High Laccase2 expression is likely involved in the formation of specific cuticular structures during soldier differentiation of the termite Reticulitermes speratus

Feeding cessation after feeding on 20-hydroxyecdysone in the Formosan subterranean termite

BACKGROUND

To control subterranean termite pests, chitin synthesis inhibitor (CSI) baits have been widely applied. Despite CSI baits having low impacts on the environment, they require a lengthy time period to eliminate colonies. 20-hydroxyecdysone (20E) was proposed to speed up the baiting process as it showed faster mortality than CSI baits. However, the efficacy of 20E has previously not been tested at the colony level prior to applying in the field.

RESULTS

We compared the effect of 20E, 20E + noviflumuron, noviflumuron and untreated control using colonies of Coptotermes formosanus. Our result revealed that both 20E and 20E + noviflumuron did not accelerate colony elimination and termite activity remained relatively stable during the observation periods. To determine the limited effects of 20E, we further investigated feeding duration and consumption amount of 20E with different concentrations (control, 100 and 1000 ppm) for 10 days. Termites ceased feeding after 1 day in 100 and 1000 ppm treatment and 100% mortality was observed within 10 days in 1000 ppm 20E, while mortality in the 100 ppm 20E treated group was much lower than that in the 1000 ppm group. Furthermore, no termites molted in the control and termites died from hyperecdysonism in 1000 ppm 20E treatment, whereas about 20% of termites molted in 100 ppm 20E.

CONCLUSION

This study demonstrated that 20E may not be suitable as a sole active ingredient to accelerate elimination of a subterranean termite colony, while CSI baits and lower concentrations of 20E may reduce the lengthy time period in colony elimination. © 2023 Society of Chemical Industry.

Caste-specific expressions and diverse roles of takeout genes in the termite Reticulitermes speratus

Acquisition of novel functions caused by gene duplication may be important for termite social evolution. To clarify this possibility, additional evidence is needed. An important example is takeout, encoding juvenile hormone binding protein. We identified 25 takeouts in the termite Reticulitermes speratus genome. RNA-seq revealed that many genes were highly expressed in specific castes. Two novel paralogs (RsTO1, RsTO2) were tandemly aligned in the same scaffold. Real-time qPCR indicated that RsTO1 and RsTO2 were highly expressed in queens and soldiers, respectively. Moreover, the highest RsTO1 expression was observed in alates during queen formation. These patterns were different from vitellogenins, encoding egg-yolk precursors, which were highly expressed in queens than alates. In situ hybridization showed that RsTO1 mRNA was localized in the alate-frontal gland, indicating that RsTO1 binds with secretions probably used for the defence during swarming flight. In contrast, increased RsTO2 expression was observed approximately 1 week after soldier differentiation. Expression patterns of geranylgeranyl diphosphate synthase, whose product functions in the terpenoid synthesis, were similar to RsTO2 expression. In situ hybridization indicated RsTO2-specific mRNA signals in the soldier-frontal gland. RsTO2 may interact with terpenoids, with a soldier-specific defensive function. It may provide additional evidence for functionalization after gene duplication in termites.

Efficient RNA interference method during caste differentiation with hormone treatment in the termite Reticulitermes speratus (Isoptera: Rhinotermitidae)

Unveiling the proximate mechanism of caste differentiation is crucial for understanding insect social evolution, and gene function analysis is an important tool in this endeavor. The RNA interference (RNAi) technique is useful in termites, but its knockdown effects may differ among species. One of the most important model species in the field of termite sociogenomics is Reticulitermes speratus Kolbe (Isoptera: Rhinotermitidae). Presoldier and worker differentiation of this species can be artificially induced by juvenile hormone and 20-hydroxyecdysone application, respectively. However, appropriate RNAi technique of genes expressed during caste differentiation has never been considered. To clarify this issue, first, we injected nine different volumes of nuclease-free water (NFW, 0-404.8 nL) into workers and found that survival and caste differentiation rates were strongly reduced by the application of the top three largest volumes. Second, we injected double-stranded (ds) RNA of ecdysone receptor homolog (RsEcR) (2.0 µg/151.8 nL NFW) into workers with hormone treatments. The expression levels of RsEcR were significantly reduced at 9 days after dsRNA injection. RsEcR RNAi strongly affected both molting events during presoldier and worker differentiation induced by hormone treatments. The present results highlight the need for caution regarding injection volumes for RNAi experiments using hormone treatments. We suggest that the injection of dsRNA solution (2 µg; approximately 100-200 nL) is suitable for RNAi experiments during caste differentiation induced by hormone application in R. speratus.

