Heat shock results in cell cycle delay and synchronisation of mitotic domains in cellularised Drosophila melanogaster embryos

Heat tolerance, oxidative stress response tuning and robust gene activation in early-stage Drosophila melanogaster embryos

In organisms with complex life cycles, life stages that are most susceptible to environmental stress may determine species persistence in the face of climate change. Early embryos of Drosophila melanogaster are particularly sensitive to acute heat stress, yet tropical embryos have higher heat tolerance than temperate embryos, suggesting adaptive variation in embryonic heat tolerance. We compared transcriptomic responses to heat stress among tropical and temperate embryos to elucidate the gene regulatory basis of divergence in embryonic heat tolerance. The transcriptomes of tropical and temperate embryos differed in both constitutive and heat-stress-induced responses of the expression of relatively few genes, including genes involved in oxidative stress. Most of the transcriptomic response to heat stress was shared among all embryos. Embryos shifted the expression of thousands of genes, including increases in the expression of heat shock genes, suggesting robust zygotic gene activation and demonstrating that, contrary to previous reports, early embryos are not transcriptionally silent. The involvement of oxidative stress genes corroborates recent reports on the critical role of redox homeostasis in coordinating developmental transitions. By characterizing adaptive variation in the transcriptomic basis of embryonic heat tolerance, this study is a novel contribution to the literature on developmental physiology and developmental genetics.

Molecular chaperoning helps safeguarding mitochondrial integrity and motor functions in the Sahara silver ant Cataglyphis bombycina

The Sahara silver ant Cataglyphis bombycina is one of the world's most thermotolerant animals. Workers forage for heat-stricken arthropods during the hottest part of the day, when temperatures exceed 50 °C. However, the physiological adaptations needed to cope with such harsh conditions remain poorly studied in this desert species. Using transcriptomics, we screened for the most heat-responsive transcripts of C. bombycina with aim to better characterize the molecular mechanisms involved with macromolecular stability and cell survival to heat-stress. We identified 67 strongly and consistently expressed transcripts, and we show evidences of both evolutionary selection and specific heat-induction of mitochondrial-related molecular chaperones that have not been documented in Formicidae so far. This indicates clear focus of the silver ant's heat-shock response in preserving mitochondrial integrity and energy production. The joined induction of small heat-shock proteins likely depicts the higher requirement of this insect for proper motor function in response to extreme burst of heat-stresses. We discuss how those physiological adaptations may effectively help workers resist and survive the scorching heat and burning ground of the midday Sahara Desert.

53BP1 and USP28 mediate p53-dependent cell cycle arrest in response to centrosome loss and prolonged mitosis

Mitosis occurs efficiently, but when it is disturbed or delayed, p53-dependent cell death or senescence is often triggered after mitotic exit. To characterize this process, we conducted CRISPR-mediated loss-of-function screens using a cell-based assay in which mitosis is consistently disturbed by centrosome loss. We identified 53BP1 and USP28 as essential components acting upstream of p53, evoking p21-dependent cell cycle arrest in response not only to centrosome loss, but also to other distinct defects causing prolonged mitosis. Intriguingly, 53BP1 mediates p53 activation independently of its DNA repair activity, but requiring its interacting protein USP28 that can directly deubiquitinate p53 in vitro and ectopically stabilize p53 in vivo. Moreover, 53BP1 can transduce prolonged mitosis to cell cycle arrest independently of the spindle assembly checkpoint (SAC), suggesting that while SAC protects mitotic accuracy by slowing down mitosis, 53BP1 and USP28 function in parallel to select against disturbed or delayed mitosis, promoting mitotic efficiency.

DOI:http://dx.doi.org/10.7554/eLife.16270.001

Virus-Surface-Mimicking Surface Clustering of AuNPs onto DNA-Entrapped Polymeric Nanoparticle for Enhanced Cellular Internalization and Nanocluster-Induced NIR Photothermal Therapy

Virus-surface-mimicking decoration of deoxyribonucleic acid (DNA)-entrapped polymeric nanoparticle with AuNPs is demonstrated to lead to enhanced cellular uptake, improved gene transfection, and particularly efficient near-infrared photothermal therapy that cannot be achieved by both of them separately. This hybrid nanosystem represents a novel paradigm of multipurpose organic-inorganic nanoplatform, especially for cancer treatments.

