Abnormal centrosomes in cold-treated Drosophila embryos

In this study we examine the effect on the centrosomes of cold treatment of early Drosophila embryos. Prolonged cold treatment during the mitotic divisions which lead to the formation of the blastoderm causes arrest at metaphase of the nuclear divisions. When examined with immunofluorescence microscopy the mitotic spindles show marked pole splitting with the formation of supernumerary and irregularly sized centers, all able to nucleate microtubules. In embryos recovered for longer periods the additional organizing centers become ring-shaped and lose their nucleating properties. Cold treatment of embryos during the cellularization of the blastoderm results in marked fragmentation of the centrosomes, but nucleating capacity is preserved. Sometimes the centrioles come away from the pericentriolar material and their structure is seen to be modified.     

Abnormal centrosomes in cold-treated Drosophila embryos

In this study we examine the effect on the centrosomes of cold treatment of early Drosophila embryos. Prolonged cold treatment during the mitotic divisions which lead to the formation of the blastoderm causes arrest at metaphase of the nuclear divisions. When examined with immunofluorescence microscopy the mitotic spindles show marked pole splitting with the formation of supernumerary and irregularly sized centers, all able to nucleate microtubules. In embryos recovered for longer periods the additional organizing centers become ring-shaped and lose their nucleating properties. Cold treatment of embryos during the cellularization of the blastoderm results in marked fragmentation of the centrosomes, but nucleating capacity is preserved. Sometimes the centrioles come away from the pericentriolar material and their structure is seen to be modified.     

Microtubule arrays present during the syncytial and cellular blastoderm stages of the early Drosophila embryo

The organization of microtubules within the surface caps of Drosophila embryos is described for the mitotic cycles of the syncytial blastoderm stage (particularly cycle 10), and for the subsequent cellularization process. Tubulin was labelled with the well characterized monoclonal antibody YL 1/2 (Kilmartin et al., J cell biol 93 (1982) 576). Each surface cap was found to contain an array of microtubules running around the nucleus. The microtubules originated at prominent centrosomes located close to the apical surface of each cap nucleus. During mitosis the spindle microtubules stained strongly for tubulin. A novel finding was that the spindle microtubules of the interzone region appeared to reduce their connections with the centrosomes at the end of anaphase. The spindle remnant remained in position during telophase but then became smaller in size, disappearing by interphase. At this phase of the cell cycle duplication of the aster centrosomes occurred. The cellular blastoderm stage was marked by a change in the main axis of microtubule orientation. The centrosomes of each cap separated somewhat and formed initiation centres for the development of a well developed basket of microtubules around each nucleus, but now perpendicular to the surface. The microtubule baskets were seen to extend in parallel with nuclear elongation, but not in concert with growth of the cell membranes, which extended some way beneath the bases of the nuclei.     

Cyclins A and B associate with chromatin and the polar regions of spindles, respectively, and do not undergo complete degradation at anaphase in syncytial Drosophila embryos

Maternally contributed cyclin A and B proteins are initially distributed uniformly throughout the syncytial Drosophila embryo. As dividing nuclei migrate to the cortex of the embryo, the A and B cyclins become concentrated in surface layers extending to depths of approximately 30-40 microns and 5-10 microns, respectively. The initiation of nuclear envelope breakdown, spindle formation, and the initial congression of the centromeric regions of the chromosomes onto the metaphase plate all take place within the surface layer occupied by cyclin B on the apical side of the blastoderm nuclei. Cyclin B is seen mainly, but not exclusively, in the vicinity of microtubules throughout the mitotic cycle. It is most conspicuous around the centrosomes. Cyclin A is present at its highest concentrations throughout the cytoplasm during the interphase periods of the blastoderm cycles, although weak punctate staining can also be detected in the nucleus. It associates with the condensing chromosomes during prophase, segregates into daughter nuclei in association with chromosomes during anaphase, to redistribute into the cytoplasm after telophase. In contrast to the cycles following cellularization, neither cyclin is completely degraded upon the metaphase-anaphase transition.     

