DMRT1 is upregulated in the gonads during female-to-male sex reversal in ZW chicken embryos

Identification, expression, and regulation of anti-Müllerian hormone type-II receptor in the embryonic chicken gonad

Anti-Müllerian hormone (AMH) signaling is required for proper development of the urogenital system in vertebrates. In male mammals, AMH is responsible for regressing the Müllerian ducts, which otherwise develop into the fallopian tubes, oviducts, and upper vagina of the female reproductive tract. This role is highly conserved across higher vertebrates. However, AMH is required for testis development in fish species that lack Müllerian ducts, implying that AMH signaling has broader roles in other vertebrates. AMH signals through two serine/threonine kinase receptors. The primary AMH receptor, AMH receptor type-II (AMHR2), recruits the type I receptor, which transduces the signal intracellularly. To enhance our understanding of AMH signaling and the potential role of AMH in gonadal sex differentiation, we cloned chicken AMHR2 cDNA and examined its expression profile during gonadal sex differentiation. AMHR2 is expressed in the gonads and Müllerian ducts of both sexes but is more strongly expressed in males after the onset of gonadal sex differentiation. In the testes, the AMHR2 protein colocalizes with AMH, within Sertoli cells of the testis cords. AMHR2 protein expression is up-regulated in female embryos treated with the estrogen synthesis inhibitor fadrozole. Conversely, knockdown of the key testis gene DMRT1 leads to disruption of AMHR2 expression in the developing seminiferous cords of males. These results indicate that AMHR2 is developmentally regulated during testicular differentiation in the chicken embryo. AMH signaling may be important for gonadal differentiation in addition to Müllerian duct regression in birds.

Over-expression of DMRT1 induces the male pathway in embryonic chicken gonads

DMRT1 encodes a conserved transcription factor with an essential role in gonadal function. In the chicken, DMRT1 in located on the Z sex chromosome and is currently the best candidate master regulator of avian gonadal sex differentiation. We previously showed that knockdown of DMRT1 expression during the period of sexual differentiation induces feminisation of male embryonic chicken gonads. This gene is therefore necessary for proper testis development in the chicken. However, whether it is sufficient to induce testicular differentiation has remained unresolved. We show here that over-expression of DMRT1 induces male pathway genes and antagonises the female pathway in embryonic chicken gonads. Ectopic DMRT1 expression in female gonads induces localised SOX9 and AMH expression. It also induces expression of the recently identified Z-linked male factor, Hemogen (HEMGN). Masculinised gonads show evidence of cord-like structures and retarded female-type cortical development. Furthermore, expression of the critical feminising enzyme, aromatase, is reduced in the presence of over-expressed DMRT1. These data indicate that DMRT1 is an essential sex-linked regulator of gonadal differentiation in avians, and that it likely acts via a dosage mechanism established through the lack of global Z dosage compensation in birds.

Sexually dimorphic and sex-independent left-right asymmetries in chicken embryonic gonads

Female birds develop asymmetric gonads: a functional ovary develops on the left, whereas the right gonad regresses. In males, however, testes develop on both sides. We examined the distribution of germ cells using Vasa/Cvh as a marker. Expression is asymmetric in both sexes: at stage 35 the left gonad contains significantly more germ cells than the right. A similar expression pattern is seen for expression of ERNI (Ens1), a gene expressed in chick embryonic stem cells while they self-renew, but downregulated upon differentiation. Other pluripotency-associated markers (PouV/Oct3/4, Nanog and Sox2) also show asymmetric expression (more expressing cells on the left) in both sexes, but this asymmetry is at least partly due to expression in stromal cells of the developing gonad, and the pattern is different for all the genes. Therefore germ cell and pluripotency-associated genes show both sex-dependent and independent left-right asymmetry and a complex pattern of expression.

Overexpression of aromatase alone is sufficient for ovarian development in genetically male chicken embryos

Estrogens play a key role in sexual differentiation of both the gonads and external traits in birds. The production of estrogen occurs via a well-characterised steroidogenic pathway, which is a multi-step process involving several enzymes, including cytochrome P450 aromatase. In chicken embryos, the aromatase gene (CYP19A1) is expressed female-specifically from the time of gonadal sex differentiation. To further explore the role of aromatase in sex determination, we ectopically delivered this enzyme using the retroviral vector RCASBP in ovo. Aromatase overexpression in male chicken embryos induced gonadal sex-reversal characterised by an enlargement of the left gonad and development of ovarian structures such as a thickened outer cortex and medulla with lacunae. In addition, the expression of key male gonad developmental genes (DMRT1, SOX9 and Anti-Müllerian hormone (AMH)) was suppressed, and the distribution of germ cells in sex-reversed males followed the female pattern. The detection of SCP3 protein in late stage sex-reversed male embryonic gonads indicated that these genetically male germ cells had entered meiosis, a process that normally only occurs in female embryonic germ cells. This work shows for the first time that the addition of aromatase into a developing male embryo is sufficient to direct ovarian development, suggesting that male gonads have the complete capacity to develop as ovaries if provided with aromatase.

Temporospatial expression of Dmrt1 in chicken urogenital system (Gallus gallus) using whole mount in situ hybridization

Doublesex and mab-3-related transcription factor 1 (Dmrt1) is a Z-linked gene that putatively determines the phenotype of gonads in birds. The sex differential expression of Dmrt1 was examined using wholemount in situ hybridization (WISH) in the urogenital systems during embryogenesis. The results revealed that Dmrt1 showed dimorphic expression in chicken gonads, which increased from day 6.5 to day 10.5. The expression of Dmrt1 in male (ZZ) gonads was not twice as much as in female (ZW) gonads, suggesting the existence of other regulatory mechanisms in addition to Z chromosome dosage effect.

