activity in the developing ovaries in some reptiles. However, other studies have not consistently supported this hypothesis. In the current study we addressed this issue by cloning P450 aromatase cDNA in the turtle, Trachemys scripta, and developing a quantitative competitive RT-PCR for aromatase. This assay was then used to quantify aromatase mRNA levels in adrenal-kidney-gonad complexes (AKG) during TSD. Aromatase mRNA was detected in the AKGs at both male- and female-producing temperatures from the earliest stage of development sampled (stage 15), through hatching (stage 26). However, levels remained relatively constant during the thermosensitive period of TSD. Further, no significant difference was detected between male- and female-producing temperatures during the thermosensitive period. After the thermosensitive period, aromatase mRNA levels increased in females (this coincides with the period during which the ovaries are differentiating). These results are consistent with those of several previous studies of certain reptiles with TSD. The current results suggest that the expression of aromatase may not be a pivotal regulatory step in the sex determination cascade of this turtle.

Role of steroidogenic factor 1 and aromatase in temperature-dependent sex determination in the red-eared slider turtle

Red-eared slider turtles are genetically bipotential for sex determination. In this species, as in many other reptiles, incubation temperature of the egg determines gonadal sex. At higher incubation temperatures females are produced and increasing temperature appears to increase estrogen production in the embryonic brain. Treatment of eggs incubating at a male-producing temperature with exogenous estrogen causes ovaries to form. At a female-biased incubation temperature, prevention of estrogen biosynthesis or administration of nonaromatizable androgens results in the development of testes. In mammals, steroidogenic factor 1 (SF-1) regulates most genes required for estrogen biosynthesis, including aromatase. In both mammals and red-eared sliders, SF-1 is differentially expressed in males and females during gonadogenesis. We have examined both SF-1 gene expression and aromatase activity in embryos incubating at different temperatures and after manipulation to change the course of gonadal development. Our findings indicate a central role for SF-1 in enacting the effect of estrogen. Estrogen treatment directly or indirectly downregulates SF-1 and, ultimately, causes development of females. The inhibition of estrogen results in upregulation of SF-1 and male hatchlings. Thus, SF-1 may lie at the center of one molecular crossroad in male versus female differentiation of the red-eared slider.