A maternal screen for genes regulating Drosophila oocyte polarity uncovers new steps in meiotic progression

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.

Assembly and Function of Gonad-Specific Non-Membranous Organelles in Drosophila piRNA Biogenesis

PIWI-interacting RNAs (piRNAs) are small non-coding RNAs that repress transposons in animal germlines. This protects the genome from the invasive DNA elements. piRNA pathway failures lead to DNA damage, gonadal development defects, and infertility. Thus, the piRNA pathway is indispensable for the continuation of animal life. piRNA-mediated transposon silencing occurs in both the nucleus and cytoplasm while piRNA biogenesis is a solely cytoplasmic event. piRNA production requires a number of proteins, the majority of which localize to non-membranous organelles that specifically appear in the gonads. Other piRNA factors are localized on outer mitochondrial membranes. In situ RNA hybridization experiments show that piRNA precursors are compartmentalized into other non-membranous organelles. In this review, we summarize recent findings about the function of these organelles in the Drosophila piRNA pathway by focusing on their assembly and function.

Genome Damage Sensing Leads to Tissue Homeostasis in Drosophila

DNA repair is a critical cellular process required for the maintenance of genomic integrity. It is now well appreciated that cells employ several DNA repair pathways to take care of distinct types of DNA damage. It is also well known that a cascade of signals namely DNA damage response or DDR is activated in response to DNA damage which comprise cellular responses, such as cell cycle arrest, DNA repair and cell death, if the damage is irreparable. There is also emerging literature suggesting a cross-talk between DNA damage signaling and several signaling networks within a cell. Moreover, cell death players themselves are also well known to engage in processes outside their canonical function of apoptosis. This chapter attempts to build a link between DNA damage, DDR and signaling from the studies mainly conducted in mammals and Drosophila model systems, with a special emphasis on their relevance in overall tissue homeostasis and development.

Mapping Second Chromosome Mutations to Defined Genomic Regions in Drosophila melanogaster

Hundreds of Drosophila melanogaster stocks are currently maintained at the Bloomington Drosophila Stock Center with mutations that have not been associated with sequence-defined genes. They have been preserved because they have interesting loss-of-function phenotypes. The experimental value of these mutations would be increased by tying them to specific genomic intervals so that geneticists can more easily associate them with annotated genes. Here, we report the mapping of 85 second chromosome complementation groups in the Bloomington collection to specific, small clusters of contiguous genes or individual genes in the sequenced genome. This information should prove valuable to Drosophila geneticists interested in processes associated with particular phenotypes and those searching for mutations affecting specific sequence-defined genes.

Brca2-Pds5 complexes mobilize persistent meiotic recombination sites to the nuclear envelope

Homologous recombination is required for reciprocal exchange between homologous chromosome arms during meiosis. Only select meiotic recombination events become chromosomal crossovers; the majority of recombination outcomes are noncrossovers. Growing evidence suggests that crossovers are repaired after noncrossovers. Here, I report that persisting recombination sites are mobilized to the nuclear envelope of Drosophila pro-oocytes during mid-pachytene. Their number correlates with the average crossover rate per meiosis. Proteomic and interaction studies reveal that the recombination mediator Brca2 associates with lamin and the cohesion factor Pds5 to secure persistent recombination sites at the nuclear envelope. In Rad51(-/-) females, all persistent DNA breaks are directed to the nuclear envelope. By contrast, a reduction of Pds5 or Brca2 levels abolishes the movement and has a negative impact on crossover rates. The data suggest that persistent meiotic DNA double-strand breaks might correspond to crossovers, which are mobilized to the nuclear envelope for their repair. The identification of Brca2-Pds5 complexes as key mediators of this process provides a first mechanistic explanation for the contribution of lamins and cohesins to meiotic recombination.

Mosaic Analysis in the Drosophila melanogaster Ovary

Drosophila melanogaster oogenesis is a versatile model system used to address many important questions of cell and developmental biology such as stem cell regulation, cell determination, cell polarization, cell-cell signaling, cell-cell adhesion, and cell-cycle regulation. The ovary is composed of germline and somatic cells of different origins and functions. Mosaic analysis using the powerful genetic tools available in Drosophila melanogaster allows deciphering the contribution of each cell type in the different processes leading to the formation of a mature egg. Germ cells and follicle cells are produced by actively dividing stem cells, which permit the use of recombinases, such as FLP, to generate genetic mosaics using mitotic recombination. This chapter summarizes the different methods used to create genetic mosaics in the germline and in somatic cells of adult ovaries. We briefly introduce the morphology and development of the adult female ovary. We then describe in practical terms how to generate mosaics with examples of cross schemes and recombining strains. We also explain how to identify the appropriate progeny and how to prepare clonal tissues for phenotypic analysis.

