Insulin signalling's role in mediating tissue-specific nutritional plasticity and robustness in the horn-polyphenic beetle Onthophagus taurus

Lactobacillus Firm-5-derived succinate prevents honeybees from having diabetes-like symptoms

The gut microbiome plays an important role in honeybee hormonal regulation and growth, but the underlying mechanisms are poorly understood. Here, we showed that the depletion of gut bacteria resulted in reduced expression of insulin-like peptide gene (ilp) in the head, accompanied by metabolic syndromes resembling those of Type 1 diabetes in humans: hyperglycemia, impaired lipid storage, and decreased metabolism. These symptoms were alleviated by gut bacterial inoculation. Gut metabolite profiling revealed that succinate, produced by Lactobacillus Firm-5, played deterministic roles in activating ilp gene expression and in regulating metabolism in honeybees. Notably, we demonstrated that succinate modulates host ilp gene expression through stimulating gut gluconeogenesis, a mechanism resembling that of humans. This study presents evidence for the role of gut metabolite in modulating host metabolism and contributes to the understanding of the interactions between gut microbiome and bee hosts.

Histone deacetylases regulate organ-specific growth in a horned beetle

BACKGROUND

Nutrient availability is among the most widespread means by which environmental variability affects developmental outcomes. Because almost all cells within an individual organism share the same genome, structure-specific growth responses must result from changes in gene regulation. Earlier work suggested that histone deacetylases (HDACs) may serve as epigenetic regulators linking nutritional conditions to trait-specific development. Here we expand on this work by assessing the function of diverse HDACs in the structure-specific growth of both sex-shared and sex-specific traits including evolutionarily novel structures in the horned dung beetle Onthophagus taurus.

RESULTS

We identified five HDAC members whose downregulation yielded highly variable mortality depending on which HDAC member was targeted. We then show that HDAC1, 3, and 4 operate in both a gene- and trait-specific manner in the regulation of nutrition-responsiveness of appendage size and shape. Specifically, HDAC 1, 3, or 4 knockdown diminished wing size similarly while leg development was differentially affected by RNAi targeting HDAC3 and HDAC4. In addition, depletion of HDAC3 transcript resulted in a more rounded shape of genitalia at the pupal stage and decreased the length of adult aedeagus across all body sizes. Most importantly, we find that HDAC3 and HDAC4 pattern the morphology and regulate the scaling of evolutionarily novel head and thoracic horns as a function of nutritional variation.

CONCLUSION

Collectively, our results suggest that both functional overlap and division of labor among HDAC members contribute to morphological diversification of both conventional and recently evolved appendages. More generally, our work raises the possibility that HDAC-mediated scaling relationships and their evolution may underpin morphological diversification within and across insect species broadly.

The diverse roles of insulin signaling in insect behavior

In insects and other animals, nutrition-mediated behaviors are modulated by communication between the brain and peripheral systems, a process that relies heavily on the insulin/insulin-like growth factor signaling pathway (IIS). Previous studies have focused on the mechanistic and physiological functions of insulin-like peptides (ILPs) in critical developmental and adult milestones like pupation or vitellogenesis. Less work has detailed the mechanisms connecting ILPs to adult nutrient-mediated behaviors related to survival and reproductive success. Here we briefly review the range of behaviors linked to IIS in insects, from conserved regulation of feeding behavior to evolutionarily derived polyphenisms. Where possible, we incorporate information from Drosophila melanogaster and other model species to describe molecular and neural mechanisms that connect nutritional status to behavioral expression via IIS. We identify knowledge gaps which include the diverse functional roles of peripheral ILPs, how ILPs modulate neural function and behavior across the lifespan, and the lack of detailed mechanistic research in a broad range of taxa. Addressing these gaps would enable a better understanding of the evolution of this conserved and widely deployed tool kit pathway.

