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

The distinct roles of insulin signaling in polyphenic development

Many insects have the ability to develop alternative morphologies in response to specific environmental signals such as photoperiod, temperature, nutrition and crowding. These signals are integrated by the brain and result in alternative patterns of secretion of developmental hormones like ecdysone, juvenile hormone and insulin-like growth factors, which, in turn, direct alternative developmental trajectories. Insulin signaling appears to be particularly important when the polyphenism involves differences in the sizes of the body, appendages and other structures, such as wings, mandibles and horns. Here we review recent advances in understanding the role of insulin signaling, and its interaction with other hormones, in the development of polyphenisms.

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.

Multiple functions of CREB-binding protein during postembryonic development: identification of target genes

Background

Juvenile hormones (JH) and ecdysteroids control postembryonic development in insects. They serve as valuable targets for pest management. Hence, understanding the molecular mechanisms of their action is of crucial importance. CREB-binding protein (CBP) is a universal transcriptional co-regulator. It controls the expression of several genes including those from hormone signaling pathways through co-activation of many transcription factors. However, the role of CBP during postembryonic development in insects is not well understood. Therefore, we have studied the role of CBP in postembryonic development in Tribolium, a model coleopteran insect.

Results

CBP is ubiquitously expressed in the red flour beetle, Tribolium castaneum. RNA interference (RNAi) mediated knockdown of CBP resulted in a decrease in JH induction of Kr-h1 gene expression in Tribolium larvae and led to a block in their development. Moreover, the injection of CBP double-stranded RNA (dsRNA) showed lethal phenotypes within 8 days of injection. RNA-seq and subsequent differential gene expression analysis identified CBP target genes in Tribolium. Knockdown of CBP caused a decrease in the expression of 1306 genes coding for transcription factors and other proteins associated with growth and development. Depletion of CBP impaired the expression of several JH response genes (e.g., Kr-h1, Hairy, early trypsin) and ecdysone response genes (EcR, E74, E75, and broad complex). Further, GO enrichment analyses of the downregulated genes showed enrichment in different functions including developmental processes, pigmentation, anatomical structure development, regulation of biological and cellular processes, etc.

Conclusion

These data suggest diverse but crucial roles for CBP during postembryonic development in the coleopteran model insect, Tribolium. It can serve as a target for RNAi mediated pest management of this stored product pest.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4373-3) contains supplementary material, which is available to authorized users.

Evaluation of ecdysteroid antisera for a competitive enzyme immunoassay and extraction procedures for the measurement of mosquito ecdysteroids

Ecdysteroid hormones regulate several aspects of insect development and reproduction. The predominant ecdysteroids produced by insects including mosquitoes are ecdysone (E) and 20-hydroxyecdysone (20E). The ability to measure E and 20E titers is essential for many studies, but few sensitive, low cost options are currently available for doing so. To address this deficiency, we developed a new enzyme-linked immunoassay (EIA). In the first part of the study, we compared the affinity of two new antisera named EAB25 and EAB27 to other available ecdysteroid antisera. EAB25 had a 27-fold higher affinity for 20E than E, while EAB27 had a four-fold higher affinity for 20E. In the second part of the study, EIA protocols were developed for analyzing E and 20E produced by the mosquito Aedes aegypti. Results indicated that pelts from fourth instar larvae and ovaries from blood-fed, adult females produced E and 20E. Methanol extraction in the presence of magnesium from whole body samples altered antibody recognition of E and 20E by EIA. However, extraction with 1-butanol and two organic/water phase separations eliminated this problem and improved assay performance. We conclude the new antisera used in the EIA provide a low-cost, flexible, and sensitive method for measuring E and 20E in insects.

Metamorphosis is induced by food absence rather than a critical weight in the solitary bee, Osmia lignaria

