Control of germline stem cell self-renewal and differentiation in the Drosophila ovary: concerted actions of niche signals and intrinsic factors

Niche Tet maintains germline stem cells independently of dioxygenase activity

Ten-eleven translocation (TET) proteins are dioxygenases that convert 5-methylcytosine (5mC) into 5-hydroxylmethylcytosine (5hmC) in DNA and RNA. However, their involvement in adult stem cell regulation remains unclear. Here, we identify a novel enzymatic activity-independent function of Tet in the Drosophila germline stem cell (GSC) niche. Tet activates the expression of Dpp, the fly homologue of BMP, in the ovary stem cell niche, thereby controlling GSC self-renewal. Depletion of Tet disrupts Dpp production, leading to premature GSC loss. Strikingly, both wild-type and enzyme-dead mutant Tet proteins rescue defective BMP signaling and GSC loss when expressed in the niche. Mechanistically, Tet interacts directly with Bap55 and Stat92E, facilitating recruitment of the Polybromo Brahma associated protein (PBAP) complex to the dpp enhancer and activating Dpp expression. Furthermore, human TET3 can effectively substitute for Drosophila Tet in the niche to support BMP signaling and GSC self-renewal. Our findings highlight a conserved novel catalytic activity-independent role of Tet as a scaffold protein in supporting niche signaling for adult stem cell self-renewal.

Dpp-mediated TGF-β signaling regulates vitellogenesis through 20-hydroxyecdysone signaling in the cabbage beetle, Colaphellus bowringi

The Dpp signaling, as one of the branches within the TGF-β superfamily, plays a crucial role in regulating various biological processes in insects. However, its impact on female reproduction through vitellogenesis remains unclear. In this study, the expression profiles implied that the Dpp signaling genes, including Dpp, Punt, Mad, and Medea, were up-regulated during reproductive development in the ovary of Colaphellus bowringi. Knockdown of these five Dpp signaling genes revealed significant effects of Dpp, Tkv, Mad, and Medea on ovarian development through vitellogenesis in the fat body. Our finding further indicated that Dpp signaling influences the expression of 20-hydroxyecdysone (20E) receptor and responsive genes in the fat body. Additionally, knockdown of 20E receptor EcR resulted in similar phenotypes as observed in the Dpp pathway genes knockdown, implying a regulatory role for Dpp signaling via EcR in vitellogenesis. Furthermore, knocking down Dpp, Tkv, and EcR in female adults led to a reduction in total dry weight and protein content, as well as the expression of mTOR, a factor linked to protein intake. These results suggest that the Dpp signaling pathway modulates vitellogenesis by impacting the AA/TOR-mediated 20E pathway in the fat body, providing novel insights into the network governing insect reproduction and offering potential targets for controlling female pest reproduction.

A feedback loop between heterochromatin and the nucleopore complex controls germ-cell-to-oocyte transition during Drosophila oogenesis

Germ cells differentiate into oocytes that launch the next generation upon fertilization. How the highly specialized oocyte acquires this distinct cell fate is poorly understood. During Drosophila oogenesis, H3K9me3 histone methyltransferase SETDB1 translocates from the cytoplasm to the nucleus of germ cells concurrently with oocyte specification. Here, we discovered that nuclear SETDB1 is required for silencing a cohort of differentiation-promoting genes by mediating their heterochromatinization. Intriguingly, SETDB1 is also required for upregulating 18 of the ∼30 nucleoporins (Nups) that compose the nucleopore complex (NPC), promoting NPC formation. NPCs anchor SETDB1-dependent heterochromatin at the nuclear periphery to maintain H3K9me3 and gene silencing in the egg chambers. Aberrant gene expression due to the loss of SETDB1 or Nups results in the loss of oocyte identity, cell death, and sterility. Thus, a feedback loop between heterochromatin and NPCs promotes transcriptional reprogramming at the onset of oocyte specification, which is critical for establishing oocyte identity.

RNAi-mediated knockdown of α-Spectrin depresses reproductive performance in female Bactrocera dorsalis

The female reproductive potential plays a crucial role in reproduction, population dynamics and population maintenance. However, the function of endogenous genes in undifferentiated germ cells has been largely unknown in Bactrocera dorsalis. In this study, the conservative analysis showed that α-Spectrin shared a similarity in B. dorsalis and other dipteral flies. Further, the differential expression of α-Spectrin was examined in B. dorsalis by RT-qPCR, and the expression pattern of α-Spectrin protein was identified in female adult ovaries by using immunostaining. During the development of ovary, the change on the number of undifferentiated germ cells was also characterized and analyzed. To understand the function of α-Spectrin in B. dorsalis ovary, the RNAi-based knockdown was conducted, and the RNAi efficiency was examined by RT-qPCR, western blot and bioassay. The results revealed that the α-Spectrin dsRNA could strikingly decrease the expression level of α-Spectrin in ovaries and diminish oviposition and ovary size as a consequence of downregulation of α-Spectrin. Overall, our study facilitates reproductive research on the function of conservative genes in B. dorsalis ovary, which may provide a new insight into seeking novel target genes for pest management control.

Single-cell RNA sequencing identifies eggplant as a regulator of germ cell development in Drosophila

Drosophila ovarian germline stem cells (GSCs) are a powerful model for stem cell research. In this study, we use single-cell RNA sequencing (scRNA-seq), an RNAi screen and bioinformatic analysis, to identify genes involved in germ cell differentiation, including 34 genes with upregulated expression during early germ cell development and 19 genes that may regulate germ cell differentiation. Among these, a gene we have named eggplant (eggpl) is highly expressed in GSCs and downregulated in early daughter cells. RNAi knockdown of eggpl causes germ cell proliferation and differentiation defects. In flies fed a rich yeast diet, the expression of eggpl is significantly lower and knockdown or knockout of eggpl phenocopies a rich diet. In addition, eggpl knockdown suppresses the reduction in germ cell proliferation caused by inhibition of the insulin effector PI3K. These findings suggest that downregulation of eggpl links nutritional status to germ cell proliferation and differentiation. Collectively, this study provides new insights into the signaling networks that regulate early germ cell development and identifies eggpl as a key player in this process.

Novel roles for RNA binding proteins squid, hephaesteus, and Hrb27C in Drosophila oogenesis

BACKGROUND

Reproductive capacity in many organisms is maintained by germline stem cells (GSCs). A complex regulatory network influences stem cell fate, including intrinsic factors, local signals, and hormonal and nutritional cues. Posttranscriptional regulatory mechanisms ensure proper cell fate transitions, promoting germ cell differentiation to oocytes. As essential RNA binding proteins with constitutive functions in RNA metabolism, heterogeneous nuclear ribonucleoproteins (hnRNPs) have been implicated in GSC function and axis specification during oocyte development. HnRNPs support biogenesis, localization, maturation, and translation of nascent transcripts. Whether and individual hnRNPs specifically regulate GSC function has yet to be explored.

RESULTS

We demonstrate that hnRNPs are expressed in distinct patterns in the Drosophila germarium. We show that three hnRNPs, squid, hephaestus, and Hrb27C are cell-autonomously required in GSCs for their maintenance. Although these hnRNPs do not impact adhesion of GSCs to adjacent cap cells, squid and hephaestus (but not Hrb27C) are necessary for proper bone morphogenetic protein signaling in GSCs. Moreover, Hrb27C promotes proper GSC proliferation, whereas hephaestus promotes cyst division.

CONCLUSIONS

We find that hnRNPs are independently and intrinsically required in GSCs for their maintenance in adults. Our results support the model that hnRNPs play unique roles in stem cells essential for their self-renewal and proliferation.

