Larval cuticular morphogenesis in the tobacco hornworm, Manduca sexta, and its hormonal regulation

A Life's Journey Through Insect Metamorphosis

This autobiographical article describes the research career of Lynn M. Riddiford from its early beginnings in a summer program for high school students at Jackson Laboratory to the present "retirement" at the Friday Harbor Laboratories. The emphasis is on her forays into many areas of insect endocrinology, supported by her graduate students and postdoctoral associates. The main theme is the hormonal regulation of metamorphosis, especially the roles of juvenile hormone (JH). The article describes the work of her laboratory first in the elucidation of the endocrinology of the tobacco hornworm, Manduca sexta, and later in the molecular aspects of the regulation of cuticular and pigment proteins and of the ecdysone-induced transcription factor cascade during molting and metamorphosis. Later studies utilized Drosophila melanogaster to answer further questions about the actions of JH.

Genome-wide annotation and comparative analysis of cuticular protein genes in the noctuid pest Spodoptera litura

Insect cuticle is considered an adaptable and versatile building material with roles in the construction and function of exoskeleton. Its physical properties are varied, as the biological requirements differ among diverse structures and change during the life cycle of the insect. Although the bulk of cuticle consists basically of cuticular proteins (CPs) associated with chitin, the degree of cuticular sclerotization is an important factor in determining its physical properties. Spodoptera litura, the tobacco cutworm, is an important agricultural pest in Asia. Compared to the domestic silkworm, Bombyx mori, another lepidopteran whose CP genes have been well annotated, S. litura has a shorter life cycle, hides in soil during daytime beginning in the 5th instar and is exposed to soil in the pupal stage without the protection of a cocoon. In order to understand how the CP genes may have been adapted to support the characteristic life style of S. litura, we searched its genome and found 287 putative cuticular proteins that can be classified into 9 CP families (CPR with three groups (RR-1, RR-2, RR-3), CPAP1, CPAP3, CPF, CPFL, CPT, CPG, CPCFC and CPLCA), and a collection of unclassified CPs named CPH. There were also 112 cuticular proteins enriched in Histidine residues with content varying from 6% to 30%, comprising many more His-rich cuticular proteins than B. mori. A phylogenetic analysis between S. litura, M. sexta and B. mori uncovered large expansions of RR-1 and RR-2 CPs, forming large gene clusters in different regions of S. litura chromosome 9. We used RNA-seq analysis to document the expression profiles of CPs in different developmental stages and tissues of S. litura. The comparative genomic analysis of CPs between S. litura and B. mori integrated with the unique behavior and life cycle of the two species offers new insights into their contrasting ecological adaptations.

Hormonal control of growth in the wing imaginal disks of Junonia coenia: the relative contributions of insulin and ecdysone

The wing imaginal disks of Lepidoptera can be grown in tissue culture, but require both insulin and ecdysone to grow normally. Here we investigate the contributions the two hormones make to growth. Ecdysone is required to maintain mitoses, whereas in the presence of insulin alone mitoses stop. Both ecdysone and insulin stimulate protein synthesis, but only ecdysone stimulates DNA synthesis. Insulin stimulates primarily cytoplasmic growth and an increase in cell size, whereas ecdysone, by virtue of its stimulation of DNA synthesis and mitosis, stimulates growth by an increase in cell number. Although both hormones stimulate protein synthesis they do so in different spatial patterns. Both hormones stimulate protein synthesis in the inter-vein regions, but ecdysone stimulates synthesis more strongly in the veins and in the margin of the wing disk. We propose that the balance of insulin and ecdysone signaling must be regulated to maintain normal growth, and when growth appears to be due primarily to an increase in cell number, or an increase in cell size, this may indicate growth occurred under conditions that favored a stronger role for ecdysone, or insulin, respectively.

Multifaceted biological insights from a draft genome sequence of the tobacco hornworm moth, Manduca sexta

Manduca sexta, known as the tobacco hornworm or Carolina sphinx moth, is a lepidopteran insect that is used extensively as a model system for research in insect biochemistry, physiology, neurobiology, development, and immunity. One important benefit of this species as an experimental model is its extremely large size, reaching more than 10 g in the larval stage. M. sexta larvae feed on solanaceous plants and thus must tolerate a substantial challenge from plant allelochemicals, including nicotine. We report the sequence and annotation of the M. sexta genome, and a survey of gene expression in various tissues and developmental stages. The Msex_1.0 genome assembly resulted in a total genome size of 419.4 Mbp. Repetitive sequences accounted for 25.8% of the assembled genome. The official gene set is comprised of 15,451 protein-coding genes, of which 2498 were manually curated. Extensive RNA-seq data from many tissues and developmental stages were used to improve gene models and for insights into gene expression patterns. Genome wide synteny analysis indicated a high level of macrosynteny in the Lepidoptera. Annotation and analyses were carried out for gene families involved in a wide spectrum of biological processes, including apoptosis, vacuole sorting, growth and development, structures of exoskeleton, egg shells, and muscle, vision, chemosensation, ion channels, signal transduction, neuropeptide signaling, neurotransmitter synthesis and transport, nicotine tolerance, lipid metabolism, and immunity. This genome sequence, annotation, and analysis provide an important new resource from a well-studied model insect species and will facilitate further biochemical and mechanistic experimental studies of many biological systems in insects.

Evidence of a hemolymph-born factor that induces onset of maturation in Manduca sexta larvae

Insect metamorphosis is a complex developmental transition determined and coordinated by hormonal signaling that begins at a critical weight late in the larval phase of life. Even though this hormonal signaling is well understood in insects, the internal factors that are assessed at the critical weight and that drive commitment to metamorphosis have remained unresolved in most species. The critical weight may represent either an autonomous decision by the neuroendocrine system without input from other developing larval tissues, or an assessment of developmental thresholds occurring throughout the body that are then integrated by the neuroendocrine tissues. The latter hypothesis predicts that there could be one or more developmental threshold signals that originate from developing tissues and ultimately induce the onset of metamorphosis. However, there is no evidence for such a signal in the organisms for which the critical weight is well described. Here we test for the evidence of this factor in Manduca sexta (Lepidoptera: Sphingidae) by transferring hemolymph from individuals that are either post- or pre-critical weight into pre-critical weight 5(th) instar larvae. We found that hemolymph from a post-critical weight donor induces a shortening of development time, though the mass at pupation is unaffected. This suggests that metamorphic commitment occurring at the critical weight is at least partially coordinated by signaling from developing tissues via a hemolymph-borne signaling factor.

A Quantitative Analysis of Growth and Size Regulation in Manduca sexta: The Physiological Basis of Variation in Size and Age at Metamorphosis

Body size and development time are important life history traits because they are often highly correlated with fitness. Although the developmental mechanisms that control growth have been well studied, the mechanisms that control how a species-characteristic body size is achieved remain poorly understood. In insects adult body size is determined by the number of larval molts, the size increment at each molt, and the mechanism that determines during which instar larval growth will stop. Adult insects do not grow, so the size at which a larva stops growing determines adult body size. Here we develop a quantitative understanding of the kinetics of growth throughout larval life of Manduca sexta, under different conditions of nutrition and temperature, and for genetic strains with different adult body sizes. We show that the generally accepted view that the size increment at each molt is constant (Dyar’s Rule) is systematically violated: there is actually a progressive increase in the size increment from instar to instar that is independent of temperature. In addition, the mass-specific growth rate declines throughout the growth phase in a temperature-dependent manner. We show that growth within an instar follows a truncated Gompertz trajectory. The critical weight, which determines when in an instar a molt will occur, and the threshold size, which determines which instar is the last, are different in genetic strains with different adult body sizes. Under nutrient and temperature stress Manduca has a variable number of larval instars and we show that this is due to the fact that more molts at smaller increments are taken before threshold size is reached. We test whether the new insight into the kinetics of growth and size determination are sufficient to explain body size and development time through a mathematical model that incorporates our quantitative findings.