Comparison of gene expression profiles among caste differentiations in the termite Reticulitermes speratus

Termite castes express specialized phenotypes for their own tasks and are a good example of insect polyphenism. To understand the comprehensive gene expression profiles during caste differentiation, RNA-seq analysis based on the genome data was performed during the worker, presoldier, and nymphoid molts in Reticulitermes speratus. In this species, artificial induction methods for each molt have already been established, and the time scale has been clarified. Three different periods (before the gut purge (GP), during the GP, and after the molt) were discriminated in each molt, and two body parts (head and other body regions) were separately sampled. The results revealed that many differentially expressed genes (head: 2884, body: 2579) were identified in each molt. Based on the independent real-time quantitative PCR analysis, we confirmed the different expression patterns of seven out of eight genes in the presoldier molt. Based on the GO and KEGG enrichment analyses, the expressions of genes related to juvenile hormone titer changes (e.g., JH acid methyltransferase), nutrition status (e.g., Acyl-CoA Delta desaturase), and cell proliferation (e.g., insulin receptor), were shown to specifically fluctuate in each molt. These differences may have a crucial impact on caste differentiation. These data are important resources for future termite sociogenomics.

Transcriptomics on Social Interactions in Termites: Effects of Soldier Presence

The organization of social insect colonies requires sophisticated mechanisms to regulate caste composition according to colony demands. In termites, the soldier caste is responsible for the inhibition of soldier differentiation, but the mechanism underlying the regulation of soldier differentiation is still unclear. In this study, we performed transcriptome analyses to identify genes expressed in workers that fluctuated in the presence of soldiers in the subterranean termite Reticulitermes speratus. First, soldier differentiation was artificially induced via juvenile hormone (JH) application, and the inhibitory effects of soldier differentiation on soldier presence were evaluated. Second, transcriptomes were prepared from workers with or without soldiers under JH treatment, and expression analyses were performed to identify differentially expressed genes (DEGs) for each treatment. The expression levels of several DEGs were verified by quantitative real-time PCR. The results indicated that only a small number of DEGs were upregulated by the presence of soldiers. A homology search of DEGs and gene ontology (GO) analysis of the DEGs showed that some genes were responsible for the regulation of hormone levels, social interaction, and response to xenobiotic substances, suggesting that they could be involved in developmental arrest and pheromonal regulation in workers. Moreover, GO analysis indicated that the expression of many genes, including those involved in hormone metabolic processes, fluctuated with JH application. Suppression of soldier differentiation in the presence of soldiers could be accomplished by the expression of a large number of genes required for soldier differentiation.

