Germ line development in the grasshopper Schistocerca gregaria: vasa as a marker

Germline progenitors and oocyte production in the honeybee queen ovary

Understanding the reproduction of honeybee queens is crucial to support populations of this economically important insect. Here we examine the structure of the honeybee ovary to determine the nature of the germline progenitors in the ovary. Using a panel of marker genes that mark somatic or germline tissue in other insects we determine which cells in the honeybee ovary are somatic and which germline. We examine patterns of cell division and demonstrate that, unlike Drosophila, there is no evidence of single germline stem cells that provide the germline in honeybees. Germline progenitors are clustered in groups of 8 cells, joined by a polyfusome, and collections of these, in each ovariole, appear to maintain the germline during reproduction. We also show that these 8-cell clusters can divide and that their division occurs such that the numbers of germline progenitors are relatively constant over the reproductive life of queen honeybees. This information helps us to understand the diversity of structures in insect reproduction, and provide information to better support honeybee reproduction.

Germline specification and axis determination in viviparous and oviparous pea aphids: conserved and divergent features

Aphids are hemimetabolous insects that undergo incomplete metamorphosis without pupation. The annual life cycle of most aphids includes both an asexual (viviparous) and a sexual (oviparous) phase. Sexual reproduction only occurs once per year and is followed by many generations of asexual reproduction, during which aphids propagate exponentially with telescopic development. Here, we discuss the potential links between viviparous embryogenesis and derived developmental features in the pea aphid Acyrthosiphon pisum, particularly focusing on germline specification and axis determination, both of which are key events of early development in insects. We also discuss potential evolutionary paths through which both viviparous and oviparous females might have come to utilize maternal germ plasm to drive germline specification. This developmental strategy, as defined by germline markers, has not been reported in other hemimetabolous insects. In viviparous females, furthermore, we discuss whether molecules that in other insects characterize germ plasm, like Vasa, also participate in posterior determination and how the anterior localization of the hunchback orthologue Ap-hb establishes the anterior-posterior axis. We propose that the linked chain of developing oocytes and embryos within each ovariole and the special morphology of early embryos might have driven the formation of evolutionary novelties in germline specification and axis determination in the viviparous aphids. Moreover, based upon the finding that the endosymbiont Buchnera aphidicola is closely associated with germ cells throughout embryogenesis, we propose presumptive roles for B. aphidicola in aphid development, discussing how it might regulate germline migration in both reproductive modes of pea aphids. In summary, we expect that this review will shed light on viviparous as well as oviparous development in aphids.

Identification and characterization of a new germline-specific marker vasa gene and its promoter in the giant freshwater prawn Macrobrachium rosenbergii

Vasa gene encodes a protein member of DEAD-box superfamily of ATP-dependent RNA helicases, which plays a key role in germline development in metazoans. In present study, we identified a new germline-specific marker Mrvasa in the prawn Macrobrachium rosenbergii, whose genomic DNA sequence consists of 14 exons and 13 introns. A 2516 bp of full-length Mrvasa cDNA encodes a protein of 603 amino acids. It contains nine conserved motifs, a zinc-finger motif, and RGG repeats. RT-PCR indicated that Mrvasa mRNA was specifically expressed in gonads. QPCR analysis further revealed that the expression of Mrvasa mRNA is much higher in testis than in ovary. In testis, the relative expression level of Mrvasa mRNA in late developing stage is significantly higher than that in early-middle developing stage. During ovarian development, no significant difference in expression was found. In situ hybridization demonstrated that Mrvasa mRNA was localized in germline cells including spermatogonia, spermatocytes, and spermatozoa in testes, and previtellogenic and vitellogenic oocytes in ovary. We then isolated the Mrvasa promoter and determined the transcription core region of this promoter. This is the first report on identification of vasa core promoter in crustaceans. Our results will provide a useful germline-specific marker Mrvasa for tracing germline cell formation and development in M. rosenbergii.

Early embryonic development of Bombyx

Decades have passed since the early molecular embryogenesis of Drosophila melanogaster was outlined. During this period, the molecular mechanisms underlying early embryonic development in other insects, particularly the flour beetle, Tribolium castaneum, have been described in more detail. The information clearly demonstrated that Drosophila embryogenesis is not representative of other insects and has highly distinctive characteristics. At the same time, this new data has been gradually clarifying ancestral operating mechanisms. The silk moth, Bombyx mori, is a lepidopteran insect and, as a representative of the order, has many unique characteristics found in early embryonic development that have not been identified in other insect groups. Herein, some of these characteristics are introduced and discussed in the context of recent information obtained from other insects.

The evolution of insect germline specification strategies

The establishment of the germline is essential for sexually reproducing organisms. In animals, there are two major strategies to specify the germline: maternal provision and zygotic induction. The molecular basis of the maternal provision mode has been well characterized in several model organisms (fly, frog, fish, and nematode), while that of the zygotic induction mode has mainly been studied in mammalian models such as the mouse. Shifts in germline determination modes occur unexpectedly frequently and many such shifts have occurred several times among insects. Given their general tractability and rapidly increasing genomic and genetic tools applicable to many species, the insects present a uniquely powerful model system for understanding major transitions in reproductive strategies, and developmental processes in general.

A model for germ cell development in a fully segmented worm

INTRODUCTION

Polychaetes are segmented marine worms with body segments separated by a complete or incomplete septum. In most polychaetes the whole body cavity is filled with gametes during the breeding season. Platynereis dumerilii (Pl. dumerilii), which has an incomplete septum was shown to develop a single gonadal structure for gamete production located in the neck region. However, in Perinereis nuntia (Pe. nuntia), which has a complete septum separating each segment, the developmental feature of gametes remains unknown. To clarify this, the marker gene vasa was used to trace the development of germ cells throughout the life stages of Pe. nuntia.

