Chimaeras of Drosophila melanogaster obtained by injection of haploid nuclei

The Enigma of Centriole Loss in the 1182-4 Cell Line

The Drosophila melanogaster cell line 1182-4, which constitutively lacks centrioles, was established many years ago from haploid embryos laid by females homozygous for the maternal haploid (mh) mutation. This was the first clear example of animal cells regularly dividing in the absence of this organelle. However, the cause of the acentriolar nature of the 1182-4 cell line remained unclear and could not be clearly assigned to a particular genetic event. Here, we detail historically the longstanding mystery of the lack of centrioles in this Drosophila cell line. Recent advances, such as the characterization of the mh gene and the genomic analysis of 1182-4 cells, allow now a better understanding of the physiology of these cells. By combining these new data, we propose three reasonable hypotheses of the genesis of this remarkable phenotype.

Quantitative microscopy uncovers ploidy changes during mitosis in live Drosophila embryos and their effect on nuclear size

Time-lapse microscopy is a powerful tool to investigate cellular and developmental dynamics. In Drosophila melanogaster, it can be used to study division cycles in embryogenesis. To obtain quantitative information from 3D time-lapse data and track proliferating nuclei from the syncytial stage until gastrulation, we developed an image analysis pipeline consisting of nuclear segmentation, tracking, annotation and quantification. Image analysis of maternal-haploid (mh) embryos revealed that a fraction of haploid syncytial nuclei fused to give rise to nuclei of higher ploidy (2n, 3n, 4n). Moreover, nuclear densities in mh embryos at the mid-blastula transition varied over threefold. By tracking synchronized nuclei of different karyotypes side-by-side, we show that DNA content determines nuclear growth rate and size in early interphase, while the nuclear to cytoplasmic ratio constrains nuclear growth during late interphase. mh encodes the Drosophila ortholog of human Spartan, a protein involved in DNA damage tolerance. To explore the link between mh and chromosome instability, we fluorescently tagged Mh protein to study its subcellular localization. We show Mh-mKO2 localizes to nuclear speckles that increase in numbers as nuclei expand in interphase. In summary, quantitative microscopy can provide new insights into well-studied genes and biological processes.

Summary: A new 3D time-lapse microscopy image analysis pipeline consisting of nuclear segmentation, tracking, annotation and quantification revealed karyotype changes in Drosophila embryos.

Drosophila I-R hybrid dysgenesis is associated with catastrophic meiosis and abnormal zygote formation

The Drosophila I-R type of hybrid dysgenesis is a sterility syndrome (SF sterility) associated with the mobilization of the I retrotransposon in female germ cells. SF sterility results from a maternal-effect embryonic lethality whose origin has remained unclear since its discovery about 40 years ago. Here, we show that meiotic divisions in SF oocytes are catastrophic and systematically fail to produce a functional female pronucleus at fertilization. As a consequence, most embryos from SF females rapidly arrest their development with aneuploid or damaged nuclei, whereas others develop as non-viable, androgenetic haploid embryos. Finally, we show that, in contrast to mutants affecting the biogenesis of piRNAs, SF egg chambers do not accumulate persistent DNA double-strand breaks, suggesting that I-element activity might perturb the functional organization of meiotic chromosomes without triggering an early DNA damage response.

Quantitative microinjection of Drosophila embryos: general strategy

INTRODUCTIONMicroinjection of Drosophila embryos is a common technique used by a wide range of investigators, but some applications require a refined strategy for handling embryos. This article outlines the general procedures for microinjection and quantification of aqueous solutions during high-resolution observation of early development in the fly embryo. It also describes the design of suitable support slides for the manipulation of Drosophila embryos under upright and inverted microscopes.

