Production of medakafish chimeras from a stable embryonic stem cell line

Profiling the Physiological Roles in Fish Primary Cell Culture

Fish primary cell culture has emerged as a valuable tool for investigating the physiological roles and responses of various cell types found in fish species. This review aims to provide an overview of the advancements and applications of fish primary cell culture techniques, focusing on the profiling of physiological roles exhibited by fish cells in vitro. Fish primary cell culture involves the isolation and cultivation of cells directly derived from fish tissues, maintaining their functional characteristics and enabling researchers to study their behavior and responses under controlled conditions. Over the years, significant progress has been made in optimizing the culture conditions, establishing standardized protocols, and improving the characterization techniques for fish primary cell cultures. The review highlights the diverse cell types that have been successfully cultured from different fish species, including gonad cells, pituitary cells, muscle cells, hepatocytes, kidney and immune cells, adipocyte cells and myeloid cells, brain cells, primary fin cells, gill cells, and other cells. Each cell type exhibits distinct physiological functions, contributing to vital processes such as metabolism, tissue regeneration, immune response, and toxin metabolism. Furthermore, this paper explores the pivotal role of fish primary cell culture in elucidating the mechanisms underlying various physiological processes. Researchers have utilized fish primary cell cultures to study the effects of environmental factors, toxins, pathogens, and pharmaceutical compounds on cellular functions, providing valuable insights into fish health, disease pathogenesis, and drug development. The paper also discusses the application of fish primary cell cultures in aquaculture research, particularly in investigating fish growth, nutrition, reproduction, and stress responses. By mimicking the in vivo conditions in vitro, primary cell culture has proven instrumental in identifying key factors influencing fish health and performance, thereby contributing to the development of sustainable aquaculture practices.

Medaka (Oryzias latipes) Olpax6.2 acquires maternal inheritance and germ cells expression, but was functionally degenerated in the eye

Gene duplication provides raw material for the evolution of genetic and phenotypic complexity. It has remained a long-standing mystery how duplicated genes evolve into new genes by neofunctionalization via the acquisition of new expression and/or activity and simultaneous loss of the old expression and activity. Fishes have many gene duplicates from whole genome duplication, making them excellent for studying the evolution of gene duplicates. In the fish medaka (Oryzias latipes), an ancestral pax6 gene has given rise to Olpax6.1 and Olpax6.2. Here we report that medaka Olpax6.2 is evolving towards neofunctionalization. A chromosomal syntenic analysis indicated that Olpax6.1 and Olpax6.2 are structurally co-homologous to the single pax6 in other organisms. Interestingly, Olpax6.2 maintains all conserved coding exons but loses the non-coding exons of Olpax6.1, and has 4 promoters versus 8 in Olpax6.1. RT-PCR revealed that Olpax6.2 maintained expression in the brain eye, pancreas as Olpax6.1. Surprisingly, Olpax6.2 also exhibited maternal inheritance and gonadal expression by RT-PCR, in situ hybridization and RNA transcriptome analysis. The expression and distribution of Olpax6.2 is not different from Olpax6.1 in the adult brain, eye and pancreas, but exhibited overlapping and distinct expression in early embryogenesis. We show that ovarian Olpax6.2 expression occurs in female germ cells. Olpax6.2 knockout showed no obvious defect in eye development, while Olpax6.1 F0 mutant have severe defects in eye development. Thus, Olpax6.2 has acquired maternal inheritance and germ cell expression, but was functionally degenerated in the eye, making this gene as an excellent model to study the neofunctionalization of duplicated genes.

Establishment of an Integrated CRISPR/Cas9 Plasmid System for Simple and Efficient Genome Editing in Medaka In Vitro and In Vivo

Although CRISPR/Cas9 has been used in gene manipulation of several fish species in vivo, its application in fish cultured cells is still challenged and limited. In this study, we established an integrated CRISPR/Cas9 plasmid system and evaluated its efficiency of gene knock-out or knock-in at a specific site in medaka (Oryzias latipes) in vitro and in vivo. By using the enhanced green fluorescent protein reporter plasmid pGNtsf1, we demonstrate that pCas9-U6sgRNA driven by endogenous U6 promoter (pCas9-mU6sgRNA) mediated very high gene editing efficiency in medaka cultured cells, but not by exogenous U6 promoters. After optimizing the conditions, the gene editing efficiencies of eight sites targeting for four endogenous genes were calculated, and the highest was up to 94% with no detectable off-target. By one-cell embryo microinjection, pCas9-mU6sgRNA also mediated efficient gene knock-out in vivo. Furthermore, pCas9-mU6sgRNA efficiently mediated gene knock-in at a specific site in medaka cultured cells as well as embryos. Collectively, our study demonstrates that the genetic relationship of U6 promoter is critical to gene editing efficiency in medaka cultured cells, and a simple and efficient system for medaka genome editing in vitro and in vivo has been established. This study provides an insight into other fish genome editing and promotes gene functional analysis.

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

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.

Defining the Pluripotent Marker Genes for Identification of Teleost Fish Cell Pluripotency During Reprogramming

Pluripotency is a transient state in early embryos, which is regulated by an interconnected network of pluripotency-related genes. The pluripotent state itself seems to be highly dynamic, which leads to significant differences in the description of induced pluripotent stem cells from different species at the molecular level. With the application of cell reprogramming technology in fish, the establishment of a set of molecular standards for defining pluripotency will be important for the research and potential application of induced pluripotent stem cells in fish. In this study, by BLAST search and expression pattern analysis, we screen out four pluripotent genes (Oct4, Nanog, Tdgf1, and Gdf3) in zebrafish (Danio rerio) and crucian carp (Carassius). These genes were highly expressed in the short period of early embryonic development, but significantly down-regulated after differentiation. Moreover, three genes (Oct4, Nanog and Tdgf1) have been verified that are suitable for identifying the pluripotency of induced pluripotent stem cells in zebrafish and crucian carp. Our study expands the understanding of the pluripotent markers of induced pluripotent stem cells in fish.

