The steroid hormone ecdysone controls systemic growth by repressing dMyc function in Drosophila fat cells

Imp/IGF2BP levels modulate individual neural stem cell growth and division through myc mRNA stability

The numerous neurons and glia that form the brain originate from tightly controlled growth and division of neural stem cells, regulated systemically by important known stem cell-extrinsic signals. However, the cell-intrinsic mechanisms that control the distinctive proliferation rates of individual neural stem cells are unknown. Here, we show that the size and division rates of Drosophila neural stem cells (neuroblasts) are controlled by the highly conserved RNA binding protein Imp (IGF2BP), via one of its top binding targets in the brain, myc mRNA. We show that Imp stabilises myc mRNA leading to increased Myc protein levels, larger neuroblasts, and faster division rates. Declining Imp levels throughout development limit myc mRNA stability to restrain neuroblast growth and division, and heterogeneous Imp expression correlates with myc mRNA stability between individual neuroblasts in the brain. We propose that Imp-dependent regulation of myc mRNA stability fine-tunes individual neural stem cell proliferation rates.

Roles of C/EBP class bZip proteins in the growth and cell competition of Rp ('Minute') mutants in Drosophila

Reduced copy number of ribosomal protein (Rp) genes adversely affects both flies and mammals. Xrp1 encodes a reportedly Drosophila-specific AT-hook, bZIP protein responsible for many of the effects including the elimination of Rp mutant cells by competition with wild type cells. Irbp18, an evolutionarily conserved bZIP gene, heterodimerizes with Xrp1 and with another bZip protein, dATF4. We show that Irbp18 is required for the effects of Xrp1, whereas dATF4 does not share the same phenotype, indicating that Xrp1/Irbp18 is the complex active in Rp mutant cells, independently of other complexes that share Irbp18. Xrp1 and Irbp18 transcripts and proteins are upregulated in Rp mutant cells by auto-regulatory expression that depends on the Xrp1 DNA binding domains and is necessary for cell competition. We show that Xrp1 is conserved beyond Drosophila, although under positive selection for rapid evolution, and that at least one human bZip protein can similarly affect Drosophila development.

Cell migration by swimming: Drosophila adipocytes as a new in vivo model of adhesion-independent motility

Several cell lineages migrate through the developing and adult tissues of our bodies utilising a variety of modes of motility to suit the different substrates and environments they encounter en route to their destinations. Here we describe a novel adhesion-independent mode of single cell locomotion utilised by Drosophila fat body cells - the equivalent of vertebrate adipocytes. Like their human counterpart, these large cells were previously presumed to be immotile. However, in the Drosophila pupa fat body cells appear to be motile and migrate in a directed way towards wounds by peristaltic swimming through the hemolymph. The propulsive force is generated from a wave of cortical actomyosin that travels rearwards along the length of the cell. We discuss how this swimming mode of motility overcomes the physical constraints of microscopic objects moving in fluids, how fat body cells switch on other "motility machinery" to plug the wound on arrival, and whether other cell lineages in Drosophila and other organisms may, under certain circumstances, also adopt swimming as an effective mode of migration.

Roles of the insulin signaling pathway in insect development and organ growth

Organismal development is a complex process as it requires coordination of many aspects to grow into fit individuals, such as the control of body size and organ growth. Therefore, the mechanisms of precise control of growth are essential for ensuring the growth of organisms at a correct body size and proper organ proportions during development. The control of the growth rate and the duration of growth (or the cessation of growth) are required in size control. The insulin signaling pathway and the elements involved are essential in the control of growth. On the other hand, the ecdysteroid molting hormone determines the duration of growth. The secretion of these hormones is controlled by environmental factors such as nutrition. Moreover, the target of rapamycin (TOR) pathway is considered as a nutrient sensing pathway. Important cross-talks have been shown to exist among these pathways. In this review, we outline the control of body and organ growth by the insulin/TOR signaling pathway, and also the interaction between nutrition via insulin/TOR signaling and ecdysteroids at the coordination of organismal development and organ growth in insects, mainly focusing on the well-studied fruit fly Drosophila melanogaster.

CRISPR disruption of TCTP gene impaired normal development in the silkworm Bombyx mori

The translationally controlled tumor protein (TCTP) is a highly conserved and multifunctional protein with activities ranging from cytoskeletal regulation to transcription regulation in numerous organisms. In insects, TCTP is essential for cell growth and proliferation. Recently, TCTP has been reported to affect the innate intestinal immune pathway in the Bombyx mori silkworm, a lepidopteran model insect. However, the comprehensive physiological roles of TCTP in the silkworm remain poorly understood. Here, we performed functional analysis of BmTCTP by using a binary transgenic CRISPR/Cas9 (clustered regularly interspaced short palindromic repeat/RNA-guided CRISPER-associated protein 9 nucleases) system. Disruption of BmTCTP led to developmental arrestment and subsequent lethality in third instar larvae. Histological analysis revealed that growth impairment originated from decreased cell size, and the proliferation and differentiation of intestinal epithelial cells were also affected. RNA-seq analysis revealed that genes involved in carbohydrate metabolism, lipid metabolism and digestive system pathways were significantly affected by BmTCTP depletion. Together, the results demonstrated that BmTCTP plays a key role in controlling larval growth and development.

Eiger/TNFα-mediated Dilp8 and ROS production coordinate intra-organ growth in Drosophila

Coordinated intra- and inter-organ growth during animal development is essential to ensure a correctly proportioned individual. The Drosophila wing has been a valuable model system to reveal the existence of a stress response mechanism involved in the coordination of growth between adjacent cell populations and to identify a role of the fly orthologue of p53 (Dmp53) in this process. Here we identify the molecular mechanisms used by Dmp53 to regulate growth and proliferation in a non-autonomous manner. First, Dmp53-mediated transcriptional induction of Eiger, the fly orthologue of TNFα ligand, leads to the cell-autonomous activation of JNK. Second, two distinct signaling events downstream of the Eiger/JNK axis are induced in order to independently regulate tissue size and cell number in adjacent cell populations. Whereas expression of the hormone dILP8 acts systemically to reduce growth rates and tissue size of adjacent cell populations, the production of Reactive Oxygen Species—downstream of Eiger/JNK and as a consequence of apoptosis induction—acts in a non-cell-autonomous manner to reduce proliferation rates. Our results unravel how local and systemic signals act concertedly within a tissue to coordinate growth and proliferation, thereby generating well-proportioned organs and functionally integrated adults.

