Telomere shortening as a mechanism of long-term cost of infectious diseases in natural animal populations

No correlative evidence of costs of infection or immunity on leucocyte telomere length in a wild population of Soay sheep

Telomere length (TL) is a biomarker hypothesized to capture evolutionarily and ecologically important physiological costs of reproduction, infection and immunity. Few studies have estimated the relationships among infection status, immunity, TL and fitness in natural systems. The hypothesis that short telomeres predict reduced survival because they reflect costly consequences of infection and immune investment remains largely untested. Using longitudinal data from a free-living Soay sheep population, we tested whether leucocyte TL was predicted by infection with nematode parasites and antibody levels against those parasites. Helminth parasite burdens were positively associated with leucocyte TL in both lambs and adults, which is not consistent with TL reflecting infection costs. We found no association between TL and helminth-specific IgG levels in either young or old individuals which suggests TL does not reflect costs of an activated immune response or immunosenescence. Furthermore, we found no support for TL acting as a mediator of trade-offs between infection, immunity and subsequent survival in the wild. Our results suggest that while variation in TL could reflect short-term variation in resource investment or environmental conditions, it does not capture costs of infection and immunity, nor does it behave like a marker of an individual's helminth-specific antibody immune response.

Telomere length is not a useful tool for chronological age estimation in animals

Telomeres are short repetitive DNA sequences capping the ends of chromosomes. Telomere shortening occurs during cell division and may be accelerated by oxidative damage or ameliorated by telomere maintenance mechanisms. Consequently, telomere length changes with age, which was recently confirmed in a large meta-analysis across vertebrates. However, based on the correlation between telomere length and age, it was concluded that telomere length can be used as a tool for chronological age estimation in animals. Correlation should not be confused with predictability, and the current data and studies suggest that telomeres cannot be used to reliably predict individual chronological age. There are biological reasons for why there is large individual variation in telomere dynamics, which is mainly due to high susceptibility to a wide range of environmental, but also genetic factors, rendering telomeres unfeasible as a tool for age estimation. The use of telomeres for chronological age estimation is largely a misguided effort, but its occasional reappearance in the literature raises concerns that it will mislead resources in wildlife conservation.

Why and when should organisms elongate their telomeres? Elaborations on the 'excess resources elongation' and 'last resort elongation' hypotheses

Telomere length and telomere shortening are thought to be critical cellular attributes and processes that are related to an individual's life span and fitness. The general pattern across most taxa is that after birth telomere length gradually decreases with age. Telomere protection and restoration mechanisms are usually assumed to reduce the rate of shortening or at most keep telomere length constant. However, here we have compiled a list of 26 articles showing that there is an increasing number of studies reporting apparent elongation of telomeres (i.e., a net increase in TL from timet to timet+1) often in a considerable proportion of the individuals studied. Moreover, the few studies which have studied telomere elongation in detail show that increases in telomere length are unlikely to be due to measurement error alone. In this article, we argue that episodes of telomere elongation deserve more attention as they could reflect individual strategies to optimise life histories and maximise fitness, which may not be reflected in the overall telomere dynamics patterns. We propose that patterns of telomere (net) elongation may be partly determined by other factors than those causing telomere shortening, and therefore deserve analyses specifically targeted to investigate the occurrence of telomere elongation. We elaborate on two ecological hypotheses that have been proposed to explain patterns of telomere elongation (the 'excess resources elongation' and the 'last resort elongation' hypothesis) and we discuss the current evidence for (or against) these hypotheses and propose ways to test them.

Leukocyte telomere length predicts subsequent infectious morbidity among Colombian schoolchildren

OBJECTIVE

Telomere length (TL) attrition is related to chronic disease risk. However, less is known on whether TL predicts infectious outcomes, especially in childhood. We examined whether leukocyte TL (LTL) was associated with subsequent infectious morbidity in schoolchildren.

METHODS

We assessed LTL in 717 Colombian children 5-12 years-old at the beginning of a school year and followed them through the year for daily occurrence of common infection symptoms and doctor visits. We estimated adjusted incidence rate ratios (IRR) with 95% confidence intervals (CI) of gastrointestinal and respiratory syndromes for quartiles of standardized LTL Z score and per unit LTL Z score.

RESULTS

A longer LTL was associated with increased incidence of all infectious morbidity syndromes considered. Adjusted IRR (95% CI) per unit LTL Z score were 1.55 (1.20, 2.00) for diarrhea with vomiting, 1.34 (1.13, 1.60) for cough with fever, 1.70 (1.28, 2.28) for ear infection, and 1.66 (1.36, 2.02) for doctor visits with symptoms.