The effects of microinjection of rhodamine-phalloidin on mitosis and cytokinesis in early stage Drosophila embryos

Rhodamine-phalloidin was microinjected into early stage Drosophila embryos, which were then allowed to develop for various times, fixed, and examined by fluorescence microscopy. A gradient of effects was seen. Close to the site of injection an area of diffuse bright fluorescence was found which included lumps and long strands of fluorescent material. Around this region particular cytoplasmic domains showed a denser F-actin distribution. These domains included the nuclear islands of the preblastoderm, the cortical caps of the syncytial blastoderm, and the contractile ring network which forms during cellularization of the blastoderm. It is proposed that these domains are regions of preferential actin polymerization under the appropriate cellular conditions and that the injected phalloidin causes incorporation of additional polymer into existing structures. Further away the pattern of phalloidin staining corresponded to that found with fixed material. In contrast to the domains of apparent additional F-actin polymerization a reduction of actin incorporated into small aggregates was found, both in syncytial blastoderm stages and during cellularization. This occurred in regions where additional actin had been incorporated into adjacent actin-rich structures. A storage role for the aggregates, which are depleted when F-actin is polymerized, is proposed. Both mitosis and cytokinesis were found to be slowed but the inhibition was only transient. However, most embryos died without differentiating. Rarely, differentiated tissues formed and the musculature was strongly stained by rh-phalloidin. When embryos were injected immediately prior to the start of cellularization cytokinesis was inhibited only locally and continued normally elsewhere. This finding argues against the hypothesis that contraction of an actomyosin network over the whole surface is the only force involved in the cellularization of the blastoderm and that local factors, e.g., plasmalemma extension, must be involved.     

The cytoskeletal involvement in cellularization of theDrosophila melanogaster embryo

Summary The distribution and arrangement of cytoskeletal components in the early embryo ofDrosophila melanogaster were examined by thin-section electron microscopy to elucidate their involvement in the formation of the cellular blastoderm, a process called cellularization. During the final nuclear division in the cortex of the syncytial blastoderm bundles of astral microtubules were closely associated with the surface plasma membrane along the midline where a new gutter was initiated. Thus the new gutter together with the pre-formed ones compartmentalized the embryo surface to reflect underlying individual daughter nuclei. Subsequently such gutters became deeper by further invagination of the plasma membrane between adjacent nuclei to form so-called cleavage furrows. Nuclei simultaneously elongated in the direction perpendicular to the embryo surface and numerous microtubules from the centrosomes ran longitudinally between the nucleus and the cleavage furrow. Microtubules often appeared to be in close association with the nuclear envelope and the cleavage furrow membrane. The plasma membrane at the advancing tip of the furrow was always undercoated with an electron-dense layer, which could be shown to be mainly composed of 5–6 nm microfilaments. These microfilaments were decorated with H-meromyosin to be identified as actin filaments. As cleavage proceeded, each nucleus with its perikaryon became demarcated by the furrow membrane, which then extended laterally to constrict the cytoplasmic connection between each newly forming cell and the central yolk region. The cytoplasmic strand thus formed possessed a prominent circular bundle of microfilaments which were also decorated with H-meromyosin and bidirectionally arranged, similar in structure to the contractile ring in cytokinesis. These observations strongly suggest that both microtubules and actin filaments play a crucial role in cellularization ofDrosophila embryos.     

Drop out: a third chromosome maternal-effect locus required for formation of the Drosophila cellular blastoderm.

drop out (dop) is a recessive maternal-effect locus identified in a screen for female-sterile mutations in Drosophila polytene region 71C-F. Phenotypic analyses of the dop mutation indicate that the gene is required for proper formation of the cellular blastoderm. In embryos derived from either homozygous or hemizygous dop mothers, cytoplasmic clearing, nuclear migration and division, and pole cell formation appear normal. However, developmental defects are observed prior to and during cellularization of the blastoderm. At the beginning of nuclear cycle 14, the distinct separation of the internal yolk mass and the cortical cytoplasm breaks down. Subsequently, a population of somatic nuclei located at the periphery of the syncytial blastoderm becomes irregularly spaced and nonuniform in their distribution. Despite a somewhat regular formation of the cortical actin network, cellularization in mutant embryos is extremely variable. Such embryos fail to gastrulate normally and produce variable amounts of defective cuticle. Overall, our analyses suggest that the dop gene functions in maintaining the separation of yolk and cortical cytoplasm and in stabilizing the distribution of somatic nuclei in the Drosophila syncytial blastoderm.     