Endocuticle is involved in caste differentiation of the lower termite

Caste differentiation in termites is one of the most conspicuous examples of facultative polyphenism in animals. It is clear that specific cuticular formation occurs in hard exocuticles during caste differentiation. However, the developmental pattern of the soft endocuticle in the differentiation pathways of castes is unknown. To reveal whether the endocuticle is involved in caste differentiation, we compared the exocuticle and endocuticle thickness of individuals in 2 pathways (nymph line and worker line) of caste differentiation in the termite Reticulitermes aculabialis. The endocuticle protein genes were identified by transcriptome analysis and the expression patterns of these genes were confirmed in caste differentiation. We found that the endocuticle structure showed dynamic changes in 2 pathways, and the first difference in endocuticle structure occurred after larvae differentiation bifurcated into workers and nymphs. The thinning of the endocuticle was a significant event from nymphs developing into alates with the thickest exocuticle and thinnest endocuticle. The thickest endocuticle layers were found in the heads of the workers and the ultrastructure of the endocuticle in the heads was more complex than that in the thorax–abdomens. Six endocuticle protein genes were identified and annotated as endocuticle structural glycoproteins SgAbd-2, SgAbd-9, and Abd-5. The expression levels of endocuticle protein genes changed dramatically during caste development and the expression levels in neotenic reproductives (secondary reproductives) were significantly higher than those in alates (primary reproductives). These results reveal the roles of endocuticles in caste differentiation and adaptation to the environment.

Reproductive Soldier Development Is Controlled by Direct Physical Interactions with Reproductive and Soldier Termites

Simple Summary

Generally, in termites, the soldier caste is sterile, and their reproductive organ formation is suppressed. However, in some primitive taxa, neotenic reproductives with soldier-like mandibles, called reproductive soldiers, occasionally appear in the incipient colony. Our first step in understanding the developmental mechanism of this unique caste was to establish efficient conditions to induce reproductive soldiers in Zootermopsis nevadensis. Removing both the king and soldiers from an incipient colony strongly stimulated the formation of male reproductive soldiers, which possessed soldier-like mandibles and developed testes. Similarly, high differentiation rates of male reproductive soldiers were observed after king-and-soldier separation treatment using wire mesh. However, no male reproductive soldiers were produced without direct interaction with the queen. These results suggest that reproductive soldier development might be initiated and controlled by direct physical interactions with both reproductives (the king and queen) and soldiers.

Abstract

In eusocial insects (e.g., ants, bees, and termites), the roles of different castes are assigned to different individuals. These castes possess unique phenotypes that are specialized for specific tasks. The acquisition of sterile individuals with specific roles is considered a requirement for social evolution. In termites, the soldier is a sterile caste. In primitive taxa (family Archotermopsidae and Stolotermitidae), however, secondary reproductives (neotenic reproductives) with their mandibles developed into weapons (so-called reproductive soldiers, also termed as soldier-headed reproductives or soldier neotenics) have been reported. To understand the developmental mechanism of this unique caste, it is necessary to understand the environmental cues and developmental processes of reproductive soldiers under natural conditions. Here, we established efficient conditions to induce reproductive soldiers in Zootermopsis nevadensis. Male reproductive soldiers frequently developed after the removal of both the king and soldiers from an incipient colony. Similarly, high differentiation rates of male reproductive soldiers were observed after king-and-soldier separation treatment using wire mesh. However, no male reproductive soldiers were produced without direct interaction with the queen. These results suggest that male reproductive soldier development is repressed by direct physical interactions with both the king and soldiers and facilitated by direct physical interaction with the queen.

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.

Diversity and function of multicopper oxidase genes in the stinkbug Plautia stali

Multicopper oxidase (MCO) genes comprise multigene families in bacteria, fungi, plants and animals. Two families of MCO genes, MCO1 (laccase1) and MCO2 (laccase2), are conserved among diverse insects and relatively well-characterized, whereas additional MCO genes, whose biological functions have been poorly understood, are also found in some insects. Previous studies reported that MCO1 participates in gut immunity and MCO2 plays important roles in cuticle sclerotization and pigmentation of insects. In mosquitoes, MCO2 was reported to be involved in eggshell sclerotization and pigmentation, on the ground that knockdown of MCO2 caused deformity and fragility of the eggshell. Here we identified a total of 7 MCO genes, including PsMCO1 and PsMCO2, and investigated their expression and function in the brown-winged green stinkbug Plautia stali. RNA interference (RNAi) knockdown of MCO genes by injecting double-stranded RNA (dsRNA) into nymphs revealed that MCO2, but not the other 6 MCOs, is required for cuticle sclerotization and pigmentation, and also for survival of P. stali. Trans-generational knockdown of MCO2 by injecting dsRNA into adult females (maternal RNAi) resulted in the production of unhatched eggs despite the absence of deformity or fragility of the eggshell. These results suggested that MCO2 plays an important role in sclerotization and pigmentation of the cuticle but not in eggshell integrity in P. stali. Maternal RNAi of any of the other 6 MCO genes and 3 tyrosinase genes affected neither survival nor eggshell integrity of P. stali. Contrary to the observations in the red flour beetle and the brown rice planthopper, RNAi knockdown of MCO6 (MCORP; Multicopper oxidase related protein) exhibited no lethal effects on P. stali. Taken together, our findings provide insight into the functional diversity and commonality of MCOs across hemipteran and other insect groups.