The coordination of growth between the parts of a given developing organ is an absolute requirement for the generation of functionally integrated structures during animal development. Although this question has fascinated biologists for centuries, the molecular mechanisms responsible have remained elusive to date. In this work, we used the developing wing primordium of Drosophila to identify the molecular mechanisms and signaling molecules that mediate communication between adjacent cell populations upon a targeted reduction of growth rate. We first present evidence that the activation of Dmp53 in the growth-depleted territory induces the expression of the fly TNF ligand Eiger, which activates the JNK stress signaling pathway in a cell-autonomous manner. While JNK-dependent expression of the systemic hormone dILP8 reduces the growth and final size of adjacent territories, the production of Reactive Oxygen Species downstream of JNK and the apoptotic machinery act locally to regulate the proliferation of adjacent epithelial cells. Our data reveal how different signals, acting both locally and systemically, can regulate tissue growth and cell proliferation in an independent manner to coordinate the tissue size and cell number of different parts of an organ, ultimately giving rise to well-proportioned adult structures.

Inter-Organ Growth Coordination Is Mediated by the Xrp1-Dilp8 Axis in Drosophila

How organs scale with other body parts is not mechanistically understood. We have addressed this question using the Drosophila imaginal disc model. When the growth of one disc domain is perturbed, other parts of the disc and other discs slow down their growth, maintaining proper inter-disc and intra-disc proportions. We show here that the relaxin-like Dilp8 is required for this inter-organ coordination. Our work also reveals that the stress-response transcription factor Xrp1 plays a key role upstream of dilp8 in linking organ growth status with the systemic growth response. In addition, we show that the small ribosomal subunit protein RpS12 is required to trigger Xrp1-dependent non-autonomous response. Our work demonstrates that RpS12, Xrp1, and Dilp8 form an independent regulatory module that ensures intra- and inter-organ growth coordination during development.

Fat Body Biology in the Last Decade

The insect fat body is analogous to vertebrate adipose tissue and liver. In this review, the new and exciting advancements made in fat body biology in the last decade are summarized. Controlled by hormonal and nutritional signals, insect fat body cells undergo mitosis during embryogenesis, endoreplication during the larval stages, and remodeling during metamorphosis and regulate reproduction in adults. Fat body tissues are major sites for nutrient storage, energy metabolism, innate immunity, and detoxification. Recent studies have revealed that the fat body plays a central role in the integration of hormonal and nutritional signals to regulate larval growth, body size, circadian clock, pupal diapause, longevity, feeding behavior, and courtship behavior, partially by releasing fat body signals to remotely control the brain. In addition, the fat body has emerged as a fascinating model for studying metabolic disorders and immune diseases. Potential future directions for fat body biology are also proposed herein.

Body size regulation by maturation steroid hormones: a Drosophila perspective

The mechanism that determines the specific body size of an animal is a fundamental biological question that remains largely unanswered. This aspect is now beginning to be understood in insect models, particularly in Drosophila melanogaster, with studies highlighting the importance of nutrient-responsive growth signaling pathways involving insulin/insulin-like growth factor signaling (IIS) and target of rapamycin (TOR) (IIS/TOR). These pathways operate in animals, from insects to mammals, adjusting the growth rate in response to the nutritional condition of the organism. Organismal growth is closely coupled with the process of developmental maturation mediated by maturation steroid hormones, which is influenced greatly by environmental and nutritional conditions. Recent Drosophila studies have been revealing the mechanisms responsible for this phenomenon. In this review, I summarize some important findings about the steroid hormone regulation of Drosophila body growth, calling attention to the influence of developmental nutritional conditions on animal size determination.

A role for Lin-28 in growth and metamorphosis in Drosophila melanogaster

Insect metamorphosis has been a classic model to understand the role of hormones in growth and timing of developmental transitions. In addition to hormones, transitions in some species are regulated by genetic programs, such as the heterochronic gene network discovered in C. elegans. However, the functional link between hormones and heterochronic genes is not clear. The heterochronic gene lin-28 is involved in the maintenance of stem cells, growth and developmental timing in vertebrates. In this work, we used gain-of-function and loss-of-function experiments to study the role of Lin-28 in larval growth and the timing of metamorphosis of Drosophila melanogaster. During the late third instar stage, Lin-28 is mainly expressed in neurons of the central nervous system and in the intestine. Loss-of-function lin-28 mutant larvae are smaller and the larval-to-pupal transition is accelerated. This faster transition correlates with increased levels of ecdysone direct target genes such as Broad-Complex (BR-C) and Ecdysone Receptor (EcR). Overexpression of Lin-28 does not affect the timing of pupariation but most animals are not able to eclose, suggesting defects in metamorphosis. Overexpression of human Lin-28 results in delayed pupariation and the death of animals during metamorphosis. Altogether, these results suggest that Lin-28 is involved in the control of growth during larval development and in the timing and progression of metamorphosis.

Steroid signaling mediates nutritional regulation of juvenile body growth via IGF-binding protein in Drosophila

Nutritional condition during the juvenile growth period considerably affects final adult size. The insulin/insulin-like growth factor signaling (IIS)/target of rapamycin (TOR) nutrient-sensing pathway is known to regulate growth and metabolism in response to nutritional conditions. However, there is limited information on how endocrine pathways communicate nutritional information to different metabolic organs to regulate organismal growth. Here, we show that Imaginal morphogenesis protein-Late 2 (Imp-L2), a Drosophila homolog of insulin-like growth factor-binding protein 7 (IGFBP7), plays a key role in the nutritional control of organismal growth. Nutritional restriction during the larval growth period causes undersized adults, which is largely diminished by Imp-L2 mutation. We delineate a pathway in which nutritional restriction increases levels of the steroid hormone ecdysone, which, in turn, triggers ecdysone signaling-dependent Imp-L2 production from the fat body, a fly adipose organ, thereby attenuating peripheral IIS and body growth. Surprisingly, this endocrine pathway operates independent of the fat-body-TOR internal nutrient sensor, long believed to be the control center for nutrition-dependent growth. Our study reveals a previously unrecognized endocrine circuit mediating nutrition-dependent juvenile growth, which could also potentially be related to the insulin resistance frequently observed in puberty.

