Evolutionary theories of ageing applied to long-lived organisms

[Theories of biological aging: An integrative review]

In this article, we review the main theories of biological aging, exploring the interaction of genetic, epigenetic, metabolic, immunological, and ecological factors in this process. For this purpose, we examine and discuss theories such as the allocation of metabolic resources, pleiotropic antagonism, genetic regulation, codon restriction, replicative senescence, action of free radicals, caloric restriction, catastrophic error, immunological theory, neuroendocrine theory, programmed aging, epigenetics of aging, grandmother and caregiver theories and ecological biophysical theory. We identify the contribution of different biological mechanisms to aging, emphasizing the complementarity of theories such as the allocation of metabolic resources, pleiotropic antagonism, and caloric restriction, providing a more comprehensive view of the phenomenon. In conclusion, we highlight the need to consider diverse perspectives in aging research, recognizing the absence of a single explanation. Integrating these theories is crucial to comprehensively understand the process and develop effective interventions in health and well-being in old age.

Speciation and development

Understanding general principles about the origin of species remains one of the foundational challenges in evolutionary biology. The genomic divergence between groups of individuals can spawn hybrid inviability and hybrid sterility, which presents a tantalizing developmental problem. Divergent developmental programs may yield either conserved or divergent phenotypes relative to ancestral traits, both of which can be responsible for reproductive isolation during the speciation process. The genetic mechanisms of developmental evolution involve cis- and trans-acting gene regulatory change, protein-protein interactions, genetic network structures, dosage, and epigenetic regulation, all of which also have roots in population genetic and molecular evolutionary processes. Toward the goal of demystifying Darwin's "mystery of mysteries," this review integrates microevolutionary concepts of genetic change with principles of organismal development, establishing explicit links between population genetic process and developmental mechanisms in the production of macroevolutionary pattern. This integration aims to establish a more unified view of speciation that binds process and mechanism.

Crosstalk between the aging intestinal microflora and the brain in ischemic stroke

Aging is an inevitable phenomenon experienced by animals and humans, and its intensity varies from one individual to another. Aging has been identified as a risk factor for neurodegenerative disorders by influencing the composition of the gut microbiota, microglia activity and cognitive performance. The microbiota-gut-brain axis is a two-way communication path between the gut microbes and the host brain. The aging intestinal microbiota communicates with the brain through secreted metabolites (neurotransmitters), and this phenomenon leads to the destruction of neuronal cells. Numerous external factors, such as living conditions and internal factors related to the age of the host, affect the condition of the intestinal microflora in the form of dysbiosis. Dysbiosis is defined as changes in the composition and function of the gut microflora that affect the pathogenesis, progress, and response to treatment of a disease entity. Dysbiosis occurs when changes in the composition and function of the microbiota exceed the ability of the microflora and its host to restore equilibrium. Dysbiosis leading to dysfunction of the microbiota-gut-brain axis regulates the development and functioning of the host’s nervous, immune, and metabolic systems. Dysbiosis, which causes disturbances in the microbiota-gut-brain axis, is seen with age and with the onset of stroke, and is closely related to the development of risk factors for stroke. The review presents and summarizes the basic elements of the microbiota-gut-brain axis to better understand age-related changes in signaling along the microbiota-gut-brain axis and its dysfunction after stroke. We focused on the relationship between the microbiota-gut-brain axis and aging, emphasizing that all elements of the microbiota-gut-brain axis are subject to age-related changes. We also discuss the interaction between microbiota, microglia and neurons in the aged individuals in the brain after ischemic stroke. Finally, we presented preclinical and clinical studies on the role of the aged microbiota-gut-brain axis in the development of risk factors for stroke and changes in the post-stroke microflora.

The importance of elders: Extending Hamilton's force of selection to include intergenerational transfers

In classical evolutionary models, the force of natural selection diminishes with age toward zero by last reproduction. However, intergenerational resource transfers and other late-life contributions in social species may select for postreproductive longevity. We present a formal framework for estimating indirect fitness contributions via production transfers in a skills-intensive foraging niche, reflecting kinship and cooperation among group members. Among contemporary human hunter-gatherers and horticulturalists, indirect fitness contributions from transfers exceed direct reproductive contributions from before menopause until ages when surpluses end, around the modal age of adult death (∼70 y). Under reasonable assumptions, these benefits are the equivalent to having up to several more offspring after age 50. Despite early independence, minimal production surplus, and a shorter lifespan, chimpanzees could theoretically make indirect contributions if they adopted reliable food-sharing practices. Our results for chimpanzees hypothetically adopting hunter-gatherer subsistence suggest that a skills-intensive foraging ecology with late independence and late peak production could select for human-like life histories via positive feedback between longevity and late-life transfers. In contrast, life history changes preceding subsistence shifts would not favor further life extension or subsistence shifts. Our results formalize the theory that longevity can be favored under socioecological conditions characterized by parental and alloparental care funded through transfers of mid- to late-life production surpluses. We also extend our analysis beyond food transfers to illustrate the potential for indirect fitness contributions from pedagogy, or information transfers. While we focus mostly on humans, our approach is adaptable to any context or species where transfers can affect fitness.

Sexual size monomorphism may evolve in lizards with a body size maximizing reproductive performance for both sexes

We used Takydromus septentrionalis, a sexually size-monomorphic lacertid lizard, as a model system to test the hypothesis that sexual size monomorphism may evolve in lizards where reproductive performance is maximized at a similar body size for both sexes. We allowed lizards housed in laboratory enclosures to lay as many clutches (for females) as they could or to mate as many times (for males) as they could in a breeding season. Size-assortative mating was weak but evident in T. septentrionalis, as revealed by the fact that male and female snout-vent lengths (SVLs) in mating pairs were significantly and positively correlated. Mating frequency (indicative of male reproductive performance) varied from 1 to 8 per breeding season, generally increasing as SVL increased in adult males smaller than 67.4 mm snout-vent length (SVL). Clutch frequency varied from 1 to 7 per breeding season, with female reproductive performance (determined by clutch frequency, annual fecundity and annual reproductive output) maximized in females with a SVL of 68.0 mm. Accordingly to our hypothesis, the reproductive performance was maximized in the intermediate-sized rather than the largest individuals in both sexes, and the body size maximizing reproductive performance was similar for both sexes. Future work could usefully investigate other lineages of lizards with sexually monomorphic species in a phylogenetic context to corroborate the hypothesis of this study.

