Involvement of androgen receptor in sex determination in an amphibian species

Gonadal expression profiles reveal the underlying mechanisms of temperature effects on sex determination in the large-scale loach (Paramisgurnus dabryanus)

The sex determination mechanism in large-scale loach (Paramisgurnus dabryanus) follows a ZZ/ZW system, with sexual differentiation regulated by both genotypic factors and temperature effects (GSD+TSD), where elevated temperatures result in a higher proportion of males. Currently, research on the sex determination mechanisms in large-scale loach is limited, and the specific gene expression profiles and the role of temperature in influencing sex remain largely unknown. This study investigated the impact of temperature on the sex ratio in cultured populations of the large-scale loach, and then identified a female-specific genetic marker by whole genome sequencing, facilitating the distinguishing of females, males, and pseudo-males within this population. Transcriptomic analysis was subsequently performed on these groups, and the data revealed a similar expression pattern between pseudo-males and true-males. The research combined differential expression analysis with WGCNA to construct a regulatory network of nine sex differentiation-related genes (SDG) (map3k4, trpv4, hsd17b12a, wt1, ar, dmrt1, bcar1, sox9a, cyp17a1), indicating that sex differentiation in large-scale loach is probably driven by the regulation of male-related genes. The transcriptomic analysis suggested that temperature significantly modified the expression of SDG in the ovaries, while in the testes, it predominantly affects metabolism-related pathways. We established a temperature-sensitive gene network in females, based on the correlation between gene expression and temperature, as well as the number of co-regulated genes in female data. We propose that, with increasing temperature, wt1 serves as a central regulator, leading to the down-regulation of foxl2a, cyp19a1a, and the cholesterol biosynthesis-related gene sqlea, ultimately resulting in the development of pseudo-males.

Disruption of Sex-Linked Sox3 Causes ZW Female-to-Male Sex Reversal in the Japanese Frog Glandirana rugosa

Sox3 is an ancestral homologous gene of the male-determining Sry in eutherian mammals and determines maleness in medaka fish. In the Japanese frog, Glandirana rugosa, Sox3 is located on the Z and W chromosomes. To assess the sex-determining function of Sox3 in this frog, we investigated its expression in gonads during early tadpole development and conducted genome-editing experiments. We found that the Sox3 mRNA levels in the gonads/mesonephroi were much higher in ZW females than that in ZZ males, and that the W-borne allele was dominantly expressed. A higher expression in ZW females preceded the onset of the sexually dimorphic expression of other autosomal sex differentiation genes. The Sox3 protein was detected by immunostaining in the somatic cells of early tadpole gonads around the boundary between the medulla and cortex in ZW females, whereas it was outside the gonads in ZZ males. Disrupting Sox3 using TALEN, which targets two distinct sites, generated sex-reversed ZW males and hermaphrodites, whereas no sex reversal was observed in ZZ males. These results suggest that the sex-linked Sox3 is involved in female determination in the ZZ-ZW sex-determining system of the frog, an exact opposite function to the male determination of medaka Sox3y and eutherian Sry.

Impacts of Exposure to Ultraviolet Radiation and an Agricultural Pollutant on Morphology and Behavior of Tadpoles (Limnodynastes tasmaniensis)