The peculiarities of piRNA expression upon heat shock exposure in Drosophila melanogaster

Different types of stress including heat shock may induce genomic instability, due to the derepression and amplification of mobile elements (MEs). It remains unclear, however, whether piRNA-machinery regulating ME expression functions normally under stressful conditions. The aim of this study was to explore the features of piRNA expression after heat shock (HS) exposure in Drosophila melanogaster. We also evaluated functioning of piRNA-machinery in the absence of major stress protein Hsp70 in this species. We analyzed the deep sequence data of piRNA expression after HS treatment and demonstrated that it modulates the expression of certain double-stranded germinal piRNA-clusters. Notable, we demonstrated significant changes in piRNA levels targeting a group of MEs after HS only in the strain containing normal set of hsp70 genes. Surprisingly, we failed to detect any correlation between the levels of piRNAs and the transcription of complementary MEs in the studied strains. We propose that modulation of certain piRNA-clusters expression upon HS exposure in D. melanogaster occurs due to HS-induced altering of chromatin state at certain chromosome regions.

Temperature-Related Reaction Norms of Gene Expression: Regulatory Architecture and Functional Implications

The environment has profound effects on the expression of many traits and reaction norms describe the expression dynamics of a trait across a broad range of environmental conditions. Here, we analyze gene expression in Drosophila melanogaster across four different developmental temperatures (13-29 °C). Gene expression is highly plastic with 83.3% of the genes being differentially expressed. We distinguished three components of plasticity: 1) Dynamics of gene expression intensity (sum of change), 2) direction of change, and 3) curvature of the reaction norm (linear vs. quadratic). Studying their regulatory architecture we found that all three plasticity components were most strongly affected by the number of different transcription factors (TFs) binding to the target gene. More TFs were found in genes with less expression changes across temperatures. Although the effect of microRNAs was weaker, we consistently noted a trend in the opposite direction. The most plastic genes were regulated by fewer TFs and more microRNAs than less plastic genes. Different patterns of plasticity were also reflected by their functional characterization based on gene ontology. Our results suggest that reaction norms provide an important key to understand the functional requirements of natural populations exposed to variable environmental conditions.

Over-expression of Ultrabithorax alters embryonic body plan and wing patterns in the butterfly Bicyclus anynana

In insects, forewings and hindwings usually have different shapes, sizes, and color patterns. A variety of RNAi experiments across insect species have shown that the hox gene Ultrabithorax (Ubx) is necessary to promote hindwing identity. However, it remains unclear whether Ubx is sufficient to confer hindwing fate to forewings across insects. Here, we address this question by over-expressing Ubx in the butterfly Bicyclus anynana using a heat-shock promoter. Ubx whole-body over-expression during embryonic and larvae development led to body plan changes in larvae but to mere quantitative changes to adult morphology, respectively. Embryonic heat-shocks led to fused segments, loss of thoracic and abdominal limbs, and transformation of head limbs to larger appendages. Larval heat-shocks led to reduced eyespot size in the expected homeotic direction, but neither additional eyespots nor wing shape changes were observed in forewings as expected of a homeotic transformation. Interestingly, Ubx was found to be expressed in a novel, non-characteristic domain - in the hindwing eyespot centers. Furthermore, ectopic expression of Ubx on the pupal wing activated the eyespot-associated genes spalt and Distal-less, known to be directly repressed by Ubx in the fly׳s haltere and leg primordia, respectively, and led to the differentiation of black wing scales. These results suggest that Ubx has been co-opted into a novel eyespot gene regulatory network, and that it is capable of activating black pigmentation in butterflies.

Study of heat shock response of human umbilical vein endothelial cells (HUVECs) using cDNA microarray

Genetic response of human umbilical vein cells (HUVECs) to heat shock was studied using gene expression analysis. HUVECs were subjected to heat treatment at 44 degrees C and 55 degrees C for 1 h, respectively. Four hours after the treatment, gene expression of the treated cells and control cells (37 degrees C) were profiled using cDNA microarray. Data were analyzed using bioinformatics tools, and the results were verified by real-time quantitative PCR experiment. It has been shown that a large number of genes were regulated indicating global response to heat treatment at the genetic level.

