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

Vitamin D3 application and factors of its activity in the adrenal cortex of andropausal rats: A functionally-histological study

INTRODUCTION AND AIM

Vitamin D supplementation in aging subjects manifests a positive effect on various health-related parameters. We performed a functionally-histological analysis of the adrenal cortex regarding the factors of vitamin D activity and corticosterone output after vitamin D3 application in a rat model of the andropause.

MATERIAL AND METHODS

Middle-aged Wistar rats were divided into sham operated (SO; n=8), orchidectomized (Orx; n=8) and vitamin D3-treated orchidectomized (Orx+vit. D; n=8) groups. Vitamin D3 (5 μg/kg b.m.) was administered subcutaneously for three weeks, while the SO and Orx groups received the vehicle alone. Set objectives were achieved using histochemistry/immunohistochemistry, stereology, ultrastructural and biochemical analyses.

RESULTS

Orchidectomy (Orx) decreased the adrenal cortex-related volume densities of vascular (p<0,0001), vitamin D receptor (VDR; p<0,0166), cytochrome P450 oxidase 2R1 (CYP 2R1; p<0,0001) and cytochrome P450 oxidase 24 (CYP 24; p<0,0001) depots, but increased the volume density of cytochrome P450 27B1 (CYP 27B1; p<0,0001) depots. In Orx+vit. D rats, increase of the adrenal cortex-related volume densities of collagen (p<0,0001), VDR (p<0,0001) and CYP 2R1 (p<0,0001) depots as well as the lipid-droplet diameter (p<0,0001) in adrenocortical outer zona fasciculata cells was observed, while a decrease of volume densities of the vascular (p<0,0001), CYP 27B1 (p<0,0001) and CYP 24 (p<0,0001) depots was registered, all versus Orx group. Plasma level of ACTH was decreased (p=0,0155) and serum concentrations of 25-hydroxyvitamin D3 and corticosterone were increased (p<0,0001 and p=0,0187, respectively), all after the same treatment.

CONCLUSIONS

Increased corticosterone output after vitamin D3 application to andropausal rats appears not to be related to increased availability of 25-hydroxyvitamin D3 and decreased degradation of 1,25-dihydroxyvitamin D3 in adrenal tissue, but rather involves the central regulatory mechanisms.

Homology in Sex Determination in Two Distant Spiny Frogs, Nanorana quadranus and Quasipaa yei

Sex determination is remarkably diverse, with frequent transitions between sex chromosomes, in amphibians. Under these transitions, some chromosomes are more likely to be recurrently co-opted as sex chromosomes, as they are often observed across deeply divergent taxa. However, little is known about the pattern of sex chromosome evolution among closely related groups. Here, we examined sex chromosome and sex determination in two spiny frogs, Nanorana quadranus and Quasipaa yei. We conducted an analysis of genotyping-by-sequencing (GBS) data from a total of 34 individuals to identify sex-specific makers, with the results verified by PCR. The results suggest that chromosome 1 is a homologous sex chromosome with an XY pattern in both species. This chromosome has been evolutionarily conserved across these closely related groups within a period of time. The DMRT1 gene is proposed to be implicated in homology across two distantly related spiny frog species as a putative candidate sex-determining gene. Harboring the DMRT1 gene, chromosome 1 would have been independently co-opted for sex determination in deeply divergent groups of anurans.

Morphological and transcriptional analysis of sexual differentiation and gonadal development in a burrowing fish, the four-eyed sleeper (Bostrychus sinensis)

Four-eyed sleeper (Bostrychus sinensis) is a commercially important sea water fish, and the male individuals exhibit significant advantages in somatic growth and stress resistance, so developing sex control strategy to create all-male progeny will produce higher economic value. However, little is known about the genetic background associated with sex differentiation in this species. In this study, we investigated gonadal development and uncovered critical window stages of sexual differentiation (about 2 mph), transition from proliferation to differentiation in female germ stem cells (GSCs) (2-3 mph) and male GSCs (3-4 mph). De novo transcriptome analysis revealed candidate genes and signaling pathways associated with sexual differentiation and gonadal development in four-eyed sleeper. The results showed that sox9 and zglp1 were the earliest sex-biased transcription factors during sex differentiation. Down-regulation of chemokine, cytokines-cytokine receptors and up-regulation of cellular senescence pathway might be involved in GSC differentiation. Weighted gene correlation network analysis showed that metabolic pathway and occludin were the hub signaling and gene in ovarian development, meanwhile the MAPK signaling pathways, cellular senescence pathway and ash1l (histone H3-lysine4 N-trimethyltransferase) were the hub pathways and gene in testicular development. The present work elucidated the developmental processes of sexual differentiation and gonadal development and revealed their associated revealed genes and signaling pathways in four-eyed sleeper, providing theoretical basis for developing sex-control techniques.

