Germ plasm in Caenorhabditis elegans, Drosophila and Xenopus

Characteristics of the Vasa Gene in Silurus asotus and Its Expression Response to Letrozole Treatment

The identification and expression of germ cells are important for studying sex-related mechanisms in fish. The vasa gene, encoding an ATP-dependent RNA helicase, is recognized as a molecular marker of germ cells and plays a crucial role in germ cell development. Silurus asotus, an important freshwater economic fish species in China, shows significant sex dimorphism with the female growing faster than the male. However, the molecular mechanisms underlying these sex differences especially involving in the vasa gene in this fish remain poorly understood. In this work, the vasa gene sequence of S. asotus (named as Savasa) was obtained through RT-PCR and rapid amplification of cDNA end (RACE), and its expression in embryos and tissues was analyzed using qRT-PCR and an in situ hybridization method. Letrozole (LT) treatment on the larvae fish was also conducted to investigate its influence on the gene. The results revealed that the open reading frame (ORF) of Savasa was 1989 bp, encoding 662 amino acids. The SaVasa protein contains 10 conserved domains unique to the DEAD-box protein family, showing the highest sequence identity of 95.92% with that of Silurus meridionalis. In embryos, Savasa is highly expressed from the two-cell stage to the blastula stage in early embryos, with a gradually decreasing trend from the gastrula stage to the heart-beating stage. Furthermore, Savasa was initially detected at the end of the cleavage furrow during the two-cell stage, later condensing into four symmetrical cell clusters with embryonic development. At the gastrula stage, Savasa-positive cells increased and began to migrate towards the dorsal side of the embryo. In tissues, Savasa is predominantly expressed in the ovaries, with almost no or lower expression in other detected tissues. Moreover, Savasa was expressed in phase I-V oocytes in the ovaries, as well as in spermatogonia and spermatocytes in the testis, implying a specific expression pattern of germ cells. In addition, LT significantly upregulated the expression of Savasa in a concentration-dependent manner during the key gonadal differentiation period of the fish. Notably, at 120 dph after LT treatment, Savasa expression was the lowest in the testis and ovary of the high concentration group. Collectively, findings from gene structure, protein sequence, phylogenetic analysis, RNA expression patterns, and response to LT suggest that Savasa is maternally inherited with conserved features, serving as a potential marker gene for germ cells in S.asotus, and might participate in LT-induced early embryonic development and gonadal development processes of the fish. This would provide a basis for further research on the application of germ cell markers and the molecular mechanisms of sex differences in S. asotus.

Vasa Is a Potential Germ Cell Marker in Leopard Coral Grouper (Plectropomus leopardus)

Vasa (Ddx4, DEAD box polypeptide 4), an extremely specific marker of germ cells in vivo, is an ATP-dependent RNA helicase that plays an essential role in germ cell development and gametogenesis. However, the expression and function information about this gene in groupers remains lacking. Here, vasa homolog termed Plvasa was isolated and identified Plvasa as a putative germ cell marker in the leopard coral grouper, (Plectropomus leopardus). Results indicated that Plvasa contained 17 exons in the genomic sequence and 9 conserved motifs of the DEAD-box protein by sequence analysis. The sequence comparison, phylogenetic analyses and synteny analyses showed that Plvasa was homologous with other teleosts. Additionally, the expression of Plvasa was significantly higher in gonads than in other tissues in adult individuals (p < 0.05). Further, the distribution of Plvasa revealed that it was only expressed in the germ cells, such as spermatids, germline stem cells and oocytes at different stages, and could not be detected in the somatic cells of gonads. The current study verified that the Plvasa gene is a valuable molecular marker of germ cells in leopard coral grouper, which potentially plays an important role in investigating the genesis and development of teleost germ cells.

Post-transcriptional regulation of factors important for the germ line

Echinoderms are a major model system for many general aspects of biology, including mechanisms of gene regulation. Analysis of transcriptional regulation (Gene regulatory networks, direct DNA-binding of proteins to specific cis-elements, and transgenesis) has contributed to our understanding of how an embryo works. This chapter looks at post-transcriptional gene regulation in the context of how the primordial germ cells are formed, and how the factors essential for this process are regulated. Important in echinoderms, as in many embryos, is that key steps of fate determination are made post-transcriptionally. This chapter highlights these steps uncovered in sea urchins and sea stars, and links them to a general theme of how the germ line may regulate its fate differently than many of the embryo's somatic cell lineages.

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.

Germline development during embryogenesis of the larvacean, Oikopleura dioica

Germ cells develop into eggs and sperms and represent a lineage that survives through multiple generations. Germ cell specification during embryogenesis proceeds through one of two basic modes: either the cell-autonomous mode or the inductive mode. In the cell-autonomous mode, specification of germ cell fate involves asymmetric partitioning of the specialized maternal cytoplasm, known as the germplasm. Oikopleura dioica is a larvacean (class Appendicularia) and a chordate. It is regarded as a promising animal model for studying chordate development because of its short life cycle (5 days) and small genome size (∼60 ​Mb). We show that their embryos possess germplasm, as observed in ascidians (class Ascidiacea). The vegetal cytoplasm shifted towards the future posterior pole before the first cleavage occurred. A bilateral pair of primordial germ cells (PGC, B11 ​cells) was formed at the posterior pole at the 32-cell stage through two rounds of unequal cleavage. These B11 ​cells did not undergo further division before hatching of the tadpole-shaped larvae. The centrosome-attracting body (CAB) is a subcellular structure that contains the germplasm and plays crucial roles in germ cell development in ascidians. The presence of CAB with germplasm was observed in the germline lineage cells of larvaceans via electron microscopy and using extracted embryos. The CAB appeared at the 8-cell stage and persisted until the middle stage of embryogenesis. The antigen for the phosphorylated histone 3 antibody was localized to the CAB and persisted in the PGC until hatching after the CAB disappeared. Maternal snail mRNA, which encodes a transcription factor, was co-localized with the antigen for the H3S28p antibody. Furthermore, we found a novel PGC-specific subcellular structure that we call the germ body (GB). This study thus highlights the conserved and non-conserved features of germline development between ascidians and larvaceans. The rapid development and short life cycle (five days) of O. dioica would open the way to genetically analyze germ cell development in the future.

