Natural sex change in fish

Developmental neuroscience: Building sex-specific adult circuitry from common larval origins

The development of sex-specific neural circuitry is critical for reproductive behaviors. A new study traces the developmental origin of female-specific neurons that underlie an adult mating behavior to larval neurons common to both sexes in Drosophila.

Gestodene causes masculinization of the western mosquitofish (Gambusia affinis): Insights from ovary metabolomics

Gestodene (GES) is a common synthetic progesterone frequently detected in aquatic environments. Chronic exposure to GES can cause masculinization of a variety of fish; however, whether metabolism is closely related to the masculinization has yet to be explored. Hence, the ovary metabolome of adult female western mosquitofish (Gambusia affinis) after exposing to GES (0.0, 5.0, 50.0, and 500.0 ng/L) for 40 days was analyzed by using high-performance liquid chromatography ionization with quadrupole time-of-flight tandem mass spectrometry (HPLC-QTOF-MS). The results showed that GES increased the levels of cysteine, taurine, ophthalmic acid and cAMP while decreased methionine, these metabolites changes may owing to the oxidative stress of the ovaries; while taurcholic acid and uric acid were decreased along with induced oocyte apopotosis. Steroids hormone metabolism was also significantly affected, with progesterone and cortisol being the most affected. Enzyme-linked immunoassay results showed that estradiol levels were decreased while testosterone levels were increased with GES exposure. In addition, correlation analysis showed that the differential metabolites of some amino acids (e.g. leucine) were strongly correlated with the levels of steroids hormones secreted by the pituitary gland. The results of this study suggest that GES affects ovarian metabolism via the hypothalamus-pituitary-gonad and hypothalamic-pituitary-adrenal axes, impair antioxidant capacity, induce apoptosis in the ovary of G. affinis, and finally caused masculinization.

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.

Adult sex change leads to extensive forebrain reorganization in clownfish

Sexual differentiation of the brain occurs in all major vertebrate lineages but is not well understood at a molecular and cellular level. Unlike most vertebrates, sex-changing fishes have the remarkable ability to change reproductive sex during adulthood in response to social stimuli, offering a unique opportunity to understand mechanisms by which the nervous system can initiate and coordinate sexual differentiation. This study explores sexual differentiation of the forebrain using single nucleus RNA-sequencing in the anemonefish Amphiprion ocellaris, producing the first cellular atlas of a sex-changing brain. We uncover extensive sex differences in cell type-specific gene expression, relative proportions of cells, baseline neuronal excitation, and predicted inter-neuronal communication. Additionally, we identify the cholecystokinin, galanin, and estrogen systems as central molecular axes of sexual differentiation. Supported by these findings, we propose a model of neurosexual differentiation in the conserved vertebrate social decision-making network spanning multiple subtypes of neurons and glia, including neuronal subpopulations within the preoptic area that are positioned to regulate gonadal differentiation. This work deepens our understanding of sexual differentiation in the vertebrate brain and defines a rich suite of molecular and cellular pathways that differentiate during adult sex change in anemonefish.

Sex diversity in the 21st century: Concepts, frameworks, and approaches for the future of neuroendocrinology

Sex is ubiquitous and variable throughout the animal kingdom. Historically, scientists have used reductionist methodologies that rely on a priori sex categorizations, in which two discrete sexes are inextricably linked with gamete type. However, this binarized operationalization does not adequately reflect the diversity of sex observed in nature. This is due, in part, to the fact that sex exists across many levels of biological analysis, including genetic, molecular, cellular, morphological, behavioral, and population levels. Furthermore, the biological mechanisms governing sex are embedded in complex networks that dynamically interact with other systems. To produce the most accurate and scientifically rigorous work examining sex in neuroendocrinology and to capture the full range of sex variability and diversity present in animal systems, we must critically assess the frameworks, experimental designs, and analytical methods used in our research. In this perspective piece, we first propose a new conceptual framework to guide the integrative study of sex. Then, we provide practical guidance on research approaches for studying sex-associated variables, including factors to consider in study design, selection of model organisms, experimental methodologies, and statistical analyses. We invite fellow scientists to conscientiously apply these modernized approaches to advance our biological understanding of sex and to encourage academically and socially responsible outcomes of our work. By expanding our conceptual frameworks and methodological approaches to the study of sex, we will gain insight into the unique ways that sex exists across levels of biological organization to produce the vast array of variability and diversity observed in nature.

