Molecular underpinnings of division of labour among workers in a socially complex termite

Division of labour characterizes all major evolutionary transitions, such as the evolution of eukaryotic cells or multicellular organisms. Social insects are characterized by reproductive division of labour, with one or a few reproducing individuals (queens) and many non-reproducing nestmates (workers) forming a colony. Among the workers, further division of labour can occur with different individuals performing different tasks such as foraging, brood care or building. While mechanisms underlying task division are intensively studied in social Hymenoptera, less is known for termites, which independently evolved eusociality. We investigated molecular mechanisms underlying task division in termite workers to test for communality with social Hymenoptera. We compared similar-aged foraging workers with builders of the fungus-growing termite Macrotermes bellicosus using transcriptomes, endocrine measures and estimators of physiological condition. Based on results for social Hymenoptera and theory, we tested the hypotheses that (i) foragers are in worse physiological conditions than builders, (ii) builders are more similar in their gene expression profile to queens than foragers are, and (iii) builders invest more in anti-ageing mechanism than foragers. Our results support all three hypotheses. We found storage proteins to underlie task division of these similar-aged termite workers and these genes also characterize reproductive division of labour between queens and workers. This implies a co-option of nutrient-based pathways to regulate division of labour across lineages of termites and social Hymenoptera, which are separated by more than 133 million years.

Does the evolution of division of labour require accelerating returns from individual specialisation?

Recent theory has overturned the assumption that accelerating returns from individual specialisation are required to favour the evolution of division of labour. Yanni et al. (2020) showed that topologically constrained groups, where cells cooperate with only direct neighbours such as for filaments or branching growths, can evolve a reproductive division of labour even with diminishing returns from individual specialisation. We developed a conceptual framework and specific models to investigate the factors that can favour the initial evolution of reproductive division of labour. We found that selection for division of labour in topologically constrained groups: (1) is not a single mechanism to favour division of labour – depending upon details of the group structure, division of labour can be favoured for different reasons; (2) always involves an efficiency benefit at the level of group fitness; and (3) requires a mechanism of coordination to determine which individuals perform which tasks. Given that such coordination is unlikely to evolve before division of labour, this limits the extent to which topological constraints could have favoured the initial evolution of division of labour. We conclude by suggesting experimental designs that could determine why division of labour is favoured in the natural world.

Suppressed eusocial reproduction supports evolutionary convergence over co-option

Eusocial insects are characterized by the presence of division of labour between reproductive (i.e. workers) and non-reproductive (i.e. queens) individuals. In eusocial insects, such as wasps, ants, honeybees and termites, such reproductive division of labour is mediated by the use of unique pheromones. In eusocial animals, pheromones control two distinct aspects of the division of labour: reproduction and division of tasks amongst workers. It has been suggested that eusociality arose independently on several occasions from pheromone-mediated co-option of pathways regulating the reproductive life cycle of solitary insects. Transcriptomic analysis has supported the co-option of similar pathways in each independently evolved eusocial insect. Using a solitary insect model, we sought to determine if these similar transcriptomic signals resulted from the co-option of similar pathways or varying pathways subject to convergent evolution. We measured the transcriptomic and physiological response of Drosophila melanogaster to pheromones from bumblebees, honey bees, and termites. In each case we observed a strong physiological response - the reduction of ovary size. However, employing conventional differential gene expression analysis and several forms of network analysis, we were unable to detect any conserved pathway or genes acting as a primer for eusociality. This strongly suggests that pheromone-mediated eusociality is the result of convergent evolution. Wherein a physiological response, such as reduced reproductive capacity, acts as the primer for eusociality and is subsequently refined.

