Abstract
The evolution of a reproductively altruistic caste contributed to the success of eusociality through the division of labour. In termites, the proportion of soldiers in the colonies increased ...throughout the group's evolution. In Nasutitermitinae, soldiers have a crucial role in defence, resource selection and foraging. However, the role of soldiers and workers in colony immunity is still poorly understood.
Here, we evaluated the role of mixed caste groups in the social immunity of
Nasutitermes corniger
(Termitidae: Nasutitermitinae) exposed to the entomopathogenic fungi
Metarhizium anisopliae
(Metsch.). We tested the hypothesis that the presence of workers and soldiers optimizes defence against pathogens, and this defence is increased when the soldier ratio is closer to what naturally occurs in colonies.
Our results showed that mixed caste groups of
N. corniger
exposed to
M. anisopliae
survive longer compared to groups with only one caste. In addition, termite groups that died from fungi were less likely to show fungal infection in mixed caste groups.
The behavioural observations suggest that in the natural proportion of soldiers, allogrooming and trophallaxis play a crucial role in the control of disease and its death hazard in termites. Soldiers may play an important role in colony immunity, being able to start the alarm signal indicating the threat of pathogens.
Our results show that the presence of the two castes promotes possible mechanisms for socially mediated immunity. This study may help in the understanding of the function of the soldiers and workers and the significance of pathogens in termite eusocial evolution.
The dominance-nutrition hypothesis predicts that nutritional intake and energetic costs in adulthood interact to drive behavioral and physiological differences between females in primitively eusocial ...insects, and thereby affect reproductive caste. We tested predictions of this hypothesis in independent-founding Mischocyttarus pallidipectus paper wasps. We measured stable isotope tissue composition to compare nutritional status before adult emergence and among adult females. Adult tissue d 15 N content (an indicator of feeding at a higher trophic level because it is enriched in animal prey relative to plant-based foods) was significantly higher in adults than at the end of pupal development, suggesting adult nutrition affects δ¹⁵N content after the end of pupal development. We then asked if behavior and nutritional status predicted ovary development. We measured ovary development, nitrogen and carbon stable isotope ratios, dominance behavior, and task performance (foraging, as indicated by time spent on the nest) for adult female wasps. We used social network analysis to quantify differences in social status between females with developed and undeveloped (filamentous) ovaries. Dominant females spent more time on the nest and were significantly more enriched in δ¹⁵N than subordinate females. These data support the dominance-nutrition hypothesis: adult behavior and energy expenditure, and access to animal-based diets, correspond to female reproductive physiology, and may play a role in adult-stage caste determination.
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.
Abstract
Caste differentiation in termites is one of the most conspicuous examples of facultative polyphenism in animals. It is clear that specific cuticular formation occurs in hard exocuticles ...during caste differentiation. However, the developmental pattern of the soft endocuticle in the differentiation pathways of castes is unknown. To reveal whether the endocuticle is involved in caste differentiation, we compared the exocuticle and endocuticle thickness of individuals in 2 pathways (nymph line and worker line) of caste differentiation in the termite Reticulitermes aculabialis. The endocuticle protein genes were identified by transcriptome analysis and the expression patterns of these genes were confirmed in caste differentiation. We found that the endocuticle structure showed dynamic changes in 2 pathways, and the first difference in endocuticle structure occurred after larvae differentiation bifurcated into workers and nymphs. The thinning of the endocuticle was a significant event from nymphs developing into alates with the thickest exocuticle and thinnest endocuticle. The thickest endocuticle layers were found in the heads of the workers and the ultrastructure of the endocuticle in the heads was more complex than that in the thorax–abdomens. Six endocuticle protein genes were identified and annotated as endocuticle structural glycoproteins SgAbd-2, SgAbd-9, and Abd-5. The expression levels of endocuticle protein genes changed dramatically during caste development and the expression levels in neotenic reproductives (secondary reproductives) were significantly higher than those in alates (primary reproductives). These results reveal the roles of endocuticles in caste differentiation and adaptation to the environment.
Desaturase enzymes play an essential role in the biosynthesis of unsaturated fatty acids (UFAs). In this study, we identified seven "first desaturase" subfamily genes (Cfor-desatA1, Cfor-desatA2-a, ...Cfor-desatA2-b, Cfor-desatB-a, Cfor-desatB-b, Cfor-desatD and Cfor-desatE) from the Formosan subterranean termite Coptotermes formosanus. These desaturases were highly expressed in the cuticle and fat body of C. formosanus. Inhibition of either the Cfor-desatA2-a or Cfor-desatA2-b gene resulted in a significant decrease in the contents of fatty acids (C16:0, C18:0, C18:1 and C18:2) in worker castes. Moreover, we observed that inhibition of most of desaturase genes identified in this study had a negative impact on the survival rate and desiccation tolerance of workers. Interestingly, when normal soldiers were reared together with dsCfor-desatA2-b-treated workers, they exhibited higher mortality, suggesting that desaturase had an impact on trophallaxis among C. formosanus castes. Our findings shed light on the novel roles of desaturase family genes in the eusocial termite C. formosanus.
