Size assessment and growth control: how adult size is determined in insects

Amino acids and TOR signaling promote prothoracic gland growth and the initiation of larval molts in the tobacco hornworm Manduca sexta

Molting in arthropods is orchestrated by a series of endocrine changes that occur towards the end of an instar. However, little is understood about the mechanisms that trigger these endocrine changes. Here, nutritional inputs were manipulated to investigate the minimal nutritional inputs required for a Manduca sexta larva to initiate a molt. Amino acids were found to be necessary for a larva to molt, indicating the involvement of an amino acid sensitive pathway. Feeding rapamycin, an inhibitor of the target of rapamycin (TOR) signaling, delayed the onset of a molt and resulted in abnormally larger larvae. Rapamycin also suppressed the growth of the prothoracic glands relative to the whole body growth, and this was accompanied by suppression of ecdysone production and secretion. Higher doses of rapamycin also slowed the growth rate, indicating that TOR signaling also plays a role in systemic growth. TOR signaling therefore couples the nutritional status of the larva to the endocrine system to regulate the timing of a molt.

Developmental expression of drop-dead is required for early adult survival and normal body mass in Drosophila melanogaster

In Drosophila melanogaster, mutations in the gene drop-dead (drd) result in early adult lethality, with flies dying within 2 weeks of eclosion. Additional phenotypes include neurodegeneration, tracheal defects, starvation, reduced body mass, and female sterility. The cause of early lethality and the function of the drd protein remain unknown. In the current study, the temporal profiles of drd expression required for adult survival and body mass regulation were investigated. Knockdown of drd expression by UAS-RNAi transgenes and rescue of drd expression on a drd mutant background by a UAS-drd transgene were controlled with the Heat Shock Protein 70 (Hsp70)-Gal4 driver. Flies were heat-shocked at different stages of their lifecycle, and the survival and body mass of the resulting adult flies were assayed. Surprisingly, the adult lethal phenotype did not depend upon drd expression in the adult. Rather, expression of drd during the second half of metamorphosis was both necessary and sufficient to prevent rapid adult mortality. In contrast, the attainment of normal adult body mass required a different temporal pattern of drd expression. In this case, manipulation of drd expression solely during larval development or metamorphosis had no effect on body mass, while knockdown or rescue of drd expression during all of pre-adult (embryonic, larval, and pupal) development did significantly alter body mass. Together, these results indicate that the adult-lethal gene drd is required only during development. Furthermore, the mutant phenotypes of body mass and lifespan are separable phenotypes arising from an absence of drd expression at different developmental stages.

How to measure maturation: a comparison of probabilistic methods used to test for genotypic variation and plasticity in the decision to mature

Maturation is a developmental trait that plays a key role in shaping organisms' life-history. However, progress in understanding how maturation phenotypes evolve has been held back by confusion over how best to model maturation decisions and a lack of studies comparing genotypic variation in maturation. Here, we fitted probabilistic maturation reaction norms (PMRNs) to data collected from five clones of Daphnia magna and five of Daphnia pulex collected from within and between different populations. We directly compared the utility of modeling approaches that assume maturation to be a process with an instantaneous rate with those that do not by fitting maturation rate and logistic regression models, and emphasize similarities and differences between them. Our results demonstrate that in Daphnia, PMRNs using a logistic regression approach were simpler to use and provided a better fit to the data. The decision to mature was plastic across a range of growth trajectories and dependent upon both body size and age. However, the age effect was stronger in D. magna than D. pulex and varied considerably between clones. Our results support the idea that maturation thresholds can evolve but also suggest that the notion of a threshold based on a single fixed state is an oversimplification that underestimates the adaptability of these important traits.

The regulation of organ size in Drosophila: physiology, plasticity, patterning and physical force

The correct regulation of organ size is a fundamental developmental process, the failure of which can compromise organ function and organismal integrity. Consequently, the mechanisms that regulate organ size have been subject to intense research. This research has highlighted four classes of mechanism that are involved in organ size regulation: physiology, plasticity, patterning and physical force. Nevertheless, how these mechanisms are integrated and converge on the cellular process that regulate organ growth is unknown. One group of animals where this integration is beginning to be achieved is in the insects. Here, I review the different mechanisms that regulate organ size in insects, and describe our current understanding of how these mechanisms interact. The genes and hormones involved are remarkably conserved in all animals, so these studies in insects provide a precedent for future research on organ size regulation in mammals.

Small GTPase Rab4b participates in the gene transcription of 20-hydroxyecdysone and insulin pathways to regulate glycogen level and metamorphosis

The insulin and 20-hydroxyecdysone (20E) pathways coordinately regulate insect growth and metamorphosis. However, the molecular mechanism of the interaction of these two pathways in regulating insect development is not well understood. In the present study, we found that a small GTPase Rab4b from a lepidopteran insect Helicoverpa armigera participates in gene transcription in the two pathways. The results show that RNA interference of Rab4b in larvae results in a decrease in glycogen levels, small pupae, abnormal metamorphic transition, or larval death. The molecular mechanisms are demonstrated that knockdown of Rab4b in the larvae suppresses the transcription of glycogen synthase (GS), as well as the metamorphic-initiating factor (Br) and hormone receptor 3 (HR3), but increases the transcription of Forkhead box class O (FOXO). Further studies in the cell line confirm that Rab4b is necessary for gene transcription in the insulin and 20E pathways. Rab4b locates in the cytoplasm and takes part in regulation on FOXO cytoplasmic location by insulin induction, but travels toward the cell membrane upon 20E induction without affecting the FOXO location. The transcription of Rab4b could be upregulated by insulin injection or glucose feeding to the larvae, but not by 20E or juvenile hormone analogy methoprene. Our data suggest that Rab4b takes part in metamorphosis by regulating gene transcription and glycogen level in the insulin and 20E pathways.

Evolution of novel mosaic castes in ants: modularity, phenotypic plasticity, and colonial buffering

Many ants have independently evolved castes with novel morphology as well as function, such as soldiers and permanently wingless (ergatoid) queens. We present a conceptual model, based on modularity in morphology and development, in which evolutionary innovation is facilitated by the ancestral ant polyphenism of winged queens and wingless workers. We suggest that novel castes evolved from rare intercastes, anomalous mosaics of winged queens and workers, erratically produced by colonies through environmental or genetic perturbations. The colonial environment is highly accommodating and buffers viable intercastes from individual selection. Their cost is limited because they are diluted by the large number of nestmates, yet some can bring disproportionate benefits to their colonies in the context of defense or reproduction (e.g., wingless intercastes able to mate). Useful intercastes will increase in frequency as their morphology is stabilized through genetic accommodation. We show that both soldiers and ergatoid queens are mosaics of winged queens and workers, and they are strikingly similar to some intercastes. Modularity and developmental plasticity together with winged/wingless polyphenism thus allow for the production of highly variable mosaic intercastes, and colonies incubate the advantageous mosaics.

