Elevated expression of P450c17 (CYP17) during testicular formation in the frog

Steroidogenic potential of the gonad during sex differentiation in the rice field frog Hoplobatrachus rugulosus (Anura: Dicroglossidae)

Prior studies demonstrated that gonadal differentiation in the rice field frog, Hoplobatrachus rugulosus, was of an undifferentiated type since all individuals had ovaries at complete metamorphosis. However, the steroidogenic potential of the gonad is still unknown. In this study, H. rugulosus were obtained by stimulating fertilization in the laboratory under natural light and temperature conditions. The gonads were collected and their steroidogenic potential was evaluated by determining the expression level of messenger RNA (mRNA) encoding for cytochrome P450 17-hydroxylase/C17-20 lyase (CYP17) and cytochrome P450 aromatase (CYP19) using quantitative real-time RT-PCR and the localization of CYP17 mRNA in tissues by in situ hybridization. The CYP17 mRNA levels in males at 4-11 weeks postmetamorphosis were higher than in female and intersex gonads. This corresponded to their localization in the gonadal tissues, where CYP17 signals were specifically detected in the Leydig cells of the testis at 5-16 weeks postmetamorphosis but was undetectable in all ovary samples. The CYP19 mRNA levels in females at 4-11 weeks postmetamorphosis was higher than in male and intersex gonads, which corresponded with gonadal development, indicating the potential steroidogenic function of the ovary. Based on the present results, the role of CYP17 and CYP19 mRNA in sex differentiation in H. rugulosus may occur after gonadal sex differentiation and the steroidogenic potential of the gonads exhibited a sexual dimorphic pattern. These results provide a crucial basis for further research on the developmental biology in anuran species.

Sex Differentiation in Amphibians: Effect of Temperature and Its Influence on Sex Reversal

The role of environmental factors in sexual differentiation in amphibians is not new. The effect of hormones or hormone-like compounds is widely demonstrated. However, the effect of temperature has traditionally been regarded as something anecdotal that occurs in extreme situations and not as a factor to be considered. The data currently available reveal a different situation. Sexual differentiation in some amphibian species can be altered even by small changes in temperature. On the other hand, although not proven, it is possible that temperature is related to the appearance of sex-reversed individuals in natural populations under conditions unrelated to environmental contaminants. According to this, temperature, through sex reversal (phenotypic sex opposed to genetic sex), could play an important role in the turnover of sex-determining genes and in the maintenance of homomorphic sex chromosomes in this group. Accordingly, and given the expected increase in global temperatures, growth and sexual differentiation in amphibians could easily be affected, altering the sex ratio in natural populations and posing major conservation challenges for a group in worldwide decline. It is therefore particularly urgent to understand the mechanism by which temperature affects sexual differentiation in amphibians.

Functional Divergence of Multiple Duplicated Foxl2 Homeologs and Alleles in a Recurrent Polyploid Fish

Evolutionary fates of duplicated genes have been widely investigated in many polyploid plants and animals, but research is scarce in recurrent polyploids. In this study, we focused on foxl2, a central player in ovary, and elaborated the functional divergence in gibel carp (Carassius gibelio), a recurrent auto-allo-hexaploid fish. First, we identified three divergent foxl2 homeologs (Cgfoxl2a-B, Cgfoxl2b-A, and Cgfoxl2b-B), each of them possessing three highly conserved alleles and revealed their biased retention/loss. Then, their abundant sexual dimorphism and biased expression were uncovered in hypothalamic-pituitary-gonadal axis. Significantly, granulosa cells and three subpopulations of thecal cells were distinguished by cellular localization of CgFoxl2a and CgFoxl2b, and the functional roles and the involved process were traced in folliculogenesis. Finally, we successfully edited multiple foxl2 homeologs and/or alleles by using CRISPR/Cas9. Cgfoxl2a-B deficiency led to ovary development arrest or complete sex reversal, whereas complete disruption of Cgfoxl2b-A and Cgfoxl2b-B resulted in the depletion of germ cells. Taken together, the detailed cellular localization and functional differences indicate that Cgfoxl2a and Cgfoxl2b have subfunctionalized and cooperated to regulate folliculogenesis and gonad differentiation, and Cgfoxl2b has evolved a new function in oogenesis. Therefore, the current study provides a typical case of homeolog/allele diversification, retention/loss, biased expression, and sub-/neofunctionalization in the evolution of duplicated genes driven by polyploidy and subsequent diploidization from the recurrent polyploid fish.

Testis Development and Differentiation in Amphibians

Sex is determined genetically in amphibians; however, little is known about the sex chromosomes, testis-determining genes, and the genes involved in testis differentiation in this class. Certain inherent characteristics of the species of this group, like the homomorphic sex chromosomes, the high diversity of the sex-determining mechanisms, or the existence of polyploids, may hinder the design of experiments when studying how the gonads can differentiate. Even so, other features, like their external development or the possibility of inducing sex reversal by external treatments, can be helpful. This review summarizes the current knowledge on amphibian sex determination, gonadal development, and testis differentiation. The analysis of this information, compared with the information available for other vertebrate groups, allows us to identify the evolutionarily conserved and divergent pathways involved in testis differentiation. Overall, the data confirm the previous observations in other vertebrates-the morphology of the adult testis is similar across different groups; however, the male-determining signal and the genetic networks involved in testis differentiation are not evolutionarily conserved.

