Reconstructed evolution of insulin receptors in insects reveals duplications in early insects and cockroaches

Body part-specific development in termite caste differentiation: crosstalk between hormonal actions and developmental toolkit genes

In social insects, interactions among colony members trigger caste differentiation with morphological modifications. During caste differentiation in termites, body parts and caste-specific morphologies are modified during postembryonic development under endocrine controls such as juvenile hormone (JH) and ecdysone. In addition to endocrine factors, developmental toolkit genes such as Hox- and appendage-patterning genes also contribute to the caste-specific body part modifications. These toolkits are thought to provide spatial information for specific morphogenesis. During social evolution, the complex crosstalks between physiological and developmental mechanisms should be established, leading to the sophisticated caste systems. This article reviews recent studies on these mechanisms underlying the termite caste differentiation and addresses implications for the evolution of caste systems in termites.

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.

Termite primary queen - ancestral, but highly specialized eusocial phenotype

Termite eusociality is accompanied by flagrant caste polyphenism manifested by the presence of several sterile (workers and soldiers) and reproductive (imaginal and neotenic kings and queens) caste phenotypes. Imaginal kings and queens are developmentally equivalent to adults of other hemimetabolous insects but display multiple adaptations inherent to their role of eusocial colony founders, such as long lifespan and high fecundity. Herein, we summarize the recent advances in understanding the biology of imaginal (primary) queens as emblematic examples of termite polyphenism acquired during social evolution. We focus on the control of queen development, on dynamics in physiology and fecundity during the queen's life, on new findings about queen fertility signaling, and on proximate mechanisms underlying queen longevity.

Comparative Evolutionary Genomics in Insects

Genome sequencing quality, in terms of both read length and accuracy, is constantly improving. By combining long-read sequencing technologies with various scaffolding techniques, chromosome-level genome assemblies are now achievable at an affordable price for non-model organisms. Insects represent an exciting taxon for studying the genomic underpinnings of evolutionary innovations, due to ancient origins, immense species-richness, and broad phenotypic diversity. Here we summarize some of the most important methods for carrying out a comparative genomics study on insects. We describe available tools and offer concrete tips on all stages of such an endeavor from DNA extraction through genome sequencing, annotation, and several evolutionary analyses. Along the way we describe important insect-specific aspects, such as DNA extraction difficulties or gene families that are particularly difficult to annotate, and offer solutions. We describe results from several examples of comparative genomics analyses on insects to illustrate the fascinating questions that can now be addressed in this new age of genomics research.

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.

Structural conservation of insulin/IGF signalling axis at the insulin receptors level in Drosophila and humans

The insulin-related hormones regulate key life processes in Metazoa, from metabolism to growth, lifespan and aging, through an evolutionarily conserved insulin signalling axis (IIS). In humans the IIS axis is controlled by insulin, two insulin-like growth factors, two isoforms of the insulin receptor (hIR-A and -B), and its homologous IGF-1R. In Drosophila, this signalling engages seven insulin-like hormones (DILP1-7) and a single receptor (dmIR). This report describes the cryoEM structure of the dmIR ectodomain:DILP5 complex, revealing high structural homology between dmIR and hIR. The excess of DILP5 yields dmIR complex in an asymmetric 'T' conformation, similar to that observed in some complexes of human IRs. However, dmIR binds three DILP5 molecules in a distinct arrangement, showing also dmIR-specific features. This work adds structural support to evolutionary conservation of the IIS axis at the IR level, and also underpins a better understanding of an important model organism.

Different neuroendocrine cell types in the pars intercerebralis of Periplaneta americana produce their own specific IGF-related peptides

Of the nine genes of the American cockroach, Periplaneta americana, coding for peptides related to insulin and insulin-like growth factor, seven show significant expression in the central nervous system as demonstrated by the polymerase chain reaction on reverse transcribed RNA. In situ hybridisation shows that five of those are expressed by cells in the pars intercerebralis. Antisera raised to the predicted peptides show that these cells are neuroendocrine in nature and project to the corpora cardiaca. Interestingly, there are at least three cell types that each express different genes. This contrasts with Drosophila where a single cell type expresses a number of genes expressing several such peptides. Whereas in Drosophila the neuroendocrine cells producing insulin-like peptides also express sulfakinins, the arthropod orthologs of gastrin and cholecystokinin, in Periplaneta the sulfakinins are produced by different cells. Other neuropeptides known to be produced by the pars intercerebralis in Periplaneta and other insect species, such as the CRF-like diuretic hormone, neuroparsin, leucokinin or myosuppressin, neither colocalize with an insulin-related peptide. The separate cellular localization of these peptides and the existence of multiple insulin receptors in this species implies a more complex regulation by insulin and IGF-related peptides in cockroaches than in the fruit fly.

