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

The onset of regenerative properties in ctenophores

Ctenophores are a clade of animals that branch off at the base of the animal tree. They have a unique and delicate body plan, and distinct pattern forming mechanisms at different life history stages. They have a stereotyped embryonic cell lineage and are highly 'mosaic' as embryos, but most have amazing capacity to regenerate as adults. Unfortunately, only a handful of ctenophore species have been studied in detail. This review summarizes the key features of the regenerative properties of adults, and the characteristics of the embryological onset of regenerative abilities. The genomes of several ctenophore species have already been sequenced, and these resources set the stage for more detailed cellular and molecular analysis of body plan patterning in this group.

An unregulated regulator: Vasa expression in the development of somatic cells and in tumorigenesis

Growing evidence in diverse organisms shows that genes originally thought to function uniquely in the germ line may also function in somatic cells, and in some cases even contribute to tumorigenesis. Here we review the somatic functions of Vasa, one of the most conserved "germ line" factors among metazoans. Vasa expression in somatic cells is tightly regulated and often transient during normal development, and appears to play essential roles in regulation of embryonic cells and regenerative tissues. Its dysregulation, however, is believed to be an important element of tumorigenic cell regulation. In this perspectives paper, we propose how some conserved functions of Vasa may be selected for somatic cell regulation, including its potential impact on efficient and localized translational activities and in some cases on cellular malfunctioning and tumorigenesis.

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.

The ontogeny of nanos homologue expression in the oligochaete annelid Tubifex tubifex

We have cloned and characterized the expression of a nanos homologue (designated Ttu-nos) from the oligochaete annelid Tubifex tubifex. Ttu-nos mRNA is distributed broadly throughout the early cleavage stages. Ttu-nos is expressed in most if not all of the early blastomeres, in which Ttu-nos RNA associates with pole plasms. Ttu-nos transcripts are concentrated to 2d and 4d cells. Shortly after 2d(111) (derived from 2d cell) divides into a bilateral pair of NOPQ proteloblasts, Ttu-nos RNA vanishes from the embryo, which is soon followed by the resumption of Ttu-nos expression in nascent primary blast cells produced by teloblasts. The resumption of Ttu-nos expression occurs only in a subset of teloblast lineages (viz., M, N and Q). After Ttu-nos expression is retained in the germ band for a while, it disappears in anterior-to-posterior progression. At the end of embryogenesis, there is no trace of Ttu-nos expression. Thereafter, growing juveniles do not show any sign of Ttu-nos expression, either. The first sign of Ttu-nos expression is detected in oocytes in the ovary of young adults (ca 40 days after hatching), and its expression continues in growing oocytes that undergo yolk deposition and maturation in the ovisac.

A piece of the pi(e): The diverse roles of animal piRNAs and their PIWI partners

Small non-coding RNAs are indispensable to many biological processes. A class of endogenous small RNAs, termed PIWI-interacting RNAs (piRNAs) because of their association with PIWI proteins, has known roles in safeguarding the genome against inordinate transposon mobilization, embryonic development, and stem cell regulation, among others. This review discusses the biogenesis of animal piRNAs and their diverse functions together with their PIWI protein partners, both in the germline and in somatic cells, and highlights the evolutionarily conserved aspects of these molecular players in animal biology.

Gonad establishment during asexual reproduction in the annelid Pristina leidyi

Animals that can reproduce by both asexual agametic reproduction and sexual reproduction must transmit or re-establish their germ line post-embryonically. Although such a dual reproductive mode has evolved repeatedly among animals, how asexually produced individuals establish their germ line remains poorly understood in most groups. We investigated germ line development in the annelid Pristina leidyi, a species that typically reproduces asexually by paratomic fission, intercalating a new tail and head in the middle of the body followed by splitting. We found that in fissioning individuals, gonads occur in anterior segments in the anterior-most individual as well as in new heads forming within fission zones. Homologs of the germ line/multipotency genes piwi, vasa, and nanos are expressed in the gonads, as well as in proliferative tissues including the posterior growth zone, fission zone, and regeneration blastema. In fissioning animals, certain cells on the ventral nerve cord express a homolog of piwi, are abundant near fission zones, and sometimes make contact with gonads. Such cells are typically undetectable near the blastema and posterior growth zone. Time-lapse imaging provides direct evidence that cells on the ventral nerve cord migrate preferentially towards fission zones. Our findings indicate that gonads form routinely in fissioning individuals, that a population of piwi-positive cells on the ventral nerve cord is associated with fission and gonads, and that cells resembling these piwi-positive cells migrate along the ventral nerve cord. We suggest that the piwi-positive ventral cells are germ cells that transmit the germ line across asexually produced individuals via migration along the ventral nerve cord.

Essential elements for translation: the germline factor Vasa functions broadly in somatic cells

Vasa is a conserved RNA-helicase found in the germ lines of all metazoans tested. Whereas Vasa presence is often indicated as a metric for germline determination in animals, it is also expressed in stem cells of diverse origin. Recent research suggests, however, that Vasa has a much broader function, including a significant role in cell cycle regulation. Results herein indicate that Vasa is utilized widely, and often induced transiently, during development in diverse somatic cells and adult precursor tissues. We identified that Vasa in the sea urchin is essential for: (1) general mRNA translation during embryogenesis, (2) developmental re-programming upon manipulations to the embryo and (3) larval wound healing. We also learned that Vasa interacted with mRNAs in the perinuclear area and at the spindle in an Importin-dependent manner during cell cycle progression. These results suggest that, when present, Vasa functions are essential to contributing to developmental regulation.