Chicken hemogen homolog is involved in the chicken-specific sex-determining mechanism

Using a comprehensive transcriptome analysis, a Z chromosome-linked chicken homolog of hemogen (cHEMGN) was identified and shown to be specifically involved in testis differentiation in early chicken embryos. Hemogen [Hemgn in mice, EDAG (erythroid differentiation-associated gene protein) in humans] was recently characterized as a hematopoietic tissue-specific gene encoding a transcription factor that regulates the proliferation and differentiation of hematopoietic cells in mammals. In chicken, cHEMGN was expressed not only in hematopoietic tissues but also in the early embryonic gonad of male chickens. The male-specific expression was identified in the nucleus of (pre)Sertoli cells after the sex determination period and before the expression of SOX9 (SRY-box 9). The expression of cHEMGN was induced in ZW embryonic gonads that were masculinized by aromatase inhibitor treatment. ZW embryos overexpressing cHEMGN, generated by infection with retrovirus carrying cHEMGN, showed masculinized gonads. These findings suggest that cHEMGN is a transcription factor specifically involved in chicken sex determination.

Organogenesis of the ovary: a comparative review on vertebrate ovary formation

The general perspective of ovary organogenesis is that the ovary is the default organ which develops in the absence of testis-promoting factors. Testis formation, on the other hand, is a male-specific event promoted by active components that override the default ovarian process. However, when comparing the sex determination mechanism among different vertebrate species, it is apparent that this default view of ovary formation can only be applied to mammals. In species such as reptiles and birds, ovary formation is an active process stimulated by estrogen. Remnants of this estrogen-dominant pathway are still present in marsupials, a close relative of eutherian mammals, like humans and mice. Although initial formation of the mammalian ovary has become strictly regulated by genetic components and is therefore independent of estrogen, the feminizing effect of estrogen regains its command in adult ovaries. When estrogen production, or its signaling, is inhibited, transdifferentiation of ovarian tissues to testis structures occur in adult females. Taken together, these observations prompt us to reconsider the process of ovary organogenesis as the default organ and question if testis development is actually the default pathway.

The molecular genetics of ovarian differentiation in the avian model

In birds as in mammals, sex is determined at fertilization by the inheritance of sex chromosomes. However, sexual differentiation - development of a male or female phenotype - occurs during embryonic development. Sex differentiation requires the induction of sex-specific developmental pathways in the gonads, resulting in the formation of ovaries or testes. Birds utilize a different sex chromosome system to that of mammals, where females are the heterogametic sex (carrying Z and W chromosomes), while males are homogametic (carrying 2 Z chromosomes). Therefore, while some genes essential for testis and ovarian development are conserved, important differences also exist. Namely, the key mammalian male-determining factor SRY does not exist in birds, and another transcription factor, DMRT1, plays a central role in testis development. In contrast to our understanding of testis development, ovarian differentiation is less well-characterized. Given the presence of a female-specific chromosome, studies in chicken will provide insight into the induction and function of female-specific gonadal pathways. In this review, we discuss sexual differentiation in chicken embryos, with emphasis on ovarian development. We highlight genes that may play a conserved role in this process, and discuss how interaction between ovarian pathways may be regulated.

Masculine epigenetic sex marks of the CYP19A1/aromatase promoter in genetically male chicken embryonic gonads are resistant to estrogen-induced phenotypic sex conversion

Sex of birds is genetically determined through inheritance of the ZW sex chromosomes (ZZ males and ZW females). Although the mechanisms of avian sex determination remains unknown, the genetic sex is experimentally reversible by in ovo exposure to exogenous estrogens (ZZ-male feminization) or aromatase inhibitors (ZW-female masculinization). Expression of various testis- and ovary-specific marker genes during the normal and reversed gonadal sex differentiation in chicken embryos has been extensively studied, but the roles of sex-specific epigenetic marks in sex differentiation are unknown. In this study, we show that a 170-nt region in the promoter of CYP19A1/aromatase, a key gene required for ovarian estrogen biosynthesis and feminization of chicken embryonic gonads, contains highly quantitative, nucleotide base-level epigenetic marks that reflect phenotypic gonadal sex differentiation. We developed a protocol to feminize ZZ-male chicken embryonic gonads in a highly quantitative manner by direct injection of emulsified ethynylestradiol into yolk at various developmental stages. Taking advantage of this experimental sex reversal model, we show that the epigenetic sex marks in the CYP19A1/aromatase promoter involving DNA methylation and histone lysine methylation are feminized significantly but only partially in sex-converted gonads even when morphological and transcriptional marks of sex differentiation show complete feminization, being indistinguishable from gonads of normal ZW females. Our study suggests that the epigenetic sex of chicken embryonic gonads is more stable than the morphologically or transcriptionally characterized sex differentiation, suggesting the importance of the nucleotide base-level epigenetic sex in gonadal sex differentiation.

The potential role of microRNAs in regulating gonadal sex differentiation in the chicken embryo

Differential gene expression regulates tissue morphogenesis. The embryonic gonad is a good example, where the developmental decision to become an ovary or testis is governed by female- or male-specific gene expression. A number of genes have now been identified that control gonadal sex differentiation. However, the potential role of microRNAs (miRNAs) in ovarian and testicular pathways is unknown. In this review, we summarise our current understanding of gonadal differentiation and the possible involvement of miRNAs, using the chicken embryo as a model system. Chickens and other birds have a ZZ/ZW sex chromosome system, in which the female, ZW, is the heterogametic sex, and the male, ZZ, is homogametic (opposite to mammals). The Z-linked DMRT1 gene is thought to direct testis differentiation during embryonic life via a dosage-based mechanism. The conserved SOX9 gene is also likely to play a key role in testis formation. No master ovary determinant has yet been defined, but the autosomal FOXL2 and Aromatase genes are considered central. No miRNAs have been definitively shown to play a role in embryonic gonadal development in chickens or any other vertebrate species. Using next generation sequencing, we carried out an expression-based screen for miRNAs expressed in embryonic chicken gonads at the time of sexual differentiation. A number of miRNAs were identified, including several that showed sexually dimorphic expression. We validated a subset of miRNAs by qRT-PCR, and prediction algorithms were used to identify potential targets. We discuss the possible roles for these miRNAs in gonadal development and how these roles might be tested in the avian model.

