28 citations found with entrez 4/28/97

Genes Dev 11 (4): 423-435 (1997)

An Egalitarian-BicaudalD complex is essential for oocyte specification and axis determination in Drosophila.

Mach JM, Lehmann R

Whitehead Institute for Biomedical Research and Department of Biology, Massachusetts Institute of Technology, Cambridge 02142, USA.

Genetic experiments suggest that polarization of the oocyte is linked directly to the initial cell fate determination that singles out the oocyte from its 15 sister cells. Specification of oocyte cell fate as well as establishment and maintenance of a polarized microtubule network within the Drosophila oocyte require the activity of the egalitarian (egl) and BicaudalD (BicD) genes. We have isolated the egl gene and show that Egl protein colocalizes with BicD protein at all stages of oogenesis. Immunoprecipitation experiments show that both proteins are part of a protein complex. Egl and BicD proteins localize to the oocyte in three stages that correlate with the stepwise polarization of the oocyte. We propose that the Egl-BicD protein complex links microtubule polarity and RNA transport. During early oogenesis, the complex is required to transport factors promoting oocyte differentiation; during later stages of oogenesis the complex directs the sorting of RNA molecules required for anterior-posterior and dorsoventral patterning of the embryo. 


Development 122 (12): 3863-3879 (1996)

The neurogenic genes egghead and brainiac define a novel signaling pathway essential for epithelial morphogenesis during Drosophila oogenesis.

Goode S, Melnick M, Chou TB, Perrimon N

Department of Genetics, Harvard Medical School, Boston, Massachusetts 02115, USA. goode@rascal.med.harvard.edu

Notch (N) and other neurogenic genes have been implicated in two fundamental processes, lateral specification of cell fates, and epithelial development. Previous studies have suggested that the neurogenic gene brainiac (brn) is specifically required for epithelial development (Goode, S., Morgan, M., Liang, Y-P. and Mahowald, A. P. (1996). Dev. Biol. 178, 35-50). In this report we show that egghead (egh), a gene with phenotypes identical to brn, encodes for a novel, putative secreted or transmembrane protein. We describe the role of egh and brn germline function in the morphogenesis of the follicular epithelium from the time it is born through the time that it migrates towards the oocyte late in oogenesis. By comparing the function of germline egh and brn to N during oogenesis, we have obtained direct evidence for the involvement of Notch in maintenance of the follicle cell epithelium, and the specificity of brn and egh in epithelial development during oogenesis. The most striking phenotype observed for all three genes is a loss of apical-basal polarity and accumulation of follicular epithelial cells in multiple layers around the oocyte. The spatiotemporal onset of this adenoma-like phenotype correlates with the differential accumulation of egh transcripts in the oocyte at stage 4 of oogenesis. In contrast to N, we find that brn and egh are essential for the organization, but not specification, of stalk and polar cells. The expression patterns and functional requirements of brn, egh, and N lead us to propose that these genes mediate follicular morphogenesis by regulating germline-follicle cell adhesion. This proposal offers explanations for (1) the involvement of egh and brn in N-mediated epithelial development, but not lateral specification, (2) why brn and egh embryonic neurogenic phenotypes are not as severe as N phenotypes, and (3) how egh and brn influence Egfr-mediated processes. The correlation between the differential expression of egh in the oocyte and the differential requirement for brn, egh, and N in maintaining the follicular epithelium around the oocyte, suggests that Egghead is a critical component of a differential oocyte-follicle cell adhesive system. 


Dev Biol 178 (1): 35-50 (1996)

Brainiac encodes a novel, putative secreted protein that cooperates with Grk TGF alpha in the genesis of the follicular epithelium.

Goode S, Morgan M, Liang YP, Mahowald AP

Department of Molecular Genetics and Cell Biology, University of Chicago, Illinois 60637, USA.

