Quantitative Differences in a Single Maternal Factor Determine Survival Probabilities among Drosophila Germ Cells

Origin and establishment of the germline in Drosophila melanogaster

The continuity of a species depends on germ cells. Germ cells are different from all the other cell types of the body (somatic cells) as they are solely destined to develop into gametes (sperm or egg) to create the next generation. In this review, we will touch on 4 areas of embryonic germ cell development in Drosophila melanogaster: the assembly and function of germplasm, which houses the determinants for germ cell specification and fate and the mitochondria of the next generation; the process of pole cell formation, which will give rise to primordial germ cells (PGCs); the specification of pole cells toward the PGC fate; and finally, the migration of PGCs to the somatic gonadal precursors, where they, together with somatic gonadal precursors, form the embryonic testis and ovary.

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

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

Increased collective migration correlates with germline stem cell competition in a basal chordate

Cell competition is a process that compares the relative fitness of progenitor cells, resulting in winners, which contribute further to development, and losers, which are excluded, and is likely a universal quality control process that contributes to the fitness of an individual. Cell competition also has pathological consequences, and can create super-competitor cells responsible for tumor progression. We are studying cell competition during germline regeneration in the colonial ascidian, Botryllus schlosseri. Germline regeneration is due to the presence of germline stem cells (GSCs) which have a unique property: a competitive phenotype. When GSCs from one individual are transplanted into another, the donor and recipient cells compete for germline development. Often the donor GSCs win, and completely replace the gametes of the recipient- a process called germ cell parasitism (gcp). gcp is a heritable trait, and winner and loser genotypes can be found in nature and reared in the lab. However, the molecular and cellular mechanisms underlying gcp are unknown. Using an ex vivo migration assay, we show that GSCs isolated from winner genotypes migrate faster and in larger clusters than losers, and that cluster size correlates with expression of the Notch ligand, Jagged. Both cluster size and jagged expression can be manipulated simultaneously in a genotype dependent manner: treatment of loser GSCs with hepatocyte growth factor increases both jagged expression and cluster size, while inhibitors of the MAPK pathway decrease jagged expression and cluster size in winner GSCs. Live imaging in individuals transplanted with labeled winner and loser GSCs reveal that they migrate to the niche, some as small clusters, with the winners having a slight advantage in niche occupancy. Together, this suggests that the basis of GSC competition resides in a combination in homing ability and niche occupancy, and may be controlled by differential utilization of the Notch pathway.

Paternal source of germ plasm determinants in the viviparous teleost, Gambusia holbrooki; dads do matter

The identity of germ cells, the progenitors of life, is thought to be acquired by two modes; either by maternal signals (preformed) or induced de novo from pluripotent cells (epigenesis) in the developing embryos. However, paternal roles seem enshrouded or completely overlooked in this fundamental biological process. Hence, we investigated the presence of germplasm transcripts in the sperm of Gambusia holbrooki, a live-bearing fish, demonstrating their presence and suggesting paternal contributions. Interestingly, not all germplasm markers were present (nanos1 and tdrd6) in the sperm, but some were conspicuous (dazl, dnd-α, piwi II, and vasa), indicating that the latter is required for establishing germ cell identity in the progeny, with a possible parent-specific role. Furthermore, there were also spatial differences in the distribution of these determinants, suggesting additional roles in sperm physiology and/or fertility. Our results support the hypothesis that dads also play a vital role in establishing the germ cell identity, especially in G. holbrooki, which shares elements of both preformation and induction modes of germline determination. This, coupled with its life history traits, makes G. holbrooki an excellent system for dissecting evolutionary relationships between the two germline determination modes, their underpinning mechanisms and ultimately the perpetuity of life.

Maternal Piwi regulates primordial germ cell development to ensure the fertility of female progeny in Drosophila

