Reproductive Caste Beats a Hasty Retreat

Seven species of Australian gall-inducing thrips have a helping caste (soldiers). This caste is morphologically distinct and has been shown to use these physical differences to be a more effective fighting force. Here we investigate behavioral differences between castes of Kladothrips intermedius (Bagnall) when faced with the destruction of the gall; that is, walking behavior rather than fighting. Two parameters, curvilinear velocity (VCL) and linearity (LIN) were used to describe the two dimensional movements of each caste in our experimental setup. In our trials the dispersers moved more directly (LIN) and quickly (VCL) than soldiers. This is consistent with the view that soldiers are behaviorally distinct and committed to life in a gall while, dispersers (second-instars and adults) seem to have somewhere to go.     

Antennal olfactory sensitivity in response to task-related odours of three castes of the ant Atta mexicana (hymenoptera: formicidae)

Abstract.  The relationship between scent composition and antennal sensitivity in different castes of Atta mexicana is investigated under laboratory conditions. Extracts of dead ants are analysed by gas chromatography-mass spectrometry to identify the compounds presumably responsible for the specific undertaking behaviour. Oleic acid is identified as one compound that triggers undertaking behaviour. To determine differences in odour reception between workers of different castes (i.e. foragers, undertakers and soldiers), further antennal sensitivity to task-related odours is tested using electroantennographic techniques. Soldiers are the most specialized caste because of their low response to all odours, except odourants related to alarm pheromones. The behavioural specialization of soldiers and their reduced behavioural repertoire are discussed.     

Building a new research framework for social evolution: intralocus caste antagonism

The breeding and non‐breeding ‘castes’ of eusocial insects provide a striking example of role‐specific selection, where each caste maximises fitness through different morphological, behavioural and physiological trait values. Typically, queens are long‐lived egg‐layers, while workers are short‐lived, largely sterile foragers. Remarkably, the two castes are nevertheless produced by the same genome. The existence of inter‐caste genetic correlations is a neglected consequence of this shared genome, potentially hindering the evolution of caste dimorphism: alleles that increase the productivity of queens may decrease the productivity of workers and vice versa, such that each caste is prevented from reaching optimal trait values. A likely consequence of this ‘intralocus caste antagonism’ should be the maintenance of genetic variation for fitness and maladaptation within castes (termed ‘caste load’), analogous to the result of intralocus sexual antagonism. The aim of this review is to create a research framework for understanding caste antagonism, drawing in part upon conceptual similarities with sexual antagonism. By reviewing both the social insect and sexual antagonism literature, we highlight the current empirical evidence for caste antagonism, discuss social systems of interest, how antagonism might be resolved, and challenges for future research. We also introduce the idea that sexual and caste antagonism could interact, creating a three‐way antagonism over gene expression. This includes unpacking the implications of haplodiploidy for the outcome of this complex interaction.     

Caste-biased locomotor activities in isolated termites

Eusocial insects are characterized by a well-developed division of labour among castes. Although the successful division of labour should stem from behavioural differentiation depending on caste identity, caste-specific intrinsic behavioural characteristics might be masked by social interactions within colonies. The present study explores caste-specific intrinsic locomotive activities of termites by quantifying them in isolation. We track individual movement trajectories of the damp-wood termite Hodotermopsis sjostedti over 30 min and extract individual locomotion parameters. Multivariate statistical analyses reveal significant differences among castes: soldiers move more actively than workers and neotenic reproductives. The morphometric data of test individuals indicate that locomotor activities reflected caste identity more strongly compared with quantitative morphological variations among individuals. We find that the different locomotor activities of soldiers compared with those of neotenics and workers probably reflect their physiological differentiation. The present study provides a basis for a deeper understanding of the roles of individual locomotor activities in social behaviours.     

