Coexistence of genotypic and temperature-dependent sex determination in pejerrey Odontesthes bonariensis

Transforming growth factor-β (TGF-β): A master signal pathway in teleost sex determination

Sex determination and differentiation in fish has always been a hot topic in genetic breeding of aquatic animals. With the advances in next-generation sequencing (NGS) in recent years, sex chromosomes and sex determining genes can be efficiently identified in teleosts. To date, master sex determination genes have been elucidated in 114 species, of which 72 species have sex determination genes belonging to TGF-β superfamily. TGF-β is the only signaling pathway that the largest proportion of components, which including ligands (amhy, gsdfy, gdf6), receptors (amhr, bmpr), and regulator (id2bby), have opportunity recognized as a sex determination gene. In this review, we focus on the recent studies about teleost sex-determination genes within TGF-β superfamily and propose several hypotheses on how these genes regulate sex determination process. Differing from other reviews, our review specifically devotes significant attention to all members of the TGF-β signal pathway, not solely the sex determination genes within the TGF-β superfamily. However, the functions of the paralogous genes of TGF superfamily are still needed ongoing research. Further studies are required to more accurately interpret the molecular mechanism of TGF-β superfamily sex determination genes.

Potential Implications of Acid-Sensing Ion Channels ASIC2 and ASIC4 in Gonadal Differentiation of Dicentrarchus labrax Subjected to Water Temperature Increase during Gonadal Development

In this study, the expression and implication of acid-sensing ion channels 2 and 4 (ASIC2 and ASIC4) in the gonadal sex differentiation of Dicentrarchus labrax (D. labrax), subjected to increasing water temperatures during gonadal development, were evaluated. Two groups were selected: a control group (CG), in which the average water temperature was maintained at 15 °C and increased to 20 °C in 20 days until weaning; and an experimental group (EG), in which the water temperature was retained at 15 °C for 60 days; thereafter, the temperature was increased daily by 0.5 °C until it reached 20 °C up to the weaning time. Ten fish from the CG and 13 fish from the EG were sampled randomly on the 335th day after hatching (dph). A higher percentage of gonad differentiation in ovaries rather than in testes was observed in the EG compared to the CG (p = 0.01). ASIC2 and ASIC4 were detected for the first time in D. labrax ovaries by indirect immunofluorescence. Both ASIC2 and ASIC4 were expressed in previtellogenic oocytes of ovaries and in scattered cells within some testes, and were most likely intratesticular previtellogenic oocytes in both the CG and EG groups. The CG group showed a higher expression of ASIC4 than the EG cohort (p < 0.05). The results gathered in this study revealed the capacity of water temperature to influence both gonadal differentiation and growth in this gonochoristic fish species, and suggests the possible role of ASIC2 and ASIC4 in gonad differentiation and gamete development in D. labrax.

Fish reproduction in a warming world: vulnerable points in hormone regulation from sex determination to spawning

Reproduction in fishes is sensitive to temperature. Elevated temperatures and anomalous 'heat waves' associated with climate change have the potential to impact fish reproductive performance and, in some cases, even induce sex reversals. Here we examine how thermal sensitivity in the hormone pathways regulating reproduction provides a framework for understanding impacts of warmer conditions on fish reproduction. Such effects will differ depending on evolved variation in temperature sensitivity of endocrine pathways regulating reproductive processes of sex determination/differentiation, gametogenesis and spawning, as well as how developmental timing of those processes varies with reproductive ecology. For fish populations unable to shift geographical range, persistence under future climates may require changes in temperature responsiveness of the hormone pathways regulating reproductive processes. How thermal sensitivity in those hormone pathways varies among populations and species, how those pathways generate temperature maxima for reproduction, and how rapidly reproductive thermal tolerances can change via adaptation or transgenerational plasticity will shape which fishes are most at risk for impaired reproduction under rising temperatures. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Endocrine disruption in teleosts and amphibians is mediated by anthropogenic and natural environmental factors: implications for risk assessment

Environmental variation in the Anthropocene involves several factors that interfere with endocrine systems of wildlife and humans, presenting a planetary boundary of still unknown dimensions. Here, we focus on chemical compounds and other impacts of anthropogenic and natural origins that are adversely affecting reproduction and development. The main sink of these endocrine disruptors (EDs) is surface waters, where they mostly endanger aquatic vertebrates, like teleost fish and amphibians. For regulatory purposes, EDs are categorized into EATS modalities (oestrogenic, androgenic, thyroidal, steroidogenesis), only addressing endocrine systems being assessable by validated tests. However, there is evidence that non-EATS modalities-and even natural sources, such as decomposition products of plants or parasitic infections-can affect vertebrate endocrine systems. Recently, the disturbance of natural circadian light rhythms by artificial light at night (ALAN) has been identified as another ED. Reviewing the knowledge about EDs affecting teleosts and amphibians leads to implications for risk assessment. The generally accepted WHO-definition for EDs, which focuses exclusively on 'exogenous substances' and neglects parasitic infections or ALAN, seems to require some adaptation. Natural EDs have been involved in coevolutionary processes for ages without resulting in a general loss of biodiversity. Therefore, to address the 'One Health'-principle, future research and regulatory efforts should focus on minimizing anthropogenic factors for endocrine disruption. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Does stress make males? An experiment on the role of glucocorticoids in anuran sex reversal

Environmentally sensitive sex determination may help organisms adapt to environmental change but also makes them vulnerable to anthropogenic stressors, with diverse consequences for population dynamics and evolution. The mechanisms translating environmental stimuli to sex are controversial: although several fish experiments supported the mediator role of glucocorticoid hormones, results on some reptiles challenged it. We tested this hypothesis in amphibians by investigating the effect of corticosterone on sex determination in agile frogs (Rana dalmatina). This species is liable to environmental sex reversal whereby genetic females develop into phenotypic males. After exposing tadpoles during sex determination to waterborne corticosterone, the proportion of genetic females with testes or ovotestes increased from 11% to up to 32% at 3 out of 4 concentrations. These differences were not statistically significant except for the group treated with 10 nM corticosterone, and there was no monotonous dose-effect relationship. These findings suggest that corticosterone is unlikely to mediate sex reversal in frogs. Unexpectedly, animals originating from urban habitats had higher sex-reversal and corticosterone-release rates, reduced body mass and development speed, and lower survival compared to individuals collected from woodland habitats. Thus, anthropogenic environments may affect both sex and fitness, and the underlying mechanisms may vary across ectothermic vertebrates.

Temperature- and genotype-dependent stress response and activation of the hypothalamus-pituitary-interrenal axis during temperature-induced sex reversal in pejerrey Odontesthes bonariensis, a species with genotypic and environmental sex determination

In the pejerrey Odontesthes bonariensis (Atheriniformes, Atherinopsidae), exposure to high and low temperatures during the critical period of sex determination (CPSD) induce testicular and ovarian differentiation, respectively, regardless of the presence or not of the sex determining gene amhy, which is crucial for testis formation only at intermediate, sexually neutral temperatures. In this study we explored the existence of genotype-specific signaling of Crh (Corticotropin Releasing Hormone) family genes and their associated carrier protein, receptors, and other stress-related genes in response to temperature during the CPSD and the potential involvement of the central nervous system via the hypothalamus-pituitary-interrenal (HPI) axis in the sex determination of this species. The Crh family genes crhb, uts1, ucn3, the receptor crhr1 and the stress-related genes gr1, gr2, nr3c2 were transiently upregulated in the heads of pejerrey larvae during the CPSD by high temperature alone or in combination with other factors. Only crhr2 transcript abundance was not influenced by temperature but independently by time and genotype. In most cases, mRNA abundance was higher in the XX heads compared to that of XY individuals. The mRNAs of some of these genes were localized in the hypothalamus of pejerrey larvae during the CPSD. XX larvae also showed higher whole-body cortisol titers than the XY, downregulation of cyp19a1a and upregulation of the testis-related genes amhy/amha in trunks (gonads) and were 100% masculinized at the high temperature. In contrast, at the low temperature, crhbp and avt were upregulated in the heads, particularly the former in XY larvae. cyp19a1a and amhy/amha were up- and downregulated, respectively, in the gonads, and fish were 100% feminized. Signaling via the HPI axis was observed simultaneously with the first molecular signs of ongoing sex determination/differentiation in the gonads. Overall, the results strongly suggest a temperature-dependent, genotype-specific regulatory action of the brain involving the Crh family of stress-related genes on the process of environmental sex determination of pejerrey.