Vasa, PL10, and Piwi gene expression during caudal regeneration of the polychaete annelid Alitta virens

Polychaetes are famous for their outstanding ability to regenerate lost body parts. Moreover, these worms possess a number of ancestral features in anatomy, development, and genetics, making them particularly suitable for comparative studies. Thus, fundamental as well as new undisclosed so far features of regenerative processes may be revealed, using polychaetes as a model. In the present work, we aimed to analyze the molecular basis of caudal regeneration in the nereid polychaete Alitta virens (formerly Nereis virens). We focused on homologues genes of RNA helicases Vasa and PL10 and ncRNA-binding proteins Piwi. These markers are suggested to play a significant role in maintenance of undifferentiated state of primordial germ cells and multipotent stem cells across invertebrates. In normal conditions, A. virens homologues of Vasa, PL10, and Piwi were differentially expressed in the subterminal growth zone and germline cells. Caudal amputation induced expression of studied genes de novo, which further accompanies all steps of regeneration. An early appearance of the transcripts in wound epithelium and internal blastemal cells suggests involvement of these genes in the well-known cell dedifferentiation events that assure polychaete regeneration. Provided interpretation of the gene expression dynamics implies the primary restoration of the pygidium and growth zone, which promotes following segment formation. Obtained results are valuable as a molecular fingerprint of the alterations occurring in regulatory state of locally regenerating tissues.

vasa and piwi are required for mitotic integrity in early embryogenesis in the spider Parasteatoda tepidariorum

Studies in vertebrate and invertebrate model organisms on the molecular basis of primordial germ cell (PGC) specification have revealed that metazoans can specify their germ line either early in development by maternally transmitted cytoplasmic factors (inheritance), or later in development by signaling factors from neighboring tissues (induction). Regardless of the mode of PGC specification, once animal germ cells are specified, they invariably express a number of highly conserved genes. These include vasa and piwi, which can play essential roles in any or all of PGC specification, development, or gametogenesis. Although the arthropods are the most speciose animal phylum, to date there have been no functional studies of conserved germ line genes in species of the most basally branching arthropod clade, the chelicerates (which includes spiders, scorpions, and horseshoe crabs). Here we present the first such study by using molecular and functional tools to examine germ line development and the roles of vasa and piwi orthologues in the common house spider Parasteatoda (formerly Achaearanea) tepidariorum. We use transcript and protein expression patterns of Pt-vasa and Pt-piwi to show that primordial germ cells (PGCs) in the spider arise during late embryogenesis. Neither Pt-vasa nor Pt-piwi gene products are localized asymmetrically to any embryonic region before PGCs emerge as paired segmental clusters in opisthosomal segments 2-6 at late germ band stages. RNA interference studies reveal that both genes are required maternally for egg laying, mitotic progression in early embryos, and embryonic survival. Our results add to the growing body of evidence that vasa and piwi can play important roles in somatic development, and provide evidence for a previously hypothesized conserved role for vasa in cell cycle progression.

History of a prolific family: the Hes/Hey-related genes of the annelid Platynereis

BACKGROUND

The Hes superfamily or Hes/Hey-related genes encompass a variety of metazoan-specific bHLH genes, with somewhat fuzzy phylogenetic relationships. Hes superfamily members are involved in a variety of major developmental mechanisms in metazoans, notably in neurogenesis and segmentation processes, in which they often act as direct effector genes of the Notch signaling pathway.

RESULTS

We have investigated the molecular and functional evolution of the Hes superfamily in metazoans using the lophotrochozoan Platynereis dumerilii as model. Our phylogenetic analyses of more than 200 Metazoan Hes/Hey-related genes revealed the presence of five families, three of them (Hes, Hey and Helt) being pan-metazoan. Those families were likely composed of a unique representative in the last common metazoan ancestor. The evolution of the Hes family was shaped by many independent lineage specific tandem duplication events. The expression patterns of 13 of the 15 Hes/Hey-related genes in Platynereis indicate a broad functional diversification. Nevertheless, a majority of these genes are involved in two crucial developmental processes in annelids: neurogenesis and segmentation, resembling functions highlighted in other animal models.

CONCLUSIONS

Combining phylogenetic and expression data, our study suggests an unusual evolutionary history for the Hes superfamily. An ancestral multifunctional annelid Hes gene may have undergone multiples rounds of duplication-degeneration-complementation processes in the lineage leading to Platynereis, each gene copies ensuring their maintenance in the genome by subfunctionalisation. Similar but independent waves of duplications are at the origin of the multiplicity of Hes genes in other metazoan lineages.

Hecate/Grip2a acts to reorganize the cytoskeleton in the symmetry-breaking event of embryonic axis induction

Maternal homozygosity for three independent mutant hecate alleles results in embryos with reduced expression of dorsal organizer genes and defects in the formation of dorsoanterior structures. A positional cloning approach identified all hecate mutations as stop codons affecting the same gene, revealing that hecate encodes the Glutamate receptor interacting protein 2a (Grip2a), a protein containing multiple PDZ domains known to interact with membrane-associated factors including components of the Wnt signaling pathway. We find that grip2a mRNA is localized to the vegetal pole of the oocyte and early embryo, and that during egg activation this mRNA shifts to an off-center vegetal position corresponding to the previously proposed teleost cortical rotation. hecate mutants show defects in the alignment and bundling of microtubules at the vegetal cortex, which result in defects in the asymmetric movement of wnt8a mRNA as well as anchoring of the kinesin-associated cargo adaptor Syntabulin. We also find that, although short-range shifts in vegetal signals are affected in hecate mutant embryos, these mutants exhibit normal long-range, animally directed translocation of cortically injected dorsal beads that occurs in lateral regions of the yolk cortex. Furthermore, we show that such animally-directed movement along the lateral cortex is not restricted to a single arc corresponding to the prospective dorsal region, but occur in multiple meridional arcs even in opposite regions of the embryo. Together, our results reveal a role for Grip2a function in the reorganization and bundling of microtubules at the vegetal cortex to mediate a symmetry-breaking short-range shift corresponding to the teleost cortical rotation. The slight asymmetry achieved by this directed process is subsequently amplified by a general cortical animally-directed transport mechanism that is neither dependent on hecate function nor restricted to the prospective dorsal axis.