尖唇散白蚁工蚁、兵蚁和繁殖蚁精子发生的比较研究

为探讨白蚁非生殖品级和生殖品级生殖细胞发育差异,采用组织学染色技术对尖唇散白蚁Reticulitermes aculabialis Tsaiet Hwang繁殖蚁、工蚁和兵蚁的精巢发育以及精子发生进行了显微观察和比较研究。结果发现3个品级间精巢发育的程度差异很大,三者精巢切面面积相对大小之比为:繁殖蚁∶工蚁=1.7∶1;繁殖蚁∶兵蚁=29.3∶1;工蚁∶兵蚁=17.1∶1。繁殖蚁和工蚁精巢管内有精子的形成,工蚁和繁殖蚁精子的发生都经历了精原细胞、初级精母细胞、次级精母细胞、精细胞和精子时期,但工蚁有大量次级精母细胞呈细胞凋亡状态。兵蚁生殖细胞发育仅有精原细胞、初级精母细胞、次级精母细胞,没有精细胞和精子产生。工蚁的生殖细胞显著小于同一时期繁殖蚁的生殖细胞,兵蚁的各时期生殖细胞均极显著小于繁殖蚁同一时期的生殖细胞。研究表明各品级之间生殖功能分化与生殖细胞发育有直接关系,工蚁有转化为补充繁殖蚁和兵蚁的能力;而兵蚁由于精巢极度退化不能产生精细胞和精子,因此是非生殖品级分化的终极形式,不具有转化成补充繁殖蚁或其它品级的能力。     

Endocrine control of cuticular hydrocarbon profiles during worker-to-soldier differentiation in the termite Reticulitermes flavipes

The social organization of termites, unlike that of other social insects, is characterized by a highly plastic caste system. With the exception of the alates, all other individuals in a colony remain at an immature stage of development. Workers in particular remain developmentally flexible; they can switch castes to become soldiers or neotenics. Juvenile hormone (JH) is known to play a key role in turning workers into soldiers. In this study, we analyzed differences in cuticular hydrocarbon (CHC) profiles among castes, paying particular attention to the transition of workers to soldiers, in the subterranean termite species Reticulitermes flavipes. CHCs have a fundamental function in social insects as they serve as cues in inter- and intraspecific recognition. We showed that (1) the CHC profiles of the different castes (workers, soldiers, nymphs and neotenics) are different and (2) when workers were experimentally exposed to a JH analog and thus induced to become soldiers, their CHC profiles were modified before and after the worker-presoldier molt and before and after the presoldier-soldier molt.     

Flight muscle breakdown in the soldier caste of the gall-inducing thrips species, Kladothrips intermedius Bagnall

Within the gall-inducing thrips of Australia, genus Kladothrips, is a single origin of a soldier caste. A subsequent radiation has led to at least seven social species, and two species that are likely to have independently lost the soldier caste. Both losses of soldiers are connected to a shift in the insects’ host plant. A third inferred host shift is correlated with life history changes in a species with soldiers, K. intermedius, which might suggest this defensive caste may be in transition. The soldiers of this species have variable wing lengths that overlap with that of the flight-capable dispersers. Our study was designed to assess the possibility that some soldiers in this species retain the ability to disperse by wing. A morphological assessment shows that a proportion of soldiers have body dimensions and wing lengths that fall within the range of dispersers, with males more so than females. However, longer wing length in soldiers (sample of primarily females) did not correspond with a disperser like walking behaviour. Furthermore, histological sections suggest that wing muscles of soldiers are deteriorated or absent, which is similar to what was observed in their foundress mothers. The presence of long wings did not correspond with being flight capable in this caste.     

Soldier‐biased gene expression in a subterranean termite implies functional specialization of the defensive caste

In a termite colony, reproduction is typically monopolized by a small number of sexuals that are supported by reproductively altruistic soldiers and workers. We expect caste differentiation to be associated with clear‐cut differences in gene expression, and for these differences to reflect caste function and development. Here, we use RNA‐Sequencing to compare the gene expression profiles of sexual nymphs and two non‐reproductive helper castes (i.e., workers and soldiers) of the Eastern subterranean termite Reticulitermes flavipes. We found that of n = 93 genes that are strictly expressed as a function of caste, a majority (78%) show a soldier‐specific pattern. This conspicuous soldier‐bias in genome‐wide expression suggests that this defensively specialized caste is functionally well‐differentiated from both the reproductive and the other non‐reproductive caste of this species, despite a shared developmental program with workers. Gene ontology analysis supports the notion of functional specialization by soldiers, as soldier‐biased gene sets are enriched for novel biological processes. Whether this pattern reflects ancient or more recent bouts of selection for caste novelty at the gene‐regulatory level is not known, but because soldiers are sterile and thus have no direct fitness, any selection for novelty must have been mediated indirectly, through reproducing relatives.     

