Dnd Is a Critical Specifier of Primordial Germ Cells in the Medaka Fish

SDF-1/CXCR4 signal is involved in the induction of Primordial Germ Cell migration in a model marine fish, Japanese anchovy (Engraulis japonicus)

Primordial germ cells (PGCs) are pivotal for gonadal development and reproductive success. Though artificial induction of sterility by targeting PGCs are gaining popularity due to its advantages in fish surrogacy and biodiversity management, it is often skill and time intensive. In this study, we have focused on understanding the role of PGCs and the chemotactic SDF-1/CXCR4 signaling on gonad development of Japanese anchovy (JA, Engraulis japonicus), an upcoming marine model organism with eco-commercial values, with an aim to develop a novel, easy, and versatile gonad sterilization method. Our data showed that PGC migration related genes, i.e., sdf-1a, sdf-1b, cxcr4a, cxcr4b and vasa, are phylogenetically closer relatives of respective herring (Clupea harengus) and zebrafish (Danio rerio) homolog. Subsequently, PGC marking and live tracing experiments confirmed that PGC migration in JA initiates from 16 hours post fertilization (hpf) followed by PGC settlement in the gonadal ridge at 44 hpf. We found that overexpression of zebrafish sdf-1a mRNA in the germ cell suppresses cxcr4a and increases cxcr4b transcription at 8 hpf, dose dependently disrupts PGC migration at 24-48 hpf, induces PGC death and upregulates sdf-1b at 5 days post hatching. 48 h of immersion treatment with CXCR4 antagonist (AMD3100, Abcam) also accelerated PGC mismigration and pushed the PGC away from gonadal ridge in a dose responsive manner, and further when grown to adulthood caused germ cell less gonad formation in some individuals. Cumulatively, our data, for the first time, suggests that JA PGC migration is largely regulated by SDF1/CXCR4 signaling, and modulation of this signaling has strong potential for sterile, germ cell less gonad preparation at a mass scale. However, further in-depth analysis is pertinent to apply this methodology in marine fish species to successfully catapult Japanese anchovy into a true marine fish model.

Direct male development in chromosomally ZZ zebrafish

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish (Danio rerio), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome, or fewer than two Z chromosomes, is essential to initiate oocyte development; and without the W factor, or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

High glucose exposure of preimplantation embryos causes glucose intolerance and insulin resistance in F1 and F2 male offspring

BACKGROUND

Although studies suggest that maternal high glucose (HG) increases offspring susceptibility to type 2 diabetes mellitus (T2DM), the underlying mechanisms are largely unclear. We studied whether glucose levels in oviducts are elevated when pregestational diabetic females get pregnant and whether the oviductal HG (OVHG) would act directly on embryos to increase offspring's T2DM susceptibility.

METHODS

We established an in vivo model of OVHG by injecting female mice with streptozotocin (STZ) during the preimplantation period and an in vitro model of embryo culture with HG (ECHG) by culturing preimplantation embryos with HG, before examining glucose tolerance and insulin resistance (IR) in F1 and F2 offspring.

FINDINGS

Injection of female mice with STZ induced a lasting significant glucose elevation in blood and oviduct fluid during the preimplantation period. The glucose tolerance test showed that both the STZ-induced OVHG and the ECHG caused glucose intolerance in F1 male and F1-sired F2 male offspring but had no effect on female offspring. Insulin tolerance test and the analysis for IR-related gene expression and glycogen contents in liver and muscle revealed significant IR in these male offspring.

INTERPRETATION

This study provided evidence that HG can act directly on preimplantation embryos to increase offspring's T2DM susceptibility suggesting that the preimplantation period is a critical stage for transmission of mother's diabetes to offspring. FUND: This study was supported by grants from the China National Natural Science Foundation (Nos. 31772599, 32072738, 31702114, and 31902160), and the Natural Science Foundation of Shandong Province (Nos. ZR2022MC036, ZR2017BC025 and ZR2020QC102).

Direct Male Development in Chromosomally ZZ Zebrafish

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish ( Danio rerio ), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB strain fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome or fewer than two Z chromosomes is essential to initiate oocyte development; and without the W factor or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

Paternal source of germ plasm determinants in the viviparous teleost, Gambusia holbrooki; dads do matter

The identity of germ cells, the progenitors of life, is thought to be acquired by two modes; either by maternal signals (preformed) or induced de novo from pluripotent cells (epigenesis) in the developing embryos. However, paternal roles seem enshrouded or completely overlooked in this fundamental biological process. Hence, we investigated the presence of germplasm transcripts in the sperm of Gambusia holbrooki, a live-bearing fish, demonstrating their presence and suggesting paternal contributions. Interestingly, not all germplasm markers were present (nanos1 and tdrd6) in the sperm, but some were conspicuous (dazl, dnd-α, piwi II, and vasa), indicating that the latter is required for establishing germ cell identity in the progeny, with a possible parent-specific role. Furthermore, there were also spatial differences in the distribution of these determinants, suggesting additional roles in sperm physiology and/or fertility. Our results support the hypothesis that dads also play a vital role in establishing the germ cell identity, especially in G. holbrooki, which shares elements of both preformation and induction modes of germline determination. This, coupled with its life history traits, makes G. holbrooki an excellent system for dissecting evolutionary relationships between the two germline determination modes, their underpinning mechanisms and ultimately the perpetuity of life.