brainiac (brn) is involved in a number of developmental events. In addition to being required zygotically for segregation of neuroblasts from epidermoblasts, it is essential for a series of critical steps during oogenesis which also depend upon gurken (grk), a TGF alpha homolog. Animals harboring strong mutations of either grk or EGF receptor tyrosine kinase (Egfr) or doubly mutant for brn and weak grk or Egfr mutations produce ovarian follicles with multiple sets of nurse cell-oocyte complexes. These follicles frequently have discontinuities in the follicular epithelium that uncover nurse cells but not the oocyte. Gaps first appear in the germarium, suggesting that some nurse cells lack affinity for invading prefollicular cells. This is the first evidence that grk, in addition to its involvement in the genesis of anterior-posterior and dorsal-ventral polarity, is also required for Egfr-dependent development of the follicular epithelium that surrounds each nurse cell/oocyte cluster to form an egg chamber. We have used restriction fragment length polymorphisms to localize brn to a 10-kb region within a 300-kb stretch of DNA on the X-chromosome, and we have identified the brn gene by means of RNA rescue. brn codes for a putative secreted protein. brn is expressed in germ cells at the time follicle cells first surround the nurse cell-oocyte complex. Our genetic data suggest that brn acts in a parallel, but partially overlapping pathway to the Grk-Egfr signaling pathway. The brn pathway may help to provide specificity to TGF alpha -Egfr function during oogenesis. 


Development 121 (11): 3809-3818 (1995)

A small predicted stem-loop structure mediates oocyte localization of Drosophila K10 mRNA.

Serano TL, Cohen RS

Department of Biochemistry, University of Kansas, Lawrence 66045, USA.

The establishment of dorsoventral polarity in the Drosophila oocyte and future embryo is dependent on the efficient transport of K10 mRNA from nurse cells into the oocyte. To investigate the cis-requirements of K10 mRNA transport, we used a transgenic fly assay to analyze the expression patterns of a series of K10 deletion variants. Such studies identify a 44 nucleotide sequence within the K10 3' untranslated region that is required and sufficient for K10 mRNA transport and subsequent localization to the oocyte's anterior cortex. An inspection of the 44 nucleotide transport/localization sequence (TLS) reveals a strong potential for the formation of a stem-loop secondary structure. Nucleotide substitutions that interfere with the predicted base-pairing of the TLS block mRNA transport and anterior localization. Conversely, mutations that alter the base composition of the TLS while maintaining predicted base-pairing do not block mRNA transport or anterior localization. We conclude that K10 mRNA transport and anterior localization is mediated by a 44 nucleotide stem-loop structure. A similar putative stem-loop structure is found in the 3' untranslated region of the Drosophila orb mRNA, suggesting that the same factors mediate the transport and anterior localization of both K10 and orb mRNAs. Apart from orb, the K10 TLS is not found in any other localized mRNA, raising the possibility that the transport and localization of other mRNAs, e.g., bicoid, oskar and gurken, are mediated by novel sets of cis- and trans-acting factors. Moreover, we find that the K10 TLS overrides the activity of oskar cis-regulatory elements that mediate the late stage movement of the mRNA to the posterior pole. We propose the existence of a family of cis-regulatory elements that mediate mRNA transport into the oocyte, only some of which are compatible with the elements that mediate late stage movements. 


Curr Biol 5 (11): 1252-1254 (1995)

Pattern formation. Gurken meets torpedo for the first time.

Gavis ER

Department of Molecular Biology, Princeton University, New Jersey 08544, USA.

Intercellular communication between oocyte and follicle cells, mediated by the gurken-torpedo/DER signalling pathway, has a crucial role in determining both anterior-posterior and dorsal-ventral polarity in Drosophila. 


Genes Dev 9 (20): 2482-2494 (1995)

Cappuccino, a Drosophila maternal effect gene required for polarity of the egg and embryo, is related to the vertebrate limb deformity locus.

Emmons S, Phan H, Calley J, Chen W, James B, Manseau L

Department of Molecular and Cellular Biology, University of Arizona, Tucson 85721, USA.

We report the molecular isolation of cappuccino (capu), a gene required for localization of molecular determinants within the developing Drosophila oocyte. The carboxy-terminal half of the capu protein is closely related to that of the vertebrate limb deformity locus, which is known to function in polarity determination in the developing vertebrate limb. In addition, capu shares both a proline-rich region and a 70-amino-acid domain with a number of other genes, two of which also function in pattern formation, the Saccharomyes cerevisiae BNI1 gene and the Aspergillus FigA gene. We also show that capu mutant oocytes have abnormal microtubule distributions and premature microtubule-based cytoplasmic streaming within the oocyte, but that neither the speed nor the timing of the cytoplasmic streaming correlates with the strength of the mutant allele. This suggests that the premature cytoplasmic streaming in capu mutant oocytes does not suffice to explain the patterning defects. By inducing cytoplasmic streaming in wild-type oocytes during mid-oogenesis, we show that premature cytoplasmic streaming can displace staufen protein from the posterior pole, but not gurken mRNA from around the oocyte nucleus. 


