The distinct roles of insulin signaling in polyphenic development

Creation of Knock-In Alleles of Insulin Receptor Tagged by Fluorescent Proteins mCherry or EYFP in Fruit Fly Drosophila melanogaster

The insulin/insulin-like growth factor-like signaling (IIS) pathway is highly conserved across metazoans and regulates numerous physiological functions, including development, metabolism, fecundity, and lifespan. The insulin receptor (InR), a crucial membrane receptor in the IIS pathway, is known to be ubiquitously expressed in various tissues, albeit at generally low levels, and its subcellular localization remains incompletely characterized. In this study, we employed CRISPR-mediated mutagenesis in the fruit fly Drosophila to create knock-in alleles of InR tagged with fluorescent proteins (InR::mCherry or InR::EYFP). By inserting the coding sequence of the fluorescent proteins mCherry or EYFP near the end of the coding sequence of the endogenous InR gene, we could trace the natural InR protein through their fluorescence. As an example, we investigated epithelial cells of the male accessory gland (AG), an internal reproductive organ, and identified two distinct patterns of InR::mCherry localization. In young AG, InR::mCherry accumulated on the basal plasma membrane between cells, whereas in mature AG, it exhibited intracellular localization as multiple puncta, indicating endocytic recycling of InR during cell growth. In the AG senescence accelerated by the mutation of Diuretic hormone 31 (Dh31), the presence of InR::mCherry puncta was more pronounced compared to the wild type. These findings raise expectations for the utility of the newly created InR::mCherry/EYFP alleles for studying the precise expression levels and subcellular localization of InR. Furthermore, this fluorescently tagged allele approach can be extended to investigate other membrane receptors with low abundance, facilitating the direct examination of their true expression and localization.

Nutrition- and hormone-controlled developmental plasticity in Blattodea

Blattodea, which includes cockroaches and termites, possesses high developmental plasticity that is mainly controlled by nutritional conditions and insect hormones. Insulin/insulin-like growth factor signaling (IIS), target of rapamycin complex 1 (TORC1), and adenosine monophosphate-activated protein complex are the three primary nutrition-responsive signals. Juvenile hormone (JH) and 20-hydroxyecdysone (20E) constitute the two most vital insect hormones that might interact with each other through the Met, Kr-h1, E93 (MEKRE93) pathway. Nutritional and hormonal signals interconnect to create a complex regulatory network. Here we summarize recent progress in our understanding of how nutritional and hormonal signals coordinately control the developmental plasticity of metamorphosis, reproduction, and appendage regeneration in cockroaches as well as caste differentiation in termites. We also highlight several perspectives that should be further emphasized in the studies of developmental plasticity in Blattodea. This review provides a general landscape in the field of nutrition- and hormone-controlled developmental plasticity in insects.

DNA methylation and expression of the egfr gene are associated with worker size in monomorphic ants

The reproductive division of labour is a hallmark of eusocial Hymenoptera. Females are either reproductive queens or non-reproductive workers. In ants, workers often display further task specialisation that is associated with variation in size and/or morphology. Because female polyphenism is typically under environmental control, it is thought epigenetic mechanisms (such as DNA methylation) play a central role since they mediate gene-by-environment interactions. Methylation of the growth-promoting gene epidermal growth factor receptor (egfr) was indeed shown to control worker size in a highly polymorphic ant. However, it remains unknown if egfr methylation could also regulate worker size in monomorphic species. By combining experimental pharmacology and molecular biology, we show that worker size is associated with egfr methylation in two monomorphic ants. Furthermore, we functionally demonstrate that EGFR signalling affects worker size. These results indicate that worker size regulation by egfr methylation has been mechanistically conserved in ants but remains unexploited in monomorphic species.

Static allometries do not reflect evolutionary allometry in exaggerated weaponry of male New Zealand sheetweb spiders (Cambridgea spp.)