RESULTS

In three-segmented juveniles, Pn-vasa was expressed in the parapodia and in the two cells localized in the pygidium. During the addition of a new segment, Pn-vasa positive cells in the pygidium increased from two to four and two new Pn-vasa positive cells were found in the newly-generated segment. In adults, Pn-vasa was expressed in a large cell cluster at the distal end of the parapodia, in smaller cell clusters (which had an elongated form in the trunk area of the parapodia), and in oocytes in the coelomic cavity. This may suggest that germ cells settle in the parapodia and later translocate into the coelomic cavity to develop into oocytes.

CONCLUSION

Our observations will help in understanding the mechanism of germ cell development in all body segments of Pe. nuntia. We hypothesize that primordial germ cells are supplied from the pygidium to every newly-generating segment which later settle in the parapodium. This will explain how polychaetes can generate gametes in each body segment, even those that are independently separated with a complete septum.

Germ plasm localisation of the HELICc of Vasa in Drosophila: analysis of domain sufficiency and amino acids critical for localisation

Formation of the germ plasm drives germline specification in Drosophila and some other insects such as aphids. Identification of the DEAD-box protein Vasa (Vas) as a conserved germline marker in flies and aphids suggests that they share common components for assembling the germ plasm. However, to which extent the assembly order is conserved and the correlation between functions and sequences of Vas remain unclear. Ectopic expression of the pea aphid Vas (ApVas1) in Drosophila did not drive its localisation to the germ plasm, but ApVas1 with a replaced C-terminal domain (HELICc) of Drosophila Vas (DmVas) became germ-plasm restricted. We found that HELICc itself, through the interaction with Oskar (Osk), was sufficient for germ-plasm localisation. Similarly, HELICc of the grasshopper Vas could be recruited to the germ plasm in Drosophila. Nonetheless, germ-plasm localisation was not seen in the Drosophila oocytes expressing HELICcs of Vas orthologues from aphids, crickets, and mice. We further identified that glutamine (Gln) 527 within HELICc of DmVas was critical for localisation, and its corresponding residue could also be detected in grasshopper Vas yet missing in the other three species. This suggests that Gln527 is a direct target of Osk or critical to the maintenance of HELICc conformation.

Germ cells of the centipede Strigamia maritima are specified early in embryonic development

We provide the first systematic description of germ cell development with molecular markers in a myriapod, the centipede Strigamia maritima. By examining the expression of Strigamia vasa and nanos orthologues, we find that the primordial germ cells are specified from at least the blastoderm stage. This is a much earlier embryonic stage than previously described for centipedes, or any other member of the Myriapoda. Using these genes as markers, and taking advantage of the developmental synchrony of Strigamia embryos within single clutches, we are able to track the development of the germ cells throughout embryogenesis. We find that the germ cells accumulate at the blastopore; that the cells do not internalize through the hindgut, but rather through the closing blastopore; and that the cells undergo a long-range migration to the embryonic gonad. This is the first evidence for primordial germ cells displaying these behaviours in any myriapod. The myriapods are a phylogenetically important group in the arthropod radiation for which relatively little developmental data is currently available. Our study provides valuable comparative data that complements the growing number of studies in insects, crustaceans and chelicerates, and is important for the correct reconstruction of ancestral states and a fuller understanding of how germ cell development has evolved in different arthropod lineages.

Reversion of developmental mode in insects: evolution from long germband to short germband in the polyembrionic wasp Macrocentrus cingulum Brischke

Germband size in insects has played a central role in our understanding of insect patterning mechanisms and their evolution. The polarity of evolutionary change in insect patterning has been viewed so far as the unidirectional shift from the ancestral short germband patterning of basal hemimetabolous insects to the long germband patterning observed in most modern Holometabola. However, some orders of holometabolic insects display both short and long germband development, though the absence of a clear phylogenetic context does not permit definite conclusions on the polarity of change. Derived hymenoptera, that is, bees and wasps, represent a classical textbook example of long germband development. Yet, in some wasps putative short germband development has been described correlating with lifestyle changes, namely with evolution of endoparasitism and polyembryony. To address the potential reversion from long to short germband, we focused on the family Braconidae, which displays ancestral long germband development, and examined the derived polyembryonic braconid Macrocentrus cingulum. Using SEM analysis of M. cingulum embryogenesis coupled with analyses of embryonic patterning markers, we show that this wasp evolved short germband embryogenesis secondarily, in a way that is reminiscent of embryogenesis in the beetle Tribolium castaneum. This work shows that the evolution of germband size in insects is a reversible process that may correlate with other life-history traits and suggests broader implications on the mechanisms and evolvability of insect development.

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.

Vasa identifies germ cells and critical stages of oogenesis in the Asian seabass

Germ cells produce sperm and eggs for reproduction and fertility. The Asian seabass (Lates calcarifer), a protandrous marine fish, undergoes male-female sex reversal and thus offers an excellent model to study the role of germ cells in sex differentiation and sex reversal. Here we report the cloning and expression of vasa as a first germ cell marker in this organism. A 2241-bp cDNA was cloned by PCR using degenerate primers of conserved sequences and gene-specific primers. This cDNA contains a polyadenylation signal and a full open reading frame for 645 amino acid residues, which was designated as Lcvasa for the seabass vasa, as its predicted protein is homologous to Vasa proteins. The Lcvasa RNA is maternally supplied and specific to gonads in adulthood. By chromogenic and fluorescent in situ hybridization we revealed germ cell-specific Lcvasa expression in both the testis and ovary. Importantly, Lcvasa shows dynamic patterns of temporospatial expression and subcellular distribution during gametogenesis. At different stages of oogenesis, for example, Lcvasa undergoes nuclear-cytoplasmic redistribution and becomes concentrated preferentially in the Balbiani body of stage-II~III oocytes. Thus, the vasa RNA identifies both female and male germ cells in the Asian seabass, and its expression and distribution delineate critical stages of gametogenesis.

