Aging and reproduction in social insects--a mini-review

Offspring nursing extends mother's longevity in a long-term maternal cared spider

Females typically outlive males in animals, especially in species that provide long-term maternal care. However, life history theory predicts that investments in reproduction, such as lactation and offspring nursing, often shorten caretakers' longevity. Aiming to interpret this paradox, we selected the lactating jumping spider Toxeus magnus to investigate the effects of reproductive activities on longevity for two sexes. We found that: (1) although "milk" provisioning reduces female's longevity, mothers who cared for offspring (provisioned "milk" and nursing) lived the longest compared to virgins and those did not provide care; (2) copulation increased female's longevity but had no effects on males; and (3) the two sexes have comparable developmental duration, but the female adult's longevity was 2.1 times that of male's. This study suggests that the time requirement for offspring dispersal might act as a key selective force favoring females' adulthood extension, which ultimately generates the longer-lived females in maternal cared species.

Age-related flexibility of energetic metabolism in the honey bee Apis mellifera

The mechanisms that underpin aging are still elusive. In this study, we suggest that the ability of mitochondria to oxidize different substrates, which is known as metabolic flexibility, is involved in this process. To verify our hypothesis, we used honey bees (Apis mellifera carnica) at different ages, to assess mitochondrial oxygen consumption and enzymatic activities of key enzymes of the energetic metabolism as well as ATP5A1 content (subunit of ATP synthase) and adenylic energy charge (AEC). We also measured mRNA abundance of genes involved in mitochondrial functions and the antioxidant system. Our results demonstrated that mitochondrial respiration increased with age and favored respiration through complexes I and II of the electron transport system (ETS) while glycerol-3-phosphate (G3P) oxidation was relatively decreased. In addition, glycolytic, tricarboxylic acid cycle and ETS enzymatic activities increased, which was associated with higher ATP5A1 content and AEC. Furthermore, we detected an early decrease in the mRNA abundance of subunits of NADH ubiquinone oxidoreductase subunit B2 (NDUFB2, complex I), mitochondrial cytochrome b (CYTB, complex III) of the ETS as well as superoxide dismutase 1 and a later decrease for vitellogenin, catalase and mitochondrial cytochrome c oxidase subunit 1 (COX1, complex IV). Thus, our study suggests that the energetic metabolism is optimized with aging in honey bees, mainly through quantitative and qualitative mitochondrial changes, rather than showing signs of senescence. Moreover, aging modulated metabolic flexibility, which might reflect an underpinning mechanism that explains lifespan disparities between the different castes of worker bees.

Effect of food restriction on survival and reproduction of a termite

Food availability affects the trade-off between maintenance and reproduction in a wide range of organisms, but its effects on social insects remain poorly understood. In social insects, the maintenance-reproduction trade-off seems to be absent in individuals but may appear at the colony level, although this is rarely investigated. In this study, we restricted food availability in a termite species to test how it affects survival and reproduction, both at the individual and colony level. Using Bayesian multivariate response models, we found very minor effects of food restriction on the survival of queens, individual workers or on the colonies. In contrast, queen fecundity was significantly reduced, whereas colony-level fecundity (i.e., the number of dispersing alates, future reproductives) increased under food restriction as workers gave up cooperation within the colony and became alates that dispersed. Our study shows that life-history trade-offs can be mitigated by individuals' social behaviours in social organisms.

Histone acetylation regulates the expression of genes involved in worker reproduction in the ant Temnothorax rugatulus

Background

In insect societies, queens monopolize reproduction while workers perform tasks such as brood care or foraging. Queen loss leads to ovary development and lifespan extension in workers of many ant species. However, the underlying molecular mechanisms of this phenotypic plasticity remain unclear. Recent studies highlight the importance of epigenetics in regulating plastic traits in social insects. Thus, we investigated the role of histone acetylation in regulating worker reproduction in the ant Temnothorax rugatulus. We removed queens from their colonies to induce worker fecundity, and either fed workers with chemical inhibitors of histone acetylation (C646), deacetylation (TSA), or the solvent (DMSO) as control. We monitored worker number for six weeks after which we assessed ovary development and sequenced fat body mRNA.

Results

Workers survived better in queenless colonies. They also developed their ovaries after queen removal in control colonies as expected, but not in colonies treated with the chemical inhibitors. Both inhibitors affected gene expression, although the inhibition of histone acetylation using C646 altered the expression of more genes with immunity, fecundity, and longevity functionalities. Interestingly, these C646-treated workers shared many upregulated genes with infertile workers from queenright colonies. We also identified one gene with antioxidant properties commonly downregulated in infertile workers from queenright colonies and both C646 and TSA-treated workers from queenless colonies.

Conclusion

Our results suggest that histone acetylation is involved in the molecular regulation of worker reproduction, and thus point to an important role of histone modifications in modulating phenotypic plasticity of life history traits in social insects.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08196-8.

Temporal and sequential order of nonoverlapping gene networks unraveled in mated female Drosophila

Mating triggers successive waves of temporal transcriptomic changes within independent gene networks in female Drosophila, suggesting a recruitment of interconnected modules that vanish in late life.

In this study, we reanalyzed available datasets of gene expression changes in female Drosophila head induced by mating. Mated females present metabolic phenotypic changes and display behavioral characteristics that are not observed in virgin females, such as repulsion to male sexual aggressiveness, fidelity to food spots selected for oviposition, and restriction to the colonization of new niches. We characterize gene networks that play a role in female brain plasticity after mating using AMINE, a novel algorithm to find dysregulated modules of interacting genes. The uncovered networks of altered genes revealed a strong specificity for each successive period of life span after mating in the female head, with little conservation between them. This finding highlights a temporal order of recruitment of waves of interconnected genes which are apparently transiently modified: the first wave disappears before the emergence of the second wave in a reversible manner and ends with few consolidated gene expression changes at day 20. This analysis might document an extended field of a programmatic control of female phenotypic traits by male seminal fluid.

Biomarkers in a socially exchanged /fluid reflect colony maturity, behavior, and distributed metabolism

In cooperative systems exhibiting division of labor, such as microbial communities, multicellular organisms, and social insect colonies, individual units share costs and benefits through both task specialization and exchanged materials. Socially exchanged fluids, like seminal fluid and milk, allow individuals to molecularly influence conspecifics. Many social insects have a social circulatory system, where food and endogenously produced molecules are transferred mouth-to-mouth (stomodeal trophallaxis), connecting all the individuals in the society. To understand how these endogenous molecules relate to colony life, we used quantitative proteomics to investigate the trophallactic fluid within colonies of the carpenter ant Camponotus floridanus. We show that different stages of the colony life cycle circulate different types of proteins: young colonies prioritize direct carbohydrate processing; mature colonies prioritize accumulation and transmission of stored resources. Further, colonies circulate proteins implicated in oxidative stress, ageing, and social insect caste determination, potentially acting as superorganismal hormones. Brood-caring individuals that are also closer to the queen in the social network (nurses) showed higher abundance of oxidative stress-related proteins. Thus, trophallaxis behavior could provide a mechanism for distributed metabolism in social insect societies. The ability to thoroughly analyze the materials exchanged between cooperative units makes social insect colonies useful models to understand the evolution and consequences of metabolic division of labor at other scales.