Across the animal kingdom, exaggerated weaponry is frequently used by one sex to contest access for potential mates. Within species, if disproportionate investment in weaponry confers an advantage to larger individuals, this may result in positive static allometry. It is predicted that the same selective pressures may also lead to positive evolutionary allometry, where larger species bear disproportionately large weapons on average, compared with smaller species. Furthermore, in species with stronger sexual selection, the static allometries of those weapons are expected to steepen. All adult males across the New Zealand sheetweb spider genus Cambridgea bear exaggerated chelicerae, which are used to compete for control of females' webs. Here, we characterize the distribution of chelicera lengths within each sex of 12 Cambridgea species to show that chelicerae almost always exhibit positive static allometry in males while female chelicera lengths are consistently isometric. We use comparative phylogenetic methods to demonstrate that the slopes of static allometries steepen in males of larger species but that the ratio of average chelicera length to cephalothorax width is tightly conserved across taxa, leading to an isometric evolutionary allometry. While static allometries indeed steepen in larger species, possibly due to stronger sexual selection, this conservation of relative trait size suggests that chelicera length is subject to other stabilizing selective pressures. Changes to species body plans might be constrained, while still allowing for disproportionate investment in weapon traits in the upper range of intraspecific body sizes.

Molecular characterization of insulin receptor (IR) in oriental fruit moth, Grapholita molesta (Lepidoptera: Tortricidae), and elucidation of its regulatory roles in glucolipid homeostasis and metamorphosis through interaction with miR-982490

As an intermediate molecule in the Insulin/Insulin-like growth factor signalling pathway (IIS), the insulin receptor (IR) plays vital roles linking nutritional signals to the downstream regulation of metabolic homeostasis, development, metamorphosis, reproduction and stress responses. In the present study, we describe the molecular characteristics of IR in the cosmopolitan fruit boring pest, Grapholita molesta, and its predicted posttranscription regulator miR-982490, and elucidate its regulatory roles in glucolipid homeostasis and metamorphosis. Phylogenetic and domain analyses indicate that lepidopteran IRs normally cluster within families, and that four main domains are conserved in GmIR and those of other Lepidoptera. Bio-informatic prediction, synchronic expression profile evaluation and dual luciferase reporter assays indicated negative regulation of GmIR by miR-982490. Injection of miR-982490 agomir into fifth instar larvae yielded effects similar to dsGmIR injection, resulting in enhanced levels of trehalose and triglyceride in haemolymph, and reduced pupation success and pupal weight, both of which could be rescued by co-injection of dsGmIR and miR-982490 antagomir. We infer that GmIR regulates glucolipid homeostasis and affects G. molesta metamorphosis via interactions with its posttranscriptional regulator miR-982490. This study expands our understanding of the regulatory network of IIS in insect nutritional homeostasis and development.

Single-cell transcriptomic analysis of honeybee brains identifies vitellogenin as caste differentiation-related factor

The honeybee (Apis mellifera) is a well-known eusocial insect. In honeybee colonies, thousands of sterile workers, including nurse and forager bees, perform various tasks within or outside the hive, respectively. The queen is the only fertile female and is responsible for reproduction. The queen and workers share similar genomes but occupy different caste statuses. We established single-cell transcriptomic atlases of brains from queens and worker subcastes and identified five major cell groups: Kenyon, optic lobe, olfactory projection, glial, and hemocyte cells. By dividing Kenyon and glial cells into multiple subtypes based on credible markers, we observed that vitellogenin (vg) was highly expressed in specific glial-cell subtypes in brains of queens. Knockdown of vg at the early larval stage significantly suppressed the development into adult queens. We demonstrate vg expression as a "molecular signature" for the queen caste and suggest involvement of vg in regulating caste differentiation.

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scRNA-seq revealed distinct gene expression in the brains of queens and workers

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Vitellogenin (vg) may represent a "molecular signature" of the queen caste

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Knockdown of vg at early larval stage suppressed development into adult queens

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Vg may be involved in regulating caste differentiation

Antibiotics Alter the Expression of Genes Related to Behavioral Development in Honey Bees (Hymenoptera: Apidae)

Honey bees, as many species of social insects, display a division of labor among colony members based on behavioral specializations related to age. Adult worker honey bees perform a series of tasks in the hive when they are young (such as brood care or nursing) and at ca. 2-3 wk of age, shift to foraging for nectar and pollen outside the hive. The transition to foraging involves changes in metabolism and neuroendocrine activities. These changes are associated with a suite of developmental genes. It was recently demonstrated that antibiotics influence behavioral development by accelerating or delaying the onset of foraging depending on timing of antibiotic exposure. To understand the mechanisms of these changes, we conducted a study on the effects of antibiotics on expression of candidate genes known to regulate behavioral development. We demonstrate a delay in the typical changes in gene expression over the lifetime of the individuals that were exposed to antibiotics during immature stage and adulthood. Additionally, we show an acceleration in the typical changes in gene expression on individuals that were expose to antibiotics only during immature stage. These results show that timing of antibiotic exposure alter the typical regulation of behavioral development by metabolic and neuroendocrine processes.

