Molecular cloning and expression profile of sex-specific genes, Figla and Dmrt1, in the protogynous hermaphroditic fish, Halichoeres poecilopterus

Molecular Characterization, Expression Pattern, DNA Methylation and Gene Disruption of Figla in Blotched Snakehead (Channa maculata)

Figla is one of the earliest expressed genes in the oocyte during ovarian development. In this study, Figla was characterized in C. maculata, one of the main aquaculture species in China, and designated as CmFigla. The length of CmFigla cDNA was 1303 bp, encoding 197 amino acids that contained a conserved bHLH domain. CmFigla revealed a female-biased expression patterns in the gonads of adult fish, and CmFigla expression was far higher in ovaries than that in testes at all gonadal development stages, especially at 60~180 days post-fertilization (dpf). Furthermore, a noteworthy inverse relationship was observed between CmFigla expression and the methylation of its promoter in the adult gonads. Gonads at 90 dpf were used for in situ hybridization (ISH), and CmFigla transcripts were mainly concentrated in oogonia and the primary oocytes in ovaries, but undetectable in the testes. These results indicated that Figla would play vital roles in the ovarian development in C. maculata. Additionally, the frame-shift mutations of CmFigla were successfully constructed through the CRISPR/Cas9 system, which established a positive foundation for further investigation on the role of Figla in the ovarian development of C. maculata. Our study provides valuable clues for exploring the regulatory mechanism of Figla in the fish ovarian development and maintenance, which would be useful for the sex control and reproduction of fish in aquaculture.

Salpa genome and developmental transcriptome analyses reveal molecular flexibility enabling reproductive success in a rapidly changing environment

Ocean warming favors pelagic tunicates, such as salps, that exhibit increasingly frequent and rapid population blooms, impacting trophic dynamics and composition and human marine-dependent activities. Salp blooms are a result of their successful reproductive life history, alternating seasonally between asexual and sexual protogynous (i.e. sequential) hermaphroditic stages. While predicting future salp bloom frequency and intensity relies on an understanding of the transitions during the sexual stage from female through parturition and subsequent sex change to male, these transitions have not been explored at the molecular level. Here we report the development of the first complete genome of S. thompsoni and the North Atlantic sister species S. aspera. Genome and comparative analyses reveal an abundance of repeats and G-quadruplex (G4) motifs, a highly stable secondary structure, distributed throughout both salp genomes, a feature shared with other tunicates that perform alternating sexual-asexual reproductive strategies. Transcriptional analyses across sexual reproductive stages for S. thompsoni revealed genes associated with male sex differentiation and spermatogenesis are expressed as early as birth and before parturition, inconsistent with previous descriptions of sequential sexual differentiation in salps. Our findings suggest salp are poised for reproductive success at birth, increasing the potential for bloom formation as ocean temperatures rise.

Whole-genome assembly and annotation for the little yellow croaker (Larimichthys polyactis) provide insights into the evolution of hermaphroditism and gonochorism

The evolutionary direction of gonochorism and hermaphroditism is an intriguing mystery to be solved. The special transient hermaphroditic stage makes the little yellow croaker (Larimichthys polyactis) an appealing model for studying hermaphrodite formation. However, the origin and evolutionary relationship between of L. polyactis and Larimichthys crocea, the most famous commercial fish species in East Asia, remain unclear. Here, we report the sequence of the L. polyactis genome, which we found is ~706 Mb long (contig N50 = 1.21 Mb and scaffold N50 = 4.52 Mb) and contains 25,233 protein-coding genes. Phylogenomic analysis suggested that L. polyactis diverged from the common ancestor, L. crocea, approximately 25.4 million years ago. Our high-quality genome assembly enabled comparative genomic analysis, which revealed several within-chromosome rearrangements and translocations, without major chromosome fission or fusion events between the two species. The dmrt1 gene was identified as the male-specific gene in L. polyactis. Transcriptome analysis showed that the expression of dmrt1 and its upstream regulatory gene (rnf183) were both sexually dimorphic. Rnf183, unlike its two paralogues rnf223 and rnf225, is only present in Larimichthys and Lates but not in other teleost species, suggesting that it originated from lineage-specific duplication or was lost in other teleosts. Phylogenetic analysis shows that the hermaphrodite stage in male L. polyactis may be explained by the sequence evolution of dmrt1. Decoding the L. polyactis genome not only provides insight into the genetic underpinnings of hermaphrodite evolution, but also provides valuable information for enhancing fish aquaculture.

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.

