Mortality rates in a genetically heterogeneous population of Caenorhabditis elegans

Life history in Caenorhabditis elegans: from molecular genetics to evolutionary ecology

Life history is defined by traits that reflect key components of fitness, especially those relating to reproduction and survival. Research in life history seeks to unravel the relationships among these traits and understand how life history strategies evolve to maximize fitness. As such, life history research integrates the study of the genetic and developmental mechanisms underlying trait determination with the evolutionary and ecological context of Darwinian fitness. As a leading model organism for molecular and developmental genetics, Caenorhabditis elegans is unmatched in the characterization of life history-related processes, including developmental timing and plasticity, reproductive behaviors, sex determination, stress tolerance, and aging. Building on recent studies of natural populations and ecology, the combination of C. elegans' historical research strengths with new insights into trait variation now positions it as a uniquely valuable model for life history research. In this review, we summarize the contributions of C. elegans and related species to life history and its evolution. We begin by reviewing the key characteristics of C. elegans life history, with an emphasis on its distinctive reproductive strategies and notable life cycle plasticity. Next, we explore intraspecific variation in life history traits and its underlying genetic architecture. Finally, we provide an overview of how C. elegans has guided research on major life history transitions both within the genus Caenorhabditis and across the broader phylum Nematoda. While C. elegans is relatively new to life history research, significant progress has been made by leveraging its distinctive biological traits, establishing it as a highly cross-disciplinary system for life history studies.

Coping with global warming: Adult thermal thresholds in four pestiferous Anastrepha species determined under experimental laboratory conditions and development/survival times of immatures and adults under natural field conditions

Climate change, particularly global warming, is disturbing biological processes in unexpected ways and forcing us to re-study/reanalyze the effects of varying temperatures, among them extreme ones, on insect functional traits such as lifespan and fecundity/fertility. Here we experimentally tested, under both laboratory and field conditions, the effects of an extreme range of temperatures (5, 10, 15, 20, 30, 40, and 45 °C, and the naturally varying conditions experienced in the field), on survivorship/lifespan, fecundity, and fertility of four pestiferous fruit fly species exhibiting contrasting life histories and belonging to two phylogenetic groups within the genus Anastrepha: A. ludens, A. obliqua, A. striata, and A. serpentina. In the field, we also measured the length of the entire life cycle (egg to adult), and in one species (A. ludens), the effect on the latter of the host plant (mango and grapefruit). Under laboratory conditions, none of the adults, independent of species, could survive a single day when exposed to a constant temperature of 45 °C, but A. striata and A. serpentina females/males survived at the highly contrasting temperatures of 5 and 40 °C at least 7 days. Maximum longevity was achieved in all species at 15 °C (375, 225, 175 and 160 days in A. ludens, A. serpentina, A. striata and A. obliqua females, respectively). Anastrepha ludens layed many eggs until late in life (368 days) at 15 °C, but none eclosed. Eclosion was only observed in all species at 20 and 30 °C. Under natural conditions, flies lived ca. 100 days less than in the laboratory at 15 °C, likely due to the physiological cost of dealing with the highly varying environmental patterns over 24 h (minimum and maximum temperatures and relative humidity of ca. 10–40 °C, and 22–100%, respectively). In the case of A. ludens, the immature’s developmental time was shorter in mango, but adult survival was longer than in grapefruit. We discuss our results considering the physiological processes regulating the traits measured and tie them to the increasing problem of global warming and its hidden effects on the physiology of insects, as well as the ecological and pest management implications.

The Replica Set Method is a Robust, Accurate, and High-Throughput Approach for Assessing and Comparing Lifespan in C. elegans Experiments

The advent of feeding based RNAi in Caenorhabditis elegans led to an era of gene discovery in aging research. Hundreds of gerogenes were discovered, and many are evolutionarily conserved, raising the exciting possibility that the underlying genetic basis for healthy aging in higher vertebrates could be quickly deciphered. Yet, the majority of putative gerogenes have still only been cursorily characterized, highlighting the need for high-throughput, quantitative assessments of changes in aging. A widely used surrogate measure of aging is lifespan. The traditional way to measure mortality in C. elegans tracks the deaths of individual animals over time within a relatively small population. This traditional method provides straightforward, direct measurements of median and maximum lifespan for the sampled population. However, this method is time consuming, often underpowered, and involves repeated handling of a set of animals over time, which in turn can introduce contamination or possibly damage increasingly fragile, aged animals. We have previously developed an alternative "Replica Set" methodology, which minimizes handling and increases throughput by at least an order of magnitude. The Replica Set method allows changes in lifespan to be measured for over one hundred feeding-based RNAi clones by one investigator in a single experiment- facilitating the generation of large quantitative phenotypic datasets, a prerequisite for development of biological models at a systems level. Here, we demonstrate through analysis of lifespan experiments simulated in silico that the Replica Set method is at least as precise and accurate as the traditional method in evaluating and estimating lifespan, and requires many fewer total animal observations across the course of an experiment. Furthermore, we show that the traditional approach to lifespan experiments is more vulnerable than the Replica Set method to experimental and measurement error. We find no compromise in statistical power for Replica Set experiments, even for moderate effect sizes, or when simulated experimental errors are introduced. We compare and contrast the statistical analysis of data generated by the two approaches, and highlight pitfalls common with the traditional methodology. Collectively, our analysis provides a standard of measure for each method across comparable parameters, which will be invaluable in both experimental design and evaluation of published data for lifespan studies.