In many animals, germline development is initiated by proteins and RNAs that are expressed maternally. PIWI proteins and their associated small noncoding PIWI-interacting RNAs (piRNAs), which guide PIWI to target RNAs by base-pairing, are among the maternal components deposited into the germline of the Drosophila early embryo. Piwi has been extensively studied in the adult ovary and testis, where it is required for transposon suppression, germline stem cell self-renewal, and fertility. Consequently, loss of Piwi in the adult ovary using piwi-null alleles or knockdown from early oogenesis results in complete sterility, limiting investigation into possible embryonic functions of maternal Piwi. In this study, we show that the maternal Piwi protein persists in the embryonic germline through gonad coalescence, suggesting that maternal Piwi can regulate germline development beyond early embryogenesis. Using a maternal knockdown strategy, we find that maternal Piwi is required for the fertility and normal gonad morphology of female, but not male, progeny. Following maternal piwi knockdown, transposons were mildly derepressed in the early embryo but were fully repressed in the ovaries of adult progeny. Furthermore, the maternal piRNA pool was diminished, reducing the capacity of the PIWI/piRNA complex to target zygotic genes during embryogenesis. Examination of embryonic germ cell proliferation and ovarian gene expression showed that the germline of female progeny was partially masculinized by maternal piwi knockdown. Our study reveals a novel role for maternal Piwi in the germline development of female progeny and suggests that the PIWI/piRNA pathway is involved in germline sex determination in Drosophila.

Diverse roles of phosphatidate phosphatases in insect development and metabolism

The conversion of the glycerophospholipid phosphatidic acid (PA) into diacylglycerol (DAG) is essential for the biosynthesis of membrane phospholipids and storage fats. Importantly, both PA and DAG can also serve signaling functions in the cell. The dephosphorylation of PA that yields DAG can be executed by two different classes of enzymes, Mg²⁺-dependent lipins and Mg²⁺-independent lipid phosphate phosphatases. Here, I will discuss the current status of research directed at understanding the roles of these enzymes in insect development and metabolism. Special emphasis will be given to studies in the model organism Drosophila melanogaster.

DNase II mediates a parthanatos-like developmental cell death pathway in Drosophila primordial germ cells

During Drosophila embryonic development, cell death eliminates 30% of the primordial germ cells (PGCs). Inhibiting apoptosis does not prevent PGC death, suggesting a divergence from the conventional apoptotic program. Here, we demonstrate that PGCs normally activate an intrinsic alternative cell death (ACD) pathway mediated by DNase II release from lysosomes, leading to nuclear translocation and subsequent DNA double-strand breaks (DSBs). DSBs activate the DNA damage-sensing enzyme, Poly(ADP-ribose) (PAR) polymerase-1 (PARP-1) and the ATR/Chk1 branch of the DNA damage response. PARP-1 and DNase II engage in a positive feedback amplification loop mediated by the release of PAR polymers from the nucleus and the nuclear accumulation of DNase II in an AIF- and CypA-dependent manner, ultimately resulting in PGC death. Given the anatomical and molecular similarities with an ACD pathway called parthanatos, these findings reveal a parthanatos-like cell death pathway active during Drosophila development.

Maternally inherited intron coordinates primordial germ cell homeostasis during Drosophila embryogenesis

Primordial germ cells (PGCs) give rise to the germline stem cells (GSCs) in the adult Drosophila gonads. Both PGCs and GSCs need to be tightly regulated to safeguard the survival of the entire species. During larval development, a non-cell autonomous homeostatic mechanism is in place to maintain PGC number in the gonads. Whether such germline homeostasis occurs during early embryogenesis before PGCs reach the gonads remains unclear. We have previously shown that the maternally deposited sisRNA sisR-2 can influence GSC number in the female progeny. Here we uncover the presence of a homeostatic mechanism regulating PGCs during embryogenesis. sisR-2 represses PGC number by promoting PGC death. Surprisingly, increasing maternal sisR-2 leads to an increase in PGC death, but no drop in PGC number was observed. This is due to ectopic division of PGCs via the de-repression of Cyclin B, which is governed by a genetic pathway involving sisR-2, bantam and brat. We propose a cell autonomous model whereby germline homeostasis is achieved by preserving PGC number during embryogenesis.

Myc plays an important role in Drosophila P-M hybrid dysgenesis to eliminate germline cells with genetic damage

Genetic damage in the germline induced by P-element mobilization causes a syndrome known as P-M hybrid dysgenesis (HD), which manifests as elevated mutation frequency and loss of germline cells. In this study, we found that Myc plays an important role in eliminating germline cells in the context of HD. P-element mobilization resulted in downregulation of Myc expression in the germline. Myc knockdown caused germline elimination; conversely, Myc overexpression rescued the germline loss caused by P-element mobilization. Moreover, restoration of fertility by Myc resulted in the production of gametes with elevated mutation frequency and reduced ability to undergo development. Our results demonstrate that Myc downregulation mediates elimination of germline cells with accumulated genetic damage, and that failure to remove these cells results in increased production of aberrant gametes. Therefore, we propose that elimination of germline cells mediated by Myc downregulation is a quality control mechanism that maintains the genomic integrity of the germline.

