Expression of the zygotic genome in blastoderm stage embryos ofDrosophila: Analysis of a specific protein

Incorporation of CENP-A/CID into centromeres during early Drosophila embryogenesis does not require RNA polymerase II-mediated transcription

In many species, centromere identity is specified epigenetically by special nucleosomes containing a centromere-specific histone H3 variant, designated as CENP-A in humans and CID in Drosophila melanogaster. After partitioning of centromere-specific nucleosomes onto newly replicated sister centromeres, loading of additional CENP-A/CID into centromeric chromatin is required for centromere maintenance in proliferating cells. Analyses with cultured cells have indicated that transcription of centromeric DNA by RNA polymerase II is required for deposition of new CID into centromere chromatin. However, a dependence of centromeric CID loading on transcription is difficult to reconcile with the notion that the initial embryonic stages appear to proceed in the absence of transcription in Drosophila, as also in many other animal species. To address the role of RNA polymerase II–mediated transcription for CID loading in early Drosophila embryos, we have quantified the effects of alpha-amanitin and triptolide on centromeric CID-EGFP levels. Our analyses demonstrate that microinjection of these two potent inhibitors of RNA polymerase II–mediated transcription has at most a marginal effect on centromeric CID deposition during progression through the early embryonic cleavage cycles. Thus, we conclude that at least during early Drosophila embryogenesis, incorporation of CID into centromeres does not depend on RNA polymerase II–mediated transcription.

Developmentally regulated H2Av buffering via dynamic sequestration to lipid droplets in Drosophila embryos

Regulating nuclear histone balance is essential for survival, yet in early Drosophila melanogaster embryos many regulatory strategies employed in somatic cells are unavailable. Previous work had suggested that lipid droplets (LDs) buffer nuclear accumulation of the histone variant H2Av. Here, we elucidate the buffering mechanism and demonstrate that it is developmentally controlled. Using live imaging, we find that H2Av continuously exchanges between LDs. Our data suggest that the major driving force for H2Av accumulation in nuclei is H2Av abundance in the cytoplasm and that LD binding slows nuclear import kinetically, by limiting this cytoplasmic pool. Nuclear H2Av accumulation is indeed inversely regulated by overall buffering capacity. Histone exchange between LDs abruptly ceases during the midblastula transition, presumably to allow canonical regulatory mechanisms to take over. These findings provide a mechanistic basis for the emerging role of LDs as regulators of protein homeostasis and demonstrate that LDs can control developmental progression.

Control of cleavage cycles in Drosophila embryos by frühstart

Early metazoan development consists of cleavage stages characterized by rapid cell cycles that successively divide the fertilized egg. The cell cycle oscillator pauses when the ratio of DNA and cytoplasm (N/C) reaches a threshold characteristic for the species. This pause requires maternal factors as well as zygotic expression of as yet unknown genes. Here we isolate the zygotic gene frühstart of Drosophila and show that it is involved in pausing the cleavage cell cycle. frs is expressed immediately after the last cleavage division. It plays a role in generating a uniform pause and it can inhibit cleavage divisions when precociously expressed. Furthermore, the expression of frs is delayed in haploid embryos and requires activity of the maternal checkpoint gene grapes. We propose that zygotic frs expression is involved in linking the N/C and the pause of cleavage cycle.

Zygotic transcription and cell proliferation during Drosophila embryogenesis

The zygotic expression of only a few Drosophila genes is known to be required for completion of the normal embryonic mitoses. Molecular genetic analyses of these genes reveal that they fall into two classes, those whose mRNA levels are regulated in a stage and/or tissue-specific fashion to control cell cycle events during embryogenesis, and those in which, in the absence of functional zygotic expression, the maternal mRNA contribution does not provide sufficient product to complete the normal embryonic mitoses. Genes that comprise the first class are involved in the developmental control of the cell cycle, while those of the second class identify components of the cell cycle machinery.

Transcription in leech: mRNA synthesis is required for early cleavages in Helobdella embryos

Zygotic transcription was analyzed in embryos of the glossiphoniid leech Helobdella triserialis by autoradiographic detection of tritiated uridine incorporated in the presence or absence of low concentrations of alpha-amanitin. RNA synthesis was first detected after the second cleavage and alpha-amanitin-sensitive RNA synthesis was first detected during the divisions yielding the embryonic stem cells, or teloblasts. RNA synthesis increased as development progressed, and the bulk of alpha-amanitin-sensitive RNA synthesis was found in two classes of cells, the blast cells, which are the progeny of the teloblasts, and the micromere-derived cells. The time during which zygotic gene products are required was determined by observing the developmental consequences of alpha-amanitin exposure. Zygotes microinjected with alpha-amanitin underwent the first several cleavages with normal timing and symmetry, but underwent aberrant cleavages and produced supernumerary large blastomeres during the time that the control embryos generated teloblasts. Once the teloblasts were formed, the microinjection of alpha-amanitin did not affect the production of blast cells by the teloblasts, but it did block the divisions and movements of the blast cells and the micromere-derived cells. These data suggest that zygotic transcription is activated during the early cleavages of Helobdella embryos and that newly synthesized transcripts are required for the generation of teloblasts. Thus, there is an early, critical period of messenger RNA synthesis essential for teloblast production that is distinct from the later phase of messenger RNA synthesis required for cell divisions and cell movements during gastrulation.

