Post-transcriptional regulation of gurken by encore is required for axis determination in Drosophila

A ribosomal protein S5 isoform is essential for oogenesis and interacts with distinct RNAs in Drosophila melanogaster

In Drosophila melanogaster there are two genes encoding ribosomal protein S5, RpS5a and RpS5b. Here, we demonstrate that RpS5b is required for oogenesis. Females lacking RpS5b produce ovaries with numerous developmental defects that undergo widespread apoptosis in mid-oogenesis. Females lacking germline RpS5a are fully fertile, but germline expression of interfering RNA targeting germline RpS5a in an RpS5b mutant background worsened the RpS5b phenotype and blocked oogenesis before egg chambers form. A broad spectrum of mRNAs co-purified in immunoprecipitations with RpS5a, while RpS5b-associated mRNAs were specifically enriched for GO terms related to mitochondrial electron transport and cellular metabolic processes. Consistent with this, RpS5b mitochondrial fractions are depleted for proteins linked to oxidative phosphorylation and mitochondrial respiration, and RpS5b mitochondria tended to form large clusters and had more heterogeneous morphology than those from controls. We conclude that RpS5b-containing ribosomes preferentially associate with particular mRNAs and serve an essential function in oogenesis.

Genetic screen identifies a requirement for SMN in mRNA localisation within the Drosophila oocyte

Objective

Spinal muscular atrophy (SMA) results from insufficient levels of the survival motor neuron (SMN) protein. Drosophila is conducive to large-scale genetic-modifier screens which can reveal novel pathways underpinning the disease mechanism. We tested the ability of a large collection of genomic deletions to enhance SMN-dependent lethality. To test our design, we asked whether our study can identify loci containing genes identified in previous genetic screens. Our objective was to find a common link between genes flagged in independent screens, which would allow us to expose novel functions for SMN in vivo.

Results

Out of 128 chromosome deficiency lines, 12 (9.4%) were found to consistently depress adult viability when crossed to SMN loss-of-function heterozygotes. In their majority, the enhancing deletions harboured genes that were previously identified as genetic modifiers, hence, validating the design of the screen. Importantly, gene overlap allowed us to flag genes with a role in post-transcriptional regulation of mRNAs that are crucial for determining the axes of the oocyte and future embryo. We find that SMN is also required for the correct localisation of gurken and oskar mRNAs in oocytes. These findings extend the role of SMN in oogenesis by identifying a key requirement for mRNA trafficking.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3496-1) contains supplementary material, which is available to authorized users.

The Complexities and Unexpected Insights of Developmental Genetic Analysis

The study of development involves many important techniques. Here I am trying to reflect on the strength of genetic analysis and its ability to uncover unexpected relationships and regulatory inputs from seemingly unrelated pathways.

Localized Translation of gurken/TGF-α mRNA during Axis Specification Is Controlled by Access to Orb/CPEB on Processing Bodies

In Drosophila oocytes, gurken/TGF-α mRNA is essential for establishing the future embryonic axes. gurken remains translationally silent during transport from its point of synthesis in nurse cells to its final destination in the oocyte, where it associates with the edge of processing bodies. Here we show that, in nurse cells, gurken is kept translationally silent by the lack of sufficient Orb/CPEB, its translational activator. Processing bodies in nurse cells have a similar protein complement and ultrastructure to those in the oocyte, but they markedly less Orb and do not associate with gurken mRNA. Ectopic expression of Orb in nurse cells at levels similar to the wild-type oocyte dorso-anterior corner at mid-oogenesis is sufficient to cause gurken mRNA to associate with processing bodies and translate prematurely. We propose that controlling the spatial distribution of translational activators is a fundamental mechanism for regulating localized translation.