Division of labor is essential for cooperation, because groups can achieve more when individuals specialize in different tasks. This happens across the natural world, from different cells in organisms performing specific roles, to the individuals in an ant colony carrying out diverse duties. In both of these systems, individuals work together to ensure the survival of the collective unit – the body or the colony – instead of competing against each other. One of the main ways division of labor is evident within these two systems is regarding reproduction. Both in the body and in an ant colony, only one or a few individual units can reproduce, while the rest provide support. In the case of ant colonies, only queens and males reproduce, while the young workers nurse the brood and older workers forage for food.

This intense cooperation requires close communication between individual units – in the case of some species of ants, by sharing fluids mouth-to-mouth. These fluids contain food but also many molecules produced by the ants themselves, including proteins. Given that both individuals and the colony as a whole change as they age – with workers acquiring new roles, and new queens and males only reared once the colony is mature – it is likely that the proteins transmitted in the fluid also change.

To better understand whether the lifecycles of individuals and the age of the colony affect the fluids shared by carpenter ants Camponotus floridanus, Hakala et al. examined the ant-produced proteins in these fluids. This revealed differences in the proteins shared by young and mature colonies, and young nurse ants and older forager ants. In young colonies, the fluids contained proteins involved in fast sugar processing; while in mature colonies, the fluids contained more proteins to store nutrients, which help insect larvae grow into larger individuals, like queens. Young worker ants, who spend their time nursing the brood, produced more anti-aging proteins. This may be because these ants are in close contact with the queen, who lives much longer than the rest of the ants in the colony. Taken together, these observations suggest that ants divide the labor of metabolism, as well as work and reproduction.

Dividing the labor of metabolism among individuals is one more similarity between ants and the cells of a multicellular organism, like a fly or a human. Division of labor allows the sharing of burden, with some individuals lightening the load of others. Understanding how ants achieve this by sharing fluids could shed new light on this complex exchange at other scales or in other organisms. By matching proteins to life stages, researchers have a starting point to examine individual molecules in more detail.

Senescence as a trade-off between successful land colonisation and longevity: critical review and analysis of a hypothesis

BACKGROUND

Most common terrestrial animal clades exhibit senescence, suggesting strong adaptive value of this trait. However, there is little support for senescence correlated with specific adaptations. Nevertheless, insects, mammals, and birds, which are the most common terrestrial animal clades that show symptoms of senescence, evolved from clades that predominantly did not show symptoms of senescence. Thus, we aimed to examine senescence in the context of the ecology and life histories of the main clades of animals, including humans, and to formulate hypotheses to explain the causes and origin of senescence in the major clades of terrestrial animals.

METHODOLOGY

We reviewed literature from 1950 to 2020 concerning life expectancy, the existence of senescence, and the adaptive characteristics of the major groups of animals. We then proposed a relationship between senescence and environmental factors, considering the biology of these groups of animals. We constructed a model showing the phylogenetic relationships between animal clades in the context of the major stages of evolution, distinguishing between senescent and biologically 'immortal' clades of animals. Finally, we synthesised current data on senescence with the most important concepts and theories explaining the origin and mechanisms of senescence. Although this categorisation into different senescent phenotypes may be simplistic, we used this to propose a framework for understanding senescence.

RESULTS

We found that terrestrial mammals, insects, and birds show senescence, even though they likely evolved from non-senescent ancestors. Moreover, secondarily aquatic animals show lower rate of senescence than their terrestrial counterparts. Based on the possible life histories of these groups and the analysis of the most important factors affecting the transition from a non-senescent to senescent phenotype, we conclude that aging has evolved, not as a direct effect, but as a correlated response of selection on developmental strategies, and that this occurred separately within each clade. Adoption of specific life history strategies could thus have far-reaching effects in terms of senescence and lifespan.

CONCLUSIONS

Our analysis strongly suggests that senescence may have emerged as a side effect of the evolution of adaptive features that allowed the colonisation of land. Senescence in mammals may be a compromise between land colonisation and longevity. This hypothesis, is supported by palaeobiological and ecological evidence. We hope that the development of new research methodologies and the availability of more data could be used to test this hypothesis and shed greater light on the evolution of senescence.

Age-related mushroom body expansion in male sweat bees and bumble bees

A well-documented phenomenon among social insects is that brain changes occur prior to or at the onset of certain experiences, potentially serving to prime the brain for specific tasks. This insight comes almost exclusively from studies considering developmental maturation in females. As a result, it is unclear whether age-related brain plasticity is consistent across sexes, and to what extent developmental patterns differ. Using confocal microscopy and volumetric analyses, we investigated age-related brain changes coinciding with sexual maturation in the males of the facultatively eusocial sweat bee, Megalopta genalis, and the obligately eusocial bumble bee, Bombus impatiens. We compared volumetric measurements between newly eclosed and reproductively mature males kept isolated in the lab. We found expansion of the mushroom bodies-brain regions associated with learning and memory-with maturation, which were consistent across both species. This age-related plasticity may, therefore, play a functionally-relevant role in preparing male bees for mating, and suggests that developmentally-driven neural restructuring can occur in males, even in species where it is absent in females.

Sex, mating and repeatability of Drosophila melanogaster longevity

Costs of reproduction are seemingly ubiquitous across the animal kingdom, and these reproductive costs are generally defined by increased reproduction leading to decreases in other fitness components, often longevity. However, some recent reports question whether reproductive costs exist in every species or population. To provide insight on this issue, we sought to determine the extent to which genetic variation might play a role in one type of reproductive cost-survival-using Drosophila melanogaster. We found, surprisingly, no costs of reproduction nor sex differences in longevity across all 15 genetic backgrounds in two cohorts. We did find significant variation within some genotypes, though these were much smaller than expected. We also observed that small laboratory changes lead to significant changes in longevity within genotypes, suggesting that longevity repeatability in flies may be difficult. We finally compared our results to previously published longevities and found that reproducibility is similar to what we saw in our own laboratory, further suggesting that stochasticity is a strong component of fruit fly lifespan. Overall, our results suggest that there are still large gaps in our knowledge about the effects of sex and mating, as well as genetic background and laboratory conditions on lifespan reproducibility.

