The pathway to femaleness: current knowledge on embryonic development of the ovary

A model to study human ovotesticular syndrome

Ovotesticular syndrome is a rare disorder of sex development characterized by the presence of testicular and ovarian tissue. The histologic characteristics of human testicular tissue are well defined by the presence of seminiferous cords or tubules containing TSPY-positive germ cells and Sox9-positive Sertoli cells surrounded by interstitial tissue containing cytochrome P450-positive Leydig cells and smooth muscle α-actin-positive peritubular myoid cells. The histological characteristics of the ovary can be defined by germ cell nests and the development of follicles. In contrast to the testis, the ovary has a paucity of defined specific protein markers, with the granulosa cell marker FOXL2 being the most widely used. In practice, defining the ovarian component of the ovotestis can be quite difficult. We developed a model of human ovotesticular syndrome by combining fetal human testis and ovary in a xenograft model. Ovotesticular xenografts were grown under the renal capsules of gonadectomized athymic nude mice for 6-32 weeks along with age matched control grafts of fetal testis and ovary. Forty ovotesticular xenografts and their controls were analyzed by histology, immunohistochemistry, and fluorescent in situ hybridization to determine the protein expression and karyotype of the cells within the grafts. The ovotesticular xenografts exhibited recognizable testicular and ovarian tissue based on testis-specific and ovary-specific markers defined above. The xenografts simulated a bipolar ovotestis in which the testicular and ovarian elements retain their separate histological characteristics and are separated by a well-defined border. This contrasts with the compartmentalized ovotestis previously described in the literature where the testicular tissue is surrounded by ovarian tissue or a mixed histology where testicular and ovarian tissues are interspersed throughout the gonad. In conclusion, we have characterized a human model of ovotestis which will allow a deeper understanding of ovotestis development in humans and facilitate a more accurate diagnosis of the ovotesticular syndrome.

Sex-specific differences in zebrafish brains

In this systematic review, we highlight the differences between the male and female zebrafish brains to understand their differentiation and their use in studying sex-specific neurological diseases. Male and female brains display subtle differences at the cellular level which may be important in driving sex-specific signaling. Sex differences in the brain have been observed in humans as well as in non-human species. However, the molecular mechanisms of brain sex differentiation remain unclear. The classical model of brain sex differentiation suggests that the steroid hormones derived from the gonads are the primary determinants in establishing male and female neural networks. Recent studies indicate that the developing brain shows sex-specific differences in gene expression prior to gonadal hormone action. Hence, genetic differences may also be responsible for differentiating the brain into male and female types. Understanding the signaling mechanisms involved in brain sex differentiation could help further elucidate the sex-specific incidences of certain neurological diseases. The zebrafish model could be appropriate for enhancing our understanding of brain sex differentiation and the signaling involved in neurological diseases. Zebrafish brains show sex-specific differences at the hormonal level, and recent advances in RNA sequencing have highlighted critical sex-specific differences at the transcript level. The differences are also evident at the cellular and metabolite levels, which could be important in organizing sex-specific neuronal signaling. Furthermore, in addition to having one ortholog for 70% of the human gene, zebrafish also shares brain structural similarities with other higher eukaryotes, including mammals. Hence, deciphering brain sex differentiation in zebrafish will help further enhance the diagnostic and pharmacological intervention of neurological diseases.

Feminization of channel catfish with 17β-oestradiol involves methylation and expression of a specific set of genes independent of the sex determination region

Exogenous oestrogen 17β-oestradiol (E2) has been shown to effectively induce feminization in teleosts. However, the molecular mechanisms underlying the process remain unclear. Here, we determined global DNA methylation and gene expression profiles of channel catfish (Ictalurus punctatus) during early sex differentiation after E2 treatment. Overall, the levels of global DNA methylation after E2 treatment were not significantly different from those of controls. However, a specific set of genes were differentially methylated, which included many sex differentiation-related pathways, such as MARK signalling, adrenergic signalling, Wnt signalling, GnRH signalling, ErbB signalling, and ECM-receptor interactions. Many genes involved in these pathways were also differentially expressed after E2 treatment. Specifically, E2 treatments resulted in upregulation of female-related genes and downregulation of male-related genes in genetic males during sex reversal. However, E2-induced sex reversal did not cause sex-specific changes in methylation profiles or gene expression within the sex determination region (SDR) on chromosome 4, suggesting that E2-induced sex reversal was a downstream process independent of the sex determination process that was regulated by sex-specific methylation within the SDR.

Screening and Identification of Differential Ovarian Proteins before and after Induced Ovulation via Seminal Plasma in Bactrian Camels

Simple Summary

Camelidae are induced ovulators whose ovulation is tightly regulated by multiple factors. Understanding the biological mechanisms underlying follicular development, hormone secretion, and ovulation requires investigating the potential molecular pathways involved in these mechanisms. However, little is known about these molecular pathways in Bactrian camels. To screen and identify candidate biomarkers after seminal plasma (SP)-induced ovulation in the ovaries, we performed comprehensive proteomic and molecular biological analyses of the ovaries from camels that were intramuscularly injected with either seminal plasma or phosphate-buffered saline. Identification of these candidate biomarkers will enable a better understanding of reproduction in Bactrian camels. Our findings suggest candidate proteins for further studies on the molecular mechanisms of induced ovulation.

Abstract

Camelidae are induced ovulators whose ovulation is tightly regulated by multiple factors. Understanding the biological mechanisms underlying follicular development, hormone secretion, and ovulation requires investigating the potential molecular pathways involved. However, little is known about these pathways in Bactrian camels. To screen and identify candidate biomarkers after inducing ovulation, this study performed comprehensive proteomic and molecular biological analyses of the ovaries from two camel groups (n = 6). We identified 5075 expressed ovarian proteins, of which 404 were differentially expressed (264 upregulated, 140 downregulated) (p < 0.05 or p < 0.01), in samples from plasma-induced versus control camels. Gene ontology annotation identified the potential functions of the differentially expressed proteins (DEPs). These results validated the differential expression for a subset of these proteins using Western blot (p < 0.05) and immunofluorescence staining. Three DEPs (FST, NR5A1, and PRL) were involved in neurochemical signal transduction, as well as endocrine and reproductive hormone regulatory processes. The Kyoto Encyclopedia of Genes and Genomes analysis indicated the involvement of several pathways, including the calcium, cAMP, gonadotropin-releasing hormone, MAPK, and neuroactive ligand–receptor signaling pathways, suggesting that induced ovulation depends on the hypothalamic–pituitary–ovarian axis. Identifying these candidate biomarkers enables a better understanding of Bactrian camel reproduction. Ovarian proteomic profiling and the measurement of selected proteins using more targeted methods is a promising approach for studying induced-ovulation mechanisms.

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.

