The soldiers in societies: defense, regulation, and evolution

Caste-biased patterns of brain investment in the subterranean termite Reticulitermes flavipes

Investment into neural tissue is expected to reflect the specific sensory and behavioral capabilities of a particular organism. Termites are eusocial insects that exhibit a caste system in which individuals can develop into one of several morphologically and behaviorally distinct castes. However, it is unclear to what extent these differences between castes are reflected in the anatomy of the brain. To address this question, we used deformation-based morphometry to conduct pairwise comparisons between the brains of different castes in the eastern subterranean termite, Reticulitermes flavipes. Workers exhibited enlargement in the antennal lobes and mushroom bodies, while reproductives showed increased investment into the optic lobes and central body. In addition, caste-specific enlargement was observed in regions that could not be mapped to distinct neuropils, most notably in soldiers. These findings demonstrate a significant influence of caste development on brain anatomy in termites alongside convergence with eusocial hymenopteran systems.

Meta-Analysis of Public RNA Sequencing Data Revealed Potential Key Genes Associated with Reproductive Division of Labor in Social Hymenoptera and Termites

Eusociality in insects has evolved independently many times. One of the most notable characteristics of eusociality is the reproductive division of labor. In social insects, the reproductive division of labor is accomplished by queens and workers. Transcriptome analyses of queens and workers have been conducted for various eusocial species. However, the genes that regulate the reproductive division of labor across all or multiple eusocial species have not yet been fully elucidated. Therefore, we conducted a meta-analysis using publicly available RNA-sequencing data from four major groups of social insects. In this meta-analysis, we collected 258 pairs (queen vs. worker) of RNA-sequencing data from 34 eusocial species. The meta-analysis identified a total of 20 genes that were differentially expressed in queens or workers. Out of these, 12 genes have not previously been reported to be involved in the reproductive division of labor. Functional annotation of these 20 genes in other organisms revealed that they could be regulators of behaviors and physiological states related to the reproductive division of labor. These 20 genes, revealed using massive datasets of numerous eusocial insects, may be key regulators of the reproductive division of labor.

Influence of Soldiers on Exploratory Foraging Behavior in the Formosan Subterranean Termite, Coptotermes formosanus (Blattodea: Rhinotermitidae)

Termites are eusocial insects that live in organized colonies consisting of reproductives, workers, and soldiers. Soldiers are specialized for defense but are expensive to maintain, as they are incapable of husbandry and must be fed and groomed by workers. The soldiers of several species influence foraging behavior by acting as scouts that initiate foraging or by mediating worker behavioral plasticity during food exploration. These behaviors imply that soldiers may play a keystone role in termite colony function, apart from defense. Subterranean termite workers tunnel through soil in search of food while accompanied by varying proportions of soldiers, depending on the species and colony conditions. Previous studies have shown that soldiers accelerate worker exploratory tunneling behavior in two Reticulitermes species, the colonies of which contain fewer than 2% soldiers. This effect, however, is unknown in other subterranean species with different soldier proportions. In this study, we examined the influence of soldiers on exploratory foraging behavior in the Formosan subterranean termite, Coptotermes formosanus Shiraki, which is an economically devastating invasive species that maintains a relatively high soldier proportion (about 10%). When 100 foraging workers were grouped with 0, 2, 10, or 30 soldiers in two-dimensional foraging arenas, we found no significant effect of soldiers on the tunnel length, branch pattern, food source interception, or food collected within 96 h. These results suggest that C. formosanus colonies maintain food exploration efficiency regardless of soldier proportion variation.

Positive geographic correlation between soldiers' weapon size and defensive prowess in a eusocial aphid, Ceratovacuna japonica

Some aphid species produce a soldier caste with enlarged forelegs and horns (weapons). It has been hypothesised that the evolution of morphological specialization by soldiers in social aphids is accelerated by high predation pressure, but this possibility has not been tested. Here, we investigated the relationship between local predator abundance and soldiers’ weapon size and aggressiveness in a prey–predator system comprising a eusocial aphid, Ceratovacuna japonica, and its predators (larvae of the butterfly Taraka hamada and of the moth Atkinsonia ignipicta) in two populations with different predator abundances. We found that the soldiers in the predator-abundant population had larger weapons and were more aggressive than those in the population with lower predator abundance. Furthermore, the soldiers’ defensive prowess (evaluated as the survival of aphids in the presence of predators) was greater in the predator-abundant population. These results provide the first evidence that a population of eusocial aphids experiencing high predation pressure has soldiers with pronounced defensive traits and defensive prowess.

