Isolation of Drosophila clones encoding maternally restricted RNAs

A fluorescent quantitative PCR approach to map gene deletions in the Drosophila genome

We report the application of TaqMan quantitative PCR (QPCR) to map Drosophila chromosome deficiencies by discrimination of twofold copy number differences. For a model system, we used this technology to confirm the X chromosomal mapping of Dspt6 given the autosomal mapping of Dspt4. We then used this technique on both preexisting deletion mutant flies and flies that we generated with deletions to demonstrate the presence or absence of Dspt6, Dspt4, and swa in various deletion mutant flies. In contrast with in situ hybridization studies, QPCR both vitiates the need to do these more intricate studies, and it is more accurate as the site of deletion can be known down to the 10(2)-bp level. We then successfully applied the technique to the analysis of transcription, demonstrating that the amount of Dspt6 or Dspt4 transcriptional product depended directly on the dosage of the Dspt6 or Dspt4 gene, respectively. The rapidity and precision of this method demonstrates its applicability in Drosophila genetics, the rapid and accurate mapping of Drosophila deletion mutants.

Role of Adducin-like (hu-li tai shao) mRNA and protein localization in regulating cytoskeletal structure and function during Drosophila Oogenesis and early embryogenesis

Adducin is a cytoskeletal protein that can function in vitro to bundle F-actin and to control the assembly of the F-actin/spectrin cytoskeletal network. We previously reported cloning of the Drosophila Adducin-like (Add) locus [Ding et al., 1993] also referred to as hu-li tai shao (hts) [Yue and Spradling, 1992], and identification of two adducin-related protein isoforms: a 95 x 10(3) Mr form (ADD-95) and an 87 x 10(3) Mr form (ADD-87) [Zaccai and Lipshitz, 1996]. ADD-87 protein is present throughout the oocyte cortex at stages 9 and 10 of oogenesis but is restricted to its anterior pole from stage 11 onward. This ADD-87 protein localization is preceded by localization of Add-hts mRNA first to the cortex and then to the anterior pole of the oocyte. Mutation of the swallow gene results in delocalization of Add-hts mRNA and ADD-87 protein from the cortex of stage 9 and 10 oocytes, and from the anterior pole of later stage oocytes. Early embryos produced by swallow or Add-hts mutant females have severe defects in the distribution of F-actin and spectrin as well as abnormalities in nuclear division, nuclear migration, and cellularization. In addition to their cytoskeletal defects, embryos produced by swallow females have an abnormal anterior pattern because bicoid mRNA is delocalized from the anterior pole. In contrast, bicoid mRNA is still found at the anterior of embryos produced by Add-hts mothers. Thus swallow functions to restrict bicoid mRNA and Add-hts mRNA to the cortex of the oocyte. Cortical restriction of Add-hts mRNA and protein is required for the normal structure and function of the early embryonic F-actin/spectrin cytoskeleton. A defective embryonic cytoskeleton can be induced in either of two ways: (1) by delocalization of functional ADD from the oocyte cortex (as in swallow mutants), or (2) by reduction of ADD function while retaining its normal cortical localization during oogenesis (as in Add-hts mutants).

Staufen protein associates with the 3'UTR of bicoid mRNA to form particles that move in a microtubule-dependent manner

Staufen protein is required in order to anchor bicoid (bcd) mRNA at the anterior pole of the Drosophila egg. Here we show that staufen protein colocalizes with bcd mRNA at the anterior, and that this localization depends upon its association with the mRNA. Upon injection into the embryo, bcd transcripts specifically interact with staufen, and we have mapped the sequences required to three regions of the 3'UTR, each of which is predicted to form a long stem-loop. The resulting staufen-bcd 3'UTR complexes form particles that show a microtubule-dependent localization. Since staufen is also transported with oskar (osk) mRNA during oogenesis, staufen associates specifically with both osk and bcd mRNAs to mediate their localizations, but at two distinct stages of development.

