bullwinkle is required for epithelial morphogenesis during Drosophila oogenesis

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Imaginal disk growth factors are Drosophila chitinase-like proteins with roles in morphogenesis and CO2 response

Chitinase-like proteins (CLPs) are members of the family 18 glycosyl hydrolases, which include chitinases and the enzymatically inactive CLPs. A mutation in the enzyme's catalytic site, conserved in vertebrates and invertebrates, allowed CLPs to evolve independently with functions that do not require chitinase activity. CLPs normally function during inflammatory responses, wound healing, and host defense, but when they persist at excessive levels at sites of chronic inflammation and in tissue-remodeling disorders, they correlate positively with disease progression and poor prognosis. Little is known, however, about their physiological function. Drosophila melanogaster has 6 CLPs, termed Imaginal disk growth factors (Idgfs), encoded by Idgf1, Idgf2, Idgf3, Idgf4, Idgf5, and Idgf6. In this study, we developed tools to facilitate characterization of the physiological roles of the Idgfs by deleting each of the Idgf genes using the CRISPR/Cas9 system and assessing loss-of-function phenotypes. Using null lines, we showed that loss of function for all 6 Idgf proteins significantly lowers viability and fertility. We also showed that Idgfs play roles in epithelial morphogenesis, maintaining proper epithelial architecture and cell shape, regulating E-cadherin and cortical actin, and remarkably, protecting these tissues against CO2 exposure. Defining the normal molecular mechanisms of CLPs is a key to understanding how deviations tip the balance from a physiological to a pathological state.

The Vast Utility of Drosophila Oogenesis

In this chapter, we highlight examples of the diverse array of developmental, cellular, and biochemical insights that can be gained by using Drosophila melanogaster oogenesis as a model tissue. We begin with an overview of ovary development and adult oogenesis. Then we summarize how the adult Drosophila ovary continues to advance our understanding of stem cells, cell cycle, cell migration, cytoplasmic streaming, nurse cell dumping, and cell death. We also review emerging areas of study, including the roles of lipid droplets, ribosomes, and nuclear actin in egg development. Finally, we conclude by discussing the growing conservation of processes and signaling pathways that regulate oogenesis and female reproduction from flies to humans.

The R-SNARE Ykt6 is required for multiple events during oogenesis in Drosophila

Ykt6 has emerged as a key protein involved in a wide array of trafficking events, and has also been implicated in a number of human pathologies, including the progression of several cancers. It is a complex protein that simultaneously exhibits a high degree of structural and functional homology, and yet adopts differing roles in different cellular contexts. Because Ykt6 has been implicated in a variety of vesicle fusion events, we characterized the role of Ykt6 in oogenesis by observing the phenotype of Ykt6 germline clones. Immunofluorescence was used to visualize the expression of membrane proteins, organelles, and vesicular trafficking markers in mutant egg chambers. We find that Ykt6 germline clones have morphological and actin defects affecting both the nurse cells and oocyte, consistent with a role in regulating membrane growth during mid-oogenesis. Additionally, these egg chambers exhibit defects in bicoid and oskar RNA localization, and in the trafficking of Gurken during mid-to-late oogenesis. Finally, we show that Ykt6 mutations result in defects in late endosomal pathways, including endo- and exocytosis. These findings suggest a role for Ykt6 in endosome maturation and in the movement of membranes to and from the cell surface.

Septate junction proteins are required for egg elongation and border cell migration during oogenesis in Drosophila

Protein components of the invertebrate occluding junction—known as the septate junction (SJ)—are required for morphogenetic developmental events during embryogenesis in Drosophila melanogaster. In order to determine whether SJ proteins are similarly required for morphogenesis during other developmental stages, we investigated the localization and requirement of four representative SJ proteins during oogenesis: Contactin, Macroglobulin complement-related, Neurexin IV, and Coracle. A number of morphogenetic processes occur during oogenesis, including egg elongation, formation of dorsal appendages, and border cell (BC) migration. We found that all four SJ proteins are expressed in egg chambers throughout oogenesis, with the highest and the most sustained levels in the follicular epithelium (FE). In the FE, SJ proteins localize along the lateral membrane during early and mid-oogenesis, but become enriched in an apical-lateral domain (the presumptive SJ) by stage 11. SJ protein relocalization requires the expression of other SJ proteins, as well as Rab5 and Rab11 like SJ biogenesis in the embryo. Knocking down the expression of these SJ proteins in follicle cells throughout oogenesis results in egg elongation defects and abnormal dorsal appendages. Similarly, reducing the expression of SJ genes in the BC cluster results in BC migration defects. Together, these results demonstrate an essential requirement for SJ genes in morphogenesis during oogenesis, and suggest that SJ proteins may have conserved functions in epithelial morphogenesis across developmental stages.

Detecting New Allies: Modifier Screen Identifies a Genetic Interaction Between Imaginal disc growth factor 3 and combover, a Rho-kinase Substrate, During Dorsal Appendage Tube Formation in Drosophila

Biological tube formation underlies organ development and, when disrupted, can cause severe birth defects. To investigate the genetic basis of tubulogenesis, we study the formation of Drosophila melanogaster eggshell structures, called dorsal appendages, which are produced by epithelial tubes. Previously we found that precise levels of Drosophila Chitinase-Like Proteins (CLPs), encoded by the Imaginal disc growth factor (Idgf) gene family, are needed to regulate dorsal-appendage tube closure and tube migration. To identify factors that act in the Idgf pathway, we developed a genetic modifier screen based on the finding that overexpressing Idgf3 causes dorsal appendage defects with ∼50% frequency. Using a library of partially overlapping heterozygous deficiencies, we scanned chromosome 3L and found regions that enhanced or suppressed the Idgf3-overexpression phenotype. Using smaller deletions, RNAi, and mutant alleles, we further mapped five regions and refined the interactions to 58 candidate genes. Importantly, mutant alleles identified combover (cmb), a substrate of Rho-kinase (Rok) and a component of the Planar Cell Polarity (PCP) pathway, as an Idgf3-interacting gene: loss of function enhanced while gain of function suppressed the dorsal appendage defects. Since PCP drives cell intercalation in other systems, we asked if cmb/+ affected cell intercalation in our model, but we found no evidence of its involvement in this step. Instead, we found that loss of cmb dominantly enhanced tube defects associated with Idgf3 overexpression by expanding the apical area of dorsal appendage cells. Apical surface area determines tube volume and shape; in this way, Idgf3 and cmb regulate tube morphology.