The Vast Utility of Drosophila Oogenesis

In this chapter, we highlight examples of the diverse array of developmental, cellular, and biochemical insights that can be gained by using Drosophila melanogaster oogenesis as a model tissue. We begin with an overview of ovary development and adult oogenesis. Then we summarize how the adult Drosophila ovary continues to advance our understanding of stem cells, cell cycle, cell migration, cytoplasmic streaming, nurse cell dumping, and cell death. We also review emerging areas of study, including the roles of lipid droplets, ribosomes, and nuclear actin in egg development. Finally, we conclude by discussing the growing conservation of processes and signaling pathways that regulate oogenesis and female reproduction from flies to humans.

Epigenetic dynamics during germline development: insights from Drosophila and C. elegans

Gametogenesis produces the only cell type within a metazoan that contributes both genetic and epigenetic information to the offspring. Extensive epigenetic dynamics are required to express or repress gene expression in a precise spatiotemporal manner. On the other hand, early embryos must be extensively reprogrammed as they begin a new life cycle, involving intergenerational epigenetic inheritance. Seminal work in both Drosophila and C. elegans has elucidated the role of various regulators of epigenetic inheritance, including (1) histones, (2) histone-modifying enzymes, and (3) small RNA-dependent epigenetic regulation in the maintenance of germline identity. This review highlights recent discoveries of epigenetic regulation during the stepwise changes of transcription and chromatin structure that takes place during germline stem cell self-renewal, maintenance of germline identity, and intergenerational epigenetic inheritance. Findings from these two species provide precedence and opportunity to extend relevant studies to vertebrates.

Niche formation and function in developing tissue: studies from the Drosophila ovary

Adult stem cells have a unique ability to self-renew and to generate differentiated daughter cells that are required in the body tissues. The identity of adult stem cells is maintained by extrinsic signals from other cell types, known as niche cells. Thus, the niche is required for appropriate tissue homeostasis. Niche is formed and recruits stem cells during tissue development; therefore, it is essential to establish niche cells and stem cells in proper numbers during development. A small niche may recruit too few stem cells and cause tissue degeneration, while a large niche may maintain too many stem cells and lead to tumorigenesis. Given that vertebrate tissues are not suitable for large-scale forward genetics studies, the Drosophila ovary stands out as an excellent model for studying how multiple niche cell types and germ cells (GCs) are coordinately regulated in vivo. Recent studies are beginning to reveal how various signaling molecules regulate niche formation and how niche cells non-autonomously influence GC number. In this review, we summarize the ovarian niche structure, the key signaling pathways for niche formation, and how niche cells generate extrinsic factors to control GC proliferation during ovarian development. Video abstract.

The expression of Catsup in escort cells affects Drosophila ovarian stem cell niche establishment and germline stem cells self-renewal via Notch signaling

Stem cell niche provides extrinsic signals to maintain stem cell renewal or initiate cell differentiation. Drosophila niche is composed of somatic terminal filament cells, cap cells and escort cells. However, the underlying mechanism for the development of stem cell niche remains largely unclear. Here we found that the expression of a zinc transporter Catsup is essential for ovary morphogenesis. Catsup knockdown in escort cells results in defects of niche establishment and germline stem cells self-renewal. These defects could be modified by altered expression of genes involved in zinc metabolism or intervention of dietary zinc levels. Further studies indicated that Catsup RNAi affected adult ovary morphogenesis by suppressing Notch signaling. Lastly, we demonstrated that the defects of Catsup RNAi could be restored by overexpression of heat shock cognate protein 70 (Hsc70). These findings expand our understanding of the mechanisms controlling adult oogenesis and niche establishment in Drosophila.

Cyromazine affects the ovarian germ cells of Drosophila via the ecdysone signaling pathway

Cyromazine, an insect growth regulator, has been extensively used against the insect pests of livestock and households. Previously, it was observed that the continuous selection of cyromazine from the larval to the adult stage decreased the number of germline stem cells (GSCs) and cystoblasts (CBs) in the adult ovary. In addition, in this study, we observed that the number of primordial germ cells (PGCs) was also decreased in the larval ovary after treatment with cyromazine. However, the mechanism by which it affects the germ cells is yet to be explored. Consequently, to deeply investigate the effects of cyromazine on the germ cells, we performed tissue-specific RNA sequencing. Bioinformatics analysis revealed that the ecdysone signaling pathway was significantly influenced under cyromazine stress. Based on that, we screened and selected 14 ecdysone signaling responsive genes and silenced their expression in the germ cells only. Results of that showed a considerable reduction in the number of germ cells. Furthermore, we mixed exogenous 20E with the cyromazine-containing diet to rescue the ecdysone signaling. Our results supported that the application of exogenous 20E significantly rescued the germ cells in the transgenic lines. Therefore, this implies that the cyromazine decreased the number of germ cells by affecting the ecdysone signaling pathway.

A dual role of lola in Drosophila ovary development: regulating stem cell niche establishment and repressing apoptosis

In Drosophila ovary, niche is composed of somatic cells, including terminal filament cells (TFCs), cap cells (CCs) and escort cells (ECs), which provide extrinsic signals to maintain stem cell renewal or initiate cell differentiation. Niche establishment begins in larval stages when terminal filaments (TFs) are formed, but the underlying mechanism for the development of TFs remains largely unknown. Here we report that transcription factor longitudinals lacking (Lola) is essential for ovary morphogenesis. We showed that Lola protein was expressed abundantly in TFCs and CCs, although also in other cells, and lola was required for the establishment of niche during larval stage. Importantly, we found that knockdown expression of lola induced apoptosis in adult ovary, and that lola affected adult ovary morphogenesis by suppressing expression of Regulator of cullins 1b (Roc1b), an apoptosis-related gene that regulates caspase activation during spermatogenesis. These findings significantly expand our understanding of the mechanisms controlling niche establishment and adult oogenesis in Drosophila.

Novel intrinsic factor Yun maintains female germline stem cell fate through Thickveins

Germline stem cells (GSCs) are critical for the reproduction of an organism. The self-renewal and differentiation of GSCs must be tightly controlled to avoid uncontrolled stem cell proliferation or premature stem cell differentiation. However, how the self-renewal and differentiation of GSCs are properly controlled is not fully understood. Here, we find that the novel intrinsic factor Yun is required for female GSC maintenance in Drosophila. GSCs undergo precocious differentiation due to de-repression of differentiation factor Bam by defective BMP/Dpp signaling in the absence of yun. Mechanistically, Yun associates with and stabilizes Thickveins (Tkv), the type I receptor of Dpp/BMP signaling. Finally, ectopic expression of a constitutively active Tkv (Tkv^QD) completely suppresses GSC loss caused by yun depletion. Collectively, these data demonstrate that Yun functions through Tkv to maintain GSC fate. Our results provide new insight into the regulatory mechanisms of how stem cell maintenance is properly controlled.

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Novel intrinsic factor Yun is required for female GSC maintenance

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Yun-defective GSCs undergo differentiation due to Bam upregulation

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Yun associates with and stabilizes Tkv to regulate GSC maintenance

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GSC loss in the absence of yun could be rescued by constitutively active Tkv

Drosophila MESR4 Gene Ensures Germline Stem Cell Differentiation by Promoting the Transcription of bag of marbles

Ovarian germline stem cells (GSCs) of Drosophila melanogaster provide a valuable in vivo model to investigate how the adult stem cell identity is maintained and the differentiation of the daughter cells is regulated. GSCs are embedded into a specialized cellular microenvironment, the so-called stem cell niche. Besides the complex signaling interactions between the germ cells and the niche cells, the germ cell intrinsic mechanisms, such as chromatin regulation and transcriptional control, are also crucial in the decision about self-renewal and differentiation. The key differentiation regulator gene is the bag of marbles (bam), which is transcriptionally repressed in the GSCs and de-repressed in the differentiating daughter cell. Here, we show that the transcription factor MESR4 functions in the germline to promote GSC daughter differentiation. We find that the loss of MESR4 results in the accumulation of GSC daughter cells which fail to transit from the pre-cystoblast (pre-CB) to the differentiated cystoblast (CB) stage. The forced expression of bam can rescue this differentiation defect. By a series of epistasis experiments and a transcriptional analysis, we demonstrate that MESR4 positively regulates the transcription of bam. Our results suggest that lack of repression alone is not sufficient, but MESR4-mediated transcriptional activation is also required for bam expression.

