The GNU subunit of PNG kinase, the developmental regulator of mRNA translation, binds BIC-C to localize to RNP granules

Control of mRNA translation is a key mechanism by which the differentiated oocyte transitions to a totipotent embryo. In Drosophila, the PNG kinase complex regulates maternal mRNA translation at the oocyte-to-embryo transition. We previously showed that the GNU activating subunit is crucial in regulating PNG and timing its activity to the window between egg activation and early embryogenesis (Hara et al., 2017). In this study, we find associations between GNU and proteins of RNP granules and demonstrate that GNU localizes to cytoplasmic RNP granules in the mature oocyte, identifying GNU as a new component of a subset of RNP granules. Furthermore, we define roles for the domains of GNU. Interactions between GNU and the granule component BIC-C reveal potential conserved functions for translational regulation in metazoan development. We propose that by binding to BIC-C, upon egg activation GNU brings PNG to its initial targets, translational repressors in RNP granules.

Early genome activation in Drosophila is extensive with an initial tendency for aborted transcripts and retained introns

Control of metazoan embryogenesis shifts from maternal to zygotic gene products as the zygotic genome becomes transcriptionally activated. In Drosophila, zygotic genome activation (ZGA) has been thought to occur in two phases, starting with a minor wave, in which a small number of genes become expressed, and progressing to the major wave, in which many more genes are activated. However, technical challenges have hampered the identification of early transcripts or obscured the onset of their transcription. Here, we develop an approach to isolate transcribed mRNAs and apply it over the course of Drosophila early genome activation. Our results increase by 10-fold the genes reported to be activated during what has been thought of as the minor wave and show that early genome activation is continuous and gradual. Transposable-element mRNAs are also produced, but discontinuously. Genes transcribed in the early and middle part of ZGA are short with few if any introns, and their transcripts are frequently aborted and tend to have retained introns, suggesting that inefficient splicing as well as rapid cell divisions constrain the lengths of early transcripts.

Activating embryonic development in Drosophila

The transition from oocyte to embryo marks the onset of development. This process requires complex regulation to link developmental signals with profound changes in mRNA translation, cell cycle control, and metabolism. This control is beginning to be understood for most organisms, and research in the fruit fly Drosophila melanogaster has generated new insights. Recent findings have increased our understanding of the roles played by hormone and Ca2+ signaling events as well as metabolic remodeling crucial for this transition. Specialized features of the structure and assembly of the meiotic spindle have been identified. The changes in protein levels, mRNA translation, and polyadenylation that occur as the oocyte becomes an embryo have been identified together with key aspects of their regulation. Here we highlight these important developments and the insights they provide on the intricate regulation of this dramatic transition.

Identification of PNG kinase substrates uncovers interactions with the translational repressor TRAL in the oocyte-to-embryo transition

The Drosophila Pan Gu (PNG) kinase complex regulates hundreds of maternal mRNAs that become translationally repressed or activated as the oocyte transitions to an embryo. In a previous paper (Hara et al., 2017), we demonstrated PNG activity is under tight developmental control and restricted to this transition. Here, examination of PNG specificity showed it to be a Thr-kinase yet lacking a clear phosphorylation site consensus sequence. An unbiased biochemical screen for PNG substrates identified the conserved translational repressor Trailer Hitch (TRAL). Phosphomimetic mutation of the PNG phospho-sites in TRAL reduced its ability to inhibit translation in vitro. In vivo, mutation of tral dominantly suppressed png mutants and restored Cyclin B protein levels. The repressor Pumilio (PUM) has the same relationship with PNG, and we also show that PUM is a PNG substrate. Furthermore, PNG can phosphorylate BICC and ME31B, repressors that bind TRAL in cytoplasmic RNPs. Therefore, PNG likely promotes translation at the oocyte-to-embryo transition by phosphorylating and inactivating translational repressors.

Variant cell cycles regulated by Notch signaling control cell size and ensure a functional blood-brain barrier

Regulation of cell size is crucial in development. In plants and animals two cell cycle variants are employed to generate large cells by increased ploidy: the endocycle and endomitosis. The rationale behind the choice of which of these cycles is implemented is unknown. We show that in the Drosophila nervous system the subperineurial glia (SPG) are unique in using both the endocycle and endomitosis to grow. In the brain, the majority of SPG initially endocycle, then switch to endomitosis during larval development. The Notch signaling pathway and the String Cdc25 phosphatase are crucial for the endocycle versus endomitosis choice, providing the means experimentally to change cells from one to the other. This revealed fundamental insights into the control of cell size and the properties of endomitotic cells. Endomitotic cells attain a higher ploidy and larger size than endocycling cells, and endomitotic SPG are necessary for the blood-brain barrier. Decreased Notch signaling promotes endomitosis even in the ventral nerve cord SPG that normally are mononucleate, but not in the endocycling salivary gland cells, revealing tissue-specific cell cycle responses.

Highlighted Article: In Drosophila brain lobes, Notch and the mitosis-activating phosphatase String regulate the switch of subperineurial glia from endocycle to endomitosis during larval development, with endomitotic cells attaining increased ploidy and size.

Control of PNG kinase, a key regulator of mRNA translation, is coupled to meiosis completion at egg activation

The oocyte-to-embryo transition involves extensive changes in mRNA translation, regulated in Drosophila by the PNG kinase complex whose activity we show here to be under precise developmental control. Despite presence of the catalytic PNG subunit and the PLU and GNU activating subunits in the mature oocyte, GNU is phosphorylated at Cyclin B/CDK1sites and unable to bind PNG and PLU. In vitro phosphorylation of GNU by CyclinB/CDK1 blocks activation of PNG. Meiotic completion promotes GNU dephosphorylation and PNG kinase activation to regulate translation. The critical regulatory effect of phosphorylation is shown by replacement in the oocyte with a phosphorylation-resistant form of GNU, which promotes PNG-GNU complex formation, elevation of Cyclin B, and meiotic defects consistent with premature PNG activation. After PNG activation GNU is destabilized, thus inactivating PNG. This short-lived burst in kinase activity links development with maternal mRNA translation and ensures irreversibility of the oocyte-to-embryo transition.

DOI:http://dx.doi.org/10.7554/eLife.22219.001

New egg cells form via a specialized kind of cell division called called meiosis, and will pause at key stages in this process before continuing their development. One of these pauses occurs before the egg cell is fertilized. At fertilization, the egg cell becomes “activated”, development resumes, and it starts forming into an embryo. Molecules deposited in the egg cell when it originally formed are used to control these earliest stages of embryonic development. These molecules include messenger RNA molecules (mRNAs for short), which can be “translated” to build proteins.

In fruit flies, an enzyme called PNG kinase regulates the translation of hundreds of mRNA molecules during the period after the pause, when the maturing egg cell is activated and the embryo begins to develop. It is not well understood what activates and inactivates the kinase to limit its activity to this period of time. However, it was known that a protein called GNU was needed to bind to the PNG kinase to make it active.

