Opposing effects of ecdysone signaling regulate neuroblast proliferation to ensure coordination of brain and organism development

Growth regulation must be robust to ensure correct final size, but also adaptative to adjust to less favorable environmental conditions. Developmental coordination between whole-organism and the brain is particularly important, as the brain is a critical organ with little adaptability. Brain growth mainly depends on neural stem cell (NSC) proliferation to generate differentiated neural cells, it is however unclear how organism developmental progression is coordinated with NSCs. Here we demonstrate that the steroid hormone ecdysone plays a multi-step, stage specific role in regulating Drosophila NSCs, the neuroblasts. We used animals that are unable to synthesize ecdysone, to show that the developmental milestone called "critical weight peak", the peak that informs the body has reached minimum viable weight to survive metamorphosis, acts a checkpoint necessary to set neuroblast cell cycle pace during larval neurogenesis. The peaks of ecdysone that occur post-critical weight are no longer required to maintain neuroblast division rate. We additionally show that in a second stage, at the onset of pupariation, ecdysone is instead required to trigger neuroblast's proliferation exit and consequently the end of neurogenesis. We demonstrate that, without this signal from ecdysone, neuroblasts lose their ability to exit proliferation. Interestingly, although these neuroblasts proliferate for a longer period, the number of differentiated neurons is smaller compared to wild-type brains, suggesting a role for ecdysone in neuron maintenance. Our study provides insights into how neural stem cells coordinate their division rate with the pace of body growth, identifying a novel coordination mechanism between animal development and NSC proliferation.

Starvation selection reduces and delays larval ecdysone production and signaling

Previous studies have shown that selection for starvation resistance in Drosophila melanogaster results in delayed eclosion and increased adult fat stores. It is assumed that these traits are caused by the starvation selection pressure, but its mechanism is unknown. We found that our starvation-selected (SS) population stores more fat during larval development and has extended larval development and pupal development time. Developmental checkpoints in the third instar associated with ecdysteroid hormone pulses are increasingly delayed. The delay in the late larval period seen in the SS population is indicative of reduced and delayed ecdysone signaling. An enzyme immunoassay for ecdysteroids (with greatest affinity to the metabolically active 20-hydroxyecdysone and the α-ecdysone precursor) confirmed that the SS population had reduced and delayed hormone production compared with that of fed control (FC) flies. Feeding third instar larvae on food supplemented with α-ecdysone partially rescued the developmental delay and reduced subsequent adult starvation resistance. This work suggests that starvation selection causes reduced and delayed production of ecdysteroids in the larval stage and affects the developmental delay phenotype that contributes to subsequent adult fat storage and starvation resistance.

Rethinking the ecdysteroid source during Drosophila pupal-adult development

Ecdysteroids, typified by 20-hydroxyecdysone (20E), are essential hormones for the development, reproduction and physiology of insects and other arthropods. For over half a century, the vinegar fly Drosophila melanogaster (Ephydroidea: Diptera) has been used as a model of ecdysteroid biology. Many aspects of the biosynthesis and regulation of ecdysteroids in this species are understood at the molecular level, particularly with respect to their secretion from the prothoracic gland (PG) cells of the ring gland, widely considered the dominant biosynthetic tissue during development. Discrete pulses of 20E orchestrate transitions during the D. melanogaster life cycle, the sources of which are generally well understood, apart from the large 20E pulse at the onset of pharate adult development, which has received little recent attention. As the source of this pharate adult pulse (PAP) is a curious blind spot in Drosophila endocrinology, we evaluate published biochemical and genetic data as they pertain to three hypotheses for the source of PAP 20E: the PG; an alternative biosynthetic tissue; or the recycling of stored 20E. Based on multiple lines of evidence, we contend the PAP cannot be derived from biosynthesis, with other data consistent with D. melanogaster able to recycle ecdysteroids before and during metamorphosis. Published data also suggest the PAP is conserved across Diptera, with evidence for pupal-adult ecdysteroid recycling occurring in other cyclorrhaphan flies. Further experimental work is required to test the ecdysteroid recycling hypothesis, which would establish fundamental knowledge of the function, regulation, and evolution of metamorphic hormones in dipterans and other insects.

The steroid hormone ecdysone regulates growth rate in response to oxygen availability

In almost all animals, physiologically low oxygen (hypoxia) during development slows growth and reduces adult body size. The developmental mechanisms that determine growth under hypoxic conditions are, however, poorly understood. Here we show that the growth and body size response to moderate hypoxia (10% O₂) in Drosophila melanogaster is systemically regulated via the steroid hormone ecdysone. Hypoxia increases level of circulating ecdysone and inhibition of ecdysone synthesis ameliorates the negative effect of low oxygen on growth. We also show that the effect of ecdysone on growth under hypoxia is through suppression of the insulin/IGF-signaling pathway, via increased expression of the insulin-binding protein Imp-L2. These data indicate that growth suppression in hypoxic Drosophila larvae is accomplished by a systemic endocrine mechanism that overlaps with the mechanism that slows growth at low nutrition. This suggests the existence of growth-regulatory mechanisms that respond to general environmental perturbation rather than individual environmental factors.