Functional Divergence of the bag-of-marbles Gene in the Drosophila melanogaster Species Group

In Drosophila melanogaster, a key germline stem cell (GSC) differentiation factor, bag of marbles (bam) shows rapid bursts of amino acid fixations between sibling species D. melanogaster and Drosophila simulans, but not in the outgroup species Drosophila ananassae. Here, we test the null hypothesis that bam's differentiation function is conserved between D. melanogaster and four additional Drosophila species in the melanogaster species group spanning approximately 30 million years of divergence. Surprisingly, we demonstrate that bam is not necessary for oogenesis or spermatogenesis in Drosophila teissieri nor is bam necessary for spermatogenesis in D. ananassae. Remarkably bam function may change on a relatively short time scale. We further report tests of neutral sequence evolution at bam in additional species of Drosophila and find a positive, but not perfect, correlation between evidence for positive selection at bam and its essential role in GSC regulation and fertility for both males and females. Further characterization of bam function in more divergent lineages will be necessary to distinguish between bam's critical gametogenesis role being newly derived in D. melanogaster, D. simulans, Drosophila yakuba, and D. ananassae females or it being basal to the genus and subsequently lost in numerous lineages.

Cyromazine Effects the Reproduction of Drosophila by Decreasing the Number of Germ Cells in the Female Adult Ovary

Simple Summary

Cyromazine, an insect growth regulator, is used to control the Dipteran pest population. Previous findings observed that treatment with cyromazine increased the larval mortality, by interfering with the ecdysone signaling. In addition, the application of exogenous 20E significantly reduced the mortality caused by cyromazine. Many studies have also supported the role of ecdysone signaling in the maintenance of germline stem cells (GSCs), where mutations in ecdysone signaling-related genes significantly decreased the number of GSCs. However, to date, no study has reported the effect of cyromazine on the GSCs of Drosophila melanogaster. In the present study, we observed that cyromazine significantly reduced the number of both GSCs and cystoblasts (CBs) in the ovary of adult female. To further understand the effect of cyromazine on germ cells, we selected some key genes related to the ecdysone signaling pathway and evaluated their expression through RT-qPCR. Additionally, we measured the ecdysone titer from the cyromazine-treated ovaries. Our results indicated a significant decrease in the expression of ecdysone signaling-related genes and also in the ecdysone titer. These results further supported our findings that cyromazine reduced the number of germ cells by interfering with the ecdysone signaling pathway.

Abstract

In the present study, we observed a 58% decrease in the fecundity of Drosophila melanogaster, after treatment with the cyromazine. To further elucidate the effects of cyromazine on reproduction, we counted the number of both germline stem cells (GSCs) and cystoblasts (CBs) in the ovary of a 3-day-old adult female. The results showed a significant decrease in the number of GSCs and CBs as compared to the control group. The mode of action of cyromazine is believed to be through the ecdysone signaling pathway. To further support this postulate, we observed the expression of key genes involved in the ecdysone signaling pathway and also determined the ecdysone titer from cyromazine-treated ovaries. Results indicated a significant decrease in the expression of ecdysone signaling-related genes as compared to the control group. Furthermore, the titer of the ecdysone hormone was also markedly reduced (90%) in cyromazine-treated adult ovaries, suggesting that ecdysone signaling was directly related to the decrease in the number of GSCs and CBs. However, further studies are required to understand the mechanism by which cyromazine affects the GSCs and CBs in female adult ovaries.

Stonewall prevents expression of ectopic genes in the ovary and accumulates at insulator elements in D. melanogaster

Germline stem cells (GSCs) are the progenitor cells of the germline for the lifetime of an animal. In Drosophila, these cells reside in a cellular niche that is required for both their maintenance (self-renewal) and differentiation (asymmetric division resulting in a daughter cell that differs from the GSC). The stem cell—daughter cell transition is tightly regulated by a number of processes, including an array of proteins required for genome stability. The germline stem-cell maintenance factor Stonewall (Stwl) associates with heterochromatin, but its molecular function is poorly understood. We performed RNA-Seq on stwl mutant ovaries and found significant derepression of many transposon families but not heterochromatic genes. We also discovered inappropriate expression of multiple classes of genes. Most prominent are testis-enriched genes, including the male germline sex-determination switch Phf7, the differentiation factor bgcn, and a large testis-specific gene cluster on chromosome 2, all of which are upregulated or ectopically expressed in stwl mutant ovaries. Surprisingly, we also found that RNAi knockdown of stwl in somatic S2 cells results in ectopic expression of these testis genes. Using parallel ChIP-Seq and RNA-Seq experiments in S2 cells, we discovered that Stwl localizes upstream of transcription start sites and at heterochromatic sequences including repetitive sequences associated with telomeres. Stwl is also enriched at bgcn, suggesting that it directly regulates this essential differentiation factor. Finally, we identify Stwl binding motifs that are shared with known insulator binding proteins. We propose that Stwl affects gene regulation, including repression of male transcripts in the female germline, by binding insulators and establishing chromatin boundaries.

Stem cells are defined by their ability to divide asymmetrically, resulting in a differentiated cell and a stem cell daughter. In fruit flies, sperm and egg production begins with germline stem cells (GSCs). The ability of a GSC to differentiate or self-renew is tightly regulated by a myriad of factors. Some of these are transcription factors, which are responsible for activating or suppressing other genes to promote one state in favor of another. Stonewall is an ovarian nuclear protein required for GSC self-renewal, whose molecular function is poorly understood. Here we show that Stonewall is responsible for preventing the activation of “male” molecular programming in the fruit fly ovary. When Stonewall is absent from the ovary, egg production is terminated and testis-specific genes become highly expressed, including the male transcript of Phf7, which induces male sexual identity in female germ cells. We also show that Stonewall is likely localizing to genomic insulators, which are regions of the genome that shield genes from nearby regulators. Our findings suggest that Stonewall helps to organize the genome in ovarian germ cells and prevent expression of male genes.

Diverse wMel variants of Wolbachia pipientis differentially rescue fertility and cytological defects of the bag of marbles partial loss of function mutation in Drosophila melanogaster

In Drosophila melanogaster, the maternally inherited endosymbiont Wolbachia pipientis interacts with germline stem cell genes during oogenesis. One such gene, bag of marbles (bam) is the key switch for differentiation and also shows signals of adaptive evolution for protein diversification. These observations have led us to hypothesize that W. pipientis could be driving the adaptive evolution of bam for control of oogenesis. To test this hypothesis, we must understand the specificity of the genetic interaction between bam and W. pipientis. Previously, we documented that the W. pipientis variant, wMel, rescued the fertility of the bamBW hypomorphic mutant as a transheterozygote over a bam null. However, bamBW was generated more than 20 years ago in an uncontrolled genetic background and maintained over a balancer chromosome. Consequently, the chromosome carrying bamBW accumulated mutations that have prevented controlled experiments to further assess the interaction. Here, we used CRISPR/Cas9 to engineer the same single amino acid bam hypomorphic mutation (bamL255F) and a new bam null disruption mutation into the w1118 isogenic background. We assess the fertility of wildtype bam, bamL255F/bamnull hypomorphic, and bamL255F/bamL255F mutant females, each infected individually with 10 W. pipientis wMel variants representing three phylogenetic clades. Overall, we find that all of the W. pipientis variants tested here rescue bam hypomorphic fertility defects with wMelCS-like variants exhibiting the strongest rescue effects. In addition, these variants did not increase wildtype bam female fertility. Therefore, both bam and W. pipientis interact in genotype-specific ways to modulate female fertility, a critical fitness phenotype.