Insulin-like growth factor 3 regulates expression of genes encoding steroidogenic enzymes and key transcription factors in the Nile tilapia gonad

Insulin-like growth factors (Igfs) are implicated in a wide variety of physiological roles in teleost gonadal development and reproduction. In the present study, igf3 mRNA expression in the tilapia ovary was found to be higher than in the testis from 5 to 40 days after hatching (dah) but was lower than that in testis from 50 to 70 dah. Consistently, Igf3 protein signal was detected in the somatic cells of XX and XY gonads from 10 dah until adulthood by immunohistochemistry, using a specific Igf3 polyclonal antibody. Incubation of ovarian and testicular cells in primary culture with recombinant Igf3 significantly increased nr5a1, foxl2, dmrt1, cyp19a1a, cyp11a1, cyp11b2, hsd3b2 , and cyp17a1 expression in a time- and dose-dependent manner. Promoter analysis using luciferase assays in HEK293 cells revealed that igf3 promoter activity was directly activated by Nr5a1 (Sf1) and further enhanced by Foxl2, Nr0b1a (Dax1), and Nr0b1b (Dax2) but repressed by Dmrt1 and estrogen receptor (Esr1, Esr2a, or Esr2b) along with 17beta-estradiol treatment. In addition, igf3 promoter activity was increased slightly by forskolin treatment alone but synergistically up-regulated by transfection with nr5a1. These in vitro results correlated well with the expression profile of igf3 during early gonad differentiation. Our results indicated that igf3 is involved in fish gonad steroidogenesis because of its ability to regulate the expression of foxl2, dmrt1, and nr5a1 and steroidogenic enzymes. The expression of igf3 is in turn regulated by transcription factors Foxl2, Dmrt1, and Nr5a1, as well as by 17beta-estradiol treatment.

Molecular mechanisms of temperature-dependent sex determination in the context of ecological developmental biology

Temperature-dependent sex determination (TSD) is a prime example of phenotypic plasticity in that gonadal sex is determined by the temperature of the incubating egg. In the red-eared slider turtle (Trachemys scripta), the effect of temperature can be overridden by exogenous ligands, i.e., sex steroid hormones and steroid metabolism enzyme inhibitors, during the temperature-sensitive period (TSP) of development. Precisely how the physical signal of temperature is transduced into a biological signal that ultimately results in sex determination remains unknown. In this review, we discuss the sex determining pathway underlying TSD by focusing on two candidate sex determining genes, Forkhead box protein L2 (FoxL2) and Doublesex mab3- related transcription factor 1 (Dmrt1). They appear to be involved in transducing the environmental temperature signal into a biological signal that subsequently determines gonadal sex. FoxL2 and Dmrt1 exhibit gonad-typical patterns of expression in response to temperature during the TSP in the red-eared slider turtle. Further, the biologically active ligands regulate the expression of FoxL2 and Dmrt1 during development to modify gonad trajectory. The precise regulatory mechanisms of expression of these genes by temperature or exogenous ligands are not clear. However, the environment often influences developmental gene expression by altering the epigenetic status in regulatory regions. Here, we will discuss if the regulation of FoxL2 and Dmrt1 expression by environment is mediated through epigenetic mechanisms during development in species with TSD.

[Mechanism of avian sex determination and differentiation]

Avian sex is determined by genes on the sex chromosomes (ZZ for male and ZW for female). In avian embryo stage, genes on one or two chromosomes control the sex differentiation. Gonad develops to testis in ZZ male and to ovary in ZW female. To date, DMRT1 (Doublesex and mab-3 related transcription factor 1) is considered to be the best candidate gene in controlling the avian gonad differentiation. However, recent study showed that avian sex might be determined by cell autonomous independent of sex hormone signal. Therefore, sex determination gene does not only control the gonadal differentiation, but also control body cells. From this sense, DMRT1 is not the switch gene of avian sex determination. What is the switch factor of avian sex determination, and what is the mechanism of avian sex determination? This review discussed the current progresses on avian sex determination and differentiation from three aspects: W chromosome and ovary development, Z chromosome and testis development, and avian sex determination and cell autonomous.

The long non-coding RNA, MHM, plays a role in chicken embryonic development, including gonadogenesis

MHM is a chicken Z chromosome-linked locus that is methylated and transcriptionally silent in male cells, but is hypomethylated and transcribed into a long non-coding RNA in female cells. MHM has been implicated in both localised dosage compensation and sex determination in the chicken embryo, but direct evidence is lacking. We investigated the potential role of MHM in chicken embryonic development, using expression analysis and retroviral-mediated mis-expression. At embryonic stages, MHM is only expressed in females. Northern blotting showed that both sense and antisense strands of the MHM locus are transcribed, with the sense strand being more abundant. Whole mount in situ hybridization confirmed that the sense RNA is present in developing female embryos, notably in gonads, limbs, heart, branchial arch and brain. Within these cells, the MHM RNA is localized to the nucleus. The antisense transcript is lowly expressed and has a cytoplasmic localization in cells. Mis-expression of MHM sense and antisense sequences results in overgrowth of tissues in which transcripts are predominantly expressed. This includes altered asymmetric ovarian development in females. In males, MHM mis-expression impairs gonadal expression of the testis gene, DMRT1. Both MHM sense and antisense mis-expression cause brain abnormalities, while MHM sense causes an increase in male-biased embryo mortality. These results indicate that MHM has a role in chicken normal embryonic development, including gonadal sex differentiation.

