Caste-specific cytochrome P450 in the damp-wood termite Hodotermopsis sjostedti (Isoptera, Termopsidae)

Genome-Wide Identification and Expression Pattern of Cytochrome P450 Genes in the Social Aphid Pseudoregma bambucicola

Cytochrome P450 monooxygenases (P450s) have a variety of functions, including involvement in the metabolism of exogenous substances and the synthesis and degradation of endogenous substances, which are important for the growth and development of insects. Pseudoregma bambucicola is a social aphid that produces genetically identical but morphologically and behaviorally distinct first-instar soldiers and normal nymphs within colonies. In this study, we identified 43 P450 genes based on P. bambucicola genome data. Phylogenetic analysis showed that these genes were classified into 4 clans, 13 families, and 23 subfamilies. The CYP3 and CYP4 clans had a somewhat decreased number of genes. In addition, differential gene expression analysis based on transcriptome data showed that several P450 genes, including CYP18A1, CYP4G332, and CYP4G333, showed higher expression levels in soldiers compared to normal nymphs and adult aphids. These genes may be candidates for causing epidermal hardening and developmental arrest in soldiers. This study provides valuable data and lays the foundation for the study of functions of P450 genes in the social aphid P. bambucicola.

Silencing of CYP6AS160 in Solenopsis invicta Buren by RNA interference enhances worker susceptibility to fipronil

Cytochrome P450 monooxygenases play a key role in pest resistance to insecticides by detoxification. Four new P450 genes, CYP6AS160, CYP6AS161, CYP4AB73 and CYP4G232 were identified from Solenopsis invicta. CYP6AS160 was highly expressed in the abdomen and its expression could be induced significantly with exposure to fipronil, whereas CYP4AB73 was not highly expressed in the abdomen and its expression could not be significantly induced following exposure to fipronil. Expression levels of CYP6AS160 and CYP4AB73 in workers were significantly higher than that in queens. RNA interference-mediated gene silencing by feeding on double-stranded RNA (dsRNA) found that the levels of this transcript decreased (by maximum to 64.6%) when they fed on CYP6AS160-specific dsRNA. Workers fed dsCYP6AS160 had significantly higher mortality after 24 h of exposure to fipronil compared to controls. Workers fed dsCYP6AS160 had reduced total P450 activity of microsomal preparations toward model substrates p-nitroanisole. However, the knockdown of a non-overexpressed P450 gene, CYP4AB73 did not lead to an increase of mortality or a decrease of total P450 activity. The knockdown effects of CYP6AS160 on worker susceptibility to fipronil, combined with our other findings, indicate that CYP6AS160 is responsible for detoxification of fipronil. Feeding insects dsRNA may be a general strategy to trigger RNA interference and may find applications in entomological research and in the control of insect pests in the field.

Expression profiles of neotropical termites reveal microbiota-associated, caste-biased genes and biotechnological targets

Termites are well recognized by their complex development trajectories, involving dynamic differentiation process between non-reproductive castes, workers and soldiers. These insects are associated with endosymbiotic microorganisms, which help in lignocellulose digestion and nitrogen metabolism. Aiming to identify genes harbouring biotechnological potential, we analyzed workers and soldiers RNA-Seq data of three neotropical termites: Heterotermes tenuis (Isoptera: Rhinotermitidae), Velocitermes heteropterus (Isoptera: Termitidae) and Cornitermes cumulans (Isoptera: Termitidae). We observed differences in the microbiota associated with each termite family, and found protists' genes in both Termitidae species. We found an opposite pattern of caste-biased gene expression between H. tenuis and the termitids studied. Moreover, the two termitids are considerably different concerning the number of differentially expressed genes (DEGs). Functional annotation indicated considerable differences in caste-biased gene content between V. heteropterus and C. cumulans, even though they share similar diet and biological niche. Among the most DEGs, we highlighted those involved in caste differentiation and cellulose digestion, which are attractive targets for studying more efficient technologies for termite control, biomass digestion and other biotechnological applications.

Methoprene-Induced Genes in Workers of Formosan Subterranean Termites (Coptotermes formosanus Shiraki)

Termites have a distinct polyphenism controlled by concise hormonal and molecular mechanisms. Workers undergo double molts to transform into soldiers (worker–presoldier–soldier). Juvenile hormone analogs, such as methoprene, can induce workers to transform into presoldiers. However, the molecular mechanism underlying the worker-to-presoldier transformation in Coptotermes formosanus Shiraki is still not clear. We sequenced the transcriptome of workers four days after they had fed on methoprene-treated filter paper and control group workers, which fed on acetone-treated filter paper. The transcriptome of C. formosanus was assembled using the de novo assembly method. Expression levels of unigenes in the methoprene-treated group and the control group were compared. The differentially expressed genes were further analyzed by Gene Ontology (GO) term enrichment analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. Tetrapyrrole binding, oxidoreductase activity, and metal ion binding were the only three enriched GO terms. Juvenile hormone synthesis was the first ranked enriched pathway. Carbohydrate, amino acid, and lipid metabolism pathways were also enriched. These three pathways may be related to fat body development, which is critical for presoldier formation. Our results have demonstrated the significance of JH synthesis pathways, and pathways related to fat body development in the artificial induction of presoldiers.

