Genetic variation and aging

The Overview of Cognitive Aging Models

To understand the cause of the age-related decline in cognitive function and its underlying mechanism, the cognitive aging model can provide us with important insights. In this section, we will introduce behavioral and neural models about age-related cognitive changes. Among behavioral models, several aging theories were discussed from the perspectives of educational, biological, and sociological factors, which could explain parts of the aging process. With the development of imaging technology, many studies have discussed the neural mechanism of aging and successively proposed neural models to explain the aging phenomenon. Behavioral models and neural mechanism models supplement each other, gradually unveiling the mystery of cognitive aging.

Terminal life history: late-life fecundity and survival in experimental populations of Drosophila melanogaster

There are two life history landmarks that can be used to define the terminal period in individual Drosophila melanogaster females: the cessation of daily oviposition, which defines the start of the retired stage, and final oviposition, which defines the start of post-ovipository survival. The terminal period is a substantial component of D. melanogaster life history. Analysis of published data on the daily fecundity and survival of 3971 individually maintained, mated female flies reveals that the terminal period is far more variable within populations than other life history components, including total adult life span. It has been reported that there is a negative correlation between fecundity and duration of the terminal state in recently collected wild stocks. Here I show that the negative correlation occurs in multiple inbred and outbred lab-adapted populations as well. In terms of proportion of adult life, lower fecundity flies spend on average twice as much time in the terminal stage as higher fecundity flies from the same population. Both high and low fecundity flies experience end-of-life plateaus in mortality, with the former exhibiting higher plateau levels. The negative correlation between fecundity and terminal survival is of sufficient magnitude to create heterogeneity among the oldest old: the final 10% of survivors are predominately flies with a history of high fecundity, but about one in five is a low fecundity fly with long terminal stage.

Apoptosis inhibition mitigates aging effects in Drosophila melanogaster

Aging is a natural biological process that results in progressive loss of cell, tissue, and organ function. One of the causing factors of the aging process is the decrease in muscle mass, which has not been fully verified in Drosophila. Apoptotic cell death may result in aberrant cell loss and can eventually diminish tissue function and muscle atrophy. If so, inhibition of apoptosis may prolong longevity and reduce motor function and muscle mass decline with age in Drosophila flies. Here, we used Drosophila melanogaster as study material, and induced the overexpression of Drosophila inhibitor of apoptosis protein 1 gene to inhibit apoptosis, and investigated the effect of apoptosis inhibition on the longevity and age-related declines in flight and climbing ability and muscle mass. As a result, the inhibition of apoptosis tended to mitigate the aging effects and prolonged longevity and reduced climbing ability decline with age. The current study suggests that apoptosis inhibition could mitigate the aging effects in D. melanogaster. Although such effects have already been known in mammals, the current results suggest that the apoptosis may play a similar role in insects as well.

Theories of Aging and the Prevalence of Alzheimer's Disease

Objective

Aging and AD are associated in some way, then it is reasonable to ask whether or not it is possible to age without AD inexorably appearing at any moment, depending on the period of life. Therefore, the goal of this review is to verify, in light of some aging theories, the prevalence of AD.

Methods

For the purpose of this manuscript, the indexers Alzheimer, aging, Alzheimer, and aging were considered; theories of aging were researched. The research was conducted using PubMed, Medline, Scopus, Elsevier, and Google Scholar.

Results

The most common subjects in the papers analyzed for this manuscript were aging and Alzheimer's disease. The association between Alzheimer and theories of aging seems inconclusive.

Conclusions

Accordingly, the general idea is that AD is associated with aging in such a way that almost all people will present this disease; however, it is plausible to consider that the increase in life expectancy will generate a high prevalence of AD. In a general sense, it seems that the theories of aging explain the origin of AD under superlative and catastrophic considerations and use more biomolecular data than social or behavioral data as the bases of analysis, which may be the problem.