Identification of 30-kDa heat shock protein gene in Trichophyton rubrum

Small heat shock proteins (sHSPs) are chaperones that are crucial in the heat shock response but also have important non-stress roles within the cell. HSP70 in Trichophyton rubrum is already detected and carefully characterised; however, no study was carried out for HSP30 in this pathogenic fungus. In the present study, T. rubrum was obtained from patients with dermatophytosis and cultured in appropriate conditions. High-molecular-weight DNA was extracted using standard extraction methods. Pairs of 21 nt primers were designed from highly conserved regions of the similar genes in other eukaryotic cells. Mentioned primers were utilised in PCR using isolated genomic DNA and extracted RNA templates of T. rubrum. The PCR fragments were then sequenced and 415 nucleotides of HSP30 in this pathogenic fungus were detected; the open reading frame had 156 nucleotides and was coding 51 amino acids. This gene (called TrHSP30) is registered in GenBank at National Center for Biotechnology Information (NIH, USA) database. Detection of TrHSP30 gene may open the way to determination of its possible role in the pathogenesis of dermatophyte infections due to T. rubrum.

Covalently linked Au nanoparticles to a viral vector: potential for combined photothermal and gene cancer therapy

Hyperthermia can be produced by near-infrared laser irradiation of gold nanoparticles present in tumors and thus induce tumor cell killing via a bystander effect. To be clinically relevant, however, several problems still need to be resolved. In particular, selective delivery and physical targeting of gold nanoparticles to tumor cells are necessary to improve therapeutic selectivity. Considerable progress has been made with respect to retargeting adenoviral vectors for cancer gene therapy. We therefore hypothesized that covalent coupling of gold nanoparticles to retargeted adenoviral vectors would allow selective delivery of the nanoparticles to tumor cells, thus feasibilizing hyperthermia and gene therapy as a combinatorial therapeutic approach. For this, sulfo-N-hydroxysuccinimide labeled gold nanoparticles were reacted to adenoviral vectors encoding a luciferase reporter gene driven by the cytomegalovirus promoter (AdCMVLuc). We herein demonstrate that covalent coupling could be achieved, while retaining virus infectivity and ability to retarget tumor-associated antigens. These results indicate the possibility of using adenoviral vectors as carriers for gold nanoparticles.

Whole-genome analysis reveals a strong positional bias of conserved dMyc-dependent E-boxes

Myc is a transcription factor with diverse biological effects ranging from the control of cellular proliferation and growth to the induction of apoptosis. Here we present a comprehensive analysis of the transcriptional targets of the sole Myc ortholog in Drosophila melanogaster, dMyc. We show that the genes that are down-regulated in response to dmyc inhibition are largely identical to those that are up-regulated after dMyc overexpression and that many of them play a role in growth control. The promoter regions of these targets are characterized by the presence of the E-box sequence CACGTG, a known dMyc binding site. Surprisingly, a large subgroup of (functionally related) dMyc targets contains a single E-box located within the first 100 nucleotides after the transcription start site. The relevance of this E-box and its position was confirmed by a mutational analysis of a selected dMyc target and by the observation of its evolutionary conservation in a different Drosophila species, Drosophila pseudoobscura. These observations raise the possibility that a subset of Myc targets share a distinct regulatory mechanism.

Heat shock reduces developmental competence and alters spindle configuration of bovine oocytes

Heat shock may enhance the thermotolerance of, or cause detrimental effects on, a variety of cell types or organisms, depending on the duration and intensity of the thermal challenge. Experiments were designed to investigate the effect of heat shock on the developmental competence and cytoskeletal structures of bovine oocytes following IVF. In Experiment 1, bovine cumulus-oocyte complexes (COCs) were subjected to standard IVM culture conditions for 20 h and were then randomly allocated to groups for heat shock at 42 degrees C for 0 (control), 1, 2, or 4h. The oocytes were fertilized after heat shock and followed by culture in KSOM for 8d. There were no significant differences in cleavage rates, but blastocyst formation (27% versus 44%) and total cell number per blastocyst (82+/-21 versus 108+/-36; mean+/-S.D.) were lower in the 4-h heat shock group compared to the control (P<0.05). Trophectoderm, but not ICM, cell numbers were decreased (P<0.05) in the 4-h heat shock group compared to the control. Alterations in the meiotic spindle of IVM oocytes (n=120-126) were examined after 1 to 4-h of heat shock in Experiments 2 and 3. The metaphase spindle became elongated or aberrant and smaller following heat shock, compared to the non-heat shock oocytes (P<0.05). The basis for changes in spindle configuration and the differential decrease in trophectoderm cell numbers after heat shock are not clear, but may lead to reduced embryonic development and perhaps the low pregnancy rate of domestic animals during hot seasons.