Transitions between sex-determining systems in reptiles and amphibians

Important technological advances in genomics are driving a new understanding of the evolution of sex determination in vertebrates. In particular, comparative chromosome mapping in reptiles has shown an intriguing distribution of homology in sex chromosomes across reptile groups. When this new understanding is combined with the widespread distribution of genetic and temperature-dependent sex-determination mechanisms among reptiles, it is apparent that transitions between modes have occurred many times, as they have for amphibians (particularly between male and female heterogamety). It is also likely that thermosensitivity in sex determination is a key factor in those transitions in reptiles, and possibly in amphibians too. New models of sex determination involving temperature thresholds are providing the framework for the investigation of transitions and making possible key predictions about the homologies and sex-determination patterns expected among taxa in these groups. Molecular cytogenetics and other genomic approaches are essential to providing the fundamental material necessary to make advances in this field.

Manipulation of estrogen synthesis alters MIR202* expression in embryonic chicken gonads

Tissue-specific patterns of microRNA (miRNA) expression contribute to organogenesis during embryonic development. Using the embryonic chicken gonads as a model for vertebrate gonadogenesis, we previously reported that miRNAs are expressed in a sexually dimorphic manner during gonadal sex differentiation. Being male biased, we hypothesised that up-regulation of microRNA 202* (MIR202*) is characteristic of testicular differentiation. To address this hypothesis, we used estrogen modulation to induce gonadal sex reversal in embryonic chicken gonads and analyzed changes in MIR202* expression. In ovo injection of estradiol-17beta at Embryonic Day 4.5 (E4.5) caused feminization of male gonads at E9.5 and reduced MIR202* expression to female levels. Female gonads treated at E3.5 with an aromatase inhibitor, which blocks estrogen synthesis, were masculinized by E9.5, and MIR202* expression was increased. Reduced MIR202* expression correlated with reduced expression of the testis-associated genes DMRT1 and SOX9, and up-regulation of ovary-associated genes FOXL2 and CYP19A1 (aromatase). Increased MIR202* expression correlated with down-regulation of FOXL2 and aromatase and up-regulation of DMRT1 and SOX9. These results confirm that up-regulation of MIR202* coincides with testicular differentiation in embryonic chicken gonads.

Identification of early transcripts related to male development in chicken embryos

Early transcripts related to male development in chicken embryos and their expression profiles were examined. A total of 89 and 127 candidate male development transcripts that represented 83 known and 119 unknown non-redundant sequences, respectively, were characterized in an embryonic day 3 (E3; Hamburger and Hamilton Stage 20: HH20) male-subtract-female complementary DNA library. Of 35 selected transcripts, quantitative reverse transcription-polymerase chain reaction validated that the expression levels of 25 transcripts were higher in male E3 whole embryos than in females (P < 0.05). Twelve of these transcripts mapped to the Z chromosome. At 72 wk of age, 20 and 4 transcripts were expressed at higher levels in the testes and brains of male than in the ovaries and brains of female chickens (P < 0.05), respectively. Whole mount and frozen cross-section in situ hybridization, as well as Western blotting analysis further corroborated that riboflavin kinase (RFK), WD repeat domain 36 (WDR36), and EY505808 transcripts; RFK and WDR36 protein products were predominantly expressed in E7 male gonads. Treatment with an aromatase inhibitor formestane at E4 affected the expression levels at E7 of the coatomer protein complex (subunit beta 1), solute carrier family 35 member F1, LOC427316 and EY505812 transcripts across both sexes (P < 0.05), similar to what was observed for the doublesex and mab-3 related transcription factor 1 gene. The interaction effects of sex by formestane treatment were observed in 15 candidate male development transcripts (P < 0.05). Taken together, we identified a panel of potentially candidate male development transcripts during early chicken embryogenesis; some might be regulated by sex hormones.

Studies on feminization, sex determination, and differentiation of the Southern catfish, Silurus meridionalis--a review

The sex ratio of the feral Southern catfish was reported to be about 1:1, while the fish obtained by artificial fertilization were always female. Hence, we examined the possible influence of the micro-environment during artificial insemination (pH of the ovarian fluid and concentration of the semen) and early development (feed, hatching temperature, and water) on the sex ratio of Southern catfish fry. In order to examine the possibility of the occurrence of gynogenesis during artificial propagation, cytological observations on the insemination processes and the artificial induction of gynogenesis were also performed. However, no male fish were obtained even in these experiments, excluding the possibilities of these micro-environmental changes on catfish sex ratio and the occurrence of gynogenesis during artificial propagation. Female-to-male sex reversal was achieved by treatment with fadrozole (an aromatase inhibitor) and tamoxifen (an estrogen receptor antagonist). Histological analyses on the gonadal development of both female and induced male fish were subsequently performed. Moreover, several genes involved in sex differentiation, such as dmrt1, foxl2, and cyp19, and three subunits of gonadotropin (gth), i.e., gthalpha, lhbeta, and fshbeta, were isolated. Their expression patterns were studied under normal gonadal development and sex reversal conditions. The results revealed that dmrt1, foxl2, and cyp19a were closely related to catfish sex differentiation, and the gth subunits were possibly related to ovarian differentiation and oocyte development. Taken together, we hypothesized that estrogen was highly responsible for the ovarian differentiation and feminization of catfish fry under artificial propagation, although the mechanism involved remains elusive.