Identification of germ cells in large yellow croaker (Larimichthys crocea) and yellow drum (Nibea albiflora) using RT-PCR and in situ hybridization analyses

Ocean-caught large yellow croaker (Larimichthys crocea) represents an important germplasm resource for the breeding of this species; however, these fish tend to show poor survival in captivity and would be unsuitable breeding purposes. As an alternative to the use of wild-caught croakers, germ cell transplantation has been proposed using the L. crocea specimens as donors and yellow drum (Nibea albiflora) as recipients. In this regard, the identification of L. crocea and N. albiflora germ cells is an essential prerequisite for establishing a germ cell transplantation protocol for these fish. In this study, we cloned the 3' untranslated regions (UTR) of the vasa, dnd, and nanos2 genes in N. albiflora using the rapid amplification of cDNA ends (RACE) method and then aligned and analyzed the sequences of the corresponding genes in L. crocea and N. albiflora. On the basis of gene sequence differences, we designed species-specific primers and probes for RT-PCR analysis and in situ hybridization. RT-PCR analysis revealed that these species-specific primers exclusively amplified DNA from gonads of the respective species, thus confirming that we had six specific primer pairs that could be used to distinguish the germ cells in L. crocea and N. albiflora. Using in situ hybridization analysis, we established that whereas Lcvasa and Nadnd probes showed high species specificity, the probes for Navasa and Lcdnd showed a less specificity. In situ hybridization using Lcvasa and Nadnd thus enabled us to visualize the germ cells in these two species. Using these species-specific primers and probes, we can reliably distinguish the germ cells of L. crocea and N. albiflora, thereby establishing an effective approach for the post-transplantation identification of germ cells when using L. crocea and N. albiflora as donors and recipients, respectively.

Infertility control of transgenic fluorescent zebrafish with targeted mutagenesis of the dnd1 gene by CRISPR/Cas9 genome editing

Transgenic technology and selective breeding have great potential for the genetic breeding in both edible fish and ornamental fish. The development of infertility control technologies in transgenic fish and farmed fish is the critical issue to prevent the gene flow with wild relatives. In this study, we report the genome editing of the dead end (dnd1) gene in the zebrafish model, using the CRISPR/Cas9 technology to achieve a loss-of-function mutation in both wild-type zebrafish and transgenic fluorescent zebrafish to develop complete infertility control technology of farmed fish and transgenic fish. We effectively performed targeted mutagenesis in the dnd1 gene of zebrafish with a single gRNA, which resulted in a small deletion (-7 bp) or insertion (+41 bp) in exon 2, leading to a null mutation. Heterozygotes and homozygotes of dnd1-knockout zebrafish were both selected by genotyping in the F 1 and F 2 generations. Based on a comparison of histological sections of the gonads between wild-type, heterozygous, and homozygous dnd1 zebrafish mutants, the dnd1 homozygous mutation (aa) resulted in the loss of germ cells. Still, there was no difference between the wild-type (AA) and dnd1 heterozygous (Aa) zebrafish. The homozygous dnd1 mutants of adult zebrafish and transgenic fluorescent zebrafish became all male, which had normal courtship behavior to induce wild-type female zebrafish spawning. However, they both had no sperm to fertilize the spawned eggs from wild-type females. Thus, all the unfertilized eggs died within 10 h. The targeted mutagenesis of the dnd1 gene using the CRISPR/Cas9 technology is stably heritable by crossing of fertile heterozygous mutants to obtain sterile homozygous mutants. It can be applied in the infertility control of transgenic fluorescent fish and genetically improved farmed fish by selective breeding to promote ecologically responsible aquaculture.

Germline stem cells in human

The germline cells are essential for the propagation of human beings, thus essential for the survival of mankind. The germline stem cells, as a unique cell type, generate various states of germ stem cells and then differentiate into specialized cells, spermatozoa and ova, for producing offspring, while self-renew to generate more stem cells. Abnormal development of germline stem cells often causes severe diseases in humans, including infertility and cancer. Primordial germ cells (PGCs) first emerge during early embryonic development, migrate into the gentile ridge, and then join in the formation of gonads. In males, they differentiate into spermatogonial stem cells, which give rise to spermatozoa via meiosis from the onset of puberty, while in females, the female germline stem cells (FGSCs) retain stemness in the ovary and initiate meiosis to generate oocytes. Primordial germ cell-like cells (PGCLCs) can be induced in vitro from embryonic stem cells or induced pluripotent stem cells. In this review, we focus on current advances in these embryonic and adult germline stem cells, and the induced PGCLCs in humans, provide an overview of molecular mechanisms underlying the development and differentiation of the germline stem cells and outline their physiological functions, pathological implications, and clinical applications.