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gurken mRNA is not silenced by known repressors during its transport

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In nurse cells, gurken mRNA is not associated with processing bodies

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In nurse cells, lack of sufficient Orb/CPEB silences gurken mRNA translation

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In oocytes, gurken mRNA is associated with Orb on processing bodies and translated

Poly(A)-binding proteins are required for diverse biological processes in metazoans

PABPs [poly(A)-binding proteins] bind to the poly(A) tail of eukaryotic mRNAs and are conserved in species ranging from yeast to human. The prototypical cytoplasmic member, PABP1, is a multifunctional RNA-binding protein with roles in global and mRNA-specific translation and stability, consistent with a function as a central regulator of mRNA fate in the cytoplasm. More limited insight into the molecular functions of other family members is available. However, the consequences of disrupting PABP function in whole organisms is less clear, particularly in vertebrates, and even more so in mammals. In the present review, we discuss current and emerging knowledge with respect to the functions of PABP family members in whole animal studies which, although incomplete, already underlines their biological importance and highlights the need for further intensive research in this area.

Drosophila ataxin 2-binding protein 1 marks an intermediate step in the molecular differentiation of female germline cysts

In the Drosophila ovary, extrinsic signaling from the niche and intrinsic translational control machinery regulate the balance between germline stem cell maintenance and the differentiation of their daughters. However, the molecules that promote the continued stepwise development of ovarian germ cells after their exit from the niche remain largely unknown. Here, we report that the early development of germline cysts depends on the Drosophila homolog of the human ataxin 2-binding protein 1 (A2BP1) gene. Drosophila A2BP1 protein expression is first observed in the cytoplasm of 4-, 8- and 16-cell cysts, bridging the expression of the early differentiation factor Bam with late markers such as Orb, Rbp9 and Bruno encoded by arrest. The expression of A2BP1 is lost in bam, sans-fille (snf) and mei-P26 mutants, but is still present in other mutants such as rbp9 and arrest. A2BP1 alleles of varying strength produce mutant phenotypes that include germline counting defects and cystic tumors. Phenotypic analysis reveals that strong A2BP1 alleles disrupt the transition from mitosis to meiosis. These mutant cells continue to express high levels of mitotic cyclins and fail to express markers of terminal differentiation. Biochemical analysis reveals that A2BP1 isoforms bind to each other and associate with Bruno, a known translational repressor protein. These data show that A2BP1 promotes the molecular differentiation of ovarian germline cysts.

The impact on microtubule network of a bracovirus IkappaB-like protein

Polydnavirus-encoded IkappaB-like proteins are similar to insect and mammalian IkappaB, and an immunosuppressive function in the host cells has been inferred to these proteins. Here we show that the expression of one of these IkappaB-like viral genes, the TnBVank1, in the Drosophila germline affects the localization of gurken, bicoid, and oskar mRNAs whose gene products are relevant for proper embryonic patterning. The altered localization of these mRNAs is suggestive of general defects in the intracellular, microtubule-based, trafficking routes. Analysis of microtubule motor proteins components such as the dynein heavy chain and the kinesin heavy chain revealed defects in the polarized microtubule network. Interestingly, the TnBVANK1 viral protein is uniformly distributed over the entire oocyte cortex, and appears to be anchored to the microtubule ends. Our data open up a very interesting issue on novel function(s) played by the ank gene family by interfering with cytoskeleton organization.

Identification of EMS-induced mutations in Drosophila melanogaster by whole-genome sequencing

Next-generation methods for rapid whole-genome sequencing enable the identification of single-base-pair mutations in Drosophila by comparing a chromosome bearing a new mutation to the unmutagenized sequence. To validate this approach, we sought to identify the molecular lesion responsible for a recessive EMS-induced mutation affecting egg shell morphology by using Illumina next-generation sequencing. After obtaining sufficient sequence from larvae that were homozygous for either wild-type or mutant chromosomes, we obtained high-quality reads for base pairs composing approximately 70% of the third chromosome of both DNA samples. We verified 103 single-base-pair changes between the two chromosomes. Nine changes were nonsynonymous mutations and two were nonsense mutations. One nonsense mutation was in a gene, encore, whose mutations produce an egg shell phenotype also observed in progeny of homozygous mutant mothers. Complementation analysis revealed that the chromosome carried a new functional allele of encore, demonstrating that one round of next-generation sequencing can identify the causative lesion for a phenotype of interest. This new method of whole-genome sequencing represents great promise for mutant mapping in flies, potentially replacing conventional methods.