CyclinB/CDK1 is another kinase, and in contrast to PNG it is highly active when the egg cell is paused. When the egg cell is activated for embryonic development, the levels of this second kinase drop sharply and meiosis is completed. Like all kinases, CyclinB/CDK1 attaches phosphate groups onto other molecules, and Hara et al. now show that CyclinB/CDK1 can modify the GNU protein in this way. The added phosphate groups prevent GNU from binding to the PNG kinase, meaning that the high levels of CyclinB/CDK1 during the pause stop GNU from activating the PNG kinase. However, when the egg cell is activated, the level of CyclinB/CDK1 declines so that there are not enough of these molecules to add phosphates onto GNU. This leaves GNU free to activate the PNG kinase, allowing this kinase to control the translation of mRNA molecules. Furthermore, the activity of PNG kinase leads to the destruction of GNU, and this feedback loop limits this kinase’s activity to the narrow window of time in which it is needed.

The fruit fly is the second example of an animal in which the activity of a kinase essential for embryonic development has been linked to the completion of meiosis (the other being the roundworm Caenorhabditis elegans). The use of this strategy in two such different animals suggests that it may also be common to many other animals, including humans. Further investigation is now needed to determine if this is indeed the case.

DOI:http://dx.doi.org/10.7554/eLife.22219.002

mRNA poly(A)-tail changes specified by deadenylation broadly reshape translation in Drosophila oocytes and early embryos

Because maturing oocytes and early embryos lack appreciable transcription, posttranscriptional regulatory processes control their development. To better understand this control, we profiled translational efficiencies and poly(A)-tail lengths throughout Drosophila oocyte maturation and early embryonic development. The correspondence between translational-efficiency changes and tail-length changes indicated that tail-length changes broadly regulate translation until gastrulation, when this coupling disappears. During egg activation, relative changes in poly(A)-tail length, and thus translational efficiency, were largely retained in the absence of cytoplasmic polyadenylation, which indicated that selective poly(A)-tail shortening primarily specifies these changes. Many translational changes depended on PAN GU and Smaug, and these changes were largely attributable to tail-length changes. Our results also revealed the presence of tail-length-independent mechanisms that maintained translation despite tail-length shortening during oocyte maturation, and prevented essentially all translation of bicoid and several other mRNAs before egg activation. In addition to these fundamental insights, our results provide valuable resources for future studies.

Understanding replication fork progression, stability, and chromosome fragility by exploiting the Suppressor of Underreplication protein

There are many layers of regulation governing DNA replication to ensure that genetic information is accurately transmitted from mother cell to daughter cell. While much of the control occurs at the level of origin selection and firing, less is known about how replication fork progression is controlled throughout the genome. In Drosophila polytene cells, specific regions of the genome become repressed for DNA replication, resulting in underreplication and decreased copy number. Importantly, underreplicated domains share properties with common fragile sites. The Suppressor of Underreplication protein SUUR is essential for this repression. Recent work established that SUUR functions by directly inhibiting replication fork progression, raising several interesting questions as to how replication fork progression and stability can be modulated within targeted regions of the genome. Here we discuss potential mechanisms by which replication fork inhibition can be achieved and the consequences this has on genome stability and copy number control.

Replication fork progression during re-replication requires the DNA damage checkpoint and double-strand break repair

Replication origins are under tight regulation to ensure activation occurs only once per cell cycle [1, 2]. Origin re-firing in a single S phase leads to the generation of DNA double-strand breaks (DSBs) and activation of the DNA damage checkpoint [2-7]. If the checkpoint is blocked, cells enter mitosis with partially re-replicated DNA that generates chromosome breaks and fusions [5]. These types of chromosomal aberrations are common in numerous human cancers, suggesting that re-replication events contribute to cancer progression. It was proposed that fork instability and DSBs formed during re-replication are the result of head-to-tail collisions and collapse of adjacent replication forks [3]. However, previously studied systems lack the resolution to determine whether the observed DSBs are generated at sites of fork collisions. Here, we utilize the Drosophila ovarian follicle cells, which exhibit re-replication under precise developmental control [8-10], to model the consequences of re-replication at actively elongating forks. Re-replication occurs from specific replication origins at six genomic loci, termed Drosophila amplicons in follicle cells (DAFCs) [10-12]. Precise developmental timing of DAFC origin firing permits identification of forks at defined points after origin initiation [13, 14]. Here, we show that DAFC re-replication causes fork instability and generates DSBs at sites of potential fork collisions. Immunofluorescence and ChIP-seq demonstrate the DSB marker γH2Av is enriched at elongating forks. Fork progression is reduced in the absence of DNA damage checkpoint components and nonhomologous end-joining (NHEJ), but not homologous recombination. NHEJ appears to continually repair forks during re-replication to maintain elongation.

When bigger is better: the role of polyploidy in organogenesis

Defining how organ size is regulated, a process controlled not only by the number of cells but also by the size of the cells, is a frontier in developmental biology. Large cells are produced by increasing DNA content or ploidy, a developmental strategy employed throughout the plant and animal kingdoms. The widespread use of polyploidy during cell differentiation makes it important to define how this hypertrophy contributes to organogenesis. I discuss here examples from a variety of animals and plants in which polyploidy controls organ size, the size and function of specific tissues within an organ, or the differentiated properties of cells. In addition, I highlight how polyploidy functions in wound healing and tissue regeneration.

DNA copy-number control through inhibition of replication fork progression

Proper control of DNA replication is essential to ensure faithful transmission of genetic material and prevent chromosomal aberrations that can drive cancer progression and developmental disorders. DNA replication is regulated primarily at the level of initiation and is under strict cell-cycle regulation. Importantly, DNA replication is highly influenced by developmental cues. In Drosophila, specific regions of the genome are repressed for DNA replication during differentiation by the SNF2 domain-containing protein SUUR through an unknown mechanism. We demonstrate that SUUR is recruited to active replication forks and mediates the repression of DNA replication by directly inhibiting replication fork progression instead of functioning as a replication fork barrier. Mass spectrometry identification of SUUR-associated proteins identified the replicative helicase member CDC45 as a SUUR-associated protein, supporting a role for SUUR directly at replication forks. Our results reveal that control of eukaryotic DNA copy number can occur through the inhibition of replication fork progression.

The role of transcription in the activation of a Drosophila amplification origin

The mechanisms that underlie metazoan DNA replication initiation, especially the connection between transcription and replication origin activation, are not well understood. To probe the role of transcription in origin activation, we exploited a specific replication origin in Drosophila melanogaster follicle cells, ori62, which coincides with the yellow-g2 transcription unit and exhibits transcription-dependent origin firing. Within a 10-kb genomic fragment that contains ori62 and is sufficient for amplification, RNA-sequencing analysis revealed that all detected RNAs mapped solely to the yellow-g2 gene. To determine whether transcription is required in cis for ori62 firing, we generated a set of tagged yellow-g2 transgenes in which we could prevent local transcription across ori62 by deletions in the yellow-g2 promoter. Surprisingly, inhibition of yellow-g2 transcription by promoter deletions did not affect ori62 firing. Our results reveal that transcription in cis is not required for ori62 firing, raising the possibility that a trans-acting factor is required specifically for the activation of ori62. This finding illustrates that a diversity of mechanisms can be used in the regulation of metazoan DNA replication initiation.