A High Soldier Proportion Encouraged the Greater Antifungal Immunity in a Subterranean Termite

Termites possess a mighty social immune system, serving as one of the key obstacles to controlling them biologically. However, the dynamic mechanism coordinating the social immunologic defense and caste distribution of the termites remains elusive. This study used the Coptotermes formosanus Shiraki and an entomopathogenic fungus as a host–pathogen system and experimentally manipulated a series of groups with different caste compositions of workers and soldiers. Then, the impact of demography on the behavior and innate immunity of termites was explored by analyzing the fungus susceptibility of the respective caste, efficiencies, and caste preferences of sanitary care, as well as the expression of the immune genes and phenoloxidase activity. Overall, to ensure the general health and survival of a group, the infected workers were found to sacrifice their survivorship for maintaining the soldier proportion of the group. If soldier proportion was limited within a threshold, both the survivorship of the workers and soldiers were not significantly affected by the infection. Correspondingly, the infected group with a higher proportion of soldiers stimulated the higher efficiency of a non-caste-biased sanitary care of the workers to the nestmate workers and soldiers. Moreover, the innate immunities of the infected workers were found to be more intensely upregulated in the group with higher soldier proportions. This suggested that the adjustable non-caste-biased sanitary care and innate immunity of the workers would contribute to the flexibility of the worker–soldier caste ratio in C. formosanus. This study, therefore, enhanced our understanding of the functional adaptation mechanism between pathogen-driven social immunity and the demography of the termites.

Molecular characterization and expression variation of the odorant receptor co-receptor in the Formosan subterranean termite

Subterranean termites live in underground colonies with a division of labor among castes (i.e., queens and kings, workers, and soldiers). The function of social colonies relies on sophisticated chemical communication. Olfaction, the sense of smell from food, pathogens, and colony members, plays an important role in their social life. Olfactory plasticity in insects can be induced by long- and short-term environmental perturbations, allowing adaptive responses to the chemical environment according to their physiological and behavioral state. However, there is a paucity of information on the molecular basis of olfaction in termites. In this study, we identified an ortholog encoding the odorant receptor co-receptor (Orco) in the Formosan subterranean termite, Coptotermes formosanus, and examined its expression variation across developmental stages and in response to social conditions. We found that C. formosanus Orco showed conserved sequence and structure compared with other insects. Spatial and temporal analyses showed that the Orco gene was primarily expressed in the antennae, and it was expressed in eggs and all postembryonic developmental stages. The antennal expression of Orco was upregulated in alates (winged reproductives) compared with workers and soldiers. Further, the expression of Orco decreased in workers after starvation for seven days, but it was not affected by the absence of soldiers or different group sizes. Our study reveals the molecular characteristics of Orco in a termite, and the results suggest a link between olfactory sensitivity and nutritional status. Further studies are warranted to better understand the role of Orco in olfactory plasticity and behavioral response.

Defense in Social Insects: Diversity, Division of Labor, and Evolution

All social insects defend their colony from predators, parasites, and pathogens. In Oster and Wilson's classic work, they posed one of the key paradoxes about defense in social insects: Given the universal necessity of defense, why then is there so much diversity in mechanisms? Ecological factors undoubtedly are important: Predation and usurpation have imposed strong selection on eusocial insects, and active defense by colonies is a ubiquitous feature of all social insects. The description of diverse insect groups with castes of sterile workers whose main duty is defense has broadened the purview of social evolution in insects, in particular with respect to caste and behavior. Defense is one of the central axes along which we can begin to organize and understand sociality in insects. With the establishment of social insect models such as the honey bee, new discoveries are emerging regarding the endocrine, neural, and gene regulatory mechanisms underlying defense in social insects. The mechanisms underlying morphological and behavioral defense traits may be shared across diverse groups, providing opportunities for identifying both conserved and novel mechanisms at work. Emerging themes highlight the context dependency of and interaction between factors that regulate defense in social insects.