Body-fat sensor triggers ribosome maturation in the steroidogenic gland to initiate sexual maturation in Drosophila

Fat stores are critical for reproductive success and may govern maturation initiation. Here, we report that signaling and sensing fat sufficiency for sexual maturation commitment requires the lipid carrier apolipophorin in fat cells and Sema1a in the neuroendocrine prothoracic gland (PG). Larvae lacking apolpp or Sema1a fail to initiate maturation despite accruing sufficient fat stores, and they continue gaining weight until death. Mechanistically, sensing peripheral body-fat levels via the apolipophorin/Sema1a axis regulates endocytosis, endoplasmic reticulum remodeling, and ribosomal maturation for the acquisition of the PG cells' high biosynthetic and secretory capacity. Downstream of apolipophorin/Sema1a, leptin-like upd2 triggers the cessation of feeding and initiates sexual maturation. Human Leptin in the insect PG substitutes for upd2, preventing obesity and triggering maturation downstream of Sema1a. These data show how peripheral fat levels regulate the control of the maturation decision-making process via remodeling of endomembranes and ribosomal biogenesis in gland cells.

Divergent mechanisms for regulating growth and development after imaginal disc damage in the tobacco hornworm, Manduca sexta

Holometabolous insects have been able to radiate to vast ecological niches as adults through the evolution of adult-specific structures such as wings, antennae and eyes. These structures arise from imaginal discs that show regenerative capacity when damaged. During imaginal disc regeneration, development has been shown to be delayed in the fruit fly Drosophila melanogaster, but how conserved the delay-inducing mechanisms are across holometabolous insects has not been assessed. The goal of this research was to develop the hornworm Manduca sexta as an alternative model organism to study such damage-induced mechanisms, with the advantage of a larger hemolymph volume enabling access to the hormonal responses to imaginal disc damage. Upon whole-body X-ray exposure, we noted that the imaginal discs were selectively damaged, as assessed by TUNEL and Acridine Orange stains. Moreover, development was delayed, predominantly at the pupal-to-adult transition, with a concomitant delay in the prepupal ecdysteroid peak. The delays to eclosion were dose dependent, with some ability for repair of damaged tissues. We noted a shift in critical weight, as assessed by the point at which starvation no longer impacted developmental timing, without a change in growth rate, which was uncoupled from juvenile hormone clearance in the body. The developmental profile was different from that of D. melanogaster, which suggests species differences may exist in the mechanisms delaying development.

Drosophila melanogaster larvae make nutritional choices that minimize developmental time

Organisms from slime moulds to humans carefully regulate their macronutrient intake to optimize a wide range of life history characters including survival, stress resistance, and reproductive success. However, life history characters often differ in their response to nutrition, forcing organisms to make foraging decisions while balancing the trade-offs between these effects. To date, we have a limited understanding of how the nutritional environment shapes the relationship between life history characters and foraging decisions. To gain insight into the problem, we used a geometric framework for nutrition to assess how the protein and carbohydrate content of the larval diet affected key life history traits in the fruit fly, Drosophila melanogaster. In no-choice assays, survival from egg to pupae, female and male body size, and ovariole number - a proxy for female fecundity - were maximized at the highest protein to carbohydrate (P:C) ratio (1.5:1). In contrast, development time was minimized at intermediate P:C ratios, around 1:2. Next, we subjected larvae to two-choice tests to determine how they regulated their protein and carbohydrate intake in relation to these life history traits. Our results show that larvae targeted their consumption to P:C ratios that minimized development time. Finally, we examined whether adult females also chose to lay their eggs in the P:C ratios that minimized developmental time. Using a three-choice assay, we found that adult females preferentially laid their eggs in food P:C ratios that were suboptimal for all larval life history traits. Our results demonstrate that D. melanogaster larvae make foraging decisions that trade-off developmental time with body size, ovariole number, and survival. In addition, adult females make oviposition decisions that do not appear to benefit the larvae. We propose that these decisions may reflect the living nature of the larval nutritional environment in rotting fruit. These studies illustrate the interaction between the nutritional environment, life history traits, and foraging choices in D. melanogaster, and lend insight into the ecology of their foraging decisions.

Respiratory metabolism of salivary glands during the late larval and prepupal development of Drosophila melanogaster

During the late larval period, the salivary glands (SG) of Drosophila show a cascade of cytological changes associated with exocytosis and the expectoration of the proteinaceous glue that is used to affix the pupariating larva to a substrate. After puparium formation (APF), SG undergo extensive cytoplasmic vacuolation due to endocytosis, vacuole consolidation and massive apocrine secretion. Here we investigated possible correlations between cytological changes, the puffing pattern in polytene chromosomes and respiratory metabolism of the SG. The carefully staged SG were explanted into small amounts (1 or 2μl) of tissue culture medium. The respiratory metabolism of single or up to 3 pairs of glands was evaluated by recording the rate of O2 consumption using a scanning microrespirographic technique sensitive to subnanoliter volumes of the respiratory O2 or CO2. The recordings were carried out at times between 8h before pupariation (BPF), until 16h APF, at which point the SG completely disintegrate. At the early wandering larval stage (8h BPF), the glands consume 2nl of O2/gland/min (=2500μl O2/g/h). This relatively high metabolic rate decreases down to 1.2-1.3nl of O2 during the endogenous peak in ecdysteroid concentration that culminates around pupariation. The metabolic decline coincides with the exocytosis of the proteinaceous glue. During and shortly after puparium formation, which is accompanied cytologically by intense vacuolation, O2 consumption in the SG temporarily increases to 1.6nl O2/gland/min. After this time, the metabolic rate of the SG decreases downward steadily until 16h APF, when the glands disintegrate and cease to consume oxygen. The SG we analyzed from Drosophila larvae were composed of 134 intrinsic cells, with the average volume of one lobe being 37nl. Therefore, a single SG cell of the wandering larva (with O2 consumption of 2nl/gland/min), consumes each about 16pl of O2/cell/min. A simultaneous analysis of the rate of protein and RNA synthesis in the SG shows a course similar to that found in respiratory metabolism.