Two Drosophilids exhibit distinct EGF pathway patterns in oogenesis

Deciphering the evolution of morphological structures is a remaining challenge in the field of developmental biology. The respiratory structures of insect eggshells, called the dorsal appendages, provide an outstanding system for exploring these processes since considerable information is known about their patterning and morphogenesis in Drosophila melanogaster and dorsal appendage number and morphology vary widely across Drosophilid species. We investigated the patterning differences that might facilitate morphogenetic differences between D. melanogaster, which produces two oar-like structures first by wrapping and then elongating the tubes via cell intercalation and cell crawling, and Scaptodrosophila lebanonensis, which produces a variable number of appendages simply by cell intercalation and crawling. Analyses of BMP pathway components thickveins and P-Mad demonstrate that anterior patterning is conserved between these species. In contrast, EGF signaling exhibits significant differences. Transcripts for the ligand encoded by gurken localize similarly in the two species, but this morphogen creates a single dorsolateral primordium in S. lebanonensis as defined by activated MAP kinase and the downstream marker broad. Expression patterns of pointed, argos, and Capicua, early steps in the EGF pathway, exhibit a heterochronic shift in S. lebanonensis relative to those seen in D. melanogaster. We demonstrate that the S. lebanonensis Gurken homolog is active in D. melanogaster but is insufficient to alter downstream patterning responses, indicating that Gurken-EGF receptor interactions do not distinguish the two species' patterning. Altogether, these results differentiate EGF signaling patterns between species and shed light on how changes to the regulation of patterning genes may contribute to different tube-forming mechanisms.

Epithelial Patterning, Morphogenesis, and Evolution: Drosophila Eggshell as a Model

Understanding the mechanisms driving tissue and organ formation requires knowledge across scales. How do signaling pathways specify distinct tissue types? How does the patterning system control morphogenesis? How do these processes evolve? The Drosophila egg chamber, where EGF and BMP signaling intersect to specify unique cell types that construct epithelial tubes for specialized eggshell structures, has provided a tractable system to ask these questions. Work there has elucidated connections between scales of development, including across evolutionary scales, and fostered the development of quantitative modeling tools. These tools and general principles can be applied to the understanding of other developmental processes across organisms.

The repertoire of epithelial morphogenesis on display: Progressive elaboration of Drosophila egg structure

Epithelial structures are foundational for tissue organization in all metazoans. Sheets of epithelial cells form lateral adhesive junctions and acquire apico-basal polarity perpendicular to the surface of the sheet. Genetic analyses in the insect model, Drosophila melanogaster, have greatly advanced our understanding of how epithelial organization is established, and how it is modulated during tissue morphogenesis. Major insights into collective cell migrations have come from analyses of morphogenetic movements within the adult follicular epithelium that cooperates with female germ cells to build a mature egg. Epithelial follicle cells progress through tightly choreographed phases of proliferation, patterning, reorganization and migrations, before they differentiate to form the elaborate structures of the eggshell. Distinct structural domains are organized by differential adhesion, within which lateral junctions are remodeled to further shape the organized epithelia. During collective cell migrations, adhesive interactions mediate supracellular organization of planar polarized macromolecules, and facilitate crawling over the basement membrane or traction against adjacent cell surfaces. Comparative studies with other insects are revealing the diversification of morphogenetic movements for elaboration of epithelial structures. This review surveys the repertoire of follicle cell morphogenesis, to highlight the coordination of epithelial plasticity with progressive differentiation of a secretory epithelium. Technological advances will keep this tissue at the leading edge for interrogating the precise spatiotemporal regulation of normal epithelial reorganization events, and provide a framework for understanding pathological tissue dysplasia.

Proteomics Analysis Identifies Orthologs of Human Chitinase-Like Proteins as Inducers of Tube Morphogenesis Defects in Drosophila melanogaster

Elevated levels of human chitinase-like proteins (CLPs) are associated with numerous chronic inflammatory diseases and several cancers, often correlating with poor prognosis. Nevertheless, there is scant knowledge of their function. The CLPs normally mediate immune responses and wound healing and, when upregulated, they can promote disease progression by remodeling tissue, activating signaling cascades, stimulating proliferation and migration, and by regulating adhesion. We identified Imaginal disc growth factors (Idgfs), orthologs of human CLPs CHI3L1, CHI3L2, and OVGP1, in a proteomics analysis designed to discover factors that regulate tube morphogenesis in a Drosophila melanogaster model of tube formation. We implemented a novel approach that uses magnetic beads to isolate a small population of specialized ovarian cells, cells that nonautonomously regulate morphogenesis of epithelial tubes that form and secrete eggshell structures called dorsal appendages (DAs). Differential mass spectrometry analysis of these cells detected elevated levels of four of the six Idgf family members (Idgf1, Idgf2, Idgf4, and Idgf6) in flies mutant for bullwinkle (bwk), which encodes a transcription factor and is a known regulator of DA-tube morphogenesis. We show that, during oogenesis, dysregulation of Idgfs (either gain or loss of function) disrupts the formation of the DA tubes. Previous studies demonstrate roles for Drosophila Idgfs in innate immunity, wound healing, and cell proliferation and motility in cell culture. Here, we identify a novel role for Idgfs in both normal and aberrant tubulogenesis processes.

Targeted Downregulation of s36 Protein Unearths its Cardinal Role in Chorion Biogenesis and Architecture during Drosophila melanogaster Oogenesis

Drosophila chorion represents a model biological system for the in vivo study of gene activity, epithelial development, extracellular-matrix assembly and morphogenetic-patterning control. It is produced during the late stages of oogenesis by epithelial follicle cells and develops into a highly organized multi-layered structure that exhibits regional specialization and radial complexity. Among the six major proteins involved in chorion’s formation, the s36 and s38 ones are synthesized first and regulated in a cell type-specific and developmental stage-dependent manner. In our study, an RNAi-mediated silencing of s36 chorionic-gene expression specifically in the follicle-cell compartment of Drosophila ovary unearths the essential, and far from redundant, role of s36 protein in patterning establishment of chorion’s regional specialization and radial complexity. Without perturbing the developmental courses of follicle- and nurse-cell clusters, the absence of s36 not only promotes chorion’s fragility but also induces severe structural irregularities on chorion’s surface and entirely impairs fly’s fertility. Moreover, we herein unveil a novel function of s36 chorionic protein in the regulation of number and morphogenetic integrity of dorsal appendages in follicles sporadically undergoing aged fly-dependent stress.

Cultivation and Live Imaging of Drosophila Ovaries

Drosophila egg chamber development depends on a number of dynamic cellular processes that contribute to the final shape and function of the egg. We can gain insight into the mechanisms underlying these events by combining the power of Drosophila genetics and ex vivo live imaging. During developmental stages 1-8, egg chambers rotate around their anterior-posterior axes due to collective migration of the follicular epithelium. This motion is required for the proper elongation of the egg chamber. Here, we describe how to prepare stage 1-8 egg chambers for live imaging. We provide alternate protocols for the use of inverted or upright microscopes and describe ways to stabilize egg chambers to reduce drift during imaging. We discuss the advantages and limitations of these methods to assist the researcher in choosing an appropriate method based on experimental need and available resources.

