Steroid signaling within Drosophila ovarian epithelial cells sex-specifically modulates early germ cell development and meiotic entry

The pioneer factor Zelda induces male-to-female somatic sex reversal in adult tissues

Somatic sex identity must be maintained throughout adulthood for tissue function. Adult somatic stem cells in the Drosophila testis (i.e., CySCs) lacking the transcription factor Chinmo are reprogrammed to their ovarian counterparts by induction of female-specific Tra F , but this is not mechanistically understood. Pioneer factors play central roles in direct reprogramming, and many upregulated genes in chinmo -/- CySCs contain binding sites for the pioneer factor Zelda (Zld). microRNAs repress zld mRNA in wild type CySCs, but they are downregulated after Chinmo loss, allowing for zld mRNA translation. Zld depletion from chinmo -/- CySCs suppresses feminization, and ectopic Zld induces Tra F and feminizes wild-type CySCs. qkr58E-2 and ecdysone receptor ( EcR ), direct Zld targets in the embryo, are female-biased in adult gonads and upregulated in chinmo -/- CySCs. The RNA-binding protein Qkr58E-2 produces Tra F , while EcR promotes female-biased gene expression. Ectopic Zld feminizes adult male adipose tissue, demonstrating that Zld can instruct female and override male identity in adult XY tissues.

Highlights

zld mRNA is repressed by microRNAs in XY somatic gonadal cells Zld is upregulated in and required for sex reversal of XY chinmo -/- cells Zld induces Qkr58E-2 and EcR, which cause Tra F and female-biased transcription Zld feminizes XY adipose cells by inducing Tra F and downregulating Chinmo.

E93 is indispensable for reproduction in ametabolous and hemimetabolous insects

Ecdysone-induced protein 93 (E93), known as the 'adult-specifier' transcription factor in insects, triggers metamorphosis in both hemimetabolous and holometabolous insects. Although E93 is conserved in ametabolous insects, its spatiotemporal expression and physiological function remain poorly understood. In this study, we first discover that, in the ametabolous firebrat Thermobia domestica, the previtellogenic ovary exhibits cyclically high E93 expression, and E93 mRNA is broadly distributed in previtellogenic ovarioles. E93 homozygous mutant females of T. domestica exhibit severe fecundity deficiency due to impaired previtellogenic development of the ovarian follicles, likely because E93 induces the expression of genes involved in ECM (extracellular matrix)-receptor interactions during previtellogenesis. Moreover, we reveal that in the hemimetabolous cockroach Blattella germanica, E93 similarly promotes previtellogenic ovarian development. In addition, E93 is also essential for vitellogenesis that is necessary to guarantee ovarian maturation and promotes the vitellogenesis-previtellogenesis switch in the fat body of adult female cockroaches. Our findings deepen the understanding of the roles of E93 in controlling reproduction in insects, and of E93 expression and functional evolution, which are proposed to have made crucial contributions to the origin of insect metamorphosis.

Conditional Inhibition of Eip75B Eliminates the Effects of Mating and Mifepristone on Lifespan in Female Drosophila

Mating in female Drosophila melanogaster causes midgut hypertrophy and reduced lifespan, and these effects are blocked by the drug mifepristone. Eip75B is a transcription factor previously reported to have pleiotropic effects on Drosophila lifespan. Because Eip75B null mutations are lethal, conditional systems and/or partial knock-down are needed to study Eip75B effects in adults. Previous studies showed that Eip75B is required for adult midgut cell proliferation in response to mating. To test the possible role of Eip75B in mediating the lifespan effects of mating and mifepristone, a tripartite FLP-recombinase-based conditional system was employed that provides controls for genetic background. Expression of a Hsp70-FLP transgene was induced in third instar larvae by a brief heat pulse. The FLP recombinase catalyzed the recombination and activation of an Actin5C-GAL4 transgene. The GAL4 transcription factor in turn activated expression of a UAS-Eip75B-RNAi transgene. Inhibition of Eip75B activity was confirmed by loss of midgut hypertrophy upon mating, and the lifespan effects of both mating and mifepristone were eliminated. In addition, the negative effects of mifepristone on egg production were eliminated. The data indicate that Eip75B mediates the effects of mating and mifepristone on female midgut hypertrophy, egg production, and lifespan.

Fecundity is optimized by levels of nutrient signal-dependent expression of Dve and EcR in Drosophila male accessory gland

Steroid hormones play various physiological roles including metabolism and reproduction. Steroid hormones in insects are ecdysteroids, and the major form in Drosophila melanogaster is ecdysone. In Drosophila males, the accessory gland is responsive to nutrient-dependent regulation of fertility/fecundity. The accessory gland is composed of two types of binucleated epithelial cells: a main cell and a secondary cell (SC). The transcription factors Defective proventriculus (Dve), Abdominal-B, and Ecdysone receptors (EcRs) are strongly expressed in adult SCs. We show that this EcR expression is regulated by parallel pathways of nutrient signaling and the Dve activity. Induction of Dve expression is also dependent on nutrient signaling, and it becomes nutrient signal-independent during a restricted period of development. Forced dve expression during the restricted period significantly increased the number of SCs. Here, we provide evidence that the level of nutrient signal-dependent Dve expression during the restricted period determines the number of SCs, and that ecdysone signaling is also crucial to optimize male fecundity through nutrient signal-dependent survival and maturation of SCs.

Microbes control Drosophila germline stem cell increase and egg maturation through hormonal pathways

Reproduction is highly dependent on environmental and physiological factors including nutrition, mating stimuli and microbes. Among these factors, microbes facilitate vital functions for host animals such as nutritional intake, metabolic regulation, and enhancing fertility under poor nutrition conditions. However, detailed molecular mechanisms by which microbes control germline maturation, leading to reproduction, remain largely unknown. In this study, we show that environmental microbes exert a beneficial effect on Drosophila oogenesis by promoting germline stem cell (GSC) proliferation and subsequent egg maturation via acceleration of ovarian cell division and suppression of apoptosis. Moreover, insulin-related signaling is not required; rather, the ecdysone pathway is necessary for microbe-induced increase of GSCs and promotion of egg maturation, while juvenile hormone contributes only to increasing GSC numbers, suggesting that hormonal pathways are activated at different stages of oogenesis. Our findings reveal that environmental microbes can enhance host reproductivity by modulating host hormone release and promoting oogenesis.