Early transcriptional events linked to induction of diapause revealed by RNAseq in larvae of drosophilid fly, Chymomyza costata

Background

Diapause is a developmental alternative to direct ontogeny in many invertebrates. Its primary adaptive meaning is to secure survival over unfavourable seasons in a state of developmental arrest usually accompanied by metabolic suppression and enhanced tolerance to environmental stressors. During photoperiodically triggered diapause of insects, the ontogeny is centrally turned off under hormonal control, the molecular details of this transition being poorly understood. Using RNAseq technology, we characterized transcription profiles associated with photoperiodic diapause induction in the larvae of the drosophilid fly Chymomyza costata with the goal of identifying candidate genes and processes linked to upstream regulatory events that eventually lead to a complex phenotypic change.

Results

Short day photoperiod triggering diapause was associated to inhibition of 20-hydroxy ecdysone (20-HE) signalling during the photoperiod-sensitive stage of C. costata larval development. The mRNA levels of several key genes involved in 20-HE biosynthesis, perception, and signalling were significantly downregulated under short days. Hormonal change was translated into downregulation of a series of other transcripts with broad influence on gene expression, protein translation, alternative histone marking by methylation and alternative splicing. These changes probably resulted in blockade of direct development and deep restructuring of metabolic pathways indicated by differential expression of genes involved in cell cycle regulation, metabolism, detoxification, redox balance, protection against oxidative stress, cuticle formation and synthesis of larval storage proteins. This highly complex alteration of gene transcription was expressed already during first extended night, within the first four hours after the change of the photoperiodic signal from long days to short days. We validated our RNAseq differential gene expression results in an independent qRT-PCR experiment involving wild-type (photoperiodic) and NPD-mutant (non-photoperiodic) strains of C. costata.

Conclusions

Our study revealed several strong candidate genes for follow-up functional studies. Candidate genes code for upstream regulators of a complex change of gene expression, which leads to phenotypic switch from direct ontogeny to larval diapause.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1907-4) contains supplementary material, which is available to authorized users.

Tissue damage disrupts developmental progression and ecdysteroid biosynthesis in Drosophila

In humans, chronic inflammation, severe injury, infection and disease can result in changes in steroid hormone titers and delayed onset of puberty; however the pathway by which this occurs remains largely unknown. Similarly, in insects injury to specific tissues can result in a global developmental delay (e.g. prolonged larval/pupal stages) often associated with decreased levels of ecdysone – a steroid hormone that regulates developmental transitions in insects. We use Drosophila melanogaster as a model to examine the pathway by which tissue injury disrupts developmental progression. Imaginal disc damage inflicted early in larval development triggers developmental delays while the effects are minimized in older larvae. We find that the switch in injury response (e.g. delay/no delay) is coincident with the mid-3rd instar transition – a developmental time-point that is characterized by widespread changes in gene expression and marks the initial steps of metamorphosis. Finally, we show that developmental delays induced by tissue damage are associated with decreased expression of genes involved in ecdysteroid synthesis and signaling.

The kinase Sgg modulates temporal development of macrochaetes in Drosophila by phosphorylation of Scute and Pannier

Evolution of novel structures is often made possible by changes in the timing or spatial expression of genes regulating development. Macrochaetes, large sensory bristles arranged into species-specific stereotypical patterns, are an evolutionary novelty of cyclorraphous flies and are associated with changes in both the temporal and spatial expression of the proneural genes achaete (ac) and scute (sc). Changes in spatial expression are associated with the evolution of cis-regulatory sequences, but it is not known how temporal regulation is achieved. One factor required for ac-sc expression, the expression of which coincides temporally with that of ac-sc in the notum, is Wingless (Wg; also known as Wnt). Wingless downregulates the activity of the serine/threonine kinase Shaggy (Sgg; also known as GSK-3). We demonstrate that Scute is phosphorylated by Sgg on a serine residue and that mutation of this residue results in a form of Sc with heightened proneural activity that can rescue the loss of bristles characteristic of wg mutants. We suggest that the phosphorylated form of Sc has reduced transcriptional activity such that sc is unable to autoregulate, an essential function for the segregation of bristle precursors. Sgg also phosphorylates Pannier, a transcriptional activator of ac-sc, the activity of which is similarly dampened when in the phosphorylated state. Furthermore, we show that Wg signalling does not act directly via a cis-regulatory element of the ac-sc genes. We suggest that temporal control of ac-sc activity in cyclorraphous flies is likely to be regulated by permissive factors and might therefore not be encoded at the level of ac-sc gene sequences.

Developmental model of static allometry in holometabolous insects

The regulation of static allometry is a fundamental developmental process, yet little is understood of the mechanisms that ensure organs scale correctly across a range of body sizes. Recent studies have revealed the physiological and genetic mechanisms that control nutritional variation in the final body and organ size in holometabolous insects. The implications these mechanisms have for the regulation of static allometry is, however, unknown. Here, we formulate a mathematical description of the nutritional control of body and organ size in Drosophila melanogaster and use it to explore how the developmental regulators of size influence static allometry. The model suggests that the slope of nutritional static allometries, the 'allometric coefficient', is controlled by the relative sensitivity of an organ's growth rate to changes in nutrition, and the relative duration of development when nutrition affects an organ's final size. The model also predicts that, in order to maintain correct scaling, sensitivity to changes in nutrition varies among organs, and within organs through time. We present experimental data that support these predictions. By revealing how specific physiological and genetic regulators of size influence allometry, the model serves to identify developmental processes upon which evolution may act to alter scaling relationships.