Obstructor A organizes matrix assembly at the apical cell surface to promote enzymatic cuticle maturation in Drosophila

Assembly and maturation of the apical extracellular matrix (aECM) is crucial for protecting organisms, but underlying molecular mechanisms remain poorly understood. Epidermal cells secrete proteins and enzymes that assemble at the apical cell surface to provide epithelial integrity and stability during developmental growth and upon tissue damage. We analyzed molecular mechanisms of aECM assembly and identified the conserved chitin-binding protein Obst-A (Obstructor A) as an essential regulator. We show in Drosophila that Obst-A is required to coordinate protein and chitin matrix packaging at the apical cell surface during development. Secreted by epidermal cells, the Obst-A protein is specifically enriched in the apical assembly zone where matrix components are packaged into their highly ordered architecture. In obst-A null mutant larvae, the assembly zone is strongly diminished, resulting in severe disturbance of matrix scaffold organization and impaired aECM integrity. Furthermore, enzymes that support aECM stability are mislocalized. As a biological consequence, cuticle architecture, integrity, and function are disturbed in obst-A mutants, finally resulting in immediate lethality upon wounding. Our studies identify a new core organizing center, the assembly zone that controls aECM assembly at the apical cell surface. We propose a genetically conserved molecular mechanism by which Obst-A forms a matrix scaffold to coordinate trafficking and localization of proteins and enzymes in the newly deposited aECM. This mechanism is essential for maturation and stabilization of the aECM in a growing and remodeling epithelial tissue as an outermost barrier.

Developmental mechanisms of body size and wing-body scaling in insects

The developmental mechanisms that control body size and the relative sizes of body parts are today best understood in insects. Size is controlled by the mechanisms that cause growth to stop when a size characteristic of the species has been achieved. This requires the mechanisms to assess size and respond by stopping the process that controls growth. Growth is controlled by two hormones, insulin and ecdysone, that act synergistically by controlling cell growth and cell division. Ecdysone has two distinct functions: At low concentration it controls growth, and at high levels it causes molting and tissue differentiation. Growth is stopped by the pulse of ecdysone that initiates the metamorphic molt. Body size is sensed by either stretch receptors or oxygen restriction, depending on the species, which stimulate the high level of ecdysone secretion that induces a molt. Wing growth occurs mostly after the body has stopped growing. Wing size is adjusted to body size by variation in both the duration and level of ecdysone secretion.

Body size determination in insects: a review and synthesis of size- and brain-dependent and independent mechanisms

Body size determination requires a mechanism for sensing size and a mechanism for linking size information to the termination of growth. Although the hormonal mechanisms that terminate growth are well elucidated, the mechanisms by which a body senses its own size are only partially understood; most of this understanding has come from the study of the mechanisms that control insect moulting and metamorphosis. We first review and discuss advances in our understanding of the physiological mechanisms by which insect larvae sense their size. Second, we present new findings on how larvae in which the size-sensing mechanism has been disrupted eventually terminate growth (in a size-independent manner). We synthesize recent insights into the genetic and molecular mechanisms of ecdysteroid regulation in Drosophila melanogaster with developmental physiology findings in Manduca sexta, paving the way for an integrated understanding of the mechanisms of body size regulation.

Mass and volume growth of larval insect tracheal system within a single instar

Organisms must accommodate oxygen delivery to developing tissues as body mass increases during growth. In insects, the growth of the respiratory system has been assumed to occur only when it molts, whereas body mass and volume increase during the larval stages between molts. This decouples whole body growth from the growth of the oxygen supply system. This assumption is derived from the observation that the insect respiratory system is an invagination of the exoskeleton, which must be shed during molts for continued growth to occur. Here, we provide evidence that this assumption is incorrect. We found that the respiratory system increases substantially in both mass and volume within the last larval instar of Manduca sexta larvae, and that the growth of the respiratory system changes with diet quality, potentially as a consequence of shifting metabolic demands.

Isolation and maintenance-free culture of contractile myotubes from Manduca sexta embryos

Skeletal muscle tissue engineering has the potential to treat tissue loss and degenerative diseases. However, these systems are also applicable for a variety of devices where actuation is needed, such as microelectromechanical systems (MEMS) and robotics. Most current efforts to generate muscle bioactuators are focused on using mammalian cells, which require exacting conditions for survival and function. In contrast, invertebrate cells are more environmentally robust, metabolically adaptable and relatively autonomous. Our hypothesis is that the use of invertebrate muscle cells will obviate many of the limitations encountered when mammalian cells are used for bioactuation. We focus on the tobacco hornworm, Manduca sexta, due to its easy availability, large size and well-characterized muscle contractile properties. Using isolated embryonic cells, we have developed culture conditions to grow and characterize contractile M. sexta muscles. The insect hormone 20-hydroxyecdysone was used to induce differentiation in the system, resulting in cells that stained positive for myosin, contract spontaneously for the duration of the culture, and do not require media changes over periods of more than a month. These cells proliferate under normal conditions, but the application of juvenile hormone induced further proliferation and inhibited differentiation. Cellular metabolism under normal and low glucose conditions was compared for C2C12 mouse and M. sexta myoblast cells. While differentiated C2C12 cells consumed glucose and produced lactate over one week as expected, M. sexta muscle did not consume significant glucose, and lactate production exceeded mammalian muscle production on a per cell basis. Contractile properties were evaluated using index of movement analysis, which demonstrated the potential of these cells to perform mechanical work. The ability of cultured M. sexta muscle to continuously function at ambient conditions without medium replenishment, combined with the interesting metabolic properties, suggests that this cell source is a promising candidate for further investigation toward bioactuator applications.

Control of body size by oxygen supply reveals size-dependent and size-independent mechanisms of molting and metamorphosis

Body size profoundly affects many aspects of animal biology, including metamorphosis, allometry, size-dependent alternative pathways of gene expression, and the social and ecological roles of individuals. However, regulation of body size is one of the fundamental unsolved problems in developmental biology. The control of body size requires a mechanism that assesses size and stops growth within a characteristic range of sizes. Under normal growth conditions in Manduca sexta, the endocrine cascade that causes the brain to initiate metamorphosis starts when the larva reaches a critical weight. Metamorphosis is initiated by a size-sensing mechanism, but the nature of this mechanism has remained elusive. Here we show that this size-sensing mechanism depends on the limited ability of a fixed tracheal system to sustain the oxygen supply to a growing individual. As body mass increases, the demand for oxygen also increases, but the fixed tracheal system does not allow a corresponding increase in oxygen supply. We show that interinstar molting has the same size-related oxygen-dependent mechanism of regulation as metamorphosis. We show that low oxygen tension induces molting at smaller body size, consistent with the hypothesis that under normal growth conditions, body size is regulated by a mechanism that senses oxygen limitation. We also found that under poor growth conditions, larvae may never attain the critical weight but eventually molt regardless. We show that under these conditions, larvae do not use the critical weight mechanism, but instead use a size-independent mechanism that is independent of the brain.