Structural Conservation of Insulin/IGF Signalling Axis at the Insulin Receptors Level inDrosophilaand humans.. [DOI: 10.1101/2023.02.17.528932] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The insulin-related hormones regulate key life processes in Metazoa, from metabolism to growth, lifespan and aging, through an evolutionarily conserved insulin signalling axis (IIS). In humans the IIS axis is controlled by insulin, two insulin-like growth factors, two isoforms of the insulin receptor (hIR-A and -B), and its homologous IGF-1R. InDrosophila, this signalling engages seven insulin-like hormones (DILP1-7) and a single receptor (dmIR). This report describes the first cryoEM structure of the dmIR ectodomain:DILP5 complex, revealing high structural homology between dmIR and hIR. The excess of DILP5 yields dmIR complex in an asymmetric ‘T’ conformation, similar to that observed in some complexes of human IRs. However, dmIR binds three DILP5 molecules in a hitherto-unseen arrangement, showing also dmIR-specific features. This work adds structural support to evolutionary conservation of the IIS axis at the IRs levels, underpinning also a better understanding of an important model organism.

Differential expression of some termite neuropeptides and insulin/IGF-related hormones and their plausible functions in growth, reproduction and caste determination

Background

Insulin-like growth factor (IGF) and other insulin-like peptides (ilps) are important hormones regulating growth and development in animals. Whereas most animals have a single female and male adult phenotype, in some insect species the same genome may lead to different final forms. Perhaps the best known example is the honeybee where females can either develop into queens or workers. More extreme forms of such polyphenism occur in termites, where queens, kings, workers and soldiers coexist. Both juvenile hormone and insulin-like peptides are known to regulate growth and reproduction as well as polyphenism. In termites the role of juvenile hormone in reproduction and the induction of the soldier caste is well known, but the role of IGF and other ilps in these processes remains largely unknown. Here the various termite ilps are identified and hypotheses regarding their functions suggested.

Methods

Genome assemblies and transcriptome short read archives (SRAs) were used to identify insulin-like peptides and neuropeptides in termites and to determine their expression in different species, tissues and castes.

Results and Discussion

Termites have seven different ilps, i.e. gonadulin, IGF and an ortholog of Drosophila insulin-like peptide 7 (dilp7), which are commonly present in insects, and four smaller peptides, that have collectively been called short IGF-related peptides (sirps) and individually atirpin, birpin, cirpin and brovirpin. Gonadulin is lost from the higher termites which have however amplified the brovirpin gene, of which they often have two or three paralogs. Based on differential expression of these genes it seems likely that IGF is a growth hormone and atirpin an autocrine tissue factor that is released when a tissue faces metabolic stress. Birpin seems to be responsible for growth and in the absence of juvenile hormone this may lead to reproductive adults or, when juvenile hormone is present, to soldiers. Brovirpin is expressed both by the brain and the ovary and likely stimulates vitellogenesis, while the function of cirpin is less clear.

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.

The hormonal and neural control of egg production in the historically important model insect, Rhodnius prolixus: A review, with new insights in this post-genomic era

Rhodnius prolixus, the blood gorging kissing bug, is a model insect, extensively used by Sir Vincent Wigglesworth and others, upon which the foundations of insect physiology, endocrinology, and development are built. It is also medically important, being a principal vector of Trypanosoma cruzi, the causative agent of Chagas disease in humans. The blood meal stimulates and enables egg production, and since an adult mated female can take several blood meals, each female can produce hundreds of offspring. Understanding the reproductive biology of R. prolixus is therefore of some critical importance for controlling the transmission of Chagas disease. The R. prolixus genome is available and so the post-genomic era has arrived for this historic model insect. This review focuses on the female reproductive system and coordination over the production of eggs, emphasizing the classical (neuro)endocrinological studies that led to a model describing inputs from feeding and mating, and the neural control of egg-laying. We then review recent insights brought about by molecular analyses, including transcriptomics, that confirm, support, and considerably extends this model. We conclude this review with an updated model describing the events leading to full expression of egg production, and also provide a consideration of questions for future exploration and experimentation.