Meiosis in male Drosophila

Meiosis entails sorting and separating both homologous and sister chromatids. The mechanisms for connecting sister chromatids and homologs during meiosis are highly conserved and include specialized forms of the cohesin complex and a tightly regulated homolog synapsis/recombination pathway designed to yield regular crossovers between homologous chromatids. Drosophila male meiosis is of special interest because it dispenses with large segments of the standard meiotic script, particularly recombination, synapsis and the associated structures. Instead, Drosophila relies on a unique protein complex composed of at least two novel proteins, SNM and MNM, to provide stable connections between homologs during meiosis I. Sister chromatid cohesion in Drosophila is mediated by cohesins, ring-shaped complexes that entrap sister chromatids. However, unlike other eukaryotes Drosophila does not rely on the highly conserved Rec8 cohesin in meiosis, but instead utilizes two novel cohesion proteins, ORD and SOLO, which interact with the SMC1/3 cohesin components in providing meiotic cohesion.

The meiotic checkpoint network: step-by-step through meiotic prophase

The generation of haploid gametes by meiosis is a highly conserved process for sexually reproducing organisms that, in almost all cases, involves the extensive breakage of chromosomes. These chromosome breaks occur during meiotic prophase and are essential for meiotic recombination as well as the subsequent segregation of homologous chromosomes. However, their formation and repair must be carefully monitored and choreographed with nuclear dynamics and the cell division program to avoid the creation of aberrant chromosomes and defective gametes. It is becoming increasingly clear that an intricate checkpoint-signaling network related to the canonical DNA damage response is deeply interwoven with the meiotic program and preserves order during meiotic prophase. This meiotic checkpoint network (MCN) creates a wide range of dependent relationships controlling chromosome movement, chromosome pairing, chromatin structure, and double-strand break (DSB) repair. In this review, we summarize our current understanding of the MCN. We discuss commonalities and differences in different experimental systems, with a particular emphasis on the emerging design principles that control and limit cross talk between signals to ultimately ensure the faithful inheritance of chromosomes by the next generation.

Transcriptional regulation of Drosophila gonad formation

The formation of the Drosophila embryonic gonad, involving the fusion of clusters of somatic gonadal precursor cells (SGPs) and their ensheathment of germ cells, provides a simple and genetically tractable model for the interplay between cells during organ formation. In a screen for mutants affecting gonad formation we identified a SGP cell autonomous role for Midline (Mid) and Longitudinals lacking (Lola). These transcriptional factors are required for multiple aspects of SGP behaviour including SGP cluster fusion, germ cell ensheathment and gonad compaction. The lola locus encodes more than 25 differentially spliced isoforms and we have identified an isoform specific requirement for lola in the gonad which is distinct from that in nervous system development. Mid and Lola work in parallel in gonad formation and surprisingly Mid overexpression in a lola background leads to additional SGPs at the expense of fat body cells. Our findings support the idea that although the transcription factors required by SGPs can ostensibly be assigned to those being required for either SGP specification or behaviour, they can also interact to impinge on both processes.

A mosaic genetic screen for genes involved in the early steps of Drosophila oogenesis

The first hours of Drosophila embryogenesis rely exclusively on maternal information stored within the egg during oogenesis. The formation of the egg chamber is thus a crucial step for the development of the future adult. It has emerged that many key developmental decisions are made during the very first stages of oogenesis. We performed a clonal genetic screen on the left arm of chromosome 2 for mutations affecting early oogenesis. During the first round of screening, we scored for defects in egg chambers morphology as an easy read-out of early abnormalities. In a second round of screening, we analyzed the localization of centrosomes and Orb protein within the oocyte, the position of the oocyte within the egg chamber, and the progression through meiosis. We have generated a collection of 71 EMS-induced mutants that affect oocyte determination, polarization, or localization. We also recovered mutants affecting the number of germline cyst divisions or the differentiation of follicle cells. Here, we describe the analysis of nine complementation groups and eight single alleles. We mapped several mutations and identified alleles of Bicaudal-D, lethal(2) giant larvae, kuzbanian, GDP-mannose 4,6-dehydratase, tho2, and eiF4A. We further report the molecular identification of two alleles of the Drosophila homolog of Che-1/AATF and demonstrate its antiapoptotic activity in vivo. This collection of mutants will be useful to investigate further the early steps of Drosophila oogenesis at a genetic level.