Spinyhead Croaker Germ Cells Gene dnd Visualizes Primordial Germ Cells in Medaka

Spinyhead croaker (Collichthys lucidus) is an economically important fish suffering from population decline caused by overfishing and habitat destruction. Researches on the development of primordial germ cell (PGC) and reproduction biology were an emergency for the long-term conservation of the involved species. Dead end (dnd) gene plays an indispensable role in PGC specification, maintenance, and development. In the current study, we report the cloning and expression patterns of dnd in C. lucidus (Cldnd). RT-PCR analysis revealed that Cldnd was specifically expressed in both sexual gonads. In the ovary, Cldnd RNA was uniformly distributed in the oocytes and abundant in oogonia, and gradually decreased with oogenesis. A similar expression pattern was also detected in testis. Dual fluorescent in situ hybridization of Cldnd and Clvasa demonstrated that they almost had the same distribution except in oocytes at stage I, in which the vasa RNA aggregated into some particles. Furthermore, Cldnd 3' UTR was sufficient to guide the Green Fluorescent Protein (GFP) specifically and stably expressed in the PGCs of medaka. These findings offer insight into that Cldnd is an evolutionarily conserved germline-specific gene and even a potential candidate for PGC manipulation in C. lucidus.

Generation of a Normal Long-Term-Cultured Chinese Hook Snout Carp Spermatogonial Stem Cell Line Capable of Sperm Production In Vitro

Simple Summary

In vitro-induced differentiation of sperm cells is a key technology for genetic resource conservation. In the past ten years, Opsariichthys bidens has become a famous and excellent aquatic species in some areas in China. However, its genetic resources have reduced dramatically. To protect against the decline of O. bidens, a long-term-cultured spermatogonial stem cell line (ObSSC) of adult O. bidens was successfully established. The result of our study showed that ObSSC had a diploid karyotype and stable growth over more than 2 years, with SSC-typical gene expression patterns. Furthermore, our research demonstrates the potential and regulation mechanism of fish spermatogonial stem cell differentiation into different cells of three germ layers. Our findings will assist further research on the genetic resource conservation of germplasm in a commercially and ecologically valuable fish species.

Abstract

Opsariichthys bidens belongs to the family Cyprinidae and is a small freshwater economic fish widely distributed in China. In recent years, the natural resources of O. bidens have been drastically reduced due to overfishing and the destruction of the water environment. The in vitro culture and long-term preservation of germ stem cells are the key technologies to keep genetic resources from degeneration. However, except for the establishment of the first long-term cultured medaka spermatogonia cell line (SSC) capable of producing sperm in vitro in 2004, no other long-term cultured SSC line has been found in other fish species. In this study, we successfully established another long-term-cultured spermatogonial stem cell line from Opsariichthys bidens (ObSSC). After more than 2 years of culture, ObSSC had a diploid karyotype and stable growth, with the typical gene expression patterns of SSC. Under in vitro culture, ObSSC could be induced to differentiate into sperm and other different types of somatic cells. In vivo, ObSSC could differentiate into different cells of three germ layers upon being transplanted into zebrafish embryos. Our research helps to explore the potential and regulation mechanism of fish SSC differentiation and spermatogenesis in vitro, provides a new way for solving the problem of fish genetic resource degradation and lays a foundation for further research on fish germ cell transplantation.

Differential Expression of Duplicate Insulin-like Growth Factor-1 Receptors (igf1rs) in Medaka Gonads

Insulin-like growth factor-1 receptors (igf1rs) play important roles in regulating development, differentiation, and proliferation in diverse organisms. In the present study, subtypes of medaka igf1r, igf1ra, and igf1rb were isolated and characterized. RT-PCR results showed that igf1ra and igf1rb mRNA were expressed in all tissues and throughout embryogenesis. Using real-time PCR, the differential expression of igf1ra and igf1rb mRNA during folliculogenesis was observed. The results of in situ hybridization (ISH) revealed that both of them were expressed in ovarian follicles at different stages, and igf1rb was also expressed in theca cells and granulosa cells. In the testis, both igf1ra and igf1rb mRNA were highly expressed in sperm, while igf1rb mRNA was also obviously detected in spermatogonia. In addition, igf1ra mRNA was also present in Leydig cells in contrast to the distribution of igf1rb mRNA in Sertoli cells. Collectively, we demonstrated that differential igf1rs RNA expression identifies medaka meiotic germ cells and somatic cells of both sexes. These findings highlight the importance of the igf system in the development of fish gonads.

