Reproduction, social behavior, and aging trajectories in honeybee workers

Social life results in social stress protection: a novel concept to explain individual life-history patterns in social insects

Resistance to and avoidance of stress slow aging and confer increased longevity in numerous organisms. Honey bees and other superorganismal social insects have two main advantages over solitary species to avoid or resist stress: individuals can directly help each other by resource or information transfer, and they can cooperatively control their environment. These benefits have been recognised in the context of pathogen and parasite stress as the concept of social immunity, which has been extensively studied. However, we argue that social immunity is only a special case of a general concept that we define here as social stress protection to include group-level defences against all biotic and abiotic stressors. We reason that social stress protection may have allowed the evolution of reduced individual-level defences and individual life-history optimization, including the exceptional aging plasticity of many social insects. We describe major categories of stress and how a colonial lifestyle may protect social insects, particularly against temporary peaks of extreme stress. We use the honey bee (Apis mellifera L.) to illustrate how patterns of life expectancy may be explained by social stress protection and how modern beekeeping practices can disrupt social stress protection. We conclude that the broad concept of social stress protection requires rigorous empirical testing because it may have implications for our general understanding of social evolution and specifically for improving honey bee health.

Telomeres and telomerase: active but complex players in life-history decisions

Studies on human telomeres have established that telomeres exert a significant influence on lifespan and health of organisms. However, recent research has indicated that the original idea that telomeres affect lifespan in a universal and central manner across all eukaryotic species is an oversimplification. Indeed, findings from a variety of animal species revealed that the role of telomere biology in aging is more subtle and intricate than previously recognized. Here, we show how telomere biology varies depending on the taxon. We also show how telomere biology corresponds to basic life history traits and affects the life table of a species and investments in growth, body size, reproduction, and lifespan; telomeres are hypothesized to shape evolutionary perspectives for species in an active but complex manner. Our evaluation is based on telomere biology data from many examples from throughout the animal kingdom that vary according to the degree of organismal complexity and life history strategies.

Advancing methods for the biodemography of aging within social contexts

Several social dimensions including social integration, status, early-life adversity, and their interactions across the life course can predict health, reproduction, and mortality in humans. Accordingly, the social environment plays a fundamental role in the emergence of phenotypes driving the evolution of aging. Recent work placing human social gradients on a biological continuum with other species provides a useful evolutionary context for aging questions, but there is still a need for a unified evolutionary framework linking health and aging within social contexts. Here, we summarize current challenges to understand the role of the social environment in human life courses. Next, we review recent advances in comparative biodemography and propose a biodemographic perspective to address socially driven health phenotype distributions and their evolutionary consequences using a nonhuman primate population. This new comparative approach uses evolutionary demography to address the joint dynamics of populations, social dimensions, phenotypes, and life history parameters. The long-term goal is to advance our understanding of the link between individual social environments, population-level outcomes, and the evolution of aging.

Seasonal changes in ultrastructure and gene expression in the fat body of worker honey bees

The anatomical, physiological, and behavioral characteristics of honey bees are affected by the season as well as division of labor. In this study, we examined the structure, ultrastructure, and gene expression of fat body cells in both long-lived winter and short-lived summer worker bees (the youngest stage of hive bees and forager bees). In contrast to hive bees, foragers and winter bees have a higher metabolism due to intensive muscle activity during their flight (foragers) or endothermic heat production (winter bees). These workers differ from hive bees in the biology of their mitochondria, peroxisomes, and lysosomes as well as in the expression of the genes involved in lipid, carbohydrate, amino acid metabolism, insulin, and TGF- β signaling. Additionally, the expression of genes related to phospholipid metabolism was higher in the hive bees. However, we found no differences between workers in the expression of genes controlling cell organelles, such as the Golgi apparatus, endoplasmic reticulum, ribosomes, nucleus, and vacuoles, as well as genes for DNA replication, cell cycle control, and autophagy. Furthermore, lysosomes, autophagic processes and lipofuscin particles were more frequently observed in winter bees using electron microscopy.

Queen loss increases worker survival in leaf-cutting ants under paraquat-induced oxidative stress

Longevity is traded off with fecundity in most solitary species, but the two traits are positively linked in social insects. In ants, the most fecund individuals (queens and kings) live longer than the non-reproductive individuals, the workers. In many species, workers may become fertile following queen loss, and recent evidence suggests that worker fecundity extends worker lifespan. We postulated that this effect is in part owing to improved resilience to oxidative stress, and tested this hypothesis in three Myrmicine ants: Temnothorax rugatulus, and the leaf-cutting ants Atta colombica and Acromyrmex echinatior. We removed the queen from colonies to induce worker reproduction and subjected workers to oxidative stress. Oxidative stress drastically reduced survival, but this effect was less pronounced in leaf-cutting ant workers from queenless nests. We also found that, irrespective of oxidative stress, outside workers died earlier than inside workers did, likely because they were older. Since At. colombica workers cannot produce fertile offspring, our results indicate that direct reproduction is not necessary to extend the lives of queenless workers. Our findings suggest that workers are less resilient to oxidative stress in the presence of the queen, and raise questions on the proximate and ultimate mechanisms underlying socially mediated variation in worker lifespan. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Reproductive activation in honeybee (Apis mellifera) workers protects against abiotic and biotic stress