Ectoparasite reproductive performance when host condition varies

Host condition can influence both the nutritive resources available to parasites and the strength of host defences. Since these factors are likely to be correlated, it is unclear whether parasites would be more successful on hosts in good, intermediate or poor conditions. For more complex parasites, like fleas, where larvae depend on adults to extract and make available some essential host resources, host condition can act at two levels. First, it can affect the investment of females into eggs, and second, it can influence offspring growth. In a two-step experiment, we first let female hen fleas Ceratophyllus gallinae feed on nestlings of reduced, control or enlarged great tit Parus major broods and secondly used the blood from these nestlings as a food source for flea larvae reared in the laboratory. We then assessed the effect of brood size manipulation on reproductive investment and survival of female fleas, and on survival, developmental time, mass and size of pre-imago larvae and adults of the first generation. Although host condition, measured as body mass controlled for body size, was significantly influenced by brood size manipulation, it did not affect the female fleas' reproductive investment and survival. Larvae fed with blood from nestlings of reduced broods lived longer, however, than larvae fed on blood from enlarged or control broods. Additionally, F1 adults grew shorter tibiae when their mother had fed on hosts of reduced broods. The finding that brood size manipulation influenced parasite reproduction suggests that it affected nutritive resources and/or host defence, but the precise mechanism or balance between the two requires further investigation.

Predicting the response to simultaneous selection: genetic architecture and physiological constraints

A great deal is known about the evolutionary significance of body size and development time. They are determined by the nonlinear interaction of three physiological traits: two hormonal events and growth rate (GR). In this study we investigate how the genetic architecture of the underlying three physiological traits affects the simultaneous response to selection on the two life-history traits in the hawkmoth Manduca sexta. The genetic architecture suggests that when the two life-history traits are both selected in the same direction (to increase or decrease) the response to selection is primarily determined by the hormonal mechanism. When the life-history traits are selected in opposite directions (one to increase and one to decrease) the response to selection is primarily determined by factors that affect the GR. To determine how the physiological traits affect the response to selection of the life-history traits, we simulated the predicted response to 10 generations of selection. A total of 83% of our predictions were supported by the simulation. The main components of this physiological framework also exist in unicellular organisms, vertebrates, and plants and can thus provide a robust framework for understanding how underlying physiology can determine the simultaneous evolution of life-history traits.

Insulin signaling as a mechanism underlying developmental plasticity: the role of FOXO in a nutritional polyphenism

We investigated whether insulin signaling, known to mediate physiological plasticity in response to changes in nutrition, also facilitates discrete phenotypic responses such as polyphenisms. We test the hypothesis that the gene FOXO--which regulates growth arrest under nutrient stress--mediates a nutritional polyphenism in the horned beetle, Onthophagus nigriventris. Male beetles in the genus Onthophagus vary their mating strategy with body size: large males express horns and fight for access to females while small males invest heavily in genitalia and sneak copulations with females. Given that body size and larval nutrition are linked, we predicted that 1) FOXO expression would differentially scale with body size (nutritional status) between males and females, and 2) manipulation of FOXO expression would affect the nutritional polyphenism in horns and genitalia. First, we found that FOXO expression varied with body size in a tissue- and sex-specific manner, being more highly expressed in the abdominal tissue of large (horned) males, in particular in regions associated with genitalia development. Second, we found that knockdown of FOXO through RNA-interference resulted in the growth of relatively larger copulatory organs compared to control-injected individuals and significant, albeit modest, increases in relative horn length. Our results support the hypothesis that FOXO expression in the abdominal tissue limits genitalia growth, and provides limited support for the hypothesis that FOXO regulates relative horn length through direct suppression of horn growth. Both results support the idea that tissue-specific FOXO expression may play a general role in regulating scaling relationships in nutritional polyphenisms by signaling traits to be relatively smaller.

Precocious metamorphosis in the juvenile hormone-deficient mutant of the silkworm, Bombyx mori

Insect molting and metamorphosis are intricately governed by two hormones, ecdysteroids and juvenile hormones (JHs). JHs prevent precocious metamorphosis and allow the larva to undergo multiple rounds of molting until it attains the proper size for metamorphosis. In the silkworm, Bombyx mori, several “moltinism” mutations have been identified that exhibit variations in the number of larval molts; however, none of them have been characterized molecularly. Here we report the identification and characterization of the gene responsible for the dimolting (mod) mutant that undergoes precocious metamorphosis with fewer larval–larval molts. We show that the mod mutation results in complete loss of JHs in the larval hemolymph and that the mutant phenotype can be rescued by topical application of a JH analog. We performed positional cloning of mod and found a null mutation in the cytochrome P450 gene CYP15C1 in the mod allele. We also demonstrated that CYP15C1 is specifically expressed in the corpus allatum, an endocrine organ that synthesizes and secretes JHs. Furthermore, a biochemical experiment showed that CYP15C1 epoxidizes farnesoic acid to JH acid in a highly stereospecific manner. Precocious metamorphosis of mod larvae was rescued when the wild-type allele of CYP15C1 was expressed in transgenic mod larvae using the GAL4/UAS system. Our data therefore reveal that CYP15C1 is the gene responsible for the mod mutation and is essential for JH biosynthesis. Remarkably, precocious larval–pupal transition in mod larvae does not occur in the first or second instar, suggesting that authentic epoxidized JHs are not essential in very young larvae of B. mori. Our identification of a JH–deficient mutant in this model insect will lead to a greater understanding of the molecular basis of the hormonal control of development and metamorphosis.

The number of larval instars in insects varies greatly across insect taxa and can even vary at the intraspecific level. However, little is known about how the number of larval instars is fixed in each species or modified by the environment. The silkworm, Bombyx mori, provides a unique bioresource for investigating this question, as there are several “moltinism” strains that exhibit variations in the number of larval molts. The present study describes the first positional cloning of a moltinism gene. We performed genetic and biochemical analyses on the dimolting (mod) mutant, which shows precocious metamorphosis with fewer larval–larval molts. We found that mod is a juvenile hormone (JH)–deficient mutant that is unable to synthesize JH, a hormone that prevents precocious metamorphosis and allows the larvae to undergo multiple rounds of larval–larval molts. This JH–deficient mutation is the first described to date in any insect species and, therefore, the mod strain will serve as a useful model for elucidating the molecular mechanism of JH action. Remarkably, precocious larval–pupal transition in mod larvae does not occur in the first or second instar, suggesting that morphostatic action of JH is not necessary for young larvae of B. mori.