Genome evolution and divergence in cis-regulatory architecture is associated with condition-responsive development in horned dung beetles

Phenotypic plasticity is thought to be an important driver of diversification and adaptation to environmental variation, yet the genomic mechanisms mediating plastic trait development and evolution remain poorly understood. The Scarabaeinae, or true dung beetles, are a species-rich clade of insects recognized for their highly diversified nutrition-responsive development including that of cephalic horns-evolutionarily novel, secondary sexual weapons that exhibit remarkable intra- and interspecific variation. Here, we investigate the evolutionary basis for horns as well as other key dung beetle traits via comparative genomic and developmental assays. We begin by presenting chromosome-level genome assemblies of three dung beetle species in the tribe Onthophagini (> 2500 extant species) including Onthophagus taurus, O. sagittarius, and Digitonthophagus gazella. Comparing these assemblies to those of seven other species across the order Coleoptera identifies evolutionary changes in coding sequence associated with metabolic regulation of plasticity and metamorphosis. We then contrast chromatin accessibility in developing head horn tissues of high- and low-nutrition O. taurus males and females and identify distinct cis-regulatory architectures underlying nutrition- compared to sex-responsive development, including a large proportion of recently evolved regulatory elements sensitive to horn morph determination. Binding motifs of known and new candidate transcription factors are enriched in these nutrition-responsive open chromatin regions. Our work highlights the importance of chromatin state regulation in mediating the development and evolution of plastic traits, demonstrates gene networks are highly evolvable transducers of environmental and genetic signals, and provides new reference-quality genomes for three species that will bolster future developmental, ecological, and evolutionary studies of this insect group.

Utilizing geometric morphometrics to investigate gene function during organ growth: Insights through the study of beetle horn shape allometry

Static allometry is a major component of morphological variation. Much of the literature on the development of allometry investigates how functional perturbations of diverse pathways affect the relationship between trait size and body size. Often, this is done with the explicit objective to identify developmental mechanisms that enable the sensing of organ size and the regulation of relative growth. However, changes in relative trait size can also be brought about by a range of other distinctly different developmental processes, such as changes in patterning or tissue folding, yet standard univariate biometric approaches are usually unable to distinguish among alternative explanations. Here, we utilize geometric morphometrics to investigate the degree to which functional genetic manipulations known to affect the size of dung beetle horns also recapitulate the effect of horn shape allometry. We reasoned that the knockdown phenotypes of pathways governing relative growth should closely resemble shape variation induced by natural allometric variation. In contrast, we predicted that if genes primarily affect alternative developmental processes, knockdown effects should align poorly with shape allometry. We find that the knockdown effects of several genes (e.g., doublesex, Foxo) indeed closely aligned with shape allometry, indicating that their corresponding pathways may indeed function primarily in the regulation of relative trait growth. In contrast, other knockdown effects (e.g., Distal-less, dachs) failed to align with allometry, implicating these pathways in potentially scaling-independent processes. Our findings moderate the interpretation of studies focusing on trait length and highlight the usefulness of multivariate approaches to study allometry and phenotypic plasticity.

Gene regulatory networks underlying the development and evolution of plasticity in horned beetles

Horned beetles have emerged as a powerful study system with which to investigate the developmental mechanisms underlying environment-responsive development and its evolution. We begin by reviewing key advances in our understanding of the diverse roles played by transcription factors, endocrine regulators, and signal transduction pathways in the regulation of horned beetle plasticity. We then explore recent efforts aimed at understanding how such condition-specific expression may be regulated in the first place, as well as how the differential expression of master regulators may instruct conditional expression of downstream target genes. Here, we focus on the significance of chromatin remodeling as a powerful but thus far understudied mechanism able to facilitate trait-, sex-, and species-specific responses to environmental conditions.

Assessing the evolutionary lability of insulin signalling in the regulation of nutritional plasticity across traits and species of horned dung beetles

Nutrition-dependent growth of sexual traits is a major contributor to phenotypic diversity, and a large body of research documents insulin signalling as a major regulator of nutritional plasticity. However, findings across studies raise the possibility that the role of individual components within the insulin signalling pathway diverges in function among traits and taxa. Here, we use RNAi-mediated transcript depletion in the gazelle dung beetle to investigate the functions of forkhead box O (Foxo) and two paralogs of the insulin receptor (InR1 and InR2) in shaping nutritional plasticity in polyphenic male head horns, exaggerated fore legs, and weakly nutrition-responsive genitalia. Our functional genetic manipulations led to three main findings: FoxoRNAi reduced the length of exaggerated head horns in large males, while neither InR1 nor InR2 knock-downs resulted in measurable horn phenotypes. These results are similar to those documented previously for another dung beetle (Onthophagus taurus), but in stark contrast to findings in rhinoceros beetles. Secondly, knockdown of Foxo, InR1, and InR2 led to an increase in the intercept or slope of the scaling relationship of genitalia size. These findings are in contrast even to results documented previously for O. taurus. Lastly, while FoxoRNAi reduces male forelegs in D. gazella and O. taurus, the effects of InR1 and InR2 knockdowns diverged across dung beetle species. Our results add to the growing body of literature indicating that despite insulin signalling's conserved role as a regulator of nutritional plasticity, the functions of its components may diversify among traits and species, potentially fuelling the evolution of scaling relationships.

