Telomere length is repeatable, shortens with age and reproductive success, and predicts remaining lifespan in a long‐lived seabird

Early-life telomeres are influenced by environments acting at multiple temporal and spatial scales

An individual's telomere length early in life may reflect or contribute to key life-history processes sensitive to environmental variation. Yet, the relative importance of genetic and environmental factors in shaping early-life telomere length is not well understood as it requires samples collected from multiple generations with known developmental histories. We used a confirmed pedigree and conducted an animal model analysis of telomere lengths obtained from nestling house sparrows (Passer domesticus) sampled over a span of 22 years. We found significant additive genetic variation for early-life telomere length, but it comprised a small proportion (9%) of the total biological variation. Three sources of environmental variation were important: among cohorts, among-breeding attempts within years, and among nestmates. The magnitude of variation among breeding attempts and among nestmates also differed by cohort, suggesting that interactive effects of environmental factors across time or spatial scales were important, yet we were unable to identify the specific causes of these interactions. The mean amount of precipitation during the breeding season positively predicted telomere length, but neither weather during a given breeding attempt nor date in the breeding season contributed to an offspring's telomere length. At the level of individual nestlings, offspring sex, size and mass at 10 days of age also did not predict telomere length. Environmental effects appear especially important in shaping early-life telomere length in some species, and more focus on how environmental factors that interact across scales may help to explain some of the variation observed among studies.

How does maternal age influence reproductive performance and offspring phenotype in the snow petrel (Pagodroma nivea)?

In wild vertebrates, the increase of breeding success with advancing age has been extensively studied through laying date, clutch size, hatching success, and fledging success. However, to better evaluate the influence of age on reproductive performance in species with high reproductive success, assessing not only reproductive success but also other proxies of reproductive performance appear crucial. For example, the quality of developmental conditions and offspring phenotype can provide robust and complementary information on reproductive performance. In long-lived vertebrate species, several proxies of developmental conditions can be used to estimate the quality of the produced offspring (i.e., body size, body condition, corticosterone levels, and telomere length), and therefore, their probability to survive. By sampling chicks reared by known-aged mothers, we investigated the influence of maternal age on reproductive performance and offspring quality in a long-lived bird species, the snow petrel (Pagodroma nivea). Older females bred and left their chick alone earlier. Moreover, older females had larger chicks that grew faster, and ultimately, those chicks had a higher survival probability at the nest. In addition, older mothers produced chicks with a higher sensitivity to stress, as shown by moderately higher stress-induced corticosterone levels. Overall, our study demonstrated that maternal age is correlated to reproductive performance (hatching date, duration of the guarding period and survival) and offspring quality (body size, growth rate and sensitivity to stress), suggesting that older individuals provide better parental cares to their offspring. These results also demonstrate that maternal age can affect the offspring phenotype with potential long-term consequences.

A Multibreed Genome-Wide Association Study for Cattle Leukocyte Telomere Length

Telomeres are terminal DNA regions of chromosomes that prevent chromosomal fusion and degradation during cell division. In cattle, leukocyte telomere length (LTL) is associated with longevity, productive lifespan, and disease susceptibility. However, the genetic basis of LTL in this species is less studied than in humans. In this study, we utilized the whole-genome resequencing data of 239 animals from 17 cattle breeds for computational leukocyte telomere length estimation and subsequent genome-wide association study of LTL. As a result, we identified 42 significant SNPs, of which eight were found in seven genes (EXOC6B, PTPRD, RPS6KC1, NSL1, AGBL1, ENSBTAG00000052188, and GPC1) when using covariates for two major breed groups (Turano-Mongolian and European). Association analysis with covariates for breed effect detected 63 SNPs, including 13 in five genes (EXOC6B, PTPRD, RPS6KC1, ENSBTAG00000040318, and NELL1). The PTPRD gene, demonstrating the top signal in analysis with breed effect, was previously associated with leukocyte telomere length in cattle and likely is involved in the mechanism of alternative lengthening of telomeres. The single nucleotide variants found could be tested for marker-assisted selection to improve telomere-length-associated traits.

Early-life telomere length predicts life-history strategy and reproductive senescence in a threatened wild songbird

Telomeres are well known for their associations with lifespan and ageing across diverse taxa. Early-life telomere length can be influenced by developmental conditions and has been shown positively affect lifetime reproductive success in a limited number of studies. Whether these effects are caused by a change in lifespan, reproductive rate or perhaps most importantly reproductive senescence is unclear. Using long-term data on female breeding success from a threatened songbird (the hihi, Notiomystis cincta), we show that the early-life telomere length of individuals predicts the presence and rate of future senescence of key reproductive traits: clutch size and hatching success. In contrast, senescence of fledging success is not associated with early-life telomere length, which may be due to the added influence of biparental care at this stage. Early-life telomere length does not predict lifespan or lifetime reproductive success in this species. Females may therefore change their reproductive allocation strategy depending on their early developmental conditions, which we hypothesise are reflected in their early-life telomere length. Our results offer new insights on the role that telomeres play in reproductive senescence and individual fitness and suggest telomere length can be used as a predictor for future life history in threatened species.