Epigenetic regulation of gonadal and brain aromatase expression in a cichlid fish with environmental sex determination

A fit animal must develop testes or ovaries, with brain and physiology to match. In species with alternative male morphs this coordination of development across tissues operates within sexes as well as between. For Pelvicachromis pulcher, an African cichlid in which early pH exposure influences both sex and alternative male morph, we sequence both copies of aromatase (cyp19a1), a key gene for sex determination. We analyze gene expression and epigenetic state, comparing gonad and brain tissue from females, alternative male morphs, and fry. Relative to brain, we find elevated expression of the A-copy in the ovaries but not testes. Methylation analysis suggests strong epigenetic regulation, with one region specifying sex and another specifying tissue. We find elevated brain expression of the B-copy with no sex or male morph differences. B-copy methylation follows that of the A-copy rather than corresponding to B-copy expression. In 30-day old fry, we see elevated B-copy expression in the head, but we do not see the expected elevated A-copy expression in the trunk that would reflect ovarian development. Interestingly, the A-copy epialleles that distinguish ovaries from testes are among the most explanatory patterns for variation among fry, suggesting epigenetic marking of sex prior to differentiation and thus laying the groundwork for mechanistic studies of epigenetic regulation of sex and morph differentiation.

Disorders of Sexual Development: Current Status and Progress in the Diagnostic Approach

Disorders of sexual development (DSD) are conditions with an atypical chromosomal, gonadal or phenotypic sex, which leads to differences in the development of the urogenital tract and different clinical phenotypes. Some genes have been implicated in the sex development during gonadal and functional differentiation where the maintenance of the somatic sex of the gonad as either male or female is achieved by suppression of the alternate route. The diagnosis of DSD requires a structured approach, involving a multidisciplinary team and different molecular techniques. We discuss the dimorphic genes and the specific pathways involved in gonadal differentiation, as well as new techniques for genetic analysis and their diagnostic value including epigenetic mechanisms, expanding the evidence in the diagnostic approach of individuals with DSD to increase knowledge of the etiology.

The transcriptional regulator CBX2 and ovarian function: A whole genome and whole transcriptome approach

The chromobox homolog 2 (CBX2) was found to be important for human testis development, but its role in the human ovary remains elusive. We conducted a genome-wide analysis based on DNA adenine methyltransferase identification (DamID) and RNA sequencing strategies to investigate CBX2 in the human granulosa cells. Functional analysis revealed that CBX2 was upstream of genes contributing to ovarian function like folliculogenesis and steroidogenesis (i.e. ESR1, NRG1, AKR1C1, PTGER2, BMP15, BMP2, FSHR and NTRK1/2). We identified CBX2 regulated genes associated with polycystic ovary syndrome (PCOS) such as TGFβ, MAP3K15 and DKK1, as well as genes implicated in premature ovarian failure (POF) (i.e. POF1B, BMP15 and HOXA13) and the pituitary deficiency (i.e. LHX4 and KISS1). Our study provided an excellent opportunity to identify genes surrounding CBX2 in the ovary and might contribute to the understanding of ovarian physiopathology causing infertility in women.

The germline-specific expression of Foxl3a and its paralogous Foxl3b are associated with male gonadal differentiation in the Japanese eel, Anguilla japonica

Unlike its paralog Foxl2, which is well known for its role in ovarian development in vertebrates, the function of Foxl3 is still unclear. Foxl3 is an ancient duplicated copy of Foxl2. It is present as a single copy in ray-finned fish. But, due to repeated losses, it is absent in most tetrapods. Our transcriptomic data, however, show that two Foxl3s (Foxl3a and its paralog Foxl3b) are present in Japanese eel. Foxl3a is predominantly expressed in the pituitary, and Foxl3b is predominantly expressed in the gills. Both Foxl3s show a sex-dimorphic expression, being higher expression in testes than in ovaries. Moreover, Foxl3a and Foxl3b were exclusively expressed during gonadal differentiation in control eels (100% male). Conversely, Foxl3a and Foxl3b significantly decreased after gonadal differentiation in E2-treated eels (100% female). Furthermore, in accordance the difference in adhesive ability between somatic cells and germline cells in testes, Foxl3s showed a high expression in suspension cells (putative germline cells) and low expression in adhesive cells (putative somatic cells). In situ hybridization further showed that Foxl3a and Foxl3b were expressed in the testicular germline cells. In addition, Foxl3s expression was not changed by sex steroids in in vitro testes culture. Taken together, our results suggest that the teleost-specific Foxl3 paralog was repeatedly lost in most fish after the third round of whole genome duplication. The two germline-expressed Foxl3s had higher expression levels in males than in females during gonadal differentiation in Japanese eel. These results demonstrated that Foxl3s might play an important role in germline sexual fate determination from ancient fish to modern fish.

Update on the genetics of differences of sex development (DSD)

Human gonadal development is regulated by the temporospatial expression of many different genes with critical dosage effects. Subsequent sex steroid hormone production requires several consecutive enzymatic steps and functional hormone receptors. Disruption of this complex process can result in atypical sex development and lead to conditions referred to as differences (disorders) of sex development (DSD). With the advent of massively parallel sequencing technologies, in silico protein modeling and innovative tools for the generation of animal models, new genes and pathways have been implicated in the pathogenesis of these conditions. Here, we provide an overview of the currently known DSD genes and mechanisms involved in the process of gonadal and phenotypical sex development and highlight phenotypic findings that may trigger further diagnostic investigations.

Hormone Production by Human First-Trimester Gonads in a Functional In Vitro System

In the past, explant tissue-culture methodologies have been used to grow gonads and study their development. Results from in vitro cultures of human gonads showed limited progress toward gonadal cell differentiation and were focused mainly on germ-cell differentiation. Thus, detailed studies focusing on human first-trimester gonadal tissue functionality in vitro are still missing. In this study we investigated the endocrine function of human first-trimester gonads in vitro. We included 27 female and 28 male gonadal samples, derived from a total of 55 cases, at postconceptional ages of 4.5 to 10.5 weeks. Tissues were cultured using an explant tissue-culture system for 14 days. Assays for testosterone (liquid chromatography-tandem mass spectrometry), anti-Müllerian hormone (AMH; ELISA), and inhibin B (ELISA) were performed using media collected after 7 and 14 days of culture. We demonstrated sex- and age-dependent secretion profiles of testosterone, AMH, and inhibin B in the culture media, which resemble the pattern of hormone production in human gonads in vivo, from the few available studies at the same age range. Our study shows that explant tissue-culture conditions are robust for culture of human first-trimester gonadal somatic cells. Thus, it can be used to study human gonadal development and related diseases as well as the effect of potentially hormone-disturbing substances in human gonads during development. However, detailed molecular studies are needed for better understanding of the mechanistic control of the endocrine function of human first-trimester gonads.

Filtered reproductive long non-coding RNAs by genome-wide analyses of goat ovary at different estrus periods

Background

The goat is an important farm animal. Reproduction is an important process of goat farming. The ovary is the most important reproductive organ for goats. In recent years, an increasing number of long non-coding RNAs (lncRNAs) have been implicated in the regulation of mammal reproduction. However, there are few studies on the function of lncRNAs in reproduction, particularly lncRNAs in the ovary.

