Sexual dimorphic expression of dnd in germ cells during sex reversal and its requirement for primordial germ cell survival in protogynous hermaphroditic grouper

Production of all-male non-transgenic zebrafish by conditional primordial germ cell ablation

Many fish species exhibit remarkable sexual dimorphism, with males possessing numerous advantageous traits for commercial production by aquaculture such as faster growth rate, more efficient food energy utilization for muscle development, and better breeding performance. Several studies have shown that a decrease in the number of primordial germ cells (PGCs) during early development leads predominantly to male progeny. In this study, we developed a method to obtain all-male zebrafish (Danio rerio) by targeted PGC ablation using the nitroreductase/metronidazole (NTR/Mtz) system. Embryos generated by female heterozygous Tg(nanos3:nfsB-mCherry-nanos3 3'UTR) and male wild-types (WTs) were treated with vehicle or Mtz. Compared to vehicle-treated controls, 5.0 and 10.0 mM Mtz treatment for 24 h significantly reduced the number of PGCs and yielded an exclusively male phenotype in adulthood. The gonads of offspring treated with 5.0 mM Mtz exhibited relatively normal morphology and histological characteristics. Furthermore, these males were able to chase females, spawn, and produce viable offspring, while about 20.0% of males treated with 10.0 mM Mtz were unable to produce viable offspring. The 5.0 mM Mtz treatment protocol may thus be suitable for large-scale production of fertile male offspring. Moreover, about half of these males were WT as evidenced by the absence of nfsB gene expression. It may thus be possible to breed an all-male WT fish population by Mtz-mediated PGC ablation.

Characterization of ddx4 and dnd Homologs in Snakeskin Gourami (Trichopodus pectoralis) and Their Expression Levels during Larval Development and in Gonads of Males and Females

The purpose of this study was to clone and characterize ddx4 and dnd1 homologs in snakeskin gourami (Trichopodus pectoralis) and to determine their expression levels during larval development and in the gonads of males and females. Both cDNAs contained predicted regions that shared consensus motifs with the ddx4 family in teleosts and the dnd family in vertebrates. Phylogenetic tree construction analysis confirmed that these two genes were clustered in the families of teleosts. Both ddx4 and dnd1 mRNAs were detectable only in the gonads, particularly in germ cells. These two genes were expressed during early larval development. The expression of ddx4 was high during early larval development and decreased with increasing developmental age, whereas dnd1 expression increased with developmental age. In adult fish, the expression levels of both genes were higher in the ovary than in the testis. Overall, these findings provide valuable molecular information on ddx4 and dnd, and can be applied in future reproductive biological studies relating to sex dimorphism in snakeskin gourami.

Establishment of a Spermatogonial Stem Cell Line with Potential of Meiosis in a Hermaphroditic Fish, Epinephelus coioides

Spermatogonial stem cells (SSCs) are unique adult stem cells capable of self-renewal and differentiation into sperm. Grouper is a protogynous hermaphroditic fish farmed widely in the tropical and subtropical seas. In this study, we established an SSC line derived from adult testis of orange-spotted grouper, Epinephelus coioides. In the presence of basic fibroblast growth factor (bFGF) and leukemia inhibitory factor (LIF), the cells could be maintained with proliferation and self-renewal over 20 months and 120 passages under in vitro culture conditions. The cells exhibited strong alkaline phosphatase activity and the characteristics of SSCs with the expression of germ cell markers, including Vasa, Dazl, and Plzf, as well as the stem cell markers Nanog, Oct4, and Ssea1. Furthermore, the cultured cells could be induced by 11-ketotestosterone treatment to highly express the meiotic markers Rec8, Sycp3, and Dmc1, and produce some spherical cells, and even sperm-like cells with a tail. The findings of this study suggested that the cultured grouper SSC line would serve as an excellent tool to study the molecular mechanisms behind SSCs self-renewal and differentiation, meiosis during spermatogenesis, and sex reversal in hermaphroditic vertebrates. Moreover, this SSC line has great application value in grouper fish aquaculture, such as germ cell transplantation, genetic manipulation, and disease research.

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.

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.

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.