Molting dynamics and juvenile hormone titer profiles in the nymphal stages of a lower termite, Cryptotermes secundus (Kalotermitidae)--signatures of developmental plasticity

Termites are social cockroaches and this sociality is founded on a high plasticity during development. Three molting types (progressive, stationary and regressive molts) are fundamental to achieve plasticity during alate/sexual development, and they make termites a major challenge to any model on endocrine regulation in insect development. As the endocrine signatures underpinning this plasticity are barely understood, we studied the developmental dynamics and their underlying juvenile hormone (JH) titers in a wood-dwelling termite, Cryptotermes secundus, which is characterized by an ancestral life style of living in dead wood and individuals being totipotent in development. The following general pattern elements could be identified during winged sexual development (i) regressive molts were accompanied by longer intermolt periods than other molting types, (ii) JH titers decreased gradually during the developmental transition from larva (immatures without wing buds), to nymph (immatures with wing buds), to winged adult, (iii) in all nymphal stages, the JH titer rose before the next molt and dropped thereafter within the first week, (iv) considerable variation in JH titers occurred in the midphase of the molting cycle of the 2nd and 3rd nymphal instar, inferring that this variation may reflect the underlying endocrine signature of each of the three molting types, (v) the 4th nymphal instar, the shortest of all, seems to be a switch point in development, as nymphs in this stage mainly developed progressively. When comparing these patterns with endocrine signatures seen in cockroaches, the developmental program of Cryptotermes can be interpreted as a co-option and repetitive use of hormonal dynamics of the post dorsal-closure phase of cockroach embryonic development.

Coordinating growth and maturation - insights from Drosophila

Adult body size in higher animals is dependent on the amount of growth that occurs during the juvenile stage. The duration of juvenile development, therefore, must be flexible and responsive to environmental conditions. When immature animals experience environmental stresses such as malnutrition or disease, maturation can be delayed until conditions improve and normal growth can resume. In contrast, when animals are raised under ideal conditions that promote rapid growth, internal checkpoints ensure that maturation does not occur until juvenile development is complete. Although the mechanisms that regulate growth and gate the onset of maturation have been investigated for decades, the emerging links between childhood obesity, early onset puberty, and adult metabolic disease have placed a new emphasis on this field. Remarkably, genetic studies in the fruit fly Drosophila melanogaster have shown that the central regulatory pathways that control growth and the timing of sexual maturation are conserved through evolution, and suggest that this aspect of animal life history is regulated by a common genetic architecture. This review focuses on these conserved mechanisms and highlights recent studies that explore how Drosophila coordinates developmental growth with environmental conditions.

A novel role for copper in Ras/mitogen-activated protein kinase signaling

Copper (Cu) is essential for development and proliferation, yet the cellular requirements for Cu in these processes are not well defined. We report that Cu plays an unanticipated role in the mitogen-activated protein (MAP) kinase pathway. Ablation of the Ctr1 high-affinity Cu transporter in flies and mouse cells, mutation of Ctr1, and Cu chelators all reduce the ability of the MAP kinase kinase Mek1 to phosphorylate the MAP kinase Erk. Moreover, mice bearing a cardiac-tissue-specific knockout of Ctr1 are deficient in Erk phosphorylation in cardiac tissue. in vitro investigations reveal that recombinant Mek1 binds two Cu atoms with high affinity and that Cu enhances Mek1 phosphorylation of Erk in a dose-dependent fashion. Coimmunoprecipitation experiments suggest that Cu is important for promoting the Mek1-Erk physical interaction that precedes the phosphorylation of Erk by Mek1. These results demonstrate a role for Ctr1 and Cu in activating a pathway well known to play a key role in normal physiology and in cancer.

Conserved role for the Dachshund protein with Drosophila Pax6 homolog Eyeless in insulin expression

Members of the insulin family peptides have conserved roles in the regulation of growth and metabolism in a wide variety of metazoans. The Drosophila genome encodes seven insulin-like peptide genes, dilp1-7, and the most prominent dilps (dilp2, dilp3, and dilp5) are expressed in brain neurosecretory cells known as "insulin-producing cells" (IPCs). Although these dilps are expressed in the same cells, the expression of each dilp is regulated independently. However, the molecular mechanisms that regulate the expression of individual dilps in the IPCs remain largely unknown. Here, we show that Dachshund (Dac), which is a highly conserved nuclear protein, is a critical transcription factor that specifically regulates dilp5 expression. Dac was strongly expressed in IPCs throughout development. dac loss-of-function analyses revealed a severely reduced dilp5 expression level in young larvae. Dac interacted physically with the Drosophila Pax6 homolog Eyeless (Ey), and these proteins synergistically promoted dilp5 expression. In addition, the mammalian homolog of Dac, Dach1/2, facilitated the promoting action of Pax6 on the expression of islet hormone genes in cultured mammalian cells. These observations indicate the conserved role of Dac/Dach in controlling insulin expression in conjunction with Ey/Pax6.

Drosophila RNA polymerase III repressor Maf1 controls body size and developmental timing by modulating tRNAiMet synthesis and systemic insulin signaling

The target-of-rapamycin pathway couples nutrient availability with tissue and organismal growth in metazoans. The key effectors underlying this growth are, however, unclear. Here we show that Maf1, a repressor of RNA polymerase III-dependent tRNA transcription, is an important mediator of nutrient-dependent growth in Drosophila. We find nutrients promote tRNA synthesis during larval development by inhibiting Maf1. Genetic inhibition of Maf1 accelerates development and increases body size. These phenotypes are due to a non-cell-autonomous effect of Maf1 inhibition in the fat body, the main larval endocrine organ. Inhibiting Maf1 in the fat body increases growth by promoting the expression of brain-derived insulin-like peptides and consequently enhanced systemic insulin signaling. Remarkably, the effects of Maf1 inhibition are reproduced in flies carrying one extra copy of the initiator methionine tRNA, tRNA(i)(Met). These findings suggest the stimulation of tRNA(i)(Met) synthesis via inhibition of dMaf1 is limiting for nutrition-dependent growth during development.

Experimental manipulation of body size to estimate morphological scaling relationships in Drosophila

The scaling of body parts is a central feature of animal morphology^1-7. Within species, morphological traits need to be correctly proportioned to the body for the organism to function; larger individuals typically have larger body parts and smaller individuals generally have smaller body parts, such that overall body shape is maintained across a range of adult body sizes. The requirement for correct proportions means that individuals within species usually exhibit low variation in relative trait size. In contrast, relative trait size can vary dramatically among species and is a primary mechanism by which morphological diversity is produced. Over a century of comparative work has established these intra- and interspecific patterns^3,4.

Perhaps the most widely used approach to describe this variation is to calculate the scaling relationship between the size of two morphological traits using the allometric equation y=bxα, where x and y are the size of the two traits, such as organ and body size^8,9. This equation describes the within-group (e.g., species, population) scaling relationship between two traits as both vary in size. Log-transformation of this equation produces a simple linear equation, log(y) = log(b) + αlog(x) and log-log plots of the size of different traits among individuals of the same species typically reveal linear scaling with an intercept of log(b) and a slope of α, called the 'allometric coefficient'^9,10. Morphological variation among groups is described by differences in scaling relationship intercepts or slopes for a given trait pair. Consequently, variation in the parameters of the allometric equation (b and α) elegantly describes the shape variation captured in the relationship between organ and body size within and among biological groups (see ^11,12).

