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PubMed1996

Regulation of embryonic cell division by a Xenopus gastrula-specific protein kinase.

We describe a novel protein kinase, Pk9.7, and its role in cell division in the Xenopus embryo. Pk9.7 is transcribed only during blastula and gastrula stages. Expression of Pk9.7 in Xenopus oocytes induces meiotic maturation, while overexpression in embryos blocks blastomere cleavage in a MAP kinase-independent fashion. In both Pk9.7-injected oocytes and mitotic cells of cleavage-blocked embryos, chromosomes appear detached from abnormal spindles, and in oocytes additional microtubule structures are formed, suggesting that one function of Pk9.7 is to regulate formation of, and chromosome attachment to, the spindle. Consistent with this, Pk9.7 co-immunoprecipitates tubulin and phosphorylates it in vitro. Pk9.7 expression coincides with the switch from maternal to zygotic control of the cell cycle, and with the switch from microtubule independence to microtubule dependence. Our results suggest that Pk9.7 plays a role in these processes.

Animals↗
PubMed1993

Delays in anaphase initiation occur in individual nuclei of the syncytial Drosophila embryo.

The syncytial divisions of the Drosophila melanogaster embryo lack some of the well established cell-cycle checkpoints. It has been suggested that without these checkpoints the divisions would display a reduced fidelity. To test this idea, we examined division error frequencies in individuals bearing an abnormally long and rearranged second chromosome, designated C(2)EN. Relative to a normal chromosome, this chromosome imposes additional structural demands on the mitotic apparatus in both the early syncytial embryonic divisions and the later somatic divisions. We demonstrate that the C(2)EN chromosome does not increase the error frequency of the late larva neuroblast divisions. However, in the syncytial embryonic nuclear divisions, the C(2)EN chromosome produces a 10-fold increase in division errors relative to embryos with a normal karyotype. During late anaphase of the neuroblast divisions, the sister C(2)EN chromosomes cleanly separate from one another. In contrast, during late anaphase of the syncytial divisions in C(2)EN-bearing nuclei, large amounts of chromatin often lag on the metaphase plate. Live analysis of C(2)EN-bearing embryos demonstrates that individual nuclei in the syncytial population of dividing nuclei often delay in their initiation of anaphase. These delays frequently lead to division errors. Eventually the products of the nuclei delayed in anaphase sink inward and are removed from the dividing population of syncytial nuclei. These results suggest that the Drosophila embryo may be equipped with mechanisms that monitor the fidelity of the syncytial nuclear divisions. Unlike checkpoints that rely on cell cycle delays to identify and correct division errors, these embryonic mechanisms rely on cell cycle delays to identify and discard the products of division errors.

Anaphase↗
PubMed2025

4D Microscopy and Tracking of Chromosomes and the Spindle in C. elegans Early Embryos.

Maintaining genomic integrity throughout successive cell divisions is essential for the proper development and functioning of organisms. Chromosome alignment and segregation occur on a microtubule-based spindle originating from centrosomes. The molecular and cellular mechanisms involved in accurate chromosome segregation during early embryonic divisions are highly conserved between worms and humans. Therefore, C. elegans serves as a robust model for investigating mitotic cell divisions within a metazoan system. Throughout early embryonic development, filming and tracking successive cell divisions becomes progressively more challenging as the number of cells increases and cell size decreases. To address this challenge, we describe a method for preparing live samples, performing 4D time-lapse imaging, and semi-automated tracking of chromosomes and spindle poles during early mitotic divisions in C. elegans embryos.

Caenorhabditis elegans↗
PubMed1987

Determination of cell division axes in the early embryogenesis of Caenorhabditis elegans.

The establishment of cell division axes was examined in the early embryonic divisions of Caenorhabditis elegans. It has been shown previously that there are two different patterns of cleavage during early embryogenesis. In one set of cells, which undergo predominantly determinative divisions, the division axes are established successively in the same orientation, while division axes in the other set, which divide mainly proliferatively, have an orthogonal pattern of division. We have investigated the establishment of these axes by following the movement of the centrosomes. Centrosome separation follows a reproducible pattern in all cells, and this pattern by itself results in an orthogonal pattern of cleavage. In those cells that divide on the same axis, there is an additional directed rotation of pairs of centrosomes together with the nucleus through well-defined angles. Intact microtubules are required for rotation; rotation is prevented by inhibitors of polymerization and depolymerization of microtubules. We have examined the distribution of microtubules in fixed embryos during rotation. From these and other data we infer that microtubules running from the centrosome to the cortex have a central role in aligning the centrosome-nuclear complex.

