Candidate target genes of the male-specific expressed Doublesex in the termite Reticulitermes speratus.

Eusocial insects such as termites, ants, bees, and wasps exhibit a reproductive division of labor. The developmental regulation of reproductive organ (ovaries and testes) is crucial for distinguishing between reproductive and sterile castes. The development of reproductive organ in insects is regulated by sex-determination pathways. The sex determination gene Doublesex (Dsx), encoding transcription factors, plays an important role in this pathway. Therefore, clarifying the function of Dsx in the developmental regulation of sexual traits is important to understand the social evolution of eusocial insects. However, no studies have reported the function of Dsx in hemimetabolous eusocial group termites. In this study, we searched for binding sites and candidate target genes of Dsx in species with available genome information as the first step in clarifying the function of Dsx in termites. First, we focused on the Reticulitermes speratus genome and identified 101 candidate target genes of Dsx. Using a similar method, we obtained 112, 39, and 76 candidate Dsx target genes in Reticulitermes lucifugus, Coptotermes formosanus, and Macrotermes natalensis, respectively. Second, we compared the candidate Dsx target genes between species and identified 37 common genes between R. speratus and R. lucifugus. These included several genes probably involved in spermatogenesis and longevity. However, only a few common target genes were identified between R. speratus and the other two species. Finally, Dsx dsRNA injection resulted in the differential expression of several target genes, including piwi-like protein and B-box type zinc finger protein ncl-1 in R. speratus. These results provide valuable resource data for future functional analyses of Dsx in termites.

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.

Gene expression profiles of chemosensory genes of termite soldier and worker antennae.

Termites have an elaborate social system that involves cooperation and division of labour among colony members. Although this social system is regulated by chemical signals produced in the colony, it remains unclear how these signals are perceived by other members. Signal transduction is well known to be triggered by the reception of odorant molecules by some binding proteins in the antennae, after which, a signal is transmitted to chemosensory receptors. However, there is insufficient information on the role of chemosensory genes involved in signal transduction in termites. Here, we identified the genes involved in chemosensory reception in the termite Reticulitermes speratus and performed a genome-wide comparative transcriptome analysis of worker and soldier antennae. First, we identified 31 odorant-binding proteins (OBPs), and three chemosensory protein A (CheA) from the genome data. Thereafter, we performed RNA sequencing to compare the expression levels of OBPs, CheAs, and previously identified chemosensory receptor genes between worker and soldier antennae. There were no receptor genes with significant differences in expression between castes. However, the expression levels of three non-receptor odorant-detection/binding proteins (OBP, CheA, and Sensory neuron membrane protein) were significantly different between castes. Real-time qPCR (RT-qPCR) analysis using antennae and other head parts confirmed that these genes were highly expressed in soldier antennae. Finally, independent RT-qPCR analysis showed that the expression patterns of these genes were altered in soldiers from different social contexts. Present results suggest that gene expression levels of some non-receptors are affected by both castes and behavioural interactions among colony members in termites.

Evolution and functionalization of vitellogenin genes in the termite Reticulitermes speratus.

Eusociality has been commonly observed in distinct animal lineages. The reproductive division of labor is a particular feature, achieved by the coordination between fertile and sterile castes within the same nest. The sociogenomic approach in social hymenopteran insects indicates that vitellogenin (Vg) has undergone neo-functionalization in sterile castes. Here, to know whether Vgs have distinct roles in nonreproductive castes in termites, we investigated the unique characteristics of Vgs in the rhinotermitid termite Reticulitermes speratus. The four Vgs were identified from R. speratus (RsVg1-4), and RsVg3 sequences were newly identified using the RACE method. Molecular phylogenetic analysis supported the monophyly of the four termite Vgs. Moreover, the termites Vg1-3 and Vg4 were positioned in two different clades. The  dN/dS ratios indicated that the branch leading to the common ancestor of termite Vg4 was under weak purifying selection. Expression analyses among castes (reproductives, workers, and soldiers) and females (nymphs, winged alates, and queens) showed that RsVg1-3 was highly expressed in fertile queens. In contrast, RsVg4 was highly expressed in workers and female nonreproductives (nymphs and winged adults). Localization of RsVg4 messenger RNA was confirmed in the fat body of worker heads and abdomens. These results suggest that Vg genes are functionalized after gene duplication during termite eusocial transition and that Vg4 is involved in nonreproductive roles 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.