Sex steroids and steroidogenesis-related genes in the sea cucumber, Holothuria scabra and their potential role in gonad maturation

The sea cucumber Holothuria scabra is an economically valuable marine species which is distributed throughout the Asia-Pacific region. With the natural population declining due to over fishing, aquaculture of this species is deemed necessary. Hence, it is essential to understand the mechanisms regulating the reproduction in order to increase their populations. Sex steroids, including estrogens, androgens and progestogens, play an important role in reproduction in most vertebrates and several invertebrates. It has been proposed that sea cucumbers have the same sex steroids as vertebrates but the steroidogenic pathway in the sea cucumbers is still unclear. In this study, we demonstrated by using liquid chromatography-tandem mass spectrometry (LC-MS/MS) that sex steroids (estradiol, progesterone, and testosterone) were present in H. scabra neural and gonadal tissues. In silico searches of available sea cucumber transcriptome data identified 26 steroidogenesis-related genes. Comparative analysis of encoded proteins for the steroidogenic acute regulatory protein (HscStAR), CYP P450 10, 17 and 3A (HscCYP10, HscCYP17, HscCYP3A) and hydroxysteroid dehydrogenases (Hsc3β-HSD, Hsc17β-HSD) with other species was performed to confirm their evolutionary conservation. Gene expression analyses revealed widespread tissue expression. Real-time PCR analysis revealed that HscStAR, HscCYP10, Hsc3β-HSD, and Hsc17β-HSD gene expressions were similar to those in ovaries and testes, which increased during the gonad maturation. HscCYP17 mRNA was increased during ovarian development and its expression declined at late stages in females but continued high level in males. The expression of the HscCYP3A was high at the early stages of ovarian development, but not at other later stages in ovaries, however it remained low in testes. Moreover, a role for steroids in reproduction was confirmed following the effect of sex steroids on vitellogenin (Vtg) expression in ovary explant culture, showing upregulation of Vtg level. Collectively, this study has confirmed the existence of steroids in an echinoderm, as well as characterizing key genes associated with the steroidogenic pathway. We propose that sex steroids might also be associated with the reproduction of H. scabra, and the identification of biosynthetic genes enables future functional studies to be performed.

A threshold dosage of estrogen for male-to-female sex reversal in the Glandirana rugosa frog

Steroid hormones play very important roles in gonadal differentiation in many vertebrate species. Previously, we have determined a threshold dosage of testosterone (T) to induce female-to-male sex reversal in Glandirana rugosa frogs. Genetic females formed a mixture of testis and ovary, the so-called ovotestis, when tadpoles of G. rugosa were reared in water containing the dosage of T, which enabled us to detect primary changes in the histology of the masculinizing gonads. In this study, we determined a threshold dosage of estradiol-17β (E2) to cause male-to-female sex reversal in this frog. We observed first signs of histological changes in the ovotestes, when tadpoles were reared in water containing the dosage of E2. Ovotestes were significantly larger than wild-type testes in size. By E2 treatment, male germ cells degenerated in the feminizing testis leading to their final disappearance. In parallel, oocytes appeared in the medulla of the ovotestis and later in the cortex as well. Quantitative polymerase chain reaction analysis revealed that the expression of sex-related genes involved in testis formation was significantly decreased in the ovotestis. In addition, immuno-positive signals of CYP17 that is involved in testis differentiation in this frog disappeared in the medulla first and then in the cortex. These results suggested that oocytes expanded in the feminizing gonad (ovary) contemporaneously with male germ cell disappearance. Primary changes in the histology of the gonads during male-to-female sex reversal occurred in the medulla and later in the cortex. This direction was opposite to that observed during female-to-male sex reversal in the G. rugosa frog.

Transcriptional changes caused by estrogenic endocrine disrupting chemicals in gonad-mesonephros complexes of genetic male Xenopus laevis: Multiple biomarkers for early detection of testis differentiation disruption

Our recent study revealed some early molecular and cellular events in which 17β-estradiol (E2) disrupted testis differentiation and resulted in feminization in Xenopus laevis (the African clawed frog), an ideal species for studying reproductive endocrine disruption by estrogenic endocrine disrupting chemicals (EDCs). On this basis, we aimed to develop multiple biomarkers for early detection of testis differentiation disruption by estrogenic EDCs in X. laevis. Tadpoles at stage 45/46 were exposed to four known estrogenic EDCs with different estrogenic activities, including E2, diethylstilbestrol (DES), mestranol (MES) and 4-n-nonyphenol (NP). At stage 53, gonadal morphological and histological changes as well as altered sex-dimorphic gene expression in gonad-mesonephros complexes (GMCs) showed that these estrogenic EDCs disrupted testis differentiation and caused feminization to different degrees. Then we measured transcriptional changes of 48 candidate genes, which are believed to be associated with E2-induced testis differentiation alterations, in GMCs at stage 50. As a result, 19 genes were found to be transcriptionally altered by all test chemicals and proposed as promising biomarkers for early detection of testis differentiation disruption by estrogenic EDCs. Finally, all biomarker responses were integrated as integrated biomarker response (IBR) index to characterize testis differentiation disruption by these estrogenic EDCs in X. laevis. Compared with the methods used in previous studies, the multiple biomarker test using X. laevis at early developmental stages largely shortens the exposure duration, thereby achieving the goal of rapid detection. Certainly, the biomarker test needs further validations in the future study.

A Phylogenetically Distinct Group of Glandirana rugosa Found in Kyushu, Japan

The Japanese wrinkled frog Glandirana rugosa is separated into five genetically different groups. One group in western Japan is further divided into three subgroups, found in Kyushu, Shikoku, and western Honshu. We collected G. rugosa frogs at 39 sites in Kyushu and determined nucleotide sequences of the mitochondrial 12S and 16S rRNA genes for phylogenetic analysis. Unexpectedly, we found a group of frogs in southeastern Kyushu that did not cluster with any of the pre-existing five groups of G. rugosa on the phylogenetic trees. The frogs in the new group and G. rugosa in Kyushu were externally similar, but there were a few significant differences in morphological features between the two populations. In addition, we observed significant differences in the frogs' calls . Thus, the group of the frogs in southeastern Kyushu may represent a new candidate species in the genus Glandirana. We discuss the possibility of a new species.

The combined effects of temperature and aromatase inhibitor on metamorphosis, growth, locomotion, and sex ratio of tiger frog (Hoplobatrachus rugulosus) tadpoles

Background

The tiger frog (Hoplobatrachus rugulosus) is widely raised by many farms in southern region of China as an economically edible frog. The growth, development, and sexual differentiation of amphibians are influenced by temperature and steroid hormone level. However, the problem of hormone residues is caused by the addition of exogenous hormones in frog breeding, it is worth considering whether non-sterol aromatase inhibitors can be used instead of hormones.

Methods

In our study, H. rugulosus tadpoles were subjected to two water temperatures (29 °C and 34 °C) and three letrozole concentrations in the feed (0, 0.1 and 1 mg/g) to examine the effects of temperature, aromatase inhibitor and their interaction on metamorphosis, locomotion, and sex ratios. A G-test and contingency table were used to analyze the metamorphosis rate of tadpoles and the survival rate of froglets after feeding for 90 days. A G-test was also used to analyze sex ratios in different treatment groups.