Imaginal discs regulate developmental timing in Drosophila melanogaster

The regulation of body size in animals involves mechanisms that terminate growth. In holometabolous insects growth ends at the onset of metamorphosis and is contingent on their reaching a critical size in the final larval instar. Despite the importance of critical size in regulating final body size, the developmental mechanisms regulating critical size are poorly understood. Here we demonstrate that the developing adult organs, called imaginal discs, are a regulator of critical size in larval Drosophila. We show that damage to, or slow growth of, the imaginal discs is sufficient to retard metamorphosis both by increasing critical size and extending the period between attainment of critical size and metamorphosis. Nevertheless, larvae with damaged and slow growing discs metamorphose at the same size as wild-type larvae. In contrast, complete removal of all imaginal tissue has no effect on critical size. These data indicate that both attainment of critical size and the timely onset of metamorphosis are regulated by the imaginal discs in Drosophila, and suggest that the termination of growth is coordinated among growing tissues to ensure that all organs attain a characteristic final size.

Size and shape: the developmental regulation of static allometry in insects

Among all organisms, the size of each body part or organ scales with overall body size, a phenomenon called allometry. The study of shape and form has attracted enormous interest from biologists, but the genetic, developmental and physiological mechanisms that control allometry and the proportional growth of parts have remained elusive. Recent progress in our understanding of body-size regulation provides a new synthetic framework for thinking about the mechanisms and the evolution of allometric scaling. In particular, insulin/IGF signaling, which plays major roles in longevity, diabetes and the regulation of cell, organ and body size, might also be centrally involved in regulating organismal shape. Here we review recent advances in the fields of growth regulation and endocrinology and use them to construct a developmental model of static allometry expression in insects. This model serves as the foundation for a research program that will result in a deeper understanding of the relationship between growth and form, a question that has fascinated biologists for centuries.

Discrete pulses of molting hormone, 20-hydroxyecdysone, during late larval development of Drosophila melanogaster: correlations with changes in gene activity

Periodic pulses of the insect steroid molting hormone 20-hydroxyecdysone (20E), acting via its nuclear receptor complex (EcR/USP), control gene expression at many stages throughout Drosophila development. However, during the last larval instar of some lepidopteran insects, subtle changes in titers of ecdysteroids have been documented, including the so-called "commitment peak." This small elevation of 20E reprograms the larva for metamorphosis to the pupa. Similar periods of ecdysteroid immunoreactivity have been observed during the last larval instar of Drosophila. However, due to low amplitude and short duration, along with small body size and staging difficulties, their timing and ecdysteroid composition have remained uncertain. Employing a rigorous regimen of Drosophila culture and a salivary gland reporter gene, Sgs3-GFP, we used RP-HPLC and differential ecdysteroid RIA analysis to determine whole body titers of 20E during the last larval instar. Three small peaks of 20E were observed at 8, 20, and 28 hr following ecdysis, prior to the well-characterized large peak around the time of pupariation. The possible regulation of 20E levels by biosynthetic P450 enzymes and the roles of these early peaks in coordinating gene expression and late larval development are discussed.

The Role of the Prothoracic Gland in Determining Critical Weight for Metamorphosis in Drosophila melanogaster

BACKGROUND

The timely onset of metamorphosis in holometabolous insects depends on their reaching the appropriate size known as critical weight. Once critical weight is reached, juvenile hormone (JH) titers decline, resulting in the release of prothoracicotropic hormone (PTTH) at the next photoperiod gate and thereby inducing metamorphosis. How individuals determine when they have reached critical weight is unknown. We present evidence that in Drosophila, a component of the ring gland, the prothoracic gland (PG), assesses growth to determine when critical weight has been achieved.

RESULTS

We used the GAL4/UAS system to suppress or enhance growth by overexpressing PTEN or Dp110, respectively, in various components of the ring gland. Suppression of the growth of the PG and CA, but not of the CA alone, produced larger-than-normal larvae and adults. Suppression of only PG growth resulted in nonviable larvae, but larvae with enlarged PGs produced significantly smaller larvae and adults. Rearing larvae with enlarged PGs under constant light enhanced these effects, suggesting a role for photoperiod-gated PTTH secretion. These larvae are smaller, in part as a result of their repressed growth rates, a phenotype that could be rescued through nutritional supplementation (yeast paste). Most importantly, larvae with enlarged PGs overestimated size so that they initiated metamorphosis before surpassing the minimal viable weight necessary to survive pupation.

CONCLUSIONS

The PG acts as a size-assessing tissue by using insulin-dependent PG cell growth to determine when critical weight has been reached.

The temporal requirements for insulin signaling during development in Drosophila

Recent studies have indicated that the insulin-signaling pathway controls body and organ size in Drosophila, and most metazoans, by signaling nutritional conditions to the growing organs. The temporal requirements for insulin signaling during development are, however, unknown. Using a temperature-sensitive insulin receptor (Inr) mutation in Drosophila, we show that the developmental requirements for Inr activity are organ specific and vary in time. Early in development, before larvae reach the "critical size" (the size at which they commit to metamorphosis and can complete development without further feeding), Inr activity influences total development time but not final body and organ size. After critical size, Inr activity no longer affects total development time but does influence final body and organ size. Final body size is affected by Inr activity from critical size until pupariation, whereas final organ size is sensitive to Inr activity from critical size until early pupal development. In addition, different organs show different sensitivities to changes in Inr activity for different periods of development, implicating the insulin pathway in the control of organ allometry. The reduction in Inr activity is accompanied by a two-fold increase in free-sugar levels, similar to the effect of reduced insulin signaling in mammals. Finally, we find that varying the magnitude of Inr activity has different effects on cell size and cell number in the fly wing, providing a potential linkage between the mode of action of insulin signaling and the distinct downstream controls of cell size and number. We present a model that incorporates the effects of the insulin-signaling pathway into the Drosophila life cycle. We hypothesize that the insulin-signaling pathway controls such diverse effects as total developmental time, total body size and organ size through its effects on the rate of cell growth, and proliferation in different organs.

