Developmental analysis of the torso-like phenotype in Drosophila produced by a maternal-effect locus

Gastrulation in Drosophila melanogaster: Genetic control, cellular basis and biomechanics

Gastrulation is generally understood as the morphogenetic processes that result in the spatial organization of the blastomere into the three germ layers, ectoderm, mesoderm and endoderm. This review summarizes our current knowledge of the morphogenetic mechanisms in Drosophila gastrulation. In addition to the events that drive mesoderm invagination and germband elongation, we pay particular attention to other, less well-known mechanisms including midgut invagination, cephalic furrow formation, dorsal fold formation, and mesoderm layer formation. This review covers topics ranging from the identification and functional characterization of developmental and morphogenetic control genes to the analysis of the physical properties of cells and tissues and the control of cell and tissue mechanics of the morphogenetic movements in the gastrula.

Divergent effects of intrinsically active MEK variants on developmental Ras signaling

Germline mutations in Ras pathway components are associated with a large class of human developmental abnormalities, known as RASopathies, that are characterized by a range of structural and functional phenotypes, including cardiac defects and neurocognitive delays. Although it is generally believed that RASopathies are caused by altered levels of pathway activation, the signaling changes in developing tissues remain largely unknown. We used assays with spatiotemporal resolution in Drosophila melanogaster (fruit fly) and Danio rerio (zebrafish) to quantify signaling changes caused by mutations in MAP2K1 (encoding MEK), a core component of the Ras pathway that is mutated in both RASopathies and cancers in humans. Surprisingly, we discovered that intrinsically active MEK variants can both increase and reduce the levels of pathway activation in vivo. The sign of the effect depends on cellular context, implying that some of the emerging phenotypes in RASopathies may be caused by increased, as well as attenuated, levels of Ras signaling.

Automated cell tracking identifies mechanically oriented cell divisions during Drosophila axis elongation

Embryos extend their anterior-posterior (AP) axis in a conserved process known as axis elongation. Drosophila axis elongation occurs in an epithelial monolayer, the germband, and is driven by cell intercalation, cell shape changes, and oriented cell divisions at the posterior germband. Anterior germband cells also divide during axis elongation. We developed image analysis and pattern-recognition methods to track dividing cells from confocal microscopy movies in a generally applicable approach. Mesectoderm cells, forming the ventral midline, divided parallel to the AP axis, while lateral cells displayed a uniform distribution of division orientations. Mesectoderm cells did not intercalate and sustained increased AP strain before cell division. After division, mesectoderm cell density increased along the AP axis, thus relieving strain. We used laser ablation to isolate mesectoderm cells from the influence of other tissues. Uncoupling the mesectoderm from intercalating cells did not affect cell division orientation. Conversely, separating the mesectoderm from the anterior and posterior poles of the embryo resulted in uniformly oriented divisions. Our data suggest that mesectoderm cells align their division angle to reduce strain caused by mechanical forces along the AP axis of the embryo.

The pea aphid uses a version of the terminal system during oviparous, but not viviparous, development

Background

In most species of aphid, female nymphs develop into either sexual or asexual adults depending on the length of the photoperiod to which their mothers were exposed. The progeny of these sexual and asexual females, in turn, develop in dramatically different ways. The fertilized oocytes of sexual females begin embryogenesis after being deposited on leaves (oviparous development) while the oocytes of asexual females complete embryogenesis within the mother (viviparous development). Compared with oviparous development, viviparous development involves a smaller transient oocyte surrounded by fewer somatic epithelial cells and a smaller early embryo that comprises fewer cells. To investigate whether patterning mechanisms differ between the earliest stages of the oviparous and viviparous modes of pea aphid development, we examined the expression of pea aphid orthologs of genes known to specify embryonic termini in other insects.

Results

Here we show that pea aphid oviparous ovaries express torso-like in somatic posterior follicle cells and activate ERK MAP kinase at the posterior of the oocyte. In addition to suggesting that some posterior features of the terminal system are evolutionarily conserved, our detection of activated ERK in the oocyte, rather than in the embryo, suggests that pea aphids may transduce the terminal signal using a mechanism distinct from the one used in Drosophila. In contrast with oviparous development, the pea aphid version of the terminal system does not appear to be used during viviparous development, since we did not detect expression of torso-like in the somatic epithelial cells that surround either the oocyte or the blastoderm embryo and we did not observe restricted activated ERK in the oocyte.

Conclusions

We suggest that while oviparous oocytes and embryos may specify posterior fate through an aphid terminal system, viviparous oocytes and embryos employ a different mechanism, perhaps one that does not rely on an interaction between the oocyte and surrounding somatic cells. Together, these observations provide a striking example of a difference in the fundamental events of early development that is both environmentally induced and encoded by the same genome.