The biology and evolution of the Dilp8-Lgr3 pathway: A relaxin-like pathway coupling tissue growth and developmental timing control

Many insects, like cockroaches, moths, and flies, can regenerate tissues by extending the growth-competent phases of their life cycle. The molecular and cellular players mediating this coordination between tissue growth and developmental timing have been recently discovered in Drosophila. The insulin/relaxin-like peptide, Dilp8, was identified as a factor communicating abnormal growth status of Drosophila larval imaginal discs to the neuroendocrine centers that control the timing of the onset of metamorphosis. Dilp8 requires a neuronal relaxin receptor for this function, the Leucine rich repeat containing G protein coupled receptor, Lgr3. A review of current data supports a model where imaginal disc-derived Dilp8 acts on four central nervous system Lgr3-positive neurons to activate cyclic-AMP signaling in an Lgr3-dependent manner. This causes a reduction in ecdysone hormone production by the larval endocrine prothoracic gland, which leads to a delay in the onset of metamorphosis and a simultaneous slowing down in the growth rates of healthy imaginal tissues, promoting the generation of proportionate individuals. We discuss reports indicating that the Dilp8-Lgr3 pathway might have other functions at different life history stages, which remain to be elucidated, and review molecular evolution data on invertebrate genes related to the relaxin-pathway. The strong conservation of the relaxin pathway throughout animal evolution contrasts with instances of its complete loss in some clades, such as lepidopterans, which must coordinate growth and developmental timing using another mechanism. Research into these areas should generate exciting new insights into the biology of growth coordination, the evolution of the relaxin signaling pathway, and likely reveal unforeseen functions in other developmental stages.

Neuronal Calcium Signaling in Metabolic Regulation and Adaptation to Nutrient Stress

All organisms can respond physiologically and behaviorally to environmental fluxes in nutrient levels. Different nutrient sensing pathways exist for specific metabolites, and their inputs ultimately define appropriate nutrient uptake and metabolic homeostasis. Nutrient sensing mechanisms at the cellular level require pathways such as insulin and target of rapamycin (TOR) signaling that integrates information from different organ systems like the fat body and the gut. Such integration is essential for coordinating growth with development. Here we review the role of a newly identified set of integrative interneurons and the role of intracellular calcium signaling within these neurons, in regulating nutrient sensing under conditions of nutrient stress. A comparison of the identified Drosophila circuit and cellular mechanisms employed in this circuit, with vertebrate systems, suggests that the identified cell signaling mechanisms may be conserved for neural circuit function related to nutrient sensing by central neurons. The ideas proposed are potentially relevant for understanding the molecular basis of metabolic disorders, because these are frequently linked to nutritional stress.

Krüppel homolog 1 represses insect ecdysone biosynthesis by directly inhibiting the transcription of steroidogenic enzymes

In insects, juvenile hormone (JH) and the steroid hormone ecdysone have opposing effects on regulation of the larval-pupal transition. Although increasing evidence suggests that JH represses ecdysone biosynthesis during larval development, the mechanism underlying this repression is not well understood. Here, we demonstrate that the expression of the Krüppel homolog 1 (Kr-h1), a gene encoding a transcription factor that mediates JH signaling, in ecdysone-producing organ prothoracic gland (PG) represses ecdysone biosynthesis by directly inhibiting the transcription of steroidogenic enzymes in both Drosophila and Bombyx Application of a JH mimic on ex vivo cultured PGs from Drosophila and Bombyx larvae induces Kr-h1 expression and inhibits the transcription of steroidogenic enzymes. In addition, PG-specific knockdown of Drosophila Kr-h1 promotes-while overexpression hampers-ecdysone production and pupariation. We further find that Kr-h1 inhibits the transcription of steroidogenic enzymes by directly binding to their promoters to induce promoter DNA methylation. Finally, we show that Kr-h1 does not affect DNA replication in Drosophila PG cells and that the reduction of PG size mediated by Kr-h1 overexpression can be rescued by feeding ecdysone. Taken together, our data indicate direct and conserved Kr-h1 repression of insect ecdysone biosynthesis in response to JH stimulation, providing insights into mechanisms underlying the antagonistic roles of JH and ecdysone.

The (ongoing) problem of relative growth

Differential growth, the phenomenon where parts of the body grow at different rates, is necessary to generate the complex morphologies of most multicellular organisms. Despite this central importance, how differential growth is regulated remains largely unknown. Recent discoveries, particularly in insects, have started to uncover the molecular-genetic and physiological mechanisms that coordinate growth among different tissues throughout the body and regulate relative growth. These discoveries suggest that growth is coordinated by a network of signals that emanate from growing tissues and central endocrine organs. Here we review these findings and discuss their implications for understanding the regulation of relative growth and the evolution of morphology.

Unravelling the diversity of mechanisms through which nutrition regulates body size in insects

Insects show impressive diversity in adult body size across species, and within species adult body size is sensitive to numerous environmental conditions, particularly to changes in nutrition. Body size in adult insects correlates with a number of important fitness-related traits such as fecundity, longevity, stress resistance, and mating success. Over the past few decades, the field of insect body size regulation has made impressive progress towards understanding the signalling pathways that regulate body size in response to nutrition. These studies have shown that conserved nutrition-sensitive signalling pathways act in animals from insects to vertebrates to regulate growth. In particular, pathways like the insulin/insulin-like growth factor signalling (IIS) pathway and the Target of rapamycin (TOR) pathway respond to the levels of dietary nutrients to adjust both the rate of growth and the duration of the growth period. They do this not only by regulating organ growth, but also by modifying the rates of synthesis and circulating concentrations of key developmental hormones. Although the mechanisms through which this occurs have been well documented in one insect, the fruit fly Drosophila melanogaster, it is becoming increasingly clear that the downstream mechanisms through which IIS and TOR signalling alter size in response to nutrition differ between organs and across species. In this review, we highlight how understanding the organ-specific effects of IIS/TOR signalling are key to revealing the diversity of size control mechanisms across insects.

Functional characterization of adaptive variation within a cis-regulatory element influencing Drosophila melanogaster growth

Gene expression variation is a major contributor to phenotypic diversity within species and is thought to play an important role in adaptation. However, examples of adaptive regulatory polymorphism are rare, especially those that have been characterized at both the molecular genetic level and the organismal level. In this study, we perform a functional analysis of the Drosophila melanogaster CG9509 enhancer, a cis-regulatory element that shows evidence of adaptive evolution in populations outside the species’ ancestral range in sub-Saharan Africa. Using site-directed mutagenesis and transgenic reporter gene assays, we determined that 3 single nucleotide polymorphisms are responsible for the difference in CG9509 expression that is observed between sub-Saharan African and cosmopolitan populations. Interestingly, while 2 of these variants appear to have been the targets of a selective sweep outside of sub-Saharan Africa, the variant with the largest effect on expression remains polymorphic in cosmopolitan populations, suggesting it may be subject to a different mode of selection. To elucidate the function of CG9509, we performed a series of functional and tolerance assays on flies in which CG9509 expression was disrupted. We found that CG9509 plays a role in larval growth and influences adult body and wing size, as well as wing loading. Furthermore, variation in several of these traits was associated with variation within the CG9509 enhancer. The effect on growth appears to result from a modulation of active ecdysone levels and expression of growth factors. Taken together, our findings suggest that selection acted on 3 sites within the CG9509 enhancer to increase CG9509 expression and, as a result, reduce wing loading as D. melanogaster expanded out of sub-Saharan Africa.