In Vitro Culturing and Live Imaging of Drosophila Egg Chambers: A History and Adaptable Method

The development of the Drosophila egg chamber encompasses a myriad of diverse germline and somatic events, and as such, the egg chamber has become a widely used and influential developmental model. Advantages of this system include physical accessibility, genetic tractability, and amenability to microscopy and live culturing, the last of which is the focus of this chapter. To provide adequate context, we summarize the structure of the Drosophila ovary and egg chamber, the morphogenetic events of oogenesis, the history of egg-chamber live culturing, and many of the important discoveries that this culturing has afforded. Subsequently, we discuss various culturing methods that have facilitated analyses of different stages of egg-chamber development and different types of cells within the egg chamber, and we present an optimized protocol for live culturing Drosophila egg chambers.We designed this protocol for culturing late-stage Drosophila egg chambers and live imaging epithelial tube morphogenesis, but with appropriate modifications, it can be used to culture egg chambers of any stage. The protocol employs a liquid-permeable, weighted "blanket" to gently hold egg chambers against the coverslip in a glass-bottomed culture dish so the egg chambers can be imaged on an inverted microscope. This setup provides a more buffered, stable, culturing environment than previously published methods by using a larger volume of culture media, but the setup is also compatible with small volumes. This chapter should aid researchers in their efforts to culture and live-image Drosophila egg chambers, further augmenting the impressive power of this model system.

Dynamin-mediated endocytosis is required for tube closure, cell intercalation, and biased apical expansion during epithelial tubulogenesis in the Drosophila ovary

Most metazoans are able to grow beyond a few hundred cells and to support differentiated tissues because they elaborate multicellular, epithelial tubes that are indispensable for nutrient and gas exchange. To identify and characterize the cellular behaviors and molecular mechanisms required for the morphogenesis of epithelial tubes (i.e., tubulogenesis), we have turned to the D. melanogaster ovary. Here, epithelia surrounding the developing egg chambers first pattern, then form and extend a set of simple, paired, epithelial tubes, the dorsal appendage (DA) tubes, and they create these structures in the absence of cell division or cell death. This genetically tractable system lets us assess the relative contributions that coordinated changes in cell shape, adhesion, orientation, and migration make to basic epithelial tubulogenesis. We find that Dynamin, a conserved regulator of endocytosis and the cytoskeleton, serves a key role in DA tubulogenesis. We demonstrate that Dynamin is required for distinct aspects of DA tubulogenesis: DA-tube closure, DA-tube-cell intercalation, and biased apical-luminal cell expansion. We provide evidence that Dynamin promotes these processes by facilitating endocytosis of cell-cell and cell-matrix adhesion complexes, and we find that precise levels and sub-cellular distribution of E-Cadherin and specific Integrin subunits impact DA tubulogenesis. Thus, our studies identify novel morphogenetic roles (i.e., tube closure and biased apical expansion), and expand upon established roles (i.e., cell intercalation and adhesion remodeling), for Dynamin in tubulogenesis.

Diversity of epithelial morphogenesis during eggshell formation in drosophilids

The eggshells of drosophilid species provide a powerful model for studying the origins of morphological diversity. The dorsal appendages, or respiratory filaments, of these eggshells display a remarkable interspecies variation in number and shape, and the epithelial patterning underlying the formation of these structures is an area of active research. To extend the analysis of dorsal appendage formation to include morphogenesis, we developed an improved 3D image reconstruction approach. This approach revealed considerable interspecies variation in the cell shape changes and neighbor exchanges underlying appendage formation. Specifically, although the appendage floor in Drosophila melanogaster is formed through spatially ordered neighbor exchanges, the same structure in Scaptodrosophila pattersoni is formed through extreme changes in cell shape, whereas Drosophila funebris appears to display a combination of both cellular mechanisms. Furthermore, localization patterns of Par3/Bazooka suggest a self-organized, cell polarity-based origin for the variability of appendage number in S. pattersoni. Our results suggest that species deploy different combinations of apically and basally driven mechanisms to convert a two-dimensional primordium into a three-dimensional structure, and provide new directions for exploring the molecular origins of interspecies morphological variation.

Culturing Drosophila Egg Chambers and Investigating Developmental Processes Through Live Imaging

Drosophila oogenesis provides many examples of essential processes in development. A myriad of genetic tools combined with recent advances in culturing egg chambers ex vivo has revealed several surprising mechanisms that govern how this tissue develops, and which could not have been determined in fixed tissues. Here we describe a straightforward protocol for dissecting ovaries, culturing egg chambers, and observing egg development in real time by fluorescent microscopy. This technique is suitable for observation of early- or late-stage egg development, and can be adapted to study a variety of cellular, molecular, or developmental processes. Ongoing analysis of oogenesis in living egg chambers has tremendous potential for discovery of new developmental mechanisms.

A discrete model of Drosophila eggshell patterning reveals cell-autonomous and juxtacrine effects

The Drosophila eggshell constitutes a remarkable system for the study of epithelial patterning, both experimentally and through computational modeling. Dorsal eggshell appendages arise from specific regions in the anterior follicular epithelium that covers the oocyte: two groups of cells expressing broad (roof cells) bordered by rhomboid expressing cells (floor cells). Despite the large number of genes known to participate in defining these domains and the important modeling efforts put into this developmental system, key patterning events still lack a proper mechanistic understanding and/or genetic basis, and the literature appears to conflict on some crucial points. We tackle these issues with an original, discrete framework that considers single-cell models that are integrated to construct epithelial models. We first build a phenomenological model that reproduces wild type follicular epithelial patterns, confirming EGF and BMP signaling input as sufficient to establish the major features of this patterning system within the anterior domain. Importantly, this simple model predicts an instructive juxtacrine signal linking the roof and floor domains. To explore this prediction, we define a mechanistic model that integrates the combined effects of cellular genetic networks, cell communication and network adjustment through developmental events. Moreover, we focus on the anterior competence region, and postulate that early BMP signaling participates with early EGF signaling in its specification. This model accurately simulates wild type pattern formation and is able to reproduce, with unprecedented level of precision and completeness, various published gain-of-function and loss-of-function experiments, including perturbations of the BMP pathway previously seen as conflicting results. The result is a coherent model built upon rules that may be generalized to other epithelia and developmental systems.