Diversity and Convergence of Sex-Determination Mechanisms in Teleost Fish

Sexual reproduction is prevalent across diverse taxa. However, sex-determination mechanisms are so diverse that even closely related species often differ in sex-determination systems. Teleost fish is a taxonomic group with frequent turnovers of sex-determining mechanisms and thus provides us with great opportunities to investigate the molecular and evolutionary mechanisms underlying the turnover of sex-determining systems. Here, we compile recent studies on the diversity of sex-determination mechanisms in fish. We demonstrate that genes in the TGF-β signaling pathway are frequently used for master sex-determining (MSD) genes. MSD genes arise via two main mechanisms, duplication-and-transposition and allelic mutations, with a few exceptions. We also demonstrate that temperature influences sex determination in many fish species, even those with sex chromosomes, with higher temperatures inducing differentiation into males in most cases. Finally, we review theoretical models for the turnover of sex-determining mechanisms and discuss what questions remain elusive.

Molecular biological, physiological, cytological, and epigenetic mechanisms of environmental sex differentiation in teleosts: A systematic review

Human activities have been exerting widespread stress and environmental risks in aquatic ecosystems. Environmental stress, including temperature rise, acidification, hypoxia, light pollution, and crowding, had a considerable negative impact on the life histology of aquatic animals, especially on sex differentiation (SDi) and the resulting sex ratios. Understanding how the sex of fish responds to stressful environments is of great importance for understanding the origin and maintenance of sex, the dynamics of the natural population in the changing world, and the precise application of sex control in aquaculture. This review conducted an exhaustive search of the available literature on the influence of environmental stress (ES) on SDi. Evidence has shown that all types of ES can affect SDi and universally result in an increase in males or masculinization, which has been reported in 100 fish species and 121 cases. Then, this comprehensive review aimed to summarize the molecular biology, physiology, cytology, and epigenetic mechanisms through which ES contributes to male development or masculinization. The relationship between ES and fish SDi from multiple aspects was analyzed, and it was found that environmental sex differentiation (ESDi) is the result of the combined effects of genetic and epigenetic factors, self-physiological regulation, and response to environmental signals, which involves a sophisticated network of various hormones and numerous genes at multiple levels and multiple gradations in bipotential gonads. In both normal male differentiation and ES-induced masculinization, the stress pathway and epigenetic regulation play important roles; however, how they co-regulate SDi is unclear. Evidence suggests that the universal emergence or increase in males in aquatic animals is an adaptation to moderate ES. ES-induced sex reversal should be fully investigated in more fish species and extensively in the wild. The potential aquaculture applications and difficulties associated with ESDi have also been addressed. Finally, the knowledge gaps in the ESDi are presented, which will guide the priorities of future research.

Homeology of sex chromosomes in Amazonian Harttia armored catfishes supports the X-fission hypothesis for the X1X2Y sex chromosome system origin

The Neotropical monophyletic catfish genus Harttia represents an excellent model to study karyotype and sex chromosome evolution in teleosts. Its species split into three phylogenetic clades distributed along the Brazilian territory and they differ widely in karyotype traits, including the presence of standard or multiple sex chromosome systems in some members. Here, we investigate the chromosomal rearrangements and associated synteny blocks involved in the origin of a multiple X1X2Y sex chromosome system present in three out of six sampled Amazonian-clade species. Using 5S and 18S ribosomal DNA fluorescence in situ hybridization and whole chromosome painting with probes corresponding to X1 and X2 chromosomes of X1X2Y system from H. punctata, we confirm previous assumptions that X1X2Y sex chromosome systems of H. punctata, H. duriventris and H. villasboas represent the same linkage groups which also form the putative XY sex chromosomes of H. rondoni. The shared homeology between X1X2Y sex chromosomes suggests they might have originated once in the common ancestor of these closely related species. A joint arrangement of mapped H. punctata X1 and X2 sex chromosomes in early diverging species of different Harttia clades suggests that the X1X2Y sex chromosome system may have formed through an X chromosome fission rather than previously proposed Y-autosome fusion.

Expression pattern analysis of anti-Mullerian hormone in testis development of pearlscale angelfish (Centropyge vrolikii)

In vertebrates, anti-Mullerian hormone (Amh) secreted by Sertoli cells (SC) performs a pivotal function in male sex differentiation. Compared with that of higher vertebrates, the expression pattern of Amh is more diversified in fish. In this study, the full-length complementary DNA (cDNA) of Amh in Centropyge vrolikii (Cv-Amh) was cloned and analysed, which was 2,470 bp, including a 238 bp 5'UTR, a 1,602 bp ORF and a 633 bp 3'UTR; the similarity of Amh between Cv-Amh and other fish is relatively high. The quantitative real-time PCR (qRT-PCR) results of healthy tissues and gonads at sex reversal stages in C. vrolikii showed that the expression level of Amh in the testis was significantly higher than that in other tissues (P < 0.05). Amh was weakly expressed in the vitellogenic stage ovary and perinucleolus stage ovary, but its expression significantly increased in the gonads at the hermaphroditic stage, and finally reached the highest in the pure testis after sexual reversal. The results of in situ hybridization indicated that the positive signal of Amh was strongly concentrated in SCs of testis. After Amh knockdown in the gonads, the effect on sex-related genes was tested using qRT-PCR. Among these, the expression of Dmrt1, Cyp11a, Hsd11b2, Sox8 and Sox9 significantly decreased, whereas that of Cyp19a, Sox4, Foxl2 and Sox3 increased. These results suggested that Amh could be the pivotal gene in reproductive regulation in C. vrolikii, and the data will contribute to sex-related research of C. vrolikii in the future.

No evidence for sex chromosomes in natural populations of the cichlid fish Astatotilapia burtoni

Sex determination (SD) is not conserved among teleost fishes and can even differ between populations of the same species. Across the outstandingly species-rich fish family Cichlidae, more and more SD systems are being discovered. Still, the picture of SD evolution in this group is far from being complete. Lake Tanganyika and its affluent rivers are home to Astatotilapia burtoni, which belongs to the extremely successful East African cichlid lineage Haplochromini. Previously, in different families of an A. burtoni laboratory strain, an XYW system and an XY system have been described. The latter was also found in a second laboratory strain. In a laboratory-reared family descending from a population of the species' southern distribution, a second XY system was discovered. Yet, an analysis of sex chromosomes for the whole species distribution is missing. Here, we examined the genomes of 11 natural populations of A. burtoni, encompassing a wide range of its distribution, for sex-linked regions. We did not detect signs of differentiated sex chromosomes and also not the previously described sex chromosomal systems present in laboratory lines, suggesting different SD systems in the same species under natural and (long-term) artificial conditions. We suggest that SD in A. burtoni is more labile than previously assumed and consists of a combination of non-genetic, polygenic, or poorly differentiated sex chromosomes.

Development of Ovaries and Sex Change in Fish: Bringing Potential into Action

BACKGROUND

Encompassing about half of the 60,000 species of vertebrates, fish display the greatest diversity of sex determination mechanisms among metazoans. As such that phylum offers a unique playground to study the impressive variety of gonadal morphogenetic strategies, ranging from gonochorism, with either genetic or environmental sex determination, to unisexuality, with either simultaneous or consecutive hermaphroditism.

SUMMARY

From the two main types of gonads, the ovaries embrace the important role to produce the larger and non-motile gametes, which is the basis for the development of a future organism. The production of the egg cells is complex and involves the formation of follicular cells, which are necessary for the maturation of the oocytes and the production of feminine hormones. In this vein, our review focuses on the development of ovaries in fish with special emphasis on the germ cells, including those that transition from one sex to the other as part of their life cycle and those that are capable of transitioning to the opposite sex depending on environmental cues.

