Eggs to die for: cell death during Drosophila oogenesis

Integrated stress response signaling acts as a metabolic sensor in fat tissues to regulate oocyte maturation and ovulation

Reproduction is an energy-intensive process requiring systemic coordination. However, the inter-organ signaling mechanisms that relay nutrient status to modulate reproductive output are poorly understood. Here, we use Drosophila melanogaster as a model to establish the integrated stress response (ISR) transcription factor, Atf4, as a fat tissue metabolic sensor that instructs oogenesis. We demonstrate that Atf4 regulates lipase activity to mediate yolk lipoprotein synthesis in the fat body. Depletion of Atf4 in the fat body also blunts oogenesis recovery after amino acid deprivation and re-feeding, suggestive of a nutrient-sensing role for Atf4. We also discovered that Atf4 promotes secretion of a fat-body-derived neuropeptide, CNMamide, which modulates neural circuits that promote egg-laying behavior (ovulation). Thus, we posit that ISR signaling in fat tissue acts as a "metabolic sensor" that instructs female reproduction-directly by impacting yolk lipoprotein production and follicle maturation and systemically by regulating ovulation.

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Size-dependent and sex-specific negative effects of micro- and nano-sized polystyrene particles in the terrestrial invertebrate model Drosophila melanogaster

Microplastic pollution is believed to be one of the most widespread and long-lasting changes on a global scale. Our understanding that microplastics significantly impact terrestrial systems and are a global change stressor continues to grow. In the present study, we investigated the negative effect of long-term (28 days of exposure in food) polystyrene particles of micro (1.0-1.9 µm, 0.4-0.6 µm) and nano (0.04-0.06 µm) scale, in low doses, on the fruit fly - representing a common, globally distributed terrestrial invertebrate, and a model species in many fields. Our observations involved such parameters as ingestion and transfer of particles, survival, reproduction, changes in ultrastructure and tissue and cell responses in midgut epithelium (the place of direct contact with plastic), ovary, and testis in adults, and transgenerational effects in larvae. These observations may indicate possible toxic effects of the tested substances, even in low doses, that can be expected in other taxa, in terrestrial ecosystems. We observed a negative impact of polystyrene particles on the fruit fly survival, midgut, ovary, and testis, involving ultrastructural alterations, such as autophagy and/or ultimately necrosis in the midgut, triggering oxidative stress and activating processes of antioxidative protection. Despite the changes, midgut function and reproduction were not altered - spermatogenesis and oogenesis proceeded normally. The effect was size-dependent - the smaller the polystyrene particles were, the more substantial was the impact they caused. Ultrastructural changes and studied parameters, i.e., generation of ROS (overproduction of which generates oxidative stress), total glutathione concentration (involved in defense against ROS, acting in distinct pathways), and total antioxidant concentration (the oxidative defense system) showed the highest levels after exposure to the smallest nanoparticles, and vice versa. The effect was also sex-dependent, with male flies being more sensitive. Negative effects in males were more substantial and more prominent, even after contact with larger particles, compared to females. The smaller particles (0.4-0.6 µm, 0.04-0.06 µm) were transferred to the ovary and accumulated in the oocytes. In this case, a transgenerational negative effect was detected in larvae. It was characterized by size-dependent alterations, with smaller particles triggering higher levels of ROS and cellular oxidative response. Only the largest particles (1.0-1.9 µm) did not pass into the gonad and did not alter the larvae. These observations together demonstrated that polystyrene particles of micro- and nanoscale, even in a low dose, can induce numerous negative effects on terrestrial invertebrates.

Adverse effects of knocking down chitin synthase A on female reproduction in Culex pipiens pallens (Diptera: Culicidae)

BACKGROUND

Current mosquito-borne disease vector control strategies, largely based on chemical insecticides, are seriously threatened by increasing resistance worldwide. There is also growing concerned about the adverse effects of insecticides on nontarget organisms and the environment, therefore effective and ecologically friendly alternative approaches are urgently needed. Targeting critical steps of reproduction is considered a potential way to control mosquito populations. Herein, we focused on the roles of chitin synthase A (encoded by chsa) in the reproduction of female mosquitoes.

RESULTS

The injection of small interfering RNA targeting Cpchsa in female Culex pipiens pallens (Diptera: Culicidae) had antireproductive effects, including decreased follicle numbers, egg-laying, and hatching rate. Scanning electron microscopy observations showed that Cpchsa silencing caused a defective egg envelope, including absence of the vitelline membrane and cracked chorion layers, which resulted in abnormal permeability. Widely distributed nurse cell apoptosis and follicular epithelial cell autophagy were observed in Cpchsa-silenced ovaries during the vitellogenesis phase. Consistent with the detective egg envelope formation during oogenesis, the exochorionic eggshell structures were also affected in eggs deposited by Cpchsa-silenced mosquitoes.

CONCLUSION

This study provided fundamental evidence for the role of chitin synthase A in the female reproductive process of mosquitoes and might result in a novel alternative strategy for mosquito control. © 2023 Society of Chemical Industry.

Adipose triglyceride lipase promotes prostaglandin-dependent actin remodeling by regulating substrate release from lipid droplets

Lipid droplets (LDs), crucial regulators of lipid metabolism, accumulate during oocyte development. However, their roles in fertility remain largely unknown. During Drosophila oogenesis, LD accumulation coincides with the actin remodeling necessary for follicle development. Loss of the LD-associated Adipose Triglyceride Lipase (ATGL) disrupts both actin bundle formation and cortical actin integrity, an unusual phenotype also seen when the prostaglandin (PG) synthase Pxt is missing. Dominant genetic interactions and PG treatment of follicles indicate that ATGL acts upstream of Pxt to regulate actin remodeling. Our data suggest that ATGL releases arachidonic acid (AA) from LDs to serve as the substrate for PG synthesis. Lipidomic analysis detects AA-containing triglycerides in ovaries, and these are increased when ATGL is lost. High levels of exogenous AA block follicle development; this is enhanced by impairing LD formation and suppressed by reducing ATGL. Together, these data support the model that AA stored in LD triglycerides is released by ATGL to drive the production of PGs, which promote the actin remodeling necessary for follicle development. We speculate that this pathway is conserved across organisms to regulate oocyte development and promote fertility.