Whole-genome sequencing reveals fine-scale environment-associated divergence near the range limits of a temperate reef fish

Environmental variation is increasingly recognized as an important driver of diversity in marine species despite the lack of physical barriers to dispersal and the presence of pelagic stages in many taxa. A robust understanding of the genomic and ecological processes involved in structuring populations is lacking for most marine species, often hindering management and conservation action. Cunner (Tautogolabrus adspersus) is a temperate reef fish with both pelagic early life-history stages and strong site-associated homing as adults; the species is also of interest for use as a cleaner fish in salmonid aquaculture in Atlantic Canada. We aimed to characterize genomic and geographic differentiation of cunner in the Northwest Atlantic. To achieve this, a chromosome-level genome assembly for cunner was produced and used to characterize spatial population structure throughout Atlantic Canada using whole-genome sequencing. The genome assembly spanned 0.72 Gbp and 24 chromosomes; whole-genome sequencing of 803 individuals from 20 locations from Newfoundland to New Jersey identified approximately 11 million genetic variants. Principal component analysis revealed four regional Atlantic Canadian groups. Pairwise FST and selection scans revealed signals of differentiation and selection at discrete genomic regions, including adjacent peaks on chromosome 10 across multiple pairwise comparisons (i.e. FST 0.5-0.75). Redundancy analysis suggested association of environmental variables related to benthic temperature and oxygen range with genomic structure. Results suggest regional scale diversity in this temperate reef fish and can directly inform the collection and translocation of cunner for aquaculture applications and the conservation of wild populations throughout the Northwest Atlantic.

Bioactive compounds from four Indian medicinal plants have different potency to induce sex reversal in Nile tilapia: A chromatographic, molecular docking and in silico analysis

ETHNOPHARMACOLOGICAL RELEVANCE

Herbal plants such as Basella alba (Family: Basellaceae), Tribulus terrestris (Family: Zygophyllaceae), Asparagus racemosus (Family: Asparagaceae) and Mucuna pruriens (Family: Fabaceae) are mentioned in Indian traditional system of medicine Ayurveda to possess androgenic activity and increase male virility. The plants have been reported to improve testosterone level and sperm production in experimental male rodents as well.

AIM OF THE STUDY

Male Nile tilapias grow more quickly than females and hence are preferred for monosex Nile tilapia culture. Ethanol extracts of B. alba leaves (EB) and T. terrestris seeds (ET), and methanol extract of A. racemosus roots (MA) and M. pruriens seeds (MM) were found effective to induce masculinization in Nile tilapia. The present study intends to evaluate the anti-aromatase activity of EB, ET, MA and MM, to identify the androgenic bioactive compounds in the extracts, and to determine their pharmacokinetics. The study may validate the use of those plant extracts and their major bioactive phytoconstituents in the field of aquaculture and pharmaceuticals.

MATERIALS AND METHODS

The four crude plant extracts were first fractioned through column and thin layer chromatography (TLC). Three days old Nile tilapia juveniles (mean weight 0.025 ± 0.009g; mean length 12.50 ± 0.12 mm; n = 50 fish/replicate, 3 replicates/treatment) were then fed diets fortified with the obtained fractions for 30 days. After 30 days, fish were sacrificed and gonad aromatase mRNA expression, and 11-ketotestosterone (11-KT) and estradiol (E₂) levels were measured. Fractions yielding the highest male percentage for each plant were subject to gas chromatography-mass spectrometry (GC-MS) analysis. The in silico docking and SwissADME study were conducted with the components showing higher peak percentage in chromatogram.

RESULTS

After column chromatography and TLC analysis, EB, ET, MM and MA yielded 6 (EB1 - EB6), 8 (ET1- ET8), 14 (MM1-MM14) and 5 (MA1- MA5) fractions, respectively. Fish fed EB2, ET2, MA2 and MM13 fraction fortified diets showed significantly (p < 0.05) higher male percentage (92.32%-98.39%) compared to other treatment groups. EB2, ET2, MA2 and MM13 fed fish showed significantly (p < 0.05) higher 11-KT level compared to control male (+247.52 - +397.76%) and lower E₂ level compared to control female (-95.92% to -90.65%). Aromatase mRNA expression was significantly (p < 0.05) down-regulated by all these four fractions (-1.32 to -5.65 fold) with respect to control female. GC-MS analysis revealed the presence of 1-Octadecene (OD) in EB2, Phenol, 2,4-bis(1,1-dimethylethyl) (PD) in ET2 and MA2, 9,12-Octadecadienoic acid (Z,Z)- (ODDA) in MM13. In silico molecular docking indicated that PD is more effective than ODDA and OD to inhibit aromatase. In addition, PD showed better pharmacokinetics and more drug-likeness compared to OD and ODDA in SwissADME analysis.