Differential DNA methylation between long-winged and short-winged adults of Nilaparvata lugens

Nilaparvata lugens, a catastrophic rice pest in South East Asia, has adults with wing dimorphism. DNA methylation has been proved to play an important role in regulation of phenotype differentiation in insects. In this study, methylation sensitive amplification polymorphism (MSAP) was used to investigate the cytosine methylation state at CCGG sites in macropterous male adults (MMA) and brachypterous male adults (BMA) of brown planthopper. In MMA, the fully methylated ratio was 2.96%, hemi-methylated ratio 3.83% and total methylated ratio 6.79%. In BMA, they were 5.53%, 4.19% and 9.72%, respectively. There were significant differences in the methylation of the target sites (CCGG) between MMA and BMA (ØST = 0.2614, P = 0.0354). Based the PCoA results, a much clear separation were also shown between MMA and BMA along the first coordinate (38.8% of variance explained). We also cloned and got nine satisfactory sequences with different methylation states between MMA and BMA. Two of them have similarity with male-specific sequence in chromosome Y and lipophorin receptor gene in N. lugens, respectively. The result showed that the methylation patterns and levels were different between two wing phenotypes of N. lugens, and will facilitate research on the epigenetic mechanism of insect wing dimorphism.

Polyphenism of a Novel Trait Integrated Rapidly Evolving Genes into Ancestrally Plastic Networks

Developmental polyphenism, the ability to switch between phenotypes in response to environmental variation, involves the alternating activation of environmentally sensitive genes. Consequently, to understand how a polyphenic response evolves requires a comparative analysis of the components that make up environmentally sensitive networks. Here, we inferred coexpression networks for a morphological polyphenism, the feeding-structure dimorphism of the nematode Pristionchus pacificus. In this species, individuals produce alternative forms of a novel trait—moveable teeth, which in one morph enable predatory feeding—in response to environmental cues. To identify the origins of polyphenism network components, we independently inferred coexpression modules for more conserved transcriptional responses, including in an ancestrally nonpolyphenic nematode species. Further, through genome-wide analyses of these components across the nematode family (Diplogastridae) in which the polyphenism arose, we reconstructed how network components have changed. To achieve this, we assembled and resolved the phylogenetic context for five genomes of species representing the breadth of Diplogastridae and a hypothesized outgroup. We found that gene networks instructing alternative forms arose from ancestral plastic responses to environment, specifically starvation-induced metabolism and the formation of a conserved diapause (dauer) stage. Moreover, loci from rapidly evolving gene families were integrated into these networks with higher connectivity than throughout the rest of the P. pacificus transcriptome. In summary, we show that the modular regulatory outputs of a polyphenic response evolved through the integration of conserved plastic responses into networks with genes of high evolutionary turnover.

Something old, something new, something borrowed, something red: the origin of ecologically relevant novelties in Hemiptera

Comparative transcriptomics, applied in an evolutionary context, has transformed the possibilities for studying phenotypic evolution in non-model taxa. We review recent discoveries about the development of novel, ecologically relevant phenotypes in hemipteran insects. These discoveries highlight the diverse genomic substrates of novelty: 'something old', when novelty results from changes in the regulation of existing genes or gene duplication; 'something new', wherein lineage-restricted genes contribute to the evolution of new phenotypes; and 'something borrowed', showcasing contributions of horizontal gene transfer to the evolution of novelty, including carotenoid synthesis (resulting in 'something red'). These findings show the power and flexibility of comparative transcriptomic approaches for expanding beyond the 'toolkit' model for the evolution of development. We conclude by raising questions about the relationship between new genes and new traits and outlining a research framework for answering them in Hemiptera.