Much of the phenotypic variation that is observed within species is thought to be caused by variation in gene expression. Variants within cis-regulatory elements, which affect the expression of nearby genes within the same DNA strand, are thought to be an abundant resource upon which natural selection can act. Understanding the functional consequences of adaptive cis-regulatory changes is important, as it can help elucidate the mechanisms underlying phenotypic evolution in general and provide insight into the development and maintenance of biodiversity. However, functional analyses of these types of changes remain rare. Here we present a functional analysis of an adaptively evolving enhancer element of a D. melanogaster gene called CG9509, of previously unknown function. We show that 3 single nucleotide polymorphisms located within the enhancer of this gene are responsible for an increase in CG9509 expression in cosmopolitan populations (outside of south and central Africa) relative to sub-Saharan populations, which include ancestral populations. We further show that CG9509 is involved in the regulation of growth rate and body size determination and propose that the CG9509 enhancer underwent positive selection to reduce wing loading as the species expanded out of sub-Saharan Africa.

Growth control through regulation of insulin-signaling by nutrition-activated steroid hormone in Drosophila

Growth and maturation are coordinated processes in all animals. Integration of internal cues, such as signalling pathways, with external cues such as nutritional status is paramount for an orderly progression of development in function of growth. In Drosophila, this involves insulin and steroid signalling, but the underlying mechanisms and their coordination are incompletely understood. We show that bioactive 20-hydroxyecdysone production by the enzyme Shade in the fat body is a nutrient-dependent process. We demonstrate that under fed conditions, Shade plays a role in growth control. We identify the trachea and the insulin-producing cells in the brain as direct targets through which 20-hydroxyecdysone regulates insulin-signaling. The identification of the trachea-dependent regulation of insulin-signaling exposes an important variable that may have been overlooked in other studies focusing on insulin-signaling in Drosophila. Our findings provide a potentially conserved, novel mechanism by which nutrition can modulate steroid hormone bioactivation, reveal an important caveat of a commonly used transgenic tool to study IPC function and yield further insights as to how steroid and insulin signalling are coordinated during development to regulate growth and developmental timing.

PWP1 Mediates Nutrient-Dependent Growth Control through Nucleolar Regulation of Ribosomal Gene Expression

Ribosome biogenesis regulates animal growth and is controlled by nutrient-responsive mTOR signaling. How ribosome biogenesis is regulated during the developmental growth of animals and how nutrient-responsive signaling adjusts ribosome biogenesis in this setting have remained insufficiently understood. We uncover PWP1 as a chromatin-associated regulator of developmental growth with a conserved role in RNA polymerase I (Pol I)-mediated rRNA transcription. We further observed that PWP1 epigenetically maintains the rDNA loci in a transcription-competent state. PWP1 responds to nutrition in Drosophila larvae via mTOR signaling through gene expression and phosphorylation, which controls the nucleolar localization of dPWP1. Our data further imply that dPWP1 acts synergistically with mTOR signaling to regulate the nucleolar localization of TFIIH, a known elongation factor of Pol I. Ribosome biogenesis is often deregulated in cancer, and we demonstrate that high PWP1 levels in human head and neck squamous cell carcinoma tumors are associated with poor prognosis.

Cell dynamics underlying oriented growth of the Drosophila wing imaginal disc

Quantitative analysis of the dynamic cellular mechanisms shaping the Drosophila wing during its larval growth phase has been limited, impeding our ability to understand how morphogen patterns regulate tissue shape. Such analysis requires explants to be imaged under conditions that maintain both growth and patterning, as well as methods to quantify how much cellular behaviors change tissue shape. Here, we demonstrate a key requirement for the steroid hormone 20-hydroxyecdysone (20E) in the maintenance of numerous patterning systems in vivo and in explant culture. We find that low concentrations of 20E support prolonged proliferation in explanted wing discs in the absence of insulin, incidentally providing novel insight into the hormonal regulation of imaginal growth. We use 20E-containing media to observe growth directly and to apply recently developed methods for quantitatively decomposing tissue shape changes into cellular contributions. We discover that whereas cell divisions drive tissue expansion along one axis, their contribution to expansion along the orthogonal axis is cancelled by cell rearrangements and cell shape changes. This finding raises the possibility that anisotropic mechanical constraints contribute to growth orientation in the wing disc.

Modeling dietary influences on offspring metabolic programming in Drosophila melanogaster

The influence of nutrition on offspring metabolism has become a hot topic in recent years owing to the growing prevalence of maternal and childhood obesity. Studies in mammals have identified several factors correlating with parental and early offspring dietary influences on progeny health; however, the molecular mechanisms that underlie these factors remain undiscovered. Mammalian metabolic tissues and pathways are heavily conserved in Drosophila melanogaster, making the fly an invaluable genetic model organism for studying metabolism. In this review, we discuss the metabolic similarities between mammals and Drosophila and present evidence supporting its use as an emerging model of metabolic programming.

Warts Signaling Controls Organ and Body Growth through Regulation of Ecdysone

Coordination of growth between individual organs and the whole body is essential during development to produce adults with appropriate size and proportions [1, 2]. How local organ-intrinsic signals and nutrient-dependent systemic factors are integrated to generate correctly proportioned organisms under different environmental conditions is poorly understood. In Drosophila, Hippo/Warts signaling functions intrinsically to regulate tissue growth and organ size [3, 4], whereas systemic growth is controlled via antagonistic interactions of the steroid hormone ecdysone and nutrient-dependent insulin/insulin-like growth factor (IGF) (insulin) signaling [2, 5]. The interplay between insulin and ecdysone signaling regulates systemic growth and controls organismal size. Here, we show that Warts (Wts; LATS1/2) signaling regulates systemic growth in Drosophila by activating basal ecdysone production, which negatively regulates body growth. Further, we provide evidence that Wts mediates effects of insulin and the neuropeptide prothoracicotropic hormone (PTTH) on regulation of ecdysone production through Yorkie (Yki; YAP/TAZ) and the microRNA bantam (ban). Thus, Wts couples insulin signaling with ecdysone production to adjust systemic growth in response to nutritional conditions during development. Inhibition of Wts activity in the ecdysone-producing cells non-autonomously slows the growth of the developing imaginal-disc tissues while simultaneously leading to overgrowth of the animal. This indicates that ecdysone, while restricting overall body growth, is limiting for growth of certain organs. Our data show that, in addition to its well-known intrinsic role in restricting organ growth, Wts/Yki/ban signaling also controls growth systemically by regulating ecdysone production, a mechanism that we propose controls growth between tissues and organismal size in response to nutrient availability.