The Microbiota-Gut-Brain Axis in Health and Disease and Its Implications for Translational Research

Over the past decades, microbiome research has evolved rapidly and became a hot topic in basic, preclinical and clinical research, for the pharmaceutical industry and for the general public. With the help of new high-throughput sequencing technologies tremendous progress has been made in the characterization of host-microbiota interactions identifying the microbiome as a major factor shaping mammalian physiology. This development also led to the discovery of the gut-brain axis as the crucial connection between gut microbiota and the nervous system. Consequently, a rapidly growing body of evidence emerged suggesting that the commensal gut microbiota plays a vital role in brain physiology. Moreover, it became evident that the communication along this microbiota-gut-brain axis is bidirectional and primarily mediated by biologically active microbial molecules and metabolites. Further, intestinal dysbiosis leading to changes in the bidirectional relationship between gut microbiota and the nervous system was linked to the pathogenesis of several psychiatric and neurological disorders. Here, we discuss the impact of the gut microbiota on the brain in health and disease, specifically as regards to neuronal homeostasis, development and normal aging as well as their role in neurological diseases of the highest socioeconomic burden such as Alzheimer's disease and stroke. Subsequently, we utilize Alzheimer's disease and stroke to examine the translational research value of current mouse models in the spotlight of microbiome research. Finally, we propose future strategies on how we could conduct translational microbiome research in the field of neuroscience that may lead to the identification of novel treatments for human diseases.

Speciation and the developmental alarm clock

New species arise as the genomes of populations diverge. The developmental 'alarm clock' of speciation sounds off when sufficient divergence in genetic control of development leads hybrid individuals to infertility or inviability, the world awoken to the dawn of new species with intrinsic post-zygotic reproductive isolation. Some developmental stages will be more prone to hybrid dysfunction due to how molecular evolution interacts with the ontogenetic timing of gene expression. Considering the ontogeny of hybrid incompatibilities provides a profitable connection between 'evo-devo' and speciation genetics to better link macroevolutionary pattern, microevolutionary process, and molecular mechanisms. Here, we explore speciation alongside development, emphasizing their mutual dependence on genetic network features, fitness landscapes, and developmental system drift. We assess models for how ontogenetic timing of reproductive isolation can be predictable. Experiments and theory within this synthetic perspective can help identify new rules of speciation as well as rules in the molecular evolution of development.

Age-related shifts in gut microbiota contribute to cognitive decline in aged rats

Cognitive function declines during the aging process, meanwhile, gut microbiota of the elderly changed significantly. Although previous studies have reported the effect of gut microbiota on learning and memory, all the reports were based on various artificial interventions to change the gut microbiota without involvement of aging biological characteristics. Here, we investigated the effect of aged gut microbiota on cognitive function by using fecal microbiota transplantation (FMT) from aged to young rats. Results showed that FMT impaired cognitive behavior in young recipient rats; decreased the regional homogeneity in medial prefrontal cortex and hippocampus; changed synaptic structures and decreased dendritic spines; reduced expression of brain-derived neurotrophic factor (BDNF), N-methyl-D-aspartate receptor NR1 subunit, and synaptophysin; increased expression of advanced glycation end products (AGEs) and receptor for AGEs (RAGE). All these behavioral, brain structural and functional alterations induced by FMT reflected cognitive decline. In addition, FMT increased levels of pro-inflammatory cytokines and oxidative stress in young rats, indicating that inflammation and oxidative stress may underlie gut-related cognitive decline in aging. This study provides direct evidence for the contribution of gut microbiota to the cognitive decline during normal aging and suggests that restoring microbiota homeostasis in the elderly may improve cognitive function.

Microbial ageing and longevity

Longevity reflects the ability to maintain homeostatic conditions necessary for life as an organism ages. A long-lived organism must contend not only with environmental hazards but also with internal entropy and macromolecular damage that result in the loss of fitness during ageing, a phenomenon known as senescence. Although central to many of the core concepts in biology, ageing and longevity have primarily been investigated in sexually reproducing, multicellular organisms. However, growing evidence suggests that microorganisms undergo senescence, and can also exhibit extreme longevity. In this Review, we integrate theoretical and empirical insights to establish a unified perspective on senescence and longevity. We discuss the evolutionary origins, genetic mechanisms and functional consequences of microbial ageing. In addition to having biomedical implications, insights into microbial ageing shed light on the role of ageing in the origin of life and the upper limits to longevity.

Molecular evolution across developmental time reveals rapid divergence in early embryogenesis

Ontogenetic development hinges on the changes in gene expression in time and space within an organism, suggesting that the demands of ontogenetic growth can impose or reveal predictable pattern in the molecular evolution of genes expressed dynamically across development. Here, we characterize coexpression modules of the Caenorhabditis elegans transcriptome, using a time series of 30 points from early embryo to adult. By capturing the functional form of expression profiles with quantitative metrics, we find fastest evolution in the distinctive set of genes with transcript abundance that declines through development from a peak in young embryos. These genes are highly enriched for oogenic function and transient early zygotic expression, are nonrandomly distributed in the genome, and correspond to a life stage especially prone to inviability in interspecies hybrids. These observations conflict with the "early conservation model" for the evolution of development, although expression-weighted sequence divergence analysis provides some support for the "hourglass model." Genes in coexpression modules that peak toward adulthood also evolve fast, being hyper-enriched for roles in spermatogenesis, implicating a history of sexual selection and relaxation of selection on sperm as key factors driving rapid change to ontogenetically distinguishable coexpression modules of genes. We propose that these predictable trends of molecular evolution for dynamically expressed genes across ontogeny predispose particular life stages, early embryogenesis in particular, to hybrid dysfunction in the speciation process.