A functional antagonism between the pgc germline repressor and torso in the development of somatic cells

Segregation of the germline is a fundamental event during early development. In Drosophila, germ cells are specified at the posterior pole of the embryo by the germplasm. As zygotic expression is activated, germ cells remain transcriptionally silent owing to the polar granule component (Pgc), a small peptide present in germ cells. Somatic cells at both the embryonic ends are specified by the torso (Tor) receptor tyrosine kinase, and in tor mutants the somatic cells closer to the germ cells fail to cellularize correctly. Here, we show that extra wild-type gene copies of pgc cause a similar cellularization phenotype, and that both excessive pgc and a lack of tor are associated with an impairment of transcription in somatic cells. Moreover, a lack of pgc partly ameliorates the cellularization defect of tor mutants, thus revealing a functional antagonism between pgc and tor in the specification of germline and somatic properties. As transcriptional quiescence is a general feature of germ cells, similar mechanisms might operate in many organisms to 'protect' somatic cells that adjoin germ cells from inappropriately succumbing to such quiescence.

Sap18 is required for the maternal gene bicoid to direct anterior patterning in Drosophila melanogaster

Development of the insect head is a complex process that in Drosophila requires the anterior determinant, Bicoid. Bicoid is present in an anterior-to-posterior concentration gradient, and binds DNA and stimulates transcription of head-specific genes. Many of these genes, including the gap-gene hunchback, are initially activated in a broad domain across the head primordium, but later retract so that their expression is cleared from the anterior-most segmented regions. Here, we show that retraction requires a Bicoid-interacting protein, Sap18, which is part of the Sin3/Rpd3 histone deacetylase complex. In sensitized-mutant backgrounds (e.g., bcdE1/+, removal of maternal sap18 results in embryos that are missing labrally derived parts of the cephalopharyngeal skeleton. These sap18 mutant embryos fail to repress hb expression, and show reduced anterior cap expression of the labral determinant cap 'n' collar. These phenotypes are enhanced by lowering the dose of rpd3, which encodes the catalytic subunit of the deacetylase complex. The results suggest a model where, in labral regions of the head, Bicoid is converted from an activator into a repressor by recruitment of a co-repressor to Bicoid-dependent promoters. Bicoid's activity, therefore, depends not only on its concentration gradient, but also on its interactions with modifier proteins within spatially restricted domains.

Functions and mechanisms of receptor tyrosine kinase Torso signaling: lessons from Drosophila embryonic terminal development

The Torso receptor tyrosine kinase (RTK) is required for cell fate specification in the terminal regions (head and tail) of the early Drosophila embryo. Torso contains a split tyrosine kinase domain and belongs to the type III subgroup of the RTK superfamily that also includes the platelet-derived growth factor receptors, stem cell or steel factor receptor c-Kit proto-oncoprotein, colony-stimulating factor-1 receptor, and vascular endothelial growth factor receptor. The Torso pathway has been a model system for studying RTK signal transduction. Genetic and biochemical studies of Torso signaling have provided valuable insights into the biological functions and mechanisms of RTK signaling during early Drosophila embryogenesis.

The Drosophila embryonic patterning determinant torsolike is a component of the eggshell

The development of the head and tail regions of the Drosophila embryo is dependent upon the localized polar activation of Torso (Tor), a receptor tyrosine kinase that is uniformly distributed in the membrane of the developing embryo. Trunk (Trk), the proposed ligand for Tor, is secreted as an inactive precursor into the perivitelline fluid that lies between the embryonic membrane and the vitelline membrane (VM), the inner layer of the eggshell. The spatial regulation of Trk processing is thought to be mediated by the secreted product of the torsolike (tsl) gene, which is expressed during oogenesis by a specialized population of follicle cells present at the two ends of the oocyte. We show here that Tsl protein is specifically localized to the polar regions of the VM in laid eggs. We further demonstrate that although Tsl can associate with nonpolar regions of the VM, the activity of polar-localized Tsl is enhanced, suggesting the existence of another spatially restricted factor acting in this pathway. The incorporation of Tsl into the VM provides a mechanism for the transfer of spatial information from the follicle cells to the developing embryo. To our knowledge, Tsl represents the first example of an embryonic patterning determinant that is a component of the eggshell.

Drosophila rab GDI mutants disrupt development but have normal Rab membrane extraction

Rab GTPases are essential for vesicular transport. Rab GDP dissociation inhibitor (GDI) binds to GDP-bound rabs, removes rabs from acceptor membranes and delivers rabs to donor membranes. We isolated lethal GDI mutations in Drosophila and analyzed their developmental phenotypes. To learn how these mutations affect GDI structure, the crystal structure of Drosophila GDI was determined by molecular replacement to a resolution of 3.0 A. Two hypomorphic, missense mutations are located in domain II of GDI at highly conserved positions, but not in previously identified sequence conserved regions. The mutant GDIs were tested for ability to extract rabs from membranes and showed wild-type levels of rab membrane extraction. The two missense alleles showed intragenic complementation, indicating that domain II of GDI may have two separable functions. This study indicates that GDI function is essential for development of a complex, multicellular organism and that puparium formation and pole cell formation are especially dependent on GDI function.