Results

The sequencing of goat ovaries generated 1,122,014,112 clean reads, and 4926 lncRNAs and 1454 TUCPs (transcripts of uncertain coding potential) were identified for further analysis by using the coding potential analysis software, CNCI, CPC and Pfam-sca. There were 115 /22 differential lncRNAs /TUCPs transcripts between the ovaries of the luteal phase and the follicular phase. We predicted the related genes of lncRNA /TUCP based on co-expression and co-localization methods. In total, 2584 /904 genes were predicted by co-expression, and 326/73 genes were predicted by co-localization. The functions of these genes were further analyzed with GO and KEGG analysis. The results showed that lncRNAs /TUCPs, which are highly expressed in goat ovaries in the luteal phase, are mainly associated with the synthesis of progesterone, and we filtered the lncRNAs /TUCPs, such as XR_001918177.1 and TUCP_001362, which may regulate the synthesis of progesterone; lncRNAs /TUCPs, which are highly expressed in goat ovaries in the follicular phase, are mainly associated with oogenesis and the maturation of oocytes, and we filtered the lncRNAs /TUCPs that may regulate the oogenesis and maturation of oocyte, such as XR_001917388.1 and TUCP_000849.

Conclusion

The present study provided the genome expression profile of lncRNAs /TUCPs in goat ovaries at different estrus periods and filtered the potential lncRNAs /TUCPs associated with goat reproduction. These results are helpful to further study the molecular mechanisms of goat reproduction.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5268-7) contains supplementary material, which is available to authorized users.

Disorders of sex development

Normal sex development depends on the precise spatio-temporal sequence and coordination of mutually antagonistic activating and repressing factors. These factors regulate the commitment of the unipotential gonad into the binary pathways governing normal sex development. Typically, the presence of the SRY gene on the Y chromosome triggers the cascade of molecular events that lead to male sex development. Disorders of sex development comprise a heterogeneous group of congenital conditions associated with atypical development of internal and external genitalia. These disorders are generally attributed to deviations from the typical progression of sex development. Disorders of sex development can be classified into several categories including chromosomal, gonadal, and anatomic abnormalities. Genetic tools such as microarray analyses and next-generation sequencing techniques have identified novel genetic variants among patients with disorders of sexual development. Most importantly, patient management needs to be individualized, especially for decisions related to sex of rearing, surgical interventions, hormone treatment, and potential for fertility preservation.

Developmental Programming of Ovarian Functions and Dysfunctions

The pathophysiological mechanisms underlying the origin of several ovarian pathologies remain unclear. In addition to the genetic basis, developmental insults are gaining attention as a basis for the origin of these pathologies. Such early insults include maternal over or under nutrition, stress, and exposure to environmental chemicals. This chapter reviews the development and physiological function of the ovary, the known ovarian pathologies, the developmental check points of ovarian differentiation impacted by developmental insults, the role played by steroidal and metabolic factors as mediaries, the epigenetic mechanisms via which these mediaries induce their effects, and the knowledge gaps for targeting future studies to ultimately aid in the development of improved treatments.

Novel population of small tumour-initiating stem cells in the ovaries of women with borderline ovarian cancer

Small stem cells with diameters of up to 5 μm previously isolated from adult human ovaries indicated pluripotency and germinal lineage, especially primordial germ cells, and developed into primitive oocyte-like cells in vitro. Here, we show that a comparable population of small stem cells can be found in the ovarian tissue of women with borderline ovarian cancer, which, in contrast to small stem cells in "healthy" ovaries, formed spontaneous tumour-like structures and expressed some markers related to pluripotency and germinal lineage. The gene expression profile of these small putative cancer stem cells differed from similar cells sorted from "healthy" ovaries by 132 upregulated and 97 downregulated genes, including some important forkhead box and homeobox genes related to transcription regulation, developmental processes, embryogenesis, and ovarian cancer. These putative cancer stem cells are suggested to be a novel population of ovarian tumour-initiating cells in humans.

Gender-specific medicine in the genomic era

This article is intended to illuminate several important changes in our concept of gender-specific medicine in the genomic era. It reviews the history of gender-specific medicine, pointing out the changes in our perception of the nature of biological sex and our expanding knowledge of how it affects the phenotype. The old debate about 'nature versus nurture' is now largely resolved; the two are inextricably intertwined as a result of epigenomic regulation of gene expression; many of the resulting phenotypic changes are inherited and affect future generations. More accurate, rapid and cheaper methods of editing genomic composition are implementing a more sophisticated understanding of how genes function and how individual components of the genome might be added or eliminated to maintain health and prevent disease. As Venter predicted, the new discipline of synthetic biology, based on the creation and use of novel 'designer' chromosomes is an inevitable expansion of our ability to decipher the naturally occurring genome and the factors that control its expression. As we move with unexpected and stunning rapidity into our exploration and manipulation of the genetic code, our investigations must acknowledge the solidly established fact that biological sex will have a profound impact on the interventions we have made and will make in the future. Unfortunately, in spite of the recent urging of the National Institutes of Health (NIH) that sex be included as an essential variable in all levels of scientific investigation, genuine issues remain to be resolved before all scientists accept not only the importance of doing this, but also how to implement it.

Morphology, sex steroid level and gene expression analysis in gonadal sex reversal of triploid female (XXX) rainbow trout (Oncorhynchus mykiss)

In non-mammalian vertebrates, estrogens and expressions of cyp19a1 and foxl2 play critical roles in maintaining ovary differentiation and development, while dmrt1 and sox9 are male-specific genes in testicular differentiation and are highly conserved. In order to deeply understand the morphological change, sex steroids level and molecular mechanism of triploid female gonadal reversal in rainbow trout, we studied the ovary morphology, tendency of estradiol-17β (E2) and testosterone (T) levels and the relative expressions of dmrt1, cyp19a1, sox9 and foxl2 in juvenile and adult fish. Our results demonstrated that the development of triploid female gonads in rainbow trout went through arrested development, oocytes dedifferentiation, ovary reconstruction and sex reversal finally. During early gonadal development (154-334 days post-fertilization), the expressions of foxl2 and cyp19a1 increased linearly, while expressions of dmrt1 and sox9 were extremely suppressed, and E2 level was higher, while T level was lower. During the mid-to-late period of triploid female gonadal development (574-964 days post-fertilization), the expressions of dmrt1 and sox9 remained high and were very close to the quantity of diploid male genes, and T levels were even reaching diploid male plasma concentrations, while expressions of cyp19a1 and foxl2 were decreased, leading to decrease in E2 level. We realized that the development model of rainbow trout triploid female gonads was extremely rare, and the regulatory mechanism was very special. Genes involved in gonadal development and endogenous estrogens are pivotal factors in fish natural sex reversal.