Oncogene 11 (6): 1033-1040 (1995)

Requirement of the ETS domain transcription factor D-ELG for egg chamber patterning and development during Drosophila oogenesis.

Gajewski KM, Schulz RA

Department of Biochemistry and Molecular Biology, University of Texas MD Anderson Cancer Center, Houston 77030, USA.

The D-elg gene encodes an ETS domain transcription factor that functions in Drosophila oogenesis. D-elg belongs to a small group of genes that are required for the formation of both the anteroposterior and dorsoventral axes of the egg chamber. During oogenesis in D-elg mutant females, the spatial localization of oskar and gurken mRNAs in the oocyte is disrupted and a follicle cell enhancer trap marker identifies dorsoventral polarity defects. Also, specialized follicle cells, called border cells, fail to migrate from their anterior location to a position adjacent to the developing oocyte. Consistent with these phenotypes, D-elg shows genetic interactions with two genes required for normal egg chamber differentiation. 


Cell 82 (5): 785-794 (1995)

An unusual mosaic protein with a protease domain, encoded by the nudel gene, is involved in defining embryonic dorsoventral polarity in Drosophila.

Hong CC, Hashimoto C

Department of Genetics, Yale University School of Medicine, New Haven, Connecticut 06520, USA.

Dorsoventral polarity of the Drosophila embryo is induced by a ventral extracellular signal, which is produced by a locally activated protease cascade within the extraembryonic perivitelline compartment. Local activation of the protease cascade depends on a positional cue that is laid down during oogenesis outside the oocyte. Here we present evidence that the nudel gene encodes an essential component of this cue. The nudel gene, which is expressed in follicle cells covering the oocyte, encodes an unusual mosaic protein resembling an extracellular matrix protein with a central serine protease domain. Our findings suggest that embryonic dorsoventral polarity is defined by a positional cue that requires the nudel protein to anchor and to trigger the protease cascade producing the polarity-inducing signal. 


Development 121 (9): 3023-3033 (1995)

Mutations in the Drosophila gene bullwinkle cause the formation of abnormal eggshell structures and bicaudal embryos.

Rittenhouse KR, Berg CA

University of Washington, Department of Genetics, Seattle 98195-7360, USA.

Subcellular localization of gene products and cell migration are both critical for pattern formation during development. The bullwinkle gene is required in Drosophila for disparate aspects of these processes. In females mutant at the bullwinkle locus, the follicle cells that synthesize the dorsal eggshell filaments do not migrate properly, creating short, broad structures. Mosaic analyses demonstrate that wild-type BULLWINKLE function is required in the germ line for these migrations. Since the mRNA for gurken, the putative ligand that signals dorsal follicle cell fate, is correctly localized in bullwinkle mutants, we conclude that our bullwinkle alleles do not affect the dorsoventral polarity of the oocyte and thus must be affecting the follicle cell migrations in some other way. In addition, the embryos that develop from bullwinkle mothers are bicaudal. A KINESIN:beta-GALACTOSIDASE fusion protein is correctly localized to the posterior pole of bullwinkle oocytes during stage 9. Thus, the microtubule structure of the oocyte and general transport along it do not appear to be disrupted prior to cytoplasmic streaming. Unlike other bicaudal mutants, oskar mRNA is localized correctly to the posterior pole of the oocyte at stage 10. By early embryogenesis, however, some oskar mRNA is mislocalized to the anterior pole. Consistent with the mislocalization of oskar mRNA, a fraction of the VASA protein and nanos mRNA are also mislocalized to the anterior pole of bullwinkle embryos. Mislocalization of nanos mRNA to the anterior is dependent on functional VASA protein. Although the mirror-image segmentation defects appear to result from the action of the posterior group genes, germ cells are not formed at the anterior pole. The bicaudal phenotype is also germ-line dependent for bullwinkle. We suspect that BULLWINKLE interacts with the cytoskeleton and extracellular matrix and is necessary for gene product localization and cell migration during oogenesis after stage 10a. 


Nature 375 (6533): 654-658 (1995)

Polarization of both major body axes in Drosophila by gurken-torpedo signalling.

Gonzalez-Reyes A, Elliott H, St Johnston D

Wellcome/CRC Institute, University of Cambridge, UK.