One of the earliest and most crucial events in animal development is the establishment of the embryonic dorsal axis. In amphibians and fish, this event depends on the transport of so-called “dorsal determinants” from one region of the egg, at the pole opposite from the site where the oocyte nucleus lies, towards the site of axis induction. There, the dorsal determinant activates the Wnt signaling pathway, which in turn triggers dorsal gene expression. Dorsal determinant transport is mediated by the reorganization of a cellular network composed of microtubules. We determine that hecate, a zebrafish gene active during egg formation that is essential for embryonic axis induction, is required for an early step in this microtubule reorganization. We find that hecate corresponds to glutamate receptor interacting protein 2a, which participates in other animal systems in Wnt-based pathways. We also show that the microtubule reorganization dependent on hecate results in a subtle symmetry-breaking event that subsequently becomes amplified by a more general transport process independent of hecate function. Our data reveal new links between glutamate receptor interacting protein 2a, Wnt signaling and axis induction, and highlights basic mechanisms by which small changes early in development translate into global changes in the embryo.

Maternal inheritance of twist and analysis of MAPK activation in embryos of the polychaete annelid Platynereis dumerilii

In this study, we aimed to identify molecular mechanisms involved in the specification of the 4d (mesentoblast) lineage in Platynereis dumerilii. We employ RT-PCR and in situ hybridization against the Platynereis dumerilii twist homolog (Pdu-twist) to reveal mesodermal specification within this lineage. We show that Pdu-twist mRNA is already maternally distributed. After fertilization, ooplasmatic segregation leads to relocation of Pdu-twist transcripts into the somatoblast (2d) lineage and 4d, indicating that the maternal component of Pdu-twist might be an important prerequisite for further mesoderm specification but does not represent a defining characteristic of the mesentoblast. However, after the primordial germ cells have separated from the 4d lineage, zygotic transcription of Pdu-twist is exclusively observed in the myogenic progenitors, suggesting that mesodermal specification occurs after the 4d stage. Previous studies on spiral cleaving embryos revealed a spatio-temporal correlation between the 4d lineage and the activity of an embryonic organizer that is capable to induce the developmental fates of certain micromeres. This has raised the question if specification of the 4d lineage could be connected to the organizer activity. Therefore, we aimed to reveal the existence of such a proposed conserved organizer in Platynereis employing antibody staining against dpERK. In contrast to former observations in other spiralian embryos, activation of MAPK signaling during 2d and 4d formation cannot be detected which questions the existence of a conserved connection between organizer function and specification of the 4d lineage. However, our experiments unveil robust MAPK activation in the prospective nephroblasts as well as in the macromeres and some micromeres at the blastopore in gastrulating embryos. Inhibition of MAPK activation leads to larvae with a shortened body axis, defects in trunk muscle spreading and improper nervous system condensation, indicating a critical function for MAPK signaling for the reorganization of embryonic tissues during the gastrulation process.

The unique stem cell system of the immortal larva of the human parasite Echinococcus multilocularis

Background

It is believed that in tapeworms a separate population of undifferentiated cells, the germinative cells, is the only source of cell proliferation throughout the life cycle (similar to the neoblasts of free living flatworms). In Echinococcus multilocularis, the metacestode larval stage has a unique development, growing continuously like a mass of vesicles that infiltrate the tissues of the intermediate host, generating multiple protoscoleces by asexual budding. This unique proliferation potential indicates the existence of stem cells that are totipotent and have the ability for extensive self-renewal.

Results

We show that only the germinative cells proliferate in the larval vesicles and in primary cell cultures that undergo complete vesicle regeneration, by using a combination of morphological criteria and by developing molecular markers of differentiated cell types. The germinative cells are homogeneous in morphology but heterogeneous at the molecular level, since only sub-populations express homologs of the post-transcriptional regulators nanos and argonaute. Important differences are observed between the expression patterns of selected neoblast marker genes of other flatworms and the E. multilocularis germinative cells, including widespread expression in E. multilocularis of some genes that are neoblast-specific in planarians. Hydroxyurea treatment results in the depletion of germinative cells in larval vesicles, and after recovery following hydroxyurea treatment, surviving proliferating cells grow as patches that suggest extensive self-renewal potential for individual germinative cells.

Conclusions

In E. multilocularis metacestodes, the germinative cells are the only proliferating cells, presumably driving the continuous growth of the larval vesicles. However, the existence of sub-populations of the germinative cells is strongly supported by our data. Although the germinative cells are very similar to the neoblasts of other flatworms in function and in undifferentiated morphology, their unique gene expression pattern and the evolutionary loss of conserved stem cells regulators suggest that important differences in their physiology exist, which could be related to the unique biology of E. multilocularis larvae.

17β-Estradiol induces supernumerary primordial germ cells in embryos of the polychaete Platynereis dumerilii

In the polychaete Platynereis dumerilii exactly four primordial germ cells (PGCs) arise in early development and are subject to a transient mitotic arrest until the animals enter gametogenesis. In order to unravel the mechanisms controlling the number of PGCs in Platynereis, we tested whether the steroid 17β-estradiol (E2) is able to induce PGC proliferation, as it had been described in other species. Our data provide strong support for such a mechanism, showing that E2 significantly increases the occurrence of larvae with supernumerary PGCs in Platynereis in a dose dependent manner. E2 responsiveness is restricted to early developmental stages, when the PGCs are specified. During these stages, embryos exhibit high expression levels of the estradiol receptor (ER). The ER transcript localizes to the yolk-free cytoplasm of unfertilized eggs and segregates into the micromeres during cleavage stages. Nuclear ER protein is found asymmetrically distributed between daughter cells. Neither transcript nor protein is detectable in PGCs at larval stages. Addition of the specific estradiol receptor inhibitor ICI-182,780 (ICI) abolishes the proliferative effect of E2, suggesting that it is mediated by ER signaling. Our study reports for the first time an ER mediated proliferative effect of E2 on PGCs in an invertebrate organism.

