New functions of the Drosophila rhomboid gene during embryonic and adult development are revealed by a novel genetic method, enhancer piracy

Phosphorylation of Ind by MAP kinase enhances Ind-dependent transcriptional repression

The Drosophila neuroectoderm is initially subdivided into three longitudinal domains that give rise to columns of neuroblasts. This subdivision is coordinately accomplished by the action of the signaling pathways, Dorsal and Epidermal Growth Factor Receptor (EGFR), in conjunction with the homeodomain proteins, Ventral nervous system defective, Intermediate neuroblasts defective (Ind) and Muscle Segment Homeobox. We previously demonstrated that Ind expression is activated in response to the EGFR pathway. Here we show that EGF signaling subsequently mediates the direct phosphorylation of Ind by MAP kinase, which enhances the capacity of Ind to repress target genes, such as achaete. Specifically, we show that reduced EGF signaling results in diminished repression of achaete in the intermediate column, despite the presence of high levels of Ind protein. We also demonstrate that ectopic activation of MAP kinase results in the lateral expansion of the Ind expression domain with a corresponding reduction in achaete expression. This regulation is also dependent on the co-repressor, Dichaete. Our data indicate that EGF signaling, acting through MAP kinase, impinges on multiple aspects of Ind regulatory activity. While it has been often demonstrated that MAP kinase phosphorylation of transcriptional repressors attenuates their repressor activity, here we provide an example of phosphorylation enhancing repressor activity.

Cysteine repeat domains and adjacent sequences determine distinct bone morphogenetic protein modulatory activities of the Drosophila Sog protein

The Drosophila short gastrulation gene (sog) encodes a large extracellular protein (Sog) that inhibits signaling by BMP-related ligands. Sog and its vertebrate counterpart Chordin contain four copies of a cysteine repeat (CR) motif defined by 10 cysteine residues spaced in a fixed pattern and a tryptophan residue situated between the first two cysteines. Here we present a structure-function analysis of the CR repeats in Sog, using a series of deletion and point mutation constructs, as well as constructs in which CR domains have been swapped. This analysis indicates that the CR domains are individually dispensable for Sog function but that they are not interchangeable. These studies reveal three different types of Sog activity: intact Sog, which inhibits signaling mediated by the ligand Glass bottom boat (Gbb), a more broadly active class of BMP antagonist referred to as Supersog, and a newly identified activity, which may promote rather than inhibit BMP signaling. Analysis of the activities of CR swap constructs indicates that the CR domains are required for full activity of the various forms of Sog but that the type of Sog activity is determined primarily by surrounding protein sequences. Cumulatively, our analysis suggests that CR domains interact physically with adjacent protein sequences to create forms of Sog with distinct BMP modulatory activities.

Vein-positioning in the Drosophila wing in response to Hh; new roles of Notch signaling

The Drosophila wing is a classical model for studying the generation of developmental patterns. Previous studies have suggested that vein primordia form at boundaries between discrete sectors of gene expression along the antero-posterior (A/P) axis in the larval wing imaginal disc. Observation that the vein marker rhomboid (rho) is expressed at the centre of wider vein-competent domains led to propose that narrow vein primordia form first, and produce secondary short-range signals activating provein genes in neighbouring cells (see Curr. Opin. Genet. Dev. 10 (2000) 393). Here, we examined how the central L3 and L4 veins are positioned relative to the limits of expression of Collier (Col), a dose-dependent Hedgehog (Hh) target activated in the wing A/P organiser. We found that rho expression is first activated in broad domains adjacent to Col-expressing cells and secondarily restricted to the centre of these domains. This restriction which depends upon Notch (N) signaling sets the L3 and L4 vein primordia off the boundaries of Col expression. N activity is also required to fix the anterior limit of Col expression by locally antagonising Hh activation, thus precisely positioning the L3 vein primordium relative to the A/P compartment boundary. Experiments using Nts mutants further indicated that these two activities of N could be temporally uncoupled. Together, these observations highlight new roles of N in topologically linking the position of veins to prepattern gene expression.