Phantom, a cytochrome P450 enzyme essential for ecdysone biosynthesis, plays a critical role in the control of border cell migration in Drosophila

The border cells of Drosophila are a model system for coordinated cell migration. Ecdysone signaling has been shown to act as the timing signal to initiate the migration process. Here we find that mutations in phantom (phm), encoding an enzyme in the ecdysone biosynthesis pathway, block border cell migration when the entire follicular epithelium of an egg chamber is mutant, even when the associated germline cells (nurse cells and oocyte) are wild-type. Conversely, mutant germline cells survive and do not affect border cell migration, as long as the surrounding follicle cells are wild-type. Interestingly, even small patches of wild-type follicle cells in a mosaic epithelium are sufficient to allow the production of above-threshold levels of ecdysone to promote border cell migration. The same phenotype is observed with mutations in shade (shd), encoding the last enzyme in the pathway that converts ecdysone to the active 20-hydroxyecdysone. Administration of high 20-hydroxyecdysone titers in the medium can also rescue the border cell migration phenotype in cultured egg chambers with an entirely phm mutant follicular epithelium. These results indicate that in normal oogenesis, the follicle cell epithelium of each individual egg chamber must supply sufficient ecdysone precursors, leading ultimately to high enough levels of mature 20-hydroxyecdysone to the border cells to initiate their migration. Neither the germline, nor the neighboring egg chambers, nor the surrounding hemolymph appear to provide threshold amounts of 20-hydroxyecdysone to do so. This "egg chamber autonomous" ecdysone synthesis constitutes a useful way to regulate the individual maturation of the asynchronous egg chambers present in the Drosophila ovary.

The utility of stage-specific mid-to-late Drosophila follicle isolation

Drosophila oogenesis or follicle development has been widely used to advance the understanding of complex developmental and cell biologic processes. This methods paper describes how to isolate mid-to-late stage follicles (Stage 10B-14) and utilize them to provide new insights into the molecular and morphologic events occurring during tight windows of developmental time. Isolated follicles can be used for a variety of experimental techniques, including in vitro development assays, live imaging, mRNA expression analysis and western blot analysis of proteins. Follicles at Stage 10B (S10B) or later will complete development in culture; this allows one to combine genetic or pharmacologic perturbations with in vitro development to define the effects of such manipulations on the processes occurring during specific periods of development. Additionally, because these follicles develop in culture, they are ideally suited for live imaging studies, which often reveal new mechanisms that mediate morphological events. Isolated follicles can also be used for molecular analyses. For example, changes in gene expression that result from genetic perturbations can be defined for specific developmental windows. Additionally, protein level, stability, and/or posttranslational modification state during a particular stage of follicle development can be examined through western blot analyses. Thus, stage-specific isolation of Drosophila follicles provides a rich source of information into widely conserved processes of development and morphogenesis.

Chorion patterning: a window into gene regulation and Drosophila species' relatedness

Changes in gene regulation are associated with the evolution of morphologies. However, the specific sequence information controlling gene expression is largely unknown and discovery is time and labor consuming. We use the intricate patterning of follicle cells to probe species' relatedness in the absence of sequence information. We focus on one of the major families of genes that pattern the Drosophila eggshell, the Chorion protein (Cp). Systematically screening for the spatiotemporal patterning of all nine Cp genes in three species (Drosophila melanogaster, D. nebulosa, and D. willistoni), we found that most genes are expressed dynamically during mid and late stages of oogenesis. Applying an annotation code, we transformed the data into binary matrices that capture the complexity of gene expression. Gene patterning is sufficient to predict species' relatedness, consistent with their phylogeny. Surprisingly, we found that expression domains of most genes are different among species, suggesting that Cp regulation is rapidly evolving. In addition, we found a morphological novelty along the dorsalmost side of the eggshell, the dorsal ridge. Our matrix analysis placed the dorsal ridge domain in a cluster of epidermal growth factor receptor associated domains, which was validated through genetic and chemical perturbations. Expression domains are regulated cooperatively or independently by signaling pathways, supporting that complex patterns are combinatorially assembled from simple domains.

Response to the dorsal anterior gradient of EGFR signaling in Drosophila oogenesis is prepatterned by earlier posterior EGFR activation

Spatially restricted epidermal growth factor receptor (EGFR) activity plays a central role in patterning the follicular epithelium of the Drosophila ovary. In midoogenesis, localized EGFR activation is achieved by the graded dorsal anterior localization of its ligand, Gurken. Graded EGFR activity determines multiple dorsal anterior fates along the dorsal-ventral axis but cannot explain the sharp posterior limit of this domain. Here, we show that posterior follicle cells express the T-box transcription factors Midline and H15, which render cells unable to adopt a dorsal anterior fate in response to EGFR activation. The posterior expression of Midline and H15 is itself induced in early oogenesis by posteriorly localized EGFR signaling, defining a feedback loop in which early induction of Mid and H15 confers a molecular memory that fundamentally alters the outcome of later EGFR signaling. Spatial regulation of the EGFR pathway thus occurs both through localization of the ligand and through localized regulation of the cellular response.

A remarkable career in science-Joseph G. Gall

A festive group of ∼150 current and former students, postdoctoral and other associates, and colleagues gathered during the weekend of April 12-14, 2013 to celebrate Joe Gall's 85th birthday. The gathering, hosted by the Carnegie Institution for Science, Department of Embryology (Allan Spradling, Director) and organized by a group of Joe's current and former students (Zehra Nizami, Alison Singer, Ji-Long Liu, Virginia Zakian, Susan Gerbi), was held in Baltimore, MD. Dinners and symposia extending over 3 days celebrated Joe's scientific findings over the years, together with those of his former students, postdoctoral fellows, and other associates (see program at https://sites.google.com/site/gallsymposium2013/ ).

Three-dimensional epithelial morphogenesis in the developing Drosophila egg

Morphogenesis of the respiratory appendages on eggshells of Drosophila species provides a powerful experimental system for studying how cell sheets give rise to complex three-dimensional structures. In Drosophila melanogaster, each of the two tubular eggshell appendages is derived from a primordium comprising two distinct cell types. Using live imaging and three-dimensional image reconstruction, we demonstrate that the transformation of this two-dimensional primordium into a tube involves out-of-plane bending followed by a sequence of spatially ordered cell intercalations. These morphological transformations correlate with the appearance of complementary distributions of myosin and Bazooka in the primordium. These distributions suggest that a two-dimensional pattern of line tensions along cell-cell edges on the apical side of the epithelium is sufficient to produce the observed changes in morphology. Computational modeling shows that this mechanism could explain the main features of tissue deformation and cell rearrangements observed during three-dimensional morphogenesis.