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-linked markers in an Australian frog Platyplectrum ornatum (Limnodynastidae) with a small genome and homomorphic sex chromosomes

Amphibians have highly diverse sex-determining modes leading to a notable interest in vertebrate sex determination and sex chromosome evolution. The identification of sex-determining systems in amphibians, however, is often difficult as a vast majority consist of homomorphic sex chromosomes making them hard to distinguish. In this study, we used Diversity Array Technology sequencing (DArTseq) to identify the sex-determining system in the ornate burrowing frog from Australia, Platyplectrum ornatum. We applied DArTseq to 44 individuals, 19 males and 25 females, collected from two locations to develop sex-linked markers. Unexpectedly, these 44 individuals were classified into two distinct population clusters based on our SNP analyses, 36 individuals in cluster 1, and 8 individuals in cluster 2. We then performed sex-linkage analyses separately in each cluster. We identified 35 sex-linked markers from cluster 1, which were all associated with maleness. Therefore, P. ornatum cluster 1 is utilising a male heterogametic (XX/XY) sex-determining system. On the other hand, we identified 210 sex-linked markers from cluster 2, of which 89 were male specific, i.e., identifying XX/XY sex determining system and 111 were female specific, i.e., identifying ZZ/ZW sex determining system, suggesting existence of either male or female heterogametic sex determining system in cluster 2. We also performed cytogenetic analyses in 1 male and 1 female from cluster 1; however, we did not detect any visible differentiation between the X and Y sex chromosomes. We also mapped sex-linked markers from the two clusters against the P. ornatum genome and our comparative analysis indicated that the sex chromosomes in both clusters shared homologies to chromosome 10 (autosome) of Rana temporaria and ZWY sex chromosome of Xenopus tropicalis. Our preliminary data suggest that it is plausible that the cluster 2 has a potential to be either male or female heterogamety in sex determination, requiring further investigation.

cyp21a2 Knockout Tadpoles Survive Metamorphosis Despite Low Corticosterone

Corticosteroids are so vital for organ maturation that reduced corticosteroid signaling during postembryonic development causes death in terrestrial vertebrates. Indeed, death occurs at metamorphosis in frogs lacking proopiomelanocortin (pomc) or the glucocorticoid receptor (GR; nr3c1). Some residual corticosteroids exist in pomc mutants to activate the wild-type (WT) GR and mineralocorticoid receptor (MR), and the elevated corticosteroids in GR mutants may activate MR. Thus, we expected a more severe developmental phenotype in tadpoles with inactivation of 21-hydroxylase, which should eliminate all interrenal corticosteroid biosynthesis. Using CRISPR/Cas9 in Xenopus tropicalis, we produced an 11-base pair deletion in cyp21a2, the gene encoding 21-hydroxylase. Growth and development were delayed in cyp21a2 mutant tadpoles, but unlike the other frog models, they survived metamorphosis. Consistent with an absence of 21-hydroxylase, mutant tadpoles had a 95% reduction of aldosterone in tail tissue, but they retained some corticosterone (∼40% of WT siblings), an amount, however, too low for survival in pomc mutants. Decreased corticosteroid signaling was evidenced by reduced expression of corticosteroid-response gene, klf9, and by impaired negative feedback in the hypothalamus-pituitary-interrenal axis with higher messenger RNA expression levels of crh, pomc, star, and cyp11b2 and an approximately 30-fold increase in tail content of progesterone. In vitro tail-tip culture showed that progesterone can transactivate the frog GR. The inadequate activation of GR by corticosterone in cyp21a2 mutants was likely compensated for by sufficient corticosteroid signaling from other GR ligands to allow survival through the developmental transition from aquatic to terrestrial life.