Sperm chromatin remodelling and Wolbachia-induced cytoplasmic incompatibility in Drosophila

Wolbachia pipientis is an obligate bacterial endosymbiont, which has successfully invaded approximately 20% of all insect species by manipulating their normal developmental patterns. Wolbachia-induced phenotypes include parthenogenesis, male killing, and, most notably, cytoplasmic incompatibility. In the future these phenotypes might be useful in controlling or modifying insect populations but this will depend on our understanding of the basic molecular processes underlying insect fertilization and development. Wolbachia-infected Drosophila simulans express high levels of cytoplasmic incompatibility in which the sperm nucleus is modified and does not form a normal male pronucleus when fertilizing eggs from uninfected females. The sperm modification is somehow rescued in eggs infected with the same strain of Wolbachia. Thus, D. simulans has become an excellent model organism for investigating the manner in which endosymbionts can alter reproductive programs in insect hosts. This paper reviews the current knowledge of Drosophila early development and particularly sperm function. Developmental mutations in Drosophila that are known to affect sperm function will also be discussed.incompatibility.

The Drosophila misfire gene has an essential role in sperm activation during fertilization

The male sterile mutation, misfire (mfr), of Drosophila melanogaster is a novel paternal effect, fertilization defective mutant that effects sperm head decondensation. mfr sperm were motile, appeared normal morphologically and were transferred to the female during copulation. However, less than 0.1% of eggs laid by females mated to mfr males hatched. Although mfr sperm entered eggs at a high frequency (93%), 99% of the inseminated eggs did not initiate the first nuclear division. Unlike wild type fertilizing sperm, the position and shape of mfr sperm tails within the egg were not constant, but varied in a seemingly random manner. The heads of inseminating mutant sperm were always located near the surface of eggs just underlying the egg plasma membrane, and maintained their needle-like shape indicating the failure of nuclear decondensation. Further observations revealed that plasma membrane of inseminating sperm appeared intact, including the head region. These phenotypes were equivalent to those of sneaky (snky), another fertilization defective male sterile mutation. Our observations strongly suggest that mfr mutant males are sterile because their inseminating sperm fail to form a male pronucleus due to the inability of the sperm to properly respond to egg factors responsible for the breakdown of the plasma membrane. Although mfr and snky mutations were phenotypically identical, they mapped to cytologically distinct genetic loci and no genetic interactions were observed, suggesting that at least two distinct paternal gene products are involved in the early stages of pronuclear formation.

Interaction of the essential Drosophila nuclear protein YA with P0/AP3 in the cytoplasm and in vitro: implications for developmental regulation of YA's subcellular location

The Drosophila nuclear lamina protein YA is essential for the transition from female meiosis to embryo mitosis. Its localization and, hence, function is under developmental and cell cycle controls. YA protein is hyperphosphorylated and cytoplasmic in ovaries. Upon egg activation, YA is partially dephosphorylated and acquires the ability to enter nuclei. Its function is first detected at this time. To investigate the cytoplasmic retention machinery that keeps YA from entering nuclei, we used affinity chromatography and blot overlay assays to identify cytoplasmic proteins that associate with YA. Drosophila P0/AP3, a ribosomal protein that is also an apurinic/apyrimidinic endonuclease, binds to YA in ovary and embryo cytoplasms. P0 and YA bind specifically and directly in vitro and are present in a 20S complex in the cytoplasmic extracts. YA protein can be phosphorylated by MAPK, but not by p34(Cdc2) kinase, in vitro. This phosphorylation increases YA's binding to P0. We propose that the P0-containing 20S cytoplasmic complex retains hyperphosphorylated ovarian YA in the cytoplasm. In response to egg activation, YA is partially dephosphorylated and its binding to the 20S complex is reduced. Hence, some YA dissociates from the complex and enters nuclei. Consistent with this model, decreasing P0 levels partially suppress a hypomorphic Ya mutant allele.

The Drosophila nuclear lamina protein YA binds to DNA and histone H2B with four domains

Dramatic changes occur in nuclear organization and function during the critical developmental transition from meiosis to mitosis. The Drosophila nuclear lamina protein YA binds to chromatin and is uniquely required for this transition. In this study, we dissected YA's binding to chromatin. We found that YA can bind to chromatin directly and specifically. It binds to DNA but not RNA, with a preference for double-stranded DNA (linear or supercoiled) over single-stranded DNA. It also binds to histone H2B. YA's binding to DNA and histone H2B is mediated by four domains distributed along the length of the YA molecule. A model for YA function at the end of Drosophila female meiosis is proposed.

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.