Proteome Analysis of Whole-Body Responses in Medaka Experimentally Exposed to Fish-Killing Dinoflagellate Karenia mikimotoi

Karenia mikimotoi is a well-known harmful algal bloom species. Blooms of this dinoflagellate have become a serious threat to marine life, including fish, shellfish, and zooplanktons and are usually associated with massive fish death. Despite the discovery of several toxins such as gymnocins and gymnodimines in K. mikimotoi, the mechanisms underlying the ichthyotoxicity of this species remain unclear, and molecular studies on this topic have never been reported. The present study investigates the fish-killing mechanisms of K. mikimotoi through comparative proteomic analysis. Marine medaka, a model fish organism, was exposed to K. mikimotoi for a three-part time period (LT25, LT50 and LT90). Proteins extracted from the whole fish were separated by using two-dimensional gel electrophoresis, and differentially expressed proteins were identified with reference to an untreated control. The change in fish proteomes over the time-course of exposure were analyzed. A total of 35 differential protein spots covering 19 different proteins were identified, of which most began to show significant change in expression levels at the earliest stage of intoxication. Among the 19 identified proteins, some are closely related to the oxidative stress responses, energy metabolism, and muscle contraction. We propose that oxidative stress-mediated muscle damage might explain the symptoms developed during the ichthyotoxicity test, such as gasping for breath, loss of balance, and body twitching. Our findings lay the foundations for more in-depth studies of the mechanisms of K. mikimotoi's ichthyotoxicity.

Fish primary embryonic pluripotent cells assemble into retinal tissue mirroring in vivo early eye development

Organoids derived from pluripotent stem cells promise the solution to current challenges in basic and biomedical research. Mammalian organoids are however limited by long developmental time, variable success, and lack of direct comparison to an in vivo reference. To overcome these limitations and address species-specific cellular organization, we derived organoids from rapidly developing teleosts. We demonstrate how primary embryonic pluripotent cells from medaka and zebrafish efficiently assemble into anterior neural structures, particularly retina. Within 4 days, blastula-stage cell aggregates reproducibly execute key steps of eye development: retinal specification, morphogenesis, and differentiation. The number of aggregated cells and genetic factors crucially impacted upon the concomitant morphological changes that were intriguingly reflecting the in vivo situation. High efficiency and rapid development of fish-derived organoids in combination with advanced genome editing techniques immediately allow addressing aspects of development and disease, and systematic probing of impact of the physical environment on morphogenesis and differentiation.

Virus susceptibility of a new cell line derived from the fin of black carp Mylopharyngodon piceus

A continuous cell line MPF derived from the fin of black carp Mylopharyngodon piceus was established and characterised in this study. Mylopharyngodon piceus fin (MPF) cells were subcultured for more than 80 passages with high viability recovery after long-term storage. The karyotyping analysis revealed that MPF had a modal diploid chromosome number (2n = 48) and identical ribosomal RNA sequence with black carp. In addition, the expression of pluripotency-associated markers including nanog, oct4 and vasa, were detected in MPF. The transient transfection efficiency of MPF reached 23% with a fluorescent reporter by modified electroporation and stable expression of red fluorescent MPF was established by the baculovirus system, indicating that MPF is an ideal platform for studying gene functions in vitro. Lastly, cytopathic effects were also observed and RNA transcripts of a viral gene increased after infection by spring viremia of carp virus (SVCV), suggesting that MPF could be an alternative tool for investigating pathogen-host interactions in black carp. In conclusion, a fin cell line that is susceptible to SVCV was established as a potential adult stem-cell line, providing a suitable tool for future genetic analyses and pathogen-host studies in black carp.

A novel evolutionary conserved mechanism of RNA stability regulates synexpression of primordial germ cell-specific genes prior to the sex-determination stage in medaka

Dmrt1 is a highly conserved transcription factor, which is critically involved in regulation of gonad development of vertebrates. In medaka, a duplicate of dmrt1-acting as master sex-determining gene-has a tightly timely and spatially controlled gonadal expression pattern. In addition to transcriptional regulation, a sequence motif in the 3' UTR (D3U-box) mediates transcript stability of dmrt1 mRNAs from medaka and other vertebrates. We show here that in medaka, two RNA-binding proteins with antagonizing properties target this D3U-box, promoting either RNA stabilization in germ cells or degradation in the soma. The D3U-box is also conserved in other germ-cell transcripts, making them responsive to the same RNA binding proteins. The evolutionary conservation of the D3U-box motif within dmrt1 genes of metazoans-together with preserved expression patterns of the targeting RNA binding proteins in subsets of germ cells-suggest that this new mechanism for controlling RNA stability is not restricted to fishes but might also apply to other vertebrates.

Efficient and Stable Delivery of Multiple Genes to Fish Cells by a Modified Recombinant Baculovirus System

The recombinant baculovirus has been widely used as an efficient tool to mediate gene delivery into mammalian cells but has barely been used in fish cells. In the present study, we constructed a recombinant baculovirus containing the dual-promoter cytomegalovirus (CMV) and white spot syndrome virus (WSSV) immediate-early gene 1 (ie1) (WSSV ie1), followed by a puromycin⁻green fluorescent protein (Puro-GFP, pf) or puromycin⁻red fluorescent protein (Puro-RFP, pr) cassette, which simultaneously allowed for easy observation, rapid titer determination, drug selection, and exogenous gene expression. This recombinant baculovirus was successfully transduced into fish cells, including Mylopharyngodon piceus bladder (MPB), fin (MPF), and kidney (MPK); Oryzias latipes spermatogonia (SG3); and Danio rerio embryonic fibroblast (ZF4) cells. Stable transgenic cell lines were generated after drug selection, which was further verified by Western blot. A cell monoclonal formation assay proved the stable heredity of transgenic MPB cells. In addition, a recombinant baculovirus containing a pr cassette and four transcription factors for induced pluripotent stem cells (iPSC) was constructed and transduced into ZF4 cells, and these exogenous genes were simultaneously delivered and transcribed efficiently in drug-selected ZF4 cells, proving the practicability of this modified recombinant baculovirus system. We also proved that the WSSV ie1 promoter had robust activity in fish cells in vitro and in vivo. Taken together, this modified recombinant baculovirus can be a favorable transgenic tool to obtain transient or stable transgenic fish cells.