Vitellogenin expression in the ovaries of adult honeybee workers provides insights into the evolution of reproductive and social traits

Social insects are notable for having two female castes that exhibit extreme differences in their reproductive capacity. The molecular basis of these differences is largely unknown. Vitellogenin (Vg) is a powerful antioxidant and insulin-signalling regulator used in oocyte development. Here we investigate how Royal Jelly (the major food of honeybee queens) and queen mandibular pheromone (a major regulator of worker fertility), affect the longevity and reproductive status of honey bee workers, the expression of Vg, its receptor VgR and associated regulatory proteins. We find that Vg is expressed in the ovaries of workers and that workers fed a queen diet of Royal Jelly have increased Vg expression in the ovaries. Surprisingly, we find that expression of Vg is not associated with ovary activation in workers, suggesting that this gene has potentially acquired non-reproductive functions. Therefore, Vg expression in the ovaries of honeybee workers provides further support for the Ovarian Ground Plan Hypothesis, which argues that genes implicated in the regulation of reproduction have been co-opted to regulate behavioural differences between queens and workers.

Antagonistic pleiotropy and the evolution of extraordinary lifespans in eusocial organisms

Queens of eusocial species live extraordinarily long compared to their workers. So far, it has been argued that these lifespan divergences are readily explained by the classical evolutionary theory of ageing. As workers predominantly perform risky tasks, such as foraging and nest defense, and queens stay in the well-protected nests, selection against harmful genetic mutations expressed in old age should be weaker in workers than in queens due to caste differences in extrinsic mortality risk, and thus, lead to the evolution of longer queen and shorter worker lifespans. However, these arguments have not been supported by formal models. Here, we present a model for the evolution of caste-specific ageing in social insects, based on Williams' antagonistic pleiotropy theory of ageing. In individual-based simulations, we assume that mutations with antagonistic fitness effects can act within castes, that is, mutations in early life are accompanied by an antagonistic effect acting in later life, or between castes, where antagonistic effects emerge due to caste antagonism or indirect genetic effects between castes. In monogynous social insect species with sterile workers, large lifespan divergences between castes evolved under all different scenarios of antagonistic effects, but regardless of the degree of caste-specific extrinsic mortality. Mutations with antagonistic fitness effects within castes reduced lifespans of both castes, while mutations with between-caste antagonistic effects decreased worker lifespans more than queen lifespans, and consequently increased lifespan divergences. Our results challenge the central explanatory role of extrinsic mortality for caste-specific ageing in eusocial organisms and suggest that antagonistic pleiotropy affects castes differently due to reproductive monopolization by queens, hence, reproductive division of labor. Finally, these findings provide new insights into the evolution of tissue-specific ageing in multicellular organisms in general.

Understanding Reproductive Aging in Wildlife to Improve Animal Conservation and Human Reproductive Health

Similar to humans and laboratory animals, reproductive aging is observed in wild species-from small invertebrates to large mammals. Aging issues are also prevalent in rare and endangered species under human care as their life expectancy is longer than in the wild. The objectives of this review are to (1) present conserved as well as distinctive traits of reproductive aging in different wild animal species (2) highlight the value of comparative studies to address aging issues in conservation breeding as well as in human reproductive medicine, and (3) suggest next steps forward in that research area. From social insects to mega-vertebrates, reproductive aging studies as well as observations in the wild or in breeding centers often remain at the physiological or organismal scale (senescence) rather than at the germ cell level. Overall, multiple traits are conserved across very different species (depletion of the ovarian reserve or no decline in testicular functions), but unique features also exist (endless reproductive life or unaltered quality of germ cells). There is a broad consensus about the need to fill research gaps because many cellular and molecular processes during reproductive aging remain undescribed. More research in male aging is particularly needed across all species. Furthermore, studies on reproductive aging of target species in their natural habitat (sentinel species) are crucial to define more accurate reproductive indicators relevant to other species, including humans, sharing the same environment. Wild species can significantly contribute to our general knowledge of a crucial phenomenon and provide new approaches to extend the reproductive lifespan.

The effect of environmental stress on ageing in a termite species with low social complexity

Social insects seem to have overcome the almost universal trade-off between fecundity and longevity as queens can be highly fecund and at the same time reach lifespans of decades. By contrast, their non-reproducing workers are often short-lived. One hypothesis to explain the long lifespan of queens is that they are better protected against stress than their workers. However, evidence is controversial and experimental studies are scarce. We aimed at manipulating environmental stress and ageing by exposing colonies of the less-socially complex termite Cryptotermes secundus to temperature regimes that differed in variance. In contrast with expectation, constant temperatures imposed more stress than variable temperatures. Survival of queens and workers as well as queens' fecundity were partly reduced under constant conditions and both castes showed signs of ageing in the transcriptome signature under constant conditions. There was a clear oxidative stress defence signal under constant conditions that was, surprisingly, stronger for workers than queens. We discuss how our results relate to social complexity. We argue that workers that are totipotent to become reproductives, like in C. secundus, should invest more in 'anti-ageing' mechanisms than sterile workers because the former can still reproduce and have not reached maturity yet. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Ageing and sociality: why, when and how does sociality change ageing patterns?

Individual lifespans vary tremendously between and also within species, but the proximate and ultimate causes of different ageing speeds are still not well understood. Sociality appears to be associated with the evolution of greater longevity and probably also with a larger plasticity of the shape and pace of ageing. For example, reproductives of several termites and ants reach lifespans that surpass those of their non-reproductive nestmates by one or two decades. In this issue, 15 papers explore the interrelations between sociality and individual longevity in both, group-living vertebrates and social insects. Here, we briefly give an overview of the contents of the various contributions, including theoretical and comparative studies, and we explore the similarities and dissimilarities in proximate mechanisms underlying ageing among taxa, with particular emphasis on nutrient-sensing pathways and, in insects, juvenile hormone. These studies point to an underestimated role of more downstream processes. We highlight the need for reliable transcriptomic markers of ageing and a comprehensive ageing theory of social animals, which includes the reproductive potential of workers, and considers the fact that social insect queens reach maturity only after a prolonged period of producing non-reproductive workers. 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?'