Termite sociogenomics: evolution and regulation of caste-specific expressed genes

Termite genomes have been sequenced in at least five species from four different families. Genome-based transcriptome analyses have identified large numbers of protein-coding genes with caste-specific expression patterns. These genes include those involved in caste-specific morphologies and roles, for example high fecundity and longevity in reproductives. Some caste-specific expressed genes belong to multi-gene families, and their genetic architecture and expression profiles indicate they have evolved via tandem gene duplication. Candidate regulatory mechanisms of caste-specific expression include epigenetic regulation (e.g. histone modification and non-coding RNA) and diversification of transcription factors and cis-regulatory elements. We review current knowledge in the area of termite sociogenomics, focussing on the evolution and regulation of caste-specific expressed genes, and discuss future research directions.

Genomic and transcriptomic analyses of the subterranean termite Reticulitermes speratus: Gene duplication facilitates social evolution

Gene duplication is a major source of evolutionary innovation and is associated with the increases in biological complexity and adaptive radiation. Termites are model social organisms characterized by a sophisticated caste system. We analyzed the genome of the Japanese subterranean termite, an ecologically and economically important insect acting as a destructive pest. The analyses revealed the significance of gene duplication in social evolution. Gene duplication associated with caste-biased gene expression was prevalent in the termite genome. Many of the duplicated genes were related to social functions, such as chemical communication, social immunity, and defense, and they were often expressed in caste-specific organs. We propose that gene duplication facilitates social evolution through regulatory diversification leading to caste-biased expression and functional specialization.

Termites are model social organisms characterized by a polyphenic caste system. Subterranean termites (Rhinotermitidae) are ecologically and economically important species, including acting as destructive pests. Rhinotermitidae occupies an important evolutionary position within the clade representing a transitional taxon between the higher (Termitidae) and lower (other families) termites. Here, we report the genome, transcriptome, and methylome of the Japanese subterranean termite Reticulitermes speratus. Our analyses highlight the significance of gene duplication in social evolution in this termite. Gene duplication associated with caste-biased gene expression was prevalent in the R. speratus genome. The duplicated genes comprised diverse categories related to social functions, including lipocalins (chemical communication), cellulases (wood digestion and social interaction), lysozymes (social immunity), geranylgeranyl diphosphate synthase (social defense), and a novel class of termite lineage–specific genes with unknown functions. Paralogous genes were often observed in tandem in the genome, but their expression patterns were highly variable, exhibiting caste biases. Some of the assayed duplicated genes were expressed in caste-specific organs, such as the accessory glands of the queen ovary and the frontal glands of soldier heads. We propose that gene duplication facilitates social evolution through regulatory diversification, leading to caste-biased expression and subfunctionalization and/or neofunctionalization conferring caste-specialized functions.

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.

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 involvement of insulin/ToR signaling pathway in reproductive performance of Rhodnius prolixus

Insulins are peptide hormones widely studied for their important regulatory roles in metabolism, growth and development. In insects, insulin signaling along with the target of rapamycin (ToR) are involved in detecting and interpreting nutrient levels. Recently, by transcriptome analysis we reported an up-regulation of transcripts involved in insulin/ToR signaling in unfed Rhodnius prolixus; however, this signaling pathway is only activated in fed insects. Here, continuing with the blood-gorging triatomine R. prolixus as a model, we report the direct effect of insulin/ToR signaling on reproductive performance. By immunofluorescence we identified cells in the brain with positive signal to the R. prolixus ILP (Rhopr-ILP1) and show that the insulin receptor and protein effectors downstream of insulin/ToR signaling activation, are differentially expressed in ovarian follicles dependent on their developmental stage. Using qPCR we find that the expression of transcripts involved in insulin signaling in the central nervous system (CNS), fat body and ovaries increase as the state of starvation progresses, promoting a more highly sensitized state to respond rapidly to ILP/IGF levels. In addition, using dsRNA injection and in vivo and ex vivo assays to promote signaling activation we demonstrate a direct participation of insulin/ToR signaling in coordinating the synthesis of the main yolk protein precursor, vitellogenin, thereby influencing the numbers of eggs laid per female. We thereby show a mechanism by which nutritional signaling regulates reproductive performance in a vector of Chagas disease. As reproduction is responsible for propagation of insect populations, this work is important for the development of innovative biocontrol methods.