Body size is an important phenotypic trait that correlates with performance and fitness. For determinate growing insects, body size variation is determined by growth rate and the mechanisms that stop growth at the end of juvenile growth. Endocrine mechanisms regulate growth cessation, and their relative timing along development shapes phenotypic variation in body size and development time. Larval insects are generally hypothesized to initiate metamorphosis once they attain a critical weight. However, the mechanisms underlying the critical weight have not been resolved even for well-studied insect species. More importantly, critical weights may or may not be generalizable across species. In this study, we characterized the developmental aspects of size regulation in the solitary bee, Osmia lignaria We demonstrate that starvation cues metamorphosis in O. lignaria and that a critical weight does not exist in this species. Larvae initiated pupation <24 h after food was absent. However, even larvae fed ad libitum eventually underwent metamorphosis, suggesting that some secondary mechanism regulates metamorphosis when provisions are not completely consumed. We show that metamorphosis could be induced by precocene treatment in the presence of food, which suggests that this decision is regulated through juvenile hormone signaling. Removing food at different larval masses produced a 10-fold difference in mass between smallest and largest adults. We discuss the implications of body size variation for insect species that are provided with a fixed quantity of provisions, including many bees which have economic value as pollinators.

Caste development and evolution in ants: it's all about size

Female ants display a wide variety of morphological castes, including workers, soldiers, ergatoid (worker-like) queens and queens. Alternative caste development within a species arises from a variable array of genetic and environmental factors. Castes themselves are also variable across species and have been repeatedly gained and lost throughout the evolutionary history of ants. Here, we propose a simple theory of caste development and evolution. We propose that female morphology varies as a function of size, such that larger individuals possess more queen-like traits. Thus, the diverse mechanisms that influence caste development are simply mechanisms that affect size in ants. Each caste-associated trait has a unique relationship with size, producing a phenotypic space that permits some combinations of worker- and queen-like traits, but not others. We propose that castes are gained and lost by modifying the regions of this phenotypic space that are realized within a species. These modifications can result from changing the size-frequency distribution of individuals within a species, or by changing the association of tissue growth and size. We hope this synthesis will help unify the literature on caste in ants, and facilitate the discovery of molecular mechanisms underlying caste development and evolution.

Fitness Trade-Off Associated With Spinosad Resistance in Frankliniella occidentalis (Thysanoptera: Thripidae)

Frankliniella occidentalis (Pergande) is an economically important pest of agricultural crops. High resistance has been detected in field populations of F. occidentalis against the insecticide spinosad. In this study, we compared life history traits, body sizes, and feeding behaviors (recorded via an electrical penetration graph) of spinosad-susceptible (Ivf03) and spinosad-resistant (NIL-R) near-isogenic lines of F. occidentalis. Life table analysis showed that NIL-R had reduced female longevity and reduced fecundity. The relative fitness of NIL-R (0.43) was less than half that of Ivf03. NIL-R individuals were smaller than Ivf03 individuals, both in body length and body width at every stage. The number and duration of feeding activities were significantly reduced in NIL-R, with the exception of total duration of long-ingestion probes. These results suggest that there is a fitness trade-off associated with spinosad resistance in F. occidentalis, and that the development of resistance in this pest species may be reduced by rotating spinosad with other pesticides lacking cross-resistance.

On gonads and gadflies: the estrus angle

The first sex steroid to be crystallized was the vertebrate ovarian hormone, estrone - a less potent metabolite of 17β-estradiol, which in mammals stimulates the female urge to mate (estrus). The gadfly (Greek oistros) lent its name to the process of estrus, as an insect that bites and torments in classical Greek mythology. With the purification and crystallization of a moult-inducing steroid (ecdysone) from insects, an interesting parallel emerged between mating and moulting in lower mammals and arthropods. Ecdysterone (potent ecdysone metabolite) has anabolic effects in mammalian muscle cells that can be blocked by selective estrogen receptor antagonists. Insects utilize ecdysteroids in similar ways that vertebrates use estrogens, including stimulation of oocyte growth and maturation. Ecdysteroids also modify precopulatory insect mating behaviour, further reinforcing the gonad-gadfly/mate-moult analogy.