Differential expression of conserved germ line markers and delayed segregation of male and female primordial germ cells in a hermaphrodite, the leech helobdella

In sexually reproducing animals, primordial germ cells (PGCs) are often set aside early in embryogenesis, a strategy that minimizes the risk of genomic damage associated with replication and mitosis during the cell cycle. Here, we have used germ line markers (piwi, vasa, and nanos) and microinjected cell lineage tracers to show that PGC specification in the leech genus Helobdella follows a different scenario: in this hermaphrodite, the male and female PGCs segregate from somatic lineages only after more than 20 rounds of zygotic mitosis; the male and female PGCs share the same (mesodermal) cell lineage for 19 rounds of zygotic mitosis. Moreover, while all three markers are expressed in both male and female reproductive tissues of the adult, they are expressed differentially between the male and female PGCs of the developing embryo: piwi and vasa are expressed preferentially in female PGCs at a time when nanos is expressed preferentially in male PGCs. A priori, the delayed segregation of male and female PGCs from somatic tissues and from one another increases the probability of mutations affecting both male and female PGCs of a given individual. We speculate that this suite of features, combined with a capacity for self-fertilization, may contribute to the dramatically rearranged genome of Helobdella robusta relative to other animals.

Expression pattern of Piwi-like genes in adult Myzostoma cirriferum (Annelida)

Piwi-like genes are a subgroup of Argonaute genes which participate as gene regulators by gene silencing. In most bilaterians, such as mouse, human, insects, and zebrafish, their expression is mostly limited to gonadal stem cells. But there are some striking exceptions to this pattern; flatworms and acoels also express Piwi-like genes in somatic stem cells, due to their unique replacement system. Annelid species like Capitella teleta and Platynereis dumerilii express these genes in cells of the posterior growth zone as well as in gonadal stem cells. To investigate the expression pattern of Piwi-like genes in another annelid, we established in situ hybridization for adult Myzostoma cirriferum. Piwi-like gene transcripts recovered in an mRNA-seq library of pooled adult stages of M. cirriferum were expanded using RACE PCR, cloned and sequenced. ML analysis confirmed the identity of both transcripts as part of the Piwi1-like or Piwi2-like subfamily of Argonaute proteins. The results of in situ hybridization studies show that the expression of both Piwi-like genes, Mc-Piwi1 and Mc-Piwi2, is clearly located only in gonadal stem cells, and as such we did not find any evidence for the existence of a posterior growth zone nor expression in somatic stem cells.

Primordial germ cells in an oligochaete annelid are specified according to the birth rank order in the mesodermal teloblast lineage

The primordial germ cells (PGCs) in the oligochaete annelid Tubifex tubifex are descentants of the mesodermal (M) teloblast and are located in the two midbody segments X and XI in which they serve as germline precursors forming the testicular gonad and the ovarian gonad, respectively. During embryogenesis, vasa-expressing cells (termed presumptive PGCs or pre-PGCs) emerge in a variable set of midbody segments including the genital segments (X and XI); at the end of embryogenesis, pre-PGCs are confined to the genital segments, where they become PGCs in the juvenile. Here, using live imaging of pre-PGCs, we have demonstrated that during Tubifex embryogenesis, pre-PGCs (defined by Vasa expression) stay in segments where they have emerged, suggesting that it is unlikely that pre-PGCs move intersegmentally during embryogenesis. Thus, it is apparent that pre-PGCs derived from the 10th and 11th M teloblast-derived primary m blast cells (designated m10 and m11) that give rise, respectively, to segments X and XI are specified in situ as PGCs and that those born in other segments become undetectable at the end of embryogenesis. To address the mechanisms for this segment-specific development of PGCs, we have performed a set of cell-transplantation experiments as well as cell-ablation experiments. When m10 and m11 that are normally located in the mid region of the embryo were placed in positions near the anterior end of the host embryo, these cells formed two consecutive segments, which exhibited Vasa-positive PGC-like cells at early juvenile stage. This suggests that in terms of PGC generation, the fates of m10 and m11 remain unchanged even if they are placed in ectopic positions along the anteroposterior axis. Nor was the fate of m10 and m11 changed even if mesodermal blast cell chains preceding or succeeding m10 and m11 were absent. In a previous study, it was shown that PGC development in segments X and XI occurs normally in the absence of the overlying ectoderm. All this strongly suggests that irrespective of their surrounding cellular environments, m10 and m11 autonomously generate PGCs. We propose that m10 and m11 are exclusively specified as precursors of PGCs at the time of their birth from the M teloblast and that the M teloblast possesses a developmental program through which the sequence of mesodermal blast cell identities is determined.

Splice variants and promoter methylation status of the Bovine Vasa Homology (Bvh) gene may be involved in bull spermatogenesis

Background

Vasa is a member of the DEAD-box protein family that plays an indispensable role in mammalian spermatogenesis, particularly during meiosis. Bovine vasa homology (Bvh) of Bos taurus has been reported, however, its function in bovine testicular tissue remains obscure. This study aimed to reveal the functions of Bvh and to determine whether Bvh is a candidate gene in the regulation of spermatogenesis in bovine, and to illustrate whether its transcription is regulated by alternative splicing and DNA methylation.

Results

Here we report the molecular characterization, alternative splicing pattern, expression and promoter methylation status of Bvh. The full-length coding region of Bvh was 2190 bp, which encodes a 729 amino acid (aa) protein containing nine consensus regions of the DEAD box protein family. Bvh is expressed only in the ovary and testis of adult cattle. Two splice variants were identified and termed Bvh-V4 (2112 bp and 703 aa) and Bvh-V45 (2040 bp and 679 aa). In male cattle, full-length Bvh (Bvh-FL), Bvh-V4 and Bvh-V45 are exclusively expressed in the testes in the ratio of 2.2:1.6:1, respectively. Real-time PCR revealed significantly reduced mRNA expression of Bvh-FL, Bvh-V4 and Bvh-V45 in testes of cattle-yak hybrids, with meiotic arrest compared with cattle and yaks with normal spermatogenesis (P < 0.01). The promoter methylation level of Bvh in the testes of cattle-yak hybrids was significantly greater than in cattle and yaks (P < 0.01).

Conclusion

In the present study, Bvh was isolated and characterized. These data suggest that Bvh functions in bovine spermatogenesis, and that transcription of the gene in testes were regulated by alternative splice and promoter methylation.

