Stress and early experience underlie dominance status and division of labour in a clonal insect

Maternal manipulation of offspring size can trigger the evolution of eusociality in promiscuous species

Eusocial organisms typically live in colonies with one reproductive queen supported by thousands of sterile workers. It is widely believed that monogamous mating is a precondition for the evolution of eusociality. Here, we present a theoretical model that simulates a realistic scenario for the evolution of eusociality. In the model, mothers can evolve control over resource allocation to offspring, affecting offspring's body size. The offspring can evolve body-size-dependent dispersal, by which they disperse to breed or stay at the nest as helpers. We demonstrate that eusociality can evolve even if mothers are not strictly monogamous, provided that they can constrain their offspring's reproduction through manipulation. We also observe the evolution of social polymorphism with small individuals that help and larger individuals that disperse to breed. Our model unifies the traditional kin selection and maternal manipulation explanations for the evolution of eusociality and demonstrates that-contrary to current consensus belief-eusociality can evolve despite highly promiscuous mating.

Resource sharing is sufficient for the emergence of division of labour

Division of labour occurs in a broad range of organisms. Yet, how division of labour can emerge in the absence of pre-existing interindividual differences is poorly understood. Using a simple but realistic model, we show that in a group of initially identical individuals, division of labour emerges spontaneously if returning foragers share part of their resources with other group members. In the absence of resource sharing, individuals follow an activity schedule of alternating between foraging and other tasks. If non-foraging individuals are fed by other individuals, their alternating activity schedule becomes interrupted, leading to task specialisation and the emergence of division of labour. Furthermore, nutritional differences between individuals reinforce division of labour. Such differences can be caused by increased metabolic rates during foraging or by dominance interactions during resource sharing. Our model proposes a plausible mechanism for the self-organised emergence of division of labour in animal groups of initially identical individuals. This mechanism could also play a role for the emergence of division of labour during the major evolutionary transitions to eusociality and multicellularity.

Genetic divergence and aggressiveness within a supercolony of the invasive ant Linepithema humile

Biological invasions constitute an opportunity to study the evolutionary processes behind species’ adaptations. The invasive potential of some species, like the Argentine ant (Linepithema humile), has likely been increasing because they show low intraspecific competition. However, multiple introductions over time or genetic divergence could increase the probability of intraspecific competition, constituting barriers for their dispersal and thus, decreasing invasive success. Here, we studied the genetic and behavioural variability of L. humile workers collected at six locations on the NW coast of the Iberian Peninsula, a possible scenario for multiple introductions and population divergence, due to its high level of maritime traffic and complex coastal geography. We analysed behaviours related to spatial navigation (exploration, wall-following), resources acquisition, and competition (inter and intraspecific aggressiveness) through two relevant seasons for the nest ecology: spring and autumn. Genetic analyses using microsatellites indicated that the nests studied belonged to the most spread supercolony in South Europe. However, we identified the existence of two genetically differentiated clusters in Galiza. Lethal interactions were found between workers from different and similar genetic clusters, but a trend suggests higher agonistic behaviours between the two genetic groups. Genetic differences were positively correlated with the geographical distance, but aggressiveness was not correlated with any of them. Ants from each of the tested nests expressed different behaviours with high plasticity through time. Ants from all nests showed more exploration and aggressiveness, less wall-following and faster detection of food in autumn than in spring, with no intraspecific aggressiveness observed in spring. Our findings suggest competition between nests of the same supercolony and behavioural seasonal variability, supporting the hypothesis of divergent evolutionary processes. The results of our work question the assumed unity of supercolonies of this species and offer insights for understanding the future adaptation of L. humile in the introduced areas.