Bicaudal-C associates with a Trailer Hitch/Me31B complex and is required for efficient Gurken secretion

Bicaudal-C (Bic-C) is a multiple KH-domain RNA-binding protein required for Drosophila oogenesis and, maternally, for embryonic patterning. In early oogenesis, Bic-C negatively regulates target mRNAs, including Bic-C, by recruiting the CCR4 deadenylase through a direct association with its NOT3 subunit. Here, we identify a novel function for Bic-C in secretion of the TGF-alpha homolog Gurken (Grk). In Bic-C mutant egg chambers, Grk is sequestered within actin-coated structures during mid-oogenesis. As a consequence, Egfr signalling is not efficiently activated in the dorsal-anterior follicle cells. This phenotype is strikingly similar to that of trailer hitch (tral) mutants. Consistent with the idea that Bic-C and Tral act together in Grk secretion, Bic-C co-localizes with Tral within cytoplasmic granules, and can be co-purified with multiple protein components of a Tral mRNP complex. Taken together, our results implicate translational regulation by Bic-C and Tral in the secretory pathway.

The conserved protein SZY-20 opposes the Plk4-related kinase ZYG-1 to limit centrosome size

Microtubules are organized by the centrosome, a dynamic organelle that exhibits changes in both size and number during the cell cycle. Here we show that SZY-20, a putative RNA-binding protein, plays a critical role in limiting centrosome size in C. elegans. SZY-20 localizes in part to centrosomes and in its absence centrosomes possess increased levels of centriolar and pericentriolar components including gamma-tubulin and the centriole duplication factors ZYG-1 and SPD-2. These enlarged centrosomes possess normal centrioles, nucleate more microtubules, and fail to properly direct a number of microtubule-dependent processes. Depletion of ZYG-1 restores normal centrosome size and function to szy-20 mutants, whereas loss of szy-20 suppresses the centrosome duplication defects in both zyg-1 and spd-2 mutants. Our results describe a pathway that determines centrosome size and implicate centriole duplication factors in this process.

Translational repression of gurken mRNA in the Drosophila oocyte requires the hnRNP Squid in the nurse cells

Establishment of the Drosophila dorsal-ventral axis depends upon the correct localization of gurken mRNA and protein within the oocyte. gurken mRNA becomes localized to the presumptive dorsal anterior region of the oocyte, but is synthesized in the adjoining nurse cells. Normal gurken localization requires the heterogeneous nuclear ribonucleoprotein Squid, which binds to the gurken 3' untranslated region. However, whether Squid functions in the nurse cells or the oocyte is unknown. To address this question, we generated genetic mosaics in which half of the nurse cells attached to a given oocyte are unable to produce Squid. In these mosaics, gurken mRNA is localized normally but ectopically translated during the dorsal anterior localization process, even though the oocyte contains abundant Squid produced by the wild type nurse cells. These data indicate that translational repression of gurken mRNA requires Squid function in the nurse cells. We propose that Squid interacts with gurken mRNA in the nurse cell nuclei and, together with other factors, maintains gurken in a translationally silent state during its transport to the dorsal anterior region of the oocyte. This translational repression is not required for gurken mRNA localization, indicating that the information repressing translation is separable from that regulating localization.

Bicaudal-C recruits CCR4-NOT deadenylase to target mRNAs and regulates oogenesis, cytoskeletal organization, and its own expression

Bicaudal-C (Bic-C) encodes an RNA-binding protein required maternally for patterning the Drosophila embryo. We identified a set of mRNAs that associate with Bic-C in ovarian ribonucleoprotein complexes. These mRNAs are enriched for mRNAs that function in oogenesis and in cytoskeletal regulation, and include Bic-C RNA itself. Bic-C binds specific segments of the Bic-C 5' untranslated region and negatively regulates its own expression by binding directly to NOT3/5, a component of the CCR4 core deadenylase complex, thereby promoting deadenylation. Bic-C overexpression induces premature cytoplasmic-streaming, a posterior-group phenotype, defects in Oskar and Kinesin heavy chain:betaGal localization as well as dorsal-appendage defects. These phenotypes are largely reciprocal to those of Bic-C mutants, and they affect cellular processes that Bic-C-associated mRNAs are known, or predicted, to regulate. We conclude that Bic-C regulates expression of specific germline mRNAs by controlling their poly(A)-tail length.