Results

Metamorphosis time and body size (snout-vent length, body mass and condition factor) were significantly different between the two temperature treatments. Metamorphosis time was longer and body size was increased at 29 °C compared to those at 34 °C. Letrozole concentration and the temperature × letrozole interaction did not affect these variables. The jumping distance of froglets following metamorphosis was positively associated with the condition factor; when controlling for condition factor, jumping distance was not affected by temperature, letrozole concentration and their interaction. Temperature and letrozole concentration also did not affect metamorphosis and survival rate. Sex ratio of the control group (0 mg/g letrozole) was 1:1 at 29 °C, but there were more males at 34 °C. The sex ratios of H. rugulosus treated with letrozole at 29 °C and 34 °C were significantly biased toward males, and male ratio increased as letrozole concentration increased. Furthermore, more males were produced at 34 °C than at 29 °C at each letrozole concentration.

Exposure to an anti-androgenic herbicide negatively impacts reproductive physiology and fertility in Xenopus tropicalis

Amphibians are threatened on a global scale and pollutants may be contributing to population declines, but how chemicals impact on their reproduction is poorly understood. We conducted a life cycle analysis to investigate the impacts of early life exposure to two anti-androgens (exposure until completion of metamorphosis;stage 66): flutamide, (50 µg/L)/linuron (9 and 45 µg/L)) on sexual development and breeding competence in Xenopus tropicalis. Our analyses included: mRNA levels of dmrt1, cyp17, amh, cyp19, foxl2 and ar (tadpoles/metamorphs), gonadal histomorphology (metamorphs/adults), mRNA levels of ar/gr (adult male brain/gonad/forelimb), testosterone/corticosterone levels (adult males), secondary sexual characteristics (forelimb width/nuptial pad: adult males) and breeding competence (amplexus/fertility: adult males). Compared to controls, feminised sex ratios and increased number of spermatogonia (adults) were observed after exposure to flutamide and the lower linuron concentration. Exposure to the lower linuron concentration also resulted in demasculinisation of secondary sexual characteristics and reduced male fertility. Flutamide exposure resulted in masculinisation of the nuptial pad and elevated mRNA levels of dmrt1, cyp17, amh and foxl2 in brains (metamorphs). Testosterone levels were higher in all treatment groups, however, overall few effects were observed in response to the higher linuron concentration. Our findings advance understanding of reproductive biology of X. tropicalis and illustrate negative effects of linuron on reproductive processes at a concentration measured in freshwater environments.

Transcriptome analysis identifies genes involved in sex determination and development of Xenopus laevis gonads

Development of the gonads is a complex process, which starts with a period of undifferentiated, bipotential gonads. During this period the expression of sex-determining genes is initiated. Sex determination is a process triggering differentiation of the gonads into the testis or ovary. Sex determination period is followed by sexual differentiation, i.e. appearance of the first testis- and ovary-specific features. In Xenopus laevis W-linked DM-domain gene (DM-W) had been described as a master determinant of the gonadal female sex. However, the data on the expression and function of other genes participating in gonad development in X. laevis, and in anurans, in general, are very limited. We applied microarray technique to analyze the expression pattern of a subset of X. laevis genes previously identified to be involved in gonad development in several vertebrate species. We also analyzed the localization and the expression level of proteins encoded by these genes in developing X. laevis gonads. These analyses pointed to the set of genes differentially expressed in developing testes and ovaries. Gata4, Sox9, Dmrt1, Amh, Fgf9, Ptgds, Pdgf, Fshr, and Cyp17a1 expression was upregulated in developing testes, while DM-W, Fst, Foxl2, and Cyp19a1 were upregulated in developing ovaries. We discuss the possible roles of these genes in development of X. laevis gonads.

Participation of androgen and its receptor in sex determination of an amphibian species

INTRODUCTION

In the Japanese frog Rana (R.) rugosa the androgen receptor (AR) gene on the W chromosome (W-AR) is barely expressed. Previously we showed that incomplete female-to-male sex-reversal occurred in Z-AR transgenic female frogs. To date, however, there is no report showing that AR with androgens can determine genetically programed male sex fate in any vertebrate species. Here, we examined whether AR together with androgens functions as a sex determinant in an amphibian species.

METHODS

To examine whether complete female-to-male sex-reversal occurs in R. rugosa frogs, we produced AR-transgenic (Tg) and -knockdown (KD) female R. rugosa frogs by the I-SceI meganuclease-mediated gene trap and CRISPR/Cas9 system, respectively. AR-Tg and -KD tadpoles were reared in water containing testosterone (T) at 0 to 7.1 ng/ml. Frozen sections were prepared from the gonads of metamorphosed frogs and immunostained for laminin, Vasa, Pat1a, CYP17 and AR. We also employed PCR analysis to examine Dmrt1, Pat1a and CYP17 expression in the gonads of KD and placebo-KD female frogs.

RESULTS

Complete female-to-male sex-reversal occurred in the AR-Tg ZW female frogs when a low dosage of T was supplied in the rearing water of tadpoles. However, no sex-reversal was observed in AR-KD ZW female frogs when the gonads were treated with dosages of T high enough to induce complete female-to-male sex-reversal even in wild type frogs.

DISCUSSION

These results suggest that AR with its androgen ligand functions as a male sex-determinant in the ZW type R. rugosa frogs.

Identification and expression of cytochrome P450 genes in the Chinese giant salamander Andrias davidianus

Cytochrome P450 (cyp) genes play vital roles in biological processes, including in metabolism of endogenous and exogenous compounds. Exogenous hormone influences on gene expression that leads to disruption of gonad development have been studied in several species, but whether exogenous hormones affect cyp genes that show sexually dimorphic expression remains to be determined. Here, we identified and characterized cyp genes from the widely-cultured Chinese giant salamander Andrias davidianus. We obtained 20 cyp genes including 11 genes with complete sequences. Phylogenetic analyses supported the classification of cyp genes similar to other vertebrates. Expression profile of female and male salamanders showed multiple cyp genes to exhibit higher expression in ovary than in testis, including cyp26a, cyp19a, cyp1a1, cyp4v2, cyp3a24, cyp2c20, cyp2d14, cyp2d15, and cyp4b; while cyp11a, cyp2b11, cyp11b1, cyp4f22, cyp2j6, cyp2k1, cyp2k4, cyp1a5 was higher in testis than in ovary. Seven sex-biased genes were detected after aromatase inhibitor injection and with exposure to high water temperatures. Cyp2k1, cyp11b1, and cyp2j6 expression were increased, while cyp26a, cyp2c20, cyp4b genes were decreased with aromatase inhibition. In ovary, cyp26a and cyp2c20 expression were significantly reduced; whereas cyp2k1, cyp2j6, and cyp2k4 expression were significantly elevated with no changes in cyp11a and cyp11b1 expression after temperature treatment. These findings provide valuable information for further study of sex differentiation mechanisms and cyp gene evolution.