Social insect reproductives exhibit exceptional longevity instead of the classic trade-off between somatic maintenance and reproduction. Even normally sterile workers experience a significant increase in life expectancy when they assume a reproductive role. The mechanisms that enable the positive relation between the antagonistic demands of reproduction and somatic maintenance are unclear. To isolate the effect of reproductive activation, honeybee workers were induced to activate their ovaries. These reproductively activated workers were compared to controls for survival and gene expression patterns after exposure to Israeli Acute Paralysis Virus or the oxidative stressor paraquat. Reproductive activation increased survival, indicating better immunity and oxidative stress resistance. After qPCR analysis confirmed our experimental treatments at the physiological level, whole transcriptome analysis revealed that paraquat treatment significantly changed the expression of 1277 genes in the control workers but only two genes in reproductively activated workers, indicating that reproductive activation preemptively protects against oxidative stress. Significant overlap between genes that were upregulated by reproductive activation and in response to paraquat included prominent members of signalling pathways and anti-oxidants known to affect ageing. Thus, while our results confirm a central role of vitellogenin, they also point to other mechanisms to explain the molecular basis of the lack of a cost of reproduction and the exceptional longevity of social insect reproductives. Thus, socially induced reproductive activation preemptively protects honeybee workers against stressors, explaining their longevity. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Comparative transcriptomics of social insect queen pheromones

Queen pheromones are chemical signals that mediate reproductive division of labor in eusocial animals. Remarkably, queen pheromones are composed of identical or chemically similar compounds in some ants, wasps and bees, even though these taxa diverged >150MYA and evolved queens and workers independently. Here, we measure the transcriptomic consequences of experimental exposure to queen pheromones in workers from two ant and two bee species (genera: Lasius, Apis, Bombus), and test whether they are similar across species. Queen pheromone exposure affected transcription and splicing at many loci. Many genes responded consistently in multiple species, and the set of pheromone-sensitive genes was enriched for functions relating to lipid biosynthesis and transport, olfaction, production of cuticle, oogenesis, and histone (de)acetylation. Pheromone-sensitive genes tend to be evolutionarily ancient, positively selected, peripheral in the gene coexpression network, hypomethylated, and caste-specific in their expression. Our results reveal how queen pheromones achieve their effects, and suggest that ants and bees use similar genetic modules to achieve reproductive division of labor.

Evolution of ageing, costs of reproduction and the fecundity-longevity trade-off in eusocial insects

Eusocial insects provide special opportunities to elucidate the evolution of ageing as queens have apparently evaded costs of reproduction and reversed the fecundity–longevity trade-off generally observed in non-social organisms. But how reproduction affects longevity in eusocial insects has rarely been tested experimentally. In this study, we took advantage of the reproductive plasticity of workers to test the causal role of reproduction in determining longevity in eusocial insects. Using the eusocial bumblebee Bombus terrestris, we found that, in whole colonies, in which workers could freely ‘choose’ whether to become reproductive, workers' level of ovarian activation was significantly positively associated with longevity and ovary-active workers significantly outlived ovary-inactive workers. By contrast, when reproductivity was experimentally induced in randomly selected workers, thereby decoupling it from other traits, workers' level of ovarian activation was significantly negatively associated with longevity and ovary-active workers were significantly less long-lived than ovary-inactive workers. These findings show that workers experience costs of reproduction and suggest that intrinsically high-quality individuals can overcome these costs. They also raise the possibility that eusocial insect queens exhibit condition-dependent longevity and hence call into question whether eusociality entails a truly reversed fecundity–longevity trade-off involving a fundamental remodelling of conserved genetic and endocrine networks underpinning ageing.

Group demography affects ant colony performance and individual speed of queen and worker aging

Background

The performance and fitness of social societies mainly depends on the efficiency of interactions between reproductive individuals and helpers. Helpers need to react to the group’s requirements and to adjust their tasks accordingly, while the reproductive individual has to adjust its reproductive rate. Social insects provide a good system to study the interrelations between individual and group characteristics. In general, sterile workers focus on brood care and foraging while the queen lays eggs. Reproductive division of labor is determined by caste and not interchangeable as, e.g., in social mammals or birds. Hence, changing social and environmental conditions require a flexible response by each caste. In the ant Cardiocondyla obscurior, worker task allocation is based on age polyethism, with young workers focusing on brood care and old workers on foraging. Here, we examine how group age demography affects colony performance and fitness in colonies consisting of only old or young workers and a single old or young queen. We hypothesized that both groups will be fully functional, but that the forced task shift affects the individuals’ performance. Moreover, we expected reduced worker longevity in groups with only young workers due to precocious foraging but no effect on queen longevity depending on group composition.

Results

Neither the performance of queens nor that of workers declined strongly with time per se, but offspring number and weight were influenced by queen age and the interaction between queen and worker age. Individual residual life expectancy strongly depended on colony demography instead of physiological age. While worker age affected queen longevity only slightly, exposing old workers to the conditions of colony founding increased their life spans by up to 50% relative to workers that had emerged shortly before colony set-up.