Primordial Germ Cell Development in the Poeciliid, Gambusia holbrooki, Reveals Shared Features Between Lecithotrophs and Matrotrophs

Metazoans exhibit two modes of primordial germ cell (PGC) specification that are interspersed across taxa. However, the evolutionary link between the two modes and the reproductive strategies of lecithotrophy and matrotrophy is poorly understood. As a first step to understand this, the spatio-temporal expression of teleostean germ plasm markers was investigated in Gambusia holbrooki, a poecilid with shared lecitho- and matrotrophy. A group of germ plasm components was detected in the ovum suggesting maternal inheritance mode of PGC specification. However, the strictly zygotic activation of dnd-β and nanos1 occurred relatively early, reminiscent of models with induction mode (e.g., mice). The PGC clustering, migration and colonisation patterns of G. holbrooki resembled those of zebrafish, medaka and mice at blastula, gastrula and somitogenesis, respectively—recapitulating features of advancing evolutionary nodes with progressive developmental stages. Moreover, the expression domains of PGC markers in G. holbrooki were either specific to teleost (vasa expression in developing PGCs), murine models (dnd spliced variants) or shared between the two taxa (germline and somatic expression of piwi and nanos1). Collectively, the results suggest that the reproductive developmental adaptations may reflect a transition from lecithotrophy to matrotrophy.

Characterization of vasa and dnd homologs in summer flounder, Paralichthys dentatus: Expression analysis and colocalization of PGCs during embryogenesis

Specification of primordial germ cells (PGCs) is particularly important for germline formation. Many maternal-effect genes such as vasa, dnd, and nanos have been identified. However, the research on distribution patterns of PGCs in marine fish is limited. Vasa has been widely used as a germ cell marker to identify its origination in teleosts because vasa RNA is a component of germ plasm. Dnd is known to be an RNA binding protein that protects germline-specific RNAs from degradation. In this study, we isolated full-length vasa and dnd cDNA from summer flounder to track germ cell origination and their expression patterns by RT-PCR and ISH. The results demonstrated that deduced amino acid sequence of Pdvas and Pddnd shared typically conserved motifs of their homologues and demonstrated high identities with other teleosts. Both vasa and dnd transcripts were exclusively detected in germ cells of the gonads. During embryogenesis, vasa and dnd RNA were located at the cleavage furrows of early cleavage stages, and then through proliferation and migration they eventually moved to a location at the predetermined genital ridge. Phylogenetic analysis revealed that summer flounder belongs to the Euteleostei species, but vasa/dnd transcripts localized at the cleavage furrows was similar to that in zebrafish (Osteriophysans). This suggests that germ cells differentiating at early embryogenesis have no direct relation with phylogenesis. At the same time, we found the spatio-temporal expression pattern of dnd was highly consistent with vasa during this process, which indicated the important function of dnd in keeping the target RNA from being degraded to maintain germ cell fate. These results will provide further understanding of germ plasm localization and PGC differentiation in teleosts, and facilitate germ cell manipulation in marine fishes.

Crucial role of dead end gene for primordial germ cell survival in rice field eel (Monopterus albus)

The dead end gene has been identified as a essential factor for Primordial germ cells (PGCs) migration and survival in many species, but its role in Monopterus albus is unclear. In order to clarify the function of dead end gene in M.albus PGCs migration and survival, we first characterized the expression profile of M.albus dead end (Madnd) in developing embryos and various tissues. qRT-PCR revealed that Madnd transcripts were exclusively detected in gonad, including ovary, testis and ovotestis.Embryos injected with a Madnd morpholino (Madnd-MO) exhibited down-regulation of the vasa gene. Furthermore, the GFP signal show the PGCs migration in control group were injected with GFP-nanos3 3'-UTR mRNA for visualization, as described in a previous study, yet it was disappeared after embryos injected with Madnd-MO.Finally, we characterized the genomics sequence of the Madnd gene and designed five gRNAs for genome editing. Three gRNAs were selected for microinjection according to the results of in vitro tests. gRNAd1 was used for microinjection with the Cas9 protein and was confirmed to be effective. Our analysis in this study suggested that Madnd play a key role in PGCs migration and survival in M. albus. These data provide the basis for the production of fast-growing and reproductively M.albus sterile.

Dead-end (dnd) protein in fish-a review

Dead end (dnd) is a germ plasm-specific maternal RNA discovered in zebrafish and then in other vertebrates. Dnd protein is essential for migration and motility of primordial germ cells (PGCs), only cells destined to transfer genetic information to offspring. PGCs arise far from somatic cells of developing gonads and they must migrate to their site of function. Migration of PGCs follows complex path by various developing tissues as their disruption impacts on the fertility. Recently, it has been found that dnd is not required for survival of PGCs and dnd-deficient zebrafish PGCs transdifferentiate into the somatic cells. In fish, targeting dnd causes removal of PGCs that ultimately affects sex differentiation. Sterility in various fish species can be achieved by knockdown or knockout of dnd. In our review, we have discussed dnd as a germ cell-specific molecular marker in fish, its interaction with miRNAs, and its use in aquaculture and fish conservation.