A Stresipteran parasite extends the lifespan of workers in a social wasp

In social wasps, female lifespan depends on caste and colony tasks: workers usually live a few weeks while queens as long as 1 year. Polistes dominula paper wasps infected by the strepsipteran parasite Xenos vesparum avoid all colony tasks, cluster on vegetation where parasite dispersal and mating occur, hibernate and infect the next generation of wasp larvae. Here, we compared the survival rate of infected and uninfected wasp workers. Workers' survival was significantly affected by parasite sex: two-third of workers parasitized by a X. vesparum female survived and overwintered like future queens did, while all workers infected by a X. vesparum male died during the summer, like uninfected workers that we used as controls. We measured a set of host and parasite traits possibly associated with the observed lifespan extension. Infected overwintering workers had larger fat bodies than infected workers that died in the summer, but they had similar body size and ovary development. Furthermore, we recorded a positive correlation between parasite and host body sizes. We hypothesize that the manipulation of worker's longevity operated by X. vesparum enhances parasite's fitness: if workers infected by a female overwinter, they can spread infective parasite larvae in the spring like parasitized gynes do, thus contributing to parasite transmission.

Immune challenge reduces gut microbial diversity and triggers fertility-dependent gene expression changes in a social insect

Background

The gut microbiome can influence life history traits associated with host fitness such as fecundity and longevity. In most organisms, these two life history traits are traded-off, while they are positively linked in social insects. In ants, highly fecund queens can live for decades, while their non-reproducing workers exhibit much shorter lifespans. Yet, when fertility is induced in workers by death or removal of the queen, worker lifespan can increase. It is unclear how this positive link between fecundity and longevity is achieved and what role the gut microbiome and the immune system play in this. To gain insights into the molecular regulation of lifespan in social insects, we investigated fat body gene expression and gut microbiome composition in workers of the ant Temnothorax rugatulus in response to an experimental induction of fertility and an immune challenge.

Results

Fertile workers upregulated several molecular repair mechanisms, which could explain their extended lifespan. The immune challenge altered the expression of several thousand genes in the fat body, including many immune genes, and, interestingly, this transcriptomic response depended on worker fertility. For example, only fertile, immune-challenged workers upregulated genes involved in the synthesis of alpha-ketoglutarate, an immune system regulator, which extends the lifespan in Caenorhabditis elegans by down-regulating the TOR pathway and reducing oxidant production. Additionally, we observed a dramatic loss in bacterial diversity in the guts of the ants within a day of the immune challenge. Yet, bacterial density did not change, so that the gut microbiomes of many immune challenged workers consisted of only a single or a few bacterial strains. Moreover, the expression of immune genes was linked to the gut microbiome composition, suggesting that the ant host can regulate the microbiome in its gut.

Conclusions

Immune system flare-ups can have negative consequence on gut microbiome diversity, pointing to a previously underrated cost of immunity. Moreover, our results provide important insights into shifts in the molecular regulation of fertility and longevity associated with insect sociality.

Intergenerational Transfer of Ageing: Parental Age and Offspring Lifespan

The extent to which the age of parents at reproduction can affect offspring lifespan and other fitness-related traits is important in our understanding of the selective forces shaping life history evolution. In this article, the widely reported negative effects of parental age on offspring lifespan (the 'Lansing effect') is examined. Outlined herein are the potential routes whereby a Lansing effect can occur, whether effects might accumulate across multiple generations, and how the Lansing effect should be viewed as part of a broader framework, considering how parental age affects offspring fitness. The robustness of the evidence for a Lansing effect produced so far, potential confounding variables, and how the underlying mechanisms might best be unravelled through carefully designed experimental studies are discussed.

The deteriorating soma and the indispensable germline: gamete senescence and offspring fitness

The idea that there is an impenetrable barrier that separates the germline and soma has shaped much thinking in evolutionary biology and in many other disciplines. However, recent research has revealed that the so-called 'Weismann Barrier' is leaky, and that information is transferred from soma to germline. Moreover, the germline itself is now known to age, and to be influenced by an age-related deterioration of the soma that houses and protects it. This could reduce the likelihood of successful reproduction by old individuals, but also lead to long-term deleterious consequences for any offspring that they do produce (including a shortened lifespan). Here, we review the evidence from a diverse and multidisciplinary literature for senescence in the germline and its consequences; we also examine the underlying mechanisms responsible, emphasizing changes in mutation rate, telomere loss, and impaired mitochondrial function in gametes. We consider the effect on life-history evolution, particularly reproductive scheduling and mate choice. Throughout, we draw attention to unresolved issues, new questions to consider, and areas where more research is needed. We also highlight the need for a more comparative approach that would reveal the diversity of processes that organisms have evolved to slow or halt age-related germline deterioration.

Age-dependent changes in cuticular color and pteridine levels in a clonal ant

Social insects are emerging models for studying aging and the longevity/fecundity trade-off. Research on the demography of colonies and populations are hampered by the lack of reliable age markers. Here we investigate the suitability of cuticular pigmentation and pteridine fluorescence for age grading individuals of the clonal ant Platythyrea punctata. We found that both traits varied with age. Cuticular color darkened with individual's age until 25-30 days after hatching. For pteridine fluorescence, we found that P. punctata workers show a decrease in head pteridine levels over time until 70-80 days of age. Together with other markers, such as age-based behavior, cuticular coloration and pteridine fluorescence may help to estimate the age structure of colonies.

Seminal fluid compromises visual perception in honeybee queens reducing their survival during additional mating flights

Queens of social insects make all mate-choice decisions on a single day, except in honeybees whose queens can conduct mating flights for several days even when already inseminated by a number of drones. Honeybees therefore appear to have a unique, evolutionarily derived form of sexual conflict: a queen’s decision to pursue risky additional mating flights is driven by later-life fitness gains from genetically more diverse worker-offspring but reduces paternity shares of the drones she already mated with. We used artificial insemination, RNA-sequencing and electroretinography to show that seminal fluid induces a decline in queen vision by perturbing the phototransduction pathway within 24–48 hr. Follow up field trials revealed that queens receiving seminal fluid flew two days earlier than sister queens inseminated with saline, and failed more often to return. These findings are consistent with seminal fluid components manipulating queen eyesight to reduce queen promiscuity across mating flights.

For social insects like honeybees it is beneficial if their queens mate with many males, because genetic diversity can protect the hive against parasites. Early in life, a honeybee queen has a short period of time in which she can fly out to mate with males before returning to the hive with all the sperm needed to last for a lifetime. Queens that have mated on their first flight may embark on additional mating flights over a few consecutive days to further increase genetic variability in their offspring. This is problematic for a male that has already mated because the more males that inseminate the queen the fewer offspring will carry on his specific genes. This results in sexual conflict between males and queens over the number of mating flights.