A bird's-eye view on the modern genetics workflow and its potential applicability to the locust problem

Genetics is an immense science and the current developments in its methods and techniques as well as the fast emerging tools make it one of the most powerful biological sciences. Indeed, from taxonomy and ecology to physiology and molecular biology, every biological science makes use of genetics techniques and methods at one time or another. In fact, development in genetics is such that it is now possible to characterize and analyze the expression of the whole set of genes of virtually every living organism, even if it is a non-model one. Locusts are notorious for the damage they cause to the ecosystems and economies of the areas affected by their recurrent population outbreaks. To prevent and deal with these outbreaks, we now count on both biological as well as chemical agents that are proving to be successful in reducing the damage that otherwise locust population outbreaks might cause. However, a better, efficient and environmentally friendly solution is still a hoped-for target. In my opinion, the ideal future pesticide should be both environmentally friendly, risk free and species-specific. To reach the knowledge needed for the development of such species-specific anti-locust agent, deep and accurate knowledge of the locusts' genetics and molecular biology is a must. Since genes and their expression levels lie at the bottom of every biological phenomenon, any species-specific solution to the locust problem requires a good knowledge of these organisms' genes as well as the quantitative and spatio-temporal dynamics of their expression. To reach such knowledge, collaborative work is needed as well as a clear workflow that, given the fast development in the genetics tools, is not always clear to all research groups. For this reason, here I describe a genetics workflow that should allow taking advantage of the most recent genetics tools and techniques to answer question relating to locust biology. My hope is that the adoption of this and other work strategies by different research groups, especially when the work is a collaborative one, would provide precious information on the biology and the biological phenomena that these economically important organisms exhibit.

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.

Transcriptome analysis of the oriental river prawn, Macrobrachium nipponense using 454 pyrosequencing for discovery of genes and markers

Background

The oriental river prawn, Macrobrachium nipponense, is an economically and nutritionally important species of the Palaemonidae family of decapod crustaceans. To date, the sequencing of its whole genome is unavailable as a non-model organism. Transcriptomic information is also scarce for this species. In this study, we performed de novo transcriptome sequencing to produce the first comprehensive expressed sequence tag (EST) dataset for M. nipponense using high-throughput sequencing technologies.

Methodology and Principal Findings

Total RNA was isolated from eyestalk, gill, heart, ovary, testis, hepatopancreas, muscle, and embryos at the cleavage, gastrula, nauplius and zoea stages. Equal quantities of RNA from each tissue and stage were pooled to construct a cDNA library. Using 454 pyrosequencing technology, we generated a total of 984,204 high quality reads (338.59Mb) with an average length of 344 bp. Clustering and assembly of these reads produced a non-redundant set of 81,411 unique sequences, comprising 42,551 contigs and 38,860 singletons. All of the unique sequences were involved in the molecular function (30,425), cellular component (44,112) and biological process (67,679) categories by GO analysis. Potential genes and their functions were predicted by KEGG pathway mapping and COG analysis. Based on our sequence analysis and published literature, many putative genes involved in sex determination, including DMRT1, FTZ-F1, FOXL2, FEM1 and other potentially important candidate genes, were identified for the first time in this prawn. Furthermore, 6,689 SSRs and 18,107 high-confidence SNPs were identified in this EST dataset.

Conclusions

The transcriptome provides an invaluable new data for a functional genomics resource and future biological research in M. nipponense. The molecular markers identified in this study will provide a material basis for future genetic linkage and quantitative trait loci analyses, and will be essential for accelerating aquaculture breeding programs with this species.

Vasa-Like DEAD-Box RNA Helicases of Schistosoma mansoni

Genome sequences are available for the human blood flukes, Schistosoma japonicum, S. mansoni and S. haematobium. Functional genomic approaches could aid in identifying the role and importance of these newly described schistosome genes. Transgenesis is established for functional genomics in model species, which can lead to gain- or loss-of-functions, facilitate vector-based RNA interference, and represents an effective forward genetics tool for insertional mutagenesis screens. Progress toward routine transgenesis in schistosomes might be expedited if germ cells could be reliably localized in cultured schistosomes. Vasa, a member of the ATP-dependent DEAD-box RNA helicase family, is a prototypic marker of primordial germ cells and the germ line in the Metazoa. Using bioinformatics, 33 putative DEAD-box RNA helicases exhibiting conserved motifs that characterize helicases of this family were identified in the S. mansoni genome. Moreover, three of the helicases exhibited vasa-like sequences; phylogenetic analysis confirmed the three vasa-like genes-termed Smvlg1, Smvlg2, and Smvlg3-were members of the Vasa/PL10 DEAD-box subfamily. Transcripts encoding Smvlg1, Smvlg2, and Smvlg3 were cloned from cDNAs from mixed sex adult worms, and quantitative real time PCR revealed their presence in developmental stages of S. mansoni with elevated expression in sporocysts, adult females, eggs, and miracidia, with strikingly high expression in the undeveloped egg. Whole mount in situ hybridization (WISH) analysis revealed that Smvlg1, Smvlg2 and Smvlg3 were transcribed in the posterior ovary where the oocytes mature. Germ cell specific expression of schistosome vasa-like genes should provide an informative landmark for germ line transgenesis of schistosomes, etiologic agents of major neglected tropical diseases.