A screen for dominant mutations applied to components in the Drosophila EGF-R pathway

The Drosophila epidermal growth factor receptor (EGF-R) controls many critical cell fate choices throughout development. Several proteins collaborate to promote localized EGF-R activation, such as Star and Rhomboid (Rho), which act sequentially to ensure the maturation and processing of inactive membrane-bound EGF ligands. To gain insights into the mechanisms underlying Rho and Star function, we developed a mutagenesis scheme to isolate novel overexpression activity (NOVA) alleles. In the case of rho, we isolated a dominant neomorphic allele, which interferes with Notch signaling, as well as a dominant-negative allele, which produces RNA interference-like flip-back transcripts that reduce endogenous rho expression. We also obtained dominant-negative and neomorphic Star mutations, which have phenotypes similar to those of rho NOVA alleles, as well as dominant-negative Egf-r alleles. The isolation of dominant alleles in several different genes suggests that NOVA mutagenesis should be widely applicable and emerge as an effective tool for generating dominant mutations in genes of unknown function.

Intracellular trafficking by Star regulates cleavage of the Drosophila EGF receptor ligand Spitz

Spitz (Spi) is a TGFalpha homolog that is a cardinal ligand for the Drosophila EGF receptor throughout development. Cleavage of the ubiquitously expressed transmembrane form of Spi (mSpi) precedes EGF receptor activation. We show that the Star and Rhomboid (Rho) proteins are necessary for Spi cleavage in Drosophila cells. Complexes between the Spi and Star proteins, as well as between the Star and Rho proteins were identified, but no Spi-Star-Rho triple complex was detected. This observation suggests a sequential activity of Star and Rho in mSpi processing. The interactions between Spi and Star regulate the intracellular trafficking of Spi. The Spi precursor is retained in the periphery of the nucleus. Coexpression of Star promotes translocation of Spi to a compartment where Rho is present both in cells and in embryos. A Star deletion construct that maintains binding to Spi and Rho, but is unable to facilitate Spi translocation, lost biological activity. These results underscore the importance of regulated intracellular trafficking in processing of a TGFalpha family ligand.

Regulation of Drosophila wing vein patterning: net encodes a bHLH protein repressing rhomboid and is repressed by rhomboid-dependent Egfr signalling

The stereotyped pattern of veins in the Drosophila wing is generated in response to local EGF signalling. Mutations in the rhomboid (rho) gene, which encodes a sevenpass membrane protein required to enhance signalling transmitted by the EGF receptor (Egfr), inhibit vein development and disrupt the vein pattern. By contrast, net mutations produce ectopic veins in intervein regions. We have cloned the net gene and show that it encodes a basic HLH protein that probably acts as a transcriptional repressor. net and rho are expressed in mutually exclusive patterns during the development of the wing imaginal disc. Lack of net activity causes rho expression to expand, and vice versa. Furthermore, ectopic expression of net or rho results in their mutual repression and thus suppresses vein formation or generates tube-like wings composed of vein-like tissue. Egfr signalling and net exert mutually antagonising activities during the specification of vein versus intervein fate. While Egfr signalling represses net transcription, net exhibits a two-tiered control by repressing rho transcription and interfering with Egfr signalling downstream of Rho. Our results further suggest that net is required to maintain intervein development by restricting Egfr signalling, which promotes vein development, to the Net-free vein regions of the wing disc.

Drawing lines in the Drosophila wing: initiation of wing vein development

It has been proposed that wing veins in Drosophila form at boundaries between discrete sectors of cells that subdivide the anterior-posterior axis of the developing wing primordium. Recently, analysis of events underlying initiation of vein formation suggests that there is a general developmental mechanism for drawing lines between adjacent domains of cells, which is referred to as 'for-export-only-signaling'. In this model, cells in one domain produce a short range signal to which they cannot respond. As a consequence of this constraint, cells lying in a narrow line immediately outside the signal-producing domain are the only cells that can respond to the signal by activating expression of vein-promoting genes.