IDH knockdown alters foraging behavior in the termite Odontotermes formosanus in different social contexts

Foraging, as an energy-consuming behavior, is very important for colony survival in termites. How energy metabolism related to glucose decomposition and adenosine triphosphate (ATP) production influences foraging behavior in termites is still unclear. Here, we analyzed the change in energy metabolism in the whole organism and brain after silencing the key metabolic gene isocitrate dehydrogenase (IDH) and then investigated its impact on foraging behavior in the subterranean termite Odontotermes formosanus in different social contexts. The IDH gene exhibited higher expression in the abdomen and head of O. formosanus. The knockdown of IDH resulted in metabolic disorders in the whole organism. The dsIDH-injected workers showed significantly reduced walking activity but increased foraging success. Interestingly, IDH knockdown altered brain energy metabolism, resulting in a decline in ATP levels and an increase in IDH activity. Additionally, the social context affected brain energy metabolism and, thus, altered foraging behavior in O. formosanus. We found that the presence of predator ants increased the negative influence on the foraging behavior of dsIDH-injected workers, including a decrease in foraging success. However, an increase in the number of nestmate soldiers could provide social buffering to relieve the adverse effect of predator ants on worker foraging behavior. Our orthogonal experiments further verified that the role of the IDH gene as an inherent factor was dominant in manipulating termite foraging behavior compared with external social contexts, suggesting that energy metabolism, especially brain energy metabolism, plays a crucial role in regulating termite foraging behavior.

Comparative Antennal Morphometry and Sensilla Organization in the Reproductive and Non-Reproductive Castes of the Formosan Subterranean Termite

Simple Summary

Insects use antennae to perceive the chemical environment, and olfaction (the sense of smell) is essential for a variety of behavioral responses. Termites are social insects that display a division of labor based on an elaborate caste system consisting of reproductive (queen and king) and non-reproductive individuals (workers and soldiers). Whether these castes have different senses of smell is poorly understood. In this study, we characterized the morphology of antennae in alates (winged reproductives), workers, and soldiers in the Formosan subterranean termite, Coptotermes formosanus, and further analyzed the diversity and abundance of the antennal sensilla (sensory receptors) in each caste. We found that both female and male alates had longer antennae and greater numbers of sensilla than workers and soldiers, but all castes possessed the same nine types of antennal sensilla. Each type of sensilla had a specific spatial distribution along the antenna. The quantitative composition of sensilla differed between the reproductive and non-reproductive castes, but few differences were found between female and male alates or between worker and soldier castes. These results suggest that the olfactory morphology is associated with the reproductive division of labor in subterranean termites.

Abstract

Antennae are the primary sensory organs in insects, where a variety of sensilla are distributed for the perception of the chemical environment. In eusocial insects, colony function is maintained by a division of labor between reproductive and non-reproductive castes, and chemosensation is essential for regulating their specialized social activities. Several social species in Hymenoptera display caste-specific characteristics in antennal morphology and diversity of sensilla, reflecting their differential tasks. In termites, however, little is known about how the division of labor is associated with chemosensory morphology among castes. Using light and scanning electron microscopy, we performed antennal morphometry and characterized the organization of sensilla in reproductive (female and male alates) and non-reproductive (worker and soldier) castes in the Formosan subterranean termite, Coptotermes formosanus Shiraki. Here, we show that the antennal sensilla in alates are twice as abundant as in workers and soldiers, along with the greater number of antennal segments and antennal length in alates. However, all castes exhibit the same types of antennal sensilla, including basiconicum, campaniformium, capitulum, chaeticum I, chaeticum II, chaeticum III, marginal, trichodeum I, and trichodeum I. The quantitative composition of sensilla diverges between reproductive and non-reproductive castes, but not between female and male alates or between worker and soldier castes. The sensilla display spatial-specific distribution, with basiconicum exclusively and capitulum predominantly found on the ventral side of antennae. In addition, the abundance of chemosensilla increases toward the distal end of antennae in each caste. This research provides morphological signatures of chemosensation and their implications for the division of labor, and suggests future neurophysiological and molecular studies to address the mechanisms of chemical communication in termites.