Eusociality through conflict dissolution

Eusociality, where largely unreproductive offspring help their mothers reproduce, is a major form of social organization. An increasingly documented feature of eusociality is that mothers induce their offspring to help by means of hormones, pheromones or behavioural displays, with evidence often indicating that offspring help voluntarily. The co-occurrence of maternal influence and offspring voluntary help may be explained by what we call the converted helping hypothesis, whereby maternally manipulated helping subsequently becomes voluntary. Such hypothesis requires that parent-offspring conflict is eventually dissolved—for instance, if the benefit of helping increases sufficiently over evolutionary time. We show that help provided by maternally manipulated offspring can enable the mother to sufficiently increase her fertility to transform parent-offspring conflict into parent-offspring agreement. This conflict-dissolution mechanism requires that helpers alleviate maternal life-history trade-offs, and results in reproductive division of labour, high queen fertility and honest queen signalling suppressing worker reproduction—thus exceptionally recovering diverse features of eusociality. As such trade-off alleviation seemingly holds widely across eusocial taxa, this mechanism offers a potentially general explanation for the origin of eusociality, the prevalence of maternal influence, and the offspring’s willingness to help. Overall, our results explain how a major evolutionary transition can happen from ancestral conflict.

Honeybee Queen mandibular pheromone induces starvation in Drosophila melanogaster, implying a role for nutrition signalling in the evolution of eusociality.

Eusocial insect societies are defined by the reproductive division of labour, a social structure that is generally enforced by the reproductive dominant or queen. Reproductive dominance is maintained through behavioural dominance in some species as well as production of queen pheromones in others, or a mixture of both. Queen mandibular pheromone (QMP) is produced by honeybee (Apis mellifera) queens and has been characterised chemically. How QMP acts to repress worker reproduction, and how it has evolved this activity, remains less well understood. Surprisingly, QMP is capable of repressing reproduction in non-target arthropods which have not co-evolved with QMP, are never exposed to QMP in nature, and are up to 530 million years diverged from the honeybee. Here we show that, in Drosophila melanogaster, QMP treatment mimics nutrient limiting conditions, leading to disrupted reproduction. Exposure to QMP induces an increase in food consumption, consistent with that observed in D. melanogaster in response to starvation conditions. This response induces the activation of two checkpoints within the ovary that inhibit oogenesis. The first is the 2a/b ovarian checkpoint in the germarium, which reduces the flow of presumptive oocytes. A stage 9 ovarian checkpoint is also activated, causing degradation of oocytes. The magnitude of activation of both checkpoints is indistinguishable between QMP treated and starved individuals. As QMP seems to trigger a starvation response in an insect highly diverged from honeybees, we propose that QMP originally evolved by co-opting nutrition signalling pathways to regulate reproduction, a key step in the evolution of eusociality.

Reproduction and signals regulating worker policing under identical hormonal control in social wasps

In social Hymenoptera, fertility and fertility signalling are often under identical hormonal control, and it has been suggested that such hormonal pleiotropies can help to maintain signal honesty. In the common wasp Vespula vulgaris, for example, fertile queens have much higher juvenile hormone (JH) titers than workers, and JH also controls the production of chemical fertility cues present on the females’ cuticle. To regulate reproductive division of labour, queens use these fertility cues in two distinct ways: as queen pheromones that directly suppress the workers’ reproduction as well as to mark queen eggs and enable the workers to recognize and police eggs laid by other workers. Here, we investigated the hormonal pleiotropy hypothesis by testing if experimental treatment with the JH analogue methoprene could enable the workers to lay eggs that evade policing. In support of this hypothesis, we find that methoprene-treated workers laid more eggs, and that the chemical profiles of their eggs were more queen-like, thereby causing fewer of their eggs to be policed compared to in the control. Overall, our results identify JH as a key regulator of both reproduction and the production of egg marking pheromones that mediate policing behaviour in eusocial wasps.