Signal transduction pathway for anterior-posterior development in Drosophila

In Drosophila, the establishment of embryonic polarity along the anterior-posterior axis of the egg is determined by the activity of maternal gene products that accumulate during oogenesis. Amongst these are the Bicoid, the Nanos, and the terminal class gene products, some of which are oncoproteins involved in signal transduction for the formation of terminal structures in the embryo. Several signal transduction pathways have been described in Drosophila, and this review explores the potential of oncogene studies using one of those pathways - the terminal class signal transduction pathway - to better understand the cellular mechanisms of proto-oncogenes that mediate cellular responses in vertebrates including humans.

Terminal pattern elements in Drosophila embryo induced by the torso-like protein

The genes torso (tor) and torso-like (tsl) are two of the Drosophila maternal group genes implicated in a receptor tyrosine kinase signalling pathway that specifies terminal cell fate (reviewed in ref. 3). Loss-of-function mutations in these loci cause an identical phenotype in which pattern elements from the anterior (acron) and posterior (telson) ends have been deleted. We have cloned the tsl gene and demonstrate here that, in agreement with previous genetic data, it encodes a protein that is secreted and whose transcription is restricted to specialized categories of follicle cells localized at the poles of the egg chamber. At early blastoderm stage, tsl protein forms a symmetrical concentration gradient at the poles on the surface of the devitellinized embryo. Unrestricted expression of the tsl protein in tsl female mutants induces terminal pattern elements and suppresses the formation of abdomen in embryos. These results suggest that the tsl protein is the ligand that binds to the torso receptor.

Biochemical analysis of torso and D-raf during Drosophila embryogenesis: implications for terminal signal transduction

Determination of anterior and posterior terminal structures of Drosophila embryos requires activation of two genes encoding putative protein kinases, torso and D-raf. In this study, we demonstrate that Torso has intrinsic tyrosine kinase activity and show that it is transiently tyrosine phosphorylated (activated) at syncytial blastoderm stages. Torso proteins causing a gain-of-function phenotype are constitutively tyrosine phosphorylated, while Torso proteins causing a loss-of-function phenotype lack tyrosine kinase activity. The D-raf gene product, which is required for Torso function, is identified as a 90-kDa protein with intrinsic serine/threonine kinase activity. D-Raf is expressed throughout embryogenesis; however, the phosphorylation state of the protein changes during development. In wild-type embryos, D-Raf is hyperphosphorylated at 1 to 2 h after egg laying, and thereafter only the most highly phosphorylated form is detected. Embryos lacking Torso activity, however, show significant reductions in D-Raf protein expression rather than major alterations in the protein's phosphorylation state. This report provides the first biochemical analysis of the terminal signal transduction pathway in Drosophila embryos.

A protein kinase similar to MAP kinase activator acts downstream of the raf kinase in Drosophila

D-raf, a Drosophila homolog of Raf-1, plays key roles in multiple signal transduction pathways. Dsor1, a putative factor downstream of D-raf, was genetically identified by screening of dominant suppressors of D-raf. Dsor1Su1 mapped on X chromosome significantly suppressed the D-raf mutant phenotypes, and the loss-of-function mutations of Dsor1 showed phenotypes similar to those of the D-raf null mutations. Dsor1Su1 also significantly suppressed the mutations of other terminal class genes acting further upstream of D-raf. Molecular cloning of Dsor1 revealed its product with striking similarity to the microtubule-associated protein (MAP) kinase activator and yeast PBS2, STE7, and byr1. Our genetic results demonstrate the connection between raf and the highly conserved protein kinase cascade involving MAP kinase in vivo.

Biochemical analysis of torso and D-raf during Drosophila embryogenesis: implications for terminal signal transduction

Determination of anterior and posterior terminal structures of Drosophila embryos requires activation of two genes encoding putative protein kinases, torso and D-raf. In this study, we demonstrate that Torso has intrinsic tyrosine kinase activity and show that it is transiently tyrosine phosphorylated (activated) at syncytial blastoderm stages. Torso proteins causing a gain-of-function phenotype are constitutively tyrosine phosphorylated, while Torso proteins causing a loss-of-function phenotype lack tyrosine kinase activity. The D-raf gene product, which is required for Torso function, is identified as a 90-kDa protein with intrinsic serine/threonine kinase activity. D-Raf is expressed throughout embryogenesis; however, the phosphorylation state of the protein changes during development. In wild-type embryos, D-Raf is hyperphosphorylated at 1 to 2 h after egg laying, and thereafter only the most highly phosphorylated form is detected. Embryos lacking Torso activity, however, show significant reductions in D-Raf protein expression rather than major alterations in the protein's phosphorylation state. This report provides the first biochemical analysis of the terminal signal transduction pathway in Drosophila embryos.