Comparative transcriptomic analysis and endocuticular protein gene expression of alate adults, workers and soldiers of the termite Reticulitermes aculabialis

Background

The insect cuticle is mainly composed of exocuticle and endocuticle layers that consist of a large number of structural proteins. The thickness and synthesis of the exocuticle depend on different castes that perform various functions in alates, workers and soldiers. However, it is not clear whether the soft endocuticle is involved in the division of labour in termite colonies. To reveal the structural characteristics of the endocuticle in different castes, we investigated the thickness of endocuticle layers in alates, workers and soldiers of the termite Reticulitermes aculabialis, and then we sequenced their transcriptome and detected the endocuticle protein genes. The differential expression levels of the endocuticular protein genes were confirmed in the three castes.

Results

We found that there was a great difference in the thickness of the endocuticle among the alates, soldiers and workers. The thickest endocuticle layers were found in the heads of the workers 7.88 ± 1.67 μm. The endocuticle layer in the head of the workers was approximately three-fold and nine-fold thicker than that in the heads of soldiers and alates, respectively. The thinnest endocuticle layers occurred in the head, thorax and abdomen of alates, which were 0.86 ± 0.15, 0.76 ± 0.24 and 0.52 ± 0.17 μm thick, respectively, and had no significant differences. A total of 43,531,650 clean sequencing reads was obtained, and 89,475 unigenes were assembled. Of the 70 identified cuticular protein genes, 10 endocuticular genes that belong to the RR-1 family were selected. qRT-PCR analysis of the five endocuticular genes (SgAbd-2, SgAbd-9, Abd-5, SgAbd-2-like and Abd-4-like) revealed that the endocuticle genes were more highly expressed in workers than in soldiers and alates.

Conclusion

These results suggest that SgAbd and Abd are the key components of the endocuticle. We infer that the thicker endocuticle in workers is helpful for them to perform their functions against environmental stress.

Mini-review an insect-specific system for terrestrialization: Laccase-mediated cuticle formation

Insects are often regarded as the most successful group of animals in the terrestrial environment. Their success can be represented by their huge biomass and large impact on ecosystems. Among the factors suggested to be responsible for their success, we focus on the possibility that the cuticle might have affected the process of insects' evolution. The cuticle of insects, like that of other arthropods, is composed mainly of chitin and structural cuticle proteins. However, insects seem to have evolved a specific system for cuticle formation. Oxidation reaction of catecholamines catalyzed by a copper enzyme, laccase, is the key step in the metabolic pathway for hardening of the insect cuticle. Molecular phylogenetic analysis indicates that laccase functioning in cuticle sclerotization has evolved only in insects. In this review, we discuss a theory on how the insect-specific "laccase" function has been advantageous for establishing their current ecological position as terrestrial animals.