Control of larval-pupal-adult molt in the moth Sesamia nonagrioides by juvenile hormone and ecdysteroids

Sesamia nonagrioides (Lepidoptera: Noctuidae) larvae reared under long day (LD; 16L:8D) conditions pupate after 5 or 6 larval instars, whereas under short day (SD; 12L:12D) conditions they undergo up to 12 additional molts before pupating. This extended period of repeated molting is maintained by high levels of juvenile hormone (JH). Previous work demonstrated that both LD and SD larvae decapitated in the 6th instar pupate but further development is halted. By contrast, about one-third of SD larvae from which only the brain has been removed, undergo first a larval molt, then pupate and subsequently developed to the adult stage. Debrained LD larvae molt to larvae exceptionally but regularly pupate and produce adults. Implanted brains may induce several larval molts in debrained recipient larvae irrespectively of the photoperiodic conditions. The results of present work demonstrate that the prothoracic glands (PGs) and the corpora allata (CA) of debrained larvae continue to produce ecdysteroids and JHs, respectively. PGs are active also in the decapitated larvae that lack JH, consistent with the paradigm that CA, which are absent in the decapitated larvae, are the only source of this hormone. Completion of the pupal-adult transformation in both LD and SD debrained insects demonstrates that brain is not crucial for the development of S. nonagrioides but is required for diapause maintenance. Application of JH to headless pupae induces molting, presumably by activating their PGs. It is likely that JH plays this role also in the induction of pupal-adult transformation in debrained insects. Application of the ecdysteroid agonist RH 2485 (methoxyfenozide) to headless pupae also elicits molting: newly secreted cuticle is in some cases thin and indifferent, in other cases it bears distinct pupal or adult features.

Scaling of caterpillar body properties and its biomechanical implications for the use of a hydrostatic skeleton

Caterpillars can increase their body mass 10,000-fold in 2 weeks. It is therefore remarkable that most caterpillars appear to maintain the same locomotion kinematics throughout their entire larval stage. This study examined how the body properties of a caterpillar might change to accommodate such dramatic changes in body load. Using Manduca sexta as a model system, we measured changes in body volume, tissue density and baseline body pressure, and the dimensions of load-bearing tissues (the cuticle and muscles) over a body mass range from milligrams to several grams. All Manduca biometrics relevant to the hydrostatic skeleton scaled allometrically but close to the isometric predictions. Body density and pressure were almost constant. We next investigated the effects of scaling on the bending stiffness of the caterpillar hydrostatic skeleton. The anisotropic non-linear mechanical response of Manduca muscles and soft cuticle has previously been quantified and modeled with constitutive equations. Using biometric data and these material laws, we constructed finite element models to simulate a hydrostatic skeleton under different conditions. The results show that increasing the internal pressure leads to a non-linear increase in bending stiffness. Increasing the body size results in a decrease in the normalized bending stiffness. Muscle activation can double this stiffness in the physiological pressure range, but thickening the cuticle or increasing the muscle area reduces the structural stiffness. These non-linear effects may dictate the effectiveness of a hydrostatic skeleton at different sizes. Given the shared anatomy and size variation in Lepidoptera larvae, these mechanical scaling constraints may implicate the diverse locomotion strategies in different species.

Developmental expression of mRNAs for epidermal and fat body proteins and hormonally regulated transcription factors in the tobacco hornworm, Manduca sexta

This paper provides a compilation of diagrammatic representations of the expression profiles of epidermal and fat body mRNAs during the last two larval instars and metamorphosis of the tobacco hornworm, Manduca sexta. Included are those encoding insecticyanin, three larval cuticular proteins, dopa decarboxylase, moling, and the juvenile hormone-binding protein JP29 produced by the dorsal abdominal epidermis, and arylphorin and the methionine-rich storage proteins made by the fat body. The mRNA profiles of the ecdysteroid-regulated cascade of transcription factors in the epidermis during the larval molt and the onset of metamorphosis and in the pupal wing during the onset of adult development are also shown. These profiles are accompanied by a brief summary of the current knowledge about the regulation of these mRNAs by ecdysteroids and juvenile hormone based on experimental manipulations, both in vivo and in vitro.

Mode of action of methoprene in affecting female reproduction in the African malaria mosquito, Anopheles gambiae

BACKGROUND

One of the most studied actions of juvenile hormone (JH) is its ability to modulate ecdysteroid signaling during insect development and metamorphosis. Previous studies in mosquitoes showed that 20-hydroxyecdysone (20E) regulates vitellogenin synthesis. However, the action of JH and its mimics, e.g. methoprene, on female reproduction of mosquitoes remains unknown.

RESULTS

Here, a major malaria vector, Anopheles gambiae Giles, was used as a model insect to study the action of methoprene on female reproduction. Ecdysteroid titers and expression profiles of ecdysone-regulated genes were determined before and after a blood meal. An ecdysteroid peak was detected at 12 h post blood meal (PBM). The maximum expression of ecdysone-regulated genes, such as ecdysone receptor (EcR), hormone receptor 3 (HR3) and vitellogenin (Vg) gene, coincided with the ecdysteroid peak. Interestingly, topical application of methoprene at 6 h PBM delayed ovarian development and egg maturation by suppressing the expression of ecdysone-regulated genes in female mosquitoes.

CONCLUSION

The data suggest that ecdysteroid titers are correlated with Vg synthesis, and methoprene affects vitellogenesis by modulating ecdysteroid action in A. gambiae.

A switch in the control of growth of the wing imaginal disks of Manduca sexta

BACKGROUND

Insulin and ecdysone are the key extrinsic regulators of growth for the wing imaginal disks of insects. In vitro tissue culture studies have shown that these two growth regulators act synergistically: either factor alone stimulates only limited growth, but together they stimulate disks to grow at a rate identical to that observed in situ. It is generally thought that insulin signaling links growth to nutrition, and that starvation stops growth because it inhibits insulin secretion. At the end of larval life feeding stops but the disks continue to grow, so at that time disk growth has become uncoupled from nutrition. We sought to determine at exactly what point in development this uncoupling occurs.

METHODOLOGY

Growth and cell proliferation in the wing imaginal disks and hemolymph carbohydrate concentrations were measured at various stages in the last larval instar under experimental conditions of starvation, ligation, rescue, and hormone treatment.

PRINCIPAL FINDINGS

Here we show that in the last larval instar of M. sexta, the uncoupling of nutrition and growth occurs as the larva passes the critical weight. Before this time, starvation causes a decline in hemolymph glucose and trehalose and a cessation of wing imaginal disks growth, which can be rescued by injections of trehalose. After the critical weight the trehalose response to starvation disappears, and the expression of insulin becomes decoupled from nutrition. After the critical weight the wing disks loose their sensitivity to repression by juvenile hormone, and factors from the abdomen, but not the brain, are required to drive continued growth.

CONCLUSIONS

During the last larval instar imaginal disk growth becomes decoupled from somatic growth at the time that the endocrine events of metamorphosis are initiated. These regulatory changes ensure that disk growth continues uninterrupted when the nutritive and endocrine signals undergo the drastic changes associated with metamorphosis.

The regulation of expression of insect cuticle protein genes

The exoskeleton of insects (cuticle) is an assembly of chitin and cuticle proteins. Its physical properties are determined largely by the proteins it contains, and vary widely with developmental stages and body regions. The genes encoding cuticle proteins are therefore good models to study the molecular mechanisms of signalling by ecdysteroids and juvenile hormones, which regulate molting and metamorphosis in insects. This review summarizes the studies of hormonal regulation of insect cuticle protein genes, and the recent progress in the analysis of the regulatory sequences and transcription factors important for their expression.

Soft-cuticle biomechanics: a constitutive model of anisotropy for caterpillar integument

The mechanical properties of soft tissues are important for the control of motion in many invertebrates. Pressurized cylindrical animals such as worms have circumferential reinforcement of the body wall; however, no experimental characterization of comparable anisotropy has been reported for climbing larvae such as caterpillars. Using uniaxial, real-time fluorescence extensometry on millimeter scale cuticle specimens we have quantified differences in the mechanical properties of cuticle to circumferentially and longitudinally applied forces. Based on these results and the composite matrix-fiber structure of cuticle, a pseudo-elastic transversely isotropic constitutive material model was constructed with circumferential reinforcement realized as a Horgan-Saccomandi strain energy function. This model was then used numerically to describe the anisotropic material properties of Manduca cuticle. The constitutive material model will be used in a detailed finite-element analysis to improve our understanding of the mechanics of caterpillar crawling.