CONCLUSION

The present results indicate that ET and MA are more potent to produce all-male tilapia by means of aromatase inhibition. PD can be an ideal compound to achieve masculinization in Nile tilapia through dietary administration, but further investigation is required.

Use of male-to-female sex reversal as a welfare scoring system in the protandrous farmed gilthead sea bream (Sparus aurata)

Gilthead sea bream is a highly cultured marine fish throughout the Mediterranean area, but new and strict criteria of welfare are needed to assure that the intensification of production has no negative effects on animal farming. Most welfare indicators are specific to a given phase of the production cycle, but others such as the timing of puberty and/or sex reversal are of retrospective value. This is of particular relevance in the protandrous gilthead sea bream, in which the sex ratio is highly regulated at the nutritional level. Social and environmental factors (e.g., contaminant loads) also alter the sex ratio, but the contribution of the genetic component remains unclear. To assess this complex issue, five gilthead sea bream families representative of slow/intermediate/fast growth were grown out with control or a plant-based diet in a common garden system from early life to the completion of their sexual maturity in 3-year-old fish. The plant-based diet highly enhanced the male-to-female sex reversal. This occurred in parallel with the progressive impairment of growth performance, which was indicative of changes in nutrient requirements as the result of the different energy demands for growth and reproduction through development. The effect of a different nutritional and genetic background on the reproductive performance was also assessed by measurements of circulating levels of sex steroids during the two consecutive spawning seasons, varying plasma levels of 17β-estradiol (E₂) and 11-ketotestosterone (11-KT) with age, gender, diet, and genetic background. Principal component analysis (PCA) of 3-year-old fish displayed a gradual increase of the E₂/11-KT ratio from males to females with the improvement of nutritional/genetic background. Altogether, these results support the use of a reproductive tract scoring system for leading farmed fish toward their optimum welfare condition, contributing to improving the productivity of the current gilthead sea bream livestock.

The ontogenic gonadal transcriptomes provide insights into sex change in the ricefield eel Monopterus albus

BACKGROUND

The ricefield eel is a freshwater protogynous hermaphrodite fish and has become an important aquaculture species in China. The sex change of ricefield eel is impeding its aquaculture practice, particularly the large-scale artificial breeding. Many studies including transcriptomes of mixed gonadal samples from different individuals have been aimed to elucidate mechanisms underlying the sex change. However, the key physiological factors involved in the initiation of sex change remain to be identified.  RESULTS: The present study performed transcriptomic analysis on gonadal samples of different sexual stages obtained through biopsy from the same fish undergoing sex change. A total of 539,764,816 high-quality reads were generated from twelve cDNA libraries of gonadal tissues at female (F), early intersexual (EI), mid-intersexual (MI), and late intersexual (LI) stages of three individual sex-changing fish. Pairwise comparisons between EI and F, MI and EI, and LI and MI identified 886, 319, and 10,767 differentially expressed genes (DEGs), respectively. Realtime quantitative PCR analysis of 12 representative DEGs showed similar expression profiles to those inferred from transcriptome data, suggesting the reliability of RNA-seq data for gene expression analysis. The expression of apoeb, csl2, and enpp2 was dramatically increased and peaked at EI while that of cyp19a1a, wnt4a, fgf16, and foxl2a significantly downregulated from F to EI and remained at very low levels during subsequent development until LI, which suggests that apoeb, csl2, enpp2, cyp19a1a, wnt4a, fgf16, and foxl2a may be closely associated with the initiation of sex change of ricefield eels.