Fecundity for free? Enhanced oviposition in longevous populations of Drosophila melanogaster

Artificial selection for increased life span in experimental populations of Drosophila melanogaster sometimes produces long-lived populations that exhibit greater fecundity than unselected controls. The absence of a trade-off between survival and reproduction in these cases might be an artefact of the rich diet of typical lab culture; if nutritional resources are not limiting then there may be no need to trade off. Here I test the rich diet hypothesis by estimating genetic correlations between survival and age-specific fecundity in three nutritional environments. Experimental material consists of 58 recombinant inbred lines derived from an artificial selection experiment. Reducing the yeast content of medium causes substantial reductions in fecundity but does not alter patterns of genetic correlation. The correlation between life span and early fecundity is non-significant in all environments, while the life span correlations with mid-life fecundity are positive and statistically significant in all environments. The rich diet hypothesis is rejected. Qualitative features of fecundity trajectories are conserved across environments, with long-lived lines exhibiting a secondary peak of oviposition in mid-life. The micro-evolution of extended life span is not a monolithic process and does not necessarily involve direct trade-offs between survival and reproduction.

Reproduction predicts shorter telomeres and epigenetic age acceleration among young adult women

Evolutionary theory predicts that reproduction entails costs that detract from somatic maintenance, accelerating biological aging. Despite support from studies in human and non-human animals, mechanisms linking 'costs of reproduction' (CoR) to aging are poorly understood. Human pregnancy is characterized by major alterations in metabolic regulation, oxidative stress, and immune cell proliferation. We hypothesized that these adaptations could accelerate blood-derived cellular aging. To test this hypothesis, we examined gravidity in relation to telomere length (TL, n = 821) and DNA-methylation age (DNAmAge, n = 397) in a cohort of young (20-22 year-old) Filipino women. Age-corrected TL and accelerated DNAmAge both predict age-related morbidity and mortality, and provide markers of mitotic and non-mitotic cellular aging, respectively. Consistent with theoretical predictions, TL decreased (p = 0.031) and DNAmAge increased (p = 0.007) with gravidity, a relationship that was not contingent upon resource availability. Neither biomarker was associated with subsequent fertility (both p > 0.3), broadly consistent with a causal effect of gravidity on cellular aging. Our findings provide evidence that reproduction in women carries costs in the form of accelerated aging through two independent cellular pathways.

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.

Hearing ability decreases in ageing locusts

Insects display signs of ageing, despite their short lifespan. However, the limited studies on senescence emphasize longevity or reproduction. We focused on the hearing ability of ageing adult locusts, Schistocerca gregaria. Our results indicate that the youngest adults (2 weeks post-maturity) have a greater overall neurophysiological response to sound, especially for low frequencies (<10 kHz), as well as a shorter latency to this neural response. Interestingly, when measuring displacement of the tympanal membrane that the receptor neurons directly attach to, we found movement is not directly correlated with neural response. Therefore, we suggest the enhanced response in younger animals is due to the condition of their tissues (e.g. elasticity). Secondly, we found the sexes do not have the same responses, particularly at 4 weeks post-adult moult. We propose female reproductive condition reduces their ability to receive sounds. Overall our results indicate older animals, especially females, are less sensitive to sounds.

Enhanced tethered-flight duration and locomotor activity by overexpression of the human gene SOD1 in Drosophila motorneurons

Mutation of the human gene superoxide dismutase (hSOD1) is associated with the fatal neurodegenerative disease familial amyotrophic lateral sclerosis (Lou Gehrig's disease). Selective overexpression of hSOD1 in Drosophila motorneurons increases lifespan to 140% of normal. The current study was designed to determine resistance to lifespan decline and failure of sensorimotor functions by overexpressing hSOD1 in Drosophila's motorneurons. First, we measured the ability to maintain continuous flight and wingbeat frequency (WBF) as a function of age (5 to 50 days). Flies overexpressing hSOD1 under the D42-GAL4 activator were able to sustain flight significantly longer than controls, with the largest effect observed in the middle stages of life. The hSOD1-expressed line also had, on average, slower wingbeat frequencies in late, but not early life relative to age-matched controls. Second, we examined locomotor (exploratory walking) behavior in late life when flies had lost the ability to fly (age ≥ 60 d). hSOD1-expressed flies showed significantly more robust walking activity relative to controls. Findings show patterns of functional decline dissimilar to those reported for other life-extended lines, and suggest that the hSOD1 gene not only delays death but enhances sensorimotor abilities critical to survival even in late life.