Conserved germ plasm characteristics across the Danio and Devario lineages

In many animal species, germ cell specification requires the inheritance of germ plasm, a biomolecular condensate containing maternally derived RNAs and proteins. Most studies of germ plasm composition and function have been performed in widely evolutionarily divergent model organisms, such as Caenorhabditis elegans, Drosophila, Xenopus laevis, and Danio rerio (zebrafish). In zebrafish, 12 RNAs localize to germ plasm at the furrows of the early embryo. Here, we tested for the presence of these RNAs in three additional species within the Danionin clade: Danio kyathit, Danio albolineatus, and Devario aequipinnatus. By visualizing nanos RNA, we find that germ plasm segregation patterns during early embryogenesis are conserved across these species. Ten additional germ plasm RNAs exhibit localization at the furrows of early embryos in all three non-zebrafish Danionin species, consistent with germ plasm localization. One component of zebrafish germ plasm, ca15b, lacked specific localization in embryos of the more distantly related D. aequipinnatus. Our findings show that within a subset of closely related Danionin species, the vast majority of germ plasm RNA components are conserved. At the same time, the lack of ca15b localization in D. aequipinnatus germ plasm highlights the potential for the divergence of germ plasm composition across a restricted phylogenetic space.

Germ plasm and the origin of the primordial germ cells in the oriental river prawn Macrobrachium nipponense

Germ plasm is a special cytoplasmic component containing special RNAs and proteins, and is located in specific regions of oocytes and embryos. Only the blastomeres inheriting the germ plasm can develop into primordial germ cells (PGCs). By using Vasa mRNA as a germline marker, we previously demonstrated that germline specification followed the preformation mode in the prawn Macrobrachium nipponense. In this study, we raised the Vasa antibody to identify germ plasm in the oocyte and trace the origin and migration of PGCs. In previtellogenic oocytes, Vasa protein was detected in the perinuclear region, in which electron-dense granules associated with numerous mitochondria were mostly visualized under a transmission electron microscope. In mature oocytes, immunosignal was localized to a large granule under the plasma membrane. During early embryogenesis, the granule was inherited by a single blastomere from 1-cell to 16-cell stages, and thereafter was segregated into two daughter blastomeres at the 32-cell stage. In gastrula, the Vasa-positive cells were large with typical PGC characteristics, containing a big round nucleus and a prominent nucleolus. The immunosignal was localized to the perinuclear region again. In the zoea stage, the Vasa-positive cells migrated toward the genital ridge and clustered in the dorsomedial region close to the yolk portion. Accordingly, we concluded that the prawn PGCs could be specified from the 16-cell stage by inheriting the germplasm. To our knowledge, this is the first report on the identification of the prawn germ plasm and PGCs. The continuous expression of Vasa protein throughout oogenesis and embryogenesis suggests that Vasa protein could be an important factor in germ plasm that functions in early germ cell specification.

H3K27me3 suppresses sister-lineage somatic gene expression in late embryonic germline cells of the ascidian, Halocynthia roretzi

Protection of the germline from somatic differentiation programs is crucial for germ cell development. In many animals, whose germline development relies on the maternally inherited germ plasm, such protection in particular at early stages of embryogenesis is achieved by maternally localized global transcriptional repressors, such as PIE-1 of Caenorhabditis elegans, Pgc of Drosophila melanogaster and Pem of ascidians. However, zygotic gene expression starts in later germline cells eventually and mechanisms by which somatic gene expression is selectively kept under repression in the transcriptionally active cells are poorly understood. By using the ascidian species Halocynthia roretzi, we found that H3K27me3, a repressive transcription-related chromatin mark, became enriched in germline cells starting at the 64-cell stage when Pem protein level and its contribution to transcriptional repression decrease. Interestingly, inhibition of H3K27me3 together with Pem knockdown resulted in ectopic expression in germline cells of muscle developmental genes Muscle actin (MA4) and Snail, and of Clone 22 (which is expressed in all somatic but not germline cells), but not of other tissue-specific genes such as the notochord gene Brachyury, the nerve cord marker ETR-1 and a heart precursor gene Mesp, at the 110-cell stage. Importantly, these ectopically expressed genes are normally expressed in the germline sister cells (B7.5), the last somatic lineage separated from the germline. Also, the ectopic expression of MA4 was dependent on a maternally localized muscle determinant Macho-1. Taken together, we propose that H3K27me3 may be responsible for selective transcriptional repression for somatic genes in later germline cells in Halocynthia embryos and that the preferential repression of germline sister-lineage genes may be related to the mechanism of germline segregation in ascidian embryos, where the germline is segregated progressively by successive asymmetric cell divisions during cell cleavage stages. Together with findings from C. elegans and D. melanogaster, our data for this urochordate animal support the proposal for a mechanism, conserved widely throughout the animal kingdom, where germline transcriptional repression is mediated initially by maternally localized factors and subsequently by a chromatin-based mechanism.

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

Evolutionary conservation and divergence of Vasa, Dazl and Nanos1 during embryogenesis and gametogenesis in dark sleeper (Odontobutis potamophila)

Germline-specific genes, Vasa, Dazl and Nanos1, have highly conserved functions in germline development and fertility across animal phyla. In this study, the full-length sequences of Opvasa, Opdazl and Opnanos1 were cloned and characterized from the dark sleeper (Odontobutis potamophila). Gonad-specific expression patterns of Opvasa and Opdazl were confirmed in adult tissues by quantitative real-time PCR (qRT-PCR). Different from Opvasa and Opdazl, the expression of Opnanos1 was ubiquitously detected in all examined tissues except for the liver and spleen. Time-course dynamic expressions during embryogenesis were assessed, and all three genes (Opvasa, Opdazl and Opnanos1) persisted at a high level until gastrulation. qRT-PCR and Western blotting analyses revealed that all three genes were highly expressed throughout gametogenesis. In testis, the expressions of all three genes at the mRNA and protein levels were down-regulated during spermatogenesis. In ovary, different expression patterns were found, and all three genes had a differential role in translational regulation during oogenesis. The expressions of Opvasa, Opdazl and Opnanos1 at the mRNA but not the protein level were high in stage IV. Different expression patterns were found in premeiotic gonads treated by HPG axis hormones (HCG and LHRH-A). Immunolocalization analysis demonstrated that in testis, Opvasa, Opdazl and Opnanos1 were detected in spermatogonia and spermatocytes but absent in the meiotic products, such as spermatids and spermatozoa. In ovary, Opvasa, Opdazl and Opnanos1 persisted at a high level throughout oogenesis. These findings indicated that Opvasa, Opdazl and Opnanos1 played an important role in mitotic and early meiotic phases of oogenesis and spermatogenesis, and they functioned as maternal factors in early embryogenesis. Their proteins could be used as three new markers for germ cells during gametogenesis in O. potamophila gonad. Our data laid a good foundation for improving the breeding efficiency of O. potamophila.