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.

Efficient genome editing using CRISPR/Cas9 ribonucleoprotein approach in cultured Medaka fish cells

Gene editing with CRISPR/Cas9 is a powerful tool to study the function of target genes. Although this technology has demonstrated wide efficiency in many species, including fertilized zebrafish and medaka fish embryos when microinjected, its application to achieve efficient gene editing in cultured fish cells have met some difficulty. Here, we report an efficient and reliable approach to edit genes in cultured medaka (Oryzias latipes) fish cells using pre-formed gRNA-Cas9 ribonucleoprotein (RNP) complex. Both medaka fish haploid and diploid cells were transfected with the RNP complex by electroporation. Efficient gene editing was demonstrated by polymerase chain reaction (PCR) amplification of the target gene from genomic DNA and heteroduplex mobility assay carried out with polyacrylamide gel electrophoresis (PAGE). The heteroduplex bands caused by RNP cleavage and non-homologous end joining could be readily detected by PAGE. DNA sequencing confirmed that these heteroduplex bands contains the mutated target gene sequence. The average gene editing efficiency in haploid cells reached 50%, enabling us to generate a clonal cell line with ntrk3b gene mutation for further study. This RNP transfection method also works efficiently in diploid medaka cells, with the highest mutation efficiency of 61.5%. The specificity of this synthetic RNP CRISPR/Cas9 approach was verified by candidate off-target gene sequencing. Our result indicated that transfection of pre-formed gRNA-Cas9 RNP into fish cells is efficient and reliable to edit target genes in cultured medaka fish cells. This method will be very useful for gene function studies using cultured fish cells.

Isolation and in vitro culture of ovarian stem cells in Chinese soft-shell turtle (Pelodiscus sinensis)

Gonadal cell lines provide valuable tools for studying gametogenesis, sex differentiation, and manipulating germ cells in reproductive biology. Female germline stem cells have been characterized and isolated from ovaries of mammalian species, including mice and human, but there has been very few studies on female germline stem cells in reptiles. Here, we described an ovarian stem cell-like line isolated and cultured from the Chinese soft-shell turtle (Pelodiscus sinensis), designated as PSO1. The cells showed high alkaline phosphatase activity with a normal diploid karyotype. As shown by reverse transcription-polymerase chain reaction, the cells were positive for the expression of germ cell-specific genes, vasa and dazl, as well as a stem cell marker, nanog, but negative for the expression of the folliculogenesis-specific gene, figla. Likewise, through fluorescent immunostaining analyses, both the Dazl and Vasa proteins were detected abundantly in the cytoplasm of perinuclear region, whereas Nanog and PCNA were dominantly observed in the nuclei in PSO1 cells. Moreover, PSO1 cells transfected with pCS2:h2b-egfp could properly express the fusion protein in the nuclei. Taken together, the findings suggested that the germline stem cells exist in the ovary of juvenile Chinese soft-shell turtle and these cells can be isolated for a long-term in vitro culture under experimental conditions. This study has provided a valuable basis for further investigations on the molecular mechanisms whereby the germline stem cells develop and differentiate into gametes in turtles. Also, it has paved the way for studies on oogenesis in turtles, even in the other reptiles.

Establishment and growth responses of Nile tilapia embryonic stem-like cell lines under feeder-free condition

Embryonic stem (ES) cells provide an invaluable tool for molecular analysis of vertebrate development and a bridge linking genomic manipulations in vitro and functional analysis of target genes in vivo. Work towards fish ES cells so far has focused on zebrafish (Danio renio) and medaka (Oryzias latipes). Here we describe the derivation, pluripotency, differentiation and growth responses of ES cell lines from Nile tilapia (Oreochromis niloticus), a world-wide commercial farmed fish. These cell lines, designated as TES1-3, were initiated from blastomeres of Nile tilapia middle blastula embryos (MBE). One representative line, TES1, showed stable growth and phenotypic characteristics of ES cells over 200 days of culture with more than 59 passages under feeder-free conditions. They exhibited high alkaline phosphatase activity and expression of pluripotency genes including pou5f3 (the pou5f1/oct4 homologue), sox2, myc and klf4. In suspension culture together with retinoic acid treatment, TES1 cells formed embryoid bodies, which exhibited expression profile of differentiation genes characteristics of all three germ cell layers. Notably, PKH26-labeled TES1 cells introduced into Nile tilapia MBE could contribute to body compartment development and led to hatched chimera formation with an efficacy of 13%. These results suggest that TES1 cells have pluripotency and differentiation potential in vitro and in vivo. In the conditioned DMEM, all of the supplements including the fetal bovine serum, fish embryonic extract, fish serum, basic fibroblast growth factor and non-protein supplement combination 5N were mitogenic for TES1 cell growth. This study will promote ES-based biotechnology in commercial fish.

Regulation of Zygotic Genome and Cellular Pluripotency

Events, manifesting transition from maternal to zygotic period of development are studied for more than 100 years, but underlying mechanisms are not yet clear. We provide a brief historical overview of development of concepts and explain the specific terminology used in the field. We further discuss differences and similarities between the zygotic genome activation and in vitro reprogramming process. Finally, we envision the future research directions within the field, where biochemical methods will play increasingly important role.

Singapore grouper iridovirus protein VP088 is essential for viral infectivity

Viral infection is a great challenge in healthcare and agriculture. The Singapore grouper iridovirus (SGIV) is highly infectious to numerous marine fishes and increasingly threatens mariculture and wildlife conservation. SGIV intervention is not available because little is known about key players and their precise roles in SGVI infection. Here we report the precise role of VP088 as a key player in SGIV infection. VP088 was verified as an envelope protein encoded by late gene orf088. We show that SGIV could be neutralized with an antibody against VP088. Depletion or deletion of VP088 significantly suppresses SGIV infection without altering viral gene expression and host responses. By precisely quantifying the genome copy numbers of host cells and virions, we reveal that VP088 deletion dramatically reduces SGIV infectivity through inhibiting virus entry without altering viral pathogenicity, genome stability and replication and progeny virus release. These results pinpoint that VP088 is a key player in SGIV entry and represents an ideal target for SGIV intervention.