Ecdysteroid Biosynthesis Halloween Gene Spook Plays an Important Role in the Oviposition Process of Spider Mite, Tetranychus urticae

In insects, the ecdysteroid hormone regulates development and reproduction. However, its function in the reproduction process of spider mites is still unclear. In this study, we investigated the effect of the Halloween gene Spook on the oviposition of the reproduction process in a spider mite, Tetranychus urticae. The expression patterns of the ecdysteroid biosynthesis and signaling pathway genes, as analyzed by RT-qPCR, showed that the expression pattern of the Halloween genes was similar to the oviposition pattern of the female mite and the expression patterns of the vitellogenesis-related genes TuVg and TuVgR, suggesting that the Halloween genes are involved in the oviposition of spider mites. To investigate the function of the ecdysteroid hormone on the oviposition of the reproduction process, we carried out an RNAi assay against the Halloween gene Spook by injection in female mites. Effective silencing of TuSpo led to a significant reduction of oviposition. In summary, these results provide an initial study on the effect of Halloween genes on the reproduction in T. urticae and may be a foundation for a new strategy to control spider mites.

Genetic and Epigenetic Regulation of Drosophila Oocyte Determination

Primary oocyte determination occurs in many organisms within a germ line cyst, a multicellular structure composed of interconnected germ cells. However, the structure of the cyst is itself highly diverse, which raises intriguing questions about the benefits of this stereotypical multicellular environment for female gametogenesis. Drosophila melanogaster is a well-studied model for female gametogenesis, and numerous genes and pathways critical for the determination and differentiation of a viable female gamete have been identified. This review provides an up-to-date overview of Drosophila oocyte determination, with a particular emphasis on the mechanisms that regulate germ line gene expression.

The loss of Halloween gene function seriously affects the development and reproduction of Diaphorina citri (Hemiptera: Liviidae) and increases its susceptibility to pesticides

The citrus industry has suffered severe losses as a result of Huanglongbing spread by Diaphorina citri. Controlling the population of D. citri is the key to preventing and controlling the spread of Huanglongbing. Ecdysteroids are key hormones that regulate insect development and reproduction. Therefore, the Halloween gene family involved in the ecdysone synthesis of D. citri is an ideal target for controlling the population growth of this insect. In this study, we successfully cloned four Halloween genes expressed during D. citri development. Silencing of one of the four genes resulted in a significant decrease in 20E titers in nymphs and significant decreases in the developmental, survival and emergence rates. Inhibiting Halloween gene expression in adults impeded the growth of the female ovary, diminished yolk formation, lowered vitellogenin transcription levels, and hence impaired female fecundity. This showed that Halloween genes were required for D. citri development and reproduction. DcCYP315A1 and DcCYP314A1 were highly expressed when D. citri was exposed to thiamethoxam and cypermethrin, and silencing these two genes made D. citri more sensitive to these two pesticides. Inhibition of DcCYP315A1 and DcCYP314A1 expression not only significantly delayed the development and reproduction of D. citri but also increased its susceptibility to pesticides. Therefore, these two genes are more suitable as potential target genes for controlling D. citri.

Bell H, Tower J. A screen of small molecule and genetic modulators of life span in female Drosophila identifies etomoxir, RH5849 and unanticipated temperature effects

Mifepristone increases life span in female Drosophila melanogaster, and its molecular target(s) remain unclear. Here small molecule and genetic interventions were tested for ability to mimic mifepristone, or to decrease life span in a way that can be rescued by mifepristone. Etomoxir inhibits lipid metabolism, and significantly increased life span in virgin and mated females, but not males, at 50 µM concentration. Pioglitazone is reported to activate both mammalian PPARγ and its Drosophila homolog Eip75B. Pioglitazone produced minor and inconsistent benefits for female Drosophila life span, and only at the lowest concentrations tested. Ecdysone is a Drosophila steroid hormone reported to regulate responses to mating, and RH5849 is a potent mimic of ecdysone. RH5849 reduced virgin female life span, and this was partly rescued by mifepristone. Mifepristone did not compete with RH5849 for activation of an ecdysone receptor (EcR)-responsive transgenic reporter, indicating that the relevant target for mifepristone is not EcR. The conditional GAL4/GAL80ts system was used in attempt to test the effect of an Eip75B RNAi construct on female life span. However, the 29°C temperature used for induction reduced or eliminated mating-induced midgut hypertrophy, the negative life span effects of mating, and the positive life span effects of mifepristone. Even when applied after mating was complete, a shift to 29°C temperature reduced mating-induced midgut hypertrophy by half, and the life span effects of mating by 4.8-fold. Taken together, these results identify promising small molecules for further analysis, and inform the design of experiments involving the GAL4/GAL80ts system.

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.

Knockdown of the Halloween Genes spook, shadow and shade Influences Oocyte Development, Egg Shape, Oviposition and Hatching in the Desert Locust

Ecdysteroids are widely investigated for their role during the molting cascade in insects; however, they are also involved in the development of the female reproductive system. Ecdysteroids are synthesized from cholesterol, which is further converted via a series of enzymatic steps into the main molting hormone, 20-hydoxyecdysone. Most of these biosynthetic conversion steps involve the activity of cytochrome P450 (CYP) hydroxylases, which are encoded by the Halloween genes. Three of these genes, spook (spo), phantom (phm) and shade (shd), were previously characterized in the desert locust, Schistocerca gregaria. Based on recent sequencing data, we have now identified the sequences of disembodied (dib) and shadow (sad), for which we also analyzed spatiotemporal expression profiles using qRT-PCR. Furthermore, we investigated the possible role(s) of five different Halloween genes in the oogenesis process by means of RNA interference mediated knockdown experiments. Our results showed that depleting the expression of SchgrSpo, SchgrSad and SchgrShd had a significant impact on oocyte development, oviposition and hatching of the eggs. Moreover, the shape of the growing oocytes, as well as the deposited eggs, was very drastically altered by the experimental treatments. Consequently, it can be proposed that these three enzymes play an important role in oogenesis.

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.

