Divergent expression patterns of Sox9 duplicates in teleosts indicate a lineage specific subfunctionalization

Expansion and Functional Diversification of Long-Wavelength-Sensitive Opsin in Anabantoid Fishes

Gene duplication is one of the most important sources of novel genotypic diversity and the subsequent evolution of phenotypic diversity. Determining the evolutionary history and functional changes of duplicated genes is crucial for a comprehensive understanding of adaptive evolution. The evolutionary history of visual opsin genes is very dynamic, with repeated duplication events followed by sub- or neofunctionalization. While duplication of the green-sensitive opsins rh2 is common in teleost fish, fewer cases of multiple duplication events of the red-sensitive opsin lws are known. In this study, we investigate the visual opsin gene repertoire of the anabantoid fishes, focusing on the five lws opsin genes found in the genus Betta. We determine the evolutionary history of the lws opsin gene by taking advantage of whole-genome sequences of nine anabantoid species, including the newly assembled genome of Betta imbellis. Our results show that at least two independent duplications of lws occurred in the Betta lineage. The analysis of amino acid sequences of the lws paralogs of Betta revealed high levels of diversification in four of the seven transmembrane regions of the lws protein. Amino acid substitutions at two key-tuning sites are predicted to lead to differentiation of absorption maxima (λmax) between the paralogs within Betta. Finally, eye transcriptomics of B. splendens at different developmental stages revealed expression shifts between paralogs for all cone opsin classes. The lws genes are expressed according to their relative position in the lws opsin cluster throughout ontogeny. We conclude that temporal collinearity of lws expression might have facilitated subfunctionalization of lws in Betta and teleost opsins in general.

Insights into Craniofacial Development and Anomalies: Exploring Fgf Signaling in Zebrafish Models

PURPOSE OF REVIEW

To illustrate the value of using zebrafish to understand the role of the Fgf signaling pathway during craniofacial skeletal development under normal and pathological conditions.

RECENT FINDINGS

Recent data obtained from studies on zebrafish have demonstrated the genetic redundancy of Fgf signaling pathway and have identified new molecular partners of this signaling during the early stages of craniofacial skeletal development. Studies on zebrafish models demonstrate the involvement of the Fgf signaling pathway at every stage of craniofacial development. They particularly emphasize the central role of Fgf signaling pathway during the early stages of the development, which significantly impacts the formation of the various structures making up the craniofacial skeleton. This partly explains the craniofacial abnormalities observed in disorders associated with FGF signaling. Future research efforts should focus on investigating zebrafish Fgf signaling during more advanced stages, notably by establishing zebrafish models expressing mutations responsible for diseases such as craniosynostoses.

Increased water temperature contributes to a chondrogenesis response in the eyes of spotted wolffish

Adult vertebrate cartilage is usually quiescent. Some vertebrates possess ocular scleral skeletons composed of cartilage or bone. The morphological characteristics of the spotted wolffish (Anarhichas minor) scleral skeleton have not been described. Here we assessed the scleral skeletons of cultured spotted wolffish, a globally threatened marine species. The healthy spotted wolffish we assessed had scleral skeletons with a low percentage of cells staining for the chondrogenesis marker sex-determining region Y-box (Sox) 9, but harboured a population of intraocular cells that co-express immunoglobulin M (IgM) and Sox9. Scleral skeletons of spotted wolffish with grossly observable eye abnormalities displayed a high degree of perochondrial activation as evidenced by cellular morphology and expression of proliferating cell nuclear antigen (PCNA) and phosphotyrosine. Cells staining for cluster of differentiation (CD) 45 and IgM accumulated around sites of active chondrogenesis, which contained cells that strongly expressed Sox9. The level of scleral chondrogenesis and the numbers of scleral cartilage PCNA positive cells increased with the temperature of the water in which spotted wolffish were cultured. Our results provide new knowledge of differing Sox9 spatial tissue expression patterns during chondrogenesis in normal control and ocular insult paradigms. Our work also provides evidence that spotted wolffish possess an inherent scleral chondrogenesis response that may be sensitive to temperature. This work also advances the fundamental knowledge of teleost ocular skeletal systems.

Ovary of Zebrafish during Spawning Season: Ultrastructure and Immunohistochemical Profiles of Sox9 and Myostatin

This study sought to examine the ovarian cellular and stromal components of the zebrafish (Danio rerio) throughout the spawning season using light and electron microscopic tools. The ovaries of zebrafish showed oocytes in all stages of follicular development and degeneration (atresia). Six stages of oogenesis were demonstrated: oogonia, early oocytes, late oocytes, vacuolated follicles, the yolk globule stage (vitellogenesis), and mature follicles. The SOX9 protein was expressed in the ooplasm of the primary and previtellogenic oocytes and the theca cell layer of the mature follicles. Myostatin was expressed in the granulosa and theca cells. Many stem cells in the ovarian stroma expressed myostatin and SOX9. During the spawning season, the EM results indicated that the zona radiata increased in thickness and was crossed perpendicularly by pore canals that contained processes from both oocytes and zona granulosa. The granulosa cells contained many mitochondria, rER, sER, and vesicles. Meanwhile, the thecal layer consisted of fibroblast-like cells. Atretic follicles could be demonstrated that involved both oocytes and their follicular walls. Several types of cells were distinguished in the ovarian stroma, including mast cells, telocytes, lymphocytes, fibroblasts, endocrine cells, macrophages, adipocytes, dendritic cells, and steroidogenic (stromal) cells. The ovary of the zebrafish serves as a model to investigate follicular development.