Comparative Genomics Studies on the dmrt Gene Family in Fish

Doublesex and mab-3-related transcription factor (dmrt) genes are widely distributed across various biological groups and play critical roles in sex determination and neural development. Here, we applied bioinformatics methods to exam cross-species changes in the dmrt family members and evolutionary relationships of the dmrt genes based on genomes of 17 fish species. All the examined fish species have dmrt1–5 while only five species contained dmrt6. Most fish harbored two dmrt2 paralogs (dmrt2a and dmrt2b), with dmrt2b being unique to fish. In the phylogenetic tree, 147 DMRT are categorized into eight groups (DMRT1–DMRT8) and then clustered in three main groups. Selective evolutionary pressure analysis indicated purifying selections on dmrt1–3 genes and the dmrt1–3–2(2a) gene cluster. Similar genomic conservation patterns of the dmrt1–dmrt3–dmrt2(2a) gene cluster with 20-kb upstream/downstream regions in fish with various sex-determination systems were observed except for three regions with remarkable diversity. Synteny analysis revealed that dmrt1, dmrt2a, dmrt2b, and dmrt3–5 were relatively conserved in fish during the evolutionary process. While dmrt6 was lost in most species during evolution. The high conservation of the dmrt1–dmrt3–dmrt2(2a) gene cluster in various fish genomes suggests their crucial biological functions while various dmrt family members and sequences across fish species suggest different biological roles during evolution. This study provides a molecular basis for fish dmrt functional analysis and may serve as a reference for in-depth phylogenomics.

Natural sex change in fish

Sexual fate can no longer be considered an irreversible deterministic process that once established during early embryonic development, plays out unchanged across an organism's life. Rather, it appears to be a dynamic process, with sexual phenotype determined through an ongoing battle for supremacy between antagonistic male and female developmental pathways. That sexual fate is not final and is actively regulated via the suppression or activation of opposing genetic networks creates the potential for flexibility in sexual phenotype in adulthood. Such flexibility is seen in many fish, where sex change is a usual and adaptive part of the life cycle. Many fish are sequential hermaphrodites, beginning life as one sex and changing sometime later to the other. Sequential hermaphrodites include species capable of female-to-male (protogynous), male-to-female (protandrous), or bidirectional (serial) sex change. These natural forms of sex change involve coordinated transformations across multiple biological systems, including behavioral, anatomical, neuroendocrine and molecular axes. Here we review the biological processes underlying this amazing transformation, focusing particularly on the molecular aspects, where new genomic technologies are beginning to help us understand how sex change is initiated and regulated at the molecular level.

The Gene Toolkit Implicated in Functional Sex in Sparidae Hermaphrodites: Inferences From Comparative Transcriptomics

Sex-biased gene expression is the mode through which sex dimorphism arises from a nearly identical genome, especially in organisms without genetic sex determination. Teleost fishes show great variations in the way the sex phenotype forms. Among them, Sparidae, that might be considered as a model family displays a remarkable diversity of reproductive modes. In this study, we sequenced and analyzed the sex-biased transcriptome in gonads and brain (the tissues with the most profound role in sexual development and reproduction) of two sparids with different reproductive modes: the gonochoristic common dentex, Dentex dentex, and the protandrous hermaphrodite gilthead seabream, Sparus aurata. Through comparative analysis with other protogynous and rudimentary protandrous sparid transcriptomes already available, we put forward common male and female-specific genes and pathways that are probably implicated in sex-maintenance in this fish family. Our results contribute to the understanding of the complex processes behind the establishment of the functional sex, especially in hermaphrodite species and set the groundwork for future experiments by providing a gene toolkit that can improve efforts to control phenotypic sex in finfish in the ever-increasingly important field of aquaculture.

Roles of Figla/figla in Juvenile Ovary Development and Follicle Formation During Zebrafish Gonadogenesis

Sex determination and differentiation are complex processes. As a juvenile hermaphrodite or undifferentiated gonochorist, zebrafish undergo a special juvenile ovarian phase during sex differentiation, making it an excellent model for studying early oogenesis and folliculogenesis. We provide lines of evidence at morphological, molecular, and genetic levels for roles of factor in the germline α (Figla), an oocyte-specific transcription factor, in early zebrafish gonadogenesis. As in mammals, Figla/figla was also expressed in the gonads and its expression in the ovary was also restricted to early oocytes. Disruption of figla gene by CRISPR/Cas9 led to an all-male phenotype in the mutant. Detailed analysis of early gonadal development showed that the germ cells in the mutant were clustered in cysts and underwent meiosis, forming oocytes at prefollicular chromatin nucleolar (CN) stage (stage IA). However, the subsequent transition from cystic CN oocytes to individual follicular perinucleolar oocytes (stage IB) was blocked, resulting in an all-male phenotype in the mutant. The phenotype of figla mutant could not be rescued by estrogen treatment, in contrast to cyp19a1a mutant, and introduction of tp53 mutation also had no effect, unlike in fancd1 and fancl mutants. Transcriptome analysis revealed that many biological processes and pathways related to germ cell development, especially oogenesis, were upregulated in the presence of Figla and that the regulation of figla expression may involve heat shock proteins. Our results strongly suggest important roles for Figla in juvenile ovary development, especially the formation of individual follicles from cystic oocytes.