Subcellular redistribution and sequential recruitment of macromolecular components during SGIV assembly

Virus infection consists of entry, synthesis of macromolecular components, virus assembly and release. Understanding of the mechanisms underlying each event is necessary for the intervention of virus infection in human healthcare and agriculture. Here we report the visualization of Singapore grouper iridovirus (SGIV) assembly in the medaka haploid embryonic stem (ES) cell line HX1. SGIV is a highly infectious DNA virus that causes a massive loss in marine aquaculture. Ectopic expression of VP88GFP, a fusion between green fluorescent protein and the envelope protein VP088, did not compromise the ES cell properties and susceptibility to SGIV infection. Although VP88GFP disperses evenly in the cytoplasm of non-infected cells, it undergoes aggregation and redistribution in SGIV-infected cells. Real-time visualization revealed multiple key stages of VP88GFP redistribution and the dynamics of viral assembly site (VAS). Specifically, VP88GFP entry into and condensation in the VAS occurred within a 6-h duration, a similar duration was observed also for the release of VP88GFP-containing SGIV out of the cell. Taken together, VP088 is an excellent marker for visualizing the SGIV infection process. Our results provide new insight into macromolecular component recruitment and SGIV assembly.

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.

Evolution and functions of Oct4 homologs in non-mammalian vertebrates

PouV class transcription factor Oct4/Pou5f1 is a central regulator of indefinite pluripotency in mammalian embryonic stem cells (ESCs) but also participates in cell lineage specification in mouse embryos and in differentiating cell cultures. The molecular basis for this versatility, which is shared between Oct4 and its non-mammalian homologs Pou5f1 and Pou5f3, is not yet completely understood. Here, I review the current understanding of the evolution of PouV class transcription factors and discuss equivalent and diverse roles of Oct4 homologs in pluripotency, differentiation, and cell behavior in different vertebrate embryos. This article is part of a Special Issue entitled: The Oct Transcription Factor Family, edited by Dr. Dean Tantin.

Zygotic Genome Activators, Developmental Timing, and Pluripotency

The transcription factors Pou5f1, Sox2, and Nanog are central regulators of pluripotency in mammalian ES and iPS cells. In vertebrate embryos, Pou5f1/3, SoxB1, and Nanog control zygotic genome activation and participate in lineage decisions. We review the current knowledge of the roles of these genes in developing vertebrate embryos from fish to mammals and suggest a model for pluripotency gene regulatory network functions in early development.

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

Primordial germ cell (PGC) specification occurs early in development. PGC specifiers have been identified in Drosophila, mouse, and human but remained elusive in most animals. Here we identify the RNA-binding protein Dnd as a critical PGC specifier in the medaka fish (Oryzias latipes). Dnd depletion specifically abolished PGCs, and its overexpression boosted PGCs. We established a single-cell culture procedure enabling lineage tracing in vitro. We show that individual blastomeres from cleavage embryos at the 32- and 64-cell stages are capable of PGC production in culture. Importantly, Dnd overexpression increases PGCs via increasing PGC precursors. Strikingly, dnd RNA forms prominent particles that segregate asymmetrically. Dnd concentrates in germ plasm and stabilizes germ plasm RNA. Therefore, Dnd is a critical specifier of fish PGCs and utilizes particle partition as a previously unidentified mechanism for asymmetric segregation. These findings offer insights into PGC specification and manipulation in medaka as a lower vertebrate model.

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The medaka RNA-binding protein Dnd specifies primordial germ cells

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Cells from medaka cleavage embryos can be singly cultured for lineage tracing

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The dnd RNA forms particles as a new mechanism for asymmetric segregation

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These findings offer new insights into PGC specification and manipulation

The genomic and genetic toolbox of the teleost medaka (Oryzias latipes)

The Japanese medaka, Oryzias latipes, is a vertebrate teleost model with a long history of genetic research. A number of unique features and established resources distinguish medaka from other vertebrate model systems. A large number of laboratory strains from different locations are available. Due to a high tolerance to inbreeding, many highly inbred strains have been established, thus providing a rich resource for genetic studies. Furthermore, closely related species native to different habitats in Southeast Asia permit comparative evolutionary studies. The transparency of embryos, larvae, and juveniles allows a detailed in vivo analysis of development. New tools to study diverse aspects of medaka biology are constantly being generated. Thus, medaka has become an important vertebrate model organism to study development, behavior, and physiology. In this review, we provide a comprehensive overview of established genetic and molecular-genetic tools that render medaka fish a full-fledged vertebrate system.

Medaka Oct4 is essential for pluripotency in blastula formation and ES cell derivation

The origin and evolution of molecular mechanisms underlying cellular pluripotency is a fundamental question in stem cell biology. The transcription factor Oct4 or Pou5f1 identified in mouse features pluripotency expression and activity in the inner cell mass and embryonic stem (ES) cells. Pou2 identified in zebrafish is the non-mammalian homolog prototype of mouse Oct4. The genes oct4 and pou2 have reportedly evolved by pou5 gene duplication in the common ancestor of vertebrates. Unlike mouse oct4, however, zebrafish pou2 lacks pluripotency expression and activity. Whether the presence of pluripotency expression and activity is specific for mammalian Oct4 or common to the ancestor of vertebrate Oct4 and Pou2 proteins has remained to be determined. Here we report that Oloct4, the medaka oct4/pou2, is essential for early embryogenesis and pluripotency maintenance. Oloct4 exists as a single copy gene and is orthologous to pou2 by sequence and chromosome synteny. Oloct4 expression occurs in early embryos, germ stem cells and ES cells like mouse oct4 but also in the brain and tail bud like zebrafish pou2. Importantly, OlOct4 depletion caused blastula lethality or blockage. We show that Oloct4 depletion abolishes ES cell derivation from midblastula embryos. Thus, Oloct4 has pluripotency expression and is essential for early embryogenesis and pluripotency maintenance. Our results demonstrate the conservation of pluripotency expression and activity in vertebrate Oct4 and Pou2 proteins. The finding that Oloct4 combines the features of mouse oct4 and zebrafish pou2 in expression and function suggests that Oloct4 might represent the ancestral prototype of vertebrate oct4 and pou2 genes.