Drosophila Keap1 xenobiotic response factor regulates developmental transcription through binding to chromatin

The Keap1-Nrf2 complex is a central regulator that mediates transcriptional responses to xenobiotic stimuli and is highly related with multiple human diseases. The molecular mechanisms and biological functions of Keap1 and Nrf2 are not fully understood. The Drosophila Keap1 homolog (dKeap1) is conserved with mammalian Keap1 except that dKeap1 contains a 156 aa C-terminal tail (CTD). A dKeap1 truncation with the CTD removed (dKeap1-ΔCTD) shows abolished nuclear localization and chromatin-binding. Expression of dKeap1-ΔCTD in the dKeap1 null background significantly rescues this mutant to the adult stage, but the files showed partial lethality, sterility and defects in adipose tissue. In the rescued flies, expression levels of ecdysone-response genes, ecdysone-synthetic genes and adipogenesis genes were down-regulated in specific tissues, indicating that the chromatin-binding of dKeap1 mediates the activation of these developmental genes. At the same time, dKeap1-ΔCTD can still suppress the basal expression of detoxifying genes and mediate the activation of these genes in response to xenobiotic stimuli, suggesting that the chromatin-binding of dKeap1 is not required for the regulation of detoxifying genes. These results support a model in which dKeap1 on one hand functions as an inhibitor for the Nrf2-mediated transcription in the xenobiotic response pathway and on the other hand functions as a chromatin-binding transcription activator in the developmental pathway. Our study reveals a novel mechanism whereby Keap1-Nrf2 xenobiotic response signaling regulates development using a mechanism independent of redox signaling.

Regulation of Mating-Induced Increase in Female Germline Stem Cells in the Fruit Fly Drosophila melanogaster

In many insect species, mating stimuli can lead to changes in various behavioral and physiological responses, including feeding, mating refusal, egg-laying behavior, energy demand, and organ remodeling, which are collectively known as the post-mating response. Recently, an increase in germline stem cells (GSCs) has been identified as a new post-mating response in both males and females of the fruit fly, Drosophila melanogaster. We have extensively studied mating-induced increase in female GSCs of D. melanogaster at the molecular, cellular, and systemic levels. After mating, the male seminal fluid peptide [e.g. sex peptide (SP)] is transferred to the female uterus. This is followed by binding to the sex peptide receptor (SPR), which evokes post-mating responses, including increase in number of female GSCs. Downstream of SP-SPR signaling, the following three hormones and neurotransmitters have been found to act on female GSC niche cells to regulate mating-induced increase in female GSCs: (1) neuropeptide F, a peptide hormone produced in enteroendocrine cells; (2) octopamine, a monoaminergic neurotransmitter synthesized in ovary-projecting neurons; and (3) ecdysone, a steroid hormone produced in ovarian follicular cells. These humoral factors are secreted from each organ and are received by ovarian somatic cells and regulate the strength of niche signaling in female GSCs. This review provides an overview of the latest findings on the inter-organ relationship to regulate mating-induced female GSC increase in D. melanogaster as a model. We also discuss the remaining issues that should be addressed in the future.

Expression of 20-hydroxyecdysone-related genes during gonadal development of Teleogryllus emma (Orthoptera: Gryllidae)

Insect gonads develop under endocrine signals. In this study, we assessed the characters of partial complementary DNAs encoding the Teleogryllus emma orthologs of 20-hydroxyecdysone (20E)-related genes (RXR, E75, HR3, Hsc70, and Hsp90) and analyzed their expression patterns in both nymph and adult crickets. 20E treatment suppressed expression of TeEcR, TeRXR, TeE75, TeHR3, TeHsc70, and TeHsp90. Temporal expression analysis demonstrated that TeERR and 20E-related genes were expressed in four stages of gonadal development from the fourth-instar nymph stage to the adult stage. The expression pattern of these genes differed in testicular and ovarian development. TeRXR, HR3, TeHsc70, and TeHsp90 were irregularly expressed in gonads of the same developmental stages, while mRNAs encoding TeERR, TeEcR, and TeE75 accumulated in higher levels in ovaries than in testes. RNA interference (RNAi) of TeEcR expression led to decrease of the expression levels of TeEcR, TeRXR, TeHR3, and TeHsc70, while it enhanced TeE75 and TeHsp90 expressions. These results demonstrate that the TeERR and 20E-related genes help regulate gonadal development, while TeEcR appears to inhibit TeE75 expression, TeE75 inhibits HR3 expression. Hsc70 indirectly regulated the expression of the primary and secondary response genes E74A, E75B, and HR3. Hsp90 regulated Usp expression with no direct regulatory relationship with EcR.

Role of Endocrine System in the Regulation of Female Insect Reproduction

The proper synthesis and functioning of ecdysteroids and juvenile hormones (JHs) are very important for the regulation of vitellogenesis and oogenesis. However, their role and function contrast among different orders, and even in the same insect order. For example, the JH is the main hormone that regulates vitellogenesis in hemimetabolous insect orders, which include Orthoptera, Blattodea, and Hemiptera, while ecdysteroids regulate the vitellogenesis among the insect orders of Diptera, some Hymenoptera and Lepidoptera. These endocrine hormones also regulate each other. Even at some specific stage of insect life, they positively regulate each other, while at other stages of insect life, they negatively control each other. Such positive and negative interaction of 20-hydroxyecdysone (20E) and JH is also discussed in this review article to better understand the role of these hormones in regulating the reproduction. Therefore, the purpose of the present review is to deeply understand the complex interaction of endocrine hormones with each other and with the insulin signaling pathway. The role of microbiomes in the regulation of the insect endocrine system is also reviewed, as the endocrine hormones are significantly affected by the compounds produced by the microbiota.

The neuropeptide allatostatin C from clock-associated DN1p neurons generates the circadian rhythm for oogenesis

Metazoan species optimize the timing of reproduction to maximize fitness. To understand how biological clocks direct reproduction, we investigated the neural substrates that produce oogenesis rhythms in the genetically amenable model organism Drosophila melanogaster. The neuropeptide allatostatin C (AstC) is an insect counterpart of the vertebrate neuropeptide somatostatin, which suppresses gonadotropin production. A subset of the brain circadian pacemaker neurons produces AstC. We have uncovered that these clock-associated AstC neurons generate the circadian oogenesis rhythm via brain insulin-producing cells and the insect gonadotropin juvenile hormone. Identification of a conserved neuropeptide pathway that links female reproduction and the biological clock offers insight into the molecular mechanisms that direct reproductive timing.

The link between the biological clock and reproduction is evident in most metazoans. The fruit fly Drosophila melanogaster, a key model organism in the field of chronobiology because of its well-defined networks of molecular clock genes and pacemaker neurons in the brain, shows a pronounced diurnal rhythmicity in oogenesis. Still, it is unclear how the circadian clock generates this reproductive rhythm. A subset of the group of neurons designated “posterior dorsal neuron 1” (DN1p), which are among the ∼150 pacemaker neurons in the fly brain, produces the neuropeptide allatostatin C (AstC-DN1p). Here, we report that six pairs of AstC-DN1p send inhibitory inputs to the brain insulin-producing cells, which express two AstC receptors, star1 and AICR2. Consistent with the roles of insulin/insulin-like signaling in oogenesis, activation of AstC-DN1p suppresses oogenesis through the insulin-producing cells. We show evidence that AstC-DN1p activity plays a role in generating an oogenesis rhythm by regulating juvenile hormone and vitellogenesis indirectly via insulin/insulin-like signaling. AstC is orthologous to the vertebrate neuropeptide somatostatin (SST). Like AstC, SST inhibits gonadotrophin secretion indirectly through gonadotropin-releasing hormone neurons in the hypothalamus. The functional and structural conservation linking the AstC and SST systems suggest an ancient origin for the neural substrates that generate reproductive rhythms.