The FOXO transcription factor controls insect growth and development by regulating juvenile hormone degradation in the silkworm, Bombyx mori

Forkhead box O (FOXO) functions as the terminal transcription factor of the insulin signaling pathway and regulates multiple physiological processes in many organisms, including lifespan in insects. However, how FOXO interacts with hormone signaling to modulate insect growth and development is largely unknown. Here, using the transgene-based CRISPR/Cas9 system, we generated and characterized mutants of the silkworm Bombyx mori FOXO (BmFOXO) to elucidate its physiological functions during development of this lepidopteran insect. The BmFOXO mutant (FOXO-M) exhibited growth delays from the first larval stage and showed precocious metamorphosis, pupating at the end of the fourth instar (trimolter) rather than at the end of the fifth instar as in the wild-type (WT) animals. However, different from previous reports on precocious metamorphosis caused by juvenile hormone (JH) deficiency in silkworm mutants, the total developmental time of the larval period in the FOXO-M was comparable with that of the WT. Exogenous application of 20-hydroxyecdysone (20E) or of the JH analog rescued the trimolter phenotype. RNA-seq and gene expression analyses indicated that genes involved in JH degradation but not in JH biosynthesis were up-regulated in the FOXO-M compared with the WT animals. Moreover, we identified several FOXO-binding sites in the promoter of genes coding for JH-degradation enzymes. These results suggest that FOXO regulates JH degradation rather than its biosynthesis, which further modulates hormone homeostasis to control growth and development in B. mori In conclusion, we have uncovered a pivotal role for FOXO in regulating JH signaling to control insect development.

Proteome Analysis of Drosophila Mutants Identifies a Regulatory Role for 14-3-3ε in Metabolic Pathways

The evolutionary conserved family of 14-3-3 proteins appears to have a role in integrating numerous intracellular pathways, including signal transduction, intracellular trafficking, and metabolism. However, little is known about how this interactive network might be affected by the direct abrogation of 14-3-3 function. The loss of Drosophila 14-3-3ε resulted in reduced survival of mutants during larval-to-adult transition, which is known to depend on an energy supply coming from the histolysis of fat body tissue. Here we report a differential proteomic analysis of larval fat body tissue at the onset of larval-to-adult transition, with the loss of 14-3-3ε resulting in the altered abundance of 16 proteins. These included proteins linked to protein biosynthesis, glycolysis, tricarboxylic acid cycle, and lipid metabolic pathways. The ecdysone receptor (EcR), which is responsible for initiating the larval-to-adult transition, colocalized with 14-3-3ε in wild-type fat body tissues. The altered protein abundance in 14-3-3ε mutant fat body tissue was associated with transcriptional deregulation of alcohol dehydrogenase, fat body protein 1, and lamin genes, which are known targets of the EcR. This study indicates that 14-3-3ε has a critical role in cellular metabolism involving either molecular crosstalk with the EcR or direct interaction with metabolic proteins.

Spastic paraplegia-linked phospholipase PAPLA1 is necessary for development, reproduction, and energy metabolism in Drosophila

The human PAPLA1 phospholipase family is associated with hereditary spastic paraplegia (HSP), a neurodegenerative syndrome characterized by progressive spasticity and weakness of the lower limbs. Taking advantage of a new Drosophila PAPLA1 mutant, we describe here novel functions of this phospholipase family in fly development, reproduction, and energy metabolism. Loss of Drosophila PAPLA1 reduces egg hatchability, pre-adult viability, developmental speed, and impairs reproductive functions of both males and females. In addition, our work describes novel metabolic roles of PAPLA1, manifested as decreased food intake, lower energy expenditure, and reduced ATP levels of the mutants. Moreover, PAPLA1 has an important role in the glycogen metabolism, being required for expression of several regulators of carbohydrate metabolism and for glycogen storage. In contrast, global loss of PAPLA1 does not affect fat reserves in adult flies. Interestingly, several of the PAPLA1 phenotypes in fly are reminiscent of symptoms described in some HSP patients, suggesting evolutionary conserved functions of PAPLA1 family in the affected processes. Altogether, this work reveals novel physiological functions of PAPLA1, which are likely evolutionary conserved from flies to humans.

MYC, Cell Competition, and Cell Death in Cancer: The Inseparable Triad

Deregulation of MYC family proteins in cancer is associated with a global reprogramming of gene expression, ultimately promoting glycolytic pathways, cell growth, and proliferation. It is well known that MYC upregulation triggers cell-autonomous apoptosis in normal tissues, while frankly malignant cells develop resistance to apoptotic stimuli, partly resulting from MYC addiction. As well as inducing cell-autonomous apoptosis, MYC upregulation is able to trigger non cell-autonomous apoptotic death through an evolutionarily conserved mechanism known as “cell competition”. With regard to this intimate and dual relationship between MYC and cell death, recent evidence obtained in Drosophila models of cancer has revealed that, in early tumourigenesis, MYC upregulation guides the clonal expansion of mutant cells, while the surrounding tissue undergoes non-cell autonomous death. Apoptosis inhibition in this context was shown to restrain tumour growth and to restore a wild-type phenotype. This suggests that cell-autonomous and non cell-autonomous apoptosis dependent on MYC upregulation may shape tumour growth in different ways, soliciting the need to reconsider the role of cell death in cancer in the light of this new level of complexity. Here we review recent literature about MYC and cell competition obtained in Drosophila, with a particular emphasis on the relevance of cell death to cell competition and, more generally, to cancer. Possible implications of these findings for the understanding of mammalian cancers are also discussed.

Genome-wide microarray screening for Bombyx mori genes related to transmitting the determination outcome of whether to produce diapause or nondiapause eggs

The bivoltine silkworm Bombyx mori (Lepidoptera: Bombycidae) exhibits a maternally controlled embryonic diapause. Maternal silkworms decide whether to lay diapause or nondiapause eggs depending on environmental factors such as the temperature and photoperiod during the egg and larval stages, and then induce diapause eggs during the pupal stage. However, little is known about the molecular mechanism that conveys the outcome of whether to produce diapause or nondiapause eggs from the egg or larval stages to the pupal stage. This study used microarray analysis to investigate differentially expressed genes in the larval brains of diapause- and nondiapause-egg producers, to which bivoltine silkworms were destined by thermal or photic stimulation during the egg stage. The cytochrome P450 18a1 and Krüppel homolog 1 genes were upregulated in producers of diapause eggs compared with those of nondiapause eggs under both experimental conditions. Cytochrome P450 18a1 encodes a key enzyme for steroid hormone inactivation and Krüppel homolog 1 is an early juvenile hormone-inducible gene that mediates the repression of metamorphosis. The upregulation of these genes during the larval stage might be involved in the signaling pathway that transmits information about the diapause program from the egg stage to the pupal stage in the silkworm.