Maternal Nanos-Dependent RNA Stabilization in the Primordial Germ Cells of Drosophila Embryos

Nanos (Nos) is an evolutionary conserved protein expressed in the germline of various animal species. In Drosophila, maternal Nos protein is essential for germline development. In the germline progenitors, or the primordial germ cells (PGCs), Nos binds to the 3' UTR of target mRNAs to repress their translation. In contrast to this prevailing role of Nos, here we report that the 3' UTR of CG32425 mRNA mediates Nos-dependent RNA stabilization in PGCs. We found that the level of mRNA expressed from a reporter gene fused to the CG32425 3' UTR was significantly reduced in PGCs lacking maternal Nos (nos PGCs) as compared with normal PGCs. By deleting the CG32425 3' UTR, we identified the region required for mRNA stabilization, which includes Nos-binding sites. In normal embryos, CG32425 mRNA was maternally supplied into PGCs and remained in this cell type during embryogenesis. However, as expected from our reporter assay, the levels of CG32425 mRNA and its protein product expressed in nos PGCs were lower than in normal PGCs. Thus, we propose that Nos protein has dual functions in translational repression and stabilization of specific RNAs to ensure proper germline development.

Convergent evolution of germ granule nucleators: A hypothesis

Germ cells have been considered "the ultimate stem cell" because they alone, during normal development of sexually reproducing organisms, are able to give rise to all organismal cell types. Morphological descriptions of a specialized cytoplasm termed 'germ plasm' and associated electron dense ribonucleoprotein (RNP) structures called 'germ granules' within germ cells date back as early as the 1800s. Both germ plasm and germ granules are implicated in germ line specification across metazoans. However, at a molecular level, little is currently understood about the molecular mechanisms that assemble these entities in germ cells. The discovery that in some animals, the gene products of a small number of lineage-specific genes initiate the assembly (also termed nucleation) of germ granules and/or germ plasm is the first step towards facilitating a better understanding of these complex biological processes. Here, we draw on research spanning over 100years that supports the hypothesis that these nucleator genes may have evolved convergently, allowing them to perform analogous roles across animal lineages.

Conserved role of Ovo in germline development in mouse and Drosophila

Ovo, which encodes a transcription factor with Zn-finger domains, is evolutionarily conserved among animals. In Drosophila, in addition to its zygotic function for egg production, maternal ovo activity is required in primordial germ cells (PGCs) for expression of germline genes such as vasa and nanos. In this study, we found that maternal Ovo accumulates in PGC nuclei during embryogenesis. In these cells, ovo serves a dual function: activation of genes expressed predominantly in PGCs, and conversely suppression of somatic genes. Reduction of ovo activity in PGCs makes them unable to develop normally into germ cells of both sexes. In mice, knockout of the ovo ortholog, Ovol2, which is expressed in PGCs, decreases the number of PGCs during early embryogenesis. These data strongly suggest that ovo acts as part of an evolutionarily conserved mechanism that regulates germline development in animals.

Mitochondrial trafficking through Rhot1 is involved in the aggregation of germinal granule components during primordial germ cell formation in Xenopus embryos

In many animals, the germ plasm is sufficient and necessary for primordial germ cell (PGC) formation. It contains germinal granules and abundant mitochondria (germline-Mt). However, the role of germline-Mt in germ cell formation remains poorly understood. In Xenopus, the germ plasm is distributed as many small islands at the vegetal pole, which gradually aggregates to form a single large mass in each of the four vegetal pole cells at the early blastula stage. Polymerized microtubules and the adapter protein kinesin are required for the aggregation of germ plasm. However, it remains unknown whether germline-Mt trafficking is important for the cytoplasmic transport of germinal granules during germ plasm aggregation. In this study, we focused on the mitochondrial small GTPase protein Rhot1 to inhibit mitochondrial trafficking during the germ plasm aggregation. Expression of Rhot1ΔC, which lacks the C-terminal mitochondrial transmembrane domain, inhibited the aggregation of germline-Mt during early development. In Rhot1-inhibited embryos, germinal granule components did not aggregate during cleavage stages, which reduced the number of PGCs on the genital ridge at tail-bud stage. These results suggest that mitochondrial trafficking is involved in the aggregation of germinal granule components, which are essential for the formation of PGCs.

A perspective on the evolution of germ-cell development and germinal mosaics of deleterious mutations

In many animals a small number of primordial germ cells (PGCs) are set aside early in development, mitosis and mitochondrial DNA syntheses are arrested, transcription is stopped or reduced, and the PGCs migrate later to the emerging gonads and become germ cells. What could be the evolutionary advantage of sequestering non-dividing PGCs early in development? A commonly cited advantage is a reduction in the number of new deleterious mutations that would occur if there were additional divisions in PGCs early in development. We would like to add to this advantage the fact that these additional mutations in PGCs give rise to germinal mosaics (i.e., premeiotic clusters of mutation) in multiple progeny of the same individual, thus having a larger detrimental effect on the evolutionary fitness of their carriers. Here, we reviewed published studies providing evidence that germinal mosaics of deleterious mutant alleles are not rare, occur for all types of genetic damage, and have been observed in all tested organisms and in nature. We propose the hypothesis that PGC sequestration during early animal development may have evolved in part in response to selection for preventing the occurrence of premeiotic clusters of deleterious mutant alleles, and describe a series of predictions that would allow the assessment of the potential role of germinal mosaics on the evolution of PGC sequestration.

Pre-meiotic transformation of germplasm-related structures during male gamete differentiation in Xenopus laevis

To highlight the ultrastructural features of transformation occurring with germplasm-related structures (GPRS), the spermatogenic cells of Xenopus laevis were studied by transmission electron microscopy and quantitative analysis. It was determined that in spermatogonia and spermatocytes, the compact germinal granules underwent fragmentation into particles comparable with inter-mitochondrial cement (IMC). Fragments of IMC agglutinated some cell mitochondria and resulted in the creation of mitochondrial clusters. Clustered mitochondria responded with loss of their membranes that occurred by the twisting of membranous protrusions around themselves until multi-layered membranes were formed. The mitochondrial affinity of multi-layered membranes was proven by an immunopositive test for mitochondrial dihydrolipoamide acetyltransferase. As a consequence of mitochondrial membrane twisting, the naked mitochondrial cores appeared and presumably underwent dispersion, which is the terminal stage of GPRS transformation. As no GPRS were observed in spermatids and sperm, it was assumed that these structures are functionally assigned to early stages of meiotic differentiation.