Screening of reference genes for expression analysis in the study of soldier caste differentiation of Formosan subterranean termite Coptotermes formosanus Shiraki

The Formosan subterranean termite, Coptotermes formosanus Shiraki, is a serious pest worldwide. Juvenile hormone analog (and its analogs such as methoprene) can induce the transformation of the worker caste into soldier caste in C. formosanus. However, several factors, such as feeding substrate and colony origin, influence the proportion of solider formation. The molecular mechanism of worker to soldier transformation of C. formosanus is still not clear. RT-qPCR is a powerful tool for molecular studies. Accurate gene quantification by the relative quantification method requires a stable expressed gene as the reference gene. However, no reference genes were available for this species in the methoprene bioassay. To study the problem of gene response to methoprene by RT-qPCR we have to first screen reference genes in C. formosanus. Workers were fed with methoprene. Termites were collected during the methoprene bioassay and separated into head and thorax+abdomen. Expression profiles of 10 candidate reference genes in the two body part types were investigated using RT-qPCR. The results were analyzed by a set of established methods (geNorm, NormFinder, BestKeeper, and RefFinder) as well as comparative ΔCt method. Our results suggest that RPS18 is the most stably-expressed gene both in the head and thorax+abdomen for expression analysis in the methoprene bioassay of C. formosanus. The screening of suitable reference genes in C. formosanus establishes the foundation for the molecular study of soldier caste differentiation in this species.

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.

Cloning and Expression of Multiple Cytochrome P450 Genes: Induction by Fipronil in Workers of the Red Imported Fire Ant (Solenopsis invicta Buren)

Both exogenous and endogenous compounds can induce the expression of cytochrome P450 genes. The insect cytochrome P450 genes related to insecticide resistance are likely to be expressed as the “first line of defense” when challenged with insecticides. In this study, four cytochrome P450 genes, SinvCYP6B1, SinvCYP6A1, SinvCYP4C1, and SinvCYP4G15, were firstly isolated from workers of the red imported fire ant (Solenopsis invicta) through rapid amplification of cDNA ends (RACE) and sequenced. The fipronil induction profiles of the four cytochrome P450 genes and the two previously isolated CYP4AB1 and CYP4AB2 were characterized in workers. The results revealed that the expression of SinvCYP6B1, SinvCYP6A1, CYP4AB2, and SinvCYP4G15, increased 1.4-fold and 1.3-fold more than those of acetone control, respectively, after 24 h exposure to fipronil at concentrations of 0.25 μg mL⁻¹ (median lethal dose) and 0.56 μg mL⁻¹ (90% lethal dose), while no significant induction of the expression of CYP4AB1 and SinvCYP4C1 was detected. Among these genes, SinvCYP6B1 was the most significantly induced, and its maximum expression was 3.6-fold higher than that in acetone control. These results might suggest that multiple cytochrome P450 genes are co-up-regulated in workers of the fire ant through induction mechanism when challenged with fipronil. These findings indicated that cytochrome P450 genes play an important role in the detoxification of insecticides and provide a theoretical basis for the mechanisms of insecticide metabolism in the fire ant.

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.

Omic research in termites: an overview and a roadmap

Many recent breakthroughs in our understanding of termite biology have been facilitated by "omics" research. Omic science seeks to collectively catalog, quantify, and characterize pools of biological molecules that translate into structure, function, and life processes of an organism. Biological molecules in this context include genomic DNA, messenger RNA, proteins, and other biochemicals. Other permutations of omics that apply to termites include sociogenomics, which seeks to define social life in molecular terms (e.g., behavior, sociality, physiology, symbiosis, etc.) and digestomics, which seeks to define the collective pool of host and symbiont genes that collaborate to achieve high-efficiency lignocellulose digestion in the termite gut. This review covers a wide spectrum of termite omic studies from the past 15 years. Topics covered include a summary of terminology, the various kinds of omic efforts that have been undertaken, what has been revealed, and to a degree, what the results mean. Although recent omic efforts have contributed to a better understanding of many facets of termite and symbiont biology, and have created important new resources for many species, significant knowledge gaps still remain. Crossing these gaps can best be done by applying new omic resources within multi-dimensional (i.e., functional, translational, and applied) research programs.

Exaggerated trait growth in insects

Animal structures occasionally attain extreme proportions, eclipsing in size the surrounding body parts. We review insect examples of exaggerated traits, such as the mandibles of stag beetles (Lucanidae), the claspers of praying mantids (Mantidae), the elongated hindlimbs of grasshoppers (Orthoptera: Caelifera), and the giant heads of soldier ants (Formicidae) and termites (Isoptera). Developmentally, disproportionate growth can arise through trait-specific modifications to the activity of at least four pathways: the sex determination pathway, the appendage patterning pathway, the insulin/IGF signaling pathway, and the juvenile hormone/ecdysteroid pathway. Although most exaggerated traits have not been studied mechanistically, it is already apparent that distinct developmental mechanisms underlie the evolution of the different types of exaggerated traits. We suggest this reflects the nature of selection in each instance, revealing an exciting link between mechanism, form, and function. We use this information to make explicit predictions for the types of regulatory pathways likely to underlie each type of exaggerated trait.