Social buffer or avoidance depends on the similarity of stress between queen ants

The association of unrelated ant queens (pleometrosis) is supposed to improve nest foundation and competitiveness under environmental stress, but its evolutionary maintenance is difficult to explain because only one of the queens survives after nest foundation. My aim was to test the potential effect of queen association as a social buffer, i.e. as a mechanism reducing stress and improving fitness due to the benefits of social contact. I analysed the survival, fecundity, and behaviour of isolated and paired Lasius flavus queens exposed and not exposed to stressors (disturbing environmental conditions). I found no difference in survivorship between isolated and paired queens or between stressed and unstressed isolated queens. Groups in which one or two paired queens were stressed showed higher mortality. Unstressed queens died similarly to their stressed nestmates, suggesting stress transmission. A trend suggested that paired queens produced eggs more quickly, but eggs were produced similarly between isolated and paired queens. Social avoidance was observed in groups with one stressed and one unstressed queen. However, the groups with two stressed queens showed the expected behaviours according to social buffering: lower mobility and more interindividual inspection. My findings suggest the synergistic effect of pleometrosis and stress and the dependence of stress level similarity between nestmates on social buffering or rejection on ant queens.

Response thresholds alone cannot explain empirical patterns of division of labor in social insects

The effects of heterogeneity in group composition remain a major hurdle to our understanding of collective behavior across disciplines. In social insects, division of labor (DOL) is an emergent, colony-level trait thought to depend on colony composition. Theoretically, behavioral response threshold models have most commonly been employed to investigate the impact of heterogeneity on DOL. However, empirical studies that systematically test their predictions are lacking because they require control over colony composition and the ability to monitor individual behavior in groups, both of which are challenging. Here, we employ automated behavioral tracking in 120 colonies of the clonal raider ant with unparalleled control over genetic, morphological, and demographic composition. We find that each of these sources of variation in colony composition generates a distinct pattern of behavioral organization, ranging from the amplification to the dampening of inherent behavioral differences in heterogeneous colonies. Furthermore, larvae modulate interactions between adults, exacerbating the apparent complexity. Models based on threshold variation alone only partially recapitulate these empirical patterns. However, by incorporating the potential for variability in task efficiency among adults and task demand among larvae, we account for all the observed phenomena. Our findings highlight the significance of previously overlooked parameters pertaining to both larvae and workers, allow the formulation of theoretical predictions for increasing colony complexity, and suggest new avenues of empirical study.

This study uses automated tracking of clonal raider ants and mathematical modeling to reveal how previously overlooked traits of larvae and workers might shape social organization in heterogeneous ant colonies. By incorporating the potential for variability in task efficiency among adults and task demand among larvae, the authors were able to account for all empirically observed phenomena.

Relaxed selection underlies genome erosion in socially parasitic ant species

Inquiline ants are highly specialized and obligate social parasites that infiltrate and exploit colonies of closely related species. They have evolved many times convergently, are often evolutionarily young lineages, and are almost invariably rare. Focusing on the leaf-cutting ant genus Acromyrmex, we compared genomes of three inquiline social parasites with their free-living, closely-related hosts. The social parasite genomes show distinct signatures of erosion compared to the host lineages, as a consequence of relaxed selective constraints on traits associated with cooperative ant colony life and of inquilines having very small effective population sizes. We find parallel gene losses, particularly in olfactory receptors, consistent with inquiline species having highly reduced social behavioral repertoires. Many of the genomic changes that we uncover resemble those observed in the genomes of obligate non-social parasites and intracellular endosymbionts that branched off into highly specialized, host-dependent niches.

Oxidative stress and senescence in social insects: a significant but inconsistent link?

The life-prolonging effects of antioxidants have long entered popular culture, but the scientific community still debates whether free radicals and the resulting oxidative stress negatively affect longevity. Social insects are intriguing models for analysing the relationship between oxidative stress and senescence because life histories differ vastly between long-lived reproductives and the genetically similar but short-lived workers. Here, we present the results of an experiment on the accumulation of oxidative damage to proteins, and a comparative analysis of the expression of 20 selected genes commonly involved in managing oxidative damage, across four species of social insects: a termite, two bees and an ant. Although the source of analysed tissue varied across the four species, our results suggest that oxidative stress is a significant factor in senescence and that its manifestation and antioxidant defenses differ among species, making it difficult to find general patterns. More detailed and controlled investigations on why responses to oxidative stress may differ across social species may lead to a better understanding of the relations between oxidative stress, antioxidants, social life history and senescence. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Comparative transcriptomic analysis of the mechanisms underpinning ageing and fecundity in social insects