A Mediator subunit imparts robustness to a polyphenism decision

Polyphenism is a type of developmental plasticity that translates continuous environmental variability into discontinuous phenotypes. Such discontinuity likely requires a switch between alternative gene-regulatory networks, a principle that has been borne out by mechanisms found to promote morph-specific gene expression. However, whether robustness is required to execute a polyphenism decision has awaited testing at the molecular level. Here, we used a nematode model for polyphenism, Pristionchus pacificus, to identify the molecular regulatory factors that ensure the development of alternative forms. This species has a dimorphism in its adult feeding structures, specifically teeth, which are a morphological novelty that allows predation on other nematodes. Through a forward genetic screen, we determined that a duplicate homolog of the Mediator subunit MDT-15/MED15, P. pacificus MDT-15.1, is necessary for the polyphenism and the robustness of the resulting phenotypes. This transcriptional coregulator, which has a conserved role in metabolic responses to nutritional stress, coordinates these processes with its effects on this diet-induced polyphenism. Moreover, this MED15 homolog genetically interacts with two nuclear receptors, NHR-1 and NHR-40, to achieve dimorphism: Single and double mutants for these three factors result in morphologies that together produce a continuum of forms between the extremes of the polyphenism. In summary, we have identified a molecular regulator that confers discontinuity to a morphological polyphenism, while also identifying a role for MED15 as a plasticity effector.

Recent progress in understanding the genomic architecture of sexual conflict

Genomic conflict between the sexes over shared traits is widely assumed to be resolved through the evolution of sex-biased expression and the subsequent emergence of sexually dimorphic phenotypes. However, while there is support for a broad relationship between genome-wide patterns of expression level and sexual conflict, recent studies suggest that sex differences in the nature and strength of interactions between loci are instead key to conflict resolution. Furthermore, the advent of new technologies for measuring and perturbing expression means we now have much more power to detect genomic signatures of sexual conflict. Here, we review our current understanding of the genomic architecture of sexual conflict in the light of these new studies and highlight the potential for novel approaches to address outstanding knowledge gaps.

Ontogenetic trajectories and early shape differentiation of treehopper pronota (Hemiptera: Membracidae)

Membracids (family: Membracidae), commonly known as treehoppers, are recognizable by their enlarged and often elaborated pronota. Much of the research investigating the development and evolution of this structure has focused on the fifth instar to adult transition, in which the pronotum undergoes the largest transformation as it takes on adult identity. However, little is known about the earlier nymphal stages, the degree to which the pronotum develops at these timepoints, and how development has changed relative to the ancestral state. Here, we studied the nymphal stages and adults of five morphologically distinct membracid species and of Aetalion reticulatum (family: Aetalionidae), the outgroup which was used as an ancestral state proxy. We found that shape differentiation in the pronotum of membracids can start as early as the second instar stage. Most shape differentiation occurs within the nymphal stages and not in the embryo since the shape of the first-instar pronotum did not differ from the outgroup species in all but one species we investigated. We found the anterior-posterior axis of the pronotum elongated at a faster relative rate in membracid species than in A. reticulatum, which contributed to the development of exaggerated pronotal size. Finally, we found differences in the morphogenesis of shape across species. We suggest this is due to the developmental and evolutionary divergence of differential growth patterning of the dorsal surface of the pronotum, not only across species, but also between stages within the same species. This lability may contribute to the evolvability and diversification of the membracid pronotum.