Regulation of centromere localization of the Drosophila Shugoshin MEI-S332 and sister-chromatid cohesion in meiosis

The Shugoshin (Sgo) protein family helps to ensure proper chromosome segregation by protecting cohesion at the centromere by preventing cleavage of the cohesin complex. Some Sgo proteins also influence other aspects of kinetochore-microtubule attachments. Although many Sgo members require Aurora B kinase to localize to the centromere, factors controlling delocalization are poorly understood and diverse. Moreover, it is not clear how Sgo function is inactivated and whether this is distinct from delocalization. We investigated these questions in Drosophila melanogaster, an organism with superb chromosome cytology to monitor Sgo localization and quantitative assays to test its function in sister-chromatid segregation in meiosis. Previous research showed that in mitosis in cell culture, phosphorylation of the Drosophila Sgo, MEI-S332, by Aurora B promotes centromere localization, whereas Polo phosphorylation promotes delocalization. These studies also suggested that MEI-S332 can be inactivated independently of delocalization, a conclusion supported here by localization and function studies in meiosis. Phosphoresistant and phosphomimetic mutants for the Aurora B and Polo phosphorylation sites were examined for effects on MEI-S332 localization and chromosome segregation in meiosis. Strikingly, MEI-S332 with a phosphomimetic mutation in the Aurora B phosphorylation site prematurely dissociates from the centromeres in meiosis I. Despite the absence of MEI-S332 on meiosis II centromeres in male meiosis, sister chromatids segregate normally, demonstrating that detectable levels of this Sgo are not essential for chromosome congression, kinetochore biorientation, or spindle assembly.

A meiosis-specific form of the APC/C promotes the oocyte-to-embryo transition by decreasing levels of the Polo kinase inhibitor matrimony

During the oocyte-to-embryo transition in Drosophila, degradation of the Polo kinase inhibitor, Matrimony, depends on Cortex, a meiosis-specific form of the Anaphase Promoting Complex/Cyclosome that is required for the oocyte's normal transition from meiosis to mitosis.

Oocytes are stockpiled with proteins and mRNA that are required to drive the initial mitotic divisions of embryogenesis. But are there proteins specific to meiosis whose levels must be decreased to begin embryogenesis properly? The Drosophila protein Cortex (Cort) is a female, meiosis-specific activator of the Anaphase Promoting Complex/Cyclosome (APC/C), an E3 ubiquitin ligase. We performed immunoprecipitation of Cortex followed by mass spectrometry, and identified the Polo kinase inhibitor Matrimony (Mtrm) as a potential interactor with Cort. In vitro binding assays showed Mtrm and Cort can bind directly. We found Mtrm protein levels to be reduced dramatically during the oocyte-to-embryo transition, and this downregulation did not take place in cort mutant eggs, consistent with Mtrm being a substrate of APC^Cort. We showed that Mtrm is subject to APC^Cort-mediated proteasomal degradation and have identified a putative APC/C recognition motif in Mtrm that when mutated partially stabilized the protein in the embryo. Furthermore, overexpression of Mtrm in the early embryo caused aberrant nuclear divisions and developmental defects, and these were enhanced by decreasing levels of active Polo. These data indicate APC^Cort ubiquitylates Mtrm at the oocyte-to-embryo transition, thus preventing excessive inhibition of Polo kinase activity due to Mtrm's presence.

Despite their many differences, the meiotic and mitotic divisions of the early embryo take place within the same cytoplasmic space. The oocyte-to-embryo transition is the process by which an oocyte, which initially undergoes meiosis, becomes “adapted” to support the rapid mitotic divisions of embryogenesis. This involves fertilization as well as the stockpiling of proteins and mRNA for the transcriptionally silent early embryo. The Anaphase Promoting Complex/Cyclosome (APC/C) is a large protein complex that is active during both mitosis and meiosis and is responsible for targeting certain proteins for degradation. The discovery of the existence of APC/C activators that are present only during meiosis hinted at the possibility that this complex also functions to regulate protein degradation during the oocyte-to-embryo transition. Here we study Cortex, a female- and meiosis-specific activator of the APC/C in the fruit fly Drosophila melanogaster. We find that Cortex activity is necessary for the degradation of Matrimony, a key regulator of female meiosis in Drosophila. Matrimony itself inhibits Polo kinase, another important regulator of both mitosis and meiosis that also functions in chromosome segregation, centrosome dynamics, and cytokinesis. When excess Matrimony protein is not removed from the early embryo, developmental defects arise. Together our findings demonstrate that the precise regulation of Matrimony levels in the egg is necessary for the switch from meiosis to mitosis.

Fundamental differences in endoreplication in mammals and Drosophila revealed by analysis of endocycling and endomitotic cells

Throughout the plant and animal kingdoms specific cell types become polyploid, increasing their DNA content to attain a large cell size. In mammals, megakaryocytes (MKs) become polyploid before fragmenting into platelets. The mammalian trophoblast giant cells (TGCs) exploit their size to form a barrier between the maternal and embryonic tissues. The mechanism of polyploidization has been investigated extensively in Drosophila, in which a modified cell cycle--the endocycle, consisting solely of alternating S and gap phases--produces polyploid tissues. During S phase in the Drosophila endocycle, heterochromatin and specific euchromatic regions are underreplicated and reduced in copy number. Here we investigate the properties of polyploidization in murine MKs and TGCs. We induced differentiation of primary MKs and directly microdissected TGCs from embryonic day 9.5 implantation sites. The copy number across the genome was analyzed by array-based comparative genome hybridization. In striking contrast to Drosophila, the genome was uniformly and integrally duplicated in both MKs and TGCs. This was true even for heterochromatic regions analyzed by quantitative PCR. Underreplication of specific regions in polyploid cells is proposed to be due to a slower S phase, resulting from low expression of S-phase genes, causing failure to duplicate late replicating genomic intervals. We defined the transcriptome of TGCs and found robust expression of S-phase genes. Similarly, S-phase gene expression is not repressed in MKs, providing an explanation for the distinct endoreplication parameters compared with Drosophila. Consistent with TGCs endocycling rather than undergoing endomitosis, they have low expression of M-phase genes.

Regulation of DNA replication during development

As development unfolds, DNA replication is not only coordinated with cell proliferation, but is regulated uniquely in specific cell types and organs. This differential regulation of DNA synthesis requires crosstalk between DNA replication and differentiation. This dynamic aspect of DNA replication is highlighted by the finding that the distribution of replication origins varies between differentiated cell types and changes with differentiation. Moreover, differential DNA replication in some cell types can lead to increases or decreases in gene copy number along chromosomes. This review highlights the recent advances and technologies that have provided us with new insights into the developmental regulation of DNA replication.