Animals↗
PubMed1998

A genetic screen for temperature-sensitive cell-division mutants of Caenorhabditis elegans.

A novel screen to isolate conditional cell-division mutants in Caenorhabditis elegans has been developed. The screen is based on the phenotypes associated with existing cell-division mutations: some disrupt postembryonic divisions and affect formation of the gonad and ventral nerve cord-resulting in sterile, uncoordinated animals-while others affect embryonic divisions and result in lethality. We obtained 19 conditional mutants that displayed these phenotypes when shifted to the restrictive temperature at the appropriate developmental stage. Eighteen of these mutations have been mapped; 17 proved to be single alleles of newly identified genes, while 1 proved to be an allele of a previously identified gene. Genetic tests on the embryonic lethal phenotypes indicated that for 13 genes, embryogenesis required maternal expression, while for 6, zygotic expression could suffice. In all cases, maternal expression of wild-type activity was found to be largely sufficient for embryogenesis. Cytological analysis revealed that 10 mutants possessed embryonic cell-division defects, including failure to properly segregate DNA, failure to assemble a mitotic spindle, late cytokinesis defects, prolonged cell cycles, and improperly oriented mitotic spindles. We conclude that this approach can be used to identify mutations that affect various aspects of the cell-division cycle.

Animals↗
PubMed1997

On cyclins, oocytes, and eggs.

Oocyte and egg are suitable model systems for studying cell division since meiotic maturation resembles a G2/M transition and early embryonic divisions are precisely timed and occur without zygotic transcription. The analysis of oocytes and eggs from different species provides the opportunity to understand the roles of proteins that the critical to the progression and maintenance of the cell cycle. Among them, cyclins are certainly worthy of investigation. Mitotic cyclins (cyclins A and B) are clearly implicated in meiosis and early embryonic cell cycles. More recent studies have revealed that G1-type cyclins (cyclins E and D) could also play a role in both processes and cyclin H has been suggesed to participate to CAK activity (cdc2-activating kinase) in oocytes. The study of cyclins in oocytes and eggs clearly offer insights into their roles during the cell cycle.

Animals↗
PubMed2001

Paternal chromosome incorporation into the zygote nucleus is controlled by maternal haploid in Drosophila.

maternal haploid (mh) is a strict maternal effect mutation that causes the production of haploid gynogenetic embryos (eggs are fertilized but only maternal chromosomes participate in development). We conducted a cytological analysis of fertilization and early development in mh eggs to elucidate the mechanism of paternal chromosome elimination. In mh eggs, as in wild-type eggs, male and female pronuclei migrate and appose, the first mitotic spindle forms, and both parental sets of chromosomes congress on the metaphase plate. In contrast to control eggs, mh paternal sister chromatids fail to separate in anaphase of the first division. As a consequence the paternal chromatin stretches and forms a bridge in telophase. During the first three embryonic divisions, damaged paternal chromosomes are progressively eliminated from the spindles that organize around maternal chromosomes. A majority of mh embryos do not survive the deleterious presence of aneuploid nuclei and rapidly arrest their development. The rest of mh embryos develop as haploid gynogenetic embryos and die before hatching. The mh phenotype is highly reminiscent of the early developmental defects observed in eggs fertilized by ms(3)K81 mutant males and in eggs produced in incompatible crosses of Drosophila harboring the endosymbiont bacteria Wolbachia.

Aneuploidy↗
PubMed2006

Heterogeneous nuclear-transferred-embryos reconstructed with camel (Camelus bactrianus) skin fibroblasts and enucleated ovine oocytes and their development H-M.