Telomeres and telomerase: three decades of progress

Many recent advances have emerged in the telomere and telomerase fields. This Timeline article highlights the key advances that have expanded our views on the mechanistic underpinnings of telomeres and telomerase and their roles in ageing and disease. Three decades ago, the classic view was that telomeres protected the natural ends of linear chromosomes and that telomerase was a specific telomere-terminal transferase necessary for the replication of chromosome ends in single-celled organisms. While this concept is still correct, many diverse fields associated with telomeres and telomerase have substantially matured. These areas include the discovery of most of the key molecular components of telomerase, implications for limits to cellular replication, identification and characterization of human genetic disorders that result in premature telomere shortening, the concept that inhibiting telomerase might be a successful therapeutic strategy and roles for telomeres in regulating gene expression. We discuss progress in these areas and conclude with challenges and unanswered questions in the field.

Age-related reproduction of female Mongolian racerunners (Eremias argus; Lacertidae): Evidence of reproductive senescence

The reproductive maturation hypothesis, the terminal investment hypothesis, and the senescence hypothesis are the most extensively evaluated hypotheses proposed to explain age-related patterns of reproduction in iteroparous organisms. Here, we evaluated these hypotheses for the Mongolian racerunner (Eremias argus), a short-lived lacertid lizard, by comparing reproductive traits between females that completed reproductive cycles under the same laboratory conditions in two consecutive years (2008 and 2009). Reproductive females gained linear size (snout-vent length) not only as they got 1 year older but also during the breeding season. Larger females generally laid eggs earlier and invested more in reproduction than did smaller ones. Females switched from laying smaller eggs in the first clutch to larger eggs in the subsequent clutches but kept clutch size and postpartum body mass constant between successive clutches in a breeding season and between years. Females that laid more clutches or eggs in 2008 did not lay fewer clutches or eggs in 2009. Of the traits examined, only clutch frequency, annual fecundity, and annual reproductive output were susceptible to ageing. Specifically, the clutch frequency was reduced by 1.1 clutches, annual fecundity by 3.1 eggs and annual reproductive output by 1.0 g in 2009 compared with 2008. Our results suggest that the reproductive maturation hypothesis better explains patterns of reproduction in young or prime-aged females of E. argus, whereas the senescence hypothesis better explains reproductive patterns in old females. The terminal investment hypothesis does not apply to any trait examined because no trait value was maximized in old females.

Biogerontology: research status, challenges and opportunities

Biogerontology is the study of the biological basis of ageing and age-related diseases. The phenomenon and the process of ageing are well understood in evolutionary and biological terms; and a conceptual framework has been established within which general principles of ageing and longevity can be formulated. The phenotype of ageing in terms of progressive loss of physical function and fitness is best seen during the period of survival after the evolution-determined essential lifespan (ELS) of a species. However, the ageing phenotype is highly heterogenous and individualistic at all levels from the whole body to the molecular one. Most significantly, the process and the progression of ageing are not determined by any specific gerontogenes. Ageing is the result of imperfect maintenance and repair systems that allow a progressive shrinkage of the homeodynamic space of an individual. The challenge is to develop and apply wholistic approaches to the complex trait of ageing for maintaining and/or improving health. One such approach is that of mild stress-induced physiological hormesis by physical, mental and nutritional hormetins. Biogerontological research offers numerous opportunities for developing evidence-based novel biomedical technologies for maintaining and improving health, for preventing the onset of age-related diseases, and for extending the health-span.

Drosophila as a model for ageing

Drosophila melanogaster has been a key model in developing our current understanding of the molecular mechanisms of ageing. Of particular note is its role in establishing the evolutionary conservation of reduced insulin and IGF-1-like signaling in promoting healthy ageing. Capitalizing on its many advantages for experimentation, more recent work has revealed how precise nutritional and genetic interventions can improve fly lifespan without obvious detrimental side effects. We give a brief summary of these recent findings as well as examples of how they may modify ageing via actions in the gut and muscle. These discoveries highlight how expanding our understanding of metabolic and signaling interconnections will provide even greater insight into how these benefits may be harnessed for anti-ageing interventions.

Slow aging in mammals-Lessons from African mole-rats and bats

Traditionally, the main mammalian models used in aging research have been mice and rats, i.e. short-lived species that obviously lack effective maintenance mechanisms to keep their soma in a functional state for prolonged periods of time. It is doubtful that life-extending mechanisms identified only in such short-lived species adequately reflect the diversity of longevity pathways that have naturally evolved in mammals, or that they have much relevance for long-lived species such as humans. Therefore, some complementary, long-lived mammalian models have been introduced to aging research in the past 15-20 years, particularly naked mole-rats (and to a lesser extent also other mole-rats) and bats. Here, I summarize and compare the most important results regarding various aspects of aging - oxidative stress, molecular homeostasis and repair, and endocrinology - that have been obtained from studies using these new mammalian models of high longevity. I argue that the inclusion of these models was an important step forward, because it drew researchers' attention to certain oversimplifications of existing aging theories and to several features that appear to be universal components of enhanced longevity in mammals. However, even among mammals with high longevity, considerable variation exists with respect to other candidate mechanisms that also must be taken into account if inadequate generalizations are to be avoided.

The reproductive advantages of a long life: longevity and senescence in wild female African elephants

Long-lived species such as elephants, whales and primates exhibit extended post-fertile survival compared to species with shorter lifespans but data on age-related fecundity and survival are limited to few species or populations. We assess relationships between longevity, reproductive onset, reproductive rate and age for 834 longitudinally monitored wild female African elephants in Amboseli, Kenya. The mean known age at first reproduction was 13.8 years; only 5 % commenced reproduction by 10 years. Early reproducers (<12.5 years) had higher age-specific fertility rates than did females who commenced reproduction late (15+ years) with no differences in survival between these groups. Age-specific reproductive rates of females dying before 40 years were reduced by comparison to same-aged survivors, illustrating a mortality filter and reproductive advantages of a long life. Overall, 95 % of fertility was completed before 50, and 95 % of mortality experienced by age 65, with a mean life expectancy of 41 years for females who survived to the minimum age at first birth (9 years). Elephant females have a relatively long period (c. 16 years) of viability after 95 % completed fertility, although reproduction does not entirely cease until they are over 65. We found no evidence of increased investment among females aged over 40 in terms of delay to next birth or calf mortality. The presence of a mother reproducing simultaneously with her daughter was associated with higher rates of daughter reproduction suggesting advantages from maternal (and grandmaternal) co-residence during reproduction.