Not all traits scale linearly with each other or with body size (e.g., ^13,14) Hence, morphological scaling relationships are most informative when the data are taken from the full range of trait sizes. Here we describe how simple experimental manipulation of diet can be used to produce the full range of body size in insects. This permits an estimation of the full scaling relationship for any given pair of traits, allowing a complete description of how shape covaries with size and a robust comparison of scaling relationship parameters among biological groups. Although we focus on Drosophila, our methodology should be applicable to nearly any fully metamorphic insect.

Population and temperature effects on Lucilia sericata (Diptera: Calliphoridae) body size and minimum development time

Understanding how ecological conditions influence physiological responses is fundamental to forensic entomology. When determining the minimum postmortem interval with blow fly evidence in forensic investigations, using a reliable and accurate model of development is integral. Many published studies vary in results, source populations, and experimental designs. Accordingly, disentangling genetic causes of developmental variation from environmental causes is difficult. This study determined the minimum time of development and pupal sizes of three populations of Lucilia sericata Meigen (Diptera: Calliphoridae; from California, Michigan, and West Virginia) at two temperatures (20 degrees C and 33.5 degrees C). Development times differed significantly between strain and temperature. In addition, California pupae were the largest and fastest developing at 20 degrees C, but at 33.5 degrees C, though they still maintained their rank in size among the three populations, they were the slowest to develop. These results indicate a need to account for genetic differences in development, and genetic variation in environmental responses, when estimating a postmortem interval with entomological data.

An FXPRLamide neuropeptide induces seasonal reproductive polyphenism underlying a life-history tradeoff in the tussock moth

The white spotted tussock moth, Orgyia thyellina, is a typical insect that exhibits seasonal polyphenisms in morphological, physiological, and behavioral traits, including a life-history tradeoff known as oogenesis-flight syndrome. However, the developmental processes and molecular mechanisms that mediate developmental plasticity, including life-history tradeoff, remain largely unknown. To analyze the molecular mechanisms involved in reproductive polyphenism, including the diapause induction, we first cloned and characterized the diapause hormone-pheromone biosynthesis activating neuropeptide (DH-PBAN) cDNA encoding the five Phe-X-Pro-Arg-Leu-NH(2) (FXPRLa) neuropeptides: DH, PBAN, and α-, β-, and γ-SGNPs (subesophageal ganglion neuropeptides). This gene is expressed in neurosecretory cells within the subesophageal ganglion whose axonal projections reach the neurohemal organ, the corpus cardiacum, suggesting that the DH neuroendocrine system is conserved in Lepidoptera. By injection of chemically synthetic DH and anti-FXPRLa antibody into female pupae, we revealed that not only does the Orgyia DH induce embryonic diapause, but also that this neuropeptide induces seasonal polyphenism, participating in the hypertrophy of follicles and ovaries. In addition, the other four FXPRLa also induced embryonic diapause in O. thyellina, but not in Bombyx mori. This is the first study showing that a neuropeptide has a pleiotropic effect in seasonal reproductive polyphenism to accomplish seasonal adaptation. We also show that a novel factor (i.e., the DH neuropeptide) acts as an important inducer of seasonal polyphenism underlying a life-history tradeoff. Furthermore, we speculate that there must be evolutionary conservation and diversification in the neuroendocrine systems of two lepidopteran genera, Orgyia and Bombyx, in order to facilitate the evolution of coregulated life-history traits and tradeoffs.

Cricket body size is altered by systemic RNAi against insulin signaling components and epidermal growth factor receptor

A long-standing problem of developmental biology is how body size is determined. In Drosophila melanogaster, the insulin/insulin-like growth factor (I/IGF) and target of rapamycin (TOR) signaling pathways play important roles in this process. However, the detailed mechanisms by which insect body growth is regulated are not known. Therefore, we have attempted to utilize systemic nymphal RNA interference (nyRNAi) to knockdown expression of insulin signaling components including Insulin receptor (InR), Insulin receptor substrate (chico), Phosphatase and tensin homologue (Pten), Target of rapamycin (Tor), RPS6-p70-protein kinase (S6k), Forkhead box O (FoxO) and Epidermal growth factor receptor (Egfr) and observed the effects on body size in the Gryllus bimaculatus cricket. We found that crickets treated with double-stranded RNA (dsRNA) against Gryllus InR, chico, Tor, S6k and Egfr displayed smaller body sizes, while Gryllus FoxO nyRNAi-ed crickets exhibited larger than normal body sizes. Furthermore, RNAi against Gryllus chico and Tor displayed slow growth and RNAi against Gryllus chico displayed longer lifespan than control crickets. Since no significant difference in ability of food uptake was observed between the Gryllus chico(nyRNAi) nymphs and controls, we conclude that the adult cricket body size can be altered by knockdown of expressions of Gryllus InR, chico, Tor, S6k, FoxO and Egfr by systemic RNAi. Our results suggest that the cricket is a promising model to study mechanisms underlying controls of body size and life span with RNAi methods.

The coordination of growth among Drosophila organs in response to localized growth-perturbation

The developmental mechanisms by which growth is coordinated among developing organs are largely unknown and yet are essential to generate a correctly proportioned adult. In particular, such coordinating mechanisms must be able to accommodate perturbations in the growth of individual organs caused by environmental or developmental stress. By autonomously slowing the growth of the developing wing discs within Drosophila larvae, we show that growing organs are able to signal localized growth perturbation to the other organs in the body and slow their growth also. Growth rate is so tightly coordinated among organs that they all show approximately the same reduction in growth rate as the developing wings, thereby maintaining their correct size relationship relative to one another throughout development. Further, we show that the systemic growth effects of localized growth-perturbation are mediated by ecdysone. Application of ecdysone to larvae with growth-perturbed wing discs rescues the growth rate of other organs in the body, indicating that ecdysone is limiting for their growth, and disrupts the coordination of their growth with growth of the wing discs. Collectively our data demonstrate the existence of a novel growth-coordinating mechanism in Drosophila that synchronizes growth among organs in response to localized growth perturbation.

Support for maternal manipulation of developmental nutrition in a facultatively eusocial bee, Megalopta genalis (Halictidae)

Developmental maternal effects are a potentially important source of phenotypic variation, but they can be difficult to distinguish from other environmental factors. This is an important distinction within the context of social evolution, because if variation in offspring helping behavior is due to maternal manipulation, social selection may act on maternal phenotypes, as well as those of offspring. Factors correlated with social castes have been linked to variation in developmental nutrition, which might provide opportunity for females to manipulate the social behavior of their offspring. Megalopta genalis is a mass-provisioning facultatively eusocial sweat bee for which production of males and females in social and solitary nests is concurrent and asynchronous. Female offspring may become either gynes (reproductive dispersers) or workers (non-reproductive helpers). We predicted that if maternal manipulation plays a role in M. genalis caste determination, investment in daughters should vary more than for sons. The mass and protein content of pollen stores provided to female offspring varied significantly more than those of males, but volume and sugar content did not. Sugar content varied more among female eggs in social nests than in solitary nests. Provisions were larger, with higher nutrient content, for female eggs and in social nests. Adult females and males show different patterns of allometry, and their investment ratio ranged from 1.23 to 1.69. Adult body weight varied more for females than males, possibly reflecting increased variation in maternal investment in female offspring. These differences are consistent with a role for maternal manipulation in the social plasticity observed in M. genalis.