Transcriptome characterization of gonadal sex differentiation in Pacific bluefin tuna, Thunnus orientalis (Temminck et Schlegel)

Tunas (genus Thunnus) are one of the most ecologically and commercially important fish worldwide. To establish a biological basis for reproduction in this globally essential species, we have recently studied crucial reproductive aspects of the Pacific bluefin tuna (T. orientalis; PBT), as a model of tuna species, based on our closed-cycle aquaculture technology. In this study, we clarified the global expression profile of the genes regulating gonadal sex differentiation in PBT, as this developmental process is vital to sexual reproduction. Based on the results of our comparative (RNA-sequencing) and temporal (qRT-PCR) transcriptome analyses using the updated genome dataset, we propose the molecular mechanisms of gonadal sex differentiation in PBT. In female gonads, foxl2 and cyp19a1a (coding aromatase) are expressed at the onset of sex differentiation. Active aromatase-mediated estrogen biosynthesis, which includes positive regulation of cyp19a1a expression by Foxl2, induces ovarian differentiation. By contrast, dmrt1 and gsdf are upregulated in differentiating male gonads lacking active estrogen synthesis. Dmrt1 and Gsdf would mainly promote testicular differentiation. Furthermore, androgen biosynthesis is upregulated in differentiating male gonad. Endogenous androgens may also be vital to testicular differentiation. This study provides the first comprehensive data clarifying the molecular basis for gonadal sex differentiation in tunas.

Inhibition of the NF-κB signaling pathway affects gonadal differentiation and leads to male bias in Paramisgurnus dabryanus

In recent years, sex-controlled breeding has emerged as an effective strategy to enhance the yields of economic animals with different growth characteristics, while increasing the economic benefits of aquaculture. It is known that the NF-κB pathway participates in gonadal differentiation and reproduction. Therefore, we used the large-scale loach as a research model for the present study and selected an effective inhibitor of the NF-κB signaling pathway (QNZ). This, to investigates the impacts of the NF-κB signaling pathway on gonadal differentiation during a critical period of gonad development and after maturation. Simultaneously, the sex ratio bias and the reproductive capacities of adult fish were analyzed. Our results indicated that the inhibition of the NF-κB signaling pathway influenced the expression of genes related to gonad development, regulated the gene expression related to the brain-gonad-liver axis of juvenile loaches, and finally impacted the gonadal differentiation of the large-scale loach and promoted a male-biased sex ratio. Meanwhile, high QNZ concentrations affected the reproductive abilities of adult loaches and inhibited the growth performance of offspring. Thus, our results deepened the exploration of sex control in fish and provided a certain research basis for the sustainable development of the aquaculture industry.

De novo gonad transcriptome analysis of elongate loach (Leptobotia elongata) provides novel insights into sex-related genes

Elongate loach (Leptobotia elongata) is endemic in middle and upper reaches of the Yangtze River in China. Because of many anthropogenic factors such as overfishing and dam construction, the loach has become a highly endangered species. So far, the genomic resources which benefit for species conservation and utilization are still lacking in elongate loach. Therefore, the first gonad transcriptome analysis of the loach was conducted in this study, providing novel insights into sex-related genes. A total of 286,800,660 clean reads with a total length of 42.02 Gb were obtained. 18,975 differentially expressed genes (DEGs) were identified, where 12,976 DEGs, especially Sox9a, Sox9b, Igf2 and Fgfr2, were upregulated in the testis, and 5999 DEGs, especially Zp3, Eg2, Plk1, Ccnb1, Cdc20 and Mos, were upregulated in the ovary. Meanwhile, some testis-specific genes (i.e. Cald1, Atp1a, Muc2 and Ca2) and ovary-specific genes (i.e. Ca4, Tuba, Acp5, Ccna, Larp6 and Nop4) were identified and verified. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses of DEGs, we found a series of enrichment pathways related to reproduction in elongate loach, such as the MAPK signaling pathway, oxytocin signaling pathway and oocyte meiosis pathway. Twelve DEGs were randomly selected to verify RNA-seq results by qPCR. In conclusion, this study provides a data source to study the molecular characteristics and regulatory mechanisms of sex-related genes in elongate loach, which has a potential to improve the resource protection and aquaculture production of the loach.

The Evolution of Sox Gene Repertoires and Regulation of Segmentation in Arachnids

The Sox family of transcription factors regulates many processes during metazoan development, including stem cell maintenance and nervous system specification. Characterizing the repertoires and roles of these genes can therefore provide important insights into animal evolution and development. We further characterized the Sox repertoires of several arachnid species with and without an ancestral whole-genome duplication and compared their expression between the spider Parasteatoda tepidariorum and the harvestman Phalangium opilio. We found that most Sox families have been retained as ohnologs after whole-genome duplication and evidence for potential subfunctionalization and/or neofunctionalization events. Our results also suggest that Sox21b-1 likely regulated segmentation ancestrally in arachnids, playing a similar role to the closely related SoxB gene, Dichaete, in insects. We previously showed that Sox21b-1 is required for the simultaneous formation of prosomal segments and sequential addition of opisthosomal segments in P. tepidariorum. We studied the expression and function of Sox21b-1 further in this spider and found that although this gene regulates the generation of both prosomal and opisthosomal segments, it plays different roles in the formation of these tagmata reflecting their contrasting modes of segmentation and deployment of gene regulatory networks with different architectures.

Transcriptome Profiling Reveals the Sexual Dimorphism of Gene Expression Patterns during Gonad Differentiation in the Half-Smooth Tongue Sole (Cynoglossus semilaevis)

The half-smooth tongue sole (Cynoglossus semilaevis), one of the most economically-important fish species in China, exhibits sexually dimorphic growth. An understanding of sex-related gene expression patterns in the tongue sole may inform sex regulation and breeding processes that increase fish production. However, the gene expression patterns during gonad development in the tongue sole remain unknown. In this study, transcriptome sequencing analyses were performed during gonad differentiation in the tongue sole, namely, at 62 days post-hatching (dph), 100 dph, 120 dph, and 150 dph. Differentially expressed genes associated with sex differentiation and gonad development were identified at each time point. Trend analysis showed that gene expression patterns varied over time. These expression patterns either explained common, non-sexually-dimorphic features or indicated significant sexual dimorphism. Transcript structure analyses identified both sex and time differences among samples. This study investigated the time-dependent expression patterns of several sex-related genes, including Dmrt1, Amh, Foxl2, aromatase encoding gene, Esr, and the Sox gene family, during gonad differentiation in the tongue sole. These results might clarify the significant sexual differences during early development in the tongue sole and might provide insight into the mechanisms controlling sex differentiation and development.