This study reconstructed heterogeneous embryos using camel skin fibroblast cells as donor karyoplasts and ovine oocytes as recipient cytoplasts for investigating the developmental potential of the reconstructed embryos. Serum-starved adult camel skin fibroblast cells were used as donor somatic cells. Ovine oocytes matured in vitro were employed as recipient cytoplasts. The fusion of fibroblast cells into recipient cytoplasm was induced by electrofusion. The fused oocytes were activated by 5mM/ml inomycin with 2mM/ml 6-dimethylaminopurine (6-DMAP). The activated reconstructed embryos were co-cultured with ovine cumulus cells in synthetic oviduct fluid supplemented with amino acid (SOFaa) and 10% fetal calf serum (FCS) for 168h. A total of 300 enucleated ovine oocytes were available for xenonuclear embryo reconstruction. The results showed that 71% of the nuclear transfer couplets were successfully fused, 55% of the fused oocytes cleaved within 48h after activation, 82% of the cleaved oocytes developed to 2-16-cell embryo stages and 18% of the cleaved nuclear transfer zygotes developed to the morula stage. This study demonstrated that the xenonuclear transfer camel embryos can undergo the first embryonic division and subsequent development to morula stage in vitro.

Animals↗
PubMed1997

The Drosophila grapes gene is related to checkpoint gene chk1/rad27 and is required for late syncytial division fidelity.

BACKGROUND: Cell cycle checkpoints maintain the fidelity of the somatic cell cycle by ensuring that one step in the cell cycle is not initiated until a previous step has been completed. The extent to which cell cycle checkpoints play a role in the initial rapid embryonic divisions of higher eukaryotes is unclear. The initial syncytial divisions of Drosophila embryogenesis provide an excellent opportunity to address this issue as they are amenable to both genetic and cellular analysis. In order to study the relevance of cell cycle checkpoints in early Drosophila embryogenesis, we have characterized the maternal-effect grapes (grp) mutation, which may affect feedback control during early syncytial divisions. RESULTS: The Drosophila grp gene encodes a predicted serine/threonine kinase and has significant homology to chk1/rad27, a gene required for a DNA damage checkpoint in Schizosaccharomyces pombe. Relative to normal embryos, embryos derived from grp-mutant mothers exhibit elevated levels of DNA damage. During nuclear cycles 12 and 13, alignment of the chromosomes on the metaphase plate was disrupted in grp-derived embryos, and the embryos underwent a progression of cytological events that were indistinguishable from those observed in normal syncytial embryos exposed to X-irradiation. The mutant embryos also failed to progress through a regulatory transition in Cdc2 activity that normally occurs during interphase of nuclear cycle 14. CONCLUSION: We propose that the primary defect in grp-derived embryos is a failure to replicate or repair DNA completely before mitotic entry during the late syncytial divisions. This suggests that wild-type grp functions in a developmentally regulated DNA replication/damage checkpoint operating during the late syncytial divisions. These results are discussed with respect to the proposed function of the chk1/rad27 gene.

Amino Acid Sequence↗
PubMed1997

Centriole and centrosome dynamics during the embryonic cell cycles that follow the formation of the cellular blastoderm in Drosophila.

We have used immunofluorescence and electron microscopy to examine centrosome dynamics during the first postblastodermic mitoses in the Drosophila embryo. The centrosomal material, as recognized by antibodies against CP190 and gamma-tubulin, does not show the typical shape changes observed in syncytial embryos, but remains compact throughout mitosis. Centrioles, however, behave as during the syncytial mitoses, with each daughter cell inheriting two separated centrioles at the end of telophase. During interphase in epithelial cells that have a distinct G1 phase, two isolated centrioles are found, suggesting that the separation of sister centrioles is tightly coupled to a mitotic oscillator in both the "abbreviated" and the "complete" embryonic division cycles. The centrioles of the Drosophila embryo sharply differed from the sperm basal body, having a cartwheel structure with nine microtubular doublets and a central tubule. This "immature" centriolar morphology was shown to persist throughout embryonic development, clearly demonstrating that these centrioles are able to replicate despite their apparently neotenic structure.

Animals↗
PubMed1997

Drosophila PLUTONIUM protein is a specialized cell cycle regulator required at the onset of embryogenesis.