KEY MESSAGES

Clearly, establishing an individual as either a female or a male is not accomplished by the sole development of two types of gonads. In most cases, that dichotomy, be it final or transient, is accompanied by coordinated transformations across the entire organism, leading to changes in the physiological sex as a whole. These coordinated transformations require both molecular and neuroendocrine networks, but also anatomical and behavioural adjustments. Remarkably, fish managed to tame the ins and outs of sex reversal mechanisms to take the most advantages of changing sex as adaptive strategies in some situations.

Atrazine alters early sexual development of the South American silverside, Odontesthes bonariensis

Atrazine (ATZ) is a frequent contaminant in freshwater ecosystems within agricultural regions. The capacity of this herbicide to interfere with the vertebrate endocrine system is broadly recognized, but the mechanisms and responses usually differ among species. In this study, ATZ effects on hypothalamus-pituitary-gonadal (HPG) axis key genes expression and early gonadal development were evaluated in Odontesthes bonariensis larvae waterborne exposed during the gonadal differentiation period. Fish were treated to 0, 0.7, 7.0, and 70 µg ATZ/L at 25 °C from the 2nd to 6th week after hatching (wah), and a group was kept in clean water until the 12th wah. Parallelly, a group was submitted to 0.05 µg/L of ethinylestradiol (EE₂) as a positive estrogenic control. From each treatment, eight larvae were sampled at 6 wah for gene expression analysis and twelve larvae at 12 wah for phenotypic sex histological determination. The expression of gnrh1, lhb, fshb, and cyp19a1b was assessed in the head, and the ones of amha, 11βhsd2, and cyp19a1a in the trunk. Fish growth was significantly higher in fish exposed to 7 and 70 µg ATZ/L in the 6 wah, but the effect vanished at the 12 wah. The expression of lhb was upregulated in both sex larvae exposed from 7 µg ATZ/L. However, a dimorphic effect was induced on cyp19a1a expression at 70 µg ATZ/L, up or downregulating mRNA transcription in males and females, respectively. Delayed ovarian development and increased number of testicular germ cells were histologically observed from 7 to 70 µg ATZ/L, respectively, and a sex inversion (genotypic male to phenotypic female) was found in one larva at 70 µg ATZ/L. The lhb expression was also upregulated by EE₂, but the cyp19a1a expression was not affected, and a complete male-to-female reversal was induced. Further, EE₂ upregulated gnrh1 in females and cyp19a1b in both sexes, but it did not alter any assessed gene in the trunk. In conclusion, ATZ disrupted HPG axis physiology and normal gonadal development in O. bonariensis larvae at environmentally relevant concentrations. The responses to ATZ only partially overlapped and were less active when compared to the model estrogenic compound EE₂.

Sex reversal of pejerrey (Odontesthes bonariensis), a species with temperature-dependent sex determination, in a seminatural environment

This study examined the changes in sex ratios and sex reversal rates in pejerrey Odontesthes bonariensis that occur with the progression of the spawning season in a seminatural setting. Four groups of hatchery-produced pejerrey larvae were stocked in floating cages in La Salada de Monasterio lake (Pampas region), a natural habitat of this species, and reared from hatching beyond gonadal sex determination with minimum human interference. Cage 1 was stocked at the beginning of the spring spawning season and the other cages were stocked with monthly delays until cage 4 in early summer. The genotypic (amhy+, XY/YY; amhy-, XX) and phenotypic (testis, male; ovary, female) sex ratios and proportions of genotype/phenotype mismatched individuals were estimated and their relation to water temperature and daylength during the experiment was analysed by generalized linear modelling. Water temperature varied between 11 and 30.5°C, and daylength duration between 11 h 22 min and 14 h 35 min. Sex genotyping revealed nearly balanced sex ratios of XY/YY (46%-49.1%) and XX (50.9%-54%) fish in cages 2-4 whereas the genotypic sex ratio in cage 1 was clearly biased towards XY/YY fish (60.6%). Phenotypic males ranged from 42% to 54.4% in cages 1-3. Cage 4, in turn, had significantly more phenotypic males (66%). The percentage of XX males (phenotypic male/genotypic female) was 23.1% in cage 1, decreased to a minimum of 5.4% in cage 2 and gradually increased in cages 3 and 4 to a maximum of 40.7% in the latter. The percentages of XY/YY females (phenotypic female/genotypic male) were highest in cage 1 (30%) and decreased progressively in the other cages to a significantly lower value (4.3%) in cage 4. These results generally support the findings of laboratory studies on the effect of temperature on the sex determination of this species and also provide novel evidence of a XX genotype-specific masculinizing effect of short daylength.

Hybridization is strongly constrained by salinity during secondary contact between silverside fishes (Odontesthes, Atheriniformes)

This study investigates a contact zone between two silverside fish species (marine Odontesthes argentinensis and freshwater O. bonariensis) in the estuarine Mar Chiquita lagoon along the Atlantic coast in Argentina (MChL), in which intermediate morphs had been reported. It has been suggested that admixture and introgression occur in MChL between these two species, but direct genetic evidence is lacking. Leveraging samples collected over several years (n = 676), we document the spatial distribution of both species and intermediate morphs within this habitat and collect landmark-based morphometric and multilocus genetic data (9876 loci for n = 110 individuals) to test the hypothesis of hybridization. Our analysis unambiguously characterizes intermediate morphs as F1 or F2 hybrids. We show that the low frequency of hybrid individuals in MChL may be explained by uneven abundance of parental species, which in turn are strongly affected by water salinity, limiting the size of the contact zone. Although hybrids seem to be fertile, their fitness may be reduced by external and intrinsic factors that may limit their success and suggest that this is an unstable hybrid zone. Genetic distinctiveness of both parental species is strongly supported by genome-wide data, explaining a known pattern of mitonuclear discordance as a consequence of hybridization followed by mitochondrial introgression. A clear signature of population genetic structure was detected in O. argentinensis, distinguishing MChL residents from marine populations of this species, that also was supported by distinctive morphometric features among these groups. Previous hypotheses of speciation in these fishes are discussed in the light of the new findings.

Y-specific amh allele, amhy, is the master sex-determining gene in Japanese flounder Paralichthys olivaceus

Japanese flounder (Paralichthys olivaceus) is an important marine fish species of both fisheries and aquaculture in Northeast Asia. The commercial interest for all-female progenies due to several sex-related traits has prompted basic research on the mechanisms of sex determination in this species. By conducting a linkage analysis of the sex-determining locus, we initially identified 12 microsatellite markers linked to sex in 11 scaffolds, whose localization was restricted to a specific region of linkage group 9. Sequence analysis of this region identified 181 genes based on the UniProt database annotations. Among them, the amh gene was considered a potential candidate for sex determination because this gene is known to have taken over the role of sex determination in many teleosts. An in-depth sequence analysis of both the coding and non-coding regions of amh in XX and XY individuals detected nine SNPs linked with maleness. However, because these substitutions were synonymous, the upstream and downstream regions of amh were also investigated and a male-specific variant with deletions in the promoter region was detected. This truncated Y-specific amh variant was named amhy, and the amh shared by both sexes was named amhx. The association analysis using both females and males of the genotypic sex inferred by the presence/absence of amhy found complete association with phenotypic sex and genotype. Gene expression analysis in larvae derived from a single-pair progeny by quantitative real-time PCR detected amhy transcripts in the larval trunks between 20 and 100 days after hatching only in XY larvae. Localization of amhy by in situ hybridization was detected in presumptive Sertoli cells of XY gonads. Expression of amhx was almost undetectable in both XX and XY genotypes. Loss of Amh function by CRISPR-Cas9 induced male-to-female sex reversal, indicating that this gene was necessary for the masculinization of XY individuals. In conclusion, the complete linkage of amhy with males, its early expression in XY gonads before testicular differentiation, and the induction of sex reversal by loss-of-function mutation support the view that amhy is the sex-determining gene in this species.