Xpd/Ercc2 regulates CAK activity and mitotic progression

General transcription factor IIH (TFIIH) consists of nine subunits: cyclin-dependent kinase 7 (Cdk7), cyclin H and MAT1 (forming the Cdk-activating-kinase or CAK complex), the two helicases Xpb/Hay and Xpd, and p34, p44, p52 and p62 (refs 1-3). As the kinase subunit of TFIIH, Cdk7 participates in basal transcription by phosphorylating the carboxy-terminal domain of the largest subunit of RNA polymerase II. As part of CAK, Cdk7 also phosphorylates other Cdks, an essential step for their activation. Here we show that the Drosophila TFIIH component Xpd negatively regulates the cell cycle function of Cdk7, the CAK activity. Excess Xpd titrates CAK activity, resulting in decreased Cdk T-loop phosphorylation, mitotic defects and lethality, whereas a decrease in Xpd results in increased CAK activity and cell proliferation. Moreover, Xpd is downregulated at the beginning of mitosis when Cdk1, a cell cycle target of Cdk7, is most active. Downregulation of Xpd thus seems to contribute to the upregulation of mitotic CAK activity and to regulate mitotic progression positively. Simultaneously, the downregulation of Xpd might be a major mechanism of mitotic silencing of basal transcription.

Armadillo levels are reduced during mitosis in Drosophila

The Armadillo protein of Drosophila melanogaster is both a structural component of adherens junctions at apical cell membranes and also a key cytoplasmic transducer of the Wingless signalling pathway. We have used the Gal4-UAS system to over-express Armadillo in the Drosophila wing: this hyperactivates the Wingless pathway and leads to the formation of ectopic, supernumerary wing bristles. Here, we report that this adult phenotype is dominantly enhanced by mutations in cdc25(string) and, conversely, is suppressed by co-expression of Cdc25(String). Furthermore, we show that the steady state levels of Armadillo protein produced from the UAS transgene are also sensitive to cdc25(string) dosage in the cells of the larval imaginal wing disc. Consistent with the role of Cdc25(String) in promoting mitosis and with our genetic interaction data, we find a strong correlation between progression through mitosis and a reduction in Armadillo levels. Significantly, this is true whether Armadillo is over-expressed or not, and both cytoplasmic (signalling) and membrane-associated (junctional) Armadillo appears to be affected. We conclude that this phenomenon may reduce the efficacy of Wingless signalling and/or intercellular adhesion during cell division.

The effect of hyperthermia on the induction of cell death in brain, testis, and thymus of the adult and developing rat

Stressful stimuli can elicit 2 distinct reactive cellular responses, the heat shock (stress) response and the activation of cell death pathways. Most studies on the effects of hyperthermia on the mammalian nervous system have focused on the heat shock response, characterized by the transient induction of Hsps, which play roles in repair and protective mechanisms. This study examines the effect of hyperthermia on the induction of cell death via apoptosis, assayed by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling and active caspase 3 cytochemistry, in the adult rat brain, testis, and thymus. Results show that a fever-like increase in temperature triggered apoptosis in dividing cell populations of testis and thymus, but not in mature, postmitotic cells of the adult cerebellum. These differential apoptotic responses did not correlate with whole-tissue levels of Hsp70 induction. We further investigated whether dividing neural cells were more sensitive to heat-induced apoptosis by examining the external granule cell layer of the cerebellum at postnatal day 7 and the neuroepithelial layers of the neocortex and tectum at embryonic day 17. These proliferative neural regions were highly susceptible to hyperthermia-induced apoptosis, suggesting that actively dividing cell populations are more prone to cell death induced by hyperthermia than fully differentiated postmitotic neural cells.

Cell cycle progression and cell division are sensitive to hypoxia in Drosophila melanogaster embryos

We and others recently demonstrated that Drosophila melanogaster embryos arrest development and embryonic cells cease dividing when they are deprived of O2. To further characterize the behavior of these embryos in response to O2 deprivation and to define the O2-sensitive checkpoints in the cell cycle, embryos undergoing nuclear cycles 3-13 were subjected to O2 deprivation and examined by confocal microscopy under control, hypoxic, and reoxygenation conditions. In vivo, real-time analysis of embryos carrying green fluorescent protein-kinesin demonstrated that cells arrest at two major points of the cell cycle, either at the interphase (before DNA duplication) or at metaphase, depending on the cell cycle phase at which O2 deprivation was induced. Immunoblot analysis of embryos whose cell divisions are synchronized by inducible String (cdc25 homolog) demonstrated that cyclin B was degraded during low O2 conditions in interphase-arrested embryos but not in those arrested in metaphase. Embryos resumed cell cycle activity within ~20 min of reoxygenation, with very little apparent change in cell cycle kinetics. We conclude that there are specific points during the embryonic cell cycle that are sensitive to the O2 level in D. melanogaster. Given the fact that O2 deprivation also influences the growth and development of other species, we suggest that similar hypoxia-sensitive cell cycle checkpoints may also exist in mammalian cells.