Showing traces of eusocial behavior,
Zethus miniatus
is an emblematic member of the primarily solitary subfamily Eumeninae. The females of this casteless species share a common nest, but each one ...provisions her own brood in a progressive manner. They express dominance when reproductive (queen-like behavior) and occasionally perform tasks that benefit others when provisioning the brood (worker-like behavior). Hence, the biology of this species has long been considered as harboring traits that could mark the transition from a solitary to social lifestyle. Here, we present for the first time measurements of the juvenile hormone (JH) titer in combination with behavioral observations and data on the females’ ovarian status. Aggressive reproductives seeking to reuse or usurp brood cells were found to possess longer oocytes than provisioners of young larvae. Brood killings by reproductive cell-seekers and JH titers were both higher when the female/open-brood-cell ratio of the nest was high—an indicator of strong competition. Females lacking brood, such as those that are victims of cell usurpation attempts by other females, were found to perform tasks relating to nest defense, nest maintenance, and/or adoption of orphaned brood. Hence, we hypothesize that competition for brood cells may have shaped the reproductive physiology of this group-living species of social wasps, and that JH is critical for this competitive behavior. Furthermore, we hypothesize that JH-mediated dominance interactions in casteless groups may have preceded and contributed to the origin of reproductive castes.
Significance statement
How, mechanistically, did caste phenotypes (a polyphenism with egg-layers and non-egg-laying helper females within groups) originate from solitary ancestors? Key to addressing this question can be casteless, group-living wasp species, where the females progressively feed their brood. The eumenine wasp
Zethus miniatus
has behavioral traits intermediate between solitary and caste-possessing wasps, and hence, has gained a model status for this transition. This is the first study of such a wasp in nature that combines data on behavior, ovarian activity, and juvenile hormone (JH) levels. The results show that the ovarian status of a
Z
.
miniatus
female is a good predictor of her behavior, and that JH titers were highest when competition for available brood cells was intense. This suggests that JH may have become linked to the origin of worker caste behaviors through its involvement in competitive interactions among nestmates.
Certain species of parasitic flies belonging to the Phoridae are known to attack Atta spp. workers foraging along trails, near nest openings used by the ants to supply the colony with plant material, ...and in the areas where the ants are actively cutting plant material. However, there have been no previous studies of phorid parasitism of non-foraging worker ants, for example excavators and soldiers. Excavators can be found on the surface around specialized nest openings, carrying and dumping soil on characteristic mounds. Soldiers can be found on the trails protecting foragers or guarding the different types of nest openings. The current study was performed to investigate the differential parasitism rates of Atta laevigata (Smith, 1858) worker castes by four species of phorids. Ants of all castes on trails and at nest entrances were collect from 18 mature colonies in the field. A total of 21,254 ants were collected from trails and 14,649 collected from the mounds of loose soil near nest openings. The captured workers were maintained under controlled laboratory conditions to evaluate the rate of parasitism. Of the ants collected from trails, 1,112 (5.23%) were found to have been parasitized, of which 1,102 were foragers and only 10 were soldiers. Of the ants collected from the soil mounds near the nest openings, only 27 (0.18%) were found to have been parasitized, of those 25 were excavators and 2 were soldiers. When evaluating parasitism of ants on the trails, 46.2% were attacked by Apocephalus attophilus Borgmeier, 1928, 22.6% by Myrmosicarius grandicornis Borgmeier, 1928, 16.6% by Eibesfeldtphora erthali (Brown, 2001) and 14.6% by Apocephalus vicosae Disney, 2000. Only two species of phorid, M. grandicornis and E. erthali, were observed parasitizing excavators, whilst only E. erthali parasitized soldiers. This is the first time that Atta spp. excavators and soldiers have been shown to be parasitized by phorids. The low rates of parasitism and specificity of certain phorid species for excavators and soldiers is discussed in relation to the behavioral interactions of hosts and their parasitoids, as well as the relationship between host and parasitoid size.
In the past few years, however, journalists, student groups and researchers have been gathering diversity data using public-information laws, and arguing for change. (Minz is the second Adivasi woman ...to hold a vice-chancellorship in India.) Samadhan says that when he started a life-science bachelor's degree in 2009, students from privileged castes often called him and other students from marginalized communities "free off" - a slur referring to students on government aid. An Adivasi PhD student from a science department at Delhi University, for instance, told Nature that in 2018, when she approached a privileged-caste professor to be her supervisor, he responded that she was a "quota candidate" and could find a supervisor anywhere. ...the Department of Science and Technology (DST), one of India's two main science-funding government agencies, did share data with Nature on postdoctoral researchers whom it had awarded INSPIRE Faculty Fellowships - positions aimed at supporting young talent, which represent an important but small part of the DST's total funding.
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.
Gender inequality in literacy is higher in rural areas compared to urban areas and also in Scheduled Castes (SCs) and Scheduled Tribes (STs) compared to other communities in India. Comprehensive ...studies across regions and social groups are required to reduce gender inequality in literacy in this country. In this context, the study attempts to analyse the nature of gender gap in rural literacy of Bankura district in West Bengal, India from 1961 to 2011. Gender gap in rural literacy in this district has been assessed using logged gender odds ratios. Time series analysis has also been used to estimate the time period to achieve 100% rural female literacy in this district. During the study period, decline in gender gap in rural literacy has been observed in Bankura district, however, this gap has remained comparatively high in the western part of the district. Expansion of education infrastructure and adequate support towards the girls of agricultural distressed families are essential for reduction in gender gap in rural literacy. Special attention is also required towards the female education in SCs and STs of Bankura district in this context.
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