The plasticity of lifespan in social insects

One of the central questions of ageing research is why lifespans of organisms differ so tremendously among related taxa and, even more surprising, among members of the same species. Social insects provide a particularly pronounced example for this. Here, we review previously published information on lifespan plasticity in social insects and provide new data on worker lifespan in the ant Cardiocondyla obscurior, which because of its relatively short lifespan is a convenient model to study ageing. We show that individual lifespan may vary within species with several reproductive and social traits, such as egg-laying rate, queen number, task, colony size and colony composition. For example, in Cardiocondyla, highly fecund queens live longer than reproductively less active queens, and workers tend to live longer when transferred into a novel social environment or, as we show with new data, into small colonies. We hypothesize that this plasticity of lifespan serves to maximize the reproductive output of the colony as a whole and thus the inclusive fitness of all individuals. The underlying mechanisms that link the social environment or reproductive status with lifespan are currently unresolved. Several studies in honeybees and ants indicate an involvement of nutrient-sensing pathways, but the details appear to differ among species. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Complex Evolution of Insect Insulin Receptors and Homologous Decoy Receptors, and Functional Significance of Their Multiplicity

Evidence accumulates that the functional plasticity of insulin and insulin-like growth factor signaling in insects could spring, among others, from the multiplicity of insulin receptors (InRs). Their multiple variants may be implemented in the control of insect polyphenism, such as wing or caste polyphenism. Here, we present a comprehensive phylogenetic analysis of insect InR sequences in 118 species from 23 orders and investigate the role of three InRs identified in the linden bug, Pyrrhocoris apterus, in wing polymorphism control. We identified two gene clusters (Clusters I and II) resulting from an ancestral duplication in a late ancestor of winged insects, which remained conserved in most lineages, only in some of them being subject to further duplications or losses. One remarkable yet neglected feature of InR evolution is the loss of the tyrosine kinase catalytic domain, giving rise to decoys of InR in both clusters. Within the Cluster I, we confirmed the presence of the secreted decoy of insulin receptor in all studied Muscomorpha. More importantly, we described a new tyrosine kinase-less gene (DR2) in the Cluster II, conserved in apical Holometabola for ∼300 My. We differentially silenced the three P. apterus InRs and confirmed their participation in wing polymorphism control. We observed a pattern of Cluster I and Cluster II InRs impact on wing development, which differed from that postulated in planthoppers, suggesting an independent establishment of insulin/insulin-like growth factor signaling control over wing development, leading to idiosyncrasies in the co-option of multiple InRs in polyphenism control in different taxa.

The origin of wing polyphenism in ants: An eco-evo-devo perspective

The evolution of eusociality, where solitary individuals integrate into a single colony, is a major transition in individuality. In ants, the origin of eusociality coincided with the origin of a wing polyphenism approximately 160 million years ago, giving rise to colonies with winged queens and wingless workers. As a consequence, both eusociality and wing polyphenism are nearly universal features of all ants. Here, we synthesize fossil, ecological, developmental, and evolutionary data in an attempt to understand the factors that contributed to the origin of wing polyphenism in ants. We propose multiple models and hypotheses to explain how wing polyphenism is orchestrated at multiple levels, from environmental cues to gene networks. Furthermore, we argue that the origin of wing polyphenism enabled the subsequent evolution of morphological diversity across the ants. We finally conclude by outlining several outstanding questions for future work.

Body size variation in bees: regulation, mechanisms, and relationship to social organization

Size polymorphism is common in bees, and is determined by environmental factors such as temperature, brood cell size, and the diet provided to developing larvae. In social bees, these factors are further influenced by intricate interactions between the queen, workers, and the developing brood which eventually determine the final size and caste of developing larvae. Environmental and social factors act in part on juvenile hormone and ecdysteroids, which are key hormonal regulators of body size and caste determination. In some social bees, body size variation is central for social organization because it structures reproductive division of labor, task allocation among workers, or both. At ecological scales, body size also impacts bee-mediated pollination services in solitary and social species by influencing floral visitation and pollination efficacy.