Development and the Genetics of Evolutionary Change Within Insect Species

▪ Abstract  Changes in genes and in developmental processes generate the phenotypic variation that is sorted by natural selection in adaptive evolution. We review several case studies in which artificial selection experiments in insects have led to divergent morphologies, and where further work has revealed information about the underlying changes at both the genetic and developmental levels. In addition, we examine several studies of phenotypic plasticity where multidisciplinary approaches are also beginning to reveal more about how developmental processes are modulated. Such integrated research will lead to a richer understanding of the changes in development that occur during evolutionary responses to natural selection, and it will also more rigorously examine how developmental processes can influence the tempo and direction of evolutionary change.

Catalase expression in Drosophila melanogaster is responsive to ecdysone and exhibits both transcriptional and post-transcriptional regulation

In the present study, we have examined catalase protein and mRNA levels and the factors that may regulate catalase expression in Drosophila melanogaster during development. Both mRNA and protein changes are in general accord with variations in ecdysteroid titer during development. Differences in mRNA and protein accumulation profiles, particularly in embryos and young adults, suggest that catalase may be regulated at both transcriptional and post-transcriptional levels. It was possible to induce catalase expression by administering exogenous 20-hydroxyecdysone (Ec) in culture at certain stages of development (usually at time points corresponding to previously observed hormone and catalase peaks). Experiments with exogenous administration of Ec, cycloheximide, and actinomycin D suggest a complex interplay of factors affecting catalase expression. In cultured third instar larvae, superinduction of catalase occurred in the presence of both Ec and cycloheximide. If ecdysteroid production was suppressed prior to antibiotic treatment by temperature upshift of the conditional mutant dre4(e55), superinduction occurred mostly at the protein level. In cultured adult abdomens, we observed induction by Ec and superinduction in the presence of hormone and translation or transcription inhibitors. Unlike what was observed in larvae, superinduction of catalase protein was dramatically more pronounced in control flies.

Relationships between protein isoforms and genetic functions demonstrate functional redundancy at the Broad-Complex during Drosophila metamorphosis

Metamorphosis in holometabolous insects is an ecdysone-dependent process by which the larval form is replaced by a reproductive, adult form. At the onset of metamorphosis ecdysone induces a set of early genes which coordinate tissue-specific responses to hormone. The Broad-Complex (BR-C) early gene, which acts as a global regulator of tissue-specific responses to ecdysone, encodes a family of zinc-finger DNA binding proteins known as Z1, Z2, Z3, and Z4. Genetically the BR-C encodes three complementing functions, br, rbp, and 2Bc, and a class of npr1 alleles that fail to complement any of the other genetic functions. The effects of BR-C mutations on metamorphic development are highly pleiotropic, yet little is known about the roles of individual BR-C proteins in directing the required responses to ecdysone. Because the BR-C is a vital regulator of metamorphosis it is essential to establish the relationships between BR-C genetic functions and protein products. We present here the first general and definitive study of these relationships. Using heat-inducible transgenes we have rescued lethality associated with each of the complementing genetic functions and have restored transcriptional activity of tissue-specific BR-C(+)-dependent target genes. Our data lead us to conclude that br+ function is only provided by the Z2 isoform. We find that Z1 transgenes provide full rbp+ function, while Z4 provides partial function. Likewise, while Z3 provides full 2Bc+ function, Z2 also provides partial function. These results indicate possible functional redundancy or regulatory dependence (via autoregulation) associated with the rbp+ and 2Bc+ functions. The establishment of these relationships between BR-C genetic functions and protein isoforms is an important step toward understanding the roles of BR-C proteins in directing metamorphic responses to ecdysone.

Sequence and developmental expression of Cyp18, a member of a new cytochrome P450 family from Drosophila

Preliminary studies of a partial cDNA clone of the Eig17-1 gene from Drosophila melanogaster have shown that it encodes a probable cytochrome P450 of unknown function. To further characterize the Eig17-1 gene product, a full-length cDNA clone was isolated from a late-larval cDNA library and sequenced. Eig17-1 encodes a protein of 538 amino acids. The predicted protein is a cytochrome P450 that has been assigned to a new family, CYP18. The CYP18 protein is most closely related to steroid and xenobiotic metabolizing P450s of family CYP2 (30-33% identity), and to vertebrate steroidogenic P450s of families CYP17 and CYP21 (25-28% identity). Developmental Northern blot analysis revealed five distinct periods of Cyp18 expression during postembryonic development. Each period lasted 12-15 h, and was tightly correlated with reported ecdysteroid pulses in the first, second and third larval instars, at the time of pupariation and in pupae. This pattern of expression is consistent with the known induction of Cyp18 transcription by 20-hydroxyecdysone at the time of pupariation and suggests that ecdysteroids are major regulators of Cyp18 expression throughout postembryonic development. Northern blot analysis of RNA isolated from different prepupal tissues indicates that Cyp18 is differentially expressed in various ecdysteroid-responsive tissues. High Cyp18 expression was observed in body wall and gut while negligible expression was observed in salivary glands and fat body.