A Threshold Dosage of Testosterone for Female-to-Male Sex Reversal in Rana rugosa Frogs

Androgens play a critical role in testicular differentiation in many species of vertebrates. While female-to-male sex reversal can be induced by testosterone (T) in some species of amphibians, the mechanism still remains largely unknown even at the histological level. In this study, we determined a threshold dosage of T to induce female-to-male sex reversal in the Japanese frog Rana (R.) rugosa. Tadpoles were allowed to metamorphose into frogs with T present in the rearing water. At 0.2 ng/mL T, female frogs formed tissue comprising a mixture of ovary and testis, the so-called ovotestis, the size of which was significantly smaller than the wild-type ovary. Histological changes occurring in the oocytes of T-treated ovaries induced oocyte degeneration in the masculinizing ovaries leading to their final disappearance. In parallel, many germ cells emerged in the cortex of the ovotestis and, later, in the medulla as well. RT-PCR analysis revealed upregulated expression of CYP17 and Dmrt1 but not 17βHSD in the ovotestis, and downregulation of Pat1a expression. Furthermore, immunohistology revealed CYP17-positive signals in the cortex of the masculinizing ovary, spreading throughout the whole area as the testis developed. These results indicate that oocytes are sensitive to T in the ovary of R. rugosa and that male-type germ cells expand in the masculinizing gonad (testis) contemporaneous with oocyte disappearance.

Sex Reversal in Amphibians

Amphibians have been widely used to study developmental biology due to the fact that embryo development takes place independently of the maternal organism and that observations and experimental approaches are easy. Some amphibians like Xenopus became model organisms in this field. In the first part of this article, the differentiation of the gonads in amphibians and the mechanisms governing this process are reviewed. In the second part, the state of the art about sex reversal, which can be induced by steroid hormones in general and by temperature in some species, is presented. Also information about pollutants found in the environment that could interfere with the development of the amphibian reproductive apparatus or with their reproductive physiology is given. Such compounds could play a part in the amphibian decline, since in the wild, many amphibians are endangered species.

Structural changes in female-to-male sex-reversing gonads of Rana RUGOSA

The phenotypic sex of many species of amphibians is subject to reversal by steroid hormones. The mechanism of this process, however, still remains largely unknown. As a step toward understanding the histological changes during sex reversal in amphibians, we analyzed two- and three-dimensional (2D and 3D) structures of sex-reversing gonads in Rana rugosa frogs. 2D views revealed that many oocytes in the wild-type ovary disappeared during female-to-male sex-reversal concomitant with the emergence of Vasa-positive small germ cells. Some of the germ cells were labeled with BrdU. BrdU-positive germ cells were few in the testosterone (T) treated ovaries at days 8 and 16, which resembled wild-type ovaries. Basement membranes became disrupted by day 24 in T-treated ovaries. However, the membranes were later reconfigured into testis-like gonadal structures 40 days after T treatment. 3D imaging of the sex-reversing gonad using serial immunostained sections showed that germ cells were organized in linear fashion extending out from where the sex-reversing gonad attached to the mesorchium 24 days after T treatment. Germ cells were increased in number by 40 days and were localized to the cortex of the gonads. In a T-untreated testis at day 24, many germ cells were distributed throughout the cortex except in the central space, while the efferent duct ran between two sheets of the mesorchium. These results, taken together, suggest that the mesorchium plays an important role in the organization of testicular structure. This is the first report showing germ cell ontogeny and organization in the female-to-male sex-reversing gonad in a vertebrate species.

Complete depletion of primordial germ cells in an All-female fish leads to Sex-biased gene expression alteration and sterile All-male occurrence

Background

Gynogenesis is one of unisexual reproduction modes in vertebrates, and produces all-female individuals with identical genetic background. In sexual reproduction vertebrates, the roles of primordial germ cells on sexual dimorphism and gonadal differentiation have been largely studied, and two distinct functional models have been proposed. However, the role of primordial germ cells remains unknown in unisexual animals, and it is also unclear whether the functional models in sexual reproduction animals are common in unisexual animals.

Results

To solve these puzzles, we attempt to utilize the gynogenetic superiority of polyploid Carassius gibelio to create a complete germ cell-depleted gonad model by a similar morpholino-mediated knockdown approach used in other examined sexual reproduction fishes. Through the germ cell-depleted gonad model, we have performed comprehensive and comparative transcriptome analysis, and revealed a complete alteration of sex-biased gene expression. Moreover, the expression alteration leads to up-regulation of testis-biased genes and down-regulation of ovary-biased genes, and results in the occurrence of sterile all-males with testis-like gonads and secondary sex characteristics in the germ cell-depleted gynogenetic Carassius gibelio.

Conclusions

Our current results have demonstrated that unisexual gynogenetic embryos remain keeping male sex determination information in the genome, and the complete depletion of primordial germ cells in the all-female fish leads to sex-biased gene expression alteration and sterile all-male occurrence.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2130-z) contains supplementary material, which is available to authorized users.