Compilation and functional classification of telomere length-associated genes in humans and other animal species

Telomeres are the terminal regions of chromosomes that ensure their stability while cell division. Telomere shortening initiates cellular senescence, which can lead to degeneration and atrophy of tissues, so the process is associated with a reduction in life expectancy and predisposition to a number of diseases. An accelerated rate of telomere attrition can serve as a predictor of life expectancy and health status of an individual. Telomere length is a complex phenotypic trait that is determined by many factors, including the genetic ones. Numerous studies (including genome-wide association studies, GWAS) indicate the polygenic nature of telomere length control. The objective of the present study was to characterize the genetic basis of the telomere length regulation using the GWAS data obtained during the studies of various human and other animal populations. To do so, a compilation of the genes associated with telomere length in GWAS experiments was collected, which included information on 270 human genes, as well as 23, 22, and 9 genes identified in the cattle, sparrow, and nematode, respectively. Among them were two orthologous genes encoding a shelterin protein (POT1 in humans and pot-2 in C. elegans). Functional analysis has shown that telomere length can be influenced by genetic variants in the genes encoding: (1) structural components of telomerase; (2) the protein components of telomeric regions (shelterin and CST complexes); (3) the proteins involved in telomerase biogenesis and regulating its activity; (4) the proteins that regulate the functional activity of the shelterin components; (5) the proteins involved in telomere replication and/or capping; (6) the proteins involved in the alternative telomere lengthening; (7) the proteins that respond to DNA damage and are responsible for DNA repair; (8) RNA-exosome components. The human genes identified by several research groups in populations of different ethnic origins are the genes encoding telomerase components such as TERC and TERT as well as STN1 encoding the CST complex component. Apparently, the polymorphic loci affecting the functions of these genes may be the most reliable susceptibility markers for telomere-related diseases. The systematized data about the genes and their functions can serve as a basis for the development of prognostic criteria for telomere length-associated diseases in humans. Information about the genes and processes that control telomere length can be used for marker-assisted and genomic selection in the farm animals, aimed at increasing the duration of their productive lifetime.

Longitudinal telomere dynamics within natural lifespans of a wild bird

Telomeres, the nucleotide sequences that protect the ends of eukaryotic chromosomes, shorten with each cell division and telomere loss may be influenced by environmental factors. Telomere length (TL) decreases with age in several species, but little is known about the sources of genetic and environmental variation in the change in TL (∆TL) in wild animals. In this study, we tracked changes in TL throughout the natural lifespan (from a few months to almost 9 years) of free-living house sparrows (Passer domesticus) in two different island populations. TL was measured in nestlings and subsequently up to four times during their lifetime. TL generally decreased with age (senescence), but we also observed instances of telomere lengthening within individuals. We found some evidence for selective disappearance of individuals with shorter telomeres through life. Early-life TL positively predicted later-life TL, but the within-individual repeatability in TL was low (9.2%). Using genetic pedigrees, we found a moderate heritability of ∆TL (h² = 0.21), which was higher than the heritabilities of early-life TL (h² = 0.14) and later-life TL measurements (h² = 0.15). Cohort effects explained considerable proportions of variation in early-life TL (60%), later-life TL (53%), and ∆TL (37%), which suggests persistent impacts of the early-life environment on lifelong telomere dynamics. Individual changes in TL were independent of early-life TL. Finally, there was weak evidence for population differences in ∆TL that may be linked to ecological differences in habitat types. Combined, our results show that individual telomere biology is highly dynamic and influenced by both genetic and environmental variation in natural conditions.

Sex-specific changes in autosomal methylation rate in ageing common terns

Senescence, an age-related decline in survival and/or reproductive performance, occurs in species across the tree of life. Molecular mechanisms underlying this within-individual phenomenon are still largely unknown, but DNA methylation changes with age are among the candidates. Using a longitudinal approach, we investigated age-specific changes in autosomal methylation of common terns, relatively long-lived migratory seabirds known to show senescence. We collected blood at 1-, 3- and/or 4-year intervals, extracted DNA from the erythrocytes and estimated autosomal DNA methylation by mapping Reduced Representative Bisulfite Sequencing reads to a de novo assembled reference genome. We found autosomal methylation levels to decrease with age within females, but not males, and no evidence for selective (dis)appearance of birds of either sex in relation to their methylation level. Moreover, although we found positions in the genome to consistently vary in their methylation levels, individuals did not show such strong consistent variance. These results pave the way for studies at the level of genome features or specific positions, which should elucidate the functional consequences of the patterns observed, and how they translate to the ageing phenotype.

Unlocking the Secrets of Longevity: Exploring the Impact of Socioeconomic Factors and Health Resources on Life Expectancy in Oman and Qatar

In an era marked by a sweeping pandemic and the encroaching shadow of an energy crisis, the well-being and lifespan of global populations have become pressing concerns for every nation. This research zeroes in on life expectancy (LE), a powerful indicator of societal health in Oman and Qatar. Our study juxtaposes 3 key factors affecting LE: health status and resources (HSR), macroeconomics (ME), and sociodemographic (SD) factors. To achieve this, we tapped into a comprehensive data set from the World Bank, encompassing a transformative 3-decade span from 1990 to 2020. The intricate interplay between these factors and LE was deciphered through robust Structural Equation Modeling (SEM). Our findings construct a compelling narrative: ME has an indirect yet considerable influence on LE-manifesting with an effect of 0.602 for Oman and 0.676 for Qatar. This influence is mediated by SD and HSR components. Similarly, SD elements impact LE indirectly-with an effect of 0.653 for Oman and 0.759 for Qatar- this effect is mediated by HSR. In contrast, HSR themselves wield a robust and direct influence on LE, indicated by an effect of 0.839 for Oman and 0.904 for Qatar. All these aforementioned effects were statistically substantial (P < .001). Our research magnifies the robust direct influence of HSR on LE in both Oman and Qatar, simultaneously highlighting the noteworthy indirect role of ME and SD factors. This emphasizes the significance of adopting an integrated policy approach that considers all the SD, ME, and HSR factors to improve the population health in both countries, which are also crucial for promoting LE growth in the Gulf Cooperation Council region.