Malagasy Conostigmus (Hymenoptera: Ceraphronoidea) and the secret of scutes

We revise the genus Conostigmus Dahlbom 1858 occurring in Madagascar, based on data from more specimens than were examined for the latest world revision of the genus. Our results yield new information about intraspecific variability and the nature of the atypical latitudinal diversity gradient (LDG) observed in Ceraphronoidea. We also investigate cellular processes that underlie body size polyphenism, by utilizing the correspondence between epidermal cells and scutes, polygonal units of leather-like microsculpture. Our results reveal that body size polyphenism in Megaspilidae is most likely related to cell number and not cell size variation, and that cell size differs between epithelial fields of the head and that of the mesosoma. Three species, Conostigmus ballescoracas Dessart, 1997, C. babaiax Dessart, 1996 and C. longulus Dessart, 1997, are redescribed. Females of C. longulus are described for the first time, as are nine new species: C. bucephalus Mikó and Trietsch sp. nov., C. clavatus Mikó and Trietsch sp. nov., C. fianarantsoaensis Mikó and Trietsch sp. nov., C. lucidus Mikó and Trietsch sp. nov., C. macrocupula, Mikó and Trietsch sp. nov., C. madagascariensis Mikó and Trietsch sp. nov., C. missyhazenae Mikó and Trietsch sp. nov., C. pseudobabaiax Mikó and Trietsch sp. nov., and C. toliaraensis Mikó and Trietsch sp. nov. A fully illustrated identification key for Malagasy Conostigmus species and a Web Ontology Language (OWL) representation of the taxonomic treatment, including specimen data, nomenclature, and phenotype descriptions, in both natural and formal languages, are provided.

Colony-Level Differences in the Scaling Rules Governing Wood Ant Compound Eye Structure

Differential organ growth during development is essential for adults to maintain the correct proportions and achieve their characteristic shape. Organs scale with body size, a process known as allometry that has been studied extensively in a range of organisms. Such scaling rules, typically studied from a limited sample, are assumed to apply to all members of a population and/or species. Here we study scaling in the compound eyes of workers of the wood ant, Formica rufa, from different colonies within a single population. Workers’ eye area increased with body size in all the colonies showing a negative allometry. However, both the slope and intercept of some allometric scaling relationships differed significantly among colonies. Moreover, though mean facet diameter and facet number increased with body size, some colonies primarily increased facet number whereas others increased facet diameter, showing that the cellular level processes underlying organ scaling differed among colonies. Thus, the rules that govern scaling at the organ and cellular levels can differ even within a single population.

The Ecdysone and Notch Pathways Synergistically Regulate Cut at the Dorsal-Ventral Boundary in Drosophila Wing Discs

Metazoan development requires coordination of signaling pathways to regulate patterns of gene expression. In Drosophila, the wing imaginal disc provides an excellent model for the study of how signaling pathways interact to regulate pattern formation. The determination of the dorsal-ventral (DV) boundary of the wing disc depends on the Notch pathway, which is activated along the DV boundary and induces the expression of the homeobox transcription factor, Cut. Here, we show that Broad (Br), a zinc-finger transcription factor, is also involved in regulating Cut expression in the DV boundary region. However, Br expression is not regulated by Notch signaling in wing discs, while ecdysone signaling is the upstream signal that induces Br for Cut upregulation. Also, we find that the ecdysone-Br cascade upregulates cut-lacZ expression, a reporter containing a 2.7 kb cut enhancer region, implying that ecdysone signaling, similar to Notch, regulates cut at the transcriptional level. Collectively, our findings reveal that the Notch and ecdysone signaling pathways synergistically regulate Cut expression for proper DV boundary formation in the wing disc. Additionally, we show br promotes Delta, a Notch ligand, near the DV boundary to suppress aberrant high Notch activity, indicating further interaction between the two pathways for DV patterning of the wing disc.

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

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

Drosophila Lgr3 Couples Organ Growth with Maturation and Ensures Developmental Stability

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

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.

Cell size versus body size in geophilomorph centipedes

Variation in animal body size is the result of a complex interplay between variation in cell number and cell size, but the latter has seldom been considered in wide-ranging comparative studies, although distinct patterns of variation have been described in the evolution of different lineages. We investigated the correlation between epidermal cell size and body size in a sample of 29 geophilomorph centipede species, representative of a wide range of body sizes, from 6 mm dwarf species to gigantic species more than 200 mm long, exploiting the marks of epidermal cells on the overlying cuticle in the form of micro-sculptures called scutes. We found conspicuous and significant variation in average scute area, both between suprageneric taxa and between genera, while the within-species range of variation is comparatively small. This supports the view that the average epidermal cell size is to some extent taxon specific. However, regression analyses show that neither body size nor the number of leg-bearing segments explain this variation, which suggests that cell size is not an usual target of change for body size evolution in this group of arthropods, although there is evidence of its correlation with other morphological variables, like cuticle thickness. Scute sizes of miniaturized geophilomorph species are well within the range of the lineage to which the species belong, suggesting recent evolutionary transitions to smaller body size.