Posterior localization of ApVas1 positions the preformed germ plasm in the sexual oviparous pea aphid Acyrthosiphon pisum

Background

Germline specification in some animals is driven by the maternally inherited germ plasm during early embryogenesis (inheritance mode), whereas in others it is induced by signals from neighboring cells in mid or late development (induction mode). In the Metazoa, the induction mode appears as a more prevalent and ancestral condition; the inheritance mode is therefore derived. However, regarding germline specification in organisms with asexual and sexual reproduction it has not been clear whether both strategies are used, one for each reproductive phase, or if just one strategy is used for both phases. Previously we have demonstrated that specification of germ cells in the asexual viviparous pea aphid depends on a preformed germ plasm. In this study, we extended this work to investigate how germ cells were specified in the sexual oviparous embryos, aiming to understand whether or not developmental plasticity of germline specification exists in the pea aphid.

Results

We employed Apvas1, a Drosophila vasa ortholog in the pea aphid, as a germline marker to examine whether germ plasm is preformed during oviparous development, as has already been seen in the viviparous embryos. During oogenesis, Apvas1 mRNA and ApVas1 protein were both evenly distributed. After fertilization, uniform expression of Apvas1 remained in the egg but posterior localization of ApVas1 occurred from the fifth nuclear cycle onward. Posterior co-localization of Apvas1/ApVas1 was first identified in the syncytial blastoderm undergoing cellularization, and later we could detect specific expression of Apvas1/ApVas1 in the morphologically identifiable germ cells of mature embryos. This suggests that Apvas1/ApVas1-positive cells are primordial germ cells and posterior localization of ApVas1 prior to cellularization positions the preformed germ plasm.

Conclusions

We conclude that both asexual and sexual pea aphids rely on the preformed germ plasm to specify germ cells and that developmental plasticity of germline specification, unlike axis patterning, occurs in neither of the two aphid reproductive phases. Consequently, the maternal inheritance mode implicated by a preformed germ plasm in the oviparous pea aphid becomes a non-canonical case in the Hemimetabola, where so far the zygotic induction mode prevails in most other studied insects.

In mouse oocytes the mitochondrion-originated germinal body-like structures accumulate mouse Vasa homologue (MVH) protein

Mouse Vasa homologue (MVH) antibodies were applied to mouse Graafian oocytes to clarify if mitochondrion-originated germinal body-like structures, described previously by conventional electron microscopy, were associated with the germ plasm. It was found that both the mitochondrion-like structures with cristae and the germinal body-like structures that lacked any signs of cristae were labelled specifically by the anti-MVH antibody. Moreover, some granules were MVH-positive ultrastructural hybrids of the mitochondria and germinal body-like structures, the presence of which clearly supported the idea of a mitochondrial origin for the germinal body-like structures. This finding is the first evidence that mitochondrion-originated germinal body-like granules represent mouse germ plasm.

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.

Restriction of the Xenopus DEADSouth mRNA to the primordial germ cells is ensured by multiple mechanisms

DEADSouth mRNA encoding the RNA helicase DDX25 is a component of the germ plasm in Xenopus laevis. We investigated the mechanisms underlying its specific mRNA expression in primordial germ cells (PGCs). Based on our previous findings of several microRNA miR-427 recognition elements (MREs) in the 3' untranslated region of the mRNA, we first examined whether DEADSouth mRNA was degraded by miR-427 targeting in somatic cells. Injection of antisense miR-427 oligomer and reporter mRNA for mutated MREs revealed that DEADSouth mRNA was potentially degraded in somatic cells via miR-427 targeting, but not in PGCs after the mid-blastula transition (MBT). The expression level of miR-427 was very low in PGCs, which probably resulted in the lack of miR-427-mediated degradation. In addition, the DEADSouth gene was expressed zygotically after MBT. Thus, the predominant expression of DEADSouth mRNA in the PGCs is ensured by multiple mechanisms including zygotic expression and prohibition from miR-427-mediated degradation.

Nuclear genes with sex bias in Ruditapes philippinarum (Bivalvia, veneridae): Mitochondrial inheritance and sex determination in DUI species

Mitochondria are inherited maternally in most metazoans, but in bivalves with Doubly Uniparental Inheritance (DUI) a mitochondrial lineage is transmitted through eggs (F-type), and another through sperm (M-type). In DUI species, a sex-ratio distortion of the progeny was observed: some females produce a female-biased offspring (female-biased family), others a male-biased progeny (male-biased family), and others a 50:50 sex-ratio. A peculiar segregation pattern of M-type mitochondria in DUI organisms appears to be correlated with the sex bias of these families. According to a proposed model for the inheritance of M-type mitochondria in DUI, the transmission of sperm mitochondria is controlled by three nuclear genes, named W, X, and Z. An additional S gene with different dosage effect would be involved in sex determination. In this study, we analyzed structure and localization of three transcripts (psa, birc, and anubl1) with specific sex and family biases in the Manila clam Ruditapes philippinarum. In situ hybridization confirmed the localization of these transcripts in gametogenic cells. In other animals, homologs of these genes are involved in reproduction and ubiquitination. We hypothesized that these genes may have a role in sex determination and could also be responsible for the maintenance/degradation of spermatozoon mitochondria during embryo development of the DUI species R. philippinarum, so that we propose them as candidate factors of the W/X/Z/S system.

Analysis of localization and reorganization of germ plasm in Xenopus transgenic line with fluorescence-labeled mitochondria

Germ plasm is found in germ-line cells of Xenopus and thought to include the determinant of primordial germ cells (PGCs). As mitochondria is abundant in germ plasm, vital staining of mitochondria was used to analyze the movement and function of germ plasm; however, its application was limited in early cleavage embryos. We made transgenic Xenopus, harboring enhanced green fluorescent protein (EGFP) fused to the mitochondria transport signal (Dria-line). Germ plasm with EGFP-labeled mitochondria was clearly distinguishable from the other cytoplasm, and retained mostly during one generation of germ-line cells in Dria-line females. Using the Dria-line, we show that germ plasm is reorganized from near the cell membrane to the perinuclear space at St. 9, dependent on the microtubule system.