In many animals, males manipulate females using molecules in seminal fluid to reduce the chances of the female mating again and honeybee males may use a similar strategy. Previous studies revealed that insemination alters the activity of genes related to vision in a honeybee queen’s brain. This could be one way for the males to prevent queens from embarking on additional mating flights.

Now, Liberti et al. find support for this idea by showing that seminal fluid can indeed trigger changes in the activity of vision-related genes in the brains of honeybee queens, which in turn reduce a queen’s opportunity to complete additional mating flights. Queens inseminated with seminal fluid were less responsive to light compared to queens that were exposed to saline instead. Electronic tracking devices affixed to queens showed that the seminal fluid-exposed queens left for mating flights sooner but were more likely to get lost and to not return to their hives compared to the saline-exposed queens.

The experiments support the idea of a sexual arms race in honeybees. Males use seminal fluid to cause rapid deteriorating vision in queens, thus reducing their likelihood of leaving the hive to mate again and to find males when they do fly again. The queens try to counteract these effects by leaving for mating flights sooner, thereby increasing offspring genetic diversity and the success of their colonies. Further studies will be needed to find out how the honeybee sexual arms race varies across seasons, bee races, and geographic ranges. Such information will be useful for honeybee breeding programs, which rely on queen mating success and hive genetic diversity to ensure hive health.

Insulin signaling mediates previtellogenic development and enhances juvenile hormone-mediated vitellogenesis in a lepidopteran insect, Maruca vitrata

BACKGROUND

Insulin/insulin-like growth peptide signaling (IIS) down-regulates hemolymph sugar level and facilitates larval growth in the soybean pod borer, Maruca vitrata. The objective of this study is to determine whether IIS of M. vitrata can mediate ovarian development of adult females.

RESULTS

A pair of ovaries consists of 8 ovarioles, each of which is separated into distal germarium and proximal vitellarium in M. vitrata. In the germarium, oocyte development occurred with active mitotic activity which was visible by incorporating bromodeoxyribose uridine. Previtellogenic development and subsequent vitellogenesis began soon after adult emergence. They continued with increase of female age. Oocyte development was facilitated by up-regulation of vitellogenin (Vg) and Vg receptor (VgR) gene expression. Larval diets significantly influenced on ovarian development of M. vitrata because oocyte development varied with pupal size derived from larvae treated with different nutritional diets. Its ovarian development was dependent on endocrine signal(s) from the head because decapitation soon after adult emergence prevented oogenesis and subsequent vitellogenesis along with marked reduction of Vg and VgR expression. Topical application of juvenile hormone (JH) significantly recovered its ovarian development whereas farnesoic acid (a precursor of JH biosynthesis) or 20-hydroxyecdysone treatment did not. JH stimulated vitellogenesis and choriogenesis, but not previtellogenic development. In contrast, insulin injection to decapitated females stimulated oocyte differentiation and vitellogenesis along with increase of Vg and VgR expression. To further analyze the effect of insulin on ovarian development, expression of four IIS components (InR, FOXO, Akt, and TOR) genes was manipulated by RNA interference. Hemocoelic injection of gene-specific double stranded RNAs significantly reduced their target gene mRNA levels and interfered with ovarian development. An addition of insulin to JH treatment against decapitated females enhanced the gonadotropic effect of JH by stimulating oogenesis.

CONCLUSIONS

IIS plays crucial role in mediating previtellogenic development of M. vitrata in response to nutrient signal. It also enhances the gonadotropic effect of JH II on vitellogenesis.

Ants as Object of Gerontological Research

Social insects with identical genotype that form castes with radically different lifespans are a promising model system for studying the mechanisms underlying longevity. The main direction of progressive evolution of social insects, in particular, ants, is the development of the social way of life inextricably linked with the increase in the colony size. Only in a large colony, it is possible to have a developed polyethism, create large food reserves, and actively regulate the nest microclimate. The lifespan of ants hugely varies among genetically similar queens, workers (unproductive females), and males. The main advantage of studies on insects is the determinism of ontogenetic processes, with a single genome leading to completely different lifespans in different castes. This high degree of determinacy is precisely the reason why some researchers (incorrectly) call a colony of ants the "superorganism", emphasizing the fact that during the development, depending on the community needs, ants can switch their ontogenetic programs, which influences their social roles, ability to learn (i.e., the brain [mushroom-like body] plasticity), and, respectively, the spectrum of tasks performed by a given individual. It has been shown that in many types of food behavior, older ants surpass young ones in both performing the tasks and transferring the experience. The balance between the need to reduce the "cost" of non-breeding individuals (short lifespan and small size of workers) and the benefit from experienced long-lived workers possessing useful skills (large size and "non-aging") apparently determines the differences in the lifespan and aging rate of workers in different species of ants. A large spectrum of rigidly determined ontogenetic trajectories in different castes with identical genomes and the possibility of comparison between "evolutionarily advanced" and "primitive" subfamilies (e.g., Formicinae and Ponerinae) make ants an attractive object in the studies of both normal aging and effects of anti-aging drugs.

Long live the queen, the king and the commoner? Transcript expression differences between old and young in the termite Cryptotermes secundus

Social insects provide promising new avenues for aging research. Within a colony, individuals that share the same genetic background can differ in lifespan by up to two orders of magnitude. Reproducing queens (and in termites also kings) can live for more than 20 years, extraordinary lifespans for insects. We studied aging in a termite species, Cryptotermes secundus, which lives in less socially complex societies with a few hundred colony members. Reproductives develop from workers which are totipotent immatures. Comparing transcriptomes of young and old individuals, we found evidence for aging in reproductives that was especially associated with DNA and protein damage and the activity of transposable elements. By contrast, workers seemed to be better protected against aging. Thus our results differed from those obtained for social insects that live in more complex societies. Yet, they are in agreement with lifespan estimates for the study species. Our data are also in line with expectations from evolutionary theory. For individuals that are able to reproduce, it predicts that aging should only start after reaching maturity. As C. secundus workers are immatures with full reproductive options we expect them to invest into anti-aging processes. Our study illustrates that the degree of aging can differ between social insects and that it may be associated with caste-specific opportunities for reproduction.

Reproductive Senescence in Drones of the Honey Bee (Apis mellifera)

In the face of high proportions of yearly colony losses, queen health and fecundity has been a major focus of industry and research. Much of the reproductive quality of the queen, though, is a function of the mating success and quality of the drones (males). Many environmental factors can negatively impact drone semen quality, but little is known about factors that impact the drones' ability to successfully mate and deliver that semen, or how widely drones vary. In our study, we observed the daily variation in honey bee drone reproductive quality over time, along with a number of morphological traits. Drones were reared in cages in bank colonies, and 20 individuals were dissected and measured daily. The number of viable spermatozoa in the seminal vesicles was zero at emergence and reached an average maximum of 7.39 ± 0.19 million around 20 days of life. Decline in spermatozoa count occurred after day 30, though viability was constant throughout life, when controlling for count. Older drones had smaller wet weights, head widths, and wing lengths. We predict that this is likely due to sampling bias due to a differential lifespan among larger, more reproductively developed drones. Our study shows that drones are more highly variable than previously suggested and that they have a significant variation in reproductive physiology as a function of age.