A transcriptomic investigation of heat-induced transgenerational plasticity in beetles

In response to environmental stressors, parents can shape the developmental outcomes of their offspring by contributing non-genetic but heritable factors. The transmission of such factors can potentially allow offspring, from the beginning of their lives, to express phenotypes that match their anticipated environments. In this study, I ask whether enhanced growth in larvae of Onthophagus taurus (the bull-headed dung beetle) is modified by parental exposure to heat or by exposure of the offspring to heat during early life. I find that, irrespective of the early environment of the offspring, individuals produced by parents exposed to heat grow larger. Furthermore, taking a transcriptomic approach, I find that ecdysone signalling might mediate the transgenerational effect and that increased insulin signalling or reduced production of heat shock proteins might be responsible for the enhanced growth in larvae derived from parents exposed to heat. Together, my results provide evidence for a thermally induced transgenerational effect and a foundation for functional testing of candidate mechanisms mediating the effect.

Mitochondria dysfunction impairs Tribolium castaneum wing development during metamorphosis

The disproportionate growth of insect appendages such as facultative growth of wings and exaggeration of beetle horns are examples of phenotypic plasticity. Insect metamorphosis is the critical stage for development of pupal and adult structures and degeneration of the larval cells. How the disproportionate growth of external appendages is regulated during tissue remodeling remains unanswered. Tribolium castaneum is used as a model to study the function of mitochondria in metamorphosis. Mitochondrial dysfunction is achieved by the knockdown of key mitochondrial regulators. Here we show that mitochondrial function is not required for metamorphosis except that severe mitochondrial dysfunction blocks ecdysis. Surprisingly, various abnormal wing growth, including short and wingless phenotypes, are induced after knocking down mitochondrial regulators. Mitochondrial activity is regulated by IIS (insulin/insulin-like growth factor signaling)/FOXO (forkhead box, sub-group O) pathway through TFAM (transcription factor A, mitochondrial). RNA sequencing and differential gene expression analysis show that wing-patterning and insect hormone response genes are downregulated, while programmed cell death and immune response genes are upregulated in insect wing discs with mitochondrial dysfunction. These studies reveal that mitochondria play critical roles in regulating insect wing growth by targeting wing development during metamorphosis, thus showing a novel molecular mechanism underlying developmental plasticity.

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.

Developmental bias in the evolution and plasticity of beetle horn shape

The degree to which developmental systems bias the phenotypic effects of environmental and genetic variation, and how these biases affect evolution, is subject to much debate. Here, we assess whether developmental variability in beetle horn shape aligns with the phenotypic effects of plasticity and evolutionary divergence, yielding three salient results. First, we find that most pathways previously shown to regulate horn length also affect shape. Second, we find that the phenotypic effects of manipulating divergent developmental pathways are correlated with each other as well as multivariate fluctuating asymmetry-a measure of developmental variability. Third, these effects further aligned with thermal plasticity, population differences and macroevolutionary divergence between sister taxa and more distantly related species. Collectively, our results support the hypothesis that changes in horn shape-whether brought about by environmentally plastic responses, functional manipulations or evolutionary divergences-converge along 'developmental lines of least resistance', i.e. are biased by the developmental system underpinning horn shape.

The Insulin Receptor: An Important Target for the Development of Novel Medicines and Pesticides

The insulin receptor (IR) is a transmembrane protein that is activated by ligands in insulin signaling pathways. The IR has been considered as a novel therapeutic target for clinical intervention, considering the overexpression of its protein and A-isoform in multiple cancers, Alzheimer's disease, and Type 2 diabetes mellitus in humans. Meanwhile, it may also serve as a potential target in pest management due to its multiple physiological influences in insects. In this review, we provide an overview of the structural and molecular biology of the IR, functions of IRs in humans and insects, physiological and nonpeptide small molecule modulators of the IR, and the regulating mechanisms of the IR. Xenobiotic compounds and the corresponding insecticidal chemicals functioning on the IR are also discussed. This review is expected to provide useful information for a better understanding of human IR-related diseases, as well as to facilitate the development of novel small-molecule activators and inhibitors of the IR for use as medicines or pesticides.