rhomboid and Star interact synergistically to promote EGFR/MAPK signaling during Drosophila wing vein development

Genes of the ventrolateral group in Drosophila are dedicated to developmental regulation of Egfr signaling in multiple processes including wing vein development. Among these genes, Egfr encodes the Drosophila EGF-Receptor, spitz (spi) and vein (vn) encode EGF-related ligands, and rhomboid (rho) and Star (S) encode membrane proteins. In this study, we show that rho-mediated hyperactivation of the EGFR/MAPK pathway is required for vein formation throughout late larval and early pupal development. Consistent with this observation, rho activity is necessary and sufficient to activate MAPK in vein primordium during late larval and early pupal stages. Epistasis studies using a dominant negative version of Egfr and a ligand-independent activated form of Egfr suggest that rho acts upstream of the receptor. We show that rho and S function in a common aspect of vein development since loss-of-function clones of rho or S result in nearly identical non-autonomous loss-of-vein phenotypes. Furthermore, mis-expression of rho and S in wild-type and mutant backgrounds reveals that these genes function in a synergistic and co-dependent manner. In contrast, spi does not play an essential role in the wing. These data indicate that rho and S act in concert, but independently of spi, to promote vein development through the EGFR/MAPK signaling pathway.

Boundaries in the Drosophila wing imaginal disc organize vein-specific genetic programs

Previous studies have suggested that vein primordia in Drosophila form at boundaries along the A/P axis between discrete sectors of the larval wing imaginal disc. Genes involved in initiating vein development during the third larval instar are expressed either in narrow stripes corresponding to vein primordia or in broader ‘provein’ domains consisting of cells competent to become veins. In addition, genes specifying the alternative intervein cell fate are expressed in complementary intervein regions. The regulatory relationships between genes expressed in narrow vein primordia, in broad provein stripes and in interveins remains unknown, however. In this manuscript, we provide additional evidence for veins forming in narrow stripes at borders of A/P sectors. These experiments further suggest that narrow vein primordia produce secondary short-range signal(s), which activate expression of provein genes in a broad pattern in neighboring cells. We also show that crossregulatory interactions among genes expressed in veins, proveins and interveins contribute to establishing the vein-versus-intervein pattern, and that control of gene expression in vein and intervein regions must be considered on a stripe-by-stripe basis. Finally, we present evidence for a second set of vein-inducing boundaries lying between veins, which we refer to as paravein boundaries. We propose that veins develop at both vein and paravein boundaries in more ‘primitive’ insects, which have up to twice the number of veins present in Drosophila. We present a model in which different A/P boundaries organize vein-specific genetic programs to govern the development of individual veins.

The knirps and knirps-related genes organize development of the second wing vein in Drosophila

The neighboring homologous knirps (kni) and knirps-related (knrl) genes in Drosophila encode transcription factors in the steroid hormone receptor superfamily. During early embryogenesis, kni functions as a gap gene to control expression of segmentation genes within the abdominal region of the embryo. In this study, we present evidence that kni and knrl link A/P positional information in larval wing imaginal discs to morphogenesis of the second longitudinal wing vein (L2). We show that kni and knrl are expressed in similar narrow stripes corresponding to the position of the L2 primordium. The kni and knrl L2 stripes abut the anterior border of the broad central expression domain of the Dpp target gene spalt major (salm). We provide evidence that radius incompletus (ri), a well-known viable mutant lacking the L2 vein, is a regulatory mutant of the kni/knrl locus. In ri mutant wing discs, kni and knrl fail to be expressed in the L2 primordium. In addition, the positions of molecular breakpoints in the kni/knrl locus indicate that the ri function is provided by cis-acting sequences upstream of the kni transcription unit. Epistasis tests reveal that the kni/knrl locus functions downstream of spalt major (salm) and upstream of genes required to initiate vein-versus-intervein differentiation. Mis-expression experiments suggest that kni and knrl expressing cells inhibit neighboring cells from becoming vein cells. Finally, kni and knrl are likely to refine the L2 position by positively auto-regulating their own expression and by providing negative feedback to repress salm expression. We propose a model in which the combined activities of kni and knrl organize development of the L2 vein in the appropriate position.