The dynamic duo of microtubule polymerase Mini spindles/XMAP215 and cytoplasmic dynein is essential for maintaining Drosophila oocyte fate

In many species, only one oocyte is specified among a group of interconnected germline sister cells. In Drosophila melanogaster, 16 interconnected cells form a germline cyst, where one cell differentiates into an oocyte, while the rest become nurse cells that supply the oocyte with mRNAs, proteins, and organelles through intercellular cytoplasmic bridges named ring canals via microtubule-based transport. In this study, we find that a microtubule polymerase Mini spindles (Msps), the Drosophila homolog of XMAP215, is essential for maintenance of the oocyte specification. mRNA encoding Msps is transported and concentrated in the oocyte by dynein-dependent transport along microtubules. Translated Msps stimulates microtubule polymerization in the oocyte, causing more microtubule plus ends to grow from the oocyte through the ring canals into nurse cells, further enhancing nurse cell-to-oocyte transport by dynein. Knockdown of msps blocks the oocyte growth and causes gradual loss of oocyte determinants. Thus, the Msps-dynein duo creates a positive feedback loop, ensuring oocyte fate maintenance by promoting high microtubule polymerization activity in the oocyte, and enhancing dynein-dependent nurse cell-to-oocyte transport.

Histone methylation regulates reproductive diapause in Drosophila melanogaster

Fluctuating environments threaten fertility and viability. To better match the immediate, local environment, many organisms adopt alternative phenotypic states, a phenomenon called "phenotypic plasticity." Natural populations that predictably encounter fluctuating environments tend to be more plastic than conspecific populations that encounter a constant environment, suggesting that phenotypic plasticity can be adaptive. Despite pervasive evidence of such "adaptive phenotypic plasticity," gene regulatory mechanisms underlying plasticity remains poorly understood. Here we test the hypothesis that environment-dependent phenotypic plasticity is mediated by epigenetic factors. To test this hypothesis, we exploit the adaptive reproductive arrest of Drosophila melanogaster females, called diapause. Using an inbred line from a natural population with high diapause plasticity, we demonstrate that diapause is determined epigenetically: only a subset of genetically identical individuals enter diapause and this diapause plasticity is epigenetically transmitted for at least three generations. Upon screening a suite of epigenetic marks, we discovered that the active histone marks H3K4me3 and H3K36me1 are depleted in diapausing ovaries. Using ovary-specific knockdown of histone mark writers and erasers, we demonstrate that H3K4me3 and H3K36me1 depletion promotes diapause. Given that diapause is highly polygenic, that is, distinct suites of alleles mediate diapause plasticity across distinct genotypes, we also investigated the potential for genetic variation in diapause-determining epigenetic marks. Specifically, we asked if these histone marks were similarly depleted in diapause of a genotypically distinct line. We found evidence of divergence in both the gene expression program and histone mark abundance. This study reveals chromatin determinants of phenotypic plasticity and suggests that these determinants may be genotype-dependent, offering new insight into how organisms may exploit and evolve epigenetic mechanisms to persist in fluctuating environments.

The RagA GTPase protects young egg chambers in Drosophila

The preservation of female fertility under unfavorable conditions is essential for animal reproduction. Inhibition of the target of rapamycin complex 1 (TORC1) is indispensable for Drosophila young egg chamber maintenance under nutrient starvation. Here, we show that knockdown of RagA results in young egg chamber death independent of TORC1 hyperactivity. RagA RNAi ovaries have autolysosomal acidification and degradation defects, which make the young egg chambers sensitive to autophagosome augmentation. Meanwhile, RagA RNAi ovaries have nuclear-localized Mitf, which promotes autophagic degradation and protects young egg chambers under stress. Interestingly, GDP-bound RagA rescues autolysosome defects, while GTP-bound RagA rescues Mitf nuclear localization in RagA RNAi young egg chambers. Moreover, Rag GTPase activity, rather than TORC1 activity, controls Mitf cellular localization in the Drosophila germ line. Our work suggests that RagA separately controls autolysosomal acidification and Mitf activity in the Drosophila young egg chambers.

Drosophila oocyte specification is maintained by the dynamic duo of microtubule polymerase Mini spindles/XMAP215 and dynein

In many species, only one oocyte is specified among a group of interconnected germline sister cells. In Drosophila melanogaster , 16-cell interconnected cells form a germline cyst, where one cell differentiates into an oocyte, while the rest become nurse cells that supply the oocyte with mRNAs, proteins, and organelles through intercellular cytoplasmic bridges named ring canals via microtubule-based transport. In this study, we find that a microtubule polymerase Mini spindles (Msps), the Drosophila homolog of XMAP215, is essential for the oocyte fate determination. mRNA encoding Msps is concentrated in the oocyte by dynein-dependent transport along microtubules. Translated Msps stimulates microtubule polymerization in the oocyte, causing more microtubule plus ends to grow from the oocyte through the ring canals into nurse cells, further enhancing nurse cell-to-oocyte transport by dynein. Knockdown of msps blocks the oocyte growth and causes gradual loss of oocyte determinants. Thus, the Msps-dynein duo creates a positive feedback loop, enhancing dynein-dependent nurse cell-to-oocyte transport and transforming a small stochastic difference in microtubule polarity among sister cells into a clear oocyte fate determination.

Significance statement

Oocyte determination in Drosophila melanogaster provides a valuable model for studying cell fate specification. We describe the crucial role of the duo of microtubule polymerase Mini spindles (Msps) and cytoplasmic dynein in this process. We show that Msps is essential for oocyte fate determination. Msps concentration in the oocyte is achieved through dynein-dependent transport of msps mRNA along microtubules. Translated Msps stimulates microtubule polymerization in the oocyte, further enhancing nurse cell-to-oocyte transport by dynein. This creates a positive feedback loop that transforms a small stochastic difference in microtubule polarity among sister cells into a clear oocyte fate determination. Our findings provide important insights into the mechanisms of oocyte specification and have implications for understanding the development of multicellular organisms.

Go with the flow - bulk transport by molecular motors

Cells are the smallest building blocks of all living eukaryotic organisms, usually ranging from a couple of micrometers (for example, platelets) to hundreds of micrometers (for example, neurons and oocytes) in size. In eukaryotic cells that are more than 100 µm in diameter, very often a self-organized large-scale movement of cytoplasmic contents, known as cytoplasmic streaming, occurs to compensate for the physical constraints of large cells. In this Review, we discuss cytoplasmic streaming in multiple cell types and the mechanisms driving this event. We particularly focus on the molecular motors responsible for cytoplasmic movements and the biological roles of cytoplasmic streaming in cells. Finally, we describe bulk intercellular flow that transports cytoplasmic materials to the oocyte from its sister germline cells to drive rapid oocyte growth.