Shedding new light on early sex determination in zebrafish

In contrast to established zebrafish gene annotations, the question of sex determination has still not been conclusively clarified for developing zebrafish, Danio rerio, larvae, 28 dpf or earlier. Recent studies indicate polygenic sex determination (PSD), with the genes being distributed throughout the genome. Early genetic markers of sex in zebrafish help unravel co-founding sex-related differences to apply to human health and environmental toxicity studies. A qPCR-based method was developed for six genes: cytochrome P450, family 17, subfamily A, polypeptide 1 (cyp17a1); cytochrome P450, family 19, subfamily A, polypeptide 1a (cyp19a1a); cytochrome P450, family 19, subfamily A, polypeptides 1b (cyp19a1b); vitellogenin 1 (vtg1); nuclear receptor subfamily 0, group B, member 1 (nr0b1), sry (sex-determining region Y)-box 9b (sox9b) and actin, beta 1 (actb1), the reference gene. Sry-box 9a (Sox9a), insulin-like growth factor 3 (igf3) and double sex and mab-3 related transcription factor 1 (dmrt1), which are also known to be associated with sex determination, were used in gene expression tests. Additionally, Next-Generation-Sequencing (NGS) sequenced the genome of two adult female and male and two juveniles. PCR analysis of adult zebrafish revealed sex-specific expression of cyp17a1, cyp19a1a, vtg1, igf3 and dmrt1, the first four strongly expressed in female zebrafish and the last one highly expressed in male conspecifics. From NGS, nine female and four male-fated genes were selected as novel for assessing zebrafish sex, 28 dpf. Differences in transcriptomes allowed allocation of sex-specific genes also expressed in juvenile zebrafish.

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.

Selenomethionine exposure affects chondrogenic differentiation and bone formation in Japanese medaka (Oryzias latipes)

Excess selenium entering the aquatic environment from anthropogenic activities has been associated with developmental abnormalities in fish including skeletal deformities of the head and spine. However, mechanisms of this developmental toxicity have not been well-characterized. In this study, Japanese medaka (Oryzias latipes) embryos were exposed to seleno-l-methionine (Se-Met) in a range of concentrations. Gene expression was evaluated for sex-determining region Y (SRY)-related box (Sox9a and Sox9b), runt-related transcription factor 2 (Runx2), and melatonin receptor (Mtr). Alterations in the length of Meckel's cartilage, tail curvature, and decreased calcification were observed in skeletal stains at 10- and 22-days post-fertilization (dpf). Embryonic exposure of Osterix-mCherry transgenic medaka resulted in fewer teeth. Sox9a and Sox9b were up-regulated, while Runx2 and Mtr were down-regulated by Se-Met prior to hatch. Whole mount in situ hybridization (WISH) localized gene expression to areas observed to be affected in vivo. In addition, Se-Met exposures of a Mtr morpholino (Mtr-MO) as well as Luzindole exposed embryos developed similar skeletal malformations, supporting involvement of Mtr. These findings demonstrate that Se-Met modulates expression of key genes involved in chondrogenic differentiation and bone formation during development.

Comparative transcriptome analysis of male and female gonads reveals sex-biased genes in spotted scat (Scatophagus argus)

Scatophagus argus is a new emerging aquaculture fish in East and Southeast Asia. To date, research on reproductive development and regulation in S. argus is lacking. Additionally, genetic and genomic information about reproduction, such as gonadal transcriptome data, is also lacking. Herein, we report the first gonadal transcriptomes of S. argus and identify genes potentially involved in reproduction and gonadal development. A total of 136,561 unigenes were obtained by sequencing of testes (n = 3) and ovaries (n = 3) at stage III. Genes upregulated in males and females known to be involved in gonadal development and gametogenesis were identified, including male-biased dmrt1, amh, gsdf, wt1a, sox9b, and nanos2, and female-biased foxl2, gdf9, bmp15, sox3, zar1, and figla. Serum estradiol-17β and 11-ketotestosterone levels were biased in female and male fish, respectively. Sexual dimorphism of serum steroid hormone levels were interpreted after expression analysis of 20 steroidogenesis-related genes, including cyp19a1a and cyp11b2. This gonadal transcript dataset will help investigate functional genes related to reproduction in S. argus.

Identification of genes involved in gonadal sex differentiation and the dimorphic expression pattern in Takifugu rubripes gonad at the early stage of sex differentiation

Quantifying the expression of mRNAs in the gonads at the critical stage of molecular sex differentiation stage might help to clarify the regulatory network during early sex differentiation and provide new information on the role of sex-related genes in gonadal function. In this study, transcriptomic analysis of sex-related genes expression profiles in fugu gonads at 60 and 90 days after hatching (dah) was conducted firstly, and a total of 112,504,991 clean reads, encompassing 28.35 Gb of sequences were retrieved. Twenty-three thousand eight hundred ten genes were found to be expressed in juvenile fugu gonads, and we mainly focused on the differentially expressed genes that have the potential to be involved in the gonadal sex differentiation. For 60-dah juveniles, we identified 1014 genes that were upregulated in the ovary and 1570 that were upregulated in the testis. For 90-dah juveniles, we identified 1287 genes that were upregulated in the ovary and 1500 that were upregulated in the testis. The dimorphic expression patterns of 15 genes in gonads at 30 and 40 dah were further investigate using qPCR. Cyp11b and star were expressed at higher levels in XY than in XX, while cyp11a1 and cyp19a1a were expressed at higher levels in XX than in XY at 30 dah. At 40 dah, the levels of gsdf, dmrt1, dmrt3, cyp11c1, star, and hsd3b expression were higher in XY, while the levels of foxl2, cyp19a1a, wnt9b, and foxD4 expression were higher in XX. Sox9, cyp11a1, cyp17a1, cyp17a2, and nr5a2 were expressed at similar levels in XX and XY at 40 dah. This is the first report of gonadal transcriptome of fugu at early sex differentiation stage, and our results provide an archive for further study on molecular mechanism underlying sex differentiation in this species.