Genetic control of cell division patterns in the Drosophila embryo

In Drosophila embryogenesis, mitotic control undergoes a significant transition during the 14th interphase. Mitoses before interphase 14 run on maternal products, and occur in metasynchronous waves. Mitoses after interphase 14 require zygotic transcription, and occur asyncronously in an intricate, highly ordered spatio-temporal pattern. Mutations at the string (stg) locus cause cell-cycle arrest during this transition, in G2 of interphase 14, yet do not arrest other aspects of development. This phenotype suggests that stg is required specifically for initiating mitosis. We describe the cloning of stg, and show that its predicted amino acid sequence is homologous to that of cdc25, a regular of mitotic initiation in the yeast S. pombe. In addition, we show that zygotic expression of stg mRNA occurs in a dynamic series of spatial patterns which anticipate the patterns of the zygotically driven cell divisions. Therefore we suggest that regulated expression of stg mRNA controls the timing and location of these embryonic cell divisions.

Drosophila genes encoding maternal-specific and maternal-differential RNAs

The unique cellular and genetic events which occur during the first few hours of Drosophila embryogenesis suggest that there are genes whose function is entirely or largely limited to this stage; this is supported by both genetic and molecular evidence. To identify some of these genes and characterize the relative contribution of specifically maternal and specifically zygotic transcription to early embryogenesis, we used competition and differential screening of a Drosophila genomic DNA library to obtain blastoderm- and maternal-differential sequences [Roark et al.: Dev Biol 109:476-488, 1985]. We describe here the Eco RI restriction fragments, chromosomal location, and size and developmental pattern of expression of the RNAs transcribed from 19 maternal-differential sequences. Five sequences encode maternal-specific transcripts (50-150-fold more abundant in maternal RNA than at any other stage). The maternal-specific and maternal-differential sequences are located at single sites on all major chromosome arms. Comparison of these sites with the sites of presently mapped maternal-effect genes shows several possible correlations, including one region containing three maternal-effect lethal mutations and two maternal-specific sequences.

Cell cycle control by the nucleo-cytoplasmic ratio in early Drosophila development

We have studied the role of the nucleo-cytoplasmic ratio in the early development of Drosophila, using mutants and experimental manipulations that alter nuclear density. Haploid embryos produced by either maternal or paternal effect mutations compensate for haploidy by an extra nuclear division during the syncytial blastoderm stage. Decreasing the nucleo-cytoplasmic ratio in wild-type embryos by ligation can cause a similar extra blastoderm division. Conversely, increasing this ratio can cause the omission of a blastoderm division. The duration of mitotic cycles is affected by the nucleo-cytoplasmic ratio four cycles before the terminal blastoderm division. Transcription patterns in haploid embryos indicate that transcriptional activation is not directly controlled by the nucleo-cytoplasmic ratio, but may be an effect of the lengthening of interphase periods.

Blastoderm-differential and blastoderm-specific genes of Drosophila melanogaster

We have isolated, by molecular cloning, genes expressed differentially at the blastoderm stage of Drosophila melanogaster. Two of the blastoderm-differential genes are reexpressed at later stages, and map to single chromosomal loci 95C and 99E. The sequence at 99E is that encoding the myosin light chain 2. Two other blastoderm-differential sequences are members of multigene families (one of which is B104, or roo) and map to multiple dispersed chromosomal loci. A gastrula-differential sequence was found which maps to 71A. Most significantly, we have identified three genes encoding transcripts expressed uniquely at the blastoderm stage; these map to single chromosomal loci: 25D3, 75C, and 99D4-8. At least some of the blastoderm-differential and blastoderm-specific loci appear to be distinct from loci involved in embryonic pattern formation that have been identified in recent genetic "saturation" screens. The procedure of identifying genes specific to the blastoderm stage may thus allow the identification of genes, not previously identified by classical genetic techniques, that are involved in important embryonic processes.

Stage dependent synthesis of heat shock induced proteins in early embryos of Drosophila melanogaster

The synthesis of heat shock proteins (hsp) has been examined during the early embryogenesis of Drosophila melanogaster. Normal protein synthesis stops after heat shock at all developmental stages, while hsp synthesis is induced only after treatment at blastoderm and later stages. The small hsps continue to be synthesised after heat shock for a longer period than the larger ones. Heat shocks at 35 degrees C, 37 degrees C and 40 degrees C were compared for their effect on hsp synthesis and the effect of heat shock on the normal course of development was analysed.