The exceptional longevity of social insect queens despite their lifelong high fecundity remains poorly understood in ageing biology. To gain insights into the mechanisms that might underlie ageing in social insects, we compared gene expression patterns between young and old castes (both queens and workers) across different lineages of social insects (two termite, two bee and two ant species). After global analyses, we paid particular attention to genes of the insulin/insulin-like growth factor 1 signalling (IIS)/target of rapamycin (TOR)/juvenile hormone (JH) network, which is well known to regulate lifespan and the trade-off between reproduction and somatic maintenance in solitary insects. Our results reveal a major role of the downstream components and target genes of this network (e.g. JH signalling, vitellogenins, major royal jelly proteins and immune genes) in affecting ageing and the caste-specific physiology of social insects, but an apparently lesser role of the upstream IIS/TOR signalling components. Together with a growing appreciation of the importance of such downstream targets, this leads us to propose the TI-J-LiFe (TOR/IIS-JH-Lifespan and Fecundity) network as a conceptual framework for understanding the mechanisms of ageing and fecundity in social insects and beyond. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Why behavioral neuroscience still needs diversity?: A curious case of a persistent need

In the past few decades, a substantial portion of neuroscience research has moved from studies conducted across a spectrum of animals to reliance on a few species. While this undoubtedly promotes consistency, in-depth analysis, and a better claim to unraveling molecular mechanisms, investing heavily in a subset of species also restricts the type of questions that can be asked, and impacts the generalizability of findings. A conspicuous body of literature has long advocated the need to expand the diversity of animal systems used in neuroscience research. Part of this need is utilitarian with respect to translation, but the remaining is the knowledge that historically, a diverse set of species were instrumental in obtaining transformative understanding. We argue that diversifying matters also because the current approach limits the scope of what can be discovered. Technological advancements are already bridging several practical gaps separating these two worlds. What remains is a wholehearted embrace by the community that has benefitted from past history. We suggest the time for it is now.

Lipid content influences division of labour in a clonal ant

The fat body, a major metabolic hub in insects, is involved in many functions, e.g. energy storage, nutrient sensing and immune response. In social insects, fat appears to play an additional role in division of labour between egg layers and workers, which specialize in non-reproductive tasks inside and outside their nest. For instance, reproductives are more resistant to starvation, and changes in fat content have been associated with the transition from inside to outside work or reproductive activities. However, most studies have been correlative and we still need to unravel the causal interrelationships between fat content and division of both reproductive and non-reproductive labour. Clonal ants, e.g. Platythyrea punctata, are ideal models for studying task partitioning without confounding variation in genotype and morphology. In this study, we examined the range of variation and flexibility of fat content throughout the lifespan of workers, the threshold of corpulence associated with foraging or reproduction and whether low fat content is a cause rather than a consequence of the transition to foraging. We found that lipid stores change with division of labour from corpulent to lean and, in reverted nurses, back to corpulent. In addition, our data show the presence of fat content thresholds that trigger the onset of foraging or egg-laying behaviour. Our study supports the view that mechanisms that regulate reproduction and foraging in solitary insects, in particular the nutritional status of individuals, have been co-opted to regulate division of labour in colonies of social insects.

Stress and early experience underlie dominance status and division of labour in a clonal insect

Cooperation and division of labour are fundamental in the 'major transitions' in evolution. While the factors regulating cell differentiation in multi-cellular organisms are quite well understood, we are just beginning to unveil the mechanisms underlying individual specialization in cooperative groups of animals. Clonal ants allow the study of which factors influence task allocation without confounding variation in genotype and morphology. Here, we subjected larvae and freshly hatched workers of the clonal ant Platythyrea punctata to different rearing conditions and investigated how these manipulations affected division of labour among pairs of oppositely treated, same-aged clonemates. High rearing temperature, physical stress, injury and malnutrition increased the propensity of individuals to become subordinate foragers rather than dominant reproductives. This is reflected in changed gene regulation: early stages of division of labour were associated with different expression of genes involved in nutrient signalling pathways, metabolism and the phenotypic response to environmental stimuli. Many of these genes appear to be capable of responding to a broad range of stressors. They might link environmental stimuli to behavioural and phenotypic changes and could therefore be more broadly involved in caste differentiation in social insects. Our experiments also shed light on the causes of behavioural variation among genetically identical individuals.