Uninvited guests: how transposable elements take advantage of Drosophila germline stem cells, and how stem cells fight back

Transposable elements (TEs) are mobile genetic parasites that spread through host genomes by replicating in germline cells. New TE copies that arise in the genomes of germline stem cells (GSCs) are of particular value, because they are potentially transmitted to multiple offspring through the plethora of gametes arising from the same progenitor GSC. However, the fidelity of GSC genomes is also of utmost importance to the host in ensuring the production of abundant and fit offspring. Here we review tactics that TEs employ to replicate in Drosophila female GSCs, as well as mechanisms those cells use to defend against TEs. We also discuss the relationship between transposition and GSC loss, which is arbitrated through reduced signaling for self renewal, increased signaling for differentiation, and DNA damage response pathways.

Germ granules in Drosophila

Germ granules are hallmarks of all germ cells. Early ultrastructural studies in Drosophila first described these membraneless granules in the oocyte and early embryo as filled with amorphous to fibrillar material mixed with RNA. Genetic studies identified key protein components and specific mRNAs that regulate germ cell‐specific functions. More recently these ultrastructural studies have been complemented by biophysical analysis describing germ granules as phase‐transitioned condensates. In this review, we provide an overview that connects the composition of germ granules with their function in controlling germ cell specification, formation and migration, and illuminate these mysterious condensates as the gatekeepers of the next generation.

Heterogeneity of primordial germ cells

Primordial germ cells (PGCs) must complete a complex and dynamic developmental program during embryogenesis to establish the germline. This process is highly conserved and involves a diverse array of tasks required of PGCs, including migration, survival, sex differentiation, and extensive epigenetic reprogramming. A common theme across many organisms is that PGC success is heterogeneous: only a portion of all PGCs complete all these steps while many other PGCs are eliminated from further germline contribution. The differences that distinguish successful PGCs as a population are not well understood. Here, we examine variation that exists in PGCs as they navigate the many stages of this developmental journey. We explore potential sources of PGC heterogeneity and their potential implications in affecting germ cell behaviors. Lastly, we discuss the potential for PGC development to function as a multistage selection process that assesses heterogeneity in PGCs to refine germline quality.

Pgc suppresses the zygotically acting RNA decay pathway to protect germ plasm RNAs in the Drosophila embryo

Specification of germ cells is pivotal to ensure continuation of animal species. In many animal embryos, germ cell specification depends on maternally supplied determinants in the germ plasm. Drosophila polar granule component (pgc) mRNA is a component of the germ plasm. pgc encodes a small protein that is transiently expressed in newly formed pole cells, the germline progenitors, where it globally represses mRNA transcription. pgc is also required for pole cell survival, but the mechanism linking transcriptional repression to pole cell survival remains elusive. We report that pole cells lacking pgc show premature loss of germ plasm mRNAs, including the germ cell survival factor nanos, and undergo apoptosis. We found that pgc^- pole cells misexpress multiple miRNA genes. Reduction of miRNA pathway activity in pgc^- embryos partially suppressed germ plasm mRNA degradation and pole cell death, suggesting that Pgc represses zygotic miRNA transcription in pole cells to protect germ plasm mRNAs. Interestingly, germ plasm mRNAs are protected from miRNA-mediated degradation in vertebrates, albeit by a different mechanism. Thus, independently evolved mechanisms are used to silence miRNAs during germ cell specification.

Distinct cis-acting elements mediate targeting and clustering of Drosophila polar granule mRNAs

Specification and development of Drosophila germ cells depend on molecular determinants within the germ plasm, a specialized cytoplasmic domain at the posterior of the embryo. Localization of numerous mRNAs to the germ plasm occurs by their incorporation, as single-transcript ribonucleoprotein (RNP) particles, into complex RNP granules called polar granules. Incorporation of mRNAs into polar granules is followed by recruitment of additional like transcripts to form discrete homotypic clusters. The cis-acting localization signals that target mRNAs to polar granules and promote homotypic clustering remain largely uncharacterized. Here, we show that the polar granule component (pgc) and germ cell-less (gcl) 3' untranslated regions contain complex localization signals comprising multiple, independently weak and partially functionally redundant localization elements (LEs). We demonstrate that targeting of pgc to polar granules and self-assembly into homotypic clusters are functionally separable processes mediated by distinct classes of LEs. We identify a sequence motif shared by other polar granule mRNAs that contributes to homotypic clustering. Our results suggest that mRNA localization signal complexity may be a feature required by the targeting and self-recruitment mechanism that drives germ plasm mRNA localization.