Identification of a candidate sex determination gene in Culaea inconstans suggests convergent recruitment of an Amh duplicate in two lineages of stickleback

Sex chromosomes vary greatly in their age and levels of differentiation across the tree of life. This variation is largely due to the rates of sex chromosome turnover in different lineages; however, we still lack an explanation for why sex chromosomes are so conserved in some lineages (e.g. mammals, birds) but so labile in others (e.g. teleosts, amphibians). To identify general mechanisms driving transitions in sex determination systems or forces which favour their conservation, we first require empirical data on sex chromosome systems from multiple lineages. Stickleback fishes are a valuable model lineage for the study of sex chromosome evolution due to variation in sex chromosome systems between closely-related species. Here, we identify the sex chromosome and a strong candidate for the master sex determination gene in the brook stickleback, Culaea inconstans. Using whole-genome sequencing of wild-caught samples and a lab cross, we identify AmhY, a male specific duplication of the gene Amh, as the candidate master sex determination gene. AmhY resides on Chromosome 20 in C. inconstans and is likely a recent duplication, as both AmhY and the sex-linked region of Chromosome 20 show little sequence divergence. Importantly, this duplicate AmhY represents the second independent duplication and recruitment of Amh as the sex determination gene in stickleback and the eighth example known across teleosts. We discuss this convergence in the context of sex chromosome turnovers and the role that the Amh/AmhrII pathway, which is crucial for sex determination, may play in the evolution of sex chromosomes in teleosts.

Effects of Anthropic Pollutants Identified in Pampas Lakes on the Development and Reproduction of Pejerrey Fish Odontesthes bonariensis

Anthropic activities can seriously affect the health of the organisms inhabiting them, and the observation of any alteration in the reproduction of fish could be associated with the presence of endocrine disruptors. In this manuscript we have collected information on the adverse effects of pollutants (heavy metals, environmental steroids, and agrochemicals), present in Chascomús lake, Argentina, either at environmentally relevant and pharmacological concentrations on reproduction, embryonic development, and larval survival of pejerrey fish Odontesthes bonariensis. During development, it has been reported that 17β-estradiol (E₂) feminized and reduced larval survival, while 17α-ethinyl-estradiol (EE₂) not only feminized but also affected both embryo and larval survival. In adult male fish, treatments with EE₂ and E₂ + EE₂ were able to increase mRNA abundance of gnrh3 and cyp19a1b and decreased those of gonadotropin receptors (fshr and lhcgr). Heavy metals such as cadmium, chromium, and copper negatively affected sperm quality, diminishing the motility. Also, a decrease in the percentage of hatching rate and larval survival was also observed with the same metals, highlighting zinc as the most detrimental metal. Furthermore, all these metals altered the expression of hypothalamic and pituitary genes related to reproduction in male pejerrey (gnrh1,2,3; cyp19a1b; fshb; lhb; fshr and, lhcgr). Moreover, in all cases pyknotic cells, corresponding to the degeneration of the germ cells, were observed in the testes of exposed fish. For agrochemicals, exposure of male pejerrey to environmental concentrations of glyphosate did not cause alterations on the endocrine reproductive axis. However, male pejerrey with gonadal abnormalities such as the presence of intersex (testis-ova) gonads were found in other Pampa´s lakes with high concentrations of atrazine and glyphosate associated with soybean and corn crops near their coasts. These types of studies demonstrate that pejerrey, an endemic species with economic importance inhabiting the Pampas shallow lakes, can be used as a sentinel species. It should be noted that increased pollution of aquatic ecosystems and the effects on the reproduction of organisms can lead to a decline in fish populations worldwide. Which, added to overfishing and other external factors such as global warming, could cause an eventual extinction of an emblematic species.

High-temperature stress will put the thermo-sensitive teleost yellow catfish (Tachysurus fulvidraco) in danger through reducing reproductivity

Recently, concerns for species that sex differentiation is influenced by temperature in the context of global warming have increased because disrupted operational sex ratios could threaten population maintenance. In contrast, little attention has been given to the reproductive ability of populations that experienced elevated temperatures. In this study, we demonstrated that high temperature (HT) would decrease population size via three different aspects of reproductive ability for the first time. We show that, in a thermo-sensitive teleost yellow catfish, a short period of HT (+3 °C) exposure during the critical period of sex differentiation leads to a different percentage of masculinization of XX genotypic females (1-23%) in wet-lab and natural water bodies. Combining the results of gonadal appearance, histology, sperm parameters, and fertilization rate, we found that XX pseudo-males induced by HT display significantly discounted fertility and reproductive performance compared to XY normal males. We demonstrate that the survival of the XY genotype is lower than XX genotype under environmental stress, including HT, hypoxia, and parasite infection, and the differential survival seems unrelated to male-biased sexual size dimorphism. The mathematical model predicts that the phenotypic female percent will be stabilized at 50% and the population will be sustainably maintained when masculinizing force is less than 0.5, while HT will put the population in danger when the masculinizing force exceeds 0.5. However, when we combine the real-world data of reproductive ability and mathematic model, our results suggest the population size decreases and the long-term survival of the studied species are threatened under the projected pace of increasing temperature. These findings will be useful for understanding the long-term effects of increasing temperature on sex ratio, reproduction and population maintenance in teleost.

Effects of endocrine disrupting chemicals on gonad development: Mechanistic insights from fish and mammals

Over the past century, evidence has emerged that endocrine disrupting chemicals (EDCs) have an impact on reproductive health. An increased frequency of reproductive disorders has been observed worldwide in both wildlife and humans that is correlated with accidental exposures to EDCs and their increased production. Epidemiological and experimental studies have highlighted the consequences of early exposures and the existence of key windows of sensitivity during development. Such early in life exposures can have an immediate impact on gonadal and reproductive tract development, as well as on long-term reproductive health in both males and females. Traditionally, EDCs were thought to exert their effects by modifying the endocrine pathways controlling reproduction. Advances in knowledge of the mechanisms regulating sex determination, differentiation and gonadal development in fish and rodents have led to a better understanding of the molecular mechanisms underlying the effects of early exposure to EDCs on reproduction. In this manuscript, we review the key developmental stages sensitive to EDCs and the state of knowledge on the mechanisms by which model EDCs affect these processes, based on the roadmap of gonad development specific to fish and mammals.

A nonfunctional copy of the salmonid sex-determining gene (sdY) is responsible for the “apparent” XY females in Chinook salmon, Oncorhynchus tshawytscha

Many salmonids have a male heterogametic (XX/XY) sex determination system, and they are supposed to have a conserved master sex-determining gene (sdY) that interacts at the protein level with Foxl2 leading to the blockage of the synergistic induction of Foxl2 and Nr5a1 of the cyp19a1a promoter. However, this hypothesis of a conserved master sex-determining role of sdY in salmonids is challenged by a few exceptions, one of them being the presence of naturally occurring “apparent” XY Chinook salmon, Oncorhynchus tshawytscha, females. Here, we show that some XY Chinook salmon females have a sdY gene (sdY-N183), with 1 missense mutation leading to a substitution of a conserved isoleucine to an asparagine (I183N). In contrast, Chinook salmon males have both a nonmutated sdY-I183 gene and the missense mutation sdY-N183 gene. The 3-dimensional model of SdY-I183N predicts that the I183N hydrophobic to hydrophilic amino acid change leads to a modification in the SdY β-sandwich structure. Using in vitro cell transfection assays, we found that SdY-I183N, like the wild-type SdY, is preferentially localized in the cytoplasm. However, compared to wild-type SdY, SdY-I183N is more prone to degradation, its nuclear translocation by Foxl2 is reduced, and SdY-I183N is unable to significantly repress the synergistic Foxl2/Nr5a1 induction of the cyp19a1a promoter. Altogether, our results suggest that the sdY-N183 gene of XY Chinook females is nonfunctional and that SdY-I183N is no longer able to promote testicular differentiation by impairing the synthesis of estrogens in the early differentiating gonads of wild Chinook salmon XY females.