The weird eusociality of polyembryonic parasites

Some parasitoid wasps possess soldier castes during their parasitic larval stage, but are often neglected from our evolutionary theories explaining caste systems in animal societies. This is primarily due to the polyembryonic origin of their societies. However, recent discoveries of polyembryonic trematodes (i.e. flatworms) possessing soldier castes require us to reconsider this reasoning. I argue we can benefit from including these polyembryonic parasites in eusocial discussions, for polyembryony and parasitism are taxonomically vast and influence the evolution of social behaviours and caste systems in various circumstances. Despite their polyembryony, their social evolution can be explained by theories of eusociality designed for parent–offspring groups, which are the subjects of most social evolution research. Including polyembryonic parasites in these theories follows the trend of major evolutionary transitions theory expanding social evolution research into all levels of biological organization. In addition, these continued discoveries of caste systems in parasites suggest social evolution may be more relevant to parasitology than currently acknowledged.

The Plasticity and Developmental Potential of Termites

Phenotypic plasticity provides organisms with the potential to adapt to their environment and can drive evolutionary innovations. Developmental plasticity is environmentally induced variation in phenotypes during development that arise from a shared genomic background. Social insects are useful models for studying the mechanisms of developmental plasticity, due to the phenotypic diversity they display in the form of castes. However, the literature has been biased toward the study of developmental plasticity in the holometabolous social insects (i.e., bees, wasps, and ants); the hemimetabolous social insects (i.e., the termites) have received less attention. Here, we review the phenotypic complexity and diversity of termites as models for studying developmental plasticity. We argue that the current terminology used to define plastic phenotypes in social insects does not capture the diversity and complexity of these hemimetabolous social insects. We suggest that terminology used to describe levels of cellular potency could be helpful in describing the many levels of phenotypic plasticity in termites. Accordingly, we propose a conceptual framework for categorizing the changes in potential of individuals to express alternative phenotypes through the developmental life stages of termites. We compile from the literature an exemplar dataset on the phenotypic potencies expressed within and between species across the phylogeny of the termites and use this to illustrate how the potencies of different life stages of different species can be described using this framework. We highlight how this conceptual framework can help exploit the rich phenotypic diversity of termites to address fundamental questions about the evolution and mechanisms of developmental plasticity. This conceptual contribution is likely to have wider relevance to the study of other hemimetabolous insects, such as aphids and gall-forming thrips, and may even prove useful for some holometabolous social insects which have high caste polyphenism.

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.

Large body size variation is associated with low communication success in tandem running ants

Abstract

Diversity in animal groups is often assumed to increase group performance. In insect colonies, genetic, behavioural and morphological variation among workers can improve colony functioning and resilience. However, it has been hypothesized that during communication processes, differences between workers, e.g. in body size, could also have negative effects. Tandem running is a common recruitment strategy in ants and allows a leader to guide a nestmate follower to resources. A substantial proportion of tandem runs fail because leader and follower lose contact. Using the ant Temnothorax nylanderi as a model system, we tested the hypothesis that tandem running success is impaired if leader and follower differ in size. Indeed, we found that the success rate of tandem pairs drops considerably as size variation increases: tandem runs were unsuccessful when the leader–follower size difference exceeded 10%, whereas ~ 80% of tandem runs were successful when ants differed less than 5% in body length. Possible explanations are that size differences are linked to differences in walking speed or sensory perception. Ants did not choose partners of similar size, but extranidal workers were larger than intranidal workers, which could reduce recruitment mistakes because it reduced the chance that very large and very small ants perform tandem runs together. Our results suggest that phenotypic differences between interacting workers can have negative effects on the efficiency of communication processes. Whether phenotypic variation has positive or negative effects is likely to depend on the task and the phenotypic trait that shows variation.