CircRNA profiling reveals circ880 functions as miR-375-3p sponge in medaka gonads

Circular RNAs (circRNAs) have been regarded as regulators in the biological processes of various species. However, there is no report about circRNAs in the gonads of model fish medaka (Oryzias latipes). In this study, 1157 and 1570 circRNAs were obtained in the ovary and testis by RNA-sequencing. The characteristics of circRNAs were explored in sequence length, exon composition, and chromosome position. 24 circRNAs were significantly up or down-regulated in the testis compared to the ovary, 9 of which were verified by qRT-PCR. Interestingly, circ452 was highly expressed in the testis while circ880 expression exhibited sexual dimorphism. In situ hybridization (ISH) revealed that circ452 and circ880 were expressed in meiotic germ cells, and circ880 was also abundant in spermatogonia. In addition, dual-luciferase reporter assay manifested that circ880 and Oldnd can combine with miR-375-3p. Overall, these results provide emerging circRNA libraries and open new avenues for future investigation of circRNAs in medaka.

Bucky ball induces primordial germ cell increase in medaka

Balbaini body (Bb) plays a vital role in germ plasm (GP) assembly and dorsoventral pattern, which is of critical important in germline specification and development. Bucky ball (buc) is reported to be essential for boosting primordial germ cell (PGC) through Bb in previous research. In the present study, a buc homolog (Olbuc) was identified in medaka (Oryzias latipes), and the roles of Olbuc on PGC development were further elucidated. The full length of Olbuc was 2148 bp, which contains a 1724 bp CDS (Coding sequence), a 167 bp 5' UTR (Untranslated region), and a 257 bp 3' UTR. By RT-PCR, the Olbuc RNA expression was maternally provided during embryogenesis and was restricted in the ovary of adult tissues. By in situ hybridization, Olbuc RNA was abundant in oocyte of meiotic stage, but gradually decreased as the oogenesis proceeded. Surprisingly, Olbuc was not co-localized with dazl, the marker gene of Bb. Interestingly, GFP can be specifically and stably expressed through the induction of Olbuc 3'UTR in PGCs. Furthermore, overexpression of Olbuc mRNA could increase PGC number and generate ectopic PGC in medaka and zebrafish embryos. In summary, our results showed that Olbuc performs a conserved function in PGC development in medaka.

Germline Specific Expression of a vasa Homologue Gene in the Viviparous Fish Black Rockfish (Sebastes schlegelii) and Functional Analysis of the vasa 3 Untranslated Region

Germ cells play a key role in gonad development. As precursors, primordial germ cells (PGCs) are particularly important for germline formation. However, the origination and migration patterns of PGCs are poorly studied in marine fish, especially for viviparous economic species. The vasa gene has been widely used as a germ cell marker to identify a germline because vasa RNA is a component of germ plasm. In this study, we described the expression pattern of black rockfish (Sebastes schlegelii) vasa (Ssvas) in gonadal formation and development by in situ hybridization. The results showed that Ssvas failed in localization at the cleavage furrows until the late gastrula stage, when PGCs appeared and migrated to the genital ridge and formed elongated gonadal primordia at 10 days after birth. This study firstly revealed the PGCs origination and migration characteristics in viviparous marine fish. Furthermore, we microinjected chimeric mRNA containing EGFP and the 3′untranslated region (3′UTR) of Ssvas into zebrafish (Danio rerio) and marine medaka (Oryzias melastigma) fertilized eggs for tracing PGCs. We found that, although Sebastes schlegelii lacked early localization, similar to red seabream (Pagrus major) and marine medaka, only the 3′UTR of Ssvas vasa 3′UTR of black rockfish was able to label both zebrafish and marine medaka PGCs. In comparison with other three Euteleostei species, besides some basal motifs, black rockfish had three specific motifs of M10, M12, and M19 just presented in zebrafish, which might play an important role in labeling zebrafish PGCs. These results will promote germ cell manipulation technology development and facilitate artificial reproduction regulation in aquaculture.

Rescue of germ cells in dnd crispant embryos opens the possibility to produce inherited sterility in Atlantic salmon

Genetic introgression of escaped farmed Atlantic salmon (Salmo salar) into wild populations is a major environmental concern for the salmon aquaculture industry. Using sterile fish in commercial aquaculture operations is, therefore, a sustainable strategy for bio-containment. So far, the only commercially used methodology for producing sterile fish is triploidization. However, triploid fish are less robust. A novel approach in which to achieve sterility is to produce germ cell-free salmon, which can be accomplished by knocking out the dead-end (dnd) gene using CRISPR-Cas9. The lack of germ cells in the resulting dnd crispants, thus, prevents reproduction and inhibits subsequent large-scale production of sterile fish. Here, we report a rescue approach for producing germ cells in Atlantic salmon dnd crispants. To achieve this, we co-injected the wild-type (wt) variant of salmon dnd mRNA together with CRISPR-Cas9 constructs targeting dnd into 1-cell stage embryos. We found that rescued one-year-old fish contained germ cells, type A spermatogonia in males and previtellogenic primary oocytes in females. The method presented here opens a possibility for large-scale production of germ-cell free Atlantic salmon offspring through the genetically sterile broodstock which can pass the sterility trait on the next generation.

