An evolutionary conserved region in the vasa 3'UTR targets RNA translation to the germ cells in the zebrafish

The journey of a generation: advances and promises in the study of primordial germ cell migration

The germline provides the genetic and non-genetic information that passes from one generation to the next. Given this important role in species propagation, egg and sperm precursors, called primordial germ cells (PGCs), are one of the first cell types specified during embryogenesis. In fact, PGCs form well before the bipotential somatic gonad is specified. This common feature of germline development necessitates that PGCs migrate through many tissues to reach the somatic gonad. During their journey, PGCs must respond to select environmental cues while ignoring others in a dynamically developing embryo. The complex multi-tissue, combinatorial nature of PGC migration is an excellent model for understanding how cells navigate complex environments in vivo. Here, we discuss recent findings on the migratory path, the somatic cells that shepherd PGCs, the guidance cues somatic cells provide, and the PGC response to these cues to reach the gonad and establish the germline pool for future generations. We end by discussing the fate of wayward PGCs that fail to reach the gonad in diverse species. Collectively, this field is poised to yield important insights into emerging reproductive technologies.

Traceability of primordial germ cells in three neotropical fish species aiming genetic conservation actions

Primordial germ cells (PGCs) are embryonic pluripotent cells that can differentiate into spermatogonia and oogonia, and therefore, PGCs are a genetic source for germplasm conservation through cryobanking and the generation of germline chimeras. The knowledge of PGC migration routes is essential for transplantation studies. In this work, the mRNA synthesized from the ddx4 3'UTR sequence of Pseudopimelodus mangurus, in fusion with gfp or dsred, was microinjected into zygotes of three neotropical species (P. mangurus, Astyanax altiparanae, and Prochilodus lineatus) for PGC labeling. Visualization of labeled PGCs was achieved by fluorescence microscopy during embryonic development. In addition, ddx4 and dnd1 expressions were evaluated during embryonic development, larvae, and adult tissues of P. mangurus, to validate their use as a PGC marker. As a result, the effective identification of presumptive PGCs was obtained. DsRed-positive PGC of P. mangurus was observed in the hatching stage, GFP-positive PGC of A. altiparanae in the gastrula stage, and GFP-positive PGCs from P. lineatus were identified at the segmentation stage, with representative labeling percentages of 29% and 16% in A. altiparanae and P. lineatus, respectively. The expression of ddx4 and dnd1 of P. mangurus confirmed the specificity of these genes in germ cells. These results point to the functionality of the P. mangurus ddx4 3'UTR sequence as a PGC marker, demonstrating that PGC labeling was more efficient in A. altiparanae and P. lineatus. The procedures used to identify PGCs in P. mangurus consolidate the first step for generating germinal chimeras as a conservation action of P. mangurus.

Primordial germ cell identification and traceability during the initial development of the Siluriformes fish Pseudopimelodus mangurus

Primordial germ cells (PGCs) are responsible for generating all germ cells. Therefore, they are essential targets to be used as a tool for the production of germline chimeras. The labeling and route of PGCs were evaluated during the initial embryonic development of Pseudopimelodus mangurus, using whole-mount in situ hybridization (WISH) and mRNA microinjection in zygotes. A specific antisense RNA probe constituted by a partial coding region from P. mangurus nanos3 mRNA was synthesized for the WISH method. RNA microinjection was performed using the GFP gene reporter regulated by translation regulatory P. mangurus buc and nanos3 3'UTR sequences, germline-specific markers used to describe in vivo migration of PGCs. Nanos3 and buc gene expression was evaluated in tissues for male and female adults and initial development phases and larvae from the first to seventh days post-hatching. The results from the WISH technique indicated the origin of PGCs in P. mangurus from the aggregations of nanos3 mRNA in the cleavage grooves and the signals obtained from nanos3 probes corresponded topographically to the migratory patterns of the PGCs reported for other fish species. Diffuse signals were observed in all blastomeres until the 16-cell stage, which could be related to the two sequences of the nanos3 3'UTR observed in the P. mangurus unfertilized egg transcriptome. Microinjection was not successful using GFP-Dr-nanos1 3'UTR mRNA and GFP-Pm-buc 3'UTR mRNA and allowed the identification of potential PGCs with less than 2% efficiency only and after hatching using GFP-Pm-nanos3 3'UTR. Nanos3 and buc gene expression was reported in the female gonads and from fertilized eggs until the blastula phase. These results provide information about the PGC migration of P. mangurus and the possible use of PGCs for the future generation of germline chimeras to be applied in the conservation efforts of Neotropical Siluriformes species. This study can contribute to establishing genetic banks, manipulating organisms, and assisting in biotechnologies such as transplanting germ cells in fish.

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.

Spatio-temporal mRNA tracking in the early zebrafish embryo

Early stages of embryogenesis depend on subcellular localization and transport of maternal mRNA. However, systematic analysis of these processes is hindered by a lack of spatio-temporal information in single-cell RNA sequencing. Here, we combine spatially-resolved transcriptomics and single-cell RNA labeling to perform a spatio-temporal analysis of the transcriptome during early zebrafish development. We measure spatial localization of mRNA molecules within the one-cell stage embryo, which allows us to identify a class of mRNAs that are specifically localized at an extraembryonic position, the vegetal pole. Furthermore, we establish a method for high-throughput single-cell RNA labeling in early zebrafish embryos, which enables us to follow the fate of individual maternal transcripts until gastrulation. This approach reveals that many localized transcripts are specifically transported to the primordial germ cells. Finally, we acquire spatial transcriptomes of two xenopus species and compare evolutionary conservation of localized genes as well as enriched sequence motifs.