The transcriptomic signature of different sexes in two protogynous hermaphrodites: Insights into the molecular network underlying sex phenotype in fish

Sex differentiation is a puzzling problem in fish due to the variety of reproductive systems and the flexibility of their sex determination mechanisms. The Sparidae, a teleost family, reflects this remarkable diversity of sexual mechanisms found in fish. Our aim was to capture the transcriptomic signature of different sexes in two protogynous hermaphrodite sparids, the common pandora Pagellus erythrinus and the red porgy Pagrus pagrus in order to shed light on the molecular network contributing to either the female or the male phenotype in these organisms. Through RNA sequencing, we investigated sex-specific differences in gene expression in both species' brains and gonads. The analysis revealed common male and female specific genes/pathways between these protogynous fish. Whereas limited sex differences found in the brain indicate a sexually plastic tissue, in contrast, the great amount of sex-biased genes observed in gonads reflects the functional divergence of the transformed tissue to either its male or female character. Α common "crew" of well-known molecular players is acting to preserve either sex identity of the gonad in these fish. Lastly, this study lays the ground for a deeper understanding of the complex process of sex differentiation in two species with an evolutionary significant reproductive system.

Expression patterns of sex differentiation-related genes during gonadal sex change in the protogynous wrasse, Halichoeres trimaculatus

The three-spot wrasse, Halichoeres trimaculatus, can change sex from female to male (i.e. protogyny) due to sharp decrease in endogenous estrogen. During the sex change, ovarian tissue degenerates and testicular tissue arises newly. Finally, ovarian tissue disappears completely and replaces into mature testis. In order to predict the molecular mechanisms controlling the processes of sex change, we investigated the expression patterns of four genes (rspo1, figla, sox9b and amh), which have been thought to be associated with ovarian/testicular differentiation in vertebrates. Expression levels of rspo1 and figla, which play important roles for ovarian differentiation in vertebrates, were stable until the middle stage of the sex change, and subsequently down-regulated. Therefore, it was indicated that decrease in rspo1 and figla could result from ovarian degeneration. On the other hand, basis on the expression pattern, it was indicated that sox9b and amh, which are involved in testicular differentiation in vertebrates, were implicated in testicular formation and spermatogenesis during the sex change as well. The present results could be fundamental information for investigating the relationship between these factors and E2 depletion, which is crucial trigger for sex change.

Identification and analysis of the β-catenin1 gene in half-smooth tongue sole (Cynoglossus semilaevis)

β-catenin is a key signalling molecule in the canonical Wnt pathway, which plays a role in cell adhesion, embryogenesis and sex determination. However, little is known about its function in teleosts. We cloned and characterized the full-length β-catenin1 gene from half-smooth tongue sole (Cynoglossus semilaevis), which was designated CS-β-catenin1. The CS-β-catenin1 cDNA consists of 2,346 nucleotides and encodes a protein with 782 amino acids. Although CS-β-catenin1 was transcribed in the gonads of both sexes, the level was significantly higher in ovaries compared to testes. Furthermore, the mRNA level of CS-β-catenin1 was significantly upregulated at 160 days and constantly increased until 2 years of age. In situ hybridization revealed that CS-β-catenin1 mRNA was mainly localized in oocyte cells, especially in stage I, II and III oocytes. When CS-β-catenin1 expression was inhibited by injection of quercetin in the ovaries, levels of CS-Figla and CS-foxl2 mRNA were significantly down-regulated, and CS-dmrt1 was up-regulated, which suggested that CS-β-catenin1 is a potential upstream gene of CS-Figla and is involved in the development of the ovaries, i.e., folliculogenesis.