CONCLUSIONS

Collectively, results of the present study confirmed that the down-regulation of female-related genes, such as cyp19a1a, wnt4a, fgf16, and foxl2a, is important for the sex change of ricefield eels. More importantly, some novel genes, including apoeb, csl2, and enpp2, were shown to be expressed with peak values at EI, which are potentially involved in the initiation of sex change. The present transcriptomic data may provide an important research resource for further unraveling the mechanisms underlying the sex change and testicular development in ricefield eels as well as other teleosts.

Individual Genetic Heterogeneity

Genetic variation has been widely covered in literature, however, not from the perspective of an individual in any species. Here, a synthesis of genetic concepts and variations relevant for individual genetic constitution is provided. All the different levels of genetic information and variation are covered, ranging from whether an organism is unmixed or hybrid, has variations in genome, chromosomes, and more locally in DNA regions, to epigenetic variants or alterations in selfish genetic elements. Genetic constitution and heterogeneity of microbiota are highly relevant for health and wellbeing of an individual. Mutation rates vary widely for variation types, e.g., due to the sequence context. Genetic information guides numerous aspects in organisms. Types of inheritance, whether Mendelian or non-Mendelian, zygosity, sexual reproduction, and sex determination are covered. Functions of DNA and functional effects of variations are introduced, along with mechanism that reduce and modulate functional effects, including TARAR countermeasures and intraindividual genetic conflict. TARAR countermeasures for tolerance, avoidance, repair, attenuation, and resistance are essential for life, integrity of genetic information, and gene expression. The genetic composition, effects of variations, and their expression are considered also in diseases and personalized medicine. The text synthesizes knowledge and insight on individual genetic heterogeneity and organizes and systematizes the central concepts.

Transcriptome Profiling and Expression Localization of Key Sex-Related Genes in a Socially-Controlled Hermaphroditic Clownfish, Amphiprion clarkii

Clownfish can be an excellent research model for investigating the socially-controlled sexual development of sequential hermaphrodite teleosts. However, the molecular cascades underlying the social cues that orchestrate the sexual development process remain poorly understood. Here, we performed a comparative transcriptomic analysis of gonads from females, males, and nonbreeders of Amphiprion clarkii, which constitute a complete social group, allowing us to investigate the molecular regulatory network under social control. Our analysis highlighted that the gonads of nonbreeders and males exhibited high similarities but were far from females, both in global transcriptomic profiles and histological characteristics, and identified numerous candidate genes involved in sexual development, some well-known and some novel. Significant upregulation of cyp19a1a, foxl2, nr5a1a, wnt4a, hsd3b7, and pgr in females provides strong evidence for the importance of steroidogenesis in ovarian development and maintenance, with cyp19a1a playing a central role. Amh and sox8 are two potential key factors that may regulate testicular tissue development in early and late stages, respectively, as they are expressed at higher levels in males than in females, but with slightly different expression timings. Unlike previous descriptions in other fishes, the unique expression pattern of dmrt1 in A. clarkii implied its potential function in both male and female gonads, and we speculated that it might play promoting roles in the early development of both testicular and ovarian tissues.

Turning the tide on sex and the microbiota in aquatic animals

Sex-based differences in animal microbiota are increasingly recognized as of biological importance. While most animal biomass is found in aquatic ecosystems and many water-dwelling species are of high economic and ecological value, biological sex is rarely included as an explanatory variable in studies of the aquatic animal microbiota. In this opinion piece, we argue for greater consideration of host sex in studying the microbiota of aquatic animals, emphasizing the many advancements that this information could provide in the life sciences, from the evolution of sex to aquaculture.

ESR1 mediates estrogen-induced feminization of genetic male Chinese soft-shelled turtle

Exogenous estrogen have shown their femininization abilities during the specific sex differentiation period in several reptiles. However, the specific regulatory mechanism and downstream regulatory genes of estrogen remain elusive. In the present study, 17β-estradiol (E2), as well as drugs of specific antagonists and/or agonists of estrogen receptors, were employed to figure out the molecular pathway involved in the E2-induced feminization in Chinese soft-shelled turtles, an important aquaculture species in China. E2 treatment led to typical female characteristics in the gonads of ZZ individuals, including thickened outer cortex containing a number of germ cells and degenerated medullary cords, as well as the disappearance of male marker SOX9, and the ectopic expression of ovarian regulator FOXL2 at the embryonic developmental stage 27 and 1 month after hatching. The specific ESR1 antagonist or a combination of three estrogen receptor antagonists could block the sex reversal of ZZ individuals induced by estrogen. In addition, specific activation of ESR1 by agonist also led to the feminization of ZZ gonads, which was similar to the effect of estrogen treatment. Furthermore, transcriptome data showed that the expression level of FOXL2 was significantly up-regulated, while mRNA levels of DMRT1, SOX9 and AMH were down-regulated after estrogen treatment. Taken together, our results indicated that E2 induced the feminization of ZZ Chinese soft-shelled turtles via ESR1, and decrease of male genes DMRT1, SOX9and AMH and increase of ovarian development regulator FOXL2 might be responsible for the initiation of E2-induced feminization.