The Drosophila gonads: models for stem cell proliferation, self-renewal, and differentiation

The male and female gonads of Drosophila melanogaster have developed into powerful model systems for both the study of stem cell behaviours, and for understanding how stem cell misregulation can lead to cancers. Using these systems, one is able to observe and manipulate the resident stem cell populations in vivo with a great deal of licence. The tractability of the testis and ovary also allow researchers to explore a range of cellular mechanisms, such as proliferation and polarity, as well as the influence exerted by the local environment through a host of highly-conserved signalling pathways. Importantly, many of the cellular behaviours and processes studied in the Drosophila testis and ovary are known to be disrupted, or otherwise misregulated, in human tumourigenic cells. Here, we review the mechanisms relating to stem cell behaviour, though we acknowledge there are many other fascinating aspects of gametogenesis, including the invasive behaviour of migratory border cells in the Drosophila ovary that, though relevant to the study of tumourigenesis, will unfortunately not be covered.

The tumour suppressor brain tumour (Brat) regulates linker histone dBigH1 expression in the Drosophila female germline and the early embryo

Linker histones H1 are essential chromatin components that exist as multiple developmentally regulated variants. In metazoans, specific H1s are expressed during germline development in a tightly regulated manner. However, the mechanisms governing their stage-dependent expression are poorly understood. Here, we address this question in Drosophila, which encodes for a single germline-specific dBigH1 linker histone. We show that during female germline lineage differentiation, dBigH1 is expressed in germ stem cells and cystoblasts, becomes silenced during transit-amplifying (TA) cystocytes divisions to resume expression after proliferation stops and differentiation starts, when it progressively accumulates in the oocyte. We find that dBigH1 silencing during TA divisions is post-transcriptional and depends on the tumour suppressor Brain tumour (Brat), an essential RNA-binding protein that regulates mRNA translation and stability. Like other oocyte-specific variants, dBigH1 is maternally expressed during early embryogenesis until it is replaced by somatic dH1 at the maternal-to-zygotic transition (MZT). Brat also mediates dBigH1 silencing at MZT. Finally, we discuss the situation in testes, where Brat is not expressed, but dBigH1 is translationally silenced too.

Mitochondrial Control of Stem Cell State and Fate: Lessons From Drosophila

Over the years, Drosophila has served as a wonderful genetically tractable model system to unravel various facets of tissue-resident stem cells in their microenvironment. Studies in different stem and progenitor cell types of Drosophila have led to the discovery of cell-intrinsic and extrinsic factors crucial for stem cell state and fate. Though initially touted as the ATP generating machines for carrying various cellular processes, it is now increasingly becoming clear that mitochondrial processes alone can override the cellular program of stem cells. The last few years have witnessed a surge in our understanding of mitochondria's contribution to governing different stem cell properties in their subtissular niches in Drosophila. Through this review, we intend to sum up and highlight the outcome of these in vivo studies that implicate mitochondria as a central regulator of stem cell fate decisions; to find the commonalities and uniqueness associated with these regulatory mechanisms.

Multiple Niche Compartments Orchestrate Stepwise Germline Stem Cell Progeny Differentiation

The niche controls stem cell self-renewal and progenitor differentiation for maintaining adult tissue homeostasis in various organisms. However, it remains unclear whether the niche is compartmentalized to control stem cell self-renewal and stepwise progeny differentiation. In the Drosophila ovary, inner germarial sheath (IGS) cells form a niche for controlling germline stem cell (GSC) progeny differentiation. In this study, we have identified four IGS subpopulations, which form linearly arranged niche compartments for controlling GSC maintenance and multi-step progeny differentiation. Single-cell analysis of the adult ovary has identified four IGS subpopulations (IGS1-IGS4), the identities and cellular locations of which have been further confirmed by fluorescent in situ hybridization. IGS1 and IGS2 physically interact with GSCs and mitotic cysts to control GSC maintenance and cyst formation, respectively, whereas IGS3 and IGS4 physically interact with 16-cell cysts to regulate meiosis, oocyte development, and cyst morphological change. Finally, one follicle cell progenitor population has also been transcriptionally defined for facilitating future studies on follicle stem cell regulation. Therefore, this study has structurally revealed that the niche is organized into multiple compartments for orchestrating stepwise adult stem cell development and has also provided useful resources and tools for further functional characterization of the niche in the future.

The apical cell - An enigmatic somatic cell in leech ovaries - Structure and putative functions

Although somatic cells play an integral role in animal gametogenesis, their organization and function are usually poorly characterized, especially in non-model systems. One such example is a peculiar cell found in leech ovaries - the apical cell (AC). A single AC can be found at the apical tip of each ovary cord, the functional unit of leech ovaries, where it is surrounded by other somatic and germline cells. The AC is easily distinguished due to its enormous size and its numerous long cytoplasmic projections that penetrate the space between neighboring cells. It is also characterized by a prominent accumulation of mitochondria, Golgi complexes and electron-dense vesicles. ACs are also enriched in cytoskeleton, mainly in form of intermediate filaments. Additionally, the AC is connected to neighboring cells via junctions that structurally resemble hemidesmosomes. In spite of numerous descriptive data about the AC, its functions remain poorly understood. Its suggested functions include a role in forming skeleton for the germline cells, and a role in defining a niche for germline stem cells. The latter is more speculative, since germline stem cells have not been identified in leech ovaries. Somatic cells with similar morphological properties to those of the AC have been found in gonads of nematodes - the distal tip cell - and in insects - Verson's cell, hub cells and cap cells. In the present article we summarize information about the AC structure and its putative functions. AC is compared with other well-described somatic cells with potentially similar roles in gametogenesis.

Dlp-mediated Hh and Wnt signaling interdependence is critical in the niche for germline stem cell progeny differentiation

Although multiple signaling pathways work synergistically in various niches to control stem cell self-renewal and differentiation, it remains poorly understood how they cooperate with one another molecularly. In the Drosophila ovary, Hh and Wnt pathways function in the niche to promote germline stem cell (GSC) progeny differentiation. Here, we show that glypican Dlp-mediated Hh and Wnt signaling interdependence operates in the niche to promote GSC progeny differentiation by preventing BMP signaling. Hh/Wnt-mediated dlp repression is essential for their signaling interdependence in niche cells and for GSC progeny differentiation by preventing BMP signaling. Mechanistically, Hh and Wnt downstream transcription factors directly bind to the same dlp regulatory region and recruit corepressors composed of transcription factor Croc and Egg/H3K9 trimethylase to repress Dlp expression. Therefore, our study reveals a novel mechanism for Hh/Wnt signaling-mediated direct dlp repression and a novel regulatory mechanism for Dlp-mediated Hh/Wnt signaling interdependence in the GSC differentiation niche.

Coordinating Proliferation, Polarity, and Cell Fate in the Drosophila Female Germline

Gametes are highly specialized cell types produced by a complex differentiation process. Production of viable oocytes requires a series of precise and coordinated molecular events. Early in their development, germ cells are an interconnected group of mitotically dividing cells. Key regulatory events lead to the specification of mature oocytes and initiate a switch to the meiotic cell cycle program. Though the chromosomal events of meiosis have been extensively studied, it is unclear how other aspects of oocyte specification are temporally coordinated. The fruit fly, Drosophila melanogaster, has long been at the forefront as a model system for genetics and cell biology research. The adult Drosophila ovary continuously produces germ cells throughout the organism’s lifetime, and many of the cellular processes that occur to establish oocyte fate are conserved with mammalian gamete development. Here, we review recent discoveries from Drosophila that advance our understanding of how early germ cells balance mitotic exit with meiotic initiation. We discuss cell cycle control and establishment of cell polarity as major themes in oocyte specification. We also highlight a germline-specific organelle, the fusome, as integral to the coordination of cell division, cell polarity, and cell fate in ovarian germ cells. Finally, we discuss how the molecular controls of the cell cycle might be integrated with cell polarity and cell fate to maintain oocyte production.