Phase transitioned nuclear Oskar promotes cell division of Drosophila primordial germ cells

Germ granules are non-membranous ribonucleoprotein granules deemed the hubs for post-transcriptional gene regulation and functionally linked to germ cell fate across species. Little is known about the physical properties of germ granules and how these relate to germ cell function. Here we study two types of germ granules in the Drosophila embryo: cytoplasmic germ granules that instruct primordial germ cells (PGCs) formation and nuclear germ granules within early PGCs with unknown function. We show that cytoplasmic and nuclear germ granules are phase transitioned condensates nucleated by Oskar protein that display liquid as well as hydrogel-like properties. Focusing on nuclear granules, we find that Oskar drives their formation in heterologous cell systems. Multiple, independent Oskar protein domains synergize to promote granule phase separation. Deletion of Oskar’s nuclear localization sequence specifically ablates nuclear granules in cell systems. In the embryo, nuclear germ granules promote germ cell divisions thereby increasing PGC number for the next generation.

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

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

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

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Maternal Tdrd6a coordinates transcript deposition into future PGCs

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

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

Stochastic Seeding Coupled with mRNA Self-Recruitment Generates Heterogeneous Drosophila Germ Granules

The formation of ribonucleoprotein assemblies called germ granules is a conserved feature of germline development. In Drosophila, germ granules form at the posterior of the oocyte in a specialized cytoplasm called the germ plasm, which specifies germline fate during embryogenesis. mRNAs, including nanos (nos) and polar granule component (pgc), that function in germline development are localized to the germ plasm through their incorporation into germ granules, which deliver them to the primordial germ cells. Germ granules are nucleated by Oskar (Osk) protein and contain varying combinations and quantities of their constituent mRNAs, which are organized as spatially distinct, multi-copy homotypic clusters. The process that gives rise to such heterogeneous yet organized granules remains unknown. Here, we show that individual nos and pgc transcripts can populate the same nascent granule, and these first transcripts then act as seeds, recruiting additional like transcripts to form homotypic clusters. Within a granule, homotypic clusters grow independently of each other but depend on the simultaneous acquisition of additional Osk. Although granules can contain multiple clusters of a particular mRNA, granule mRNA content is dominated by cluster size. These results suggest that the accumulation of mRNAs in the germ plasm is controlled by the mRNAs themselves through their ability to form homotypic clusters; thus, RNA self-association drives germ granule mRNA localization. We propose that a stochastic seeding and self-recruitment mechanism enables granules to simultaneously incorporate many different mRNAs while ensuring that each becomes enriched to a functional threshold.

Domain-specific control of germ cell polarity and migration by multifunction Tre1 GPCR

Migrating cells encounter directional cues to reach their destinations, often using G protein–coupled receptors (GPCRs) to interpret such cues. LeBlanc and Lehmann show that two highly conserved domains in the GPCR Tre1 mediate distinct migratory responses in germ cells via separate signaling pathways, one regulating cell polarization and the other directional migration.

The migration of primordial germ cells (PGCs) from their place of origin to the embryonic gonad is an essential reproductive feature in many animal species. In Drosophila melanogaster, a single G protein–coupled receptor, Trapped in endoderm 1 (Tre1), mediates germ cell polarization at the onset of active migration and directs subsequent migration of PGCs through the midgut primordium. How these different aspects of cell behavior are coordinated through a single receptor is not known. We demonstrate that two highly conserved domains, the E/N/DRY and NPxxY motifs, have overlapping and unique functions in Tre1. The Tre1-NRY domain via G protein signaling is required for reading and responding to guidance and survival cues controlled by the lipid phosphate phosphatases Wunen and Wunen2. In contrast, the Tre1-NPIIY domain has a separate role in Rho1- and E-cadherin–mediated polarization at the initiation stage independent of G protein signaling. We propose that this bifurcation of the Tre1 G protein–coupled receptor signaling response via G protein–dependent and independent branches enables distinct spatiotemporal regulation of germ cell migration.