The Drosophila 63F early puff contains E63-1, an ecdysone-inducible gene that encodes a novel Ca(2+)-binding protein

Pulses of ecdysone at the end of Drosophila larval development dramatically reprogram gene expression as they signal the onset of metamorphosis. Ecdysone directly induces several early puffs in the salivary gland polytene chromosomes that, in turn, activate many late puffs. Three early puffs, at 2B5, 74EF, and 75B, have been studied at the molecular level. Each contains a single ecdysone primary-response gene that encodes a family of widely expressed transcription factors. We report here a molecular characterization of the 63F early puff. Unexpectedly, we have found this locus to be significantly different from the previously characterized early puff loci. First, the 63F puff contains a pair of ecdysone-inducible genes that are transcribed in the larval salivary glands: E63-1 and E63-2. Second, E63-1 induction in late third instar larvae appears to be highly tissue-specific, restricted to the salivary gland. Third, E63-1 encodes a novel Ca(2+)-binding protein related to calmodulin. The discovery of an ecdysone-inducible Ca(2+)-binding protein provides a foundation for integrating steroid hormone and calcium second messenger signaling pathways and generates an additional level for potential regulation of the ecdysone response.

On the control of ecdysone biosynthesis by the central nervous system of blowfly larvae

Ecdysone was found to be the major secreted steroid of ring glands dissected from blowfly larvae and incubated in vitro. Other secretory products such as 3-dehydroecdysone and 20-deoxy-makisterone A could not be detected when the glands were labelled with tritiated cholesterol. Ecdysone synthesis and secretion were found to be tightly coupled. The highest rate of secretion was observed a few hours before pupariation. In vitro, the rate of ecdysone secretion by ring glands was affected significantly by coincubation with the central nervous system (CNS). Modulating effects from the CNS to the gland were mediated both by culture medium and by nerve connections. Distinct parts of the CNS revealed multiple and partially opposite effects on ecdysone secretion, suggesting a more complex control than had been anticipated. Multiple neural control systems appear to be involved. Moreover, the observed effects changed with development during the second half of the third instar, reflecting a significant plasticity of neural control.

Drosophila salivary glands exhibit a regional reprogramming of gene expression during the third larval instar

In D. virilis salivary glands transcripts of two early gland protein genes, Egp-1 and Egp-2, which encode putative secretory proteins, accumulate in all cells from the first to mid third larval instar. Subsequently the transcripts disappear from the cytoplasm of the corpus cells, but not from their nuclei, where they accumulate at the chromosomal site of their synthesis. In the collum cells, however, Egp-transcripts continue to be detectable in the cytoplasm until the end of larval life. In the salivary glands of transgenic D. melanogaster the presence of a Egp-1/lacZ fusion protein shows the same regional shift as the cytoplasmic Egp-transcripts in D. virilis. We predict that the expression of Egp-genes is related to an early secretory function of the larval salivary glands which is executed by all cells during earlier larval stages but becomes restricted exclusively to the collum cells during the third larval instar.

Effect of ecd1 mutation on the expression of genes mapped at the Drosophila melanogaster 3C11-12 intermoult puff

The Drosophila melanogaster ecd1 mutation causes a severe temperature-sensitive deficiency in the titre of the steroid hormone ecdysone. This mutation was used to investigate the role of ecdysone in both the transcription of the genes mapped at the 3C11-12 intermoult puff region and the puff formation. Thoroughly synchronized ecd1 larvae were shifted to the non-permissive temperature at various times of the development; after 24 or 48 h, the levels of the transcripts derived from Sgs-4, Pig-1 and ng-1, the three genes located at the 3C11-12 polytene bands, were determined. The results showed that the levels of the transcripts encoded by Pig-1 and ng-1 are unaffected by the drop in the ecdysone titre occurring in non-permissive conditions whereas the amount of Sgs-4 mRNA is greatly reduced. These data clearly indicate that transcription of the three genes mapped within the puff region is affected differently by the hormone. Furthermore, ecd1 larvae cultured at the non-permissive temperature show a prominent puff at the 3C11-12 polytene bands, indicating that ecdysone is not essential for puff induction and that puff size is not simply correlated with high-level Sgs-4 transcription.

Ultrastructural analysis of polytene chromatin of Drosophila melanogaster reveals clusters of tightly linked co-expressed genes

Patterns of gene activity on individual chromatids of polytene chromosomes of Drosophila melanogaster white prepupae were ultrastructurally characterized by electron microscopy. The band-interband structure of salivary gland polytene chromosomes is lost when they are dispersed in a low ionic strength detergent solution. Morphologically similar, active genes in close proximity to one another were seen in dispersed white prepupal chromatin. The arrays of genes almost certainly represent sister copies of the same locus. Although lateral register between gene copies on multiple strands was not maintained, analysis of sister transcriptional units of unknown identity was achieved at the periphery of the chromatin arrays. Juxtaposed genes with divergent transcriptional polarity were prevalent. The morphology, size and transcriptional polarity of multiple copies of short, tandemly organized, RNA polymerase dense, co-expressed gene clusters is reported. One highly transcriptionally active region, designated the white prepupal locus (WPP locus), composed of a co-expressed tandem cluster of ten genes within an approximately 50 kb region was analyzed on six separate chromatids. The transcriptional map suggests that the pattern of gene activity for at least one gene within the cluster may not be identical on all homologous strands. The survey of active polytene genes provides ultrastructural correlation with previous molecular data that demonstrate tight linkage of certain developmentally co-regulated Drosophila genes. Our findings are discussed in relation to Drosophila gene organization, clustering, and regulation of gene expression.