Developmentally regulated nuclear transport of transcription factors in Drosophila embryos enable the heat shock response

Hsp70 is a broadly conserved thermotolerance factor, but inhibits growth at normal temperatures and cannot be induced in early embryos. We report that in Drosophila embryos the temporal and spatial patterns of Hsp70 inducibility were unexpectedly complex, with striking differences between the soma and the germline. In both, regulation occurred at the level of transcription. During the refractory period for Hsp70 induction, HSF (heat-shock transcription factor) exhibited specific DNA-binding activity characteristic of activation in extracts of heated embryos. Remarkably, however, HSF was restricted to the cytoplasm in intact embryos even after heat shock. HSF moved from the cytoplasm to the nucleus in the absence of heat precisely when the capacity to induce Hsp70 was acquired (cycle 12 of the germline, cycle 13 in the soma). During oogenesis, Hsp70 inducibility was lost in nurse cells around stage 10, in a posterior-to-anterior gradient and HSF redistributed from nucleus to cytoplasm in the same spatiotemporal pattern. In a highly inbred derivative of the Samarkind strain, HSF moved into embryonic nuclei earlier than in our standard wild-type strain. Correspondingly, Hsp70 was inducible earlier, confirming that nuclear transport of HSF controls the inducibility of Hsp70 in early embryos. We also report for the first time the nuclear import patterns of two general transcription factors, RNA polymerase subunit Ilc and TATA binding protein (TBP). Both enter nuclei in a highly synchronous manner, independently of each other and of HSF. The import of TBP coincides with the first reported appearance of transcripts in the embryo. We suggest that the potentiation of general and heat shock-specific transcription in Drosophila embryos is controlled by the developmentally programmed relocalization of general and heat shock-specific transcription factors. Restricted nuclear entry of HSF represents a newly described mechanism for regulating the heat-shock response.

Imaginal tissues of Drosophila melanogaster exhibit different modes of cell proliferation control

The highly conserved regulatory mechanisms that control progression of a cell through the cell cycle do not, alone, explain the programmed control of cell proliferation during animal development. Additional controls must coordinate the cell cycle regulators with developmental regulatory events. Here we report studies of cell cycle control in the imaginal tissues of Drosophila melanogaster, specifically in situations where cell cycle progression is regulated by varying the length of the G2 phase. We show that G2-phase arrest in late larval wing imaginal disks requires transcriptional control of stg, a mitotic inducer that encodes a D. melanogaster homologue of the Schizosaccharomyces pombe p80cdc25 phosphatase. In a second study, string transcriptional regulation was also shown to be important for G2-phase regulation in eye disk cells posterior to the morphogenetic furrow. Finally, unlike all other situations described to date, string transcriptional regulation was found not to be the cause of G2 arrest in abdominal histoblasts, these cells being refractory to ectopic expression of stg. This study further establishes string as an important regulator of G2 phase during D. melanogaster development, but also reveals that at least one additional mechanism is utilized to control G2-phase length and thus cell proliferation in different developmental contexts.

Targeted gene expression without a tissue-specific promoter: creating mosaic embryos using laser-induced single-cell heat shock

We have developed a method to target gene expression in the Drosophila embryo to a specific cell without having a promoter that directs expression in that particular cell. Using a digitally enhanced imaging system to identify single cells within the living embryo, we apply a heat shock to each cell individually by using a laser microbeam. A 1- to 2-min laser treatment is sufficient to induce a heat-shock response but is not lethal to the heat-shocked cells. Induction of heat shock was measured in a variety of cell types, including neurons and somatic muscles, by the expression of beta-galactosidase from an hsp26-lacZ reporter construct or by expression of a UAS target gene after induction of hsGAL4. We discuss the applicability of this technique to ectopic gene expression studies, lineage tracing, gene inactivation studies, and studies of cells in vitro. Laser heat shock is a versatile technique that can be adapted for use in a variety of research organisms and is useful for any studies in which it is desirable to express a given gene in only a distinct cell or clone of cells, either transiently or constitutively, at a time point of choice.