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.

Non-autonomous cell death induced by the Draper phagocytosis receptor requires signaling through the JNK and SRC pathways

The last step of cell death is cell clearance, a process critical for tissue homeostasis. For efficient cell clearance to occur, phagocytes and dead cells need to reciprocally signal to each other. One important phenomenon that is under-investigated, however, is that phagocytes not only engulf corpses but contribute to cell death progression. The aims of this study were to determine how the phagocytic receptor Draper non-autonomously induces cell death, using the Drosophila ovary as a model system. We found that Draper, expressed in epithelial follicle cells, requires its intracellular signaling domain to kill the adjacent nurse cell population. Kinases Src42A, Shark and JNK (Bsk) were required for Draper-induced nurse cell death. Signs of nurse cell death occurred prior to apparent engulfment and required the caspase Dcp-1, indicating that it uses a similar apoptotic pathway to starvation-induced cell death. These findings indicate that active signaling by Draper is required to kill nurse cells via the caspase Dcp-1, providing novel insights into mechanisms of phagoptosis driven by non-professional phagocytes.

A novel mechanism of bulk cytoplasmic transport by cortical dynein in Drosophila ovary

Cytoplasmic dynein, a major minus-end directed microtubule motor, plays essential roles in eukaryotic cells. Drosophila oocyte growth is mainly dependent on the contribution of cytoplasmic contents from the interconnected sister cells, nurse cells. We have previously shown that cytoplasmic dynein is required for Drosophila oocyte growth and assumed that it simply transports cargoes along microtubule tracks from nurse cells to the oocyte. Here, we report that instead of transporting individual cargoes along stationary microtubules into the oocyte, cortical dynein actively moves microtubules within nurse cells and from nurse cells to the oocyte via the cytoplasmic bridges, the ring canals. This robust microtubule movement is sufficient to drag even inert cytoplasmic particles through the ring canals to the oocyte. Furthermore, replacing dynein with a minus-end directed plant kinesin linked to the actin cortex is sufficient for transporting organelles and cytoplasm to the oocyte and driving its growth. These experiments show that cortical dynein performs bulk cytoplasmic transport by gliding microtubules along the cell cortex and through the ring canals to the oocyte. We propose that the dynein-driven microtubule flow could serve as a novel mode of fast cytoplasmic transport.

Enhanced germline stem cell longevity in Drosophila diapause

In many species including humans, aging reduces female fertility. Intriguingly, some animals preserve fertility longer under specific environmental conditions. For example, at low temperature and short day-length, Drosophila melanogaster enters a state called adult reproductive diapause. As in other stressful conditions, ovarian development arrests at the yolk uptake checkpoint; however, mechanisms underlying fertility preservation and post-diapause recovery are largely unknown. Here, we report that diapause causes more complete arrest than other stresses yet preserves greater recovery potential. During dormancy, germline stem cells (GSCs) incur DNA damage, activate p53 and Chk2, and divide less. Despite reduced niche signaling, germline precursor cells do not differentiate. GSCs adopt an atypical, suspended state connected to their daughters. Post-diapause recovery of niche signaling and resumption of division contribute to restoring GSCs. Mimicking one feature of quiescence, reduced juvenile hormone production, enhanced GSC longevity in non-diapausing flies. Thus, diapause mechanisms provide approaches to GSC longevity enhancement.

Drosophila enter adult reproductive diapause in low temperatures and short day, halting ovarian development yet preserving fertility. Here the authors show that ovarian arrest in diapause is distinct from other stress responses and that despite DNA damage and decreased division, germline stem cells recover.

An RNAi screen of the kinome in epithelial follicle cells of the Drosophila melanogaster ovary reveals genes required for proper germline death and clearance

Programmed cell death and cell corpse clearance are an essential part of organismal health and development. Cell corpses are often cleared away by professional phagocytes such as macrophages. However, in certain tissues, neighboring cells known as nonprofessional phagocytes can also carry out clearance functions. Here, we use the Drosophila melanogaster ovary to identify novel genes required for clearance by nonprofessional phagocytes. In the Drosophila ovary, germline cells can die at multiple time points. As death proceeds, the epithelial follicle cells act as phagocytes to facilitate the clearance of these cells. We performed an unbiased kinase screen to identify novel proteins and pathways involved in cell clearance during two death events. Of 224 genes examined, 18 demonstrated severe phenotypes during developmental death and clearance while 12 demonstrated severe phenotypes during starvation-induced cell death and clearance, representing a number of pathways not previously implicated in phagocytosis. Interestingly, it was found that several genes not only affected the clearance process in the phagocytes, but also non-autonomously affected the process by which germline cells died. This kinase screen has revealed new avenues for further exploration and investigation.

Murder on the Ovarian Express: A Tale of Non-Autonomous Cell Death in the Drosophila Ovary

Throughout oogenesis, Drosophila egg chambers traverse the fine line between survival and death. After surviving the ten early and middle stages of oogenesis, egg chambers drastically change their size and structure to produce fully developed oocytes. The development of an oocyte comes at a cost, the price is the lives of the oocyte’s 15 siblings, the nurse cells. These nurse cells do not die of their own accord. Their death is dependent upon their neighbors—the stretch follicle cells. Stretch follicle cells are nonprofessional phagocytes that spend the final stages of oogenesis surrounding the nurse cells and subsequently forcing the nurse cells to give up everything for the sake of the oocyte. In this review, we provide an overview of cell death in the ovary, with a focus on recent findings concerning this phagocyte-dependent non-autonomous cell death.