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.

DEADSouth protein localizes to germ plasm and is required for the development of primordial germ cells in Xenopus laevis

DEADSouth mRNA is a component of germ plasm in Xenopus laevis and encodes a DDX25 DEAD-box RNA helicase. To determine the intracellular localization of DEADSouth protein, we injected mRNA encoding DEADSouth tagged with mCherry fluorescent protein into fertilized eggs from transgenic Xenopus expressing EGFP fused with a mitochondrial targeting signal. The DEADSouth-mCherry fusion protein was localized to the germ plasm, a mitochondria-rich region in primordial germ cells (PGCs). DEADSouth overexpression resulted in a reduction of PGC numbers after stage 20. Conversely, DEADSouth knockdown using an antisense locked nucleic acid gapmer inhibited movement of the germ plasm from the cortex to the perinuclear region, resulting in inhibition of PGC division at stage 12 and a decrease in PGC numbers at later stages. The knockdown phenotype was rescued by intact DEADSouth mRNA, but not mutant mRNA encoding inactive DEADSouth helicase. Surprisingly, it was also rescued by mouse vasa homolog and Xenopus vasa-like gene 1 mRNAs that encode DDX4 RNA helicases. The rescue was dependent on the 3' untranslated region (3'UTR) of DEADSouth mRNA, which was used for PGC-specific expression. The 3'UTR contributed to localization of the injected mRNA to the germ plasm, resulting in effective localization of DEADSouth protein. These results demonstrate that localization of DEADSouth helicase to the germ plasm is required for proper PGC development in Xenopus laevis.

Localization of germline marker vasa homolog RNA to a single blastomere at early cleavage stages in the oriental river prawn Macrobrachium nipponense: evidence for germ cell specification by preformation

Germ cells are specified by the inheritance of maternal germline determinants (preformation mode) or inductive signals from somatic cells (epigenesis mode) during embryogenesis. However, the germline specification in decapod crustaceans is unclear so far. Using vasa homolog (MnVasa) as a germ cell marker, here we probed the early events of germline specification in the oriental river prawn Macrobrachium nipponense. Quantitative RT-PCR analysis of unfertilized eggs and embryos demonstrated that the prawn MnVasa mRNA is a maternal factor. Whole-mount in situ hybridization further indicated that MnVasa transcripts are maternally supplied to only one blastomere at the very early cleavage stages. As cleavage proceeds, the MnVasa-positive blastomere undergoes proliferation and increases in number. During gastrulation, the MnVasa-positive cells are found to be around a blastopore and could migrate into an embryo through the blastopore. At the zoea stage, clusters of the MnVasa-positive cells distribute not only in the gonad rudiment in the cephalothorax but also at an extragonadic site, dorsal to the posterior hindgut in the abdomen, suggesting that MnVasa-positive cells could migrate anteriorly to the genital rudiment through the hindgut. Based on the dynamic localization and number of MnVasa-positive cells during embryogenesis, we concluded that the MnVasa-positive cells are primordial germ cells (PGC) or founder cells of PGC that are separated from soma at the early cleavage stage. MnVasa mRNA might have a key function in the specification of the prawn germline cells as a maternal determinant. These results provide the first evidence that the germline specification in decapod crustaceans follows a preformation mode.

Cleavage pattern and fate map of the mesentoblast, 4d, in the gastropod Crepidula: a hallmark of spiralian development

Background

Animals with a spiral cleavage program, such as mollusks and annelids, make up the majority of the superphylum Lophotrochozoa. The great diversity of larval and adult body plans in this group emerges from this highly conserved developmental program. The 4d micromere is one of the most conserved aspects of spiralian development. Unlike the preceding pattern of spiral divisions, cleavages within the 4d teloblastic sublineages are bilateral, representing a critical transition towards constructing the bilaterian body plan. These cells give rise to the visceral mesoderm in virtually all spiralians examined and in many species they also contribute to the endodermal intestine. Hence, the 4d lineage is an ideal one for studying the evolution and diversification of the bipotential endomesodermal germ layer in protostomes at the level of individual cells. Little is known of how division patterns are controlled or how mesodermal and endodermal sublineages diverge in spiralians. Detailed modern fate maps for 4d exist in only a few species of clitellate annelids, specifically in glossiphoniid leeches and the sludge worm Tubifex. We investigated the 4d lineage in the gastropod Crepidula fornicata, an established model system for spiralian biology, and in a closely related direct-developing species, C. convexa.

Results

High-resolution cell lineage tracing techniques were used to study the 4d lineage of C. fornicata and C. convexa. We present a new nomenclature to name the progeny of 4d, and report the fate map for the sublineages up through the birth of the first five pairs of teloblast daughter cells (when 28 cells are present in the 4d sublineage), and describe each clone’s behavior during gastrulation and later stages as these undergo differentiation. We identify the precise origin of the intestine, two cells of the larval kidney complex, the larval retractor muscles and the presumptive germ cells, among others. Other tissues that arise later in the 4d lineage include the adult heart, internal foot tissues, and additional muscle and mesenchymal cells derived from later-born progeny of the left and right teloblasts. To test whether other cells can compensate for the loss of these tissues (that is, undergo regulation), specific cells were ablated in C. fornicata.

Conclusions

Our results present the first fate map of the 4d micromere sublineages in a mollusk. The fate map reveals that endodermal and mesodermal fates segregate much later than previously thought. We observed little evidence of regulation between sublineages, consistent with a lineage-driven cell specification process. Our results provide a framework for comparisons with other spiralians and lay the groundwork for investigation of the molecular mechanisms of endomesoderm formation, germ line segregation and bilateral differentiation in Crepidula.

Germ cell development in the scleractinian coral Euphyllia ancora (Cnidaria, Anthozoa)

Sexual reproduction of scleractinian coral is among the most important means of establishing coral populations. However, thus far, little is known about the mechanisms underlying coral gametogenesis. To better understand coral germ cell development, we performed a histological analysis of gametogenesis in Euphyllia ancora and characterized the coral homolog of the Drosophila germline marker gene vasa. The histological analysis revealed that E. ancora gametogenesis occurs in the mesenterial mesoglea between the mesenterial filaments and the retractor muscle bands. The development of germ cells takes approximately one year in females and half a year in males. Staining of tissue sections with an antibody against E. ancora Vasa (Eavas) revealed anti-Eavas immunoreactivity in the oogonia, early oocyte, and developing oocyte, but only faint or undetectable reactivity in developing oocytes that were >150 µm in diameters. In males, Eavas could be detected in the spermatogonia and primary spermatocytes but was only faintly detectable in the secondary spermatocytes, spermatids, and sperms. Furthermore, a reverse transcription-polymerase chain reaction analysis and Western blotting analysis of unfertilized mature eggs proved the presence of Eavas transcripts and proteins, suggesting that Eavas may be a maternal factor. Vasa may represent a germ cell marker for corals, and would allow us to distinguish germ cells from somatic cells in coral bodies that have no distinct organs.