Following the 'tracks': Tramtrack69 regulates epithelial tube expansion in the Drosophila ovary through Paxillin, Dynamin, and the homeobox protein Mirror

Epithelial tubes are the infrastructure for organs and tissues, and tube morphogenesis requires precise orchestration of cell signaling, shape, migration, and adhesion. Follicle cells in the Drosophila ovary form a pair of epithelial tubes whose lumens act as molds for the eggshell respiratory filaments, or dorsal appendages (DAs). DA formation is a robust and accessible model for studying the patterning, formation, and expansion of epithelial tubes. Tramtrack69 (TTK69), a transcription factor that exhibits a variable embryonic DNA-binding preference, controls DA lumen volume and shape by promoting tube expansion; the tramtrack mutation twin peaks (ttk(twk)) reduces TTK69 levels late in oogenesis, inhibiting this expansion. Microarray analysis of wild-type and ttk(twk) ovaries, followed by in situ hybridization and RNAi of candidate genes, identified the Phospholipase B-like protein Lamina ancestor (LAMA), the scaffold protein Paxillin, the endocytotic regulator Shibire (Dynamin), and the homeodomain transcription factor Mirror, as TTK69 effectors of DA-tube expansion. These genes displayed enriched expression in DA-tube cells, except lama, which was expressed in all follicle cells. All four genes showed reduced expression in ttk(twk) mutants and exhibited RNAi phenotypes that were enhanced in a ttk(twk)/+ background, indicating ttk(twk) genetic interactions. Although previous studies show that Mirror patterns the follicular epithelium prior to DA tubulogenesis, we show that Mirror has an independent, novel role in tube expansion, involving positive regulation of Paxillin. Thus, characterization of ttk(twk)-differentially expressed genes expands the network of TTK69 effectors, identifies novel epithelial tube-expansion regulators, and significantly advances our understanding of this vital developmental process.

Co-option of a coordinate system defined by the EGFr and Dpp pathways in the evolution of a morphological novelty

Background

Morphological innovation is an elusive and fascinating concept in evolutionary biology. A novel structure may open up an array of possibilities for adaptation, and thus is fundamental to the evolution of complex multicellular life. We use the respiratory appendages on the dorsal-anterior side of the Drosophila eggshell as a model system for morphological novelty. To study the co-option of genetic pathways in the evolution of this novelty we have compared oogenesis and eggshell patterning in Drosophila melanogaster with Ceratitis capitata, a dipteran whose eggs do not bear dorsal appendages.

Results

During the final stages of oogenesis, the appendages are formed by specific groups of cells in the follicular epithelium of the egg chamber. These cells are defined via signaling activity of the Dpp and EGFr pathways, and we find that both pathways are active in C. capitata oogenesis. The transcription factor gene mirror is expressed downstream of EGFr activation in a dorsolateral domain in the D. melanogaster egg chamber, but could not be detected during C. capitata oogenesis. In D. melanogaster, mirror regulates the expression of two important genes: broad, which defines the appendage primordia, and pipe, involved in embryonic dorsoventral polarity. In C. capitata, broad remains expressed ubiquitously throughout the follicular epithelium, and is not restricted to the appendage primordia. Interestingly pipe expression did not differ between the two species.

Conclusions

Our analysis identifies both broad and mirror as important nodes that have been redeployed in the Drosophila egg chamber patterning network in the evolution of a morphologically novel feature. Further, our results show how pre-existing signals can provide an epithelium with a spatial coordinate system, which can be co-opted for novel patterns.

The Capicua repressor--a general sensor of RTK signaling in development and disease

Receptor tyrosine kinase (RTK) signaling pathways control multiple cellular decisions in metazoans, often by regulating the expression of downstream genes. In Drosophila melanogaster and other systems, E-twenty-six (ETS) transcription factors are considered to be the predominant nuclear effectors of RTK pathways. Here, we highlight recent progress in identifying the HMG-box protein Capicua (CIC) as a key sensor of RTK signaling in both Drosophila and mammals. Several studies have shown that CIC functions as a repressor of RTK-responsive genes, keeping them silent in the absence of signaling. Following the activation of RTK signaling, CIC repression is relieved, and this allows the expression of the targeted gene in response to local or ubiquitous activators. This regulatory switch is essential for several RTK responses in Drosophila, from the determination of cell fate to cell proliferation. Furthermore, increasing evidence supports the notion that this mechanism is conserved in mammals, where CIC has been implicated in cancer and neurodegeneration. In addition to summarizing our current knowledge on CIC, we also discuss the implications of these findings for our understanding of RTK signaling specificity in different biological processes.

Evolution of BMP signaling in Drosophila oogenesis: a receptor-based mechanism

The bone morphogenetic protein (BMP) signaling pathway is a conserved regulator of cellular and developmental processes in animals. The mechanisms underlying BMP signaling activation differ among tissues and mostly reflect changes in the expression of pathway components. BMP signaling is one of the major pathways responsible for the patterning of the Drosophila eggshell, a complex structure derived from a layer of follicle cells (FCs) surrounding the developing oocyte. Activation of BMP signaling in the FCs is dynamic. Initially, signaling is along the anterior-posterior (A/P) axis; later, signaling acquires dorsal-ventral (D/V) polarity. These dynamics are regulated by changes in the expression pattern of the type I BMP receptor thickveins (tkv). We recently found that signaling dynamics and TKV patterning are highly correlated in the FCs of multiple Drosophila species. In addition, we showed that signaling patterns are spatially different among species. Here, we use a mathematical model to simulate the dynamics and differences of BMP signaling in numerous species. This model predicts that qualitative and quantitative changes in receptor expression can lead to differences in the spatial pattern of BMP signaling. We tested these predications experimentally in three different Drosophila species and through genetic perturbations of BMP signaling in D. melanogaster. On the basis of our results, we concluded that changes in tkv patterning can account for the experimentally observed differences in the patterns of BMP signaling in multiple Drosophila species.

EGFR-dependent network interactions that pattern Drosophila eggshell appendages

Similar to other organisms, Drosophila uses its Epidermal Growth Factor Receptor (EGFR) multiple times throughout development. One crucial EGFR-dependent event is patterning of the follicular epithelium during oogenesis. In addition to providing inductive cues necessary for body axes specification, patterning of the follicle cells initiates the formation of two respiratory eggshell appendages. Each appendage is derived from a primordium comprising a patch of cells expressing broad (br) and an adjacent stripe of cells expressing rhomboid (rho). Several mechanisms of eggshell patterning have been proposed in the past, but none of them can explain the highly coordinated expression of br and rho. To address some of the outstanding issues in this system, we synthesized the existing information into a revised mathematical model of follicle cell patterning. Based on the computational model analysis, we propose that dorsal appendage primordia are established by sequential action of feed-forward loops and juxtacrine signals activated by the gradient of EGFR signaling. The model describes pattern formation in a large number of mutants and points to several unanswered questions related to the dynamic interaction of the EGFR and Notch pathways.

Unit operations of tissue development: epithelial folding

The development of multicellular organisms relies on a small set of construction techniques-assembly, sculpting, and folding-that are spatially and temporally regulated in a combinatorial manner to produce the diversity of tissues within the body. These basic processes are well conserved across tissue types and species at the level of both genes and mechanisms. Here we review the signaling, patterning, and biomechanical transformations that occur in two well-studied model systems of epithelial folding to illustrate both the complexity and modularity of tissue development. In particular, we discuss the possibility of a spatial code specifying morphogenesis. To decipher this code, engineers and scientists need to establish quantitative experimental systems and to develop models that address mechanisms at multiple levels of organization, from gene sequence to tissue biomechanics. In turn, quantitative models of embryogenesis can inspire novel methods for creating synthetic organs and treating degenerative tissue diseases.