From microbes to mammals: The experimental evolution of aging and longevity across species

Senescence, the functional deterioration of cells or organisms associated with increased age, is pervasive across the tree of life. Yet our understanding of the genetic and physiological basis underlying age-related declines in health and reproduction remains limited. Experimental evolution allows empirical examination of the question of why aging occurs; imposing selection for age-specific fitness traits shifts patterns of aging in experimental populations, enabling investigations of the variation underlying senescence and the mechanisms governing it. Whole-genome sequencing of experimentally evolved populations may reveal candidate genomic variants underlying particular aging patterns; unfortunately, most study systems suffer from limitations that weaken associations between genotypes and phenotypes. In this review, we provide a survey of experimental evolution studies that have altered population-level patterns of reproductive timing and senescence in a variety of species. We discuss the specific selection conditions that have increased longevity, the phenotypic responses and trade-offs that accompany these increases, and examine genomic data collected from these experiments. Additionally, we consider how selected field studies complement laboratory experiments on life-history evolution. Finally, we address the strengths and weaknesses of existing study systems, and evaluate which model organisms appear most promising for future genomic investigations of the evolutionary biology of aging.

University of Southern California and buck institute nathan shock center: multidimensional models of aging

The USC-Buck Nathan Shock Center of Excellence in the Biology of Aging is a new and fully integrated multi-institutional center focused on training the next generation of geroscientists and providing access to cutting-edge geroscience technologies to investigators across the nation. The USC-Buck NSC is devoted to forging a deeper understanding of how and why aging processes cause disease in order to advance the translation of basic research on aging into effective preventions and therapies. Including more than 61 NIA-supported investigators, six NIA-funded research centers, four NIA T32s, and several additional aging research centers of excellence, the USC-Buck NSC constitutes one of the largest collections of leaders in geroscience research within the USA; the unique nature of the USC-Buck NSC research infrastructure ensures an integrated organization that is representative of the wide breadth of topics encompassed by the biology of aging field. By leveraging the 25-year-long relationship, current collaborations and joint administrational activities of the University of Southern California and the Buck Institute for Aging Research, the USC-Buck NSC aims to enhance and expand promising research in the biology of aging at both at the and to make a positive impact across California, the nation and throughout the world. Specialized cores provide services to all Shock Center members, as well as provide support for services to the community at large.

A universal transcriptomic signature of age reveals the temporal scaling of Caenorhabditis elegans aging trajectories

We collected 60 age-dependent transcriptomes for C. elegans strains including four exceptionally long-lived mutants (mean adult lifespan extended 2.2- to 9.4-fold) and three examples of lifespan-increasing RNAi treatments. Principal Component Analysis (PCA) reveals aging as a transcriptomic drift along a single direction, consistent across the vastly diverse biological conditions and coinciding with the first principal component, a hallmark of the criticality of the underlying gene regulatory network. We therefore expected that the organism's aging state could be characterized by a single number closely related to vitality deficit or biological age. The "aging trajectory", i.e. the dependence of the biological age on chronological age, is then a universal stochastic function modulated by the network stiffness; a macroscopic parameter reflecting the network topology and associated with the rate of aging. To corroborate this view, we used publicly available datasets to define a transcriptomic biomarker of age and observed that the rescaling of age by lifespan simultaneously brings together aging trajectories of transcription and survival curves. In accordance with the theoretical prediction, the limiting mortality value at the plateau agrees closely with the mortality rate doubling exponent estimated at the cross-over age near the average lifespan. Finally, we used the transcriptomic signature of age to identify possible life-extending drug compounds and successfully tested a handful of the top-ranking molecules in C. elegans survival assays and achieved up to a +30% extension of mean lifespan.

Genetic cartography of longevity in humans and mice: Current landscape and horizons

Aging is a complex and highly variable process. Heritability of longevity among humans and other species is low, and this finding has given rise to the idea that it may be futile to search for DNA variants that modulate aging. We argue that the problem in mapping longevity genes is mainly one of low power and the genetic and environmental complexity of aging. In this review we highlight progress made in mapping genes and molecular networks associated with longevity, paying special attention to work in mice and humans. We summarize 40 years of linkage studies using murine cohorts and 15 years of studies in human populations that have exploited candidate gene and genome-wide association methods. A small but growing number of gene variants contribute to known longevity mechanisms, but a much larger set have unknown functions. We outline these and other challenges and suggest some possible solutions, including more intense collaboration between research communities that use model organisms and human cohorts. Once hundreds of gene variants have been linked to differences in longevity in mammals, it will become feasible to systematically explore gene-by-environmental interactions, dissect mechanisms with more assurance, and evaluate the roles of epistasis and epigenetics in aging. A deeper understanding of complex networks-genetic, cellular, physiological, and social-should position us well to improve healthspan.

Experimental Evolution with Caenorhabditis Nematodes

The hermaphroditic nematode Caenorhabditis elegans has been one of the primary model systems in biology since the 1970s, but only within the last two decades has this nematode also become a useful model for experimental evolution. Here, we outline the goals and major foci of experimental evolution with C. elegans and related species, such as C. briggsae and C. remanei, by discussing the principles of experimental design, and highlighting the strengths and limitations of Caenorhabditis as model systems. We then review three exemplars of Caenorhabditis experimental evolution studies, underlining representative evolution experiments that have addressed the: (1) maintenance of genetic variation; (2) role of natural selection during transitions from outcrossing to selfing, as well as the maintenance of mixed breeding modes during evolution; and (3) evolution of phenotypic plasticity and its role in adaptation to variable environments, including host-pathogen coevolution. We conclude by suggesting some future directions for which experimental evolution with Caenorhabditis would be particularly informative.

MORTALITY PLATEAUS AND THE EVOLUTION OF SENESCENCE: WHY ARE OLD-AGE MORTALITY RATES SO LOW?