Insulin and 20-hydroxyecdysone action in Bombyx mori: Glycogen content and expression pattern of insulin and ecdysone receptors in fat body

Insulin and ecdysone signaling play a critical role on the growth and development of insects including Bombyx mori. Our previous study showed that Bombyx larvae reached critical weight for metamorphosis between day 3.5 and 4 of the fifth larval instar. The present study showed that the effect of insulin on the accumulation of glycogen in fat body of Bombyx larvae depends on the critical growth period. When larvae are in active growth period (before reaching critical weight), insulin caused increased accumulation of glycogen, while its treatment in larvae at terminal growth period (after critical period) resulted in an increased mobilization of glycogen. During terminal growth period, insulin and 20-hydroxyecdysone (20E) showed an antagonistic effect on the accumulation of fat body glycogen in fed, food deprived and decapitated larvae as well as in isolated abdomens. Insulin treatment decreased the glycogen content, whereas, 20E increased it. Food deprivation and decapitation caused an increase in the transcript levels of insulin receptor (InR) and this increase in InR expression might be attributed to a decrease in synthesis/secretion of insulin-like peptides, as insulin treatment in these larvae showed a down-regulation in InR expression. However, insulin showed an up-regulation in InR in isolated abdomens and it suggests that in food deprived and decapitated larvae, the exogenous insulin may interact with some head and/or thoracic factors in modulating the expression of InR. Moreover, in fed larvae, insulin-mediated increase in InR expression indicates that its regulation by insulin-like peptides also depends on the nutritional status of the larvae. The treatment of 20E in fed larvae showed an antagonistic effect on the transcript levels since a down-regulation in InR expression was observed. 20E treatment also led to a decreased expression of InR in food deprived and decapitated larvae as well as in isolated abdomens. Insulin and 20E also modulated the expression level of ecdysone receptors (EcRB1 and USP1). 20E treatment showed an up-regulation in expression of ecdysone receptors, but only in fed larvae, whereas insulin treatment showed a down-regulation in the expression of EcRB1 and USP1 in all the experimental larvae studied. Further, the data indicates that an up-regulation of ecdysone receptors is associated with an increase in fat body glycogen content, whereas an up-regulation of insulin receptor expression causes glycogen mobilization. The study, therefore, suggests that the insulin and ecdysone signaling are linked to each other and that both insulin and ecdysone are involved in regulating the carbohydrate reserves in B. mori.

Sexual Dimorphism of Body Size Is Controlled by Dosage of the X-Chromosomal Gene Myc and by the Sex-Determining Gene tra in Drosophila

Drosophila females are larger than males. In this article, we describe how X-chromosome dosage drives sexual dimorphism of body size through two means: first, through unbalanced expression of a key X-linked growth-regulating gene, and second, through female-specific activation of the sex-determination pathway. X-chromosome dosage determines phenotypic sex by regulating the genes of the sex-determining pathway. In the presence of two sets of X-chromosome signal elements (XSEs), Sex-lethal (Sxl) is activated in female (XX) but not male (XY) animals. Sxl activates transformer (tra), a gene that encodes a splicing factor essential for female-specific development. It has previously been shown that null mutations in the tra gene result in only a partial reduction of body size of XX animals, which shows that other factors must contribute to size determination. We tested whether X dosage directly affects animal size by analyzing males with duplications of X-chromosomal segments. Upon tiling across the X chromosome, we found four duplications that increase male size by >9%. Within these, we identified several genes that promote growth as a result of duplication. Only one of these, Myc, was found not to be dosage compensated. Together, our results indicate that both Myc dosage and tra expression play crucial roles in determining sex-specific size in Drosophila larvae and adult tissue. Since Myc also acts as an XSE that contributes to tra activation in early development, a double dose of Myc in females serves at least twice in development to promote sexual size dimorphism.

BmATG5 and BmATG6 mediate apoptosis following autophagy induced by 20-hydroxyecdysone or starvation

Autophagy and apoptosis, which could be induced by common stimuli, play crucial roles in development and disease. The functional relationship between autophagy and apoptosis is complex, due to the dual effects of autophagy. In the Bombyx Bm-12 cells, 20-hydroxyecdysone (20E) treatment or starvation-induced cell death, with autophagy preceding apoptosis. In response to 20E or starvation, BmATG8 was rapidly cleaved and conjugated with PE to form BmATG8-PE; subsequently, BmATG5 and BmATG6 were cleaved into BmATG5-tN and BmATG6-C, respectively. Reduction of expression of BmAtg5 or BmAtg6 by RNAi decreased the proportion of cells undergoing both autophagy and apoptosis after 20E treatment or starvation. Overexpression of BmAtg5 or BmAtg6 induced autophagy but not apoptosis in the absence of the stimuli, but promoted both autophagy and apoptosis induced by 20E or starvation. Notably, overexpression of cleavage site-deleted BmAtg5 or BmAtg6 increased autophagy but not apoptosis induced by 20E or starvation, whereas overexpression of BmAtg5-tN and BmAtg6-C was able to directly trigger apoptosis or promote the induced apoptosis. In conclusion, being cleaved into BmATG5-tN and BmATG6-C, BmATG5 and BmATG6 mediate apoptosis following autophagy induced by 20E or starvation in Bombyx Bm-12 cells, reflecting that autophagy precedes apoptosis in the midgut during Bombyx metamorphosis.

The role of the microbial environment in Drosophila post-embryonic development

Development, growth and maturation of animals are under genetic and environmental control. Multicellular organisms interact throughout their lives with a variety of environment- and body-associated microorganisms. It has now been appreciated that the very conspicuous and varied microbial population associated with the food and the gastro-intestinal tract is a critical factor that can influence growth. Beyond the phenomenology, the mechanisms underlying the beneficial effects of microbes on development are being revealed from studies in Drosophila melanogaster, a particularly well suited system for a mechanistic understanding of host/microbiota interactions. Association of otherwise germ-free eggs with specific bacterial strains isolated from Drosophila gut samples can accelerate growth in larvae raised on restrictive diets. We review advances made possible by the exploitation of such simplified gnotobiotic systems in the search for the genes, molecules and physiological adaptations responsible for this effect in both host and microbes. Transposon mutagenesis and gene-trait match studies in bacteria can identify the key microbial genes and metabolites required for the beneficial effect, acetic acid being one of them. In the fly, functional genomic analysis, transcriptomics and metabolomics point to the modulation of systemic insulin and steroid hormone signalling as well as the regulation of intestinal physiology, including the enhancement of intestinal protease activity, as crucial mediators of the host's response.