Horizons in the evolution of aging

Between the 1930s and 50s, evolutionary biologists developed a successful theory of why organisms age, firmly rooted in population genetic principles. By the 1980s the evolution of aging had a secure experimental basis. Since the force of selection declines with age, aging evolves due to mutation accumulation or a benefit to fitness early in life. Here we review major insights and challenges that have emerged over the last 35 years: selection does not always necessarily decline with age; higher extrinsic (i.e., environmentally caused) mortality does not always accelerate aging; conserved pathways control aging rate; senescence patterns are more diverse than previously thought; aging is not universal; trade-offs involving lifespan can be 'broken'; aging might be 'druggable'; and human life expectancy continues to rise but compressing late-life morbidity remains a pressing challenge.

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.

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.

Royal Darwinian Demons: Enforced Changes in Reproductive Efforts Do Not Affect the Life Expectancy of Ant Queens

One of the central tenets of life-history theory is that organisms cannot simultaneously maximize all fitness components. This results in the fundamental trade-off between reproduction and life span known from numerous animals, including humans. Social insects are a well-known exception to this rule: reproductive queens outlive nonreproductive workers. Here, we take a step forward and show that under identical social and environmental conditions the fecundity-longevity trade-off is absent also within the queen caste. A change in reproduction did not alter life expectancy, and even a strong enforced increase in reproductive efforts did not reduce residual life span. Generally, egg-laying rate and life span were positively correlated. Queens of perennial social insects thus seem to maximize at the same time two fitness parameters that are normally negatively correlated. Even though they are not immortal, they best approach a hypothetical "Darwinian demon" in the animal kingdom.

Mating and longevity in ant males

Across multicellular organisms, the costs of reproduction and self‐maintenance result in a life history trade‐off between fecundity and longevity. Queens of perennial social Hymenoptera are both highly fertile and long‐lived, and thus, this fundamental trade‐off is lacking. Whether social insect males similarly evade the fecundity/longevity trade‐off remains largely unstudied. Wingless males of the ant genus Cardiocondyla stay in their natal colonies throughout their relatively long lives and mate with multiple female sexuals. Here, we show that Cardiocondyla obscurior males that were allowed to mate with large numbers of female sexuals had a shortened life span compared to males that mated at a low frequency or virgin males. Although frequent mating negatively affects longevity, males clearly benefit from a “live fast, die young strategy” by inseminating as many female sexuals as possible at a cost to their own survival.

Worker lifespan is an adaptive trait during colony establishment in the long-lived ant Lasius niger

Eusociality has been recognized as a strong driver of lifespan evolution. While queens show extraordinary lifespans of 20years and more, worker lifespan is short and variable. A recent comparative study found that in eusocial species with larger average colony sizes the disparities in the lifespans of the queen and the worker are also greater, which suggests that lifespan might be an evolved trait. Here, we tested whether the same pattern holds during colony establishment: as colonies grow larger, worker lifespan should decrease. We studied the mortality of lab-reared Lasius niger workers from colonies at two different developmental stages (small and intermediate-sized) in a common garden experiment. Workers were kept in artificial cohorts that differed only with respect to the stage of the colony they were born in. We found that the stage of the birth colony affected the body size and the survival probability of the workers. The workers that had emerged from early stage colonies were smaller and had lower mortality during the first 400days of their life than the workers born in colonies at a later stage. Our results suggest that early stage colonies produce small workers with an increased survival probability. These workers are gradually augmented by larger workers with a decreased survival probability that serve as a redundant workforce with easily replaceable individuals. We doubt that the observed differences in lifespan are driven by differences in body size. Rather, we suspect that physiological mechanisms are the basis for the observed differences in lifespan.

Life history evolution in social insects: a female perspective

Social insects are known for their unusual life histories with fecund, long-lived queens and sterile, short-lived workers. We review ultimate factors underlying variation in life history strategies in female social insects, whose social life reshapes common trade-offs, such as the one between fecundity and longevity. Interspecific life history variation is associated with colony size, mediated by changes in division of labour and extrinsic mortality. In addition to the ratio of juvenile to adult mortality, social factors such as queen number influence life history trajectories. We discuss two hypotheses explaining why queen fecundity and lifespan is higher in single-queen societies and suggest further research directions on the evolution of life history variation in social insects.

Patterns of longevity across a sociality gradient in vespid wasps

The reversal of the fecundity/longevity tradeoff in social insects is striking, but we lack understanding of when and how this reversal evolved. Vespid wasps are excellent models for studying social evolution because species show different levels of sociality from solitary to primitively to advanced eusocial. We provide the first synthesis of existing, but scanty, data available on longevity in vespids. We explore whether the fecundity/longevity tradeoff reversal is exaggerated in species with more derived sociality. Although the reversal is evident in primitively social wasps, the available data suggest it may be stronger in large-colonied species, which show a trend toward shorter worker longevity. More data are needed on additional species and underlying mechanisms, but vespids hold promise for understanding the evolution of the fecundity/longevity tradeoff reversal.

Facultative social insects can provide insights into the reversal of the longevity/fecundity trade-off across the eusocial insects

In eusocial insects, reversal of the fecundity/longevity trade-off and extreme differences in life histories between castes of the same species garner scientific and public interest. Facultative social species at the threshold of sociality, in which individuals are socially plastic, provide an excellent opportunity to understand the causes and mechanisms underlying this reversal in life history trade-off associated with eusociality. We briefly present the ultimate factors favoring sociality and the association between fecundity and longevity in facultative eusocial insects, including kin selection and disposable soma, as well as proximate mechanisms observed in such species, such as differences in hormone titers and functions. Potential genetic underpinnings of lifespan and fecundity differences between castes are discussed and future research directions are proposed.