When the end modifies its means: the origins of novelty and the evolution of innovation

The origin of novel complex traits constitutes a central yet largely unresolved challenge in evolutionary biology. Intriguingly, many of the most promising breakthroughs in understanding the genesis of evolutionary novelty in recent years have occurred not in evolutionary biology itself, but through the comparative study of development and, more recently, the interface of developmental biology and ecology. Here, I discuss how these insights are changing our understanding of what matters in the origin of novel, complex traits in ontogeny and evolution. Specifically, my essay has two major objectives. First, I discuss how the nature of developmental systems biases the production of phenotypic variation in the face of novel or stressful environments toward functional, integrated and, possibly, adaptive variants. This, in turn, allows the production of novel phenotypes to precede (rather than follow) changes in genotype and allows developmental processes that are the product of past evolution to shape evolutionary change that has yet to occur. Second, I explore how this nature of developmental systems has itself evolved over time, increasing the repertoire of ontogenies to pursue a wider range of objectives across an expanding range of conditions, thereby creating an increasingly extensive affordance landscape in development and developmental evolution. Developmental systems and their evolution can thus be viewed as dynamic processes that modify their own means across ontogeny and phylogeny. The study of these dynamics necessitates more than the strict reductionist approach that currently dominates the fields of developmental and evolutionary developmental biology.

Sex-specific regulation of development, growth and metabolism

Adult females and males of most species differ in many aspects of their morphology, physiology and behavior, in response to sex-specific selective pressures that maximize fitness. While we have an increasingly good understanding of the genetic mechanisms that initiate these differences, the sex-specific developmental trajectories that generate them are much less well understood. Here we review recent advances in the sex-specific regulation of development focusing on two models where this development is increasingly well understood: Sexual dimorphism of body size in the fruit fly Drosophila melanogaster and sexual dimorphism of horns in the horned beetle Onthophagus taurus. Because growth and development are also supported by metabolism, the regulation of sex-specific metabolism during and after development is an important aspect of the generation of female and male phenotypes. Hitherto, the study of sex-specific development has largely been independent of the study of sex-specific metabolism. Nevertheless, as we discuss in this review, recent research has begun to reveal considerable overlap in the cellular and physiological mechanisms that regulate sex-specific development and metabolism.

Conserved and convergent mechanisms underlying performance-life-history trade-offs

Phenotypic trade-offs are inevitable in nature, but the mechanisms driving them are poorly understood. Movement and oxygen are essential to all animals, and as such, the common ancestor to all living animals passed on mechanisms to acquire oxygen and contract muscle, sometimes at the expense of other activities or expression of traits. Nevertheless, convergent pathways have also evolved to deal with critical trade-offs that are necessary to survive ubiquitous environmental challenges. We discuss how whole-animal performance traits, such as locomotion, are important to fitness, yet costly, resulting in trade-offs with other aspects of the phenotype via specific conserved and convergent mechanistic pathways across all animals. Specifically, we discuss conserved pathways involved in muscle structure and signaling, insulin/insulin-like signaling, sirtuins, mitochondria and hypoxia-inducible factors, as well as convergent pathways involved in energy regulation, development, reproductive investment and energy storage. The details of these mechanisms are only known from a few model systems, and more comparative studies are needed. We make two main recommendations as a framework for future studies of animal form and function. First, studies of performance should consider the broader life-history context of the organism, and vice versa, as performance expression can require a large portion of acquired resources. Second, studies of life histories or mechanistic pathways that measure performance should do so in meaningful and standardized ways. Understanding proximate mechanisms of phenotypic trade-offs will not only better explain the phenotypes of the organisms we study, but also allow predictions about phenotypic variation at the evolutionary scale.

Neofunctionalization of a second insulin receptor gene in the wing-dimorphic planthopper, Nilaparvata lugens

A single insulin receptor (InR) gene has been identified and extensively studied in model species ranging from nematodes to mice. However, most insects possess additional copies of InR, yet the functional significance, if any, of alternate InRs is unknown. Here, we used the wing-dimorphic brown planthopper (BPH) as a model system to query the role of a second InR copy in insects. NlInR2 resembled the BPH InR homologue (NlInR1) in terms of nymph development and reproduction, but revealed distinct regulatory roles in fuel metabolism, lifespan, and starvation tolerance. Unlike a lethal phenotype derived from NlInR1 null, homozygous NlInR2 null mutants were viable and accelerated DNA replication and cell proliferation in wing cells, thus redirecting short-winged–destined BPHs to develop into long-winged morphs. Additionally, the proper expression of NlInR2 was needed to maintain symmetric vein patterning in wings. Our findings provide the first direct evidence for the regulatory complexity of the two InR paralogues in insects, implying the functionally independent evolution of multiple InRs in invertebrates.