An autoregulatory cascade of EGF receptor signaling patterns the Drosophila egg

Intercellular signaling through the EGF receptor (EGFR) patterns the Drosophila egg. The TGF alpha-like ligand Gurken signals from the oocyte to the receptor in the overlying somatic follicle cells. We show that in the dorsal follicle cells this initial paracrine signaling event triggers an autocrine amplification by two other EGFR ligands, Spitz and Vein. Spitz only becomes an effective ligand in the presence of the multitransmembrane domain protein Rhomboid. Consequent high-level EGFR activation leads to localized expression of the diffusible inhibitor Argos, which alters the profile of signaling. This sequential activation, amplification, and local inhibition of the EGFR forms an autoregulatory cascade that leads to the splitting of an initial single peak of signaling into two, thereby patterning the egg.

Signalling by the Drosophila epidermal growth factor receptor is required for the specification and diversification of embryonic muscle progenitors

Muscle development initiates in the Drosophila embryo with the segregation of single progenitor cells, from which a complete set of myofibres arises. Each progenitor is assigned a unique fate, characterized by the expression of particular identity genes. We now demonstrate that the Drosophila epidermal growth factor receptor provides an inductive signal for the specification of a large subset of muscle progenitors. In the absence of the receptor or its ligand, SPITZ, specific progenitors fail to segregate. The resulting unspecified mesodermal cells undergo programmed cell death. In contrast, receptor hyperactivation generates supernumerary progenitors, as well as the duplication of at least one SPITZ-dependent myofibre. The development of individual muscles is differentially sensitive to variations in the level of signalling by the epidermal growth factor receptor. Such graded myogenic effects can be influenced by alterations in the functions of Star and rhomboid. In addition, muscle patterning is dependent on the generation of a spatially restricted, activating SPITZ signal, a process that may rely on the localized mesodermal expression of RHOMBOID. Thus, the epidermal growth factor receptor contributes both to muscle progenitor specification and to the diversification of muscle identities.

Localized activation of RTK/MAPK pathways during Drosophila development

Receptor tyrosine kinase (RTK) signaling is mediated by a signaling cascade culminating in activation of mitogen-activated protein kinase (MAPK) by double phosphorylation on threonine and tyrosine residues. The pattern of MAPK activation can now be directly visualized in situ during embryonic and adult development using an antiserum is specific for the double phosphorylated form of MAPK (db-P MAPK). The pattern of MAPK activation detected by this antiserum in developing embryos and larval imaginal discs conforms remarkably well to the inferred pattern of known RTK function. In addition, db-P MAPK staining directly reveals features of signaling such as the range of signal spreading and the kinetics of RTK activation, which would be difficult to measure by other methods. The ability to visualize the output of RTK signaling also permits detailed establishment of epistatic relationships between signaling components of RTK cascades.

Sequential activation of the EGF receptor pathway during Drosophila oogenesis establishes the dorsoventral axis

Previous work has demonstrated a role for the Drosophila EGF receptor (Torpedo/DER) and its ligand, Gurken, in the determination of anterioposterior and dorsoventral axes of the follicle cells and oocyte. The roles of DER in establishing the polarity of the follicle cells were examined further, by following the expression of DER-target genes. One class of genes (e.g. kekon) is induced by the DER pathway at all stages. Broad expression of kekon at the stage in which the follicle cells migrate posteriorly over the oocyte, demonstrates the capacity of the pathway to pattern all follicle cells except the ventral-most rows. This may provide the spatial coordinates for the ventral-most follicle cell fates. A second group of target genes (e.g. rhomboid (rho)) is induced only at later stages of oogenesis, and may require additional inputs by signals emanating from the anterior, stretch follicle cells. The function of Rho was analyzed by ectopic expression in the stretch follicle cells, and shown to induce a non-autonomous dorsalizing activity that is independent of Gurken. Rho thus appears to be involved in processing a DER ligand in the follicle cells, to pattern the egg chamber and allow persistent activation of the DER pathway during formation of the dorsal appendages.