Amphibians are the most threatened vertebrate class globally. Multiple factors have been implicated in their global decline, and it has been hypothesized that interactions between stressors may be a major cause. Increased ultraviolet (UV) radiation, as a result of ozone depletion, has been identified as one such stressor. Exposure to UV radiation has been shown to have detrimental effects on amphibians and can exacerbate the effects of other stressors, such as chemical pollutants. Chemical pollution has likewise been recognized as a major factor contributing to amphibian declines, particularly, endocrine-disrupting chemicals. In this regard, 17β-trenbolone is a potent anabolic steroid used in the agricultural industry to increase muscle mass in cattle and has been repeatedly detected in the environment where amphibians live and breed. At high concentrations, 17β-trenbolone has been shown to impact amphibian survival and gonadal development. In the present study, we investigated the effects of environmentally realistic UV radiation and 17β-trenbolone exposure, both in isolation and in combination, on the morphology and behavior of tadpoles (Limnodynastes tasmaniensis). We found that neither stressor in isolation affected tadpoles, nor did we find any interactive effects. The results from our 17β-trenbolone treatment are consistent with recent research suggesting that, at environmentally realistic concentrations, tadpoles may be less vulnerable to this pollutant compared to other vertebrate classes. The absence of UV radiation-induced effects found in the present study could be due to species-specific variation in susceptibility, as well as the dosage utilized. We suggest that future research should incorporate long-term studies with multiple stressors to accurately identify the threats to, and subsequent consequences for, amphibians under natural conditions. Environ Toxicol Chem 2024;43:1615-1626. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Heteromorphic ZZ/ZW sex chromosomes sharing gene content with mammalian XX/XY are conserved in Madagascan chameleons of the genus Furcifer

Chameleons are well-known lizards with unique morphology and physiology, but their sex determination has remained poorly studied. Madagascan chameleons of the genus Furcifer have cytogenetically distinct Z and W sex chromosomes and occasionally Z1Z1Z2Z2/Z1Z2W multiple neo-sex chromosomes. To identify the gene content of their sex chromosomes, we microdissected and sequenced the sex chromosomes of F. oustaleti (ZZ/ZW) and F. pardalis (Z1Z1Z2Z2/Z1Z2W). In addition, we sequenced the genomes of a male and a female of F. lateralis (ZZ/ZW) and F. pardalis and performed a comparative coverage analysis between the sexes. Despite the notable heteromorphy and distinctiveness in heterochromatin content, the Z and W sex chromosomes share approximately 90% of their gene content. This finding demonstrates poor correlation of the degree of differentiation of sex chromosomes at the cytogenetic and gene level. The test of homology based on the comparison of gene copy number variation revealed that female heterogamety with differentiated sex chromosomes remained stable in the genus Furcifer for at least 20 million years. These chameleons co-opted for the role of sex chromosomes the same genomic region as viviparous mammals, lacertids and geckos of the genus Paroedura, which makes these groups excellent model for studies of convergent and divergent evolution of sex chromosomes.

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.

Bigger and bolder: Widespread agricultural pollutant 17β-trenbolone increases growth and alters behaviour in tadpoles (Litoria ewingii)

Endocrine-disrupting chemicals-compounds that directly interfere with the endocrine system of exposed animals-are insidious environmental pollutants that can disrupt hormone function, even at very low concentrations. The dramatic impacts that some endocrine-disrupting chemicals can have on the reproductive development of wildlife are well documented. However, the potential of endocrine-disrupting chemicals to disrupt animal behaviour has received far less attention, despite the important links between behavioural processes and population-level fitness. Accordingly, we investigated the impacts of 14 and 21-day exposure to two environmentally realistic levels of 17β-trenbolone (4.6 and 11.2 ng/L), a potent endocrine-disrupting steroid and agricultural pollutant, on growth and behaviour in tadpoles of an anuran amphibian, the southern brown tree frog (Litoria ewingii). We found that 17β-trenbolone altered morphology, baseline activity and responses to a predatory threat, but did not affect anxiety-like behaviours in a scototaxis assay. Specifically, we found that tadpoles exposed to our high-17β-trenbolone treatment were significantly longer and heavier at 14 and 21 days. We also found that tadpoles exposed to 17β-trenbolone showed higher levels of baseline activity, and significantly reduced their activity following a simulated predator strike. These results provide insights into the wider repercussions of agricultural pollutants on key developmental and behavioural traits in aquatic species, and demonstrate the importance of behavioural studies in the ecotoxicological field.