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.

Both of marine fish species Oryzias melastigma and Pagrus major all failing in early localization at embryo stage by vasa RNA

Germ cells are essential for gonadal development. As precursors of germ cells, primordial germ cells (PGCs) are particularly important for germline formation. However, the research on distribution patterns of PGCs in marine fish is very limited, especially for economic species. The vasa gene has been widely used as marker to identify PGCs origination and migration because of vasa RNA is a component of germ plasm. In this study, we isolated full-length vasa cDNA (Omvas and Pmvas) from marine medaka (Oryzias melastigma) and red seabream (Pagrus major), detected vasa transcripts in different tissues by RT-PCR and described vasa expression patterns during embryogenesis and gametogenesis by in situ hybridization. At the same time, we also explored the relationship between early distribution of germ plasm components and species evolution. The results demonstrated that deduced amino acid sequence of Omvas and Pmvas shared several conserved motifs of Vasa homologues and high identity with other teleost, and vasa transcripts were exclusively detected in early germ cells of gonad. During embryogenesis, vasa RNA of both fishes, like medaka (Oryzias latipes), failed to localize at cleavage furrows and distributed uniformly throughout each blastomere. This study firstly discovered that the marine economic fish, red seabream, lost vasa RNA early specific localization at cleavage furrows and distinctive distribution in germ cells. In addition, compared with other teleost, we found that early distribution of germ plasm might not relate to species evolution. This will improve our understanding of vasa localization modes in teleost, and facilitate fish germ cell manipulation.

Identifying difference in primordial germ cells between XX female and XY male yellow catfish embryos

Primordial germ cells (PGCs) are singled out from somatic cells very early during embryogenesis, then they migrate towards the genital ridge and differentiate into gametes through oogenesis or spermatogenesis. Labeling PGCs with Localized RNAexpression (LRE) technique by fluorescent proteins has been widely applied among teleost species to study the germ cell development and gonad differentiation. In this study, we first cloned and characterized the 3' untranslated regions (3'UTRs) of nanos homolog 1-like (nos1l), dead end (dnd), and vasa in yellow catfish (Pelteobagrus fulvidraco), and then synthesized the GFP-nos1l/dnd/vasa 3'UTR mRNAs. Each of these three 3'UTRs could label PGCs in yellow catfish embryos, of which, vasa 3'UTR exhibited the highest labeling efficiency. To identify the differences in PGCs at embryonic stage, XX all-female and XY all-male yellow catfish embryos were produced and injected with GFP-vasa 3'UTR mRNA. We observed the PGC migration route in these two monosex embryos from 24 hpf to 7 dpf, and found there was no difference between them. Besides, the PGC number was counted at 48 hpf, and the result showed that the average PGC number in XX females (11.3) was significantly larger than that in XY males (8.1).These findings provide an insight into the development of PGCs in yellow catfish embryos and the relationship between embryonicPGCnumberand thelatergonaddifferentiation.

Targeted RNA Knockdown by a Type III CRISPR-Cas Complex in Zebrafish

RNA interference is a powerful experimental tool for RNA knockdown, but not all organisms are amenable. Here, we provide a proof of principle demonstration that a type III Csm effector complex can be used for programmable mRNA transcript degradation in eukaryotes. In zebrafish, Streptococcus thermophilus Csm complex (StCsm) proved effective for knockdown of maternally expressed EGFP in germ cells of Tg(ddx4:ddx4-EGFP) fish. It also led to significant, albeit less drastic, fluorescence reduction at one day postfertilization in Tg(myl7:GFP) and Tg(fli1:EGFP) fish that express EGFP zygotically. StCsm targeted against the endogenous tdgf1 elicited the characteristic one-eyed phenotype with greater than 50% penetrance, and hence with similar efficiency to morpholino-mediated knockdown. We conclude that Csm-mediated knockdown is very efficient for maternal transcripts and can also be used for mixed maternal/early zygotic and early zygotic transcripts, in some cases reaching comparable efficiency to morpholino-based knockdown without significant off-target effects.

The role of the cytoskeleton in germ plasm aggregation and compaction in the zebrafish embryo

The transmission of genetic information from one generation to another is crucial for survival of animal species. This is accomplished by the induction of primordial germ cells (PGCs) that will eventually establish the germline. In some animals the germline is induced by signals in gastrula, whereas in others it is specified by inheritance of maternal determinants, known as germ plasm. In zebrafish, aggregation and compaction of maternally derived germ plasm during the first several embryonic cell cycles is essential for generation of PGCs. These processes are controlled by cellular functions associated with the cellular division apparatus. Ribonucleoparticles containing germ plasm components are bound to both the ends of astral microtubules and a dynamic F-actin network through a mechanism integrated with that which drives the cell division program. In this chapter we discuss the role that modifications of the cell division apparatus, including the cytoskeleton and cytoskeleton-associated proteins, play in the regulation of zebrafish germ plasm assembly.