The Drosophila actin regulator ENABLED regulates cell shape and orientation during gonad morphogenesis

Organs develop distinctive morphologies to fulfill their unique functions. We used Drosophila embryonic gonads as a model to study how two different cell lineages, primordial germ cells (PGCs) and somatic gonadal precursors (SGPs), combine to form one organ. We developed a membrane GFP marker to image SGP behaviors live. These studies show that a combination of SGP cell shape changes and inward movement of anterior and posterior SGPs leads to the compaction of the spherical gonad. This process is disrupted in mutants of the actin regulator, enabled (ena). We show that Ena coordinates these cell shape changes and the inward movement of the SGPs, and Ena affects the intracellular localization of DE-cadherin (DE-cad). Mathematical simulation based on these observations suggests that changes in DE-cad localization can generate the forces needed to compact an elongated structure into a sphere. We propose that Ena regulates force balance in the SGPs by sequestering DE-cad, leading to the morphogenetic movement required for gonad compaction.

Polo-mediated phosphorylation of Maelstrom regulates oocyte determination during oogenesis in Drosophila

In Drosophila, Maelstrom is a conserved component of the perinuclear nuage, a germline-unique structure that appears to serve as a site for Piwi-interacting RNA (piRNA) production to repress deleterious transposons. Maelstrom also functions in the nucleus as a transcriptional regulator to repress the expression of microRNA-7, a process that is essential for the proper differentiation of germline stem cells. In this paper, we report another function of Maelstrom in regulating oocyte determination independently of its transposon silencing and germline stem cell differentiation activities. In Drosophila, the conserved serine 138 residue in Maelstrom is required for its phosphorylation, an event that promotes oocyte determination. Phosphorylation of Maelstrom is required for the repression of the pachytene checkpoint protein Sir2, but not for transposon silencing or for germline stem cell differentiation. We identify Polo as a kinase that mediates the phosphorylation of Maelstrom. Our results suggest that the Polo-mediated phosphorylation of Maelstrom may be a mechanism that controls oocyte determination by inactivating the pachytene checkpoint via the repression of Sir2 in Drosophila ovaries.

Next generation organelles: structure and role of germ granules in the germline

Germ cells belong to a unique class of stem cells that gives rise to eggs and sperm, and ultimately to an entire organism after gamete fusion. In many organisms, germ cells contain electron-dense structures that are also known as nuage or germ granules. Although germ granules were discovered more than 100 years ago, their composition, structure, assembly, and function are not fully understood. Germ granules contain non-coding RNAs, mRNAs, and proteins required for germline development. Here we review recent studies that highlight the importance of several protein families in germ granule assembly and function, including germ granule inducers, which initiate the granule formation, and downstream components, such as RNA helicases and Tudor domain-Piwi protein-piRNA complexes. Assembly of these components into one granule is likely to result in a highly efficient molecular machine that ensures translational control and protects germline DNA from mutations caused by mobile genetic elements. Furthermore, recent studies have shown that different somatic cells, including stem cells and neurons, produce germ granule components that play a crucial role in stem cell maintenance and memory formation, indicating a much more diverse functional repertoire for these organelles than previously thought.

The conserved kinase SRPK regulates karyosome formation and spindle microtubule assembly in Drosophila oocytes

In Drosophila oocytes, after the completion of recombination, meiotic chromosomes form a compact cluster called the karyosome within the nucleus, and later assemble spindle microtubules without centrosomes. Although these oocyte-specific phenomena are also observed in humans, their molecular basis is not well understood. Here, we report essential roles for the conserved kinase SRPK in both karyosome formation and spindle microtubule assembly in oocytes. We have identified a female-sterile srpk mutant through a cytological screen for karyosome defects. Unlike most karyosome mutants, the karyosome defect is independent of the meiotic recombination checkpoint. Heterochromatin clustering found within the wild-type karyosome is disrupted in the mutant. Strikingly, a loss of SRPK severely prevents microtubule assembly for acentrosomal spindles in mature oocytes. Subsequently, bi-orientation and segregation of meiotic chromosomes are also defective. Therefore, this study demonstrates new roles of this conserved kinase in two independent meiotic steps specific to oocytes.