Morphological evolution in the ant reproductive caste

The evolution of eusociality has led to considerable changes in the general hymenopteran body plan. In particular, the evolution of reproductive division of labour caused the worker caste to be largely freed from the demands involved in reproduction. As a consequence, workers were able to evolve highly specialized morphologies for foraging and colony maintenance, whereas the reproductive caste became specialized for reproduction. Despite these important changes, little is known about the general patterns of morphological evolution within the ant reproductive caste. Our goals here were to characterize morphological variation in the ant reproductive caste and to test whether different sexes display variation in their evolutionary rates. We obtained measurements of 897 specimens from a total of 678 ant species. The shapes of the size distributions were similar between sexes, with queens being larger than males in all traits except for eye length. Contrary to the expectation based on Rensch’s rule, although queens were larger, the degree of dimorphism increased with body size. Finally, there was strong evidence for an accelerated tempo of morphological evolution in queens in relation to males. These results represent the first comprehensive treatment of morphological variation in the ant reproductive caste and provide important new insights into their evolution.

Morphological evolution in the ant reproductive caste

The evolution of eusociality led to severe changes in the general hymenopteran body plan. In particular, the evolution of reproductive division of labour caused the worker caste to be largely freed from the demands involved in reproduction. As a consequence, workers were able to evolve highly specialized morphologies for foraging and colony maintenance, whereas the reproductive caste became specialized for reproduction. Despite these important changes, little is known about general patterns of morphological evolution within the ant reproductive caste. Our goals were to characterize morphological variation in the ant reproductive caste and to test whether different sexes display variation in their evolutionary rates. We obtained measurements of 897 specimens from a total of 678 ant species. The shapes of the size distributions were similar between sexes, with queens being larger than males in all traits except for eye length. Contrary to the expectation based on Rensch’s rule, although queens were larger, the degree of dimorphism increased with body size. Finally, there is strong evidence for an accelerated tempo of morphological evolution in queens in relation to males. These results represent the first comprehensive treatment of morphological variation in the ant reproductive caste and provide important new insights into their evolution.

Cooperative policing behaviour regulates reproductive division of labour in a termite

Reproductive conflicts are common in insect societies where helping castes retain reproductive potential. One of the mechanisms regulating these conflicts is policing, a coercive behaviour that reduces direct reproduction by other individuals. In eusocial Hymenoptera (ants, bees and wasps), workers or the queen act aggressively towards fertile workers, or destroy their eggs. In many termite species (order Blattodea), upon the death of the primary queen and king, workers and nymphs can differentiate into neotenic reproductives and inherit the breeding position. During this process, competition among neotenics is inevitable, but how this conflict is resolved remains unclear. Here, we report a policing behaviour that regulates reproductive division of labour in the eastern subterranean termite, Reticulitermes flavipes . Our results demonstrate that the policing behaviour is a cooperative effort performed sequentially by successful neotenics and workers. A neotenic reproductive initiates the attack of the fellow neotenic by biting and displays alarm behaviour. Workers are then recruited to cannibalize the injured neotenic. Furthermore, the initiation of policing is age-dependent, with older reproductives attacking younger ones, thereby inheriting the reproductive position. This study provides empirical evidence of policing behaviour in termites, which represents a convergent trait shared between eusocial Hymenoptera and Blattodea.

Mechanisms to divide labour in social microorganisms

Division of labour, where cooperating individuals specialise to perform complementary tasks, plays a fundamental role at all levels of biological complexity, from genes to complex animal societies. Different labour-dividing species employ different mechanisms to determine how tasks are allocated between individuals, including coordinated, random, and genetic determination of phenotype (caste). It is not clear if this diversity is adaptive—arising because different mechanisms are favoured in different environments—or is merely the result of non-adaptive, historical artifacts of evolution. We analyse theoretically the relative advantages of the two dominant mechanisms employed for reproductive division of labour in social microorganisms: coordinated and random specialisation. We show that coordinated specialisation is more likely to evolve over random specialisation when: (i) social groups are small; (ii) cooperation has a greater relative impact on fitness (i.e., it is more “essential”); and (iii) there is a low cost to coordination. We also show how a fully coordinated group of cells can evolve step-wise from less precise forms of coordination. Our predictions are in broad qualitative agreement with empirical patterns across microorganism species that divide labour. More generally, our results show how evolutionary theory can help explain both why between-individual coordination evolves, and why there is mechanistic variation between species.