The maternal effect mutation sésame affects the formation of the male pronucleus in Drosophila melanogaster

After entering the oocyte and before the formation of the diploid zygote, the sperm nucleus is transformed into a male pronucleus, a process that involves a series of conserved steps in sexually reproducing animals. Notably, a major modification of the male gamete lies in the decondensation of the highly compact sperm chromatin. We present here the phenotype of sésame (ssm), a maternal effect mutation which affects the formation of the male pronucleus in Drosophila melanogaster. Homozygous ssm(185b) females produce haploid embryos which develop with only the maternally derived chromosomes. These haploid embryos die at the end of embryogenesis. Cytological analyses of the fertilization in eggs laid by ssm(185b) mutant females showed that both pronuclear migration and pronuclear apposition occurred normally. However, a dramatic alteration of the male pronucleus by which its chromatin failed to fully decondense was systematically observed. Consequently, the affected male pronucleus does not enter the first mitotic spindle, which is organized around only the maternally derived chromosomes. Immunodetection of lamina antigens indicates that a male pronuclear envelope is able to form around the partially decondensed paternal chromatin. This suggests that the maternally provided sésame(+) function is required for a late stage of sperm chromatin remodeling.

Sex determination in Drosophila melanogaster: X-linked genes involved in the initial step of sex-lethal activation

Sex determination is the commitment of an embryo to either the female or the male developmental pathway. The ratio of X chromosomes to sets of autosomes is the primary genetic signal that determines sex in Drosophila, by triggering the functional state of the gene Sex-lethal: in females (2X;2A) Sxl will be ON, whereas in males (X;2A) Sxl will be OFF. Genetic and molecular studies have defined a set of genes involved in the formation of the X:A signal, as well as other genes, with either maternal or zygotic effects, which are also involved in regulating the initial step of Sex-lethal activation. We review these data and present new data on two more regions of the X chromosome that define other genes needed for Sxl activation. In addition, we report on the interaction between some of the genes regulating Sxl activation.

The Drosophila maternal-effect gene fs(1)Ya encodes a cell cycle-dependent nuclear envelope component required for embryonic mitosis

The maternal-effect gene fs(1)Ya is specifically required for embryonic mitosis in Drosophila. fs(1)Ya is involved in the initiation of the first embryonic mitosis and may also be necessary for subsequent embryonic mitotic divisions. fs(1)Ya encodes a 91.3 kd hydrophilic protein containing two putative MPF phosphorylation target sites and two potential nuclear localization signals. This protein is synthesized during postoogenic maturation from its maternal RNA and persists throughout embryogenesis. In early embryos, the fs(1)Ya protein is localized to the nuclear envelope from interphase to metaphase. During anaphase and telophase, it is dispersed in the nucleoplasm and cytoplasm, a behavior that is different from that of both the nuclear envelope and lamins. These results suggest that the fs(1)Ya protein is a cell cycle-dependent component of the nuclear envelope that specifically functions in embryonic mitosis.

Cloning and analysis of fs(1) Ya, a maternal effect gene required for the initiation of Drosophila embryogenesis

The maternal effect locus fs(1) Ya is required for the fusion of the apposed sperm and egg pronuclei (syngamy) following fertilization in Drosophila. It is tightly linked to another complementation group, fs(1) Yb, needed for both oogenesis and embryogenesis. We have isolated a set of overlapping cloned sequences in the 3B4-6 region of the X chromosome encompassing the fs(1) Ya-fs(1) Yb region. A single 2.4 kb maternal transcript is encoded within this region, and an 8.5 kb DNA fragment that contains this transcript complements both fs(1) Ya and fs(1) Yb mutations. Northern and in situ hybridization analyses show that the maternal transcript is only present in nurse cells and oocytes beginning in previtellogenic stages, and is evenly distributed in the cytoplasm of 0-2 h syncytial embryos. The transcript is not detected in later stages of embryonic development. This expression pattern correlates closely with the genetic and developmental characteristics expected of the fs(1) Ya gene product.