Elephants born in the high stress season have faster reproductive ageing

Senescent declines in reproduction and survival are found across the tree of life, but little is known of the factors causing individual variation in reproductive ageing rates. One contributor may be variation in early developmental conditions, but only a few studies quantify the effects of early environment on reproductive ageing and none concern comparably long-lived species to humans. We determine the effects of ‘stressful’ birth conditions on lifetime reproduction in a large semi-captive population of Asian elephants (Elephas maximus). We categorise birth month into stressful vs. not-stressful periods based on longitudinal measures of glucocorticoid metabolites in reproductive-aged females, which peak during heavy workload and the start of the monsoon in June-August. Females born in these months exhibit faster reproductive senescence in adulthood and have significantly reduced lifetime reproductive success than their counterparts born at other times of year. Improving developmental conditions could therefore delay reproductive ageing in species as long-lived as humans.

Genetics and pharmacology of longevity: the road to therapeutics for healthy aging

Aging can be defined as the progressive decline in tissue and organismal function and the ability to respond to stress that occurs in association with homeostatic failure and the accumulation of molecular damage. Aging is the biggest risk factor for human disease and results in a wide range of aging pathologies. Although we do not completely understand the underlying molecular basis that drives the aging process, we have gained exceptional insights into the plasticity of life span and healthspan from the use of model organisms such as the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Single-gene mutations in key cellular pathways that regulate environmental sensing, and the response to stress, have been identified that prolong life span across evolution from yeast to mammals. These genetic manipulations also correlate with a delay in the onset of tissue and organismal dysfunction. While the molecular genetics of aging will remain a prosperous and attractive area of research in biogerontology, we are moving towards an era defined by the search for therapeutic drugs that promote healthy aging. Translational biogerontology will require incorporation of both therapeutic and pharmacological concepts. The use of model organisms will remain central to the quest for drug discovery, but as we uncover molecular processes regulated by repurposed drugs and polypharmacy, studies of pharmacodynamics and pharmacokinetics, drug-drug interactions, drug toxicity, and therapeutic index will slowly become more prevalent in aging research. As we move from genetics to pharmacology and therapeutics, studies will not only require demonstration of life span extension and an underlying molecular mechanism, but also the translational relevance for human health and disease prevention.

Gametic selection, developmental trajectories, and extrinsic heterogeneity in Haldane's rule

Deciphering the genetic and developmental causes of the disproportionate rarity, inviability, and sterility of hybrid males, Haldane's rule, is important for understanding the evolution of reproductive isolation between species. Moreover, extrinsic and prezygotic factors can contribute to the magnitude of intrinsic isolation experienced between species with partial reproductive compatibility. Here, we use the nematodes Caenorhabditis briggsae and C. nigoni to quantify the sensitivity of hybrid male viability to extrinsic temperature and developmental timing, and test for a role of mito-nuclear incompatibility as a genetic cause. We demonstrate that hybrid male inviability manifests almost entirely as embryonic, not larval, arrest and is maximal at the lowest rearing temperatures, indicating an intrinsic-by-extrinsic interaction to hybrid inviability. Crosses using mitochondrial substitution strains that have reciprocally introgressed mitochondrial and nuclear genomes show that mito-nuclear incompatibility is not a dominant contributor to postzygotic isolation and does not drive Haldane's rule in this system. Crosses also reveal that competitive superiority of X-bearing sperm provides a novel means by which postmating prezygotic factors exacerbate the rarity of hybrid males. These findings highlight the important roles of gametic, developmental, and extrinsic factors in modulating the manifestation of Haldane's rule.

Longevity Is Linked to Mitochondrial Mutation Rates in Rockfish: A Test Using Poisson Regression

The mitochondrial theory of ageing proposes that the cumulative effect of biochemical damage in mitochondria causes mitochondrial mutations and plays a key role in ageing. Numerous studies have applied comparative approaches to test one of the predictions of the theory: That the rate of mitochondrial mutations is negatively correlated with longevity. Comparative studies face three challenges in detecting correlates of mutation rate: Covariation of mutation rates between species due to ancestry, covariation between life-history traits, and difficulty obtaining accurate estimates of mutation rate. We address these challenges using a novel Poisson regression method to examine the link between mutation rate and lifespan in rockfish (Sebastes). This method has better performance than traditional sister-species comparisons when sister species are too recently diverged to give reliable estimates of mutation rate. Rockfish are an ideal model system: They have long life spans with indeterminate growth and little evidence of senescence, which minimizes the confounding tradeoffs between lifespan and fecundity. We show that lifespan in rockfish is negatively correlated to rate of mitochondrial mutation, but not the rate of nuclear mutation. The life history of rockfish allows us to conclude that this relationship is unlikely to be driven by the tradeoffs between longevity and fecundity, or by the frequency of DNA replications in the germline. Instead, the relationship is compatible with the hypothesis that mutation rates are reduced by selection in long-lived taxa to reduce the chance of mitochondrial damage over its lifespan, consistent with the mitochondrial theory of ageing.

A new hypothesis of aging

INTRODUCTION AND AIMS

There are over 300 hypotheses of aging, but none of them has enough predictive power to explain most experiments and observations on this process. On the basis of a critical analysis of the most relevant data on aging, especially on the factors that influences its rhythm, we present a new hypothesis, as well as the way the hypothesis' predictions explain some of the phylogenetic implications of the aging process.

METHODS

The hypothesis starts from a new, biochemical view on evolution and the behavior of living matter. According to this view, life is a fabric of chemical reactions that sustain each other. Reactants and energy support are needed for these reactions to take place in a cell. Given this, aging stems from a leftward shift of the global equilibrium of some biochemical reactions involved in cell differentiation and repair, which take place at a high level during the organism's growth period. In time, for species with evident aging, some reactions lose their specificity, which affects cell division and differentiation. This, in turn, influences cell energy metabolism.

RESULTS

Cell and tissue degeneration appears when, while some specific reactions are absent, non-specific reactions such as those of cell proliferation receive additional energy support.