Genetic analysis of the cellularization of the Drosophila embryo.

The synchronous cellularization of the Drosophila embryo at the blastoderm stage provides a unique system for studying the molecular mechanisms involved in cytokinesis, using genetical and biochemical approaches. The cellularization process requires the major components of the embryonic cytoskeleton that are deposited into the egg during oogenesis. Genetical analysis indicates that it requires also the products of additional maternally-acting genes, as well as that of a limited set of zygotically-acting genes. The cellularization defective phenotypes associated with small deficiencies uncovering these latter loci reveal specific steps within this complex process. The molecular analysis of these genes will ultimately provide meaningful insights into the normal process of cellularization. Among them, the serendipity alpha gene encodes a membrane-associated protein, which is exclusively accumulated during cellularization, and is required for the reorganization of the microfilaments as the onset of cellularization.     

Isolation and characterization of a Drosophila gene essential for early embryonic development and formation of cortical cleavage furrows

We have isolated a new female sterile mutant from Drosophila melanogaster, which arrests the embryonic development during the transition from syncytial to cellular blastoderm. Cytological analysis of the mutant embryos indicates that pseudocleavage furrows in the syncytial blastoderm are abnormal but not completely disrupted. However, cleavage furrows during cellularization are totally disorganized, and no embryos can develop beyond this stage. Consistent with this observation, the expression of this gene peaks around the cellular blastoderm and not in any later developmental stages. Based on immunofluorescence experiments, the protein product of this gene is localized in both pseudocleavage furrows at the syncytial blastoderm and in the cleavage furrows during the cellularization stage. Sequence homology analysis demonstrates a modest, but statistically significant, similarity of this protein with the carboxyl-terminal domains of dystrophin and a family of proteins collectively known as apodystrophins. It is possible that this protein may play an essential role in organizing and maintaining a specialized cytoskeletal structure, a function also suggested for dystrophin and apodystrophins.     

Female Sterile Mutations on the Second Chromosome of Drosophila Melanogaster. I. Maternal Effect Mutations

In mutagenesis screens for recessive female sterile mutations on the second chromosome of Drosophila melanogaster 529 chromosomes were isolated which allow the homozygous females to survive, but cause them to be sterile. In 136 of these lines, mutant females produce morphologically normal eggs which cannot support normal embryonic development. These "maternal-effect" mutations fall into 67 complementation groups which define 23 multiply hit and 44 singly hit loci. In eggs from 14 complementation groups development is blocked before the formation of a syncytial blastoderm. In eggs from 12 complementation groups development is abnormal before cellularization, 17 complementation groups cause abnormal cellularization, 12 complementation groups cause changes in cellular morphology in early gastrulation stages, and 12 complementation groups seem to affect later embryonic development.     

The secretory membrane system in the Drosophila syncytial blastoderm embryo exists as functionally compartmentalized units around individual nuclei

Drosophila melanogaster embryogenesis begins with 13 nuclear division cycles within a syncytium. This produces >6,000 nuclei that, during the next division cycle, become encased in plasma membrane in the process known as cellularization. In this study, we investigate how the secretory membrane system becomes equally apportioned among the thousands of syncytial nuclei in preparation for cellularization. Upon nuclear arrival at the cortex, the endoplasmic reticulum (ER) and Golgi were found to segregate among nuclei, with each nucleus becoming surrounded by a single ER/Golgi membrane system separate from adjacent ones. The nuclear-associated units of ER and Golgi across the syncytial blastoderm produced secretory products that were delivered to the plasma membrane in a spatially restricted fashion across the embryo. This occurred in the absence of plasma membrane boundaries between nuclei and was dependent on centrosome-derived microtubules. The emergence of secretory membranes that compartmentalized around individual nuclei in the syncytial blastoderm is likely to ensure that secretory organelles are equivalently partitioned among nuclei at cellularization and could play an important role in the establishment of localized gene and protein expression patterns within the early embryo.     