Low concentrations of dihydrotestosterone induce female-to-male sex reversal in the frog Pelophylax nigromaculatus

Previous studies have demonstrated that some amphibian species can be sex-reversed by high concentrations of androgens. Little attention has focused on the effects of androgenic endocrine-disrupting chemicals (EDCs) on amphibians. The present study aimed to investigate the effects of lower concentrations of the androgenic EDC 5α-dihydrotestosterone (DHT) on gonadal differentiation and development in Pelophylax nigromaculatus, a true frog distributed widely in East Asia. Tadpoles at Gosner stage 24/25 were exposed to nominal concentrations of 40 ng/L, 400 ng/L, and 4000 ng/L DHT to complete metamorphosis. In all DHT treatment groups, males and ambiguous sexes were identified based on gonadal morphology, whereas no females were found; thus, all treatment groups exhibited male-skewed ratios compared with the control group. Gonadal histological examination revealed that ambiguous sexes displayed overall testicular structure with certain ovarian characteristics, demonstrating that DHT-induced sex-ambiguous gonads were incomplete ovary-to-testis reversals (IOTTRs). The expression levels of some ovary-biased genes in the IOTTRs were significantly higher than in the control testes but lower than in the control ovaries. These results show that low concentrations of DHT induced complete or incomplete female-to-male sex reversal in P. nigromaculatus, and incomplete sex reversal retained certain ovarian characteristics not only at gonadal morphological and histological levels but also at the molecular level. They present study highlights potential risks of DHT and other androgenic EDCs for P. nigromaculatus.

Molecular cloning and characterization of oocyte-specific Pat1a in Rana rugosa frogs

The Pat1 gene is expressed in the immature oocytes of Xenopus, and is reportedly involved in regulating the translation of maternal mRNAs required for oocyte-maturation. However, it is still unknown when Pat1a first appears in the differentiating ovary of amphibians. To address this issue, we isolated the full-length Pat1a cDNA from the frog Rana rugosa and examined its expression in the differentiating ovary of this frog. Among eight different tissues examined, the Pat1a mRNA was detectable in only the ovary. When frozen sections from the ovaries of tadpoles at various stages of development were immunostained for Vasa-a germ cell-specific protein-and Pat1a, Vasa-immunopositive signals were observed in all of the germ cells, whereas Pat1a signals were confined to the growing oocytes (50-200 μm in diameter), and absent from small germ cells (<50 μm in diameter). Forty days after testosterone injection into tadpoles to induce female-to-male sex-reversal, Pat1a-immunoreactive oocytes had disappeared completely from the sex-reversed gonad, but Vasa-positive small germ cells persisted. Thus, Pat1a would be a good marker for identifying the sexual status of the sex-reversing gonad in amphibians. In addition, fluorescence in situ hybridization analysis showed Pat1a to have an autosomal locus, suggesting that Pat1a transcription is probably regulated by a tissue-specific transcription factor in R. rugosa.

Molecular cloning and characterization of anti-Müllerian hormone (AMH) from the Japanese wrinkled frog, Rana rugosa

The role of anti-Müllerian hormone (AMH) during gonad development has been studied extensively in many species of mammal, bird, reptile, and fish but remains unresolved in amphibians. In male mammalian embryos, Sox9 activates AMH expression, which initiates regression of the Müllerian ducts. However, Sox9 (Sry-related HMG box 9) is unlikely to initiate AMH in chicken, because AMH precedes Sox9 expression in this species. To clarify whether AMH is involved in testicular differentiation in amphibians, we cloned the full-length AMH cDNA from the Japanese wrinkled frog, Rana rugosa. The AMH gene, which appears to be autosomal, is exclusively expressed in the testis of adult frog among 8 different tissues examined; Sertoli cells are probably responsible for its expression. AMH expression was found in the undifferentiated gonad of both male and female tadpoles, increasing in the differentiating testis. Moreover, we observed consensus binding sites for Sox9 in the 5'-flanking region of the AMH gene. Sox9 stimulated statistically significant AMH expression in luciferase reporter assays when coexpressed in Xenopus kidney-derived A6 cells. However, Sox9 expression showed no sexual dimorphism when AMH expression was up-regulated in the developing testis. These results, taken together, suggest that AMH is probably involved in testicular differentiation in R. rugosa, although an additional, perhaps tissue-specific, transcription factor may be required for the regulation of AMH transcription.

Cell-mass structures expressing the aromatase gene Cyp19a1 lead to ovarian cavities in Xenopus laevis

The African clawed frog, Xenopus laevis, has a ZZ/ZW-type sex-determination system. We previously reported that a W-linked gene, Dm-W, can determine development as a female. However, the mechanisms of early sex differentiation remain unclear. We used microarrays to screen for genes with sexually dimorphic expression in ZZ and ZW gonads during early sex differentiation in X laevis and found several steroidogenic genes. Importantly, the steroid 17α-hydroxylase gene Cyp17a1 and the aromatase gene Cyp19a1 were highly expressed in ZZ and ZW gonads, respectively, just after sex determination. At this stage, we found that Cyp17a1, Cyp19a1, or both were expressed in the ZZ and ZW gonads in a unique mass-in-line structure, in which several masses of cells, each surrounded by a basement membrane, were aligned along the anteroposterior axis. In fact, during sex differentiation, ovarian cavities formed inside each mass of Cyp17a1- and Cyp19a1-positive cells in the ZW gonads. However, the mass-in-line structure disappeared during testicular development in the ZZ testes. These results suggested that the mass-in-line structure found in both ZZ and ZW gonads just after sex determination might be formed in advance to produce ovarian cavities and then oocytes. Consequently, we propose a view that the default sex may be female in the morphological aspect of gonads in X laevis.

Involvement of androgen receptor in sex determination in an amphibian species

In mice and humans, the androgen receptor (AR) gene, located on the X chromosome, is not known to be involved in sex determination. In the Japanese frog Rana rugosa the AR is located on the sex chromosomes (X, Y, Z and W). Phylogenetic analysis shows that the AR on the X chromosome (X-AR) of the Korean R. rugosa is basal and segregates into two clusters: one containing W-AR of Japanese R. rugosa, the other containing Y-AR. AR expression is twice as high in ZZ (male) compared to ZW (female) embryos in which the W-AR is barely expressed. Higher AR-expression may be associated with male sex determination in this species. To examine whether the Z-AR is involved in sex determination in R. rugosa, we produced transgenic (Tg) frogs carrying an exogenous Z-AR. Analysis of ZW Tg frogs revealed development of masculinized gonads or 'ovotestes'. Expression of CYP17 and Dmrt1, genes known to be activated during normal male gonadal development, were up-regulated in the ZW ovotestis. Testosterone, supplied to the rearing water, completed the female-to-male sex-reversal in the AR-Tg ZW frogs. Here we report that Z-AR is involved in male sex-determination in an amphibian species.