Torso receptor activity is regulated by a diffusible ligand produced at the extracellular terminal regions of the Drosophila egg

torso encodes a receptor tyrosine kinase (torso) required for anterior and posterior terminal development of the Drosophila embryo. Injecting eggs with in vitro synthesized torso mRNAs revealed that torso activation is governed by an extracellular molecule, presumably the torso ligand, produced at terminal regions of the egg during early embryogenesis. In the absence of torso, the ligand shows no apparent localization, indicating that it is diffusible and normally bound by an excess of torso receptor at the egg poles. Mutant ligand-binding torso proteins can suppress telson formation in a dominant negative manner, suggesting that the ligand is limited in amount. Analysis of torso mutations indicates that torso functions as a tyrosine kinase and that gain-of-function mutations causing ligand-independent activation are located in the extracellular domain.

corkscrew encodes a putative protein tyrosine phosphatase that functions to transduce the terminal signal from the receptor tyrosine kinase torso

We describe the characterization of the Drosophila gene, corkscrew (csw), which is maternally required for normal determination of cell fates at the termini of the embryo. Determination of terminal cell fates is mediated by a signal transduction pathway that involves a receptor tyrosine kinase, torso, a serine/threonine kinase, D-raf, and the transcription factors, tailless and huckebein. Double mutant and cellular analyses between csw, torso, D-raf, and tailless indicate that csw acts downstream of torso and in concert with D-raf to positively transduce the torso signal via tailless, to downstream terminal genes. The csw gene encodes a putative nonreceptor protein tyrosine phosphatase covalently linked to two N-terminal SH2 domains, which is similar to the mammalian PTP1C protein.

Generating lineage-specific markers to study Drosophila development

To generate cell- and tissue-specific expression patterns of the reporter gene lacZ in Drosophila, we have generated and characterized 1,426 independent insertion strains using four different P-element constructs. These four transposons carry a lacZ gene driven either by the weak promoter of the P-element transposase gene or by partial promoters from the even-skipped, fushi-tarazu, or engrailed genes. The tissue-specific patterns of beta-galactosidase expression that we are able to generate depend on the promoter utilized. We describe in detail 13 strains that can be used to follow specific cell lineages and demonstrate their utility in analyzing the phenotypes of developmental mutants. Insertion strains generated with P-elements that carry various sequences upstream of the lacZ gene exhibit an increased variety of expression patterns that can be used to study Drosophila development.

Zygotic genes that mediate torso receptor tyrosine kinase functions in the Drosophila melanogaster embryo

The developmental signal that specifies the fates of cells at the anterior and posterior termini of the Drosophila embryo is transmitted by the torso receptor tyrosine kinase. This paper presents the results of a genetic interaction test for zygotic loci that act downstream of torso in the terminal genetic hierarchy. Tests of 26 zygotic mutants with defects in terminal development indicate that at least 14 reside in this hierarchy. The phenotypes associated with these genes fall into three classes, each of which represents a distinct aspect of terminal development and evolution. Four of the genes have been molecularly cloned and their products include an intercellular communication factor and three kinds of transcription factors.

Analysis of cytoplasmic activity dependent on the Drosophila terminal pattern gene torso

Cytoplasm from wildtype Drosophila embryos was transplanted into torso (tor) mutant embryos to determine the distribution of terminal rescuing activity at the cleavage stage. Although posterior and lateral wildtype cytoplasm contained rescuing activity that restored posterior terminal (telson) structures Klingler et al. (1988, Nature (London) 335, 275-277) this rescuing activity was not found in anterior cytoplasm. Similarly, transplantation of anterior and lateral wildtype cytoplasm into the anterior of tor embryos rescued anterior terminal (acron) structures, whereas posterior cytoplasm did not. This failure of reciprocal rescue is due to the presence of the products of the anterior and posterior classes of genes, because anterior cytoplasm from bicoid mutant embryos restored the telson in the posterior as well as the acron in the anterior of tor embryos, and because posterior cytoplasm from nanos embryos rescued the acron in the anterior as well as the telson in the posterior of tor embryos. Therefore terminal rescuing activity is evenly distributed throughout the cleavage stage embryo as anticipated from molecular studies.