Doublesex- and Mab-3-related transcription factor-1 repression of aromatase transcription, a possible mechanism favoring the male pathway in tilapia

Doublesex- and Mab-3-related transcription factor-1 (Dmrt1) is an important transcription factor implicated in early testicular differentiation in vertebrates, but its target genes are largely unknown. In the Nile tilapia, estrogen is the natural inducer of ovarian differentiation. Our recent studies have shown that Forkhead-l2 up-regulated transcription of the Cyp19a1a gene (aromatase) in the gonads in a female-specific manner. However, the upstream factor(s) down-regulating Cyp19a1a expression during testicular differentiation remains unclear. In the present study, we used in vitro (promoter analysis) and in vivo (transgenesis and in situ hybridization) approaches to examine whether Dmrt1 inhibits Cyp19a1a's transcriptional activity. The in vitro analysis using luciferase assays revealed that Dmrt1 repressed basal as well as Ad4BP/SF-1-activated Cyp19a1a transcription in HEK 293 cells. Luciferase assays with various deletions of Dmrt1 also showed that the Doublesex and Mab-3 domain is essential for the repression. In vitro-translated Dmrt1 and the nuclear extract from tilapia testis could directly bind to the palindrome sequence ACATATGT in the Cyp19a1a promoter, as determined by EMSAs. Transgenic overexpression of Dmrt1 in XX fish resulted in decreased aromatase gene expression, reduced serum estradiol-17beta levels, retardation of the ovarian cavity's development, varying degrees of follicular degeneration, and even a partial to complete sex reversal. Our results indicate that aromatase is one of the targets of Dmrt1. Dmrt1 suppresses the female pathway by repressing aromatase gene transcription and estrogen production in the gonads of tilapia and possibly other vertebrates.

Estrogen represses SOX9 during sex determination in the red-eared slider turtle Trachemys scripta

Production of male offspring in viviparous eutherian mammals requires a sex-determining mechanism resistant to maternal hormones. This constraint is relaxed in egg-laying species, which are sensitive to hormones during sex determination and often use an increase in aromatase, the estrogen-synthesizing enzyme, as a key feminizing signal. In the turtle Trachemys scripta, sex is normally determined by temperature, but estrogen treatment overrides this cue and leads exclusively to female development. We assessed whether the expression of SOX9, a central male sex-determining gene in mammals, or three other conserved transcription factors (WT1, GATA4, and LHX9) was regulated by estrogen signaling in the turtle. As in mice, all somatic cell types in the immature turtle gonad initially expressed WT1 and GATA4, whereas SOX9 was restricted to the Sertoli precursors and LHX9 to the coelomic epithelium and interstitium. After the bipotential period, SOX9 was abruptly down-regulated at the female temperature. Strikingly, embryos treated with beta-estradiol at the male temperature lost SOX9 expression more than two stages earlier than controls, though WT1, GATA4, and LHX9 were unaffected. Conversely, inhibition of estrogen synthesis and signaling prevented or delayed SOX9 down-regulation at the female temperature. These results suggest that endogenous estrogen feminizes the medulla of the bipotential turtle gonad by inhibiting SOX9 expression. This mechanism may be involved in the male-to-female sex reversal in wild populations exposed to environmental estrogens, and is consistent with results showing that the estrogen receptor represses Sox9 to block transdifferentiation of granulosa cells into Sertoli-like cells in the adult mouse ovary.

Estradiol-17beta treatment induces intersexual gonadal development in the pufferfish, Takifugu rubripes

Estrogens are responsible for most characteristics of the female sex of a species, such as metabolic, behavioral, and morphological changes during reproduction. Artificial estradiol-17beta (E2) treatment Induces sex reversal in some fish. The Japanese pufferfish (Takifugu rubripes) has the most compact genome among vertebrates and great pottial for comparative genome analysis. In this paper, we describe the Influence of E2 treatment during gonadal development in the pufferfish. After hatching, fry were treated with no (control) or a 0.1, 1, 10, or 100 microg/g diet from 21 to 80 days after hatching (dah). Doublesex-mab3-related transcription factor (DMRT1) is Involved in testicular development. VASA is responsible for germ cell development, and CYP19A plays a role in E2 biosynthesis during ovarian development across animal phyla as well as in gonadal morphology after E2 treatment. DMRT1, VASA, and CYP19A were Investigated in the gonads of E2-treated pufferfish. Fish fed with the highest dose (E2 100 microg/g diet) developed Intersexual gonads in the testis; the majority of germ cells were oocytes, but some spermatocytes were detected. RT-PCR results showed the expression of VASA and CYP19A in all intersexual gonads and DMRT1 in some. Furthermore, abnormalities in the epithelium-tunica layer were detected, and gonadal somatic cells (e.g., granulosa cells, theca cells, or germinal epithelium) proliferated extensively in the intersexual gonad. These results suggest that E2 treatment Induces ovarian development in the bipotential gonads of genetic males by modification of gonadal somatic cells and E2 production, mediated by CYP19A.

Avian sex chromosomes: dosage compensation matters

In 2001 it was established that, contrary to our previous understanding, a mechanism exists that equalises the expression levels of Z chromosome genes found in male (ZZ) and female (ZW) birds (McQueen et al. 2001). More recent large scale studies have revealed that avian dosage compensation is not a chromosome-wide phenomenon and that the degree of dosage compensation can vary between genes (Itoh et al. 2007; Ellegren et al. 2007). Although, surprisingly, dosage compensation has recently been described as absent in birds (Mank and Ellegren 2009b), this interpretation is not supported by the accumulated evidence, which indicates that a significant proportion of Z chromosome genes show robust dosage compensation and that a particular cluster of such dosage compensated genes can be found on the short arm of the Z chromosome. The implications of this new picture of avian dosage compensation for avian sex determination are discussed, along with a possible mechanism of avian dosage compensation.