An X-linked body color gene of the frog Rana rugosa and its application to the molecular analysis of gonadal sex differentiation

We identified a sex-linked, recessive body color gene, presently designated w (whitish-yellow), in the frog Rana rugosa from the Iwakuni population in Western Japan. This is the first time a sex-linked body color gene was found in amphibians so far. In this population of R. rugosa, males are the heterogametic sex, but the sex chromosomes are still homomorphic. When heterozygous males (Ww), which were produced by crossing a whitish-yellow female (ww) found in the field and a wild-type male (WW) of the same population, were backcrossed to the homozygous whitish-yellow female (ww), the resultant male offspring were all wild-type, whereas the females were all whitish-yellow. This result definitely indicates that w is recessive and X-linked, and its wild-type allele W is located on the Y chromosome. Using this strain (X(w)X(w) female and X(w)Y(W) male), we found that expression of Dmrt1 and Rspo1, which are involved in testicular and ovarian differentiation in vertebrates, was higher in males and females, respectively, prior to the onset of the sexually dimorphic expression of Cyp17 and Cyp19, which are involved in biosynthesis of sex steroids and are critical markers of gonadal sex differentiation.

Up-regulation of FSHR expression during gonadal sex determination in the frog Rana rugosa

In vertebrates, gonadal production of steroid hormones is regulated by follicle-stimulating hormone (FSH) and luteinizing hormone (LH) via their receptors designated FSHR and LHR, respectively. We have shown recently that steroid hormones are synthesized in the differentiating gonad of tadpoles during sex determination in the frog Rana rugosa. To elucidate the role of gonadotropins (GTHs) and their receptors in the production of gonadal steroid hormones during sex determination, we isolated the full-length FSHβ, LHβ, FSHR and LHR cDNAs from R. rugosa and determined gonadal expression of FSHR (FSH receptor) and LHR (LH receptor) as well as brain expression of FSHβ and LHβ during sex determination in this species. The molecular structures of these four glycoproteins are conserved among different classes of vertebrates. FSHβ expression was observed at similar levels in the whole brain (including the pituitary) of tadpoles, but it showed no sexual dimorphism during gonadal sex determination. By contrast, LHβ mRNA was undetectable in the whole brain of tadpoles. FSHβ-immunopositive cells were observed in the pituitary of female tadpoles with a differentiating gonad. Furthermore, FSHR expression was significantly higher in the gonad of female tadpoles during sex determination than in that of males, whereas LHR was expressed at similar levels in males and females. The results collectively suggest that FSHR, probably in conjunction with FSH, is involved in the steroid-hormone production during female-sex determination in R. rugosa.

Isolation and characterization of the CYP17A1 gene and its processed pseudogene in Rana rugosa

CYP17A1 expression is up-regulated in the gonad in Rana (Glandirana) rugosa tadpoles treated with androgens to induce female-to-male sex-reversal. In this study, we isolated the CYP17A1 gene and its processed pseudogene from R. rugosa. The former was found to consist of 8 exons, and the latter a single-exon gene, designated CYP17A1P. The sequence of the promoter region of CYP17A1 differed from that of CYP17A1P. We found several consensus binding-sites for candidate transcription factors including androgen receptor (AR), Sox and FoxL2 in the CYP17A1 promoter region, but an AR-binding site was absent from CYP17A1P. When AR was over-expressed in Xenopus A6 cells, it did not increase CYP17A1 transcription in luciferase assays. CYP17A1 was strongly expressed in indifferent male gonads during sex determination and exclusively in testis, among eight adult tissues of R. rugosa. By contrast, CYP17A1P was expressed at very low, and similar levels in the adult tissues of both sexes. Fluorescent In-Situ Hybridization (FISH) analysis showed that CYP17A1P is localized to chromosome 4, while CYP17A1 is on chromosome 9. These results collectively suggest that CYP17A1, but not CYP17A1P is involved in male sex-determination in R. rugosa, and that androgens may not have a direct effect on the CYP17A1 transcription.

Expression of cytochrome P450 side-chain cleavage enzyme mRNA and production of pregnenolone in the brain of the red-bellied newt Cynops pyrrhogaster

It is becoming clear that the vertebrate brain has the capability of forming steroids de novo, the so-called "neurosteroids". To understand neurosteroidogenesis in the brain, it is essential to demonstrate the formation of pregnenolone, a main precursor of neurosteroids. In amphibians, the pregnenolone formation from cholesterol is still unclear, although the brain accumulates pregnenolone, pregnenolone sulfate and 7α-hydroxypregnenolone. This study was addressed to obtain basic information about pregnenolone formation in the newt brain. Firstly, we demonstrated that the newt brain produces pregnenolone from cholesterol. Subsequently, cDNA encoding cytochrome P450 side-chain cleavage enzyme (P450scc), a key steroidogenic enzyme catalyzing pregnenolone formation, was isolated from the newt. The sequence analysis showed that the isolated P450scc cDNA contained a putative coding region consisting of 1569 bp, which encoded 523 amino acids. The steroid- and heme-binding domains of P450scc were highly shared in amino acids among vertebrates. RT-PCR analysis amplified the authentic fragment corresponding to newt P450scc showed its transcription in the brain. However, the transcription level in the brain was lower than those of the gonad and the kidney including adrenals. The restricted cells in the four major regions of the newt brain, such as the telencephalon, diencephalon, mesencephalon, and rhombencephalon, were demonstrated to express P450scc transcripts by RT-PCR and in situ hybridization. Taken together, these results indicate that the newt brain expresses P450scc mRNA and produces pregnenolone from cholesterol.

Estrogen biosynthesis in the gonad of the frog Rana rugosa

In certain species of amphibians gonadal differentiation is influenced by steroid hormones. In the case of the frog Rana rugosa testosterone given to tadpoles reverses sex from female to male, while the opposite reversal - male to female - can be achieved using estradiol-17β. In this study, we investigated whether CYP19 (P450 aromatase), the enzyme responsible for a production of estradiol-17β, was present in the differentiating gonad of R. rugosa. Initially, we immunized rabbits against frog CYP19 peptides and performed immunostaining using specific antibodies purified from that serum. CYP19-reactive signals were observed in gonadal somatic cells of the female, but not male tadpoles at stage (St.) I (the stage prior to phenotypic sex determination in tadpoles of R. rugosa). Immunopositive signals were also produced in ovarian somatic cells froglets at St. XXV (just after the completion of metamorphosis). We also examined the enzymatic activity of CYP19 in the differentiating gonad of R. rugosa. Reverse-phase HPLC (high performance liquid chromatography) analysis revealed that [(3)H]testosterone was converted to [(3)H]estradiol-17β in the gonad of tadpoles at St. I. Interestingly, the rate of conversion was much higher in females than in males. To the best of our knowledge, this is the first report on the biosynthesis of estradiol-17β in the gonad of amphibians, and the co-incident identification of active CYP19 enzyme in the differentiating gonad of R. rugosa. Based on our results, we conclude that estradiol-17β may be involved in ovarian differentiation in this species.