A role for SOX9 in post-transcriptional processes: insights from the amphibian oocyte

Sox9 is a member of the gene family of SOX transcription factors, which is highly conserved among vertebrates. It is involved in different developmental processes including gonadogenesis. In all amniote species examined thus far, Sox9 is expressed in the Sertoli cells of the male gonad, suggesting an evolutionarily conserved role in testis development. However, in the anamniotes, fishes and amphibians, it is also expressed in the oocyte but the significance of such an expression remains to be elucidated. Here, we have investigated the nuclear localization of the SOX9 protein in the oocyte of three amphibian species, the urodelan Pleurodeles waltl, and two anurans, Xenopus laevis and Xenopus tropicalis. We demonstrate that SOX9 is associated with ribonucleoprotein (RNP) transcripts of lampbrush chromosomes in an RNA-dependent manner. This association can be visualized by Super-resolution Structured Illumination Microscopy (SIM). Our results suggest that SOX9, known to bind DNA, also carries an additional function in the posttranscriptional processes. We also discuss the significance of the acquisition or loss of Sox9 expression in the oocyte during evolution at the transition between anamniotes and amniotes.

Expansion by whole genome duplication and evolution of the sox gene family in teleost fish

It is now recognized that several rounds of whole genome duplication (WGD) have occurred during the evolution of vertebrates, but the link between WGDs and phenotypic diversification remains unsolved. We have investigated in this study the impact of the teleost-specific WGD on the evolution of the sox gene family in teleostean fishes. The sox gene family, which encodes for transcription factors, has essential role in morphology, physiology and behavior of vertebrates and teleosts, the current largest group of vertebrates. We have first redrawn the evolution of all sox genes identified in eleven teleost genomes using a comparative genomic approach including phylogenetic and synteny analyses. We noticed, compared to tetrapods, an important expansion of the sox family: 58% (11/19) of sox genes are duplicated in teleost genomes. Furthermore, all duplicated sox genes, except sox17 paralogs, are derived from the teleost-specific WGD. Then, focusing on five sox genes, analyzing the evolution of coding and non-coding sequences, as well as the expression patterns in fish embryos and adult tissues, we demonstrated that these paralogs followed lineage-specific evolutionary trajectories in teleost genomes. This work, based on whole genome data from multiple teleostean species, supports the contribution of WGDs to the expansion of gene families, as well as to the emergence of genomic differences between lineages that might promote genetic and phenotypic diversity in teleosts.

Genome-wide identification and expression profiling of the SOX gene family in a bivalve mollusc Patinopecten yessoensis

SOX family is composed of transcription factors that play vital roles in various developmental processes. Comprehensive understanding on evolution of the SOX family requires full characterization of SOX genes in different phyla. Mollusca is the second largest metazoan phylum, but till now, systematic investigation on the SOX family is still lacking in this phylum. In this study, we conducted genome-wide identification of the SOX family in Yesso scallop Patinopecten yessoensis and profiled their tissue distribution and temporal expression patterns in the ovaries and testes during gametogenesis. Seven SOX genes were identified, including SOXB1, B2, C, D, E, F and H, representing the first record in protostomes with SOX members identical to that proposed to exist in the last common ancestor of chordates. Genomic structure analysis identified relatively conserved exon-intron structures, accompanied by intron insertion. Quantitative real-time PCR analysis revealed possible involvement of scallop SOX in various functions, including neuro-sensory cell differentiation, hematopoiesis, myogenesis and gametogenesis. This study represents the first systematic characterization of SOX gene family in Mollusca. It will assist in a better understanding of the evolution and function of SOX family in metazoans.

Non-canonical expression patterns and evolutionary rates of sex-biased genes in a seasonal fish

Sex determination is a highly variable process that utilizes many different mechanisms to initiate the cascade of differentiation processes. The molecular pathways controlling sexual development are less conserved than previously assumed, and appear to require active maintenance in some species; indeed, the developmental decision of gonad phenotype in gonochoristic species is not fixed at an early developmental stage. Much of the knowledge about sex determination mechanisms was derived from research on gonochoristic, non-seasonal breeders. In this study, the transcriptome of resting adult gonads of a seasonal breeder, the endangered Iberian cyprinid fish Squalius pyrenaicus, was analyzed to assess the expression patterns and evolutionary rates of sex-biased genes that could be involved in maintenance of gonad identity as well as in sex determination. Remarkably, some crucial female genes-such as aromatase cyp19a1a, estrogen receptor esr1a, and foxl2-were expressed more abundantly in S. pyrenaicus testis than in ovaries. Moreover, contrary to the higher evolutionary rate changes observed in male-biased genes, higher d_N /d_S ratios were observed for female-biased genes than for male-biased genes in S. pyrenaicus. These results help unravel the impact of seasonality in sex determination mechanisms and the evolution of genes, and highlight the need to study fish at different gonadal maturation states to understand the function of sex-biased genes. Mol. Reprod. Dev. 83: 1102-1115, 2016. © 2016 Wiley Periodicals, Inc.

SOXE neofunctionalization and elaboration of the neural crest during chordate evolution

During chordate evolution, two genome-wide duplications facilitated acquisition of vertebrate traits, including emergence of neural crest cells (NCCs), in which neofunctionalization of the duplicated genes are thought to have facilitated development of craniofacial structures and the peripheral nervous system. How these duplicated genes evolve and acquire the ability to specify NC and their derivatives are largely unknown. Vertebrate SoxE paralogues, most notably Sox9/10, are essential for NC induction, delamination and lineage specification. In contrast, the basal chordate, amphioxus, has a single SoxE gene and lacks NC-like cells. Here, we test the hypothesis that duplication and divergence of an ancestral SoxE gene may have facilitated elaboration of NC lineages. By using an in vivo expression assay to compare effects of AmphiSoxE and vertebrate Sox9 on NC development, we demonstrate that all SOXE proteins possess similar DNA binding and homodimerization properties and can induce NCCs. However, AmphiSOXE is less efficient than SOX9 in transactivation activity and in the ability to preferentially promote glial over neuronal fate, a difference that lies within the combined properties of amino terminal and transactivation domains. We propose that acquisition of AmphiSoxE expression in the neural plate border led to NCC emergence while duplication and divergence produced advantageous mutations in vertebrate homologues, promoting elaboration of NC traits.