Germ cell specification requires zygotic mechanisms rather than germ plasm in a basally branching insect

BACKGROUND

Primordial germ cell (PGC) specification is a universal process across animals, but the molecular mechanisms specifying PGCs are remarkably diverse. In Drosophila, PGCs are specified by maternally provided, asymmetrically localized cytoplasmic factors (germ plasm). In contrast, historical literature on most other arthropods reports that PGCs arise from mesoderm during midembryogenesis, suggesting that an arthropod last common ancestor may have specified PGCs via zygotic mechanisms. However, there has been no direct experimental evidence to date for germ plasm-independent arthropod PGC specification.

RESULTS

Here we show that in a basally branching insect, the cricket Gryllus bimaculatus, conserved germ plasm molecules are ubiquitously, rather than asymmetrically, localized during oogenesis and early embryogenesis. Molecular and cytological analyses suggest that Gryllus PGCs arise from abdominal mesoderm during segmentation, and twist RNAi embryos that lack mesoderm fail to form PGCs. Using RNA interference we show that vasa and piwi are not required maternally or zygotically for PGC formation but rather are required for primary spermatogonial divisions in adult males.

CONCLUSIONS

These observations suggest that Gryllus lacks a maternally inherited germ plasm, in contrast with many holometabolous insects, including Drosophila. The mesodermal origin of Gryllus PGCs and absence of instructive roles for vasa and piwi in PGC formation are reminiscent of mouse PGC specification and suggest that zygotic cell signaling may direct PGC specification in Gryllus and other Hemimetabola.

Pheromone evolution, reproductive genes, and comparative transcriptomics in mediterranean earthworms (annelida, oligochaeta, hormogastridae)

Animals inhabiting cryptic environments are often subjected to morphological stasis due to the lack of obvious agents driving selection, and hence chemical cues may be important drivers of sexual selection and individual recognition. Here, we provide a comparative analysis of de novo-assembled transcriptomes in two Mediterranean earthworm species with the objective to detect pheromone proteins and other reproductive genes that could be involved in cryptic speciation processes, as recently characterized in other earthworm species. cDNA libraries of unspecific tissue of Hormogaster samnitica and three different tissues of H. elisae were sequenced in an Illumina Genome Analyzer II or Hi-Seq. Two pheromones, Attractin and Temptin were detected in all tissue samples and both species. Attractin resulted in a reliable marker for phylogenetic inference. Temptin contained multiple paralogs and was slightly overexpressed in the digestive tissue, suggesting that these pheromones could be released with the casts. Genes involved in sexual determination and fertilization were highly expressed in reproductive tissue. This is thus the first detailed analysis of the molecular machinery of sexual reproduction in earthworms.

Notch and Nodal control forkhead factor expression in the specification of multipotent progenitors in sea urchin

Indirect development, in which embryogenesis gives rise to a larval form, requires that some cells retain developmental potency until they contribute to the different tissues in the adult, including the germ line, in a later, post-embryonic phase. In sea urchins, the coelomic pouches are the major contributor to the adult, but how coelomic pouch cells (CPCs) are specified during embryogenesis is unknown. Here we identify the key signaling inputs into the CPC specification network and show that the forkhead factor foxY is the first transcription factor specifically expressed in CPC progenitors. Through dissection of its cis-regulatory apparatus we determine that the foxY expression pattern is the result of two signaling inputs: first, Delta/Notch signaling activates foxY in CPC progenitors; second, Nodal signaling restricts its expression to the left side, where the adult rudiment will form, through direct repression by the Nodal target pitx2. A third signal, Hedgehog, is required for coelomic pouch morphogenesis and institution of laterality, but does not directly affect foxY transcription. Knockdown of foxY results in a failure to form coelomic pouches and disrupts the expression of virtually all transcription factors known to be expressed in this cell type. Our experiments place foxY at the top of the regulatory hierarchy underlying the specification of a cell type that maintains developmental potency.

Stem cell systems and regeneration in planaria

Planarians are members of the Platyhelminthes (flatworms). These animals have evolved a remarkable stem cell system. A single pluripotent adult stem cell type ("neoblast") gives rise to the entire range of cell types and organs in the planarian body plan, including a brain, digestive-, excretory-, sensory- and reproductive systems. Neoblasts are abundantly present throughout the mesenchyme and divide continuously. The resulting stream of progenitors and turnover of differentiated cells drive the rapid self-renewal of the entire animal within a matter of weeks. Planarians grow and literally de-grow ("shrink") by the food supply-dependent adjustment of organismal turnover rates, scaling body plan proportions over as much as a 50-fold size range. Their dynamic body architecture further allows astonishing regenerative abilities, including the regeneration of complete and perfectly proportioned animals even from tiny tissue remnants. Planarians as an experimental system, therefore, provide unique opportunities for addressing a spectrum of current problems in stem cell research, including the evolutionary conservation of pluripotency, the dynamic organization of differentiation lineages and the mechanisms underlying organismal stem cell homeostasis. The first part of this review focuses on the molecular biology of neoblasts as pluripotent stem cells. The second part examines the fascinating mechanistic and conceptual challenges posed by a stem cell system that epitomizes a universal design principle of biological systems: the dynamic steady state.

The 3'UTR of nanos2 directs enrichment in the germ cell lineage of the sea urchin

Nanos is a translational regulator required for the survival and maintenance of primordial germ cells during embryogenesis. Three nanos homologs are present in the genome of the sea urchin Strongylocentrotus purpuratus (Sp), and each nanos mRNA accumulates specifically in the small micromere (sMic) lineage. We found that a highly conserved element in the 3' UTR of nanos2 is sufficient for reporter expression selectively in the sMic lineage: microinjection into a Sp fertilized egg of an RNA that contains the GFP open reading frame followed by Sp nanos2 3'UTR leads to selective reporter enrichment in the small micromeres in blastulae. The same result was seen with nanos2 from the sea urchin Hemicentrotus pulcherrimus (Hp). In both species, the 5'UTR alone is not sufficient for the sMic localization but it always increased the sMic reporter enrichment when present with the 3'UTR. We defined an element conserved between Hp and Sp in the nanos2 3'UTR which is necessary and sufficient for protein enrichment in the sMic, and refer to it as GNARLE (Global Nanos Associated RNA Lability Element). We also found that the nanos2 3'UTR is essential for the selective RNA retention in the small micromeres; GNARLE is required but not sufficient for this process. These results show that a combination of selective RNA retention and translational control mechanisms instills nanos accumulation uniquely in the sMic lineage.