RNA-binding proteins, RNA granules, and gametes: is unity strength?

Changes in mRNA translation and degradation represent post-transcriptional processes operating during gametogenesis and early embryogenesis to ensure regulated protein synthesis. Numerous mRNA-binding proteins (RBPs) have been described in multiple animal models that contribute to the control of mRNA translation and decay during oogenesis and spermatogenesis. An emerging view from studies performed in germ cells and somatic cells is that RBPs associate with their target mRNAs in RNA–protein (or ribonucleoprotein) complexes (mRNPs) that assemble in various cytoplasmic RNA granules that communicate with the translation machinery and control mRNA storage, triage, and degradation. In comparison withXenopus, Caenorhabditis elegans, orDrosophila, the composition and role of cytoplasmic RNA-containing granules in mammalian germ cells are still poorly understood. However, regained interest for these structures has emerged with the recent discovery of their role in small RNA synthesis and transposon silencing through DNA methylation. In this review, we will briefly summarize our current knowledge on cytoplasmic RNA granules in murine germ cells and describe the role of some of the RBPs they contain in regulating mRNA metabolism and small RNA processing during gametogenesis.

Doubly uniparental inheritance of mitochondria as a model system for studying germ line formation

BACKGROUND

Doubly Uniparental Inheritance (DUI) of mitochondria occurs when both mothers and fathers are capable of transmitting mitochondria to their offspring, in contrast to the typical Strictly Maternal Inheritance (SMI). DUI was found in some bivalve molluscs, in which two mitochondrial genomes are inherited, one through eggs, the other through sperm. During male embryo development, spermatozoon mitochondria aggregate in proximity of the first cleavage furrow and end up in the primordial germ cells, while they are dispersed in female embryos.

METHODOLOGY/PRINCIPAL FINDINGS

We used MitoTracker, microtubule staining and transmission electron microscopy to examine the mechanisms of this unusual distribution of sperm mitochondria in the DUI species Ruditapes philippinarum. Our results suggest that in male embryos the midbody deriving from the mitotic spindle of the first division concurs in positioning the aggregate of sperm mitochondria. Furthermore, an immunocytochemical analysis showed that the germ line determinant Vasa segregates close to the first cleavage furrow.

CONCLUSIONS/SIGNIFICANCE

In DUI male embryos, spermatozoon mitochondria aggregate in a stable area on the animal-vegetal axis: in organisms with spiral segmentation this zone is not involved in cleavage, so the aggregation is maintained. Moreover, sperm mitochondria reach the same embryonic area in which also germ plasm is transferred. In 2-blastomere embryos, the segregation of sperm mitochondria in the same region with Vasa suggests their contribution in male germ line formation. In DUI male embryos, M-type mitochondria must be recognized by egg factors to be actively transferred in the germ line, where they become dominant replacing the Balbiani body mitochondria. The typical features of germ line assembly point to a common biological mechanism shared by DUI and SMI organisms. Although the molecular dynamics of the segregation of sperm mitochondria in DUI species are unknown, they could be a variation of the mechanism regulating the mitochondrial bottleneck in all metazoans.

Premeiotic transformation of germ plasm-related structures during the sea urchin spermatogenesis

The germ plasm-related structures (GPRS) and the transformation that occurs to them during the spermatogenesis of the sea urchin Anthocidaris crassispina were studied by electron microscopy and morphometry. The GPRS were observed in spermatogonia and spermatocytes, but not in spermatids and sperm, which suggests an important role for these structures during the onset of meiosis. It was proposed that the germinal granules are fragmented into the compact electron-dense nuage, and fragments of the latter penetrate into the periphery of the compact electron-lucent nuage. The process of nuage integration is completed with the formation of the combined nuage, which aggregates some mitochondria into clusters. Once formed, the mitochondrial clusters undergo dissemination and assume the appearance of the dispersed nuage with mitochondrial derivatives, which in turn develops into the scattered nuage. The scattered nuage, which presumably presents the composite mixture saturated with mitochondrial matrix, terminates the GPRS transformation.

Toxicity and differential protein analysis following destruxin A treatment of Spodoptera litura (Lepidoptera: Noctuidae) SL-1 cells

The cytotoxicity of a destruxin A (DA) treatment of Spodoptera litura SL-1 cells was investigated. An MTT assay showed that DA was highly toxic to SL-1 cells in a concentration- and time-dependent manner. The IC(50) values of DA, after 24 h and 48 h of treatment, were 17.86 μg/mL and 7.80 μg/mL, respectively. Under inverted phase contrast microscopy (IPCM), it was found that prolonged treatment with DA could induce cell rounding, cellular membrane shrinking, formation of apoptotic bodies, vacuole appearance and cytoplasm leak out. Apoptosis induced by DA was further confirmed by fluorescence microscopy (FM) and flow cytometry (FCM) studies. SL-1 cells entered early apoptosis following a treatment with 2.5 μg/mL DA and entered late apoptosis following a treatment with increasing concentrations of DA. Furthermore, two-dimensional gel electrophoresis (2-DE) analysis was used to identify 22 proteins which were differentially expressed (≥2-fold difference) between control cells and DA-treated cells, and the expression level of these proteins was significantly different between the treated and untreated cells. Our results suggest that these differentially expressed proteins may help explain the diverse biological effects caused by the destruxin A treatment of cells; additionally, some of the identified proteins may have roles in SL-1 cellular proliferation and apoptosis.

piRNAs, transposon silencing, and Drosophila germline development

Transposons are prominent features of most eukaryotic genomes and mobilization of these elements triggers genetic instability. Transposon silencing is particularly critical in the germline, which maintains the heritable genetic complement. Piwi-interacting RNAs (piRNAs) have emerged as central players in transposon silencing and genome maintenance during germline development. In particular, research on Drosophila oogenesis has provided critical insights into piRNA biogenesis and transposon silencing. In this system, the ability to place piRNA mutant phenotypes within a well-defined developmental framework has been instrumental in elucidating the molecular mechanisms underlying the connection between piRNAs and transposon control.