Divergent expression regulation of gonad development genes in medaka shows incomplete conservation of the downstream regulatory network of vertebrate sex determination

Genetic control of male or female gonad development displays between different groups of organisms a remarkable diversity of "master sex-determining genes" at the top of the genetic hierarchies, whereas downstream components surprisingly appear to be evolutionarily more conserved. Without much further studies, conservation of sequence has been equalized to conservation of function. We have used the medaka fish to investigate the generality of this paradigm. In medaka, the master male sex-determining gene is dmrt1bY, a highly conserved downstream regulator of sex determination in vertebrates. To understand its function in orchestrating the complex gene regulatory network, we have identified targets genes and regulated pathways of Dmrt1bY. Monitoring gene expression and interactions by transgenic fluorescent reporter fish lines, in vivo tissue-chromatin immunoprecipitation and in vitro gene regulation assays revealed concordance but also major discrepancies between mammals and medaka, notably amongst spatial, temporal expression patterns and regulations of the canonical Hedgehog and R-spondin/Wnt/Follistatin signaling pathways. Examination of Foxl2 protein distribution in the medaka ovary defined a new subpopulation of theca cells, where ovarian-type aromatase transcriptional regulation appears to be independent of Foxl2. In summary, these data show that the regulation of the downstream regulatory network of sex determination is less conserved than previously thought.

Epigenetic control of gonadal aromatase (cyp19a1) in temperature-dependent sex determination of red-eared slider turtles

In the red-eared slider turtle (Trachemys scripta), a species with temperature-dependent sex determination (TSD), the expression of the aromatase gene during gonad development is strictly limited to the female-producing temperature. The underlying mechanism remains unknown. In this study, we identified the upstream 5'-flanking region of the aromatase gene, gonad-specific promoter, and the temperature-dependent DNA methylation signatures during gonad development in the red-eared slider turtle. The 5'-flanking region of the slider aromatase exhibited sequence similarities to the aromatase genes of the American alligator, chicken, quail, and zebra finch. A putative TATA box was located 31 bp upstream of the gonad-specific transcription start site. DNA methylation at the CpG sites between the putative binding sites of the fork head domain factor (FOX) and vertebrate steroidogenic factor 1 (SF1) and adjacent TATA box in the promoter region were significantly lower in embryonic gonads at the female-producing temperature compared the male-producing temperature. A shift from male- to female-, but not from female- to male-, producing temperature changed the level of DNA methylation in gonads. Taken together these results indicate that the temperature, particularly female-producing temperature, allows demethylation at the specific CpG sites of the promoter region which leads the temperature-specific expression of aromatase during gonad development.

Diversity and plasticity of sex determination and differentiation in fishes

Among vertebrates, fishes show an exceptional range of reproductive strategies regarding the expression of their sexuality. Fish sexualities were categorized into gonochorism, synchronous/sequential hermaphrodite, or unisexual reproduction. In gonochoristic fishes, sex is determined genetically or by environmental factors. After sex determination, the gonads are differentiated into ovary or testis, with the sex remaining fixed for the entire life cycle. In contrast, some sequential hermaphrodite fishes can change their sex from male to female (protandrous), female to male (protogynous), or serially (bi-directional sex change) in their life cycle. In many cases, sex change is cued by social factors such as the disappearance of a male or female from a group. This unique diversity in fishes provides an ideal animal model to investigate sex determination and differentiation in vertebrates. This review first discusses genetic-orientated sex determination mechanisms. Then, we address the gonadal sex differentiation process in a gonochoristic fish, using an example of the Nile tilapia. Finally, we discuss various types of sex change that occur in hermaphrodite fishes.

Adult mammalian stem cells: the role of Wnt, Lgr5 and R-spondins

After its discovery as oncogen and morphogen, studies on Wnt focused initially on its role in animal development. With the finding that the colorectal tumour suppressor gene APC is a negative regulator of the Wnt pathway in (colorectal) cancer, attention gradually shifted to the study of the role of Wnt signalling in the adult. The first indication that adult Wnt signalling controls stem cells came from a Tcf4 knockout experiment: mutant mice failed to build crypt stem cell compartments. This observation was followed by similar findings in multiple other tissues. Recent studies have indicated that Wnt agonists of the R-spondin family provide potent growth stimuli for crypts in vivo and in vitro. Independently, Lgr5 was found as an exquisite marker for these crypt stem cells. The story has come full circle with the finding that the stem cell marker Lgr5 constitutes the receptor for R-spondins and occurs in complex with Frizzled/Lrp.

Gene-environment interactions: the potential role of contaminants in somatic growth and the development of the reproductive system of the American alligator

Developing organisms interpret and integrate environmental signals to produce adaptive phenotypes that are prospectively suited for probable demands in later life. This plasticity can be disrupted when embryos are impacted by exogenous contaminants, such as environmental pollutants, producing potentially deleterious and long-lasting mismatches between phenotype and the future environment. We investigated the ability for in ovo environmental contaminant exposure to alter the growth trajectory and ovarian function of alligators at five months after hatching. Alligators collected as eggs from polluted Lake Apopka, FL, hatched with smaller body masses but grew faster during the first five months after hatching, as compared to reference-site alligators. Further, ovaries from Lake Apopka alligators displayed lower basal expression levels of inhibin beta A mRNA as well as decreased responsiveness of aromatase and follistatin mRNA expression levels to treatment with follicle stimulating hormone. We posit that these differences predispose these animals to increased risks of disease and reproductive dysfunction at adulthood.

Altered gonadal expression of TGF-β superfamily signaling factors in environmental contaminant-exposed juvenile alligators

Environmental contaminant exposure can influence gonadal steroid signaling milieus; however, little research has investigated the vulnerability of non-steroidal signaling pathways in the gonads. Here we use American alligators (Alligator mississippiensis) hatched from field-collected eggs to analyze gonadal mRNA transcript levels of the activin-inhibin-follistatin gene expression network and growth differentiation factor 9. The eggs were collected from Lake Woodruff National Wildlife Refuge, a site with minimal anthropogenic influence, and Lake Apopka, a highly contaminated lake adjacent to a former EPA Superfund site. The hatchling alligators were raised for 13 months under controlled conditions, thus limiting differences to embryonic origins. Our data reveal sexually dimorphic mRNA expression in 13-month-old alligator gonads similar to patterns established in vertebrates with genetic sex determination. In addition, we observed a relationship between lake of origin and mRNA expression of activin/inhibin subunits α and βB, follistatin, and growth differentiation factor 9. Our study suggests that embryonic exposure to environmental contaminants can affect future non-steroidal signaling patterns in the gonads of a long-lived species.

Current concepts in disorders of sexual development

Disorders of sex development (DSD) with or without ambiguous genitalia require medical attention to reach a definite diagnosis. Advances in identification of molecular causes of abnormal sex, heightened awareness of ethical issues and this necessitated a re-evaluation of nomenclature. The term DSD was proposed for congenital conditions in which chromosomal, gonadal or anatomical sex is atypical. In general, factors influencing sex determination are transcriptional regulators, whereas factors important for sex differentiation are secreted hormones and their receptors. The current intense debate on the management of patients with intersexuality and related conditions focus on four major issues: 1) aetiological diagnosis, 2) assignment of gender, 3) indication for and timing of genital surgery, 4) the disclosure of medical information to the patient and his/her parents. The psychological and social implications of gender assignment require a multidisciplinary approach and a team which includes ageneticist, neonatologist, endocrinologist, gynaecologist, psychiatrist, surgeon and a social worker. Each patient should be evaluated individually by multidisciplinary approach.