Termites as targets and models for biotechnology

Termites have many unique evolutionary adaptations associated with their eusocial lifestyles. Recent omics research has created a wealth of new information in numerous areas of termite biology (e.g., caste polyphenism, lignocellulose digestion, and microbial symbiosis) with wide-ranging applications in diverse biotechnological niches. Termite biotechnology falls into two categories: (a) termite-targeted biotechnology for pest management purposes, and (b) termite-modeled biotechnology for use in various industrial applications. The first category includes several candidate termiticidal modes of action such as RNA interference, digestive inhibition, pathogen enhancement, antimicrobials, endocrine disruption, and primer pheromone mimicry. In the second category, termite digestomes are deep resources for host and symbiont lignocellulases and other enzymes with applications in a variety of biomass, industrial, and processing applications. Moving forward, one of the most important approaches for accelerating advances in both termite-targeted and termite-modeled biotechnology will be to consider host and symbiont together as a single functional unit.

Effects of caste on the expression of genes associated with septic injury and xenobiotic exposure in the Formosan subterranean termite

As social insects, termites live in densely populated colonies with specialized castes under conditions conducive to microbial growth and transmission. Furthermore, termites are exposed to xenobiotics in soil and their lignocellulose diet. Therefore, termites are valuable models for studying gene expression involved in response to septic injury, immunity and detoxification in relation to caste membership. In this study, workers and soldiers of the Formosan subterranean termite, Coptotermes formosanus, were challenged by bacterial injection or by no-choice feeding with a sublethal concentration (0.5%) of phenobarbital. Constitutive and induced expression of six putative immune response genes (two encoding for lectin-like proteins, one for a ficolin-precursor, one for the Down syndrome cell adhesion molecule, one for a chitin binding protein, and one for the gram-negative binding protein 2) and four putative detoxification genes (two encoding for cytochrome P450s, one for glutathione S-transferase, and one for the multi antimicrobial extrusion protein), were measured via quantitative real time polymerase chain reaction and compared within and among 1) colonies, 2) treatment types and 3) castes via ANOVA. Eight genes were inducible by septic injury, feeding with phenobarbital or both. Colony origin had no effect on inducibility or differential gene expression. However, treatment type showed significant effects on the expression of the eight inducible genes. Caste effects on expression levels were significant in five of the eight inducible genes with constitutive and induced expression of most target genes being higher in workers than in soldiers.

The scent of royalty: a p450 gene signals reproductive status in a social insect

Cooperation requires communication; this applies to animals and humans alike. The main communication means differ between taxa and social insects (ants, termites, and some bees and wasps) lack the cognitive abilities of most social vertebrates. Central to the regulation of the reproductive harmony in insect societies is the production of a royalty scent which signals the fertility status of the reproducing queen to the nonreproducing workers. Here, we revealed a central genetic component underlying this hallmark of insect societies in the termite Cryptotermes secundus. Communication between queens and workers relied upon the expression of a gene, Neofem4, which belongs to the cytochrome P450 genes. We inhibited Neofem4 in queens by RNA interference. This resulted in the loss of the royalty scent in queens and the workers behaved as though the queen were absent. The queen's behavior was not generally affected by silencing Neofem4. This suggests that the lack of the royalty scent lead to workers not recognizing her anymore as queen. P450 genes are known to be involved in the production of chemical signals in cockroaches and their expression has been linked to a major fertility regulator, juvenile hormone. This makes P450 genes, both a suitable and available evolutionary substrate in the face of natural selection for production of a queen substance. Our data suggest that in an organism without elaborate cognitive abilities communication has been achieved by the exploitation of a central gene that links the fertility network with the chemical communication pathway. As termites and social Hymenoptera seem to share the same class of compounds in signaling fertility, this role of P450 genes might be more widespread across social insects.

Social interactions affecting caste development through physiological actions in termites

A colony of social insects is not only an aggregation of individuals but also a functional unit. To achieve adaptive social behavior in fluctuating environmental conditions, in addition to coordination of physiological status in each individual, the whole colony is coordinated by interactions among colony members. The study on the regulation of social-insect colonies is termed "social physiology." Termites, a major group of social insects, exhibit many interesting phenomena related to social physiology, such as mechanisms of caste regulation in a colony. In their colonies, there are different types of individuals, i.e., castes, which show distinctive phenotypes specialized in specific colony tasks. Termite castes comprise reproductives, soldiers and workers, and the caste composition can be altered depending on circumstances. For the regulation of caste compositions, interactions among individuals, i.e., social interactions, are thought to be important. In this article, we review previous studies on the adaptive meanings and those on the proximate mechanisms of the caste regulation in termites, and try to understand those comprehensively in terms of social physiology. Firstly, we summarize classical studies on the social interactions. Secondly, previous studies on the pheromone substances that mediate the caste regulatory mechanisms are overviewed. Then, we discuss the roles of a physiological factor, juvenile hormone (JH) in the regulation of caste differentiation. Finally, we introduce the achievements of molecular studies on the animal sociality (i.e., sociogenomics) in terms of social physiology. By comparing the proximate mechanisms of social physiology in termites with those in hymenopterans, we try to get insights into the general principles of social physiology in social animals.