Warmer incubation temperatures and later lay-orders lead to shorter telomere lengths in wood duck (Aix sponsa) ducklings

The environment that animals experience during development shapes phenotypic expression. In birds, two important aspects of the early-developmental environment are lay-order sequence and incubation. Later-laid eggs tend to produce weaker offspring, sometimes with compensatory mechanisms to accelerate their growth rate to catch-up to their siblings. Further, small decreases in incubation temperature slow down embryonic growth rates and lead to wide-ranging negative effects on many posthatch traits. Recently, telomeres, noncoding DNA sequences at the end of chromosomes, have been recognized as a potential proxy for fitness because longer telomeres are positively related to lifespan and individual quality in many animals, including birds. Although telomeres appear to be mechanistically linked to growth rate, little is known about how incubation temperature and lay-order may influence telomere length. We incubated wood duck (Aix sponsa) eggs at two ecologically-relevant temperatures (34.9°C and 36.2°C) and measured telomere length at hatch and 1 week after. We found that ducklings incubated at the lower temperature had longer telomeres than those incubated at the higher temperature both at hatch and 1 week later. Further, we found that later-laid eggs produced ducklings with shorter telomeres than those laid early in the lay-sequence, although lay-order was not related to embryonic developmental rate. This study contributes to our broader understanding of how parental effects can affect telomere length early in life. More work is needed to determine if these effects on telomere length persist until adulthood, and if they are associated with effects on fitness in this precocial species.

Alternative telomere maintenance mechanism in Alligator sinensis provides insights into aging evolution

Lifespan is a life-history trait that undergoes natural selection. Telomeres are hallmarks of aging, and shortening rate predicts species lifespan, making telomere maintenance mechanisms throughout different lifespans a worthy topic for study. Alligators are suitable for the exploration of anti-aging molecular mechanisms, because they exhibit low or even negligible mortality in adults and no significant telomere shortening. Telomerase reverse transcriptase (TERT) expression is absent in the adult Alligator sinensis, as in humans. Selection analyses on telomere maintenance genes indicated that ATM, FANCE, SAMHD1, HMBOX1, NAT10, and MAP3K4 experienced positive selection on A. sinensis. Repressed pleiotropic ATM kinase in A. sinensis suggests their fitness optimum shift. In ATM downstream, Alternative Lengthening of Telomeres (ALT)-related genes were clustered in a higher expression pattern in A. sinensis, which covers 10-15% of human cancers showing no telomerase activities. In summary, we demonstrated how telomere shortening, telomerase activities, and ALT contributed to anti-aging strategies.

Telomeres in ecology and evolution: A review and classification of hypotheses

Research on telomeres in the fields of ecology and evolution has been rapidly expanding over the last two decades. This has resulted in the formulation of a multitude of, often name-given, hypotheses related to the associations between telomeres and life-history traits or fitness-facilitating processes (and the mechanisms underlying them). However, the differences (or similarities) between the various hypotheses, which can originate from different research fields, are often not obvious. Our aim here is therefore to give an overview of the hypotheses that are of interest in ecology and evolution and to provide two frameworks that help discriminate among them. We group the hypotheses (i) based on their association with different research questions, and (ii) using a hierarchical approach that builds on the assumptions they make, such as about causality of telomere length/shortening and/or the proposed functional consequences of telomere shortening on organism performance. Both our frameworks show that there exist parallel lines of thoughts in different research fields. Moreover, they also clearly illustrate that there are in many cases competing hypotheses within clusters, and that some of these even have contradictory assumptions and/or predictions. We also touch upon two topics in telomere research that would benefit from further conceptualization. This review should help researchers, both those familiar with and those new to the subject, to identify future avenues of research.

Telomere length is heritable and genetically correlated with lifespan in a wild bird

Telomeres are protective caps at the end of eukaryotic chromosomes that shorten with age and in response to stressful or resource-demanding conditions. Their length predicts individual health and lifespan across a wide range of animals, but whether the observed positive association between telomere length and lifespan is environmentally induced, or set at conception due to a shared genetic basis, has not been tested in wild animals. We applied quantitative genetic "animal models" to longitudinal telomere measurements collected over a 10-year period from individuals of a wild seabird (common tern; Sterna hirundo) with known pedigree. We found no variation in telomere shortening with age among individuals at the phenotypic and genetic level, and only a small permanent environmental effect on adult telomere length. Instead, we found telomere length to be highly heritable and strongly positively genetically correlated with lifespan. Such heritable differences between individuals that are set at conception may present a hitherto underappreciated component of variation in somatic state.

Telomere dynamics in the first year of life, but not later in life, predict lifespan in a wild bird

Telomeres are protective, nucleoprotein structures at the end of chromosomes that have been associated with lifespan across taxa. However, the extent to which these associations can be attributed to absolute length vs. the rate of telomere shortening prior to sampling remains unresolved. In a longitudinal study, we examined the relationship between lifespan, telomere length and the rate of telomere shortening in wild, purple-crowned fairy-wrens (Malurus coronatus coronatus). To this end, we measured telomere length using quantitative polymerase chain reaction in the blood of 59 individuals sampled as nestlings and 4-14 months thereafter, and in 141 known-age individuals sampled on average three times across adulthood. We applied within-subject centring analyses to simultaneously test for associations between lifespan and average telomere length and telomere shortening. We reveal that the rate of telomere shortening and to a lesser extent telomere length in the first year of life independently predicted lifespan, with individuals with faster shortening rates and/or shorter telomeres living less long. In contrast, in adulthood neither telomere shortening nor telomere length predicted lifespan, despite a considerably larger data set. Our results suggest that telomere length measured very early in life (during development) and longitudinal assessments of telomere shortening during the first year of life constitute more useful biomarkers of total life expectancy than either telomere length measured after development, or telomere shortening later in adulthood.

The association between female reproductive performance and leukocyte telomere length in wild Soay sheep

Telomere length (TL), typically measured across a sample of blood cells, has emerged as an exciting potential marker of physiological state and of the costs of investment in growth and reproduction within evolutionary ecology. While there is mounting evidence from studies of wild vertebrates that short TL predicts raised subsequent mortality risk, the relationship between reproductive investment and TL is less clear cut, and previous studies report both negative and positive associations. In this study, we examined the relationship between TL and different aspects of maternal reproductive performance in a free-living population of Soay sheep. We find evidence for shorter TL in females that bred, and thus paid any costs of gestation, compared to females that did not breed. However, we found no evidence for any association between TL and litter size. Furthermore, females that invested in gestation and lactation actually had longer TL than females who only invested in gestation because their offspring died shortly after birth. We used multivariate models to decompose these associations into among- and within-individual effects, and discovered that within-individual effects were driving both the negative association between TL and gestation, and the positive association between TL and lactation. This suggests that telomere dynamics may reflect recent physiologically costly investment or variation in physiological condition, depending on the aspect of reproduction being investigated. Our results highlight the physiological complexity of vertebrate reproduction, and the need to better understand how and why different aspects of physiological variation underpinning life histories impact blood cell TL.