CONCLUSIONS

Longer LTL is related to increased incidence of common infectious morbidities in middle childhood.

Taking flight: An ecological, evolutionary and genomic perspective on bat telomeres

Over 20% of all living mammals are bats (order Chiroptera). Bats possess extraordinary adaptations including powered flight, laryngeal echolocation and a unique immune system that enables them to tolerate a diversity of viral infections without presenting clinical disease symptoms. They occupy multiple trophic niches and environments globally. Significant physiological and ecological diversity occurs across the order. Bats also exhibit extreme longevity given their body size with many species showing few signs of ageing. The molecular basis of this extended longevity has recently attracted attention. Telomere maintenance potentially underpins bats' extended healthspan, although functional studies are still required to validate the causative mechanisms. In this review, we detail the current knowledge on bat telomeres, telomerase expression, and how these relate to ecology, longevity and life-history strategies. Patterns of telomere shortening and telomerase expression vary across species, and comparative genomic analyses suggest that alternative telomere maintenance mechanisms evolved in the longest-lived bats. We discuss the unique challenges faced when working with populations of wild bats and highlight ways to advance the field including expanding long-term monitoring across species that display contrasting life-histories and occupy different environmental niches. We further review how new high quality, chromosome-level genome assemblies can enable us to uncover the molecular mechanisms governing telomere dynamics and how phylogenomic analyses can reveal the adaptive significance of telomere maintenance and variation in bats.

Telomeres, the loop tying cancer to organismal life-histories

Recent developments in telomere and cancer evolutionary ecology demonstrate a very complex relationship between the need of tissue repair and controlling the emergence of abnormally proliferating cells. The trade-off is balanced by natural and sexual selection and mediated via both intrinsic and environmental factors. Here, we explore the effects of telomere-cancer dynamics on life history traits and strategies as well as on the cumulative effects of genetic and environmental factors. We show that telomere-cancer dynamics constitute an incredibly complex and multifaceted process. From research to date, it appears that the relationship between telomere length and cancer risk is likely nonlinear with good evidence that both (too) long and (too) short telomeres can be associated with increased cancer risk. The ability and propensity of organisms to respond to the interplay of telomere dynamics and oncogenic processes, depends on the combination of its tissue environments, life history strategies, environmental challenges (i.e., extreme climatic conditions), pressure by predators and pollution, as well as its evolutionary history. Consequently, precise interpretation of telomere-cancer dynamics requires integrative and multidisciplinary approaches. Finally, incorporating information on telomere dynamics and the expression of tumour suppressor genes and oncogenes could potentially provide the synergistic overview that could lay the foundations to study telomere-cancer dynamics at ecosystem levels.

Causes and consequences of telomere lengthening in a wild vertebrate population

Telomeres have been advocated to be important markers of biological age in evolutionary and ecological studies. Telomeres usually shorten with age and shortening is frequently associated with environmental stressors and increased subsequent mortality. Telomere lengthening - an apparent increase in telomere length between repeated samples from the same individual - also occurs. However, the exact circumstances, and consequences, of telomere lengthening are poorly understood. Using longitudinal data from the Seychelles warbler (Acrocephalus sechellensis), we tested whether telomere lengthening - which occurs in adults of this species - is associated with specific stressors (reproductive effort, food availability, malarial infection and cooperative breeding) and predicts subsequent survival. In females, telomere shortening was observed under greater stress (i.e., low food availability, malaria infection), while telomere lengthening was observed in females experiencing lower stress (i.e., high food availability, assisted by helpers, without malaria). The telomere dynamics of males were not associated with the key stressors tested. These results indicate that, at least for females, telomere lengthening occurs in circumstances more conducive to self-maintenance. Importantly, both females and males with lengthened telomeres had improved subsequent survival relative to individuals that displayed unchanged, or shortened, telomeres - indicating that telomere lengthening is associated with individual fitness. These results demonstrate that telomere dynamics are bidirectionally responsive to the level of stress that an individual faces, but may poorly reflect the accumulation of stress over an individuals lifetime.