Endocrine uncoupling of the trade-off between reproduction and somatic maintenance in eusocial insects

In most animals reproduction trades off with somatic maintenance and survival. Physiologically this trade-off is mediated by hormones with opposite effects on reproduction and maintenance. In many insects, this regulation is achieved by an endocrine network that integrates insulin-like/IGF-1 signaling (IIS), juvenile hormone (JH), and the yolk precursor vitellogenin (Vg) (or, more generally, yolk proteins [YPs]). Downregulation of this network promotes maintenance and survival at the expense of reproduction. Remarkably, however, queens of highly eusocial social insects exhibit both enormous reproductive output and longevity, thus escaping the trade-off. Here we argue - based on recent evidence - that the proximate reason for why eusocial insects can decouple this trade-off is that they have evolved a different 'wiring' of the IIS-JH-Vg/YP circuit.

Chapter 6. Comparative genomic approaches to investigate molecular traits specific to social insects

Ageing is a feature of nearly all known organisms and, by its connection to survival, appears to trade off with fecundity. However, in some organisms such as in queens of social insects, this negative relation appears reversed and individuals live long and reproduce much. Since new experimental techniques, transcriptomes and genomes of many social insects have recently become available, a comparison of these data in a phylogenetic framework becomes feasible. This allows the study of general trends, species specific oddities and evolutionary dynamics of the molecular properties and changes which underlie ageing, fecundity and the reversal of this negative association. In the framework of social insect evolution, we review the most important recent insights, computational methods, their applications and data resources which are available.

Lifespan divergence between social insect castes: challenges and opportunities for evolutionary theories of aging

The extraordinarily long lifespans of queens (and kings) in eusocial insects and the strikingly large differences in life expectancy between workers and queens challenge our understanding of the evolution of aging and provide unique opportunities for studying the causes underlying adaptive variation in lifespan within species. Here we review the major evolutionary theories of aging, focusing on their scope and limitations when applied to social insects. We show that reproductive division of labor, interactions between kin, caste-specific gene regulation networks, and the integration of colony-level trade-offs with individual-level trade-offs provide challenges to the classical theories We briefly indicate how these challenges could be met in future models of adaptive phenotypic plasticity in lifespan between and within different castes.

Ant Genetics: Reproductive Physiology, Worker Morphology, and Behavior

Many exciting studies have begun to elucidate the genetics of the morphological and physiological diversity of ants, but as yet few studies have investigated the genetics of ant behavior directly. Ant genomes are marked by extreme rates of gene turnover, especially in gene families related to olfactory communication, such as the synthesis of cuticular hydrocarbons and the perception of environmental semiochemicals. Transcriptomic and epigenetic differences are apparent between reproductive and sterile females, males and females, and workers that differ in body size. Quantitative genetic approaches suggest heritability of task performance, and population genetic studies indicate a genetic association with reproductive status in some species. Gene expression is associated with behavior including foraging, response to queens attempting to join a colony, circadian patterns of task performance, and age-related changes of task. Ant behavioral genetics needs further investigation of the feedback between individual-level physiological changes and socially mediated responses to environmental conditions.

Transcriptomic response to injury sheds light on the physiological costs of reproduction in ant queens

The trade-off between reproduction and longevity is widespread among multicellular organisms. As an important exception, the reproductive females of perennial social insects (ants, honeybees, termites) are simultaneously highly fertile and very long-lived relative to their nonreproductive nestmates. The observation that increased fecundity is not coupled with decreased lifespan suggests that social insect queens do not have to reallocate resources between reproduction and self-maintenance. If queens have to compensate for the costs of reproduction on the level of the individual, the activation of other energy-demanding physiological processes might force them to reduce the production of eggs. To test this hypothesis in ant queens, we increased immunity costs by injury and measured the effect of this treatment on egg-laying rates and genomewide gene expression. Amputation of both middle legs led to a temporary decrease in egg-laying rates and affected the expression of 947 genes corresponding to 9% of the transcriptome. The changes comprised the upregulation of the immune and wound healing response on the one hand, and the downregulation of germ cell development, central nervous system development and learning ability on the other hand. Injury strongly influenced metabolism by inducing catabolism and repressing amino acid and nitrogen compound metabolism. By comparing our results to similar transcriptomic studies in insects, we found a highly consistent upregulation of immune genes due to sterile and septic wounding. The gene expression changes, complemented by the temporary decline of egg-laying rates, clearly reveal a trade-off between reproduction and the immune response in social insect queens.

Ant Colonies Do Not Trade-Off Reproduction against Maintenance

The question on how individuals allocate resources into maintenance and reproduction is one of the central questions in life history theory. Yet, resource allocation into maintenance on the organismic level can only be measured indirectly. This is different in a social insect colony, a "superorganism" where workers represent the soma and the queen the germ line of the colony. Here, we investigate whether trade-offs exist between maintenance and reproduction on two levels of biological organization, queens and colonies, by following single-queen colonies of the ant Cardiocondyla obscurior throughout the entire lifespan of the queen. Our results show that maintenance and reproduction are positively correlated on the colony level, and we confirm results of an earlier study that found no trade-off on the individual (queen) level. We attribute this unexpected outcome to the existence of a positive feedback loop where investment into maintenance (workers) increases the rate of resource acquisition under laboratory conditions. Even though food was provided ad libitum, variation in productivity among the colonies suggests that resources can only be utilized and invested into additional maintenance and reproduction by the colony if enough workers are available. The resulting relationship between per-capita and colony productivity in our study fits well with other studies conducted in the field, where decreasing per-capita productivity and the leveling off of colony productivity have been linked to density dependent effects due to competition among colonies. This suggests that the absence of trade-offs in our laboratory study might also be prevalent under natural conditions, leading to a positive association of maintenance, (= growth) and reproduction. In this respect, insect colonies resemble indeterminate growing organisms.

Transcriptomic Signatures Mirror the Lack of the Fecundity/Longevity Trade-Off in Ant Queens

Life-history theory predicts a trade-off between reproductive investment and self-maintenance. The negative association between fertility and longevity found throughout multicellular organisms supports this prediction. As an important exception, the reproductives of many eusocial insects (ants, bees, and termites) are simultaneously very long-lived and highly fertile. Here, we examine the proximate basis for this exceptional relationship by comparing whole-body transcriptomes of differently aged queens of the ant Cardiocondyla obscurior. We show that the sets of genes differentially expressed with age significantly overlap with age-related expression changes previously found in female Drosophila melanogaster. We identified several developmental processes, such as the generation of neurons, as common signatures of aging. More generally, however, gene expression in ant queens and flies changes with age mainly in opposite directions. In contrast to flies, reproduction-associated genes were upregulated and genes associated with metabolic processes and muscle contraction were downregulated in old relative to young ant queens. Furthermore, we searched for putative C. obscurior longevity candidates associated with the previously reported lifespan-prolonging effect of mating by comparing the transcriptomes of queens that differed in mating and reproductive status. We found 21 genes, including the putative aging candidate NLaz (an insect homolog of APOD), which were consistently more highly expressed in short-lived, unmated queens than in long-lived, mated queens. Our study provides clear evidence that the alternative regulation of conserved molecular pathways that mediate the interplay among mating, egg laying, and aging underlies the lack of the fecundity/longevity trade-off in ant queens.