Telomere heritability and parental age at conception effects in a wild avian population

Individual variation in telomere length is predictive of health and mortality risk across a range of species. However, the relative influence of environmental and genetic variation on individual telomere length in wild populations remains poorly understood. Heritability of telomere length has primarily been calculated using parent-offspring regression which can be confounded by shared environments. To control for confounding variables, quantitative genetic "animal models" can be used, but few studies have applied animal models in wild populations. Furthermore, parental age at conception may also influence offspring telomere length, but most studies have been cross-sectional. We investigated within- and between-parental age at conception effects and heritability of telomere length in the Seychelles warbler using measures from birds caught over 20 years and a multigenerational pedigree. We found a weak negative within-paternal age at conception effect (as fathers aged, their offspring had shorter telomeres) and a weak positive between-maternal age at conception effect (females that survived to older ages had offspring with longer telomeres). Animal models provided evidence that heritability and evolvability of telomere length were low in this population, and that variation in telomere length was not driven by early-life effects of hatch period or parental identities. Quantitative polymerase chain reaction plate had a large influence on telomere length variation and not accounting for it in the models would have underestimated heritability. Our study illustrates the need to include and account for technical variation in order to accurately estimate heritability, as well as other environmental effects, on telomere length in natural populations.

Genetic architecture and heritability of early-life telomere length in a wild passerine

Early-life telomere length (TL) is associated with fitness in a range of organisms. Little is known about the genetic basis of variation in TL in wild animal populations, but to understand the evolutionary and ecological significance of TL it is important to quantify the relative importance of genetic and environmental variation in TL. In this study, we measured TL in 2746 house sparrow nestlings sampled across 20 years and used an animal model to show that there is a small heritable component of early-life TL (h²  = 0.04). Variation in TL among individuals was mainly driven by environmental (annual) variance, but also brood and parental effects. Parent-offspring regressions showed a large maternal inheritance component in TL ( h maternal 2  = 0.44), but no paternal inheritance. We did not find evidence for a negative genetic correlation underlying the observed negative phenotypic correlation between TL and structural body size. Thus, TL may evolve independently of body size and the negative phenotypic correlation is likely to be caused by nongenetic environmental effects. We further used genome-wide association analysis to identify genomic regions associated with TL variation. We identified several putative genes underlying TL variation; these have been inferred to be involved in oxidative stress, cellular growth, skeletal development, cell differentiation and tumorigenesis in other species. Together, our results show that TL has a low heritability and is a polygenic trait strongly affected by environmental conditions in a free-living bird.

Decline in telomere length with increasing age across nonhuman vertebrates: A meta-analysis

The prediction that telomere length (TL) shortens with increasing age is a major element in considering the role of telomeres as a key player in evolution. While telomere attrition is found in humans both in vitro and in vivo, the increasing number of studies reporting diverse age-specific patterns of TL challenges the hypothesis of a universal decline of TL with increasing age. Here, we performed a meta-analysis to estimate the relationship between TL and age across 175 estimates encompassing 98 species of vertebrates. We found that, on average, TL does decline with increasing age during adulthood. However, this decline was weak and variable across vertebrate classes, and we also found evidence for a publication bias that might weaken our current evidence of decreasing TL with increasing age. We found no evidence for a faster decline in TL with increasing age when considering the juvenile stage (from birth to age at first reproduction) compared to the adult stage. Heterogeneity in TL ageing rates was explained by the method used to measure telomeres: detectable TL declines with increasing age were found only among studies using TRF with in-gel hybridisation and qFISH methods, but not in studies using qPCR and Southern blot-based TRF methods. While we confirmed that TL declines with increasing age in most adult vertebrates, our results identify an influence of telomere measurement methodology, which highlights the need to examine more thoroughly the effect of the method of measurement on TL estimates.

Associations between DNA methylation and telomere length during early life: Insight from wild zebra finches (Taeniopygia guttata)

Telomere length and DNA methylation (DNAm) are two promising biomarkers of biological age. Environmental factors and life history traits are known to affect variation in both these biomarkers, especially during early life, yet surprisingly little is known about their reciprocal association, especially in natural populations. Here, we explore how variation in DNAm, growth rate, and early-life conditions are associated with telomere length changes during development. We tested these associations by collecting data from wild, nestling zebra finches in the Australian desert. We found that increases in the level of DNAm were negatively correlated with telomere length changes across early life. We also confirm previously documented effects of post hatch growth rate and clutch size on telomere length in a natural ecological context for a species that has been extensively studied in the laboratory. However, we did not detect any effect of ambient temperature during developmental on telomere length dynamics. We also found that the absolute telomere length of wild zebra finches, measured using the in-gel TRF method, was similar to that of captive birds. Our findings highlight exciting new opportunities to link and disentangle potential relationships between DNA based biomarkers of ageing, and of physiological reactions to environmental change.