On the comparative biology of mammalian telomeres: Telomere length co-evolves with body mass, lifespan and cancer risk

Telomeres, the short repetitive DNA sequences that cap the ends of linear chromosomes, shorten during cell division and are implicated in senescence in most species. Telomerase can rebuild telomeres but is repressed in many mammals that exhibit replicative senescence, presumably as a tumour suppression mechanism. It is therefore important to understand the co-evolution of telomere biology and life-history traits that has shaped the diversity of senescence patterns across species. Gomes et al. previously produced a large data set on telomere length (TL), telomerase activity, body mass and lifespan among 57 mammal species. We re-analysed their data using the same phylogenetic multiple regressions and with several additional analyses to test the robustness of the findings. We found substantial inconsistencies in our results compared to Gomes et al.'s. Consistent with Gomes et al. we found an inverse association between TL and lifespan. Contrary to the analyses in Gomes et al., we found a generally robust inverse association between TL and mass, and only weak nonrobust evidence for an association between telomerase activity and mass. These results suggest that shorter TL may have been selected for in larger and longer lived species, probably as a mechanism to suppress cancer. We support this hypothesis by showing that longer telomeres predict higher cancer risk across 22 species. Furthermore, we find that domesticated species have longer telomeres. Our results call into question past interpretations of the co-evolution of telomere biology and life-history traits and stress the need for careful attention to model construction.

Telomerase activity, relative telomere length, and longevity in alfalfa (Medicago sativa L.)

Background

Medicago sativa L. ‘Qingshui’ is a valuable rhizomatous forage germplasm resource. We previously crossed Qingshui with the high-yielding Medicago sativa L. ‘WL168’ and obtained novel rhizomatous hybrid strains (RSA-01, RSA-02, and RSA-03). Telomere dynamics are more accurate predictors of survival and mortality than chronological age. Based on telomere analyses, we aimed to identify alfalfa varieties with increased stamina and longevity for the establishment of artificial grazing grasslands.

Methods

In this study, we performed longitudinal analysis of telomerase activity and relative telomere length in five alfalfa varieties (Qingshui, WL168, RSA-01, RSA-02, and RSA-03) at the age of 1 year and 5 years to examine the relationship among telomerase activity, rate of change in relative telomere length, and longevity. We further aimed to evaluate the longevity of the examined varieties. Telomerase activity and relative telomere length were measured using enzyme-linked immunosorbent assay and real-time polymerase chain reaction, respectively.

Results

We observed significant differences in telomerase activity between plants aged 1 year and those aged 5 years in all varieties except WL168, and the rate of change in telomerase activity does not differ reliably with age. As telomerase activity and relative telomere length are complex phenomena, further studies examining the molecular mechanisms of telomere-related proteins are needed. Relative telomere lengths of Qingshui, WL168, RSA-01, RSA-02, and RSA-03 in plants aged 5 years were higher than those aged 1 year by 11.41, 11.24, 9.21, 10.23, and 11.41, respectively. Relative telomere length of alfalfa tended to increase with age. Accordingly, alfalfa varieties can be classified according to rate of change in relative telomere length as long-lived (Qingshui, WL168, and RSA-03), medium-lived (RSA-02) and short-lived (RSA-01). The differences in relative telomere length distances of Qingshui, WL168, RSA-01, RSA-02, and RSA-03 between plants aged 1 and 5 years were 10.40, 13.02, 12.22, 11.22, and 13.25, respectively. The largest difference in relative telomere length was found between Qingshui and RSA-02 at 2.20. Our findings demonstrated that relative telomere length in alfalfa is influenced by genetic variation and age, with age exerting a greater effect.

Causes and consequences of variation in early-life telomere length in a bird metapopulation

Environmental conditions during early-life development can have lasting effects shaping individual heterogeneity in fitness and fitness-related traits. The length of telomeres, the DNA sequences protecting chromosome ends, may be affected by early-life conditions, and telomere length (TL) has been associated with individual performance within some wild animal populations. Thus, knowledge of the mechanisms that generate variation in TL, and the relationship between TL and fitness, is important in understanding the role of telomeres in ecology and life-history evolution. Here, we investigate how environmental conditions and morphological traits are associated with early-life blood TL and if TL predicts natal dispersal probability or components of fitness in 2746 wild house sparrow (Passer domesticus) nestlings from two populations sampled across 20 years (1994-2013). We retrieved weather data and we monitored population fluctuations, individual survival, and reproductive output using field observations and genetic pedigrees. We found a negative effect of population density on TL, but only in one of the populations. There was a curvilinear association between TL and the maximum daily North Atlantic Oscillation index during incubation, suggesting that there are optimal weather conditions that result in the longest TL. Dispersers tended to have shorter telomeres than non-dispersers. TL did not predict survival, but we found a tendency for individuals with short telomeres to have higher annual reproductive success. Our study showed how early-life TL is shaped by effects of growth, weather conditions, and population density, supporting that environmental stressors negatively affect TL in wild populations. In addition, shorter telomeres may be associated with a faster pace-of-life, as individuals with higher dispersal rates and annual reproduction tended to have shorter early-life TL.