Transposon-induced epigenetic silencing in the X chromosome as a novel form of dmrt1 expression regulation during sex determination in the fighting fish

Background

Fishes are the one of the most diverse groups of animals with respect to their modes of sex determination, providing unique models for uncovering the evolutionary and molecular mechanisms underlying sex determination and reversal. Here, we have investigated how sex is determined in a species of both commercial and ecological importance, the Siamese fighting fish Betta splendens.

Results

We conducted association mapping on four commercial and two wild populations of B. splendens. In three of the four commercial populations, the master sex determining (MSD) locus was found to be located in a region of ~ 80 kb on LG2 which harbours five protein coding genes, including dmrt1, a gene involved in male sex determination in different animal taxa. In these fish, dmrt1 shows a male-biased gonadal expression from undifferentiated stages to adult organs and the knockout of this gene resulted in ovarian development in XY genotypes. Genome sequencing of XX and YY genotypes identified a transposon, drbx1, inserted into the fourth intron of the X-linked dmrt1 allele. Methylation assays revealed that epigenetic changes induced by drbx1 spread out to the promoter region of dmrt1. In addition, drbx1 being inserted between two closely linked cis-regulatory elements reduced their enhancer activities. Thus, epigenetic changes, induced by drbx1, contribute to the reduced expression of the X-linked dmrt1 allele, leading to female development. This represents a previously undescribed solution in animals relying on dmrt1 function for sex determination. Differentiation between the X and Y chromosomes is limited to a small region of ~ 200 kb surrounding the MSD gene. Recombination suppression spread slightly out of the SD locus. However, this mechanism was not found in the fourth commercial stock we studied, or in the two wild populations analysed, suggesting that it originated recently during domestication.

Conclusions

Taken together, our data provide novel insights into the role of epigenetic regulation of dmrt1 in sex determination and turnover of SD systems and suggest that fighting fish are a suitable model to study the initial stages of sex chromosome evolution.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01205-y.

Involvement of Transforming Growth Factor Beta Family Genes in Gonadal Differentiation in Japanese Eel, Anguilla japonica, According to Sex-Related Gene Expressions

The gonochoristic feature with environmental sex determination that occurs during the yellow stage in the eel provides an interesting model to investigate the mechanisms of gonadal development. We previously studied various sex-related genes during gonadal sex differentiation in Japanese eels. In the present study, the members of transforming growth factor beta (TGF-β) superfamily were investigated. Transcript levels of anti-Müllerian hormone, its receptor, gonadal soma-derived factor (amh, amhr2, and gsdf, respectively) measured by real-time polymerase chain reaction (qPCR) showed a strong sexual dimorphism. Transcripts were dominantly expressed in the testis, and their levels significantly increased with testicular differentiation. In contrast, the expressions of amh, amhr2, and gsdf transcripts were low in the ovary of E2-feminized female eels. In situ hybridization detected gsdf (but not amh) transcript signals in undifferentiated gonads. amh and gsdf signals were localized to Sertoli cells and had increased significantly with testicular differentiation. Weak gsdf and no amh signals were detected in early ovaries of E2-feminized female eels. Transcript levels of amh and gsdf (not amhr2) decreased during human chorionic gonadotropin (HCG)-induced spermatogenesis in males. This study suggests that amh, amhr2, and especially gsdf might be involved in the gene pathway regulating testicular differentiation of Japanese eels.

Advances in Reproductive Endocrinology and Neuroendocrine Research Using Catfish Models

Catfishes, belonging to the order siluriformes, represent one of the largest groups of freshwater fishes with more than 4000 species and almost 12% of teleostean population. Due to their worldwide distribution and diversity, catfishes are interesting models for ecologists and evolutionary biologists. Incidentally, catfish emerged as an excellent animal model for aquaculture research because of economic importance, availability, disease resistance, adaptability to artificial spawning, handling, culture, high fecundity, hatchability, hypoxia tolerance and their ability to acclimate to laboratory conditions. Reproductive system in catfish is orchestrated by complex network of nervous, endocrine system and environmental factors during gonadal growth as well as recrudescence. Lot of new information on the molecular mechanism of gonadal development have been obtained over several decades which are evident from significant number of scientific publications pertaining to reproductive biology and neuroendocrine research in catfish. This review aims to synthesize key findings and compile highly relevant aspects on how catfish can offer insight into fundamental mechanisms of all the areas of reproduction and its neuroendocrine regulation, from gametogenesis to spawning including seasonal reproductive cycle. In addition, the state-of-knowledge surrounding gonadal development and neuroendocrine control of gonadal sex differentiation in catfish are comprehensively summarized in comparison with other fish models.

Developmental aspects of the hypothalamic-pituitary network related to reproduction in teleost fish

The hypothalamic-pituitary-gonadal axis is the main system that regulates reproduction in vertebrates through a complex network that involves different neuropeptides, neurotransmitters, and pituitary hormones. Considering that this axis is established early on life, the main goal of the present work is to gather information on its development and the actions of its components during early life stages. This review focuses on fish because their neuroanatomical characteristics make them excellent models to study neuroendocrine systems. The following points are discussed: i) developmental functions of the neuroendocrine components of this network, and ii) developmental disruptions that may impact adult reproduction. The importance of the components of this network and their susceptibility to external/internal signals that can alter their specific early functions and/or even the establishment of the reproductive axis, indicate that more studies are necessary to understand this complex and dynamic network.

A brief review of vertebrate sex evolution with a pledge for integrative research: towards 'sexomics'

Triggers and biological processes controlling male or female gonadal differentiation vary in vertebrates, with sex determination (SD) governed by environmental factors or simple to complex genetic mechanisms that evolved repeatedly and independently in various groups. Here, we review sex evolution across major clades of vertebrates with information on SD, sexual development and reproductive modes. We offer an up-to-date review of divergence times, species diversity, genomic resources, genome size, occurrence and nature of polyploids, SD systems, sex chromosomes, SD genes, dosage compensation and sex-biased gene expression. Advances in sequencing technologies now enable us to study the evolution of SD at broader evolutionary scales, and we now hope to pursue a sexomics integrative research initiative across vertebrates. The vertebrate sexome comprises interdisciplinary and integrated information on sexual differentiation, development and reproduction at all biological levels, from genomes, transcriptomes and proteomes, to the organs involved in sexual and sex-specific processes, including gonads, secondary sex organs and those with transcriptional sex-bias. The sexome also includes ontogenetic and behavioural aspects of sexual differentiation, including malfunction and impairment of SD, sexual differentiation and fertility. Starting from data generated by high-throughput approaches, we encourage others to contribute expertise to building understanding of the sexomes of many key vertebrate species. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

The replaceable master of sex determination: bottom-up hypothesis revisited

Different group of vertebrates and invertebrates demonstrate an amazing diversity of gene regulations not only at the top but also at the bottom of the sex determination genetic network. As early as 1995, based on emerging findings in Drosophila melanogaster and Caenorhabditis elegans, Wilkins suggested that the evolution of the sex determination pathway evolved from the bottom to the top of the hierarchy. Based on our current knowledge, this review revisits the 'bottom-up' hypothesis and applies its logic to vertebrates. The basic operation of the determination network is through the dynamics of the opposing male and female pathways together with a persistent need to maintain the sexual identity of the cells of the gonad up to the reproductive stage in adults. The sex-determining trigger circumstantially acts from outside the genetic network, but the regulatory network is not built around it as a main node, thus maintaining the genetic structure of the network. New sex-promoting genes arise either through allelic diversification or gene duplication and act specially at the sex-determination period, without integration into the complete network. Due to this peripheral position the new regulator is not an indispensable component of the sex-determining network and can be easily replaced. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Evolution of master sex determiners: TGF-β signalling pathways at regulatory crossroads