Age-specific mortality rates level off far below 100% at advanced ages in experimental populations of Drosophila melanogaster and other organisms. This observation is inconsistent with the equilibrium predictions of both the antagonistic pleiotropy and mutation accumulation models of senescence, which, under a wide variety of assumptions, predict a "wall" of mortality rates near 100% at postreproductive ages. Previous models of age-specific mortality patterns are discussed in light of recent demographic data concerning late-age mortality deceleration and age-specific properties of new mutations. The most recent theory (Mueller and Rose 1996) argues that existing evolutionary models can easily and robustly explain the demographic data. Here we discuss the sensitivity of that analysis to different types of mutational effects, and demonstrate that its conclusion is very sensitive to assumptions about mutations. A legitimate resolution of evolutionary theory and demographic data will require experimental observations on the age-specificity of mutational effects for new mutations and the degree to which mortality rates in adjacent ages are constrained to be similar (positive pleiotropy), as well as consideration of redundancy and heterogeneity models from demographic theory.

Two forms of death in ageing Caenorhabditis elegans

Ageing generates senescent pathologies, some of which cause death. Interventions that delay or prevent lethal pathologies will extend lifespan. Here we identify life-limiting pathologies in Caenorhabditis elegans with a necropsy analysis of worms that have died of old age. Our results imply the presence of multiple causes of death. Specifically, we identify two classes of corpse: early deaths with a swollen pharynx (which we call 'P deaths'), and later deaths with an atrophied pharynx (termed 'p deaths'). The effects of interventions on lifespan can be broken down into changes in the frequency and/or timing of either form of death. For example, glp-1 mutation only delays p death, while eat-2 mutation reduces P death. Combining pathology and mortality analysis allows mortality profiles to be deconvolved, providing biological meaning to complex survival and mortality profiles.

Patterns of physiological decline due to age and selection in Drosophila melanogaster

In outbred sexually reproducing populations, age-specific mortality rates reach a plateau in late life following the exponential increase in mortality rates that marks aging. Little is known about what happens to physiology when cohorts transition from aging to late life. We measured age-specific values for starvation resistance, desiccation resistance, time-in-motion, and geotaxis in ten Drosophila melanogaster populations: five populations selected for rapid development and five control populations. Adulthood was divided into two stages, the aging phase and the late-life phase according to demographic data. Consistent with previous studies, we found that populations selected for rapid development entered the late-life phase at an earlier age than the controls. Age-specific rates of change for all physiological phenotypes showed differences between the aging phase and the late-life phase. This result suggests that late life is physiologically distinct from aging. The ages of transitions in physiological characteristics from aging to late life statistically match the age at which the demographic transition from aging to late life occurs, in all cases but one. These experimental results support evolutionary theories of late life that depend on patterns of decline and stabilization in the forces of natural selection.

Retired flies, hidden plateaus, and the evolution of senescence in Drosophila melanogaster

Late-life plateaus in age-specific mortality have been an evolutionary and biodemographic puzzle for decades. Although classic theory on the evolution of senescence predicts late-life walls of death, observations in experimental organisms document the opposite trend: a slowing in the rate of increase of mortality at advanced ages. Here, I analyze published life-history data on individual Drosophila melanogaster females and argue for a fundamental change in our understanding of mortality in this important model system. Mortality plateaus are not, as widely assumed, exclusive to late life, and are not explained by population heterogeneity-they are intimately connected to individual fecundity. Female flies begin adult life in the working stage, a period of active oviposition and low but accelerating mortality. Later they transition to the retired stage, a terminal period characterized by limited fecundity and relatively constant mortality. Because ages of transition differ between flies, age-synchronized cohorts contain a mix of working and retired flies. Early- and mid-life plateaus are obscured by the presence of working flies, but can be detected when cohorts are stratified by retirement status. Stage-specificity may be an important component of Drosophila life-history evolution.

Do Insect Populations Die at Constant Rates as They Become Older? Contrasting Demographic Failure Kinetics with Respect to Temperature According to the Weibull Model

Temperature implies contrasting biological causes of demographic aging in poikilotherms. In this work, we used the reliability theory to describe the consistency of mortality with age in moth populations and to show that differentiation in hazard rates is related to extrinsic environmental causes such as temperature. Moreover, experiments that manipulate extrinsic mortality were used to distinguish temperature-related death rates and the pertinence of the Weibull aging model. The Newton-Raphson optimization method was applied to calculate parameters for small samples of ages at death by estimating the maximum likelihoods surfaces using scored gradient vectors and the Hessian matrix. The study reveals for the first time that the Weibull function is able to describe contrasting biological causes of demographic aging for moth populations maintained at different temperature regimes. We demonstrate that at favourable conditions the insect death rate accelerates as age advances, in contrast to the extreme temperatures in which each individual drifts toward death in a linear fashion and has a constant chance of passing away. Moreover, slope of hazard rates shifts towards a constant initial rate which is a pattern demonstrated by systems which are not wearing out (e.g. non-aging) since the failure, or death, is a random event independent of time. This finding may appear surprising, because, traditionally, it was mostly thought as rule that in aging population force of mortality increases exponentially until all individuals have died. Moreover, in relation to other studies, we have not observed any typical decelerating aging patterns at late life (mortality leveling-off), but rather, accelerated hazard rates at optimum temperatures and a stabilized increase at the extremes.In most cases, the increase in aging-related mortality was simulated reasonably well according to the Weibull survivorship model that is applied. Moreover, semi log- probability hazard rate model illustrations and maximum likelihoods may be usefully in defining periods of mortality leveling off and provide clear evidence that environmental variability may affect parameter estimates and insect population failure rate. From a reliability theory standpoint, failure rates vary according to a linear function of age at the extremes indicating that the life system (i.e., population) is able to eliminate earlier failure and/or to keep later failure rates constant. The applied model was able to identify the major correlates of extended longevity and to suggest new ideas for using demographic concepts in both basic and applied population biology and aging.