Cloning and expression of candidate sexual development genes in the cane toad (Bufo marinus)

The development of the reproductive system in bufonids (true toads) is unique in several respects: sexual differentiation occurs later than in other anurans, and toads develop a Bidder's organ, a rudimentary ovary that can be manipulated in males to produce mature oocytes. To illuminate the genesis of this unusual reproductive system, we isolated from the cane toad (Bufo marinus) the orthologues of several known vertebrate sex-determining genes, determined their primary structure, and studied their expression by reverse transcriptase-polymerase chain reaction and in situ hybridization of tissue sections. We report here that cane toad Sox9, Dmrt1, and p450aromatase (Cyp19a1) are highly homologous to their counterparts in other vertebrates. They show profiles of expression that generally follow patterns observed in other taxa, but with some novel features. Our data suggest that these genes likely play key roles in sex determination and early gonad development in bufonids.

OVEX1, a novel chicken endogenous retrovirus with sex-specific and left-right asymmetrical expression in gonads

BACKGROUND

In chickens, as in most birds, female gonad morphogenesis is asymmetrical. Gonads appear first rather similarly, but only the left one undergoes full differentiation and gives rise to a functional ovary. The right gonad, in which the cortex does not develop, remains restricted to the medulla and finally regresses. Opportunity was taken of this left-right asymmetry to perform a suppression subtractive hybridization screening to select for transcripts preferentially expressed in the developing left ovary as compared to the right one, and thus identify genes that are potentially involved in the process of ovarian differentiation.

RESULTS

One of these transcripts, named Ovex1 according to its expression profile, corresponds to an endogenous retrovirus that has not been previously characterized. It is transcribed as full-length and singly spliced mRNAs and contains three uninterrupted open reading frames coding potentially for proteins with homology to Gag and Pro-Pol retroviral polyproteins and a third protein showing only a weak similarity with Env glycoproteins. Ovex1 is severely degenerated; it is devoid of typical long terminal repeats and displays some evidence of recombination. An orthologous Ovex1 locus was identified in the genome of zebra finch, a member of a different bird order, and similar sequences were detected in turkey, guinea fowl, and duck DNA. The relationship between these sequences follows the bird phylogeny, suggesting vertical transmission of the endogenous retrovirus for more than 100 million years. Ovex1 is transcribed in chicken gonads with a sex-dependent and left-right asymmetrical pattern. It is first expressed in the cortex of the left indifferent gonads of both sexes. Expression is transient in the left testis and absent in the right one. In developing ovaries, Ovex1 transcription increases sharply in the left cortex and is weakly detected in the medulla. After folliculogenesis, Ovex1-expressing cells constitute the follicular granulosa cell layer. Ovex1 expression highlights a striking desquamation process that leads to profound cortical remodeling associated with follicle morphogenesis.

CONCLUSION

Evidence for a selection pressure at the protein level suggests that this endogenous retrovirus, expressed in the ovarian supporting cell lineage, might play an active role in bird ovarian physiology.

Expression and evolutionary conservation of the tescalcin gene during development

The tescalcin gene (Tesc) encodes an EF-hand calcium-binding protein that interacts with the sodium/hydrogen exchanger, NHE1. Previous studies indicated that Tesc was expressed in mouse embryonic testis, but not in ovary, during the critical period of testis and ovary determination. In this paper we compared the expression of Tesc in embryonic tissues of chicken and mouse. Tesc expression was sexually dimorphic in the embryonic gonads of both mouse and chicken. Tescalcin (TESC) was detected in both Sertoli cells and germ cells. In the embryonic brain of both mouse and chicken, Tesc was highly expressed in the nasal placode and in fibers extending from the olfactory epithelium to the primordial olfactory bulb. Tesc was expressed in the embryonic heart of both chicken and mouse. In mouse Tesc expression was also detected in embryonic adrenal. These studies indicate very specific expression of Tesc in various tissues in chicken and mouse during embryologic development, and conservation of Tesc expression in both species.

Isolation and characterization of testis-specific DMRT1 in the tropical abalone (Haliotis asinina)

The Doublesex Male abnormal-3 Related Transcription factor-1 (DMRT1) gene encodes a protein containing the DNA-binding motif called the DM domain, involved in the sexual development of various species. To gain insight into its implications for gonadal differentiation in the tropical abalone (Haliotis asinina), a DMRT1 homolog was identified and characterized. The full length cDNA of HADMRT1 (1,740 bp with an ORF of 732 bp corresponding to a putative polypeptide of 243 amino acids) and its DM domain-less variant (HADMRT1-like, 1,430 bp with an ORF of 312 bp, 103 amino acids) were successfully isolated and reported for the first time in molluscs. HADMRT1 was specifically expressed in the testes of adult H. asinina (N = 16) but not in whole juveniles (2, 3, 5 months old, N = 6 for each group) and ovaries (N = 16), and pooled hemocytes (from 50 individuals) of adults. Tissue distribution analysis further revealed testis-specific expression of HADMRT1. Semiquantitative RT-PCR illustrated that the relative expression level of HADMRT1 in developed testes (stages II, III, and IV) was significantly greater than that in undeveloped testes (stage I) of abalone broodstock (P < 0.05).

Onset of meiosis in the chicken embryo; evidence of a role for retinoic acid

Background

Meiosis in higher vertebrates shows a dramatic sexual dimorphism: germ cells enter meiosis and arrest at prophase I during embryogenesis in females, whereas in males they enter mitotic arrest during embryogenesis and enter meiosis only after birth. Here we report the molecular analysis of meiosis onset in the chicken model and provide evidence for conserved regulation by retinoic acid.