Biased gene expression in early honeybee larval development

Background

Female larvae of the honeybee (Apis mellifera) develop into either queens or workers depending on nutrition. This nutritional stimulus triggers different developmental trajectories, resulting in adults that differ from each other in physiology, behaviour and life span.

Results

To understand how these trajectories are established we have generated a comprehensive atlas of gene expression throughout larval development. We found substantial differences in gene expression between worker and queen-destined larvae at 6 hours after hatching. Some of these early changes in gene expression are maintained throughout larval development, indicating that caste-specific developmental trajectories are established much earlier than previously thought. Within our gene expression data we identified processes that potentially underlie caste differentiation. Queen-destined larvae have higher expression of genes involved in transcription, translation and protein folding early in development with a later switch to genes involved in energy generation. Using RNA interference, we were able to demonstrate that one of these genes, hexamerin 70b, has a role in caste differentiation. Both queen and worker developmental trajectories are associated with the expression of genes that have alternative splice variants, although only a single variant of a gene tends to be differentially expressed in a given caste.

Conclusions

Our data, based on the biases in gene expression early in development together with published data, supports the idea that caste development in the honeybee consists of two phases; an initial biased phase of development, where larvae can still switch to the other caste by differential feeding, followed by commitment to a particular developmental trajectory.

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

Termite soldiers are morphologically specialized for colony defense. Analysis of the mechanisms of soldier differentiation is important for understanding the establishment of termite societies. Soldiers differentiate from workers through a presoldier stage and have well-sclerotized and pigmented cuticles. These characteristics are important for nest defense and are likely to be caused by soldier-specific mechanisms of cuticular tanning. The molecular mechanisms leading to cuticular tanning have not been elucidated. Laccase2 (Lac2) plays important roles in this process in insects, and we hypothesized that Lac2 expression may be involved in soldier-specific cuticular tanning. We observed inner and outer head cuticle changes and compared the Lac2 expression patterns among three molts (worker-worker, worker-presoldier and presoldier-soldier) in the termite Reticulitermes speratus. Quantitative analyses of head cuticle colors showed that the color properties changed more conspicuously in presoldier-soldier molts than in the other two molts. Histological observations showed that the exocuticles of soldier heads were substantially thicker than those of worker and presoldier heads, underwent tanning before or just after ecdysis, and were pigmented at earlier time points than other molts. Finally, markedly higher Lac2 expression levels were observed just before and after ecdysis only in the presoldier-soldier molt. These results suggest that specific cuticular formation occurs in the exocuticles during soldier differentiation, and that the high level of Lac2 expression during the presoldier-soldier molt is related to soldier-specific cuticular tanning. We speculate that evolution of the regulatory mechanisms of Lac2 expression were important for the acquisition of soldier-specific cuticles.

Soldier morphogenesis in the damp-wood termite is regulated by the insulin signaling pathway

Eusocial insects exhibit various morphological castes associated with the division of labor within a colony. Termite soldiers possess defensive traits including mandibles that are greatly exaggerated and enlarged, as compared to termite reproductives and workers. The enlarged mandibles of soldiers are known to result from dynamic morphogenesis during soldier differentiation that can be induced by juvenile hormone and its analogs. However, the detailed developmental mechanisms still remain unresolved. Because the insulin/insulin-like growth factor signaling (IIS) pathway has been shown to regulate the relative sizes of organs (i.e., allometry) in other insects, we examined the expression profiles of major IIS factors in the damp-wood termite Hodotermopsis sjostedti, during soldier differentiation. The relative expression patterns of orthologs for termite InR (HsjInR), PKB/Akt (HsjPKB/Akt), and FOXO (HsjFOXO) suggest that HsjInR and HsjPKB/Akt were up-regulated in the period of elongation of mandibles during soldier development. In situ hybridization showed that HsjInR was strongly expressed in the mandibular epithelial tissues, and RNA interference (RNAi) for HsjInR disrupted soldier-specific morphogenesis including mandibular elongation. These results suggest that signaling through the IIS pathway is required for soldier-specific morphogenesis. In addition, up-regulation of the IIS pathway in other body tissues occurred at earlier stages of development, indicating that there is tissue-specific IIS regulation. Because the IIS pathway is generally thought to act upstream of JH in insects, our results suggest the damp-wood termite may have evolved a novel feedback loop between JH and IIS that enables social interactions, rather than nutrition, to regulate caste determination.