Association between caste and genotype in the termite Mastotermes darwiniensis Froggatt (Isoptera: Mastotermitidae)

Abstract Termite workers and soldiers differ markedly in their morphology and behaviour. We sought evidence for genetic influences on caste determination in the giant northern termite, Mastotermes darwiniensis , by investigating if workers and soldiers from the same colony differed genetically. The genotypes of 795 termites from 11 distinct colonies were assayed at six polymorphic microsatellite loci. We found that the multilocus genotypes of workers and soldiers from 8 of the 11 colonies did not differ significantly. Thus, the majority of the data provided no evidence for a genetic association with caste in workers and soldiers of M. darwiniensis . However, the genotype frequencies of workers and soldiers from three colonies differed, suggesting that genotype is occasionally associated with caste in this species. The genetic differentiation of castes within these colonies could reflect differences in the propensities of termites with distinct genotypes to develop into particular castes and provide a selective advantage to colonies headed by multiple reproductives.     

Physiological variation as a mechanism for developmental caste-biasing in a facultatively eusocial sweat bee

Social castes of eusocial insects may have arisen through an evolutionary modification of an ancestral reproductive ground plan, such that some adults emerge from development physiologically primed to specialize on reproduction (queens) and others on maternal care expressed as allo-maternal behaviour (workers). This hypothesis predicts that variation in reproductive physiology should emerge from ontogeny and underlie division of labour. To test these predictions, we identified physiological links to division of labour in a facultatively eusocial sweat bee, Megalopta genalis. Queens are larger, have larger ovaries and have higher vitellogenin titres than workers. We then compared queens and workers with their solitary counterparts-solitary reproductive females and dispersing nest foundresses-to investigate physiological variation as a factor in caste evolution. Within dyads, body size and ovary development were the best predictors of behavioural class. Queens and dispersers are larger, with larger ovaries than their solitary counterparts. Finally, we raised bees in social isolation to investigate the influence of ontogeny on physiological variation. Body size and ovary development among isolated females were highly variable, and linked to differences in vitellogenin titres. As these are key physiological predictors of social caste, our results provide evidence for developmental caste-biasing in a facultatively eusocial bee.     

Caste‐biases in gene expression are specific to developmental stage in the ant Formica exsecta

Understanding how a single genome creates and maintains distinct phenotypes is a central goal in evolutionary biology. Social insects are a striking example of co‐opted genetic backgrounds giving rise to dramatically different phenotypes, such as queen and worker castes. A conserved set of molecular pathways, previously envisioned as a set of ‘toolkit’ genes, has been hypothesized to underlie queen and worker phenotypes in independently evolved social insect lineages. Here, we investigated the toolkit from a developmental point of view, using RNA‐Seq to compare caste‐biased gene expression patterns across three life stages (pupae, emerging adult and old adult) and two female castes (queens and workers) in the ant Formica exsecta. We found that the number of genes with caste‐biased expression increases dramatically from pupal to old adult stages. This result suggests that phenotypic differences between queens and workers at the pupal stage may derive from a relatively low number of caste‐biased genes, compared to higher number of genes required to maintain caste differences at the adult stage. Gene expression patterns were more similar among castes within developmental stages than within castes despite the extensive phenotypic differences between queens and workers. Caste‐biased expression was highly variable among life stages at the level of single genes, but more consistent when gene functions (gene ontology terms) were investigated. Finally, we found that a large part of putative toolkit genes were caste‐biased at least in some life stages in F. exsecta, and the caste‐biases, but not their direction, were more often shared between F. exsecta and other ant species than between F. exsecta and bees. Our results indicate that gene expression should be examined across several developmental stages to fully reveal the genetic basis of polyphenisms.     

Worker aggression of ant Lasius japonicus enhanced by termite soldier–specific secretion as an alarm pheromone of Reticulitermes speratus

The subterranean termite Reticulitermes speratus usually nests in rotten wood trunks, which may also be occupied by the Japanese garden ant Lasius japonicus. Few battles were observed between them under ordinary circumstances because they inhabit separate nesting sites. However, once the termite nesting sites were artificially broken, the ant workers invaded and hunted the termites, although the termite soldiers fought against the ants. This study aims to confirm intra‐ and inter‐specific chemical interactions between the termite and ant. Solid phase microextraction–gas chromatograph (SPME‐GC) analyses revealed that R. speratus soldiers secreted caste‐specific sesquiterpene hydrocarbon when they were irritated. Both the hexane extract of the soldiers and its hydrocarbon fraction, as well as the crushed soldier bodies, attracted the soldiers but dispersed the workers when presented on the trails. We also confirmed that the soldier chemicals enhanced aggressiveness of L. japonicus, which rushed around the odor sources and hunted any termites that were present. These findings suggest that: (i) the soldier–specific secretion might serve as an alarm pheromone in termite chemical communication, in which components recruit soldiers and also warn the other colony members away; and (ii) termite communication is eavesdropped on by L. japonicus workers to locate and hunt the termites.     