Anterior-posterior polarity in Drosophila arises from the movement of the oocyte to the posterior of the egg chamber, and the subsequent acquisition of posterior fate by the adjacent somatic follicle cells. We demonstrate that gurken is necessary in the oocyte and torpedo/DER in the follicle cells for the induction of posterior fate. As the gurken-torpedo/DER pathway also establishes dorsoventral polarity later in oogenesis, Drosophila uses the same germline to soma signalling pathway to determine both embryonic axes. 


Cell 81 (6): 967-978 (1995)

cornichon and the EGF receptor signaling process are necessary for both anterior-posterior and dorsal-ventral pattern formation in Drosophila.

Roth S, Neuman-Silberberg FS, Barcelo G, Schupbach T

Department of Molecular Biology, Howard Hughes Medical Institute, Princeton University, New Jersey 08544, USA.

In Drosophila, the dorsal-ventral polarity of the egg chamber depends on the localization of the oocyte nucleus and the gurken RNA to the dorsal-anterior corner of the oocyte. Gurken protein presumably acts as a ligand for the Drosophila EGF receptor (torpedo/DER) expressed in the somatic follicle cells surrounding the oocyte. cornichon is a gene required in the germline for dorsal-ventral signaling. cornichon, gurken, and torpedo also function in an earlier signaling event that establishes posterior follicle cell fates and specifies the anterior-posterior polarity of the egg chamber. Mutations in all three genes prevent the formation of a correctly polarized microtubule cytoskeleton required for proper localization of the anterior and posterior determinants bicoid and oskar and for the asymmetric positioning of the oocyte nucleus. 


Mech Dev 51 (2-3): 183-192 (1995)

The role of fs(1)K10 in the localization of the mRNA of the TGF alpha homolog gurken within the Drosophila oocyte.

Serano TL, Karlin-McGinness M, Cohen RS

Department of Biochemistry, University of Kansas, Lawrence 66045, USA.

A critical step in Drosophila dorsoventral patterning is the movement of gurken mRNA from the anterior cortex of the oocyte to the oocyte's anterodorsal corner at stage 8 of oogenesis. Such movement is dependent on fs(1)K10. It has been proposed that fs(1)K10 mediates gurken mRNA movement by down-regulating gurken mRNA levels, thus ensuring that gurken mRNA does not saturate its receptors located in the oocyte's anterodorsal corner. In contradiction to this model, we show here--both genetically and immunocytochemically--that GRK protein levels are lower in the anterodorsal region of fs(1)K10 mutant oocytes than in the anterodorsal region of fs(1)K10+ oocytes. From this and other data, we propose a more direct role for fs(1)K10 in the gurken mRNA localization process. 


Annu Rev Genet 29: 371-399 (1995)

Signaling pathways that establish the dorsal-ventral pattern of the Drosophila embryo.

Morisato D, Anderson KV

Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA.

The dorsal-ventral pattern of the Drosophila embryo is established by three sequential signaling pathways. Each pathway transmits spatial information by localizing the activity of an extracellular signal, which acts as a ligand for a broadly distributed transmembrane receptor. The components of the first two pathways are encoded by maternal effect genes, while the third pathway is specified by genes expressed in the zygote. During oogenesis, the oocyte transmits a signal to the surrounding follicle cells by the gurken-torpedo pathway. After fertilization, the initial asymmetry of the egg chamber is used by the spatzle-Toll pathway to generate within the embryo a nuclear gradient of the transcription factor Dorsal, which regulates the regional expression of a set of zygotic genes. On the dorsal side of the embryo, the decapentaplegic-punt/thick veins pathway then establishes patterning of the amnioserosa and dorsal ectoderm. Each pathway uses a distinct strategy to achieve spatial localization of signaling activity. 


Genes Dev 8 (24): 2986-2995 (1994)

RNA localization along the anteroposterior axis of the Drosophila oocyte requires PKA-mediated signal transduction to direct normal microtubule organization.

Lane ME, Kalderon D

Department of Biological Sciences, Columbia University, New York, New York 10027.

Microtubule polarity has been implicated as the basis for polarized localization of morphogenetic determinants that specify the anteroposterior axis in Drosophila oocytes. We describe mutation affecting Protein Kinase A (PKA) that act in the germ line to disrupt both microtubule distribution and RNA localization along this axis. In normal oocytes, the site of microtubule nucleation shifts from posterior to anterior immediately prior to polarized localization of bicoid and oskar RNAs. In PKA-deficient oocytes, posterior microtubules are present during this transition, oskar RNA fails to accumulate at the posterior, and bicoid RNA accumulates at both ends of the oocyte. Similar RNA mislocalization patterns previously reported for Notch and Delta mutants suggest that PKA transduces a signal for microtubule reorganization that is sent by posteriorly located follicle cells. 