The phylogenetic origin of oskar coincided with the origin of maternally provisioned germ plasm and pole cells at the base of the Holometabola

The establishment of the germline is a critical, yet surprisingly evolutionarily labile, event in the development of sexually reproducing animals. In the fly Drosophila, germ cells acquire their fate early during development through the inheritance of the germ plasm, a specialized maternal cytoplasm localized at the posterior pole of the oocyte. The gene oskar (osk) is both necessary and sufficient for assembling this substance. Both maternal germ plasm and oskar are evolutionary novelties within the insects, as the germline is specified by zygotic induction in basally branching insects, and osk has until now only been detected in dipterans. In order to understand the origin of these evolutionary novelties, we used comparative genomics, parental RNAi, and gene expression analyses in multiple insect species. We have found that the origin of osk and its role in specifying the germline coincided with the innovation of maternal germ plasm and pole cells at the base of the holometabolous insects and that losses of osk are correlated with changes in germline determination strategies within the Holometabola. Our results indicate that the invention of the novel gene osk was a key innovation that allowed the transition from the ancestral late zygotic mode of germline induction to a maternally controlled establishment of the germline found in many holometabolous insect species. We propose that the ancestral role of osk was to connect an upstream network ancestrally involved in mRNA localization and translational control to a downstream regulatory network ancestrally involved in executing the germ cell program.

Germ plasm-like Dot cells maintain their wound regenerative function after in vitro expansion

1. Wounds in fetal skin heal without scarring; however, the mechanism for this is unknown. We have identified a novel group of protein and nucleotides-positive particles in fetal and adult mouse blood and in human blood, and termed them 'Dot cells'. Freshly isolated Dot cells regenerate wounds with less scarring and can be cultured without feeder layers. 2. Because the morphology of Dot cells has never been described, in the present study we describe the specific characterizations of Dot cells, including their growth pattern in vitro, and their expressions of stem cell markers using fluorescent cell sorting analyses and immunofluorescent histology. Our data indicates that cultured Dot cells express stem cell surface markers and embryonic stem cell transcription markers, such as Oct4, Nanog and Sox-2. In addition, Dot cells express VASA, the germ plasm specific marker. 3. To confirm whether Dot cells maintain their wound regenerative activity after in vitro expansion, in vitro cultured Dot cells were transplanted to wounded mice. Dot cells from albino mice maintain their wound regenerative activities after intravenous transplantation to black-background diabetic mice. In addition, Dot cells regenerate both the epithelial and dermal cells in the wounds of wild-type mice. The regenerated hair follicles, smooth muscle and dermal tissues express transiently to VASA. 4. Our data demonstrate that Dot cells are newly identified organisms located in the blood and bone marrow of mammals. They express germ cell, embryonic stem cell and adult stem cell markers. Dot cells maintain their regenerative function after in vitro expansion.

Germ cell specification and ovary structure in the rotifer Brachionus plicatilis

Background

The segregation of the germline from somatic tissues is an essential process in the development of all animals. Specification of the primordial germ cells (PGCs) takes place via different strategies across animal phyla; either specified early in embryogenesis by the inheritance of maternal determinants in the cytoplasm of the oocyte ('preformation') or selected later in embryonic development from undifferentiated precursors by a localized inductive signal ('epigenesis'). Here we investigate the specification and development of the germ cells in the rotifer Brachionus plicatilis, a member of the poorly-characterized superphyla Lophotrochozoa, by isolating the Brachionus homologues of the conserved germ cell markers vasa and nanos, and examining their expression using in situ hybridization.

Results

Bpvasa and Bpnos RNA expression have very similar distributions in the Brachionus ovary, showing ubiquitous expression in the vitellarium, with higher levels in the putative germ cell cluster. Bpvas RNA expression is present in freshly laid eggs, remaining ubiquitous in embryos until at least the 96 cell stage after which expression narrows to a small cluster of cells at the putative posterior of the embryo, consistent with the developing ovary. Bpnos RNA expression is also present in just-laid eggs but expression is much reduced by the four-cell stage and absent by the 16-cell stage. Shortly before hatching of the juvenile rotifer from the egg, Bpnos RNA expression is re-activated, located in a subset of posterior cells similar to those expressing Bpvas at the same stage.

Conclusions

The observed expression of vasa and nanos in the developing B. plicatilis embryo implies an epigenetic origin of primordial germ cells in Rotifer.

Sequence and expression pattern of the germ line marker vasa in honey bees and stingless bees

Queens and workers of social insects differ in the rates of egg laying. Using genomic information we determined the sequence of vasa, a highly conserved gene specific to the germ line of metazoans, for the honey bee and four stingless bees. The vasa sequence of social bees differed from that of other insects in two motifs. By RT-PCR we confirmed the germ line specificity of Amvasa expression in honey bees. In situ hybridization on ovarioles showed that Amvasa is expressed throughout the germarium, except for the transition zone beneath the terminal filament. A diffuse vasa signal was also seen in terminal filaments suggesting the presence of germ line cells. Oocytes showed elevated levels of Amvasa transcripts in the lower germarium and after follicles became segregated. In previtellogenic follicles, Amvasa transcription was detected in the trophocytes, which appear to supply its mRNA to the growing oocyte. A similar picture was obtained for ovarioles of the stingless bee Melipona quadrifasciata, except that Amvasa expression was higher in the oocytes of previtellogenic follicles. The social bees differ in this respect from Drosophila, the model system for insect oogenesis, suggesting that changes in the sequence and expression pattern of vasa may have occurred during social evolution.