Localized requirement for torso-like expression in follicle cells for development of terminal anlagen of the Drosophila embryo

The torso (tor) gene, one of six identified maternal genes essential for the development of the anterior and posterior terminal structures in the Drosophila embryo, is likely to function as a transmembrane receptor tyrosine kinase. Although tor protein is uniformly distributed in the membrane of the egg cell and syncytial embryo, genetic and molecular data suggest that tor is locally activated at the ends of the embryo by a ligand present in the perivitelline space. Local activation of tor could be achieved if the ligand were expressed by a subpopulation of the follicle cells that surround the developing oocyte. Here we describe torso-like (tsl), the sixth member of the terminal gene class, and show that it is unique among these genes in that its expression is required in the somatic follicle cells rather than in the germ line. Moreover, mosaic analysis demonstrates that tsl expression is necessary only in subpopulations of follicle cells located at the poles of the oocyte. Thus, the spatially regulated expression of tsl in the follicle cell layer may generate a localized signal that is transduced by tor, ultimately resulting in the formation of the terminal structures of the embryo.

Posterior localization of vasa protein correlates with, but is not sufficient for, pole cell development

The protein product of the Drosophila maternal-effect posterior group gene vasa is localized to the posterior pole of the oocyte and is sequestered by the pole cells as they form. It is, however, present at easily detectable levels throughout the oocyte and pre-blastoderm embryo. The protein is present in the pole cells and their germ line derivatives throughout all stages of development. An antiserum against this protein recognizes a pole-cell-specific antigen in seven other Drosophila species. Of six other maternal-effect loci essential for embryonic pole cell development, none affects expression of vasa, mutations in four abolish vasa protein localization, and mutations in two, tudor and valois, have little, if any, effect on vasa expression or localization. This indicates that vasa protein, when properly localized, is not sufficient for induction of pole cell development, and that at least the tudor and valois wild-type functions are also required specifically for this process. These results are discussed with respect to the multiple functions of the vasa gene.

The egg came first, of course! Anterior-posterior pattern formation in Drosophila embryogenesis and oogenesis

The anterior-posterior body pattern of the Drosophila embryo is initiated through the action of maternal gene products. In particular, three groups of maternally acting genes (the anterior, posterior and terminal groups) have been shown to direct the synthesis and spatial restriction of the three major organizing activities in the egg. The initial spatial localizations of the maternal organizing activities are established during oogenesis. After fertilization these activities regulate zygotic gene activity along the anterior-posterior axis of the egg.

Requirement of the Drosophila raf homologue for torso function

In Drosophila the correct formation of the most anterior and posterior regions of the larva, acron and telson is dependent on the maternally expressed terminal class of genes. In their absence, the anterior head skeleton is truncated and all the structures posterior to the abdominal segment seven are not formed. The protein predicted to be encoded by one of these genes, torso (tor), seems to be a transmembrane protein with an extracytoplasmic domain acting as a receptor and a cytoplasmic domain containing tyrosine kinase activity. Here we report that another member of the terminal-genes class, l(1)polehole (l(1)ph), which is also zygotically expressed, is the Drosophila homologue of the v-raf oncogene and encodes a potential serine-and-threonine kinase. We also show that functional l(1)ph gene product is required for the expression of a gain-of-function tor mutant phenotype, indicating that l(1)ph acts downstream of tor. Together, these results support the idea that the induction of terminal development occurs through a signal transduction system, involving the local activation of the tor-encoded tyrosine kinase at the anterior and posterior egg poles, resulting in the phosphorylation of the l(1)ph gene product. In turn, downstream target proteins may be phosphorylated, ultimately leading to the regionalized expression of zygotic target genes. Such a process is in agreement with the finding that both tor and l(1)ph messenger RNAs are evenly distributed.

Multiple functions of a Drosophila homeotic gene, zeste-white 3, during segmentation and neurogenesis

Lack of both maternal and zygotic gene activity at the zeste-white 3 (zw3) locus causes severe developmental transformations. Embryos derived from germ cells that lack zw3+ gene activity die during embryogenesis and have a phenotype that is similar to that of embryos mutant for the segment polarity gene naked (nkd). In both nkd and germ line clone-derived zw3 embryos the pattern elements derived from the anterior-most part of each segment, the denticle belts, are deleted. Similar abnormal patterns of the zygotically expressed genes engrailed and Ultrabithorax are detected in both mutants, suggesting that the two genes are involved in the same developmental process. Additionally, the induction of clones of zw3 mutant cells in imaginal discs causes homeotic transformations of noninnervated hair cells into innervated sensory bristles. The multiple roles of zw3 during development and its possible interactions with the zygotic gene nkd are discussed.