The functioning of the Drosophila CPEB protein Orb is regulated by phosphorylation and requires casein kinase 2 activity

The Orb CPEB protein regulates translation of localized mRNAs in Drosophila ovaries. While there are multiple hypo- and hyperphosphorylated Orb isoforms in wild type ovaries, most are missing in orb^F303, which has an amino acid substitution in a buried region of the second RRM domain. Using a proteomics approach we identified a candidate Orb kinase, Casein Kinase 2 (CK2). In addition to being associated with Orb in vivo, we show that ck2 is required for orb functioning in gurken signaling and in the autoregulation of orb mRNA localization and translation. Supporting a role for ck2 in Orb phosphorylation, we find that the phosphorylation pattern is altered when ck2 activity is partially compromised. Finally, we show that the Orb hypophosphorylated isoforms are in slowly sedimenting complexes that contain the translational repressor Bruno, while the hyperphosphorylated isoforms assemble into large complexes that co-sediment with polysomes and contain the Wisp poly(A) polymerase.

Biogenesis pathways of piRNAs loaded onto AGO3 in the Drosophila testis

PIWI-interacting RNAs (piRNAs) silence transposable elements in animal germ cells. In Drosophila ovaries, piRNAs are produced by two distinct pathways: the "ping-pong" amplification cycle that operates in germ cells and a ping-pong-independent pathway termed the primary pathway that mainly operates in somatic cells. AGO3, one of three PIWI proteins in flies, is involved in the ping-pong cycle in ovaries. We characterized AGO3-associated piRNAs in fly testes and found that like in ovaries, AGO3 functions in the ping-pong cycle with Aubergine (Aub) for piRNA production from transposon transcripts. In contrast, most AGO3-associated piRNAs corresponding to Suppressor of Stellate [Su(Ste)] genes are antisense-oriented and bound to Aub. In addition, the vast majority of AGO3-bound piRNAs derived from the AT-chX locus on chromosome X are antisense-oriented and are also found among Aub-associated piRNAs. The presence of very few sense Su(Ste) and AT-chX piRNAs suggests that biogenesis of both Su(Ste) and AT-chX piRNAs by a ping-pong mechanism only is highly unlikely. Nevertheless, the mutual interdependence of AGO3 and Aub for the accumulation of these piRNAs shows that their production relies on both AGO3 and Aub. Analysis of piRNA pathway mutants revealed that although the requirements for piRNA factors for Su(Ste)- and AT-chX-piRNA levels mostly overlap and resemble those for the ping-pong mechanism in the ovaries, Armitage (armi) is not required for the accumulation of AT-chX-1 piRNA. These findings suggest that the impacts of armi mutants on the operation of the piRNA pathway are variable in germ cells of fly testes.

Building RNA-protein granules: insight from the germline

The germline originates from primordial embryonic germ cells which give rise to sperm and egg cells and consequently, to the next generation. Germ cells of many organisms contain electron-dense granules that comprise RNA and proteins indispensable for germline development. Here we review recent reports that provide important insights into the structure and function of crucial RNA and protein components of the granules, including DEAD-box helicases, Tudor domain proteins, Piwi/Argonaute proteins and piRNA. Collectively, these components function in translational control, remodeling of ribonucleoprotein complexes and transposon silencing. Furthermore, they interact with each other by means of conserved structural modules and post-translationally modified amino acids. These data suggest a widespread use of several protein motifs in germline development and further our understanding of other ribonucleoprotein structures, for example, processing bodies and neuronal granules.

closca, a new gene required for both Torso RTK activation and vitelline membrane integrity. Germline proteins contribute to Drosophila eggshell composition

The Drosophila eggshell is a specialised extracellular matrix (ECM) that surrounds and protects the oocyte and the embryo until its eclosion. In addition, the vitelline membrane, the innermost layer of the eggshell, holds the local determinant required to activate the Torso RTK pathway, which establishes the embryonic terminal regions. Here we report the identification and characterisation of closca, a gene encoding a new member of a group of proteins that act non-redundantly in vitelline membrane biogenesis and in Torso signalling. We also show that the Nasrat protein, another member of this group, is incorporated into the vitelline membrane, thereby indicating that the eggshell is a shared ECM that receives contributions from both follicle cells and the germline. This observation also provides a new scenario that accounts for the long known contribution of germline products to vitelline membrane biogenesis and to the follicle cell-dependent activation of the Torso receptor.