Identification, visualization and clonal analysis of intestinal stem cells in fish

Recently, a stochastic model of symmetrical stem cell division followed by neutral drift has been proposed for intestinal stem cells (ISCs), which has been suggested to represent the predominant mode of stem cell progression in mammals. In contrast, stem cells in the retina of teleost fish show an asymmetric division mode. To address whether the mode of stem cell division follows phylogenetic or ontogenetic routes, we analysed the entire gastrointestinal tract of the teleost medaka (Oryzias latipes). X-ray microcomputed tomography shows a correlation of 3D topography with the functional domains. Analysis of ISCs in proliferation assays and via genetically encoded lineage tracing highlights a stem cell niche in the furrow between the long intestinal folds that is functionally equivalent to mammalian intestinal crypts. Stem cells in this compartment are characterized by the expression of homologs of mammalian ISC markers – sox9, axin2 and lgr5 – emphasizing the evolutionary conservation of the Wnt pathway components in the stem cell niche of the intestine. The stochastic, sparse initial labelling of ISCs ultimately resulted in extended labelled or unlabelled domains originating from single stem cells in the furrow niche, contributing to both homeostasis and growth. Thus, different modes of stem cell division co-evolved within one organism, and in the absence of physical isolation in crypts, ISCs contribute to homeostatic growth.

Summary: Adult medaka intestinal stem cells (ISCs) proliferate within a niche functionally equivalent to that in the mammal. Like mammalian ISCs, but unlike medaka retinal stem cells, their mode of division is largely symmetric.

Dimerization and Transactivation Domains as Candidates for Functional Modulation and Diversity of Sox9

Sox9 plays an important role in a large variety of developmental pathways in vertebrates. It is composed of three domains: high-mobility group box (HMG box), dimerization (DIM) and transactivation (TAD). One of the main processes for regulation and variability of the pathways involving Sox9 is the self-gene expression regulation of Sox9. However, the subsequent roles of the Sox9 domains can also generate regulatory modulations. Studies have shown that TADs can bind to different types of proteins and its function seems to be influenced by DIM. Therefore, we hypothesized that both domains are directly associated and can be responsible for the functional variability of Sox9. We applied a method based on a broad phylogenetic context, using sequences of the HMG box domain, to ensure the homology of all the Sox9 copies used herein. The data obtained included 4,921 sequences relative to 657 metazoan species. Based on coevolutionary and selective pressure analyses of the Sox9 sequences, we observed coevolutions involving DIM and TADs. These data, along with the experimental data from literature, indicate a functional relationship between these domains. Moreover, DIM and TADs may be responsible for the functional plasticity of Sox9 because they are more tolerant for molecular changes (higher Ka/Ks ratio than the HMG box domain). This tolerance could allow a differential regulation of target genes or promote novel targets during transcriptional activation. In conclusion, we suggest that DIM and TADs functional association may regulate differentially the target genes or even promote novel targets during transcription activation mediated by Sox9 paralogs, contributing to the subfunctionalization of Sox9a and Sox9b in teleosts.

Expression analysis of Sox9 genes during annual reproductive cycles in gonads and after nanodelivery of LHRH in Clarias batrachus

Transcription factor Sox9 plays a crucial role in determining the fate of several cell types and is a primary factor in regulation of gonadal development. Present study reports full-length cDNA sequence of Sox9a gene and partial coding sequence (cds) of Sox9b (two duplicate orthologs of Sox9 gene) from Clarias batrachus. The coding region of Sox9a gene encoded a peptide of 460 amino acids. The partial cds of Sox9b with the length of 558bp was amplified that codes for 186 amino acids. Quantitative Real-time PCR (qRT-PCR) analysis revealed that Sox9a and Sox9b mRNA expression was significantly higher in gonads and brain tissues. Furthermore Sox9a and Sox9b mRNA expression levels were high during preparatory and pre-spawning phases and decreased gradually with onset of spawning and post-spawning phases of reproductive cycles in gonads. Chitosan nanoconjugated sLHRH (CsLHRH) of particle size 133.0nm and zeta potential of 34.3mV were synthesized and evaluated against naked sLHRH (salmon luteinizing hormone-releasing hormone). The entrapment efficiency of CsLHRH was 63%. CsLHRH nanoparticles increased the expression level of Sox9 transcripts in gonads and steroid hormonal levels in blood of male and female. Thus, our findings clearly indicate that Sox9 genes play essential role during seasonal variation of gonads. Besides, the current study reports that sustained release delivery-system will be helpful for proper gonadal development of fish. To the best of our knowledge, till date no study has been reported on nanodelivery of sLHRH and their effect on reproductive gene expression in fish.

Molecular cloning and expression analysis of dmrt1 and sox9 during gonad development and male reproductive cycle in the lambari fish, Astyanax altiparanae

BACKGROUND

The dmrt1 and sox9 genes have a well conserved function related to testis formation in vertebrates, and the group of fish presents a great diversity of species and reproductive mechanisms. The lambari fish (Astyanax altiparanae) is an important Neotropical species, where studies on molecular level of sex determination and gonad maturation are scarce.

METHODS

Here, we employed molecular cloning techniques to analyze the cDNA sequences of the dmrt1 and sox9 genes, and describe the expression pattern of those genes during development and the male reproductive cycle by qRT-PCR, and related to histology of the gonad.

RESULTS

Phylogenetic analyses of predicted amino acid sequences of dmrt1 and sox9 clustered A. altiparanae in the Ostariophysi group, which is consistent with the morphological phylogeny of this species. Studies of the gonad development revealed that ovary formation occurred at 58 days after hatching (dah), 2 weeks earlier than testis formation. Expression studies of sox9 and dmrt1 in different tissues of adult males and females and during development revealed specific expression in the testis, indicating that both genes also have a male-specific role in the adult. During the period of gonad sex differentiation, dmrt1 seems to have a more significant role than sox9. During the male reproductive cycle dmrt1 and sox9 are down-regulated after spermiation, indicating a role of these genes in spermatogenesis.

CONCLUSIONS

For the first time the dmrt1 and sox9 were cloned in a Characiformes species. We show that both genes have a conserved structure and expression, evidencing their role in sex determination, sex differentiation and the male reproductive cycle in A. altiparanae. These findings contribute to a better understanding of the molecular mechanisms of sex determination and differentiation in fish.

Genetics of sexual development: an evolutionary playground for fish

Teleost fishes are the most species-rich clade of vertebrates and feature an overwhelming diversity of sex-determining mechanisms, classically grouped into environmental and genetic systems. Here, we review the recent findings in the field of sex determination in fish. In the past few years, several new master regulators of sex determination and other factors involved in sexual development have been discovered in teleosts. These data point toward a greater genetic plasticity in generating the male and female sex than previously appreciated and implicate novel gene pathways in the initial regulation of the sexual fate. Overall, it seems that sex determination in fish does not resort to a single genetic cascade but is rather regulated along a continuum of environmental and heritable factors.