Effects of the agricultural pollutant 17β-trenbolone on morphology and behaviour of tadpoles (Limnodynastes tasmaniensis)

Pollutants, such as endocrine disrupting chemicals (EDCs), are increasingly being detected in organisms and ecosystems globally. Agricultural activities, including the use of hormonal growth promotants (HGPs), are a major source of EDC contamination. One potent EDC that enters into the environment through the use of HGPs is 17β-trenbolone. Despite EDCs being repeatedly shown to affect reproduction and development, comparatively little is known regarding their effects on behaviour. Amphibians, one of the most imperilled vertebrate taxa globally, are at particular risk of exposure to such pollutants as they often live and breed near agricultural operations. Yet, no previous research on amphibians has explored the effects of 17β-trenbolone exposure on foraging or antipredator behaviour, both of which are key fitness-related behavioural traits. Accordingly, we investigated the impacts of 28-day exposure to two environmentally realistic concentrations of 17β-trenbolone (average measured concentrations: 10 and 66 ng/L) on the behaviour and growth of spotted marsh frog tadpoles (Limnodynastes tasmaniensis). Contrary to our predictions, there was no significant effect of 17β-trenbolone exposure on tadpole growth, antipredator response, anxiety-like behaviour, or foraging. We hypothesise that the differences in effects found between this study and those conducted on fish may be due to taxonomic differences and/or the life stage of the animals used, and suggest further research is needed to investigate the potential for delayed manifestation of the effects of 17β-trenbolone exposure.

MiR-34b/c play a role in early sex differentiation of Amur sturgeon, Acipenser schrenckii

Background

Sex differentiation can be viewed as a controlled regulatory balance between sex differentiation-related mRNAs and post-transcriptional mechanisms mediated by non-coding RNAs. In mammals, increasing evidence has been reported regarding the importance of gonad-specific microRNAs (miRNAs) in sex differentiation. Although many fishes express a large number of gonadal miRNAs, the effects of these sex-biased miRNAs on sex differentiation in teleost fish remain unknown. Previous studies have shown the exclusive and sexually dimorphic expression of miR-34b/c in the gonads of the Amur sturgeon (Acipenser schrenckii), suggesting its potential role in the sex differentiation process.

Results

Using quantitative real-time PCR (qPCR), we observed that miR-34b/c showed consistent spatiotemporal expression patterns; the expression levels significantly increased during early sex differentiation. Using in situ hybridization, miR-34c was found to be located in the germ cells. In primary germ cells in vitro, the group subjected to overexpression and inhibition of miR-34c showed significantly higher proliferation ability and lower apoptosis, respectively, compared to the corresponding control group. Luciferase reporter assays using the ar-3′UTR-psiCHECK-2 luciferase vector suggested a targeted regulatory interaction between miR-34b/c and the 3′UTR of the androgen receptor (ar) mRNA. Furthermore, miR-34b/c and ar showed negative expression patterns during early sex differentiation. Additionally, a negative feedback regulation pattern was observed between foxl2 expression in the ovaries and amh and sox9 expression in the testes during early sex differentiation.

Conclusions

This study sheds new light on the roles of miR-34b/c in gonad development of Amur sturgeon, and provides the first comprehensive evidence that the gonad-predominant microRNAs may have a major role in sex differentiation in teleost fish.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-022-00469-6.