Comprehensive Characterization of Circular RNAs in Ovary and Testis From Nile Tilapia

Circular RNA (circRNA) is an endogenous biomolecule in eukaryotes. It has tissue- and cell-specific expression patterns and can act as a microRNA sponge or competitive endogenous RNA. Although circRNA has been found in several species in recent years, the expression profiles in fish gonad are still not fully understood. We detected the expression of circRNA in the ovary, testis, and sex-changed gonad of tilapia by high-throughput deep sequencing, and circRNA-specific computing tools. A total of 20,607 circRNAs were obtained, of which 141 were differentially expressed in the testis and ovary. Among these circRNAs, 135 circRNAs were upregulated and 6 circRNAs were downregulated in female fish. In addition, GO annotation and KEGG pathway analysis of the host genes of circRNAs indicated that these host genes were mainly involved in adherens junction, androgen production, and reproductive development, such as ZP3, PLC, delta 4a, ARHGEF10, and HSD17b3. It is worth noting that we found that circRNAs in tilapia gonads have abundant miRNA-binding sites. Among them, 935 circRNAs have a regulatory effect on miR-212, 856 circRNAs have a regulatory effect on miR-200b-3p, and 529 circRNAs have a regulatory effect on miR-200b-5p. Thus, our findings provide a new evidence for circRNA–miRNA networks in the gonads in tilapia.

Identification of SF-1 and FOXL2 and Their Effect on Activating P450 Aromatase Transcription via Specific Binding to the Promoter Motifs in Sex Reversing Cheilinus undulatus

The giant wrasse Cheilinus undulatus is a protogynous socially hermaphroditic fish. However, the physiological basis of its sex reversal remains largely unknown. cyp19 is a key gender-related gene encoding P450 aromatase, which converts androgens to estrogens. cyp19 transcription regulation is currently unknown in socially sexually reversible fish. We identified NR5A1 by encoding SF-1, and FOXL2 from giant wrasse cDNA and cyp19a1a and cyp19a1b promoter regions were cloned from genomic DNA to determine the function of both genes in cyp19a1 regulation. Structural analysis showed that SF-1 contained a conserved DNA-binding domain (DBD) and a C-terminal ligand-binding domain (LBD). FOXL2 was comprised of an evolutionarily conserved Forkhead domain. In vitro transfection assays showed that SF-1 could upregulate cyp19a1 promoter activities, but FOXL2 could only enhance cyp19a1b promoter transcriptional activity in the HEK293T cell line. Furthermore, HEK293T and COS-7 cell lines showed that co-transfecting the two transcription factors significantly increased cyp19a1 promoter activity. The -120 to -112 bp (5'-CAAGGGCAC-3') and -890 to -872 bp (5'-AGAGGAGAACAAGGGGAG-3') regions of the cyp19a1a promoter were the core regulatory elements for SF-1 and FOXL2, respectively, to regulate cyp19a1b promoter transcriptional activity. Collectively, these results suggest that both FOXL2 and SF-1 are involved in giant wrasse sex reversal.

Unraveling the genotype by environment interaction in a thermosensitive fish with a polygenic sex determination system