Drosophila Syncrip binds the gurken mRNA localisation signal and regulates localised transcripts during axis specification

In the Drosophila oocyte, mRNA transport and localised translation play a fundamental role in axis determination and germline formation of the future embryo. gurken mRNA encodes a secreted TGF-α signal that specifies dorsal structures, and is localised to the dorso-anterior corner of the oocyte via a cis-acting 64 nucleotide gurken localisation signal. Using GRNA chromatography, we characterised the biochemical composition of the ribonucleoprotein complexes that form around the gurken mRNA localisation signal in the oocyte. We identified a number of the factors already known to be involved in gurken localisation and translational regulation, such as Squid and Imp, in addition to a number of factors with known links to mRNA localisation, such as Me31B and Exu. We also identified previously uncharacterised Drosophila proteins, including the fly homologue of mammalian SYNCRIP/hnRNPQ, a component of RNA transport granules in the dendrites of mammalian hippocampal neurons. We show that Drosophila Syncrip binds specifically to gurken and oskar, but not bicoid transcripts. The loss-of-function and overexpression phenotypes of syncrip in Drosophila egg chambers show that the protein is required for correct grk and osk mRNA localisation and translational regulation. We conclude that Drosophila Syncrip is a new factor required for localisation and translational regulation of oskar and gurken mRNA in the oocyte. We propose that Syncrip/SYNCRIP is part of a conserved complex associated with localised transcripts and required for their correct translational regulation in flies and mammals.

A very simple mode of follicular cell diversification in Euborellia fulviceps (Dermaptera, Anisolabididae) involves actively migrating cells

The ovaries of Euborellia fulviceps are composed of five elongated ovarioles of meroistic-polytrophic type. The individual ovariole has three discernible regions: the terminal filament, germarium, and vitellarium. The terminal filament is a stalk of flattened, disc-shaped somatic cells. In the germarium, germline cells in subsequent stages of differentiation are located, and the vitellarium comprises numerous ovarian follicles arranged linearly. The individual ovarian follicles within the vitellarium are separated by prominent interfollicular stalks. The follicles are composed by two germline cells only: an oocyte and a single, polyploid nurse cell, which are surrounded by a monolayer of somatic follicular cells (FCs). During subsequent stages of oogenesis, initially uniform follicular epithelium begins to diversify into morphologically and physiologically distinct subpopulations. In E. fulviceps, the FC diversification mode is rather simple and leads to the formation of only three different FC subpopulations: (1) cuboidal FCs covering the oocyte, (2) stretched FCs surrounding the nurse cell and (3) FCs actively migrating between oocyte and a nurse cell. We found that FCs from the latter subpopulation send long and thin filopodium-like and microtubule-rich processes penetrating between the oocyte and nurse cell membranes. This suggests that, in E. fulviceps, cells from at least one FCs subpopulation show the ability to change position within an ovarian follicle by means of active migration.

Transcriptional interpretation of the EGF receptor signaling gradient

Epidermal growth factor receptor (EGFR) controls a wide range of developmental events, from body axes specification in insects to cardiac development in humans. During Drosophila oogenesis, a gradient of EGFR activation patterns the follicular epithelium. Multiple transcriptional targets of EGFR in this tissue have been identified, but their regulatory elements are essentially unknown. We report the regulatory elements of broad (br) and pipe (pip), two important targets of EGFR signaling in Drosophila oogenesis. br is expressed in a complex pattern that prefigures the formation of respiratory eggshell appendages. We found that this pattern is generated by dynamic activities of two regulatory elements, which display different responses to Pointed, Capicua, and Mirror, transcription factors involved in the EGFR-mediated gene expression. One of these elements is active in a pattern similar to pip, a gene repressed by EGFR and essential for establishing the dorsoventral polarity of the embryo. We demonstrate that this similarity of expression depends on a common sequence motif that binds Mirror in vitro and is essential for transcriptional repression in vivo.

Shining light on Drosophila oogenesis: live imaging of egg development

Drosophila oogenesis is a powerful model for the study of numerous questions in cell and developmental biology. In addition to its longstanding value as a genetically tractable model of organogenesis, recently it has emerged as an excellent system in which to combine genetics and live imaging. Rapidly improving ex vivo culture conditions, new fluorescent biosensors and photo-manipulation tools, and advances in microscopy have allowed direct observation in real time of processes such as stem cell self-renewal, collective cell migration, and polarized mRNA and protein transport. In addition, entirely new phenomena have been discovered, including revolution of the follicle within the basement membrane and oscillating assembly and disassembly of myosin on a polarized actin network, both of which contribute to elongating this tissue. This review focuses on recent advances in live-cell imaging techniques and the biological insights gleaned from live imaging of egg chamber development.

Pattern formation by receptor tyrosine kinases: analysis of the Gurken gradient in Drosophila oogenesis

Spatial patterns of cell differentiation in developing tissues can be controlled by receptor tyrosine kinase (RTK) signaling gradients, which may form when locally secreted ligands activate uniformly expressed receptors. Graded activation of RTKs can span multiple cell diameters, giving rise to spatiotemporal patterns of signaling through the Extracellular Signal Regulated/Mitogen Activated Protein Kinase (ERK/MAPK), which connects receptor activation to multiple aspects of tissue morphogenesis. This general mechanism has been identified in numerous developmental contexts, from body axis specification in insects to patterning of the mammalian neocortex. We review recent quantitative studies of this mechanism in Drosophila oogenesis, an established genetic model of signaling through the Epidermal Growth Factor Receptor (EGFR), a highly conserved RTK.

Long-term live imaging provides new insight into stem cell regulation and germline-soma coordination in the Drosophila ovary

The Drosophila ovariole tip produces new ovarian follicles on a 12-hour cycle by controlling niche-based germline and follicle stem cell divisions and nurturing their developing daughters. Static images provide a thumbnail view of folliculogenesis but imperfectly capture the dynamic cellular interactions that underlie follicle production. We describe a live-imaging culture system that supports normal ovarian stem cell activity, cyst movement and intercellular interaction over 14 hours, which is long enough to visualize all the steps of follicle generation. Our results show that live imaging has unique potential to address diverse aspects of stem cell biology and gametogenesis. Stem cells in cultured tissue respond to insulin and orient their mitotic spindles. Somatic escort cells, the glial-like partners of early germ cells, do not adhere to and migrate along with germline stem cell daughters as previously proposed. Instead, dynamic, microtubule-rich cell membranes pass cysts from one escort cell to the next. Additionally, escort cells are not replenished by the regular division of escort stem cells as previously suggested. Rather, escort cells remain quiescent and divide only to maintain a constant germ cell:escort cell ratio.