CONCLUSIONS

This hypothesis explains phylogenetic differences related to lifespan and longevity, and body-size differences between species and within the same species.

Molecular adaptation of telomere associated genes in mammals

BACKGROUND

Placental mammals display a huge range of life history traits, including size, longevity, metabolic rate and germ line generation time. Although a number of general trends have been proposed between these traits, there are exceptions that warrant further investigation. Species such as naked mole rat, human and certain bat species all exhibit extreme longevity with respect to body size. It has long been established that telomeres and telomere maintenance have a clear role in ageing but it has not yet been established whether there is evidence for adaptation in telomere maintenance proteins that could account for increased longevity in these species.

RESULTS

Here we carry out a molecular investigation of selective pressure variation, specifically focusing on telomere associated genes across placental mammals. In general we observe a large number of instances of positive selection acting on telomere genes. Although these signatures of selection overall are not significantly correlated with either longevity or body size we do identify positive selection in the microbat species Myotis lucifugus in functionally important regions of the telomere maintenance genes DKC1 and TERT, and in naked mole rat in the DNA repair gene BRCA1.

CONCLUSION

These results demonstrate the multifarious selective pressures acting across the mammal phylogeny driving lineage-specific adaptations of telomere associated genes. Our results show that regardless of the longevity of a species, these proteins have evolved under positive selection thereby removing increased longevity as the single selective force driving this rapid rate of evolution. However, evidence of molecular adaptations specific to naked mole rat and Myotis lucifugus highlight functionally significant regions in genes that may alter the way in which telomeres are regulated and maintained in these longer-lived species.

Genomic response to selection for postponed senescence in Drosophila

Limited lifespan and senescence are quantitative traits, controlled by many interacting genes with individually small and environmentally plastic effects, complicating genetic analysis. We performed genome wide analysis of gene expression for two Drosophila melanogaster lines selected for postponed senescence and one control, unselected line to identify candidate genes affecting lifespan as well as variation in lifespan. We obtained gene expression profiles for young flies of all lines, all lines at the time only 10% of the control lines survived, and the time at which 10% of the selected lines survived. Transcriptional responses to aging involved 19% of the genome. The transcriptional signature of aging involved the down-regulation of genes affecting proteolysis, metabolism, oxidative phosphorylation, and mitochrondrial function; and the up-regulation of genes affecting protein synthesis, immunity, defense responses, and the detoxification of xenobiotic substances. The transcriptional signature of postponed senescence involved the up-regulation of proteases and phosphatases and genes affecting detoxification of xenobiotics; and the down-regulation of genes affecting immunity, defense responses, metabolism and muscle function. Functional tests of 17 mutations confirmed 12 novel genes affecting Drosophila lifespan. Identification of genes affecting longevity by analysis of gene expression changes in lines selected for postponed senescence thus complements alternative genetic approaches.

Bats and birds: Exceptional longevity despite high metabolic rates

Bats and birds live substantially longer on average than non-flying mammals of similar body size. The combination of small body size, high metabolic rates, and long lifespan in bats and birds would not seem to support oxidative theories of ageing that view senescence as the gradual accumulation of damage from metabolic byproducts. However, large-scale comparative analyses and laboratory studies on a few emerging model species have identified multiple mechanisms for resisting oxidative damage to mitochondrial DNA and cellular structures in both bats and birds. Here we review these recent findings, and suggest areas in which additional progress on ageing mechanisms can be made using bats and birds as novel systems. New techniques for determining the age of free-living, wild individuals, and robustly supported molecular phylogenies, are under development and will improve the efforts of comparative biologists to identify ecological and evolutionary factors promoting long lifespan. In the laboratory, greater development of emerging laboratory models and comparative functional genomic approaches will be needed to identify the molecular pathways of longevity extension in birds and bats.

Comparative analyses of time-course gene expression profiles of the long-lived sch9Delta mutant

In an attempt to elucidate the underlying longevity-promoting mechanisms of mutants lacking SCH9, which live three times as long as wild type chronologically, we measured their time-course gene expression profiles. We interpreted their expression time differences by statistical inferences based on prior biological knowledge, and identified the following significant changes: (i) between 12 and 24 h, stress response genes were up-regulated by larger fold changes and ribosomal RNA (rRNA) processing genes were down-regulated more dramatically; (ii) mitochondrial ribosomal protein genes were not up-regulated between 12 and 60 h as wild type were; (iii) electron transport, oxidative phosphorylation and TCA genes were down-regulated early; (iv) the up-regulation of TCA and electron transport was accompanied by deep down-regulation of rRNA processing over time; and (v) rRNA processing genes were more volatile over time, and three associated cis-regulatory elements [rRNA processing element (rRPE), polymerase A and C (PAC) and glucose response element (GRE)] were identified. Deletion of AZF1, which encodes the transcriptional factor that binds to the GRE element, reversed the lifespan extension of sch9Δ. The significant alterations in these time-dependent expression profiles imply that the lack of SCH9 turns on the longevity programme that extends the lifespan through changes in metabolic pathways and protection mechanisms, particularly, the regulation of aerobic respiration and rRNA processing.

Integrating evolutionary and molecular genetics of aging

Aging or senescence is an age-dependent decline in physiological function, demographically manifest as decreased survival and fecundity with increasing age. Since aging is disadvantageous it should not evolve by natural selection. So why do organisms age and die? In the 1940s and 1950s evolutionary geneticists resolved this paradox by positing that aging evolves because selection is inefficient at maintaining function late in life. By the 1980s and 1990s this evolutionary theory of aging had received firm empirical support, but little was known about the mechanisms of aging. Around the same time biologists began to apply the tools of molecular genetics to aging and successfully identified mutations that affect longevity. Today, the molecular genetics of aging is a burgeoning field, but progress in evolutionary genetics of aging has largely stalled. Here we argue that some of the most exciting and unresolved questions about aging require an integration of molecular and evolutionary approaches. Is aging a universal process? Why do species age at different rates? Are the mechanisms of aging conserved or lineage-specific? Are longevity genes identified in the laboratory under selection in natural populations? What is the genetic basis of plasticity in aging in response to environmental cues and is this plasticity adaptive? What are the mechanisms underlying trade-offs between early fitness traits and life span? To answer these questions evolutionary biologists must adopt the tools of molecular biology, while molecular biologists must put their experiments into an evolutionary framework. The time is ripe for a synthesis of molecular biogerontology and the evolutionary biology of aging.