TGFβ signaling related genes are involved in hormonal mediation during termite soldier differentiation

A working knowledge of the proximate factors intrinsic to sterile caste differentiation is necessary to understand the evolution of eusocial insects. Genomic and transcriptomic analyses in social hymenopteran insects have resulted in the hypothesis that sterile castes are generated by the novel function of co-opted or recruited universal gene networks found in solitary ancestors. However, transcriptome analysis during caste differentiation has not been tested in termites, and evolutionary processes associated with acquiring the caste are still unknown. Termites possess the soldier caste, which is regarded as the first acquired permanently sterile caste in the taxon. In this study, we performed a comparative transcriptome analysis in termite heads during 3 molting processes, i.e., worker, presoldier and soldier molts, under natural conditions in an incipient colony of the damp-wood termite Zootermopsis nevadensis. Although similar expression patterns were observed during each molting process, more than 50 genes were shown to be highly expressed before the presoldier (intermediate stage of soldier) molt. We then performed RNA interference (RNAi) of the candidate 13 genes, including transcription factors and uncharacterized protein genes, during presoldier differentiation induced by juvenile hormone (JH) analog treatment. Presoldiers induced after RNAi of two genes related to TGFβ (Transforming growth factor beta) signaling were extremely unusual and possessed soldier-like phenotypes. These individuals also displayed aggressive behaviors similar to natural soldiers when confronted with Formica ants as hypothetical enemies. These presoldiers never molted into the next instar, presumably due to the decreased expression levels of the molting hormone (20-hydroxyecdysone; 20E) signaling genes. These results suggest that TGFβ signaling was acquired for the novel function of regulating between JH and 20E signaling during soldier differentiation in termites.

Suppression of Laccase 2 severely impairs cuticle tanning and pathogen resistance during the pupal metamorphosis of Anopheles sinensis (Diptera: Culicidae)

Background

Phenol oxidases (POs) catalyze the oxidation of dopa and dopamine to melanin, which is crucial for cuticle formation and innate immune maintenance in insects. Although, Laccase 2, a member of the PO family, has been reported to be a requirement for melanin-mediated cuticle tanning in the development stages of some insects, whether it participates in cuticle construction and other physiological processes during the metamorphosis of mosquito pupae is unclear.

Methods

The association between the phenotype and the expression profile of Anopheles sinensis Laccase 2 (AsLac2) was assessed from pupation to adult eclosion. Individuals showing an expression deficiency of AsLac2 that was produced by RNAi and their phenotypic defects and physiological characterizations were compared in detail with the controls.

Results

During the dominant expression period, knockdown of AsLac2 in pupae caused the cuticle to be unpigmented, and produced thin and very soft cuticles, which further impeded the eclosion rate of adults as well as their fitness. Moreover, melanization immune responses in the pupae were sharply decreased, leading to poor resistance to microorganism infection. Both the high conservation among Laccase 2 homologs and a very similar genomic synteny of the neighborhood in Anopheles genus implies a conservative function in the pupal stage.

Conclusions

To our knowledge, this is the first study to report the serious phenotypic defects in mosquito pupae caused by the dysfunction of Laccase 2. Our findings strongly suggest that Laccase 2 is crucial for Anopheles cuticle construction and melanization immune responses to pathogen infections during pupal metamorphosis. This irreplaceability provides valuable information on the application of Lacccase 2 and/or other key genes in the melanin metabolism pathway for developing mosquito control strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2118-4) contains supplementary material, which is available to authorized users.

Gene expression changes in the tyrosine metabolic pathway regulate caste-specific cuticular pigmentation of termites

In social insects, all castes have characteristic phenotypes suitable for their own tasks and to engage in social behavior. The acquisition of caste-specific phenotypes was a key event in the course of social insect evolution. However, understanding of the genetic basis and the developmental mechanisms that produce these phenotypes is still very limited. In particular, termites normally possess more than two castes with specific phenotypes (i.e. workers, soldiers, and reproductives), but proximate developmental mechanisms are far from being fully understood. In this study, we focused on the pigmentation of the cuticle as a model trait for caste-specific phenotypes, during the molts of each caste; workers, soldiers, presoldiers (intermediate stage of soldiers), and alates (primary reproductives) in Zootermopsis nevadensis. Expression patterns of cuticular tanning genes (members of the tyrosine metabolic pathway) were different among each molt, and high expression levels of several "key genes" were observed during each caste differentiation. For the differentiation of castes with well-tanned cuticles (i.e. soldiers and alates), all focal genes except DDC in the former were highly expressed. On the other hand, high expression levels of yellow and aaNAT were observed during worker and presoldier molts, respectively, but most other genes in the pathway were expressed at low levels. RNA interference (RNAi) of these key genes affected caste-specific cuticular pigmentation, leading to soldiers with yellowish-white heads and pigmented mandibular tips, presoldiers with partly pigmented head cuticles, and alates with the yellow head capsules. These results suggest that the pigmentation of caste-specific cuticles is achieved by the regulation of gene expression in the tyrosine metabolic pathway.