Interactions between the EGF receptor and DPP pathways establish distinct cell fates in the tracheal placodes

The formation of the tracheal network in Drosophila is driven by stereotyped migration of cells from the tracheal pits. No cell divisions take place during tracheal migration and the number of cells in each branch is fixed. This work examines the basis for the determination of tracheal branch fates, prior to the onset of migration. We show that the EGF receptor pathway is activated by localized processing of the ligand SPITZ in the tracheal placodes and is responsible for the capacity to form the dorsal trunk and visceral branch. The DPP pathway, on the contrary, is induced in the tracheal pit by local presentation of DPP from the adjacent dorsal and ventral ectodermal cells. This pathway patterns the dorsal and lateral branches. Elimination of both pathways blocks migration of all tracheal branches. Antagonistic interactions between the two pathways are demonstrated. The opposing activities of two pathways may refine the final determination of tracheal branch fates.

In situ activation pattern of Drosophila EGF receptor pathway during development

Signaling cascades triggered by receptor tyrosine kinases (RTKs) participate in diverse developmental processes. The active state of these signaling pathways was monitored by examination of the in situ distribution of the active, dual phosphorylated form of mitogen-activated protein kinase (ERK) with a specific monoclonal antibody. Detection of the active state of the Drosophila epidermal growth factor receptor (DER) pathway allowed the visualization of gradients and boundaries of receptor activation, assessment of the distribution of activating ligands, and analysis of interplay with the inhibitory ligand Argos. This in situ approach can be used to monitor other receptor-triggered pathways in a wide range of organisms.

Notch signalling regulates veinlet expression and establishes boundaries between veins and interveins in the Drosophila wing

The veins in the Drosophila wing have a characteristic width, which is regulated by the activity of the Notch pathway. The expression of the Notch-ligand Delta is restricted to the developing veins, and coincides with places where Notch transcription is lower. We find that this asymmetrical distribution of ligand and receptor leads to activation of Notch on both sides of each vein within a territory of Delta-expressing cells, and to the establishment of boundary cells that separate the vein from adjacent interveins. In these cells, the expression of the Enhancer of split gene m beta is activated and the transcription of the vein-promoting gene veinlet is repressed, thus restricting vein differentiation. We propose that the establishment of vein thickness utilises a combination of mechanisms that include: (1) independent regulation of Notch and Delta expression in intervein and vein territories, (2) Notch activation by Delta in cells where Notch and Delta expression overlaps, (3) positive feedback on Notch transcription in cells where Notch has been activated and (4) repression of veinlet transcription by E(spl)m beta and maintenance of Delta expression by veinlet/torpedo activity.

The spalt gene links the A/P compartment boundary to a linear adult structure in the Drosophila wing

During Drosophila embryogenesis, each segment is subdivided into an anterior and a posterior compartment through the action of the engrailed gene. Compartmental boundaries bisect imaginal disc primordia which give rise to adult appendages. In early larval development, a short-range Hedgehog signal originating from the posterior compartment of the imaginal wing disc activates expression of genes including decapentaplegic (dpp) in a stripe running along the anterior-posterior compartment boundary. Secreted Dpp emanating from the A/P boundary of wing discs then acts as a secondary signal to organize the wing over large distances. The transcription factor encoded by spalt major (salm) gene, which is expressed in a broad wedge centered over the dpp stripe, is one target of Dpp signaling. In this manuscript, we show that the anterior edge of the salm expression domain abuts a narrow stripe of rhomboid (rho)-expressing cells corresponding to the L2 longitudinal vein primordium. hh mis-expression along the anterior wing margin induces a surrounding domain of salm expression, the anterior edge of which abuts a displaced rho L2 stripe. salm plays a key role in defining the position of the L2 vein since loss of salm function in mosaic patches induces the formation of ectopic L2 branches, which comprise salm- cells running along clone borders where salm- cells confront salm+ cells. These data suggest that salm determines the position of the L2 vein primordium by activating rho expression in neighboring cells through a locally non-autonomous mechanism. rho then functions to initiate and maintain vein differentiation. We discuss how these data provide the final link connecting the formation of a linear adult structure to the establishment of a boundary by the maternal Bicoid morphogen gradient in the blastoderm embryo.