To date, more than 20 different vertebrate master sex-determining genes have been identified on different sex chromosomes of mammals, birds, frogs and fish. Interestingly, six of these genes are transcription factors (Dmrt1- or Sox3- related) and 13 others belong to the TGF-β signalling pathway (Amh, Amhr2, Bmpr1b, Gsdf and Gdf6). This pattern suggests that only a limited group of factors/signalling pathways are prone to become top regulators again and again. Although being clearly a subordinate member of the sex-regulatory network in mammals, the TGF-β signalling pathway made it to the top recurrently and independently. Facing this rolling wave of TGF-β signalling pathways, this review will decipher how the TGF-β signalling pathways cope with the canonical sex gene regulatory network and challenge the current evolutionary concepts accounting for the diversity of sex-determining mechanisms. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Where the Ends Meet: An Overview of Sex Determination in Atheriniform Fishes

Atheriniform fishes have recently emerged as attractive models for evolutionary, ecological, and molecular/physiological studies on sex determination. Many species in this group have marked temperature-dependent sex determination (TSD) and yet many species also have a sex determinant gene that provides a strong drive for male differentiation. Thus, in these species the 2 forms of sex determination that were once considered to be mutually exclusive, environmental (ESD) and genotypic (GSD) sex determination, can coexist at environmentally relevant conditions. Here, we review the current knowledge on sex determination in atheriniform fishes with emphasis on the molecular and physiological mechanisms of ESD and GSD, the coexistence and cross-talk between these 2 mechanisms, the possibility of extragonadal transduction of environmental information and/or extragonadal onset of sex determination, and the results of field studies applying novel tools such as otolith increment analysis and molecular markers of genetic sex developed for selected New World and Old World atheriniform species. We also discuss the existence of molecular and histological mechanisms to prevent the discrepant differentiation in parts of the gonads because of ambiguous or conflicting environmental and genetic signals and particularly the possibility that the female is the default state in these species.

Oryzias curvinotus in Sanya Does Not Contain the Male Sex-Determining Gene dmy

Hainan medaka (Oryzias curvinotus) is distributed in the coastal waters of the South China Sea and is able to adapt to a wide range of salinities. In this study, we characterized O. curvinotus in Sanya River (SY-medaka), which lacks dmy (a male sex-determining gene in O. latipes and O. curvinotus). In a comparison of SY-medaka and Gaoqiao medaka (GQ-medaka), the morphological difference between the two populations does not reach the subspecies level and they can be considered two geographic populations of O. curvinotus. A mitochondrial cytochrome oxidase subunit I (CoI) sequence alignment showed that the sequence identities between SY-medaka and other geographic populations of O. curvinotus are as high as 95%. A phylogenetic analysis of the mitochondrial genome also indicated that SY-medaka belongs to O. curvinotus. Molecular marker-based genetic sex assays and whole genome re-sequencing showed that SY-medaka does not contain dmy. Further, in RNA-Seq analyses of the testis and ovaries of sexually mature SY-medaka, dmy expression was not detected. We speculate that high temperatures resulted in the loss of dmy in SY-medaka during evolution, or the lineage has another sex-determining gene. This study provides a valuable dataset for elucidating the mechanism underlying sex determination in Oryzias genus and advances research on functional genomics or reproduction biology in O. curvinotus.

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.

Activation of stress response axis as a key process in environment-induced sex plasticity in fish

The determination of sex is an important hallmark in the life cycle of organisms, in which the fate of gonads and then the individual sex are defined. In gonochoristic teleost fish, this process is characterized by a high plasticity, considering that in spite of genotypic sex many environmental factors can cause shifts from one to another molecular pathway, resulting in organisms with mismatching genotypic and phenotypic sexes. Interestingly, in most instances, both female-to-male or male-to-female sex-reversed individuals develop functional gonads with normal gametogenesis and respective progenies with full viability. The study of these mechanisms is being spread to other non-model species or to those inhabiting more extreme environmental conditions. Although water temperature is an important mechanism involved in sex determination, there are other environmental stressors affected by the climate change which are also implicated in stress response-induced masculinization in fish. In this regard, the brain has emerged as the transducer of the environment input that can influence the gonadal fate. Furthermore, the evaluation of other environmental stressors or their synergic effect on sex determination at conditions that simulate the natural environments is growing gradually. Within such scope, the concerns related to climate change impacts rely on the fact that many of biotic and abiotic parameters reported to affect sex ratios are expected to increase concomitantly as a result of increased greenhouse gas emissions and, particularly worrying, many of them are related to male bias in the populations, such as high temperature, hypoxia, and acidity. These environmental changes can also generate epigenetic changes in sex-related genes affecting their expression, with implications on sex differentiation not only of exposed individuals but also in following generations. The co-analysis of multi-stressors with potential inter- and transgenerational effects is essential to allow researchers to perform long-term predictions on climate change impacts in wild populations and for establishing highly accurate monitoring tools and suitable mitigation strategies.

Molecular and morphological sex differentiation in sablefish (Anoplopoma fimbria), a marine teleost with XX/XY sex determination

Phenotypic sex of an organism is determined by molecular changes in the gonads, so-called molecular sex differentiation, which should precede the rise of cellular or anatomical sex-distinguishing features. This study characterized molecular and morphological sex differentiation in sablefish (Anoplopoma fimbria), a marine teleost with established XX/XY genotypic sex determination. Next generation sequencing was conducted on sablefish ovarian and testicular mRNAs to obtain sequences for transcripts associated with vertebrate sex determination and differentiation and early reproductive development. Gene-specific PCRs were developed to determine the distribution and ontogenetic gonadal expression of transcription, growth, steroidogenic and germline factors, as well as gonadotropin and steroid receptors. Molecular changes associated with sex differentiation were first apparent in both XY- and XX-genotype sablefish at ~ 60 mm in body length and prior to histological signs of sex differentiation. The earliest and most robust markers of testicular differentiation were gsdf, amh, dmrt1, cyp11b, star, sox9a, and fshr. Markedly elevated mRNA levels of several steroidogenesis-related genes and ar2 in differentiating testes suggested that androgens play a role in sablefish testicular differentiation. The earliest markers of ovarian differentiation were cyp19a1a, lhcgr, foxl2, nr0b1, and igf3. Other transcripts such as figla, zp3, and pou5f3 were expressed predominantly in XX-genotype fish and significantly increased with the first appearance and subsequent development of primary oocytes. This study provides valuable insight to the developmental sequence of events associated with gonadal sex differentiation in marine teleosts with XX/XY sex determination. It also implicates particular genes in processes of male and female development and establishes robust molecular markers for phenotypic sex in sablefish, useful for ongoing work related to sex control and reproductive sterilization.

Amh dominant expression in Sertoli cells during the testicular differentiation and development stages in the olive flounder Paralichthys olivaceus

The olive flounder Paralichthys olivaceus, an important marine fish, shows gender differences in growth. The mechanism on its gonadal differentiation direction affected with exogenous factors still needs to be clarified. The anti-Müllerian hormone (amh) gene is involved in fish testicular differentiation and maintenance. The aim of this study was to investigate the expression of the flounder amh in tissues and the gonads. The quantitative expression analysis results showed that it was highly expressed in the testis, especially in the testis at stages I - IV (P < 0.05). Also, amh was detected in Sertoli cells surrounding spermatogonia and peripheral seminiferous lobule of the testis with in situ hybridization (ISH) and immunohistochemistry (IHC). During the differentiation period, the amh expression in the testis of the tamoxifen treatment group (100 ppm) was higher than that in the ovary of the 17β-estradiol (E2, 5 ppm) group, and the expression levels of amh during process of the male differentiation in the tamoxifen group were higher than those in the 17ɑ-methyltestosterone (MT, 5 ppm) group (P < 0.05). ISH results also exhibited that amh was expressed in the somatic cells that surrounded the germ cells of juvenile flounder similar to adult ones. Furthermore, the flounder gonads in the tamoxifen group maintained more germ cells and somatic cells than those in the MT group from 20 to 80 mm total length (TL). Especially, at 60 and 80 mm TL, the numbers of germ and somatic cells in the tamoxifen group were significantly higher than those in the MT group (P < 0.05). In summary, amh might initiate the process of testicular differentiation, and is involved in the early development and maintenance of testis.