Worms under Pressure: Bulk Mechanical Properties of C. elegans Are Independent of the Cuticle

The mechanical properties of cells and tissues play a well-known role in physiology and disease. The model organism Caenorhabditis elegans exhibits mechanical properties that are still poorly understood, but are thought to be dominated by its collagen-rich outer cuticle. To our knowledge, we use a novel microfluidic technique to reveal that the worm responds linearly to low applied hydrostatic stress, exhibiting a volumetric compression with a bulk modulus, κ = 140 ± 20 kPa; applying negative pressures leads to volumetric expansion of the worm, with a similar bulk modulus. Surprisingly, however, we find that a variety of collagen mutants and pharmacological perturbations targeting the cuticle do not impact the bulk modulus. Moreover, the worm exhibits dramatic stiffening at higher stresses-behavior that is also independent of the cuticle. The stress-strain curves for all conditions can be scaled onto a master equation, suggesting that C. elegans exhibits a universal elastic response dominated by the mechanics of pressurized internal organs.

Assessment of selenium toxicity on the life cycle of Caenorhabditis elegans

Selenium (Se) is a growing problem of global concern. Se can cause adverse effects on reproductive systems, which have been linked to declines in animal populations. The soil nematode Caenorhabditis elegans (C. elegans) is a ubiquitous soil organism that is increasingly utilized as a model organism in aquatic and soil toxicology. In the present study, the experimental data for individual body length, survival rate, brood size, and hatching rate were used to evaluate the possible effects of selenite [Se(IV)] on C. elegans. A stage-classified matrix model was applied to the experimental data to provide information on the population dynamics of C. elegans and to assess the Se(IV)-affected asymptotic population growth rate. Estimates of the survival probability showed significant decreases in survival at all stages when C. elegans was exposed to Se(IV). The growth probability of C. elegans in the L1 stage showed the most significant decline, from 0.11 h(-1) (for the control) to 0.04 h(-1) [for exposure to 3 mM Se(IV)]. These results showed that Se(IV) has a profound impact on C. elegans population dynamics. The asymptotic population growth rate (λ) was found to range from 1.00 to 0.64 h(-1) for increasing Se(IV) concentrations, implying a potential risk of population decrease for C. elegans exposure to a Se(IV)-contaminated environment. Our study shows how a mechanistic view based on the Se(IV) effects on the soil nematode C. elegans can promote a life cycle toxicity assessment. An important implication of this analysis is that mathematical models can be used to produce a population stage structure, to give clarity to the analysis of the key population-level endpoint (the asymptotic population growth rate) of population dynamics, and to evaluate the influences for the response of other species to environmental Se. These models sequentially provide candidate environmental criteria for the evaluation of the population impact of Se.

Evolution of aging: individual life history trade-offs and population heterogeneity account for mortality patterns across species

A broad range of mortality patterns has been documented across species, some even including decreasing mortality over age. Whether there exist a common denominator to explain both similarities and differences in these mortality patterns remains an open question. The disposable soma theory, an evolutionary theory of aging, proposes that universal intracellular trade-offs between maintenance/lifespan and reproduction would drive aging across species. The disposable soma theory has provided numerous insights concerning aging processes in single individuals. Yet, which specific population mortality patterns it can lead to is still largely unexplored. In this article, we propose a model exploring the mortality patterns which emerge from an evolutionary process including only the disposable soma theory core principles. We adapt a well-known model of genomic evolution to show that mortality curves producing a kink or mid-life plateaus derive from a common minimal evolutionary framework. These mortality shapes qualitatively correspond to those of Drosophila melanogaster, Caenorhabditis elegans, medflies, yeasts and humans. Species evolved in silico especially differ in their population diversity of maintenance strategies, which itself emerges as an adaptation to the environment over generations. Based on this integrative framework, we also derive predictions and interpretations concerning the effects of diet changes and heat-shock treatments on mortality patterns.

Age-related micro-RNA abundance in individual C. elegans

Non-coding small RNAs of the micro-RNA class (miRNA) are conserved regulators of gene function with a broad impact on biological processes. We screened miRNA levels for age-related changes in individual worms and investigated their influence on the lifespan of the nematode C. elegans. We measured the abundance of 69 miRNAs expressed in individual animals at different ages with over thirty five thousand discrete quantitative nano-fluidic polymerase chain reactions. We found that miRNA abundance was highly variable between individual worms raised under identical conditions and that expression variability generally increased with age. To identify expression differences associated with either reproductive or somatic tissues, we analyzed wild type and mutants that lacked germlines. miRNAs from the mir-35-41 cluster increased in abundance with age in wild type animals, but were nearly absent from mutants lacking a germline, suggesting their age-related increase originates from the germline. Most miRNAs with age-dependent levels did not have a major effect on lifespan, as corresponding deletion mutants exhibited wild-type lifespans. The major exception to this was mir-71, which increased in abundance with age and was required for normal longevity. Our genetic characterization indicates that mir-71 acts at least partly in parallel to insulin/IGF like signals to influence lifespan.