Within-individual repeatability in telomere length: A meta-analysis in nonmammalian vertebrates

Telomere length is increasingly used as a biomarker of long-term somatic state and future survival prospects. While most studies have overlooked this aspect, biological interpretations based on a given telomere length will benefit from considering the level of within-individual repeatability of telomere length through time. Therefore, we conducted a meta-analysis on 74 longitudinal studies in nonmammalian vertebrates, with the aim to establish the current pattern of within-individual repeatability in telomere length and to identify the methodological (e.g., qPCR/TRF) and biological factors (e.g., age class, phylogeny) that may affect it. While the median within-individual repeatability of telomere length was moderate to high (R = 0.55; 95% CI: 0.05-0.95; N = 82), marked heterogeneity between studies was evident. Measurement method affected the repeatability estimate strongly, with TRF-based studies exhibiting high repeatability (R = 0.80; 95% CI: 0.34-0.96; N = 25), while repeatability of qPCR-based studies was markedly lower and more variable (R = 0.46; 95% CI: 0.04-0.82; N = 57). While phylogeny explained some variance in repeatability, phylogenetic signal was not significant (λ = 0.32; 95% CI: 0.00-0.83). None of the biological factors investigated here significantly explained variation in the repeatability of telomere length, being potentially obscured by methodological differences. Our meta-analysis highlights the high variability in within-individual repeatability estimates between studies and the need to put more effort into separating technical and biological explanations. This is important to better understand to what extent biological factors can affect the repeatability of telomere length and thus the interpretation of telomere length data.

A meta-analysis on the heritability of vertebrate telomere length

Telomere dynamics are linked with both cellular and organismal senescence, and life history, individual quality and health. Telomere dynamics, particularly telomere length, have therefore garnered much research interest in evolutionary biology. To examine the evolution of telomere length, it is important to quantify its heritability, the proportion of total variation explained by additive genetic effects. Many studies have quantified telomere length heritability, but estimates are varied, and no general conclusion has been drawn. Additionally, it is unclear whether biological and methodological factors influence telomere length heritability estimates. We present the first meta-analysis of telomere length heritability, using 104 estimates from 43 studies over 18 vertebrate species. We calculated an overall mean heritability and examined how estimates varied by study, phylogeny, species-specific ecology, environmental setting, age at sampling, laboratory methods, statistical methods, sex and repeated measurements. Overall heritability was moderate (44.9%, 95% CI: 25.2-64.7%), and there was considerable heterogeneity in heritability estimates, in particular among studies and estimates. Laboratory method influenced heritability estimates, with in-gel hybridization TRF yielding higher heritabilities than qPCR and Southern blot TRF. There was also an effect from statistical method, with twin-based and SNP-based estimates lower than correlation-based or pedigree-based estimates. Our results highlight an overall heritable basis of telomere length, and we recommend future research on a wider range of taxa, and the use of variance-partitioning methods with relatedness or SNP data over correlation methods to minimize heritability estimation bias.

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 correlates with physiological and behavioural responses of a long-lived seabird to an ecologically relevant challenge

Determinants of individual variation in reallocation of limited resources towards self-maintenance versus reproduction are not well known. We tested the hypothesis that individual heterogeneity in long-term 'somatic state' (i) explains variation in endocrine and behavioural responses to environmental challenges, and (ii) is associated with variation in strategies for allocating to self-maintenance versus reproduction. We used relative telomere length as an indicator of somatic state and experimentally generated an abrupt short-term reduction of food availability (withdrawal of food supplementation) for free-living seabirds (black-legged kittiwakes, Rissa tridactyla). Incubating male kittiwakes responded to withdrawal by increasing circulating corticosterone and losing more weight compared to continuously supplemented controls. Males with longer telomeres increased time in directed travel regardless of treatment, while experiencing smaller increases in corticosterone. Males with longer telomeres fledged more chicks in the control group and tended to be more likely to return regardless of treatment. This study supports the hypothesis that somatic state can explain variation in short-term physiological and behavioural responses to challenges, and longer-term consequences for fitness. Male kittiwakes with longer telomeres appear to have prioritized investment in self over investment in offspring under challenging conditions.

Telomere dynamics in female Columbian ground squirrels: recovery after emergence and loss after reproduction

Telomeres are specialized non-coding DNA sequences located at the end of chromosomes and that protect genetic information. Telomere loss over lifespan is generally viewed as a phenomenon associated with aging in animals. Recently, telomere elongation after hibernation has been described in several mammals. Whether this pattern is an adaptation to repair DNA damage caused during rewarming from torpor or if it coevolved as a mechanism to promote somatic maintenance in preparation for the upcoming reproductive effort remains unclear. In a longitudinal study measuring telomere length using buccal swabs, we tested if telomere elongation was related to reproductive success in wild adult female Columbian ground squirrels (Urocitellus columbianus) that were monitored from emergence from hibernation to the end of the reproductive season. We found three key results. First, female telomere length increased at the start of the breeding season, both in breeding and non-breeding individuals. Second, post-emergence telomere lengthening was unrelated to female future reproductive output. Third, telomere length decreased in breeding females during lactation, but remained stable in non-breeding females over a similar period. Within breeders, telomeres shortened more in females producing larger and heavier litters. We concluded that telomere lengthening after hibernation did not constrain immediate female reproductive capacities. It was more likely to be part of the body recovery process that takes place after hibernation. Telomere erosion that occurs after birth may constitute a physiological cost of female reproduction.

Telomere length positively correlates with pace-of-life in a sex- and cohort-specific way and elongates with age in a wild mammal

Understanding ageing and the diversity of life histories is a cornerstone in biology. Telomeres, the protecting caps of chromosomes, are thought to be involved in ageing, cancer risks and life-history strategies. They shorten with cell division and age in somatic tissues of most species, possibly limiting lifespan. The resource allocation trade-off hypothesis predicts that short telomeres have thus co-evolved with early reproduction, proactive behaviour and reduced lifespan, i.e. a fast Pace-of-Life Syndrome (POLS). Conversely, since short telomeres may also reduce the risks of cancer, the anti-cancer hypothesis advances that they should be associated with slow POLS. Conclusion on which hypothesis best supports the role of telomeres as mediators of life-history strategies is hampered by a lack of study on wild short-lived vertebrates, apart from birds. Using seven years of data on wild Eastern chipmunks Tamias striatus, we highlighted that telomeres elongate with age (n = 204 and n = 20) and do not limit lifespan in this species (n = 51). Furthermore, short telomeres correlated with a slow POLS in a sex-specific way (n = 37). Females with short telomeres had a delayed age at first breeding and a lower fecundity rate than females with long telomeres, while we found no differences in males. Our findings support most predictions adapted from the anti-cancer hypothesis, but none of those from the resource allocation trade-off hypothesis. Results are in line with an increasing body of evidence suggesting that other evolutionary forces than resource allocation trade-offs shape the diversity of telomere length in adult somatic cells and the relationships between telomere length and life-histories.