Significance statement

Diversity is often assumed to increase colony performance in social insects. However, phenotypic differences among workers could also have negative effects, e.g. during communication. Tandem running is a common recruitment strategy in ants, but tandem runs often fail when ants lose contact. We used the ant Temnothorax nylanderi to test the hypothesis that body size differences between tandem leader and follower impair tandem communication. We show that the success rate of tandem pairs drops considerably as size variation increases, possibly because ants of varying size also differ in walking speed. Our study supports the hypothesis that phenotypic variation among workers might not always be beneficial and can negatively impact the efficiency of communication processes.

Poplar Tree Response to Feeding by the Petiole Gall Aphid Pemphigus spyrothecae Pass

Pemphigus spyrothecae Pass. which is a member of the subfamily Pemphiginae is one of the gall-inducing aphids that occurs on poplar trees. Phloem feeding of a founding mother on leaf petiole results in the formation of a new organ, i.e., the spiral gall. This study documents aphid development inside the galls during the growing season and the effect of their feeding on leaf architecture and physiology of the host plant. In particular, leaf length, width, and area were measured, as well as hydrogen peroxide (H₂O₂) content, electrolyte leakage (E_L), malondialdehyde (MDA) concentration, and the activity of ascorbate (APX) and guaiacol peroxidase (GPX) were determined in galls and galled leaves. The presence of petiole galls significantly decreased the length, width, and leaf area. Aphid activity increased H₂O₂ concentration in galls and E_L from galls and leaf tissues, which was accompanied by a strong decrease in MDA content and both peroxidase activities, especially in gall tissues. It can be suggested that P. spyrothecae can manipulate physiological machinery of the host plant for its own benefit.

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.

Trait evolution is reversible, repeatable, and decoupled in the soldier caste of turtle ants

The scope of adaptive phenotypic change within a lineage is shaped by how functional traits evolve. Castes are defining functional traits of adaptive phenotypic change in complex insect societies, and caste evolution is expected to be phylogenetically conserved and developmentally constrained at broad phylogenetic scales. Yet how castes evolve at the species level has remained largely unaddressed. Turtle ant soldiers (genus Cephalotes), an iconic example of caste specialization, defend nest entrances by using their elaborately armored heads as living barricades. Across species, soldier morphotype determines entrance specialization and defensive strategy, while head size sets the specific size of defended entrances. Our species-level comparative analyses of morphotype and head size evolution reveal that these key ecomorphological traits are extensively reversible, repeatable, and decoupled within soldiers and between soldier and queen castes. Repeated evolutionary gains and losses of the four morphotypes were reconstructed consistently across multiple analyses. In addition, morphotype did not predict mean head size across the three most common morphotypes, and head size distributions overlapped broadly across all morphotypes. Concordantly, multiple model-fitting approaches suggested that soldier head size evolution is best explained by a process of divergent pulses of change. Finally, while soldier and queen head size were broadly coupled across species, the level of head size disparity between castes was decoupled from both queen head size and soldier morphotype. These findings demonstrate that caste evolution can be highly dynamic at the species level, reshaping our understanding of adaptive morphological change in complex social lineages.

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.

Changing of the guard: mixed specialization and flexibility in nest defense (Tetragonisca angustula)

Task allocation is a central challenge of collective behavior in a variety of group-living species, and this is particularly the case for the allocation of social insect workers for group defense. In social insects, both benefits and considerable costs are associated with the production of specialized soldiers. We asked whether colonies mitigate costs of production of specialized soldiers by simultaneously employing behavioral flexibility in nonspecialist workers that can augment defense capabilities at short time scales. We studied colonies of the stingless bee Tetragonisca angustula, a species that has 2 discrete nest-guarding tasks typically performed by majors: hovering guarding and standing guarding. Majors showed age polyethism across nest-guarding tasks, first hovering and then changing to the task of standing guarding after 1 week. Colonies were also able to reassign minors to guarding tasks when majors were experimentally removed. Replacement guards persisted in nest defense tasks until colonies produced enough majors to return to their initial state. Tetragonisca angustula colonies thus employed a coordinated set of specialization strategies in nest defense: morphologically specialized soldiers, age polyethism among soldiers within specific guarding tasks, and rapid flexible reallocation of nonspecialists to guarding during soldier loss. This mixed strategy achieves the benefits of a highly specialized defensive force while maintaining the potential for rapid reinforcement when soldiers are lost or colonies face unexpectedly intense attack.