Gatekeeper function for Short stop at the ring canals of the Drosophila ovary

Growth of the Drosophila oocyte requires transport of cytoplasmic materials from the interconnected sister cells (nurse cells) through ring canals, the cytoplasmic bridges that remained open after incomplete germ cell division. Given the open nature of the ring canals, it is unclear how the direction of transport through the ring canal is controlled. In this work, we show that a single Drosophila spectraplakin Short stop (Shot) controls the direction of flow from nurse cells to the oocyte. Knockdown of shot changes the direction of transport through the ring canals from unidirectional (toward the oocyte) to bidirectional. After shot knockdown, the oocyte stops growing, resulting in a characteristic small oocyte phenotype. In agreement with this transport-directing function of Shot, we find that it is localized at the asymmetric actin baskets on the nurse cell side of the ring canals. In wild-type egg chambers, microtubules localized in the ring canals have uniform polarity (minus ends toward the oocyte), while in the absence of Shot, these microtubules have mixed polarity. Together, we propose that Shot functions as a gatekeeper directing transport from nurse cells to the oocyte via the organization of microtubule tracks to facilitate the transport driven by the minus-end-directed microtubule motor cytoplasmic dynein. VIDEO ABSTRACT.

Mitochondrial Respiration in Drosophila Ovaries after a Full Cycle of Oogenesis under Simulated Microgravity

Studies of the function of the female reproductive system in zero gravity are urgent for the future exploration of deep space. Female reproductive cells, oocytes, are rich in mitochondria, which allow oocytes to produce embryos. The rate of cellular respiration was determined to assess the functional state of the mitochondrial apparatus in Drosophila melanogaster ovaries in which the full cycle of oogenesis took place under simulated microgravity. Since cellular respiration depends on the state of the cytoskeleton, the contents of the main cytoskeletal proteins were determined by Western blotting. To modulate the structure of the cytoskeleton, essential phospholipids were administered per os at a dosage of 500 mg/kg in medium. The results of this study show that after a full cycle of oogenesis under simulated microgravity, the rate of cellular respiration in the fruit fly ovaries increases, apparently due to complex II of the respiratory chain. At the same time, we did not find any changes in the area of oocytes or in the content of proteins in the respiratory chain. However, changes were found in the relative contents of proteins of the actin cytoskeleton. There were no changes of essential phospholipids and no increase in the rate of cellular respiration of the ovaries after exposure to simulated microgravity. However, in the control, the administration of essential phospholipids led to a decrease in the efficiency of oxygen consumption in the flies’ ovaries due to complexes IV–V.

A region of SLBP outside the mRNA-processing domain is essential for deposition of histone mRNA into the Drosophila egg

Replication-dependent histone mRNAs are the only cellular mRNAs that are not polyadenylated, ending in a stemloop instead of a polyA tail, and are normally regulated coordinately with DNA replication. Stemloop-binding protein (SLBP) binds the 3' end of histone mRNA, and is required for processing and translation. During Drosophila oogenesis, large amounts of histone mRNAs and proteins are deposited in the developing oocyte. The maternally deposited histone mRNA is synthesized in stage 10B oocytes after the nurse cells complete endoreduplication. We report that in wild-type stage 10B oocytes, the histone locus bodies (HLBs), formed on the histone genes, produce histone mRNAs in the absence of phosphorylation of Mxc, which is normally required for histone gene expression in S-phase cells. Two mutants of SLBP, one with reduced expression and another with a 10-amino-acid deletion, fail to deposit sufficient histone mRNA in the oocyte, and do not transcribe the histone genes in stage 10B. Mutations in a putative SLBP nuclear localization sequence overlapping the deletion phenocopy the deletion. We conclude that a high concentration of SLBP in the nucleus of stage 10B oocytes is essential for histone gene transcription.This article has an associated First Person interview with the first author of the paper.

Modulation of Cell-Cell Interactions in Drosophila Oocyte Development

The Drosophila ovary offers a suitable model system to study the mechanisms that orchestrate diverse cellular processes. Oogenesis starts from asymmetric stem cell division, proper differentiation and the production of fully patterned oocytes equipped with all the maternal information required for embryogenesis. Spatial and temporal regulation of cell-cell interaction is particularly important to fulfill accurate biological outcomes at each step of oocyte development. Progress has been made in understanding diverse cell physiological regulation of signaling. Here we review the roles of specialized cellular machinery in cell-cell communication in different stages of oogenesis.

Transmission of Functional, Wild-Type Mitochondria and the Fittest mtDNA to the Next Generation: Bottleneck Phenomenon, Balbiani Body, and Mitophagy

The most important role of mitochondria is to supply cells with metabolic energy in the form of adenosine triphosphate (ATP). As synthesis of ATP molecules is accompanied by the generation of reactive oxygen species (ROS), mitochondrial DNA (mtDNA) is highly vulnerable to impairment and, consequently, accumulation of deleterious mutations. In most animals, mitochondria are transmitted to the next generation maternally, i.e., exclusively from female germline cells (oocytes and eggs). It has been suggested, in this context, that a specialized mechanism must operate in the developing oocytes enabling escape from the impairment and subsequent transmission of accurate (devoid of mutations) mtDNA from one generation to the next. Literature survey suggest that two distinct and irreplaceable pathways of mitochondria transmission may be operational in various animal lineages. In some taxa, the mitochondria are apparently selected: functional mitochondria with high inner membrane potential are transferred to the cells of the embryo, whereas those with low membrane potential (overloaded with mutations in mtDNA) are eliminated by mitophagy. In other species, the respiratory activity of germline mitochondria is suppressed and ROS production alleviated leading to the same final effect, i.e., transmission of undamaged mitochondria to offspring, via an entirely different route.

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.

Hijacking Oogenesis Enables Massive Propagation of LINE and Retroviral Transposons

Although animals have evolved multiple mechanisms to suppress transposons, "leaky" mobilizations that cause mutations and diseases still occur. This suggests that transposons employ specific tactics to accomplish robust propagation. By directly tracking mobilization, we show that, during a short and specific time window of oogenesis, retrotransposons achieve massive amplification via a cell-type-specific targeting strategy. Retrotransposons rarely mobilize in undifferentiated germline stem cells. However, as oogenesis proceeds, they utilize supporting nurse cells-which are highly polyploid and eventually undergo apoptosis-as factories to massively manufacture invading products. Moreover, retrotransposons rarely integrate into nurse cells themselves but, instead, via microtubule-mediated transport, they preferentially target the DNA of the interconnected oocytes. Blocking microtubule-dependent intercellular transport from nurse cells significantly alleviates damage to the oocyte genome. Our data reveal that parasitic genomic elements can efficiently hijack a host developmental process to propagate robustly, thereby driving evolutionary change and causing disease.