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

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

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

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

Extra-pair paternity in the wood-feeding cockroach Cryptocercus punctulatus Scudder: Social but not genetic monogamy.

Subsocial Cryptocercus cockroaches are the sister group to termites and considered to be socially monogamous. Because genetic monogamy is a suggested requirement for evolution of cooperative breeding/eusociality, particularly in hymenopterans, clarification of the mating biology of Cryptocercus would help illuminate evolutionary trends in eusocial insects. To investigate possible extra-pair paternity in C. punctulatus, microsatellite markers were used to analyse offspring parentage, the stored sperm in females and results of experimental manipulation of sperm competition. Extra-pair paternity was common in field-collected families, but a lack of maternal alleles in several nymphs suggests sampling error or adoption. Isolating prereproductive pairs and assaying subsequently produced nymphs confirmed that nymphs lacked alleles from the pair male in 40% of families, with extra-pair male(s) siring 27%-77% of nymphs. Sperm of extra-pair males was detected in the spermatheca of 51% of paired prereproductive females. Mate switching and surgical manipulation of male mating ability indicated a tendency towards last male sperm precedence. Overall, the results demonstrate that about half of young females are serially monogamous during their maturational year, but bond, overwinter and produce their only set of offspring in company of the last mated male (=pair male). Repeated mating by the pair male increases the number of nymphs sired, but because many females use stored sperm of previous copulatory partners to fertilize eggs, pair males extend parental care to unrelated nymphs. The results suggest that genetic monogamy either developed in the termite ancestor after splitting from the Cryptocercus lineage, or that genetic monogamy may not be a strict prerequisite for the evolution of termite eusociality.

Evolutionary transition of doublesex regulation from sex-specific splicing to male-specific transcription in termites.

The sex determination gene doublesex (dsx) encodes a transcription factor with two domains, oligomerization domain 1 (OD1) and OD2, and is present throughout insects. Sex-specific Dsx splicing isoforms regulate the transcription of target genes and trigger sex differentiation in all Holometabola examined to date. However, in some hemimetabolous insects, dsx is not spliced sexually and its sequence is less conserved. Here, to elucidate evolutionary changes in dsx in domain organisation and regulation in termites, we searched genome and/or transcriptome databases for the dsx OD1 and OD2 in seven termite species and their sister group (Cryptocercus woodroaches). Molecular phylogenetic and synteny analyses identified OD1 sequences of termites and C. punctulatus that clustered with dsx of Holometabola and regarded them as dsx orthologues. The Cryptocercus dsx orthologue containing OD2 was spliced sexually, as previously shown in other insects. However, OD2 was not found in all termite dsx orthologues. These orthologues were encoded by a single exon in three termites for which genome information is available; they were not alternatively spliced but transcribed in a male-specific manner in two examined species. Evolution of dsx regulation from sex-specific splicing to male-specific transcription may have occurred at an early stage of social evolution in termites.

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

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.

Duplication and soldier-specific expression of geranylgeranyl diphosphate synthase genes in a nasute termite Nasutitermes takasagoensis.