Visualization of Turbot (Scophthalmus maximus) Primordial Germ Cells in vivo Using Fluorescent Protein Mediated by the 3' Untranslated Region of nanos3 or vasa Gene

Primordial germ cells (PGCs) as the precursors of germ cells are responsible for transmitting genetic information to the next generation. Visualization of teleost PGCs in vivo is essential to research the origination and development of germ cells and facilitate further manipulation on PGCs isolation, cryopreservation, and surrogate breeding. In this study, artificially synthesized mRNAs constructed by fusing fluorescent protein coding region to the 3' untranslated region (3'UTR) of nanos3 or vasa (mCherry-Smnanos3 3'UTR or mCherry-Smvasa 3'UTR mRNA) were injected into turbot (Scophthalmus maximus) fertilized eggs for tracing PGCs. The results demonstrated that the fluorescent PGCs differentiated from somatic cells and aligned on both sides of the trunk at the early segmentation period, then migrated and located at the dorsal part of the gut where the gonad would form. In the same way, we also found that the zebrafish (Danio rerio) vasa 3'UTR could trace turbot PGCs, while the vasa 3'UTR s of marine medaka (Oryzias melastigma) and red seabream (Pagrus major) failed, although they could label the marine medaka PGCs. In addition, through comparative analysis, we discovered that some potential sequence elements in the3 'UTRs of nanos3 and vasa, such as GCACs, 62-bp U-rich regions and nucleotide 187-218 regions might be involved in PGCs stabilization. The results of this study provided an efficient, rapid, and specific non-transgenic approach for visualizing PGCs of economical marine fish in vivo.

Embryonic Exposure to Bisphenol A Impairs Primordial Germ Cell Migration without Jeopardizing Male Breeding Capacity

A large amount of chemicals are released to the environment each year. Among them, bisphenol A (BPA) is of utmost concern since it interferes with the reproductive system of wild organisms due to its capacity to bind to hormone receptors. Additionally, BPA epigenotoxic activity is known to affect basic processes during embryonic life. However, its effects on primordial germ cells (PGCs) proliferation and migration, both mechanisms being crucial for gametogenesis, remain unknown. To investigate the effects of BPA on PGCs migration and eventual testicle development, zebrafish embryos were exposed to 100, 2000 and 4000 µg/L BPA during the first 24 h of development. Vasa immunostaining of PGCs revealed that exposure to 2000 and 4000 µg/L BPA impaired their migration to the genital ridge. Two pivotal genes of PGCs migration (cxcr4b and sdf1a) were highly dysregulated in embryos exposed to these doses, whereas DNA methylation and epigenetic marks in PGCs and their surrounding somatic cells were not altered. Once embryos reached adulthood, the morphometric study of their gonads revealed that, despite the reduced number of PGCs which colonized the genital ridges, normal testicles were developed. Although H3K9ac decreased in the sperm from treated fishes, it did not affect the progeny development.

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.

Tdrd6a Regulates the Aggregation of Buc into Functional Subcellular Compartments that Drive Germ Cell Specification

Phase separation represents an important form of subcellular compartmentalization. However, relatively little is known about how the formation or disassembly of such compartments is regulated. In zebrafish, the Balbiani body (Bb) and the germ plasm (Gp) are intimately linked phase-separated structures essential for germ cell specification and home to many germ cell-specific mRNAs and proteins. Throughout development, these structures occur as a single large aggregate (Bb), which disperses throughout oogenesis and upon fertilization accumulates again into relatively large assemblies (Gp). Formation of the Bb requires Bucky ball (Buc), a protein with prion-like properties. We found that the multi-tudor domain-containing protein Tdrd6a interacts with Buc, affecting its mobility and aggregation properties. Importantly, lack of this regulatory interaction leads to significant defects in germ cell development. Our work presents insights into how prion-like protein aggregations can be regulated and highlights the biological relevance of such regulatory events.

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Tdrd6a is required for Bucky ball mobility within aggregates, and for PGC formation

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Maternal Tdrd6a coordinates transcript deposition into future PGCs

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A dimethylated tri-RG motif in Bucky ball mediates interaction with Tdrd6a

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The tri-RG motif is essential for Balbiani body and germ cell formation

The translational regulation of maternal mRNAs in time and space

Since their discovery, the study of maternal mRNAs has led to the identification of mechanisms underlying their spatiotemporal regulation within the context of oogenesis and early embryogenesis. Following synthesis in the oocyte, maternal mRNAs are translationally silenced and sequestered into storage in cytoplasmic granules. At the same time, their unique distribution patterns throughout the oocyte and embryo are tightly controlled and connected to their functions in downstream embryonic processes. At certain points in oogenesis and early embryogenesis, maternal mRNAs are translationally activated to perform their functions in a timely manner. The cytoplasmic polyadenylation machinery is responsible for the translational activation of maternal mRNAs, and its role in initiating the maternal to zygotic transition events has recently come to light. Here, we summarize the current knowledge on maternal mRNA regulation, with particular focus on cytoplasmic polyadenylation as a mechanism for translational regulation.

Mechanisms of Vertebrate Germ Cell Determination

Two unique characteristics of the germ line are the ability to persist from generation to generation and to retain full developmental potential while differentiating into gametes. How the germ line is specified that allows it to retain these characteristics within the context of a developing embryo remains unknown and is one focus of current research. Germ cell specification proceeds through one of two basic mechanisms: cell autonomous or inductive. Here, we discuss how germ plasm driven germ cell specification (cell autonomous) occurs in both zebrafish and the frog Xenopus. We describe the segregation of germ cells during embryonic development of solitary and colonial ascidians to provide an evolutionary context to both mechanisms. We conclude with a discussion of the inductive mechanism as exemplified by both the mouse and axolotl model systems. Regardless of mechanism, several general themes can be recognized including the essential role of repression and posttranscriptional regulation of gene expression.