Science 266 (5185): 639-642 (1994)

Role of oocyte position in establishment of anterior-posterior polarity in Drosophila.

Gonzalez-Reyes A, St Johnston D

Wellcome/CRC Institute, University of Cambridge, England.

The polarized microtubule cytoskeleton of the Drosophila oocyte directs the localization of the maternal determinants which establish the anterior-posterior (AP) axis of the embryo. Because the formation of this microtubule array is dependent on signals from the follicle cells that surround the oocyte, it has been proposed that AP polarity originates in the follicle cells. Here it is shown that the movement of the oocyte to the posterior of the egg chamber early in oogenesis determines AP polarity in the follicle cell layer, and also in the oocyte. Moreover, the generation of AP asymmetry requires signaling from the germ line to the soma and back again. 


Curr Opin Genet Dev 4 (4): 502-507 (1994)

Dorsoventral patterning in Drosophila oogenesis.

Schupbach T, Roth S

Department of Molecular Biology, Princeton University, New Jersey 08544.

Dorsoventral polarity in the egg chamber of Drosophila involves the localization of maternal gurken RNA to the dorsal side of the oocyte. The gurken protein has homology to secreted growth factors and may bind to the torpedo/DER receptor tyrosine kinase present on the adjacent follicle cells. This localized signal from the oocyte to the follicle cells appears to initiate a cascade of events leading to dorsal follicle cell differentiation, and delimiting and orienting the future dorsoventral axis of the embryo. 


Development 120 (5): 1233-1242 (1994)

Null alleles reveal novel requirements for Bic-D during Drosophila oogenesis and zygotic development.

Ran B, Bopp R, Suter B

Department of Biology, McGill University, Montreal, PQ, Canada.

In the Drosophila ovary, the Bicaudal-D (Bic-D) gene is required for the differentiation of one of 16 interconnected cystocyte sister cells into an oocyte. A new class of Bic-Dnull alleles reveals a novel requirement for Bic-D for zygotic viability. In the germ line, the null mutations show that developmental processes that take place in germarial region 1, even those that create asymmetry, are independent of Bic-D function. Bic-D is then required to establish oocyte identity in one cystocyte and is essential, not only for the oocyte-specific accumulation of all oocyte markers that we have tested so far, but also for the posterior migration of the oocyte. In addition, normal polarity amongst the nurse cells requires Bic-D, indicating that the creation of different nurse cell identities may depend on oocyte determination. Our results show that different processes in early oogenesis require different amounts of Bic-D in a process-specific way and certain later processes can proceed at low levels of Bic-D. This suggests that the patterning of the female germ line and the development of an oocyte depend on differential responses to a single activity that is capable of initiating distinct oogenesis processes and can establish different cell fates. 


Curr Biol 4 (4): 289-300 (1994)

Transient posterior localization of a kinesin fusion protein reflects anteroposterior polarity of the Drosophila oocyte.

Clark I, Giniger E, Ruohola-Baker H, Jan LY, Jan YN

Howard Hughes Medical Institute, University of California at San Francisco 94143-0724.

BACKGROUND: During oogenesis in Drosophila, determinants that will dictate abdomen and germline formation are localized to the 'polar plasm' in the posterior of the oocyte. Assembly of the polar plasm involves the sequential localization of several messenger RNAs and proteins to the posterior of the oocyte, beginning with the localization of oskar mRNA and Staufen protein during stages 8 and 9 of oogenesis. The mechanism by which these two early components accumulate at the posterior is not known. We have investigated whether directed transport along microtubules could be used to accomplish this localization. RESULTS: We have made a fusion protein composed of the bacterial beta-galactosidase enzyme as a reporter, joined to part of the plus-end-directed microtubule motor, kinesin, and have found that the fusion protein transiently localizes to the posterior of the oocyte during stages 8 and 9 of oogenesis. Treatment with the microtubule-depolymerizing agent colchicine prevents both the localization of the fusion protein and the posterior transport of oskar mRNA and Staufen protein. Furthermore, the fusion protein localizes normally in oocytes mutant for either oskar and staufen, but not in other mutants in which oskar mRNA and Staufen protein are mislocalized. CONCLUSIONS: Association with a plus-end-directed microtubule motor can promote posterior localization of a reporter protein during oogenesis. The genetic requirements for this localization and its sensitivity to colchicine, both of which are shared with the posterior transport of oskar mRNA and Staufen protein, suggest that similar mechanism may function in both processes. 