Age-specific reproduction and survival of individually marked Wood Thrushes, Hylocichla mustelina

Several nonexclusive, sometimes contradictory, hypotheses have been employed to explain age-related changes in reproduction and survival among vertebrates. We evaluated these hypotheses with observations collected from a 31-year study of individually marked Wood Thrushes (Hylocichla mustelina). Age-specific fledgling production, survival probabilities, and derived life table columns are presented here. Additionally, we report "genetic value," or the number of young remaining to be recruited into the breeding population by individuals of age x. Controlling for age of mate, the presence of some individuals in multiple years, and the effect of year of breeding, reproduction and survival peaked in middle-aged birds of each sex and declined later in life for the oldest individuals. We could not exclude the selection hypothesis (removal of inferior individuals) as an explanation for improved female reproduction until middle age. The age at last reproduction was a significant determinant of age-related reproductive success, indicating a disappearance of low-quality individuals. Yearling females that did not successfully reproduce had greater within-season emigration than all other age classes of each sex, regardless of the reproductive success of other classes. For males, we favored the constraint hypothesis (improved reproduction with experience) to explain greater reproductive output with increasing age until at least middle age. For each sex, the age of the mate independently influenced all measures of reproductive performance, but there was generally a linear or quadratic, rather than interactive, effect of the mate's age on reproduction. For both sexes, hypotheses of age-related reproduction were only partially supported because of effects of senescence, or a decline in reproductive output or survival probabilities with increasing age. Senescence may confound attempts to examine or apply general hypotheses of age-related reproduction that assume a monotonic relationship between age and reproduction or survival.

The impact of experimentally elevated energy expenditure on oxidative stress and lifespan in the short-tailed field vole Microtus agrestis

Life-history theory assumes that animal life histories are a consequence of trade-offs between current activities and future reproductive performance or survival, because resource supply is limited. Empirical evidence for such trade-offs in the wild are common, yet investigations of the underlying mechanisms are rare. Life-history trade-offs may have both physiological and ecological mediated costs. One hypothesized physiological mechanism is that elevated energy metabolism may increase reactive oxygen species production, leading to somatic damage and thus compromising future survival. We investigated the impact of experimentally elevated energy expenditure on oxidative damage, protection and lifespan in short-tailed field voles (Microtus agrestis) maintained in captivity to remove any confounding ecological factor effects. Energy expenditure was elevated via lifelong cold exposure (7+/-2 degrees C), relative to siblings in the warm (22+/-2 degrees C). No treatment effect on cumulative mortality risk was observed, with negligible effects on oxidative stress and antioxidant protection. These data suggest that in captive animals physiologically mediated costs on life history do not result from increased energy expenditure and consequent elevations in oxidative stress and reduced survival.

Significant and systematic expression differentiation in long-lived yeast strains

Background

Recent studies suggest that the regulation of longevity may be partially conserved in many eukaryotes ranging from yeast to mammals. The three yeast mutants sch9Δ, ras2Δ, tor1Δ show extended chronological life span up to three folds. Our aim is to dissect the mechanisms that lead to the yeast life span extension.

Methodology/Principal Findings

We obtain gene expression profiles of sch9Δ, ras2Δ, tor1Δ as well as that for a wild type at day 2.5 in SDC medium using Affymetrix Yeast2.0 arrays. To accurately estimate the expression differentiation between the wild type and the long-lived mutants, we use sub-array normalization followed by a variant of the median-polishing summarization. The results are validated by the probe sets of S. pombe on the same chips. To translate the differentiation into changes of biological activities, we make statistical inference by integrating the expression profiles with biological gene subsets defined by Gene Ontology, KEGG pathways, and cellular localization of proteins. Other than subset-versus-other comparisons, we also make local comparisons between two directly-related gene subsets such as cytosolic and mitochondrial ribosomes. Our consensus is obtained by cross-examination of these inferences. The significant and systematic differentiation in the three long-lived strains includes: lower transcriptional activities; down-regulation of TCA cycle and oxidative phosphorylation versus up-regulation of the KEGG pathway Glycolysis/Gluconeogenesis; the overall reduction of mitochondrial activities. We also report some different expression patterns such as reduction of the activities relating to mitosis in ras2Δ.

Conclusions/Significance

The modification of energy pathways and modification of compartment activities such as down-regulation of mitochondrial ribosome proteins versus up-regulation of cytosolic ribosome proteins are directly associated with the life span extension in yeast. The results provide a new and systematic S. cerevisiae version of the free radical theory from the perspective of functional genomics.

Telomere length inheritance and aging

Telomere shortening accompanies human aging, and premature aging syndromes are often associated with short telomeres. These two observations are central to the hypothesis that telomere length directly influences longevity. If true, genetically determined mechanisms of telomere length homeostasis should significantly contribute to variations of longevity in the human population. On the other hand, telomere shortening is also observed in the course of many aging-associated disorders but determining whether it is a cause or a consequence is not an easy task. Here, we review the most relevant experimental and descriptive data relating telomere length, as a quantitative trait, to aging and longevity.

Environment-dependent trade-offs between ectoparasite resistance and larval competitive ability in the Drosophila–Macrocheles system

Costs of resistance are expected to contribute to the maintenance of genetic variation for resistance in natural host populations. In the present study, we experimentally test for genetic trade-offs between parasite resistance and larval competitive ability expressed under varying levels of crowding and temperature. Artificial selection for increased behavioral resistance was applied against an ectoparasitic mite (Macrocheles subbadius) in replicate lines of the fruit fly Drosophila nigrospiracula. We then measured correlated responses to selection in larval competitive ability by contrasting replicate selected and control (unselected) lines in the absence of parasitism. Experiments were conducted under variable environmental conditions: two temperatures and three levels of larval density. Our results reveal a negative genetic correlation between resistance and larval-adult survival under conditions of moderate and severe intra-specific competition. At both low and high temperature, percent emergence was significantly higher among control lines than selected lines. This divergence in larval competitive ability was magnified under high levels of competition, but only at low temperature. Hence, the interaction between selection treatment and larval density was modified by temperature. As predicted, larvae experiencing medium and high levels of competition exhibited an overall reduction in female body size compared to larvae at low levels of competition. Female flies emerging from selected lines were significantly smaller than those females from control lines, but this effect was only significant under conditions of moderate to severe competition. These results provide evidence of environment-dependent trade-offs between ectoparasite resistance and larval competitive ability, a potential mechanism maintaining genetic polymorphism for resistance.