cappuccino and spire: two unique maternal-effect loci required for both the anteroposterior and dorsoventral patterns of the Drosophila embryo

cappuccino and spire are unique Drosophila maternal-effect loci that participate in pattern formation in both the anteroposterior and dorsoventral axes of the early embryo. Mutant females produce embryos lacking pole cells, polar granules, and normal abdominal segmentation. They share these defects with the posterior group of maternal-effect genes. Although embryos are defective in abdominal segmentation, in double mutant combinations with Bicaudal D, abdominal segments can be formed in the anterior half of the egg. This indicates that embryos produced by mutant females contain the 'posterior determinant' required for abdominal segmentation (Nüsslein-Volhard et al. 1987) and suggests that the wild-type gene products are not required for production of the posterior determinant but, rather, for its localization or stabilization. The vasa protein, a component of polar granules, is not localized at the posterior pole of mutant egg chambers or embryos, providing additional support for the hypothesis that localization to or stabilization of substances at the posterior pole of the egg chamber is defective in mutant females. Females mutant for the strongest alleles also produce dorsalized embryos. Phenotypic analysis reveals that these dorsalized embryos also have abdominal segmentation defects. The mutant phenotypes can be ordered in a series of increasing severity. Pole cell formation is most sensitive to loss of functional gene products, followed by abdominal segmentation, whereas normal dorsoventral patterning is the least sensitive to loss of functional gene products. In addition, mutant females contain egg chambers that appear to be dorsalized, resulting in the production of eggs with dorsalized eggshells. Germ-line mosaics indicate that cappuccino and spire are required in the oocyte-nurse cell complex. This suggests that the eggshell phenotype results from altered pattern in the underlying germ cell. Also, we defined the epistatic relationships between several early patterning loci, on the basis of an analysis of the eggs and embryos produced by females doubly mutant for cappuccino or spire and other loci that affect the pattern of both the egg and the embryo. On the basis of our current knowledge of the genes involved in this process, we formulated a working model for the early steps in dorsoventral patterning.

The Drosophila gene torso encodes a putative receptor tyrosine kinase

The maternal gene torso, required for determination of anterior and posterior terminal structures in the Drosophila embryo, was cloned using P-element tagging. Genetic evidence suggests that the action of the gene product is spatially restricted to the terminal regions; the torso messenger RNA, however, is evenly distributed. Structural similarities of the predicted torso protein with growth-factor receptor tyrosine kinases suggest that the spatial restriction of torso activity results from a localized activation of the torso protein at the anterior and posterior egg pole.

Reciprocal effects of hyper- and hypoactivity mutations in the Drosophila pattern gene torso

In Drosophila, five "terminal" polarity genes must be active in females in order for them to produce embryos with normal anterior and posterior ends. Hypoactivity mutations in one such gene, torso, result in the loss of the most posterior domain of fushi tarazu expression and the terminal cuticular structures. In contrast, a torso hyperactivity mutation causes the loss of central fushi tarazu expression and central cuticular structures. Cytoplasmic leakage, transplantation, and temperature-shift experiments suggest that the latter effect is caused by abnormal persistence of the torso product in the central region of the embryo during early development. Thus, the amount and timing of torso activity is key to distinguishing the central and terminal regions of the embryo. Mutations in the tailless terminal gene act as dominant maternal suppressors of the hyperactive torso allele, indicating that the torso product acts through, or in concert with, the tailless product.

Zygotic lethals with specific maternal effect phenotypes in Drosophila melanogaster. I. Loci on the X chromosome

In order to identify all X-linked zygotic lethal loci that exhibit a specific maternal effect on embryonic development, germline clonal analyses of X-linked zygotic lethal mutations have been performed. Two strategies were employed. In Screen A germline clonal analysis of 441 mutations at 211 previously mapped X-linked loci within defined regions was performed. In Screen B germline clonal analysis of 581 larval and pupal mutations distributed throughout the entire length of the X chromosome was performed. These approaches provide an 86% level of saturation for X-linked late zygotic lethals (larval and pupal) with specific maternal effect embryonic lethal phenotypes. The maternal effect phenotypes of these mutations are described.

Identification of the binding sites for potential regulatory proteins in the upstream enhancer element of the Drosophila fushi tarazu gene

With a view to identifying proteins that regulate the expression of the Drosophila ftz gene we have sequenced its enhancer-like upstream element (USE) and determined the binding sites for embryonic nuclear proteins within this region by in vitro DNAaseI footprinting. We find that greater than 50% of this element is bound by nuclear protein. By footprinting and gel-retardation studies in embryonic extracts from different developmental stages, we have characterised a number of USE/protein complexes whose nature alters in concert with changes in the ftz expression pattern, suggesting that these USE-binding proteins may be involved in the regulation of gene activity. In some cases this suggestion is substantiated by the observation that the protected DNA sequences show homology to the binding sites for ftz regulating DNA-binding proteins such as the pair-rule gene product even-skipped.