Novel method for mass producing genetically sterile fish from surrogate broodstock via spermatogonial transplantation†

A stable system for producing sterile domesticated fish is required to prevent genetic contamination to native populations caused by aquaculture escapees. The objective of this study was to develop a system to mass produce stock for aquaculture that is genetically sterile by surrogate broodstock via spermatogonial transplantation (SGTP). We previously discovered that female medaka carrying mutations on the follicle-stimulating hormone receptor (fshr) gene become sterile. In this study, we demonstrated that sterile hybrid recipient females that received spermatogonia isolated from sex-reversed XX males (fshr (-/-)) recovered their fertility and produced only donor-derived fshr (-) X eggs. Natural mating between these females and fshr (-/-) sex-reversed XX males successfully produced large numbers of sterile fshr (-/-) female offspring. In conclusion, we established a new strategy for efficient mass production of sterile fish. This system can be applied to any aquaculture species for which SGTP and methods for producing sterile recipients can be established.

A High-Density Genetic Linkage Map and QTL Mapping for Sex and Growth-Related Traits of Large-Scale Loach (Paramisgurnus dabryanus)

Large-scale loach (Paramisgurnus dabryanus) is a commercially important species in East Asia; however, the cultured population that exhibited degradation of germplasm resource cannot meet the market needs, and the genome resources for P. dabryanus are still lacking. In this study, the first high-density genetic map of P. dabryanus was constructed using 15,830 SNP markers based on high-throughput sequencing with an improved SLAF-seq strategy. The quantitative trait locus (QTL) mapping for sex, growth, and morphology traits was performed for the first time. The genetic map spanned 4,657.64 cM in length with an average inter-marker distance of 0.30 cM. QTL mapping and association analysis identified eight QTLs of growth traits, nine QTLs of morphology traits, and five QTLs of sex-related traits, respectively. Interestingly, the most significant QTLs for almost all the traits were concentrated on the same linkage group LG11. Seven candidate markers and 12 potentially key genes, which were associated with sex determination and growth, were identified within the overlapped QTL regions on LG11. Further, the first genome survey analysis of P. dabryanus was performed which represents the first step toward fully decoding the P. dabryanus genome. The genome scaffolds were anchored to the high-density linkage map, spanning 960.27 Mb of P. dabryanus reference genome. The collinearity analysis revealed a high level of collinearity between the genetic map and the reference genome of P. dabryanus. Moreover, a certain degree of homology was observed between large-scale loach and zebrafish using comparative genomic analysis. The constructed high-density genetic map was an important basis for QTL fine mapping, genome assembly, and genome comparison. The present study will provide a valuable resource for future marker-assisted breeding, and further genetic and genomic researches in P. dabryanus.

Heat shock factor 1 protects germ cell proliferation during early ovarian differentiation in medaka

The heat shock response is important for the viability of all living organisms. It involves the induction of heat shock proteins whose expression is mainly regulated by heat shock factor 1 (HSF1). Medaka (Oryzias latipes) is a teleost fish with an XX/XY sex determination system. High water temperature (HT) inhibits the female-type proliferation of germ cells and induces the masculinisation of XX medaka in some cases during gonadal sex differentiation. Here, we investigated the roles of HSF1 on the proliferation of germ cells using HSF1 knockout medaka. Loss of HSF1 function under HT completely inhibited the female-type proliferation of germ cells, induced the expression of the anti-Mullerian hormone receptor type 2 (amhr2) and apoptosis-related genes, and suppressed that of the dead end (dnd) and heat shock protein-related genes. Moreover, the loss of HSF1 and AMHR2 function under HT recovered female-type proliferation in germ cells, while loss of HSF1 function under HT induced gonadal somatic cell apoptosis during early sex differentiation. These results strongly suggest that HSF1 under the HT protects the female-type proliferation of germ cells by inhibiting amhr2 expression in gonadal somatic cells. These findings provide new insights into the molecular mechanisms underlying environmental sex determination.

Non-coding RNA regulation in reproduction: Their potential use as biomarkers

Non-coding RNAs (ncRNAs) are crucial regulatory elements in most biological processes and reproduction is also controlled by them. The different types of ncRNAs, as well as the high complexity of these regulatory pathways, present a complex scenario; however, recent studies have shed some light on these questions, discovering the regulatory function of specific ncRNAs on concrete reproductive biology processes. This mini review will focus on the role of ncRNAs in spermatogenesis and oogenesis, and their potential use as biomarkers for reproductive diseases or for reproduction success.