Results

Meiosis in the chicken embryo is initiated late in embryogenesis (day 15.5), relative to gonadal sex differentiation (from day 6). Meiotic germ cells are first detectable only in female gonads from day 15.5, correlating with the expression of the meiosis marker, SCP3. Gonads isolated from day 10.5 female embryos and grown in serum-free medium could still initiate meiosis at day 16.5, suggesting that this process is controlled by an endogenous clock in the germ cells themselves, and/or that germ cells are already committed to meiosis at the time of explantation. Early commitment is supported by the analysis of chicken STRA8, a pre-meiotic marker shown to be essential for meiosis in mouse. Chicken STRA8 is expressed female-specifically from embryonic day 12.5, preceding morphological evidence of meiosis at day 15.5. Previous studies have shown that, in the mouse embryo, female-specific induction of STRA8 and meiosis are triggered by retinoic acid. A comprehensive analysis of genes regulating retinoic acid metabolism in chicken embryos reveals dynamic expression in the gonads. In particular, the retinoic acid-synthesising enzyme, RALDH2, is expressed in the left ovarian cortex at the time of STRA8 up-regulation, prior to meiosis.

Conclusion

This study presents the first molecular analysis of meiosis onset in an avian embryo. Although aspects of avian meiosis differ from that of mammals, a role for retinoic acid may be conserved.

Cloning and expression of R-Spondin1 in different vertebrates suggests a conserved role in ovarian development

Background

R-Spondin1 (Rspo1) is a novel regulator of the Wnt/β-catenin signalling pathway. Loss-of-function mutations in human RSPO1 cause testicular differentiation in 46, XX females, pointing to a role in ovarian development. Here we report the cloning and comparative expression analysis of R-SPONDIN1 orthologues in the mouse, chicken and red-eared slider turtle, three species with different sex-determining mechanisms. Evidence is presented that this gene is an ancient component of the vertebrate ovary-determining pathway.

Results

Gonadal RSPO1 gene expression is female up-regulated in the embryonic gonads in each species at the onset of sexual differentiation. In the mouse gonad, Rspo1 mRNA is expressed in the somatic cell lineage at the time of ovarian differentiation (E12.5–E15.5), with little expression in germ cells. However, the protein is localised in the cytoplasm and at the cell surface of both somatic (pre-follicular) and germ cells. In the chicken embryo, RSPO1 expression becomes elevated in females at the time of ovarian differentiation, coinciding with female-specific activation of the FOXL2 gene and estrogen synthesis. RSPO1 protein in chicken is localised in the outer cortical zone of the developing ovary, the site of primordial follicle formation and germ cell differentiation. Inhibition of estrogen synthesis with a specific aromatase inhibitor results in a decline in chicken RSPO1 expression, indicating that RSPO1 is influenced by estrogen. In the red-eared slider turtle, which exhibits temperature-dependent sex determination, up-regulation of RSPO1 occurs during the temperature-sensitive period, when gonadal development is responsive to temperature. Accordingly, RSPO1 expression is temperature-responsive, and is down-regulated in embryos shifted from female- to male-producing incubation temperatures.

Conclusion

These results indicate that RSPO1 is up-regulated in the embryonic gonads of female vertebrates with different sex-determining mechanisms. In all instances, RSPO1 is expressed in the incipient ovary. These findings suggest that R-SPONDIN1 is an ancient, conserved part of the vertebrate ovary-determining pathway.

Sexual dimorphic expression of DMRT1 and Sox9a during gonadal differentiation and hormone-induced sex reversal in the teleost fish Nile tilapia (Oreochromis niloticus)

We examined the expression profiles of tDMRT1 and Sox9a during gonadal sex differentiation and hormone-induced sex reversal. tDMRT1 was detected in the gonial germ-cell-surrounding cells in XY fry specifically before the appearance of any signs of morphological sex differentiation, that is, sex differences in germ cell number and histogenesis, such as differentiation into intratesticular efferent duct or ovarian cavity. The signals became localized in the Sertoli and epithelial cells comprising the efferent duct during gonadal differentiation. After the induction of XY sex reversal with estrogen, tDMRT1 decreased and then disappeared completely. In contrast, tDMRT1 was expressed in the germ-cell-surrounding cells in XX sex reversal with androgen. On the other hand, Sox9a did not show sexual dimorphism before the appearance of sex differences in histogenesis and was not expressed in the efferent duct in the testis. These results suggest that tDMRT1 is a superior testicular differentiation marker in tilapia.

Molecular cloning and quantitative expression of sexually dimorphic markers Dmrt1 and Foxl2 during female-to-male sex change in Epinephelus merra

The honeycomb grouper (Epinephelus merra) is one of the smallest members of the Serranidae family and is often used to study protogynous sex change. To determine the role of the male-determining gene Dmrt1 and the ovarian-specific gene Foxl2 in sex change, we cloned these two markers from E. merra gonads by reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). Two isoforms, Dmrt1a and Dmrt1b, resulted from alternative splicing in the coding region, causing the insertion of one glutamine residue in Dmrt1b. RT-PCR revealed that Dmrt1 was expressed only in the gonads, with higher levels in the testis than in the ovary. cDNA encoding Foxl2 was isolated from the ovary; Foxl2 was expressed extensively in the brain, pituitary, gonads, and gill, with its highest level in the ovary, indicating a potential role for Foxl2 in the brain-pituitary-gonad axis. Real-time quantitative RT-PCR analyses showed that Foxl2 mRNA expression was significantly downregulated from the late transitional phase to the completion of sex change. Conversely, Dmrt1 expression increased with the progression of spermatogenesis and continued until the formation of the testis. The expression profiles of these two sex-specific marker genes corresponded closely with the histological process of sex change. The down-regulation of Foxl2 most likely facilitates oocyte degeneration, whereas the up-regulation of Dmrt1 causes the proliferation of gonial germ cells into spermatogina and initiates sex change.