Cleavage and gastrulation in the Kuruma shrimp Penaeus (Marsupenaeus) japonicus (Bate): a revised cell lineage and identification of a presumptive germ cell marker

A previous study suggested that mesendoderm (ME) cell arrest occurred at the 64-cell stage and a ring of eight presumptive naupliar mesoderm cells or crown cells surrounded the blastopore in the Kuruma shrimp Penaeus (Marsupenaeus) japonicus. Since this varied from the pattern observed in other penaeoidean shrimp, cleavage and gastrulation was re-examined in P. japonicus using the nucleic acid stain Sytox Green and confocal microscopy. In contrast to the earlier study, cleavage and gastrulation followed the pattern observed in other penaeoidean shrimp. The ME cells arrested at the 32-cell stage, ingressed into the blastocoel, and resumed division after a three cell cycle delay. Nine naupliar mesoderm or crown cells surrounded the blastopore and their descendants invaginated during gastrulation. An intracellular body (ICB) was detected by Sytox Green and SYTO RNASelect staining to be segregated to one ME cell in P. japonicus, as described previously in Penaeus monodon. Staining of the ICB was eliminated by pre-treatment with RNase but not DNase. The ICB was also found in two other penaeoidean shrimp, Penaeus vannamei (Family Penaeidae) and Sicyonia ingentis (Family Sicyoniidae). The results support the hypothesis that the ICB is a germ granule found in the Dendrobranchiata.

Temporal expression and steroidal regulation of piRNA pathway genes (mael, piwi, vasa) during Silurana (Xenopus) tropicalis embryogenesis and early larval development

It has been extensively documented that exposure of amphibians and teleost fish to exogenous steroid hormones like estrogen, androgen, xenoestrogen or steroid biosynthesis inhibitors can impair their gonadal development or induce sex reversal against genotypic sex. However, the molecular pathways underlying sexual development and the effects of sex steroids or other exogenous hormones in these aquatic vertebrates remain elusive. Recently, a germ plasm-associated piRNA (piwi-interacting RNA) pathway has been shown to be a determinant in the development of animal gonadal germline cells. In the current study, we examined whether this piRNA pathway is involved in the regulation of sex steroid hormones in gonadal development. We firstly established developmental expression patterns of three key piRNA pathway genes (mael, piwi and vasa), during Silurana (Xenopus) tropicalis embryogenesis and early larval development. All three genes exhibit high expression at early developmental stages and have significantly decreased expression thereafter, indicating a very active involvement of piRNA pathway at the beginning of embryogenesis. We further examined gene expression changes of those genes in frog larvae exposed to two sex steroid biosynthesis inhibitors, fadrozole and finasteride, both of which are known to result in male-biased or female-biased phenotypes, respectively. We found that fadrozole and finasteride exposures increased the expression of piRNA pathway genes such as mael and vasa at the larval stage when the expression of piRNA pathway genes is programmed to be very low. Therefore, our results indicate that the piRNA pathway is likely a common pathway by which different sex steroid hormones regulate gonadal sex differentiation.

Cloning and differential expression pattern of vasa in the developing and recrudescing gonads of catfish, Clarias gariepinus

Vasa gene codes for a DEAD box family protein, which plays a crucial role in primordial germ cell proliferation. In this study, we report cloning of vasa from gonads of air-breathing catfish, Clarias gariepinus, a seasonally reproducing teleost fish. We studied the expression pattern of vasa during gametogenesis using real-time PCR. We also examined the hormonal regulation on vasa in gonads of catfish. RT-PCR analysis revealed that vasa was detectable only in the gonads. Further, real-time PCR results showed that expression of vasa was seen throughout the development from embryonic stage to adult. However, the expression was more in ovary than in testis during gonadal development. In adult testis, the vasa transcripts were significantly high during spermatogenesis and it declined during spermiation. On the other hand, during ovarian recrudescence, vasa transcripts were high in immature oocytes (stages I and II oocytes) when compared to mature oocytes (stages III and IV oocytes). Human chorionic gonadotropin treatment in recrudescing ovary (in vivo) as well as in testicular slices (in vitro) resulted in up regulation of vasa mRNA in a time-dependent manner. These results together suggest that vasa gene has got an important role to play in spermatogenesis and oogenesis during recrudescence in addition to development.

Fish germ cells

Fish, like many other animals, have two major cell lineages, namely the germline and soma. The germ-soma separation is one of the earliest events of embryonic development. Germ cells can be specifically labeled and isolated for culture and transplantation, providing tools for reproduction of endangered species in close relatives, such as surrogate production of trout in salmon. Haploid cell cultures, such as medaka haploid embryonic stem cells have recently been obtained, which are capable of mimicking sperm to produce fertile offspring, upon nuclear being directly transferred into normal eggs. Such fish originated from a mosaic oocyte that had a haploid meiotic nucleus and a transplanted haploid mitotic cell culture nucleus. The first semi-cloned fish is Holly. Here we review the current status and future directions of understanding and manipulating fish germ cells in basic research and reproductive technology.

Different requirements for conserved post-transcriptional regulators in planarian regeneration and stem cell maintenance

Planarian regeneration depends on the presence and precise regulation of pluripotent adult somatic stem cells named neoblasts, which differentiate to replace cells of any missing tissue. A characteristic feature of neoblasts is the presence of large perinuclear nonmembranous organelles named "chromatoid bodies", which are comparable to ribonucleoprotein structures found in germ cells of organisms across different phyla. In order to better understand regulation of gene expression in neoblasts, and potentially the function and composition of chromatoid bodies, we characterized homologues to known germ and soma ribonucleoprotein granule components from other organisms and analyzed their function during regeneration of the planarian Dugesia japonica. Expression in neoblasts was detected for 49 of 55 analyzed genes, highlighting the prevalence of post-transcriptional regulation in planarian stem cells. RNAi-mediated knockdown of two factors [ago-2 and bruli] lead to loss of neoblasts, and consequently loss of regeneration, corroborating with results previously reported for a bruli ortholog in the planarian Schmidtea mediterranea (Guo et al., 2006). Conversely, depletion mRNA turnover factors [edc-4 or upf-1], exoribonucleases [xrn-1 or xrn-2], or DEAD box RNA helicases [Djcbc-1 or vas-1] inhibited planarian regeneration, but did not reduce neoblast proliferation or abundance. We also found that depletion of cap-dependent translation initiation factors eIF-3A or eIF-2A interrupted cell cycle progression outside the M-phase of mitosis. Our results show that a set of post-transcriptional regulators is required to maintain the stem cell identity in neoblasts, while another facilitates proper differentiation. We propose that planarian neoblasts maintain pluripotency by employing mechanisms of post-transcriptional regulation exhibited in germ cells and early development of most metazoans.