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.

Redox signaling as a modulator of germline stem cell behavior: Implications for regenerative medicine

Germline stem cells (GSCs) are crucial for the generation of gametes and propagation of the species. Both intrinsic signaling pathways and environmental cues are employed in order to tightly control GSC behavior, including mitotic divisions, the choice between self-renewal or onset of differentiation, and survival. Recently, oxidation-reduction (redox) signaling has emerged as an important regulator of GSC and gamete behavior across species. In this review, we will highlight the primary mechanisms through which redox signaling acts to influence GSC behavior in different model organisms (Caenorhabditis elegans, Drosophila melanogaster and Mus musculus). In addition, we will summarize the latest research on the use of antioxidants to support mammalian spermatogenesis and discuss potential strategies for regenerative medicine in humans to enhance reproductive fitness.

Hormone receptor 4 is required in muscles and distinct ovarian cell types to regulate specific steps of Drosophila oogenesis

The conserved nuclear receptor superfamily has crucial roles in many processes, including reproduction. Nuclear receptors with known roles in oogenesis have been studied mostly in the context of their ovary-intrinsic requirement. Recent studies in Drosophila, however, have begun to reveal new roles of nuclear receptor signaling in peripheral tissues in controlling reproduction. Here, we identified Hormone receptor 4 (Hr4) as an oogenesis regulator required in the ovary and muscles. Global Hr4 knockdown leads to increased germline stem cell (GSC) loss, reduced GSC proliferation, early germline cyst death, slowed follicle growth and vitellogenic follicle degeneration. Tissue-specific knockdown experiments uncovered ovary-intrinsic and peripheral tissue requirements for Hr4 In the ovary, Hr4 is required in the niche for GSC proliferation and in the germline for GSC maintenance. Hr4 functions in muscles to promote GSC maintenance and follicle growth. The specific tissues that require Hr4 for survival of early germline cysts and vitellogenic follicles remain unidentified. These results add to the few examples of muscles controlling gametogenesis and expand our understanding of the complexity of nuclear receptor regulation of various aspects of oogenesis.

A Progressive Somatic Cell Niche Regulates Germline Cyst Differentiation in the Drosophila Ovary

In the germarium of the Drosophila ovary, developing germline cysts are surrounded by a population of somatic escort cells that are known to function as the niche cells for germline differentiation;¹ however, the underlying molecular mechanisms of this niche function remain poorly understood. Through single-cell gene expression profiling combined with genetic analyses, we here demonstrate that the escort cells can be spatially and functionally divided into two successive domains. The anterior escort cells (aECs) specifically produce ecdysone, which acts on the cystoblast to promote synchronous cell division, whereas the posterior escort cells (pECs) respond to ecdysone signaling and regulate soma-germline cell adhesion to promote the transition from 16-cell cyst-to-egg chamber formation. The patterning of the aEC and pEC domains is independent of the germline but is dependent on JAK/STAT signaling activity, which emanates from the posterior. Thus, a heterogeneous population of escort cells constitutes a stepwise niche environment to orchestrate cystoblast division and differentiation toward egg chamber formation.

Nuclear receptors linking physiology and germline stem cells in Drosophila

Maternal nutrition and physiology are intimately associated with reproductive success in diverse organisms. Despite decades of study, the molecular mechanisms linking maternal diet to the production and quality of oocytes remain poorly defined. Nuclear receptors (NRs) link nutritional signals to cellular responses and are essential for oocyte development. The fruit fly, Drosophila melanogaster, is an excellent genetically tractable model to study the relationship between NR signaling and oocyte production. In this review, we explore how NRs in Drosophila regulate the earliest stages of oocyte development. Long-recognized as an essential mediator of developmental transitions, we focus on the intrinsic roles of the Ecdysone Receptor and its ligand, ecdysone, in oogenesis. We also review recent studies suggesting broader roles for NRs as regulators of maternal physiology and their impact specifically on oocyte production. We propose that NRs form the molecular basis of a broad physiological surveillance network linking maternal diet with oocyte production. Given the functional conservation between Drosophila and humans, continued experimental investigation into the molecular mechanisms by which NRs promote oogenesis will likely aid our understanding of human fertility.

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 Nuclear Receptor Seven Up Regulates Genes Involved in Immunity and Xenobiotic Response in the Adult Drosophila Female Fat Body

The physiology of organisms depends on inter-organ communication in response to changes in the environment. Nuclear receptors are broadly expressed transcription factors that respond to circulating molecules to control many biological processes, including immunity, detoxification, and reproduction. Although the tissue-intrinsic roles of nuclear receptors in reproduction have been extensively studied, there is increasing evidence that nuclear receptor signaling in peripheral tissues can also influence oogenesis. We previously showed that the Drosophila nuclear receptor Seven up (Svp) is required in the adult fat body to regulate distinct steps of oogenesis; however, the relevant downstream targets of Svp remain unknown. Here, we took an RNA sequencing approach to identify candidate Svp targets specifically in the adult female fat body that might mediate this response. svp knockdown in the adult female fat body significantly downregulated immune genes involved in the first line of pathogen defense, suggesting a role for Svp in stimulating early immunity. In addition, we found that Svp transcriptionally regulates genes involved in each step of the xenobiotic detoxification response. Based on these findings, we propose a testable model in which Svp functions in the adult female fat body to stimulate early defense against pathogens and facilitate detoxification as part of its mechanisms to promote oogenesis.