Associations between nucleosome phasing, sequence asymmetry, and tissue-specific expression in a set of inbred Medaka species

BACKGROUND

Transcription start sites (TSSs) with pronounced and phased nucleosome arrays downstream and nucleosome-depleted regions upstream of TSSs are observed in various species.

RESULTS

We have characterized sequence variation and expression properties of this set of TSSs (which we call "Nucleocyclic TSSs") using germline and somatic cells of three medaka (Oryzias latipes) inbred isolates from different locations. We found nucleocyclic TSSs in medaka to be associated with higher gene expression and characterized by a clear boundary in sequence composition with potentially-nucleosome-destabilizing A/T-enrichment upstream (p < 10(-60)) and nucleosome- accommodating C/G-enrichment downstream (p < 10(-40)) that was highly conserved from an ancestor. A substantial genetic distance between the strains facilitated the in-depth analysis of patterns of fixed mutations, revealing a localization-specific equilibrium between the rates of distinct mutation categories that would serve to maintain the conserved sequence anisotropy around TSSs. Downstream of nucleocyclic TSSs, C to T, T to C, and other mutation rates on the sense strand increased around first nucleosome dyads and decreased around first linkers, which contrasted with genomewide mutational patterns around nucleosomes (p < 5 %). C to T rates are higher than G to A rates around nucleosome associated with germline nucleocyclic TSS sites (p < 5 %), potentially due to the asymmetric effect of transcription-coupled repair.

CONCLUSIONS

Our results demonstrate an atypical evolutionary process surrounding nucleocyclic TSSs.

Gene transfer and genome-wide insertional mutagenesis by retroviral transduction in fish stem cells

Retrovirus (RV) is efficient for gene transfer and integration in dividing cells of diverse organisms. RV provides a powerful tool for insertional mutagenesis (IM) to identify and functionally analyze genes essential for normal and pathological processes. Here we report RV-mediated gene transfer and genome-wide IM in fish stem cells from medaka and zebrafish. Three RVs were produced for fish cell transduction: rvLegfp and rvLcherry produce green fluorescent protein (GFP) and mCherry fluorescent protein respectively under control of human cytomegalovirus immediate early promoter upon any chromosomal integration, whereas rvGTgfp contains a splicing acceptor and expresses GFP only upon gene trapping (GT) via intronic in-frame integration and spliced to endogenous active genes. We show that rvLegfp and rvLcherry produce a transduction efficiency of 11~23% in medaka and zebrafish stem cell lines, which is as 30~67% efficient as the positive control in NIH/3T3. Upon co-infection with rvGTgfp and rvLcherry, GFP-positive cells were much fewer than Cherry-positive cells, consistent with rareness of productive gene trapping events versus random integration. Importantly, rvGTgfp infection in the medaka haploid embryonic stem (ES) cell line HX1 generated GTgfp insertion on all 24 chromosomes of the haploid genome. Similar to the mammalian haploid cells, these insertion events were presented predominantly in intergenic regions and introns but rarely in exons. RV-transduced HX1 retained the ES cell properties such as stable growth, embryoid body formation and pluripotency gene expression. Therefore, RV is proficient for gene transfer and IM in fish stem cells. Our results open new avenue for genome-wide IM in medaka haploid ES cells in culture.

Gene replacement by zinc finger nucleases in medaka embryos

Gene replacement (GR) via homologous recombination is a powerful tool for genome editing. Recently, direct GR is achieved successfully by coinjection of mRNAs for engineered endonucleases such as zinc finger nucleases (ZFNs) and donor DNA in developing embryos of diverse organisms. Here, we report the procedures and efficiency for direct GR by using ZFNs in the fish medaka. Upon zygotic coinjection of mRNAs encoding ZFNs that target the gonad-specifically expressed gsdf locus, linear DNA of GR vector pGRgsdf containing the red fluorescent protein (rfp) gene flanked by two homology arms of ~1-kb each underwent GR via homologous recombination. Specifically, 15 of 231 adults from manipulated embryos contained a GR allele in the caudal fin, producing an efficiency of ~7 % for somatic GR. Progeny test revealed that two out of nine fertile fish containing the GR allele in the fin were capable of transmitting the GR allele to ~6 % of F1 generation at adulthood, generating an efficiency of ~22 % for germline transmission. Sequencing and Southern blotting validated precise GR. We show that the GR allele expressed a chimeric gsdf:rfp RNA between gsdf and cointegrated rfp specifically in the gonad, demonstrating recapitulation of endogenous RNA expression as predicted for the defined GR allele. Most importantly, RFP expression coincides faithfully with the gonad-specific gsdf expression in developing embryos and adults. These results demonstrate, for the first time, the feasibility and efficiency of ZFN-mediated precise GR directly in the developing embryo of medaka as a lower vertebrate model.