The Drosophila decapentaplegic and short gastrulation genes function antagonistically during adult wing vein development

TGF-beta-related signaling pathways play diverse roles during vertebrate and invertebrate development. A common mechanism for regulating the activity of TGF-beta family members is inhibition by extracellular antagonists. Recently, the Drosophila short gastrulation (sog) gene was shown to encode a predicted diffusible factor which antagonizes signaling mediated by the TGF-beta-like Decapentaplegic (Dpp) pathway in the early blastoderm embryo. sog and dpp, which are among the earliest zygotic genes to be activated, are expressed in complementary dorsal-ventral domains. The opposing actions of sog and dpp in the early embryo have been highly conserved during evolution as their vertebrate counterparts, chordin and BMP-4, function homologously to define neural versus non-neural ectoderm in Xenopus. Here we exploit the genetically sensitive adult wing vein pattern to investigate the generality of the antagonistic relationship between sog and dpp. We show that dpp is expressed in vein primordia during pupal wing development and functions to promote vein formation. In contrast, sog is expressed in complementary intervein cells and suppresses vein formation. sog and dpp function during the same phenocritical periods (i.e. 16–28 hours after pupariation) to influence the vein versus intervein cell fate choice. The conflicting activities of dpp and sog are also revealed by antagonistic dosage-sensitive interactions between these two genes during vein development. Analysis of vein and intervein marker expression in dpp and sog mutant wings suggests that dpp promotes vein fates indirectly by activating the vein gene rhomboid (rho), and that sog functions by blocking an autoactivating Dpp feedback loop. These data support the view that Sog is a dedicated Dpp antagonist.

Alternate functions of the single-minded and rhomboid genes in development of the Drosophila ventral neuroectoderm

We have investigated the roles of the single-minded (sim) and rhomboid (rho) genes in generating distinct cell fates in the Drosophila embryonic neuroectoderm. We show that sim functions to repress ventral ectodermal cell fates, as in sim mutants mesectodermal cells adopt the fates of neighboring ventral ectodermal cells and targeted sim expression in P[paired.Gal4]/P[UAS-sim] embryos results in loss of epidermal cells. We also find that rho is not required for early expression of sim or ventral nervous system defective in mesectodermal or ventral ectodermal cells; targeted rho expression in P[paired-Gal4]/P[UAS-rho] embryos results in lateral-to-ventral cell fate shifts in the developing neuroectoderm; and midline-targeted rho expression can rescue the medial denticle fusions in rho mutant cuticles.

MADR1, a MAD-related protein that functions in BMP2 signaling pathways

Components of the signaling pathways that lie downstream of Ser/Thr kinase receptors and are required for signaling by the TGF beta superfamily have been poorly defined. The Drosophila gene Mothers against dpp (MAD) and the C. elegans sma genes are implicated in these signaling pathways. We show that MAD functions downstream of DPP receptors and is required for receptor signaling. Phosphorylation of MADR1, a human homolog of MAD, is tightly regulated and rapidly induced by BMP2, but not TGF beta or activin. This phosphorylation is necessary for function, since a point mutant that yields a null phenotype in Drosophila is not phosphorylated. BMP2 treatment results in accumulation of MADR1 in the nucleus. MAD proteins may thus define a novel class of signaling molecules with nuclear function in Ser/Thr kinase receptor signaling pathways.