Squid, Cup, and PABP55B function together to regulate gurken translation in Drosophila

During Drosophila melanogaster oogenesis, the proper localization of gurken (grk) mRNA and protein is required for the establishment of the dorsal-ventral axis of the egg and future embryo. Squid (Sqd) is an RNA-binding protein that is required for the correct localization and translational regulation of the grk message. We show that Cup and polyA-binding protein (PABP) interact physically with Sqd and with each other in ovaries. We show that cup mutants lay dorsalized eggs, enhance dorsalization of weak sqd alleles, and display defects in grk mRNA localization and Grk protein accumulation. In contrast, pAbp mutants lay ventralized eggs and enhance grk haploinsufficiency. PABP also interacts genetically and biochemically with Encore. These data predict a model in which Cup and Sqd mediate translational repression of unlocalized grk mRNA, and PABP and Enc facilitate translational activation of the message once it is fully localized to the dorsal-anterior region of the oocyte. These data also provide the first evidence of a link between the complex of commonly used trans-acting factors and Enc, a factor that is required for grk translation.

Microtubule polarity and axis formation in the Drosophila oocyte

The body axes of the fruit fly are established in mid-oogenesis by the localization of three mRNA determinants, bicoid, oskar, and gurken, within the oocyte. General mechanisms of RNA localization and cell polarization, applicable to many cell types, have emerged from investigation of these determinants in Drosophila oogenesis. Localization of these RNAs is dependent on the germline microtubules, which reorganize to form a polarized array at mid-oogenesis in response to a signaling relay between the oocyte and the surrounding somatic follicle cells. Here we describe what is known about this microtubule reorganization and the signaling relay that triggers it. Recent studies have identified a number of ubiquitous RNA binding proteins essential for this process. So far, no targets for any of these proteins have been identified, and future work will be needed to illuminate how they function to reorganize microtubes and whether similar mechanisms also exist in other cell types.

Genetic control of germline sexual dimorphism in Drosophila

Females produce eggs and males produce sperm. Work in Drosophila is helping to elucidate how this sex-specific germline differentiation is genetically encoded. While important details remain somewhat controversial, it is clear that signals generated by somatic cells, probably in the embryonic gonads, are required as extrinsic factors for germline sex determination. It is equally clear that the sex chromosome karyotype of the germ cell is an intrinsic factor for germline sex determination. There is also extensive somatic signaling required for differentiation of germline cells in the adult gonads. Mismatched germline and somatic line sexual identities place germ cells in an inappropriate signaling milieu, which results in either failed maintenance of germline stems cells when female germ cells are in a male soma or overproliferation of germline cells when male germ cells are in a female soma. The well-studied somatic sex determination genes including transformer, transformer-2, and doublesex are clearly involved in the nonautonomous signaling from somatic cells, while the autonomous functions of genes including ovo, ovarian tumor, and Sex-lethal are involved in the germline. The integration of these two pathways is not yet clear.

Bruno regulates gurken during Drosophila oogenesis

Translational regulation of localized transcripts is a powerful mechanism to control the precise timing and localization of protein expression within a cell. In the Drosophila germline, oskar transcript must be translationally repressed until its localization at the posterior pole of the oocyte, as ectopic production of Oskar causes severe patterning defects. Translational repression of oskar mRNA is mediated by the RNA-binding protein Bruno, which binds to specific motifs in the oskar 3'UTR. Here we show that Bruno over-expression causes defects in antero-posterior and dorso-ventral patterning, consistent with a role of Bruno in both oskar and gurken mRNA regulation. We also show that Bruno and gurken interact genetically. Finally, we show that Bruno binds specifically to the gurken 3'UTR and that the dorso-ventral defects caused by Bruno over-expression are due to a reduction of Gurken levels in the oocyte. We conclude that Bruno plays similar roles in translational regulation of gurken and oskar.