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.

Juvenile hormone as a physiological regulator mediating phenotypic plasticity in pancrustaceans

Phenotypic plasticity and polyphenism, in which phenotypes can be changed depending on environmental conditions, are common in insects. Several studies focusing on physiological, developmental, and molecular processes underlying the plastic responses have revealed that similar endocrine mechanisms using juvenile hormone (JH) are used to coordinate the flexible developmental processes. This review discusses accumulated knowledge on the caste polyphenism in social insects (especially termites), the wing and the reproductive polyphenisms in aphids, and the nutritional polyphenism and sexual dimorphism in stag beetles. For the comparison with non-insect arthropods, extensive studies on the inducible defense (and reproductive polyphenism) in daphnids (crustacean) are also addressed. In all the cases, JH (and methyl farnesoate in daphnids) plays a central role in mediating environmental stimuli with morphogenetic processes. Since the synthetic pathways for juvenoids, i.e., the mevalonate pathway and downstream pathways to sesquiterpenoids, are conserved across pancrustacean lineages (crustaceans and hexapods including insects), the evolution of developmental regulation by juvenoids that control molting (ecdysis) and metamorphosis is suggested to have occurred in the ancestral arthropods. The discontinuous postembryonic development (i.e., molting) and the regulatory physiological factors (juvenoids) would have enabled plastic developmental systems observed in many arthropod lineages.

Termite soldiers contribute to social immunity by synthesizing potent oral secretions

The importance of soldiers to termite society defence has long been recognized, but the contribution of soldiers to other societal functions, such as colony immunity, is less well understood. We explore this issue by examining the role of soldiers in protecting nestmates against pathogen infection. Even though they are unable to engage in grooming behaviour, we find that the presence of soldiers of the Darwin termite, Mastotermes darwiniensis, significantly improves the survival of nestmates following entomopathogenic infection. We also show that the copious exocrine oral secretions produced by Darwin termite soldiers contain a high concentration of proteins involved in digestion, chemical biosynthesis, and immunity. The oral secretions produced by soldiers are sufficient to protect nestmates against infection, and they have potent inhibitory activity against a broad spectrum of microbes. Our findings support the view that soldiers may play an important role in colony immunity, and broaden our understanding of the possible function of soldiers during the origin of soldier-first societies.

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.

Sex-specific inhibition and stimulation of worker-reproductive transition in a termite

In social insects, the postembryonic development of individuals exhibits strong phenotypic plasticity in response to the environment, thus generating the caste system. Different from eusocial Hymenoptera, in which queens dominate reproduction and inhibit worker fertility, the primary reproductive caste in termites (kings and queens) can be replaced by neotenic reproductives derived from functionally sterile individuals. Feedback regulation of nestmate differentiation into reproductives has been suggested, but the sex specificity remains inconclusive. In the eastern subterranean termite, Reticulitermes flavipes, we tested the hypothesis that neotenic reproductives regulate worker-reproductive transition in a sex-specific manner. With this R. flavipes system, we demonstrate a sex-specific regulatory mechanism with both inhibitory and stimulatory functions. Neotenics inhibit workers of the same sex from differentiating into additional reproductives but stimulate workers of the opposite sex to undergo this transition. Furthermore, this process is not affected by the presence of soldiers. Our results highlight the reproductive plasticity of termites in response to social cues and provide insights into the regulation of reproductive division of labor in a hemimetabolous social insect.