Visualizing primordial germ cell migration in embryos of rice field eel (Monopterus albus) using fluorescent protein tagged 3' untranslated regions of nanos3, dead end and vasa

In rice field eel (Monopterus albus), germ cell development in the developing gonad has been revealed in detail. However, it is unclear how primordial germ cells (PGCs) migrate to the somatic part of the gonad (genital ridge). This study visualized PGC migration by injecting a chimeric mRNA containing a fluorescent protein fused to the 3' untranslated region (3'UTR) of three different genes, nanos3 of zebrafish (Danio rerio) and dead end (dnd) and vasa of rice field eel. The mRNAs were injected either alone or in pairs into embryos at the one-cell stage. The results showed that mRNAs containing nanos3 and dnd 3'UTRs labeled PGCs over a wider time frame than those containing vasa 3'UTR, suggesting that nanos3 and dnd 3'UTRs are suitable for visualizing PGCs in rice field eel. Using this direct visualization method, the normal migration route of PGCs was observed from the 50%-epiboly stage to hatching stage for the first time, and the ectopic PGCs were also visualized during this period in rice field eel. These findings extend our knowledge of germ cell development, and lay a foundation for further research on the relationship between PGCs and sex differentiation, and on incubation conditions for embryos in rice field eel.

Molecular characterization and expression patterns of stem-loop binding protein (SLBP) genes in protogynous hermaphroditic grouper, Epinephelus coioides

Stem-loop binding protein (SLBP) binds a stem-loop structure of the mRNA, which is important for the stability of histone mRNAs and translation process. In the present study, two slbp cDNAs (Ecslbp1 and Ecslbp2) were cloned from a protogynous hermaphroditic orange-spotted grouper, Epinephelus coioides. Ecslbp1 cDNA contained a 678 base pair (bp) open reading frame (ORF), encoding a predicted polypeptide of 225 amino acids. Ecslbp2 cDNA contained a 1041 bp, encoding a predicted protein of 346 amino acids. The result of real-time PCR revealed that Ecslbp2 mRNA was exclusively detected in the ovary. Moreover, it was found to be restricted to oocytes according to in situ hybridization (ISH) analysis. Ecslbp2 was found to be hardly detected in gonia and significantly increase in the cytoplasm of primary-growth stage oocytes, but decreased during the process of vitellogenesis. Interestingly, Ecslbp2 expression centralized as a perinuclear speckle in early-primary-growth stage oocytes, which appeared to form into the Balbiani body (Bb) in late-primary-growth stage oocytes. These data indicated that Ecslbp2 might play an important role in the process of oocyte development, and could serve as an oocyte-specific molecular marker for the study of ovary development and sex reversal in groupers.

The germ cell marker dead end reveals alternatively spliced transcripts with dissimilar expression

Since the late 19th century, the Amazon species Colossoma macropomum (tambaqui) has been exploited commercially and the climate change has contributed to decline in tambaqui numbers. Although germ cell cryopreservation and transplantation can help preserve the species’ genetic resources semipermanently, its germ cell behavior has not been analyzed to date. In this study, we isolated the tambaqui’s dead end gene (dnd) homolog (tdnd) and used it as a molecular marker for germ cells to obtain basic information essential for transplantation. The amino acid sequence showed 98% similarity and 53% identity with the zebrafish dnd. Phylogenetic analysis and the presence of consensus motifs known for dnd revealed that tdnd encodes the dnd ortholog and its transcript is detectable only in the testes and ovaries, showing a strong positive signal in oocytes and spermatogonia. The tambaqui possesses, at least, three different transcripts of tdnd which show dissimilar expression profile in undifferentiated and sexually mature animals, suggesting that they play distinct roles in germline development and they may influence the choice of donors for the cell transplantation study.