Mre11-Rad50-Nbs complex is required to cap telomeres during Drosophila embryogenesis

Using Drosophila as a model system, we identified here a stringent requirement for Mre11-Rad50-Nbs (MRN) function in telomere protection during early embryonic development. Animals homozygous for hypomorphic mutations in either mre11 or nbs develop normally with minimal telomere dysfunction. However, they produce inviable embryos that succumb to failure of mitosis caused by covalent fusion of telomeric DNA. Interestingly, the molecular defect is not the absence of MRN interaction or of Mre11 nuclease activities, but the depletion of the maternal pool of Nbs protein in these embryos. Because of Nbs depletion, Mre11 and Rad50 (MR) are excluded from chromatin. This maternal effect lethality in Drosophila is similar to that seen in mice carrying hypomorphic mrn mutations found in human patients, suggesting a common defect in telomere maintenance because of the loss of MRN integrity.

Specialized piRNA pathways act in germline and somatic tissues of the Drosophila ovary

In Drosophila gonads, Piwi proteins and associated piRNAs collaborate with additional factors to form a small RNA-based immune system that silences mobile elements. Here, we analyzed nine Drosophila piRNA pathway mutants for their impacts on both small RNA populations and the subcellular localization patterns of Piwi proteins. We find that distinct piRNA pathways with differing components function in ovarian germ and somatic cells. In the soma, Piwi acts singularly with the conserved flamenco piRNA cluster to enforce silencing of retroviral elements that may propagate by infecting neighboring germ cells. In the germline, silencing programs encoded within piRNA clusters are optimized via a slicer-dependent amplification loop to suppress a broad spectrum of elements. The classes of transposons targeted by germline and somatic piRNA clusters, though not the precise elements, are conserved among Drosophilids, demonstrating that the architecture of piRNA clusters has coevolved with the transposons that they are tasked to control.

Hedgehog does not guide migrating Drosophila germ cells

In many species, the germ cells, precursors of sperm and egg, migrate during embryogenesis. The signals that regulate this migration are thus essential for fertility. In flies, lipid signals have been shown to affect germ cell guidance. In particular, the synthesis of geranylgeranyl pyrophosphate through the 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (Hmgcr) pathway is critical for attracting germ cells to their target tissue. In a genetic analysis of signaling pathways known to affect cell migration of other migratory cells, we failed to find a role for the Hedgehog (Hh) pathway in germ cell migration. However, previous reports had implicated Hh as a germ cell attractant in flies and suggested that Hh signaling is enhanced through the action of the Hmgcr pathway. We therefore repeated several critical experiments and carried out further experiments to test specifically whether Hh is a germ cell attractant in flies. In contrast to previously reported findings and consistent with findings in zebrafish our data do not support the notion that Hh has a direct role in the guidance of migrating germ cells in flies.

Tre1 GPCR initiates germ cell transepithelial migration by regulating Drosophila melanogaster E-cadherin

Despite significant progress in identifying the guidance pathways that control cell migration, how a cell starts to move within an intact organism, acquires motility, and loses contact with its neighbors is poorly understood. We show that activation of the G protein–coupled receptor (GPCR) trapped in endoderm 1 (Tre1) directs the redistribution of the G protein Gβ as well as adherens junction proteins and Rho guanosine triphosphatase from the cell periphery to the lagging tail of germ cells at the onset of Drosophila melanogaster germ cell migration. Subsequently, Tre1 activity triggers germ cell dispersal and orients them toward the midgut for directed transepithelial migration. A transition toward invasive migration is also a prerequisite for metastasis formation, which often correlates with down-regulation of adhesion proteins. We show that uniform down-regulation of E-cadherin causes germ cell dispersal but is not sufficient for transepithelial migration in the absence of Tre1. Our findings therefore suggest a new mechanism for GPCR function that links cell polarity, modulation of cell adhesion, and invasion.

Differential requirements of a mitotic acetyltransferase in somatic and germ line cells

During mitosis different types of cells can have differential requirements for chromosome segregation. We isolated two new alleles of the separation anxiety gene (san). san was previously described in both Drosophila and in humans to be required for centromeric sister chromatid cohesion (Hou et al., 2007; Williams et al., 2003). Our work confirms and expands the observation that san is required in vivo for normal mitosis of different types of somatic cells. In addition, we suggest that san is also important for the correct resolution of chromosomes, implying a more general function of this acetyltransferase. Surprisingly, during oogenesis we cannot detect mitotic defects in germ line cells mutant for san. We hypothesize the female germ line stem cells have differential requirements for mitotic sister chromatid cohesion.