Glucose metabolism during embryogenesis of the hard tick Boophilus microplus

Glucose metabolism plays an essential role in the physiology and development of almost all living organisms. In the present study we investigated glucose metabolism during the embryogenesis of the hard tick Boophilus microplus. An increase in glucose and glycogen content during the embryonic development of B. microplus was detected and shown to be due to the high enzyme activity of both gluconeogenesis and glycolytic pathways. Glucose 6-phosphate (G-6P), formed by hexokinase, is driven mainly to pentose-phosphate pathway, producing fundamental substrates for cellular biosynthesis. We detected an increase in glucose 6-phosphate dehydrogenase and pyruvate kinase activities after embryo cellularization. Accumulation of key metabolites such as glycogen and glucose was monitored and revealed that glycogen content decreases from day 1 up to day 6, as the early events of embryogenesis take place, and increases after the formation of embryo cellular blastoderm on day 6. Glucose and guanine (a sub-product of amino acids degradation in arachnids) accumulate almost concomitantly. The activity of phosphoenolpyruvate carboxykinase was increased after embryo cellularization. Taken together these data indicate that glycogen and glucose, formed during B. microplus embryogenesis after blastoderm formation, are produced by intense gluconeogenesis.     

How one becomes many: blastoderm cellularization in Drosophila melanogaster

Embryonic development in Drosophila melanogaster begins with a rapid series of mitotic nuclear divisions, unaccompanied by cytokinesis, to produce a multi-nucleated single cell embryo, the syncytial blastoderm. The syncytium then undergoes a process of cell formation, in which the individual nuclei become enclosed in individual cells. This process of cellularization involves integrating mechanisms of cell polarity, cell-cell adhesion and a specialized form of cytokinesis. The detailed molecular mechanism, however, is highly complex and, despite extensive analysis, remains poorly understood. Nevertheless, new insights are emerging from recent studies on aspects of membrane polarization and insertion, which show that membrane components from intracellular organelles are involved. In addition, actin and actin-associated proteins have been heavily implicated while new evidence shows that microtubule cytoskeletal elements are mechanistically involved in all aspects of cellularization. This review will draw on both the traditional models and the new data to provide a current perspective on the nature of cellular blastoderm formation in Drosophila melanogaster.     

Centrosomes and the Scrambled protein coordinate microtubule-independent actin reorganization

In Drosophila syncytial blastoderm embryos, centrosomes specify the position of actin-based interphase caps and mitotic furrows. Mutations in the scrambled locus prevent assembly of mitotic furrows, but do not block actin cap formation. The scrambled gene encodes a protein that localizes to the mitotic furrows and centrosomes. Sced localization, actin reorganization from caps into mitotic furrows, and centrosome-coordinated assembly of actin caps are not blocked by microtubule disruption. Our results indicate that centrosomes may coordinate assembly of cortical actin caps through a microtubule-independent mechanism, and that Scrambled mediates a second microtubule-independent process that drives mitotic furrow assembly.     

src64 and tec29 are required for microfilament contraction during Drosophila cellularization

Formation of the Drosophila cellular blastoderm involves both membrane invagination and cytoskeletal regulation. Mutations in src64 and tec29 reveal a novel role for these genes in controlling contraction of the actin-myosin microfilament ring during this process. Although membrane invagination still proceeds in mutant embryos, its depth is not uniform, and basal closure of the cells does not occur during late cellularization. Double-mutant analysis between scraps, a mutation in anillin that eliminates microfilament rings, and bottleneck suggests that microfilaments can still contract even though they are not organized into rings. However, the failure of rings to contract in the src64 bottleneck double mutant suggests that src64 is required for microfilament ring contraction even in the absence of Bottleneck protein. Our results suggest that src64-dependent microfilament ring contraction is resisted by Bottleneck to create tension and coordinate membrane invagination during early cellularization. The absence of Bottleneck during late cellularization allows src64-dependent microfilament ring constriction to drive basal closure.     