Pattern formation by a moving morphogen source

During Drosophila melanogaster oogenesis, the follicular epithelium that envelops the germline cyst gives rise to an elaborate eggshell, which houses the future embryo and mediates its interaction with the environment. A prominent feature of the eggshell is a pair of dorsal appendages, which are needed for embryo respiration. Morphogenesis of this structure depends on broad, a zinc-finger transcription factor, regulated by the EGFR pathway. While much has been learned about the mechanisms of broad regulation by EGFR, current understanding of processes that shape the spatial pattern of broad expression is incomplete. We propose that this pattern is defined by two different phases of EGFR activation: an early, posterior-to-anterior gradient of EGFR signaling sets the posterior boundary of broad expression, while the anterior boundary is set by a later phase of EGFR signaling, distributed in a dorsoventral gradient. This model can explain the wild-type pattern of broad in D. melanogaster, predicts how this pattern responds to genetic perturbations, and provides insight into the mechanisms driving diversification of eggshell patterning. The proposed model of the broad expression pattern can be used as a starting point for the quantitative analysis of a large number of gene expression patterns in Drosophila oogenesis.

BMP signaling dynamics in the follicle cells of multiple Drosophila species

The dorsal anterior region of the follicle cells (FCs) in the developing Drosophila egg gives rise to the respiratory eggshell appendages. These tubular structures display a wide range of qualitative and quantitative variations across Drosophila species, providing a remarkable example of a rapidly evolving morphology. In D. melanogaster, the bone morphogenetic protein (BMP) signaling pathway is an important regulator of FCs patterning and dorsal appendages morphology. To explore the mechanisms underlying the diversification of eggshell patterning, we analyzed BMP signaling in the FCs of 16 Drosophila species that span 45 million years of evolution. We found that the spatial patterns of BMP signaling in the FCs are dynamic and exhibit a range of interspecies' variations. In most of the species examined, the dynamics of BMP signaling correlate with the expression of the type I BMP receptor thickveins (tkv). This correlation suggests that interspecies' variations of tkv expression are responsible for the diversification of BMP signaling during oogenesis. This model was supported by genetic manipulations of tkv expression in the FCs of D. melanogaster that successfully recapitulated the signaling diversities found in the other species. Our results suggest that regulation of receptor expression mediates spatial diversification of BMP signaling in Drosophila oogenesis, and they provide insight into a mechanism underlying the evolution of eggshell patterning.

Division of labor: subsets of dorsal-appendage-forming cells control the shape of the entire tube

The function of an organ relies on its form, which in turn depends on the individual shapes of the cells that create it and the interactions between them. Despite remarkable progress in the field of developmental biology, how cells collaborate to make a tissue remains an unsolved mystery. To investigate the mechanisms that determine organ structure, we are studying the cells that form the dorsal appendages (DAs) of the Drosophila melanogaster eggshell. These cells consist of two differentially patterned subtypes: roof cells, which form the outward-facing roof of the lumen, and floor cells, which dive underneath the roof cells to seal off the floor of the tube. In this paper, we present three lines of evidence that reveal a further stratification of the DA-forming epithelium. Laser ablation of only a few cells in the anterior of the region causes a disproportionately severe shortening of the appendage. Genetic alteration through the twin peaks allele of tramtrack69 (ttk(twk)), a female-sterile mutation that leads to severely shortened DAs, causes no such shortening when removed from a majority of the DA-forming cells, but rather, produces short appendages only when removed from cells in the very anterior of the tube-forming tissue. Additionally we show that heterotrimeric G-protein function is required for DA morphogenesis. Like TTK69, Gbeta 13F is not required in all DA-forming follicle cells but only in the floor and leading roof cells. The different phenotypes that result from removal of Gbeta 13F from each region demonstrate a striking division of function between different DA-forming cells. Gbeta mutant floor cells are unable to control the width of the appendage while Gbeta mutant leading roof cells fail to direct the elongation of the appendage and the convergent-extension of the roof-cell population.

Control in time and space: Tramtrack69 cooperates with Notch and Ecdysone to repress ectopic fate and shape changes during Drosophila egg chamber maturation

Organ morphogenesis requires cooperation between cells, which determine their course of action based upon location within a tissue. Just as important, cells must synchronize their activities, which requires awareness of developmental time. To understand how cells coordinate behaviors in time and space, we analyzed Drosophila egg chamber development. We found that the transcription factor Tramtrack69 (TTK69) controls the fates and shapes of all columnar follicle cells by integrating temporal and spatial information, restricting characteristic changes in morphology and expression that occur at stage 10B to appropriate domains. TTK69 is required again later in oogenesis: it controls the volume of the dorsal-appendage (DA) tubes by promoting apical re-expansion and lateral shortening of DA-forming follicle cells. We show that TTK69 and Notch compete to repress each other's expression and that a local Ecdysone signal is required to shift the balance in favor of TTK69. We hypothesize that TTK69 then cooperates with spatially restricted co-factors to define appropriate responses to a globally available (but as yet unidentified) temporal signal that initiates the S10B transformations.

Feedback control of the EGFR signaling gradient: superposition of domain-splitting events in Drosophila oogenesis

The morphogenesis of structures with repeated functional units, such as body segments and appendages, depends on multi-domain patterns of cell signaling and gene expression. We demonstrate that during Drosophila oogenesis, the two-domain expression pattern of Broad, a transcription factor essential for the formation of the two respiratory eggshell appendages, is established by a single gradient of EGFR activation that induces both Broad and Pointed, which mediates repression of Broad. Two negative-feedback loops provided by the intracellular inhibitors of EGFR signaling, Kekkon-1 and Sprouty, control the number and position of Broad-expressing cells and in this way influence eggshell morphology. Later in oogenesis, the gradient of EGFR activation is split into two smaller domains in a process that depends on Argos, a secreted antagonist of EGFR signaling. In contrast to the previously proposed model of eggshell patterning, we show that the two-domain pattern of EGFR signaling is not essential for specifying the number of appendages. Thus, the processes that define the two-domain patterns of Broad and EGFR activation are distinct; their actions are separated in time and have different effects on eggshell morphology.

Graded Egfr activity patterns the Drosophila eggshell independently of autocrine feedback

The pattern of the Drosophila eggshell is determined by the establishment of a complex and stereotyped pattern of cell fates in the follicular epithelium of the ovary. Localized activation of the Epidermal growth factor receptor (Egfr) is essential for this patterning. Modulation of Egfr pathway activity in time and space determines distinct fates at their appropriate locations, but the details of how Egfr signaling is regulated and how the profile of Egfr activity corresponds to cell fate remain unclear. Here we analyze the effect of loss of various Egfr regulators and targets on follicle cell patterning, using a marker for follicle cell fate, and on the mature eggshell phenotype, using a novel eggshell marker. We show, contrary to current patterning models, that feedback regulation of Egfr activity by the autocrine ligand Spitz and the inhibitor Argos is not necessary for patterning. Given the cell-autonomous nature of the mutant phenotypes we observed, we propose instead that the pattern of cell fates is generated by spatial information derived directly from the germline ligand Gurken, without a requirement for subsequent patterning by diffusible Egfr regulators in the follicular epithelium.