Trends Genet 10 (3): 89-94 (1994)

The role of gene cassettes in axis formation during Drosophila oogenesis.

Ruohola-Baker H, Jan LY, Jan YN

Howard Hughes Medical Institute, University of California, San Francisco 94143-0724.

Establishment of the anteroposterior and dorsoventral axes of the fly originates during oogenesis and relies on signaling between the oocyte and the surrounding somatic follicle cells. Some genes originally identified as playing a role in signaling during embryonic development also mediate cell-cell communication during oogenesis. These genes have previously been grouped on the basis of their functions during embryogenesis, and this classification is largely maintained in oogenesis. The EGF receptor, the transmembrane protein rhomboid and proteins in the ras signal transduction pathway are required to initiate dorsoventral polarity, whereas the products of the neurogenic genes Notch and Delta are necessary for formation of the anteroposterior axis in the oocyte. 


Genes Dev 8 (5): 598-613 (1994)

The Drosophila orb RNA-binding protein is required for the formation of the egg chamber and establishment of polarity.

Lantz V, Chang JS, Horabin JI, Bopp D, Schedl P

Department of Molecular Biology, Moffett Laboratory, Princeton University, New Jersey 08544.

The orb gene of Drosophila encodes sex-specific germ-line proteins that contain two RRM-type RNA-binding domains. Here we report the distribution of Orb protein in wild-type, tumorous, and orb mutant ovaries. The wild-type distribution of Orb protein during oogenesis resembles that of its RNA, preferentially accumulating in the cytoplasm of the developing oocyte shortly after the formation of the 16-cell cyst. As anticipated from its germ-line expression, mutations in orb lead to female sterility. Analysis of the effect of orb mutants on the distribution of RNAs known to be required for oocyte differentiation and polarity suggests that orb functions in RNA localization at multiple points during oogenesis. In addition, phenotypic characterization of the orb mutants indicates that the gene is required early in oogenesis for formation of the 16-cell cyst. It then functions in the differentiation of the oocyte and is required for the three-dimensional reorganization of the germ cells in the cyst as well as for the establishment of normal germ-line-soma interactions in the egg chamber. 


Cell 75 (1): 165-174 (1993)

The Drosophila dorsoventral patterning gene gurken produces a dorsally localized RNA and encodes a TGF alpha-like protein.

Neuman-Silberberg FS, Schupbach T

Department of Molecular Biology, Princeton University, New Jersey 08544.

Cell-cell interactions in the Drosophila ovary play a crucial role in the establishment of dorsoventral polarity of both the egg shell and the future embryo. Torpedo/DER (top/DER), a homolog of the vertebrate epidermal growth factor receptor, is required for this signaling process in the somatic cells of the ovary. In contrast, gurken (grk), which also functions in this pathway, is required in the germline. We cloned the grk gene and found that it encodes a TGF alpha-like protein. Grk is, therefore, likely to be a ligand of top/DER, activating the receptor during oogenesis. During oogenesis, the grk transcript becomes asymmetrically localized to the dorsal corner of the oocyte. We propose that the dorsal localization of grk RNA results in a spatially restricted ligand that asymmetrically activates the receptor. 


Ann Genet 36 (1): 5-15 (1993)

Determination of the dorso-ventral polarity of the Drosophila embryo

Mohier E

CNRS, Institut Jacques-Monod, Paris, France.

Embryonic pattern formation has been studied extensively in many organisms. In Drosophila, the powerful combination of genetics cytoplasm transplantation experiments, as well as recent molecular data, have helped to elucidate the mechanisms responsible for the establishment of embryonic polarity. A small number of genes, most of them maternally expressed, are involved in this process and participate in four independent systems--three for the antero-posterior axis (A/P) and one for dorsoventral axis (D/V)--which define various embryonic territories by specifically localized cues. This review concerns the definition of the dorsoventral polarity responsible for the establishment of the germ layers of the embryo. Dorsoventral development is regulated by a single group of maternally expressed genes: the "dorsal group" of genes. It includes 11 genes, the loss of function of any of which results in a dorsalized development, whereas mutation of the 12th gene, cactus, results in a ventralized development. These genes are arranged according to a functional hierarchy, and have been shown to cooperate in the formation of a graded nuclear concentration of the dorsal gene product. The dorsal product corresponds to the dorsoventral morphogen and is homologous to the transcription factor NF-kappa B. Among the 11 genes of the dorsal group, 3 are required in the somatic line. This suggests the existence of inductive signals originating during oogenesis from the follicle cells that surround the developing oocyte. This somatically expressed spatial information probably controls dorsoventral development by defining the polarity of a signal transducing pathway that specifically activates the nuclear uptake of the dorsal product. This model, highlights the importance of the polarity of the egg chamber, and suggests that it is the oocyte nucleus due to its asymmetrical localization, that determines the dorsoventral pattern formation of the embryo. 