Stonewall and Brickwall: Two Partially Redundant Determinants Required for the Maintenance of Female Germline in Drosophila

Proper specification of germline stem cells (GSCs) in Drosophila ovaries depends on niche derived non-autonomous signaling and cell autonomous components of transcriptional machinery. Stonewall (Stwl), a MADF-BESS family protein, is one of the cell intrinsic transcriptional regulators involved in the establishment and/or maintenance of GSC fate in Drosophila ovaries. Here we report identification and functional characterization of another member of the same protein family, CG3838/ Brickwall (Brwl) with analogous functions. Loss of function alleles of brwl exhibit age dependent progressive degeneration of the developing ovarioles and loss of GSCs. Supporting the conclusion that the structural deterioration of mutant egg chambers is a result of apoptotic cell death, activated caspase levels are considerably elevated in brwl^- ovaries. Moreover, as in the case of stwl mutants, on several instances, loss of brwl activity results in fusion of egg chambers and misspecification of the oocyte. Importantly, brwl phenotypes can be partially rescued by germline specific over-expression of stwl arguing for overlapping yet distinct functional capabilities of the two proteins. Taken together with our phylogenetic analysis, these data suggest that brwl and stwl likely share a common MADF-BESS ancestor and they are expressed in overlapping spatiotemporal domains to ensure robust development of the female germline.

Redefining reproductive dormancy in Drosophila as a general stress response to cold temperatures

Organisms regularly encounter unfavorable conditions and the genetic adaptations facilitating survival have been of long-standing interest to evolutionary biologists. Winter is one particularly stressful condition for insects, during which they encounter low temperatures and scarcity of food. Despite dormancy being a well-studied adaptation to facilitate overwintering, there is still considerable controversy about the distribution of dormancy among natural populations and between species in Drosophila. The current definition of dormancy as developmental arrest of oogenesis at the previtellogenic stage (stage 7) distinguishes dormancy from general stress related block of oogenesis at early vitellogenic stages (stages 8 - 9). In an attempt to resolve this, we scrutinized reproductive dormancy in D. melanogaster and D. simulans. We show that dormancy shows the same hallmarks of arrest of oogenesis at stage 9, as described for other stressors and propose a new classification for dormancy. Applying this modified classification, we show that both species express dormancy in cosmopolitan and African populations, further supporting that dormancy uses an ancestral pathway induced by environmental stress. While we found significant differences between individuals and the two Drosophila species in their sensitivity to cold temperature stress, we also noted that extreme temperature stress (8 °C) resulted in very strong dormancy incidence, which strongly reduced the differences seen at less extreme temperatures. We conclude that dormancy in Drosophila should not be considered a special trait, but is better understood as a generic stress response occurring at low temperatures.

Scrambled Eggs: Apoptotic Cell Clearance by Non-Professional Phagocytes in the Drosophila Ovary

For half of a century, it has been known that non-professional phagocytes, such as fibroblasts, endothelial, and epithelial cells, are capable of efferocytosis (engulfment of apoptotic cells). Non-professional phagocytes differ from professional phagocytes in the range and efficiency of engulfment. Much of the recognition and underlying signaling machinery between non-professional and professional phagocytes is the same, but it is not known how the engulfment capacity of non-professional phagocytes is controlled. Moreover, the signaling networks involved in cell corpse recognition, engulfment, and phagosome maturation are only partially understood. The Drosophila ovary provides an excellent system to investigate the regulation of phagocytic activity by epithelial cells, a major class of non-professional phagocytes. During Drosophila oogenesis, mid-stage egg chambers undergo apoptosis of the germline in response to nutrient deprivation. Epithelial follicle cells then undergo major cell shape changes and concomitantly engulf the germline material. Our previous work has established that Draper and the integrin α-PS3/β-PS heterodimer are required in follicle cells for germline cell clearance. In addition, we have characterized phagosome maturation pathways, and found that the JNK pathway amplifies the engulfment response. In this review, we discuss recent advances on the interplay between engulfment pathways in the follicular epithelium for cell clearance in the Drosophila ovary. We also provide a comparison to apoptotic cell clearance mechanisms in C. elegans and mammals, illustrating strong conservation of efferocytosis mechanisms by non-professional phagocytes.

Effects of Wolbachia on ovarian apoptosis in Culex quinquefasciatus (Say, 1823) during the previtellogenic and vitellogenic periods

Background

Apoptosis is programmed cell death that ordinarily occurs in ovarian follicular cells in various organisms. In the best-studied holometabolous insect, Drosophila, this kind of cell death occurs in all three cell types found in the follicles, sometimes leading to follicular atresia and egg degeneration. On the other hand, egg development, quantity and viability in the mosquito Culex quinquefasciatus are disturbed by the infection with the endosymbiont Wolbachia. Considering that Wolbachia alters reproductive traits, we hypothesised that such infection would also alter the apoptosis in the ovarian cells of this mosquito. The goal of this study was to comparatively describe the occurrence of apoptosis in Wolbachia-infected and uninfected ovaries of Cx. quinquefasciatus during oogenesis and vitellogenesis. For this, we recorded under confocal microscopy the occurrence of apoptosis in all three cell types of the ovarian follicle. In the first five days of adult life we observed oogenesis and, after a blood meal, the initiation step of vitellogenesis.

Results

Apoptoses in follicular cells were found at all observation times during both oogenesis and vitellogenesis, and less commonly in nurse cells and the oocyte, as well as in atretic follicles. Our results suggested that apoptosis in follicular cells occurred in greater numbers in infected mosquitoes than in uninfected ones during the second and third days of adult life and at the initiation step of vitellogenesis.

Conclusions

The presence of Wolbachia leads to an increase of apoptosis occurrence in the ovaries of Cx. quinquefasciatus. Future studies should investigate if this augmented apoptosis frequency is the cause of the reduction in the number of eggs laid by Wolbachia-infected females. Follicular atresia is first reported in the previtellogenic period of oogenesis. Our findings may have implications for the use of Wolbachia as a mosquito and pathogens control strategy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2332-0) contains supplementary material, which is available to authorized users.