Unfertilized eggs and fertilized embryos from Drosophila mothers mutant for the plutonium (plu) gene contain giant polyploid nuclei resulting from unregulated S-phase. The PLU protein, a 19-kDa ankyrin repeat protein, is present in oocytes and early embryos but is not detectable after the completion of the initial rapid S-M cycles of the embryo. The persistence of the protein during the early embryonic divisions is consistent with a direct role in linking S-phase and M-phase. When ectopically expressed in the eye disc, PLU did not perturb the cell cycle, suggesting that PLU regulates S-phase only in early embryonic development. The pan gu (png) and giant nuclei (gnu) genes also affect the S-phase in the unfertilized egg and early embryo. We show that functional png is needed for the presence of PLU protein. By analyzing png mutations of differing severity, we find that the extent of the png mutant phenotype inversely reflects the level of PLU protein. Our data suggest that PLU protein is required at the time of egg activation and the completion of meiosis.

Animals↗
PubMed1991

Independent roles of centrosomes and DNA in organizing the Drosophila cytoskeleton.

The early embryonic divisions of Drosophila melanogaster are characterized by rapid, synchronized changes of the nuclei and surrounding cytoskeleton. We report evidence that these changes are carried out by two separately organized systems. DNA was sufficient to cause assembly of nuclear lamina and the formation of nuclear membrane with pore structures. Free centrosomes were correlated with the formation of microtubule, microfilament and spectrin networks in the absence of nuclei. In addition, we found that the morphology of the cytoskeleton associated with the free centrosomes cycled in response to the embryonic cell cycle cues. These observations suggest that the centrosomes may be responsible for the organization of this extensive cytoskeleton. The early divisions may therefore result from the independent cycling of two systems, the nucleus and the surrounding cytoskeleton, that respond separately to the mitotic cues in the embryo and function together to give the synchronized early divisions. The Drosophila embryo has an "intermediate" mitotic system in which the nuclear membrane does not break down completely during mitosis. We speculate that the principles of cytoskeleton organization in this system may be different from those of the Xenopus "open" mitotic system.

Actin Cytoskeleton↗
PubMed1989

Centrosome movement in the early divisions of Caenorhabditis elegans: a cortical site determining centrosome position.

In Caenorhabditis elegans embryos, early blastomeres of the P cell lineage divide successively on the same axis. This axis is a consequence of the specific rotational movement of the pair of centrosomes and nucleus (Hyman, A. A., and J. G. White. 1987. J. Cell Biol. 105:2123-2135). A laser has been used to perturb the centrosome movements that determine the pattern of early embryonic divisions. The results support a previously proposed model in which a centrosome rotates towards its correct position by shortening of connections, possibly microtubules, between a centrosome and a defined site on the cortex of the embryo.

Animals↗
PubMed1993

Mutations affecting the cytoskeletal organization of syncytial Drosophila embryos.

Cytoplasmic organization, nuclear migration, and nuclear division in the early syncytial Drosophila embryo are all modulated by the cytoskeleton. In an attempt to identify genes involved in cytoskeletal functions, we have examined a collection of maternal-effect lethal mutations induced by single P-element transposition for those that cause defects in nuclear movement, organization, or morphology during the syncytial embryonic divisions. We describe three mutations, grapes, scrambled, and nuclear-fallout, which define three previously uncharacterized genes. Females homozygous for these mutations produce embryos that exhibit extensive mitotic division errors only after the nuclei migrate to the surface. Analysis of the microfilament and microtubule organization in embryos derived from these newly identified mutations reveal disruptions in the cortical cytoskeleton. Each of the three mutations disrupts the actin-based pseudocleavage furrows and the cellularization furrows in a distinct fashion. In addition to identifying new genes involved in cytoskeletal organization, these mutations provide insights into cytoskeletal function during early Drosophila embryogenesis.

Animals↗
PubMed2002

Arp2/3-dependent pseudocleavage [correction of psuedocleavage] furrow assembly in syncytial Drosophila embryos.