Dynamic changes in the distribution of cytoplasmic myosin during Drosophila embryogenesis

Dramatic changes in the localization of conventional non-muscle myosin characterize early embryogenesis in Drosophila melanogaster. During cellularization, myosin is concentrated around the furrow canals that form the leading margin of the plasma membrane as it plunges inward to package each somatic nucleus into a columnar epithelial cell. During gastrulation, there is specific anti-myosin staining at the apical ends of those cells that change shape in regions of invagination. Both of these localizations appear to result from a redistribution of a cortical store of maternal myosin. In the preblastoderm embryo, myosin is localized to the egg cortex, sub-cortical arrays of inclusions, and, diffusely, the yolk-free periplasm. At the syncytial blastoderm stage, myosin is found within cytoskeletal caps associated with the somatic nuclei at the embryonic surface. Following the final syncytial division, these myosin caps give rise to the myosin rings observed during cellularization. These distributions are observed with both whole immune serum and affinity-purified antibodies directed against Drosophila non-muscle myosin heavy chain. They are not detected in embryos stained with anti-Drosophila muscle myosin antiserum or with preimmune serum. Although immunolocalization can only suggest possible function, these myosin localizations and the coincident changes in cell morphology are consistent with a key role for non-muscle myosin in powering cellularization and gastrulation during embryogenesis.     

Glucose metabolism during embryogenesis of the hard tick Boophilus microplus

Glucose metabolism plays an essential role in the physiology and development of almost all living organisms. In the present study we investigated glucose metabolism during the embryogenesis of the hard tick Boophilus microplus. An increase in glucose and glycogen content during the embryonic development of B. microplus was detected and shown to be due to the high enzyme activity of both gluconeogenesis and glycolytic pathways. Glucose 6-phosphate (G-6P), formed by hexokinase, is driven mainly to pentose-phosphate pathway, producing fundamental substrates for cellular biosynthesis. We detected an increase in glucose 6-phosphate dehydrogenase and pyruvate kinase activities after embryo cellularization. Accumulation of key metabolites such as glycogen and glucose was monitored and revealed that glycogen content decreases from day 1 up to day 6, as the early events of embryogenesis take place, and increases after the formation of embryo cellular blastoderm on day 6. Glucose and guanine (a sub-product of amino acids degradation in arachnids) accumulate almost concomitantly. The activity of phosphoenolpyruvate carboxykinase was increased after embryo cellularization. Taken together these data indicate that glycogen and glucose, formed during B. microplus embryogenesis after blastoderm formation, are produced by intense gluconeogenesis.     

Conserved Domains of the Nullo Protein Required for Cell-Surface Localization and Formation of Adherens Junctions

During cellularization, the Drosophila melanogaster embryo undergoes a transition from syncytial to cellular blastoderm with the de novo generation of a polarized epithelial sheet in the cortex of the embryo. This process couples cytokinesis with the establishment of apical, basal, and lateral membrane domains that are separated by two spatially distinct adherens-type junctions. In nullo mutant embryos, basal junctions fail to form at the onset of cellularization, leading to the failure of cleavage furrow invagination and the generation of multinucleate cells. Nullo is a novel protein that appears to stabilize the initial accumulation of cadherins and catenins as they form a mature basal junction. In this article we characterize a nullo homologue from D. virilis and identify conserved domains of Nullo that are required for basal junction formation. We also demonstrate that Nullo is a myristoylprotein and that the myristate group acts in conjunction with a cluster of basic amino acids to target Nullo to the plasma membrane. The membrane association of Nullo is required in vivo for its role in basal junction formation and for its ability to block apical junction formation when ectopically expressed during late cellularization.     

Microtubule distribution reveals superficial metameric patterns in the early Drosophila embryo

Microtubule distribution was examined in whole mounts of Drosophila embryos from the cellularization of the syncytial blastoderm (stage 6) to the completion of the gastrulation (stage 7) by fluorescence microscopy. During ventral furrow formation, the fluorescence of tubulin network was not uniform, but disposed in zebra stripes. Antibodies against alpha-tubulin showed 14 alternating pairs of darker and brighter transverse areas. The possible significance of this pattern is discussed.     

Cellular morphogenesis: slow-as-molasses accelerates polarized membrane growth

Cellularization of the early Drosophila embryo is a modified form of cytokinesis that gives rise to the blastoderm epithelium through polarized membrane growth. The gene slow-as-molasses encodes a novel protein essential for the formation of a plasma membrane domain that initiates membrane growth during cellularization.