Why ageing stops: heterogeneity explains late-life mortality deceleration in nematodes

While ageing is commonly associated with exponential increase in mortality with age, mortality rates paradoxically decelerate late in life resulting in distinct mortality plateaus. Late-life mortality plateaus have been discovered in a broad variety of taxa, including humans, but their origin is hotly debated. One hypothesis argues that deceleration occurs because the individual probability of death stops increasing at very old ages, predicting the evolution of earlier onset of mortality plateaus under increased rate of extrinsic mortality. By contrast, heterogeneity theory suggests that mortality deceleration arises from individual differences in intrinsic lifelong robustness and predicts that variation in robustness between populations will result in differences in mortality deceleration. We used experimental evolution to directly test these predictions by independently manipulating extrinsic mortality rate (high or low) and mortality source (random death or condition-dependent) to create replicate populations of nematodes, Caenorhabditis remanei that differ in the strength of selection in late-life and in the level of lifelong robustness. Late-life mortality deceleration evolved in response to differences in mortality source when mortality rate was held constant, while there was no consistent response to differences in mortality rate. These results provide direct experimental support for the heterogeneity theory of late-life mortality deceleration.

25 years after age-1: genes, interventions and the revolution in aging research

This communication will briefly review more than 30 years of research on aging using the nematode Caenorhabditis elegans ("The Worm") as carried out in the labs of Tom Johnson. We will highlight research directions initiated in the 1980's, which were exciting for those of us trying to turn over a new leaf in aging research. In this narrative, I will discuss primarily the science that I and my lab have been involved with for the last 30 years. This area has been fascinating to those studying the sociology of science as modern aging research has moved to replace the simplistic, poorly controlled and outright fictitious approaches seen in much of the previous aging research.

How evolving heterogeneity distributions of resource allocation strategies shape mortality patterns

It is well established that individuals age differently. Yet the nature of these inter-individual differences is still largely unknown. For humans, two main hypotheses have been recently formulated: individuals may experience differences in aging rate or aging timing. This issue is central because it directly influences predictions for human lifespan and provides strong insights into the biological determinants of aging. In this article, we propose a model which lets population heterogeneity emerge from an evolutionary algorithm. We find that whether individuals differ in (i) aging rate or (ii) timing leads to different emerging population heterogeneity. Yet, in both cases, the same mortality patterns are observed at the population level. These patterns qualitatively reproduce those of yeasts, flies, worms and humans. Such findings, supported by an extensive parameter exploration, suggest that mortality patterns across species and their potential shapes belong to a limited and robust set of possible curves. In addition, we use our model to shed light on the notion of subpopulations, link population heterogeneity with the experimental results of stress induction experiments and provide predictions about the expected mortality patterns. As biology is moving towards the study of the distribution of individual-based measures, the model and framework we propose here paves the way for evolutionary interpretations of empirical and experimental data linking the individual level to the population level.

Paradoxical physiological transitions from aging to late life in Drosophila

In a variety of organisms, adulthood is divided into aging and late life, where aging is a period of exponentially increasing mortality rates and late life is a period of roughly plateaued mortality rates. In this study we used ∼57,600 Drosophila melanogaster from six replicate populations to examine the physiological transitions from aging to late life in four functional characters that decline during aging: desiccation resistance, starvation resistance, time spent in motion, and negative geotaxis. Time spent in motion and desiccation resistance declined less quickly in late life compared to their patterns of decline during aging. Negative geotaxis declined at a faster rate in late life compared to its rate of decline during aging. These results yield two key findings: (1) Late-life physiology is distinct from the physiology of aging, in that there is not simply a continuation of the physiological trends which characterize aging; and (2) late life physiology is complex, in that physiological characters vary with respect to their stabilization, deceleration, or acceleration in the transition from aging to late life. These findings imply that a correct understanding of adulthood requires identifying and appropriately characterizing physiology during properly delimited late-life periods as well as aging periods.

MicroRNA predictors of longevity in Caenorhabditis elegans

Neither genetic nor environmental factors fully account for variability in individual longevity: genetically identical invertebrates in homogenous environments often experience no less variability in lifespan than outbred human populations. Such variability is often assumed to result from stochasticity in damage accumulation over time; however, the identification of early-life gene expression states that predict future longevity would suggest that lifespan is least in part epigenetically determined. Such "biomarkers of aging," genetic or otherwise, nevertheless remain rare. In this work, we sought early-life differences in organismal robustness in unperturbed individuals and examined the utility of microRNAs, known regulators of lifespan, development, and robustness, as aging biomarkers. We quantitatively examined Caenorhabditis elegans reared individually in a novel apparatus and observed throughout their lives. Early-to-mid-adulthood measures of homeostatic ability jointly predict 62% of longevity variability. Though correlated, markers of growth/muscle maintenance and of metabolic by-products ("age pigments") report independently on lifespan, suggesting that graceful aging is not a single process. We further identified three microRNAs in which early-adulthood expression patterns individually predict up to 47% of lifespan differences. Though expression of each increases throughout this time, mir-71 and mir-246 correlate with lifespan, while mir-239 anti-correlates. Two of these three microRNA "biomarkers of aging" act upstream in insulin/IGF-1-like signaling (IIS) and other known longevity pathways, thus we infer that these microRNAs not only report on but also likely determine longevity. Thus, fluctuations in early-life IIS, due to variation in these microRNAs and from other causes, may determine individual lifespan.

Cytogenetic perspective of ageing and longevity in men and women

Analysis of relationships between the ageing cell phenotype and the age of cell donors is one of the ways towards understanding the link between cellular and organismal ageing. Cytogenetically, ageing is associated with a number of gross cellular changes, including altered size and morphology, genomic instability, and changes in expression and proliferation. Genomic instability can be easily assessed by analyzing the level of cytogenetic aberrations. In this review, we focus on the differences in the level and profile of cytogenetic aberrations observed in donors of different age and gender. Centenarians are a small fraction of the population at the extreme of human longevity. Their inclusion in such studies may shed light on one of the basic questions: whether genome stability is better maintained in successfully aged individuals compared to the rest of the population. At the same time, comparing the profile of age-related amount of chromosomal aberrations in men and women may help explaining the commonly observed gender differences in longevity.