Expression profile and estrogenic regulation of Amh during gonadal sex differentiation in northern snakehead (Channa argus)

BACKGROUND

Anti-Müllerian hormone (Amh) plays a critical role in both early sex determination and later gonad development in vertebrate species. However, it remains unknown in northern snakehead (Channa argus), which is economically important freshwater fish with sexual dimorphism.

OBJECTIVE

This study aimed to identify the expression profiles and estrogenic regulation of CaAmh during gonadal sex differentiation in C. argus.

METHODS

The cDNA and genomic DNA sequences of CaAmh were identified by PCR and RACE techniques. The expression patterns of CaAmh were detected by qRT-PCR during the gonadal sex differentiation and after 17α-ethinyloestradiol (EE2) treatments.

RESULTS

CaAmh is composed of seven exons and six introns, and its full-length cDNA is 2413 bp in length, with 1635 bp open reading frame (ORF) that encodes a 544 amino acid protein. Tissues expression patterns revealed that CaAmh display the highest expression in testis of XY males (40.36 folds, p < 0.01). The spatio-temporal expression patterns during gonadal sex differentiation indicated that CaAmh expression differed between XX females and XY males at 30 day after hatching (dah), and reached to the peak (36.03 folds, p < 0.01) at 90 dah in XY gonads. However, CaAmh expression in XX gonads remained low throughout the sampling period. Furthermore, CaAmh expression in the gonads (ovaries) of the sex-reversed XY fish (XY-F) by the administration of estrogen EE2 was downregulated to low level, similar to that in ovaries of normal XX females (XX-F).

CONCLUSIONS

These results show that Amh plays a critical role in testicular differentiation of C. argus and it is apparently modulated by estrogens in this species.

Crowding stress during the period of sex determination causes masculinization in pejerrey Odontesthes bonariensis, a fish with temperature-dependent sex determination

The pejerrey is an atherinopsid species from South America that presents a combination of genotypic and environmental (temperature-dependent) sex determination whereby low and high temperatures induce feminization and masculinization, respectively. Masculinization involves a heat-induced stress response leading to increased circulating cortisol and androgens. We tested whether crowding would elicit a similar response as high temperature and affect the sex ratios of pejerrey. Larvae with XX and XY genotypes were reared at 15, 62 and 250 larvae/L in 0.4, 1.6, and 6.4 L containers during a period considered critical for sex determination at 25 °C, a mixed-sex promoting temperature. Fish were analysed at 3-7 weeks for whole-body cortisol and 11-ketotestosterone (11-KT) titer and hydroxy-steroid dehydrogenase (hsd11b2) mRNA transcript abundance, and after completion of gonadal sex differentiation (10-14 weeks) for determination of phenotypic and genotypic sex mismatches. Crowding was associated with depressed growth, higher cortisol and 11-KT titers, increased hsd11b2 transcription, and increased frequency of masculinization compared to intermediate and/or low rearing densities. Perceived crowding (by rearing in containers with mirror-finish, reflecting walls) also caused masculinization. These results suggest the possibility that other environmental factors besides temperature can also affect sex determination in pejerrey and that a stress response leading to increased cortisol and androgen levels, which is potentially perceived by the brain, may be a common feature among different forms of environmental sex determination in this species.

Alternative splicing and thermosensitive expression of Dmrt1 during urogenital development in the painted turtle, Chrysemys picta

BACKGROUND

The doublesex and mab-3 related transcription factor 1 (Dmrt1) is a highly conserved gene across numerous vertebrates and invertebrates in sequence and function. Small aminoacid changes in Dmrt1 are associated with turnovers in sex determination in reptiles. Dmrt1 is upregulated in males during gonadal development in many species, including the painted turtle, Chrysemys picta, a reptile with temperature-dependent sex determination (TSD). Dmrt1 is reported to play different roles during sex determination and differentiation, yet whether these functions are controlled by distinct Dmrt1 spliceoforms remains unclear. While Dmrt1 isoforms have been characterized in various vertebrates, no study has investigated their existence in any turtle.

METHODS

We examine the painted turtle to identify novel Dmrt1 isoforms that may be present during urogenital development using PCR, profile their expression by RNA-seq across five embryonic stages at male- and female-producing temperatures, and validate their expression pattern via qPCR with transcript-specific fluorescent probes.

RESULTS

A novel Dmrt1 spliceoform was discovered for the first time in chelonians, lacking exons 2 and 3 (Dmrt1 ΔEx2Ex3). Dmrt1 canonical and ΔEx2Ex3 transcripts were differentialy expressed by temperature at stages 19 and 22 in developing gonads of painted turtles, after the onset of sex determination, and displayed a significant male-biased expression pattern. This transcriptional pattern differs from studies in other turtles and vertebrates that reported Dmrt1 differential expression before or at the onset of sex determination. This study provides the first insight into Dmrt1 transcriptional diversity in turtles and opens the door for future functional studies of the alternative Dmrt1 transcript uncovered here.

CONCLUSIONS

The discovery of an isoform in turtles indicate that alternative splicing may be a common feature of Dmrt1 across vertebrates, as isoforms are also found in crocodilians, birds, mammals and fish, and this variation remains unexplained. The relatively late-onset of Dmrt1 expression observed here contrasts with other turtles, indicating that Dmrt1 is not the topmost male sex -determining factor in C. picta. When placed in a phylogenetic context, this discrepancy underscores the divergent regulation of Dmrt1, and of sexual development more generally, across vertebrates.

Evidence of a landlocked reproducing population of the marine pejerrey Odontesthes argentinensis (Actinopterygii; Atherinopsidae)

In South America, the order Atheriniformes includes the monophyletic genus Odontesthes with 20 species that inhabit freshwater, estuarine and coastal environments. Pejerrey Odontesthes argentinensis is widely distributed in coastal and estuarine areas of the Atlantic Ocean and is known to foray into estuaries of river systems, particularly in conditions of elevated salinity. However, to our knowledge, a landlocked self-sustaining population has never been recorded. In this study, we examined the pejerrey population of Salada de Pedro Luro Lake (south-east of Buenos Aires Province, Argentina) to clarify its taxonomic identity. An integrative taxonomic analysis based on traditional meristic, landmark-based morphometrics and genetic techniques suggests that the Salada de Pedro Luro pejerrey population represents a novel case of physiological and morphological adaptation of a marine pejerrey species to a landlocked environment and emphasises the environmental plasticity of this group of fishes.

Differential Expression of Genes Related to Sexual Determination Can Modify the Reproductive Cycle of Astyanax scabripinnis (Characiformes: Characidae) in B Chromosome Carrier Individuals

The species complex Astyanax scabripinnis is one of the most studied with respect to origin, distribution, and frequency of B chromosomes, and is considered a model organism for evolutionary studies. Research using population inferences about the occurrence and frequency of the B chromosome shows seasonal variation between sexes, which is associated with the presence of this supernumerary element. We hypothesized that the B chromosome could influence the sex ratio of these animals. Based on this assumption, the present work aimed to investigate if differences exist among levels of gene expression with qRT-PCR of the amh (associated with testicular differentiation) and foxl2a (associated with ovarian differentiation) genes between B-carrier and non-B-carrier individuals. The results showed that for the amh gene, the difference in expression between animals with B chromosomes was not accentuated compared to that in animals without this chromosome. Expression of foxl2a in B-carrier females, however, was reduced by 73.56% compared to females that lacked the B chromosome. Males had no difference in expression of the amh and foxl2a genes between carriers and non-carriers of the B chromosome. Results indicate that the presence of B chromosomes is correlated with the differential expression of sex-associated genes. An analysis of these results integrated with data from other studies on the reproductive cycle in the same species reveals that this difference in expression may be expanding the reproductive cycle of the species.