The highly conserved insulin/insulin-like growth factor signaling pathway plays a pivotal role in growth, development, and various physiological processes across a wide phylogeny of organisms. Unlike a single InR in the model species such as the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans, most insect lineages have two or even three InR copies. However, the function of the alternative InRs remains elusive. Here, we created a homozygous mutation for a second insulin receptor (InR2) in the wing-dimorphic brown planthopper (BPH), Nilaparvata lugens, using the clustered regularly interspaced palindromic repeats/CRISPR-associated (CRISPR/Cas9) system. Our findings revealed that InR2 possesses functions distinct from the BPH InR homologue (NlInR1), indicating that multiple InR paralogues may have evolved independently and may have functionally diversified in ways more complex than previously expected in invertebrates.

Doublesex mediates species-, sex-, environment- and trait-specific exaggeration of size and shape

Context-dependent trait exaggeration is a major contributor to phenotypic diversity. However, the genetic modifiers instructing development across multiple contexts remain largely unknown. We use the arthropod tibia, a hotspot for segmental differentiation, as a paradigm to assess the developmental mechanisms underlying the context-dependent structural exaggeration of size and shape through nutritional plasticity, sexual dimorphism and segmental differentiation. Using an RNAseq approach in the sexually dimorphic and male-polyphenic dung beetle Digitonthophagus gazella, we find that only a small portion (3.7%) of all transcripts covary positively in expression level with trait size across contexts. However, RNAi-mediated knockdown of the conserved sex-determination gene doublesex suggests that it functions as a context-dependent master mediator of trait exaggeration in D. gazella as well as the closely related dung beetle Onthophagus taurus. Taken together, our findings suggest (i) that the gene networks associated with trait exaggeration are highly dependent on the precise developmental context, (ii) that doublesex differentially shapes morphological exaggeration depending on developmental contexts and (iii) that this context-specificity of dsx-mediated trait exaggeration may diversify rapidly. This mechanism may contribute to the resolution of conflict arising from environment-dependent antagonistic selection among sexes and divergent developmental contexts in a wide range of animals.

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.

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.

Sex and tissue-specific evolution of developmental plasticity in Drosophila melanogaster

Developmental plasticity influences the size of adult tissues in insects. Tissues can have unique responses to environmental perturbation during development; however, the prevalence of within species evolution of tissue-specific developmental plasticity remains unclear. To address this, we studied the effects of temperature and nutrition on wing and femur size in D. melanogaster populations from a temperate and tropical region. Wings were more sensitive to temperature, while wings and femurs were equally responsive to nutrition in both populations and sexes. The temperate population was larger under all conditions, except for femurs of starved females. In line with this, we observed greater femur size plasticity in response to starvation in temperate females, leading to differences in sexual dimorphism between populations such that the slope of the reaction norm of sexual dimorphism in the tropical population was double that of the temperate population. Lastly, we observed a significant trend for steeper slopes of reaction norms in temperate than in tropical females, but not in males. These findings highlight that plasticity divergence between populations can evolve heterogeneously across sexes and tissues and that nutritional plasticity can alter sexual dimorphism in D. melanogaster.

Genetic assimilation and accommodation: Models and mechanisms

Genetic assimilation and genetic accommodation are mechanisms by which novel phenotypes are produced and become established in a population. Novel characters may be fixed and canalized so they are insensitive to environmental variation, or can be plastic and adaptively responsive to environmental variation. In this review we explore the various theories that have been proposed to explain the developmental origin and evolution of novel phenotypes and the mechanisms by which canalization and phenotypic plasticity evolve. These theories and models range from conceptual to mathematical and have taken different views of how genes and environment contribute to the development and evolution of the properties of phenotypes. We will argue that a deeper and more nuanced understanding of genetic accommodation requires a recognition that phenotypes are not static entities but are dynamic system properties with no fixed deterministic relationship between genotype and phenotype. We suggest a mechanistic systems-view of development that allows one to incorporate both genes and environment in a common model, and that enables both quantitative analysis and visualization of the evolution of canalization and phenotypic plasticity.