How does early-life adversity shape telomere dynamics during adulthood? Problems and paradigms

Although early-life adversity has been associated with negative consequences during adulthood, growing evidence shows that such adversity can also lead to subsequent stress resilience and positive fitness outcomes. Telomere dynamics are relevant in this context because of the link with developmental conditions and longevity. However, few studies have assessed whether the effects of early-life adversity on developmental telomere dynamics may relate to adult telomere dynamics. We propose that the potential links between early-life adversity and adult telomere dynamics could be driven by developmental constraints (the Constraint hypothesis), by the nature/severity of developmental adversity (the Resilience hypothesis), or by developmental-mediated changes in individual life-history strategies (the Pace of Life hypothesis). We discuss these non-mutually exclusive hypotheses, explore future research directions, and propose specific studies to test these hypotheses. Our article aims to expand our understanding of the evolutionary role of developmental conditions on adult telomere dynamics, stress resilience and ageing.

The influence of phylogeny and life history on telomere lengths and telomere rate of change among bird species: A meta-analysis

Longevity is highly variable among animal species and has coevolved with other life-history traits, such as body size and rates of reproduction. Telomeres, through their erosion over time, are one of the cell mechanisms that produce senescence at the cell level and might even have an influence on the rate of aging in whole organisms. However, uneroded telomeres are also risk factors of cell immortalization. The associations of telomere lengths, their rate of change, and life-history traits independent of body size are largely underexplored for birds. To test associations of life-history traits and telomere dynamics, we conducted a phylogenetic meta-analysis using studies of 53 species of birds. We restricted analyses to studies that applied the telomere restriction fragment length (TRF) method, and examined relationships between mean telomere length at the chick (Chick TL) and adult (Adult TL) stages, the mean rate of change in telomere length during life (TROC), and life-history traits. We examined 3 principal components of 12 life-history variables that represented: body size (PC1), the slow-fast continuum of pace of life (PC2), and postfledging parental care (PC3). Phylogeny had at best a small-to-medium influence on Adult and Chick TL (r ² = .190 and .138, respectively), but a substantial influence on TROC (r ² = .688). Phylogeny strongly influenced life histories: PC1 (r ² = .828), PC2 (.838), and PC3 (.613). Adult TL and Chick TL were poorly associated with the life-history variables. TROC, however, was negatively and moderate-to-strongly associated with PC2 (unadjusted r = -.340; with phylogenetic correction, r = -.490). Independent of body size, long-lived species with smaller clutches, and slower embryonic rate of growth may exhibit less change in telomere length over their lifetimes. We suggest that telomere lengths may have diverged, even among closely avian-related species, yet telomere dynamics are strongly linked to the pace of life.

The influence of phylogeny and life history on telomere lengths and telomere rate of change among bird species: A meta-analysis

Longevity is highly variable among animal species and has coevolved with other life-history traits, such as body size and rates of reproduction. Telomeres, through their erosion over time, are one of the cell mechanisms that produce senescence at the cell level and might even have an influence on the rate of aging in whole organisms. However, uneroded telomeres are also risk factors of cell immortalization. The associations of telomere lengths, their rate of change, and life-history traits independent of body size are largely underexplored for birds. To test associations of life-history traits and telomere dynamics, we conducted a phylogenetic meta-analysis using studies of 53 species of birds. We restricted analyses to studies that applied the telomere restriction fragment length (TRF) method, and examined relationships between mean telomere length at the chick (Chick TL) and adult (Adult TL) stages, the mean rate of change in telomere length during life (TROC), and life-history traits. We examined 3 principal components of 12 life-history variables that represented: body size (PC1), the slow-fast continuum of pace of life (PC2), and postfledging parental care (PC3). Phylogeny had at best a small-to-medium influence on Adult and Chick TL (r 2 = .190 and .138, respectively), but a substantial influence on TROC (r 2 = .688). Phylogeny strongly influenced life histories: PC1 (r 2 = .828), PC2 (.838), and PC3 (.613). Adult TL and Chick TL were poorly associated with the life-history variables. TROC, however, was negatively and moderate-to-strongly associated with PC2 (unadjusted r = -.340; with phylogenetic correction, r = -.490). Independent of body size, long-lived species with smaller clutches, and slower embryonic rate of growth may exhibit less change in telomere length over their lifetimes. We suggest that telomere lengths may have diverged, even among closely avian-related species, yet telomere dynamics are strongly linked to the pace of life.

Longer telomeres during early life predict higher lifetime reproductive success in females but not males

The mechanisms that contribute to variation in lifetime reproductive success are not well understood. One possibility is that telomeres, conserved DNA sequences at chromosome ends that often shorten with age and stress exposures, may reflect differences in vital processes or influence fitness. Telomere length often predicts longevity, but longevity is only one component of fitness and little is known about how lifetime reproductive success is related to telomere dynamics in wild populations. We examined the relationships between telomere length beginning in early life, telomere loss into adulthood and lifetime reproductive success in free-living house sparrows (Passer domesticus). We found that females, but not males, with longer telomeres during early life had higher lifetime reproductive success, owing to associations with longevity and not reproduction per year or attempt. Telomeres decreased with age in both sexes, but telomere loss was not associated with lifetime reproductive success. In this species, telomeres may reflect differences in quality or condition rather than the pace of life, but only in females. Sexually discordant selection on telomeres is expected to influence the stability and maintenance of within population variation in telomere dynamics and suggests that any role telomeres play in mediating life-history trade-offs may be sex specific.