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

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

Structural changes in gonadal basement membranes during sex differentiation in the frog Rana rugosa

Here we report that structural changes in gonadal basement membranes during sex differentiation in the frog Rana rugosa are revealed using an antibody to its laminin component. Immunohistochemical staining indicated that the first sexual dimorphism appeared in testicular cords and ovarian cavities in differentiating gonads of tadpoles at St. 25-3W, three weeks after they reached St. 25. During development, as the testis enlarged, testicular cord partitions appeared to form by invagination of the testicular epithelium. Ovarian cavities also increased in volume. Laminin-positive basement membranes initially surrounded a partial surface of oocytes close to the ovarian cavity, fully covering growing oocytes by St. X. Laminin-reactive signals were present in somatic cells outside seminiferous tubules in the testis and outside oocytes in one-year-old frogs. BrdU-labeling showed that the number of dividing germ cells increased continuously in male gonads but increased in females only up to St. V, declining at St. X and thereafter. The number of dividing germ cells declined when the basement membranes had fully covered the oocytes. Together, these findings suggest that the first sexual dimorphism in the gonad of R. rugosa first appears as a structural change in the basement membranes. Finally, we speculate that the basement membrane on the surface of oocytes may affect their proliferation in this species.

Androgen receptor of the frog Rana rugosa: molecular cloning and its characterization

The androgen receptor(AR) gene is located on the Z and W sex chromosomes in the frog Rana rugosa, designated Z- and W-AR, respectively. Among various tissues of an adult frog, AR expression levels were highest in the testis and brain. In the testis, AR was expressed in germ cells. AR expression occured in developing embryos from stage 21 and was very high in the gonad of a male tadpole before the onset of sex determination. When Z- and W-AR were expressed in Xenopus A6 cells, they activated androgen-dependent transcription of a luciferase reporter gene. By contrast, estrogen receptor (ER) alpha and beta showed no sexually dimorphic expression during sex determination, but their expressions became much higher in the gonad of a female tadpole after sex determination. In addition, AR transcripts in the ZZ-tadpoles were twice as abundant as in the ZW genotype. In contrast, W-AR expression was extremely low although when W-AR was expressed in A6 cells, it activated transcription in the luciferase assay. In this regard it is worth noting that the promoter regions of Z- and W-AR are not identical. The results suggest that Z-AR plays an important role in the testis formation in a R. rugosa tadpole, whereas ERbeta is involved in ovary differentiation. Very low expression of W-AR may be due to its promoter region having mutations in key transcription factor binding sites, although these remain to be identified. Thus, it is proposed that AR could be a candidate for a male-determining gene in R. rugosa.

Sox3: a transcription factor for Cyp19 expression in the frog Rana rugosa

Cyp19 is expressed at a high level in the gonad of the female tadpole of the frog Rana rugosa during sex determination. To identify sequence elements important for expression of Cyp19, we isolated a genomic clone (approximately 40 kbp) carrying R. rugosa Cyp19 and analyzed the nucleotide sequence of the 5'-flanking region to search for potential transcription factor binding sites. Sox (SRY-related HMG box) protein and Sf1 binding sites were found in the ovary-specific promoter region of Cyp19. Because Sox3 is located on the sex chromosome in R. rugosa, we conducted the luciferase reporter assay in Xenopus A6 cells using the promoter region. Sox3 drove the reporter gene in the cells, but Sf1 did not. When sequential deletion of the 2.7 kbp Cyp19-promoter region was undertaken, a fragment spanning nucleotides -191 to +48 was sufficient to drive the transcription of the reporter gene. In site-directed mutagenesis, the binding site at -57 in the region was critical for Sox3 responsiveness. Sox3 lacking the HMG box had no ability to promote Cyp19 transcription. In addition, a chromatin immunoprecipitation (ChIP) assay showed that DNA fragments were enriched 8-fold, as determined by real-time PCR, when chromatin was immunoprecipitated with the anti-His antibody against His-tagged Sox3. The results, taken together, suggest that Sox3 activates Cyp19 transcription by its direct binding to the binding site of the Cyp19 promoter region. Sox3 appears to be a factor that directs indifferent gonads to develop into an ovary in R. rugosa.

Molecular cloning and gene expression of Foxl2 in the frog Rana rugosa

Foxl2 is a transcription factor that plays a crucial role in the regulation of the early development of the female gonad in mammals and fish. However, little is known regarding its role in ovarian differentiation in amphibians. In this study, we isolated a Foxl2 cDNA from the ovary of the frog Rana rugosa and Xenopuslaevis and examined its expression during gonadal sex differentiation in R. rugosa. Alignment of known Foxl2 sequences from vertebrates showed high identity of the Foxl2 open reading frame and protein sequences, in particular the forkhead domain and C-terminal region, with other vertebrate sequences. Among different adult tissues, Foxl2 was expressed at its highest level in the ovary. Real-time RT-PCR analysis showed that Foxl2 expression was sexually dimorphic during gonadal sex differentiation in R. rugosa. In addition, Foxl2, which was detected immunochemically in somatic cells surrounding oocytes in the ovary, promoted R. rugosaCYP19 transcription in luciferase promoter assays conducted in A6 cells. We also found by FISH analysis that Foxl2 was an autosomal gene. Altogether, these results suggest that Foxl2 probably plays a very important role in ovarian differentiation of R. rugosa by possibly regulating CYP19 expression. The factor that up-regulates Foxl2 expression in female gonads still remains to be identified.