Ecdysone steroid hormone remote controls intestinal stem cell fate decisions via the PPARγ-homolog Eip75B in Drosophila

Developmental studies revealed fundamental principles on how organ size and function is achieved, but less is known about organ adaptation to new physiological demands. In fruit flies, juvenile hormone (JH) induces intestinal stem cell (ISC) driven absorptive epithelial expansion balancing energy uptake with increased energy demands of pregnancy. Here, we show 20-Hydroxy-Ecdysone (20HE)-signaling controlling organ homeostasis with physiological and pathological implications. Upon mating, 20HE titer in ovaries and hemolymph are increased and act on nearby midgut progenitors inducing Ecdysone-induced-protein-75B (Eip75B). Strikingly, the PPARγ-homologue Eip75B drives ISC daughter cells towards absorptive enterocyte lineage ensuring epithelial growth. To our knowledge, this is the first time a systemic hormone is shown to direct local stem cell fate decisions. Given the protective, but mechanistically unclear role of steroid hormones in female colorectal cancer patients, our findings suggest a tumor-suppressive role for steroidal signaling by promoting postmitotic fate when local signaling is deteriorated.

Two distinct pathways of pregranulosa cell differentiation support follicle formation in the mouse ovary

This paper improves knowledge of the somatic and germ cells of the developing mouse ovary that assemble into ovarian follicles, by determining cellular gene expression, and tracing lineage relationships. The study covers the last week of fetal development through the first five days of postnatal development. During this time, many critically important processes take place, including sex determination, follicle assembly, and the initial events of meiosis. We report expression differences between pregranulosa cells of wave 1 follicles that function at puberty and wave 2 follicles that sustain fertility. These studies illuminate ovarian somatic cells and provide a resource to study the development, physiology, and evolutionary conservation of mammalian ovarian follicle formation.

We sequenced more than 52,500 single cells from embryonic day 11.5 (E11.5) postembryonic day 5 (P5) gonads and performed lineage tracing to analyze primordial follicles and wave 1 medullar follicles during mouse fetal and perinatal oogenesis. Germ cells clustered into six meiotic substages, as well as dying/nurse cells. Wnt-expressing bipotential precursors already present at E11.5 are followed at each developmental stage by two groups of ovarian pregranulosa (PG) cells. One PG group, bipotential pregranulosa (BPG) cells, derives directly from bipotential precursors, expresses Foxl2 early, and associates with cysts throughout the ovary by E12.5. A second PG group, epithelial pregranulosa (EPG) cells, arises in the ovarian surface epithelium, ingresses cortically by E12.5 or earlier, expresses Lgr5, but delays robust Foxl2 expression until after birth. By E19.5, EPG cells predominate in the cortex and differentiate into granulosa cells of quiescent primordial follicles. In contrast, medullar BPG cells differentiate along a distinct pathway to become wave 1 granulosa cells. Reflecting their separate somatic cellular lineages, second wave follicles were ablated by diptheria toxin treatment of Lgr5-DTR-EGFP mice at E16.5 while first wave follicles developed normally and supported fertility. These studies provide insights into ovarian somatic cells and a resource to study the development, physiology, and evolutionary conservation of mammalian ovarian follicles.

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.

The nuclear receptor seven up functions in adipocytes and oenocytes to control distinct steps of Drosophila oogenesis

Reproduction is intimately linked to the physiology of an organism. Nuclear receptors are widely expressed transcription factors that mediate the effects of many circulating molecules on physiology and reproduction. While multiple studies have focused on the roles of nuclear receptors intrinsically in the ovary, it remains largely unknown how the actions of nuclear receptors in peripheral tissues influence oogenesis. We identified the nuclear receptor encoded by svp as a novel regulator of oogenesis in adult Drosophila. Global somatic knockdown of svp reduces egg production by increasing GSC loss, death of early germline cysts, and degeneration of vitellogenic follicles. Tissue-specific knockdown experiments revealed that svp remotely controls these different steps of oogenesis through separate mechanisms involving distinct tissues. Specifically, adipocyte-specific svp knockdown impairs GSC maintenance and early germline cyst survival, whereas oenocyte-specific svp knockdown increases the death of vitellogenic follicles without any effects on GSCs or early cysts. These results illustrate that nuclear receptors can control reproduction through a variety of mechanisms involving peripheral tissues.

Novel cis-regulatory regions in ecdysone responsive genes are sufficient to promote gene expression in Drosophila ovarian cells

The insect steroid hormone ecdysone is a key regulator of oogenesis in Drosophila melanogaster and many other species. Despite the diversity of cellular functions of ecdysone in oogenesis, the molecular regulation of most ecdysone-responsive genes in ovarian cells remains largely unexplored. We performed a functional screen using the UAS/Gal4 system to identify non-coding cis-regulatory elements within well-characterized ecdysone-response genes capable of driving transcription of an indelible reporter in ovarian cells. Using two publicly available transgenic collections (the FlyLight and Vienna Tiles resources), we tested 62 Gal4 drivers corresponding to ecdysone-response genes EcR, usp, E75, br, ftz-f1 and Hr3. We observed 31 lines that were sufficient to drive a UAS-lacZ reporter in discrete cell populations in the ovary. Reporter expression was reproducibly observed in both somatic and germ cells at distinct stages of oogenesis, including those previously characterized as critical points of ecdysone regulation. Our studies identified several useful new reagents, adding to the UAS/Gal4 toolkit available for genetic analysis of oogenesis in Drosophila. Further, our study provides novel insight into the molecular regulation of ecdysone signaling in oogenesis.

Involvement of Ecdysone Signaling in the Expression of the doublesex Gene during Embryonic Development in the Silkworm, Bombyx mori

Steroid hormones, represented by estrogen and testosterone, act as sex hormones that play an essential role in the sexual differentiation of vertebrates. However, it remains unclear whether ecdysteroids, typical steroid hormones in insects, function as sex hormones. In this study, we investigated whether ecdysteroids or ecdysone signals are involved in the sexual differentiation of the silkworm (Bombyx mori) embryo. Quantitative analysis using LC-MS/MS demonstrated that there was no significant difference in the 20-hydroxyecdysone (20E) titer between sexes during embryonic development. Consistent with this result, expression levels of 2 genes encoding ecdysteroid-phosphate phosphatase (EPPase) and ecdysone 20-hydroxylase (E20OHase), which are essential for the biosynthesis of ecdysone and 20E in eggs, did not show a significant difference between male and female embryos. Expression levels of ecdysone receptor (EcR) and E75, which is one of a small set of genes induced directly by 20E, were also similar between the 2 sexes. However, knockdown of EPPase and one isoform of EcR (EcR-A) resulted in decreased expression of Bombyx doublesex (Bmdsx), a master regulatory gene for sexual differentiation of the silkworm in both male and female embryos. In vitro analysis with cultured testes revealed that expression levels of Bmdsx were increased in a dose-dependent manner of the ecdysone analog, ponasterone A. These results suggest that ecdysone signaling may play a role in indirectly regulating the expression of some genes involved in sexual differentiation through inducing expression of Bmdsx in the silkworm.