Dung beetle species introductions: when an ecosystem service provider transforms into an invasive species

Dung beetle introduction programmes were designed to accelerate exotic livestock dung degradation and to control dung breeding pestiferous flies and livestock parasites. The introduction programmes provided exotic dung beetle species with an opportunity to cross natural barriers and spread beyond their native range. There are no reports that explain what probable adaptation mechanisms enable particular dung beetle species to be the most successful invader. Here we identify the morphological, biological, physiological, ecological and behavioural attributes of the four most widespread and successful dung beetle species in introduced areas on a global scale in relation to the assumption that these species are different from other exotic and native dung beetles. We have recognised Digitonthophagus gazella (Fabricius), Onthophagus taurus (Schreber), Euoniticellus intermedius (Reiche) and Aphodius fimetarius (Linnaeus) as the most successful invaders based on their spread, predominance, distribution range and the reports of invasion. Each of these four species has different natural history traits that increase their fitness making them successful invaders. D. gazella has high fecundity and spreading ability, can instantly locate and colonise fresh and nutritious dung, and has a broad thermal window. O. taurus has morphological plasticity, high fecundity, high brood survival rate due to bi-parenting, and is adapted to extreme thermal and moisture conditions. E. intermedius has remnant-dung feeding abilities, a wide thermal window, functioning best at upper-temperature levels, and successful breeding and survival abilities at extremely low soil moisture conditions. A. fimetarius is small-sized, has high breeding and dispersal abilities, and is adapted to lower thermal and upper moisture extremes and variable soil conditions. Discussed here are perspectives on adaptive attributes of dung beetle species that are important to consider during their selection for redistributions. We have elaborated on the fitness and success characteristics of the four species individually. Further, we recommend a prior-introduction baseline monitoring of native dung beetle assemblages so as to evaluate the future impact of exotic dung beetle introductions on the recipient ecosystem.

Identification of candidate loci for adaptive phenotypic plasticity in natural populations of spadefoot toads

Phenotypic plasticity allows organisms to alter their phenotype in direct response to changes in the environment. Despite growing recognition of plasticity's role in ecology and evolution, few studies have probed plasticity's molecular bases-especially using natural populations. We investigated the genetic basis of phenotypic plasticity in natural populations of spadefoot toads (Spea multiplicata). Spea tadpoles normally develop into an "omnivore" morph that is favored in long-lasting, low-density ponds. However, if tadpoles consume freshwater shrimp or other tadpoles, they can alternatively develop (via plasticity) into a "carnivore" morph that is favored in ephemeral, high-density ponds. By combining natural variation in pond ecology and morph production with population genetic approaches, we identified candidate loci associated with each morph (carnivores vs. omnivores) and loci associated with adaptive phenotypic plasticity (adaptive vs. maladaptive morph choice). Our candidate morph loci mapped to two genes, whereas our candidate plasticity loci mapped to 14 genes. In both cases, the identified genes tended to have functions related to their putative role in spadefoot tadpole biology. Our results thereby form the basis for future studies into the molecular mechanisms that mediate plasticity in spadefoots. More generally, these results illustrate how diverse loci might mediate adaptive plasticity.

Notch signaling patterns head horn shape in the bull-headed dung beetle Onthophagus taurus

Size and shape constitute fundamental aspects in the description of morphology. Yet while the developmental-genetic underpinnings of trait size, in particular with regard to scaling relationships, are increasingly well understood, those of shape remain largely elusive. Here we investigate the potential function of the Notch signaling pathway in instructing the shape of beetle horns, a highly diversified and evolutionarily novel morphological structure. We focused on the bull-headed dung beetle Onthophagus taurus due to the wide range of horn sizes and shapes present among males in this species, in order to assess the potential function of Notch signaling in the specification of horn shape alongside the regulation of shape changes with allometry. Using RNA interference-mediated transcript depletion of Notch and its ligands, we document a highly conserved role of Notch signaling in general appendage formation. By integrating our functional genetic approach with a geometric morphometric analysis, we find that Notch signaling moderately but consistently affects horn shape, and does so differently for the horns of minor, intermediate-sized, and major males. Our results suggest that the function of Notch signaling during head horn formation may vary in a complex manner across male morphs, and highlights the power of integrating functional genetic and geometric morphometric approaches in analyzing subtle but nevertheless biologically important phenotypes in the face of significant allometric variation.