Closing the circle of germline and stem cells: the Primordial Stem Cell hypothesis

Background

Germline determination is believed to occur by either preformation or epigenesis. Animals that undergo germ cell specification by preformation have a continuous germline. However, animals with germline determination by epigenesis have a discontinuous germline, with somatic cells intercalated. This vision is contrary to August Weismann’s Germ Plasm Theory and has led to several controversies. Recent data from metazoans as diverse as planarians, annelids and sea urchins reveal the presence of pluripotent stem cell populations that express germ plasm components, despite being considered to be somatic. These data also show that germ plasm is continuous in some of these animals, despite their discontinuous germline.

Presentation of the hypothesis

Here, based on recent molecular data on germ plasm components, I revise the germline concept. I introduce the concept of primordial stem cells, which are evolutionarily conserved stem cells that carry germ plasm components from the zygote to the germ cells. These cells, delineated by the classic concept of the Weismann barrier, can contribute to different extents to somatic tissues or be present in a rudimentary state. The primordial stem cells are a part of the germline that can drive asexual reproduction.

Testing the hypothesis

Molecular information on the expression of germ plasm components is needed during early development of non-classic model organisms, with special attention to those capable of undergoing asexual reproduction and regeneration. The cell lineage of germ plasm component-containing cells will also shed light on their position with respect to the Weismann barrier. This information will help in understanding the germline and its associated stem cells across metazoan phylogeny.

Implications of the hypothesis

This revision of the germline concept explains the extensive similarities observed among stem cells and germline cells in a wide variety of animals, and predicts the expression of germ plasm components in many others. The life history of these animals can be simply explained by changes in the extent of self-renewal, proliferation and developmental potential of the primordial stem cells. The inclusion of the primordial stem cells as a part of the germline, therefore, solves many controversies and provides a continuous germline, just as originally envisaged by August Weismann.

Autonomy in specification of primordial germ cells and their passive translocation in the sea urchin

The process of germ line determination involves many conserved genes, yet is highly variable. Echinoderms are positioned at the base of Deuterostomia and are crucial to understanding these evolutionary transitions, yet the mechanism of germ line specification is not known in any member of the phyla. Here we demonstrate that small micromeres (SMics), which are formed at the fifth cell division of the sea urchin embryo, illustrate many typical features of primordial germ cell (PGC) specification. SMics autonomously express germ line genes in isolated culture, including selective Vasa protein accumulation and transcriptional activation of nanos; their descendants are passively displaced towards the animal pole by secondary mesenchyme cells and the elongating archenteron during gastrulation; Cadherin (G form) has an important role in their development and clustering phenotype; and a left/right integration into the future adult anlagen appears to be controlled by a late developmental mechanism. These results suggest that sea urchin SMics share many more characteristics typical of PGCs than previously thought, and imply a more widely conserved system of germ line development among metazoans.

Molecular and expression characterization of a nanos1 homologue in Chinese sturgeon, Acipenser sinensis

The nanos gene family was essential for germ line development in diverse organisms. In the present study, the full-length cDNA of a nanos1 homologue in A. sinensis, Asnanos1, was isolated and characterized. The cDNA sequence of Asnanos1 was 1489 base pairs (bp) in length and encoded a peptide of 228 amino acid residues. Multiple sequence alignment showed that the zinc-finger motifs of Nanos1 were highly conserved in vertebrates. By RT-PCR analysis, Asnanos1 mRNAs were ubiquitously detected in all tissues examined except for the fat, including liver, spleen, heart, ovary, kidney, muscle, intestines, pituitary, hypothalamus, telencephalon, midbrain, cerebellum, and medulla oblongata. Moreover, a specific polyclonal antibody was prepared from the in vitro expressed partial AsNanos1 protein. Western blot analysis revealed that the tissue expression pattern of AsNanos1 was not completely coincided with that of its mRNAs, which was not found in fat, muscle and intestines. Additionally, by immunofluoresence localization, it was observed that AsNanos1 protein was in the cytoplasm of primary oocytes and spermatocytes. The presented results indicated that the expression pattern of Asnanos1 was differential conservation and divergence among diverse species.

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.

vasa is expressed in somatic cells of the embryonic gonad in a sex-specific manner in Drosophila melanogaster

Vasa is a DEAD box helicase expressed in the Drosophila germline at all stages of development. vasa homologs are found widely in animals and vasa has become the gene of choice in identifying germ cells. I now show that Drosophila vasa expression is not restricted to the germline but is also expressed in a somatic lineage, the embryonic somatic gonadal precursor cells. This expression is sexually dimorphic, being maintained specifically in males, and is regulated post-transcriptionally. Although somatic Vasa expression is not required for gonad coalescence, these data support the notion that Vasa is not solely a germline factor.

Evolutionary crossroads in developmental biology: annelids

Annelids (the segmented worms) have a long history in studies of animal developmental biology, particularly with regards to their cleavage patterns during early development and their neurobiology. With the relatively recent reorganisation of the phylogeny of the animal kingdom, and the distinction of the super-phyla Ecdysozoa and Lophotrochozoa, an extra stimulus for studying this phylum has arisen. As one of the major phyla within Lophotrochozoa, Annelida are playing an important role in deducing the developmental biology of the last common ancestor of the protostomes and deuterostomes, an animal from which &gt;98% of all described animal species evolved.

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.