Arrested development: coordinating regeneration with development and growth in Drosophila melanogaster

The capacity for tissues to regenerate often varies during development. A better understanding how developmental context regulates regenerative capacity will be an important step towards enhancing the regenerative capacity of tissues to repair disease or damage. Recent work examining the regeneration of imaginal discs in the fruit fly, Drosophila melanogaster, has begun to identify mechanisms by which developmental progress restricts regeneration, and elucidate how Drosophila coordinates regenerative repair with the growth and development of the entire organism. Here we review recent advances in describing the interplay between development and tissue regeneration in Drosophila and identify questions that arise from these findings.

Effects of starvation and hormones on DNA synthesis in silk gland cells of the silkworm, Bombyx mori

Silk gland cells of silkworm larvae undergo multiple cycles of endomitosis for the synthesis of silk proteins during the spinning phase. In this paper, we analyzed the endomitotic DNA synthesis of silk gland cells during larval development, and found that it was a periodic fluctuation, increasing during the vigorous feeding phase and being gradually inhibited in the next molting phase. That means it might be activated by a self-regulating process after molting. The expression levels of cyclin E, cdt1 and pcna were consistent with these developmental changes. Moreover, we further examined whether these changes in endomitotic DNA synthesis resulted from feeding or hormonal stimulation. The results showed that DNA synthesis could be inhibited by starvation and re-activated by re-feeding, and therefore appears to be dependent on nutrition. DNA synthesis was suppressed by in vivo treatment with 20-hydroxyecdysone (20E). However, there was no effect on DNA synthesis by in vitro 20E treatment or by either in vivo or in vitro juvenile hormone treatment. The levels of Akt and 4E-BP phosphorylation in the silk glands were also reduced by starvation and in vivo treatment with 20E. These results indicate that the activation of endomitotic DNA synthesis during the intermolt stages is related to feeding and DNA synthesis is inhibited indirectly by 20E.

Torso, a Drosophila receptor tyrosine kinase, plays a novel role in the larval fat body in regulating insulin signaling and body growth

Torso is a receptor tyrosine kinase whose localized activation at the termini of the Drosophila embryo is mediated by its ligand, Trunk. Recent studies have unveiled a second function of Torso in the larval prothoracic gland (PG) as the receptor for the prothoracicotropic hormone, which triggers pupariation. As such, inhibition of Torso in the PG prolongs the larval growth period, thereby increasing the final pupa size. Here, we report that Torso also acts in the larval fat body, regulating body size in a manner opposite from that of Torso in PG. We confirmed the expression of torso mRNA in the larval fat body and its reduction by RNA interference (RNAi). Fat body-specific knockdown of torso, by either of the two independent RNAi transgenes, significantly decreased the final pupal size. We found that torso knockdown suppresses insulin/target of rapamycin (TOR) signaling in the fat body, as confirmed by repression of Akt and S6K. Notably, the decrease in insulin/TOR signaling and decrease of pupal size induced by the knockdown of torso were rescued by the expression of a constitutively active form of the insulin receptor or by the knockdown of FOXO. Our study revealed a novel role for Torso in the fat body with respect to regulation of insulin/TOR signaling and body size. This finding exemplifies the contrasting effects of the same gene expressed in two different organs on organismal physiology.

Ecdysteroid promotes cell cycle progression in the Bombyx wing disc through activation of c-Myc

Developmental switching from growth to metamorphosis in imaginal primordia is an essential process of adult body planning in holometabolous insects. Although it is disciplined by a sequential action of the ecdysteroid, molecular mechanisms linking to cell proliferation are poorly understood. In the present study, we investigated the expression control of cell cycle-related genes by the ecdysteroid using the wing disc of the final-instar larvae of the silkworm, Bombyx mori. We found that the expression level of c-myc was remarkably elevated in the post-feeding cell proliferation phase, which coincided with a small increase in ecdysteroid titer. An in vitro wing disc culture showed that supplementation of the moderate level of the ecdysteroid upregulated c-myc expression within an hour and subsequently increased the expression of cell cycle core regulators, including A-, B-, D-, and E-type cyclin genes, Cdc25 and E2F1. We demonstrated that c-myc upregulation by the ecdysteroid was not inhibited in the presence of a protein synthesis inhibitor, suggesting a possibility that the ecdysteroid directly stimulates c-myc expression. Finally, results from the administration of a c-Myc inhibitor demonstrated that c-Myc plays an essential role in 20E-inducible cell proliferation. These findings suggested a novel pathway for ecdysteroid-inducible cell proliferation in insects, and it is likely to be conserved between insects and mammals in terms of steroid hormone regulation.

The Sex Determination Gene transformer Regulates Male-Female Differences in Drosophila Body Size

Almost all animals show sex differences in body size. For example, in Drosophila, females are larger than males. Although Drosophila is widely used as a model to study growth, the mechanisms underlying this male-female difference in size remain unclear. Here, we describe a novel role for the sex determination gene transformer (tra) in promoting female body growth. Normally, Tra is expressed only in females. We find that loss of Tra in female larvae decreases body size, while ectopic Tra expression in males increases body size. Although we find that Tra exerts autonomous effects on cell size, we also discovered that Tra expression in the fat body augments female body size in a non cell-autonomous manner. These effects of Tra do not require its only known targets doublesex and fruitless. Instead, Tra expression in the female fat body promotes growth by stimulating the secretion of insulin-like peptides from insulin producing cells in the brain. Our data suggest a model of sex-specific growth in which body size is regulated by a previously unrecognized branch of the sex determination pathway, and identify Tra as a novel link between sex and the conserved insulin signaling pathway.

Female-biased sexual size dimorphism is common in invertebrates, yet the mechanisms underlying increased female body size remain unclear. We uncovered a key role for sex determination gene transformer (tra) in promoting increased growth in females. Interestingly, we found that sex differences in body size are regulated by Tra in a pathway that is separate of the canonical sex determination pathway, and of other aspects of sexual dimorphism. Instead, Tra function in the fat body regulates growth in a non cell-autonomous manner by regulating the secretion of insulin-like peptides from the brain. This novel Tra-insulin link we describe may have implications for other sexually dimorphic phenotypes in Drosophila (eg. lifespan, stress resistance), many of which are also regulated by insulin.