Neuroendocrine regulation of Drosophila metamorphosis requires TGFbeta/Activin signaling

In insects, initiation of metamorphosis requires a surge in the production of the steroid hormone 20-hydroxyecdysone from the prothoracic gland, the primary endocrine organ of juvenile larvae. Here, we show that blocking TGFβ/Activin signaling, specifically in the Drosophila prothoracic gland, results in developmental arrest prior to metamorphosis. The terminal, giant third instar larval phenotype results from a failure to induce the large rise in ecdysteroid titer that triggers metamorphosis. We further demonstrate that activin signaling regulates competence of the prothoracic gland to receive PTTH and insulin signals, and that these two pathways act at the mRNA and post-transcriptional levels, respectively, to control ecdysone biosynthetic enzyme expression. This dual regulatory circuitry may provide a cross-check mechanism to ensure that both developmental and nutritional inputs are synchronized before initiating the final genetic program leading to reproductive adult development. As steroid hormone production in C. elegans and mammals is also influenced by TGFβ/Activin signaling, this family of secreted factors may play a general role in regulating developmental transitions across phyla.

Molecular basis of adaptive shift in body size in Drosophila melanogaster: functional and sequence analyses of the Dca gene

Latitudinal body size clines in animals conforming to Bergmann's rule occur on many continents but isolating their underlying genetic basis remains a challenge. In Drosophila melanogaster, the gene Dca accounts for approximately 5-10% of the natural wing size variation (McKechnie SW, Blacket MJ, Song SV, Rako L, Carroll X, Johnson TK, Jensen LT, Lee SF, Wee CW, Hoffmann AA. 2010. A clinally varying promoter polymorphism associated with adaptive variation in wing size in Drosophila. Mol Ecol. 19:775-784). We present here functional evidence that Dca is a negative regulator of wing size. A significant negative latitudinal cline of Dca gene expression was detected in synchronized third instar larvae. In addition, we clarified the evolutionary history of the three most common Dca promoter alleles (Dca237-1, Dca237-2, and Dca247) and showed that the insertion allele (Dca247), whose frequency increases with latitude, is associated with larger wing centroid size and higher average cell number in male flies. Finally, we showed that the overall linkage disequilibrium (LD) was low in the Dca promoter and that the insertion/deletion polymorphism that defines the Dca alleles was in strong LD with two other upstream sites. Our results provide strong support that Dca is a candidate for climatic adaptation in D. melanogaster.

Population-based resequencing of experimentally evolved populations reveals the genetic basis of body size variation in Drosophila melanogaster

Body size is a classic quantitative trait with evolutionarily significant variation within many species. Locating the alleles responsible for this variation would help understand the maintenance of variation in body size in particular, as well as quantitative traits in general. However, successful genome-wide association of genotype and phenotype may require very large sample sizes if alleles have low population frequencies or modest effects. As a complementary approach, we propose that population-based resequencing of experimentally evolved populations allows for considerable power to map functional variation. Here, we use this technique to investigate the genetic basis of natural variation in body size in Drosophila melanogaster. Significant differentiation of hundreds of loci in replicate selection populations supports the hypothesis that the genetic basis of body size variation is very polygenic in D. melanogaster. Significantly differentiated variants are limited to single genes at some loci, allowing precise hypotheses to be formed regarding causal polymorphisms, while other significant regions are large and contain many genes. By using significantly associated polymorphisms as a priori candidates in follow-up studies, these data are expected to provide considerable power to determine the genetic basis of natural variation in body size.

Understanding the causes and consequences of natural genetic variation is crucial to the characterization of biological evolution. Moreover, natural genetic variation is comprised of millions of perturbations, which are partially randomized across genotypes such that a small number of individuals can be used to combinatorially analyze a large number of differences, facilitating mechanistic understanding of biological systems. Here we demonstrate a powerful technique to parse genomic variation using artificial selection. By selecting replicate populations of Drosophila flies to become bigger and smaller, and then determining the evolutionary response at the genomic level, we have mapped hundreds of genes that respond to selection on body size. As our approach is powerful and cost-effective compared to existing approaches, we expect it to be a major component of diverse future efforts.

Relationship between larval-pupal metamorphosis and transcript expression of insulin-like peptide and insulin receptor in Spodoptera littoralis

Insulin-like peptides (ILPs) affect a wide variety of biological events, such as metabolism, lifespan, growth and reproduction. Two ILPs (Spoli-ILP1 and Spoli-ILP2) were identified in the cotton leafworm, Spodoptera littoralis, while the functions and developmental characters are not fully understood. In the present study, we identified the partial sequence of a putative S. littoralis insulin receptor (Spoli-InR) and investigated the stage (age)- and tissue-dependent expression profile of Spoli-InR in addition to Spoli-ILPs during the last larval development and larval-pupal metamorphosis. Spoli-ILP1 and Spoli-ILP2 were specifically expressed in the brain, and their gene expressions were gradually decreased in concert with larval-pupal development. On the other hand, Spoli-InR was expressed in all the selected tissues (brain, testis, fat body, Malpighian tubules, prothoracic glands and midgut), though the gene expression pattern was different among the tissues. Interestingly, the transcript expression pattern of Spoli-InR in the fat body seemed to relate with larval-pupal development. In a parallel experiment, the juvenile hormone mimetic methoprene was able to prolong the larval period when applied before the commitment peak of ecdysteroids titer in the hemolymph, and in this case the expression of Spoli-ILPs and Spoli-InR was affected. These results demonstrated first a relationship between transcript expression of Spoli-ILPs and larval-pupal development, and second they suggested the effect of ILPs may be controlled by not only Spoli-ILPs expression but also Spoli-InR expression.

MAP4K3 is a component of the TORC1 signalling complex that modulates cell growth and viability in Drosophila melanogaster

Background

MAP4K3 is a conserved Ser/Thr kinase that has being found in connection with several signalling pathways, including the Imd, EGFR, TORC1 and JNK modules, in different organisms and experimental assays. We have analyzed the consequences of changing the levels of MAP4K3 expression in the development of the Drosophila wing, a convenient model system to characterize gene function during epithelial development.

Methodology and Principal Findings

Using loss-of-function mutants and over-expression conditions we find that MAP4K3 activity affects cell growth and viability in the Drosophila wing. These requirements are related to the modulation of the TORC1 and JNK signalling pathways, and are best detected when the larvae grow in a medium with low protein concentration (TORC1) or are exposed to irradiation (JNK). We also show that MAP4K3 display strong genetic interactions with different components of the InR/Tor signalling pathway, and can interact directly with the GTPases RagA and RagC and with the multi-domain kinase Tor.