Derivation and long-term culture of an embryonic stem cell-like line from zebrafish blastomeres under feeder-free condition

Existing zebrafish embryonic stem (ES) cell lines are derived and maintained using feeder layers. We describe here the derivation and long-term culture of an ES cell-like line derived from zebrafish blastomeres without the use of feeder cells. This line, designated as ZES1, has been maintained for more than 800 days in defined Dulbecco's modified Eagle's medium supplemented with fetal bovine serum, zebrafish embryo extract, trout serum, and human basic fibroblast growth factor. ZES1 cells possessed a morphology typical of ES cells, being round or polygonal in shape with a large nucleus and sparse cytoplasm and were mostly diploid. The cells formed individual colonies consisting of tightly packed cells that stained positively for alkaline phosphatase. ZES1 cells also formed embryoid bodies when transferred onto uncoated wells. The pluripotent nature of ZES1 cells was confirmed when they could be induced to differentiate in vitro into several cell types, through low- or high-density culture conditions. Treatment with retinoic acid also induced the differentiation of ZES1 cells into primarily neuronal cells. Using immunostaining and real-time polymerase chain reaction, we showed that Sox2, a known pluripotent marker in mammalian ES cells, was also present in ZES1 cells. Chimera experiments revealed that fluorescent-labeled ZES1 cells microinjected into zebrafish blastulas participated in the formation of all three germ layers. Using GFP-labeled ZES1 cells, chimera germline transmission was also demonstrated at the F1 generation. In conclusion, ZES1 cells possess both in vitro and in vivo pluripotency characteristics, indicating that nonmammalian ES cells can be readily derived and maintained for a long term under feeder-free culture conditions.

Direct gene disruption by TALENs in medaka embryos

Targeted gene disruption (GD) is powerful for generating genetic alterations in animal genomes. Engineered endonucleases such as zinc finger nucleases and transcription activator-like effector nucleases (TALENs) allow for GD directly in animal embryos to achieve germline transmission. Here we report procedures and parameters of TALEN-mediated GD in the fish medaka by using a germ cell-specific gene dnd as a model. Embryos at the 1-cell stage were microinjected with synthetic TALEN mRNAs and examined for the survival rate and GD efficiency. Medaka embryos can tolerate a high dosage of TALEN-mRNA injection and exhibit a steadily increasing GD efficiency with increasing mRNA dosages before peaking at 100 ng/μl. This dosage produced ~24% efficiency for somatic GD. Some of the animals from manipulated embryos developed into fertile female and male. Most importantly, four fish (3 males and 1 female) examined by progeny-test were able to produce GD-bearing male and female gametes for germline transmission to F1 generation at ~10% efficiency. Therefore, TALEN is proficient for somatic and germline GD in medaka embryos, and disruption of one dnd copy does not compromise somatic development and gamete production.

Parameters and efficiency of direct gene disruption by zinc finger nucleases in medaka embryos

Zinc finger nucleases (ZFNs) can generate targeted gene disruption (GD) directly in developing embryos of zebrafish, mouse and human. In the fish medaka, ZFNs have been attempted on a transgene. Here, we developed procedures and parameters for ZFN-mediated direct GD on the gonad-specifically expressed gsdf locus in medaka. A pair of ZFNs was designed to target the first exon of gsdf and their synthetic mRNAs were microinjected into 1-cell stage embryos. We reveal dose-dependent survival rate and GD efficiency. In fry, ZFN mRNA injection at 10 ng/μl led to a GD efficiency of 30 %. This value increased up to nearly 100 % when the dose was enhanced to 40 ng/μl. In a typical series of experiments of ZFN mRNA injection at 10 ng/μl, 420 injected embryos developed into 94 adults, 4 of which had altered gsdf alleles. This leads to a GD efficacy of ∼4 % in the adulthood. Sequencing revealed a wide variety of subtle allelic alterations including additions and deletions of 1∼18 bp in length in ZFN-injected samples. Most importantly, one of the 4 adults examined was capable of germline transmission to 15.2 % of its F1 progeny. Interestingly, ontogenic analyses of the allelic profile revealed that GD commenced early in development, continued during subsequent stages of development and in primordia for different adult organs of the three germ layers. These results demonstrate the feasibility and--for the first time to our knowledge--the efficacy of ZFN-mediated direct GD on a chromosomal gene in medaka embryos.

Derivation and characterization of a ES-like cell line from indian catfish Heteropneustes fossilis blastulas

A cell line designated as HFB-ES was established from blastula stage embryos of H. fossilis (Singhi). The embryonic cells were harvested and maintained in Leibovitz's medium supplemented with 15% fetal bovine serum. The cell line had been subcultured for more than 90 passages in a period of 24 months. HFB-ES cells were able to grow at temperatures between 25 and 35°C with an optimum temperature of 28°C. The growth rate of HFB-ES was proportional to FBS concentration, with optimum growth seen at 15% FBS concentration. The originality of the cell line was confirmed by sequencing of cytochrome oxidase c subunit I (COI), cytochrome b gene, and microsatellite DNA profile. Results of chromosome complements of HFB showed normal karyo-morphology with 56 (2n) diploid number of chromosomes after 40 passages which indicated that the developed cell line is chromosomally stable. The pluripotency of HFB was demonstrated by alkaline phosphatase activity and Oct-4 gene expression. Expression of GFP reporter gene was successful in HFB-ES. These results indicated that HFB-ES could be utilized for future gene expression studies.

Differential evolution of duplicated medakafish mitf genes

Gene duplication is a major force of evolution. One whole genome duplication (WGD) event in the fish ancestor generated genome-wide duplicates in all modern species. Coloration and patterning on the animal body surface exhibit enormous diversity, representing a mysterious and ideal system for understanding gene evolution. Surface colors and patterns are determined primarily by pigment cells in the skin and eye. Thus, microphthalmia-associated transcription factor (Mitf) as a master regulator of melanocyte development is excellent for studying the evolution of WGD-derived gene duplicates. Here we report the evolution of mitf duplicate, mitf1 and mitf2, in the fish medaka (Oryzias latipes), which encode medaka co-homologs Mitf1 and Mitf2 of the mouse Mitf. Compared to mitf1, mitf2 exhibits an accelerated sequence divergence and loses melanocytic expression in embryos at critical developmental stages. Compared to a Xiphophorus counterpart, the medaka Mitf2 displayed a reduced activity in activating melanogenic gene expression by reporter assays and RT-PCR analyses. We show that the medaka Mitf2 has the ability to induce melanocyte differentiation in medaka embryonic stem cells but at a remarkably reduced efficiency compared to the Xiphophorus counterpart. Our data suggest differential evolution of the medaka mitf duplicate, with mitf1 adopting conservation and mitf2 employing degeneration, which is different from the duplication-degeneration-complementation proposed as the mechanism to preserve many gene duplicates in zebrafish. Our finding reveals species-specific variations for mitf duplicate evolution, in agreement with enormous diversity of body coloration and patterning.