Responsiveness of four gender-specific genes, figla, foxl2, scp3 and sox9a to 17α-ethinylestradiol in adult rare minnow Gobiocypris rarus

Proteins encoded by figla, foxl2, scp3 and sox9a play important roles in gonad differentiation and reproduction. In the present study, we aimed to determine the responsiveness of figla, foxl2, scp3 and sox9a to 17α-ethinylestradiol (EE2) in the gonads of adult Gobiocypris rarus. Full-length cDNAs of figla, scp3 and sox9a were cloned and characterized by RT-PCR and RACE methods. Expression patterns in adult tissues were investigated. Results indicated that figla was predominantly expressed in adult ovaries and scp3 was restrictively expressed in the male testes and sox9a was principally expressed in the brains of both genders and the testes of males. Gene expression profiles of figla, foxl2, scp3 and sox9a were analyzed in the gonads of adult G. rarus exposed to EE2 at 1, 5, 25, and 125ng/L for 3 and 6days. Three-day EE2 treatment at 1-125ng/L all caused a significant increase of figla transcript in testes and foxl2 transcript in ovaries. However, six-day EE2 exposure at 1-125ng/L repressed figla and scp3 transcript in testes and foxl2 transcript in ovaries. The present study indicates that the testicular transcripts of figla and scp3 in males and the ovarian foxl2 transcript in females have high responsiveness to EE2 and they can be used as sensitive molecular biomarkers for early warning to monitor the environmental estrogenic chemicals in fresh water environment. The present study also suggests that the effective EE2 dosage for feminization in male G. rarus might be at least 25ng/L.

Molecular players involved in temperature-dependent sex determination and sex differentiation in Teleost fish

The molecular mechanisms that underlie sex determination and differentiation are conserved and diversified. In fish species, temperature-dependent sex determination and differentiation seem to be ubiquitous and molecular players involved in these mechanisms may be conserved. Although how the ambient temperature transduces signals to the undifferentiated gonads remains to be elucidated, the genes downstream in the sex differentiation pathway are shared between sex-determining mechanisms. In this paper, we review recent advances on the molecular players that participate in the sex determination and differentiation in fish species, by putting emphasis on temperature-dependent sex determination and differentiation, which include temperature-dependent sex determination and genetic sex determination plus temperature effects. Application of temperature-dependent sex differentiation in farmed fish and the consequences of temperature-induced sex reversal are discussed.

Osterix/Sp7 regulates biomineralization of otoliths and bone in medaka (Oryzias latipes)

Osterix/Sp7 is a zinc finger transcription factor and critical regulator of osteoblast differentiation, maturation and activity. Osterix expression has also been described in non-skeletal tissues but functional analyses are lacking. In the present study, we show that in the teleost model medaka, osterix is present as two alternatively spliced transcripts, osx_tv1 and osx_tv2. Knock-down of osx_tv1 and/or osx_tv2 results in mineralization loss in early intramembranous bones while cartilage formation is mostly unaffected. Formation of the parasphenoid, the earliest mineralized bone in the medaka skeleton, is impaired and fails to recover at later stages. Ossification of later bones, such as the operculum and cleithrum, is delayed but recovers during further development. In the axial skeleton, formation of the neural arches and centra is strongly delayed. In vivo analyses using osterix:nlGFP and osteocalcin:GFP transgenic medaka and whole mount in situ hybridization suggest that bone defects observed after knock-down of osterix are caused by a delay of osteoblast maturation and activity. Furthermore, we analyzed expression profile and function of osterix during ear and otolith formation. We show that osterix is expressed in otic placodes at the otic vesicle stage and that its knock-down results in a loss of otoliths. Taken together, we show that osterix is required for bone formation in a teleost fish and that its important regulatory functions are conserved between teleosts and mammals. Furthermore, we provide the first functional evidence for a role of Osterix in a non-skeletal tissue, i.e. the otoliths.

The first transcriptome and genetic linkage map for Asian arowana

Asian arowana or dragonfish (Scleropages formosus) is an important fish species due to its unusual breeding biology and high economic value in the ornamental fish markets. In the present study, we aimed to (i) create the first transcriptome by Roche 454 pyrosequencing of Asian arowana brain and gonad samples; (ii) identify differentially expressed genes between the two sexes and develop microsatellite (SSR) markers; and (iii) construct a first-generation SSR-based genetic linkage map. A total of over 1.3 million reads were obtained from the brain and gonad of adult Asian arowana individuals through pyrosequencing. These reads were assembled into 16,242 contigs that were further grouped into 13,639 isogroups. BLASTX annotation identified a total of 8316 unique proteins from this data set. Many genes with sexually dimorphic expression levels and some putatively involved in sex development were identified. A total of 316 EST-SSRs and 356 new genomic-SSRs were developed by screening through the current transcriptome data set and SSR-enriched genomic libraries. The first genetic linkage map of the species was constructed based on these markers. Linkage analysis allowed for mapping of 308 markers to 28 linkage groups (LGs), ranging in size from 14.9 to 160.6 cM. The potentially sex-associated gene sox9 was mapped to LG4 on the consensus linkage map. Pairwise putative conserved syntenies between the Asian arowana, zebrafish, and three-spined stickleback were also established. These resources will help the conservation of the species through better understanding of its phylogenetics, genomics and biology, and comparative genome analysis within the Osteoglossidae family.

Thyroid hormones in male reproductive development: evidence for direct crosstalk between the androgen and thyroid hormone axes

Thyroid hormones (THs) exert a broad range of effects on development in vertebrate species, demonstrating connections in nearly every biological endocrine system. In particular, studies have shown that THs play a role in sexual differentiation and gonadal development in mammalian and non-mammalian species. There is considerable evidence that the effects of THs on reproductive development are mediated through the female hormonal axis; however, recent findings suggest a more direct crosstalk between THs and the androgen axis. These findings demonstrate that THs have considerable influence in the sexual ontogeny of male vertebrates, through direct interactions with select sex-determining-genes and regulation of gonadotropin production in the hypothalamus-pituitary-gonad axis. THs also regulate androgen biosynthesis and signaling through direct and indirect regulation of steroidogenic enzyme expression and activity. Novel promoter analysis presented in this work demonstrates the potential for direct and vertebrate wide crosstalk at the transcriptional level in mice (Mus musculus), Western clawed frogs (Silurana tropicalis) and medaka (Oryzias latipes). Cumulative evidence from previous studies; coupled with novel promoter analysis suggests mechanisms for a more direct crosstalk between the TH and male reproductive axes across vertebrate species.