Age‐dependent responses to alarm calls depend on foraging activity in Willow Tits Poecile montanus

Survivorship in animals depends on both foraging activities and avoidance of predation, and thus behavioural decisions often reflect a trade‐off between predation risk and foraging efficiency. In this experimental study, we compared behavioural responses of free‐living adult and juvenile Willow Tits Poecile montanus to a conspecific alarm call in two treatments. The alarm call was played back when a focal bird was either not feeding, or feeding on a sunflower seed on the middle part of a spruce branch. When feeding at the time of the alarm call, juveniles more often stayed motionless or moved shorter distances than adults. Our results suggest that in hierarchical groups, juveniles are forced to take greater risks to maintain access to food or lack experience to optimize between food and safety.     

The effect of nonnestmates on foraging trail caste ratios ofNasutitermes corniger (Isoptera: Termitidae)

The mean number of workers on foraging trails ofNasutitermes corniger Motschulsky decreases after the introduction of nonnestmate conspecifics or congeneric heterospecifics. The mean number of soldiers on foraging trais does not vary when nestmates, conspecific nonnestmates or heterospecific workers are introduced. When heterospecific soldiers are introduced to the trail, however, the number ofN. corniger soldiers increases. Consequently, the ratio of soldiers to workers increases. These findings demonstrate that the addition of nonnestmates temporarily alters caste ratios on foraging trails, representing a shift in the distribution of castes in response to environmental perturbation.     

X‐ray computed tomography reveals that intraspecific competition promotes soldier differentiation in a one‐piece nesting termite

Investment in soldier production in eusocial lineages involves a trade‐off between maintenance costs and defense benefits. Termites are eusocial insects that live in colonies organized into three castes: primary reproductives, soldiers, and workers or pseudergates. Neotermes chilensis (Blanchard) (Isoptera: Kalotermitidae) is a one‐piece nesting termite that nests and forages in a single piece of wood. Two scenarios may be of importance in a defense context of one‐piece nesting termites: during swarms, when colonies may be invaded by winged termites (alates) in search of a place to found a new colony, and when colonies of conspecifics are present within the same substrate. It was hypothesized that the ratio of soldiers to non‐soldiers would be higher at the onset of the swarming period and in substrates bearing more than one termite colony. A method based on X‐ray computed tomography (CT) was developed to study gallery connectivity in colonies of N. chilensis and caste composition within colonies. Computed tomography allowed the digital reconstruction of the galleries within the substrate, even when they belonged to different colonies, and was effective in distinguishing termites from substrate, and soldiers from reproductives and pseudergates. Using CT, the ratio of soldiers to non‐soldiers was shown to be highest in colonies within multicolonial scapes (i.e., neighboring colonies were present in the same substrate) during the swarming season, thus supporting our initial hypotheses. These results constitute a unique example of induced defenses arising from intraspecific interactions in termites.     

Social organization in a flatworm: trematode parasites form soldier and reproductive castes

In some of the most complex animal societies, individuals exhibit a cooperative division of labour to form castes. The most pronounced types of caste formation involve reproductive and non-reproductive forms that are morphologically distinct. In colonies comprising separate or mobile individuals, this type of caste formation has been recognized only among the arthropods, sea anemones and mole-rats. Here, we document physical and behavioural caste formation in a flatworm. Trematode flatworm parasites undergo repeated clonal reproduction of ‘parthenitae’ within their molluscan hosts forming colonies. We present experimental and observational data demonstrating specialization among trematode parthenitae to form distinct soldier and reproductive castes. Soldiers do not reproduce, have relatively large mouthparts, and are much smaller and thinner than reproductives. Soldiers are also more active, and are disproportionally common in areas of the host where invasions occur. Further, only soldiers readily and consistently attack heterospecifics and conspecifics from other colonies. The division of labour described here for trematodes is strongly analogous to that characterizing other social systems with a soldier caste. The parallel caste formation in these systems, despite varying reproductive mode and taxonomic affiliation, indicates the general importance of ecological factors in influencing the evolution of social behaviour. Further, the ‘recognition of self’ and the defence of the infected host body from invading parasites are comparable to aspects of immune defence. A division of labour is probably widespread among trematodes and trematode species encompass considerable taxonomic, life history and environmental diversity. Trematodes should therefore provide new, fruitful systems to investigate the ecology and evolution of sociality.     