RNA-sequencing elucidates the regulation of behavioural transitions associated with the mating process in honey bee queens

Background

Mating is a complex process, which is frequently associated with behavioural and physiological changes. However, understanding of the genetic underpinnings of these changes is limited. Honey bees are both a model system in behavioural genomics, and the dominant managed pollinator of human crops; consequently understanding the mating process has both pure and applied value. We used next-generation transcriptomics to probe changes in gene expression in the brains of honey bee queens, as they transition from virgin to mated reproductive status. In addition, we used CO₂-narcosis, which induces oviposition without mating, to isolate the process of reproductive maturation.

Results

The mating process produced significant changes in the expression of vision, chemo-reception, metabolic, and immune-related genes. Differential expression of these genes maps clearly onto known behavioural and physiological changes that occur during the transition from being a virgin queen to a newly-mated queen. A subset of these changes in gene expression were also detected in CO₂-treated queens, as predicted from previous physiological studies. In addition, we compared our results to previous studies that used microarray techniques across a range of experimental time-points. Changes in expression of immune- and vision-related genes were common to all studies, supporting an involvement of these groups of genes in the mating process.

Conclusions

Our study is an important step in understanding the molecular mechanisms regulating post-mating behavioural transitions in a natural system. The weak overlap in patterns of gene expression with previous studies demonstrates the high sensitivity of genome-wide approaches. Thus, while we build on previous microarray studies that explored post-mating changes in honey bees, the broader experimental design, use of RNA-sequencing, and focus on Australian honey bees, which remain free from the devastating parasite Varroa destructor, in the current study, provide unique insights into the biology of the mating process in honey bees.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1750-7) contains supplementary material, which is available to authorized users.

Reproduction, aging and mortality rate in social subterranean mole voles (Ellobius talpinus Pall.)

Eusocial subterranean rodents of the Bathyergidae family have enormous longevity. The long lifespan of these species is associated with negligible senescence, that is, an absence of the signs of age-related deterioration in physical condition. The question arises as to whether these features are unique to eusocial Bathyergids or typical of other social subterranean rodents as well. In the present study, we analysed data from observations of a social subterranean Microtinae rodent, the northern mole vole (Ellobius talpinus Pall.), which, like mole-rats, has reproductive skew. Among the individuals captured in the wild and maintained in captivity, females that reproduced lived significantly longer than non-breeding females. We did not find any changes in muscle strength with age in any of the demographic groups studied. Faecal glucocorticoid concentrations before death were significantly higher in non-breeding females than in breeding females and males. Increased adrenocortical activity may be one mechanism responsible for the decreased lifespan of non-reproducing individuals of social subterranean rodents. We conclude that the patterns of aging, although different in some respects, are generally common for social subterranean rodents of different taxonomic groups.

Worker senescence and the sociobiology of aging in ants

Senescence, the decline in physiological and behavioral function with increasing age, has been the focus of significant theoretical and empirical research in a broad array of animal taxa. Preeminent among invertebrate social models of aging are ants, a diverse and ecologically dominant clade of eusocial insects characterized by reproductive and sterile phenotypes. In this review, we critically examine selection for worker lifespan in ants and discuss the relationship between functional senescence, longevity, task performance, and colony fitness. We did not find strong or consistent support for the hypothesis that demographic senescence in ants is programmed, or its corollary prediction that workers that do not experience extrinsic mortality die at an age approximating their lifespan in nature. We present seven hypotheses concerning how selection could favor extended worker lifespan through its positive relationship to colony size and predict that large colony size, under some conditions, should confer multiple and significant fitness advantages. Fitness benefits derived from long worker lifespan could be mediated by increased resource acquisition, efficient division of labor, accuracy of collective decision-making, enhanced allomaternal care and colony defense, lower infection risk, and decreased energetic costs of workforce maintenance. We suggest future avenues of research to examine the evolution of worker lifespan and its relationship to colony fitness, and conclude that an innovative fusion of sociobiology, senescence theory, and mechanistic studies of aging can improve our understanding of the adaptive nature of worker lifespan in ants.

What is the main driver of ageing in long-lived winter honeybees: antioxidant enzymes, innate immunity, or vitellogenin?

To date five different theories compete in explaining the biological mechanisms of senescence or ageing in invertebrates. Physiological, genetical, and environmental mechanisms form the image of ageing in individuals and groups. Social insects, especially the honeybee Apis mellifera, present exceptional model systems to study developmentally related ageing. The extremely high phenotypic plasticity for life expectancy resulting from the female caste system provides a most useful system to study open questions with respect to ageing. Here, we used long-lived winter worker honeybees and measured transcriptional changes of 14 antioxidative enzyme, immunity, and ageing-related (insulin/insulin-like growth factor signaling pathway) genes at two time points during hibernation. Additionally, worker bees were challenged with a bacterial infection to test ageing- and infection-associated immunity changes. Gene expression levels for each group of target genes revealed that ageing had a much higher impact than the bacterial challenge, notably for immunity-related genes. Antimicrobial peptide and antioxidative enzyme genes were significantly upregulated in aged worker honeybees independent of bacterial infections. The known ageing markers vitellogenin and IlP-1 were opposed regulated with decreasing vitellogenin levels during ageing. The increased antioxidative enzyme and antimicrobial peptide gene expression may contribute to a retardation of senescence in long-lived hibernating worker honeybees.

Life span evolution in eusocial workers--a theoretical approach to understanding the effects of extrinsic mortality in a hierarchical system

While the extraordinary life span of queens and division of labor in eusocial societies have been well studied, it is less clear which selective forces act on the short life span of workers. The disparity of life span between the queen and the workers is linked to a basic issue in sociobiology: How are the resources in a colony allocated between colony maintenance and reproduction? Resources for somatic maintenance of the colony can either be invested into quality or quantity of workers. Here, we present a theoretical optimization model that uses a hierarchical trade-off within insect colonies and extrinsic mortality to explain how different aging phenotypes could have evolved to keep resources secure in the colony. The model points to the significance of two factors. First, any investment that would generate a longer intrinsic life span for workers is lost if the individual dies from external causes while foraging. As a consequence, risky environments favor the evolution of workers with a shorter life span. Second, shorter-lived workers require less investment than long-lived ones, allowing the colony to allocate these resources to sexual reproduction or colony growth.