Aromatase plays a key role during normal and temperature-induced sex differentiation of tilapia Oreochromis niloticus

Evidence that estrogen regulates the sex change of honeycomb grouper (Epinephelus merra), a protogynous hermaphrodite fish

Circulating estradiol-17beta (E2) levels decrease precipitously during female to male (protogynous) sex change in fish. Whether this drop in E2 levels is a cause or consequence of sex change is still largely unknown. The present study treated adult female honeycomb groupers (Epinephelus merra) with aromatase inhibitor (AI, Fadrozole), either alone or in combination with E2, to investigate the role of estrogen in protogynous sex change. Control fish had ovaries undergoing active vitellogenesis; the gonads of AI-treated fish had already developed into testes, which produced sperm capable of fertilization. In contrast, co-treatment of fish with E2 completely blocked AI-induced sex reversal. AI treatment significantly reduced circulating levels of E2, whereas the addition of E2 to AI prevented the loss. The plasma androgen (testosterone and 11-ketotestosterone) levels were increased in the AI-treated fish, while the levels in the E2-supplemented fish were low compared to controls. Present results show that E2 plays an important role in maintaining female sex of hermaphrodite fishes, and that the inhibition of E2 synthesis causes oocyte degeneration leading to testicular differentiation in the ovary.

Aromatase inhibitor induces complete sex change in the protogynous honeycomb grouper (Epinephelus merra)

The protogynous hermaphrodite fish change sex from female to male at the certain stages of life cycle. The endocrine mechanisms involved in gonadal restructuring throughout protogynous sex change are not clearly understood. In the present study, we implanted maturing female honeycomb groupers with nonsteroidal aromatase inhibitor (AI), Fadrozole (0, 1, and 10 mg/fish) and examined changes in gonadal structures and serum levels of sex steroid hormones 2(1/2) months after implantation. The ovaries of control females had oocytes undergoing active vitellogenesis, whereas AI caused females to develop into functional males. These males had testes, which were indistinguishable in structure from those of normal males, but bigger in size, and completed all stages of spermatogenesis including accumulation of large amount of sperm in the seminiferous tubules. AI significantly reduced the serum levels of estradiol-17beta (E2) and increased levels of testosterone (T), 11-ketotestosterone (11-KT), and 17alpha, 20beta-dihydroxy-4-pregnen-3-one (DHP). Further, AI suppressed in vitro production of E2, and stimulated the production of T and 11-KT in the ovarian fragments of mature female. In the honeycomb grouper, suppression of both in vitro and in vivo production of E2 and degeneration of oocytes by AI suggests that AI induces complete sex change through inhibition of estrogen biosynthesis, and perhaps, subsequent induction of androgen function.

Environmental versus genetic sex determination: a possible factor in dinosaur extinction?

This study examined the possibility that genetically based sex-determination mechanisms have evolved to ensure a balanced male/female ratio and that this temperature-independent checkpoint might have been unavailable to long-extinct reptiles, notably the dinosaurs. A review of the literature on molecular and phylogenetic relationships between modes of reproduction and sex determination in extant animals was conducted. Mammals, birds, all snakes and most lizards, amphibians, and some gonochoristic fish use specific sex-determining chromosomes or genes (genetic sex determination, GSD). Some reptiles, however, including all crocodilians studied to date, many turtle and tortoise species, and some lizards, use environmental or temperature-dependent sex determination (TSD). We show that various modes of GSD have evolved many times, independently in different orders. Animals using TSD would be at risk of rapid reproductive failure due to a skewed sex ratio favoring males in response to sustained environmental temperature change and favoring the selection of sex-determining genes. The disadvantage to the evolving male sex-determining chromosome, however, is its decay due to nonrecombination and the subsequent loss of spermatogenesis genes. Global temperature change can skew the sex ratio of TSD animals and might have played a significant role in the demise of long-extinct species, notably the dinosaurs, particularly if the temperature change resulted in a preponderance of males. Current global warming also represents a risk for extant TSD species.

Sex Inversion of Sexually Immature Honeycomb Grouper (Epinephelus merra) by Aromatase Inhibitor

Previous studies have shown that estrogen plays an important role in sex change of protogynous honeycomb grouper, and that the treatments with aromatase inhibitor (AI) cause estrogen depletion and complete sex inversion of pre-spawning females into functional males. In the present study, we examined whether AI causes sex inversion of sexually immature females. Female honeycomb groupers were implanted with various doses of Fadrozole (0, 100, 500 and 1000 microg/fish) in the non-breeding season, and resultant changes in the gonadal structures and the plasma levels of sex steroid hormones (estradiol-17 beta, E2; testosterone, T; 11-ketotestosterone, 11-KT) were examined three months after implantation. Vehicle-implanted groups did not change sex, while 100 and 500 microg AI-implanted groups had turned into transitionals with intersex gonad. In contrast, the highest dose receiving group exhibited both transitional and male phases. Transitional phase gonad had atretic oocytes and spermatogenic germ cells at the late stages of spermatogenesis, while male phase testis contained spermatozoa accumulated in the seminiferous tubules. All males released sperm upon slight pressure on the abdomen. In the AI-implanted fish, plasma levels of E2 decreased in a dose-dependent manner, while the levels of 11-KT were high in the highest dose receiving group. Present results suggest that estrogen plays an important role in sex change of protogynous honeycomb grouper, and that treatments with AI potentially inhibits endogenous E2 production in vivo, causing oocyte degeneration and subsequently the sex inversion from female to male. The Fadrozole could be an important tool for manipulating the sex of hermaphrodite fishes.