The Duplicated Y-specific amhy Gene Is Conserved and Linked to Maleness in Silversides of the Genus Odontesthes

Sex-determining genes have been successively isolated in several teleosts. In Odontesthes hatcheri and O. bonariensis, the amhy gene has been identified as a master sex-determining gene. However, whether this gene is conserved along related species is still unknown. In this study, the presence of amhy and its association with phenotypic sex was analyzed in 10 species of Odontesthes genus. The primer sets from O. hatcheri that amplify both amhs successfully generated fragments that correspond to amha and amhy in all species. The full sequences of amhy and amha isolated for four key species revealed higher identity values among presumptive amhy, including the 0.5 Kbp insertion in the third intron and amhy-specific insertions/deletions. Amha was present in all specimens, regardless of species and sex, whereas amhy was amplified in most but not all phenotypic males. Complete association between amhy-homologue with maleness was found in O. argentinensis, O. incisa, O. mauleanum, O. perugiae, O. piquava, O. regia, and O. smitti, whereas O. humensis, O. mirinensis, and O. nigricans showed varied degrees of phenotypic/genotypic sex mismatch. The conservation of amhy gene in Odontesthes provide an interesting framework to study the evolution and the ecological interactions of genotypic and environmental sex determination in this group.

Identification of the master sex determining gene in Northern pike (Esox lucius) reveals restricted sex chromosome differentiation

Teleost fishes, thanks to their rapid evolution of sex determination mechanisms, provide remarkable opportunities to study the formation of sex chromosomes and the mechanisms driving the birth of new master sex determining (MSD) genes. However, the evolutionary interplay between the sex chromosomes and the MSD genes they harbor is rather unexplored. We characterized a male-specific duplicate of the anti-Müllerian hormone (amh) as the MSD gene in Northern Pike (Esox lucius), using genomic and expression evidence as well as by loss-of-function and gain-of-function experiments. Using RAD-Sequencing from a family panel, we identified Linkage Group (LG) 24 as the sex chromosome and positioned the sex locus in its sub-telomeric region. Furthermore, we demonstrated that this MSD originated from an ancient duplication of the autosomal amh gene, which was subsequently translocated to LG24. Using sex-specific pooled genome sequencing and a new male genome sequence assembled using Nanopore long reads, we also characterized the differentiation of the X and Y chromosomes, revealing a small male-specific insertion containing the MSD gene and a limited region with reduced recombination. Our study reveals an unexpectedly low level of differentiation between a pair of sex chromosomes harboring an old MSD gene in a wild teleost fish population, and highlights both the pivotal role of genes from the amh pathway in sex determination, as well as the importance of gene duplication as a mechanism driving the turnover of sex chromosomes in this clade.

In stark contrast to mammals and birds, a high proportion of teleosts have homomorphic sex chromosomes and display a high diversity of sex determining genes. Yet, population level knowledge of both the sex chromosome and the master sex determining gene is only available for the Japanese medaka, a model species. Here we identified and provided functional proofs of an old duplicate of anti-Müllerian hormone (Amh), a member of the Tgf- β family, as the male master sex determining gene in the Northern pike, Esox lucius. We found that this duplicate, named amhby (Y-chromosome-specific anti-Müllerian hormone paralog b), was translocated to the sub-telomeric region of the new sex chromosome, and now amhby shows strong sequence divergence as well as substantial expression pattern differences from its autosomal paralog, amha. We assembled a male genome sequence using Nanopore long reads and identified a restricted region of differentiation within the sex chromosome pair in a wild population. Our results provide insight on the conserved players in sex determination pathways, the mechanisms of sex chromosome turnover, and the diversity of levels of differentiation between homomorphic sex chromosomes in teleosts.

Transcriptome Profiling Insights the Feature of Sex Reversal Induced by High Temperature in Tongue Sole Cynoglossus semilaevis

Sex reversal induced by temperature change is a common feature in fish. Usually, the sex ratio shift occurs when temperature deviates too much from normal during embryogenesis or sex differentiation stages. Despite decades of work, the mechanism of how temperature functions during early development and sex reversal remains mysterious. In this study, we used Chinese tongue sole as a model to identify features from gonad transcriptomic and epigenetic mechanisms involved in temperature induced masculinization. Some of genetic females reversed to pseudomales after high temperature treatment which caused the sex ratio imbalance. RNA-seq data showed that the expression profiles of females and males were significantly different, and set of genes showed sexually dimorphic expression. The general transcriptomic feature of pesudomales was similar with males, but the genes involved in spermatogenesis and energy metabolism were differentially expressed. In gonads, the methylation level of cyp19a1a promoter was higher in females than in males and pseudomales. Furthermore, high-temperature treatment increased the cyp19a1a promoter methylation levels of females. We observed a significant negative correlation between methylation levels and expression of cyp19ala. In vitro study showed that CpG within the cAMP response element (CRE) of the cyp19a1a promoter was hypermethylated, and DNA methylation decreased the basal and forskolin-induced activities of cyp19a1a promoter. These results suggested that epigenetic change, i.e., DNA methylation, which regulate the expression of cyp19a1a might be the mechanism for the temperature induced masculinization in tongue sole. It may be a common mechanism in teleost that can be induced sex reversal by temperature.

Increase of cortisol levels after temperature stress activates dmrt1a causing female-to-male sex reversal and reduced germ cell number in medaka

In vertebrates, there is accumulating evidence that environmental factors as triggers for sex determination and genetic sex determination are not two opposing alternatives but that a continuum of mechanisms bridge those extremes. One prominent example is the model fish species Oryzias latipes which has a stable XX/XY genetic sex determination system, but still responds to environmental cues, where high temperatures lead to female-to-male sex reversal. However, the mechanisms behind are still unknown. We show that high temperatures increase primordial germ cells (PGC) numbers before they reach the genital ridge, which, in turn, regulates the germ cell proliferation. Complete ablation of PGCs led to XX males with germ cell less testis, whereas experimentally increased PGC numbers did not reverse XY genotypes to female. For the underlying molecular mechanism, we provide support for the explanation that activation of the dmrt1a gene by cortisol during early development of XX embryos enables this autosomal gene to take over the role of the male determining Y-chromosomal dmrt1bY.

Profiling expression changes of genes associated with temperature and sex during high temperature-induced masculinization in the Nile tilapia brain

Fish sex-determining mechanisms can be classified as genotypic (GSD), temperature (TSD), or genotypic plus temperature effects (GSD+TE). Previous studies have shown that culturing water temperature during thermosensitive periods (TSP) could affect the expression of many genes in the gonad in some fish. However, few studies have focused on gene expression changes in the brain after temperature treatment during TSP in fish species. In this study, three families were developed by crossing XX neomales with XX females and one of them was used for transcriptome analysis. The results showed that a total of 105, 3164 and 4666 DEGs were respectively obtained in FC (female control) vs. FT (high temperature-treated females at TSP), FC vs. MC (male control), and MC vs. FT comparison groups. By profiling analysis, we show that the mRNA expression levels of 16 differentially expressed genes (DEGs) exhibited significant downregulation or upregulation after high temperature treatment and reached a similar level as that in MC. Among the 16 DEGs, LOC100699848 (lysine specific demethylase 6A) and Jarid2 contained JmjC domain, showing the possible important role of JmjC domain in response to temperature treatment in Nile tilapia. Kdm6b (lysine demethylase 6B) and Jarid2 have been shown to play important roles in reptile TSD, showing the relative conservation of underlying regulation mechanisms between TSD in reptile and TSD or GSD+TE in fish species. Finally, the transcriptome profiling was validated by quantitative real-time PCR in nine selected genes. These results provide a direction for investigating the GSD+TE molecular mechanism in fish species.

Genetic regulation of sex determination and maintenance in zebrafish (Danio rerio)

Over the last several decades zebrafish (Danio rerio) has become a major model organism for the study of vertebrate development and physiology. Given this, it may be surprising how little is known about the mechanism that zebrafish use to determine sex. While zebrafish are a gonochoristic species (having two sexes) that do not switch sex as adults, it was appreciated early on that sex ratios obtained from breeding lab domesticated lines were not typically a 1:1 ratio of male and female, suggesting that sex was not determined by a strict chromosomal mechanism. Here we will review the recent progress toward defining the genetic mechanism for sex determination in both wild and domesticated zebrafish.