Regulation and expression of sexual differentiation factors in embryonic and extragonadal tissues of Atlantic salmon

Background

The products of cyp19, dax, foxl2, mis, sf1 and sox9 have each been associated with sex-determining processes among vertebrates. We provide evidence for expression of these regulators very early in salmonid development and in tissues outside of the hypothalamic-pituitary-adrenal/gonadal (HPAG) axis. Although the function of these factors in sexual differentiation have been defined, their roles in early development before sexual fate decisions and in tissues beyond the brain or gonad are essentially unknown.

Results

Bacterial artificial chromosomes containing salmon dax1 and dax2, foxl2b and mis were isolated and the regulatory regions that control their expression were characterized. Transposon integrations are implicated in the shaping of the dax and foxl2 loci. Splice variants for cyp19b1 and mis in both embryonic and adult tissues were detected and characterized. We found that cyp19b1 transcripts are generated that contain 5'-untranslated regions of different lengths due to cryptic splicing of the 3'-end of intron 1. We also demonstrate that salmon mis transcripts can encode prodomain products that present different C-termini and terminate before translation of the MIS hormone. Regulatory differences in the expression of two distinct aromatases cyp19a and cyp19b1 are exerted, despite transcription of their transactivators (ie; dax1, foxl2, sf1) occurring much earlier during embryonic development.

Conclusions

We report the embryonic and extragonadal expression of dax, foxl2, mis and other differentiation factors that indicate that they have functions that are more general and not restricted to steroidogenesis and gonadogenesis. Spliced cyp19b1 and mis transcripts are generated that may provide regulatory controls for tissue- or development-specific activities. Selection of cyp19b1 transcripts may be regulated by DAX-1, FOXL2 and SF-1 complexes that bind motifs in intron 1, or by signals within exon 2 that recruit splicing factors, or both. The potential translation of proteins bearing only the N-terminal MIS prodomain may modulate the functions of other TGF β family members in different tissues. The expression patterns of dax1 early in salmon embryogenesis implicate its role as a lineage determination factor. Other roles for these factors during embryogenesis and outside the HPAG axis are discussed.

FOXL2 and BMP2 act cooperatively to regulate follistatin gene expression during ovarian development

Follistatin is a secreted glycoprotein required for female sex determination and early ovarian development, but the precise mechanisms regulating follistatin (Fst) gene expression are not known. Here, we investigate the roles of bone morphogenetic protein 2 (BMP2) and forkhead-domain transcription factor L2 (FOXL2) in the regulation of Fst expression in the developing mouse ovary. Bmp2 and Fst showed similar temporal profiles of mRNA expression, whereas FOXL2 protein and Fst mRNA were coexpressed in the same ovarian cells. In a cell culture model, both FOXL2 and BMP2 up-regulated Fst expression. In ex vivo mouse fetal gonad culture, exogenous BMP2 increased Fst expression, but this effect was counteracted by the BMP antagonist Noggin. Moreover, in Foxl2-null mice, Fst expression was reduced throughout fetal ovarian development, and Bmp2 expression was also reduced. Our data support a model in which FOXL2 and BMP2 cooperate to ensure correct expression of Fst in the developing ovary. Further, Wnt4-knockout mice showed reduced expression of Fst limited to early ovarian development, suggesting a role for WNT4 in the initiation, but not the maintenance, of Fst expression.

Expression and localization of forkhead transcriptional factor 2 (Foxl2) in the gonads of protogynous wrasse, Halichoeres trimaculatus

BACKGROUND

Three-spot wrasse, Halichoeres trimaculatus, is a marine protogynous hermaphrodite fish. Individuals mature either as initial phase (IP) males or females. Appropriate social cues induce the sex change from IP female to terminal phase (TP) male. However, the molecular mechanisms behind such a sex change remain largely unknown. Recently, the forkhead transcription factor 2 (Foxl2) was identified as an essential regulator of vertebrate ovarian development/function/phenotype. Inspired by this information, we characterized the expression patterns of Foxl2 in the protogynous wrasse assuming Foxl2 as the female-specific marker in this species.

METHODS

First, we clonedFoxl2 cDNA from ovary by reverse transcription polymerase chain reaction (RT-PCR) followed by rapid amplification of cDNA ends (RACE). Next, we analysed expression pattern of Foxl2 messenger RNA (mRNA) and protein in gonads of different sexual phases by real time quantitative PCR assay and flour fluorescence immunohistochemical method, respectively. Additionally, we studied the changes in Foxl2 expression pattern during aromatase inhibitor (AI)-induced sex change.

RESULTS

The amino acid sequence (306 AA) of wrasse Foxl2, especially the forkhead domain, shows high identity with that of other reported teleost Foxl2s. Quite unexpectedly, no sexual dimorphism was observable between the testes and ovary in the expression pattern of Foxl2. In female phase fish, signals for Foxl2 protein were detectable in the granulosa cells, but not the theca cells. Transcript levels of Foxl2 in the testes of IP and TP males were identical to that in the ovaries of females and, further, Foxl2 protein was found to be localized in the interstitial cells including tubules and Leydig cells. Treatment with AI induced sex change in male gonads and an up-regulation was seen in the expression of Foxl2 in these gonads.

CONCLUSIONS

Unlike in other vertebrates, including teleosts, Foxl2 may have a different role in the naturally sex changing fishes.

Epigenetic effects of endocrine-disrupting chemicals on female reproduction: an ovarian perspective

The link between in utero and neonatal exposure to environmental toxicants, such as endocrine-disrupting chemicals (EDCs) and adult female reproductive disorders is well established in both epidemiological and animal studies. Recent studies examining the epigenetic mechanisms involved in mediating the effects of EDCs on female reproduction are gathering momentum. In this review, we describe the developmental processes that are susceptible to EDC exposures in female reproductive system, with a special emphasis on the ovary. We discuss studies with select EDCs that have been shown to have physiological and correlated epigenetic effects in the ovary, neuroendocrine system, and uterus. Importantly, EDCs that can directly target the ovary can alter epigenetic mechanisms in the oocyte, leading to transgenerational epigenetic effects. The potential mechanisms involved in such effects are also discussed.