Distribution of swallow protein in egg chambers and embryos of Drosophila melanogaster

The Drosophila maternal effect gene swallow has a role in localizing bicoid mRNA at the anterior margin of the oocyte during oogenesis, and a poorly characterized role in nuclear divisions in early embryogenesis. We have examined the distribution of swallow protein during oogenesis and embryogenesis using anti-swallow antibodies. During oogenesis, high levels of swallow protein are present in basal nurse cell cytoplasm, although small amounts are also present at the anterior oocyte margin, the site of bicoid RNA localization. Only a small fraction of swallow protein is in a position to interact directly with bicoid RNA during localization. The asymmetric distribution of swallow protein is disrupted in swallow ovaries, in which bicoid RNA becomes unlocalized late in oogenesis. swallow protein is uniformly distributed in eggs, but becomes localized to nuclei during early mitotic divisions in early embryogenesis. swallow protein enters each nucleus at the beginning of mitosis, occupies a position complementary to that of condensed chromatin, and leaves each nucleus at the end of mitosis. We show examples of nuclear division defects in swallow mutant embryos, and suggest that the abnormal nuclear divisions in early swallow embryos reflect a second function for swallow protein that contributes to abdominal segmentation defects common in swallow embryos.

Genes required for embryonic muscle development in Drosophila melanogaster A survey of the X chromosome

We have begun a genetic analysis to dissect the process of myogenesis by surveying the X chromosome of Drosophila melanogaster for mutations that affect embryonic muscle development. Using polarised light microscopy and antibody staining techniques we analysed embryos hemizygous for a series of 67 deletion mutations that together cover an estimated 85% of the X chromosome, or 16.5% of the genome. Whereas the mature wild type embryo has a regular array of contractile muscles that insert into the epidermis, 31 of the deletion mutants have defects in muscle pattern, contractility or both, that cannot be attributed simply to epidermal defects and identify functions required for wild type muscle development. We have defined mutant pattern phenotypes that can be described in terms of muscle absences, incomplete myoblast fusion, failure of attachment of the muscle to the epidermis or mispositioning of attachment sites. Thus muscle development can be mutationally disrupted in characteristic and interpretable ways. The areas of overlap of the 31 deletions define 19 regions of the X chromosome that include genes whose products are essential for various aspects of myogenesis. We conclude that our screen can usefully identify loci coding for gene products essential in muscle development.

Actin in the Drosophila embryo: is there a relationship to developmental cue localization?

Recent genetic manipulations have revealed that the cytoplasm of the early Drosophila embryo contains localized information that specifies the future embryonic axes. It is the restricted distribution or activity of particular gene products, either messenger RNA or protein, that is crucial for this specification. While some of the genes responsible for this information have been sequenced and the nature and distribution of their products examined, it is not known how this localization is established or maintained. The actin-based cytoskeleton is a likely candidate for the formation of a cytomatrix that would allow such distributions and yet no direct evidence has yet been found that implicates actin in positional cue localization. In this review I summarize what is known about actin filament behavior in Drosophila embryos and compare it to the distribution of positional cues. My purpose is to juxtapose these two bodies of information such that the relationship between them may be revealed.

Sequence of swallow, a gene required for the localization of bicoid message in Drosophila eggs

We report the sequence of the Drosophila maternal effect gene swallow, one of the genes whose product is required for the localization of bicoid message during Drosophila oogenesis. The inferred swallow protein contains a domain that is predicted to be an amphipathic alpha-helix similar to those implicated in protein:protein associations in other systems. Another part of the predicted protein appears to be a diverged RNA-binding motif. We discuss these structural features in light of the function of the swallow protein in the bicoid message localization process.

Expression in the central nervous system of a subset of the yema maternally acting genes during Drosophila embryogenesis. Post-embryonic expression extends to imaginal discs and spermatocytes

The yema gene region of Drosophila melanogaster is a cluster of maternally acting genes isolated in differential screens. At least ten transcripts are encoded by the yema gene region; most of them are produced by independent transcription units (eight different transcription units). Using RNA dot-blot analysis and in situ hybridization to tissue sections, we have realized a comprehensive survey of the temporal and spatial expression of the yema transcripts. All these transcripts are maternally expressed. Five of them display a strict maternal expression. They are found exclusively in the female germ line (nurse cells and oocyte). These transcripts are still present in the embryo as maternal information. However, a subset of the yema genes also shows an embryonic and a post-embryonic expression. Interestingly, this expression is essentially restricted to the central nervous system (CNS) throughout the fly development, to the larval and pupal imaginal discs and to a subset of cells in the male gonad, the spermatocytes. Strikingly, these expression sites mainly contain proliferating and/or differentiating cells.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

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.