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.

Dynamic expression patterns of vasa during embryogenesis in the cricket Gryllus bimaculatus

The specification of germ cells during embryogenesis is an important issue in the development of metazoans. In insects, the mode of germ cell specification appears to be highly variable among species and molecular data are not sufficient to provide an evolutionary perspective to this issue. Expression of vasa can be used as a germ line marker. Here, we report the isolation of a vasa-like gene in a hemimetabolous insect, the cricket Gryllus bimaculatus (Gb'vas), and its expression patterns during oogenesis and embryogenesis. Gb'vas is preferentially expressed in the germarium and the expression of Gb'vas is detectable throughout vitellogenesis including mature eggs subjected to oviposition, suggesting that Gb'vas is maternally contributed to the cricket eggs. The zygotic expression of Gb'vas appears to start at the mid blastoderm stage in the posterior region of the egg, expanding in a developing germ anlage. In early germbands, an intense expression of Gb'vas is restricted to the posterior end. In later embryos, Gb'vas expression extends over the whole body and then distinctly localized to the embryonic gonad at the stage immediately before hatching. These results suggest that, in the cricket, germ cells are specified early in development at the posterior end of an early germband, as proposed by Heymons (1895) based on cytological criteria.

Vasa protein expression is restricted to the small micromeres of the sea urchin, but is inducible in other lineages early in development

Vasa is a DEAD-box RNA helicase that functions in translational regulation of specific mRNAs. In many animals it is essential for germ line development and may have a more general stem cell role. Here we identify vasa in two sea urchin species and analyze the regulation of its expression. We find that vasa protein accumulates in only a subset of cells containing vasa mRNA. In contrast to vasa mRNA, which is present uniformly throughout all cells of the early embryo, vasa protein accumulates selectively in the 16-cell stage micromeres, and then is restricted to the small micromeres through gastrulation to larval development. Manipulating early embryonic fate specification by blastomere separations, exposure to lithium, and dominant-negative cadherin each suggest that, although vasa protein accumulation in the small micromeres is fixed, accumulation in other cells of the embryo is inducible. Indeed, we find that embryos in which micromeres are removed respond by significant up-regulation of vasa protein translation, followed by spatial restriction of the protein late in gastrulation. Overall, these results support the contention that sea urchins do not have obligate primordial germ cells determined in early development, that vasa may function in an early stem cell population of the embryo, and that vasa expression in this embryo is restricted early by translational regulation to the small micromere lineage.

Exploration of embryonic origins of germline stem cells and neoblasts in Enchytraeus japonensis (Oligochaeta, Annelida)

An oligochaete annelid species, Enchytraeus japonensis, reproduces not only asexually but also sexually. It has been reported that putative mesodermal stem cells called neoblasts contribute to blastema formation and that Ej-piwi(+) germline stem cells participate in gonadal regeneration. To delineate the origin and formation of both of these stem cells, we isolated two vasa-related genes (Ej-vlg1 and Ej-vlg2) and analyzed the expression of each along with that of germline marker gene Ej-piwi. In adults, Ej-vlg1 and Ej-vlg2 were expressed in Ej-piwi(+) germline stem cells and germ cells in gonads, while only Ej-vlg2 mRNAs were detected in neoblasts. Expression analysis during embryogenesis indicated that clusters of Ej-vlg1(+)/Ej-vlg2(+) cells, located at the posterior ventral region in late embryos, became Ej-vlg1(+)/Ej-vlg2(+)/Ej-piwi(+) germline stem cells just after embryogenesis. On the other hand, Ej-vlg2 single positive cells with morphological characteristics of neoblasts became detectable much later after embryogenesis at the ventral position on each septum where adult neoblasts exist, although these early detected cells were much smaller in size than adult neoblasts. The present results suggest that (1) germline stem cells specified just after embryogenesis are derived from Ej-vlg1(+)/Ej-vlg2(+) cells which appear at the posterior ventral region in late embryos, and that (2) neoblasts appear much later in development.

Transient occurrence of vasa-expressing cells in nongenital segments during embryonic development in the oligochaete annelid Tubifex tubifex

The primordial germ cells (PGCs) in the oligochaete annelid Tubifex tubifex are mesodermal in origin and are located in the two midbody segments X and XI in which the testis and the ovary are formed, respectively. To identify a molecular marker for the Tubifex PGCs, we isolated the Tubifex homologue (Ttu-vas) of the Drosophila vasa gene. Using whole-mount in situ hybridization, we examined the spatial expression patterns of Ttu-vas from one-cell stage through juvenile stage. Ttu-vas messenger ribonucleic acid (RNA) is present as a maternal transcript distributed broadly throughout the early stages. Ttu-vas is expressed in all of the early cleavage blastomeres, in which Ttu-vas RNA associates with mitotic spindles and pole plasms. Expression of Ttu-vas gradually becomes restricted, first to teloblasts, then to their blast cell progeny comprising the germ bands (GBs), and finally to a set of large ventral cells (termed VE cells) in a variable set of midbody segments including the genital segments (X and XI). At the end of embryogenesis, VE cells are confined to genital segments where they are presumably germline precursors in the juvenile. Staining with a cross-reacting anti-Vasa antibody suggested that VE cells express Ttu-vas protein to the same extent irrespective of their positions along the anteroposterior axis. A set of cell ablation experiments suggested that VE cells are derived from the mesodermal teloblast lineage and that the emergence of VE cells takes place independently of the presence of the ectodermal GBs that normally overlay the mesoderm. These results suggest that T. tubifex generates supernumerary presumptive PGCs during embryogenesis whose number is variable among embryos.