Drosophila YBX1 homolog YPS promotes ovarian germ line stem cell development by preferentially recognizing 5-methylcytosine RNAs

5-Methylcytosine (m⁵C) is a RNA modification that exists in tRNAs and rRNAs and was recently found in mRNAs. Although it has been suggested to regulate diverse biological functions, whether m⁵C RNA modification influences adult stem cell development remains undetermined. In this study, we show that Ypsilon schachtel (YPS), a homolog of human Y box binding protein 1 (YBX1), promotes germ line stem cell (GSC) maintenance, proliferation, and differentiation in the Drosophila ovary by preferentially binding to m⁵C-containing RNAs. YPS is genetically demonstrated to function intrinsically for GSC maintenance, proliferation, and progeny differentiation in the Drosophila ovary, and human YBX1 can functionally replace YPS to support normal GSC development. Highly conserved cold-shock domains (CSDs) of YPS and YBX1 preferentially bind to m⁵C RNA in vitro. Moreover, YPS also preferentially binds to m⁵C-containing RNAs, including mRNAs, in germ cells. The crystal structure of the YBX1 CSD-RNA complex reveals that both hydrophobic stacking and hydrogen bonds are critical for m⁵C binding. Overexpression of RNA-binding-defective YPS and YBX1 proteins disrupts GSC development. Taken together, our findings show that m⁵C RNA modification plays an important role in adult stem cell development.

A single-cell atlas of the developing Drosophila ovary identifies follicle stem cell progenitors

Addressing the complexity of organogenesis at a system-wide level requires a complete understanding of adult cell types, their origin, and precursor relationships. The Drosophila ovary has been a model to study how coordinated stem cell units, germline, and somatic follicle stem cells maintain and renew an organ. However, lack of cell type-specific tools have limited our ability to study the origin of individual cell types and stem cell units. Here, we used a single-cell RNA sequencing approach to uncover all known cell types of the developing ovary, reveal transcriptional signatures, and identify cell type-specific markers for lineage tracing. Our study identifies a novel cell type corresponding to the elusive follicle stem cell precursors and predicts subtypes of known cell types. Altogether, we reveal a previously unanticipated complexity of the developing ovary and provide a comprehensive resource for the systematic analysis of ovary morphogenesis.

Nuclear architecture as an intrinsic regulator of Drosophila female germline stem cell maintenance

Homeostasis of Drosophila germline stem cells (GSC) depends upon the integration of intrinsic and extrinsic signals. This review highlights emerging data that support nuclear architecture as an intrinsic regulator of GSC maintenance and germ cell differentiation. Here, we focus on the nuclear lamina (NL) and the nucleolus, two compartments that undergo alterations in composition upon germ cell differentiation. Loss of NL or nucleolar components leads to GSC loss, resulting from activation of GSC quality control checkpoint pathways. We suggest that the NL and nucleolus integrate signals needed for the switch between GSC maintenance and germ cell differentiation, and propose regulation of nuclear actin pools as one mechanism that connects these compartments.

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.

Special vulnerability of somatic niche cells to transposable element activation in Drosophila larval ovaries

In the Drosophila ovary, somatic escort cells (ECs) form a niche that promotes differentiation of germline stem cell (GSC) progeny. The piRNA (Piwi-interacting RNA) pathway, which represses transposable elements (TEs), is required in ECs to prevent the accumulation of undifferentiated germ cells (germline tumor phenotype). The soma-specific piRNA cluster flamenco (flam) produces a substantial part of somatic piRNAs. Here, we characterized the biological effects of somatic TE activation on germ cell differentiation in flam mutants. We revealed that the choice between normal and tumorous phenotypes of flam mutant ovaries depends on the number of persisting ECs, which is determined at the larval stage. Accordingly, we found much more frequent DNA breaks in somatic cells of flam larval ovaries than in adult ECs. The absence of Chk2 or ATM checkpoint kinases dramatically enhanced oogenesis defects of flam mutants, in contrast to the germline TE-induced defects that are known to be mostly suppressed by сhk2 mutation. These results demonstrate a crucial role of checkpoint kinases in protecting niche cells against deleterious TE activation and suggest substantial differences between DNA damage responses in ovarian somatic and germ cells.

Molecular profiling of stem cell-like female germ line cells in Drosophila delineates networks important for stemness and differentiation

Stem cells can self-renew and produce daughter cells destined for differentiation. The precise control of the balance between these two outcomes is essential to ensure tissue homeostasis and to prevent uncontrolled proliferation resulting in tumor formation. As self-renewal and differentiation are likely to be controlled by different gene expression programs, unraveling the underlying gene regulatory networks is crucial for understanding the molecular logic of this system. In this study, we have characterized by next generation RNA sequencing (RNA-seq) the transcriptome of germline stem cell (GSC)-like cells isolated from bag of marbles (bam) mutant Drosophila ovaries and compared it to the transcriptome of germ line cells isolated from wild-type ovaries. We have complemented this dataset by utilizing an RNA-immunoprecipitation strategy to identify transcripts bound to the master differentiation factor Bam. Protein complex enrichment analysis on these combined datasets allows us to delineate known and novel networks essential for GSC maintenance and differentiation. Further comparative transcriptomics illustrates similarities between GSCs and primordial germ cells and provides a molecular footprint of the stem cell state. Our study represents a useful resource for functional studies on stem cell maintenance and differentiation.

Summary: Fruit fly germline stem cell differentiation is accompanied by major changes of the transcriptome that may be regulated at the post-transcriptional level.

Defining the expression of piRNA and transposable elements in Drosophila ovarian germline stem cells and somatic support cells

Comprehensive transcriptional characterization of mRNA and small RNA in early Drosophila germline stem cells reveals novel piRNA clusters, transposon dynamics, and alternative splicing events.

Piwi-interacting RNAs (piRNAs) are important for repressing transposable elements (TEs) and modulating gene expression in germ cells, thereby maintaining genome stability and germ cell function. Although they are also important for maintaining germline stem cells (GSCs) in the Drosophila ovary by repressing TEs and preventing DNA damage, piRNA expression has not been investigated in GSCs or their early progeny. Here, we show that the canonical piRNA clusters are more active in GSCs and their early progeny than late germ cells and also identify more than 3,000 new piRNA clusters from deep sequencing data. The increase in piRNAs in GSCs and early progeny can be attributed to both canonical and newly identified piRNA clusters. As expected, piRNA clusters in GSCs, but not those in somatic support cells (SCs), exhibit ping-pong signatures. Surprisingly, GSCs and early progeny express more TE transcripts than late germ cells, suggesting that the increase in piRNA levels may be related to the higher levels of TE transcripts in GSCs and early progeny. GSCs also have higher piRNA levels and lower TE levels than SCs. Furthermore, the 3′ UTRs of 171 mRNA transcripts may produce sense, antisense, or dual-stranded piRNAs. Finally, we show that alternative promoter usage and splicing are frequently used to modulate gene function in GSCs and SCs. Overall, this study has provided important insight into piRNA production and TE repression in GSCs and SCs. The rich information provided by this study will be a beneficial resource to the fields of piRNA biology and germ cell development.