Translational regulation of the cell cycle: when, where, how and why?

Translational regulation contributes to the control of archetypal and specialized cell cycles, such as the meiotic and early embryonic cycles. Late meiosis and early embryogenesis unfold in the absence of transcription, so they particularly rely on translational repression and activation of stored maternal mRNAs. Here, we present examples of cell cycle regulators that are translationally controlled during different cell cycle and developmental transitions in model organisms ranging from yeast to mouse. Our focus also is on the RNA-binding proteins that affect cell cycle progression by recognizing special features in untranslated regions of mRNAs. Recent research highlights the significance of the cytoplasmic polyadenylation element-binding protein (CPEB). CPEB determines polyadenylation status, and consequently translational efficiency, of its target mRNAs in both transcriptionally active somatic cells as well as in transcriptionally silent mature Xenopus oocytes and early embryos. We discuss the role of CPEB in mediating the translational timing and in some cases spindle-localized translation of critical regulators of Xenopus oogenesis and early embryogenesis. We conclude by outlining potential directions and approaches that may provide further insights into the translational control of the cell cycle.

Polyploidization of glia in neural development links tissue growth to blood-brain barrier integrity

Proper development requires coordination in growth of the cell types composing an organ. Many plant and animal cells are polyploid, but how these polyploid tissues contribute to organ growth is not well understood. We found the Drosophila melanogaster subperineurial glia (SPG) to be polyploid, and ploidy is coordinated with brain mass. Inhibition of SPG polyploidy caused rupture of the septate junctions necessary for the blood-brain barrier. Thus, the increased SPG cell size resulting from polyploidization is required to maintain the SPG envelope surrounding the growing brain. Polyploidization likely is a conserved strategy to coordinate tissue growth during organogenesis, with potential vertebrate examples.

Developmental control of gene copy number by repression of replication initiation and fork progression

Precise DNA replication is crucial for genome maintenance, yet this process has been inherently difficult to study on a genome-wide level in untransformed differentiated metazoan cells. To determine how metazoan DNA replication can be repressed, we examined regions selectively under-replicated in Drosophila polytene salivary glands, and found they are transcriptionally silent and enriched for the repressive H3K27me3 mark. In the first genome-wide analysis of binding of the origin recognition complex (ORC) in a differentiated metazoan tissue, we find that ORC binding is dramatically reduced within these large domains, suggesting reduced initiation as one mechanism leading to under-replication. Inhibition of replication fork progression by the chromatin protein SUUR is an additional repression mechanism to reduce copy number. Although repressive histone marks are removed when SUUR is mutated and copy number restored, neither transcription nor ORC binding is reinstated. Tethering of the SUUR protein to a specific site is insufficient to block replication, however. These results establish that developmental control of DNA replication, at both the initiation and elongation stages, is a mechanism to change gene copy number during differentiation.

Developmental control of oocyte maturation and egg activation in metazoan models

Production of functional eggs requires meiosis to be coordinated with developmental signals. Oocytes arrest in prophase I to permit oocyte differentiation, and in most animals, a second meiotic arrest links completion of meiosis to fertilization. Comparison of oocyte maturation and egg activation between mammals, Caenorhabditis elegans, and Drosophila reveal conserved signaling pathways and regulatory mechanisms as well as unique adaptations for reproductive strategies. Recent studies in mammals and C. elegans show the role of signaling between surrounding somatic cells and the oocyte in maintaining the prophase I arrest and controlling maturation. Proteins that regulate levels of active Cdk1/cyclin B during prophase I arrest have been identified in Drosophila. Protein kinases play crucial roles in the transition from meiosis in the oocyte to mitotic embryonic divisions in C. elegans and Drosophila. Here we will contrast the regulation of key meiotic events in oocytes.

Developmental control of the DNA replication and transcription programs

Polyploid or polytene cells, which have more than 2C DNA content, are widespread throughout nature and present in most differentiated Drosophila tissues. These cells also can display differential replication, that is, genomic regions of increased or decreased DNA copy number relative to overall genomic ploidy. How frequently differential replication is used as a developmental strategy remains unclear. Here, we use genome-wide array-based comparative genomic hybridization (aCGH) to profile differential DNA replication in isolated and purified larval fat body and midgut tissues of Drosophila, and we compare them with recent aCGH profiles of the larval salivary gland. We identify sites of euchromatic underreplication that are common to all three tissues and others that are tissue specific. We demonstrate that both common and tissue-specific underreplicated sites are dependent on the Suppressor of Underreplication protein, SUUR. mRNA-seq profiling shows that whereas underreplicated regions are generally transcriptionally silent in the larval midgut and salivary gland, transcriptional silencing and underreplication have been uncoupled in the larval fat body. In addition to revealing the prevalence of differential replication, our results show that transcriptional silencing and underreplication can be mechanistically uncoupled.

Regulation of APC/C activators in mitosis and meiosis

The anaphase-promoting complex/cyclosome (APC/C) is a multisubunit E3 ubiquitin ligase that triggers the degradation of multiple substrates during mitosis. Cdc20/Fizzy and Cdh1/Fizzy-related activate the APC/C and confer substrate specificity through complex interactions with both the core APC/C and substrate proteins. The regulation of Cdc20 and Cdh1 is critical for proper APC/C activity and occurs in multiple ways: targeted protein degradation, phosphorylation, and direct binding of inhibitory proteins. During the specialized divisions of meiosis, the activity of the APC/C must be modified to achieve proper chromosome segregation. Recent studies show that one way in which APC/C activity is modified is through the use of meiosis-specific APC/C activators. Furthermore, regulation of the APC/C during meiosis is carried out by both mitotic regulators of the APC/C as well as meiosis-specific regulators. Here, we review the regulation of APC/C activators during mitosis and the role and regulation of the APC/C during female meiosis.

Developmental role and regulation of cortex, a meiosis-specific anaphase-promoting complex/cyclosome activator

During oogenesis in metazoans, the meiotic divisions must be coordinated with development of the oocyte to ensure successful fertilization and subsequent embryogenesis. The ways in which the mitotic machinery is specialized for meiosis are not fully understood. cortex, which encodes a putative female meiosis-specific anaphase-promoting complex/cyclosome (APC/C) activator, is required for proper meiosis in Drosophila. We demonstrate that CORT physically associates with core subunits of the APC/C in ovaries. APC/C(CORT) targets Cyclin A for degradation prior to the metaphase I arrest, while Cyclins B and B3 are not targeted until after egg activation. We investigate the regulation of CORT and find that CORT protein is specifically expressed during the meiotic divisions in the oocyte. Polyadenylation of cort mRNA is correlated with appearance of CORT protein at oocyte maturation, while deadenylation of cort mRNA occurs in the early embryo. CORT protein is targeted for degradation by the APC/C following egg activation, and this degradation is dependent on an intact D-box in the C terminus of CORT. Our studies reveal the mechanism for developmental regulation of an APC/C activator and suggest it is one strategy for control of the female meiotic cell cycle in a multicellular organism.