Heritable variation in telomere length predicts mortality in Soay sheep

Telomere length (TL) is considered an important biomarker of whole-organism health and aging. Across humans and other vertebrates, short telomeres are associated with increased subsequent mortality risk, but the processes responsible for this correlation remain uncertain. A key unanswered question is whether TL-mortality associations arise due to positive effects of genes or early-life environment on both an individual's average lifetime TL and their longevity, or due to more immediate effects of environmental stressors on within-individual TL loss and increased mortality risk. Addressing this question requires longitudinal TL and life history data across the entire lifetimes of many individuals, which are difficult to obtain for long-lived species like humans. Using longitudinal data and samples collected over nearly two decades, as part of a long-term study of wild Soay sheep, we dissected an observed positive association between TL and subsequent survival using multivariate quantitative genetic models. We found no evidence that telomere attrition was associated with increased mortality risk, suggesting that TL is not an important marker of biological aging or exposure to environmental stress in our study system. Instead, we find that among-individual differences in average TL are associated with increased lifespan. Our analyses suggest that this correlation between an individual's average TL and lifespan has a genetic basis. This demonstrates that TL has the potential to evolve under natural conditions, and suggests an important role of genetics underlying the widespread observation that short telomeres predict mortality.

Clutch quality is related to embryonic development duration, hatchling body size and telomere length in the spined toad (Bufo spinosus)

Reproductive success is often related to parental quality, a parameter expressed through various traits, such as site selection, mate selection and energetic investment in the eggs or progeny. Owing to the complex interactions between environmental and parental characteristics occurring at various stages of the reproductive event, it is often complicated to tease apart the relative contributions of these different factors to reproductive success. Study systems where these complex interactions are simplified (e.g. absence of parental care) can help us to understand how metrics of parental quality (e.g. gamete and egg quality) influence reproductive success. Using such a study system in a common garden experiment, we investigated the relationships between clutch hatching success (a proxy of clutch quality) and offspring quality in an amphibian species lacking post-oviposition parental care. We found a relationship between clutch quality and embryonic development duration and hatchling phenotype. We found that hatchling telomere length was linked to hatching success. These results suggest that clutch quality is linked to early life traits in larval amphibians and that deciphering the influence of parental traits on the patterns we detected is a promising avenue of research.

Telomere attrition rates are associated with weather conditions and predict productive lifespan in dairy cattle

Telomere length is predictive of adult health and survival across vertebrate species. However, we currently do not know whether such associations result from among-individual differences in telomere length determined genetically or by early-life environmental conditions, or from differences in the rate of telomere attrition over the course of life that might be affected by environmental conditions. Here, we measured relative leukocyte telomere length (RLTL) multiple times across the entire lifespan of dairy cattle in a research population that is closely monitored for health and milk production and where individuals are predominantly culled in response to health issues. Animals varied in their change in RLTL between subsequent measurements and RLTL shortened more during early life and following hotter summers which are known to cause heat stress in dairy cows. The average amount of telomere attrition calculated over multiple repeat samples of individuals predicted a shorter productive lifespan, suggesting a link between telomere loss and health. TL attrition was a better predictor of when an animal was culled than their average TL or the previously for this population reported significant TL at the age of 1 year. Our present results support the hypothesis that TL is a flexible trait that is affected by environmental factors and that telomere attrition is linked to animal health and survival traits. Change in telomere length may represent a useful biomarker in animal welfare studies.

The changing face of genome assemblies: Guidance on achieving high-quality reference genomes

The quality of genome assemblies has improved rapidly in recent years due to continual advances in sequencing technology, assembly approaches, and quality control. In the field of molecular ecology, this has led to the development of exceptional quality genome assemblies that will be important long-term resources for broader studies into ecological, conservation, evolutionary, and population genomics of naturally occurring species. Moreover, the extent to which a single reference genome represents the diversity within a species varies: pan-genomes will become increasingly important ecological genomics resources, particularly in systems found to have considerable presence-absence variation in their functional content. Here, we highlight advances in technology that have raised the bar for genome assembly and provide guidance on standards to achieve exceptional quality reference genomes. Key recommendations include the following: (a) Genome assemblies should include long-read sequencing except in rare cases where it is effectively impossible to acquire adequately preserved samples needed for high molecular weight DNA standards. (b) At least one scaffolding approach should be included with genome assembly such as Hi-C or optical mapping. (c) Genome assemblies should be carefully evaluated, this may involve utilising short read data for genome polishing, error correction, k-mer analyses, and estimating the percent of reads that map back to an assembly. Finally, a genome assembly is most valuable if all data and methods are made publicly available and the utility of a genome for further studies is verified through examples. While these recommendations are based on current technology, we anticipate that future advances will push the field further and the molecular ecology community should continue to adopt new approaches that attain the highest quality genome assemblies.

Telomere dynamics from hatching to sexual maturity and maternal effects in the 'multivariate egg'

Avian eggs contain a large number of molecules deposited by the mother that provide the embryo with energy but also potentially influence its development via the effects of maternally derived hormones and antibodies: the avian egg is thus 'multivariate'. Multivariate effects on offspring phenotype were evaluated in a study on captive zebra finches, by simultaneously manipulating maternally derived antibodies (MAb) by lipopolysaccharide (LPS) treatment of mothers and injection of testosterone into the egg yolk. LPS treatment had a positive effect on body mass growth at 30 days after hatching and immune response at sexual maturity, while egg testosterone treatment positively influenced immune response at fledging and courtship behaviour in sexually mature male offspring. Maternal effects are known to modulate offspring telomere length (TL). However, the multivariate effects of egg-derived maternal components on offspring telomere dynamics from hatching to sexual maturity are undefined. Here, we tested: (1) the effects of LPS and testosterone treatments on TL from hatching to sexual maturity (day 82); (2) how LPS treatment modulated TL over reproduction in adult females; and (3) the relationship between maternal and offspring TL. We predicted that TL would be shorter in LPS fledglings (as a cost of faster growth) and that TL would be longer in sexually mature adults after yolk testosterone treatment (as a proxy of individual quality). In adult females, there was an overall negative relationship between laying and rearing investments and TL, this relationship was weaker in LPS-treated females. In chicks, there was an overall negative effect of LPS treatment on TL measured at fledging and sexual maturity (day 25-82). In addition, at fledging, there was a Sex×LPS×Testosterone interaction, suggesting the existence of antagonistic effects of our treatments. Our data partially support the hypothesis that telomeres are proxies of individual quality and that individual differences in TL are established very early in life.