Methods to study maternal regulation of germ cell specification in zebrafish

The process by which the germ line is specified in the zebrafish embryo is under the control of maternal gene products that were produced during oogenesis. Zebrafish are highly amenable to microscopic observation of the processes governing maternal germ cell specification because early embryos are transparent, and the germ line is specified rapidly (within 4-5h post fertilization). Advantages of zebrafish over other models used to study vertebrate germ cell formation include their genetic tractability, the large numbers of progeny, and the easily manipulable genome, all of which make zebrafish an ideal system for studying the genetic regulators and cellular basis of germ cell formation and maintenance. Classical molecular biology techniques, including expression analysis through in situ hybridization and forward genetic screens, have laid the foundation for our understanding of germ cell development in zebrafish. In this chapter, we discuss some of these classic techniques, as well as recent cutting-edge methodologies that have improved our ability to visualize the process of germ cell specification and differentiation, and the tracking of specific molecules involved in these processes. Additionally, we discuss traditional and novel technologies for manipulating the zebrafish genome to identify new components through loss-of-function studies of putative germ cell regulators. Together with the numerous aforementioned advantages of zebrafish as a genetic model for studying development, we believe these new techniques will continue to advance zebrafish to the forefront for investigation of the molecular regulators of germ cell specification and germ line biology.

Generation of Transparent Zebrafish with Fluorescent Ovaries: A Living Visible Model for Reproductive Biology

The transparent zebrafish enables researchers to study the morphology and distribution of cells and tissues in vivo. To capture the dynamic processes of germ cell proliferation and juvenile ovarian development in zebrafish in vivo, we established transgenic (TG) lines to allow us to monitor the changes in the ovaries of living fish. The original transgenic line with ovarian fluorescence was occasionally established. Although the cDNA integrated in the strain was constructed for the expression of enhanced green fluorescent protein (EGFP) driven by the medaka β-actin promoter, expression of EGFP is restricted to the oocytes and gills in adult fish. Mutant strains with transparent bodies, roy and ruby, were isolated in zebrafish. In this study, we crossed the TG strain with fluorescent ovary with transparent strains and established the TG (β-actin:EGFP);ruby strain. The strain is highly transparent, and the oocytes are easily observed in living fish. We identified a fluorescent tissue that might contain the undifferentiated germ cells close to the cloaca in the strain. This strain can be used for analysis of ovarian development in vivo.

Primordial Germ Cell Specification and Migration

Primordial germ cells are the progenitor cells that give rise to the gametes. In some animals, the germline is induced by zygotic transcription factors, whereas in others, primordial germ cell specification occurs via inheritance of maternally provided gene products known as germ plasm. Once specified, the primordial germ cells of some animals must acquire motility and migrate to the gonad in order to survive. In all animals examined, perinuclear structures called germ granules form within germ cells. This review focuses on some of the recent studies, conducted by several groups using diverse systems, from invertebrates to vertebrates, which have provided mechanistic insight into the molecular regulation of germ cell specification and migration.

Primordial germ cells: the first cell lineage or the last cells standing?

Embryos of many animal models express germ line determinants that suppress transcription and mediate early germ line commitment, which occurs before the somatic cell lineages are established. However, not all animals segregate their germ line in this manner. The 'last cell standing' model describes primordial germ cell (PGC) development in axolotls, in which PGCs are maintained by an extracellular signalling niche, and germ line commitment occurs after gastrulation. Here, we propose that this 'stochastic' mode of PGC specification is conserved in vertebrates, including non-rodent mammals. We postulate that early germ line segregation liberates genetic regulatory networks for somatic development to evolve, and that it therefore emerged repeatedly in the animal kingdom in response to natural selection.

In vitro generation of zebrafish PGC-like cells

The possibility of generating primordial germ cells (PGCs) in vitro from noncommitted embryonic cells represents an extremely useful tool in current research. Primordial germ cell in vitro differentiation has been successfully reported in mammals. However, contrary to fish, PGC specification in mammals is an inductive mechanism. This study is the first to date to describe a rapid method for PGC in vitro differentiation in teleosts. Primordial germ cell-like cells were characterized by several lines of evidence, including gene expression, cell complexity, size, and image analysis for the quantification of fluorescence under vasa promoter. Moreover, differentiated cells were able to colonize the genital ridge after transplantation. Differentiation treatments increased the number of PGCs in culture, causing differentiation of cells rather than inducing their proliferation. These results open up the possibility of differentiating genetically modified embryonic cells to PGC-like cells to ensure their transmission to the progeny and could be crucial for an in-depth understanding of germline differentiation in teleosts.

Conserved mechanisms for germ cell-specific localization of nanos3 transcripts in teleost species with aquaculture significance

The importance of the aquaculture production is increasing with the declining global fish stocks, but early sexual maturation in several farmed species reduces muscle growth and quality, and escapees could have a negative impact on wild populations. A possible solution to these problems is the production of sterile fish by ablation of the embryonic primordial germ cells (PGCs), a technique developed in zebrafish. Cell-specific regulation of mRNA stability is crucial for proper specification of the germ cell lineage and commonly involves microRNA (miRNA)-mediated degradation of targeted mRNAs in somatic cells. This study reports on the functional roles of conserved motifs in the 3' untranslated region (UTR) of the miRNA target gene nanos3 identified in Atlantic cod, Atlantic salmon, and zebrafish. The 3'UTR of cod nanos3 was sufficient for targeting the expression of green fluorescent protein (GFP) to the presumptive PGCs in injected embryos of the three phylogenetically distant species. 3'UTR elements of importance for PGC-specific expression were further examined by fusing truncated 3'UTR variants of cod nanos3 to GFP followed by injections in zebrafish embryos. The expression patterns of the GFP constructs in PGCs and somatic cells suggested that the proximal U-rich region is responsible for the PGC-specific stabilization of the endogenous nanos3 mRNA. Morpholino-mediated downregulation of the RNA-binding protein Dead end (DnD), a PGC-specific inhibitor of miRNA action, abolished the fluorescence of the PGCs in cod and zebrafish embryos, suggesting a conserved DnD-dependent mechanism for germ cell survival and migration.