Local differences in parasitism and competition shape defensive investment in a polymorphic eusocial bee

Many colonial animals rely for their defense on a soldier caste. Adaptive colony demography theory predicts that colonies should flexibly adjust the investment in different worker castes depending on the colony needs. For example, colonies should invest more in defensive workers (e.g., soldiers) in dangerous environments. However, evidence for this prediction has been mixed. We combined descriptive and experimental approaches to examine whether defensive investment and worker size are adjusted to local ecology in the only known bee with polymorphic workers, Tetragonisca angustula. Colonies of this species are defended by a morphologically specialized soldier caste. Our study included three populations that differed in the density of food competition and the occurrence of a parasitic robber bee. We found that colonies coexisting with robber bees had on average 43% more soldiers defending the nest entrance, while colonies facing stronger foraging competition had soldiers that were -6-7% smaller. We then experimentally relocated colonies to areas with different levels of competition. When released from intense food competition, body sizes of guards and foragers increased. After introducing chemical robber bee cues at nest entrances, we found both a short-term and a long-term up-regulation of the number of soldiers defending the colony. Active soldier numbers remained high after the experiment for a duration equivalent to 2-3 worker life spans. How information about past parasite threat is stored in the colony is currently unknown. In summary, T. angustula adjusts both the number and the body size of active soldiers to local ecological conditions. Competitor density also affects forager (or minor) size, an important colony trait with potential community ecological consequences. Our study supports adaptive colony demography theory in a eusocial bee and highlights the importance of colony threats and competition as selective forces shaping colony phenotype.

FOXO links wing form polyphenism and wound healing in the brown planthopper, Nilaparvata lugens

Polyphenisms such as wing dimorphisms and caste determination are important in allowing animals to adapt to changing environments. The brown planthopper Nilaparvata lugens, one of the most serious insect agricultural pests, includes two wing forms, the long wing form (macropterous) and the short wing form (brachypterous). Long wings are specialized for migration, while short wings are found in individuals specialized for reproduction. While studying wing form polyphenism in the brown planthopper, we excised single wing pads from 4th instar nymphs in order to preserve transcriptional records to correlate with adult wing form. Surprisingly, we found that excision of one wing pad from a pair of the forewings changed the wing morph of the other wing after development to the adult, resulting in the short wing morph. Further experiments showed that not only excision or slicing of the wing pad, but also needle punctures in the abdomen all caused a significant increase in the proportion of nymphs developing into short winged adults. Thus wounding appears to cause a shift to short wing development. We then tested the transcriptional expression in N. lugens of the transcription factor FOXO, which has been shown to help mediate both wing polyphenism in brown planthoppers and wound healing in mice, after excision of the wing pad. Both NlFOXO and its downstream target Nl4EBP increased significantly after wing pad excision. These results indicate that FOXO mediates both wing development and wound healing in N. lugens, which results in an interesting linkage of these two physiological processes.

Inter-caste communication in social insects

Social insect colonies function as highly integrated units despite consisting of many individuals. This requires the different functional parts of the colony (e.g. different castes) to exchange information that aid in colony functioning and ontogeny. Here we discuss inter-caste communication in three contexts, firstly, the communication between males and females during courtship, secondly, the communication between queens and workers that regulate reproduction and thirdly, the communication between worker castes that allows colonies to balance the number of different worker types. Some signals show surprising complexity in both their chemistry and function, whereas others are simple compounds that were probably already used as pheromones in the solitary ancestors of several social insect lineages.

Chemical alarm in the termite Termitogeton planus (Rhinotermitidae)

Effective defense is a common characteristic of insect societies. Indeed, the occurrence of specialized defenders, soldiers, has been the first step toward eusociality in several independent lineages, including termites. Among the multitude of defensive strategies used by termite soldiers, defense by chemicals plays a crucial role. It has evolved with complexity in advanced isopteran lineages, whose soldiers are equipped with a unique defensive organ, the frontal gland. Besides direct defense against predators, competitors, and pathogens, the chemicals emitted by soldiers from the frontal gland are used as signals of alarm. In this study, we investigated the chemical composition of the defensive secretion produced by soldiers of the termite Termitogeton planus (Isoptera: Rhinotermitidae), from West Papua, and the effects of this secretion on the behavior of termite groups. Detailed two-dimensional gas chromatography/mass spectrometry analyses of the soldier defensive secretion revealed the presence of four linear and nine monoterpene hydrocarbons. Soldier head extracts, as well as synthetic mixtures of the monoterpenes found in these extracts, elicited alarm behavior in both soldiers and pseudergates. Our results suggest that the alarm is not triggered by a single monoterpene from the defensive blend, but by a multi-component signal combining quantitatively major and minor compounds.