In the evolutionarily-derived termite subfamily Nasutitermitinae (family Termitidae), soldiers defend their nestmates by discharging polycyclic diterpenes from a head projection called the "nasus." The diterpenes are synthesised in the frontal gland from the precursor geranylgeranyl diphosphate (GGPP), which is generally used for post-translational modification of proteins in animals. In this study, we constructed a comprehensive gene catalogue to search for genes involved in the diterpene biosynthesis by assembling RNA sequencing reads of Nasutitermes takasagoensis, identifying eight gene copies for GGPP synthase (GGPPS). The number of gene copies is much larger in contrast to other related insects. Gene cloning by reverse transcription-PCR and rapid amplification of cDNA ends confirmed that seven GGPPS genes (NtGGPPS1 to NtGGPPS7) have highly variable untranslated regions. Molecular phylogenetic analysis showed that the NtGGPPS7 gene was grouped with homologs obtained from ancestral termites that have only a single copy of the gene, and the NtGGPPS6 gene was grouped with homologs obtained from a basal lineage of termitids, in which soldiers do not synthesise diterpenes. As the sister group to this clade, furthermore, a monophyletic clade included all the other NtGGPPS genes (NtGGPPS1 to NtGGPPS5). Expression analyses revealed that NtGGPPS7 gene was expressed in all the examined castes and tissues, whereas all the other genes were expressed only in the soldier head. These results suggest that gene duplication followed by subfunctionalisation of the GGPPS genes might have accompanied the evolution of chemical defence in the nasute termite lineage.

Histone modifying genes are involved in the molting period during soldier differentiation in Zootermopsis nevadensis.

Caste differentiation in eusocial insects is an outstanding example of phenotypic plasticity. Recent studies indicate that epigenetic regulation, including DNA methylation and histone modification, play a role in the morphological and behavioral polyphenism observed in the caste differentiation of hymenopteran insects. The role of epigenetic regulation in termite caste differentiation, however, is still obscure. In this study, we performed a functional analysis of epigenetic-related genes during soldier differentiation in Zootermopsis nevadensis, for which the entire genome sequence is available. In an incipient colony of this species, the oldest 3rd instar larva (No. 1 larva) always differentiates into a presoldier (intermediate stage of soldier), and the next-oldest 3rd instar larva (No. 2 larva) molts into a 4th instar (which functions as a worker). First, we detected seven epigenetic-related genes with significantly increased expression levels in the soldier-destined No. 1 larvae using RNA-seq data. Second, RNA interference (RNAi) of these seven genes was performed in the No. 1 larvae. RNAi of three histone modifying genes extended the presoldier molting period. Furthermore, these RNAi treatments reduced the expression levels of genes involved in juvenile hormone (JH) synthesis, binding and signaling. These results indicate that epigenetic-related genes do not directly affect termite soldier differentiation; nonetheless, some histone modifying genes have an effect on molting periods, possibly due to the regulation of JH action during soldier differentiation.

Transcriptomic changes during caste development through social interactions in the termite Zootermopsis nevadensis.

One of the most striking examples of phenotypic plasticity is the different phenotypes (i.e., castes) within a same nest of social insects. Castes usually derive from a single genotype initially by receiving social cues among individuals during development. Specific gene expression changes may be involved in caste differentiation, and thus, the regulatory mechanism of these changes should be clarified in order to understand social maintenance and evolution. The damp-wood termite Zootermopsis nevadensis is one of the most important model termite species, due to not only the availability of genomic, transcriptomic, and epigenomic information but also evidence that soldier- and worker-destined individuals can be identified in natural conditions. Given that the nutritional intakes via social interactions are crucial for caste differentiation in this species, there is a possibility that transcriptomic changes are influenced by the nutritional difference among these individuals. Here, whole body RNA-seq analysis of 3rd-instar larvae with biological replications and Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses were conducted. We found the drastic expression differences during caste developments between soldier- and worker-destined individuals. The results indicated that there are several key signaling pathways responsible for caste formations, which are involved in developments and social interactions. Particularly, the nutritional sensitive signaling was upregulated in soldier-destined individuals, while some metabolic pathways were identified in worker-destined individuals. These bioinformatic data obtained should be utilized to examine the molecular mechanisms of caste determination in social insects.

A lipocalin protein, Neural Lazarillo, is key to social interactions that promote termite soldier differentiation.