Conclusions and Significance

We suggest that MAP4K3 has two independent functions during wing development, one related to the activation of the JNK pathway in response to stress and other in the assembling or activation of the TORC1 complex, being critical to modulate cellular responses to changes in nutrient availability.

Timing is Everything: PTTH Mediated DHR4 Nucleocytoplasmic Trafficking Sets the Tempo of Drosophila Steroid Production

During development, multicellular organisms must become sexually mature in order to reproduce. The developmental timing of this transition is controlled by pulses of steroid hormones, but how these pulses are generated have remained unclear? A recent paper shows that in Drosophila larvae, nucleocytoplasmic trafficking of DHR4, a nuclear receptor, in response to prothoracicotropic hormone signaling, is critical for producing the correct temporal pulses of steroid hormones that coordinate the juvenile-adult transition.

Genetic mapping of adaptive wing size variation in Drosophila simulans

Many ecologically important traits exhibit latitudinal variation. Body size clines have been described repeatedly in insects across multiple continents, suggesting that similar selective forces are shaping these geographical gradients. It is unknown whether these parallel clinal patterns are controlled by the same or different genetic mechanism(s). We present here, quantitative trait loci (QTL) analysis of wing size variation in Drosophila simulans. Our results show that much of the wing size variation is controlled by a QTL on Chr 3L with relatively minor contribution from other chromosome arms. Comparative analysis of the genomic positions of the QTL indicates that the major QTL on Chr 3 are distinct in D. simulans and D. melanogaster, whereas the QTL on Chr 2R might overlap between species. Our results suggest that parallel evolution of wing size clines could be driven by non-identical genetic mechanisms but in both cases involve a major QTL as well as smaller effects of other genomic regions.

Analysis of molecular markers for metamorphic competency and their response to starvation or feeding in the mosquito, Aedes aegypti (Diptera: Culicidae)

The nutritional condition of fourth instar larvae of the yellow fever mosquito, Aedes aegypti, governs female longevity and egg production, both are key determinants of pathogen transmission. As well, nutrition provisions larval growth and development and attains its greatest pace in the last larval instar in preparation for metamorphosis to an adult. These developmental processes are regulated by a complex endocrine interplay of juvenile hormone, neuropeptides, and ecdysteroids that is nutrition sensitive. We previously determined that feeding for only 24h post-ecdysis was sufficient for fourth instar Ae. aegypti larvae to reach critical weight and accumulate sufficient nutritional stores to commit to metamorphosis. To understand the genetic basis of metamorphic commitment in Ae. aegypti, we profiled the expression of 16 genes known to be involved in the endocrine and nutritional regulation of insect metamorphosis in two ways. The first set is a developmental profile from the beginning of the fourth instar to early pupae, and the second set is for fourth instars starved or fed for up to 36 h. By comparing the two sets, we found that seven of the genes (AaegCYP302, AaegJHE43357, AaegBrCZ4, AaegCPF1-2, AaegCPR-7, AaegPpl, and AaegSlif) were expressed during metamorphic commitment in fourth instars and in fed but not starved larvae. Based on these results, the seven genes alone or in combination may serve as molecular indicators of nutritional and metamorphic status of fourth instar Ae. aegypti larvae and possibly other mosquito species in field and laboratory studies to gauge sub-lethal effects of novel and traditional cultural or chemical controls.

The Drosophila gap gene giant regulates ecdysone production through specification of the PTTH-producing neurons

In Drosophila melanogaster, hypomorphic mutations in the gap gene giant (gt) have long been known to affect ecdysone titers resulting in developmental delay and the production of large (giant) larvae, pupae and adults. However, the mechanism by which gt regulates ecdysone production has remained elusive. Here we show that hypomorphic gt mutations lead to ecdysone deficiency and developmental delay by affecting the specification of the PG neurons that produce prothoracicotropic hormone (PTTH). The gt¹ hypomorphic mutation leads to random loss of PTTH production in one or more of the 4 PG neurons in the larval brain. In cases where PTTH production is lost in all four PG neurons, delayed development and giant larvae are produced. Since immunostaining shows no evidence for Gt expression in the PG neurons once PTTH production is detectable, it is unlikely that Gt directly regulates PTTH expression. Instead, we find that innervation of the prothoracic gland by the PG neurons is absent in gt hypomorphic larvae that do not express PTTH. In addition, PG neuron axon fasciculation is abnormal in many gt hypomorphic larvae. Since several other anteriorly expressed gap genes such as tailless and orthodenticle have previously been found to affect the fate of the cerebral labrum, a region of the brain that gives rise to the neuroendocrine cells that innervate the ring gland, we conclude that gt likely controls ecdysone production indirectly by contributing the peptidergic phenotype of the PTTH-producing neurons in the embryo.

Survival strategies of a sterol auxotroph

The high sterol concentration in eukaryotic cell membranes is thought to influence membrane properties such as permeability, fluidity and microdomain formation. Drosophila cannot synthesize sterols, but do require them for development. Does this simply reflect a requirement for sterols in steroid hormone biosynthesis, or is bulk membrane sterol also essential in Drosophila? If the latter is true, how do they survive fluctuations in sterol availability and maintain membrane homeostasis? Here, we show that Drosophila require both bulk membrane sterol and steroid hormones in order to complete adult development. When sterol availability is restricted, Drosophila larvae modulate their growth to maintain membrane sterol levels within tight limits. When dietary sterol drops below a minimal threshold, larvae arrest growth and development in a reversible manner. Strikingly, membrane sterol levels in arrested larvae are dramatically reduced (dropping sixfold on average) in most tissues except the nervous system. Thus, sterols are dispensable for maintaining the basic membrane biophysical properties required for cell viability; these functions can be performed by non-sterol lipids when sterols are unavailable. However, bulk membrane sterol is likely to have essential functions in specific tissues during development. In tissues in which sterol levels drop, the overall level of sphingolipids increases and the proportion of different sphingolipid variants is altered. These changes allow survival, but not growth, when membrane sterol levels are low. This relationship between sterols and sphingolipids could be an ancient and conserved principle of membrane homeostasis.

TRPM channels mediate zinc homeostasis and cellular growth during Drosophila larval development

TRPM channels have emerged as key mediators of diverse physiological functions. However, the ionic permeability relevant to physiological function in vivo remains unclear for most members. We report that the single Drosophila TRPM gene (dTRPM) generates a conductance permeable to divalent cations, especially Zn(2+) and in vivo a loss-of-function mutation in dTRPM disrupts intracellular Zn(2+) homeostasis. TRPM deficiency leads to profound reduction in larval growth resulting from a decrease in cell size and associated defects in mitochondrial structure and function. These phenotypes are cell-autonomous and can be recapitulated in wild-type animals by Zn(2+) depletion. Both the cell size and mitochondrial defect can be rescued by extracellular Zn(2+) supplementation. Thus our results implicate TRPM channels in the regulation of cellular Zn(2+) in vivo. We propose that regulation of Zn(2+) homeostasis through dTRPM channels is required to support molecular processes that mediate class I PI3K-regulated cell growth.