Molecular characterization of vasa homologue in marbled goby, Oxyeleotris marmorata: Transcription and localization analysis during gametogenesis and embryogenesis

Identification of germ cell markers is important for investigating reproduction biology in fish. Vasa is one of the most studied germ cell markers in mammals and lower vertebrates including fish. Here, we characterized a vasa homologue from the fish marbled goby (Oxyeleotris marmorata), termed omvasa. The full length of omvasa cDNA is 2344 bp and encodes 658 amino acids, sharing high identities with Vasa homologues of other vertebrates. OmVasa protein contains 15 RG/RGG repeats at N-terminus, 2 ATPase motifs, as well as RNA unwinding and RNA binding domains at C-terminus. Phylogenetic tree showed that omVasa had the closest relationship with the Vasa homologue from the fish Boleophthalmus pectinirostris, the great blue-spotted mudskipper. qRT-PCR analysis indicated that omvasa was specifically transcribed in gonads, and the transcription level was gradually increased during oocyte development. The germ cell-specific distribution of omvasa mRNA was revealed by fluorescent in situ hybridization. In ovary, the signal of omvasa RNA displayed strong-weak-strong dynamics from oogonia over pre-vitellogenic oocytes to vitellogenic oocytes. In testis, omvasa signal was strong in spermatogonia, modest in spermatocytes but undetectable in spermatids and somatic cells. During embryogenesis, the transcription of omvasa mRNA was high at blastula stage, gradually decreased from gastrula stage and maintained at a low level in later developmental stages. Whole mount in situ hybridization indicated that omvasa mRNA was specific to primordial germ cells (PGCs). In summary, marbled goby vasa is a germ cell-specific transcript during gametogenesis, and can be used as an ideal marker for tracing PGC formation and migration, which is pivotal to germ cell manipulation in this species.

Divergent Expression Patterns and Function of Two cxcr4 Paralogs in Hermaphroditic Epinephelus coioides

Chemokine receptor Cxcr4 evolved two paralogs in the teleost lineage. However, cxcr4a and cxcr4b have been characterized only in a few species. In this study, we identified two cxcr4 paralogs from the orange-spotted grouper, Epinephelus coioides. The phylogenetic relationship and gene structure and synteny suggest that the duplicated cxcr4a/b should result from the teleost-specific genome duplication (Ts3R). The teleost cxcr4 gene clusters in two paralogous chromosomes exhibit a complementary gene loss/retention pattern. Ec_cxcr4a and Ec_cxcr4b show differential and biased expression patterns in grouper adult tissue, gonads, and embryos at different stages. During embryogenesis, Ec_cxcr4a/b are abundantly transcribed from the neurula stage and mainly expressed in the neural plate and sensory organs, indicating their roles in neurogenesis. Ec_Cxcr4a and Ec_Cxcr4b possess different chemotactic migratory abilities from the human SDF-1α, Ec_Cxcl12a, and Ec_Cxcl12b. Moreover, we uncovered the N-terminus and TM5 domain as the key elements for specific ligand⁻receptor recognition of Ec_Cxcr4a-Ec_Cxcl12b and Ec_Cxcr4b-Ec_Cxcl12a. Based on the biased and divergent expression patterns of Eccxcr4a/b, and specific ligand⁻receptor recognition of Ec_Cxcl12a/b⁻Ec_Cxcr4b/a, the current study provides a paradigm of sub-functionalization of two teleost paralogs after Ts3R.

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.

Sex steroid production associated with puberty is absent in germ cell-free salmon

In all vertebrates studied so far, germ cells are not required for pubertal maturation of the gonadal steroidogenic system, subsequent development of secondary sex characteristics and reproductive behavior. To explore if the absence of germ cells affects puberty or growth in Atlantic salmon, germ cell-free (GCF), dnd knockout and wild type (WT) postsmolts were stimulated to enter puberty. No GCF fish entered puberty, whereas 66.7% (males) and 30% (females) WT fish completed or entered puberty, respectively. Expression of genes related to steroidogenesis (star, cyp17a1, cyp11β, cyp19a1a), gonadal somatic cells (insl3, amh, igf3), oocytes (bmp15), gonadotropin receptors (fshr, lhcgr), and pituitary gonadotropic cells (fshb, lhb, gnrhr4) showed an immature status and failure to up-regulate gonadal sex steroid production in male and female GCF fish was also reflected in low or undetectable plasma sex steroids (11-ketotestosterone, estradiol-17β and testosterone). A gender difference (high in females, low in males) was found in the expression of star and cyp17a1 in GCF fish. No clear difference in growth was detected between GCF and immature WT fish, while growth was compromised in maturing WT males. We demonstrate for the first time in a vertebrate that germ cells are required for pubertal activation of the somatic steroidogenic cells.