The role of 20-hydroxyecdysone and juvenile hormone in pupal commitment of the epidermis of the silkworm, Bombyx mori

During the pupal metamorphosis in insects, cellular commitment for pupal differentiation must precede before its differentiation. The pupal commitment of Bombyx mori epidermis occurred from day 3 to day 6 last (5th) instar larvae in response to the gradual increase in ecdysteroid titer in the presence of a small amount of juvenile hormone (JH). Yet the concealed preparatory process of the commitment had begun in the newly synthesized 5th instar larval epidermis (approximately 6 h before the ecdysis) as a competence phase, in which pupal commitment in vitro was induced by 20-hydroxyecdysone (20E) but inhibited by JH. This competence phase continued until day 2 5th instar, and the decrease and increase in cellular sensitivity to JH and 20E, respectively, occurred gradually during this period. In early day 3, autonomous pupal commitment began in vitro and 20E stimulated the commitment, but JH could only partially prevent the commitment in both cases. This apparent reversible to irreversible transition ended in early day 6 by the completion of pupal commitment, when the cells completely lost their sensitivity to JH and no longer expressed the larval cuticle protein gene 30. The expression of the transcription factor, broad, closely followed the commitment, so that we could use this gene expression as a molecular marker for pupal commitment. These results indicate that exposure to 20E and loss of the sensitivity of the epidermal cells to JH are required for the completion of pupal commitment, and suggest that the unusually long process over 3 days could be due to the presence of the detectable JH during the commitment.

Correlation of hemocyte counts with different developmental parameters during the last larval instar of the tobacco hornworm, Manduca sexta

We determined the changes in hemocyte titer and in the abundance of hemocyte types of the tobacco hornworm Manduca sexta during the fourth and fifth larval stadium and the beginning of the pupal stadium. As we analyzed the samples of individual insects at daily intervals, we were able to correlate phenotypical features, body weight, as well as total protein content and lysozyme activity in the hemolymph with the observations on hemocytes. In the course of the fifth larval stadium, the hemocyte titer decreased slightly and declined further after pupation. Using calculated values for total hemocyte numbers, females had about five times and males three times more hemocytes in the circulating population at the beginning of the wandering stage (in the middle of the fifth larval stadium) than immediately after the last larval--larval molt (from the fourth to the fifth larval stadium). This sexual difference was mainly due to an increase in the number of plasmatocytes, which was more prominent in females than in males. Granular cells were dominant in early fifth larval stadium while plasmatocytes were the most abundant cells in pupae. Oenocytoids and spherule cells disappeared during the wandering stage. Lysozyme activity in the hemolymph rose to a maximum during the wandering stage, with females having lysozyme values twice as high as those for males. These changes in lysozyme activity, however, did not correlate with the increase of total hemolymph protein titer which occurred already at the beginning of the wandering stage. We postulate that changes in hemocyte titers are under direct hormonal control, which has to be proven in future experiments.

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.

Critical period for pupal commitment in the yellow fever mosquito, Aedes aegypti

Changes in ecdysteroid levels that lead to commitment of pupal and adult development were studied in the yellow fever mosquito, Aedes aegypti. Application of juvenile hormone at the time of pupal commitment usually results in delay or blockage of pupal and adult development. With methoprene, a juvenile hormone mimic, the JH sensitive period was found to be within 19 h after ecdysis to the fourth instar, at which time methoprene treatment caused a delay in pupation. Consistent with this observation, there was a small peak of ecdysteroid levels between 14 and 28 h after ecdysis to the fourth instar. Therefore, the commitment to pupal development occurs most likely between 14 and 19 h after ecdysis to the fourth instar. Levels of transcription of the ecdysone receptor gene show a small peak between 12 and 24 h, and a larger peak between 46 and 66 h after ecdysis to the fourth instar.

Commencement of pupal commitment in late penultimate instar and its hormonal control in wing imaginal discs of the silkworm, Bombyx mori

Pupal commitment of the wing imaginal disc of the silkworm, Bombyx mori, is completed shortly after the final (fifth) larval ecdysis. Pupal commitment was induced by in vitro culture with 20-hydroxyecdysone (20E). Shortly after the head capsule slippage (HCS) that occurs approximately 24 h before the final larval ecdysis, the discs become competent to respond to 20E, indicating that the process of pupal commitment begins in the late penultimate (fourth) instar. The simultaneous presence of methoprene (JHA) with 20E suppressed the pupal commitment at 4 ng/ml for the discs at 12 h after HCS and at 240 ng/ml for the discs at the ecdysis. Thus, the discs rapidly lose their sensitivity to JH at the end of the fourth instar. Day 0 fourth wing discs were not pupally committed by 20E when freshly dissected discs were exposed to 20E. By contrast, exposure to 20E after a pre-culture in a hormone free medium induced the pupal commitment. In those discs, the effective JHA concentration to suppress the 20E effects was 0.1 ng/ml. The present data suggest that pupal commitment proceeds through two stages from a reversible state that begins at around HCS to an irreversible state early in the fifth instar. The loss of sensitivity to JH is the primary impetus to begin the process and 20E is the factor that drives the discs to enter the reversible state.

Regulation of prothoracic gland ecdysteroidogenic activity leading to pupal metamorphosis

The prothoracic glands of early last (fifth) instar larvae of the silkworm are inactive with regard to ecdysteroidogenesis and unresponsive to prothoracicotropic hormone (PTTH) [J. Insect Physiol. 31 (1985) 455]. In an attempt to elucidate the hormonal mechanisms that cause the inactivity, we compared the effects of PTTH, dibutyryl cyclic AMP (dbcAMP), a cAMP phosphodiesterase inhibitor (IBMX), juvenile hormone analogue (JHA) and 20-hydroxyecdysone (20E) on secretory activity of the third, fourth and fifth instar glands. Among the factors examined, feedback inhibition by 20E was indicated to be the most likely factor. Inhibition was moderate in the third and early fourth instars while 20E strongly inhibited the glands of middle fourth instar larvae. The inhibitory effect of 20E was reduced by removal of the brain and corpora allata. Once the glands were suppressed by 20E to the degree of exhibiting neither secretory activity nor responsiveness to PTTH, dbcAMP or IBMX did not elicit ecdysone secretion at all. Thus the feedback inhibition may shut down ecdysteroidogenesis although it is obscure whether it affects the intracellular transductory cascade from the PTTH receptor through cAMP. Taken together, this evidence suggests that inactivity of the gland in the early fifth instar is brought about by feedback inhibition of the glands by 20E occurring in the late fourth instar, and that this inactivity is maintained by the juvenile hormone found in the early fifth instar.