Hormonal control of gene expression: interactions between two trans-acting regulators in Drosophila

The steroid hormone 20-hydroxyecdysone (20HE) and the Broad-Complex locus (BRC) are involved in regulating developmental changes in gene expression around the time of metamorphosis in Drosophila. We have investigated the regulatory interactions between 20HE, BRC, and a set of genes expressed in the fat body of third-instar Drosophila larvae. RNA levels for two hormone-inducible genes, Larval Serum Protein-2 and P1, accumulate to normal levels in BRC-mutant larvae. In contrast, RNA levels for the P6 gene were affected by mutations at BRC. On the basis of the results of experiments in which hormone concentrations were varied in BRC-mutant or wild-type larvae, we conclude that 20HE can both increase and decrease P6 RNA levels in the absence of BRC product(s). BRC appears to be a trans-acting modulator of the response of P6 to the hormone. We propose that BRC attenuates the repressive effect of the hormone, expanding the range of hormone concentrations that induce the gene, thus allowing P6 RNA to reach high levels during the third larval instar. The results are discussed in relation to other genes that are regulated by the same two trans-acting factors. A model is presented that refines the model of Ashburner et al. (1974, Cold Spring Harbor Symp. Quant. Biol. 38: 655-662) for the hormonal regulation of gene activity.

Drosophila glue protein gene expression. A proposal for its ecdysone-dependent developmental control

The primary targets of steroid hormones are genes. For the ecdysone-controlled genes of Drosophila larval glue proteins proximal and distal control elements were identified by mutagenesis and sequence comparison. Their presence is required for the correct stage- and tissue-specific expression of these genes. The supposed function of these elements is described in a working model.

Effects of juvenile hormone on the ecdysone response of Drosophila Kc cells

Drosophila Kc cells are ecdysone-responsive: hormone treatment leads rapidly to increased synthesis of several ecdysone-inducible polypeptides (EIPs) and to commitment to eventual proliferative arrest. Later, the treated cells undergo morphological transformation, cease to proliferate, and develop new enzymatic activities, notably, acetylcholinesterase (AChE) activity. These responses have proven useful as models for studying ecdysone action. Here we report the sensitivity of Kc cells to another important insect developmental regulator--juvenile hormone (JH). We find that JH inhibits some, but not all, aspects of the ecdysone response. When Kc cells are treated with ecdysone in the presence of either natural JHs or synthetic analogues, the morphological and proliferative responses are inhibited and AChE induction is blocked. Most striking is that JHs protect the cells from the rapid proliferative commitment induced by ecdysone alone. The JH effects exhibit reasonable dose-response curves with half-maximal responses occurring at very low JH concentrations. Nonetheless, even at high JH concentrations the inhibitory effects are incomplete. It is interesting that EIP induction appears to be refractory to JH. It seems clear that JH is not simply a generalized inhibitor of ecdysone-induced responses.

Drosophila glue gene Sgs-3: sequences required for puffing and transcriptional regulation

The 68C intermolt puff of Drosophila melanogaster contains a cluster of three glue protein genes, Sgs-3, Sgs-7, and Sgs-8. By analysis of chromosomal rearrangements which break near the glue gene cluster, we have established that a region of no more than 20 kb is required for normal expression of the glue genes and for formation of the 68C puff. Using P element-mediated transformation, we have introduced defined segments of the 68C region into the fly genome and assayed the expression of the Sgs-3 gene. Based on the criteria of correct tissue- and stage-specific expression, transcription of an RNA of appropriate size and abundance, and production of an sgs-3 protein, the correctly regulated expression of the Sgs-3 gene requires less than 3.4 kb of total flanking sequences, approximately 2.3 kb 5' and 1.1 kb 3'. Formation of a new intermolt puff at the site of insertion is not observed for all transformants which produce high levels of Sgs-3 RNA. Only transformants in which the introduced DNA from 68C also contains the Sgs-7 and Sgs-8 genes cause a new intermolt puff at the chromosomal location of the insert.

Pupal and larval cuticle proteins of Drosophila melanogaster

Proteins, soluble in 7 M urea, were extracted from third-instar larval and pupal cuticles of Drosophila melanogaster. Both extracts contain a limited number of polypeptides resolved by one- or two-dimensional electrophoresis. The five major larval proteins have low molecular weights (less than 20000) and are not glycosylated. The major pupal cuticle proteins fall into two size classes: two with apparent molecular weights of 56K and 82K and four with molecular weights between 15K and 25K. The proteins with high apparent molecular weights are glycosylated. In nondenaturing gels, no components of the larval and pupal cuticle extracts comigrate. One-dimensional "fingerprints" indicate that cuticle proteins from these two stages have unique primary structures. Immunological results indicate that the major low molecular weight larval and pupal cuticle proteins are comprised of two families of proteins that share antigenic determinants. The high molecular weight pupal cuticle proteins are immunologically unrelated to the low molecular weight components. We conclude that the pupal and larval proteins are encoded in part by multigene families that have arisen by gene duplication and evolutionary divergence.

A trans-acting regulatory product necessary for expression of the Drosophila melanogaster 68C glue gene cluster

The mutation l(1)npr-1 is located at cytological location 2B5 on the X chromosome in Drosophila melanogaster. We have found that this mutation causes absence of the normal product of the 2B5 locus and that it has the following phenotypes: the 68C glue puff on the third chromosome does not regress when mutant salivary glands are cultured in the presence of ecdysterone; the three 68C glue protein mRNAs are not synthesized; and a transformed Drosophila strain carrying both a normal resident 68C Sgs-3 gene and an introduced functional Sgs-3 gene with only a few kb of flanking sequences expresses neither Sgs-3 RNA if the l(1)npr-1 mutation is crossed into the stock. Thus the normal product of the l(1)npr-1 gene is required for regression of the 68C puff, and the l(1)npr-1 gene product allows expression of the Sgs-3 gene by interacting, either directly or indirectly, with DNA sequences near this glue protein gene.