Function of torso in determining the terminal anlagen of the Drosophila embryo

The formation of the unsegmented terminal regions of the Drosophila larva, acron and telson requires the function of at least five maternal genes (terminal genes class). In their absence, the telson and acron are not formed. One of them, torso (tor), has gain-of-function alleles which have an opposite phenotype to the lack-of-function (tor-) alleles: the segmented regions of the larval body, thorax and abdomen, are missing, whereas the acron is not affected and the telson is enlarged. In strong gain-of-function mutants, the pair-rule gene fushi tarazu (ftz) is not expressed, demonstrating the suppression of the segmentation process in an early stage of development. The tor gain-of-function effect is neutralized, and segmentation is restored in double mutants with the zygotic gene tailless (tll), which has a phenotype similar (but not identical) to that of tor-. This suggests that tor acts through tll, and that in the gain-of-function alleles of tor, the tll gene product is ectopically expressed at middle positions of the embryo, where it inhibits the expression of segmentation genes like ftz.

Maternal-effect genes that alter the fate map of the Drosophila blastoderm embryo

The pattern of segmentation in the Drosophila embryo is controlled by at least 25 zygotically active genes and at least 20 maternally active genes. We have examined the pattern of expression of the protein product of the zygotically active segmentation gene fushi tarazu (ftz) at the cellular blastoderm stage in progeny of mutant females homozygous for each of six maternal-effect segmentation genes to observe the early effects of the maternal-effect genes on zygotic gene expression. The genes included exuperantia (a member of the anterior class of maternal-effect segmentation genes); staufen and vasa (members of the posterior class); and torso, trunk, and fs(1)N (members of the terminal class). Mutations in the genes caused a disruption of the normal pattern of ftz stripes in regions of the embryo where gene activity is known to be required. The ftz stripes provide a marker for segmental determination at the cellular blastoderm stage, making it possible to correlate aberrant patterns of ftz protein with defects in cuticle morphology at the end of embryogenesis. ftz protein expression in progeny of females mutant for combinations of the above genes was also examined. The changes in the ftz pattern in progeny of females doubly mutant for genes of the anterior and terminal classes or of the posterior and terminal classes can largely be understood as the result of the additive effects of the single mutations. In contrast, clearly nonadditive effects on the ftz pattern were seen when a mutation in a gene of the anterior class (exuperantia) was combined with mutations in posterior class genes.

Maternal effects of general and regional specificity on embryos of Drosophila melanogaster caused by dunce and rutabaga mutant combinations

The developmental patterns of embryos produced by female germ line cells homozygous for null-enzyme mutations of dunce and for dunce in combination with each of three different rutabaga mutations are compared with the normal pattern. At least three discrete developmental defects at progressive stages following fertilization can be identified and correlated with the loss of adenylate cyclase activity caused by rutabaga mutations, suggesting that the defects are caused by elevated cyclic AMP levels in female germ line cells. The earliest defect occurs soon after fertilization and affects DNA replication and mitosis, prevents nuclear migration, and leads to large polyploid nuclei. A later defect prevents cleavage nuclei from migrating into, or dividing in, the posterior region of the egg. The last affects the developmental behavior or fate of blastoderm cells. Some of these defects mimic those produced by previously described maternal-effect mutations.

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.

Structure of two genes at the gooseberry locus related to the paired gene and their spatial expression during Drosophila embryogenesis

The gooseberry (gsb) locus contains two closely linked genes, BSH9 and BSH4, which are structurally related to each other and to the paired (prd) gene. Sequence analysis of genomic DNA and cDNA shows that BSH9 and BSH4 can encode proteins of 427 and 452 amino acids, respectively. The structural homology between these two putative proteins and the prd protein consists essentially of two domains forming most of the amino-terminal halves of the proteins: the prd domain of 128 amino acids and a prd-type homeo domain of 60 amino acids, which is extended by 18 amino acids at its amino-terminal end. The temporal profiles of BSH9 and BSH4 transcripts, as characterized by Northern analysis, show a peak shortly after the peak of prd transcripts. The spatial distributions of BSH9 and BSH4 transcripts have been analyzed by in situ hybridization to whole-mount and sectioned embryos. BSH9 transcripts appear in the posterior ventrolateral part of each primordial segment throughout the embryo, including head and tail segments. Transcripts are initially restricted to the ectoderm, in which they arise as two spatially shifted and temporally delayed waves exhibiting double-segment periodicity and anteroposterior polarity. During germ-band extension, BSH9 is induced in the mesoderm in register with the ectoderm and neurectoderm and in the tail segments A9-A11. In contrast, BSH4 transcripts appear with a single-segment repeat, first, in the neurectoderm during germ-band extension and, later, in single neurons during neuronal differentiation. BSH9, BSH4, and prd are activated in cells that are in register along the anteroposterior axis of the embryo in the posterior parts of primordial segments comprising the posterior compartments of engrailed expression.