A novel gene in the takeout gene family is regulated by hormones and nutrients in Manduca larval epidermis

A novel gene, moling, was cloned from epidermal RNA of the tobacco hornworm, Manduca sexta, using PCR-based suppression subtractive hybridization. moling belongs to a gene family that includes several lepidopteran hemolymph juvenile hormone (JH) binding proteins and takeout of Drosophila melanogaster. The mRNA first appears in the epidermis on day 0 of the fifth instar and rises to its peak expression by mid-day 2, then declines rapidly and is gone by the onset of wandering. moling is expressed exclusively in the last instar larval epidermis and not in the imaginal discs or any other tissues. Allatectomy early in the fourth instar induces precocious metamorphosis and causes the appearance of moling mRNA by 33 h. Allatectomy after the critical period for JH in the final larval molt had no effect on the timing of the onset of moling expression in the final instar but caused a more rapid up-regulation once begun. The JH mimic pyriproxifen given at the outset of the final instar suppressed the expression of moling mRNA to low levels, in both intact and allatectomized larvae. Starvation immediately after ecdysis to the fifth instar prevented the onset of expression. Thus, initiation of transcription requires both nutrient intake and decline in JH. Infusion of 20-hydroxyecdysone (20E) into ligated abdomens of day 2 fifth instar larvae and culture of the day 2 fifth instar larval abdominal epidermis with 20E in vitro both caused a rapid decline of moling mRNA. The slower and variable decline that occurred in mid-day 2 fifth instar larval epidermis in the ligated abdomens or when incubated in hormone-free medium indicated that the increase of 20E on day 2 had already initiated the decline of expression. The role of Moling may be to stabilize JH in the epidermal cell during the final intermolt when the JH esterase activity increases.

E74 exhibits stage-specific hormonal regulation in the epidermis of the tobacco hornworm, manduca sexta

The transcription factor E74 is one of the early genes induced by ecdysteroids during metamorphosis of Drosophila melanogaster. Here, we report the cloning and hormonal regulation of E74 from the tobacco hornworm, Manduca sexta (MsE74). MsE74 is 98% identical to that of D. melanogaster within the DNA-binding ETS domain of the protein. The 5'-isoform-specific regions of MsE74A and MsE74B share significantly lower sequence similarity (30-40%). Developmental expression by Northern blot analysis reveals that, during the 5th larval instar, MsE74B expression correlates with pupal commitment on day 3 and is induced to maximal levels within 12h by low levels of 20-hydroxyecdysone (20E) and repressed by physiologically relevant levels of juvenile hormone I (JH I). Immunocytochemical analysis shows that MsE74B appears in the epidermis before the 20E-induced Broad transcription factor that is correlated with pupal commitment (Zhou and Riddiford, 2001). In contrast, MsE74A is expressed late in the larval and the pupal molts when the ecdysteroid titer has declined to low levels and in the adult molt just as the ecdysteroid titer begins to decline. This change in timing during the adult molt appears not to be due to the absence of JH as there was no change during the pupal molt of allatectomized animals. When either 4th or 5th instar larval epidermis was explanted and subjected to hormonal manipulations, MsE74A induction occurred only after exposure to 20E followed by its removal. Thus, MsE74B appears to have a similar role at the onset of metamorphosis in Manduca as it does in Drosophila, whereas MsE74A is regulated differently at pupation in Manduca than at pupariation in Drosophila.

Characterization and cDNA cloning of three major proteins from pharate pupal cuticle of Manduca sexta

Three proteins, MsCP20, MsCP27 and MsCP36, that are secreted in greatest quantity into the pharate pupal cuticle of Manduca sexta ( Hopkins et al., 2000) were purified and their amino acid sequences determined by mass spectrometry and Edman degradation. Although these proteins become sclerotized and insoluble in the pupal exoskeleton, their sequences contain features characteristic for proteins occurring in less sclerotized pliable cuticles, such as arthrodial membranes and soft larval cuticles. These proteins carry a secondary modification attached to a threonine residue, presumably an O-linked sugar moiety. cDNA clones of the genes for MsCP20, MsCP27 and MsCP36 were constructed from pharate pupal integument RNA. Close agreement was found between the amino acid sequences determined by Edman degradation and sequences deduced from the cDNA clones. The molecular masses determined by protein sequencing for MsCP20, MsCP27, and MsCP36 were 17713, 17448, and 29582 Da, respectively, in close agreement with the masses deduced from the corresponding cDNA clones (17711, 17410, and 29638 Da). Temporal expression analysis indicates that MsCP20 and MsCP36 transcripts are present at low levels early in the fifth larval stadium, followed by a large increase in abundance prior to pupal ecdysis. MsCP27 was not detected during development of the fifth larval instar, but its transcript, like those of MsCP20 and MsCP36, increased to a peak level just before pupal ecdysis. Only the MsCP36 transcript was detected in adults. These results support the hypothesis that these proteins are synthesized by the epidermis and are subsequently deposited into the cuticle during the larval-pupal transformation of M. sexta where they become sclerotized in the formation of pupal exocuticle.

Bombyxin is a growth factor for wing imaginal disks in Lepidoptera

The mechanisms that control the growth rate of internal tissues during postembryonic development are poorly understood. In insects, the growth rate of imaginal disks varies with nutrition and keeps pace with variation in somatic growth. We describe here a mechanism by which the growth of wing imaginal disks is controlled. When wing imaginal disks of the butterfly Precis coenia are removed from the larva and placed in a standard nutrient-rich tissue culture medium they stop growing, suggesting that nutrients alone are not sufficient to support normal growth. Such disks can be made to grow at a normal rate by supplementing the culture medium with an optimal concentration of the steroid hormone 20-hydroxyecdysone and with hemolymph taken from growing larvae. The growth-promoting activity of the hemolymph is caused by a heat-stable factor that can be extracted from the CNS and appears to be identical to the neurohormone bombyxin, a member of the insulin family of proteins. Synthetic bombyxin stimulates growth at concentrations as low as 30 ngml, and specific antibodies to bombyxin completely remove growth-promoting activity from the hemolymph. Bombyxin evidently acts together with 20-hydroxyecdysone to stimulate cell division and growth of wing imaginal disks. It appears that the level of bombyxin in the hemolymph is modulated by the brain in response to variation in nutrition and is part of the mechanism that coordinates the growth of internal organs with overall somatic growth.

Localization of a cuticular protein during the postembryonal development of Manduca sexta

The pattern of cuticle protein synthesis by the epidermis of insects changes during the last larval, pupal and adult development, leading to an alteration in cuticular stucture and feature. We have isolated a protein that had an apparent molecular mass of 33.1 kD from larval cuticle of Manduca sexta. Synthesis, transport and accumulation of MsCP33.1 were followed during metamorphosis by immunoblots and immunocytochemical methods using the antibody developed against this protein. Our data prove that the presence of MsCP33.1 in the larval cuticle is general while its appearance in the pupal or adult integument is restricted only in the cuticle of wings and apodemes. We established that the synthesis of 33.1 kD protein is negatively regulated by moulting hormone (20-hydroxyecdysone). Possible roles for this cuticular protein are discussed.

Isolation and developmental expression of two nuclear receptors, MHR4 and betaFTZ-F1, in the tobacco hornworm, Manduca sexta

The cDNAs for two members of the nuclear receptor superfamily were isolated from the tobacco hornworm, Manduca sexta. The deduced amino acid sequence of MHR4 shows 93-95% identity in the DNA-binding domain and the first portion of the hinge (D) region with the germ cell nuclear factor (GCNF)-related factors (GRFs) of the silkworm, Bombyx mori, and the mealworm, Tenebrio molitor, and with a genomic sequence from the fruit fly, Drosophila melanogaster. Northern blot hybridization showed that a 7.5 kb MHR4 mRNA appeared in Manduca abdominal epidermis just as the ecdysteroid titer began to decline during the larval molt, disappeared about 12 h later, then transiently reappeared shortly before larval ecdysis. During the pupal and adult molts, a similar pattern of expression was seen (the very end of the adult molt was not studied). At peak times of expression in the epidermis, MHR4 mRNA was also present in fat body and the central nervous system (CNS). The deduced amino acid sequence of Manduca FTZ-F1 is 100% and 96% identical to that of B. mori and Drosophila betaFTZ-F1, respectively, in the DNA-binding domain and the adjacent hinge region including the FTZ-F1 box. Northern blot analysis showed that the >9.5 kb betaFTZ-F1 mRNA appeared in Manduca epidermis during the decline of the ecdysteroid titer in the larval, pupal and adult molts as the first peak of MHR4 mRNA declined, then it disappeared in the larval and pupal molts before the second peak of MHR4 appeared. betaFTZ-F1 mRNA was also found in fat body and the CNS at the time of peak expression in the epidermis during the larval and pupal molts. Both MHR4 and betaFTZ-F1 mRNAs were found in the testis during the onset of spermatogenesis in the prepupal period.