Immunohistochemical detection and biological activities of CYP17 (P450c17) in the indifferent gonad of the frog Rana rugosa

Sex steroids play a crucial role in the gonad differentiation in various species of vertebrates. However, little is known regarding the localization and biological activity of steroid-metabolizing enzymes during gonadal sex differentiation in amphibians. In the present study, we showed by real-time RT-PCR analysis that the expression of CYP17, one of the key steroidogenic enzymes, was higher in the indifferent gonad during sex differentiation in male than in female tadpoles of Rana rugosa but that there was no difference detected in the 3betaHSD mRNA level between the male and female gonads. We next examined the localization of CYP17, 3betaHSD and 17betaHSD in the indifferent and differentiating gonads by using three kinds of antibodies specific for CYP17, 3betaHSD and 17betaHSD, respectively. Positive signals for CYP17, 3betaHSD and 17betaHSD were observed in somatic cells of the indifferent gonad of males and in the interstitial cell of the testis. The enzymatic activity of CYP17 was also examined in the gonad during sex differentiation in this species. [(3)H]Progesterone (Prog) was converted to [(3)H]androstenedione (AE) in the indifferent gonad in males and females, but the rate of its conversion was higher in males than in females. Moreover, fluorescence in situ hybridization (FISH) analysis revealed that the CYP17 gene was located on the q arm of chromosome 9, indicating that CYP17 was autosomal in R. rugosa. Taken together, the results demonstrate that the CYP17 protein is synthesized in somatic cells of the indifferent gonad during gonadal sex differentiation in R. rugosa and that it is more active in converting Prog to AE in males than in females. The data suggest that CYP17 may be involved in testicular formation during sex differentiation in this species.

Sex determination in amphibians

The heterogametic sex is male in all mammals, whereas it is female in almost all birds. By contrast, there are two heterogametic types (XX/XY and ZZ/ZW) for genetic sex determination in amphibians. Though the original heterogametic sex was female in amphibians, the two heterogametic types were probably interchangeable, suggesting that sex chromosomes evolved several times in this lineage. Indeed, the frog Rana rugosa has the XX/XY and ZZ/ZW sex-determining systems within a single species, depending on the local population in Japan. The XY and ZW geographic forms with differentiated sex chromosomes probably have a common origin as undifferentiated sex chromosomes resulted from the hybridization between the primary populations of West Japan and Kanto forms. It is clear that the sex chromosomes are still undergoing evolution in this species group. Regardless of the presence of a sex-determining gene in amphibians, the gonadal sex of some species can be changed by sex steroids. Namely, sex steroids can induce the sex reversal, with estrogens inducing the male-to-female sex reversal, whereas androgens have the opposite effect. In R. rugosa, gonadal activity of CYP19 (P450 aromatase) is correlated with the feminization of gonads. Of particular interest is that high levels of CYP19 expression are observed in indifferent gonads at time before sex determination. Increases in the expression of CYP19 in female gonads and CYP17 (P450 17alpha-hydroxylase/C17-20 lyase) in male gonads suggest that the former plays an important role in phenotypic female determination, whereas the latter is needed for male determination. Thus, steroids could be the key factor for sex determination in R. rugosa. In addition to the role of sex steroids in gonadal sex determination in this species, Foxl2 and Sox3 are capable of promoting CYP19 expression. Since both the genes are autosomal, another factor up-regulating CYP19 expression must be recruited. The factor, which may be located on the X or W chromosome, intervenes directly or indirectly, in the transcriptional regulation of the CYP19 gene for feminization in amphibians. A factor up-regulating CYP17 expression remains to be identified.

Steroidogenic gene expression during sex determination in the frog Rana rugosa

Rana rugosa is unique among frog species in that it has two distinct types of sex chromosomes in two separate forms (XX/XY and ZZ/ZW). Treatment with sex steroids can reverse its gender from female to male or male to female. This phenomenon makes it a novel model for studying gonadal differentiation. The physiological role of sex steroids in sex differentiation in amphibians is yet unclear, however. To address this issue, we cloned the cDNAs of 17betaHSD types 8 (17betaHSD8) and 12 (17betaHSD12), 5alpha-reductase type 1 (5alphaRed1), and the steroidogenic acute regulatory protein known as StAR in the steroidogenic pathway. Then, we measured the mRNA levels of these genes during sex differentiation by real-time RT-PCR. The levels of CYP11A1, 3betaHSD, CYP17 and CYP19 mRNA were also measured by real-time RT-PCR. As a result, we detected transcripts of all such genes except for that of 17betaHSD8 in the indifferent gonad before the onset of sex determination. The expression of CYP17 occurred in indifferent gonads in both sexes; and its transcript levels were much higher in the male gonads. By contrast, the levels for CYP19 were much higher in the female gonads. CYP11A1, 3betaHSD, 17betaHSD12, 5alphaRed1 and StAR showed no sexually dimorphic expression during gonadal sex differentiation. Taken together, the results suggest that CYP17 has a major influence on testis development and that CYP19 plays a similar role in ovary development. However, the factors that up-regulate their expression remain to be identified.

Differential display analysis of gene expression in female-to-male sex-reversing gonads of the frog Rana rugosa

Sex steroids play pivotal roles in gonadal differentiation in many species of vertebrates. The sex can be reversed from female to male by testosterone in the Japanese wrinkled frog Rana rugosa, but it is still unclear what genes are up- or down-regulated during the XX sex-reversal in this species. To search the genes for the female-to-male sex-reversal, we employed differential display and 5'/3'-RACE. Consequently, we isolated from the gonads at day 8 after testosterone injection 24 different cDNA fragments showing a testosterone treatment-related change and then obtained three full-length cDNAs, which we termed Zfp64, Zfp112, and Rrp54. The former two cDNAs encoded different proteins with zinc-finger domains, whereas the latter cDNA encoded an unknown protein. Transcripts of the three genes were hardly detectable in the sex-reversing gonads at day 24 after the injection; at this time few growing oocytes were observed in the sex-reversing gonad. Besides, in situ hybridization analysis showed positive signals of the three genes in the cytoplasm of growing oocytes of an ovary when testosterone was injected into a tadpole. Thus, the decrease in expression of these three genes was probably due to the disappearance of growing oocytes and not to their direct involvement in the testis formation. To find the key-gene for testis formation, it will be necessary to analyze, by the differential display method, more genes showing a change in expression pattern during sex reversal.