Diet-Induced Plasticity of Linear Static Allometry Is Not So Simple for Grasshoppers: Genotype–Environment Interaction in Ontogeny Is Masked by Convergent Growth

Grasshoppers, Melanoplus sanguinipes (Orthoptera: Acrididae), develop larger head width (HW) and shorter leg length, relative to body size, when fed low nutrient, lignin-rich grasses compared to sibs fed a diet of high nutrient grasses. To elucidate how underlying genetic variation and plasticity of growth generate plasticity of this linear static allometry within coarse-grained environments, I measured head and leg size of three nymphal instars and adult grasshoppers raised on either a low or high nutrient diet within a half-sib quantitative genetic experiment. Doubly-multivariate repeated measures multiple analysis of variance (MANOVA) of head, mandible, and hind leg size and their rate of growth (mm/period) and growth period (days) through ontogeny were used to analyze how the ontogeny of diet-induced plasticity for these variables and additive genetic variation for plasticity (genotype × environment interaction [G×E]) contribute to plasticity in functional linear static allometry. Genetic variation for diet-induced plasticity (G×E) of head and leg size varied through ontogeny, as did genetic variation for plasticity of growth in third and fourth instar nymphs. Despite extensive genetic variation in plasticity of HW and leg length in fourth instar nymphs, the static allometry between head and leg was stable within each diet because the patterns of G×E were similar for HW, leg length and their coordinated growth. Nutrient sensitive plasticity in growth shifted the intercept but not the slope of static allometry, a result consistent with one outcome of a graphical model of the relationships between G× E and plasticity of within environment static allometry. In addition, G×E of fourth instar head and leg size was reduced in adults by negatively size-dependent, convergent growth in the last period of ontogeny. Consequently, the bivariate reaction norms of head and leg size for adults exhibited no G×E and, again, plasticity in the intercept but not in the slope of static allometry. The ontogeny of seemingly simple diet-induced linear static allometry between functional body parts in grasshoppers arises from a complex combination of differing patterns of nutrient-sensitive growth, duration of growth, convergent growth, and G×E, all relevant to understanding the development and evolution of functional allometry in hemimetabolous insects.

Exploring the Role of Insulin Signaling in Relative Growth: A Case Study on Wing-Body Scaling in Lepidoptera

Adult forms emerge from the relative growth of the body and its parts. Each appendage and organ has a unique pattern of growth that influences the size and shape it attains. This produces adult size relationships referred to as static allometries, which have received a great amount of attention in evolutionary and developmental biology. However, many questions remain unanswered, for example: What sorts of developmental processes coordinate growth? And how do these processes change given variation in body size? It has become increasingly clear that nutrition is one of the strongest influences on size relationships. In insects, nutrition acts via insulin/TOR signaling to facilitate inter- and intra-specific variation in body size and appendage size. Yet, the mechanism by which insulin signaling influences the scaling of growth remains unclear. Here we will discuss the potential roles of insulin signaling in wing-body scaling in Lepidoptera. We analyzed the growth of wings in animals reared on different diet qualities that induce a range of body sizes not normally present in our laboratory populations. By growing wings in tissue culture, we survey how perturbation and stimulation of insulin/TOR signaling influences wing growth. To conclude, we will discuss the implications of our findings for the development and evolution of organismal form.

Manipulation of insulin signaling phenocopies evolution of a host-associated polyphenism

Plasticity, the capacity of an organism to respond to its environment, is thought to evolve through changes in development altering the integration of environmental cues. In polyphenism, a discontinuous plastic response produces two or more phenotypic morphs. Here we describe evolutionary change in wing polyphenism and its underlying developmental regulation in natural populations of the red-shouldered soapberry bug, Jadera haematoloma (Insecta: Hemiptera: Rhopalidae) that have adapted to a novel host plant. We find differences in the fecundity of morphs in both sexes and in adult expression of insulin signaling components in the gonads. Further, the plastic response of ancestral-state bugs can be shifted to resemble the reaction norm of derived bugs by the introduction of exogenous insulin or RNA interference targeting the insulin signaling component encoded by FoxO. These results suggest that insulin signaling may be one pathway involved in the evolution of this polyphenism, allowing adaptation to a novel nutritional environment.