Regulating translation of maternal messages: multiple repression mechanisms

The dowry of mRNAs and proteins that mothers provide their progeny as part of a common developmental strategy to permit rapid embryogenesis necessitates precise translational regulation of the deposited mRNAs. Recent studies with Drosophila uncovered diverse mechanisms to control translation of the transcripts for genes that control the cell cycle and embryonic patterning. The newly delineated mechanisms include: alternative ways to disrupt eIF4E action and the formation of the preinitiation complex b y the eIF4E homologous protein, d4EHP; recruitment of the deadenylase complex by the SMAUG and PUMILIO proteins; both poly(A)-dependent and -independent promotion of translation by the PNG kinase complex; and 5' cap-independent translational regulation b y BRUNO.

Split ends antagonizes the Notch and potentiates the EGFR signaling pathways during Drosophila eye development

The Notch and Epidermal Growth Factor Receptor (EGFR) signaling pathways interact cooperatively and antagonistically to regulate many aspects of Drosophila development, including the eye. How output from these two signaling networks is fine-tuned to achieve the precise balance needed for specific inductive interactions and patterning events remains an open and important question. Previously, we reported that the gene split ends (spen) functions within or parallel to the EGFR pathway during midline glial cell development in the embryonic central nervous system. Here, we report that the cellular defects caused by loss of spen function in the developing eye imaginal disc place spen as both an antagonist of the Notch pathway and a positive contributor to EGFR signaling during retinal cell differentiation. Specifically, loss of spen results in broadened expression of Scabrous, ectopic activation of Notch signaling, and a corresponding reduction in Atonal expression at the morphogenetic furrow. Consistent with Spen's role in antagonizing Notch signaling, reduction of spen levels is sufficient to suppress Notch-dependent phenotypes. At least in part due to loss of Spen-dependent down-regulation of Notch signaling, loss of spen also dampens EGFR signaling as evidenced by reduced activity of MAP kinase (MAPK). This reduced MAPK activity in turn leads to a failure to limit expression of the EGFR pathway antagonist and the ETS-domain transcriptional repressor Yan and to a corresponding loss of cell fate specification in spen mutant ommatidia. We propose that Spen plays a role in modulating output from the Notch and EGFR pathways to ensure appropriate patterning during eye development.

The Drosophila PNG kinase complex regulates the translation of cyclin B

The Drosophila PAN GU (PNG) kinase complex regulates the developmental translation of cyclin B. cyclin B mRNA becomes unmasked during oogenesis independent of PNG activity, but PNG is required for translation from egg activation. We find that although polyadenylation of cyclin B augments translation, it is not essential, and a fully elongated poly(A) is not required for translation to proceed. In fact, changes in poly(A) tail length are not sufficient to account for PNG-mediated control of cyclin B translation and of the early embryonic cell cycles. We present evidence that PNG functions instead as an antagonist of PUMILIO-dependent translational repression. Our data argue that changes in poly(A) tail length are not a universal mechanism governing embryonic cell cycles, and that PNG-mediated derepression of translation is an important alternative mechanism in Drosophila.

Developmentally regulated histone modifications in Drosophila follicle cells: initiation of gene amplification is associated with histone H3 and H4 hyperacetylation and H1 phosphorylation

We have used gene amplification in Drosophila follicle cells as a model of metazoan DNA replication to address whether changes in histone modifications are associated with replication origin activation. We observe that replication initiation is associated with distinct histone modifications. Acetylated lysines K5, K8, and K12 on histone H4 and K14 on histone H3 are specifically enriched during replication initiation at the amplification origins. Strikingly, H4 acetylation persists at an amplification origin well after replication forks have progressed significantly outward from the origin, indicating that H4 acetylation is associated with origin regulation and not histone deposition at the replication forks. Origin recognition complex subunit 2 (orc2) mutants with severe amplification defects do not abolish H4 acetylation, whereas the dup/cdt1 mutant delays the appearance of acetylation foci, and mutants in rbf result in temporal persistence. These data indicate that core histone acetylation is associated with origin activity. Furthermore, follicle cells undergoing gene amplification exhibit high levels of histone H1 phosphorylation. The patterns of H1 phosphorylation provide insights into cell cycle states during amplification, as H1 kinase activity in follicle cells is responsive to high Cyclin E activity, and it can be abolished by overexpressing the retinoblastoma homolog, Rbf, that represses Cyclin E. These data suggest that amplification origins are able to initiate when the cells are in a late S-phase, when the genome is normally not licensed for replication.

Histone Modifications and the Chromatin Scaffold for Meiotic Chromosome Architecture

Chromosomes are capable of remarkable structural adaptability that enables their diverse functions. Histone modifications play pivotal roles in conferring structural diversity to chromosomes by influencing the compactness of chromatin. Several multi-protein complexes bind to chromatin and affect chromosome dynamics, including cohesin, condensin, the chromosome passenger complex, and the synaptonemal complex. The roles of these complexes in promoting chromosome functions include cohesion, condensation and synapsis. It is now crucial to define the relationship between the protein complexes that affect chromosome architecture and the underlying state of the chromatin. During meiosis chromosomes undergo striking morphological changes, including alignment of homologous chromosomes, double-strand break formation and repair, and establishment of meiosis-specific chromosome structures. These dynamic chromosome arrangements are accompanied by the recruitment and expulsion of multi-protein complexes from chromatin. Meiotic chromosome dynamics ensure proper chromosome segregation and production of healthy gametes. Meiosis thus affords an excellent opportunity to determine how histone modifications impact higher order chromosome dynamics by affecting localization and function of chromosome protein complexes. A meiotic mutation in the Drosophila histone kinase, NHK-1, uncovered a critical requirement for histone modifications in chromosome architecture, underscoring the power of this approach.

INCENP and Aurora B promote meiotic sister chromatid cohesion through localization of the Shugoshin MEI-S332 in Drosophila

The chromosomal passenger complex protein INCENP is required in mitosis for chromosome condensation, spindle attachment and function, and cytokinesis. Here, we show that INCENP has an essential function in the specialized behavior of centromeres in meiosis. Mutations affecting Drosophila incenp profoundly affect chromosome segregation in both meiosis I and II, due, at least in part, to premature sister chromatid separation in meiosis I. INCENP binds to the cohesion protector protein MEI-S332, which is also an excellent in vitro substrate for Aurora B kinase. A MEI-S332 mutant that is only poorly phosphorylated by Aurora B is defective in localization to centromeres. These results implicate the chromosomal passenger complex in directly regulating MEI-S332 localization and, therefore, the control of sister chromatid cohesion in meiosis.

Sister chromatid cohesion at the centromere: confrontation between kinases and phosphatases?

Accurate chromosome segregation in mitosis and meiosis requires that the cohesin complex be protected at the centromere by the Shugoshin/MEI-S332 protein family. Recent studies show that Sgo directly binds the phosphatase PP2A, tethering it to the centromere where it can protect cohesin subunits from phosphorylation, and that localization of Sgo/MEI-S332 itself is regulated by phosphorylation.