Impaired negative feedback and death following acute stress in glucocorticoid receptor knockout Xenopus tropicalis tadpoles

Blood glucocorticoid levels are regulated by the hypothalamo-pituitary-adrenal/interrenal axis (HPA axis in mammals, HPI axis in amphibians), and negative feedback by glucocorticoid signaling is a key player in that regulation. Glucocorticoid and mineralocorticoid receptors (GR and MR) mediate negative feedback in mammals, but little is known about nuclear receptor-mediated feedback in amphibians. Because amphibians have only one corticosteroidogenic cell type responsible for glucocorticoid and mineralocorticoid production, we hypothesized that GR knockout (GRKO) tadpoles have elevated levels of glucocorticoids and mineralocorticoids as well as axis components regulating their production. We also examined the response to stress and potential for increased aldosterone signaling in GRKO tadpoles. We found that GRKO tadpoles have severe hyperactivity of the HPI axis, namely high mRNA expression levels of pomc, cyp17a1, cyp21a2, cyp11b2, and star, and high tissue content of corticosterone, aldosterone, 17-hydroxyprogesterone, 21-deoxycortisol, and progesterone. Such aberrant HPI activity was accompanied by reduced survival after acute temperature shock and shaking stress. Like mammalian models of HPA hyperactivity, GRKO tadpoles have high MR mRNA expression levels in brain, kidney, heart, and skin and high levels of the inflammatory cytokine tnf-α and the profibrotic factor tgf-β in kidneys. This study showed GR is critical for negative feedback to the amphibian HPI axis and for survival from acute stressors. This study also showed GRKO tadpoles exhibit altered expression/overproduction of regulators of salt-water homeostasis and associated biomarkers of kidney disease.

Highly rapid and diverse sex chromosome evolution in the Odorrana frog species complex

Sex chromosomes in poikilothermal vertebrates are characterized by rapid and diverse evolution at the species or population level. Our previous study revealed that the Taiwanese frog Odorrana swinhoana (2n = 26) has a unique system of multiple sex chromosomes created by three sequential translocations among chromosomes 1, 3, and 7. To reveal the evolutionary history of sex chromosomes in the Odorrana species complex, we first identified the original, homomorphic sex chromosomes, prior to the occurrence of translocations, in the ancestral-type population of O. swinhoana. Then, we extended the investigation to a closely related Japanese species, Odorrana utsunomiyaorum, which is distributed on two small islands. We used a high-throughput nuclear genomic approach to analyze single-nucleotide polymorphisms and identify the sex-linked markers. Those isolated from the O. swinhoana ancestral-type population were found to be aligned to chromosome 1 and showed male heterogamety. In contrast, almost all the sex-linked markers isolated from O. utsunomiyaorum were heterozygous in females and homozygous in males and were aligned to chromosome 9. Morphologically, we confirmed chromosome 9 to be heteromorphic in females, showing a ZZ-ZW sex determination system, in which the W chromosomes were heterochromatinized in a stripe pattern along the chromosome axis. These results indicated that after divergence of the two species, the ancestral homomorphic sex chromosome 1 underwent highly rapid and diverse evolution, i.e., sequential translocations with two autosomes in O. swinhoana, and turnover to chromosome 9 in O. utsunomiyaorum, with a transition from XY to ZW heterogamety and change to heteromorphy.

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.

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 determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Gonad-Specific Transcriptomes Reveal Differential Expression of Gene and miRNA Between Male and Female of the Discus Fish (Symphysodon aequifasciatus)

The discus fish (Symphysodon aequifasciatus) is an ornamental fish that is well-known around the world. In artificial reproduction, they must be matched by one male and one female, whereas phenotype investigation indicated that there are no significant differences in appearance between males and females, which causes great difficulties in the mating during artificial reproduction. So, it is of great importance to establish artificial sex identification methods for the discus fish. The molecular mechanism of the sexual dimorphism of the discus fish was previously unknown. In this study, we constructed six cDNA libraries from three adult testes and three adult ovaries and performed RNA sequencing for identifying sex-biased candidate genes and microRNAs (miRNAs). A total of 50,082 non-redundant genes (unigenes) were identified, of which 18,570 unigenes were significantly overexpressed in testes, and 11,182 unigenes were significantly overexpressed in ovaries. A total of 551 miRNAs were identified, of which 47 miRNAs were differentially expressed between testes and ovaries. Eight differentially expressed unigenes, seven differentially expressed miRNAs and one non-differential miRNA were validated by quantitative real-time polymerase chain reaction. Twenty-four of these differentially expressed miRNAs and their 15 predicted target genes constituted 41 miRNA–mRNA interaction pairs, and some of vital sex-related metabolic pathways were also identified. These results revealed these differentially expressed genes and miRNAs between ovary and testis might be involved in regulating gonadal development, sex determination, gametogenesis, and physiological function maintenance, and there are complex regulatory networks between genes and miRNAs. It can help us understand the molecular mechanism of the sexual dimorphism and obtain a high-efficiency sex identification method in the artificial reproduction process of the discus fish.