Genetic (Co)variation for life span in rhabditid nematodes: role of mutation, selection, and history

The evolutionary mechanisms maintaining genetic variation in life span, particularly post-reproductive life span, are poorly understood. We characterized the effects of spontaneous mutations on life span in the rhabditid nematodes Caenorhabditis elegans and C. briggsae and standing genetic variance for life span and correlation of life span with fitness in C. briggsae. Mutations decreased mean life span, a signature of directional selection. Mutational correlations between life span and fitness were consistently positive. The average selection coefficient against new mutations in C. briggsae was approximately 2% when homozygous. The pattern of phylogeographic variation in life span is inconsistent with global mutation-selection balance (MSB), but MSB appears to hold at the local level. Standing genetic correlations in C. briggsae reflect mutational correlations at a local scale but not at a broad phylogeographic level. At the local scale, results are broadly consistent with predictions of the "mutation accumulation" hypothesis for the evolution of aging.

Evolution of ageing since Darwin

In the late 19th century, the evolutionary approach to the problem of ageing was initiated by August Weismann, who argued that natural selection was more important for ageing than any physiological mechanism. In the mid-twentieth century, J. B. S. Haldane, P. B. Medawar and G. C. Williams informally argued that the force of natural selection falls with adult age. In 1966, W. D. Hamilton published formal equations that showed mathematically that two 'forces of natural selection' do indeed decline with age, though his analysis was not genetically explicit. Brian Charlesworth then developed the required mathematical population genetics for the evolution of ageing in the 1970's. In the 1980's, experiments using Drosophila showed that the rate of ageing evolves as predicted by Hamilton's 'forces of natural selection'. The discovery of the cessation of ageing late in life in the 1990's was followed by its explanation in terms of evolutionary theory based on Hamilton's forces. Recently, it has been shown that the cessation of ageing can also be manipulated experimentally using Hamilton's 'forces of natural selection'. Despite the success of evolutionary research on ageing, mainstream gerontological research has largely ignored both this work and the opportunity that it provides for effective intervention in ageing.

Compounds that confer thermal stress resistance and extended lifespan

The observation that long-lived and relatively healthy animals can be obtained by simple genetic manipulation prompts the search for chemical compounds that have similar effects. Since aging is the most important risk factor for many socially and economically important diseases, the discovery of a wide range of chemical modulators of aging in model organisms could prompt new strategies for attacking age-related disease such as diabetes, cancer and neurodegenerative disorders [Collins, J.J., Evason, K., Kornfeld, K., 2006. Pharmacology of delayed aging and extended lifespan of Caenorhabditis elegans. Exp. Gerontol.; Floyd, R.A., 2006. Nitrones as therapeutics in age-related diseases. Aging Cell 5, 51-57; Gill, M.S., 2006. Endocrine targets for pharmacological intervention in aging in Caenorhabditis elegans. Aging Cell 5, 23-30; Hefti, F.F., Bales, R., 2006. Regulatory issues in aging pharmacology. Aging Cell 5, 3-8]. Resistance to multiple types of stress is a common trait in long-lived genetic variants of a number of species; therefore, we have tested compounds that act as stress response mimetics. We have focused on compounds with antioxidant properties and identified those that confer thermal stress resistance in the nematode Caenorhabditis elegans. Some of these compounds (lipoic acid, propyl gallate, trolox and taxifolin) also extend the normal lifespan of this simple invertebrate, consistent with the general model that enhanced stress resistance slows aging.

Reduced Expression of the Caenorhabditis elegans p53 Ortholog cep-1 Results in Increased Longevity

Hyperactivation of mammalian p53 has been shown to result in segmental progeria and decreased survivorship. Repression of the p53 homolog in Drosophila melanogaster has also been shown to increase survival. We show that RNA interference (RNAi) or genetic knockout of the Caenorhabditis elegans p53 ortholog, cep-1, leads to increased life span, which is dependent upon functional daf-16. Furthermore, one other DNA damage-responsive C. elegans mutant, hus-1(op241), exhibits a life-span increase. The cep-1(gk138) knockout mutant does not show increased resistance to heat, oxidative, or ultraviolet stress; nor to bacterial pathogenicity. cep-1 RNAi does not extend the life span of a sir-2.1(geIn3) overexpressing strain. cep-1 RNAi does not alter dauer formation propensity or nuclear-localization of DAF-16::GFP, even under heat stress; nor does it change nuclear-persistence and/or retention of DAF-16::GFP. This study clarifies the inverse relationship between cep-1 expression and C. elegans life span, and, by extrapolation, that between p53 expression and mammalian life span.

Age specificity of inbreeding load in Drosophila melanogaster and implications for the evolution of late-life mortality plateaus

Current evolutionary theories explain the origin of aging as a byproduct of the decline in the force of natural selection with age. These theories seem inconsistent with the well-documented occurrence of late-life mortality plateaus, since under traditional evolutionary models mortality rates should increase monotonically after sexual maturity. However, the equilibrium frequencies of deleterious alleles affecting late life are lower than predicted under traditional models, and thus evolutionary models can accommodate mortality plateaus if deleterious alleles are allowed to have effects spanning a range of neighboring age classes. Here we test the degree of age specificity of segregating alleles affecting fitness in Drosophila melanogaster. We assessed age specificity by measuring the homozygous fitness effects of segregating alleles across the adult life span and calculated genetic correlations of these effects across age classes. For both males and females, we found that allelic effects are age specific with effects extending over 1-2 weeks across all age classes, consistent with modified mutation-accumulation theory. These results indicate that a modified mutation-accumulation theory can both explain the origin of senescence and predict late-life mortality plateaus.