Transcriptional regulation of the insulin signaling pathway genes by starvation and 20-hydroxyecdysone in the Bombyx fat body

Genetic studies in the fruitfly, Drosophila melanogaster, have uncovered a conserved insulin/insulin growth factor signaling (IIS) pathway that regulates nutrition-dependent growth rates of insects. From the silkworm, Bombyx mori, we have identified and characterized several key genes involved in the IIS pathway, including InR, IRS, PI3K110, PI3K60, PTEN, PDK, and Akt. Tissue distribution analysis showed that most of these genes were highly expressed in the fat body implying that the IIS pathway is functionally important within insect adipose tissue. Developmental profile studies revealed that the expression levels of InR, IRS, PI3K110, and PDK were elevated in the fat body during molting and pupation, periods when animals ceased feeding and hemolymph levels of 20-hydroxyecdysone (20E) were high. Starvation rapidly up-regulated the mRNA levels of these same genes in the fat body, while 20E slowly induced their transcription. We conclude that 20E slowly reduces food consumption and then indirectly induces a state of starvation resulting in the elevation of the mRNA levels of InR, IRS, PI3K110, and PDK in the Bombyx fat body during molting and pupation.

Developmental timing of a sensory-mediated larval surfacing behavior correlates with cessation of feeding and determination of final adult size

Controlled organismal growth to an appropriate adult size requires a regulated balance between nutrient resources, feeding behavior and growth rate. Defects can result in decreased survival and/or reproductive capability. Since Drosophila adults do not grow larger after eclosion, timing of feeding cessation during the third and final larval instar is critical to final size. We demonstrate that larval food exit is preceded by a period of increased larval surfacing behavior termed the Intermediate Surfacing Transition (IST) that correlates with the end of larval feeding. This behavioral transition occurred during the larval Terminal Growth Period (TGP), a period of constant feeding and exponential growth of the animal. IST behavior was dependent upon function of a subset of peripheral sensory neurons expressing the Degenerin/Epithelial sodium channel (DEG/ENaC) subunit, Pickpocket1(PPK1). PPK1 neuron inactivation or loss of PPK1 function caused an absence of IST behavior. Transgenic PPK1 neuron hyperactivation caused premature IST behavior with no significant change in timing of larval food exit resulting in decreased final adult size. These results suggest a peripheral sensory mechanism functioning to alter the relationship between the animal and its environment thereby contributing to the length of the larval TGP and determination of final adult size.

Lethality and developmental delay in Drosophila melanogaster larvae after ingestion of selected Pseudomonas fluorescens strains

Background

The fruit fly, Drosophila melanogaster, is a well-established model organism for probing the molecular and cellular basis of physiological and immune system responses of adults or late stage larvae to bacterial challenge. However, very little is known about the consequences of bacterial infections that occur in earlier stages of development. We have infected mid-second instar larvae with strains of Pseudomonas fluorescens to determine how infection alters the ability of larvae to survive and complete development.

Methodology/Principal Findings

We mimicked natural routes of infection using a non-invasive feeding procedure to study the toxicity of the three sequenced P. fluorescens strains (Pf0-1, SBW25, and Pf-5) to Drosophila melanogaster. Larvae fed with the three strains of P. fluorescens showed distinct differences in developmental trajectory and survival. Treatment with SBW25 caused a subset of insects to die concomitant with a systemic melanization reaction at larval, pupal or adult stages. Larvae fed with Pf-5 died in a dose-dependent manner with adult survivors showing eye and wing morphological defects. In addition, larvae in the Pf-5 treatment groups showed a dose-dependent delay in the onset of metamorphosis relative to control-, Pf0-1-, and SBW25-treated larvae. A functional gacA gene is required for the toxic properties of wild-type Pf-5 bacteria.

Conclusions/Significance

These experiments are the first to demonstrate that ingestion of P. fluorescens bacteria by D. melanogaster larvae causes both lethal and non-lethal phenotypes, including delay in the onset of metamorphosis and morphological defects in surviving adult flies, which can be decoupled.

Peristalsis in the junction region of the Drosophila larval midgut is modulated by DH31 expressing enteroendocrine cells

Background

The underlying cellular and molecular mechanisms that coordinate the physiological processes in digestion are complex, cryptic, and involve the integration of multiple cellular and organ systems. In all intestines, peristaltic action of the gut moves food through the various stages of digestion from the anterior end towards the posterior, with the rate of flow dependent on signals, both intrinsic and extrinsic to the gut itself.

Results

We have identified an enteroendocrine cell type that regulates gut motility in the Drosophila melanogaster larval midgut. These cells are located at the junction of the anterior and the acidic portions of the midgut and are a group of enteroendocrine cells that express the peptide hormone Diuretic Hormone 31 in this region of the gut. Using cell ablation and ectopic activation via expression of the Chlamydomonas reinhardtii blue light-activated channelopsin, we demonstrate that these enteroendocrine cells are both necessary and sufficient for the peristalsis in the junction region of the midgut and require the Diuretic Hormone 31 to affect normal peristalsis in this region. Within the same junction region of the midgut, we have also identified morphological features suggesting that this region acts as a valve that regulates the transit of food from the anterior midgut into the acidic portion of the gut.

Conclusions

We have characterized and described a set of enteroendocrine cells called the Midgut Junction DH31 expressing cells that are required for peristaltic movement in the junction region between the anterior portion and acidic region of the larval midgut of Drosophila melanogaster. We have shown that the Midgut Junction DH31 expressing cells are necessary and sufficient for motility and that the peptide hormone DH31 is required for peristalsis in the junction region of the midgut. The Drosophila model system will allow for a further dissection of the digestion process and provide a better understanding of the mechanisms that regulate digestion in all organisms.

Non-molting glossy/shroud encodes a short-chain dehydrogenase/reductase that functions in the 'Black Box' of the ecdysteroid biosynthesis pathway

In insects, the precise timing of molting and metamorphosis is strictly guided by a principal steroid hormone, ecdysone. Among the multiple conversion steps for synthesizing ecdysone from dietary cholesterol, the conversion of 7-dehydrocholesterol to 5beta-ketodiol, the so-called 'Black Box', is thought to be the important rate-limiting step. Although a number of genes essential for ecdysone synthesis have recently been revealed, much less is known about the genes that are crucial for functioning in the Black Box. Here we report on a novel ecdysteroidgenic gene, non-molting glossy (nm-g)/shroud (sro), which encodes a short-chain dehydrogenase/reductase. This gene was first isolated by positional cloning of the nm-g mutant of the silkworm Bombyx mori, which exhibits a low ecdysteroid titer and consequently causes a larval arrest phenotype. In the fruit fly, Drosophila melanogaster, the closest gene to nm-g is encoded by the sro locus, one of the Halloween mutant members that are characterized by embryonic ecdysone deficiency. The lethality of the sro mutant is rescued by the overexpression of either sro or nm-g genes, indicating that these two genes are orthologous. Both the nm-g and the sro genes are predominantly expressed in tissues producing ecdysone, such as the prothoracic glands and the ovaries. Furthermore, the phenotypes caused by the loss of function of these genes are restored by the application of ecdysteroids and their precursor 5beta-ketodiol, but not by cholesterol or 7-dehydrocholesterol. Altogether, we conclude that the Nm-g/Sro family protein is an essential enzyme for ecdysteroidogenesis working in the Black Box.