A specific type of insulin-like peptide regulates the conditional growth of a beetle weapon

Evolutionarily conserved insulin/insulin-like growth factor (IGF) signaling (IIS) has been identified as a major physiological mechanism underlying the nutrient-dependent regulation of sexually selected weapon growth in animals. However, the molecular mechanisms that couple nutritional state with weapon growth remain largely unknown. Here, we show that one specific subtype of insulin-like peptide (ILP) responds to nutrient status and thereby regulates weapon size in the broad-horned flour beetle Gnatocerus cornutus. By using transcriptome information, we identified five G. cornutus ILP (GcorILP1-5) and two G. cornutus insulin-like receptor (GcorInR1, -2) genes in the G. cornutus genome. RNA interference (RNAi)-mediated gene silencing revealed that a certain subtype of ILP, GcorILP2, specifically regulated weapon size. Importantly, GcorILP2 was highly and specifically expressed in the fat body in a condition-dependent manner. We further found that GcorInR1 and GcorInR2 are functionally redundant but that the latter is partially specialized for regulating weapon growth. These results strongly suggest that GcorILP2 is an important component of the developmental mechanism that couples nutritional state to weapon growth in G. cornutus. We propose that the duplication and subsequent diversification of IIS genes played a pivotal role in the evolution of the complex growth regulation of secondary sexual traits.

Evaluating testosterone as a phenotypic integrator: From tissues to individuals to species

Hormones have the potential to bring about rapid phenotypic change; however, they are highly conserved over millions of years of evolution. Here, we examine the evolution of hormone-mediated phenotypes, and the extent to which regulation is achieved via independence or integration of the many components of endocrine systems. We focus on the sex steroid testosterone (T), its cognate receptor (androgen receptor) and related endocrine components. We pose predictions about the mechanisms underlying phenotypic integration, including coordinated sensitivity to T within and among tissues and along the HPG axis. We then assess these predictions with case studies from wild birds, asking whether gene expression related to androgenic signaling naturally co-varies among individuals in ways that would promote phenotypic integration. Finally, we review how mechanisms of integration and independence vary over developmental or evolutionary time, and we find limited support for integration.

Developmental bias in horned dung beetles and its contributions to innovation, adaptation, and resilience

Developmental processes transduce diverse influences during phenotype formation, thereby biasing and structuring amount and type of phenotypic variation available for evolutionary processes to act on. The causes, extent, and consequences of this bias are subject to significant debate. Here we explore the role of developmental bias in contributing to organisms' ability to innovate, to adapt to novel or stressful conditions, and to generate well integrated, resilient phenotypes in the face of perturbations. We focus our inquiry on one taxon, the horned dung beetle genus Onthophagus, and review the role developmental bias might play across several levels of biological organization: (a) gene regulatory networks that pattern specific body regions; (b) plastic developmental mechanisms that coordinate body wide responses to changing environments and; (c) developmental symbioses and niche construction that enable organisms to build teams and to actively modify their own selective environments. We posit that across all these levels developmental bias shapes the way living systems innovate, adapt, and withstand stress, in ways that can alternately limit, bias, or facilitate developmental evolution. We conclude that the structuring contribution of developmental bias in evolution deserves further study to better understand why and how developmental evolution unfolds the way it does.

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.

On the Reciprocally Causal and Constructive Nature of Developmental Plasticity and Robustness

Exposure to environmental variation is a characteristic feature of normal development, one that organisms can respond to during their lifetimes by actively adjusting or maintaining their phenotype in order to maximize fitness. Plasticity and robustness have historically been studied by evolutionary biologists through quantitative genetic and reaction norm approaches, while more recent efforts emerging from evolutionary developmental biology have begun to characterize the molecular and developmental genetic underpinnings of both plastic and robust trait formation. In this review, we explore how our growing mechanistic understanding of plasticity and robustness is beginning to force a revision of our perception of both phenomena, away from our conventional view of plasticity and robustness as opposites along a continuum and toward a framework that emphasizes their reciprocal, constructive, and integrative nature. We do so in three sections. Following an introduction, the first section looks inward and reviews the genetic, epigenetic, and developmental mechanisms that enable organisms to sense and respond to environmental conditions, maintaining and adjusting trait formation in the process. In the second section, we change perspective and look outward, exploring the ways in which organisms reciprocally shape their environments in ways that influence trait formation, and do so through the lens of behavioral plasticity, niche construction, and host-microbiota interactions. In the final section, we revisit established plasticity and robustness concepts in light of these findings, and highlight research opportunities to further advance our understanding of the causes, mechanisms, and consequences of these ubiquitous, and interrelated, phenomena.