The repertoire of epithelial morphogenesis on display: Progressive elaboration of Drosophila egg structure

Epithelial structures are foundational for tissue organization in all metazoans. Sheets of epithelial cells form lateral adhesive junctions and acquire apico-basal polarity perpendicular to the surface of the sheet. Genetic analyses in the insect model, Drosophila melanogaster, have greatly advanced our understanding of how epithelial organization is established, and how it is modulated during tissue morphogenesis. Major insights into collective cell migrations have come from analyses of morphogenetic movements within the adult follicular epithelium that cooperates with female germ cells to build a mature egg. Epithelial follicle cells progress through tightly choreographed phases of proliferation, patterning, reorganization and migrations, before they differentiate to form the elaborate structures of the eggshell. Distinct structural domains are organized by differential adhesion, within which lateral junctions are remodeled to further shape the organized epithelia. During collective cell migrations, adhesive interactions mediate supracellular organization of planar polarized macromolecules, and facilitate crawling over the basement membrane or traction against adjacent cell surfaces. Comparative studies with other insects are revealing the diversification of morphogenetic movements for elaboration of epithelial structures. This review surveys the repertoire of follicle cell morphogenesis, to highlight the coordination of epithelial plasticity with progressive differentiation of a secretory epithelium. Technological advances will keep this tissue at the leading edge for interrogating the precise spatiotemporal regulation of normal epithelial reorganization events, and provide a framework for understanding pathological tissue dysplasia.

The temporally controlled expression of Drongo, the fruit fly homolog of AGFG1, is achieved in female germline cells via P-bodies and its localization requires functional Rab11

To achieve proper RNA transport and localization, RNA viruses exploit cellular vesicular trafficking pathways. AGFG1, a host protein essential for HIV-1 and Influenza A replication, has been shown to mediate release of intron-containing viral RNAs from the perinuclear region. It is still unknown what its precise role in this release is, or whether AGFG1 also participates in cytoplasmic transport. We report for the first time the expression patterns during oogenesis for Drongo, the fruit fly homolog of AGFG1. We find that temporally controlled Drongo expression is achieved by translational repression of drongo mRNA within P-bodies. Here we show a first link between the recycling endosome pathway and Drongo, and find that proper Drongo localization at the oocyte's cortex during mid-oogenesis requires functional Rab11.

Drosophila glob1 is required for the maintenance of cytoskeletal integrity during oogenesis

BACKGROUND

Hemoglobins (Hbs) are evolutionarily conserved heme-containing metallo-proteins of the Globin protein family that harbour the characteristic "globin fold." Hemoglobins have been functionally diversified during evolution and their usual property of oxygen transport is rather a recent adaptation. Drosophila genome possesses three globin genes (glob1, glob2, and glob3), and we have reported earlier that adequate expression of glob1 is required for various aspects of development, as well as to regulate the cellular level of reactive oxygen species (ROS). The present study illustrates the explicit role of Drosophila globin1 in progression of oogenesis.

RESULTS

We demonstrate a dynamic expression pattern of glob1 in somatic and germ cell derivatives of developing egg chambers during various stages of oogenesis, which largely confines around the F-actin-rich cellular components. Reduced expression of glob1 leads to various types of abnormalities during oogenesis, which were primarily mediated by the inappropriately formed F-actin-based cytoskeleton. Our subsequent analysis in the somatic and germ line clones shows cell autonomous role of glob1 in the maintenance of the integrity of F-actin-based cytoskeleton components in the somatic and germ cell derivatives.

CONCLUSIONS

Our study establishes a novel role of glob1 in maintenance of F-actin-based cytoskeleton during progression of oogenesis in Drosophila. Developmental Dynamics 245:1048-1065, 2016. © 2016 Wiley Periodicals, Inc.

Ribosomal RNA gene functioning in avian oogenesis

Despite long-term exploration into ribosomal RNA gene functioning during the oogenesis of various organisms, many intriguing problems remain unsolved. In this review, we describe nucleolus organizer region (NOR) activity in avian oocytes. Whereas oocytes from an adult avian ovary never reveal the formation of the nucleolus in the germinal vesicle (GV), an ovary from juvenile birds possesses both nucleolus-containing and non-nucleolus-containing oocytes. The evolutionary diversity of oocyte NOR functioning and the potential non-rRNA-related functions of the nucleolus in oocytes are also discussed.

Maternal AP determinants in the Drosophila oocyte and embryo

An animal embryo cannot initiate its journey of forming a new life on its own. It must rely on maternally provided resources and inputs to kick-start its developmental process. In Drosophila, the initial polarities of the embryo along both the anterior-posterior (AP) and dorsal-ventral (DV) axes are also specified by maternal determinants. Over the past several decades, genetic and molecular studies have identified and characterized such determinants, as well as the zygotic genetic regulatory networks that control patterning in the early embryo. Extensive studies of oogenesis have also led to a detailed knowledge of the cellular and molecular interactions that control the formation of a mature egg. Despite these efforts, oogenesis and embryogenesis have been studied largely as separate problems, except for qualitative aspects with regard to maternal regulation of the asymmetric localization of maternal determinants. Can oogenesis and embryogenesis be viewed from a unified perspective at a quantitative level, and can that improve our understanding of how robust embryonic patterning is achieved? Here, we discuss the basic knowledge of the regulatory mechanisms controlling oogenesis and embryonic patterning along the AP axis. We explore properties of the maternal Bicoid gradient in relation to embryo size in search for a unified framework for robust AP patterning. WIREs Dev Biol 2016, 5:562-581. doi: 10.1002/wdev.235 For further resources related to this article, please visit the WIREs website.

Exclusion of dysfunctional mitochondria from Balbiani body during early oogenesis of Thermobia

Oocytes of many invertebrate and vertebrate species contain a characteristic organelle complex known as the Balbiani body (Bb). Until now, three principal functions have been ascribed to this complex: delivery of germ cell determinants and localized RNAs to the vegetal cortex/posterior pole of the oocyte, transport of the mitochondria towards the germ plasm, and participation in the formation of lipid droplets. Here, we present the results of a computer-aided 3D reconstruction of the Bb in the growing oocytes of an insect, Thermobia domestica. Our analyses have shown that, in Thermobia, the central part of each fully developed Bb comprises a single intricate mitochondrial network. This “core” network is surrounded by several isolated bean-shaped mitochondrial units that display lowered membrane potential and clear signs of degeneration. In light of the above results and recent theoretical models of mitochondrial quality control, the role of the Bb is discussed. We suggest that, in addition to the aforementioned functions, the Bb is implicated in the selective elimination of dysfunctional mitochondria during oogenesis.