The Drosophila gene RanBPM functions in the mushroom body to regulate larval behavior

BACKGROUND

In vertebrates, Ran-Binding Protein in the Microtubule Organizing Center (RanBPM) appears to function as a scaffolding protein in a variety of signal transduction pathways. In Drosophila, RanBPM is implicated in the regulation of germ line stem cell (GSC) niche organization in the ovary. Here, we addressed the role of RanBPM in nervous system function in the context of Drosophila larval behavior.

METHODOLOGY/PRINCIPAL FINDINGS

We report that in Drosophila, RanBPM is required for larval feeding, light-induced changes in locomotion, and viability. RanBPM is highly expressed in the Kenyon cells of the larval mushroom body (MB), a structure well studied for its role in associative learning in Drosophila and other insects. RanBPM mutants do not display major disruption in nervous system morphology besides reduced proliferation. Expression of the RanBPM gene in the Kenyon cells is sufficient to rescue all behavioral phenotypes. Through genetic epistasis experiments, we demonstrate that RanBPM participates with the Drosophila orthologue of the Fragile X Mental Retardation Protein (FMRP) in the development of neuromuscular junction (NMJ).

CONCLUSIONS/SIGNIFICANCE

We demonstrate that the RanBPM gene functions in the MB neurons for larval behavior. Our results suggest a role for this gene in an FMRP-dependent process. Taken together our findings point to a novel role for the MB in larval behavior.

Atmospheric oxygen level and the evolution of insect body size

Insects are small relative to vertebrates, possibly owing to limitations or costs associated with their blind-ended tracheal respiratory system. The giant insects of the late Palaeozoic occurred when atmospheric PO(2) (aPO(2)) was hyperoxic, supporting a role for oxygen in the evolution of insect body size. The paucity of the insect fossil record and the complex interactions between atmospheric oxygen level, organisms and their communities makes it impossible to definitively accept or reject the historical oxygen-size link, and multiple alternative hypotheses exist. However, a variety of recent empirical findings support a link between oxygen and insect size, including: (i) most insects develop smaller body sizes in hypoxia, and some develop and evolve larger sizes in hyperoxia; (ii) insects developmentally and evolutionarily reduce their proportional investment in the tracheal system when living in higher aPO(2), suggesting that there are significant costs associated with tracheal system structure and function; and (iii) larger insects invest more of their body in the tracheal system, potentially leading to greater effects of aPO(2) on larger insects. Together, these provide a wealth of plausible mechanisms by which tracheal oxygen delivery may be centrally involved in setting the relatively small size of insects and for hyperoxia-enabled Palaeozoic gigantism.

A developmental perspective on the evolution of sexual size dimorphism of a moth

Males and females of almost all organisms exhibit sexual differences in body size, a phenomenon called sexual size dimorphism (SSD). How the sexes evolve to be different sizes, despite sharing the same genes that control growth and development, and hence a common genetic architecture, has remained elusive. Here, we show that the genetic architecture (heritabilities and genetic correlations) of the physiological mechanism that regulates size during the last stage of larval development of a moth, differs between the sexes, and thus probably facilitates, rather than hinders, the evolution of SSD. We further show that the endocrine system plays a critical role in generating SSD. Our results demonstrate that knowledge of the genetic architecture underlying the physiological process during development that ultimately produces SSD in adults can elucidate how males and females of organisms evolve to be of different sizes.

Ovarian development and insulin-signaling pathways during reproductive differentiation in the queenless ponerine ant Diacamma sp

In many social hymenopteran species, workers possess functional ovaries that are physiologically inactive in the presence of queens. We investigated the ovarian regulatory mechanism of workers and reproductives in a queenless ponerine ant, Diacamma sp., using histological and molecular techniques. In this ant, clear reproductive differentiation occurs via a highly sophisticated dominance behavioral interaction called "gemmae mutilation". This clear and rapid bifurcation of reproductive physiology allows us to elucidate the detailed ovarian differentiation process. Histological characteristics of functional ovaries (fusomes and ring canals) were found in both workers and reproductives, suggesting that early oogenesis is not blocked in workers. Since insulin/insulin-like growth factor signaling (IIS) is known to control insect reproduction, orthologs of 2 positive IIS regulators, insulin receptor and serine-threonine kinase Akt (protein kinase B), were cloned in Diacamma (DiaInR, DiaAkt); their expression patterns during reproductive differentiation were examined by real-time quantitative polymerase chain reaction; DiaInR and DiaAkt were strongly expressed in the gasters of reproductives. Whole-mount in situ hybridization of ovaries indicated that DiaInR and DiaAkt were expressed in nurse cells, oocytes, and upper germarial regions of reproductives but not of workers. Our data suggest that the IIS pathway accounts for reproductive differentiation in late oogenesis.

A Drosophila insulin-like peptide promotes growth during nonfeeding states

In metazoans, tissue growth relies on the availability of nutrients--stored internally or obtained from the environment--and the resulting activation of insulin/IGF signaling (IIS). In Drosophila, growth is mediated by seven Drosophila insulin-like peptides (Dilps), acting through a canonical IIS pathway. During the larval period, animals feed and Dilps produced by the brain couple nutrient uptake with systemic growth. We show here that, during metamorphosis, when feeding stops, a specific DILP (Dilp6) is produced by the fat body and relays the growth signal. Expression of DILP6 during pupal development is controlled by the steroid hormone ecdysone. Remarkably, DILP6 expression is also induced upon starvation, and both its developmental and environmental expression require the Drosophila FoxO transcription factor. This study reveals a specific class of ILPs induced upon metabolic stress that promotes growth in conditions of nutritional deprivation or following developmentally induced cessation of feeding.

A fat body-derived IGF-like peptide regulates postfeeding growth in Drosophila

Members of the insulin family of peptides have conserved roles in the regulation of growth and metabolism in a wide variety of metazoans. Here we show that Drosophila insulin-like peptide 6 (DILP6), which is structurally similar to vertebrate insulin-like growth factor (IGF), is predominantly expressed in the fat body, a functional equivalent of the vertebrate liver and adipocytes. This expression occurs during the postfeeding stage under the direct regulation of ecdysteroid. We further reveal that dilp6 mutants show growth defects during the postfeeding stage, which results in reduced adult body size through a decrease in cell number. This phenotype is rescued by fat body-specific expression of dilp6. These data indicate that DILP6 is a functional, as well as a structural, counterpart of vertebrate IGFs. Our data provide in vivo evidence for a role of ILPs in determining adult body size through the regulation of postfeeding growth.