Molecular characterization, sexually dimorphic expression, and functional analysis of 3'-untranslated region of vasa gene in half-smooth tongue sole (Cynoglossus semilaevis)

Vasa is a highly conserved ATP-dependent RNA helicase expressed mainly in germ cells. The vasa gene plays a crucial role in the development of germ cell lineage and has become an excellent molecular marker in identifying germ cells in teleosts. However, little is known about the structure and function of the vasa gene in flatfish. In this study, the vasa gene (Csvasa) was isolated and characterized in half-smooth tongue sole (Cynoglossus semilaevis), an economically important flatfish in China. In the obtained 6425-bp genomic sequence, 23 exons and 22 introns were identified. The Csvasa gene encodes a 663-amino acid protein, including highly conserved domains of the DEAD-box protein family. The amino acid sequence also shared a high homology with other teleosts. Csvasa expression was mainly restricted to the gonads, with little or no expression in other tissues. Real-time quantitative polymerase chain reaction analysis revealed that Csvasa expression levels decreased during embryonic and early developmental stages and increased with the primordial germ cell proliferation. A typical sexually dimorphic expression pattern of Csvasa was observed during early development and sex differentiation, suggesting that the Csvasa gene might play a differential role in the proliferation and differentiation of male and female primordial germ cells (PGCs). Csvasa mRNA expression levels in neomales were significantly lower than those in normal males and females, indicating that the Csvasa gene might be implicated in germ cell development after sex reversal by temperature treatment. In addition, medaka (Oryzias latipes) PGCs could be transiently labeled by microinjection of synthesized mRNA containing the green fluorescence protein gene and 3'-untranslated region of Csvasa, which confirmed that the Csvasa gene has the potential to be used as a visual molecular marker of germ cells and laid a foundation for manipulation of PGCs in tongue sole reproduction.

Solea senegalensis vasa transcripts: molecular characterisation, tissue distribution and developmental expression profiles

The Vasa protein is an RNA helicase belonging the DEAD (Asp-Glu-Ala-Asp)-box family. The crucial role played by the vasa gene in the germ-cell lineage of both vertebrates and invertebrates has made this gene a useful molecular marker for germinal cells and a useful tool in surrogate broodstock production using primordial germ cell transplantation. With the aim of establishing a novel approach to improving Solea senegalensis broodstock management, the vasa gene in this species was characterised. Four S. senegalensis vasa transcripts were isolated: Ssvasa1, Ssvasa2, Ssvasa3 and Ssvasa4. Their phylogenetic relationship with other vasa homologues was determined confirming the high degree of conservation of this helicase throughout evolution. Our qPCR results showed that S. senegalensis vasa transcripts are prevalently expressed in gonads, with ovary-specific expression for Ssvasa3 and Ssvasa4. During embryonic and larval development, a switch between the longest and the shortest transcripts was observed. While Ssvasa1 and Ssvasa2 were maternally supplied, Ssvasa3 and Ssvasa4 depended on the de novo expression program of the growing juveniles, suggesting that vasa mRNA could be involved in Senegalese sole gonad differentiation. In situ hybridisation and immunohistochemical analysis performed in 150-days after hatching (DAH) larvae showed vasa product expression in the germinal region of early gonads. In our work we demonstrated the usefulness of Ssvasa mRNAs as molecular markers for primordial germ cells and germinal cells during embryonic development, larval ontogenesis and gonad differentiation. Furthermore, our results confirmed the potential of vasa to help investigate germinal cell biotechnology for Senegalese sole reproduction.

Targeted expression in zebrafish primordial germ cells by Cre/loxP and Gal4/UAS systems

In zebrafish and other vertebrates, primordial germ cells (PGCs) are a population of embryonic cells that give rise to sperm and eggs in adults. Any type of genetically manipulated lines have to be originated from the germ cells of the manipulated founders, thus it is of great importance to establish an effective technology for highly specific PGC-targeted gene manipulation in vertebrates. In the present study, we used the Cre/loxP recombinase system and Gal4/UAS transcription system for induction and regulation of mRFP (monomer red fluorescent protein) gene expression to achieve highly efficient PGC-targeted gene expression in zebrafish. First, we established two transgenic activator lines, Tg(kop:cre) and Tg(kop:KalTA4), to express the Cre recombinases and the Gal4 activator proteins in PGCs. Second, we generated two transgenic effector lines, Tg(kop:loxP-SV40-loxP-mRFP) and Tg(UAS:mRFP), which intrinsically showed transcriptional silence of mRFP. When Tg(kop:cre) females were crossed with Tg(kop:loxP-SV40-loxP-mRFP) males, the loxP flanked SV40 transcriptional stop sequence was 100 % removed from the germ cells of the transgenic hybrids. This led to massive production of PGC-specific mRFP transgenic line, Tg(kop:loxP-mRFP), from an mRFP silent transgenic line, Tg(kop:loxP-SV40-loxP-mRFP). When Tg(kop:KalTA4) females were crossed with Tg(UAS:mRFP) males, the hybrid embryos showed PGC specifically expressed mRFP from shield stage till 25 days post-fertilization (pf), indicating the high sensitivity, high efficiency, and long-lasting effect of the Gal4/UAS system. Real-time PCR analysis showed that the transcriptional amplification efficiency of the Gal4/UAS system in PGCs can be about 300 times higher than in 1-day-pf embryos. More importantly, when the UAS:mRFP-nos1 construct was directly injected into the Tg(kop:KalTA4) embryos, it was possible to specifically label the PGCs with high sensitivity, efficiency, and persistence. Therefore, we have established two targeted gene expression platforms in zebrafish PGCs, which allows us to further manipulate the PGCs of zebrafish at different levels.