Evolutionarily conserved vasa identifies embryonic and gonadal germ cells in spinyhead croaker Collichthys lucidus

In this study, a 2198 bp full-length cDNA of spinyhead croaker Collichthys lucidus vasa gene encoding 616 amino-acid residues was obtained. Multiple alignment revealed that C. lucidus vasa has eight conserved characteristic motifs of the DEAD box protein family and has the highest identity to large yellow croaker Larimichthys croceas. Reverse-transcription (RT)-PCR and Western blot analyses indicated that the vasa messenger (m)RNA and Vasa protein are specifically expressed in the gonads in both sexes. In situ hybridisation (ISH) demonstrated that vasa RNA is exclusively detected in the germ cells in C. lucidus gonads and its temporospatial expression reveals a dynamic pattern during oogenesis. Surprisingly, C. lucidus vasa 3'UTR can direct stable and specific GFP expression in the primordial germ cells (PGC) of medaka Oryzias latipes embryos. Taken together, these results suggest that because C. lucidus vasa expression delineates critical stages of oogenesis, it may be a useful molecular marker for the identification of gonadal germ cells, facilitating the isolation and utilization of germ cells in future study.

Dnd is required for primordial germ cell specification in Oryzias celebensis

Dead end (dnd) is a germ plasm component that plays an essential role for primordial germ cell (PGC) development in vertebrates. Previously, we have found that dnd is the first fish PGC specifier in medaka. Here, we present an additional evidence that dnd is the determinant for PGC specification in Oryzias celebensis. In adult tissues, the O. celebensis dnd (Ocdnd) RNA shows germ cells specific expression in gonads. In the testis, Ocdnd RNA is strongly detected in spermatogonia and meiotic cells and gradually decreases during the spermatogenesis. In the ovary, Ocdnd RNA is present throughout oogenesis. In the embryos, Ocdnd RNA is maternally provided and asymmetrically localized to prominent particles of presumptive PGCs before gastrulation stage and restricted to PGCs subsequently. In addition, Ocdnd 3' UTR can induce specific and stabilized GFP reporter expression in PGCs. Furthermore, knockdown of Ocdnd by morpholino (MO) injection abolishes the PGCs formation and this can be rescued by co-injection of medaka dnd (Oldnd) mRNA. More importantly, overexpression of Oldnd mRNA surprisingly boosts PGCs number. These results provide insights into function of dnd as a conserved specifier of PGCs in the genus Oryzias.

Identification and expression analysis of microRNAs in medaka gonads

Gonad development is a highly regulated, coordinated biological process and increasing evidences have indicated that microRNA (miRNA) may be involved in this dynamic program. Medaka (Oryzias latipes) is a good model for reproductive research as it has distinct sex determining genes, however, research in gonadal miRNAs is lacked. In this study, two small RNA libraries from the ovaries and testes were constructed and sequenced. A total of 285 conserved and 388 novel miRNAs were obtained, among which 142 mature miRNAs were significantly (> two-fold change) up or down regulated in the testis compared to the ovary. Quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) analysis showed that miR-430c, miR-26a and miR-202-5p were expressed in a gonad-specific or sex-biased pattern. Fluorescence in situ hybridization (FISH) indicated that miR-202-5p was present throughout spermatogenesis and was only detected at the early stages of oogenesis, this sex biased expression pattern suggested that miR-202-5p might be a crucial candidate in male differentiation and development. Our study provides the repertoire, a comprehensive annotation of miRNAs from gonads and a reference for functional studies of miRNAs in medaka.

Generation of biallelic F0 mutants in medaka using the CRISPR/Cas9 system

Several animal models generated by genome editing methods develop somatic mosaic mutations including wild-type genome sequence in F0 generation because it is difficult to use editing tools at the one-cell stage. Producing complete knockout animals quickly is a great advantage in determining the function of target genes. This study investigated the generation of F0 knockout medaka using the CRISPR/Cas9 system. To determine whether this editing system induced mutations in the medaka genome at the one-cell stage, recombinant Cas9 protein, tracrRNA and crRNA for dead end (dnd), which is essential for germ cell development, were injected into one-cell stage embryos of olvas-DsRedExpress transgenic medaka. This allowed germ cells to be visualized by DsRed fluorescence. Genomic DNA extracted from embryos at the one-cell stage was analyzed by sequencing. Predictably, biallelic mutated sequence patterns in the target sites of dnd were found in the injected embryos. To investigate the phenotypes of the mutated fish, fluorescent and histological observations of germ cells were carried out using fry and adults. The mutations resulted in a complete loss of germ cells, suggesting loss of function of dnd in the injected embryos. Therefore, this system appears to be extremely effective for the production of F0 knockout medaka.