What role do annelid neoblasts play? A comparison of the regeneration patterns in a neoblast-bearing and a neoblast-lacking enchytraeid oligochaete

The term ‘neoblast’ was originally coined for a particular type of cell that had been observed during annelid regeneration, but is now used to describe the pluripotent/totipotent stem cells that are indispensable for planarian regeneration. Despite having the same name, however, planarian and annelid neoblasts are morphologically and functionally distinct, and many annelid species that lack neoblasts can nonetheless substantially regenerate. To further elucidate the functions of the annelid neoblasts, a comparison was made between the regeneration patterns of two enchytraeid oligochaetes, Enchytraeus japonensis and Enchytraeus buchholzi, which possess and lack neoblasts, respectively. In E. japonensis, which can reproduce asexually by fragmentation and subsequent regeneration, neoblasts are present in all segments except for the eight anterior-most segments including the seven head-specific segments, and all body fragments containing neoblasts can regenerate a complete head and a complete tail, irrespective of the region of the body from which they were originally derived. In E. japonensis, therefore, no antero-posterior gradient of regeneration ability exists in the trunk region. However, when amputation was carried out within the head region, where neoblasts are absent, the number of regenerated segments was found to be dependent on the level of amputation along the body axis. In E. buchholzi, which reproduces only sexually and lacks neoblasts in all segments, complete heads were never regenerated and incomplete (hypomeric) heads could be regenerated only from the anterior region of the body. Such an antero-posterior gradient of regeneration ability was observed for both the anterior and posterior regeneration in the whole body of E. buchholzi. These results indicate that the presence of neoblasts correlates with the absence of an antero-posterior gradient of regeneration ability along the body axis, and suggest that the annelid neoblasts are more essential for efficient asexual reproduction than for the regeneration of missing body parts.

Hidden in the crowd: primordial germ cells and somatic stem cells in the mesodermal posterior growth zone of the polychaete Platynereis dumerillii are two distinct cell populations

Background

In the polychaete Platynereis, the primordial germ cells (PGCs) emerge from the vasa, piwi, and PL10 expressing mesodermal posterior growth zone (MPGZ) at the end of larval development, suggesting a post-embryonic formation from stem cells.

Methods

In order to verify this hypothesis, embryos and larvae were pulse labeled with the proliferation marker 5-ethynyl-2'-deoxyuridine (EdU) at different stages of development. Subsequently, the PGCs were visualized in 7-day-old young worms using antibodies against the Vasa protein.

Results

Surprisingly, the primordial germ cells of Platynereis incorporate EdU only shortly before gastrulation (6-8 hours post fertilization (hpf)), which coincides with the emergence of four small blastomeres from the mesoblast lineage. We conclude that these so-called 'secondary mesoblast cells' constitute the definitive PGCs in Platynereis. In contrast, the cells of the MPGZ incorporate EdU only from the pre-trochophore stage onward (14 hpf).

Conclusion

While PGCs and the cells of the MPGZ in Platynereis are indistinguishable in morphology and both express the germline markers vasa, nanos, and piwi, a distinct cluster of PGCs is detectable anterior of the MPGZ following EdU pulse-labeling. Indeed the PGCs form independently from the stem cells of the MPGZ prior to gastrulation. Our data suggest an early PGC formation in the polychaete by preformation rather than by epigenesis.

PRMT5 and the role of symmetrical dimethylarginine in chromatoid bodies of planarian stem cells

Planarian flatworms contain a population of adult stem cells (neoblasts) that proliferate and generate cells of all tissues during growth, regeneration and tissue homeostasis. A characteristic feature of neoblasts is the presence of chromatoid bodies, large cytoplasmic ribonucleoprotein (RNP) granules morphologically similar to structures present in the germline of many organisms. This study aims to reveal the function, and identify additional components, of planarian chromatoid bodies. We uncover the presence of symmetrical dimethylarginine (sDMA) on chromatoid body components and identify the ortholog of protein arginine methyltransferase PRMT5 as the enzyme responsible for sDMA modification in these proteins. RNA interference-mediated depletion of planarian PRMT5 results in defects in homeostasis and regeneration, reduced animal size, reduced number of neoblasts, fewer chromatoid bodies and increased levels of transposon and repetitive-element transcripts. Our results suggest that PIWI family member SMEDWI-3 is one sDMA-containing chromatoid body protein for which methylation depends on PRMT5. Additionally, we discover an RNA localized to chromatoid bodies, germinal histone H4. Our results reveal new components of chromatoid bodies and their function in planarian stem cells, and also support emerging studies indicative of sDMA function in stabilization of RNP granules and the Piwi-interacting RNA pathway.

Role of Vasa, Piwi, and Myc-expressing coelomic cells in gonad regeneration of the colonial tunicate, Botryllus primigenus

In the colonial tunicate, Botryllus primigenus Oka, gonads consist of indifferent germline precursor cells, the primordial testis and ovary, and mature gonads, of which the immature gonad components can be reconstructed de novo in vascular buds that arise from the common vascular system, although the mechanism is uncertain. In this study, we investigated how and what kinds of cells regenerated the gonad components. We found that few Vasa-positive cells in the hemocoel entered the growing vascular bud, where their number increased, and finally developed exclusively into female germ cells. Simultaneously, small cell aggregates consisting of Vasa(-) and Vasa(±) cells appeared de novo in the lateral body cavity of developing vascular buds. Double fluorescent in situ hybridization showed that these cell aggregates were both Piwi- and Myc-positive. They could form germline precursor cells and a primordial testis and ovary that strongly expressed Vasa. Myc knockdown by RNA interference conspicuously lowered Piwi expression and resulted in the loss of germline precursor cells without affecting Vasa(+) oocyte formation. Myc may contribute to gonad tissue formation via Piwi maintenance. When human recombinant BMP 4 was injected in the test vessel, coelomic Piwi(+) cells were induced to express Vasa in the blood. We conclude, therefore, that in vascular buds of B. primigenus, female germ cells can develop from homing Vasa(+) cells in the blood, and that other gonad components can arise from coelomic Vasa(-)/Piwi(+)/Myc(+) cells.