Improved functional abilities of the life-extended Drosophila mutant Methuselah are reversed at old age to below control levels

Methuselah (mth) is a chromosome 3 Drosophila mutant with an increased lifespan. A large number of studies have investigated the genetic, molecular, and biochemical mechanisms of the mth gene. Much less is known about the effects of mth on preservation of sensorimotor abilities throughout Drosophila's lifespan, particularly in late life. The current study investigated functional senescence in mth and its parental-control line (w1118) in two experiments that measured age-dependent changes in flight functions and locomotor activity. In experiment 1, a total of 158 flies (81 mth and 77 controls) with an age range from 10 to 70 days were individually tethered under an infrared laser-sensor system that allowed monitoring of flight duration during phototaxic flight. We found that mth has a statistically significant advantage in maintaining continuous flight over control flies at age 10 days, but not during middle and late life. At age 70 days, the trend reversed and parental control flies had a small but significant advantage, suggesting an interaction between age and genotype in the ability to sustain flight. In experiment 2, a total of 173 different flies (97 mth and 76 controls) with an age range from 50 to 76 days were individually placed in a large well-lit arena (60 × 45 cm) and their locomotor activity quantified as the distance walked in a 1-min period. Results showed that mth flies had lower levels of locomotor activity relative to controls at ages 50 and 60 days. These levels converged for the two genotypes at the oldest ages tested. Findings show markedly different patterns of functional decline for the mth line relative to those previously reported for other life-extended genotypes, suggesting that different life-extending genes have dissimilar effects on preservation of sensory and motor abilities throughout an organism's lifespan.

Pleiotropy and life history evolution in Drosophila melanogaster: uncoupling life span and early fecundity

Populations of Drosophila melanogaster that have been artificially selected for late age of reproduction evolve longer life spans and, in some cases, reduced early fecundity. The negative correlation is widely interpreted as evidence of antagonistic pleiotropy. Here, we show that the correlation breaks down in recombinant genomes. A major quantitative trait locus that increases adult life span by 20% has no detectable effect on early fecundity. Several recombinant genotypes are superflies, exhibiting both elevated early fecundity and long life. The genetic correlation of early fecundity and life span is not different from zero, while the midlife fecundity correlation is positive and statistically significant, suggesting age-specific adaptation. The results are not consistent with a dominant role for negative pleiotropy, but can be understood in terms of a mixture of pleiotropic and recombining nonpleiotropic elements. Life span and early fecundity can be genetically uncoupled.

Register-based research on twins

INTRODUCTION

The Danish Twin Registry (DTR) has for more than 50 years been based on surveys and clinical investigations and over the two last decades also on register linkage. Currently these two approaches are merged within Statistics Denmark.

RESEARCH TOPICS

Here we report on three major groups of register-based research in the DTR that used the uniqueness of twinning. First, we focus on the ''long-term prognosis'' of being a twin compared with being a singleton and show that Danish twins have health trajectories in adulthood similar to singletons, which is a result of interest for twins and their families as well as a test of the fetal origins hypothesis that states that fetal growth restriction has long-term health consequences. Secondly, we summarise some of the most important register-based ''classical twin studies'', e.g. heritability studies on lifespan and exceptional longevity. Finally, we illustrate how the co-twin control method in a register setting can be used to control for the effect of rearing environment and genetic factors in studies of the association between exposures and health.

CONCLUSION

The spectrum of register-based twin studies is very wide and have changed in accordance with methodological and data resource developments.

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.