Gonadal differentiation and its sensitivity to androgens during development of Pelophylax nigromaculatus

Our previous observations proposed Pelophylax nigromaculatus as a model species for studying the masculinizing effects of androgenic EDCs in amphibians. To better develop this model species, we studied the process of the gonadal differentiation/development and the sensitive stage to androgens. We found that the earliest sexual dimorphism in gonads at morphological and histological levels occurred at stages 38-40 and stage 36 respectively. Further examination of molecular markers for testicular and ovarian differentiation during development revealed that the cyp17 and cyp19 expressions were sexually dimorphic from stage 32 and stage 36 respectively. Further, we investigated the sex-reversal induced by 100 ng/L 5α-dihydrotestosterone (DHT) when exposures were initiated at stages 24, 26 and 28. We found that when exposed from stage 24, DHT resulted masculinization of all tadpoles with no typical ovaries, whereas exposures from stage 26 or 28 dramatically reduced the effect of DHT. Our findings show that gonads of P. nigromaculatus are bipotential at stage 24, in the process of differentiation at stage 26 and determined to become either testis or ovary at stage 28. Altogether, exposure of P. nigromaculatus should begin at stage 24 in order to sensitively detect masculinizing effects of EDCs. Present study provides useful information about the gonadal differentiation and development in P. nigromaculatus for effectively evaluating masculinizing effects of EDCs on 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.

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.

Sex-dependent expression of anti-Müllerian hormone (amh) and amh receptor 2 during sex organ differentiation and characterization of the Müllerian duct development in Xenopus tropicalis

Amphibian gonadal differentiation involves the action of sex steroids. Recent research indicates that the anti-Müllerian hormone (AMH) is involved in testicular development in some lower vertebrate species. For amphibians there is a lack of data on ontogenetic expression of the AMH receptor AMHR2/amhr2 and of progesterone receptors (PGRS/pgrs). Here we expand the knowledge on amphibian sex differentiation by characterizing ontogenetic mRNA levels of amh, amhr2, intracellular and membrane pgrs (ipgr and mpgr beta) and cytochrome P450 19a1 (cyp19a1) (ovarian marker) in the urogenital complex of the model species Xenopus (Silurana) tropicalis. Furthermore, we characterized the ontogenetic development of the Müllerian ducts (precursors of the female reproductive tract) histologically. The developmental period investigated spanned from beginning of gonadal differentiation, Nieuwkoop and Faber (NF) stage 51, to 4weeks post-metamorphosis. The Müllerian ducts were first observed at NF 64 in both sexes. Male-enhanced amh mRNA levels from NF 53/54 to 6days post-metamorphosis and female-enhanced cyp19a1 levels from NF 53 to 4weeks post-metamorphosis were noted. The sexually dimorphic mRNA level profile was more distinct for amh than for cyp19a1. The pgrs mRNA levels increased over the studied period and showed no sex differences. At later developmental stages, the amhr2 mRNA level was increased in putative females compared with males. Our findings suggest that AMH has a role in gonadal differentiation in X. tropicalis. We propose relative gonadal amh mRNA level as a testicular marker during early gonadal development in amphibians.

Pattern of gonadal differentiation and development up to sexual maturity in the frogs, Microhyla ornata and Hylarana malabarica: A comparative study