Glued participates in distinct microtubule-based activities in Drosophila eye development

A C-terminal truncation of Glued, the Drosophila homolog of the cytoplasmic dynein activating protein, dynactin, results in a severe and complex retinal phenotype, including a roughening of the facet array, malformation of the photosensitive rhabdomeres, and a general deficit and disorder of retinal cells. We have characterized the developmental phenotype in Glued1 and found defects in multiple stages of eye development, including mitosis, nuclear migration, cell fate determination, rhabdomere morphogenesis and cell death. Transgenic flies that express dominant negative Glued under heat-shock control reproduce distinct features of the original Glued1 phenotype depending on the stage of development. The multiple phenotypes effected by truncated Glued point to the multiple roles served by dynactin/dynein during eye development.

Distinct modes of cyclin E/cdc2c kinase regulation and S-phase control in mitotic and endoreduplication cycles of Drosophila embryogenesis

Drosophila cyclin E (DmcycE) is required in embryos for S phase of mitotic and endoreduplication cycles. Here, we describe regulatory differences characteristic for these two cell cycle types. While DmcycE transcript levels decline in DmcycE mutant cells programmed for mitotic proliferation, they are maintained and no longer restricted to transient pulses in DmcycE mutant cells programmed for endoreduplication. Moreover, DmcycE expression in endoreduplicating cells is down-regulated by ectopic expression of a heat-inducible cyclin E transgene. DmcycE expression in endoreduplicating tissues, therefore, is restricted by a negative feedback to the transient pulse triggering entry into S-phase. Conversely, during mitotic cycles, where S phase entry is not only dependent on cyclin E but also on progression through M phase, cyclin E and associated Dmcdc2c kinase activity are present throughout the cell cycle. Reinitiation of DNA replication during the G2 phase of the mitotic cell cycle, therefore, is prevented by cyclin E/Dmcdc2c kinase-independent regulation. Observations in cyclin A mutants implicate G2 cyclins in this regulation. Our results suggest molecular explanations for the different rules governing S phase during mitotic and endoreduplication cycles.

Expression of N-terminally truncated cyclin B in the Drosophila larval brain leads to mitotic delay at late anaphase

We have introduced an N-terminally truncated form of cyclin B into the Drosophila germ-line downstream of the yeast upstream activator that responds to GAL4. When such lines of flies are crossed to lines in which GAL4 is expressed in imaginal discs and larval brain, the majority of the resulting progeny die at the late pupal stage of development. Very rarely (&lt; 0.1% of progeny) adults emerge that have a mutant phenotype typical of flies with mutations in genes required for the cell cycle; they have rough eyes, deformed wings, abnormal bristles, and die within hours of emergence. The brains of third instar larval progeny show an abnormally high proportion of mitotic cells containing overcondensed chromatids that have undergone anaphase separation, together with cells that cannot be assigned to a particular mitotic stage. Immunostaining indicates that these anaphase cells contain moderate levels of cyclin B, suggesting that persistent p34cdc2 kinase activity can prevent progression from anaphase into telophase.

Dissecting the temporal requirements for homeotic gene function

Homeotic genes confer identity to the different segments of Drosophila. These genes are expressed in many cell types over long periods of time. To determine when the homeotic genes are required for specific developmental events we have expressed the Ultrabithorax, abdominal-A and Abdominal-Bm proteins at different times during development using the GAL4 targeting technique. We find that early transient homeotic gene expression has no lasting effects on the differentiation of the larval epidermis, but it switches the fate of other cell types irreversibly (e.g. the spiracle primordia). We describe one cell type in the peripheral nervous system that makes sequential, independent responses to homeotic gene expression. We also provide evidence that supports the hypothesis of in vivo competition between the bithorax complex proteins for the regulation of their down-stream targets.

Cyclin E controls S phase progression and its down-regulation during Drosophila embryogenesis is required for the arrest of cell proliferation

Most cells of the dorsal epidermis exit from the mitotic cycle after division 16 in Drosophila embryogenesis. This exit is dependent on the down-regulation of Drosophila cyclin E (DmcycE) during the final mitotic cycle. Ectopic expression of DmcycE after the final mitosis induces entry into S phase and reaccumulation of G2 cyclins and results in progression through a complete additional cell cycle. Conversely, analyses in DmcycE mutant embryos indicate that cyclin E is required for progression through S phase of the mitotic cycle. Moreover, endoreplication, which occurs in late wild-type embryos in the same pattern as DmcycE expression, is not observed in the mutant embryos. Therefore, Drosophila cyclin E, which forms a complex with the Dmcdc2c kinase, controls progression through S phase and its down-regulation limits embryonic proliferation.