Regulation of Drosophila life-span: effect of genetic background, sex, mating and social status

During the past decade, model organisms such as Drosophila have made it possible to identify individual genes and pathways that regulate organismal life-span. However, despite the progress made in Drosophila aging research, many longevity studies have often yielded controversial results that can be attributed to differences both in genetic background and in experimental design. Here, we describe the results of a systematic analysis of life-span comparisons in two laboratory wild-type strains. The main goal of these studies is to clarify the effects of social status, mating and sex on life-span with the aim of defining the optimal experimental design whereby the influence of these factors would be minimized. We find that differences in environmental factors and genetic background can be minimized by measuring the life-span of flies that are maintained as mixed-sex groups that allow for regular sexual and social contacts and seems to be more physiologically relevant for estimation of population's life-span. Taken together, these results may be especially important for screens designed to search for genes that may be involved in longevity as well as for comparative analysis of strains in which the genetic background is unknown or in those cases where it is very difficult to equilibrate.

Evidence for a genetic basis of aging in two wild vertebrate populations

Aging, or senescence, defined as a decline in physiological function with age, has long been a focus of research interest for evolutionary biologists. How has natural selection failed to remove genetic effects responsible for such reduced fitness among older individuals? Current evolutionary theory explains this phenomenon by showing that, as a result of the risk of death from environmental causes that individuals experience, the force of selection inevitably weakens with age. This in turn means that genetic mutations having detrimental effects that are only felt late in life might persist in a population. Although widely accepted, this theory rests on the assumption that there is genetic variation for aging in natural systems, or (equivalently), that genotype-by-age interactions (GxA) occur for fitness. To date, empirical support for this assumption has come almost entirely from laboratory studies on invertebrate systems, most notably Drosophila and C. elegans, whereas tests of genetic variation for aging are largely lacking from natural populations. By using data from two wild mammal populations, we perform quantitative genetic analyses of fitness and provide the first evidence for a genetic basis of senescence to come from a study in the natural environment. We find evidence that genetic differences among individuals cause variation in their rates of aging and that additive genetic variance for fitness increases with age, as predicted by the evolutionary theory of senescence.

Aging mechanism as the "down side" of adaptation: a network approach

Many diverse hypotheses on aging are in play. All from "aging genes" over decreasing telomere length to increased level of gene mutations has been suggested to determine an organism's lifespan, but no unifying theory exists. As part of a growing interest toward more integrative approaches in the field we propose a simplistic model based on the "use-it-or-lose-it" concept: we hypothesize that biological aging is a systemic property and the down side of adaptation in complex biological networks at various levels of organization: from brain over the immune system to specialized tissues or organs. The simple dynamical model undergoes three phases during its lifetime: (1) general plasticity (childhood), (2) optimization/adaptation to given conditions (youth and adolescence) and (3) steady state associated with high rigidity (aging). Furthermore, our model mimics recent data on the dynamics of the immune system during aging and, although simplistic, thus captures essential characteristics of the aging process. Finally, we discuss the abstract model in relation to current knowledge on aging and propose experimental setups for testing some of the theoretical predictions.

Age-related changes in climbing behavior and neural circuit physiology in Drosophila

Identifying the cellular and molecular basis for functional decline remains key to understanding aging. To this end, we have characterized age-dependent changes in climbing and the electrophysiology of the giant fiber circuitry in wild type (Wt) and mutant flies with altered lifespan (methuselah and fragile-X). Our data demonstrate a gradual decline in climbing in Wt and methuselah flies aged 5-45 days. In contrast, fragile-X flies climbed poorly even at 5 days and failed completely at 45 days. We then examined whether synaptic transmission to indirect flight muscles along the giant fiber circuit was altered with aging. At 5 days, the dorsal longitudinal muscle (DLM) in Wt flies followed high frequency stimulation well (at 130 Hz or above). At 35 and 45 days, these flies only followed 60-80 Hz. Methuselah flies did not follow stimuli as well as the Wt flies did at 5 and 25 days, but they were similar to Wt flies at older ages. Fragile-X flies responded poorly even at 5 days (40 Hz) and worsened at 35 days (30 Hz). Unlike DLMs, the tergotrochanteral muscle followed high frequency stimuli relatively well in all genotypes, suggesting that the peripheral interneuron along the DLM pathway or the DLM muscular synapse is prone to age-dependent functional decline. These studies reveal subcellular structures as potential targets of aging, indicating that the giant fiber pathway can be used as a model circuit for quantitative studies of aging in flies as well as fly models of age-related human neurological disorders.