Development 116 (1): 177-192 (1992)

The neurogenic locus brainiac cooperates with the Drosophila EGF receptor to establish the ovarian follicle and to determine its dorsal-ventral polarity.

Goode S, Wright D, Mahowald AP

University of Chicago, Department of Molecular Genetics and Cell Biology, Cummings Life Science Center, IL 60637.

We have characterized the function of a new neurogenic locus, brainiac (brn), during oogenesis. Homozygous brn females lay eggs with fused dorsal appendages, a phenotype associated with torpedo (top) alleles of the Drosophila EGF receptor (DER) locus. By constructing double mutant females for both brn and top, we have found that brn is required for determining the dorsal-ventral polarity of the ovarian follicle. However, embryos from mature brn eggs develop a neurogenic phenotype which can be zygotically rescued if a wild-type sperm fertilizes the egg. This is the first instance of a Drosophila gene required for determination of dorsal-ventral follicle cell fates that is not required for determination of embryonic dorsal-ventral cell fates. The temperature-sensitive period for brn dorsal-ventral patterning begins at the inception of vitellogenesis. The interaction between brn and DER is also required for at least two earlier follicle cell activities which are necessary to establish the ovarian follicle. Prefollicular cells fail to migrate between each oocyte/nurse cell complex, resulting in follicles with multiple sets of oocytes and nurse cells. brn and DER function is also required for establishing and/or maintaining a continuous follicular epithelium around each oocyte/nurse cell complex. These brn functions as well as the brn requirement for determination of dorsal-ventral polarity appear to be genetically separable functions of the brn locus. Genetic mosaic experiments show that brn is required in the germline during these processes whereas the DER is required in the follicle cells. We propose that brn may be part of a germline signaling pathway differentially regulating successive DER-dependent follicle cell activities of migration, division and/or adhesion and determination during oogenesis. These experiments indicate that brn is required in both tyrosine kinase and neurogenic intercellular signaling pathways. Moreover, the functions of brn in oogenesis are distinct from those of Notch and Delta, two other neurogenic loci that are known to be required for follicular development. 


In Vivo 5 (5): 443-456 (1991)

Models of pattern formation in insect oocytes.

Kunkel JG

Zoology Department, University of Massachussets, Amherst 01003.

Pattern formation in early insect development is dominated by coordination of the germ lines polarity with the polarity of the follicle cell layer. The production of an elaborate protective chorion, covering the ovulated oocyte, has made establishing parallel polarity of germ line and soma absolutely essential. Genetics and molecular biology, particularly on Drosophila melanogaster, have identified numerous signals passed from follicle cell to oocyte and vice versa. The physiological basis of this communication is beginning to be established with the identification of several membrane receptors and potential signal transduction steps. The contributions of three physiological models of pattern formation are discussed as they relate to the growing genetic model. Evidence for and against ionic currents as factors in polarity determinations is particularly emphasized. 


Development 107 Suppl: 169-180 (1989)

Models for positional signalling with application to the dorsoventral patterning of insects and segregation into different cell types.

Meinhardt H

Max-Planck-Institut fur Entwicklungsbiologie, Tubingen, FRG.