Regulation of transcription factors MHR4 and betaFTZ-F1 by 20-hydroxyecdysone during a larval molt in the tobacco hornworm, Manduca sexta

During the last larval molt in Manduca sexta, a number of transcription factors are sequentially expressed. Unlike E75A and MHR3, whose mRNAs are induced when the ecdysteroid titer increases, the expression of MHR4 mRNA occurs transiently at the onset of the decline of ecdysteroid titer followed by betaFTZ-F1 mRNA expression when the ecdysteroid titer becomes low. When day 2 fourth epidermis was exposed to 20-hydroxyecdysone (20E) in vitro, MHR4 mRNA appeared between 12 and 21 h, peaked at 24 h, and then declined. Using the protein synthesis inhibitors cycloheximide and anisomycin both in vivo and in vitro, we found that the MHR4 transcript was directly induced by 20E and required the presence of 20E for its expression. The accumulation of MHR4 mRNA, however, did not occur until a 20E-induced inhibitory protein(s) disappeared. This control of MHR4 expression is unique among the ecdysone-induced transcription factors. When the epidermis was cultured with 20E, betaFTZ-F1 mRNA was not induced until after the removal of 20E as previously found for Drosophila and the silkworm Bombyx mori. The presence of juvenile hormone had no effect on accumulation of either transcript.

Studies on proteins in post-ecdysial nymphal cuticle of locust, Locusta migratoria, and cockroach, Blaberus craniifer

Proteins were extracted from the cuticle of mid-instar nymphs of locusts, Locusta migratoria, and cockroaches, Blaberus craniifer. Seven proteins were purified from the locust extract and five from the cockroach extract, and their amino acid sequences were determined. Polyacrylamide gel electrophoresis indicates that the proteins are present only in the post-ecdysially deposited layer of the nymphal cuticles. One of the locust and one of the cockroach nymphal proteins contain a 68-residue motif, the RR-2 sequence, which has been reported for several proteins from the solid cuticles of other insect species. Two of the cockroach proteins contain a 75-residue motif, which is also present in a protein from the larval/pupal cuticle of a beetle, Tenebrio molitor, and in proteins from the exoskeletons of a lobster, Homarus americanus, and a spider, Araneus diadematus. The motif contains a variant of the Rebers-Riddiford consensus sequence, and is called the RR-3 motif. One of the locust and three of the cockroach post-ecdysial proteins contain one or more copies of an 18-residue motif, previously reported in a protein from Bombyx mori pupal cuticle. The nymphal post-ecdysial proteins from both species have features in common with pre-ecdysial proteins (pharate proteins) in cuticles destined to be sclerotised; they show little similarity to the post-ecdysial cuticular proteins from adult locusts or to proteins from soft, pliable cuticles. Possible roles for post-ecdysial cuticular proteins are discussed in relation to the reported structures.

Regulation of ecdysteroid signalling: molecular cloning, characterization and expression of 3-dehydroecdysone 3 alpha-reductase, a novel eukaryotic member of the short-chain dehydrogenases/reductases superfamily from the cotton leafworm, Spodoptera littoralis

One route of inactivation of ecdysteroids in insects involves ecdysone oxidase-catalysed conversion into 3-dehydroecdysone (3DE), followed by irreversible reduction by 3DE 3 alpha-reductase to 3-epiecdysone. The 3DE 3 alpha-reductase has been purified and subjected to limited amino acid sequencing. It occurs as two distinct forms, including a probable trimer of subunit molecular mass of approx. 26 kDa. A reverse-transcriptase PCR-based approach has been used to clone the cDNA (1.2 kb) encoding the 26 kDa protein. Northern blotting showed that the mRNA transcript was expressed in Malpighian tubules during the early stage of the last larval instar. Conceptual translation of the 3DE 3 alpha-reductase cDNA and database searching revealed that the enzyme belongs to the short-chain dehydrogenases/reductases superfamily. Furthermore, the enzyme is a novel eukaryotic 3-dehydrosteroid 3 alpha-reductase member of that family, whereas vertebrate 3-dehydrosteroid 3 alpha-reductases belong to the aldo-keto reductase (AKR) superfamily. Enzymically active recombinant 3DE 3 alpha-reductase has been produced using a baculovirus expression system. Surprisingly, we observed no similarity between this 3DE 3 alpha-reductase and a previously reported 3DE 3 beta-reductase, which acts on the same substrate and belongs to the AKR family.

Juvenile hormone acid and ecdysteroid together induce competence for metamorphosis of the Verson's gland in Manduca sexta

In the last larval instar of Lepidoptera, ecdysteroid in the absence of juvenile hormone (JH) is believed to cause the shift from larval to pupal development. In Manduca sexta, tissues such as the Verson's gland and crochet epidermis become pupally committed before the earliest pulse of ecdysteroid that occurs on day 2. What causes the change in commitment in these tissues? First it was necessary to determine at what stage these tissues become competent to express the pupal program. Last instar larvae of different ages were induced to molt prematurely by feeding the ecdysteroid analog RH5992 and Verson's gland proteins were analyzed by SDS-polyacrylamide gel electrophoresis. Glands became competent to make pupal proteins between 24 and 32 h after the last larval ecdysis. Next, hormonal regulation of competence was examined in ligated abdomens of 12h last instar larvae. Treatment with JH II acid or methoprene acid plus a low dose (1/50th of the molt inducing dose) of RH5992 induced competence, whereas RH5992 alone, methoprene acid alone or methoprene plus RH5992 did not. Verson's glands maintained in vitro produced pupal proteins in response to methoprene acid together with RH5992 but not with RH5992 alone. Likewise, crochet epidermis lost the ability to make crochets (metamorphic change) only in isolated abdomens treated with JH II acid or methoprene acid and low doses of RH5992. In conclusion, JH acid in the presence of basal levels of ecdysteroid induces tissue competence for metamorphosis. Metamorphic competence is followed by commitment, induced by a small pulse of ecdysteroid in the absence of JH, and finally by expression caused by a high titer of ecdysteroid. It is proposed that JH acid is an essential metamorphic hormone.

Insect cuticle, an in vivo model of protein trafficking

In the course of this study more than 20 proteins have been isolated from the larval cuticle of Manduca sexta. Synthesis, secretion, transport and accumulation of four particular proteins, representative members of four characteristic groups, were followed during metamorphosis by immunoblot and immuncytochemical methods and are described in detail in this paper. We established that only some of the proteins of the soft cuticle of Lepidopteran larvae are synthesized in epidermal cells at the beginning of the larval stages and are digested during the moulting period (MsCP29). Other proteins (MsCP30/11) are secreted into the cuticle by the epidermal cells in different forms during various developmental stages. Some proteins are secreted apically during the feeding period, but before ecdysis they are then taken up by epidermal cells and transported in a basolateral direction back into the hemolymph and saved in an immunologically intact form by the fat body cells (MsCP12.3). Some cuticle proteins have a non-epidermal origin. They are transported from the hemolymph into the cuticle. Before and during ecdysis these molecules reappear in the hemolymph and are detectable again in the pupal cuticle (MsCP78). Our data prove that the cuticle is not a non-living part of the insect body: it is not only an inert, protective armor, but maintains a continuous and dynamic metabolic connection with the other organs of the organism.