In most animals, sex determination occurs at conception, when sex chromosomes are segregated following Mendelian laws. However, in multiple reptiles and fishes, this genetic sex can be overridden by external factors after fertilization or birth. In some species, the genetic sex may also be governed by multiple genes, further limiting our understanding of sex determination in such species. We used the European sea bass (Dicentrarchus labrax) as a model and combined genomic (using a single nucleotide polymorphism chip) and transcriptomic (RNA-Sequencing) approaches to thoroughly depict this polygenic sex determination system and its interaction with temperature. We estimated genetic sex tendency (eGST), defined as the estimated genetic liability to become a given sex under a liability threshold model for sex determination, which accurately predicts the future phenotypic sex. We found evidence that energetic pathways, concerning the regulation of lipids and glucose, are involved in sex determination and could explain why females tend to exhibit higher energy levels and improved growth compared to males. Besides, early exposure to high-temperature up-regulated sox3, followed by sox9a in individuals with intermediate eGST, but not in individuals showing highly female-biased eGST, providing the most parsimonious explanation for temperature-induced masculinization. This gonadal state was maintained likely by DNA methylation and the up-regulation of several genes involved in histone modifications, including jmjd1c Overall, we describe a sex determination system resulting from continuous genetic and environmental influences in an animal. Our results provide significant progress in our understanding of the mechanisms underlying temperature-induced masculinization in fish.

A new experimental model for the investigation of sequential hermaphroditism

The stunning sexual transformation commonly triggered by age, size or social context in some fishes is one of the best examples of phenotypic plasticity thus far described. To date our understanding of this process is dominated by studies on a handful of subtropical and tropical teleosts, often in wild settings. Here we have established the protogynous New Zealand spotty wrasse, Notolabrus celidotus, as a temperate model for the experimental investigation of sex change. Captive fish were induced to change sex using aromatase inhibition or manipulation of social groups. Complete female-to-male transition occurred over 60 days in both cases and time-series sampling was used to quantify changes in hormone production, gene expression and gonadal cellular anatomy. Early-stage decreases in plasma 17β-estradiol (E2) concentrations or gonadal aromatase (cyp19a1a) expression were not detected in spotty wrasse, despite these being commonly associated with the onset of sex change in subtropical and tropical protogynous (female-to-male) hermaphrodites. In contrast, expression of the masculinising factor amh (anti-Müllerian hormone) increased during early sex change, implying a potential role as a proximate trigger for masculinisation. Collectively, these data provide a foundation for the spotty wrasse as a temperate teleost model to study sex change and cell fate in vertebrates.

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 sex-linked SNP mutation in amhr2 is responsible for male differentiation in obscure puffer (Takifugu obscurus)

BACKGROUND

Anti-Mullerian hormone receptor type II (Amhr2) is a key receptor of Amh signaling in regulating gonad development. The amhr2 gene has been identified in numerous species, including a few teleost fishes. However, the roles of Amhr2 in Amh signaling in fish are poorly studied.

METHODS AND RESULTS

In this study, an amhr2 homolog from obscure puffer (Takifugu obscurus) was identified, and its molecular characteristics were systematically analyzed. Expression analysis revealed that amhr2 was highly expressed in the gonads of adult pufferfish and significantly upregulated during sex differentiation. Significantly, a sex-linked SNP site was verified in obscure puffer amhr2. Females exhibited a homozygous genotype (C/C), while males possessed a heterozygous genotype (C/G), resulting in an amino acid variation (His/Asp384) in the kinase domain of Amhr2. Then, the functions of the different Amhr2 genotypes were further investigated. The male genotype protein (Amhr2^D384) showed an enhanced ability to interact with the type I receptor (Bmpr1a) compared to the female genotype (Amhr2^H384). The phosphorylation levels of Smads and activity of the target gene (id3) induced by the male genotype were also much higher than those induced by the female genotype. These results confirmed that the male genotype had an enhanced effect on the Amh signaling pathway compared with the female genotype.

CONCLUSIONS

This study provides direct experimental evidence for the roles of different Amhr2 genotypes in pufferfish and suggests that amhr2 is responsible for male sex differentiation in obscure puffer.

Sex differences in the cognitive abilities of a sex-changing fish species Labroides dimidiatus

Males and females of the same species are known to differ at least in some cognitive domains, but such differences are not systematic across species. As a consequence, it remains unclear whether reported differences generally reflect adaptive adjustments to diverging selective pressures, or whether differences are mere side products of physiological differences necessary for reproduction. Here, we show that sex differences in cognition occur even in a sex-changing species, a protogynous hermaphroditic species where all males have previously been females. We tested male and female cleaner fish Labroides dimidiatus in four cognitive tasks to evaluate their learning and inhibitory control abilities first in an abstract presentation of the tasks, then in more ecologically relevant contexts. The results showed that males were better learners than females in the two learning tasks (i.e. reversal learning as an abstract task and a food quantity assessment task as an ecologically relevant task). Conversely, females showed enhanced abilities compared with males in the abstract inhibitory control task (i.e. detour task); but both sexes performed equally in the ecologically relevant inhibitory control task (i.e. 'audience effect' task). Hence, sex-changing species may offer unique opportunities to study proximate and/or ultimate causes underlying sex differences in cognitive abilities.