Comparative analysis of a teleost skeleton transcriptome provides insight into its regulation

An articulated endoskeleton that is calcified is a unifying innovation of the vertebrates, however the molecular basis of the structural divergence between terrestrial and aquatic vertebrates, such as teleost fish, has not been determined. In the present study long-read next generation sequencing (NGS, Roche 454 platform) was used to characterize acellular perichondral bone (vertebrae) and chondroid bone (gill arch) in the gilthead sea bream (Sparus auratus). A total of 15.97 and 14.53Mb were produced, respectively from vertebrae and gill arch cDNA libraries and yielded 32,374 and 28,371 contigs (consensus sequences) respectively. 10,455 contigs from vertebrae and 10,625 contigs from gill arches were annotated with gene ontology terms. Comparative analysis of the global transcriptome revealed 4249 unique transcripts in vertebrae, 4201 unique transcripts in the gill arches and 3700 common transcripts. Several core gene networks were conserved between the gilthead sea bream and mammalian skeleton. Transcripts for putative endocrine factors were identified in acellular gilthead sea bream bone suggesting that in common with mammalian bone it can act as an endocrine tissue. The acellular bone of the vertebra, in contrast to current opinion based on histological analysis, was responsive to a short fast and significant (p<0.05) down-regulation of several transcripts identified by NGS, osteonectin, osteocalcin, cathepsin K and IGFI occurred. In gill arches fasting caused a significant (p<0.05) down-regulation of osteocalcin and up-regulation of MMP9.

Validation of reference genes for expression studies during craniofacial development in arctic charr

Arctic charr (Salvelinus alpinus) is a highly polymorphic species and in Lake Thingvallavatn, Iceland, four phenotypic morphs have evolved. These differences in morphology, especially in craniofacial structures are already apparent during embryonic development, indicating that genes important in the formation of the craniofacial features are expressed differentially between the morphs. In order to generate tools to examine these expression differences in Arctic charr, the aim of the present study was to identify reference genes for quantitative real-time PCR (qPCR). The specific aim was to select reference genes which are able to detect very small expression differences among different morphs. We selected twelve candidate reference genes from the literature, identified corresponding charr sequences using data derived from transcriptome sequencing (RNA-seq) and examined their expression using qPCR. Many of the candidate reference genes were found to be stably expressed, yet their quality-rank as reference genes varied considerably depending on the type of analysis used. In addition to commonly used software for reference gene validation, we used classical statistics to evaluate expression profiles avoiding a bias for reference genes with similar expression patterns (co-regulation). Based on these analyses we chose three reference genes, ACTB, UB2L3 and IF5A1 for further evaluation. Their consistency was assessed in an expression study of three known craniofacially expressed genes, sparc (or osteonectin), matrix metalloprotease 2 (mmp2) and sox9 (sex-determining region Y box 9 protein) using qPCR in embryo heads derived from four charr groups at three developmental time points. The three reference genes were found to be very suitable for studying expression differences between the morphotypes, enabling robust detection of small relative expression changes during charr development. Further, the results showed that sparc and mmp2 are differentially expressed in embryos of different Arctic charr morphotypes.

Multiple enhancers associated with ACAN suggest highly redundant transcriptional regulation in cartilage

The chondroitin sulfate proteoglycan core protein aggrecan is the major protein constituent of cartilage aside from collagen, and is largely responsible for its distinctive mechanical properties. Aggrecan is required both for proper cartilage formation in development and maintenance of mature cartilage. Prominent ACAN transcription is a conserved feature of vertebrate cartilage, although little is known about its specific transcriptional regulation. We examined the genomic interval containing human ACAN for transcriptional enhancers directing expression to cartilage, using a functional assay in transgenic zebrafish. We tested 24 conserved non-coding sequences, representing ~6% of the total sequence in the interval, and identified eleven independently capable of regulating reporter gene expression in cartilage. These enhancers were widely spaced, from >100kb upstream of the gene to within the first intron. While the majority displayed broad cartilage expression in zebrafish larvae, several were restricted to a subset of cartilage cells in the craniofacial skeleton. In older fish, the enhancers displayed differential activity; some maintained expression, either in all cartilage or preferentially in articular cartilage at the joints, while others were not active. This remarkable degree of overlapping regulatory control has been highly conserved; we identified clear orthologues of six enhancers at the chicken ACAN locus, arranged in the same order relative to the gene. These were also functional in directing expression to cartilage in transgenic zebrafish. Several enhancers contain potential binding sites for Sox9, consistent with its described role as an upstream regulator of ACAN expression. However, others lacked Sox9 consensus binding sites, implicating additional pathways and transcription factors as regulators of ACAN expression in cartilage, either in development or adult tissue. Our identification of these enhancer sequences is the necessary first step in detailed examination of the upstream regulators of ACAN expression.

Sex differentiation in Atlantic cod (Gadus morhua L.): morphological and gene expression studies

BACKGROUND

In differentiated gonochoristic species, a bipotential gonad develops into an ovary or testis during sex differentiation. Knowledge about this process is necessary to improve methods for masculinizing genetically female Atlantic cod for the subsequent purpose of producing all-female populations.

METHODS

Gonads were examined histologically in juveniles from 14 to 39 mm total body length (TL). Number and size of germ cells were determined in a subset of the samples. Relevant genes were cloned, and mRNA levels determined by qPCR of amh, cyp19a1a; dax1 (nr0b2); shp (nr0b2a) and sox9b in a mixed-sex and an all-female population ranging from 12-49 mm TL.