Replication of heterochromatin: insights into mechanisms of epigenetic inheritance

Heterochromatin is composed of tightly condensed chromatin in which the histones are deacetylated and methylated, and specific nonhistone proteins are bound. Additionally, in vertebrates and plants, the DNA within heterochromatin is methylated. As the heterochromatic state is stably inherited, replication of heterochromatin requires not only duplication of the DNA but also a reinstallment of the appropriate protein and DNA modifications. Thus replication of heterochromatin provides a framework for understanding mechanisms of epigenetic inheritance. In recent studies, roles have been identified for replication factors in reinstating heterochromatin, particularly functions for origin recognition complex, proliferating cell nuclear antigen, and chromatin-assembly factor 1 in recruiting the heterochromatin binding protein HP1, a histone methyltransferase, a DNA methyltransferase, and a chromatin remodeling complex. Potential mechanistic links between these factors are discussed. In some cells, replication of the heterochromatin is blocked, and in Drosophila this inhibition is mediated by a chromatin binding protein SuUR.

A histone code in meiosis: the histone kinase, NHK-1, is required for proper chromosomal architecture in Drosophila oocytes

To promote faithful propagation of the genetic material during sexual reproduction, meiotic chromosomes undergo specialized morphological changes that ensure accurate segregation of homologous chromosomes. The molecular mechanisms that establish the meiotic chromosomal structures are largely unknown. We describe a mutation in a recently identified Histone H2A kinase, nhk-1, in Drosophila that leads to female sterility due to defects in the formation of the meiotic chromosomal structures. The metaphase I arrest and the karyosome, a critical prophase I chromosomal structure, require nucleosomal histone kinase-1 (NHK-1) function. The defects are a result of failure to disassemble the synaptonemal complex and to load condensin onto the mutant chromosomes. Embryos laid by nhk-1-/- mutant females arrest with aberrant polar bodies and mitotic spindles, revealing that mitosis is affected as well. We analyzed the role of Histone H2A phosphorylation with respect to the histone code hypothesis and found that it is required for acetylation of Histone H3 and Histone H4 in meiosis. These studies reveal a critical role for histone modifications in chromosome dynamics in meiosis and mitosis.

Developmental gene amplification: insights into DNA replication and gene expression

In the formation of a complex organism and the differentiation of specific cell types, there are often demands for high levels of particular gene products. These demands can be met by increasing transcription or translation, or by decreasing the rate of mRNA or protein turnover. Although these are the most common means to increase expression levels, there is another mechanism: gene amplification. Developmental gene amplification is a DNA replication-based process whereby specific genes are replicated above the copy number of surrounding sequences, resulting in an increase in the template available for transcription. Recent microarray studies in Drosophila melanogaster have identified two additional amplicons, suggesting that developmental gene amplification might be more widely used than was previously thought. Furthermore, work in both Drosophila and the related fly, Sciara coprophila, has yielded insights into the mechanisms, regulatory sequences and proteins controlling DNA replication during gene amplification, including a link between transcription factors and origin usage.

Roles and regulation of the Drosophila centromere cohesion protein MEI-S332 family

In meiosis, a physical attachment, or cohesion, between the centromeres of the sister chromatids is retained until their separation at anaphase II. This cohesion is essential for ensuring accurate segregation of the sister chromatids in meiosis II and avoiding aneuploidy, a condition that can lead to prenatal lethality or birth defects. The Drosophila MEI-S332 protein localizes to centromeres when sister chromatids are attached in mitosis and meiosis, and it is required to maintain cohesion at the centromeres after cohesion along the sister chromatid arms is lost at the metaphase I/anaphase I transition. MEI-S332 is the founding member of a family of proteins that protect centromeric cohesion but whose members also affect kinetochore behaviour and spindle microtubule dynamics. We compare the Drosophila MEI-S332 family members, evaluate the role of MEI-S332 in mitosis and meiosis I, and discuss the regulation of localization of MEI-S332 to the centromere and its dissociation at anaphase. We analyse the relationship between MEI-S332 and cohesin, a protein complex that is also necessary for sister-chromatid cohesion in mitosis and meiosis. In mitosis, centromere localization of Collapse

Key Words

• meiosis
• sister chromatids
• centromere
• kinetochore
• chromosome segregation

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MESH Headings

• Amino Acid Sequence
• Animals
• Cell Cycle/physiology
• Cell Cycle Proteins/genetics
• Cell Cycle Proteins/metabolism
• Cell Cycle Proteins/physiology
• Centromere/metabolism
• Chromatids/metabolism
• Chromosomal Proteins, Non-Histone
• Chromosome Segregation/physiology
• Drosophila/metabolism
• Drosophila/physiology
• Drosophila Proteins/genetics
• Drosophila Proteins/metabolism
• Drosophila Proteins/physiology
• Fungal Proteins
• Kinetochores/metabolism
• Molecular Sequence Data
• Multigene Family/physiology
• Nuclear Proteins/metabolism
• Nuclear Proteins/physiology
• Protein Structure, Tertiary
• Sequence Alignment
• Sex Factors
• Species Specificity
• Spindle Apparatus/metabolism
• Cohesins

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Grants Collapse

Affiliation(s)

Janice Y Lee
Aki Hayashi-Hagihara
Terry L Orr-Weaver Whitehead Institute and Department of Biology, Massachusetts Institute of TechnologyCambridge, MA 02142, USA

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Mutations in the Drosophila condensin subunit dCAP-G: defining the role of condensin for chromosome condensation in mitosis and gene expression in interphase

Chromosomes are dynamic structures that are reorganized during the cell cycle to optimize them for distinct functions. SMC and non-SMC condensin proteins associate into complexes that have been implicated in the process of chromosome condensation. The roles of the individual non-SMC subunits of the complex are poorly understood, and mutations in the CAP-G subunit have not been described in metazoans. Here we elucidate a role for dCAP-G in chromosome condensation and cohesion in Drosophila. We illustrate the requirement of dCAP-G for condensation during prophase and prometaphase; however, we find that alternate mechanisms ensure that replicated chromosomes are condensed prior to metaphase. In contrast, dCAP-G is essential for chromosome condensation in metaphase of single, unreplicated sister chromatids, suggesting that there is an interplay between replicated chromatids and the condensin complex. In the dcap-g mutants, defects in sister-chromatid separation are also observed. Chromatid arms fail to resolve in prophase and are unable to separate at anaphase, whereas sister centromeres show aberrant separation in metaphase and successfully move to spindle poles at anaphase. We also identified a role for dCAP-G during interphase in regulating heterochromatic gene expression.