The gnu mutation of Drosophila causes inappropriate DNA synthesis in unfertilized and fertilized eggs

Drosophila melanogaster embryos whose mothers are homozygous for the maternal effect lethal mutation gnu (GNU embryos) under DNA synthesis but no nuclear division; this leads to the formation of a small number of giant nuclei in the syncytial blastoderm. We have shown previously that many components of the mitotic apparatus are present and functional in GNU embryos, and the primary lesion of the gnu mutation has therefore remained obscure. Here, we report that fertilization is not necessary for GNU eggs to develop. Giant nuclei originate from the products of female meiosis, and we see autonomously replicating centrosomes that must be maternally derived. If GNU eggs are inseminated, however, the male pronucleus also undergoes DNA replication. Our observations suggest that the GNU cytoplasm permits DNA synthesis in a relatively unregulated manner. In embryos from females homozygous for gnu and the female sterile haploid mutation mh, we find replication of DNA derived from the male pronucleus. This contrasts with embryos from mothers homozygous for mh alone, in which this does not occur. We propose that the gnu gene product participates in the repression of DNA synthesis found in unfertilized eggs.

Evolution of karyotype in haploid cell lines of Drosophila melanogaster

Seven continuous cell lines have been established in vitro from lethal embryos produced by the female sterile mutant mh 1182 of Drosophila melanogaster. Six lines show haploid metaphases. Karyotype analysis revealed a high level of aneuploid cells with frequent chromosome fragments. In three lines, haploid cells were quickly overgrown by diploid cells. Two lines were more stable but the proportion of haploid cells decreased with time. One line was stable, showing 80-90% of haploid cells for over 1 000 cell generations. Stable haploid clones have been isolated from two lines. Crossing of mh 1182/mh 1182 females with males bearing a ring X chromosome shows that the haploid genome retained in the cells is of maternal origin and that the diploid cells derive from pre-existing haploid cells. The appearance of the diploid cells and the conditions of karyotypic stability are analysed.

Analysis of haploid mosaics in Drosophila

Adult chimeric epidermal structures were obtained following transplantation of haploid nuclei from haploid donor embryos of Drosophila into genetically marked diploid embryos. The haploid nuclei either remained haploid or became diploid. Where possible, physical measurements indicated that the haploid cells were smaller and produced smaller cuticular structures than did diploid cells. An increase in the number of pattern elements was observed in many patches which, by various criteria, were judged to be formed by haploid cells. The observation of altered pattern element spacing in haploid patches is in agreement with the conclusion, reached by L. I. Held (1979, Wilhelm Roux's Arch. 187, 105-127) in triploid flies, that bristle spacing is a function of cell size.

The interaction between daughterless and sex-lethal in triploids: a lethal sex-transforming maternal effect linking sex determination and dosage compensation in Drosophila melanogaster

Regulation of Drosophila sex determination and X-chromosome dosage compensation in response to the X-chromosome/autosome (X/A) balance of the zygote is shown to require proper functioning of both the da+ gene in the mother and the Sxl+ gene in the zygote. Previous studies led to the hypothesis that zygotic Sxl+ alleles are differentially active in females (XXAA) vs males (XYAA) in response to the X/A balance, and that maternal da+ gene product acts as a positive regulator in this connection. Sxl+ activity was proposed to impose the female developmental sequence on cells which would follow the male sequence in its absence. Important predictions of this proposal are verified. This study focuses primarily on the phenotype of triploid intersexes (XXAAA, X/A = 0.67). They are shown here to survive effects of da and Sxl mutations that would be lethal to diploids. The ambiguous X/A signal of intersexes normally causes them to develop as phenotypic mosaics of male and female tissue. Loss of maternal da+ or zygotic Sxl+ gene function shifts their somatic sexual phenotype to the male alternative. A gain-of-function mutation at Sxl has the opposite effect, imposing female development regardless of the maternal genotype with respect to da. It also reduces their rate of X-linked gene expression. The effects of a duplication of Sxl+ resemble those of the constitutive Sxl allele, but are less extreme. The role of these genes in the process of X-chromosome dosage compensation is inferred indirectly from the strict dependence of the mutations' lethal effects on the X/A balance in haploids, diploids, and triploids, and more directly from the effects of the mutations on the phenotypes of the X-linked neomorphic mutations, Bar and Hairy-wing. The relationship of da+ and Sxl+ gene functions to those of other sex-specific lethal loci in D. melanogaster, and to sex determination mechanisms in other species, is discussed.