Differential gene expression in response to juvenile hormone analog treatment in the damp-wood termite Hodotermopsis sjostedti (Isoptera, Archotermopsidae)

Termite societies are characterized by a highly organized division of labor among conspicuous castes, groups of individuals with various morphological specializations. Termite caste differentiation is under control of juvenile hormone (JH), but the molecular mechanism underlying the response to JH and early events triggering caste differentiation are still poorly understood. In order to profile candidate gene expression during early soldier caste differentiation of the damp-wood termite, Hodotermopsis sjostedti, we treated pseudergates (workers) with a juvenile hormone analog (JHA) to induce soldier caste differentiation. We then used Suppressive Subtractive Hybridization to create two cDNA libraries enriched for transcripts that were either up- or downregulated at 24h after treatment. Finally, we used quantitative PCR to confirm temporal expression patterns. Hexamerins represent a large proportion of the genes upregulated following JHA treatment and have an expression pattern that shows roughly an inverse correlation to intrinsic JH titers. This data is consistent with the role of a JH "sink", which was demonstrated for hexamerins in another termite, Reticulitermes flavipes. A putative nuclear protein was also upregulated a few hours after JHA treatment, which suggests a role in the early response to JH and subsequent regulation of transcriptional events associated with soldier caste differentiation. Some digestive enzymes, such as endogenous beta-endoglucanase and chymotrypsin, as well as a protein associated to digestion were identified among genes downregulated after JHA treatment. This suggests that JH may directly influence the pseudergate-specific digestive system.

Cyp15F1: a novel cytochrome P450 gene linked to juvenile hormone-dependent caste differention in the termite Reticulitermes flavipes

Termites are eusocial insects that jointly utilize juvenile hormone (JH), pheromones, and other semiochemicals to regulate caste differentiation and achieve caste homeostasis. Prior EST sequencing from the symbiont-free gut transcriptome of Reticulitermes flavipes unexpectedly revealed a number of unique cytochrome P450 (Cyp) transcripts, including fragments of a Cyp15 family gene (Cyp15F1) with homology to other insect Cyp15s that participate in JH biosynthesis. The present study investigated the role of Cyp15F1 in termite caste polyphenism and specifically tested the hypothesis that it plays a role in JH-dependent caste differentiation. After assembling the full-length Cyp15F1 cDNA sequence, we (i) determined its mRNA tissue expression profile, (ii) investigated mRNA expression changes in response to JH and the caste-regulatory primer pheromones γ-cadinene (CAD) and γ-cadinenal (ALD), and (iii) used RNA interference (RNAi) in combination with caste differentiation bioassays to investigate gene function at the phenotype level. Cyp15F1 has ubiquitous whole-body expression (including gut tissue); is rapidly and sustainably induced from 3 h to 48 h by JH, CAD, and ALD; and functions at least in part by facilitating JH-dependent soldier caste differentiation. These findings provide the second example of a termite caste regulatory gene identified through the use of RNAi, and significantly build upon our understanding of termite caste homeostatic mechanisms. These results also reinforce the concept of environmental caste determination in termites by revealing how primer pheromones, as socioenvironmental factors, can directly influence Cyp15 expression and caste differentiation.

Two novel cytochrome P450 genes CYP6CS1 and CYP6CW1 from Nilaparvata lugens (Hemiptera: Delphacidae): cDNA cloning and induction by host resistant rice

Two novel full-length P450 cDNAs, CYP6CS1 and CYP6CW1, were cloned from the fourth instar nymphs of brown planthopper Nilaparvata lugens Stål (Hemiptera: Delphacidae) reared on its susceptible rice variety Taichung Native 1 (TN1) plants. The deduced proteins are typical microsomal P450s sharing conserved structural and functional domains with other insect CYP6 members. Temporal expression analysis by northern blot hybridization indicated pre-exposure to N. lugens moderately resistant rice Minghui 63 (MH63) seedlings caused a time course-dependent induction of CYP6CS1 which peaked after 24 h of treatment; in contrast, CYP6CW1 was induced and remained at a constant time course from 0-72 h. CYP6CS1 and CYP6CW1 are dramatically induced in gut tissues and, slightly upregulated in carcass and fat bodies as revealed in spatial gene expression analysis. Whole mount in situ hybridizaion revealed that the two genes are expressed at a basal level in gut tissue and Malpighian tubules in nymphs fed with TN1 rice. After exposure to MH63, the expression of CYP6CW1 was found to be high in the whole gut, including Malpighian tubules. Expression of CYP6CS1 was significantly increased in midgut, and slightly increased in foregut, hindgut and Malpighian tubules. These data suggest a potential role of the two P450s in determining patterns of N. lugens-rice relationships through allelochemical detoxification.

Comparative analysis of expressed sequence tags from three castes and two life stages of the termite Reticulitermes flavipes

Background

Termites (Isoptera) are eusocial insects whose colonies consist of morphologically and behaviorally specialized castes of sterile workers and soldiers, and reproductive alates. Previous studies on eusocial insects have indicated that caste differentiation and behavior are underlain by differential gene expression. Although much is known about gene expression in the honey bee, Apis mellifera, termites remain relatively understudied in this regard. Therefore, our objective was to assemble an expressed sequence tag (EST) data base for the eastern subterranean termite, Reticulitermes flavipes, for future gene expression studies.