Aromatase activity in the European sea bass (Dicentrarchus labrax L.) brain. Distribution and changes in relation to age, sex, and the annual reproductive cycle

Cytochrome P450 aromatase activity (AA) was measured in different tissues of the sea bass (Dicentrarchus labrax L.) using a tritiated water release assay that was previously optimized and validated for this species. In adult fish entering the reproductive season, AA was highest on a per mg protein basis, in the brain (2.04+/-0.4 pmol/mg prot/h; mean+/-SEM), followed by the ovary (0.59+/-0.1) and was detectable in visceral fat (0.21+/-0.05), liver (0.08+/-0.009), and head kidney (0.03+/-0.004). However, AA was negligible in the rest of the tissues tested: heart, testis, muscle, and spleen. Consistent with results obtained in other species, dissection of the brain into its major constitutive parts revealed that AA was concentrated in areas implicated in the control of reproduction, including the olfactory bulb, telencephalon, and hypothalamus (range: 2.6-16.2 pmol/mg prot/h), as well as the pituitary gland (6.2-9.3 pmol/mg prot/h). Lower AA was noted in the optic bulb, cerebellum, and medulla. However, in contrast to some previously published reports concerning the content and distribution of neural aromatase in fish, males consistently exhibited higher AA than females. In one-year-old juvenile fish completing the process of gonadal sex differentiation, brain AA (0.63 pmol/mg prot/h) was similar in both sexes and ten times lower than that measured in the brain of first time spawners (6.52 pmol/mg prot/h), in this case with males showing an overall higher (24%) activity than females. When surveyed throughout the year, brain AA dramatically changed during the reproductive cycle. Maximum average values of approximately 7 pmol/mg prot/h were obtained that coincided with the spawning season. The peak in brain AA was preceded by two and one months by the peak of plasma testosterone and the peak of the gonadosomatic index, respectively. This is the first measurement of the distribution of the activity of a steroidogenic enzyme in the sea bass, an established model in comparative endocrinology. Together, these results demonstrate sex- and seasonally-related variations in AA and establish the basis for further comparative studies of certain androgen-mediated actions through locally formed estrogen in both central and peripheral targets.

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.

Differential expression of P450 aromatase during gonadal sex differentiation and sex reversal of the newt Pleurodeles waltl

A better understanding of vertebrate sexual differentiation could be provided by a study of models in which genetic sex determination (GSD) of gonads can be reversed by temperature. In the newt Pleurodeles waltl, a P450 aromatase cDNA was isolated from adult gonads, and the nucleotide or deduced amino acid sequences showed a high level of identity with various vertebrate species. In adults, aromatase expression was found in gonads and brain. In developing gonads, the expression was found to fit with the thermo-sensitive period (TSP) and was detected in both ZZ and ZW larvae, as well as in ZW submitted during the whole TSP to a masculinizing temperature. In the latter individuals, in situ hybridization and semi quantitative RT-PCR showed that, at the end of TSP, aromatase expression was at the same level than in normal ZZ larvae and was significantly lower than in normal ZW ones. Furthermore, temperature-induced down regulation did not occur when heating was performed at the end of TSP. Our results confirm the importance of aromatase regulation in female versus male differentiation and demonstrate that a down regulation of aromatase expression is involved in the process of sex reversal.

Characterization of aromatase activity in the sea bass: effects of temperature and different catalytic properties of brain and ovarian homogenates and microsomes