Expression of ladybird-like homeobox 2 (LBX2) during ovarian development and folliculogenesis in the mouse

The Ladybird-like homeobox gene 2 (Lbx2) belongs to the homeodomain-containing family of transcription factor that are known to play crucial role in various developmental processes. During early mouse embryogenesis, Lbx2 was shown to be expressed in the developing eye, brain and urogenital system. Although Lbx2 was detected in the testis and epididymis throughout development, no data was available regarding its expression in the female gonad. Here we have determined Lbx2 expression throughout mouse ovarian development by in situ hybridization. In contrast to the strong expression in the male fetal gonad, no Lbx2 signal could be detected in the fetal ovary. Soon after birth, however, Lbx2 expression was detected at different levels in various ovarian compartments (oocyte, granulosa cells, theca cells) where its expression was highly dynamic depending on the stage of follicular maturation. Our data would be consistent with a role for LBX2 in ovarian maturation and folliculogenesis.

Genetic network underlying temperature-dependent sex determination is endogenously regulated by temperature in isolated cultured Trachemys scripta gonads

In reptiles with temperature-dependent sex determination, gonadogenesis is initially directed by the incubation temperature of the egg during the middle third of embryonic development. The mechanism by which temperature is transduced into a sex-determining molecular signal remains a mystery, and here we examine the molecular network underlying sex determination in gonads in vitro. We use a whole organ culture system to show that expression of putative members of the sex-determining network (Dmrt1, Sox9, Mis, and FoxL2) are regulated by temperature endogenously within cells in the bipotential gonad and do not require other embryonic tissues to be expressed in a normal pattern in the red-eared slider turtle, Trachemys scripta. Furthermore, following a change in temperature, these factors exhibit temperature-responsive expression patterns that last for the duration of gonadogenesis. Finally, mosaic misexpression of a fusion Sox9 construct demonstrates the ability to functionally manipulate the gonad at the molecular level.

Gonadal mRNA expression levels of TGFbeta superfamily signaling factors correspond with post-hatching morphological development in American alligators

Paracrine factor signaling regulates many aspects of vertebrate gonadal development. We investigated key ovarian and testicular morphological markers of the American alligator (Alligator mississippiensis) during the first 5 months post-hatching and correlated gonadal development with mRNA expression levels of a suite of regulatory factors. In both sexes, we observed significant morphology changes, including ovarian follicle assembly and meiotic progression of testicular germ cells. Concomitant with these changes were sexually dimorphic and ontogenetically variable mRNA expressions. In ovaries, FOXL2, aromatase, and follistatin mRNA expression was greater than in testes at all ages. At one week after hatching, we observed ovarian medullary remodeling in association with elevated activin/inhibin beta A subunit, follistatin, and aromatase mRNA expressions. Three and 5 months following hatching and concomitant with follicle assembly, ovaries showed increased mRNA expression levels of GDF9 and the mitotic factor PCNA. In testes, the activin/inhibin alpha and beta B subunit transcript levels were greater than in ovaries at all ages. Elevated testicular expression of GDF9 mRNA levels at 5 months after hatching aligned with increased spermatogenic activity. We propose that the mRNA expression levels and concomitant morphological changes observed here affect the establishment of alligator reproductive health and later fertility.

Building pathways for ovary organogenesis in the mouse embryo

Despite its significant role in oocyte generation and hormone production in adulthood, the ovary, with regard to its formation, has received little attention compared to its male counterpart, the testis. With the exception of germ cells, which undergo a female-specific pattern of meiosis, morphological changes in the fetal ovary are subtle. Over the past 40 years, a number of hypotheses have been proposed for the organogenesis of the mammalian ovary. It was not until the turn of the millennium, thanks to the advancement of genetic and genomic approaches, that pathways for ovary organogenesis that consist of positive and negative regulators have started to emerge. Through the action of secreted factors (R-spondin1, WNT4, and follistatin) and transcription regulators (beta-catenin and FOXL2), the developmental fate of the somatic cells is directed toward ovarian, while testicular components are suppressed. In this chapter, we review the history of studying ovary organogenesis in mammals and present the most recent discoveries using the mouse as the model organism.

Determination and stability of gonadal sex

The discovery that the SRY gene induces male sex in humans and other mammals led to speculation about a possible equivalent for female sex. But females are proving to be more complicated. Several master genes appear to be autonomously involved, and female sex determination seems to remain relatively labile. Partial loss of function of the transcription factor FOXL2 leads to premature ovarian failure in women; and in animal models, Foxl2 is required for folliculogenesis as well as for maintenance, and possibly induction, of female sex determination. In the germ line, oocytes apparently form normally even in the absence of Foxl2, dependent on genes that include female-specific factors such as Fig-alpha, Nobox, etc. In the soma, ablation of Foxl2 or the independently expressed gene Wnt4 (likely downstream of Rspo1) can produce partial testis differentiation in XX mice, and the double knockout results in the formation of tubules and spermatogonia. This indicates that at least 2 autonomous ovarian pathways are required to antagonize testis differentiation in females, a finding that is being increasingly corroborated by studies in goats and nonmammalian vertebrates. In recent expression profiling of mouse ovaries that lack Foxl2 alone or in combination with Wnt4 or Kit/c-Kit, we found that following Foxl2 loss, early testis genes (including the downstream effector of Sry, Sox9) and several novel ovarian genes were consistently dysregulated during embryo-fetal development. The results support the proposal of dose-dependent Foxl2 function and antitestis action. A partial working model for somatic development and sex determination is presented in which Sox9 is a direct antagonist of Foxl2 in the supporting cell lineage.

N- and E-cadherin expression in human ovarian and urogenital duct development

OBJECTIVE

To investigate expression of N- and E-cadherin in the developing human ovary.

DESIGN

The expression of N- and E-cadherin was analyzed in 18 human fetal ovaries between 8 and 20 weeks' gestation using immunohistochemistry. Fetal human male and rat urogenital tracts were used for comparison of expression.

SETTING

Academic research institute.

PATIENT(S)

Women undergoing termination of pregnancy.

INTERVENTION(S)

Immunofluorescent analysis of cadherin expression.

RESULT(S)

In fetal ovary, N- and E-cadherins were expressed at all gestations with overlapping but not identical patterns. Expression was associated with germ cells and adjacent somatic cells, including within newly formed primordial follicles, but neither cadherin was expressed in the somatic cell cords. The epithelia of the müllerian and wolffian ducts expressed only N- and E-cadherin, respectively, in a mutually exclusive fashion. This pattern of cadherin expression was found to be conserved between human and rat fetuses of both genders.

CONCLUSION(S)

The demonstration of N- and E-cadherin expression in the human fetal ovary indicates likely roles in gonadal development from germ cell proliferation to primordial follicle formation, as well as in the development of the urogenital ducts of both genders. This is consistent with animal studies identifying cadherins as key regulators of early germ cell development.