Gonadogenesis was studied in Microhyla ornata (Family: Microhylidae) and Hylarana malabarica (Family: Ranidae) up to sexual maturity. Indifferent gonads of M. ornata directly differentiated into either testes or ovaries while those of H. malabarica differentiated into ovaries in all the individuals followed by testicular differentiation in males through an ovarian phase. In some tadpoles of M. ornata, formation of a central cavity at Gosner stage 27 marked ovary differentiation while meiosis was initiated at stage 29. Folliculogenesis was evident at stage 39. Vitellogenesis was initiated in females 9 months post-metamorphosis that attained maturity around 11 months after the completion of metamorphosis. Gonads of males with uniformly distributed germ and somatic cells remained undifferentiated until stage 41. Germ and somatic cells reorganized into seminiferous cords at stage 42. One month after completing metamorphosis, testes contained seminiferous tubules while those of 3 months old males exhibited all spermatogenic stages. In H. malabarica, germ cells entry into meiosis marked ovary differentiation at stage 29 while, ovarian cavity was discernable around stage 35. Post-metamorphosis, ovaries of 1-6 month old females contained pre-diplotene oocytes. Females were immature even 1 year after the completion of metamorphosis. In all the tadpoles, ovaries with central cavity and meiocytes were present up to the completion of metamorphosis. Gonads of prospective males displayed an obliterating ovarian cavity along with degenerating oocytes at the end of metamorphosis. Germ and somatic cells reorganized into seminiferous cords in males 3 months after the completion of metamorphosis. Testes of 4 months old males exhibited distinct seminiferous tubules while those of 6 months old exhibited meiosis. All spermatogenic stages were observed in testes of 9 months old males indicating maturity.

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.

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.

Gonadal sex differentiation, development up to sexual maturity and steroidogenesis in the skipper frog, Euphlyctis cyanophlyctis

Gonadal sex differentiation, development up to sexual maturity and steroidogenesis were studied in the Indian skipper frog, Euphlyctis cyanophlyctis. In stage 25 tadpoles, gonads contained a few yolk laden germ cells and somatic cells. Ovarian differentiation occurred at stage 27 with the initiation of meiosis. Interestingly, meiosis preceded the formation of a central lumen that was discernible at stage 28. Folliculogenesis in the developing ovary was observed at stage 29. Vitellogenesis was observed in the 3 months old frogs and the females attained sexual maturity around 4 months. Testicular differentiation occurred indirectly through an ovarian phase. In some animals, from stage 37 onwards, oocyte degeneration was observed that was completed around metamorphic climax. Concurrently, large numbers of mesonephric cells were invading the gonads. Around metamorphosis, reorganization of the germ and somatic cells into testicular cords was observed. Following metamorphosis, the formation of seminiferous tubules was observed in the 2 weeks old males. Meiosis in the developing testes was observed in 1.5 months old males. In 3 months old males, the testes contained all stages spermatogenesis including spermatozoa. Steroidogenesis in the developing gonads was studied by immunohistochemical localization of 3β-HSD enzyme. At stage 26, a few immunoreactive cells were seen in the kidneys (interrenal cells). However, during and after differentiation, gonads failed to show positive immunoreaction. In the developing ovary at stage 37, follicular cells surrounding the oocytes were positive for 3β-HSD immunoreactivity. In the ovaries of 3 months old females, follicular cells surrounding the vitellogenic oocytes and stromal cells were positive for 3β-HSD immunoreaction. E. cyanophlyctis exhibits undifferentiated type of gonadal differentiation, in which gonadal differentiation precedes steroidogenesis.

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.

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.

A role for cytochrome P450 17alpha-hydroxylase/c17-20 lyase during shift in steroidogenesis occurring in ovarian follicles prior to oocyte maturation

Cytochrome P450 17alpha-hydroxylase/c17-20 lyase (P450c17) is regarded as one of the key enzymes involved in the steroidogenic shift that occurs prior to oocyte maturation in teleosts. Role of P450c17 in the shift in steroidogenesis during oocyte maturation is a contentious issue even after identification of a novel type of P450c17 that lacks lyase activity. To understand the role of P450c17 in steroidogenic shift explicitly, a full length cDNA encoding p450c17 from ovary of air-breathing catfish, Clarias gariepinus was cloned. p450c17 transiently expressed in COS-7 cells converted progesterone to androstenedione through 17alpha-hydroxyprogesterone and catfish p450c17 was found to be expressed ubiquitously with relatively higher levels in gonads, brain, kidney and gills. Immunocytochemical analysis revealed the presence of P450C17 in follicular layer of ovarian follicle, interstitial cells and spermatocytes of testis. p450c17 expression and ratio of lyase to hydroxylase was high in preparatory and pre-spawning phases of ovary and low in spawning phase. Expression of p450c17 correlated well with testicular recrudescence with maximum expression in preparatory and spawning phases. Neither protein expression nor lyase/hydroxylase activity changed significantly during hCG-induced oocyte maturation, in vitro and in vivo though mRNA levels increased. These results tend to suggest that the ovarian follicles attains capacity to produce maximum precursor steroid levels before spawning that might contribute to the shift in steroidogenesis.

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.