Control of testes mass by androgen receptor paralogs in a cichlid

Steroid hormones play numerous important and diverse roles in the differentiation and development of vertebrate primary and secondary reproductive characteristics. However, the exact role of androgen receptors-which bind circulating androgens-in this regulatory pathway is unclear. Teleost fishes further complicate this question by having two paralogs of the androgen receptor (ARα and ARβ) resulting from a duplication of their ancestral genome. We investigated the functional role of these two ARs on adult testes mass, by eliminating receptor function of one or both paralogs using CRISPR/Cas9 genome edited Astatotilapia burtoni, an African cichlid fish. Fish with two or more functional AR alleles were more likely to be male compared to fish with one or fewer, suggesting that the two paralogs may play redundant roles in the A. burtoni sex determination system. We replicated previous work showing that fish lacking functional ARβ possess testes smaller than wild-type fish, while fish lacking ARα possess testes larger than wild-type fish. However, we found novel evidence supporting a complex relationship between the two AR paralogs in the regulation of testes mass. For instance, the effects of ARα mutation on testes mass are eliminated in homozygous ARβ mutants but the reverse is not true. These results suggest a dynamic relationship between these two AR paralogs where ARβ functions may be permissive to ARα functions in the control of testes mass. This mechanism may contribute to the robust physiological plasticity displayed by A. burtoni and other social teleost fishes.

Characterization, expression, and regulatory effects of nr0b1a and nr0b1b in spotted scat (Scatophagus argus)

Nuclear receptor subfamily 0 group B member 1 (Nr0b1) belongs to the nuclear receptor (NR) superfamily. It plays critical roles in sex determination, sex differentiation, and gonadal development in mammals. In this study, the duplicated genes nr0b1a and nr0b1b were identified in spotted scat (Scatophagus argus). Phylogenetic and synteny analyses revealed that, unlike nr0b1a, nr0b1b was retained in several species of teleosts after an nr0b1 gene duplication event but was secondarily lost in other fish species, amphibians, reptiles, birds, and mammals. In a sequence analysis, only 1.5 LXXLL-related repeat motifs were identified in spotted scat Nr0b1a, Nr0b1b, and non-mammalian Nr0b1a/Nr0b1, different from the 3.5 repeat motifs in mammalian Nr0b1. By qPCR, nr0b1a and nr0b1b were highly expressed in testes from stages IV to V and in ovaries from stages II to IV, respectively. Male-to-female sex reversal was induced in XY spotted scat by the administration of exogenous E2. A qPCR analysis showed that nr0b1b mRNA expression was higher in sex-reversed XY fish than in control XY fish, with no difference in nr0b1a. A luciferase assay showed that spotted scat Nr0b1a and Nr0b1b did not individually activate cyp19a1a gene transcription. As in mammals, spotted scat Nr0b1a suppressed Nr5a1-mediated cyp19a1a expression, despite containing only 1.5 LXXLL-related repeat motifs in its N-terminal region, while Nr0b1b stimulated Nr5a1-mediated cyp19a1a transcription. These results demonstrated that nr0b1a and nr0b1b in spotted scat have distinct expression patterns and regulatory effects and further indicate that nr0b1b might be involved in ovarian development by regulating Nr5a1-mediated cyp19a1a expression.

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.

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.

Assigning the Sex-Specific Markers via Genotyping-by-Sequencing onto the Y Chromosome for a Torrent Frog Amolops mantzorum

We used a genotyping-by-sequencing (GBS) approach to identify sex-linked markers in a torrent frog (Amolops mantzorum), using 21 male and 19 female wild-caught individuals from the same population. A total of 141 putatively sex-linked markers were screened from 1,015,964 GBS-tags via three approaches, respectively based on sex differences in allele frequencies, sex differences in heterozygosity, and sex-limited occurrence. With validations, 69 sex-linked markers were confirmed, all of which point to male heterogamety. The male specificity of eight sex markers was further verified by PCR amplifications, with a large number of additional individuals covering the whole geographic distribution of the species. Y chromosome (No. 5) was microdissected under a light microscope and amplified by whole-genome amplification, and a draft Y genome was assembled. Of the 69 sex-linked markers, 55 could be mapped to the Y chromosome assembly (i.e., 79.7%). Thus, chromosome 5 could be added as a candidate to the chromosomes that are particularly favored for recruitment in sex-determination in frogs. Three sex-linked markers that mapped onto the Y chromosome were aligned to three different promoter regions of the Rana rugosa CYP19A1 gene, which might be considered as a candidate gene for triggering sex-determination in A. mantzorum.