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.

Generation of Inducible Gene-Switched GAL4 Expressed in the Drosophila Female Germline Stem Cell Niche

The stem cell niche, a regulatory microenvironment, houses and regulates stem cells for maintenance of tissues throughout an organism's lifespan. While it is known that stem cell function declines with age, the role of niche cells in this decline is not completely understood. Drosophila exhibits a short lifespan with well-characterized ovarian germline stem cells (GSCs) and niche compartments, providing a good model with which to study stem cell biology. However, no inducible tools for temporal and spatial control of gene expression in the GSC-niche unit have been previously developed for aging studies. The current UAS-GAL4 systems are not ideal for aging studies because fly physiological aging may be affected by the temperature shifts used to manipulate GAL4 activity. Additionally, the actual needs of the aged niche may be masked by continuously driven gene expression. Since GeneSwitch GAL4 is conveniently activated by the steroid RU486 (mifepristone), we conducted an enhancer-trap screen to isolate GeneSwitch GAL4 lines with expression in the GSC-niche unit. We identified six lines with expression in germarial somatic cells, and two lines (#2305 and #2261) with expression in niche cap cells, the major constituent of the GSC niche. The use of lines #2305 or #2261 to overexpress Drosophila insulin-like peptide 2, which maintains GSC lifespan, in aged niche cap cells significantly delayed age-dependent GSC loss. These results support the notion that insulin signaling is beneficial for maintaining aged stem cells and also validate the utility of our GeneSwitch GAL4 lines for studying stem cell aging.

An update on ecdysone signaling during insect oogenesis

An overview is presented of the different functions of ecdysone signaling during insect oogenesis. An extensive genetic toolkit allowed analysis with unprecedented temporal and spatial detail in Drosophila where functions were revealed in stem cell proliferation and niche maintenance, germline cyst differentiation and follicle formation, integration of nutrient and lipid signaling, follicle maturation and ovulation. Besides putative autocrine/paracrine signaling, hormonal networks were identified that integrate ecdysone with other endocrine signaling pathways. In other insects, progress in oogenesis has lagged behind although recently RNAi emerged as a new tool to analyze gene function in ovaries in hemimetabolous insects and Tribolium.

Turning food into eggs: insights from nutritional biology and developmental physiology of Drosophila

Nutrition plays a central role in fecundity, regulating the onset of reproductive maturity, egg production, and the survival and health of offspring from insects to humans. Although decades of research have worked to uncover how nutrition mediates these effects, it has proven difficult to disentangle the relative role of nutrients as the raw material for egg and offspring development versus their role in stimulating endocrine cascades necessary to drive development. This has been further complicated by the fact that both nutrients and the signalling cascades they regulate interact in complex ways to control fecundity. Separating the two effects becomes important when trying to understand how fecundity is regulated, and in devising strategies to offset the negative effects of nutrition on reproductive health. In this review, we use the extensive literature on egg development in the fruit fly Drosophila melanogaster to explore how the nutrients from food provide the building blocks and stimulate signalling cascades necessary for making an egg.

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.

Midgut-derived neuropeptide F controls germline stem cell proliferation in a mating-dependent manner

Stem cell maintenance is established by neighboring niche cells that promote stem cell self-renewal. However, it is poorly understood how stem cell activity is regulated by systemic, tissue-extrinsic signals in response to environmental cues and changes in physiological status. Here, we show that neuropeptide F (NPF) signaling plays an important role in the pathway regulating mating-induced germline stem cell (GSC) proliferation in the fruit fly Drosophila melanogaster. NPF expressed in enteroendocrine cells (EECs) of the midgut is released in response to the seminal-fluid protein sex peptide (SP) upon mating. This midgut-derived NPF controls mating-induced GSC proliferation via ovarian NPF receptor (NPFR) activity, which modulates bone morphogenetic protein (BMP) signaling levels in GSCs. Our study provides a molecular mechanism that describes how a gut-derived systemic factor couples stem cell behavior to physiological status, such as mating, through interorgan communication.

Communication between different organs is essential to respond quickly to environmental cues or changes in the physiological status of an organism. Recent studies have shown the existence of humoral factors or hormones, which are transported by the circulatory system to different organs and achieve coordination between them. Here, we have analyzed the communication mechanism between organs that regulates proliferation of germline stem cells (GSCs) in the ovary of the fruit fly Drosophila melanogaster. We show that a peptide hormone called neuropeptide F (NPF) is a key player in this process. This peptide is produced in both the brain and the midgut, and, remarkably, we find that only NPF released from the midgut is crucial for controlling post-mating GSC proliferation. Our data suggest that mating stimulates the release of NPF from the endocrine cells of the midgut stimulated by the presence of a seminal peptide. Midgut-derived NPF is then transduced through an NPF-specific G-protein–coupled receptor expressed in the ovary, and this triggers GSC proliferation. Our study identifies an essential interaction between the digestive system and the ovary that regulates the size of stem cell populations in flies depending on mating.

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.

A hormonal cue promotes timely follicle cell migration by modulating transcription profiles

Cell migration is essential during animal development. In the Drosophila ovary, the steroid hormone ecdysone coordinates nutrient sensing, growth, and the timing of morphogenesis events including border cell migration. To identify downstream effectors of ecdysone signaling, we profiled gene expression in wild-type follicle cells compared to cells expressing a dominant negative Ecdysone receptor or its coactivator Taiman. Of approximately 400 genes that showed differences in expression, we validated 16 candidate genes for expression in border and centripetal cells, and demonstrated that seven responded to ectopic ecdysone activation by changing their transcriptional levels. We found a requirement for seven putative targets in effective cell migration, including two other nuclear hormone receptors, a calcyphosine-encoding gene, and a prolyl hydroxylase. Thus, we identified multiple new genetic regulators modulated at the level of transcription that allow cells to interpret information from the environment and coordinate cell migration in vivo.

Escort cells generate a dynamic compartment for germline stem cell differentiation via combined Stat and Erk signalling

Two different compartments support germline stem cell (GSC) self-renewal and their timely differentiation: the classical niche provides maintenance cues, while a differentiation compartment, formed by somatic escort cells (ECs), is required for proper GSC differentiation. ECs extend long protrusions that invade between tightly packed germ cells, and alternate between encapsulating and releasing them. How ECs achieve this dynamic balance has not been resolved. By combining live imaging and genetic analyses in Drosophila, we have characterised EC shapes and their dynamic changes. We show that germ cell encapsulation by ECs is a communal phenomenon, whereby EC-EC contacts stabilise an extensive meshwork of protrusions. We further show that Signal Transducer and Activator of Transcription (Stat) and Epidermal Growth Factor Receptor (Egfr) signalling sustain EC protrusiveness and flexibility by combinatorially affecting the activity of different RhoGTPases. Our results reveal how a complex signalling network can determine the shape of a cell and its dynamic behaviour. It also explains how the differentiation compartment can establish extensive contacts with germ cells, while allowing a continual posterior movement of differentiating GSC daughters.