Functional characterization of the insulin signaling pathway in the hard tick Ixodes ricinus

Ticks are blood-feeding arachnids transmitting a variety of pathogens to humans and animals. A unique trait in tick physiology is their ability to engorge and digest large amounts of host blood, ensuring their high reproductive potential. Activation of the blood digestive machinery in the tick gut, as well as processes controlling maturation of ovaries, are triggered upon blood meal uptake by still largely unknown mechanisms. Sensing of the nutritional status in metazoan organisms is facilitated by the evolutionarily conserved Insulin Signaling Pathway (ISP) and the interlinked Target of Rapamycin (TOR) pathway. Recently, we have identified three components of these pathways in the hard tick Ixodes ricinus midgut transcriptome, namely a putative insulin receptor (InR), and the downstream intracellular serine/threonine kinases AKT and TOR. In this study, we primarily focus on the molecular and functional characterization of the I. ricinus insulin receptor (IrInR), the first InR characterized in Chelicerates. A phylogenetic analysis across the major Arthropod lineages demonstrated that ticks possess only one gene encoding an InR-related molecule. Tissue expression profiling by quantitative PCR in semi-engorged I. ricinus females revealed that the IrInR, as well as AKT (IrAKT) and TOR (IrTOR) are expressed in various organs, with the highest expression being detected in ovaries. We have further evaluated the impact of RNAi-mediated knock-down (KD) of IrInR, IrAKT, and IrTOR on tick blood-feeding and reproductive capacity. Weights of engorged IrInR KD females and laid egg clutches were reduced compared to the control group, and these quantitative parameters clearly correlated with the efficiency of RNAi-KD achieved in individual ticks. The most striking phenotype was observed for IrAKT KD that impaired tick feeding and completely aborted egg production. A recombinant extracellular fragment of the IrInR α-subunit was used to produce antibodies in experimental rabbits to assess its potential as a protective antigen against tick feeding and reproduction. Our data clearly indicate the functionality of the ISP in ticks and demonstrate the need for further investigation of specific roles played by the endogenous insulin-like peptides in tick physiological processes.

Identification of Gonadulin and Insulin-Like Growth Factor From Migratory Locusts and Their Importance in Reproduction in Locusta migratoria

Many insect species have several genes coding for insulin-related peptides (IRPs), but so far only a single IRP gene has been identified in migratory locusts. Here, we report and characterize two other genes coding for peptides that are related to insulin, namely gonadulin and arthropod insulin-like growth factor (aIGF); peptides postulated to be orthologs of Drosophila melanogaster insulin-like peptides 8 and 6 respectively. In Locusta migratoria the aIGF transcript is expressed in multiple tissues as was previously reported for IRP in both L. migratoria and Schistocerca gregaria, but there are significant differences in expression patterns between the two species. The gonadulin transcript, however, seems specific to the ovary, whereas its putative receptor transcript is expressed most abundantly in the ovary, fat body and the central nervous system. Since the central nervous system-fat body-ovary axis is essential for successful reproduction, we studied the influence of gonadulin on vitellogenesis and oocyte growth. A reduction in the gonadulin transcript (via RNA interference) led to a significant reduction in vitellogenin mRNA levels in the fat body and a strong oocyte growth inhibition, thus suggesting an important role for gonadulin in reproduction in this species.

Arthropod IGF, relaxin and gonadulin, putative orthologs of Drosophila insulin-like peptides 6, 7 and 8, likely originated from an ancient gene triplication

BACKGROUND

Insects have several genes coding for insulin-like peptides and they have been particularly well studied in Drosophila. Some of these hormones function as growth hormones and are produced by the fat body and the brain. These act through a typical insulin receptor tyrosine kinase. Two other Drosophila insulin-like hormones are either known or suspected to act through a G-protein coupled receptor. Although insulin-related peptides are known from other insect species, Drosophila insulin-like peptide 8, one that uses a G-protein coupled receptor, has so far only been identified from Drosophila and other flies. However, its receptor is widespread within arthropods and hence it should have orthologs. Such putative orthologs were recently identified in decapods and have been called gonadulins.

METHODOLOGY

In an effort to identify gonadulins in other arthropods public genome assemblies and short-read archives from insects and other arthropods were explored for the presence of genes and transcripts coding insulin-like peptides and their putative receptors.

