Colony-age-dependent variation in cuticular hydrocarbon profiles in subterranean termite colonies

Us, them, and the others: Testing for discrimination amongst outgroups in a single‐piece nesting termite, Zootermopsis angusticollis

Recognition of group members is an important adaptation in social organisms because it allows help to be directed toward kin or individuals that are likely to reciprocate, and harm to be directed toward members of competing groups. Evidence in a wide range of animals shows that responses to outgroups vary with context, suggesting that cues to group membership also depend on the social or environmental context. In termites, intergroup encounters are frequent and their outcomes highly variable, ranging from destruction of a colony to colony fusion. As well as genetic factors, nestmate recognition in social insects commonly relies on cues that are mediated by environmental factors such as food source. However, single‐piece nesting termite colonies share nesting material and food source with rival colonies (their wood substrate serves as both). In principle, the shared environment of single‐piece nesting termite colonies could constrain their ability to identify non‐nestmates, contributing to some of the variation seen in encounters, but this has not been investigated. In this study, we raised incipient colonies of a single‐piece nesting termite, Zootermopsis angusticollis, on two different wood types and conducted behavioral assays to test whether nestmate discrimination can be constrained by common environmental conditions. We found that non‐nestmates elicited higher rates of identity checking and defense behavior compared to nestmates, but there was no effect of wood type on the strength of behavioral responses to non‐nestmates. We also found that one key cooperative behavior (allogrooming) was performed equally toward both nestmates and non‐nestmates. These findings offer no support for the hypothesis that common wood type constrains the nestmate recognition system of single piece nesting termites. We suggest that where groups encounter each other frequently in a common environment, selection will favor discrimination based on genetic and/or higher resolution environmentally mediated cues.

Consistency of Cuticular Hydrocarbon Mixtures of Five Reticulitermes (Blattodea: Rhinotermitidae) Taxa From Northern California: Similarity Among Colonies and Seasonal Variation

Cuticular hydrocarbon (CHC) mixtures from workers of five distinct CHC phenotypes of Reticulitermes Holmgren 1913 from two locations in northern California were examined from monthly collections taken over a 3-yr period. The objectives of this study were (1) to identify and quantify variations of the CHCs of multiple colonies of each of these phenotypes (= species or subspecies) to demonstrate consistency, (2) to assess the potential of CHC mixtures to separate or identify colonies within each phenotype, and (3) to detect any temporal changes in each of the hydrocarbons in the CHC mixtures. Nonmetric multidimensional scaling of all CHC mixtures of all samples collected at both locations separated the samples into five clearly visible, different groups of CHC phenotypes (taxa or species) of Reticulitermes. The degree of variability of the CHC mixtures among colonies of each phenotype was such that nonmetric multidimensional scaling did not separate or identify colonies. Strong seasonal fluctuations were evident in some of the CHCs of all five phenotypes and were significantly consistent with a sine curve. Maximum proportions of seasonal CHCs within a phenotype occurred in all seasons of the year but occurred mostly in the winter and summer. In general, the CHCs displaying maximum values in the winter were short-chained (C23-C27) methyl-branched alkanes, whereas the CHCs displaying maximum values in the summer were long-chained (C35-C43) methyl-branched alkanes, which likely influences water retention. These consistent chemical fingerprints are probably responsible for inter-phenotype recognition patterns and are thus useful for chemical taxonomy.

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.

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.

Distinct chemical blends produced by different reproductive castes in the subterranean termite Reticulitermes flavipes

The production of royal pheromones by reproductives (queens and kings) enables social insect colonies to allocate individuals into reproductive and non-reproductive roles. In many termite species, nestmates can develop into neotenics when the primary king or queen dies, which then inhibit the production of additional reproductives. This suggests that primary reproductives and neotenics produce royal pheromones. The cuticular hydrocarbon heneicosane was identified as a royal pheromone in Reticulitermes flavipes neotenics. Here, we investigated the presence of this and other cuticular hydrocarbons in primary reproductives and neotenics of this species, and the ontogeny of their production in primary reproductives. Our results revealed that heneicosane was produced by most neotenics, raising the question of whether reproductive status may trigger its production. Neotenics produced six additional cuticular hydrocarbons absent from workers and nymphs. Remarkably, heneicosane and four of these compounds were absent in primary reproductives, and the other two compounds were present in lower quantities. Neotenics therefore have a distinct 'royal' blend from primary reproductives, and potentially over-signal their reproductive status. Our results suggest that primary reproductives and neotenics may face different social pressures. Future studies of these pressures should provide a more complete understanding of the mechanisms underlying social regulation in termites.

A Review of Termite Pheromones: Multifaceted, Context-Dependent, and Rational Chemical Communications

Termite colonies, composed of large numbers of siblings, develop an important caste-based division of labor; individuals in these societies interact via intra- or intercaste chemical communications. For more than 50 years, termites have been known to use a variety of pheromones to perform tasks necessary for maintenance of their societies, similar to eusocial hymenopterans. Although trail-following pheromones have been chemically identified in various termites, other types of pheromones have not been elucidated chemically or functionally. In the past decade, however, chemical compositions and biological functions have been successfully identified for several types of termite pheromones; accordingly, the details of the underlying pheromone communications have been gradually revealed. In this review, we summarize both the functions of all termite pheromones identified so far and the chemical interactions among termites and other organisms. Subsequently, we argue how termites developed their sophisticated pheromone communication. We hypothesize that termites have diverted defensive and antimicrobial substances to pheromones associated in caste recognition and caste-specific roles. Furthermore, termites have repeatedly used a pre-existing pheromone or have added supplementary compounds to it in accordance with the social context, leading to multifunctionalization of pre-existing pheromones and emergence of new pheromones. These two mechanisms may enable termites to transmit various context-dependent information with a small number of chemicals, thus resulting in formation of coordinated, complex, and rational chemical communication systems.