Nanos3 gene targeting in medaka ES cells

Gene targeting (GT) by homologous recombination offers the best precision for genome editing in mice. nanos3 is a highly conserved gene and encodes a zinc-finger RNA binding protein essential for germ stem cell maintenance in Drosophila, zebrafish and mouse. Here we report nanos3 GT in embryonic stem (ES) cells of the fish medaka as a lower vertebrate model organism. A vector was designed for GT via homologous recombination on the basis of positive-negative selection (PNS). The ES cell line MES1 after gene transfer and PNS produced 56 colonies that were expanded into ES cell sublines. Nine sublines were GT-positive by PCR genotyping, 4 of which were homologous recombinants as revealed by Southern blot. We show that one of the 4, A15, contains a precisely targeted nanos3 allele without any random events, demonstrating the GT feasibility in medaka ES cells. Importantly, A15 retained all features of undifferentiated ES cells, including stable self-renewal, an undifferentiated phenotype, pluripotency gene expression and differentiation during chimeric embryogenesis. These results provide first evidence that the GT procedure and genuine GT on a chromosomal locus such as nanos3 do not compromise pluripotency in ES cells of a lower vertebrate.

p53 gene targeting by homologous recombination in fish ES cells

Background

Gene targeting (GT) provides a powerful tool for the generation of precise genetic alterations in embryonic stem (ES) cells to elucidate gene function and create animal models for human diseases. This technology has, however, been limited to mouse and rat. We have previously established ES cell lines and procedures for gene transfer and selection for homologous recombination (HR) events in the fish medaka (Oryzias latipes).

Methodology and Principal Findings

Here we report HR-mediated GT in this organism. We designed a GT vector to disrupt the tumor suppressor gene p53 (also known as tp53). We show that all the three medaka ES cell lines, MES1∼MES3, are highly proficient for HR, as they produced detectable HR without drug selection. Furthermore, the positive-negative selection (PNS) procedure enhanced HR by ∼12 folds. Out of 39 PNS-resistant colonies analyzed, 19 (48.7%) were positive for GT by PCR genotyping. When 11 of the PCR-positive colonies were further analyzed, 6 (54.5%) were found to be bona fide homologous recombinants by Southern blot analysis, sequencing and fluorescent in situ hybridization. This produces a high efficiency of up to 26.6% for p53 GT under PNS conditions. We show that p53 disruption and long-term propagation under drug selection conditions do not compromise the pluripotency, as p53-targeted ES cells retained stable growth, undifferentiated phenotype, pluripotency gene expression profile and differentiation potential in vitro and in vivo.

Conclusions

Our results demonstrate that medaka ES cells are proficient for HR-mediated GT, offering a first model organism of lower vertebrates towards the development of full ES cell-based GT technology.

Identification of the RNAs for transcription factor Mitf as a component of the Balbiani body

Balbiani body (BB) is a large distinctive organelle aggregate uniquely present in developing oocytes of diverse animal species. BB is thought as a stage-specific structure that resembles germ plasm, the cytoplasmic organelle of germ cells. The role and function of BB have remained speculative because of a highly dynamic structure and a lack of genetic and molecular data. BB has been found to contain proteins and RNAs, none of them--except the zebrafish foxH1 RNA, is or encodes a transcription factor. Here we report in the fish medaka (Oryzias latipes) that RNAs encoding microphthalmia-associated transcription factor (Mitf) are prominent components of the BB. By fluorescence in situ hybridization on ovarian section, we revealed that the transcripts of both mitf1 and mitf2 genes concentrated in the BB, in which they co-localized with the dazl RNA, a definitive BB marker highly conserved in vertebrates. Therefore, the mitf product may play dual roles in germ gene transcription and BB formation and/or function in this organism. Our data provide the second evidence that the RNA of a transcription factor can be a prominent component of the BB in a vertebrate.

oct4-EGFP reporter gene expression marks the stem cells in embryonic development and in adult gonads of transgenic medaka

Maintenance of pluripotency in stem cells is tightly regulated among vertebrates. One of the key genes in this process is oct4, also referred to as pou5f1 in mammals and pou2 in teleosts. Pou5f1 evolved by duplication of pou2 early in the tetrapod lineage, but only monotremes and marsupials retained both genes. Either pou2 or pou5f1 was lost from the genomes of the other tetrapods that have been analyzed to date. Consequently, these two homologous genes are often designated oct4 in functional studies. In most vertebrates oct4 is expressed in pluripotent cells of the early embryo until the blastula stage, and later persist in germline stem cells until adulthood. The isolation and analysis of stem cells from embryo or adult individuals is hampered by the need for reliable markers that can identify and define the cell populations. Here, we report the faithful expression of EGFP under the control of endogenous pou2/oct4 promoters in transgenic medaka (Oryzias latipes). In vivo imaging in oct4-EGFP transgenic medaka reveals the temporal and spatial expression of pou2 in embryos and adults alike. We describe the temporal and spatial patterns of endogenous pou2 and oct4-EGFP expression in medaka with respect to germline and adult stem cells, and discuss applications of oct4-EGFP transgenic medaka in reproductive and stem cell biology.