Telomere length declines with age, but relates to immune function independent of age in a wild passerine

Telomere length (TL) shortens with age but telomere dynamics can relate to fitness components independent of age. Immune function often relates to such fitness components and can also interact with telomeres. Studying the link between TL and immune function may therefore help us understand telomere-fitness associations. We assessed the relationships between erythrocyte TL and four immune indices (haptoglobin, natural antibodies (NAbs), complement activity (CA) and heterophil-lymphocyte (HL) ratio; n = 477-589), from known-aged individuals of a wild passerine (Malurus coronatus). As expected, we find that TL significantly declined with age. To verify whether associations between TL and immune function were independent of parallel age-related changes (e.g. immunosenescence), we statistically controlled for sampling age and used within-subject centring of TL to separate relationships within or between individuals. We found that TL positively predicted CA at the between-individual level (individuals with longer average TL had higher CA), but no other immune indices. By contrast, age predicted the levels of NAbs and HL ratio, allowing inference that respective associations between TL and age with immune indices are independent. Any links existing between TL and fitness are therefore unlikely to be strongly mediated by innate immune function, while TL and immune indices appear independent expressions of individual heterogeneity.

Gut microbiome and telomere length in gull hatchlings

In many animals, recent evidence indicates that the gut microbiome may be acquired during early development, with possible consequences on newborns' health. Thus, it has been hypothesized that a healthy microbiome protects telomeres and genomic integrity against cellular stress. However, the link between the early acquired microbiome and telomere dynamics has not hitherto been investigated. In birds, this link may also be potentially modulated by the transfer of maternal glucocorticoids, since these substances dysregulate microbiome composition during postnatal development. Here, we examined the effect of the interplay between the microbiome and stress hormones on the telomere length of yellow-legged gull hatchlings by using a field experiment in which we manipulated the corticosterone content in eggs. We found that the hatchling telomere length was related to microbiome composition, but this relationship was not affected by the corticosterone treatment. Hatchlings with a microbiome dominated by potential commensal bacteria (i.e. Catellicoccus and Cetobacterium) had larger telomeres, suggesting that an early establishment of the species-specific microbiome during development may have important consequences on offspring health and survival.

The Accumulating Costs Hypothesis—to Better Understand Delayed “Hidden” Costs of Seemingly Mild Disease and Other Moderate Stressors

Mild diseases and moderate stressors are seemingly harmless and are therefore often assumed to have negligible impact on Darwinian fitness. Here we argue that the effects of “benign” parasites and other moderate stressors may have a greater impact on lifespan and other fitness traits than generally thought. We outline the “accumulating costs” hypothesis which proposes that moderate strains on the body caused by mild diseases and other moderate stressors that occur throughout life will result in small irreversible “somatic lesions” that initially are invisible (i.e., induce “hidden” costs). However, over time these somatic lesions accumulate until their summed effect reaches a critical point when cell senescence and malfunction begin to affect organ functionality and lead to the onset of degenerative diseases and aging. We briefly discuss three potential mechanisms through which the effects of moderate strains (e.g., mild diseases) could accumulate: Accelerated telomere shortening, loss of repetitious cell compartments and other uncorrected DNA damage in the genome. We suggest that telomere shortening may be a key candidate for further research with respect to the accumulating costs hypothesis. Telomeres can acquire lesions from moderate strains without immediate negative effects, lesions can be accumulated over time and lead to a critically short telomere length, which may eventually cause severe somatic malfunctioning, including aging. If effects of mild diseases, benign parasites and moderate stressors accrued throughout life can have severe delayed consequences, this might contribute to our understanding of life history strategies and trade-offs, and have important implications for medicine, including consideration of treatment therapies for mild (chronic) infections such as malaria.

