Lifespan differences between queens and workers are not explained by rates of molecular damage

Eusociality and Senescence: Neuroprotection and Physiological Resilience to Aging in Insect and Mammalian Systems

Are eusociality and extraordinary aging polyphenisms evolutionarily coupled? The remarkable disparity in longevity between social insect queens and sterile workers-decades vs. months, respectively-has long been recognized. In mammals, the lifespan of eusocial naked mole rats is extremely long-roughly 10 times greater than that of mice. Is this robustness to senescence associated with social evolution and shared mechanisms of developmental timing, neuroprotection, antioxidant defenses, and neurophysiology? Focusing on brain senescence, we examine correlates and consequences of aging across two divergent eusocial clades and how they differ from solitary taxa. Chronological age and physiological indicators of neural deterioration, including DNA damage or cell death, appear to be decoupled in eusocial insects. In some species, brain cell death does not increase with worker age and DNA damage occurs at similar rates between queens and workers. In comparison, naked mole rats exhibit characteristics of neonatal mice such as protracted development that may offer protection from aging and environmental stressors. Antioxidant defenses appear to be regulated differently across taxa, suggesting independent adaptations to life history and environment. Eusocial insects and naked mole rats appear to have evolved different mechanisms that lead to similar senescence-resistant phenotypes. Careful selection of comparison taxa and further exploration of the role of metabolism in aging can reveal mechanisms that preserve brain functionality and physiological resilience in eusocial species.

Social organization and the evolution of life-history traits in two queen morphs of the ant Temnothorax rugatulus

During the evolution of social insects, not only did life-history traits diverge, with queens becoming highly fecund and long lived compared with their sterile workers, but also individual traits lost their importance compared with colony-level traits. In solitary animals, fecundity is largely influenced by female size, whereas in eusocial insects, colony size and queen number can affect the egg-laying rate. Here, we focused on the ant Temnothorax rugatulus, which exhibits two queen morphs varying in size and reproductive strategy, correlating with their colony's social organization. We experimentally tested the influence of social structure, colony and body size on queen fecundity and investigated links between body size, metabolic rate and survival under paraquat-induced oxidative stress. To gain insight into the molecular physiology underlying the alternative reproductive strategies, we analysed fat body transcriptomes. Per-queen egg production was lower in polygynous colonies when fecundity was limited by worker care. Colony size was a determinant of fecundity rather than body size or queen number, highlighting the super-organismal properties of these societies. The smaller microgynes were more frequently fed by workers and exhibited an increase in metabolic activity, yet they were similarly resistant to oxidative stress. Small queens differentially expressed metabolic genes in the fat body, indicating that shifts in molecular physiology and resource availability allow microgyne queens to compensate for their small size with a more active metabolism without paying increased mortality costs. We provide novel insights into how life-history traits and their associations were modified during social evolution and adapted to queen reproductive strategies.

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?'

Experimental increase in fecundity causes upregulation of fecundity and body maintenance genes in the fat body of ant queens

In most organisms, fecundity and longevity are negatively associated and the molecular regulation of these two life-history traits is highly interconnected. In addition, nutrient intake often has opposing effects on lifespan and reproduction. In contrast to solitary insects, the main reproductive individual of social hymenopterans, the queen, is also the most long-lived. During development, queen larvae are well-nourished, but we are only beginning to understand the impact of nutrition on the queens' adult life and the molecular regulation and connectivity of fecundity and longevity. Here, we used two experimental manipulations to alter queen fecundity in the ant Temnothorax rugatulus and investigated associated changes in fat body gene expression. Egg removal triggered a fecundity increase, leading to expression changes in genes with functions in fecundity such as oogenesis and body maintenance. Dietary restriction lowered the egg production of queens and altered the expression of genes linked to autophagy, Toll signalling, cellular homeostasis and immunity. Our study reveals that an experimental increase in fecundity causes the co-activation of reproduction and body maintenance mechanisms, shedding light on the molecular regulation of the link between longevity and fecundity in social insects.

Plastic Senescence in the Honey Bee and the Disposable Soma Theory

The demonstration of life span plasticity in natural populations would provide a powerful test of evolutionary theories of senescence. Plastic senescence is not easily explained by mutation accumulation or antagonistic pleiotropy but is a corollary of the disposable soma theory. The life span differences among castes of the eusocial Hymenoptera are potentially some of the most striking and extreme examples of life span plasticity. Although these differences are often assumed to be plastic, this has never been demonstrated conclusively because differences in life span may be caused by the proximate effects of different levels of environmental hazard experienced by castes. Here age-dependent and age-independent components of instantaneous mortality rates of the honey bee (Apis mellifera) were estimated from published life tables for natural and seminatural populations to determine whether differences in life span between queens and workers and between different types of workers are indeed plastic. These differences in life span were found to be due to differences in the rate of actuarial senescence, which correlate positively with the rate of extrinsic mortality, in accordance with the central prediction of evolutionary theories of senescence. Although all three evolutionary theories of senescence could in principle explain such plastic senescence, given differential gene expression between castes or life stages, only the disposable soma theory adequately explains the adaptive regulation of somatic maintenance in response to different environmental conditions that appears to underlie life span plasticity.