Environmental Cues and Mechanisms Underpinning Sex Change in Fish

Fishes are the only vertebrates that undergo sex change during their lifetime, but even within this group, a unique reproductive strategy is displayed by only 1.5% of the teleosts. This lability in alternating sexual fate is the result of the simultaneous suppression and activation of opposing male and female networks. Here, we provide a brief review summarizing recent advances in our understanding of the environmental cues that trigger sex change and their perception, integration, and translation into molecular cascades that convert the sex of an individual. We particularly focus on molecular events underpinning the complex behavioral and morphological transformation involved in sex change, dissecting the main molecular players and regulatory networks that shape the transformation of one sex into the opposite. We show that histological changes and molecular pathways governing gonadal reorganization are better described than the neuroendocrine basis of sex change and that, despite important advances, information is lacking for the majority of hermaphrodite species. We highlight significant gaps in our knowledge of how sex change takes place and suggest future research directions.

The call of the wild: using non-model systems to investigate microbiome-behaviour relationships

On and within most sites across an animal's body live complex communities of microorganisms. These microorganisms perform a variety of important functions for their hosts, including communicating with the brain, immune system and endocrine axes to mediate physiological processes and affect individual behaviour. Microbiome research has primarily focused on the functions of the microbiome within the gastrointestinal tract (gut microbiome) using biomedically relevant laboratory species (i.e. model organisms). These studies have identified important connections between the gut microbiome and host immune, neuroendocrine and nervous systems, as well as how these connections, in turn, influence host behaviour and health. Recently, the field has expanded beyond traditional model systems as it has become apparent that the microbiome can drive differences in behaviour and diet, play a fundamental role in host fitness and influence community-scale dynamics in wild populations. In this Review, we highlight the value of conducting hypothesis-driven research in non-model organisms and the benefits of a comparative approach that assesses patterns across different species or taxa. Using social behaviour as an intellectual framework, we review the bidirectional relationship between the gut microbiome and host behaviour, and identify understudied mechanisms by which these effects may be mediated.

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.

Molecular and cellular regulation on sex change in hermaphroditic fish, with a special focus on protandrous black porgy, Acanthopagrus schlegelii

In teleost fish, sex can be determined by genetic factors, environmental factors, or both. Unlike in gonochoristic fish, in which sex is fixed in adults, sex can change in adults of hermaphroditic fish species. Thus, sex is generated during the initial gonadal differentiation stage (primary sex differentiation) and later during sexual fate alternation (secondary sex differentiation) in hermaphroditic fish species. Depending on the species, sex phase alternation can be induced by endogenous cues (such as individual age and body size) or by social cues (such as sex ratio or relative body size within the population). In general, the fluctuation in plasma estradiol-17β (E2) levels is correlated with the sexual fate alternation in hermaphroditic fish. Hormonal treatments can artificially induce sexual phase alternation in sequential hermaphroditic fishes, but in a transient and reversible manner. This is the case for the E2-induced female phase in protandrous black porgy and the methyltestosterone (MT)- or aromatase inhibitor (AI)-induced male phase in protogynous grouper. Recent reviews have focused on the different forms of sex change in fish who undergo sequential sex change, especially in terms of gene expression and the role of hormones. In this review, we use the protandrous black porgy, a nonsocial cue-influenced hermaphroditic species, with digonic gonads (ovarian and testis separated by a connective tissue), as a model to describe our findings and discuss the molecular and cellular regulation of sexual fate determination in hermaphroditic fish.