Coordinate regulation of sugar flux and translation by PAS kinase

PAS kinase is a serine/threonine kinase regulated in cis by a PAS domain. A genetic study of the two PAS kinase genes in budding yeast gave evidence of the involvement of these enzymes in the control of sugar metabolism and translation. Using a biochemical screen for PAS kinase substrates, three translation factors were identified as direct phosphorylation targets. PAS kinase was also found to phosphorylate UDP-glucose pyrophosphorylase and glycogen synthase, the enzymes catalyzing the two final steps in the glycogen biosynthetic pathway. Genetic, biochemical, and physiological data provide evidence that both of these enzymes are inhibited by PAS kinase-dependent phosphorylation, thereby downregulating carbohydrate storage. These studies provide evidence of a cell-autonomous signaling system that both controls and connects the balance of fuel consumption/storage to protein synthesis.

Half pint regulates alternative splice site selection in Drosophila

Alternative splicing is used by metazoans to increase protein diversity and to alter gene expression during development. However, few factors that control splice site choice in vivo have been identified. Here we describe a factor, Half pint (Hfp), that regulates RNA splicing in Drosophila. Females harboring hypomorphic mutations in hfp lay short eggs and show defects in germline mitosis, nuclear morphology, and RNA localization during oogenesis. We find that hfp encodes the Drosophila ortholog of human PUF60 and functions in both constitutive and alternative splicing in vivo. In particular, hfp mutants display striking defects in the developmentally regulated splicing of ovarian tumor (otu). Furthermore, transgenic expression of the missing otu splice form can rescue the ovarian phenotypes of hfp.

Translational regulation and RNA localization in Drosophila oocytes and embryos

Translational control is a prevalent means of gene regulation during Drosophila oogenesis and embryogenesis. Multiple maternal mRNAs are localized within the oocyte, and this localization is often coupled to their translational regulation. Subsequently, translational control allows maternally deposited mRNAs to direct the early stages of embryonic development. In this review we outline some general mechanisms of translational regulation and mRNA localization that have been uncovered in various model systems. Then we focus on the posttranscriptional regulation of four maternal transcripts in Drosophila that are localized during oogenesis and are critical for embryonic patterning: bicoid (bcd), nanos (nos), oskar (osk), and gurken (grk). Cis- and trans-acting factors required for the localization and translational control of these mRNAs are discussed along with potential mechanisms for their regulation.

Assembly of the Drosophila germ plasm

The Drosophila melanogaster germ plasm has become the paradigm for understanding both the assembly of a specific cytoplasmic localization during oogenesis and its function. The posterior ooplasm is necessary and sufficient for the induction of germ cells. For its assembly, localization of gurken mRNA and its translation at the posterior pole of early oogenic stages is essential for establishing the posterior pole of the oocyte. Subsequently, oskar mRNA becomes localized to the posterior pole where its translation leads to the assembly of a functional germ plasm. Many gene products are required for producing the posterior polar plasm, but only oskar, tudor, valois, germcell-less and some noncoding RNAs are required for germ cell formation. A key feature of germ cell formation is the precocious segregation of germ cells, which isolates the primordial germ cells from mRNA turnover, new transcription, and continued cell division. nanos is critical for maintaining the transcription quiescent state and it is required to prevent transcription of Sex-lethal in pole cells. In spite of the large body of information about the formation and function of the Drosophila germ plasm, we still do not know what specifically is required to cause the pole cells to be germ cells. A series of unanswered problems is discussed in this chapter.

RNA localization and translational regulation during axis specification in the Drosophila oocyte

The major axes of the oocyte-antero-posterior and dorso-ventral-are established over a one-day period during mid-oogenesis in Drosophila. The same molecule, GURKEN (GRK), functions to initiate signaling between the oocyte and the surrounding, somatically derived follicle cells. This results first in specification of the antero-posterior axis and, later, the dorso-ventral axis of the oocyte and surrounding follicle cells. Central to specification of both axes is a combination of cytoplasmic localization and translational regulation of the grk RNA. Here we discuss the mechanisms by which the grk RNA is localized within the oocyte and the role of translational regulation in spatially restricting the production of GRK protein. We then discuss the generality of these mechanisms during oogenesis by focusing on a second transcript, oskar, whose function is also regulated through a combination of transcript localization and translational control.