Development and maturation in the nereidid polychaetes Platynereis dumerilii and Nereis succinea exposed to xenoestrogens

Few studies link biochemical, cellular and whole animal effects of toxic compounds with growth and reproductive output on invertebrate model organisms. Thus, we explore the effects of xenoestrogens on nereid worms. Larvae of Platynereis dumerilii exposed to estradiol (E(2)) ethynylestradiol (EE(2)) and nonyplhenol (NP) observing the effects on growth, primordial germ cell (PGC) proliferation and maturation. In addition, a single exposure study was performed with a 50 days latency period on adult worms of Nereis succinea. Since reduced glutathione (GSH) is required in detoxification of NP and is the precursor of the spawning pheromone nereithione (CSSG) in N. succinea, we analysed how the estrogenic chemical NP affects GSH concentrations. PGC were not affected by exposure to E(2) and EE(2) from 24hpf to 6 days. Chronic exposure of P. dumerilii with NP over the full life cycle did not influence segment proliferation. Mature females that developed, even at high concentrations, were able to spawn and successful fertilization occurred. However, at high NP levels no P. dumerilii males matured. A significant decline of GSH can be seen in N. succinea males upon treatment with NP, but not in females, indicating that females stabilize GSH levels even in stress situations. This study shows some results that link the foundation to causally integrate toxic exposure to xenoestrogens with development, growth and reproductive outputs in nereidid polychaetes.

Somatic and germline expression of piwi during development and regeneration in the marine polychaete annelid Capitella teleta

Background

Stem cells have a critical role during adult growth and regeneration. Germline stem cells are specialized stem cells that produce gametes during sexual reproduction. Capitella teleta (formerly Capitella sp. I) is a polychaete annelid that reproduces sexually, exhibits adult growth and regeneration, and thus, is a good model to study the relationship between somatic and germline stem cells.

Results

We characterize expression of the two C. teleta orthologs of piwi, genes with roles in germline development in diverse organisms. Ct-piwi1 and Ct-piwi2 are expressed throughout the life cycle in a dynamic pattern that includes both somatic and germline cells, and show nearly identical expression patterns at all stages examined. Both genes are broadly expressed during embryonic and larval development, gradually becoming restricted to putative primordial germ cells (PGCs) and the posterior growth zone. In juveniles, Ct-piwi1 is expressed in the presumptive gonads, and in reproductive adults, it is detected in gonads and the posterior growth zone. In addition, Ct-piwi1 is expressed in a population of putative PGCs that persist in sexually mature adults, likely in a stem cell niche. Ct-piwi1 is expressed in regenerating tissue, and once segments differentiate, it becomes most prominent in the posterior growth zone and immature oocytes in regenerating ovaries of regenerating segments.

Conclusions

In C. teleta, piwi genes may have retained an ancestral role as genetic regulators of both somatic and germline stem cells. It is likely that piwi genes, and associated stem cell co-regulators, became restricted to the germline in some taxa during the course of evolution.

The DEAD-box RNA helicase Vasa functions in embryonic mitotic progression in the sea urchin

Vasa is a broadly conserved ATP-dependent RNA helicase that functions in the germ line of organisms from cnidarians to mammals. Curiously, Vasa is also present in the somatic cells of many animals and functions as a regulator of multipotent cells. Here, we report a mitotic function of Vasa revealed in the sea urchin embryo. We found that Vasa protein is present in all blastomeres of the early embryo and that its abundance oscillates with the cell cycle. Vasa associates with the spindle and the separating sister chromatids at metaphase, and then quickly disappears after telophase. Inhibition of Vasa protein synthesis interferes with proper chromosome segregation, arrests cells at M-phase, and delays overall cell cycle progression. Cdk activity is necessary for the proper localization of Vasa, implying that Vasa is involved in the cyclin-dependent cell cycle network, and Vasa is required for the efficient translation of cyclinB mRNA. Our results suggest an evolutionarily conserved role of Vasa that is independent of its function in germ line determination.

Evolution of RNA-binding proteins in animals: insights from genome-wide analysis in the sponge Amphimedon queenslandica

RNA-binding proteins (RBPs) are key players in various biological processes, most notably regulation of gene expression at the posttranscriptional level. Although many RBPs have been carefully studied in model organisms, very few studies have addressed the evolution of these proteins at the scale of the animal kingdom. We identified a large set of putative RBPs encoded by the genome of the demosponge Amphimedon queenslandica, a species representing a basal animal lineage. We compared the Amphimedon RBPs with those encoded by the genomes of two bilaterians (human and Drosophila), representatives of two other basal metazoan lineages (a placozoan and a cnidarian), a choanoflagellate (probable sister group of animals), and two fungi. We established the evolutionary history of 32 families of RBPs and found that most of the diversity of RBPs present in contemporary metazoans, including humans, was already established in the last common ancestor (LCA) of animals. This includes RBPs known to be involved in key processes in bilaterians, such as development, stem and/or germ cells properties, and noncoding RNA pathways. From this analysis, we infer that a complex toolkit of RBPs was present in the LCA of animals and that it has been recruited to perform new functions during early animal evolution, in particular in relation to the acquisition of multicellularity.

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

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

Germline cell formation and gonad regeneration in solitary and colonial ascidians

The morphology of ascidian gonad is very similar among species. The testis consists of variable number of testicular follicles; the ovary consists of ovarian tubes that are thickened forming the germinal epithelium with stem cells for female germ cells with the exception of botryllid ascidians. Peculiar accessory cells that would be germline in origin accompany the oocytes. Using vasa homologues as a molecular marker, germline precursor cells can be traced back to the embryonic posterior-most blastomeres and are found in the tail of tailbud embryo in some solitary and colonial ascidians. In Ciona, they are subsequently located in the larval tail, while in colonial botryllid ascidians vasa-expressing cells become obscure in the tail. Recent evidence suggests that ascidian germ cells can regenerate from cells other than embryonic germline. An ensemble of the embryonic stringency of germ cell lineage and the postembryonic flexibility of gonad formation is discussed.