POLO kinase regulates the Drosophila centromere cohesion protein MEI-S332

Accurate segregation of chromosomes is critical to ensure that each daughter cell receives the full genetic complement. Maintenance of cohesion between sister chromatids, especially at centromeres, is required to segregate chromosomes precisely during mitosis and meiosis. The Drosophila protein MEI-S332, the founding member of a conserved protein family, is essential in meiosis for maintaining cohesion at centromeres until sister chromatids separate at the metaphase II/anaphase II transition. MEI-S332 localizes onto centromeres in prometaphase of mitosis or meiosis I, remaining until sister chromatids segregate. We elucidated a mechanism for controlling release of MEI-S332 from centromeres via phosphorylation by POLO kinase. We demonstrate that POLO antagonizes MEI-S332 cohesive function and that full POLO activity is needed to remove MEI-S332 from centromeres, yet this delocalization is not required for sister chromatid separation. POLO phosphorylates MEI-S332 in vitro, POLO and MEI-S332 bind each other, and mutation of POLO binding sites prevents MEI-S332 dissociation from centromeres.

Drosophila Genome-Scale Screen for PAN GU Kinase Substrates Identifies Mat89Bb as a Cell Cycle Regulator

Although traditional organism-based mutational analysis is powerful in identifying genes involved in specific biological processes, limitations include incomplete coverage and time required for gene identification. Biochemical screens using cell transfection or yeast two-hybrid methods are rapid, but they are limited by cDNA library quality. The recent establishment of "uni-gene sets" has made it feasible to biochemically screen an organism's entire genome. Radiolabeled protein pools prepared from the Drosophila Gene Collection were used in a Drosophila in vitro expression cloning ("DIVEC") screen for substrates of PAN GU kinase, which is crucial for S-M embryonic cell cycles. Ablation of one identified substrate, Mat89Bb, by RNAi produces a polyploid phenotype similar to that of pan gu mutants. Xenopus embryos injected with Mat89Bb morpholinos arrest with polyploid nuclei, and Mat89Bb RNAi in HeLa cells gives rise to multinucleated cells. Thus, Mat89Bb plays an evolutionarily conserved role as a crucial regulator of both cell cycle and development.

Control of centromere localization of the MEI-S332 cohesion protection protein

In mitosis and meiosis, cohesion is maintained at the centromere until sister-chromatid separation. Drosophila MEI-S332 is essential for centromeric cohesion in meiosis and contributes to, though is not absolutely required for, cohesion in mitosis. It localizes specifically to centromeres in prometaphase and delocalizes at the metaphase-anaphase transition. In mei-S332 mutants, centromeric sister-chromatid cohesion is lost at anaphase I, giving meiosis II missegregation. MEI-S332 is the founding member of a family of proteins important for chromosome segregation. One likely activity of these proteins is to protect the cohesin subunit Rec8 from cleavage at the metaphase I-anaphase I transition. Although the family members do not show high sequence identity, there are two short stretches of homology, and mutations in conserved residues affect protein function. Here we analyze the cis- and trans-acting factors required for MEI-S332 localization. We find a striking correlation between domains necessary for MEI-S332 centromere localization and conserved regions within the protein family. Drosophila MEI-S332 expressed in human cells localizes to mitotic centromeres, further highlighting this functional conservation. MEI-S332 can localize independently of cohesin, assembling even onto unreplicated chromatids. However, the separase pathway that regulates cohesin dissociation is needed for MEI-S332 delocalization at anaphase.

Gene amplification as a developmental strategy: isolation of two developmental amplicons in Drosophila

Gene amplification is known to be critical for upregulating gene expression in a few cases, but the extent to which amplification is utilized in the development of diverse organisms remains unknown. By quantifying genomic DNA hybridization to microarrays to assay gene copy number, we identified two additional developmental amplicons in the follicle cells of the Drosophila ovary. Both amplicons contain genes which, following their amplification, are expressed in the follicle cells, and the expression of three of these genes becomes restricted to specialized follicle cells late in differentiation. Genetic analysis establishes that at least one of these genes, yellow-g, is critical for follicle cell function, because mutations in yellow-g disrupt eggshell integrity. Thus, during follicle cell differentiation the entire genome is overreplicated as the cells become polyploid, and subsequently specific genomic intervals are overreplicated to facilitate gene expression.

A proposed role for the Polycomb group protein dRING in meiotic sister-chromatid cohesion

ORD protein is required for accurate chromosome segregation during male and female meiosis in Drosophila melanogaster. Null ord mutations result in random segregation of sister chromatids during both meiotic divisions because cohesion is completely abolished prior to kinetochore capture of microtubules during meiosis I. Previous analyses of mutant ord alleles have led us to propose that the C-terminal half of the ORD protein mediates protein-protein interactions that are essential for sister-chromatid cohesion. To identify proteins that interact with ORD, we conducted a yeast two-hybrid screen using an ORD bait and isolated dRING, a core subunit of the Drosophila Polycomb repressive complex 1. We show that a missense mutation in ORD completely ablates the two-hybrid interaction with dRING and prevents nuclear retention of the mutant ORD protein in male meiotic cells. Using affinity-purified antibodies generated against full-length recombinant dRING, we demonstrate that dRING protein is expressed in the male and female gonads and colocalizes extensively with ORD on the chromatin of primary spermatocytes during G2 of meiosis. Our results suggest a novel role for the Polycomb group protein dRING and are consistent with the model that interaction of dRING and ORD is required to promote the proper segregation of meiotic chromosomes.

The Drosophila cell cycle kinase PAN GU forms an active complex with PLUTONIUM and GNU to regulate embryonic divisions

Early embryonic cell cycles in Drosophila consist of rapidly alternating S and M phases. Three genes, pan gu (png), plutonium (plu), and giant nuclei (gnu) coordinate these early S-M cycles by ensuring adequate Cyclin B protein levels. Mutations in any of these genes result in unregulated DNA replication and a lack of mitosis ("giant nuclei" phenotype). png encodes a serine/threonine protein kinase, and plu and gnu encode small, novel proteins. We show that PNG, PLU, and GNU constitute a novel protein kinase complex that specifically regulates S-M cell cycles. All three proteins are required for PNG kinase activity and are phosphorylated by PNG in vitro. Yeast two-hybrid screening revealed a direct interaction between PNG and PLU, and their co-expression is required for physical association and activation of PNG kinase. Artificial dimerization of PLU via fusion to either GST or FK506 binding protein (in the presence of dimerizing agent) abrogates the requirement for GNU to activate PNG kinase. We propose a model in which GNU normally regulates embryonic cell cycles by promoting transient dimerization of a core PNG/PLU complex, thereby stimulating PNG kinase activity.

The Drosophila MOS ortholog is not essential for meiosis

In metazoan oocytes, a metaphase arrest coordinates the completion of meiosis with fertilization. Vertebrate mos maintains the metaphase II arrest of mature oocytes and prevents DNA replication between the meiotic divisions. We identified a Drosophila homolog of mos and showed it to be the mos ortholog by two additional criteria. The dmos transcripts are present in Drosophila oocytes but not embryos, and injection of dmos into Xenopus embryos blocks mitosis and elevates active MAPK levels. In Drosophila, MAPK is activated in oocytes, consistent with a role in meiosis. We generated deletions of dmos and found that, as in vertebrates, dmos is responsible for the majority of MAPK activation. Unexpectedly, the oocytes that do mature complete meiosis normally and produce fertilized embryos that develop, although there is a reduction in female fertility and loss of some oocytes by apoptosis. Therefore, Drosophila contains a mos ortholog that activates a MAPK cascade during oogenesis and is nonessential for meiosis. This could be because there are redundant pathways regulating meiosis, because residual, low levels of active MAPK are sufficient, or because active MAPK is dispensable for meiosis in Drosophila. These results highlight the complexity of meiotic regulation that evolved to ensure accurate control over the reproductive process.