Steroid Hormones and the Physiological Regulation of Tissue-Resident Stem Cells: Lessons from the Drosophila Ovary

PURPOSE OF REVIEW

Stem cells respond to local paracrine signals; more recently, however, systemic hormones have also emerged as key regulators of stem cells. This review explores the role of steroid hormones in stem cells, using the Drosophila germline stem cell as a centerpiece for discussion.

RECENT FINDINGS

Stem cells sense and respond directly and indirectly to steroid hormones, which regulate diverse sets of target genes via interactions with nuclear hormone receptors. Hormone-regulated networks likely integrate the actions of multiple systemic signals to adjust the activity of stem cell lineages in response to changes in physiological status.

SUMMARY

Hormones are inextricably linked to animal physiology, and can control stem cells and their local niches. Elucidating the molecular mechanisms of hormone signaling in stem cells is essential for our understanding of the fundamental underpinnings of stem cell biology, and for informing new therapeutic interventions against cancers or for regenerative medicine.

Ovarian ecdysteroid biosynthesis and female germline stem cells

The germline stem cells (GSCs) are critical for gametogenesis throughout the adult life. Stem cell identity is maintained by local signals from a specialized microenvironment called the niche. However, it is unclear how systemic signals regulate stem cell activity in response to environmental cues. In our previous article, we reported that mating stimulates GSC proliferation in female Drosophila. The mating-induced GSC proliferation is mediated by ovarian ecdysteroids, whose biosynthesis is positively controlled by Sex peptide signaling. Here, we characterized the post-eclosion and post-mating expression pattern of the genes encoding the ecdysteroidogenic enzymes in the ovary. We further investigated the biosynthetic functions of the ovarian ecdysteroid in GSC maintenance in the mated females. We also briefly discuss the regulation of the ecdysteroidogenic enzyme-encoding genes and the subsequent ecdysteroid biosynthesis in the ovary of the adult Drosophila.

A visual screen for diet-regulated proteins in the Drosophila ovary using GFP protein trap lines

The effect of diet on reproduction is well documented in a large number of organisms; however, much remains to be learned about the molecular mechanisms underlying this connection. The Drosophila ovary has a well described, fast and largely reversible response to diet. Ovarian stem cells and their progeny proliferate and grow faster on a yeast-rich diet than on a yeast-free (poor) diet, and death of early germline cysts, degeneration of early vitellogenic follicles and partial block in ovulation further contribute to the ∼60-fold decrease in egg laying observed on a poor diet. Multiple diet-dependent factors, including insulin-like peptides, the steroid ecdysone, the nutrient sensor Target of Rapamycin, AMP-dependent kinase, and adipocyte factors mediate this complex response. Here, we describe the results of a visual screen using a collection of green fluorescent protein (GFP) protein trap lines to identify additional factors potentially involved in this response. In each GFP protein trap line, an artificial GFP exon is fused in frame to an endogenous protein, such that the GFP fusion pattern parallels the levels and subcellular localization of the corresponding native protein. We identified 53 GFP-tagged proteins that exhibit changes in levels and/or subcellular localization in the ovary at 12-16 hours after switching females from rich to poor diets, suggesting them as potential candidates for future functional studies.

Control of Germline Stem Cell Lineages by Diet and Physiology

Tight coupling of reproduction to environmental factors and physiological status is key to long-term species survival. In particular, highly conserved pathways modulate germline stem cell lineages according to nutrient availability. This chapter focuses on recent in vivo studies in genetic model organisms that shed light on how diet-dependent signals control the proliferation, maintenance, and survival of adult germline stem cells and their progeny. These signaling pathways can operate intrinsically in the germ line, modulate the niche, or act through intermediate organs to influence stem cells and their differentiating progeny. In addition to illustrating the extent of dietary regulation of reproduction, findings from these studies have implications for fertility during aging or disease states.

CONSERVED AND EXAPTED FUNCTIONS OF NUCLEAR RECEPTORS IN ANIMAL DEVELOPMENT

The nuclear receptor gene family includes 18 members that are broadly conserved among multiple disparate animal phyla, indicating that they trace their evolutionary origins to the time at which animal life arose. Typical nuclear receptors contain two major domains: a DNA-binding domain and a C-terminal domain that may bind a lipophilic hormone. Many of these nuclear receptors play varied roles in animal development, including coordination of life cycle events and cellular differentiation. The well-studied genetic model systems of Drosophila, C. elegans, and mouse permit an evaluation of the extent to which nuclear receptor function in development is conserved or exapted (repurposed) over animal evolution. While there are some specific examples of conserved functions and pathways, there are many clear examples of exaptation. Overall, the evolutionary theme of exaptation appears to be favored over strict functional conservation. Despite strong conservation of DNA-binding domain sequences and activity, the nuclear receptors prove to be highly-flexible regulators of animal development.

A Genetic Mosaic Screen Reveals Ecdysone-Responsive Genes Regulating Drosophila Oogenesis

Multiple aspects of Drosophila oogenesis, including germline stem cell activity, germ cell differentiation, and follicle survival, are regulated by the steroid hormone ecdysone. While the transcriptional targets of ecdysone signaling during development have been studied extensively, targets in the ovary remain largely unknown. Early studies of salivary gland polytene chromosomes led to a model in which ecdysone stimulates a hierarchical transcriptional cascade, wherein a core group of ecdysone-sensitive transcription factors induce tissue-specific responses by activating secondary branches of transcriptional targets. More recently, genome-wide approaches have identified hundreds of putative ecdysone-responsive targets. Determining whether these putative targets represent bona fide targets in vivo, however, requires that they be tested via traditional mutant analysis in a cell-type specific fashion. To investigate the molecular mechanisms whereby ecdysone signaling regulates oogenesis, we used genetic mosaic analysis to screen putative ecdysone-responsive genes for novel roles in the control of the earliest steps of oogenesis. We identified a cohort of genes required for stem cell maintenance, stem and progenitor cell proliferation, and follicle encapsulation, growth, and survival. These genes encode transcription factors, chromatin modulators, and factors required for RNA transport, stability, and ribosome biogenesis, suggesting that ecdysone might control a wide range of molecular processes during oogenesis. Our results suggest that, although ecdysone target genes are known to have cell type-specific roles, many ecdysone response genes that control larval or pupal cell types at developmental transitions are used reiteratively in the adult ovary. These results provide novel insights into the molecular mechanisms by which ecdysone signaling controls oogenesis, laying new ground for future studies.