Pattern formation by dynamically interacting network motifs

Systematic validation of pattern formation mechanisms revealed by molecular studies of development is essentially impossible without mathematical models. Models can provide a compact summary of a large number of experiments that led to mechanism formulation and guide future studies of pattern formation. Here, we realize this program by analyzing a mathematical model of epithelial patterning by the highly conserved EGFR and BMP signaling pathways in Drosophila oogenesis. The model accounts for the dynamic interaction of the feedforward and feedback network motifs that control the expression of Broad, a zinc finger transcription factor expressed in the cells that form the upper part of the respiratory eggshell appendages. Based on the combination of computational analysis and genetic experiments, we show that the model accounts for the key features of wild-type pattern formation, correctly predicts patterning defects in multiple mutants, and guides the identification of additional regulatory links in a complex pattern formation mechanism.

Glycosphingolipids control the extracellular gradient of the Drosophila EGFR ligand Gurken

Glycosphingolipids (GSLs) are present in all eukaryotic membranes and are implicated in neuropathologies and tumor progression in humans. Nevertheless, their in vivo functions remain poorly understood in vertebrates, partly owing to redundancy in the enzymes elongating their sugar chains. In Drosophila, a single GSL biosynthetic pathway is present that relies on the activity of the Egghead and Brainiac glycosyltransferases. Mutations in these two enzymes abolish GSL elongation and yield oogenesis defects, providing a unique model system in which to study GSL roles in signaling in vivo. Here, we use egghead and brainiac mutants to show that GSLs are necessary for full activation of the EGFR pathway during oogenesis in a time-dependent manner. In contrast to results from in vitro studies, we find that GSLs are required in cells producing the TGFalpha-like ligand Gurken, but not in EGFR-expressing cells. Strikingly, we find that GSLs are not essential for Gurken trafficking and secretion. However, we characterize for the first time the extracellular Gurken gradient and show that GSLs affect its formation by controlling Gurken planar transport in the extracellular space. This work presents the first in vivo evidence that GSLs act in trans to regulate the EGFR pathway and shows that extracellular EGFR ligand distribution is tightly controlled by GSLs. Our study assigns a novel role for GSLs in morphogen diffusion, possibly through regulation of their conformation.

Tube formation in Drosophila egg chambers

The reorganization of epithelial sheets into tubes is a fundamental process in the formation of many organs, such as the lungs, kidneys, gut, and neural tube. This process involves the patterning of distinct cell types and the coordination of those cells during the shape changes and rearrangements that produce the tube. A better understanding of the cellular and genetic mechanisms that regulate tube formation is necessary for tissue engineers to develop functional organs in vitro. The Drosophila egg chamber has emerged as an outstanding model for studying tubulogenesis. Synthesis of the dorsal respiratory appendages by the follicular epithelium resembles primary neurulation in vertebrates. This review summarizes work on the patterning and morphogenesis of the dorsal-appendage tubes and highlights key areas where mathematical modeling could contribute to our understanding of these processes.

Tissue remodeling: a mating-induced differentiation program for the Drosophila oviduct

BACKGROUND

In both vertebrates and invertebrates, the oviduct is an epithelial tube surrounded by visceral muscles that serves as a conduit for gamete transport between the ovary and uterus. While Drosophila is a model system for tubular organ development, few studies have addressed the development of the fly's oviduct. Recent studies in Drosophila have identified mating-responsive genes and proteins whose levels in the oviduct are altered by mating. Since many of these molecules (e.g. Muscle LIM protein 84B, Coracle, Neuroglian) have known roles in the differentiation of muscle and epithelia of other organs, mating may trigger similar differentiation events in the oviduct. This led us to hypothesize that mating mediates the last stages of oviduct differentiation in which organ-specific specializations arise.

RESULTS

Using electron- and confocal-microscopy we identified tissue-wide post-mating changes in the oviduct including differentiation of cellular junctions, remodeling of extracellular matrix, increased myofibril formation, and increased innervation. Analysis of once- and twice-mated females reveals that some mating-responsive proteins respond only to the first mating, while others respond to both matings.

CONCLUSION

We uncovered ultrastructural changes in the mated oviduct that are consistent with the roles that mating-responsive proteins play in muscle and epithelial differentiation elsewhere. This suggests that mating triggers the late differentiation of the oviduct. Furthermore, we suggest that mating-responsive proteins that respond only to the first mating are involved in the final maturation of the oviduct while proteins that remain responsive to later matings are also involved in maintenance and ongoing function of the oviduct. Taken together, our results establish the oviduct as an attractive system to address mechanisms that regulate the late stages of differentiation and maintenance of a tubular organ.

A combinatorial code for pattern formation in Drosophila oogenesis

Two-dimensional patterning of the follicular epithelium in Drosophila oogenesis is required for the formation of three-dimensional eggshell structures. Our analysis of a large number of published gene expression patterns in the follicle cells suggests that they follow a simple combinatorial code based on six spatial building blocks and the operations of union, difference, intersection, and addition. The building blocks are related to the distribution of inductive signals, provided by the highly conserved epidermal growth factor receptor and bone morphogenetic protein signaling pathways. We demonstrate the validity of the code by testing it against a set of patterns obtained in a large-scale transcriptional profiling experiment. Using the proposed code, we distinguish 36 distinct patterns for 81 genes expressed in the follicular epithelium and characterize their joint dynamics over four stages of oogenesis. The proposed combinatorial framework allows systematic analysis of the diversity and dynamics of two-dimensional transcriptional patterns and guides future studies of gene regulation.

Expression patterns of cadherin genes in Drosophila oogenesis

In Drosophila oogenesis, the follicular epithelium that envelops the oocyte is patterned by a small set of inductive signals and gives rise to an elaborate three-dimensional eggshell. Several eggshell structures provide sensitive readouts of the patterning signals, but the formation of these structures is still poorly understood. In other systems, epithelial morphogenesis is guided by the spatial patterning of cell adhesion and cytoskeleton genes. As a step towards developing a comprehensive description of patterning events leading to eggshell morphogenesis, we report the expression of Drosophila cadherins, calcium-dependent adhesion molecules that are repeatedly used throughout development. We found that 9/17 of Drosophila cadherins are expressed in the follicular epithelium in dynamic patterns during oogenesis. In late oogenesis, the expression patterns of cadherin genes in the main body follicle cells is summarized using a compact set of simple geometric shapes, reflecting the integration of the EGFR and DPP inductive signals. The multi-layered composite patterning of the cadherins is hypothesized to play a key role in the formation of the eggshell. Of particular note is the complex patterning of the region of the follicular epithelium that gives rise to the dorsal appendages, which are tubular structures that serve as respiratory organs for the developing embryo.