RESULTS

Gonadulins were detected in a number of arthropods. In those species for which transcriptome data from the gonads is available insect gonadulin genes are expressed in the ovaries and at least in some species also in the testes. In some insects differences in gonadulin expression in the ovary between actively reproducing and non-reproducing females differs more than 100-fold. Putative orthologs of Drosophila ilp 6 were also identified. In several non-Dipteran insects these peptides have C-terminally extensions that are alternatively spliced. The predicted peptides have been called arthropod insulin-like growth factors. In cockroaches, termites and stick insects genes coding for the arthropod insulin-like growth factors, gonadulin and relaxin, a third insulin-like peptide, are encoded by genes that are next to one another suggesting that they are the result of a local gene triplication. Such a close chromosomal association was also found for the arthropod insulin-like growth factor and gonadulin genes in spiders. Phylogenetic tree analysis of the typical insulin receptor tyrosine kinases from insects, decapods and chelicerates shows that the insulin signaling pathway evolved differently in these three groups. The G-protein coupled receptors that are related to the Drosophila ilp 8 receptor similarly show significant differences between those groups.

CONCLUSION

A local gene triplication in an early ancestor likely yielded three genes coding gonadulin, arthropod insulin-like growth factor and relaxin. Orthologs of these genes are now commonly present in arthropods and almost certainly include the Drosophila insulin-like peptides 6, 7 and 8.

The making of the defensive caste: Physiology, development, and evolution of the soldier differentiation in termites

Termites (Blattodea, Termitoidea, or Isoptera) constitute one of the major lineages of eusocial insects. In termite societies, multiple types of functional individuals, that is, castes, perform divisions of labors to coordinate social behaviors. Among other castes, the soldier caste is distinctive since it is sterile and exclusively specialized into defensive behavior with largely modified morphological features. Therefore, many of the previous studies have been focused on soldiers, in terms of ecology, behavior, and evolution as well as developmental and physiological mechanisms. This article overviews the accumulation of studies especially focusing on the developmental and physiological mechanisms underlying the soldier differentiation in termites. Furthermore, the evolutionary trajectories that have led the acquisition of soldier caste and have diversified the soldier characteristics in association with the social evolution are discussed.

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.

The expression of cockroach insulin-like peptides is differentially regulated by physiological conditions and affected by compensatory regulation

In insects, the insulin receptor (InR) pathway is involved in regulating key physiological processes, including juvenile hormone (JH) synthesis, vitellogenin production, and oocyte growth. This raises the question about which ligand (or ligands) binds to InR to trigger the above effects. We have cloned seven insulin-like peptides (BgILP1 to 7) from female Blattella germanica cockroaches and found that the brain expresses BgILP1 to 6, the fat body BgILP7, and the ovary BgILP2. Starvation induces the reduction of BgILP3, 5, and 6 mRNA levels in the brain, and the various BgILPs are differentially expressed during the gonadotrophic cycle. In addition, by knocking down the BgILPs we were able to identify compensatory regulation at transcriptional level between the different BgILPs, although none of the BgILP knockdown assays, including the knockdown of the seven BgILPs, produced the same phenotypes that we achieved by depleting InR. Taken together, the results indicate that B. germanica ILPs are differentially expressed in tissues and in response to physiological conditions, and that they are affected by compensatory regulation.

The genome of the water strider Gerris buenoi reveals expansions of gene repertoires associated with adaptations to life on the water

BACKGROUND

Having conquered water surfaces worldwide, the semi-aquatic bugs occupy ponds, streams, lakes, mangroves, and even open oceans. The diversity of this group has inspired a range of scientific studies from ecology and evolution to developmental genetics and hydrodynamics of fluid locomotion. However, the lack of a representative water strider genome hinders our ability to more thoroughly investigate the molecular mechanisms underlying the processes of adaptation and diversification within this group.

RESULTS

Here we report the sequencing and manual annotation of the Gerris buenoi (G. buenoi) genome; the first water strider genome to be sequenced thus far. The size of the G. buenoi genome is approximately 1,000 Mb, and this sequencing effort has recovered 20,949 predicted protein-coding genes. Manual annotation uncovered a number of local (tandem and proximal) gene duplications and expansions of gene families known for their importance in a variety of processes associated with morphological and physiological adaptations to a water surface lifestyle. These expansions may affect key processes associated with growth, vision, desiccation resistance, detoxification, olfaction and epigenetic regulation. Strikingly, the G. buenoi genome contains three insulin receptors, suggesting key changes in the rewiring and function of the insulin pathway. Other genomic changes affecting with opsin genes may be associated with wavelength sensitivity shifts in opsins, which is likely to be key in facilitating specific adaptations in vision for diverse water habitats.

CONCLUSIONS

Our findings suggest that local gene duplications might have played an important role during the evolution of water striders. Along with these findings, the sequencing of the G. buenoi genome now provides us the opportunity to pursue exciting research opportunities to further understand the genomic underpinnings of traits associated with the extreme body plan and life history of water striders.