Phagocytosis genes nonautonomously promote developmental cell death in the Drosophila ovary

Programmed cell death (PCD) is usually considered a cell-autonomous suicide program, synonymous with apoptosis. Recent research has revealed that PCD is complex, with at least a dozen cell death modalities. Here, we demonstrate that the large-scale nonapoptotic developmental PCD in the Drosophila ovary occurs by an alternative cell death program where the surrounding follicle cells nonautonomously promote death of the germ line. The phagocytic machinery of the follicle cells, including Draper, cell death abnormality (Ced)-12, and c-Jun N-terminal kinase (JNK), is essential for the death and removal of germ-line-derived nurse cells during late oogenesis. Cell death events including acidification, nuclear envelope permeabilization, and DNA fragmentation of the nurse cells are impaired when phagocytosis is inhibited. Moreover, elimination of a small subset of follicle cells prevents nurse cell death and cytoplasmic dumping. Developmental PCD in the Drosophila ovary is an intriguing example of nonapoptotic, nonautonomous PCD, providing insight on the diversity of cell death mechanisms.

Cracking the nodule worm code advances knowledge of parasite biology and biotechnology to tackle major diseases of livestock

Many infectious diseases caused by eukaryotic pathogens have a devastating, long-term impact on animal health and welfare. Hundreds of millions of animals are affected by parasitic nematodes of the order Strongylida. Unlocking the molecular biology of representatives of this order, and understanding nematode-host interactions, drug resistance and disease using advanced technologies could lead to entirely new ways of controlling the diseases that they cause. Oesophagostomum dentatum (nodule worm; superfamily Strongyloidea) is an economically important strongylid nematode parasite of swine worldwide. The present article reports recent advances made in biology and animal biotechnology through the draft genome and developmental transcriptome of O. dentatum, in order to support biological research of this and related parasitic nematodes as well as the search for new and improved interventions. This first genome of any member of the Strongyloidea is 443 Mb in size and predicted to encode 25,291 protein-coding genes. Here, we review the dynamics of transcription throughout the life cycle of O. dentatum, describe double-stranded RNA interference (RNAi) machinery and infer molecules involved in development and reproduction, and in inducing or modulating immune responses or disease. The secretome predicted for O. dentatum is particularly rich in peptidases linked to interactions with host tissues and/or feeding activity, and a diverse array of molecules likely involved in immune responses. This research progress provides an important resource for future comparative genomic and molecular biological investigations as well as for biotechnological research toward new anthelmintics, vaccines and diagnostic tests.

The insulator protein Suppressor of Hairy wing is required for proper ring canal development during oogenesis in Drosophila

Chromatin insulators orchestrate gene transcription during embryo development and cell differentiation by stabilizing interactions between distant genomic sites. Mutations in genes encoding insulator proteins are generally lethal, making in vivo functional analyses of insulator proteins difficult. In Drosophila, however, mutations in the gene encoding the Suppressor of Hairy wing insulator protein [Su(Hw)] are viable and female sterile, providing an opportunity to study insulator function during oocyte development. Whereas previous reports suggest that the function of Su(Hw) in oogenesis is independent of its insulator activity, many aspects of the role of Su(Hw) in Drosophila oogenesis remain unexplored. Here we show that mutations in su(Hw) result in smaller ring canal lumens and smaller outer ring diameters, which likely obstruct molecular and vesicle passage from nurse cells to the oocyte. Fluorescence microscopy reveals that lack of Su(Hw) leads to excess accumulation of Kelch (Kel) and Filament-actin (F-actin) proteins in the ring canal structures of developing egg chambers. Furthermore, we found that misexpression of the Src oncogene at 64B (Src64B) may cause ring canal development defects as microarray analysis and real-time RT-PCR revealed there is a three fold decrease in Src64B expression in su(Hw) mutant ovaries. Restoration of Src64B expression in su(Hw) mutant female germ cells rescued the ring phenotype but did not restore fertility. We conclude that loss of su(Hw) affects expression of many oogenesis related genes and down-regulates Src64B, resulting in ring canal defects potentially contributing to obstruction of molecular flow and an eventual failure of egg chamber organization.

Quantitative proteomics reveals the dynamics of protein changes during Drosophila oocyte maturation and the oocyte-to-embryo transition

The onset of development is marked by two major, posttranscriptionally controlled, events: oocyte maturation (release of the prophase I primary arrest) and egg activation (release from the secondary meiotic arrest). Using quantitative mass spectrometry, we previously described proteome remodeling during Drosophila egg activation. Here, we describe our quantitative mass spectrometry-based analysis of the changes in protein levels during Drosophila oocyte maturation. This study presents the first quantitative survey, to our knowledge, of proteome changes accompanying oocyte maturation in any organism and provides a powerful resource for identifying both key regulators and biological processes driving this critical developmental window. We show that Muskelin, found to be up-regulated during oocyte maturation, is required for timely nurse cell nuclei clearing from mature egg chambers. Other proteins up-regulated at maturation are factors needed not only for late oogenesis but also completion of meiosis and early embryogenesis. Interestingly, the down-regulated proteins are predominantly involved in RNA processing, translation, and RNAi. Integrating datasets on the proteome changes at oocyte maturation and egg activation uncovers dynamics in proteome remodeling during the change from oocyte to embryo. Notably, 66 proteins likely act uniquely during late oogenesis, because they are up-regulated at maturation and down-regulated at activation. We find down-regulation of this class of proteins to be mediated partially by APC/C(CORT), a meiosis-specific form of the E3 ligase anaphase promoting complex/cyclosome (APC/C).