COSTS OF RESISTANCE IN THE DROSOPHILA?MACROCHELES SYSTEM: A NEGATIVE GENETIC CORRELATION BETWEEN ECTOPARASITE RESISTANCE AND REPRODUCTION

Genetic variation for parasite resistance occurs in most host populations. Costs of resistance, manifested as reduced fitness of resistant genotypes in the absence of parasitism, can be an important factor contributing to the maintenance of this variation. One powerful tool for detecting costs of resistance is the study of correlated responses to artificial selection. Provided that experimental lines are recently derived from large outbreeding populations, and that inbreeding is minimized during the experiment, correlated responses to selection are expected to be strong indicators of pleiotropy. We artificially selected for elevated behavioral resistance against an ectoparasitic mite (Macrocheles subbadius) in replicate populations of the fly Drosophila nigrospiracula. Resistance was modeled as a threshold trait, and the realized heritability of resistance was estimated to be 12.3% (1.4% SE) across three replicate lines recently derived from nature. We contrasted the longevity and fecundity of resistant and control (unselected) flies under a variable thermal environment. We report that reduced fecundity is a correlated response to artificial selection for increased resistance, and that the strength of this effect increases from 25 degrees to 29 degrees C. In contrast, longevity differences were not detected between resistant and control lines at either temperature. These findings are robust as they were confirmed with an independent set of experimental lines. Thus, our results identify a negative genetic correlation between ectoparasite resistance and an important life-history trait. That a correlated response was only detected for fecundity, and not longevity, suggests that the genetic correlation is attributable to pleiotropic effects with narrower effects than reallocation of a general resource pool within the organism, although other interpretations are discussed. Combined with fluctuating parasite-mediated selection and temperature, the presence of this trade-off may contribute to the maintenance of genetic variation for resistance in natural populations.

Effect of deleterious mutations on life span in Drosophila melanogaster

Evolutionary theories of aging assume that the accumulation of deleterious mutations will reduce life span. We tested this assumption in Drosophila melanogaster by a newly designed mating scheme, in which mutations accumulate on the Binscy balancer X chromosome in heterozygous females in the absence of selection and recombination. We found that the life span of Binscy/RY(L) males from this cross decreased faster than the life span of their sibling controls over time in two of three runs, and that there was an age-specific increase in mortality in the Binscy/RY(L) males with time in one of three runs. Therefore, the accumulation of deleterious mutations can decrease life span by increasing fragility and can cause age-specific changes in mortality. These results support the evolutionary theory of aging.

Senescence in relation to latitude and migration in birds

Senescence is the age-related deterioration of the phenotype, explained by accumulation of mutations, antagonistic pleiotropy, free radicals or other mechanisms. I investigated patterns of actuarial senescence in a sample of 169 species of birds in relation to latitude and migration, by analysing longevity records adjusted for sampling effort, survival rate and body mass. Senescence might decrease at low latitudes because of elevated adult survival rates and generally slow life histories. Alternatively, the rate of senescence might increase at low latitudes because of the greater impact of biological interactions such as parasitism, predation and competition on fitness through differential effects of age-specific mortality (e.g. because immunologically naïve young individuals and immuno-senescent old individuals might die more frequently than individuals belonging to intermediate age classes). Bird migration entails extensive exercise twice annually, with migrants spending more time in benign environments with little abiotic mortality than residents, migrants having higher adult survival rate and lower annual fecundity than residents, and migrants suffering more from the consequences of oxidative stress than residents. The rate of senescence increased with latitude, as expected because of slow life histories at low latitudes. Independently, rate of senescence decreased with increasing migration distance. These findings were robust to control for potentially confounding effects of body mass, age of first reproduction and phenotypic similarity among species because of common descent.

Consequences of heterogeneity in survival probability in a population of Florida scrub-jays

1. Using data on breeding birds from a 35-year study of Florida scrub-jays Aphelocoma coerulescens (Bosc 1795), we show that survival probabilities are structured by age, birth cohort, and maternal family, but not by sex. Using both accelerated failure time (AFT) and Cox proportional hazard models, the data are best described by models incorporating variation among birth cohorts and greater mortality hazard with increasing age. AFT models using Weibull distributions with the shape parameter > 1 were always the best-fitting models. 2. Shared frailty models allowing for family structure greatly reduce model deviance. The best-fitting models included a term for frailty shared by maternal families. 3. To ask how long a data set must be to reach qualitatively the same conclusions, we repeated the analyses for all possible truncated data sets of 2 years in length or greater. Length of the data set affects the parameter estimates, but not the qualitative conclusions. In all but three of 337 truncated data sets the best-fitting models pointed to same conclusions as the full data set. Shared frailty models appear to be quite robust. 4. The data are not adequate for testing hypotheses as to whether variation in frailty is heritable. 5. Substantial structured heterogeneity for survival exists in this population. Such structured heterogeneity has been shown to have substantial effects in reducing demographic stochasticity.

A reassessment of sexual dimorphism in human senescence: theory, evidence, and causation

Age-specific mortality rates of men are higher than those of women, and men have shorter average life spans than women. This has been interpreted as evidence of sexual dimorphism in rates of senescence. However, because mortality can be caused by numerous factors in addition to senescence, higher mortality rates do not necessarily indicate more rapid senescence. In this paper, we (1) emphasize the necessity of decoupling mortality and senescence when considering sexual dimorphism in senescence, (2) present a theoretical framework for the hypothesis that selection affects senescence in human males and females differently due to different life history characteristics, (3) consider phenotypic evidence from the literature that human males show a later onset of senescence than human females, despite exhibiting higher mortality rates, and (4) discuss the potential roles of mutation accumulation and antagonistic pleiotropy in the evolution of sexual dimorphism in senescence.