Alternative splicing associated with phenotypic plasticity in the bumble bee Bombus terrestris

Phenotypic plasticity is when one genome can produce more than one phenotype. The caste system found in many social insects is an important example of plasticity. Several studies have examined gene expression in social insect developmental and caste differences. Changes in gene expression, however, are not the only source of phenotypic plasticity. Here, we investigate the role of alternative splicing in the buff‐tailed bumble bee Bombus terrestris. We found that 5,458 genes in B. terrestris (40%) express more than one isoform. Larvae have the lowest level of splicing events, followed by adults and then pupae. We found that when an isoform is expressed in a given caste in the larval stage, it tends to be expressed in all castes at the larval stage. The same is true at the pupal stage. However, we see more complicated interactions between the adult castes with reproductive females showing different isoform expression compared to nonreproductive females and male adults showing the most distinct patterns. We found 455 isoform switching genes, that is genes, where one developmental stage, sex or caste uses a specific isoform and another type uses a different isoform. Among genes displaying isoform switching are some involved in the ecdysteriod pathway, an important system in insect behaviour.     

Clonal mixing in the soldier-producing aphid Pemphigus spyrothecae (Hemiptera: Aphididae)

Illuminating the genetic relationships within soldier-producing aphid colonies is an essential element of any attempt to explain the evolution of the altruistic soldier caste. Pemphigus spyrothecae is a soldier-producing aphid that induces galls on the leaf petioles of its host (trees of the genus Populus). At least a quarter of the aphids within the clonally produced gall population are morphologically and behaviourally distinct first-instar soldiers that defend the gall population from predation. Using field trapping and microsatellites, we investigated the degree of clonal mixing within natural gall populations. Field trapping in the UK showed that all the migrants of P. spyrothecae and of two other Pemphigus species were wingless first-instar soldiers. The average degree of mixing estimated from trapping P. spyrothecae migrants was 0.68% (range = 0-15%). Microsatellite genotyping of 277 aphids from 13 galls collected in Italy revealed an average mixing level of 10.4% (range = 0-59%). Six galls contained more than one clone (range = 2-5 clones). Non-kin aphids were not restricted to the soldier caste but were evenly distributed across instars. An additional gall, from which 527 occupants were genotyped, contained 12 non-kin aphids distributed among nine clones, showing that clonal diversity can be high even when mixing is very low. These observations suggest that although soldiers migrate regularly and can moult and reproduce within foreign galls, clonal mixing in this species is generally low and is unlikely to provide a barrier to the evolution of investment by the aphid clones in an altruistic soldier caste.     

Intracolonial differences in gut bacterial community between worker and soldier castes of Coptotermes formosanus

Abstract The establishment of symbiotic relationships with intestinal microorganisms enables termites to thrive on recalcitrant substrates such as cellulose and wood. A termite colony is composed of several different castes which have distinct feeding habits. The soldiers, for example, cannot feed by themselves and depend on workers, who feed them with digested or semi‐digested foods. To investigate the influence of feeding habits on the bacterial symbionts, a comparative study of gut bacteria between worker and soldier castes of the termite Coptotermes formosanus was conducted. The bacterial communities of both castes were investigated using denaturing gradient gel electrophoresis (DGGE) and clonal analysis of 16S ribosomal DNA (rDNA). Both methods indicated Bacteroidetes was the common predominant group; the common dominant phylotype was affiliated with a reported uncultured Bacteroidetes phylotype (BCf1–03). There were significant differences in Bacteroidetes and Spirochaetes between two castes. Compared to the gut bacteria of workers, those of soldiers were lower in abundance and diversity of Bacteroidetes and slightly higher in Spirochaetes. Two phylotypes (W8, W11) affiliated to Bacteroidetes and two (W26, W29) affiliated to Spirochaetes were exclusively found in the DGGE profile of the worker caste. Bacteroidetes are assumed to be involved in fermentation of sugars and nitrogenous compounds as well as degradation of uric acid. Spirochaetes are supposed to aid in the functions of acetogenesis and N₂‐fixation. The different feeding habits between workers and soldiers of C. formosanus may explain the observed differences in the gut bacterial community.