Effects of gestational diethylstilbestrol treatment on male and female gonads during early embryonic development

To study the effects of gestational exposure to estrogen on early gonadal differentiation, pregnant mice were treated by sc injection of diethylstilbestrol (DES) or vehicle from embryonic day (E) 8.5 to E14.5, and gonads at E11.5, E12.5, and E14.5 were examined. Quantitative real-time RT-PCR and in situ hybridization revealed that mRNA levels of steroidogenic factor 1 (SF-1), a key regulator of gonadal differentiation, and several male gonad-specific genes, including Müllerian-inhibiting substance (MIS), steroidogenic acute regulatory protein, cholesterol side-chain cleavage cytochrome P450, and Cerebellin 1 precursor protein, were significantly decreased in the DES-treated testis, compared with the control testis at E12.5 and/or E14.5. Immunohistochemistry demonstrated that the staining intensities for SF-1 and MIS in Sertoli cells were apparently reduced in the DES-treated testis, compared with those of the controls, at E12.5 and E14.5. Because MIS, steroidogenic acute regulatory protein, cholesterol side-chain cleavage cytochrome P450, and Cerebellin 1 precursor protein are activated under the regulation of SF-1, the down-regulation of these factors may be due to reduced SF-1 expression. Immunohistochemistry for laminin-1 demonstrated that ovigerous cords in the DES-treated ovary were smaller than those in controls at E14.5. Moreover, the number of 5-bromo-2'deoxyuridine-5-monophosphate-labeled cells in the DES-treated testis was significantly reduced at E12.5 and E14.5, compared with controls, and that in the DES-treated ovary remained higher than that in the control ovary at E14.5. The results suggest that exogenous estrogens can alter sex-specific genetic pathways governing early differentiation and cell proliferation of both male and female gonads.

Dual roles of cyp19a1a in gonadal sex differentiation and development in the protandrous black porgy, Acanthopagrus schlegeli

Protandrous black porgy fish, Acanthopagrus schlegeli, have a striking life cycle, with male sex differentiation at the juvenile stage, a bisexual gonad during first 2 yr of life, and a male-to-female sex change (with vitellogenic oocytes) at 3 yr of age. The present study investigated the role of aromatase (cyp19a1a/Cyp19a1a) in gonadal development in this species, especially in relation to sexual differentiation and sex change. Fish of various ages were treated with estradiol (E2) or aromatase inhibitor (AI) to determine whether manipulation of the hormonal environment has an impact on these processes. We report an integrative immunohistochemical, cellular, and molecular data set describing these interesting phenomena. During male sex differentiation, high levels of cyp19a1a/Cyp19a1a expression were observed in the undifferentiated gonad (4 mo of age), in marked contrast to the low cyp19a1a/Cyp19a1a levels detected in the differentiated testis at the age of 5-6 mo. A low dose of E2 (0.25 mg/kg feed) stimulated testicular growth and function in sexually differentiated fish, whereas a high dose of E2 (6 mg/kg feed) induced female development. Furthermore, administration of AI suppressed male development and promoted female sexual differentiation. An increased number of figla transcripts (an oocyte-specific gene) were observed prior to cyp19a1a expression, concomitant with the development of oogonia and early primary oocytes in the ovaries of both E2- and AI-treated groups. Immunohistochemical Pcna staining showed that the regression of testicular tissue occurred prior to the development of ovarian tissue in both E2- and AI-induced females. The importance of cyp19a1a in female development was further demonstrated by the increase in cyp19a1a transcripts during the naturally occurring sex change. Transcripts of foxl2 increased in the gonads of 2- to 3-yr-old black porgy during the early stages of the natural sex change, followed by a gradual elevation of cyp19a1a levels. The levels of both genes peaked in the resulting ovarian tissue. Thus, cyp19a1a/Cyp19a1a plays dual roles in the gonadal development, namely, in testicular development during the initial period of sexual differentiation and later in ovarian development during the natural sex change.

SERKAL syndrome: an autosomal-recessive disorder caused by a loss-of-function mutation in WNT4

The WNT-signaling pathway plays a major role during mammalian embryogenesis. We report a novel autosomal-recessive syndrome that consists of female to male sex reversal and renal, adrenal, and lung dysgenesis and is associated with additional developmental defects. Using a candidate-gene approach, we identified a disease-causing homozygous missense mutation in the human WNT4 gene. The mutation was found to result in markedly reduced WNT4 mRNA levels in vivo and in vitro and to downregulate WNT4-dependent inhibition of beta-catenin degradation. Taken together with previous observations in animal models, the present data attribute a pivotal role to WNT4 signaling during organogenesis in humans.

Characterization of early molecular sex differentiation in rainbow trout, Oncorhynchus mykiss

Early differentiation in rainbow trout gonads was investigated by expression profiling and in situ hybridization (ISH). Expression of cyp19a1 and fst in females and sox9a1 in males were sexually dimorphic between 32 to 35 days post-fertilization (dpf). After 35 dpf, the differentiation proceeded with sexually dimorphic profiles for sox9a2, dmrt1, cyp11b2.1, amh in males and foxl2a, foxl2b, hsd3b1, inha in females. cyp17a1, cyp11a1, star, nr5a1b increased only after 40 dpf in both sexes with a slightly higher expression in females. cyp19a1 expression was localized in a cluster of somatic cells in the ventral side of female gonads, and sox9a2 and amh in somatic cells surrounding the germ cells, at 28 dpf and thereafter, both in male and female gonads. cyp11b2.1, cyp17a1, and cyp11a1 expressions were only detected in scattered somatic cells in males after 46 dpf. This confirms the early implication of cyp19a1 in trout ovarian differentiation and suggests that early testicular differentiation does not need androgen production.

Regulation of the gonadal transcriptome during sex determination and testis morphogenesis: comparative candidate genes

Gene expression profiles during sex determination and gonadal differentiation were investigated to identify new potential regulatory factors. Embryonic day 13 (E13), E14, and E16 rat testes and ovaries were used for microarray analysis, as well as E13 testis organ cultures that undergo testis morphogenesis and develop seminiferous cords in vitro. A list of 109 genes resulted from a selective analysis for genes present in male gonadal development and with a 1.5-fold change in expression between E13 and E16. Characterization of these 109 genes potentially important for testis development revealed that cytoskeletal-associated proteins, extracellular matrix factors, and signaling factors were highly represented. Throughout the developmental period (E13-E16), sex-enriched transcripts were more prevalent in the male with 34 of the 109 genes having testis-enriched expression during sex determination. In ovaries, the total number of transcripts with a 1.5-fold change in expression between E13 and E16 was similar to the testis, but none of those genes were both ovary enriched and regulated during the developmental period. Genes conserved in sex determination were identified by comparing changing transcripts in the rat analysis herein, to transcripts altered in previously published mouse studies of gonadal sex determination. A comparison of changing mouse and rat transcripts identified 43 genes with species conservation in sex determination and testis development. Profiles of gene expression during E13-E16 rat testis and ovary development are presented and candidate genes for involvement in sex determination and testis differentiation are identified. Analysis of cellular pathways did not reveal any specific pathways involving multiple candidate genes. However, the genes and gene network identified influence numerous cellular processes with cellular differentiation, proliferation, focal contact, RNA localization, and development being predominant.