Long-lived Temnothorax ant queens switch from investment in immunity to antioxidant production with age

Senescence is manifested by an increase in molecular damage and a deterioration of biological functions with age. In most organisms, body maintenance is traded-off with reproduction. This negative relationship between longevity and fecundity is also evident on the molecular level. Exempt from this negative trait association, social insect queens are both extremely long-lived and highly fecund. Here, we study changes in gene expression with age and fecundity in ant queens to understand the molecular basis of their long lifespan. We analyse tissue-specific gene expression in young founding queens and old fecund queens of the ant Temnothorax rugatulus. More genes altered their expression with age in the fat body than in the brain. Despite strong differences in ovary development, few fecundity genes were differentially expressed. Young founding queens invested in immunity (i.e. activation of Toll signalling pathway) and resistance against environmental and physiological stress (i.e. down-regulation of TOR pathway). Conversely, established older queens invested into anti-aging mechanisms through an overproduction of antioxidants (i.e. upregulation of catalase, superoxide dismutase). Finally, we identified candidate genes and pathways, potentially involved in the association between fertility and longevity in social insects and its proximate basis.

Sex-specific stress tolerance, proteolysis, and lifespan in the invertebrate Tigriopus californicus

Because stress tolerance and longevity are mechanistically and phenotypically linked, the sex with higher acute stress tolerance might be expected to also live longer. On the other hand, the association between stress tolerance and lifespan may be complicated by tradeoffs between acute tolerance and long-term survival. Here we use the copepod Tigriopus californicus to test for sex differences in stress resistance, proteolytic activity and longevity. Unlike many model organisms, this species does not have sex chromosomes. However, substantial sex differences were still observed. Females were found to have superior tolerance to a range of acute stressors (high temperature, high salinity, low salinity, copper and bisphenol A (BPA)) across a variety of treatments including different populations, pure vs. hybrid crosses, and different shading environments. Upregulation of proteolytic capacity - one molecular mechanism for responding to acute stress - was also found to be sexually dimorphic. In the combined stress treatment of chronic copper exposure followed by acute heat exposure, proteolytic capacity was suppressed for males. Females, however, maintained a robust proteolytic stress response. While females consistently showed greater tolerance to short-term stress, lifespan was largely equivalent between the two sexes under both benign conditions and mild thermal stress. Our findings indicate that short-term stress tolerance does not predict long-term survival under relatively mild conditions.

Elevated expression of ageing and immunity genes in queens of the black garden ant

Studies in model organisms have identified a variety of genes whose expression can be experimentally modulated to produce changes in longevity, but whether these genes are the same as those involved in natural variation in lifespan remains unclear. Social insects boast some of the largest lifespan differences known between plastic phenotypes, with queen and worker lifespans differing by an order of magnitude despite no systematic nucleotide sequence differences between them. The contrasting lifespans of queens and workers are thus the result of differences in gene expression. We used RNA sequencing of brains and legs in 1-day-old and 2-month-old individuals of the ant Lasius niger to determine whether genes with queen-biased expression are enriched for genes linked to ageing in model organisms. Because the great longevity of queens may require investment into immune processes, we also investigated whether queen-biased genes are enriched for genes with known roles in immunity. Queen-biased genes in legs were enriched for ageing genes and for genes associated with increasing rather than decreasing lifespan. Queen-biased genes in legs were also enriched for immune genes, but only in 1-day-old individuals, perhaps linked to the changing roles of workers with age. Intriguingly, the single most differentially expressed gene between 1-day-old queen and worker brains was an extra-cellular form of CuZn Superoxide Dismutase (SOD3), raising the possibility of an important role of anti-oxidant genes in modulating lifespan.

Biomonitoring of genotoxicity of industrial fertilizer pollutants in Aiolopus thalassinus (Orthoptera: Acrididae) using alkaline comet assay

Phosphate fertilizer industry is considered as one of the main sources of environmental pollutants. Besides solid waste products, e.g. phosphates, sulphates, and heavy metals, also atmospheric pollutants, such as hydrofluoric acid fumes (HF), sulphur dioxide (SO₂), nitrogen oxides (NO₂), and particulate matter with diameter up to 10 μm (PM₁₀) can be dangerous. Genotoxic effect of these pollutants was monitored by assessing the DNA damage using alkaline comet assay on cells from brain, thoracic muscles and gut of Aiolopus thalassinus collected at three sites (A-C) located at 1, 3, and 6 km away from Abu-Zaabal Company for Fertilizers and Chemical Industries. Control site was established 32 km from the source of pollution, at the Cairo University Campus. The level of the DNA damage was significantly higher in insects from polluted sites comparing to that from the control site. A strong negative correlation between percentage of cells with visible DNA damage (% of severed cells) and the distance of the sites from Abu-Zaabal Company was found. The best parameter for monitoring of fertilizer pollutants is % of severed cells. Possible impact of Abu-Zaabal Company (extremely high concentration of phosphates and sulphates in all the polluted sites) on DNA integrity in A. thalassinus tissues was discussed. The potential use of the comet assay as a biomonitoring method of the environmental pollution caused by fertilizer industry was proposed. Specific pollution resulting from the activity of the fertilizer industry can cause comparable adverse effects in the organisms inhabiting areas up to 6 km from the source of contamination.