Multiple regulatory regions control the transcription of medaka germ gene vasa

Numerous regulatory DNA regions and trans-acting protein factors controlling transcription have been characterized for many genes that are expressed in somatic cells. Little is known about the transcriptional control of germ genes, and no cell culture system has been explored for quantitative reporter assay of germ gene transcription in vitro. Here we report the development of such an in vitro system and the identification of regulatory regions in the medaka germ gene vasa. We established the medaka germ cell line SG3 as a suitable in vitro system for analyzing germ gene transcription. Transgenic production revealed that VAS, a 5.1-kb genomic fragment of medaka vasa, possessed regulatory regions essential for germ cell-specific transcription. Importantly, reporter assays revealed 11 positive and negative regulatory regions alternatively positioned throughout VAS including the first intron. Strikingly, the regulatory regions may act in additive, non-additive and dependent manners. We show that a 39-bp element within one regulatory region is able to interact with the nuclear factor(s) of vasa-expressing embryos and testes. These results demonstrate the complexity of transcriptional control of medaka vasa and provide important insights into opposing mechanisms underlying germ gene transcription.

Medaka embryonic stem cells are capable of generating entire organs and embryo-like miniatures

Embryonic stem (ES) cells have the potency to produce many cell types of the embryo and adult body. Upon transplantation into early host embryos, ES cells are able to differentiate into various specialized cells and contribute to host tissues and organs of all germ layers. Here we present data in the fish medaka (Oryzias latipes) that ES cells have a novel ability to form extra organs and even embryo-like miniatures. Upon transplantation as individual cells according to the standard procedure, ES cells distributed widely to various organ systems of 3 germ layers. Upon transplantation as aggregates, ES cells were able to form extra organs, including the hematopoietic organ and contracting heart. We show that localized ES cell transplantation often led to the formation of extra axes that comprised essentially of either host cells or donor ES cells. These extra axes were associated with the head region of the embryo proper or formed at ectopic sites on the yolk sac. Surprisingly, certain ectopic axes were even capable of forming embryo-like miniatures. We conclude that ES cells have the ability to form entire organs and even embryo-like miniatures under proper environmental conditions. This finding points to a new possibility to generate ES cell-derived axes and organs.

Efficient detection, quantification and enrichment of subtle allelic alterations

Gene targeting (GT) can introduce subtle alterations into a particular locus and represents a powerful tool for genome editing. Engineered zinc finger nucleases (ZFNs) are effective for generating minor allelic alterations. Efficient detection of such minor alterations remains one of the challenges in ZFN-mediated GT experiments. Here, we report the establishment of procedures allowing for efficient detection, quantification and enrichment of such subtle alterations. In a biallelic model, polyacrylamide gel electrophoresis (PAGE) is capable of detecting rare allelic variations in the form of DNA heteroduplexes at a high efficiency of ~0.4% compared with ~6.3% by the traditional T7 endonuclease I-digestion and agarose gel electrophoresis. In a multiple allelic model, PAGE could discriminate different alleles bearing addition or deletion of 1-18 bp as distinct bands that were easily quantifiable by densitometry. Furthermore, PAGE enables enrichment for rare alleles. We show for the first time that direct endogenous GT is possible in medaka by ZFN RNA injection, whereas PAGE allows for detection and cloning of ZFN-targeted alleles in adults arising from ZFN-injected medaka embryos. Therefore, PAGE is effective for detection, quantification and enrichment of multiple fine allelic differences and thus offers a versatile tool for screening targeted subtle gene alterations.

Genome-wide genetic variations are highly correlated with proximal DNA methylation patterns

5-methyl-cytosines at CpG sites frequently mutate into thymines, accounting for a large proportion of spontaneous point mutations. The repair system would leave substantial numbers of errors in neighboring regions if the synthesis of erased gaps around deaminated 5-methyl-cytosines is error-prone. Indeed, we identified an unexpected genome-wide role of the CpG methylation state as a major determinant of proximal natural genetic variation. Specifically, 507 Mbp (∼18%) of the human genome was within 10 bp of a CpG site; in these regions, the single nucleotide polymorphism (SNP) rate significantly increased by ∼50% (P < 10⁻⁵⁶⁶ by a two-proportion z-test) if the neighboring CpG sites are methylated. To reconfirm this finding in another vertebrate, we compared six single-base resolution methylomes in two inbred medaka (Oryzias latipes) strains with sufficient genetic divergence (3.4%). We found that the SNP rate also increased by ∼50% (P < 10⁻²¹⁷⁰), and the substitution rates in all dinucleotides increased simultaneously (P < 10⁻⁴⁴¹) around methylated CpG sites. In the hypomethylated regions, the “CGCG” motif was significantly enriched (P < 10⁻⁶⁸⁰) and evolutionarily conserved (P = ∼ 0.203%), and slow CpG deamination rather than fast CpG gain was seen, indicating a possible role of CGCG as a candidate cis-element for the hypomethylation state. In regions that were hypermethylated in germline-like tissues but were hypomethylated in somatic liver cells, the SNP rate was significantly smaller than that in hypomethylated regions in both tissues, suggesting a positive selective pressure during DNA methylation reprogramming. This is the first report of findings showing that the CpG methylation state is significantly correlated with the characteristics of evolutionary change in neighboring DNA.

Production of zebrafish offspring from cultured spermatogonial stem cells

Germ-line stem cells have the potential to be a very powerful tool for modifying the genetic information of individual animals. As a first step to use spermatogonial stem cells (SSCs) to enable genetic modification, we here describe effective long-term culture conditions for propagating zebrafish SSCs and for the production of offspring from these cultured SSCs after their differentiation into sperm in transplanted testicular cell aggregates. Dissociated testicular cells were cultured in specific medium with some modified supplements, including several mammalian growth factors. The spermatogonia actively proliferated and retained the expression of exogenous green fluorescent protein under the control of vas and sox17 promoters and also of promyelocytic leukemia zinc finger (Plzf), a marker of undifferentiated spermatogonia, after 1 month in culture. This is a longer period than the entire natural spermatogenic cycle (from SSCs to sperm). The use of subcutaneously grafted aggregates of these cultured spermatogonia and freshly dissociated testicular cells showed that these SSCs could undergo self-renewal and differentiation into sperm. Artificial insemination of these grafted aggregates successfully produced offspring. This culture method will facilitate the identification of new factors for the maintenance of SSCs and enable the future enrichment of genetically modified SSCs that will produce offspring in zebrafish.