Isolation of mesoderm-specific genes expressed in the Drosophila embryo

Tissue differentiation during embryonic development involves activation of specific genes. To isolate genes selectively expressed in mesoderm and nervous system in the Drosophila embryo, we have screened a cDNA library with molecular probes enriched in specific gene sequences from both tissues. In this way, we have isolated six mesoderm-specific genes, as demonstrated by in situ hybridization to embryo sections. Two of these genes, expressed during muscle differentiation, are described here for the first time. These genes have been localized in the 17A region of the first chromosome and in the 60A region of the second chromosome, respectively. No neural-specific genes were identified using this approach, most probably because of the low sensitivity of detection methods which combine filter hybridization techniques with the use of complex probes.

The product of the Drosophila gene vasa is very similar to eukaryotic initiation factor-4A

The vasa gene product of Drosophila melanogaster is required only in the female germ line. Progeny of females homozygous for vasa mutations lack posterior structures and pole cells. Isolation and characterization of vasa genomic and complementary DNA clones show that the transcript is abundant in the female germ line and early embryos only. The predicted amino acid sequence is very similar to those of the translation initiation factor eIF-4A and the human nuclear antigen p68.

Differential regulation of the two transcripts from the Drosophila gap segmentation gene hunchback

The Drosophila gap gene hunchback (hb) is required for the establishment of the anterior segment pattern of the embryo, and also for a small region of the posterior segment pattern. The hb gene encodes two transcripts from two promoters which show a differential regulation, although they code for the same protein product. The 3.2-kb transcript is expressed during oogenesis and forms an anterior-posterior gradient during the early stages of development. The first zygotic expression of hb during cleavage stages 11-12 is due to the 2.9-kb transcript. Its expression is under the control of the anterior pattern organizer gene bicoid (bcd) and it appears to be necessary and sufficient for the anterior segmentation. The 3.2-kb transcript is expressed again at syncytial blastoderm stage in the anterior yolk nuclei, as well as in an anterior stripe which is posteriorly adjacent to the domain of the 2.9-kb transcript, and as a posterior stripe. Using hb-promoter/lacZ fusion gene constructs in combination with germ line transformation, we have delimited a regulatory region for the 2.9-kb transcript to approximately 300 bp upstream of the site of transcription initiation and show that this region is sufficient to confer the full regulation by bcd.

The bicoid protein determines position in the Drosophila embryo in a concentration-dependent manner

The bicoid (bcd) protein in a Drosophila embryo is derived from an anteriorly localized mRNA and comes to be distributed in an exponential concentration gradient along the anteroposterior axis. To determine whether the levels of bcd protein are directly related to certain cell fates, we manipulated the density and distribution of bcd mRNA by genetic means, measured the resultant alterations in height and shape of the bcd protein gradient, and correlated the gradient with the fate map of the respective embryos. Increases or decreases in bcd protein levels in a given region of the embryo cause a corresponding posterior or anterior shift of anterior anlagen in the embryo. The bcd protein thus has the properties of a morphogen that autonomously determines positions in the anterior half of the embryo.

The role of localization of bicoid RNA in organizing the anterior pattern of the Drosophila embryo

The organization of the anterior pattern in the Drosophila embryo is mediated by the maternal effect gene bicoid. bcd has been identified in an 8.7-kb genomic fragment by germ line transformants that completely rescue the mutant phenotype. The major transcript of 2.6 kb includes a homeobox with low homology to previously known homeoboxes, a PRD-repeat and a M-repeat. In situ hybridizations reveal that bcd is transcribed in the nurse cells. The mRNA is localized at the anterior tip of oocyte and early embryo until the cellular blastoderm stage. The localization of the transcript requires the function of the maternal effect genes exuperantia and swallow while transcript stability is reduced by functions depending on posterior group genes.

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.

Determination of anteroposterior polarity in Drosophila

The principles of pattern formation in embryogenesis can be studied in Drosophila by means of a powerful combination of genetic and transplantation experiments. The segmented pattern of the Drosophila embryo is organized by two activities localized at the anterior and posterior egg poles. Both activities exert inducing and polarizing effects on the pattern when transplanted to other egg regions. A small set of maternal genes have been identified that are required for these activities. Mutants in these genes lack either the anterior or posterior part of the segmented pattern. The unsegmented terminal embryonic regions require a third class of genes and form independently of the anterior and posterior centers.