Nest ornamentation by female spotless starlings in response to a male display: an experimental study

1. The use of behavioural traits by females in signalling condition has been practically ignored in evolutionary theory. However, females may also exhibit ornaments and behavioural displays, although less elaborated than those of males. 2. In this study we suggest that the carrying of feathers by spotless starlings Sturnus unicolor Temminck females to decorate the nest represents an elaborated and costly behaviour that is displayed in response to a courtship male behaviour: the carrying of nest green plants. 3. By experimentally increasing the amount of green plants in the nests, to give the appearance that highly attractive males defended them, we induced females to increase their feather carrying rates. 4. The amount of feathers carried to the nest was correlated to female reproductive experience and laying date, two variables correlated with female body condition. These results suggests that this behaviour may work as an honest indicator of female quality. 5. We conclude that male carrying plants and female carrying feathers can be viewed as two sex-specific functionally related signalling behaviours involved in mutual courtship or status signalling.

The effect of number of births on women's mortality: systematic review of the evidence for women who have completed their childbearing

Mortality in women who have completed their childbearing may increase with the number of births experienced because of maternal depletion or a trade-off between reproduction and mortality. We report a systematic review of the evidence on this association. We searched Medline, Embase, Popline, and the Science Citation Index for published and unpublished studies up to September 2003, and the book catalogues of relevant London libraries. Where necessary we also contacted authors for additional information. Mortality declined with increasing numbers of births in twelve historical cohorts, but in eight contemporary cohorts the highest mortality was seen in the nulliparous and in women with more than four births. All effects seen were small and there were few statistically significant results. Studies examining the relationship in other ways (such as by linear trends or by mean number of births by age at death) found inconsistent associations. We discuss methodological, social, and biological factors that may have affected these associations.

Diet Influences Life Span in Parrots (Psittaciformes)

Evolutionary hypotheses regarding longevity predict that life span should increase as extrinsic mortality rates decrease. Specifically, a decline in age-specific survival and fertility should evolve and decrease life span in proportion to the magnitude of the mortality risk. We examined these ideas using a new data set on maximum longevity, ecology, and life history of 162 parrot species (Psittaciformes). Parrots are generally long-lived but exhibit remarkable variation in life span between similar-sized genera, with particularly long-lived species occurring in the Cacatua, Calyptorhynchus, and Amazona. After controlling for both body size and phylogenetic ancestry using a phylogenetic supertree of all 352 parrot species, type of diet and communal roosting explain significant variation in parrot life span, but the influence of communal roosting is statistically dependent on an association with diet type. We suggest that extreme longevity in parrots has evolved in response to species-specific characteristics of diet, habitat, and behavior that influence extrinsic mortality rates.

La Dieta Influencia la Longevidad en los Psittaciformes

Rapid Senescence in Pacific Salmon

Any useful evolutionary theory of senescence must be able to explain variation within and among natural populations and species. This requires a careful characterization of age-specific mortality rates in nature as well as the intrinsic and extrinsic factors that influence these rates. We perform this task for two populations of semelparous Pacific salmon. During the breeding season, estimated daily mortality rates increased from 0 to 0.2-0.5 (depending on the year) over the course of several weeks. Early-arriving individuals had a later onset and/or a lower rate of senescence in each breeding season, consistent with adaptive expectations based on temporal variation in selection. Interannual variation in senescence was large, in part because of extrinsic factors (e.g., water temperature). Predation rates were higher in Pick Creek sockeye salmon (anadromous Oncorhynchus nerka) than in Meadow Creek kokanee (nonanadromous O. nerka), but in contrast to evolutionary theory, senescence was not more rapid in the former. Interannual variation may have obscured interpopulation divergence in senescence. Pacific salmon are a promising system for further studies on the physiological, evolutionary, and genetic bases of senescence. In particular, we encourage further research to disentangle the relative importance of adaptive and nonadaptive variation in senescence.

The mouse cerebellum from 1 to 34 months: parallel fibers

Synaptic losses are critical events in the aging brain. In the cerebellum, synapses between the parallel fibers and Purkinje cells are strategic elements of its neuronal circuitry. We have examined the number of synaptic varicosities per parallel fiber in the anterior lobe of the cerebellum in young (1, 2.5 and 5 months), mature (10 and 18 months), and old (26 and 34 months) C57BL/6 mice. This number peaks at 2.5 months and starts to decrease immediately. This decrease is due to a reduction of the linear density of synaptic varicosities along the parallel fibers and a shortening of the parallel fibers. The fastest and most dramatic decline occurs during young adulthood between 2.5 and 5 months when 59% of the synaptic varicosities are lost. Accelerated loss also occurs in old age between 26 and 34 months. Changes between 5 and 26 months are the slowest. Between 2.5 and 34 months, mice lose 87% of their parallel fiber synaptic varicosities. Unlike many post-reproductive processes of aging, the early-onset decline beginning at 2.5 months may be under strong natural selection pressure.

Inbreeding depression and male survivorship in Drosophila: implications for senescence theory

The extent to which inbreeding depression affects longevity and patterns of survivorship is an important issue from several research perspectives, including evolutionary biology, conservation biology, and the genetic analysis of quantitative traits. However, few previous inbreeding depression studies have considered longevity as a focal life-history trait. We maintained laboratory populations of Drosophila melanogaster at census population sizes of 2 and 10 male-female pairs for up to 66 generations and performed repeated assays of male survivorship throughout this time period. On average, significant levels of inbreeding depression were observed for median life span and age-specific mortality. For age-specific mortality, the severity of inbreeding depression increased over the life span. We found that a baseline inbreeding load of 0.307 lethal equivalents per gamete affected age-specific mortality, and that this value increased at a rate of 0.046 per day of the life span. With respect to some survivorship parameters, the differentiation of lineages was nonlinear with respect to the inbreeding coefficient, which suggested that nonadditive genetic variation contributed to variation among lineages. These findings provide insights into the genetic basis of longevity as a quantitative trait and have implications regarding the mutation-accumulation evolutionary explanation of senescence.