Juvenile hormone prevents ecdysteroid-induced expression of broad complex RNAs in the epidermis of the tobacco hornworm, Manduca sexta

A cDNA homolog of the Drosophila melanogaster Broad Complex (BRC) gene was isolated from the tobacco hornworm, Manduca sexta, which shows a predicted 88% amino acid identity with Drosophila BRC in the N-terminal BTB domain. Three zinc finger domains encoding homologs of the Drosophila Z2, Z3, and Z4 domains (93, 100, and 85% identity, respectively) were obtained by RT-PCR. In Manduca dorsal abdominal epidermis, BRC RNAs were not observed during the larval molt. Three BRC transcripts-6.0, 7.0, and 9.0 kb-first appeared at the end of the feeding stage of the fifth (final) instar when the epidermis is exposed to ecdysteroids in the absence of juvenile hormone (JH) and becomes committed to pupal differentiation. These RNAs were induced in day 2 fifth larval epidermis in vitro by 20-hydroxyecdysone (20E) in the absence of JH with dose-response and time courses similar to the induction of pupal commitment. This induction by 20E in vitro was prevented by the presence of JH I at levels seen in vivo during the larval molt. In the wing discs, the BRC RNAs appeared shortly after ecdysis to the fifth instar and coincided with the onset of metamorphic competence of these discs. Application of a JH analogue pyriproxifen during the fourth instar molt delayed and reduced the levels of BRC mRNAs seen in the wing discs in the early fifth instar, but did not completely prevent their appearance in this tissue that first differentiates at metamorphosis. The expression of the BRC transcription factors thus appears to be one of the first molecular indications of the genetic reprogramming of the epidermis necessary for insect metamorphosis. How JH prevents BRC expression in this epidermis may provide the key to understanding how this hormone controls metamorphosis.

Hemolymph ecdysteroid titer and ecdysteroid-dependent developmental events in the last-larval stadium of the silkworm, Bombyx mori: role of low ecdysteroid titer in larval-pupal metamorphosis and a reappraisal of the head critical period

The endocrine regulation of larval-pupal metamorphosis was studied in the silkworm, Bombyx mori, by measuring the following changes: hemolymph ecdysteroid titer, the secretory activity of prothoracic glands and the responsiveness of larvae to ecdysteroids and prothoracicotropic hormone (PTTH), with regard to developmental events such as the occurrence of spinneret pigmentation, initiation of cocoon spinning and onset of wandering stage as indicated by gut purge. These measurements were concentrated especially on the time before and after the head critical period (HCP) which falls 3-4 days before the gut purge ([Sakurai, 1984]). A small increase in the hemolymph ecdysteroid titer was first found during the HCP, and then the titer increased with daily fluctuations. Small but significant titer peaks were found prior to the occurrence of both spinneret pigmentation and gut purge, indicating that an individual titer peak could possess a specific role in development. Responsiveness of larvae to exogenous 20-hydroxyecdysone (20E) after the HCP was markedly higher than that before the HCP. The sensitivity of the prothoracic gland to PTTH also changed during the HCP. The results thus showed that the HCP is not the period after which an additional PTTH release is not required for the developmental events occurring on schedule, but rather it is the period during which complex events occur not only in the endocrine glands but also in the peripheral tissues. In addition, various developmental phenomena before gut purge are brought about by the hemolymph ecdysteroid whose concentration gradually increased with daily fluctuations, and these precise changes in the titer appeared to be important for the sequential occurrence of developmental events in the larval-pupal metamorphosis.

Juvenile hormone acid: evidence for a hormonal function in induction of vitellogenin in larvae of Manduca sexta

In the tobacco hornworm (Manduca sexta), vitellogenin (Vg), the major yolk protein precursor, and its mRNA are first detectable in the prepupal stage; and production of both can be enhanced by methoprene, a juvenile hormone (JH) analog. Competence to respond to methoprene is acquired after ecdysteroid-initiated commitment for metamorphosis. Here we show that acquisition of competence requires prior exposure to JH-II acid in addition to ecdysteroid. Application of 20-hydroxyecdysone or RH5992, an ecdysteroid analog, to isolated abdomens from feeding larvae (precommitment) results in exposure of the dorsal vessel (EDV), a sign of metamorphic commitment--but such abdomens do not make Vg in response to methoprene. However, injection of JH-II acid along with 20-hydroxyecdysone into isolated abdomens causes Vg production in response to methoprene. Methoprene acid similarly induces competence to respond to methoprene. Northern blot analysis confirmed that Vg transcripts are present in fat body only if isolated abdomens were pretreated with both ecdysteroid, and JH-II acid or methoprene acid. The latter two can induce competence even in precocious prepupae resulting from removal of the corpora allata (the glands that produce JH) from early penultimate larvae. JH-III acid and related metabolites such as farnesol, farnesoic acid, and methyl farnesoate do not induce competence. Hitherto, JH acids have been regarded as precursors or catabolites of JHs. Here we show for the first time that JH acid has a hormonal function that cannot be performed by JH itself.

Action of different ecdysteroids on the regulation of mRNAs for the ecdysone receptor, MHR3, dopa decarboxylase, and a larval cuticle protein in the larval epidermis of the tobacco hornworm, Manduca sexta

To determine which ecdysteroids may be biologically active in the larval epidermis of the tobacco hornworm, Manduca sexta, we studied the action of several known ecdysteroids and metabolites on the expression of the genes encoding the ecdysone receptor (EcR), Manduca hormone receptor 3 (MHR3), dopa decarboxylase (DDC), and a larval cuticle protein (LCP-14). Both Day 2 fourth- and Day 2 fifth-instar larval epidermis contained significant 3 beta-reductase activity which metabolized 3-dehydroecdysone (3DE) and 3-dehydro-20-hydroxyecdysone (3D20E) to ecdysone (E) and 20-hydroxyecdysone (20E), respectively, but had only very low amounts of ecdysone oxidase activity (E to 3DE) and no detectable ecdysone 20-monooxygenase activity (E to 20E). When the expression of the various genes was studied in the epidermis in vitro, 20E and 3D20E had similar effects, whereas E, 3DE, 26-hydroxyecdysone and 20,26-dihydroxyecdysone were ineffective. Exposure of Day 2 fifth-instar epidermis to 500 ng/ml of either 20E or 3D20E for 24 hr caused a rapid, biphasic increase in EcR-B1 mRNA. By contrast, EcR-A mRNA showed a less rapid initial increase followed by a slow steady rise and was less responsive to 3D20E. Ecdysone in a 1:1 mixture with 20E effectively halved the concentration of 20E needed to induce EcR-B1 mRNA but showed no synergism in the induction of EcR-A mRNA. The induction of MHR3 mRNA and of DDC mRNA in Day 2 fourth-instar epidermis as well as the suppression of DDC and LCP-14 gene expression by 3D20E was indistinguishable from that of 20E. Therefore, for Manduca larval epidermis, only 20E and 3D20E are biologically active ecdysteroids. Since the 3D20E can be converted to 20E by the epidermis, its effects are likely mediated by 20E.