Social communication among castes is a crucial component of insect societies. However, the genes involved in soldier determination through the regulation of inter-individual interactions are largely unknown. In an incipient colony of the damp-wood termite Zootermopsis nevadensis, the first larva to develop into a third instar always differentiates into a soldier via frequent trophallactic feeding from the reproductives. Here, by performing RNA-seq analysis of third instar larvae, a homologue of Neural Lazarillo (named ZnNLaz1) was found to be the most differentially expressed gene in these soldier-destined larvae, compared with worker-destined larvae. This gene encodes a lipocalin protein related to the transport of small hydrophobic molecules. RNAi-induced knockdown of ZnNLaz1 significantly inhibited trophallactic interactions with the queen and decreased the soldier differentiation rates. This protein is localized in the gut, particularly in the internal wall, of soldier-destined larvae, suggesting that it is involved in the integration of social signals from the queen through frequent trophallactic behaviours. Based on molecular phylogenetic analysis, we suggest that a novel function of termite NLaz1 has contributed to social evolution from the cockroach ancestors of termites. These results indicated that a high larval NLaz1 expression is crucial for soldier determination through social communication in termites.

A Crucial Caste Regulation Gene Detected by Comparing Termites and Sister Group Cockroaches.

Sterile castes are a defining criterion of eusociality; investigating their evolutionary origins can critically advance theory. In termites, the soldier caste is regarded as the first acquired permanently sterile caste. Previous studies showed that juvenile hormone (JH) is the primary factor inducing soldier differentiation, and treatment of workers with artificial JH can generate presoldier differentiation. It follows that a shift from a typical hemimetabolous JH response might be required for soldier formation during the course of termite evolution within the cockroach clade. To address this possibility, analysis of the role of JH and its signaling pathway was performed in the termite Zootermopsis nevadensis and compared with the wood roach Cryptocercus punctulatus, a member of the sister group of termites. Treatment with a JH analog (JHA) induced a nymphal molt in C. punctulatus RNA interference (RNAi) of JH receptor Methoprene tolerant (Met) was then performed, and it inhibited the presoldier molt in Z. nevadensis and the nymphal molt in C. punctulatus Knockdown of Met in both species inhibited expression of 20-hydroxyecdysone (20E; the active form of ecdysone) synthesis genes. However, in Z. nevadensis, several 20E signaling genes were specifically inhibited by Met RNAi. Consequently, RNAi of these genes were performed in JHA-treated termite individuals. Knockdown of 20E signaling and nuclear receptor gene, Hormone receptor 39 (HR39/FTZ-F1ß) resulted in newly molted individuals with normal worker phenotypes. This is the first report of the JH-Met signaling feature in termites and Cryptocercus JH-dependent molting activation is shared by both taxa and mediation between JH receptor and 20E signalings for soldier morphogenesis is specific to termites.

TGFß 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 TGFß (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 TGFß signaling was acquired for the novel function of regulating between JH and 20E signaling during soldier differentiation in termites.

Distribution of Karyotypes of the Cryptocercus punctulatus Species Complex (Blattodea: Cryptocercidae) in Great Smoky Mountains National Park.

During the period between 1999 and 2006, wood-feeding cockroaches in the Cryptocercus punctulatus Scudder species complex were collected throughout Great Smoky Mountains National Park, USA. The chromosome numbers of insects from 59 sites were determined, and phylogenetic analyses were performed based on mitochondrial COII and nuclear ITS2 DNA. The distribution of the three male karyotypes found in the park (2n = 37, 39, and 45) is mapped and discussed in relation to recent disturbances and glacial history. Clades of the three karyotype groups meet near the ridgeline separating North Carolina from Tennessee in the center of the park, suggesting that these may have originated from separate lower elevation refugia after the last glacial maximum. The timing of divergence and a significant correlation between elevation difference and genetic distance in two of the clades supports this hypothesis. The ecological role of the cockroaches in the park is discussed.