The DEAD-box helicase Vasa: evidence for a multiplicity of functions in RNA processes and developmental biology

DEAD-box helicases related to the Drosophila protein Vasa (also known as Ddx4) are found throughout the animal kingdom. They have been linked to numerous processes in gametogenesis, germ cell specification, and stem cell biology, and alterations in Vasa expression are associated with malignancy of tumor cells and with some human male infertility syndromes. Experimental results indicating how Vasa contributes to all these different cellular and developmental processes are discussed, using examples from planarians, Caenorhabditis elegans, Drosophila, sea urchin, zebrafish, Xenopus, mouse, and human. Molecular, cellular, and developmental functions of Vasa and its orthologs are reviewed in this article. Evidence linking Vasa to translational regulation, to biogenesis of small RNAs, and to chromosome condensation is examined. Finally, potential overlapping functions between Vasa and related DEAD-box helicases (Belle, or Ddx3, and DEADSouth, or Ddx25) are explored. This article is part of a Special Issue entitled: The biology of RNA helicases - Modulation for life.

A high-throughput fluorescence-based assay system for appetite-regulating gene and drug screening

The increasing number of people suffering from metabolic syndrome and obesity is becoming a serious problem not only in developed countries, but also in developing countries. However, there are few agents currently approved for the treatment of obesity. Those that are available are mainly appetite suppressants and gastrointestinal fat blockers. We have developed a simple and rapid method for the measurement of the feeding volume of Danio rerio (zebrafish). This assay can be used to screen appetite suppressants and enhancers. In this study, zebrafish were fed viable paramecia that were fluorescently-labeled, and feeding volume was measured using a 96-well microplate reader. Gene expression analysis of brain-derived neurotrophic factor (bdnf), knockdown of appetite-regulating genes (neuropeptide Y, preproinsulin, melanocortin 4 receptor, agouti related protein, and cannabinoid receptor 1), and the administration of clinical appetite suppressants (fluoxetine, sibutramine, mazindol, phentermine, and rimonabant) revealed the similarity among mechanisms regulating appetite in zebrafish and mammals. In combination with behavioral analysis, we were able to evaluate adverse effects on locomotor activities from gene knockdown and chemical treatments. In conclusion, we have developed an assay that uses zebrafish, which can be applied to high-throughput screening and target gene discovery for appetite suppressants and enhancers.

In vitro oocyte culture-based manipulation of zebrafish maternal genes

In animals, females deposit gene products into developing oocytes, which drive early cellular events in embryos immediately after fertilization. As maternal gene products are present before fertilization, the functional manipulation of maternal genes is often challenging to implement, requiring gene expression or targeting during oogenesis. Maternal expression can be achieved through transgenesis, but transgenic approaches are time consuming and subject to undesired epigenetic effects. Here, we have implemented in vitro culturing of experimentally manipulated immature oocytes to study maternal gene contribution to early embryonic development in the zebrafish. We demonstrate phenotypic rescue of a maternal-effect mutation by expressing wild-type product in cultured oocytes. We also generate loss-of-function phenotypes in embryos through either the expression of a dominant-negative transcript or injection of translation-blocking morpholino oligonucleotides. Finally, we demonstrate subcellular localization during the early cell divisions immediately after fertilization of an exogenously provided maternal product fused to a fluorescent protein. These manipulations extend the potential to carry out genetic and imaging studies of zebrafish maternal genes during the egg-to-embryo transition.

Germ line specific expression of a vasa homologue gene in turbot (Scophthalmus maximus): evidence for vasa localization at cleavage furrows in euteleostei

Specification of primordial germ cells during early embryogenesis is a critical biological issue in reproduction and development. Yet, little is known in marine economic fish species. Vasa, a component of germ plasm, is the most-documented germ cell marker in teleosts. We isolated a full-length vasa cDNA (Smvas) from turbot (Scophthalmus maximus), a marine Euteleostei species, and investigated its expression patterns by RT-PCR and in situ hybridization during embryogenesis and gametogenesis to identify the germ cell lineage in this species. The deduced amino acid sequence of the isolated cDNA shared typical characteristics of Vasa protein and high identity to Vasa homologues in medaka (76.9%) and zebrafish (68.5%). The Smvas transcripts were exclusively detected in germ cells of testis and ovary, and exhibited an interesting dynamic localization pattern during oogenesis. The distribution pattern of Smvas during embyogenesis in this Euteleostei closely resembled the pattern observed in zebrafish (belonging to Osteriophysans) rather than medaka (belonging to Euteleostei). Thus, it is concluded that Smvas isolated in this study is a germ cell specific molecular marker in turbot. Furthermore, we hypothesize that Euteleostei could localize vasa mRNA by a special mode. The results not only facilitate the germ cell manipulation of the turbot, but also improve our understanding of germline development and evolution of vasa localization in teleost.