Wing: A suitable nonlethal tissue type for repeatable and rapid telomere length estimates in bats

Telomeres are used increasingly in ecology and evolution as biomarkers for ageing and environmental stress, and are typically measured from DNA extracted from nonlethally sampled blood. However, obtaining blood is not always possible in field conditions and only limited amounts can be taken from small mammals, such as bats, which moreover lack nucleated red blood cells and hence yield relatively low amounts of DNA. As telomere length can vary within species according to age and tissue, it is important to determine which tissues serve best as a representation of the organism as a whole. Here, we investigated whether wing tissue biopsies, a rapid and relatively noninvasive tissue collection method, could serve as a proxy for other tissues when measuring relative telomere length (rTL) in the Egyptian fruit bat (Rousettus aegyptiacus). Telomeres were measured from blood, brain, heart, kidney, liver lung, muscle and wing, and multiple wing biopsies were taken from the same individuals to determine intra-individual repeatability of rTL measured by using qPCR. Wing rTL correlated with rTL estimates from most tissues apart from blood. Blood rTL was not significantly correlated with rTL from any other tissue. Blood and muscle rTLs were significantly longer compared with other tissues, while lung displayed the shortest rTLs. Individual repeatability of rTL measures from wing tissue was high (>70%). Here we show the relationships between tissue telomere dynamics for the first time in a bat, and our results provide support for the use of wing tissue for rTL measurements.

Short-term telomere dynamics is associated with glucocorticoid levels in wild populations of roe deer

While evidence that telomere length is associated with health and mortality in humans and birds is accumulating, a large body of research is currently seeking to identify factors that modulate telomere dynamics. We tested the hypothesis that high levels of glucocorticoids in individuals under environmental stress should accelerate telomere shortening in two wild populations of roe deer (Capreolus capreolus) living in different ecological contexts. From two consecutive annual sampling sessions, we found that individuals with faster rates of telomere shortening had higher concentrations of fecal glucocorticoid metabolites, suggesting a functional link between glucocorticoid levels and telomere attrition rate. This relationship was consistent for both sexes and populations. This finding paves the way for further studies of the fitness consequences of exposure to environmental stressors in wild vertebrates.

Pace and stability of embryonic development affect telomere dynamics: an experimental study in a precocial bird model

Prenatal effects on telomere length are increasingly recognized as a potential contributor to the developmental origin of health and adult disease. While it is becoming clear that telomere length is influenced by prenatal conditions, the factors affecting telomere dynamics during embryogenesis remain poorly understood. We manipulated both the pace and stability of embryonic development through varying incubation temperature and its stability in Japanese quail. We investigated the impact on telomere dynamics from embryogenesis to adulthood, together with three potential drivers of telomere shortening, growth rate, oxidative damage and prenatal glucocorticoid levels. Telomere length was not affected by our prenatal manipulation for the first 75% of embryogenesis, but was reduced at hatching in groups experiencing faster (i.e. high temperature) or less stable embryonic development. These early life differences in telomere length persisted until adulthood. The effect of developmental instability on telomere length at hatching was potentially mediated by an increased secretion of glucocorticoid hormones during development. Both the pace and the stability of embryo development appear to be key factors determining telomere length and dynamics into adulthood, with fast and less stable development leading to shorter telomeres, with the potential for adverse associated outcomes in terms of reduced longevity.

The genome of the Xingu scale-backed antbird (Willisornis vidua nigrigula) reveals lineage-specific adaptations

Antbirds (Thamnophilidae) are a large neotropical family of passerine bird renowned for the ant-following foraging strategies of several members of this clade. The high diversity of antbirds provides ample opportunity for speciation studies, however these studies can be hindered by the lack of an annotated antbird reference genome. In this study, we produced a high-quality annotated reference genome for the Xingu Scale-backed Antbird (Willisornis vidua nigrigula) using 10X Genomics Chromium linked-reads technology. The assembly is 1.09 Gb, with a scaffold N50 of 12.1 Mb and 17,475 annotated protein coding genes. We compare the proteome of W. v. nigrigula to several other passerines, and produce annotations for two additional antbird genomes in order to identify genes under lineage-specific positive selection and gene families with evidence for significant expansions in antbirds. Several of these genes have functions potentially related to the lineage-specific traits of antbirds, including adaptations for thermoregulation in a humid tropical environment.

Sex-specific telomere length and dynamics in relation to age and reproductive success in Cory's shearwaters

Individuals in free‐living animal populations generally differ substantially in reproductive success, lifespan and other fitness‐related traits, but the molecular mechanisms underlying this variation are poorly understood. Telomere length and dynamics are candidate traits explaining this variation, as long telomeres predict a higher survival probability and telomere loss has been shown to reflect experienced “life stress.” However, telomere dynamics among very long‐lived species are unresolved. Additionally, it is generally not well understood how telomeres relate to reproductive success or sex. We measured telomere length and dynamics in erythrocytes to assess their relationship to age, sex and reproduction in Cory's shearwaters (Calonectris borealis), a long‐lived seabird, in the context of a long‐term study. Adult males had on average 231 bp longer telomeres than females, independent of age. In females, telomere length changed relatively little with age, whereas male telomere length declined significantly. Telomere shortening within males from one year to the next was three times higher than the interannual shortening rate based on cross‐sectional data of males. Past long‐term reproductive success was sex‐specifically reflected in age‐corrected telomere length: males with on average high fledgling production were characterized by shorter telomeres, whereas successful females had longer telomeres, and we discuss hypotheses that may explain this contrast. In conclusion, telomere length and dynamics in relation to age and reproduction are sex‐dependent in Cory's shearwaters and these findings contribute to our understanding of what characterises individual variation in fitness.