Parasitism-induced Effects on Host Growth and Metabolic Efficiency in Tobacco Hornworm Larvae Parasitized by Cotesia congregata

Parasitism by the braconid wasp Cotesia congregata affects the growth of Manduca sexta larvae in a parasitoid 'dose-dependent' fashion. Following parasitization of fourth-instar larvae, more heavily parasitized larvae grew larger compared to those containing fewer parasitoids due to an increase in host dry weight. The differences in host mass appeared to arise after oviposition. A 'dose-dependent' enhancement of host dry weight would appear nutritionally beneficial for the parasitoids developing in more 'crowded' hosts. The efficiencies of conversion of ingested and digested food to body mass and the approximate digestibility of the diet ingested by the host caterpillar did not vary significantly with clutch size although parasitoids took slightly longer to develop in the more heavily parasitized hosts. Larval parasitoids developing in the presence of many competitors weighed up to 50% less than those developing in hosts with fewer endoparasitoids, although the weight of adult female parasitoids did not vary significantly with wasp clutch size. The maximum number of emerging wasps was 200 parasitoids, possibly representing the host's 'carrying capacity' for larvae parasitized in the fourth-instar. The ratio of emerging to non-emerging parasitoids decreased as parasitoid clutch size increased, with few or none emerging from very heavily parasitized hosts containing more than 400 parasitoids. Copyright 1997 Elsevier Science Ltd. All right reserved

Up-regulation of an adult cuticular gene by 20-hydroxyecdysone in insect metamorphosing epidermis cultured in vitro

Pupal forewing epidermis of the Coleoptera, Tenebrio molitor, was used to develop an in vitro system to study the hormonal control of metamorphosis at the cellular and molecular levels. Exposure to 1 microM 20-hydroxyecdysone for 48 h caused the formation of a typical adult cuticle. Under these conditions the expression of ACP-20, an adult-specific cuticular gene, was fivefold higher than in absence of exogenous hormone. This stimulation was also observed when a higher level of 20-hydroxyecdysone was maintained, and prevented by protein inhibitors, indicating that 20-hydroxyecdysone does not act directly on this gene. Exposure to 20-hydroxyecdysone followed by exposure in hormone-free medium caused the cessation of this stimulation, showing the requirement of the 20-hydroxyecdysone continuous presence for stimulating ACP-20 gene expression. Thus, unlike the other cuticular protein genes so far studied, its expression is not repressed by 20-hydroxyecdysone, and does not need the decline in ecdysteroids titer.

Dopaminergic control of corpora allata activity in the larval tobacco hornworm, Manduca sexta

The corpora allata (CA) of insects are innervated by axons of non-neurosecretory cerebral neurons, and of the various known neurotransmitters in the brain of the tobacco hornworm, Manduca sexta, only dopamine is detected in the CA by electrochemical detection HPLC. This neurotransmitter stimulates the biosynthetic activity of the CA in vitro for the first 2 days of the last larval stadium, but inhibits CA from day 3 through day 6, the beginning of the prepupal period. Stimulation of JH synthesis has previously been linked with an increase in the production of cyclic AMP (cAMP) in the CA, and dopamine stimulates the adenylyl cyclase system of CA from larvae early in the fifth stadium, while on day 6, its effect is inhibitory. These results suggest: (1) the existence in the CA of both D1- and D2-like dopamine receptors, which in vertebrates stimulate and inhibit, respectively, adenylyl cyclase; and (2) the developmental control of their expression. A potent D1 agonist, (+/-)-SKF 82958-HBr, did not stimulate JH biosynthesis by day 0 CA as expected, but appeared to inhibit it at a concentration of 10(-5)M. Thus the apparent D1-like receptor in Manduca CA may be pharmacologically distinct from vertebrate D1 receptors. The existence of D2-like receptors is supported by the finding that a vertebrate D2 receptor agonist, (+/-) PPHT-HCl, and an antagonist, eticlopride, have the predicted effects on JH acid biosynthesis and cAMP production by day 6 Manduca CA. However, the D1 agonist also significantly reduces JH acid biosynthesis and cAMP production, indicating that while the Manduca D2-like receptor is pharmacologically similar to the vertebrate D2, it shares some characteristics with D1 receptors. The developmental regulation of these receptors by ecdysteroids is suggested by the fact that when day 0 larvae are treated in vivo with exogenous ecdysone:20-hydroxyecdysone, the biosynthetic activity of the CA in vitro 24 h later is no longer stimulated by dopamine.

Proteins of crustacean exoskeleton: IV. Partial amino acid sequences of exoskeletal proteins from the Bermuda land crab, Gecarcinus lateralis, and comparisons to certain insect proteins

As in all decapod Crustacea, the exoskeleton of the land crab Gecarcinus lateralis consists of four layers. Prior electrophoretic analysis of proteins extracted from these layers revealed an abundance of small M(r) proteins with acidic pIs are found in insect cuticle (O'Brien et al. [1991 Biol. Bull., 181:427-441). Further, immunological cross-reactivity between crab exoskeletal proteins and insect cuticular proteins has been demonstrated (Kumari and Skinner [1993] J. Exp. Zool., 265:195-210). Partial amino acid sequences of a number of proteins from the four exoskeletal layers are described here. Proteins were electrophoresed on two-dimensional (2D) gels, transferred to polyvinylidene difluoride (PVDF) membranes, and stained; individual spots were recovered and their N-termini were sequenced. In addition, a 14-kDa protein (pI = 5.4) from membranous layer (ML14) was eluted from 2D gels and digested with endoproteinase Lys-C; N-termini of its constituent peptides were sequenced. The two epicuticular proteins differed from each other. Three proteins with identical electrophoretic mobility isolated from exocuticle, endocuticle, and membranous layer appeared to have identical N termini, while another electrophoretically identical set from the three layers appeared identical with each other but differed in three positions from the first set. Two proteins from the membranous layer both had a mass of 25 kDa but different isoelectric points. Their sequences were indistinguishable from each other but clearly distinct from another membranous layer protein. Another distinct sequence was found in a 14-kDa protein from endocuticle, while a less acidic pair of 14-kDa proteins from endocuticle and membranous layer were quite similar to one another. The three internal peptide fragments from ML14 were distinct, but one had regions similar to the ML14 N terminus. One crab exoskeletal protein sequence was similar to some structural proteins of vertebrates, whereas others had motifs found in insect cuticular proteins. The sequence similarities identified did not account for the antibody cross-reactivity.

Insect cuticular proteins

Insect cuticles are composite structural materials with mechanical properties optimal for their biological functions. The bulk properties of cuticles are to a large extent determined by the interactions between the various components, mainly the chitin filament system and the proteins. The various cuticular types show pronounced differences in mechanical properties, and it is suggested that these differences can be related to the properties of the individual proteins and to the degree of secondary stabilization (sclerotization). The amino acid sequences, which have been obtained for insect cuticular proteins either by direct sequencing of purified proteins or by deduction from corresponding DNA-sequences, are listed according to insect order and species. Extensive sequence similarity is observed among several cuticular proteins obtained from different insect orders. Other cuticular proteins are characterized by repeated occurrence of a few small motifs consisting mainly of hydrophobic residues. The latter group of proteins has so far only been reported from stiff cuticles. The possible relevance of the various motifs and repeats for protein interaction and the mechanical properties of cuticles is discussed.

Regulation and consequences of cellular changes in the prothoracic glands of Manduca sexta during the last larval instar: a review

The prothoracic glands of the tobacco hornworm, Manduca sexta, respond to prothoracicotropic hormone (PTTH) by a regulatory pathway involving cAMP, protein phosphorylation, protein synthesis, and enhanced secretion of ecdysteroids including ecdysone and 3-dehydroecdysone. Recent investigations have revealed that PTTH acts by this general mechanism throughout the fifth larval instar, i.e., during the transition from larva to pupa. However, the glands undergo developmental changes in size, steroidogenic capacity, and in elements of the signalling pathway associated with synthesis, degradation, and intracellular action of cAMP. The present review describes such changes, and their possible regulation and consequences, in the general context of endocrine events underlying larval-pupal metamorphosis during the fifth larval stage.