Germ cells are essential for sexual dimorphism in the medaka gonad

To further elucidate the roles of germ cells in the sex differentiation of gonads, we have used the medaka, a teleost fish, to generate mutants that lack germ cells from the onset of gonadogenesis by the morpholino-mediated knockdown of cxcr4. The resulting germ-cell-deficient medaka show female-to-male sex reversal of their secondary sex characteristics, accompanied by increased levels of androgen and reduced levels of estrogen. A failure to maintain granulosa cells or estrogen-producing cells also occurs at early stages of sex differentiation in the cxcr4 morphants, before the initiation of gonadal morphogenesis. In contrast, androgen-producing cells are unaffected in germ-cell-deficient medaka of either sex. In addition, a single tube-like gonad that expresses male-specific genes is formed in these mutants irrespective of the genetic sex. Significantly, each of these mutant phenotypes occurs in a somatic cell-autonomous manner, suggesting that gonadal somatic cells are predisposed toward male development in the absence of germ cells. This highlights the importance of germ cells in the sexual dimorphism of the gonads.

Androgen-induced masculinization in rainbow trout results in a marked dysregulation of early gonadal gene expression profiles

Background

Fish gonadal sex differentiation is affected by sex steroids treatments providing an efficient strategy to control the sexual phenotype of fish for aquaculture purposes. However, the biological effects of such treatments are poorly understood. The aim of this study was to identify the main effects of an androgen masculinizing treatment (11β-hydroxyandrostenedione, 11βOHΔ4, 10 mg/kg of food for 3 months) on gonadal gene expression profiles of an all-female genetic population of trout. To characterize the most important molecular features of this process, we used a large scale gene expression profiling approach using rainbow trout DNA microarrays combined with a detailed gene ontology (GO) analysis.

Results

2,474 genes were characterized as up-regulated or down-regulated in trout female gonads masculinized by androgen in comparison with control male or female gonads from untreated all-male and all-female genetic populations. These genes were classified in 13 k-means clusters of temporally correlated expression profiles. Gene ontology (GO) data mining revealed that androgen treatment triggers a marked down-regulation of genes potentially involved in early oogenesis processes (GO 'mitotic cell cycle', 'nucleolus'), an up-regulation of the translation machinery (GO 'ribosome') along with a down-regulation of proteolysis (GO 'proteolysis', 'peptidase' and 'metallopeptidase activity'). Genes considered as muscle fibres markers (GO 'muscle contraction') and genes annotated as structural constituents of the extracellular matrix (GO 'extracellular matrix') or related to meiosis (GO 'chromosome' and 'meiosis') were found significantly enriched in the two clusters of genes specifically up-regulated in androgen-treated female gonads. GO annotations 'Sex differentiation' and 'steroid biosynthesis' were enriched in a cluster of genes with high expression levels only in control males. Interestingly none of these genes were stimulated by the masculinizing androgen treatment.

Conclusion

This study provides evidence that androgen masculinization results in a marked dysregulation of early gene expression profiles when compared to natural testicular or ovarian differentiation. Based on these results we suggest that, in our experimental conditions, androgen masculinization proceeds mainly through an early inhibition of female development.

Sex-specific differences in mouse DMRT1 expression are both cell type- and stage-dependent during gonad development

Immunohistochemistry was used to examine GCNA1, a germ cell-specific protein, together with DMRT1 (Doublesex and Mab-3-related transcription factor-1), a transcription factor implicated in Sertoli cell and germ cell function, in order to resolve DMRT1's cellular profile during pre- and postnatal gonad development in the mouse. In the indifferent gonad (10.5-11.5 days postcoitus [dpc]), DMRT1 localized to somatic cells and GCNA1(+) germ cells and was indistinguishable in males and females. By 12.5 dpc, a clear sexual preference for DMRT1 in male somatic cells was observed, with male DMRT1 localized to testicular cords and more abundant in Sertoli cells than in germ cells and female DMRT1 diffusely labeled and markedly lower in somatic cells than in germ cells. A male somatic preference continued throughout development, with DMRT1 evident in Sertoli cells at all ages examined and absent in ovarian somatic cells from 13.5 dpc onward. In contrast, expression in primordial germ cells was not sexually distinct, and both sexes showed DMRT1 increasing through 13.5 dpc and absent by 15.5 dpc. Notably, sexual differences in germ cell DMRT1 were detected after birth, when it was detected only in spermatogonia of the testis. Colocalization of DMRT1 with proliferation markers KI67 and proliferating cell nuclear antigen (PCNA) and stem cell markers OCT4 (also known as POU5F1) and NGN3 indicated that, in postnatal testes, DMRT1 was present in both stem and proliferating spermatogonia. Together, the findings implicate opposite functions for DMRT1 in somatic and germ cells of the testis. In Sertoli cells, DMRT1 expression correlated with differentiation, whereas in germ cells, it suggested a role in expansion and maintenance of undifferentiated spermatogonia.

Loss of Wnt4 and Foxl2 leads to female-to-male sex reversal extending to germ cells

The discovery that the SRY gene induces male sex in humans and other mammals led to speculation about a possible equivalent for female sex. However, only partial effects have been reported for candidate genes experimentally tested so far. Here we demonstrate that inactivation of two ovarian somatic factors, Wnt4 and Foxl2, produces testis differentiation in XX mice, resulting in the formation of testis tubules and spermatogonia. These genes are thus required to initiate or maintain all major aspects of female sex determination in mammals. The two genes are independently expressed and show complementary roles in ovary morphogenesis. In addition, forced expression of Foxl2 impairs testis tubule differentiation in XY transgenic mice, and germ cell-depleted XX mice lacking Foxl2 and harboring a Kit mutation undergo partial female-to-male sex reversal. The results are all consistent with an anti-testis role for Foxl2. The data suggest that the relative autonomy of the action of Foxl2, Wnt4 and additional ovarian factor(s) in the mouse should facilitate the dissection of their respective contributions to female sex determination.

Response of candidate sex-determining genes to changes in temperature reveals their involvement in the molecular network underlying temperature-dependent sex determination

Gonadogenesis, the process of forming an ovary or a testis from a bipotential gonad, is critical to the development of sexually reproducing adults. Although the molecular pathway underlying vertebrate gonadogenesis is well characterized in organisms exhibiting genotypic sex determination, it is less well understood in vertebrates whose sex is determined by environmental factors. We examine the response of six candidate sex-determining genes to sex-reversing temperature shifts in a species with temperature-dependent sex determination (TSD). For the first time, we report the regulation of FoxL2, Wnt4, Dmrt1, and Mis by temperature, confirming their involvement in the molecular pathway underlying TSD and placing them downstream of the action of temperature. We find evidence that FoxL2 plays an ovarian-specific role in development, whereas Wnt4 appears to be involved in both testis and ovary formation. Dmrt1 expression shows rapid activation in response to a shift to male-producing temperature, whereas Mis up-regulation is delayed. Furthermore, early repression of Mis appears critical to ovarian development. We also investigate Dax1 and Sox9 and reveal that at the level of gene expression, response to temperature is comparatively later in gonadogenesis. By examining the role of these genes in TSD, we can begin to elucidate elements of conservation and divergence between sex-determining mechanisms.