Local and Physiological Control of Germline Stem Cell Lineages in Drosophila melanogaster

The long-term survival of any multicellular species depends on the success of its germline in producing high-quality gametes and maximizing survival of the offspring. Studies in Drosophila melanogaster have led our growing understanding of how germline stem cell (GSC) lineages maintain their function and adjust their behavior according to varying environmental and/or physiological conditions. This review compares and contrasts the local regulation of GSCs by their specialized microenvironments, or niches; discusses how diet and diet-dependent factors, mating, and microorganisms modulate GSCs and their developing progeny; and briefly describes the tie between physiology and development during the larval phase of the germline cycle. Finally, it concludes with broad comparisons with other organisms and some future directions for further investigation.

The exocyst functions in niche cells to promote germline stem cell differentiation by directly controlling EGFR membrane trafficking

The niche controls stem cell self-renewal and differentiation in animal tissues. Although the exocyst is known to be important for protein membrane trafficking and secretion, its role in stem cells and niches has never been reported. Here, this study shows that the exocyst functions in the niche to promote germline stem cell (GSC) progeny differentiation in the Drosophila ovary by directly regulating EGFR membrane trafficking and signaling. Inactivation of exocyst components in inner germarial sheath cells, which form the differentiation niche, causes a severe GSC differentiation defect. The exocyst is required for maintaining niche cells and preventing BMP signaling in GSC progeny by promoting EGFR membrane targeting and signaling through direct association with EGFR. Finally, it is also required for EGFR membrane targeting, recycling and signaling in human cells. Therefore, this study reveals a novel function of the exocyst in niche cells to promote stem cell progeny differentiation by directly controlling EGFR membrane trafficking and signaling in vivo, and also provides important insight into how the niche controls stem cell progeny differentiation at the molecular level.

Loss of putzig in the germline impedes germ cell development by inducing cell death and new niche like microenvironments

Germline stem cell development and differentiation is tightly controlled by the surrounding somatic cells of the stem cell niche. In Drosophila females, cells of the niche emit various signals including Dpp and Wg to balance stem cell renewal and differentiation. Here, we show that the gene pzg is autonomously required in cells of the germline to sustain the interplay between niche and stem cells. Loss of pzg impairs stem cell differentiation and provokes the death of cells in the germarium. As a consequence of pzg loss, increased growth signalling activity predominantly of Dpp and Wg/Wnt, was observed, eventually disrupting the balance of germ cell self-renewal and differentiation. Whereas in the soma, apoptosis-induced compensatory growth is well established, the induction of self-renewal signals during oogenesis cannot compensate for dying germ cells, albeit inducing a new niche-like microenvironment. Instead, they impair the further development of germ cells and cause in addition a forward and feedback loop of cell death.

Effects of cadmium on fecundity and defence ability of Drosophila melanogaster

Cadmium (chemical symbol, Cd) is an extremely common pollutant that poses a toxicity threat to organisms. Therefore, we tested Drosophila melanogaster fecundity, Cd accumulation, and activity of two enzymes following Cd stress and used quantitative real-time polymerase chain reaction (qPCR) to quantify the mRNA expression levels of several genes involved in fecundity and defence. D. melanogaster was placed in a medium containing different concentrations of Cd (13, 26, and 52 mg L^-1), following which, inductively coupled plasma atomic emission spectroscopy showed that Cd accumulation in Drosophila increased with the increase in its dietary intake. We also observed that Cd at these concentrations significantly prolonged the mating latency in females and reduced the number of eggs laid. However, the same Cd concentrations did not affect male fecundity. Acetylcholinesterase activity was only detected at 52 mg L^-1 Cd in both sexes, whereas glutathione S-transferase activity was inhibited at 26 and 52 mg L^-1 Cd in females. The results of qPCR indicated that exposure to 13-52 mg L^-1 Cd affected the expression of reproduction-related genes, including downregulation of enok and upregulation of dally and dpp. The same level of exposure also induced transcriptional responses from three defence-related genes (hsp70, gstd2, and gstd6). Taken together, the results revealed that Cd exposure might negatively affect the expression of genes associated with D. melanogaster reproduction and trigger the transcription of defence-related genes. We suggest that further analyses of fecundity and defence responses may help develop indicators of Cd toxicity and improve our understanding of antitoxin defences.

Regulation of Drosophila germline stem cells

The asymmetric division of adult stem cells into one self-renewing stem cell and one differentiating cell is critical for maintaining homeostasis in many tissues. One paradigmatic model of this division is the Drosophila male and female germline stem cell, which provides two model systems not only sharing common features but also having distinct characteristics for studying asymmetric stem cell division in vivo. This asymmetric division is controlled by a combination of extrinsic signaling molecules and intrinsic factors that are either asymmetrically segregated or regulated differentially following division. In this review, we will discuss recent advances in understanding the molecular and cellular mechanisms guiding this asymmetric outcome, including extrinsic cues, intrinsic factors governing cell fate specification, and cell cycle control.

Srlp is crucial for the self-renewal and differentiation of germline stem cells via RpL6 signals in Drosophila testes

Self-renewal and differentiation in germline stem cells (GSCs) are tightly regulated by the stem cell niche and via multiple approaches. In our previous study, we screened the novel GSC regulatory gene Srlp in Drosophila testes. However, the underlying mechanistic links between Srlp and the stem cell niche remain largely undetermined. Here, using genetic manipulation of the Drosophila model, we systematically analyze the function and mechanism of Srlp in vivo and in vitro. In Drosophila, Srlp is an essential gene that regulates the self-renewal and differentiation of GSCs in the testis. In the in vitro assay, Srlp is found to control the proliferation ability and cell death in S2 cells, which is consistent with the phenotype observed in Drosophila testis. Furthermore, results of the liquid chromatography-tandem mass spectrometry (LC-MS/MS) reveal that RpL6 binds to Srlp. Srlp also regulates the expression of spliceosome and ribosome subunits and controls spliceosome and ribosome function via RpL6 signals. Collectively, our findings uncover the genetic causes and molecular mechanisms underlying the stem cell niche. This study provides new insights for elucidating the pathogenic mechanism of male sterility and the formation of testicular germ cell tumor.

Heterochromatin protein 1 (HP1) is intrinsically required for post-transcriptional regulation of Drosophila Germline Stem Cell (GSC) maintenance

A very important open question in stem cells regulation is how the fine balance between GSCs self-renewal and differentiation is orchestrated at the molecular level. In the past several years much progress has been made in understanding the molecular mechanisms underlying intrinsic and extrinsic controls of GSC regulation but the complex gene regulatory networks that regulate stem cell behavior are only partially understood. HP1 is a dynamic epigenetic determinant mainly involved in heterochromatin formation, epigenetic gene silencing and telomere maintenance. Furthermore, recent studies have revealed the importance of HP1 in DNA repair, sister chromatid cohesion and, surprisingly, in positive regulation of gene expression. Here, we show that HP1 plays a crucial role in the control of GSC homeostasis in Drosophila. Our findings demonstrate that HP1 is required intrinsically to promote GSC self-renewal and progeny differentiation by directly stabilizing the transcripts of key genes involved in GSCs maintenance.

Defining gene networks controlling the maintenance and function of the differentiation niche by an in vivo systematic RNAi screen

In the Drosophila ovary, escort cells (ECs) extrinsically control germline stem cell (GSC) maintenance and progeny differentiation. However, the underlying mechanisms remain poorly understood. In this study, we identified 173 EC genes for their roles in controlling GSC maintenance and progeny differentiation by using an in vivo systematic RNAi approach. Of the identified genes, 10 and 163 are required in ECs to promote GSC maintenance and progeny differentiation, respectively. The genes required for progeny differentiation fall into different functional categories, including transcription, mRNA splicing, protein degradation, signal transduction and cytoskeleton regulation. In addition, the GSC progeny differentiation defects caused by defective ECs are often associated with BMP signaling elevation, indicating that preventing BMP signaling is a general functional feature of the differentiation niche. Lastly, exon junction complex (EJC) components, which are essential for mRNA splicing, are required in ECs to promote GSC progeny differentiation by maintaining ECs and preventing BMP signaling. Therefore, this study has identified the major regulators of the differentiation niche, which provides important insights into how stem cell progeny differentiation is extrinsically controlled.