Natural variation in plant telomere length is associated with flowering time

Telomeres are highly repetitive DNA sequences found at the ends of chromosomes that protect the chromosomes from deterioration duringcell division. Here, using whole-genome re-sequencing and terminal restriction fragment assays, we found substantial natural intraspecific variation in telomere length in Arabidopsis thaliana, rice (Oryza sativa), and maize (Zea mays). Genome-wide association study (GWAS) mapping in A. thaliana identified 13 regions with GWAS-significant associations underlying telomere length variation, including a region that harbors the telomerase reverse transcriptase (TERT) gene. Population genomic analysis provided evidence for a selective sweep at the TERT region associated with longer telomeres. We found that telomere length is negatively correlated with flowering time variation not only in A. thaliana, but also in maize and rice, indicating a link between life-history traits and chromosome integrity. Our results point to several possible reasons for this correlation, including the possibility that longer telomeres may be more adaptive in plants that have faster developmental rates (and therefore flower earlier). Our work suggests that chromosomal structure itself might be an adaptive trait associated with plant life-history strategies.

Pathogen burden and leukocyte telomere length in the United States

BACKGROUND

Prior studies in humans have suggested that telomere shortening may be accelerated by infection, but research on multiple pathogens and use of large population-based study samples has been limited. We estimated cross-sectional associations between seropositivity to five persistent pathogens (Herpes Simplex Virus Type-1 (HSV-1), Herpes Simplex Virus Type-2 (HSV-2), cytomegalovirus (CMV), Helicobacter pylori (H.pylori), and Hepatitis B) as well as total pathogen burden and leukocyte telomere length. Data were derived from the National Health and Nutrition Examination Survey (1999-2000) for individuals 20-49 years of age, N = 1708. We analyzed the influence of each pathogen separately, a pathogen count score and a latent class model of pathogen burden on log telomere length using linear regression models, adjusted for covariates.

RESULTS

Individuals in a latent pathogen burden class characterized by high probabilities of infection with HSV-1, CMV, and H. pylori, had significantly decreased log telomere length (- 0.30 [95% CI: - 0.36, - 0.24]) compared to those in a latent class characterized by low probabilities of all five infections. There were limited significant associations using other pathogen measures.

CONCLUSIONS

These results suggest that infection with specific combinations of pathogens may be one mechanism contributing to accelerated cellular senescence with possible origins early in the life course.

Direct and indirect effects of multiple environmental stressors on fish health in human-altered rivers

Freshwater fish face multiple challenges in human-altered rivers such as trace metal contamination, temperature increase and parasitism. These multiple stressors could have unexpected interactive effects on fish health due to shared physiological pathways, but few studies investigated this question in wild fish populations. In this study, we compared 16 populations of gudgeon (Gobio occitaniae) distributed along perturbation gradients in human-altered rivers in the South of France. We tested the effects of single and combined stressors (i.e., metal contamination, temperature, parasitism) on key traits linked to fish health across different biological levels using a Structural Equation Modelling approach. Parasitism and temperature alone had limited deleterious effects on fish health. In contrast, fish living in metal-contaminated sites had higher metal bioaccumulation and higher levels of cellular damage in the liver through the induction of an inflammatory response. In addition, temperature and contamination had interactive negative effects on growth. These results suggest that trace metal contamination has deleterious effects on fish health at environmentally realistic concentrations and that temperature can modulate the effects of trace metals on fish growth. With this study, we hope to encourage integrative approaches in realistic field conditions to better predict the effects of natural and anthropogenic stressors on aquatic organisms.

Impact of ageing and soil contaminants on telomere length in the land snail

Telomeres (TLs) are non-coding DNA sequences that are usually shortened with ageing and/or chemical exposure. Bioindicators such as the land snail can be used to assess the environmental risk of contaminated soils. As for most invertebrates, the evolution of TLs with ageing or exposure to contaminants is unknown in this mollusc. The aims of this study were to explore the relationships between ageing, contaminant exposure, sublethal effects and TL length in the terrestrial gastropod Cantareus aspersus. TL length was investigated in haemocytes from five age classes of C. aspersus. The impact of contaminants on sub-adult snails exposed to Cd, Hg or a mixture of polycyclic aromatic hydrocarbons (PAHs) in soils for one or two months was studied. Bioaccumulation, growth, sexual maturity and TLs were measured. TL attrition was significant for the juvenile and sub-adult stages, but not later. Exposure to Cd increased the mortality (around 30%). Exposure to polluted soils inhibited growth (19-40%) and sexual maturity (6-100%). Although the health of the snails exposed to Cd, Hg and PAHs was altered, TL length in haemocytes was not disturbed, suggesting a high capacity of this snail species to maintain its TLs in haemocytes under chemical stress. These results first address TL length in snails and reveal that the relationship commonly proposed for vertebrates between TL shortening and ageing or exposure to contaminants cannot be generalized.