Asymmetric germ cell division and oocyte determination during Drosophila oogenesis

Early oogenesis in Drosophila, with a stereotypic pattern of germ cell division and differentiation, provides an attractive model for studying cell lineage and patterning. Drosophila oogenesis is initiated when a germline stem cell divides asymmetrically to produce a daughter stem cell and a cytoblast. The cystoblast then undergoes four rounds of incomplete mitoses to form a 16-cell cyst, accompanied by the formation of the fusome. Within the cyst, one of the two cells with four intercellular bridges differentiates into an oocyte while the rest become nurse cells. The oocyte then translocates within the cyst to a posterior position, which defines the anterio-posterior axis of the future embryo. Recent studies have shown that the asymmetric germline stem cell division is controlled by somatic signaling involving piwi, fs(1)Yb, and the dpp pathway as well as by intrinsic mechanisms involving pumilio, nanos, arrest, bag-of-marbles, and the spectrosome-the fusome precursor in the stem cells and the cystoblast. The spectrosome in the cystoblast appears to play an important role in polarized fusome growth during cyst formation. The fusome may guide the formation of a polarized microtubule network for the intracyst transport of certain RNAs and proteins to the cystocyte destined to become the oocyte. Genes such as egalitarian, Bicaudal D, stonewall, and encore are important for oocyte determination, while differential adhesion between the oocyte and its surrounding prefollicle cells, as mediated by armadillo, alpha-catenin, shotgun, and the spindle genes, is crucial for oocyte translocation. Early oogenesis shares many parallel features to early spermatogenesis, although distinct differences are also observed at both the phenomenological and mechanistic levels. The study of oogenesis, progressing at an exciting rate, contributes significantly to our understanding of the mechanisms underlying proliferation, differentiation, and patterning.

Encore is a member of a novel family of proteins and affects multiple processes in Drosophila oogenesis

Mutations in the encore (enc) gene of Drosophila melanogaster cause one extra round of mitosis in the germline, resulting in the formation of egg chambers with extra nurse cells. In addition, enc mutations affect the accumulation of Gurken protein within the oocyte, leading to the production of ventralized eggs. Here we show that enc mutants also exhibit abnormalities in karyosome morphology, similar to other ventralizing mutants such as okra and spindle B. Unlike these mutants, however, the defects in Gurken accumulation and karyosome formation do not result from activation of a meiotic checkpoint. Furthermore, we demonstrate that the requirement for enc in these processes is temporally distinct from its role in germline mitosis. Cloning of the enc locus and generation of anti-Enc antibodies reveal that enc encodes a large novel protein that accumulates within the oocyte cytoplasm and colocalizes with grk mRNA. We argue that the enc mutant phenotypes reflect a role for Enc in the regulation of several RNA targets.

Mechanisms of RNA localization and translational regulation

Transcript localization and translational regulation are two post-transcriptional mechanisms for the spatial and temporal regulation of protein production. During the past year, two transcript localization mechanisms have been elaborated in some detail. Where localization involves directional transport on cytoskeletal tracks, links between the transcripts and the cytoskeletal molecular motors have been elaborated. In the case of localization by generalized transcript degradation combined with localized protection, trans-acting pathways and cis-acting elements for degradation and protection have been identified. A third transcript localization mechanism, vectorial transport out of the nucleus into a particular cytoplasmic domain, was initially thought to localize pair-rule transcripts in Drosophila. However, these have now been shown to be localized by directional transport in the cytoplasm. Transcript localization and translational regulation can be intimately linked in that, for certain messenger RNAs, only the localized fraction of transcripts is translated whereas unlocalized transcripts are translationally repressed. Cis-acting sequences and trans-acting factors that function in translational repression have been identified along with factors involved in relief of translational repression at the site of localization.