Drosophila Lgr3 Couples Organ Growth with Maturation and Ensures Developmental Stability

Early transplantation and grafting experiments suggest that body organs follow autonomous growth programs [1-3], therefore pointing to a need for coordination mechanisms to produce fit individuals with proper proportions. We recently identified Drosophila insulin-like peptide 8 (Dilp8) as a relaxin and insulin-like molecule secreted from growing tissues that plays a central role in coordinating growth between organs and coupling organ growth with animal maturation [4, 5]. Deciphering the function of Dilp8 in growth coordination relies on the identification of the receptor and tissues relaying Dilp8 signaling. We show here that the orphan receptor leucine-rich repeat-containing G protein-coupled receptor 3 (Lgr3), a member of the highly conserved family of relaxin family peptide receptors (RXFPs), mediates the checkpoint function of Dilp8 for entry into maturation. We functionally identify two Lgr3-positive neurons in each brain lobe that are required to induce a developmental delay upon overexpression of Dilp8. These neurons are located in the pars intercerebralis, an important neuroendocrine area in the brain, and make physical contacts with the PTTH neurons that ultimately control the production and release of the molting steroid ecdysone. Reducing Lgr3 levels in these neurons results in adult flies exhibiting increased fluctuating bilateral asymmetry, therefore recapitulating the phenotype of dilp8 mutants. Our work reveals a novel Dilp8/Lgr3 neuronal circuitry involved in a feedback mechanism that ensures coordination between organ growth and developmental transitions and prevents developmental variability.

The ecdysteroidome of Drosophila: influence of diet and development

Ecdysteroids are the hormones regulating development, physiology and fertility in arthropods, which synthesize them exclusively from dietary sterols. But how dietary sterol diversity influences the ecdysteroid profile, how animals ensure the production of desired hormones and whether there are functional differences between different ecdysteroids produced in vivo remains unknown. This is because currently there is no analytical technology for unbiased, comprehensive and quantitative assessment of the full complement of endogenous ecdysteroids. We developed a new LC-MS/MS method to screen the entire chemical space of ecdysteroid-related structures and to quantify known and newly discovered hormones and their catabolites. We quantified the ecdysteroidome in Drosophila melanogaster and investigated how the ecdysteroid profile varies with diet and development. We show that Drosophila can produce four different classes of ecdysteroids, which are obligatorily derived from four types of dietary sterol precursors. Drosophila makes makisterone A from plant sterols and epi-makisterone A from ergosterol, the major yeast sterol. However, they prefer to selectively utilize scarce ergosterol precursors to make a novel hormone 24,28-dehydromakisterone A and trace cholesterol to synthesize 20-hydroxyecdysone. Interestingly, epi-makisterone A supports only larval development, whereas all other ecdysteroids allow full adult development. We suggest that evolutionary pressure against producing epi-C-24 ecdysteroids might explain selective utilization of ergosterol precursors and the puzzling preference for cholesterol.

The Systemic Control of Growth

Growth is a complex process that is intimately linked to the developmental program to form adults with proper size and proportions. Genetics is an important determinant of growth, as exemplified by the role of local diffusible molecules setting up organ proportions. In addition, organisms use adaptive responses allowing modulating the size of individuals according to environmental cues, for example, nutrition. Here, we describe some of the physiological principles participating in the determination of final individual size.

Ecdysone promotes growth of imaginal discs through the regulation of Thor in D. melanogaster

Animals have a determined species-specific body size that results from the combined action of hormones and signaling pathways regulating growth rate and duration. In Drosophila, the steroid hormone ecdysone controls developmental transitions, thereby regulating the duration of the growth period. Here we show that ecdysone promotes the growth of imaginal discs in mid-third instar larvae, since imaginal discs from larvae with reduced or no ecdysone synthesis are smaller than wild type due to smaller and fewer cells. We show that insulin-like peptides are produced and secreted normally in larvae with reduced ecdysone synthesis, and upstream components of insulin/insulin-like signaling are activated in their discs. Instead, ecdysone appears to regulate the growth of imaginal discs via Thor/4E-BP, a negative growth regulator downstream of the insulin/insulin-like growth factor/Tor pathways. Discs from larvae with reduced ecdysone synthesis have elevated levels of Thor, while mutations in Thor partially rescue their growth. The regulation of organ growth by ecdysone is evolutionarily conserved in hemimetabolous insects, as shown by our results obtained using Blattella germanica. In summary, our data provide new insights into the relationship between components of the insulin/insulin-like/Tor and ecdysone pathways in the control of organ growth.

The ecdysone receptor coactivator Taiman links Yorkie to transcriptional control of germline stem cell factors in somatic tissue

The Hippo pathway is a conserved signaling cascade that modulates tissue growth. Although its core elements are well defined, factors modulating Hippo transcriptional outputs remain elusive. Here we show that components of the steroid-responsive ecdysone (Ec) pathway modulate Hippo transcriptional effects in imaginal disc cells. The Ec receptor coactivator Taiman (Tai) interacts with the Hippo transcriptional coactivator Yorkie (Yki) and promotes expression of canonical Yki-responsive genes. Tai enhances Yki-driven growth, while Tai loss, or a form of Tai unable to bind Yki, suppresses Yki-driven tissue growth. This growth suppression is not correlated with impaired induction of canonical Hippo-responsive genes but with suppression of a distinct pro-growth program of Yki-induced/Tai-dependent genes, including the germline stem cell factors nanos and piwi. These data reveal Hippo/Ec pathway crosstalk in the form a Yki-Tai complex that collaboratively induces germline genes as part of a transcriptional program that is normally repressed in developing somatic epithelia.

Nitric Oxide Synthase Regulates Growth Coordination During Drosophila melanogaster Imaginal Disc Regeneration

Mechanisms that coordinate growth during development are essential for producing animals with proper organ proportion. Here we describe a pathway through which tissues communicate to coordinate growth. During Drosophila melanogaster larval development, damage to imaginal discs activates a regeneration checkpoint through expression of Dilp8. This both produces a delay in developmental timing and slows the growth of undamaged tissues, coordinating regeneration of the damaged tissue with developmental progression and overall growth. Here we demonstrate that Dilp8-dependent growth coordination between regenerating and undamaged tissues, but not developmental delay, requires the activity of nitric oxide synthase (NOS) in the prothoracic gland. NOS limits the growth of undamaged tissues by reducing ecdysone biosynthesis, a requirement for imaginal disc growth during both the regenerative checkpoint and normal development. Therefore, NOS activity in the prothoracic gland coordinates tissue growth through regulation of endocrine signals.

Size control: the developmental physiology of body and organ size regulation

The developmental regulation of final body and organ size is fundamental to generating a functional and correctly proportioned adult. Research over the last two decades has identified a long list of genes and signaling pathways that, when perturbed, influence final body size. However, body and organ size are ultimately a characteristic of the whole organism, and how these myriad genes and pathways function within a physiological context to control size remains largely unknown. In this review, we first describe the major size-regulatory signaling pathways: the Insulin/IGF-, RAS/RAF/MAPK-, TOR-, Hippo-, and JNK-signaling pathways. We then explore what is known of how these pathways regulate five major aspects of size regulation: growth rate, growth duration, target size, negative growth and growth coordination. While this review is by no means exhaustive, our goal is to provide a conceptual framework for integrating the mechanisms of size control at a molecular-genetic level with the mechanisms of size control at a physiological level.