High temperature stress response is not sexually dimorphic at the whole-body level and is dependent on androgens to induce sex reversal

The understanding of the molecular and endocrine mechanisms behind environmentally-induced sex reversal in fish is of great importance in the context of predicting the potential effects of climate change, especially increasing temperature. Here, we demonstrate the global effects of high temperature on genome-wide transcription in medaka (Oryzias latipes) during early development. Interestingly, data analysis did not show sexual dimorphic changes, demonstrating that thermal stress is not dependent on genotypic sex. Additionally, our results revealed significant changes in several pathways under high temperature, such as stress response from brain, steroid biosynthesis, epigenetic mechanisms, and thyroid hormone biosynthesis, among others. These microarray data raised the question of what the exact molecular and hormonal mechanisms of action are for female-to-male sex reversal under high temperatures in fish. Complementary gene expression analysis revealed that androgen-related genes increase in females (XX) experiencing high water temperature. To test the involvement of androgens in thermal-induced sex reversal, an androgen antagonist was used to treat XX medaka under a high-temperature setup. Data clearly demonstrated failure of female-to-male sex reversal when androgen action is inhibited, corroborating the importance of androgens in environmentally-induced sex reversal.

An in vitro ovarian explant culture system to examine sex change in a hermaphroditic fish

Many teleost fishes undergo natural sex change, and elucidating the physiological and molecular controls of this process offers unique opportunities not only to develop methods of controlling sex in aquaculture settings, but to better understand vertebrate sexual development more broadly. Induction of sex change in some sequentially hermaphroditic or gonochoristic fish can be achieved in vivo through social manipulation, inhibition of aromatase activity, or steroid treatment. However, the induction of sex change in vitro has been largely unexplored. In this study, we established an in vitro culture system for ovarian explants in serum-free medium for a model sequential hermaphrodite, the New Zealand spotty wrasse (Notolabrus celidotus). This culture technique enabled evaluating the effect of various treatments with 17β-estradiol (E₂), 11-ketotestosterone (11KT) or cortisol (CORT) on spotty wrasse ovarian architecture for 21 days. A quantitative approach to measuring the degree of ovarian atresia within histological images was also developed, using pixel-based machine learning software. Ovarian atresia likely due to culture was observed across all treatments including no-hormone controls, but was minimised with treatment of at least 10 ng/mL E₂. Neither 11KT nor CORT administration induced proliferation of spermatogonia (i.e., sex change) in the cultured ovaries indicating culture beyond 21 days may be needed to induce sex change in vitro. The in vitro gonadal culture and analysis systems established here enable future studies investigating the paracrine role of sex steroids, glucocorticoids and a variety of other factors during gonadal sex change in fish.

17α-ethynylestradiol prevents the natural male-to-female sex change in gilthead seabream (Sparus aurata L.)

Exposure to 17α-ethynylestradiol (EE2, 5 μg/g food) impairs some reproductive events in the protandrous gilthead seabream and a short recovery period does not allow full recovery. In this study, spermiating seabream males in the second reproductive cycle (RC) were fed a diet containing 5 or 2.5 μg EE2/g food for 28 days and then a commercial diet without EE2 for the remaining RC. Individuals were sampled at the end of the EE2 treatment and then at the end of the RC and at the beginning of the third RC, 146 and 333 days after the cessation of treatment, respectively. Increased hepatic transcript levels of the gene coding for vitellogenin (vtg) and plasma levels of Vtg indicated both concentrations of EE2 caused endocrine disruption. Modifications in the histological organization of the testis, germ cell proliferation, plasma levels of the sex steroids and pituitary expression levels of the genes coding for the gonadotropin β-subunits, fshβ and lhβ were detected. The plasma levels of Vtg and most of the reproductive parameters were restored 146 days after treatments. However, although 50% of the control fish underwent sex reversal as expected at the third RC, male-to female sex change was prevented by both EE2 concentrations.

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.

The Genetics and Epigenetics of Sex Change in Fish

Fish show extraordinary sexual plasticity, changing sex naturally as part of their life cycle or reversing sex because of environmental stressors. This plasticity shows that sexual fate is not an irreversible process but the result of an ongoing tug-of-war for supremacy between male and female signaling networks. The behavioral, gonadal, and morphological changes involved in this process are well described, yet the molecular events that underpin those changes remain poorly understood. Epigenetic modifications emerge as a critical link between environmental stimuli, the onset of sex change, and subsequent maintenance of sexual phenotype. Here we synthesize current knowledge of sex change, focusing on the genetic and epigenetic processes that are likely involved in the initiation and regulation of sex change. We anticipate that better understanding of sex change in fish will shed new light on sex determination and development in vertebrates and on how environmental perturbations affect sexual fate.