Ovaries absent links dLsd1 to HP1a for local H3K4 demethylation required for heterochromatic gene silencing

Heterochromatin Protein 1 (HP1) is a conserved chromosomal protein in eukaryotic cells that has a major role in directing heterochromatin formation, a process that requires co-transcriptional gene silencing mediated by small RNAs and their associated argonaute proteins. Heterochromatin formation requires erasing the active epigenetic mark, such as H3K4me2, but the molecular link between HP1 and H3K4 demethylation remains unclear. In a fertility screen in female Drosophila, we identified ovaries absent (ova), which functions in the stem cell niche, downstream of Piwi, to support germline stem cell differentiation. Moreover, ova acts as a suppressor of position effect variegation, and is required for silencing telomeric transposons in the germline. Biochemically, Ova acts to link the H3K4 demethylase dLsd1 to HP1a for local histone modifications. Therefore, our study provides a molecular connection between HP1a and local H3K4 demethylation during HP1a-mediated gene silencing that is required for ovary development, transposon silencing, and heterochromatin formation.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

The complete set of genetic material within a cell is known as a genome. The genomes of human and other animal cells have regions of active genes interspersed with ‘dark’ regions known as heterochromatin, which contain genes and other types of genetic material that have been inactivated.

Heterochromatin commonly contains sections of genetic material known as transposons. When a transposon is active it is able to move around the genome, therefore, inactivating (or ‘silencing’) transposons helps to maintain the integrity of the genetic material in a cell. It is particularly important to silence transposons in the stem cells that produce sperm and egg cells – known as germline stem cells – to ensure genetic information is faithfully passed on to the next generation.

A protein called HP1a plays a major role in directing where heterochromatin forms in the genome. This process requires an enzyme called dLsd1 to remove a small tag from the genetic material but it is not clear how HP1a regulates the activity of dLsd1. To address this question, Yang et al. studied how egg cells form in fruit flies, which are often used as models of animal biology in experiments.

The team screened a population of fruit flies that carried mutations in many different genes to identify genes that affect the fertility of female flies. This revealed a gene named as ovaries absent (or ova for short) is required for egg cells to form. In germline stem cells ova silences transposons and in the surrounding tissue it represses a specific signal that usually maintains stem cells to allow the stem cells to divide to make egg cells. Further experiments using biochemical techniques found that the protein encoded by ova acts as a bridge to bring HP1a and dLsd1 together to silence genes in heterochromatin.

The next step would be to identify the functional counterpart of the ova gene in mammals, including humans, which may help to discover causes of infertility and develop new fertility treatment.

Spatiotemporal Coordination of FGF and Shh Signaling Underlies the Specification of Myoblasts in the Zebrafish Embryo

Somitic cells give rise to a variety of cell types in response to Hh, BMP, and FGF signaling. Cell position within the developing zebrafish somite is highly dynamic: how, when, and where these signals specify cell fate is largely unknown. Combining four-dimensional imaging with pathway perturbations, we characterize the spatiotemporal specification and localization of somitic cells. Muscle formation is guided by highly orchestrated waves of cell specification. We find that FGF directly and indirectly controls the differentiation of fast and slow-twitch muscle lineages, respectively. FGF signaling imposes tight temporal control on Shh induction of slow muscles by regulating the time at which fast-twitch progenitors displace slow-twitch progenitors from contacting the Shh-secreting notochord. Further, we find a reciprocal regulation of fast and slow muscle differentiation, morphogenesis, and migration. In conclusion, robust cell fate determination in the developing somite requires precise spatiotemporal coordination between distinct cell lineages and signaling pathways.

Division-independent differentiation mandates proliferative competition among stem cells

Cancer-initiating gatekeeper mutations that arise in stem cells would be especially potent if they stabilize and expand an affected stem cell lineage. It is therefore important to understand how different stem cell organization strategies promote or prevent variant stem cell amplification in response to different types of mutation, including those that activate proliferation. Stem cell numbers can be maintained constant while producing differentiated products through individually asymmetrical division outcomes or by population asymmetry strategies in which individual stem cell lineages necessarily compete for niche space. We considered alternative mechanisms underlying population asymmetry and used quantitative modeling to predict starkly different consequences of altering proliferation rate: A variant, faster proliferating mutant stem cell should compete better only when stem cell division and differentiation are independent processes. For most types of stem cells, it has not been possible to ascertain experimentally whether division and differentiation are coupled. However, Drosophila follicle stem cells (FSCs) provided a favorable system with which to investigate population asymmetry mechanisms and also for measuring the impact of altered proliferation on competition. We found from detailed cell lineage studies that division and differentiation of an individual FSC are not coupled. We also found that FSC representation, reflecting maintenance and amplification, was highly responsive to genetic changes that altered only the rate of FSC proliferation. The FSC paradigm therefore provides definitive experimental evidence for the general principle that relative proliferation rate will always be a major determinant of competition among stem cells specifically when stem cell division and differentiation are independent.

A switch in the mode of Wnt signaling orchestrates the formation of germline stem cell differentiation niche in Drosophila

Germline stem cell (GSC) self-renewal and differentiation into gametes is regulated by both intrinsic factors in the germ line as well as extrinsic factors from the surrounding somatic niche. dWnt4, in the escort cells of the adult somatic niche promotes GSC differentiation using the canonical β-catenin-dependent transcriptional pathway to regulate escort cell survival, adhesion to the germ line and downregulation of self-renewal signaling. Here, we show that in addition to the β-catenin-dependent canonical pathway, dWnt4 also uses downstream components of the Wnt non-canonical pathway to promote escort cell function earlier in development. We find that the downstream non-canonical components, RhoA, Rac1 and cdc42, are expressed at high levels and are active in escort cell precursors of the female larval gonad compared to the adult somatic niche. Consistent with this expression pattern, we find that the non-canonical pathway components function in the larval stages but not in adults to regulate GSC differentiation. In the larval gonad, dWnt4, RhoA, Rac1 and cdc42 are required to promote intermingling of escort cell precursors, a function that then promotes proper escort cell function in the adults. We find that dWnt4 acts by modulating the activity of RhoA, Rac1 and cdc42, but not their protein levels. Together, our results indicate that at different points of development, dWnt4 switches from using the non-canonical pathway components to using a β-catenin-dependent canonical pathway in the escort cells to facilitate the proper differentiation of GSCs.

Germ line association with the somatic cells is critical for various aspects of germ cell biology, including migration, self-renewal and differentiation. In Drosophila females, soma–germ line association begins during embryogenesis and continues until the mature egg is formed. In the adult, the somatic escort cells promote differentiation of the germline stem cell daughter using Wnt signaling. dWnt4, a Wnt ligand, acts in an autocrine manner in these escort cells, using the canonical pathway to regulate survival, division and encapsulation of the stem cell daughter, a function critical for differentiation. Here, we show at an earlier stage, in the larvae, the same ligand uses components of Wnt non-canonical pathway, RhoA, Rac1 and cdc42, to regulate proper mingling of escort cell precursors between the germ cells. Thus, dWnt4 uses different modules of signaling at different points in development to promote cell movement and control cytoplasmic protrusions. As Wnts have been associated with cancers, understanding how Wnts modulate cell movement by switching on and off different modules may lead to insights into the etiology and progression of cancers.