Sexual difference in juvenile-hormone titer in workers leads to sex-biased soldier differentiation in termites

In termites, the soldier caste, with its specialized defensive morphology, is one of the most important characteristics for sociality. Most of the basal termite species have both male and female soldiers, and the soldier sex ratio is almost equal or only slightly biased. However, in the apical lineages (especially family Termitidae), there are many species that have soldiers with strongly biased sex ratio. Generally in termites, since high juvenile hormone (JH) titer is required for soldier differentiation from a worker via a presoldier stage, it was hypothesized that the biased soldier-sex ratio was caused by differences in JH sensitivity and/or JH titer between male and female workers. Therefore, we focused on the presoldier differentiation and the worker JH titer in species with only male soldiers (Nasutitermes takasagoensis) and with both male and female soldiers (Reticulitermes speratus) in natural conditions. In the former species, there are four types of workers; male minor, male medium, female medium and female major workers, and presoldiers differentiate from male minor workers. First, we tried to artificially induce presoldiers from male and female workers. In N. takasagoensis, the presoldier differentiation rate and mortality was significantly higher in male minor workers. Morphological analyses showed that both male and female induced presoldiers possessed normal soldier-specific morphologies. It was suggested that female workers, from which soldiers do not differentiate under natural conditions, also maintained the physiological and developmental potential for soldier differentiation. In R. speratus, however, no differences were observed in solder differentiation rate and mortality between male and female workers. Second, the JH titers of each sex/type of workers were quantified by high performance liquid chromatography-mass spectrometry in two different seasons (April and December). The results showed that, in N. takasagoensis, JH titer in male minor workers was consistently higher than those in other worker types. In R. speratus, in contrast, there were no significant differences in JH titers between male and female workers. These results suggested that, in N. takasagoensis, male minor workers maintain JH titers at a high level throughout a year, and this may cause the male-biased presoldier differentiation.

Dopamine regulates termite soldier differentiation through trophallactic behaviours

Caste polyphenism in social insects is regulated by social interactions among colony members. Trophallaxis is one of the most frequently observed interactions, but no studies have been conducted identifying the intrinsic factors involved in this behaviour and caste differentiation. Dopamine (DA) has multiple roles in the modulation of behaviours and physiology, and it produces species-specific behaviours in animals. Here, to verify the role of DA in termite soldier differentiation, we focused on the first soldier in an incipient colony of Zootermopsis nevadensis, which always differentiates from the oldest 3rd instar (No. 1 larva) via a presoldier. First, brain DA levels of the No. 1 larva at day 3 after its appearance were significantly higher than day 0. Second, DA synthesis gene expression levels were extraordinarily high in the No. 1 larva at day 0-1 after appearance. Finally, injection of a DA receptor antagonist into the No. 1 larva resulted in the inhibition of presoldier differentiation. Behavioural observations of the antagonist or control-injected larvae suggested that brain DA and signalling activity regulate the frequencies of trophallaxis from reproductives and presoldier differentiation. Because trophallaxis is a social behaviour frequently observed in natural conditions, the role of DA should be investigated in other social insects with frequent trophallactic and allogrooming behaviour.