The p53 tumor suppressor-like protein nvp63 mediates selective germ cell death in the sea anemone Nematostella vectensis

Here we report the identification and molecular function of the p53 tumor suppressor-like protein nvp63 in a non-bilaterian animal, the starlet sea anemone Nematostella vectensis. So far, p53-like proteins had been found in bilaterians only. The evolutionary origin of p53-like proteins is highly disputed and primordial p53-like proteins are variably thought to protect somatic cells from genotoxic stress. Here we show that ultraviolet (UV) irradiation at low levels selectively induces programmed cell death in early gametes but not somatic cells of adult N. vectensis polyps. We demonstrate with RNA interference that nvp63 mediates this cell death in vivo. Nvp63 is the most archaic member of three p53-like proteins found in N. vectensis and in congruence with all known p53-like proteins, nvp63 binds to the vertebrate p53 DNA recognition sequence and activates target gene transcription in vitro. A transactivation inhibitory domain at its C-terminus with high homology to the vertebrate p63 may regulate nvp63 on a molecular level. The genotoxic stress induced and nvp63 mediated apoptosis in N. vectensis gametes reveals an evolutionary ancient germ cell protective pathway which relies on p63-like proteins and is conserved from cnidarians to vertebrates.

Apvasa marks germ-cell migration in the parthenogenetic pea aphid Acyrthosiphon pisum (Hemiptera: Aphidoidea)

In the parthenogenetic and viviparous pea aphid Acyrthosiphon pisum, germline specification depends on the germ plasm localized to the posterior region of the egg chamber before the formation of the blastoderm. During blastulation, germline segregation occurs at the egg posterior, and in early gastrulation germ cells are pushed inward by the invaginating germ band. Previous studies suggest that germ cells remain dorsal in the embryo in subsequent developmental stages. In fact, though, it is not known whether germ cells remain in place or migrate dynamically during katatrepsis and germ-band retraction. We cloned Apvasa, a pea aphid homologue of Drosophila vasa, and used it as a germline marker to monitor the migration of germ cells. Apvasa messenger RNA (mRNA) was first restricted to morphologically identifiable germ cells after blastoderm formation but that expression soon faded. Apvasa transcripts were again identified in germ cells from the stage when the endosymbiotic bacteria invaded the embryo, and after that, Apvasa mRNA was present in germ cells throughout all developmental stages. At the beginning of katatrepsis, germ cells were detected at the anteriormost region of the egg chamber as they were migrating into the body cavity. During the early period of germ-band retraction, germ cells were separated into several groups surrounded by a layer of somatic cells devoid of Apvasa staining, suggesting that the coalescence between migrating germ cells and the somatic gonadal mesoderm occurs between late katatrepsis and early germ-band retraction.

Vasa unveils a common origin of germ cells and of somatic stem cells from the posterior growth zone in the polychaete Platynereis dumerilii

To elucidate the evolution of germ cell specification in Metazoa, recent comparative studies focus on ancestral animal groups. Here, we followed the germline throughout the life cycle of the polychaete annelid Platynereis dumerilii, by examining mRNA and protein expression of vasa and other germline-specific factors in combination with lineage tracing experiments. In the fertilised egg, maternal Vasa protein localises to the yolk-free cytoplasm at the animal pole. It then asymmetrically segregates first into the micromeres, then into the founder cells of the mesodermal posterior growth zone (MPGZ). Vasa transcripts initially show ubiquitous distribution, but then become progressively restricted to the MPGZ. The cells of the MPGZ are highly proliferative, as evidenced by BrdU pulse labelling experiments. Besides vasa, they express nanos along with the stem cell-specific genes piwi, and PL10. At 4 days of development, four primordial germ cells are singled out from within the MPGZ, and migrate into the anterior segments to colonise a newly discovered "primary gonad". Our data suggest a common origin of germ cells and of somatic stem cells, similar to the situation found in planarians and cnidarians, which may constitute the ancestral mode of germ cell specification in Metazoa.

Germ cell development in the Honeybee (Apis mellifera); vasa and nanos expression

Background

Studies of specification of germ-cells in insect embryos has indicated that in many taxa the germ cells form early in development, and their formation is associated with pole plasm, germ plasm or an organelle called the oosome. None of these morphological features associated with germ cell formation have been identified in the Honeybee Apis mellifera. In this study I report the cloning and expression analysis of Honeybee homologues of vasa and nanos, germ cell markers in insects and other animals.

Results

Apis vasa and nanos RNAs are present in early honeybee embryos, but the RNAs clear rapidly, without any cells expressing these germ cell markers past stage 2. These genes are then only expressed in a line of cells in the abdomen from stage 9 onwards. These cells are the developing germ cells that are moved dorsally by dorsal closure and are placed in the genital ridge.

Conclusion

This study of the expression of germ cell markers in the honeybee implies that in this species either germ cells are formed by an inductive event, late in embryogenesis, or they are formed early in development in the absence of vasa and nanos expression. This contrasts with germ cell development in other members of the Hymenoptera, Diptera and Lepidoptera.