Mating-Induced Increase in Germline Stem Cells via the Neuroendocrine System in Female Drosophila

Mating and gametogenesis are two essential components of animal reproduction. Gametogenesis must be modulated by the need for gametes, yet little is known of how mating, a process that utilizes gametes, may modulate the process of gametogenesis. Here, we report that mating stimulates female germline stem cell (GSC) proliferation in Drosophila melanogaster. Mating-induced increase in GSC number is not simply owing to the indirect effect of emission of stored eggs, but rather is stimulated by a male-derived Sex Peptide (SP) and its receptor SPR, the components of a canonical neuronal pathway that induces a post-mating behavioral switch in females. We show that ecdysteroid, the major insect steroid hormone, regulates mating-induced GSC proliferation independently of insulin signaling. Ovarian ecdysteroid level increases after mating and transmits its signal directly through the ecdysone receptor expressed in the ovarian niche to increase the number of GSCs. Impairment of ovarian ecdysteroid biosynthesis disrupts mating-induced increase in GSCs as well as egg production. Importantly, feeding of ecdysteroid rescues the decrease in GSC number caused by impairment of neuronal SP signaling. Our study illustrates how female GSC activity is coordinately regulated by the neuroendocrine system to sustain reproductive success in response to mating.

The Genetic Architecture of Natural Variation in Recombination Rate in Drosophila melanogaster

Meiotic recombination ensures proper chromosome segregation in many sexually reproducing organisms. Despite this crucial function, rates of recombination are highly variable within and between taxa, and the genetic basis of this variation remains poorly understood. Here, we exploit natural variation in the inbred, sequenced lines of the Drosophila melanogaster Genetic Reference Panel (DGRP) to map genetic variants affecting recombination rate. We used a two-step crossing scheme and visible markers to measure rates of recombination in a 33 cM interval on the X chromosome and in a 20.4 cM interval on chromosome 3R for 205 DGRP lines. Though we cannot exclude that some biases exist due to viability effects associated with the visible markers used in this study, we find ~2-fold variation in recombination rate among lines. Interestingly, we further find that recombination rates are uncorrelated between the two chromosomal intervals. We performed a genome-wide association study to identify genetic variants associated with recombination rate in each of the two intervals surveyed. We refined our list of candidate variants and genes associated with recombination rate variation and selected twenty genes for functional assessment. We present strong evidence that five genes are likely to contribute to natural variation in recombination rate in D. melanogaster; these genes lie outside the canonical meiotic recombination pathway. We also find a weak effect of Wolbachia infection on recombination rate and we confirm the interchromosomal effect. Our results highlight the magnitude of population variation in recombination rate present in D. melanogaster and implicate new genetic factors mediating natural variation in this quantitative trait.

During meiosis, homologous chromosomes exchange genetic material through recombination. In most sexually reproducing species, recombination is necessary for chromosomes to properly segregate. Recombination defects can generate gametes with an incorrect number of chromosomes, which is devastating for organismal fitness. Despite the central role of recombination for chromosome segregation, recombination is highly variable process both within and between species. Though it is clear that this variation is due at least in part to genetics, the specific genes contributing to variation in recombination within and between species remain largely unknown. This is particularly true in the model organism, Drosophila melanogaster. Here, we use the D. melanogaster Genetic Reference Panel to determine the scale of population-level variation in recombination rate and to identify genes significantly associated with this variation. We estimated rates of recombination on two different chromosomes in 205 strains of D. melanogaster. We also used genome-wide association mapping to identify genetic factors associated with recombination rate variation. We find that recombination rate on the two chromosomes are independent traits. We further find that population-level variation in recombination is mediated by many loci of small effect, and that the genes contributing to variation in recombination rate are outside of the well-characterized meiotic recombination pathway.

The ecdysone receptor signalling regulates microvilli formation in follicular epithelial cells

Epithelial morphogenesis contributes greatly to the development and homeostasis of the organs and body parts. Here, we analysed the consequences of impaired ecdysone receptor (EcR) signalling in the Drosophila follicular epithelium. Besides governing cell growth, the three EcR isoforms act redundantly in controlling follicle cell positioning. Flattening of the microvilli and an aberrant actin cytoskeleton arise from defective EcR signalling in follicle cells, and these defects impact on the organisation of the oocyte membrane. We found that this signalling governs a complex molecular network since its impairment affects key molecules as atypical protein kinase C and activated Moesin. Interestingly, the activity of the transcription factor Tramtrack69 isoform is required for microvilli and their actin core morphogenesis as well as for follicle cell positioning. In conclusion, our findings provide evidence of novel roles for EcR signalling and Tramtrack69 transcription factor in controlling stage-specific differentiation events that take place in the follicular epithelium.

OXIDIZED LIPIDS DID NOT REDUCE LIFESPAN IN THE FRUIT FLY, Drosophila melanogaster

Aging is often associated with accumulation of oxidative damage in proteins and lipids. However, some studies do not support this view, raising the question of whether high levels of oxidative damage are associated with lifespan. In the current investigation, Drosophila melanogaster flies were kept on diets with 2 or 10% of either glucose or fructose. The lifespan, fecundity, and feeding as well as amounts of protein carbonyls (PC) and lipid peroxides (LOOH), activities of superoxide dismutase (SOD), catalase, glutathione-S-transferase (GST), and glutathione reductase activity of thioredoxin reductase (TrxR) were measured in "young" (10-day old) and "aged" (50-day old) flies. Flies maintained on diets with 10% carbohydrate lived longer than those on the 2% diets. However, neither lifespan nor fecundity was affected by the type of carbohydrate. The amount of PC was unaffected by diet and age, whereas flies fed on diets with 10% carbohydrate had about fivefold higher amounts of LOOH compared to flies maintained on the 2% carbohydrate diets. Catalase activity was significantly lower in flies fed on diets with 10% carbohydrates compared to flies on 2% carbohydrate diets. The activities of SOD, GST, and TrxR were not affected by the diet or age of the flies. The higher levels of LOOH in flies maintained on 10% carbohydrate did not reduce their lifespan, from which we infer that oxidative damage to only one class of biomolecules, particularly lipids, is not sufficient to influence lifespan.