Molecular Mechanism of the Two-Component Suicidal Weapon of Neocapritermes taracua Old Workers

In termites, as in many social insects, some individuals specialize in colony defense, developing diverse weaponry. As workers of the termite Neocapritermes taracua (Termitidae: Termitinae) age, their efficiency to perform general tasks decreases, while they accumulate defensive secretions and increase their readiness to fight. This defensive mechanism involves self-sacrifice through body rupture during which an enzyme, stored as blue crystals in dorsal pouches, converts precursors produced by the labial glands into highly toxic compounds. Here, we identify both components of this activated defense system and describe the molecular basis responsible for the toxicity of N. taracua worker autothysis. The blue crystals are formed almost exclusively by a specific protein named BP76. By matching N. taracua transcriptome databases with amino acid sequences, we identified BP76 to be a laccase. Following autothysis, the series of hydroquinone precursors produced by labial glands get mixed with BP76, resulting in the conversion of relatively harmless hydroquinones into toxic benzoquinone analogues. Neocapritermes taracua workers therefore rely on a two-component activated defense system, consisting of two separately stored secretions that can react only after suicidal body rupture, which produces a sticky and toxic cocktail harmful to opponents.

Knockdown of the juvenile hormone receptor gene inhibits soldier-specific morphogenesis in the damp-wood termite Zootermopsis nevadensis (Isoptera: Archotermopsidae)

The Methoprene-tolerant (Met) protein has been established as a juvenile hormone (JH) receptor. Knockdown of the Met gene caused precocious metamorphosis and suppression of ovarian development. However, the function of Met in caste development of social insects is unclear. In termites, JH acts as a central factor for caste development, especially for soldier differentiation, which involves two molts from workers via a presoldier stage. Increased JH titer in workers is needed for the presoldier molt, and the high JH titer is maintained throughout the presoldier period. Although presoldiers have the fundamental morphological features of soldiers, the nature of the cuticle is completely different from that of soldiers. We expected that JH signals via Met are involved in soldier-specific morphogenesis of the head and mandibles during soldier differentiation, especially in the presoldier period, in natural conditions. To test this hypothesis, we focused on soldier differentiation in an incipient colony of the damp-wood termite Zootermopsis nevadensis. Met homolog (ZnMet) expression in heads increased just after the presoldier molt. This high expression was reduced by ZnMet double stranded (dsRNA) injection before the presoldier molt. Although this treatment did not cause any morphological changes in presoldiers, it caused strong effects on soldiers, their mandibles being significantly shorter and head capsules smaller than those of control soldiers. Injection of ZnMet dsRNA throughout the presoldier stage did not affect the formation of soldier morphology, including cuticle formation. These results suggested that the rapid increase in ZnMet expression and subsequent activation of JH signaling just after the presoldier molt are needed for the formation of soldier-specific weapons. Therefore, besides its established role in insect metamorphosis, the JH receptor signaling also underlies soldier development in termites.

The multicopper oxidase gene family in the brown planthopper, Nilaparvata lugens

The multicopper oxidase (MCO) family of enzymes includes laccases, ascorbate oxidases, bilirubin oxidases and a subgroup of metal oxidases. On the basis of a bioinformatics investigation, we identified 7 genes encoding putative multicopper oxidase proteins in the genome of the brown planthopper (BPH), Nilaparvata lugens (Hemiptera: Delphacidae). MCO1 and MCO2 are conserved, while others diverse in insects. Analysis of developmental and tissue-specific expression patterns revealed the following: NlMCO2 was mainly expressed in the integument, and its expression peaked periodically during molting; NlMCO3 was an ovary-specific MCO gene with a high expression level only at the adult stage; NlMCO4 was a salivary gland-specific MCO gene that was expressed at all developmental stages; NlMCO5 only had short-term expression in the middle of the fourth instar stage and was expressed mainly in the gut; NlMCO6 had a developmental expression pattern similar to that of NlMCO2 and was expressed in most N. lugens tissues; and NlMCO1 was expressed in most N. lugens tissues except for the testis, whereas NlMCO7 was mainly expressed in the gut and the Malpighian tube. BPHs injected with double-stranded RNA (dsRNA) targeting NlMCO2 failed to pigment and sclerotize, were colorless and soft-bodied and subsequently died in a short time. Lethal phenotypes were also observed in insects challenged by dsRNA targeting NlMCO6. However, no observable morphological or internal structural abnormality was obtained in the insects treated with dsRNA for NlMCO1, NlMCO3, NlMCO4, NlMCO5 or NlMCO7.