The replicative histone chaperone CAF-1 is essential for the maintenance of identity and genome integrity in adult stem cells

Chromatin packaging and modifications are important to define the identity of stem cells. How chromatin properties are retained over multiple cycles of stem cell replication, while generating differentiating progeny at the same time, remains a challenging question. The chromatin assembly factor CAF-1 is a conserved histone chaperone, which assembles histones H3 and H4 onto newly synthesized DNA during replication and repair. Here, we investigated the role of CAF-1 in the maintenance of germline stem cells (GSCs) in Drosophila ovaries. We depleted P180, the large subunit of CAF-1, in germ cells and found that it was required in GSCs to maintain their identity. In the absence of P180, GSCs still harbor stem cell properties but concomitantly express markers of differentiation. In addition, P180-depleted germ cells exhibit elevated levels of DNA damage and de-repression of the transposable I-element. These DNA damages activate p53- and Chk2-dependent checkpoints pathways, leading to cell death and female sterility. Altogether, our work demonstrates that chromatin dynamics mediated by CAF-1 play an important role in both the regulation of stem cell identity and genome integrity.

Engrailed acts with Nejire to control decapentaplegic expression in the Drosophila ovarian stem cell niche

Homeostasis of adult tissues is maintained by a small number of stem cells, which are sustained by their niches. In the Drosophila female germline stem cell (GSC) niche, Decapentaplegic (Dpp) is the primary factor that promotes GSC self-renewal. However, the mechanism regulating dpp expression in the niche is largely unknown. Here, we identify a 2.0 kb fragment located in a 5' cis-regulatory region of the dpp locus containing enhancer activity that drives its expression in the niche. This region is distinct from a previously characterized 3' cis-regulatory enhancer responsible for dpp expression in imaginal discs. Our data demonstrate that Engrailed, a homeodomain-containing transcription factor that serves as a cap cell marker, binds to this region and regulates dpp expression in cap cells. Further data suggest that En forms a complex with Nejire (Nej), the Drosophila ortholog of histone acetyltransferase CBP/p300, and directs Nej to this cis-regulatory region where Nej functions as the co-activator for dpp expression. Therefore, our study defines the molecular pathway controlling dpp expression in the Drosophila ovarian stem cell niche.

Splice variants of the SWR1-type nucleosome remodeling factor Domino have distinct functions during Drosophila melanogaster oogenesis

SWR1-type nucleosome remodeling factors replace histone H2A by variants to endow chromatin locally with specialized functionality. In Drosophila melanogaster a single H2A variant, H2A.V, combines functions of mammalian H2A.Z and H2A.X in transcription regulation and the DNA damage response. A major role in H2A.V incorporation for the only SWR1-like enzyme in flies, Domino, is assumed but not well documented in vivo. It is also unclear whether the two alternatively spliced isoforms, DOM-A and DOM-B, have redundant or specialized functions. Loss of both DOM isoforms compromises oogenesis, causing female sterility. We systematically explored roles of the two DOM isoforms during oogenesis using a cell type-specific knockdown approach. Despite their ubiquitous expression, DOM-A and DOM-B have non-redundant functions in germline and soma for egg formation. We show that chromatin incorporation of H2A.V in germline and somatic cells depends on DOM-B, whereas global incorporation in endoreplicating germline nurse cells appears to be independent of DOM. By contrast, DOM-A promotes the removal of H2A.V from stage 5 nurse cells. Remarkably, therefore, the two DOM isoforms have distinct functions in cell type-specific development and H2A.V exchange.

Temporal remodeling of the cell cycle accompanies differentiation in the Drosophila germline

Development of multicellular organisms relies upon the coordinated regulation of cellular differentiation and proliferation. Growing evidence suggests that some molecular regulatory pathways associated with the cell cycle machinery also dictate cell fate; however, it remains largely unclear how the cell cycle is remodeled in concert with cell differentiation. During Drosophila oogenesis, mature oocytes are created through a series of precisely controlled division and differentiation steps, originating from a single tissue-specific stem cell. Further, germline stem cells (GSCs) and their differentiating progeny remain in a predominantly linear arrangement as oogenesis proceeds. The ability to visualize the stepwise events of differentiation within the context of a single tissue make the Drosophila ovary an exceptional model for study of cell cycle remodeling. To describe how the cell cycle is remodeled in germ cells as they differentiate in situ, we used the Drosophila Fluorescence Ubiquitin-based Cell Cycle Indicator (Fly-FUCCI) system, in which degradable versions of GFP::E2f1 and RFP::CycB fluorescently label cells in each phase of the cell cycle. We found that the lengths of the G1, S, and G2 phases of the cell cycle change dramatically over the course of differentiation, and identified the 4/8-cell cyst as a key developmental transition state in which cells prepare for specialized cell cycles. Our data suggest that the transcriptional activator E2f1, which controls the transition from G1 to S phase, is a key regulator of mitotic divisions in the early germline. Our data support the model that E2f1 is necessary for proper GSC proliferation, self-renewal, and daughter cell development. In contrast, while E2f1 degradation by the Cullin 4 (Cul4)-containing ubiquitin E3 ligase (CRL4) is essential for developmental transitions in the early germline, our data do not support a role for E2f1 degradation as a mechanism to limit GSC proliferation or self-renewal. Taken together, these findings provide further insight into the regulation of cell proliferation and the acquisition of differentiated cell fate, with broad implications across developing tissues.

Coordination of cellular differentiation, polarity, mitosis and meiosis - New findings from early vertebrate oogenesis

A mechanistic dissection of early oocyte differentiation in vertebrates is key to advancing our knowledge of germline development, reproductive biology, the regulation of meiosis, and all of their associated disorders. Recent advances in the field include breakthroughs in the identification of germline stem cells in Medaka, in the cellular architecture of the germline cyst in mice, in a mechanistic dissection of chromosomal pairing and bouquet formation in meiosis in mice, in tracing oocyte symmetry breaking to the chromosomal bouquet of meiosis in zebrafish, and in the biology of the Balbiani body, a universal oocyte granule. Many of the major events in early oogenesis are universally conserved, and some are co-opted for species-specific needs. The chromosomal events of meiosis are of tremendous consequence to gamete formation and have been extensively studied. New light is now being shed on other aspects of early oocyte differentiation, which were traditionally considered outside the scope of meiosis, and their coordination with meiotic events. The emerging theme is of meiosis as a common groundwork for coordinating multifaceted processes of oocyte differentiation. In an accompanying manuscript we describe methods that allowed for investigations in the zebrafish ovary to contribute to these breakthroughs. Here, we review these advances mostly from the zebrafish and mouse. We discuss oogenesis concepts across established model organisms, and construct an inclusive paradigm for early oocyte differentiation in vertebrates.

Yorkie and Hedgehog independently restrict BMP production in escort cells to permit germline differentiation in the Drosophila ovary

Multiple signaling pathways guide the behavior and differentiation of both germline stem cells (GSCs) and somatic follicle stem cells (FSCs) in the Drosophila germarium, necessitating careful control of signal generation, range and responses. Signal integration involves escort cells (ECs), which promote differentiation of the GSC derivatives they envelop, provide niche signals for FSCs and derive directly from FSCs in adults. Hedgehog (Hh) signaling induces the Hippo pathway effector Yorkie (Yki) to promote proliferation and maintenance of FSCs, but Hh also signals to ECs, which are quiescent. Here, we show that in ECs both Hh and Yki limit production of BMP ligands to allow germline differentiation. Loss of Yki produced a more severe germarial phenotype than loss of Hh signaling and principally induced a different BMP ligand. Moreover, Yki activity reporters and epistasis tests showed that Yki does not mediate the key actions of Hh signaling in ECs. Thus, both the coupling and output of the Hh and Yki signaling pathways differ between FSCs and ECs despite their proximity and the fact that FSCs give rise directly to ECs.