Age-dependent stability of sensorimotor functions in the life-extended Drosophila mutant methuselah

Methuselah is a Drosophila mutant with a 35% increased lifespan. We examined the robustness of methuselah's sensorimotor abilities in tethered flight as a function of age in experiments designed to test visuomotor synchronization and phototaxis in simulated flight. A total of 282 flies from different age groups (4 hours to 70 days) and genotypes (mth and w1118) were individually tethered under an infrared laser-sensor system that digitally recorded wing-beat frequency (WBF). We found that mth has a higher average WBF throughout most of its lifespan compared to parental control flies (w1118) and develops flight ability at a younger age. Its WBF at late life, however, is not significantly different than that of its parental control line. We further found that mth entrains during flight to motion of a visual grating significantly better than its parental line. These findings suggest that the mth gene not only delays chronological aging but enhances sensorimotor abilities critical to survival during early and middle, but not late life.

Methods for nutrigenomics and longevity studies in Drosophila: effects of diets high in sucrose, palmitic acid, soy, or beef

Nutrigenomics is the study of gene-nutrient interactions and how they affect the health and metabolism of an organism. Combining nutrigenomics with longevity studies is a natural extension and promises to help identify mechanisms whereby nutrients affect the aging process, life span, and, with the incorporation of age-dependent functional measures, health span. The topics we discuss in this chapter are genetic techniques, dietary manipulations, metabolic studies, and microarray analysis methods to investigate how nutrition affects gene expression, life span, triglyceride levels, total protein levels, and live weight in Drosophila. To better illustrate nutrigenomic techniques, we analyzed Drosophila larvae or adults fed control diets (high sucrose) and compared these with larvae or adults fed diets high in the saturated fat palmitic acid, soy, or 95% lean ground beef. The main results of these studies are, surprisingly, that triglyceride and total protein levels are significantly decreased by the beef diet in all adults, and total protein levels are significantly increased in male flies fed the soy diet. Furthermore, and less surprisingly, we found that all three experimental diets significantly decreased longevity and increased the length of time to develop from egg to adult. We also describe preliminary microarray results with adult flies fed the different diets, which suggest that only about 2-3% of the approx 18,000 genes have significantly altered mRNA expression levels compared with flies fed a control sucrose diet. The significance of these results and other types of nutrigenomics and longevity analyses is discussed.

Embryo development and ageing in birds and mammals

The rate of ageing is a genetically influenced feature of an individual's life history that responds to selection on lifespan. Various costs presumably constrain the evolution of prolonged life, but these have not been well characterized and their general nature is unclear. The analyses presented here demonstrate a correlation among birds and mammals between rates of embryonic growth and ageing-related mortality, which are quantified by the exponents of fitted power functions. This relationship suggests that rapid early development leads to accelerated ageing, presumably by influencing some aspect of the quality of the adult individual. Although the mechanisms linking embryo growth rate and ageing are not known, a simple model of life-history optimization shows that the benefits of longer life can be balanced by connected costs of extended development.

Longevity differences among lines artificially selected for developmental time and wing length in Drosophila buzzatii

We assessed the indirect response of longevity in lines selected for wing length (WL) and developmental time (DT). Longevity in selection lines was compared to laboratory control lines and the offspring of recently collected females. Wild flies (W lines), flies from lines selected for fast development (F lines), and for fast development and large wing length (L lines) outlived control laboratory lines (C lines) and lines selected for fast development and short wing (S lines). The decline in longevity in S lines is in line with the idea that body size and longevity are correlated and may be the result of the fixation of alleles at loci affecting pleiotropically the two traits under selection and longevity. In addition, inbreeding and artificial selection affected the correlation between wing length and longevity that occurs in natural populations of Drosophila buzzatii, suggesting that correlations between traits are not a perdurable feature in a population.