An Opposite Pattern to the Conventional Thermal Hypothesis: Temperature-Dependent Variation in Coloration of Adults of Saccharosydne procerus (Homoptera: Delphacidae)

Melanism is a common polymorphism in many insect species that also influences immune function. According to the thermal melanin hypothesis, ectothermic individuals from cooler environments have darker cuticles and higher polyphenol oxidase (PO) levels, which represent a better immunocompetence. In this study, the links among environmental temperature, melanism, and PO activity of Saccharosydne procerus (Matsumura) were examined. Most S. procerus have a black spot on their forewings at high temperatures in the field and in the laboratory. In PO activity assay, a positive association between PO level and temperature was found. Our results showed that a diversification of melanism occurred under different temperatures and that melanism in S. procerus presented an opposite pattern to the one proposed by the thermal hypothesis.

Cuticular protein with a low complexity sequence becomes cross-linked during insect cuticle sclerotization and is required for the adult molt

In the insect cuticle, structural proteins (CPs) and the polysaccharide chitin are the major components. It has been hypothesized that CPs are cross-linked to other CPs and possibly to chitin by quinones or quinone methides produced by the laccase2-mediated oxidation of N-acylcatechols. In this study we investigated functions of TcCP30, the third most abundant CP in protein extracts of elytra (wing covers) from Tribolium castaneum adults. The mature TcCP30 protein has a low complexity and highly polar amino acid sequence. TcCP30 is localized with chitin in horizontal laminae and vertically oriented columnar structures in rigid cuticles, but not in soft and membranous cuticles. Immunoblot analysis revealed that TcCP30 undergoes laccase2-mediated cross-linking during cuticle maturation in vivo, a process confirmed in vitro using recombinant rTcCP30. We identified TcCPR27 and TcCPR18, the two most abundant proteins in the elytra, as putative cross-linking partners of TcCP30. RNAi for the TcCP30 gene had no effect on larval and pupal growth and development. However, during adult eclosion, ~70% of the adults were unable to shed their exuvium and died. These results support the hypothesis that TcCP30 plays an integral role as a cross-linked structural protein in the formation of lightweight rigid cuticle of the beetle.

Re-examination of a α-chymotrypsin-solubilized laccase in the pupal cuticle of the silkworm, Bombyx mori: Insights into the regulation system for laccase activation during the ecdysis process

The laccase in the pupal cuticle of the silkworm, Bombyx mori, is thought to accumulate as an inactive precursor that can be activated stage-dependently. In this study we isolated an 81-kDa laccase from cuticular extract of B. mori that was prepared by digestion of the pupal cuticles with α-chymotrypsin. The mass spectrometric analysis of the purified protein indicates that this 81-kDa laccase is a product of the Bombyx laccase2 gene. The purified 81-kDa laccase (α-chymotrypsin-solubilized Bombyx laccase2: Bm-clac2) has an N-terminal sequence of RNPADS that corresponds to Arg146 to Ser151 of the deduced protein sequence of Bmlaccase2 cDNA, indicating that Bm-clac2 lacks the N-terminal part upstream from residue Arg146. Bm-clac2 shows enzymatic activity, but its specific activity is increased around 17-fold after treatment with trypsin, which involves cleavage of peptide bonds at the C-terminal region. We also found that the activity of Bm-clac2 is increased in the presence of isopropanol. In previous reports, proteolytic processing has been hypothesized as a system for laccase activation in vivo, but the present result implies that this type of processing is not the only way to convert Bm-clac2 to the high-activity enzyme.