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.

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.

Nanos3 of the frog Rana rugosa: Molecular cloning and characterization

Nanos is expressed in the primordial germ cells (PGCs) and also the germ cells of a variety of organisms as diverse as Drosophila, medaka fish, Xenopus and mouse. In Nanos3-deficient mice, PGCs fail to incorporate into the gonad and the size of the testis and ovary is thereby dramatically reduced. To elucidate the role of Nanos in an amphibian species, we cloned Nanos3 cDNA from the testis of the R. rugosa frog. RT-PCR analysis showed strong expression of Nanos3 mRNA in the testis of adult R. rugosa frogs, but expression was not sexually dimorphic during gonadal differentiation. In Nanos3-knockdown tadpoles produced by the CRISPR/Cas9 system, the number of germ cells decreased dramatically in the gonads of both male and female tadpoles before sex determination and thereafter. This was confirmed by three dimensional imaging of wild-type and Nanos3 knockdown gonads using serial sections immunostained for Vasa, a marker specific to germ cells. Taken together, these results suggest that Nanos3 protein function is conserved between R. rugosa and mouse.

Origin of sex chromosomes in six groups of Rana rugosa frogs inferred from a sex-linked DNA marker

Each vertebrate species, as a general rule, has either the XX/XY or ZZ/ZW chromosomes by which sex is determined. However, the Japanese Rana (R.) rugosa frog is an exception, possessing both sex-determining combinations within one species, varying with region of origin. We collected R. rugosa frogs from 104 sites around Japan and South Korea and determined the nucleotide sequences of the mitochondrial 12S ribosomal RNA gene. Based on the sequences, R. rugosa frogs were divided into four groups from Japan and one from South Korea. The ZZ/ZW type is reportedly derived from the XX/XY type, although recently a new ZZ/ZW type of R. rugosa was reported. However, it still remains unclear from where the sex chromosomes in the five groups of this species were derived. In this study, we successfully isolated a sex-linked DNA maker and used it to classify R. rugosa frogs into several groupings. From the DNA marker as well as from nucleotide analysis of the promoter region of the androgen receptor (AR) gene, we identified another female heterogametic group, designated, West-Central. The sex chromosomes in the West-Central originated from the West and Central groups. The results indicate that a sex-linked DNA marker is a verifiable tool to determine the origin of the sex chromosomes in R. rugosa frogs in which the sex-determining system has changed, during two independent events, from the male to female heterogamety.

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.

Sequential Turnovers of Sex Chromosomes in African Clawed Frogs (Xenopus) Suggest Some Genomic Regions Are Good at Sex Determination

Sexual differentiation is fundamentally important for reproduction, yet the genetic triggers of this developmental process can vary, even between closely related species. Recent studies have uncovered, for example, variation in the genetic triggers for sexual differentiation within and between species of African clawed frogs (genus Xenopus). Here, we extend these discoveries by demonstrating that yet another sex determination system exists in Xenopus, specifically in the species Xenopus borealis. This system evolved recently in an ancestor of X. borealis that had the same sex determination system as X. laevis, a system which itself is newly evolved. Strikingly, the genomic region carrying the sex determination factor in X. borealis is homologous to that of therian mammals, including humans. Our results offer insights into how the genetic underpinnings of conserved phenotypes evolve, and suggest an important role for cooption of genetic building blocks with conserved developmental roles.

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.