Flies and their golden apples: the effect of dietary restriction on Drosophila aging and age-dependent gene expression

Reduced nutrient availability (dietary restriction) extends lifespan in species as diverse as yeast, nematode worms, Daphnia, Drosophila, and mammals. Recent demographic experiments have shown that moderate nutrient manipulation in adult Drosophila affects current mortality rate in a completely reversible manner, which suggests that dietary restriction in Drosophila increases lifespan through a reduction of the current risk of death rather than a slowing of aging-related damage. When examined in the light of the new demographic data, age-dependent changes in gene expression in normal and diet-restricted flies can provide unique insight into the biological processes affected by aging and may help identify molecular pathways that regulate it.

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.

Survival analysis of life span quantitative trait loci in Drosophila melanogaster

We used quantitative trait loci (QTL) mapping to evaluate the age specificity of naturally segregating alleles affecting life span. Estimates of age-specific mortality rates were obtained from observing 51,778 mated males and females from a panel of 144 recombinant inbred lines (RILs). Twenty-five QTL were found, having 80 significant effects on life span and weekly mortality rates. Generation of RILs from heterozygous parents enabled us to contrast effects of QTL alleles with the means of RIL populations. Most of the low-frequency alleles increased mortality, especially at younger ages. Two QTL had negatively correlated effects on mortality at different ages, while the remainder were positively correlated. Chromosomal positions of QTL were roughly concordant with estimates from other mapping populations. Our findings are broadly consistent with a mix of transient deleterious mutations and a few polymorphisms maintained by balancing selection, which together contribute to standing genetic variation in life span.

Age-specific mortality rates of reproducing and non-reproducing males of Drosophila melanogaster

The proximate and evolutionary causes of the levelling of mortality rates at late ages, observed in several species, remain obscure. To investigate the causes of mortality levelling late in life in Drosophila melanogaster, we examined the effect of reproduction on mortality patterns, by conducting population cage experiments with a total of more than 45,000 individuals. Several different genotypes of reproducing and non-reproducing males from F(1) crosses among isogenic lines were studied. Our results suggest that significant mortality levelling can occur even in non-reproducing males, but that reproduction also significantly affects mortality patterns. The results show that mortality levelling is strongly affected by the Gompertz initial mortality rate and exponential rate of increase parameters, probably through the effects of heterogeneity in mortality risks.

Longevity and metabolism in Drosophila melanogaster: genetic correlations between life span and age-specific metabolic rate in populations artificially selected for long life

We measured age-specific metabolic rates in 2861 individual Drosophila melanogaster adult males to determine how genetic variation in metabolism is related to life span. Using recombinant inbred (RI) lines derived from populations artificially selected for long life, resting metabolic rates were measured at 5, 16, 29, and 47 days posteclosion, while life spans were measured in the same genotypes in mixed-sex population cages and in single-sex vials. We observed much heritable variation between lines in age-specific metabolic rates, evidence for genotype x age interaction, and moderate to large heritabilities at all ages except the youngest. Four traits exhibit evidence of coordinate genetic control: day 16 and day 29 metabolic rates, life span in population cages, and life span in vials. Quantitative trait loci (QTL) for those traits map to the same locations on three major chromosomes, and additive genetic effects are all positively correlated. In contrast, metabolic rates at the youngest and oldest ages are unrelated to metabolic rates at other ages and to survival. We suggest that artificial selection for long life via delayed reproduction also selects for increased metabolism at intermediate ages. Contrary to predictions of the "rate of living" theory, we find no evidence that metabolic rate varies inversely with survival, at the level of either line means or additive effects of QTL.

Maintenance of aphid clonal lineages: images of immortality?