In vitro activity of dipteran ring glands and activation by the prothoracicotropic hormone

The relationship between hemolymph ecdysteroid titer, ring gland (RG) activity, and prothoracicotropic hormone (PTTH) activation of RG in vitro has been examined during the postfeeding larval, prepupal, and pupal stages of Sarcophaga bullata. Using the ecdysteroid radioimmunoassay (RIA), two significant peaks were recorded during the red spiracular stage and during the first few hours after the formation of the white prepupa and a third large peak 9 hr later. It is postulated that these increases in ecdysteroid titer are involved in the processes of pupariation, puparial tanning, and pupation, respectively. Ring glands isolated from Sarcophaga of known ages were incubated in vitro and the secreted ecdysone was quantified by RIA. Ring glands from early red spiracular stage larvae proved to be the most active and subsequent secretory activity of the RG oscillated every 4 hr with the oscillations gradually decreasing in amplitude. RG activity returned to a basal level 24 hr after formation of the white prepupa, about the time that the hemolymph ecdysteroid titer fell to its basal level. To demonstrate PTTH activity in vitro, brains from 3- to 4-hr prepupae were chosen to activate ring glands from postfeeding larvae. Using a graded series of dilutions of PTTH extract it was shown that a dose-response relationship could be obtained for Sarcophaga similar to that demonstrated or the Manduca sexta PTTH-prothoracic gland system. In Sarcophaga maximal activation resulted in a 10-fold increase in ecdysone synthesis and secretion by ring glands stimulated with 0.5 brain eq. Half-maximal stimulation was attained with 0.2 brain eq of PTTH extract.

Analysis of metamorphosis in Drosophila melanogaster: characterization of giant, an ecdysteroid-deficient mutant

The mutant allele giant of Drosophila melanogaster affects the timing and the level of increase in ecdysteroid titer normally occurring at puparium formation. The third larval instar is extended by 4 days in phenotypically "giant" individuals during which the imaginal discs mature slower than normal and finally take on the folding pattern characteristic of maturity at a time when normal individuals have already formed puparia. After puparium formation, development occurs at the same rate in giant and wild-type animals. Feeding 20-hydroxyecdysone at 94 hr after oviposition allows giant larvae to develop at the same rate as wild-type larvae and to produce normal-sized adults (although at 94 hr the imaginal discs of giant lack much of the folding pattern of mature discs). Radioimmunological determination of ecdysteroid titers in giant and normal individuals indicates that the peak of ecdysteroid activity associated with puparium formation is lower in giant and occurs 4 days later than normal. These results indicate that giant is an ecdysteroid-deficient mutant with major effects on metamorphosis. Unlike previously reported ecdysteroid-deficient mutants, however, giant larvae eventually develop into adults and may be induced to undergo complete metamorphosis at the same time as wild type by feeding 20-hydroxyecdysone.

Ecdysteroids during the third larval instar in 1(3)ecd-1ts, a temperature-sensitive mutant of Drosophila melanogaster

The temperature-sensitive 1(3)ecd-1ts mutation (A. Garen, L. Kauvar, and J.A. Lepesant (1977). Proc. Natl. Acad. Sci USA 74, 5099-5103.) has been used in several laboratories to obtain Drosophila larvae deprived of moulting hormone. The development of mutants and controls during the third larval instar at permissive (20 degrees C) and restrictive temperatures (29 degrees C) was compared. Pupariation was inhibited when larvae were shifted to the restrictive temperature immediately at the second moult. The permanent larvae obtained remained active, did not leave the food, and reached a maximum weight superior to the weight of controls. Ecdysteroids were studied during the third larval instar by HPLC analysis and radioimmunoassays. A careful synchronization of the larvae at the second moult enabled the confirmation that at least one ecdysteroid peak occurs during the third larval instar, prior to the wandering stage in controls (20 or 29 degrees C). Ecdysone was then the predominant moulting hormone, whereas 20-hydroxyecdysone was the main ecdysteroid at the time of pupariation. Low levels of ecdysteroid were measured in mutant larvae shifted to 29 degrees C immediately at the second moult but larvae completely deprived of immunoreactive material were never observed. Nearly normal levels of ecdysteroids appeared at 27.5 degrees C. Feeding ecd-1 larvae maintained at restrictive temperature on 20-hydroxyecdysone-yeast mixture for 16 hr triggered abortive pupariation. Ecdysteroid levels were measured after the return of the larvae to the standard medium; normal levels were restored 24 hr later. The mutant ecd-1 appears to present interesting opportunities for the detailed study of the hormonal induction of a developmental process during the third larval instar.(ABSTRACT TRUNCATED AT 250 WORDS)

Steroid regulation of RNAs transcribed from the Drosophila 68c polytene chromosome puff

The 68C region of the Drosophila melanogaster salivary gland polytene chromosomes harbors the structural genes for the three salivary gland glue proteins sgs-3, sgs-7, and sgs-8. This region is puffed during the third larval instar, the stage when glue proteins are being produced in the salivary glands. The puff regresses near the end of the third instar as a result of an increased titer of the steroid hormone ecdysterone in the larval hemolymph. The experiments reported here were designed to determine whether the ecdysterone effect on puffing at 68C is correlated with hormone effects on expression of the three puff RNAs. In the first series of experiments, it is shown that there is a more rapid disappearance of 68C RNA transcripts from salivary glands cultured in the presence of ecdysterone than from glands cultured in its absence. The second set of experiments, in which 68C transcripts were pulse-labeled in salivary glands cultured in the presence or absence of hormone, demonstrates that one effect of ecdysterone is to cause a sharp reduction in the rate at which newly synthesized 68C transcripts accumulate. The final experiments follow the time required for ecdysterone to produce this effect, and show that it occurs in salivary glands exposed to the hormone for as little as 15 min. In all of the experiments, the RNA products of the Sgs-3, Sgs-7, and Sgs-8 genes acted coordinately.