A demographic analysis of the fitness cost of extended longevity in Caenorhabditis elegans

We monitored survival and reproduction of 1000 individuals of Caenorhabditis elegans wild type (N2) and 800 individuals of clk-1 and daf-2, and used biodemographic analysis to address fitness as the integrative consequence of the entire age-specific schedules of survival and reproduction. Relative to N2, the mutants clk-1 and daf-2 extended average life span by 27% and 111%, respectively, but reduced net reproductive rate by 44% and 18%. The net result of differences in survival and fertility was a significant differential in fitness, with both clk-1 (lambda = 2.74) and daf-2 (lambda = 3.78) at a disadvantage relative to N2 (lambda = 3.85). Demographic life table response experiment (LTRE) analysis revealed that the fitness differentials were due to negative effects in mutants on reproduction in the first 6-7 days of life. Fitness costs in clk-1 and daf-2 of C. elegans are consistent with the theory of antagonistic pleiotropy for the evolution of senescence.

Public and private mechanisms of life extension in Caenorhabditis elegans

Model organisms have been widely used to study the ageing phenomenon in order to learn about human ageing. Although the phylogenetic diversity between vertebrates and some of the most commonly used model systems could hardly be greater, several mechanisms of life extension are public (common characteristic in divergent species) and likely share a common ancestry. Dietary restriction, reduced IGF-signaling and, seemingly, reduced ROS-induced damage are the best known mechanisms for extending longevity in a variety of organisms. In this review, we summarize the knowledge of ageing in the nematode Caenorhabditis elegans and compare the mechanisms of life extension with knowledge from other model organisms.

Survival and longevity improvements at extreme ages: an interpretation assuming an ecological stress theory of aging

The primary determinant of survival during aging is the energetic efficiency and metabolic stability required to counter the accumulated internal and external stresses of a lifetime. Hence, genetically stress-resistant individuals should accumulate with age; frailer, less robust, less energetically efficient and less metabolically stable individuals should succumb in parallel. This selection process implies the accumulation of energetically efficient stress-resistant individuals with age to the exclusion of all others. High additive genetic variability for survival is expected under extreme circumstances, however there is limited evidence close to the absolute extremes of life that diversity may fall. At this stage, only a few highly adaptive, oxidative-stress-resistant and presumably somewhat homozygous genotypes should remain. Therefore a fall in variability may occur in these outliers, when frailer individuals are unable to cope and are eliminated at extreme ages. This process could provide an explanation of mortality-rate declines in domesticated (laboratory) and free-living populations of the extremely old. That is, mortality-rate declines may be an expectation from a process of genetic sorting resulting from the accumulated responses to environmental stress over time. Application of an ecological stress theory of aging, which combines the external stresses to which organisms are exposed with internal stresses, appears to be the prerequisite for this conclusion.

Lifelong heterogeneity in fecundity is insufficient to explain late-life fecundity plateaus in Drosophila melanogaster

Previous studies have demonstrated that fecundity, like mortality, plateaus at late ages in cohorts of Drosophila melanogaster. Although evolutionary theory can explain the decline and plateau in cohort fecundity at late ages, it is conceivable that lifelong heterogeneity in individual female fecundity is producing these plateaus. For example, consistently more fecund females may die at earlier ages, leaving only females that always laid a low number of eggs preponderant at later ages. We simulated fecundity within a cohort, assuming the two phenotypes described above, and tested these predictions by measuring age of death and age-specific fecundity for individual females from three large cohorts. We statistically tested whether there was enough lifelong heterogeneity in fecundity to produce a late-life plateau by testing whether early female fecundity could predict whether that female would live to lay eggs after the onset of the population fecundity plateau. Our results indicate that heterogeneity in fecundity is not lifelong and thus not likely to cause late-life fecundity plateaus. Because lifelong heterogeneity models for fecundity are based on the same underlying assumptions as heterogeneity models for late-life mortality rates, our test of this hypothesis is also an experimental test of lifelong heterogeneity models of late life generally.

Evolution of late-life fecundity in Drosophila melanogaster

Late-life fecundity has been shown to plateau at late ages in Drosophila analogously to late-life mortality rates. In this study, we test an evolutionary theory of late life based on the declining force of natural selection that can explain the occurrence of these late-life plateaus in Drosophila. We also examine the viability of eggs laid by late-age females and test a population genetic mechanism that may be involved in the evolution of late-life fecundity: antagonistic pleiotropy. Together these experiments demonstrate that (i) fecundity plateaus at late ages, (ii) plateaus evolve according to the age at which the force of natural selection acting on fecundity reaches zero, (iii) eggs laid by females in late life are viable and (iv) antagonistic pleiotropy is involved in the evolution of late-life fecundity. This study further supports the evolutionary theory of late life based on the age-specific force of natural selection.

Late life: a new frontier for physiology

Late life is a distinct phase of life that occurs after the aging period and is now known to be general among aging organisms. While aging is characterized by a deterioration in survivorship and fertility, late life is characterized by the cessation of such age-related deterioration. Thus, late life presents a new and interesting area of research not only for evolutionary biology but also for physiology. In this article, we present the theoretical and experimental background to late life, as developed by evolutionary biologists and demographers. We discuss the discovery of late life and the two main theories developed to explain this phase of life: lifelong demographic heterogeneity theory and evolutionary theory based on the force of natural selection. Finally, we suggest topics for future physiological research on late life.