Mammalian X homolog acts as sex chromosome in lacertid lizards

Among amniotes, squamate reptiles are especially variable in their mechanisms of sex determination; however, based largely on cytogenetic data, some lineages possess highly evolutionary stable sex chromosomes. The still very limited knowledge of the genetic content of squamate sex chromosomes precludes a reliable reconstruction of the evolutionary history of sex determination in this group and consequently in all amniotes. Female heterogamety with a degenerated W chromosome typifies the lizards of the family Lacertidae, the widely distributed Old World clade including several hundreds of species. From the liver transcriptome of the lacertid Takydromus sexlineatus female, we selected candidates for Z-specific genes as the loci lacking single-nucleotide polymorphisms. We validated the candidate genes through the comparison of the copy numbers in the female and male genomes of T. sexlineatus and another lacertid species, Lacerta agilis, by quantitative PCR that also proved to be a reliable technique for the molecular sexing of the studied species. We suggest that this novel approach is effective for the detection of Z-specific and X-specific genes in lineages with degenerated W, respectively Y chromosomes. The analyzed gene content of the Z chromosome revealed that lacertid sex chromosomes are not homologous with those of other reptiles including birds, but instead the genes have orthologs in the X-conserved region shared by viviparous mammals. It is possible that this part of the vertebrate genome was independently co-opted for the function of sex chromosomes in viviparous mammals and lacertids because of its content of genes involved in gonad differentiation.

Whole genome sequencing of turbot (Scophthalmus maximus; Pleuronectiformes): a fish adapted to demersal life

The turbot is a flatfish (Pleuronectiformes) with increasing commercial value, which has prompted active genomic research aimed at more efficient selection. Here we present the sequence and annotation of the turbot genome, which represents a milestone for both boosting breeding programmes and ascertaining the origin and diversification of flatfish. We compare the turbot genome with model fish genomes to investigate teleost chromosome evolution. We observe a conserved macrosyntenic pattern within Percomorpha and identify large syntenic blocks within the turbot genome related to the teleost genome duplication. We identify gene family expansions and positive selection of genes associated with vision and metabolism of membrane lipids, which suggests adaptation to demersal lifestyle and to cold temperatures, respectively. Our data indicate a quick evolution and diversification of flatfish to adapt to benthic life and provide clues for understanding their controversial origin. Moreover, we investigate the genomic architecture of growth, sex determination and disease resistance, key traits for understanding local adaptation and boosting turbot production, by mapping candidate genes and previously reported quantitative trait loci. The genomic architecture of these productive traits has allowed the identification of candidate genes and enriched pathways that may represent useful information for future marker-assisted selection in turbot.

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.

Genome-wide identification, evolution and expression analysis of nuclear receptor superfamily in Nile tilapia, Oreochromis niloticus

The nuclear receptor (NR) superfamily, which is divided into 7 subfamilies, constitutes one of the largest classes of transcription factors. In this study, through comprehensive database search, we identified all NRs (including 4 novel members) from the tilapia (75), common carp (137), zebrafish (73), fugu (73), tetraodon (72), stickleback (70), medaka (69), coelacanth (55), spotted gar (51) and elephant shark (50). For 21 NRs, two duplicates were found in teleosts, while only one in tetrapods. These duplicates, except those of DAX1, SHP and GCNF found in the elephant shark, were derived from 3R (third round of genome duplication). The linkage duplication of 5 syntenic blocks (comprising 14 duplicated NR couples) in teleosts further supported their 3R origin. Based on transcriptome data from adult tilapia, 53 NRs were found to be expressed in more than one tissue (brain, head kidney, heart, liver, kidney, muscle, ovary and testis), and 4 were tissue-specific, indicating their essential roles in the corresponding tissue. Based on the XX and XY gonadal transcriptome data from four developmental stages, 65 NRs were detected in gonads, with 21, 31, 11 and 29 expressed sexual dimorphically at 5, 30, 90 and 180days after hatching, respectively. The expression of four selected genes was examined by in situ hybridization (ISH) and quantitative PCR (qPCR) to validate the spatial and temporal expression profiles of NRs. Comparative analyses of the expression profiles of duplicated NRs revealed divergence in gene expression as well as gene function. Our results demonstrated that NRs may play important roles in sex determination and gonadal development in teleosts.