Conclusions

The social environment strongly affected the tempo of aging and senescence in C. obscurior, highlighting the plasticity of life expectancy in social insects. Furthermore, colonies obtained the highest reproductive output when consisting of same-aged queens and workers independent of their physiological age. However, workers appeared to be able to adjust their behavior to the colony’s needs and not to suffer from age-dependent restrictions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-1026-8) contains supplementary material, which is available to authorized users.

Intrinsic worker mortality depends on behavioral caste and the queens' presence in a social insect

According to the classic life history theory, selection for longevity depends on age-dependant extrinsic mortality and fecundity. In social insects, the common life history trade-off between fecundity and longevity appears to be reversed, as the most fecund individual, the queen, often exceeds workers in lifespan several fold. But does fecundity directly affect intrinsic mortality also in social insect workers? And what is the effect of task on worker mortality? Here, we studied how social environment and behavioral caste affect intrinsic mortality of ant workers. We compared worker survival between queenless and queenright Temnothorax longispinosus nests and demonstrate that workers survive longer under the queens' absence. Temnothorax ant workers fight over reproduction when the queen is absent and dominant workers lay eggs. Worker fertility might therefore increase lifespan, possibly due to a positive physiological link between fecundity and longevity, or better care for fertile workers. In social insects, division of labor among workers is age-dependant with young workers caring for the brood and old ones going out to forage. We therefore expected nurses to survive longer than foragers, which is what we found. Surprisingly, inactive inside workers showed a lower survival than nurses but comparable to that of foragers. The reduced longevity of inactive workers could be due to them being older than the nurses, or due to a positive effect of activity on lifespan. Overall, our study points to behavioral caste-dependent intrinsic mortality rates and a positive association between fertility and longevity not only in queens but also in ant workers.

Early life stress affects mortality rate more than social behavior, gene expression or oxidative damage in honey bee workers

Early life stressors can affect aging and life expectancy in positive or negative ways. Individuals can adjust their behavior and molecular physiology based on early life experiences but relatively few studies have connected such mechanisms to demographic patterns in social organisms. Sociality buffers individuals from environmental influences and it is unclear how much early life stress affects later life history. Workers of the honey bee (Apis mellifera L.) were exposed to two stressors, Varroa parasitism and Paraquat exposure, early in life. Consequences were measured at the molecular, behavioral, and demographic level. While treatments did not significantly affect levels of oxidative damage, expression of select genes, and titers of the common deformed wing virus, most of these measures were affected by age. Some of the age effects, such as declining levels of deformed wing virus and oxidative damage, were opposite to our predictions but may be explained by demographic selection. Further analyses suggested some influences of worker behavior on mortality and indicated weak treatment effects on behavior. The latter effects were inconsistent among the two experiments. However, mortality rate was consistently reduced by Varroa mite stress during development. Thus, mortality was more responsive to early life stress than our other response variables. The lack of treatment effects on these measures may be due to the social organization of honey bees that buffers the individual from the impact of stressful developmental conditions.

Ties between ageing plasticity and reproductive physiology in honey bees (Apis mellifera) reveal a positive relation between fecundity and longevity as consequence of advanced social evolution

Honey bees (Apis mellifera) are the best studied model of ageing among the social insects. As in other social insects, the reproductive queen far outlives her non-reproductive workers despite developing from the same genome in the same colony environment. Thus, the different social roles of the two female castes are critical for the profound phenotypic plasticity. In several special cases, such as the reproductive workers of Apis mellifera capensis, within-caste plasticity enables further studies of the fecundity-longevity syndrome in honey bees. At present, molecular evidence suggests that a reorganization of physiological control pathways may facilitate longevity of reproductive individuals. However, the social role and social environment of the different colony members are also very important and one of the key future questions is how much social facilitation versus internal regulation is responsible for the positive association between fecundity and longevity in honey bees.

Intrinsic survival advantage of social insect queens depends on reproductive activation

The central trade-off between reproduction and longevity dominates most species' life history. However, no mortality cost of reproduction is apparent in eusocial species, particularly social insects in the order Hymenoptera: one or a few individuals (typically referred to as queens) in a group specialize on reproduction and are generally longer lived than all other group members (typically referred to as workers), despite having the same genome. However, it is unclear whether this survival advantage is due to social facilitation by the group or an intrinsic, individual property. Furthermore, it is unknown whether the correlation between reproduction and longevity is due to a direct mechanistic link or an indirect consequence of the social role of the reproductives. To begin addressing these questions, we performed a comparison of queen and worker longevity in the ant Cardiocondyla obscurior under social isolation conditions. Survival of single queens and workers was compared under laboratory conditions, monitoring and controlling for brood production. Our results indicate that there is no intrinsic survival advantage of queens relative to workers unless individuals are becoming reproductively active. This interactive effect of caste and reproduction on life expectancy outside of the normal social context suggests that the positive correlation between reproduction and longevity in social insect queens is due to a direct link that can activate intrinsic survival mechanisms to ensure queen longevity.