A role for the DP subunit of the E2F transcription factor in axis determination during Drosophila oogenesis

The E2F family of transcription factors contributes to cell cycle control by regulating the transcription of DNA replication factors. Functional ‘E2F’ is a DNA-binding heterodimer composed of E2F and DP proteins. Drosophila contains two E2F genes (dE2F, dE2F2) and one DP gene (dDP). Mutation of either dE2F or dDP eliminates G(1)-S transcription of known replication factors during embryogenesis and compromises DNA replication. However, the analysis of these mutant phenotypes is complicated by the perdurance of maternally supplied gene function. To address this and to further analyze the role of E2F transcription factors in development we have phenotypically characterized mitotic clones of dDP mutant cells in the female germline. Our analysis indicates that dDP is required for several essential processes during oogenesis. In a fraction of the mutant egg chambers the germ cells execute one extra round of mitosis, suggesting that in this tissue dDP is uniquely utilized for cell cycle arrest rather than cell cycle progression. Mutation of dDP in the germline also prevents nurse cell cytoplasm transfer to the oocyte, resulting in a ‘dumpless’ phenotype that blocks oocyte development. This phenotype likely results from both disruption of the actin cytoskeleton and a failure of nurse cell apoptosis, each of which are required for normal cytoplasmic transfer. Lastly, we found that dDP is required for the establishment of the dorsal-ventral axis, as loss of dDP function prevents the localized expression of the EGFR ligand Gurken in the oocyte, which initiates dorsal-ventral polarity in the egg chamber. Thus we have uncovered new functions for E2F transcription factors during development, including an unexpected role in pattern formation.

Cyclin A associates with the fusome during germline cyst formation in the Drosophila ovary

Regulated changes in the cell cycle underlie many aspects of growth and differentiation. Prior to meiosis, germ cell cycles in many organisms become accelerated, synchronized, and modified to lack cytokinesis. These changes cause cysts of interconnected germ cells to form that typically contain 2(n) cells. In Drosophila, developing germ cells during this period contain a distinctive organelle, the fusome, that is required for normal cyst formation. We find that the cell cycle regulator Cyclin A transiently associates with the fusome during the cystocyte cell cycles, suggesting that fusome-associated Cyclin A drives the interconnected cells within each cyst synchronously into mitosis. In the presence of a normal fusome, overexpression of Cyclin A forces cysts through an extra round of cell division to produce cysts with 32 germline cells. Female sterile mutations in UbcD1, encoding an E2 ubiquitin-conjugating enzyme, have a similar effect. Our observations suggest that programmed changes in the expression and cytoplasmic localization of key cell cycle regulatory proteins control germline cyst production.

Complexity of EGF receptor signalling revealed in Drosophila

The multiple roles of the Drosophila epidermal growth factor receptor (EGFR) require that its activation is regulated precisely. Recent work has highlighted two important control mechanisms: the existence of multiple ligands with distinct properties and the interaction between EGFR pathway and other signalling pathways. The integration of signalling pathways into networks is beginning to be understood.

vasa is required for GURKEN accumulation in the oocyte, and is involved in oocyte differentiation and germline cyst development

The Drosophila gene vasa is required for pole plasm assembly and function, and also for completion of oogenesis. To investigate the role of vasa in oocyte development, we generated a new null mutation of vasa, which deletes the entire coding region. Analysis of vasa-null ovaries revealed that the gene is involved in the growth of germline cysts. In vasa-null ovaries, germaria are atrophied, and contain far fewer developing cysts than do wild-type germaria; a phenotype similar to, but less severe than, that of a null nanos allele. The null mutant also revealed roles for vasa in oocyte differentiation, anterior-posterior egg chamber patterning, and dorsal-ventral follicle patterning, in addition to its better-characterized functions in posterior embryonic patterning and pole cell specification. The anterior-posterior and dorsal-ventral patterning phenotypes resemble those observed in gurken mutants. vasa-null oocytes fail to efficiently accumulate many localized RNAs, such as Bicaudal-D, orb, oskar, and nanos, but still accumulate gurken RNA. However, GRK accumulation in the oocyte is severely reduced in the absence of vasa function, suggesting a function for VASA in activating gurken translation in wild-type ovaries.