Results

Soldier, worker, and alate caste and two larval cDNA libraries were constructed, and approximately 15,000 randomly chosen clones were sequenced to compile an EST data base. Putative gene functions were assigned based on a BLASTX Swissprot search. Categorical in silico expression patterns for each library were compared using the R-statistic. A significant proportion of the ESTs of each caste and life stages had no significant similarity to those in existing data bases. All cDNA libraries, including those of non-reproductive worker and soldier castes, contained sequences with putative reproductive functions. Genes that showed a potential expression bias among castes included a putative antibacterial humoral response and translation elongation protein in soldiers and a chemosensory protein in alates.

Conclusions

We have expanded upon the available sequences for R. flavipes and utilized an in silico method to compare gene expression in different castes of an eusocial insect. The in silico analysis allowed us to identify several genes which may be differentially expressed and involved in caste differences. These include a gene overrepresented in the alate cDNA library with a predicted function of neurotransmitter secretion or cholesterol absorption and a gene predicted to be involved in protein biosynthesis and ligase activity that was overrepresented in the late larval stage cDNA library. The EST data base and analyses reported here will be a valuable resource for future studies on the genomics of R. flavipes and other termites.

The homolog of Ciboulot in the termite (Hodotermopsis sjostedti): a multimeric beta-thymosin involved in soldier-specific morphogenesis

Background

Caste differentiation in social insects is a type of polyphenism that enables division of labor among members of a colony. This elaborate social integration has attracted broad interest, although little is known about its regulatory mechanisms, especially in Isoptera (termites). In this study, we analyzed soldier differentiation in the damp-wood termite Hodotermopsis sjostedti, focusing on a possible effector gene for caste development. The gene for an actin-binding protein, HsjCib, which shows a high level of expression in developing mandibles during soldier differentiation, is characterized in detail.

Results

To examine the HsjCib gene, full-length cDNAs were obtained by rapid amplification of cDNA ends-polymerase chain reaction (RACE-PCR) and sequencing. Multiple isoforms were identified, and on the basis of the results of northern and Southern hybridization analyses, these isoforms were considered to be transcriptional variants from a single gene. On the basis of their sequence similarity to homologous genes of other organisms, functions in actin assembly were assumed to be different among isoforms. Expression analysis revealed high expression in the head during soldier differentiation, which was consistent with their allometric growth. Although isoform expression was observed in various tissues, different expression levels were observed among tissues, suggesting the possibility of tissue-specific morphogenetic regulation by HsjCib isoforms.

Conclusion

This study revealed the characteristics and dynamics of the HsjCib gene during soldier differentiation as a potential representative of downstream effector genes in caste-specific morphogenesis. From the expression patterns observed, this gene is considered to be involved in cephalic morphogenesis and neural reorganization, resulting in the establishment of caste-specific morphology and behavior.

Gene expression changes during caste-specific neuronal development in the damp-wood termite Hodotermopsis sjostedti

BACKGROUND

One of the key characters of social insects is the division of labor, in which different tasks are allocated to various castes. In termites, one of the representative groups of social insects, morphological differences as well as behavioral differences can be recognized among castes. However, very little is known about the neuronal and molecular bases of caste differentiation and caste-specific behavior. In almost all termite species, soldiers play defensive roles in their colonies, and their morphology and behavior are largely different from workers (or pseudergates). Therefore, we predicted that some genes linked to defensive behavior and/or those required for neuronal changes are differentially expressed between workers and soldiers, or during the soldier differentiation, respectively.

RESULTS

Using the brain and suboesophageal ganglion (SOG) of the damp-wood termite Hodotermopsis sjostedti, we first screened genes specifically expressed in soldiers or during soldier differentiation by the differential display method, followed by quantitative real-time polymerase chain reaction. No distinctive differences in expression patterns were detected between pseudergates and soldiers. In the course of soldier differentiation, however, five genes were found to be up-regulated in brain and/or SOG: 14-3-3 epsilon, fibrillin2, beta-tubulin, ciboulot, and a hypothetical protein containing a SAP motif. Some of these genes are thought to be associated with cytoskeletal structure or motor-associated proteins in neuronal tissues.

CONCLUSION

The identified five genes could be involved in soldier-specific neuronal modifications, resulting in defensive behaviors in termite soldiers. The temporal expression patterns of these genes were consistent with the neuronal changes during soldier differentiation, suggesting that molecular machineries, in which the identified factors would participate, play important roles in behavioral differentiation of termite soldiers.

Comparison of queen-specific gene expression in related lower termite species

The molecular mechanisms regulating caste determination and reproductive division of labor, the hallmarks of insect societies, are poorly defined. The identification of key genes involved in these developmentally important processes will be essential to gain a better understanding of the mechanisms controlling one of the most impressive examples of polyphenism, the caste structure of eusocial species. Here, we applied representational difference analysis of cDNAs, to study differential gene expression between queens (female neotenics) and workers in the dry wood termite Cryptotermes cynocephalus and identified 13 genes that were highly expressed in queens. In addition, we partially cloned several homologous genes of the related termite species Cryptotermes secundus and compared the expression profiles of 10 homologous genes. In most cases, the preferential expression in female neotenics was not conserved between species, despite the close phylogenetic relationship of both Cryptotermes species. It is possible that these genes are associated with known species-specific differences in caste development modes. Only three genes (Neofem1, 2, and 3) showed a conserved and highly preferential expression in female neotenics, suggesting that their products may play important roles in female reproductives, in particular in controlling caste determination and reproductive division of labor.