RESULTS

Individuals between 14-20 mm TL could be separated in two subgroups based on gonad size and germ cell number. Ovarian cavity formation was observed in some individuals from 18-20 mm TL. The mixed sex population displayed bimodal expression patterns as regards cyp19a1a (starting at 12 mm TL) and amh (starting at 20 mm TL) mRNA levels. After approximately 30 mm TL, cyp19a1a and amh displayed a gradual increase in both sexes. No apparent, sex-dependent expression patterns were found for dax1, shp or sox9b transcripts. However, shp levels were high until the larvae reached around 35 mm TL and then dropped to low levels, while dax1 remained low until 35 mm TL, and then increased sharply.

CONCLUSIONS

The morphological sex differentiation in females commenced between 14-20 mm TL, and ovarian cavities were evident by 18-20 mm TL. Testis development occurred later, and was morphologically evident after 30 mm TL. This pattern was corroborated with sexually dimorphic expression patterns of cyp19a1a from 12-13 mm TL, and a male-specific increase in amh from 20 mm TL.

Evolution of the osteoblast: skeletogenesis in gar and zebrafish

BACKGROUND

Although the vertebrate skeleton arose in the sea 500 million years ago, our understanding of the molecular fingerprints of chondrocytes and osteoblasts may be biased because it is informed mainly by research on land animals. In fact, the molecular fingerprint of teleost osteoblasts differs in key ways from that of tetrapods, but we do not know the origin of these novel gene functions. They either arose as neofunctionalization events after the teleost genome duplication (TGD), or they represent preserved ancestral functions that pre-date the TGD. Here, we provide evolutionary perspective to the molecular fingerprints of skeletal cells and assess the role of genome duplication in generating novel gene functions. We compared the molecular fingerprints of skeletogenic cells in two ray-finned fish: zebrafish (Danio rerio)--a teleost--and the spotted gar (Lepisosteus oculatus)--a "living fossil" representative of a lineage that diverged from the teleost lineage prior to the TGD (i.e., the teleost sister group). We analyzed developing embryos for expression of the structural collagen genes col1a2, col2a1, col10a1, and col11a2 in well-formed cartilage and bone, and studied expression of skeletal regulators, including the transcription factor genes sox9 and runx2, during mesenchymal condensation.

RESULTS

Results provided no evidence for the evolution of novel functions among gene duplicates in zebrafish compared to the gar outgroup, but our findings shed light on the evolution of the osteoblast. Zebrafish and gar chondrocytes both expressed col10a1 as they matured, but both species' osteoblasts also expressed col10a1, which tetrapod osteoblasts do not express. This novel finding, along with sox9 and col2a1 expression in developing osteoblasts of both zebrafish and gar, demonstrates that osteoblasts of both a teleost and a basally diverging ray-fin fish express components of the supposed chondrocyte molecular fingerprint.

CONCLUSIONS

Our surprising finding that the "chondrogenic" transcription factor sox9 is expressed in developing osteoblasts of both zebrafish and gar can help explain the expression of chondrocyte genes in osteoblasts of ray-finned fish. More broadly, our data suggest that the molecular fingerprint of the osteoblast, which largely is constrained among land animals, was not fixed during early vertebrate evolution.

Analysis of medaka sox9 orthologue reveals a conserved role in germ cell maintenance

The sex determining gene is divergent among different animal species. However, sox9 is up-regulated in the male gonads in a number of species in which it is the essential regulator of testis determination. It is therefore often discussed that the sex determining gene-sox9 axis functions in several vertebrates. In our current study, we show that sox9b in the medaka (Oryzias latipes) is one of the orthologues of mammalian Sox9 at syntenic and expression levels. Medaka sox9b affects the organization of extracellular matrices, which represents a conserved role of sox9, but does not directly regulate testis determination. We made this determination via gene expression and phenotype analyses of medaka with different copy numbers of sox9b. Sox9b is involved in promoting cellular associations and is indispensible for the proper proliferation and survival of germ cells in both female and male medaka gonads. Medaka mutants that lack sox9b function exhibit a seemingly paradoxical phenotype of sex reversal to male. This is explained by a reduction in the germ cell number associated with aberrant extracellular matrices. Together with its identified roles in other vertebrate gonads, a testis-determining role for Sox9 in mammals is likely to have been neofunctionalized and appended to its conserved role in germ cell maintenance.

Molecular cloning and expression of the col2a1a and col2a1b genes in the medaka, Oryzias latipes

The Col2a1 gene is expressed in notochord, otic vesicle, cartilaginous tissue and the anlage of endochondral bone during development in higher vertebrates. Type II collagen, a homotrimeric product of the Col2a1 gene, functions as a key regulatory protein for cartilage development and endochondral ossification. In medaka and zebrafish, a single homolog of the col2a1 gene has been identified. However, it is necessary to note that many genes are duplicated in teleost fishes. To clarify function of col2a1 genes in teleost fishes and to further understand the process of cartilage development and endochondral ossification, we cloned and mapped the gene loci of two col2a1 orthologs in medaka. The proteins encoded by both medaka col2a1a and col2a1b genes were highly conserved (85.3% and 82.6%) relative to human COL2A1, but synteny was not observed. We also examined the expression patterns of col2a1a and col2a1b during embryonic development. Whole-mount insitu hybridization data suggests that expression patterns of both medaka co2a1a and col2a1b genes are similar to that of zebrafish co2a1 in the early embryonic stages. In medaka, the two col2a1 genes show a closely correlated pattern of spatial and temporal expression. In late embryonic stages, however, there were differences in both expression patterns in the pectoral fin. This study is the first report of two homologs of col2a1 in teleosts and also the first examination of col2a1a and col2a1b expression patterns in this group.

Ovarian germline stem cells in the teleost fish, medaka (Oryzias latipes)

In the mammalian testis germline stem cells keep producing many sperms, while there is no direct evidence for the presence of germline stem cells in the ovary. It is widely accepted in mammals that the mature oocytes are supplied from a pool of primordial follicles in the adult ovary. In other vertebrates, such as fish, however, there has been no investigation on the mechanism underlying the high egg-producing ability. In this review, we introduce the recently identified ovarian germline stem cells and the surrounding unique structure in teleost fish, medaka (Oryzias latipes) [Nakamura S et al. Science. 2010; 328: 1561-1563]. We also discuss about the expression and function of sox9 that characterizes this unique structure.