Associations between PIWI proteins and TDRD1/MTR-1 are critical for integrated subcellular localization in murine male germ cells

The mouse Piwi family proteins (MILI, MIWI and MIWI2) play pivotal roles in spermatogenesis through transcriptional and post-transcriptional gene regulation. To reveal the molecular functions of these proteins, we investigate the proteins that bind to MILI in adult mouse testes. We found that both MILI and MIWI bind to TDRD1/MTR-1, which is also an essential protein for spermatogenesis. Co-immunoprecipitation assays and subcellular localization of the proteins and mutants thereof revealed a complex formation involving MILI, MIWI and TDRD1/MTR-1. In addition, the subcellular localizations of MILI and TDRD1/MTR-1 were altered, and chromatoid body formation was impaired in the MIWI-null round spermatids. These data suggest that the formation of complexes between MILI, MIWI and TDRD1/MTR-1 is critical for the integrated subcellular localizations of these proteins, and is presumably essential for spermatogenesis.

Doubly uniparental inheritance: two mitochondrial genomes, one precious model for organelle DNA inheritance and evolution

Eukaryotes have exploited several mechanisms for organelle uniparental inheritance, so this feature arose and evolved independently many times in their history. Metazoans' mitochondria commonly experience strict maternal inheritance; that is, they are only transmitted by females. However, the most noteworthy exception comes from some bivalve mollusks, in which two mitochondrial lineages (together with their genomes) are inherited: one through females (F) and the other through males (M). M and F genomes show up to 30% sequence divergence. This inheritance mechanism is known as doubly uniparental inheritance (DUI), because both sexes inherit uniparentally their mitochondria. Here, we review what we know about this unusual system, and we propose a model for evolution of DUI that might account for its origin as sex determination mechanism. Moreover, we propose DUI as a choice model to address many aspects that should be of interest to a wide range of biological subfields, such as mitochondrial inheritance, mtDNA evolution and recombination, genomic conflicts, evolution of sex, and developmental biology. Actually, as research proceeds, mitochondria appear to have acquired a central role in many fundamental processes of life, which are not only in their metabolic activity as cellular power plants, such as cell signaling, fertilization, development, differentiation, ageing, apoptosis, and sex determination. A function of mitochondria in the origin and maintenance of sex has been also proposed.

Postembryonic epigenesis of Vasa-positive germ cells from aggregated hemoblasts in the colonial ascidian, Botryllus primigenus

We investigated whether Vasa was a germline-specific marker in the colonial ascidian Botryllus primigenus, and whether it was inducible epigenetically in the adult life span. We cloned a Botryllus Vasa homologue (BpVas). The deduced open reading frame encoded 687 amino acid residues. It was expressed specifically by germline cells such as the loose cell mass, oogonia and juvenile oocytes in the ovary, and the primordial testis (compact cell mass), spermatogonia and juvenile spermatocytes in the testis. The loose cell mass, the most primitive germline cells, showed an ultrastructure of undifferentiated cells known as hemoblasts. The hemoblasts did not contain electron-dense materials or a mitochondrial assembly in the cytoplasm. These organelles appeared later in the oogonia and oocytes. When the loose cell mass and developing germ cells were eliminated by extirpating all zooids and buds from the colonies, BpVas transcripts disappeared completely from the vascularized colonies. After 14 days, when the colonies regenerated by vascular budding, BpVas-positive cells reappeared in some cases, and in 30 day colonies, BpVas-positive germ cells were observed in all the regenerated colonies. These results show that in B. primigenus, germ cells are inducible de novo from the Vasa-negative cells even at postembryonic stages.

Functional insight into Maelstrom in the germline piRNA pathway: a unique domain homologous to the DnaQ-H 3'-5' exonuclease, its lineage-specific expansion/loss and evolutionarily active site switch

Abstract

Maelstrom (MAEL) plays a crucial role in a recently-discovered piRNA pathway; however its specific function remains unknown. Here a novel MAEL-specific domain characterized by a set of conserved residues (Glu-His-His-Cys-His-Cys, EHHCHC) was identified in a broad range of species including vertebrates, sea squirts, insects, nematodes, and protists. It exhibits ancient lineage-specific expansions in several species, however, appears to be lost in all examined teleost fish species. Functional involvement of MAEL domains in DNA- and RNA-related processes was further revealed by its association with HMG, SR-25-like and HDAC_interact domains. A distant similarity to the DnaQ-H 3'–5' exonuclease family with the RNase H fold was discovered based on the evidence that all MAEL domains adopt the canonical RNase H fold; and several protist MAEL domains contain the conserved 3'–5' exonuclease active site residues (Asp-Glu-Asp-His-Asp, DEDHD). This evolutionary link together with structural examinations leads to a hypothesis that MAEL domains may have a potential nuclease activity or RNA-binding ability that may be implicated in piRNA biogenesis. The observed transition of two sets of characteristic residues between the ancestral DnaQ-H and the descendent MAEL domains may suggest a new mode for protein function evolution called "active site switch", in which the protist MAEL homologues are the likely evolutionary intermediates due to harboring the specific characteristics of both 3'–5' exonuclease and MAEL domains.

Reviewers

This article was reviewed by L Aravind, Wing-Cheong Wong and Frank Eisenhaber. For the full reviews, please go to the Reviewers' Comments section.

Historical survey on chromatoid body research

The chromatoid body (CB) is a male reproductive cell-specific organelle that appears in spermatocytes and spermatids. The cytoplasmic granule corresponding to the CB was first discovered some 130 years ago by von Brunn in 1876. Thirty years later the German term "chromatoide Körper" (chromatoid body) was introduced to describe this granule and is still used today. In this review, first, the results obtained by light microscopic studies on the CB for the first 60 years are examined. Next, many findings revealed by electron microscopic studies are reviewed. Finally, recent molecular cell biological studies concerning the CB are discussed. The conclusion obtained by exploring the papers on CB published during the past 130 years is that many of the modern molecular cell biological studies are undoubtedly based on information accumulated by vast amounts of early studies.