Extensive alternative polyadenylation during zebrafish development

The post-transcriptional fate of messenger RNAs (mRNAs) is largely dictated by their 3′ untranslated regions (3′ UTRs), which are defined by cleavage and polyadenylation (CPA) of pre-mRNAs. We used poly(A)-position profiling by sequencing (3P-seq) to map poly(A) sites at eight developmental stages and tissues in the zebrafish. Analysis of over 60 million 3P-seq reads substantially increased and improved existing 3′ UTR annotations, resulting in confidently identified 3′ UTRs for >79% of the annotated protein-coding genes in zebrafish. mRNAs from most zebrafish genes undergo alternative CPA, with those from more than a thousand genes using different dominant 3′ UTRs at different stages. These included one of the poly(A) polymerase genes, for which alternative CPA reinforces its repression in the ovary. 3′ UTRs tend to be shortest in the ovaries and longest in the brain. Isoforms with some of the shortest 3′ UTRs are highly expressed in the ovary, yet absent in the maternally contributed RNAs of the embryo, perhaps because their 3′ UTRs are too short to accommodate a uridine-rich motif required for stability of the maternal mRNA. At 2 h post-fertilization, thousands of unique poly(A) sites appear at locations lacking a typical polyadenylation signal, which suggests a wave of widespread cytoplasmic polyadenylation of mRNA degradation intermediates. Our insights into the identities, formation, and evolution of zebrafish 3′ UTRs provide a resource for studying gene regulation during vertebrate development.

Transient and stable GFP expression in germ cells by the vasa regulatory sequences from the red seabream (Pagrus major)

Primordial germ cells (PGCs) are the precursors of gametes responsible for genetic transmission to the next generation. They provide an ideal system for cryopreservation and restoration of biodiversity. Recently, considerable attention has been raised to visualize, isolate and transplant PGCs within and between species. In fish, stable PGC visualization in live embryo and individual has been limited to laboratory fish models such as medaka and zebrafish. One exception is the rainbow trout, which represents the only species with aquaculture importance and has GFP-labeled germ cells throughout development. PGCs can be transiently labeled by embryonic injection of mRNA containing green fluorescence protein gene (GFP) and 3'-untranslated region (3'-UTR) of a maternal germ gene such as vasa, nos1, etc. Stable PGC labeling can be achieved through production of transgenic animals by some transcriptional regulatory sequences from germ genes, such as the vasa promoter and 3'-UTR. In this study, we reported the functional analyses of the red seabream vasa (Pmvas) regulatory sequences, using medaka as a model system. It was showed that injection of GFP-Pmvas3'UTR mRNA was able to label medaka PGCs during embryogenesis. Besides, we have constructed pPmvasGFP transgenic vector, and established a stable transgenic medaka line exhibiting GFP expression in germ cells including PGCs, mitotic and meiotic germ cells of both sexes, under control of the Pmvas transcriptional regulatory sequences. It is concluded that the Pmvas regulatory sequences examined in this study are sufficient for germ cell expression and labeling.

An E-cadherin-mediated hitchhiking mechanism for C. elegans germ cell internalization during gastrulation

Gastrulation movements place endodermal precursors, mesodermal precursors and primordial germ cells (PGCs) into the interior of the embryo. Somatic cell gastrulation movements are regulated by transcription factors that also control cell fate, coupling cell identity and position. By contrast, PGCs in many species are transcriptionally quiescent, suggesting that they might use alternative gastrulation strategies. Here, we show that C. elegans PGCs internalize by attaching to internal endodermal cells, which undergo morphogenetic movements that pull the PGCs into the embryo. We show that PGCs enrich HMR-1/E-cadherin at their surfaces to stick to endoderm. HMR-1 expression in PGCs is necessary and sufficient to ensure internalization, suggesting that HMR-1 can promote PGC-endoderm adhesion through a mechanism other than homotypic trans interactions between the two cell groups. Finally, we demonstrate that the hmr-1 3' untranslated region promotes increased HMR-1 translation in PGCs. Our findings reveal that quiescent PGCs employ a post-transcriptionally regulated hitchhiking mechanism to internalize during gastrulation, and demonstrate a morphogenetic role for the conserved association of PGCs with the endoderm.

Targeted disruption of exogenous EGFP gene in medaka using zinc-finger nucleases

Zinc-finger nucleases (ZFNs) are artificial enzymes that create site-specific double-strand breaks and thereby induce targeted genome editing. Here, we demonstrated successful gene disruption in somatic and germ cells of medaka (Oryzias latipes) using ZFN to target exogenous EGFP genes. Embryos that were injected with an RNA sequence pair coding for ZFNs showed mosaic loss of green fluorescent protein fluorescence in skeletal muscle. A number of mutations that included both deletions and insertions were identified within the ZFN target site in each embryo, whereas no mutations were found at the non-targeted sites. In addition, ZFN-induced mutations were introduced in germ cells and efficiently transmitted to the next generation. The mutation frequency varied (6-100%) in the germ cells from each founder, and a founder carried more than two types of mutation in germ cells. Our results have introduced the possibility of targeted gene disruption and reverse genetics in medaka.

Cardiac developmental toxicity

Congenital heart disease (CHD) is a highly prevalent problem with mostly unknown origins. Many cases of CHD likely involve an environmental exposure coupled with genetic susceptibility, but practical and ethical considerations make nongenetic causes of CHD difficult to assess in humans. The development of the heart is highly conserved across all vertebrate species, making animal models an excellent option for screening potential cardiac teratogens. This review will discuss exposures known to cause cardiac defects, stages of heart development that are most sensitive to teratogen exposure, benefits and limitations of animal models of cardiac development, and future considerations for cardiac developmental toxicity research.