Oral magnetite nanoparticles disturb the development of Drosophila melanogaster from oogenesis to adult emergence

The potential impacts of nanomaterials (NMs) on fetal development have attracted great concerns because of the increased potential exposure to NMs during pregnancy. Drosophila melanogaster oogenesis and developmental transitions may provide an attractive system to study the biological and environmental effects of NMs on the embryonic development. In this study, the effects of three types of magnetite (Fe3O4) nanoparticles (MNPs): UN-MNPs (pristine), CA-MNPs (citric acid modified) and APTS-MNPs (3-aminopropyltriethoxylsilane coated) on the development of Drosophila at 300 and 600 μg/g dosage were studied. The uptake of MNPs by female and male flies caused obvious reduction in the female fecundity, and the developmental delay at the egg-pupae and pupae-adult transitions, especially in those treated by the positive APTS-MNPs. Further investigation demonstrates that the parental uptake of MNPs disturbs the oogenesis period, induces ovarian defect, reduces the length of eggs, decreases the number of nurse cells and delays egg chamber development, which may contribute to the decrease of fecundity of female Drosophila and the development delay of their offspring. Using the synchrotron radiation-based micro-X-ray fluorescence (SR-μXRF), the dyshomeostasis of trace elements such as Fe, Ca and Cu along the anterior-posterior axis of the fertilized eggs was found, which may be an important reason for the development delay of Drosophila.

Nuclear positioning by actin cables and perinuclear actin: Special and general?

Nuclear positioning is an important process during development and homeostasis. Depending on the affected tissue, mislocalized nuclei can alter cellular processes such as polarization, differentiation, or migration and lead ultimately to diseases. Many cells actively control the position of their nucleus using their cytoskeleton and motor proteins. We have recently shown that during Drosophila oogenesis, nurse cells employ cytoplasmic actin cables in association with perinuclear actin to position their nucleus. Here, we briefly summarize our work and discuss why nuclear positioning in nurse cells is specialized but the molecular mechanisms are likely to be more generally used.

Diversity of cell death pathways: insight from the fly ovary

Multiple types of cell death exist including necrosis, apoptosis, and autophagic cell death. The Drosophila ovary provides a valuable model to study the diversity of cell death modalities, and we review recent progress to elucidate these pathways. At least five distinct types of cell death occur in the ovary, and we focus on two that have been studied extensively. Cell death of mid-stage egg chambers occurs through a novel caspase-dependent pathway that involves autophagy and triggers phagocytosis by surrounding somatic epithelial cells. For every egg, 15 germline nurse cells undergo developmental programmed cell death, which occurs independently of most known cell death genes. These forms of cell death are strikingly similar to cell death observed in the germlines of other organisms.

Parental age, gametic age, and inbreeding interact to modulate offspring viability in Drosophila melanogaster

In principle, parental relatedness, parental age, and the age of parental gametes can all influence offspring fitness through inbreeding depression and the parental effects of organismal and postmeiotic gametic senescence. However, little is known about the extent to which these factors interact and contribute to fitness variation. Here, we show that, in Drosophila melanogaster, offspring viability is strongly affected by a three-way interaction between parental relatedness, parental age, and gametic age at successive developmental stages. Overall egg-to-adult viability was lowest for offspring produced with old gametes of related, young parents. This overall effect was largely determined at the pupa-adult stage, although three-way interactions between parental relatedness, parental age and gametic age also explained variation in egg hatchability and larva-pupa survival. Controlling for the influence of parental and gametic age, we show that inbreeding depression is negligible for egg hatchability but significant at the larva-pupa and pupa-adult stages. At the pupa-adult stage, where offspring could be sexed, parental relatedness, parental age, and gametic age interacted differently in male and female offspring, with daughters suffering higher inbreeding depression than sons. Collectively, our results demonstrate that the architecture of offspring fitness is strongly influenced by a complex interaction between parental effects, inbreeding depression and offspring sex.

Diet controls Drosophila follicle stem cell proliferation via Hedgehog sequestration and release

Dietary cholesterol levels control follicle stem cell proliferation in the Drosophila ovary via regulation of Hedgehog protein localization.

A healthy diet improves adult stem cell function and delays diseases such as cancer, heart disease, and neurodegeneration. Defining molecular mechanisms by which nutrients dictate stem cell behavior is a key step toward understanding the role of diet in tissue homeostasis. In this paper, we elucidate the mechanism by which dietary cholesterol controls epithelial follicle stem cell (FSC) proliferation in the fly ovary. In nutrient-restricted flies, the transmembrane protein Boi sequesters Hedgehog (Hh) ligand at the surface of Hh-producing cells within the ovary, limiting FSC proliferation. Upon feeding, dietary cholesterol stimulates S6 kinase–mediated phosphorylation of the Boi cytoplasmic domain, triggering Hh release and FSC proliferation. This mechanism enables a rapid, tissue-specific response to nutritional changes, tailoring stem cell divisions and egg production to environmental conditions sufficient for progeny survival. If conserved in other systems, this mechanism will likely have important implications for studies on molecular control of stem cell function, in which the benefits of low calorie and low cholesterol diets are beginning to emerge.

Germ cell apoptosis and DNA damage responses

In the past 12 years, since the first description of C. elegans germ cell apoptosis, this area of research rapidly expanded. It became evident that multiple genetic pathways lead to the apoptotic demise of germ cells. We are only beginning to understand how these pathways that all require the CED-9/Bcl-2, Apaf-1/CED-4 and CED-3 caspase core apoptosis components are regulated. Physiological apoptosis, which likely accounts for the elimination of more than 50% of all germ cells, even in unperturbed conditions, is likely to be required to maintain tissue homeostasis. The best-studied pathways lead to DNA damage-induced germ cell apoptosis in response to a variety of genotoxic stimuli. This apoptosis appears to be regulated similar to DNA damage-induced apoptosis in the mouse germ line and converges on p53 family transcription factors. DNA damage response pathways not only lead to apoptosis induction, but also directly affect DNA repair, and a transient cell cycle arrest of mitotic germ cells. Finally, distinct pathways activate germ cell apoptosis in response to defects in meiotic recombination and meiotic chromosome pairing.

Drosophila egg chamber elongation: insights into how tissues and organs are shaped

As tissues and organs are formed, they acquire a specific shape that plays an integral role in their ability to function properly. A relatively simple system that has been used to examine how tissues and organs are shaped is the formation of an elongated Drosophila egg. While it has been known for some time that Drosophila egg elongation requires interactions between a polarized intracellular basal actin network and a polarized extracellular network of basal lamina proteins, how these interactions contribute to egg elongation remained unclear. Recent studies using live imaging have revealed two novel processes, global tissue rotation and oscillating basal actomyosin contractions, which have provided significant insight into how the two polarized protein networks cooperate to produce an elongated egg. This review summarizes the proteins involved in Drosophila egg elongation and how this recent work has contributed to our current understanding of how egg elongation is achieved.