BACKGROUND: In syncytial blastoderm Drosophila embryos, actin caps assemble during telophase. As the cell cycle progresses through interphase, these small caps expand and fuse to form pseudocleavage furrows that are structurally related to the cleavage furrows that assemble during somatic cell division. The molecular mechanism driving cell cycle coordinated actin reorganization from the caps to the furrows is not understood. RESULTS: We show that Drosophila embryos contain a typical Arp2/3 complex and that components of this complex localize to the margins of the expanding caps, to mature pseudocleavage furrows, and to somatic cell cleavage furrows during the postcellularization embryonic divisions. A mutation that disrupts the arpc1 subunit of Arp2/3 leads to spindle fusions that are characteristic of pseudocleavage furrow disruption. By contrast, this mutation does not significantly affect nuclear positioning during interphase, which is dependent on actin cap function. In vivo analysis of actin reorganization demonstrates that the arpc1 mutation does not prevent assembly of small actin caps but blocks cap expansion and furrow assembly as the cell cycle progresses through interphase. The scrambled gene is also required for cap expansion and furrow assembly, and Scrambled is required for Arp2/3 localization to the cap margins. CONCLUSIONS: The Drosophila Arp2/3 complex and Scrambled protein are required for actin cap expansion and pseudocleavage furrow formation during the syncytial blastoderm divisions. We propose that Scrambled-dependent localization of Arp2/3 to the margins of the expanding caps triggers local actin polymerization that drives cap expansion and pseudocleavage furrow assembly.

Actin-Related Protein 2↗
PubMed2002

Chromosomal association of Ran during meiotic and mitotic divisions.

Recent studies in Xenopus egg extracts indicate that the small G protein Ran has a central role in spindle assembly and nuclear envelope reformation. We determined Ran localization and dynamics in cells during M phase. By immunofluorescence, Ran is accumulated on the chromosomes of meiosis-II-arrested Xenopus eggs. In living cells, fluorescently labeled Ran associated with the chromosomes in Xenopus and remained associated during anaphase when eggs were artificially activated. Fluorescent Ran associated with chromosomes in mouse eggs, during meiotic maturation and early embryonic divisions in starfish, and to a lesser degree during mitosis of a cultured mammalian cell line. Chromosomal Ran undergoes constant flux. From photobleach experiments in immature starfish oocytes, chromosomal Ran has a k(off) of approximately 0.06 second(-1), and binding analysis suggests that there is a single major site. The chromosomal interactions may serve to keep Ran-GTP in the vicinity of the chromosomes for spindle assembly and nuclear envelope reformation.

Animals↗
PubMed2000

Microspore culture in Corchorus olitorius: effect of growth regulators, temperature and sucrose on callus formation.

Culture of isolated microspores and of anthers on media containing IAA directed free microspore development to an embryogenic pathway in C. olitorius. The first division of microspores on transfer to culture media was symmetrical in contrast to the asymmetrical division seen in normal development in vivo. Initially, 10-30% microspores divided symmetrically, but only 0.2-1% of the dividing microspores continued dividing and produced multicellular microcalli. About 30% of these microcalli produced callus but only on medium with 2.0 mg/L zeatin and 0.1 mg/L IAA. Incubation in the dark at temperatures of 35 degrees C for 1 day and then 25 degrees C was found effective for induction of first embryonic division in Corchorus. The frequency of microspore callus formation was higher on medium containing either 3% or 5% sucrose. Addition of colchicine and addition of activated charcoal to the above medium did not enhance microspore division in Corchorus olitorius. On transfer to different media most calli produced roots but regeneration of shoots and embryos was not induced.

Breeding↗
PubMed1980

[Different behaviour of paternal and maternal genomes during embryogenesis in the fern, Marsilea (author's transl)].

The DNA of gametes of the water fern, Marsilea vestita, has been radioactively labelled during gemetogenesis. The distribution of the originally present paternal and maternal genomes could thus be followed during the development of the embryo. It has been found that the maternal chromatin is uniformly distributed among the daughter cells of the several successive cell generations. However, the paternal chromatin divides equally only during the first three embryonic divisions. From this stage on, the partition varies according to the different areas of the embryo. Only the organogenic cells retain a high degree of radioactivity. This pattern of inheritance concerns chiefly the apical cells of shoots and roots. This peculiar behaviour of the paternal genome can be explained in different ways. At present, a non-random distribution of paternal chromatids, i. e. a selective segregation of old and new strands of DNA to opposite poles of embryonic mitoses, appears most likely.

Anaphase↗