Mechanism of asymmetric ovarian development in chick embryos

In most animals, the gonads develop symmetrically, but most birds develop only a left ovary. A possible role for estrogen in this asymmetric ovarian development has been proposed in the chick, but the mechanism underlying this process is largely unknown. Here, we identify the molecular mechanism responsible for this ovarian asymmetry. Asymmetric PITX2 expression in the left presumptive gonad leads to the asymmetric expression of the retinoic-acid (RA)-synthesizing enzyme, RALDH2, in the right presumptive gonad. Subsequently, RA suppresses expression of the nuclear receptors Ad4BP/SF-1 and estrogen receptor alpha in the right ovarian primordium. Ad4BP/SF-1 expressed in the left ovarian primordium asymmetrically upregulates cyclin D1 to stimulate cell proliferation. These data suggest that early asymmetric expression of PITX2 leads to asymmetric ovarian development through up- or downregulation of RALDH2, Ad4BP/SF-1, estrogen receptor alpha and cyclin D1.

Avian sex determination: what, when and where?

Sex is determined genetically in all birds, but the underlying mechanism remains unknown. All species have a ZZ/ZW sex chromosome system characterised by female (ZW) heterogamety, but the chromosomes themselves can be heteromorphic (in most birds) or homomorphic (in the flightless ratites). Sex in birds might be determined by the dosage of a Z-linked gene (two in males, one in females) or by a dominant ovary-determining gene carried on the W sex chromosome, or both. Sex chromosome aneuploidy has not been conclusively documented in birds to differentiate between these possibilities. By definition, the sex chromosomes of birds must carry one or more sex-determining genes. In this review of avian sex determination, we ask what, when and where? What is the nature of the avian sex determinant? When should it be expressed in the developing embryo, and where is it expressed? The last two questions arise due to evidence suggesting that sex-determining genes in birds might be operating prior to overt sexual differentiation of the gonads into testes or ovaries, and in tissues other than the urogenital system.

Potential application of sperm bearing female-specific chromosome in chickens

This paper reviews studies on sex reversal experiments in chickens, production of sperm bearing a female-specific chromosome, its application for poultry resources and finally a mechanism of sex differentiation of gonads in the chicken.

Eukaryotic regulatory RNAs: an answer to the 'genome complexity' conundrum

A large portion of the eukaryotic genome is transcribed as noncoding RNAs (ncRNAs). While once thought of primarily as "junk," recent studies indicate that a large number of these RNAs play central roles in regulating gene expression at multiple levels. The increasing diversity of ncRNAs identified in the eukaryotic genome suggests a critical nexus between the regulatory potential of ncRNAs and the complexity of genome organization. We provide an overview of recent advances in the identification and function of eukaryotic ncRNAs and the roles played by these RNAs in chromatin organization, gene expression, and disease etiology.

Differential and spermatogenic cell-specific expression of DMRT1 during sex reversal in protogynous hermaphroditic groupers

DMRT1 has been suggested to play different roles in sex determination and gonad differentiation, because different expression patterns have been reported among different vertebrates. The groupers, since their gonads first develop as ovary and then reverse into testis, have been thought as good models to study sex differentiation and determination. In this study, we cloned the full-length cDNAs of DMRT1 gene from orange-spotted grouper (Epinephelus coioides), and prepared corresponding anti-EcDMRT1 antiserum to study the relationship of DMRT1 to sex reversal. One important finding is that the grouper DMRT1 is not only differentially expressed in different stage gonads, but also restricted to specific stages and specific cells of spermatogenesis. Grouper DMRT1 protein exists only in spermatogonia, primary spermatocytes and secondary spermatocytes, but not in the supporting Sertoli cells. Moreover, we confirmed that EcSox3 is expressed not only in oogonia and different stage oocytes, but also in Sertoli cells and spermatogonia, and EcSox9 is expressed only in Sertoli cells. The data suggested that grouper DMRT1 might be a more specific sex differentiation gene for spermatogenesis, and play its role at the specific stages from spermatogonia to spermatocytes. In addition, no introns were found in the grouper DMRT1, and no duplicated DMRT1 genes were detected. The finding implicates that the intronless DMRT1 that is able to undergo rapid transcriptional turnover might be a significant gene for stimulating spermatogenesis in the protogynous hermaphroditic gonad.

Multiple alternative splicing in gonads of chicken DMRT1

Many basic cellular processes are shared across vast phylogenetic distances, whereas sex-determining mechanisms are highly variable between phyla, although the existence of two sexes is nearly universal in the animal kingdom. However, the evolutionarily conserved DMRT1/dsx/mab3 with a common zinc finger-like DNA-binding motif, DM domain, share both similar structure and function between phyla. Here we report that six transcripts of the chicken DMRT1 were generated in gonads by multiple alternative splicing. By cDNA cloning and genomic structure analysis, we found that there were nine exons of DMRT1, which were involved in alternatively splicing to generate the DMRT1 transcripts. Northern blotting and reverse transcription (RT) PCR analysis revealed that the expression of chicken DMRT1 was testis-specific in adults. Whole-mount in situ hybridizations and RT-PCR indicated that DMRT1 b was specially expressed in embryo gonads and higher in male than female gonads at stage 31. The female gonad had stronger DMRT1 c expression than the male one, whereas DMRT1 f was detectable only in the male gonad at stage 31 of the key time of sex gonadal differentiation. The differential expression of these transcripts during gonadal differentiation provides new insight into roles of alternative splicing of DMRT1 in governing sex differentiation of the chicken.