Expression pattern of Bombyx vasa-like (BmVLG) protein and its implications in germ cell development

Germ cell development in the silkworm Bombyx mori is interesting in that the species has no recognizable germ plasm, and its germ cells appear first on the ventral side of the embryo, not on the posterior pole as in Drosophila melanogaster. We previously reported the isolation of a vasa homologue (BmVLG) from B. mori and revealed the specific expression of transcript in the germ cells. In this paper, we describe the embryonic expression pattern of BmVLG protein. Consistent with the lack of recognizable germ plasm, the protein is not localized in freshly laid eggs, and its specific expression is first detectable several hours after energids penetrate the periplasm. This is in contrast to D. melanogaster, where germ cell lineage can be traced with anti-vasa antibody just after the formation of pole cells as they sequester vasa-positive germ (pole) plasm during cellularization. It is also revealed that, within the first few hours of their appearance when extensive cell movement does not seem to occur, stained cells are sometimes widely dispersed along the midline, which eventually may lead to the formation of ectopic germ cells. The implications of these results for germ cell development are discussed.

Differential expression of vasa RNA and protein during spermatogenesis and oogenesis in the gibel carp (Carassius auratus gibelio), a bisexually and gynogenetically reproducing vertebrate

The RNA helicase Vasa is a germ cell marker in animals, and its homolog in vertebrates to date has been limited to bisexual reproduction. We cloned and characterized CagVasa, a Vasa homolog from the gibel carp, a fish that reproduces bisexually or gynogenetically. CagVasa possesses 14 RGG repeats and eight conserved motifs of Vasa proteins. In bisexually reproducing gibel carp, vasa is maternally supplied and its zygotic expression is restricted to gonads. By in situ hybridization on testicular sections, vasa is low in spermatogonia, high in primary spermatocytes, reduced in secondary spermatocytes, but disappears in spermatids and sperm. In contrast, vasa persists throughout oogenesis, displaying low-high-low levels from oogonia over vitellogenic oocytes to maturing oocytes. A rabbit anti-Vasa antibody (alphaVasa) was raised against the N-terminal CagVasa for fluorescent immunohistochemistry. On testicular sections, Vasa is the highest in spermatogonia, reduced in spermatocytes, low in spermatids, and absent in sperm. In the ovary, Vasa is the highest in oogonia but persists throughout oogenesis. Subcellular localization of vasa and its protein changes dynamically during oogenesis. The alphaVasa stains putative primordial germ cells in gibel carp fry. It detects gonadal germ cells also in several other teleosts. Therefore, Cagvasa encodes a Vasa ortholog that is differentially expressed in the testis and ovary. Interestingly, the alphaVasa in combination with a nuclear dye can differentiate critical stages of spermatogenesis and oogenesis in fish. The cross-reactivity and the ability to stain stage-specific germ cells make this antibody a useful tool to identify fish germ cell development and differentiation.

vasa and nanos expression patterns in a sea anemone and the evolution of bilaterian germ cell specification mechanisms

Most bilaterians specify primordial germ cells (PGCs) during early embryogenesis using either inherited cytoplasmic germ line determinants (preformation) or induction of germ cell fate through signaling pathways (epigenesis). However, data from nonbilaterian animals suggest that ancestral metazoans may have specified germ cells very differently from most extant bilaterians. Cnidarians and sponges have been reported to generate germ cells continuously throughout reproductive life, but previous studies on members of these basal phyla have not examined embryonic germ cell origin. To try to define the embryonic origin of PGCs in the sea anemone Nematostella vectensis, we examined the expression of members of the vasa and nanos gene families, which are critical genes in bilaterian germ cell specification and development. We found that vasa and nanos family genes are expressed not only in presumptive PGCs late in embryonic development, but also in multiple somatic cell types during early embryogenesis. These results suggest one way in which preformation in germ cell development might have evolved from the ancestral epigenetic mechanism that was probably used by a metazoan ancestor.

Exploring embryonic germ line development in the water flea, Daphnia magna, by zinc-finger-containing VASA as a marker

VASA is an ATP-dependent RNA helicase belonging to the DEAD-box family that, in many organisms, is specifically expressed in germ line cells throughout the life cycle, making it a powerful molecular marker to study germ line development. To obtain further information on germ line development in crustaceans, we cloned VASA cDNAs from three branchiopod species: water fleas Daphnia magna and Moina macrocopa, and brine shrimp Artemia franciscana. RNA helicase domains in branchiopod VASA were highly conserved among arthropod classes. However, N-terminal RNA-binding domains in branchiopod VASA were highly diverged and, unlike other arthropod VASA reported so far, possessed repeats of retroviral-type zinc finger (CCHC) motifs. Raising specific antibodies against Daphnia VASA revealed that the primordial germ cells (PGCs) in this organism segregate at a very early cleavage stage of embryogenesis in parthenogenetic and sexual eggs. Clusters of PGCs then start to migrate inside the embryo and finally settle at both sides of the intestine, the site of future gonad development. RNA analyses suggested that maternally supplied vasa mRNA was responsible for early VASA expression, while zygotic expression started during blastodermal stage of development.