Isolation of Drosophila clones encoding maternally restricted RNAs

We have isolated molecular clones encoding RNAs which appear to be transcribed predominantly or exclusively during oogenesis in Drosophila melanogaster. These clones were isolated by virtue of their homology to polyadenylated RNAs present in very early embryos and absent from older embryos. An analysis of the developmental expression of three cloned genes is presented. The genomic position of one of these clones coincides closely with the position of the maternal effect lethal gene swallow, providing strong evidence that we have cloned this locus.

The zygotic segmentation mutant tailless alters the blastoderm fate map of the Drosophila embryo

The well-characterized spatial distributions of the transcripts from several Drosophila segmentation genes provide molecular markers which can be used to examine the determination of the segment pattern in early embryos. Tailless (tll) is a zygotic lethal mutation, the phenotype of which is observed by 9 hr of embryogenesis and includes the absence of segments A8, A9, and A10 and a decrease in the procephalic lobe (Strecker et al., Dev. Biol. 113, 64-76, 1986). To establish whether this effect of the tll mutation is due, as proposed previously by Strecker et al., to a reprogramming of the blastoderm fate map, we hybridized probes for the segmentation genes fushi tarazu (ftz) and hairy (h) to whole embryos. The transcripts of these genes show an altered distribution in tll embryos as early as nuclear cycle 14, indicating that the tll gene acts on cellular determination at the blastoderm stage, and is required for normal expression of the ftz and h genes. We obtained more precise information about the alterations in the blastoderm fate map by measuring the position of ftz protein stripes in wild-type and tll embryos. From the results reported here and previously, we conclude that the tll mutation results in a deletion of anterior and posterior ectodermal positional values, concomitant with an expansion of the remaining fate map.

Germ line and soma cooperate during oogenesis to establish the dorsoventral pattern of egg shell and embryo in Drosophila melanogaster

Mutations in gurken and torpedo cause a ventralization in the follicle cell epithelium during Drosophila oogenesis and in the pattern of the embryo that develops in the resultant egg. Both genes lie midway in an epistatic series between fs(1)K10 and dorsal; the mutations block the dorsalization normally observed in K10 eggs but have no effect on the phenotype of embryos derived from dorsal mothers. Analysis of germ-line mosaics demonstrates that both ovarian and embryonic phenotypes will be produced when either the gurken+ gene is removed from the germ line or torpedo+ is removed from the soma. This shows that the dorsoventral pattern of the Drosophila egg chamber depends on the transfer of spatial information from the germ line to the somatic follicle cells, and from somatic cells to the oocyte.

Region-specific defects in l(1)giant embryos of Drosophila melanogaster

Lack of zygotic expression of the l(1)giant locus (l(1)gt;3A1), produces embryos with defects in abdominal A5, 6, and 7 and within the head. Scanning electron microscopy at the time of segment formation reveals two regions of defects in the segmentation pattern: anteriorly the labial lobe and thoracic segments T1 and T2 are fused; posteriorly, abdominal segments A5-7 are disrupted. The mature embryo shows incomplete head involution and defects within A5-7; fusion of T1 and T2 is no longer observed. Localized cell death within neural and mesodermal tissues is observed at 7 hr of development; later ventral ganglia, A5-7, are missing. Double-mutant analyses of l(1)gt with maternal effect lethal mutations and mutations that generate homeotic, segment number, gap, or segment polarity phenotypes indicate that normal activity of l(1)gt is required for differentiation of two embryonic domains: one corresponding to labial, T1 and T2 segments, and the second corresponding to abdominal segments 5, 6, and 7.

l(1)hopscotch, A larval-pupal zygotic lethal with a specific maternal effect on segmentation in Drosophila

The maternal and zygotic effect phenotypes of mutations at the l(1)hopscotch (l(1)hop) locus are described. l(1)hop is located in 10B6-8 on the salivary gland chromosome map and 17 alleles have been characterized. A complex complementation pattern is observed among the 17 alleles. The lethal phase of null alleles of l(1)hop occurs at the larval-pupal interface associated with a small disc phenotype. Embryos produced from homozygous l(1)hop germline clones show segment specific defects. The extent of these defects depends upon both the strength of the allele and the paternal contribution. In the most extreme case embryos exhibit defects associated with five segments T2, T3, A4, A5, and A8. In the less extreme phenotype defects are only associated with A5. Thus, activity of l(1)hop+ is required both for the maintenance and continued cell division of diploid imaginal precursors and for the establishment of the full array of segments.