Two aromatase genes have been discovered in the brain and ovary of some teleosts. However, data on native aromatase enzyme kinetics and thus actual catalytic activity are scarce in fish, impeding comparison of aromatase activity (AA) from different organs within and between species. In the present study, the tritiated water assay was optimized and validated to measure AA in the sea bass using 1 beta-[3H]-androstenedione as a substrate in crude homogenates and microsomes. Optimized assay variables included pH, temperature, buffer strength, incubation time, amount of fresh tissue, substrate, and cofactor concentration. Specificity of the assay was verified by using known inhibitors, inappropriate substrates, and heat-inactivation. Subcellular fractionation revealed ten-fold more activity in the microsomal over the cytosolic fraction. The assay was also validated by comparing results from the direct product isolation method. The validated assay described allows measurement of AA to levels as low as < 10 fmol/mg protein/hr. Sex differentiation is temperature-dependent in the sea bass. It was found that in the physiological range of temperatures where the sea bass can live, 10-30 degrees C, AA is highly dependent on temperature in a linear fashion (brain: r2 = 0.92; P < 0.001; ovary: r2 = 0.94; P < 0.001). When AA levels from brain and ovarian homogenates obtained from the same fish during the spawning season were compared, the respective Michaelis-Menten constant (Km) values were 7.3 nM vs. 4.6 nM, with no significant differences detected between the two tissues. Thus, sea bass aromatase has a very high affinity for androstenedione, similar to what has been found in goldfish, but much higher than other piscine or mammalian aromatases (30-435 nM). In contrast, the brain maximum reaction rate (Vmax 7.8 pmol/mg protein/hr) was four-fold higher (P < 0.001) than the ovarian Vmax (2.1 pmol/mg protein/hr). Consistent results were found using purified microsomes. Although this is the first time that the kinetic parameters are reported for a native piscine aromatase in two different tissues within the same fish, it remains to be determined whether this is a reflection of two distinct isoforms in this particular species.

Environment and sex determination in farmed fish

A plasticity of gonadal sex differentiation was reported in the 1930s following exogenous steroid treatments in fish, but demonstration that environmental factors (temperature, pH, density and social interactions) could influence the sex ratio in gonochoristic species has been relatively recent. In fish, as in reptiles and amphibians displaying environmental sex determination, the main environmental factor influencing sex seems to be temperature (TSD=Temperature Sex Determination). In most thermosensitive species (some Atherinids, Poecilids, Cichlids: tilapias, goldfish, a Siluriform, a flatfishellipsis) male to female ratio increases with temperature and/or ovarian differentiation is induced by low temperatures. Conversely, in some rare species (Dicentrarchus labrax, Ictalurus punctatus), high temperatures may produce female-biased sex ratios and/or low temperatures promote male-biased sex ratios. In the hirame Paralichthys olivaceus, both high and low temperatures induce monosex male populations while intermediate temperatures yield a 1:1 sex ratio (U-shape curve). Fish show particularities in their TSD patterns since mono-sex populations are generally not produced at extreme temperatures, suggesting the existence of strong temperature/genotype interactions. In reptiles, amphibians and fish displaying TSD, temperature treatments must be applied at a critical sensitive period, relatively similar to the hormone sensitive period. In gonochoristic fish, steroid hormones with estrogens in females and 11-oxygenated androgens in males, are probably key physiological steps in the regulation of gonadal sex differentiation. Cytochrome P450-aromatase, enzyme catalysing conversion of androgens to estrogens, seems to be a critical enzyme for ovarian differentiation. Molecular mechanisms of thermosensitivity have been addressed in two species tilapia Oreochromis niloticus and the hirame, where aromatase gene expression is down-regulated by masculinizing temperature treatments. Furthermore, in tilapia the gene expression of 11 beta-hydroxylase (a key enzyme involved in the synthesis of 11-oxygenated androgens) does not appear to be affected by temperature treatments.

Search for genes involved in the temperature-induced gonadal sex differentiation in the tilapia, Oreochromis niloticus

In the tilapia, Oreochromis niloticus, sex is determined by genetic factors (XX/XY) but temperature can also influence the gonadal sex differentiation. Elevated temperatures of 35 degrees C can generate functional male phenotypes if applied before and during sexual differentiation. The genes and mechanisms by which temperature acts on the cascade leading to sex differentiation have been investigated. Two strategies have been followed: 1) Search for novel genes by differential display, and 2) Expression studies of candidate genes. Genetically all-female and all-male progenies were reared at 27 degrees C (natural temperature) and at 35 degrees C (masculinizing treatment) and gonads dissected. Using differential display, we isolated a 300 bp cDNA (MM20C) from temperature-masculinized females. Virtual northern analysis revealed a 1.2 kb transcript in 35 degrees C treated females and males, but hardly any expression in natural females (27 degrees C). Semi-quantitative RT-PCR established a several-fold increase in MM20C expression in 35 degrees C masculinized fry. Elevated expression was observed in natural males (27 degrees C) with higher levels detected in those reared at 35 degrees C. Furthermore, we have analyzed as a candidate gene the P450 11beta-hydroxylase, an important androgen steroidogenic enzyme. Low levels of expression were found in natural males. This coincides with low concentrations of 11 ketotestosterone in the gonads before and during gonadal sex differentiation. Higher expression levels of 11beta-hydroxylase were detected in male gonads at 35 degrees C but levels in phenotypic males were similar to those found for natural females. Previous results reported that expression of aromatase is repressed by masculinizing treatments. Our study demonstrated that masculinizing-temperature can also stimulate the expression of other gene(s).