The fate of isolated blastomeres with respect to germ cell formation in the amphipod crustacean Parhyale hawaiensis

Germ cells may be specified through the localization of germ line determinants to specific cells in early embryogenesis, or by inductive signals from neighboring cells to germ cell precursors in later embryogenesis. Such determinants can be produced and localized during or after oogenesis, either autonomously by oocytes or by associated nutritive cells. In Drosophila, each oocyte is connected to nurse cells by cytoplasmic bridges, and determinants synthesized in nurse cells are transported through these bridges to the oocyte. However, the Drosophila model may not be applicable to all arthropods, since in many species of all four extant arthropod classes, gametogenesis functions without nurse cells. In this paper, I use immunodetection of Vasa protein to study germ cell development in the amphipod crustacean Parhyale hawaiensis, a species whose ovaries lack nurse cells and whose eggs lack obvious polarity. Previous cell lineage analyses have shown that all three germ layers and the germ line are exclusively specified by third cleavage. In the present study, I use a molecular marker to follow germ cell development during P. hawaiensis embryogenesis. I determine the capacity of individual blastomeres to form germ cells by isolating blastomeres at early cleavage stages and provide experimental evidence for localized germ cell determinants at the two-cell stage in P. hawaiensis. These experiments indicate that many aspects of early amphipod development, including timing and symmetry of cell division, the transition from holoblastic to superficial cleavage, and possibly some gastrulation movements, are cell autonomous following first cleavage.

Patterning of the branched head appendages in Schistocerca americana and Tribolium castaneum

Much of our understanding of arthropod limb development comes from studies on the leg imaginal disc of Drosophila melanogaster. The fly limb is a relatively simple unbranched (uniramous) structure extending out from the body wall. The molecular basis for this outgrowth involves the overlap of two signaling molecules, Decapentaplegic (Dpp) and Wingless (Wg), to create a single domain of distal outgrowth, clearly depicted by the expression of the Distal-less gene (Dll). The expression of wg and dpp during the development of other arthropod thoracic limbs indicates that these pathways might be conserved across arthropods for uniramous limb development. The appendages of crustaceans and the gnathal appendages of insects, however, exhibit a diverse array of morphologies, ranging from those with no distal elements, such as the mandible, to appendages with multiple distal elements. Examples of the latter group include branched appendages or those that possess multiple lobes; such complex morphologies are seen for many crustacean limbs as well as the maxillary and labial appendages of many insects. It is unclear how, if at all, the known patterning genes for making a uniramous limb might be deployed to generate these diverse appendage forms. Experiments in Drosophila have shown that by forcing ectopic overlaps of Wg and Dpp signaling it is possible to generate artificially branched legs. To test whether naturally branched appendages form in a similar manner, we detailed the expression patterns of wg, dpp, and Dll in the development of the branched gnathal appendages of the grasshopper, Schistocerca americana, and the flour beetle, Tribolium castaneum. We find that the branches of the gnathal appendages are not specified through the redeployment of the Wg-Dpp system for distal outgrowth, but our comparative studies do suggest a role for Dpp in forming furrows between tissues.

Caste determination in a polyembryonic wasp involves inheritance of germ cells

Social insects are characterized by the development of castes in which some colony members reproduce whereas others function as altruistic helpers. The conditional switch controlling caste formation usually involves environmental stimuli that act on processes that regulate development of individuals. Unlike other social species, embryos of polyembryonic wasps develop clonally to produce large numbers of genetically identical offspring and two morphologically distinct castes. All embryos in a clone exist in an identical environment, the host, yet develop into either reproductive larvae that mature into adult wasps or soldier larvae whose function is defense. Here, we report that caste determination in Copidosoma floridanum involves inheritance of germ cells. Expression of a C. floridanum homolog (Cf-vas) of the germ cell marker Vasa indicated that the B(4) blastomere in four cell-stage embryos is specified as a primordial germ cell. Vas expression later in development further indicated that embryos developing into reproductive larvae possess primordial germ cells whereas embryos developing into soldier larvae do not. Ablation of the B(4) blastomere resulted in most broods containing only soldiers whereas ablation of other blastomeres produced broods containing both castes. These results indicate that soldier larvae are obligately sterile and reveal a previously unknown role for germ cells in caste formation.

Developmental origin and evolution of bacteriocytes in the aphid-Buchnera symbiosis

Symbiotic relationships between bacteria and insect hosts are common. Although the bacterial endosymbionts have been subjected to intense investigation, little is known of the host cells in which they reside, the bacteriocytes. We have studied the development and evolution of aphid bacteriocytes, the host cells that contain the endosymbiotic bacteria Buchnera aphidicola. We show that bacteriocytes of Acyrthosiphon pisum express several gene products (or their paralogues): Distal-less, Ultrabithorax/Abdominal-A, and Engrailed. Using these markers, we find that a subpopulation of the bacteriocytes is specified prior to the transmission of maternal bacteria to the embryo. In addition, we discovered that a second population of cells is recruited to the bacteriocyte fate later in development. We experimentally demonstrate that bacteriocyte induction and proliferation occur independently of B. aphidicola. Major features of bacteriocyte development, including the two-step recruitment of bacteriocytes, have been conserved in aphids for 80–150 million years. Furthermore, we have investigated two cases of evolutionary loss of bacterial symbionts: in one case, where novel extracellular, eukaryotic symbionts replaced the bacteria, the bacteriocyte is maintained; in another case, where symbionts are absent, the bacteriocytes are initiated but not maintained. The bacteriocyte represents an evolutionarily novel cell fate, which is developmentally determined independently of the bacteria. Three of five transcription factors we examined show novel expression patterns in bacteriocytes, suggesting that bacteriocytes may have evolved to express many additional transcription factors. The evolutionary transition to a symbiosis in which bacteria and an aphid cell form a functional unit, similar to the origin of plastids, has apparently involved extensive molecular adaptations on the part of the host cell.

Molecular markers show that bacteriocytes, the aphid cells that house the bacterial endosymbionts, are specified in a conserved two-step process that does not depend on the presence of the bacteria