Models of pattern formation and possible molecular realizations are discussed and compared with recent experimental observations. In application to the dorsoventral patterning of insects, it is shown that a superposition of two pattern-forming reactions is required. The first system generates the overall dorsoventral polarity of the oocyte, the second generates the positional information proper with a stripe-like region of high concentration along the ventral side of the embryo. A single reaction would be insufficient since the two reactions require different parameters. The model accounts for the orientation of the DV axes of the oocytes in the ovary of Musca domestica and Sarcophaga, independent of the DV axis of the mother, for the formation of several ventral furrows in the absence of the primary gurken/torpedo system in Drosophila, as well as for the good size regulation of the dorsoventral axis as observed in some insect species. Segregation of a homogeneous cell population into different cell types requires autocatalytic processes that saturate at relatively low concentrations and nondiffusible substances responsible for the autocatalytic feed-back loops. Thus, these loops can be realized directly on the gene level via their gene products, for instance, by the mutual repression of two genes. A balance of the two cell types is achieved by a long-ranging substance interfering with the self-enhancing process. This substance is expected to have a more or less homogeneous distribution. This model accounts for the reestablishment of the correct proportion after an experimental interference and the change of determination after transplantation. Applications to the segregation of pre-stalk and prespore cells in Dictyostelium and of neuroblast cells from the ventral ectoderm in Drosophila are provided. 


Development 107 Suppl: 13-19 (1989)

Multiple steps in the localization of bicoid RNA to the anterior pole of the Drosophila oocyte.

St Johnston D, Driever W, Berleth T, Richstein S, Nusslein-Volhard C

Max Planck Institut fur Entwicklungsbiologie, Abteilung Genetik, Tubingen, FRG.

The anterior region of the Drosophila embryonic pattern is determined by a gradient of the bicoid (bcd) protein. The correct formation of this gradient requires the localization of bcd RNA to the anterior pole of the egg. Here we use a wholemount in situ technique to examine the process of bcd RNA localization during oogenesis and embryogenesis. While bcd protein becomes distributed in a gradient that extends throughout the anterior two thirds of the early embryo, bcd RNA remains restricted to a much smaller region at the anterior pole. The difference between these distributions indicates that the shape of the protein gradient must depend to some extent on the posterior movement of the protein after it has been synthesized. Four distinct phases of bcd RNA localization can be distinguished during oogenesis. Between stages 6 and 9 of oogenesis, the RNA accumulates in a ring at the anterior end of the oocyte. During the second phase, in stage 9-10a follicles, the RNA also localizes to the apical regions of the nurse cells, demonstrating that the nurse cells possess an intrinsic polarity. As the nurse cells contract during stages 10b-11, all of the bcd RNA becomes localized to the cortex at the anterior end of the oocyte. During a final phase that must occur between stage 12 of oogenesis and egg deposition, the RNA becomes localized to a spherical region that occupies a slightly dorsal position at the anterior pole. 


Nature 335 (6185): 68-70 (1988)

Drosophila nurse cells produce a posterior signal required for embryonic segmentation and polarity.

Sander K, Lehmann R

Biol. Institut I (Zoologie), Freiburg, FRG.

The segmental pattern of insect embryos depends on influences from morphogenetic centres near each of the egg poles. In Drosophila, maternal effect mutations are known that impair the normal function of each centre. Injection of wild-type cytoplasm into mutant eggs has revealed that morphogenetic signals localized at the anterior and posterior pole of eggs can be transplanted. We show here that these activities can also be detected during oogenesis. Posterior activity can be recovered at an early stage (stage 10, ref. 5) from the oocyte-nurse cell complex, but anterior activity can only be detected in the mature oocytes (stage 14). We conclude that the bicoid-dependent anterior signal, although produced by the nurse cells, does not become active before it is localized to the anterior egg pole, whereas posterior activity can be detected in the nurse cells before, and therefore independently of, its localization to the posterior egg pole. 


Biol Reprod 32 (1): 27-42 (1985)

Roles of cell-to-cell communication in development.

Schultz RM

Possible roles of cell-to-cell communication mediated by intercellular bridges and gap junctions in development of the female gamete and embryo are discussed. Synchronization of cell cycle events is presumably a role for intercellular bridges between germ cells. The follicle of the Cecropia moth reveals that an electrical polarity exists between nurse cells and oocytes which are connected by intercellular bridges and this polarity may generate differences that result in differentiation of the oogonia to become either the oocyte or nurse cells. Gap junction-mediated transfer of cyclic AMP, made in response to gonadotropin stimulation, between granulosa cells is discussed as a mechanism that allows cells within a tissue to respond to an external stimulus even though all cells in that tissue may not be exposed to the stimulus. A nutritional role for heterologous cell communication between follicle cells and the oocyte in oocyte growth is presented as an example of how gap junction-mediated communication can allow one cell type to influence the behavior of another cell type. During development, a restriction in communication between differentiating cells is frequently observed. Examples of this phenomenon in a mammal and an insect are presented.