Life history and development--a framework for understanding developmental plasticity in lower termites

Termites (Isoptera) are the phylogenetically oldest social insects, but in scientific research they have always stood in the shadow of the social Hymenoptera. Both groups of social insects evolved complex societies independently and hence, their different ancestry provided them with different life-history preadaptations for social evolution. Termites, the 'social cockroaches', have a hemimetabolous mode of development and both sexes are diploid, while the social Hymenoptera belong to the holometabolous insects and have a haplodiploid mode of sex determination. Despite this apparent disparity it is interesting to ask whether termites and social Hymenoptera share common principles in their individual and social ontogenies and how these are related to the evolution of their respective social life histories. Such a comparison has, however, been much hampered by the developmental complexity of the termite caste system, as well as by an idiosyncratic terminology, which makes it difficult for non-termitologists to access the literature. Here, we provide a conceptual guide to termite terminology based on the highly flexible caste system of the "lower termites". We summarise what is known about ultimate causes and underlying proximate mechanisms in the evolution and maintenance of termite sociality, and we try to embed the results and their discussion into general evolutionary theory and developmental biology. Finally, we speculate about fundamental factors that might have facilitated the unique evolution of complex societies in a diploid hemimetabolous insect taxon. This review also aims at a better integration of termites into general discussions on evolutionary and developmental biology, and it shows that the ecology of termites and their astounding phenotypic plasticity have a large yet still little explored potential to provide insights into elementary evo-devo questions.

Juvenile hormone titers and caste differentiation in the damp-wood termite Hodotermopsis sjostedti (Isoptera, Termopsidae)

Termites are social insects, presenting morphologically distinct castes, performing specific tasks in the colony. The developmental processes underlying caste differentiation are mainly controlled by juvenile hormone (JH). Although many fragmentary data support this fact, there was no comparative work on JH titers during the caste differentiation processes. In this study, JH titer variation was investigated using a liquid chromatography-mass spectrometry (LC-MS) quantification method in all castes of the Japanese damp-wood termite Hodotermopsis sjostedti, especially focusing on the soldier caste differentiation pathway, which was induced by treatment with a JH analog. Hemolymph JH titers fluctuated between 20 and 720pg/microl. A peak of JH was observed during molting events for the pseudergate stationary molt and presoldier differentiation, but this peak was absent prior to the imaginal molt. Soldier caste differentiation was generally associated with high JH titers and nymph to alate differentiation with low JH titers. However, JH titer rose in females during alate maturation, probably in relation to vitellogenesis. In comparison, JH titer was surprisingly low in neotenics. On the basis of these results in both natural and artificial conditions, the current model for JH action on termite caste differentiation is discussed and re-appraised.

Differential expression of CYP6A5 and CYP6A5v2 in pyrethroid-resistant house flies, Musca domestica

Two cytochrome P450 alleles, CYP6A5 and CYP6A5v2, were isolated from a pyrethroid-resistant house fly stain, ALHF. The two alleles shared 98% similarity in amino acid sequence. To understand the importance of these two alleles in resistance and examine the expression profile of the two alleles between resistant and susceptible strains, quantitative real-time PCR (qRT-PCR) was performed and compared with the Northern blot analysis. We found that qRT-PCR was an efficient method to characterize the expression profiles between these two sequence-closely-related P450 genes between resistant and susceptible houses flies. One of them, CYP6A5v2, was constitutively overexpressed in ALHF house flies compared with susceptible house fly strains. Moreover, this gene was predominantly expressed in the abdominal tissues of ALHF, in which the primary detoxification organs of insects are located. However, there was no significant difference in the expression of CYP6A5 between ALHF and susceptible house flies. The genetic linkage analysis was conducted to determine the possible link between the constitutively overexpressed CYP6A5v2 and insecticide resistance. CYP6A5v2 was mapped on autosome 5, which is correlated with the linkage of resistance in ALHF. Taken together, the study suggests the importance of CYP6A5v2 in increasing metabolic detoxification of insecticides in ALHF. The distinct expression of CYP6A5 and CYP6A5v2 in resistant and susceptible house flies implies the functional difference of theses two genes in house flies and suggests that they are two recently diverged P450 genes presented in a single organism.

Characterization of two novel cytochrome P450 genes in insecticide-resistant house-flies

Two novel P450 cDNAs, CYP6A36 and CYP6A37, were isolated from house-flies. Putative protein sequences deduced from the cDNA sequences shared 58% identity. Predicted protein sequences of CYP6A36 and CYP6A37 from pyrethroid-resistant ALHF house-flies were identical to their corresponding orthologues in susceptible aabys flies. Expression of CYP6A36 was developmentally regulated with significant overexpression in ALHF compared with susceptible CS flies. Overexpression of CYP6A36 was detected in ALHF abdomen, where the primary detoxification organs of the midgut and fat body are located. CYP6A37, however, was expressed in all tested life stages, with no significant difference in expression between ALHF and CS. Genetic linkage analysis located CYP6A36 on autosome 5; overexpression of CYP6A36 was linked to the factors on autosomes 1 and 2, corresponding to the linkage of P450-mediated resistance in ALHF. This evidence suggests the importance of CYP6A36 in detoxification of insecticides and evolution of insecticide resistance in ALHF.