Identification of differentially-expressed genes in early developmental ovary of Yellow River carp (Cyprinus carpio var) using Suppression Subtractive Hybridization

Ovary development appears to be under polygenic control, and is influenced by multiple genetic factors that may vary from organism to organism. To gain a better insight into the molecular mechanisms of carp ovary development, Suppression Subtractive Hybridization (SSH) DNA libraries in two species of Yellow River carp were analyzed. Primordial gonads and stage II ovaries were used as testers, and adult ovaries as drivers. One hundred and fifty differentially-expressed candidate genes were examined by Southern blot microarray hybridization. We identified 41 differentially-expressed genes in the PG (Primordial gonad) library and 37 in the stage II ovary library. Gene Ontology Biological Pathway analysis showed the genes were involved in signal transduction, proteolysis process, cell differentiation, TGF-β signal and other biological responses. Twenty-two candidate genes were selected and further characterized using qRT-PCR. Pvalb, epd, and MYH were found specifically expressed in PG, while bmp2b, desmin and fp1 were specifically expressed in stage II ovary. Our results indicate that these genes could be used as biomarkers of the early development of carp ovary. This finding will provide a basis for further understanding of the complex gonad developmental molecular mechanisms in Yellow River carp.

Germline replacement by blastula cell transplantation in the fish medaka

Primordial germ cell (PGC) specification early in development establishes the germline for reproduction and reproductive technologies. Germline replacement (GR) is a powerful tool for conservation of valuable or endangered animals. GR is achievable by germ cell transplantation into the PGC migration pathway or gonads. Blastula cell transplantation (BCT) can also lead to the chimeric germline containing PGCs of both donor and host origins. It has remained largely unknown whether BCT is able to achieve GR at a high efficiency. Here we report efficient GR by BCT into blastula embryos in the fish medaka (Oryzias latipes). Specifically, dnd depletion completely ablated host PGCs and fertility, and dnd overexpression remarkably boosted PGCs in donor blastulae. BCT between normal donor and host produced a germline transmission rate of ~4%. This rate was enhanced up to ~30% upon PGC boosting in donors. Most importantly, BCT between PGC-boosted donors and PGC-ablated hosts led to more than 90% fertility restoration and 100% GR. Therefore, BCT features an extremely high efficiency of fertility recovery and GR in medaka. This finding makes medaka an ideal model to analyze genetic and physiological donor-host compatibilities for BCT-mediated surrogate production and propagation of endangered lower vertebrates and biodiversity.

Molecular characterization and expression pattern of a germ cell marker gene dnd in gibel carp (Carassius gibelio)

As a germ cell marker gene, Dead end (dnd) has been identified and characterized in many vertebrates. Recently, we created a complete germ cell-depleted gonad model by the dnd-specific morpholino-mediated knockdown approach, and revealed sex-biased gene expression alteration through utilizing unisexual gynogenetic superiority in polyploid gibel carp. However, dnd and its expression pattern are still unclear in the gibel carp. In this study, we further analyzed molecular characterization of gibel carp dnd and its dynamic expression pattern during gametogenesis and embryogenesis. Similar to other homologs in vertebrates, gibel carp dnd contains a conserved RRM motif and five other motifs, and is highly evolutionary conserved in genomic organization and neighborhood gene synteny. RT-PCR and Western blot analyses showed its gonad-specific expression intensively in testis and ovary. Section in situ hybridization (SISH) and immunofluorescence localization revealed its dynamic expression pattern specific to oogenic cells and spermatogenetic cells during oogenesis and spermatogenesis. Moreover, its temporal and spatial distribution specific to PGCs were also demonstrated by RT-PCR and whole mount in situ hybridization (WISH) during embryogenesis. Therefore, gibel carp Dnd is a conserved germ cell marker during gametogenesis, and its maternal transcript is also a useful marker for tracing PGC specification and migration.

Dazl is a critical player for primordial germ cell formation in medaka

The DAZ family genes boule, daz and dazl have conserved functions in primordial germ cell (PGC) migration, germ stem cell proliferation, differentiation and meiosis progression. It has remained unknown whether this family is required for PGC formation in developing embryos. Our recent study in the fish medaka (Oryzias latipes) has defined dnd as the critical PGC specifier and predicted the presence of additional factors essential for PGC formation. Here we report that dazl is a second key player for medaka PGC formation. Dazl knockdown did not prevent PGC formation even in the absence of normal somatic structures. It turned out that a high level of Dazl protein was maternally supplied and persisted until gastrulation, and hardly affected by two antisense morpholino oligos targeting the dazl RNA translation. Importantly, microinjection of a Dazl antibody remarkably reduced the number of PGCs and even completely abolished PGC formation without causing detectable somatic abnormality. Therefore, medaka PGC formation requires the Dazl protein as maternal germ plasm component, offering first evidence that dazl is a critical player in PGC formation in vivo. Our results demonstrate that antibody neutralization is a powerful tool to study the roles of maternal protein factors in PGC development in vivo.