Multiple signaling pathways establish both the individuation and the polarity of the oocyte follicle in Drosophila

The development of the Drosophila oocyte depends upon a sequential series of interactions between the germline cells and the somatically derived follicle cells to produce individual follicles with appropriate polarities. In the germarium the control of germline cell division depends upon a proper interaction with somatic cells adjacent to the germline stem cells. Both gurken and brainiac are required in the germline, and the Egfr, daughterless, Notch, and Delta genes are required in the somatic cells to produce individual egg chambers with a continuous follicular epithelium. After a follicle forms, components in these same signaling pathways, plus additional genes, are then required for the establishment of the anterior-posterior polarity, followed by the dorsal-ventral polarity of the developing follicle. Initially, gurken mRNA is localized to the posterior edge of the oocyte, where it signals the posterior polar follicle cells to differentiate as posterior. The anterior-posterior assymmetry of the oocyte is then established by a reorganization of the microtubule network, which require a Notch-Delta-dependent signal sent from the posterior polar follicle cells to the oocyte and the activity of protein kinase A in the germ line. This reorganization leads to the localization of the maternal anterior-posterior determinants bicoid and oskar to opposite poles of the oocyte and the repositioning of the oocyte nucleus to the anterior-dorsal surface of the oocyte, gurken mRNA and protein are now concentrated between the oocyte nucleus and the adjacent anterior-dorsal follicle cells, where, in combination with Rhomboid, it locally activates the EGF receptor and its downstream cascade to direct the adjoining cells to adopt a dorsal fate. This process is thought to restrict the action of three follicle cell gene functions, encoded by windbeutel, nudal, and, pipe, to the ventral follicle cells, where they lead to the localized activation of a serine protease cascade required to produce the active Spätzle ligand to activate the Toll receptor. Finally, the termini of the embryo are dependent upon the activation of the Torso receptor, and this requires the localized expression of torso-like in a subset of follicle cells at the anterior and posterior poles of the follicle, which leads to the activation of Trunk, the putative ligand for Torso. In summary, the normal development of the oocyte requires a continuous sequence of germline-follicle cell interactions to provide the polarities responsible for normal development.

Secreted Spitz triggers the DER signaling pathway and is a limiting component in embryonic ventral ectoderm determination

The spitz gene encoding a TGF-alpha homolog, has been shown to affect a subset of developmental processes that are similar to those regulated by DER, the Drosophila EGF receptor homolog. This work demonstrates that Spitz triggers the DER signaling cascade. Addition of a secreted, but not the membrane-associated form of Spitz to S2 Drosophila cells expressing DER gives rise to a rapid tyrosine autophosphorylation of DER. Following autophosphorylation, DER associates with the Drk adapter protein. Consequently, activation of MAP kinase is observed. The profile of MAP kinase activation provides a quantitative assay for DER activation. A dose response between the levels of Spitz and MAP kinase activity was observed. The secreted Spitz protein was expressed in embryos to assess its biological activity. An alteration in cell fates was observed in the ventral ectoderm, such that lateral cells acquired the ventral-most fates. The result indicates that graded activation of the DER pathway may normally give rise to a repertoire of discrete cell fates in the ventral ectoderm. Spatially restricted processing of Spitz may be responsible for this graded activation. The Rhomboid (Rho) and Star proteins were suggested, on the basis of genetic interactions, to act as modulators of DER signaling. No alteration in DER autophosphorylation or the pattern of MAP kinase activation by secreted Spitz was observed when the Rho and Star proteins were coexpressed with DER in S2 cells. In embryos mutant for rho or Star the ventralizing effect of secreted Spitz is epistatic, suggesting that Rho and Star may normally facilitate processing of the Spitz precursor.

Analysis of the genetic hierarchy guiding wing vein development in Drosophila

The Drosophila rhomboid (rho) and Egf-r genes are members of a small group of genes required for the differentiation of various specific embryonic and adult structures. During larval and early pupal development expression of rho in longitudinal vein primordia mediates the localized formation of wing veins. In this paper we investigate the genetic hierarchy guiding vein development, by testing for genetic interactions between rho alleles and a wide variety of wing vein mutations and by examining the pattern of rho expression in mutant developing wing primordia. We identify a small group of wing vein mutants that interact strongly with rho. Examination of rho expression in these and other key vein mutants reveals when vein development first becomes abnormal. Based on these data and on previous genetic analyses of vein formation we present a sequential model for establishment and differentiation of wing veins.