Bending Genders: The Biology of Natural Sex Change in Fish

Sexual fate is no longer seen as an irreversible deterministic switch set during early embryonic development but as an ongoing battle for primacy between male and female developmental trajectories. That sexual fate is not final and must be actively maintained via continuous suppression of the opposing sexual network creates the potential for flexibility into adulthood. In many fishes, sexuality is not only extremely plastic, but sex change is a usual and adaptive part of the life cycle. Sequential hermaphrodites begin life as one sex, changing sometime later to the other, and include species capable of protandrous (male-to-female), protogynous (female-to-male), or serial (bidirectional) sex change. Natural sex change involves coordinated transformations across multiple biological systems, including behavioural, anatomical, neuroendocrine, and molecular axes. We here review the biological processes underlying this amazing transformation, focussing particularly on its molecular basis, which remains poorly understood, but where new genomic technologies are significantly advancing our understanding of how sex change is initiated and progressed at the molecular level. Knowledge of how a usually committed developmental process remains plastic in sequentially hermaphroditic fishes is relevant to understanding the evolution and functioning of sexual developmental systems in vertebrates generally, as well as pathologies of sexual development in humans.

Sexual Fate Reprogramming in the Steroid-Induced Bi-Directional Sex Change in the Protogynous Orange-Spotted Grouper, Epinephelus coioides

Androgen administration has been widely used for masculinization in fish. The mechanism of the sex change in sexual fate regulation is not clear. Oral administration or pellet implantation was applied. We orally applied an aromatase inhibitor (AI, to decrease estrogen levels) and 17α-methyltestosterone (MT, to increase androgen levels) to induce masculinization to clarify the mechanism of the sex change in the protogynous orange-spotted grouper. After 3 mo of AI/MT administration, male characteristics were observed in the female-to-male sex change fish. These male characteristics included increased plasma 11-ketotestosterone (11-KT), decreased estradiol (E2) levels, increased male-related gene (dmrt1, sox9, and cyp11b2) expression, and decreased female-related gene (figla, foxl2, and cyp19a1a) expression. However, the reduced male characteristics and male-to-female sex change occurred after AI/MT-termination in the AI- and MT-induced maleness. Furthermore, the MT-induced oocyte-depleted follicle cells (from MT-implantation) had increased proliferating activity, and the sexual fate in a portion of female gonadal soma cells was altered to male function during the female-to-male sex change. In contrast, the gonadal soma cells were not proliferative during the early process of the male-to-female sex change. Additionally, the male gonadal soma cells did not alter to female function during the male-to-female sex change in the AI/MT-terminated fish. After MT termination in the male-to-female sex-changed fish, the differentiated male germ cells showed increased proliferating activities together with dormancy and did not show characteristics of both sexes in the early germ cells. In conclusion, these findings indicate for the first time in a single species that the mechanism involved in the replacement of soma cells is different between the female-to-male and male-to-female sex change processes in grouper. These results also demonstrate that sexual fate determination (secondary sex determination) is regulated by endogenous sex steroid levels.

Large-scale transcriptome sequencing reveals novel expression patterns for key sex-related genes in a sex-changing fish

Background

Teleost fishes exhibit remarkably diverse and plastic sexual developmental patterns. One of the most astonishing is the rapid socially controlled female-to-male (protogynous) sex change observed in bluehead wrasses (Thalassoma bifasciatum). Such functional sex change is widespread in marine fishes, including species of commercial importance, yet its underlying molecular basis remains poorly explored.

Methods

RNA sequencing was performed to characterize the transcriptomic profiles and identify genes exhibiting sex-biased expression in the brain (forebrain and midbrain) and gonads of bluehead wrasses. Functional annotation and enrichment analysis were carried out for the sex-biased genes in the gonad to detect global differences in gene products and genetic pathways between males and females.

Results

Here we report the first transcriptomic analysis for a protogynous fish. Expression comparison between males and females reveals a large set of genes with sex-biased expression in the gonad, but relatively few such sex-biased genes in the brain. Functional annotation and enrichment analysis suggested that ovaries are mainly enriched for metabolic processes and testes for signal transduction, particularly receptors of neurotransmitters and steroid hormones. When compared to other species, many genes previously implicated in male sex determination and differentiation pathways showed conservation in their gonadal expression patterns in bluehead wrasses. However, some critical female-pathway genes (e.g., rspo1 and wnt4b) exhibited unanticipated expression patterns. In the brain, gene expression patterns suggest that local neurosteroid production and signaling likely contribute to the sex differences observed.

Conclusions

Expression patterns of key sex-related genes suggest that sex-changing fish predominantly use an evolutionarily conserved genetic toolkit, but that subtle variability in the standard sex-determination regulatory network likely contributes to sexual plasticity in these fish. This study not only provides the first molecular data on a system ideally suited to explore the molecular basis of sexual plasticity and tissue re-engineering, but also sheds some light on the evolution of diverse sex determination and differentiation systems.

Electronic supplementary material

The online version of this article (doi:10.1186/s13293-015-0044-8) contains supplementary material, which is available to authorized users.