Molecular basis for the reproductive division of labour in a lower termite

Background

Polyphenism, the expression of different phenotypes with the same genetic background, is well known for social insects. The substantial physiological and morphological differences among the castes generally are the result of differential gene expression. In lower termites, workers are developmentally flexible to become neotenic replacement reproductives via a single moult after the death of the founding reproductives. Thus, both castes (neotenics and workers) are expected to differ mainly in the expression of genes linked to reproductive division of labour, which constitutes the fundamental basis of insect societies.

Results

Representational difference analysis of cDNAs was used to study differential gene expression between neotenics and workers in the drywood termite Cryptotermes secundus (Kalotermitidae). We identified and, at least partially cloned five novel genes that were highly expressed in female neotenics. Quantitative real-time PCR analysis of all five genes in different castes (neotenics, founding reproductives, winged sexuals and workers of both sexes) confirmed the differential expression patterns. In addition, the relative expression of these genes was determined in three body parts of female neotenics (head, thorax, and abdomen) using quantitative real-time PCR.

Conclusion

The identified genes could be involved in the control and regulation of reproductive division of labour. Interestingly, this study revealed an expression pattern partly similar to social Hymenoptera indicating both common and species-specific regulatory mechanisms in hemimetabolous and holometabolous social insects.

Hexamerin-based regulation of juvenile hormone-dependent gene expression underlies phenotypic plasticity in a social insect

Worker termites of the genus Reticulitermes are temporally-arrested juvenile forms that can terminally differentiate into adultsoldier- or reproductive-caste phenotypes. Soldier-caste differentiation is a developmental transition that is induced by high juvenile hormone (JH) titers. Recently, a status quo hexamerin mechanism was identified, which reduces JH efficacy and maximizes colony fitness via the maintenance of high worker-caste proportions. Our goal in these studies was to investigate more thoroughly the influences of the hexamerins on JH-dependent gene expression in termite workers. Our approach involved RNA interference (RNAi), bioassays and quantification of gene expression. We first investigated the expression of 17 morphogenesis-associated genes in response to RNAi-based hexamerin silencing. Hexamerin silencing resulted in significant downstream impacts on 15 out of the 17 genes, suggesting that these genes are members of a JH-responsive genomic network. Next, we compared gene-expression profiles in workers after RNAi-based hexamerin silencing to that of (i) untreated workers that were held away from the colony; and (ii) workers that were also held away from the colony, but with ectopic JH. Here, although there was no correlation between hexamerin silencing and colony-release effects, we observed a significant correlation between hexamerin silencing and JH-treatment effects. These findings provide further evidence supporting the hypothesis that the hexamerins modulate JH availability, thus limiting the impacts of JH on termite caste polyphenism. Results are discussed in a context relative to outstanding questions on termite developmental biology, particularly on regulatory gene networks that respond to JH-, colony- and environmental-cues.

Cloning and expression of a geranylgeranyl diphosphate synthase gene: insights into the synthesis of termite defence secretion

In Nasutitermes takasagoensis, a termite in which soldiers perform specialized chemical defence, Nts19-1 gene is highly expressed exclusively in soldier head. In this study, two types of transcripts for this gene were obtained, and the full-length cDNAs were determined by rapid amplification of cDNA ends (RACE). These transcripts were putative homologues of the geranylgeranyl diphosphate (GGPP) synthase gene, involved in the condensation of dimethylallyl diphosphate with isopentenyl diphosphate in the isoprenoid biosynthetic pathway. The genes were thus termed NtGGPPS1. GGPP is a precursor of diterpenes in plants. In situ hybridization localized NtGGPPS1 expression to the epidermal secretory cells of the frontal gland reservoir where many kinds of diterpenes are produced, suggesting that NtGGPPS1 is involved in the biosynthesis of defence secretion.

Juvenile hormone and colony conditions differentially influence cytochrome P450 gene expression in the termite Reticulitermes flavipes

In lower termites, the worker caste is a totipotent immature stage that is capable of differentiating into other adult caste phenotypes. We investigated the diversity of family 4 cytochrome P450 (CYP4) genes in Reticulitermes flavipes workers, with the specific goal of identifying P450s potentially involved in regulating caste differentiation. Seven novel CYP4 genes were identified. Quantitative real-time PCR revealed the tissue distribution of expression for the seven CYP4s, as well as temporal expression changes in workers in association with a release from colony influences and during juvenile hormone (JH)-induced soldier caste differentiation. Several fat-body-related CYP4 genes were differentially expressed after JH treatment. Still other genes changed expression in association with removal from colony influences, suggesting that primer pheromones and/or other colony influences impact their expression. These findings add to a growing database of candidate termite caste-regulatory genes, and provide explicit evidence that colony factors influence termite gene expression.