Regulation of Cell Cycles in Drosophila Development: Intrinsic and Extrinsic Cues

An intriguing aspect of cell cycle regulation is how cell growth and division are coordinated with developmental signals to produce properly patterned organisms of the appropriate size. Using the foundation laid by a detailed understanding of the regulators that intrinsically control progression through the cell cycle, links between developmental signals and the cell cycle are being elucidated. Considerable progress has been made using Drosophila melanogaster, both in identifying new cell cycle regulators that respond to developmental cues and in defining the impact of extrinsic signals on homologs of mammalian oncogenes and tumor suppressors. In this review, we discuss each cell cycle phase, highlighting differences between archetypal and variant cell cycles employed for specific developmental strategies. We emphasize the interplay between developmental signals and cell cycle transitions. Developmental control of checkpoints, cell cycle exit, and cell growth are also addressed.

The E2F cell cycle regulator is required for Drosophila nurse cell DNA replication and apoptosis

During Drosophila oogenesis nurse cells become polyploid, enabling them to provide the developing oocyte with vast amounts of maternal messages and products. The nurse cells then die by apoptosis. In nurse cells, as in many other polyploid or polytene tissues, replication is differentially controlled and the heterochromatin is underreplicated. The nurse cell chromosomes also undergo developmentally induced morphological changes from being polytene, with tightly associated sister chromatids, to polyploid, with dispersed sister chromatids. We used female-sterile dE2F1 and dDP mutants to assess the role of the E2F cell cycle regulator in oogenesis and the relative contributions of transcriptional activation versus repression during nurse cell development. We report here that E2F1 transcriptional activity in nurse cells is essential for the robust synthesis of S-phase transcripts that are deposited into the oocyte. dE2F1 and dDP are needed to limit the replication of heterochromatin in nurse cells. In dE2F1 mutants the nurse cell chromosomes do not properly undergo the transition from polyteny to polyploidy. We also find that dDP and dE2F1 are needed for nurse cell apoptosis, implicating transcriptional activation of E2F target genes in this process.

Visualization of replication initiation and elongation in Drosophila

Chorion gene amplification in the ovaries of Drosophila melanogaster is a powerful system for the study of metazoan DNA replication in vivo. Using a combination of high-resolution confocal and deconvolution microscopy and quantitative realtime PCR, we found that initiation and elongation occur during separate developmental stages, thus permitting analysis of these two phases of replication in vivo. Bromodeoxyuridine, origin recognition complex, and the elongation factors minichromosome maintenance proteins (MCM)2-7 and proliferating cell nuclear antigen were precisely localized, and the DNA copy number along the third chromosome chorion amplicon was quantified during multiple developmental stages. These studies revealed that initiation takes place during stages 10B and 11 of egg chamber development, whereas only elongation of existing replication forks occurs during egg chamber stages 12 and 13. The ability to distinguish initiation from elongation makes this an outstanding model to decipher the roles of various replication factors during metazoan DNA replication. We utilized this system to demonstrate that the pre-replication complex component, double-parked protein/cell division cycle 10-dependent transcript 1, is not only necessary for proper MCM2-7 localization, but, unexpectedly, is present during elongation.

The anaphase promoting complex/cyclosome is required during development for modified cell cycles

Animals and plants use modified cell cycles to achieve particular developmental strategies. In one common example, most animals and plants have tissues in which the cells become polyploid or polytene by means of an S-G cycle, but the mechanism by which mitosis is inhibited in the endo cycle is not understood. The Drosophila morula (mr) gene regulates variant cell cycles, because in addition to disrupting the archetypal cycle (G1-S-G2-M), mr mutations affect the rapid embryonic (S-M) divisions as well as the endo cycle (S-G) that produces polyploid cells. In dividing cells mr mutations cause a metaphase arrest, and endo cycling nurse cells inappropriately reenter mitosis in mr mutants. We show mr encodes the APC2 subunit of the anaphase promoting complex/cyclosome. This finding demonstrates that anaphase promoting complex/cyclosome is required not only in proliferating cells but also to block mitosis in some endo cycles. The mr mutants further indicate that transient mitotic functions in endo cycles change chromosome morphology from polytene to polyploid.

The sister-chromatid cohesion protein ORD is required for chiasma maintenance in Drosophila oocytes

Accurate chromosome partitioning during cell division requires that cohesion hold sister chromatids together until kinetochores correctly attach to spindle microtubules. In 1932, Darlington noted that sister-chromatid cohesion distal to the site of exchange also could play a vital role in maintaining the association of chiasmate homologs during meiosis. Cohesion linking a recombinant chromatid with a sister of each homologous pair would resist spindle forces that separate kinetochores of homologous chromosomes (see Figure 1). Although centromeric cohesion must be retained to ensure proper segregation during meiosis II, dissolution of arm cohesion would be required for anaphase I to occur. This hypothesis is supported by recent evidence in yeast and C. elegans that separase activity is essential for the segregation of recombinant homologs during meiosis I. We present evidence that Drosophila oocytes require sister-chromatid cohesion to maintain a physical attachment between recombinant chromosomes. Using FISH to monitor cohesion directly, we confirm that oocytes lacking ORD activity exhibit cohesion defects, consistent with previous genetic results. We also show that ord(null) oocytes that have undergone recombination are unable to arrest at metaphase I, indicating that chiasmata are unstable in the absence of cohesion. Our results support the model that arm cohesion provides a conserved mechanism that ensures physical attachment between recombinant homologs until anaphase I.

The molecular basis of sister-chromatid cohesion

The replicated copies of each chromosome, the sister chromatids, are attached prior to their segregation in mitosis and meiosis. This association or cohesion is critical for each sister chromatid to bind to microtubules from opposite spindle poles and thus segregate away from each other at anaphase of mitosis or meiosis II. The cohesin protein complex is essential for cohesion in both mitosis and meiosis, and cleavage of one of the subunits is sufficient for loss of cohesion at anaphase. The localization of the cohesin complex and other cohesion proteins permits evaluation of the positions of sister-chromatid associations within the chromosome structure, as well as the relationship between cohesion and condensation. Recently, two key riddles in the mechanism of meiotic chromosome segregation have yielded to molecular answers. First, analysis of the cohesin complex in meiosis provides molecular support for the long-standing hypothesis that sister-chromatid cohesion links homologs in meiosis I by stabilizing chiasmata. Second, the isolation of the monopolin protein that controls kinetochore behavior in meiosis I defines a functional basis by which sister kinetochores are directed toward the same pole in meiosis I.