Circular DNA intermediate in the duplication of Nile tilapia vasa genes

vasa is a highly conserved RNA helicase involved in animal germ cell development. Among vertebrate species, it is typically present as a single copy per genome. Here we report the isolation and sequencing of BAC clones for Nile tilapia vasa genes. Contrary to a previous report that Nile tilapia have a single copy of the vasa gene, we find evidence for at least three vasa gene loci. The vasa gene locus was duplicated from the original site and integrated into two distant novel sites. For one of these insertions we find evidence that the duplication was mediated by a circular DNA intermediate. This mechanism of gene duplication may explain the origin of isolated gene duplicates during the evolution of fish genomes. These data provide a foundation for studying the role of multiple vasa genes in the development of tilapia gonads, and will contribute to investigations of the molecular mechanisms of sex determination and evolution in cichlid fishes.

Polarizing animal cells via mRNA localization in oogenesis and early development

The localization of mRNAs in developing animal cells is essential for establishing cellular polarity and setting up the body plan for subsequent development. Cellular and molecular mechanisms by which maternal mRNAs are localized during oogenesis have been extensively studied in Drosophila and Xenopus. In contrast, evidence for mechanisms used in the localization of mRNAs encoded by developmentally important genes has also been accumulating in several other organisms. This offers the opportunity to unravel the fundamental mechanisms of mRNA localization shared among many species, as well as unique mechanisms specifically acquired or retained by animals based on their developmental needs. In addition to maternal mRNAs, the localization of zygotically expressed mRNAs in the cells of cleaving embryos is also important for early development. In this review, mRNA localization dynamics in the oocytes/eggs of Drosophila and Xenopus are first summarized, and evidence for localized mRNAs in the oocytes/eggs and cleaving embryos of other organisms is then presented.

Vasa genes: emerging roles in the germ line and in multipotent cells

Sexually reproducing metazoans establish a cell lineage during development that is ultimately dedicated to gamete production. Work in a variety of animals suggests that a group of conserved molecular determinants act in this germ line maintenance and function. The most universal of these genes are Vasa and Vasa-like DEAD-box RNA helicase genes. However, recent evidence indicates that Vasa genes also function in other cell types, distinct from the germ line. Here we evaluate our current understanding of Vasa function and its regulation during development, addressing Vasa's emerging role in multipotent cells. We also explore the evolutionary diversification of the N-terminal domain of this gene and how this impacts the association of Vasa with nuage-like perinuclear structures.

Evolution of the germ line-soma relationship in vertebrate embryos

The germ line and soma together maintain genetic lineages from generation to generation: the germ line passes genetic information between generations; the soma is the vehicle for germ line transmission, and is shaped by natural selection. The germ line and somatic lineages arise simultaneously in early embryos, but how their development is related depends on how primordial germ cells (PGC) are specified. PGCs are specified by one of two means. Epigenesis describes the induction of PGCs from pluripotent cells by signals from surrounding somatic tissues. In contrast, PGCs in many species are specified cell-autonomously by maternally derived molecules, known as germ plasm, and this is called preformation. Germ plasm inhibits signaling to PGCs; thus, they are specified cell-autonomously. Germ plasm evolved independently in many animal lineages, suggesting convergent evolution, and therefore it would be expected to convey a selective advantage. But, what this is remains unknown. We propose that the selective advantage that drives the emergence of germ plasm in vertebrates is the disengagement of germ line specification from somatic influences. This liberates the evolution of gene regulatory networks (GRNs) that govern somatic development, and thereby enhances species evolvability, a well-recognized selective advantage. We cite recent evidence showing that frog embryos, which contain germ plasm, have modified GRNs that are not conserved in axolotls, which represent more basal amphibians and employ epigenesis. We also present the correlation of preformation with enhanced species radiations, and we discuss the mutually exclusive trajectories influenced by germ plasm or pluripotency, which shaped chordate evolution.

Post-translational regulation by gustavus contributes to selective Vasa protein accumulation in multipotent cells during embryogenesis

Vasa is a broadly conserved DEAD-box RNA helicase associated with germ line development and is expressed in multipotent cells in many animals. During embryonic development of the sea urchin Strongylocentrotus purpuratus, Vasa protein is enriched in the small micromeres despite a uniform distribution of vasa transcript. Here we show that the Vasa coding region is sufficient for its selective enrichment and find that gustavus, the B30.2/SPRY and SOCS box domain gene, contributes to this phenomenon. In vitro binding analyses show that Gustavus binds the N-terminal and DEAD-box portions of Vasa protein independently. A knockdown of Gustavus protein reduces both Vasa protein abundance and its propensity for accumulation in the small micromeres, whereas overexpression of the Vasa-interacting domain of Gustavus (GusΔSOCS) results in Vasa protein accumulation throughout the embryo. We propose that Gustavus has a conserved, positive regulatory role in Vasa protein accumulation during embryonic development.

In vitro developmental toxicology assays: A review of the state of the science of rodent and zebrafish whole embryo culture and embryonic stem cell assays

In vitro developmental model systems have been an important tool for advancing basic research in the embryology and teratology fields. The rat and zebrafish embryo models have had broad utility in both fields for many decades. Furthermore embryonic stem cells, applied as a basic research tool, have broad applications across the development fields and many other fields including cancer, regeneration and epigenetic research. These models have historically been applied in mechanistic studies but were also considered promising for evaluating teratogenic potential of test substances. In recent years, in vitro teratogenicity assays have become an area of interest for supporting the 3 Rs (reduction, refinement, and replacement of animal use). Generation of such assays also provides a means to facilitate early assessment of test agents at a higher throughput without excessive use of animals. In this review, the three models are described with an emphasis of how they are being developed and/or refined to support teratogenicity assessment as screening tools. An overview of the state of the science and future directions are described.