Testing whether male age or high nutrition causes the cessation of reproductive aging in female Drosophila melanogaster populations

Fecundity seems to stop declining and plateaus at low levels very late in Drosophila melanogaster populations. Here we test whether this apparent cessation of reproductive aging by a population, herein referred to as fecundity plateaus, is robust under various environmental influences: namely, male age and nutrition. The effect of male age on late age fecundity patterns was tested by supplying older females with young males before average population fecundity declined to plateau levels. The second possible environmental influence we tested was nutrition and whether late-life fecundity plateaus arise from a decline in the calories available for reproduction. This hypothesis was tested by comparing average daily female fecundity with both low- and high-lifetime nutrition. Both hypotheses were tested by measuring mid- and late-life fecundity for each cohort under the various environmental influences, and statistically testing whether fecundity stops declining and plateaus at late ages. These experiments demonstrate that mid- and late-life population fecundity patterns are significantly affected by the age of males and nutrition level. However, male age and nutrition level did not affect the existence of late-life fecundity plateaus, which demonstrates the robustness of our earlier findings. These results do not address any issue pertaining to the possible role, if any, of lifelong inter-individual heterogeneity in Drosophila fecundity.

Re-evaluating a test of the heterogeneity explanation for mortality plateaus

[Drapeau, M.D., Gass, E.K., Simison, M.D., Mueller, L.D., Rose, M.R., 2000. Testing the heterogeneity theory of late-life mortality plateaus by using cohorts of Drosophila melanogaster, Experimental Gerontology, 35 71-84.] tested, in populations of Drosophila melanogaster, a prediction of the heterogeneity explanation for mortality plateaus. They concluded that heterogeneity could not explain their results. We contend here that the statistical analysis was flawed. It was declared that there was no difference between the mortality plateaus of three different strains, on the basis of averaged outcomes. In fact, the results for the different strains were quite different. Most trials showed the expected lowering of the mortality plateaus for the flies selected for robustness, but these effects were washed out by a small number of very large opposing deviations. There is ample reason to believe that the opposing deviations are artifacts of fitting an overly restrictive hazard-rate model. When we fit more appropriate models, the evidence points toward a rejection of the null hypothesis (of identical plateaus), hence toward modest support for the heterogeneity explanation.

Evolutionary genetics of lifespan and mortality rates in two populations of the seed beetle, Callosobruchus maculatus

The age at which individuals die varies substantially within and between species, but we still have little understanding of why there is such variation in life expectancy. We examined sex-specific and genetic variation in adult lifespan and the shape of mortality curves both within and between two populations of the seed beetle, Callosobruchus maculatus, that differ in a suite of life history characters associated with adaptation to different host species. Mean adult lifespan and the shape of the logistic mortality curves differed substantially between males and females (males had lower initial mortality rates, but a faster increase in the rate of mortality with increasing age) and between populations (they differed in the rate of increase in mortality with age). Larger individuals lived longer than smaller individuals, both because they had lower initial mortality rates and a slower increase in the rate of mortality with increasing age. However, differences in body size were not adequate to explain the differences in mortality between the sexes or populations. Both lifespan and mortality rates were genetically variable within populations and genetic variance/covariance matrices for lifespan differed between the populations and sexes. This study thus demonstrated substantial genetic variation in lifespan and mortality rates within and between populations of C. maculatus.

The quest for a general theory of aging and longevity

Extensive studies of phenomena related to aging have produced many diverse findings, which require a general theoretical framework to be organized into a comprehensive body of knowledge. As demonstrated by the success of evolutionary theories of aging, quite general theoretical considerations can be very useful when applied to research on aging. In this theoretical study, we attempt to gain insight into aging by applying a general theory of systems failure known as reliability theory. Considerations of this theory lead to the following conclusions: (i) Redundancy is a concept of crucial importance for understanding aging, particularly the systemic nature of aging. Systems that are redundant in numbers of irreplaceable elements deteriorate (that is, age) over time, even if they are built of elements that do not themselves age. (ii) An apparent aging rate or expression of aging is higher for systems that have higher levels of redundancy. (iii) Redundancy exhaustion over the life course explains a number of observations about mortality, including mortality convergence at later life (when death rates are becoming relatively similar at advanced ages for different populations of the same species) as well as late-life mortality deceleration, leveling off, and mortality plateaus. (iv) Living organisms apparently contain a high load of initial damage from the early stages of development, and therefore their life span and aging patterns may be sensitive to early-life conditions that determine this initial damage load. Thus, the reliability theory provides a parsimonious explanation for many important aging-related phenomena and suggests a number of interesting testable predictions. We therefore suggest adding the reliability theory to the arsenal of methodological approaches applied to research on aging.

Use of a mathematical model in the analysis of survival curves of Daphnia magna exposed to toxicants

Chronic toxicity tests carried out on species of the genus Daphnia (Crustacea: Cladocera) are bioassays commonly used in ecotoxicology. Mortality in the cohorts exposed to toxicants can be examined by the analysis of survival curves. The shape of these curves may be very different because of inter-individual heterogeneity: the less rectangular is the shape the more different is the probability of dying of daphnids in the cohort. Aim of this paper is to analyze the characteristics-in particular the shape-of survival curves of cohorts of Daphnia magna exposed to heavy metals in chronic toxicity tests. Experimental curves were fitted by a mathematical model recently proposed, which puts emphasis just on this heterogeneity, described by the parameter of the model S(0). The information contained in S(0) is very useful for the present purposes: the higher the value of S(0) the less rectangular the shape of the curve and, as a consequence, the higher the age-at-death heterogeneity of the cohort is thought to be. The model contains a second parameter, omega, representing the maximum potential ability of the individuals to survive in a specific environment and is related with the maximal life span. The model fitted well the survival curves in most cases and both S(0) and omega showed statistically different values between treatments, useful for comparisons. It was concluded that S(0) provides a quantitative estimation of curve "rectangularization", useful to check different sensitivities to a specific toxicant concentration among daphnids belonging to the same cohort, while omega provides an estimate of maximal life span.