Artificial cloning and ancient asexuals have impacted upon both scientific and lay thinking in applied and theoretical fields as diverse as medicine and evolution. Hence, this is an opportune time to promote debate and discussion on what maintains a clonal lineage. The genetic fidelity of a clone has been discussed in detail elsewhere [Genet. Res. 79 (2002) 1; Biol. J. Linnean Soc. 79 (2003) 3]. In this paper, we focus on the lineage integrity (=longevity), or physiological lifespan of a clone with respect to senesce in relation to factors controlling telomere functioning. Aspects of cell line research pertinent to eukaryotic clonal lineages are discussed and, in particular, we try to extrapolate aspects of this research and apply it to apomictic (=mitotic) aphid lineages to suggest how they may be maintained. Analogies are made between single cells and individual aphids that senescence through a generation, whilst the respective lineages persist for finite periods, unless that is, compensatory mechanisms have evolved allowing immortality in the one and ancient asexuality in the other. Such comparison may allow fresh insights into the mechanisms of clonal lineage maintenance and evolution. We hypothesise that: (1). the cause of extinction in eukaryotic clonal lineages is due to deleterious effects on key regions of the genome, the chromosomal telomere being one such site; (2). recombination acts as a common mechanism to reset telomere functioning, perhaps more fundamental than its utility to reduce genetic load and maintain adaptability; and (3). ancient lineages persist through time as a function of group-specific compensatory mechanisms that maintain telomere integrity.

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 role of somatic and germline mutations in aging and a mutation interaction model of aging

Mutations with a deleterious effect that is expressed after the average reproductive period are not effectively selected against and can accumulate in the germline. A conservative estimate is that at least 1-2% of new deleterious mutations affect some aspect of DNA replication, repair, or chromosome segregation. Since deleterious mutations can have an effect even as heterozygotes, this mutation accumulation can create an inherited background of late-acting mutations that themselves enhance mutation rate. This can have an interactive effect, in that it may increase the rate of somatic mutation during an individual's lifetime. The aging individual therefore becomes increasingly mosaic for somatic mutations, which in turn could potentially contribute to the gradual deterioration of biological processes and influence what we experience as senescence. Interventions that reduce somatic and germ cell mutations should, therefore, reduce the aging process in present and future generations.

Evolutionary experimentation through hybridization under laboratory condition in Drosophila: evidence for recombinational speciation

BACKGROUND

Drosophila nasuta nasuta (2n = 8) and Drosophila nasuta albomicans (2n = 6) are a pair of sibling allopatric chromosomal cross-fertile races of the nasuta subgroup of immigrans species group of Drosophila. Interracial hybridization between these two races has given rise to new karyotypic strains called Cytorace 1 and Cytorace 2 (first phase). Further hybridization between Thailand strain of D. n. albomicans and D. n. nasuta of Coorg strain has resulted in the evolution of two more Cytoraces, namely Cytorace 3 and Cytorace 4 (second phase). The third phase Cytoraces (Cytorace 5 to Cytorace 16) have evolved through interracial hybridization among first, second phase Cytoraces along with parental races. Each of these Cytoraces is composed of recombined genomes of the parental races. Here, we have made an attempt to systematically assess the impact of hybridization on karyotypes, morphometric and life history traits in all 16 Cytoraces.

RESULTS

The results reveal that in most cases, the newly evolved Cytoraces, with different chromosome constitutions, exhibit decreased body size, better fitness and live longer than their parents. Particularly, Cytorace 5, 6 and 8 have evolved with very much higher range values of quantitative traits than the parents and other Cytoraces, which suggests the role of transgressive segregation in the evolution of these Cytoraces.

CONCLUSION

Thus, the rapid divergence recorded in the chromosomes, karyotypes, body size and fitness traits of Cytoraces exhibit the early event of recombinational raciation / speciation in the evolution of the Cytoraces under laboratory conditions.

Interpreting interactions between treatments that slow aging

A major challenge in current research into aging using model organisms is to establish whether different treatments resulting in slowed aging involve common or distinct mechanisms. Such treatments include gene mutation, dietary restriction (DR), and manipulation of reproduction, gonadal signals and temperature. The principal method used to determine whether these treatments act through common mechanisms is to compare the magnitude of the effect on aging of each treatment separately with that when two are applied simultaneously. In this discussion we identify five types of methodological shortcomings that have marred such studies. These are (1) submaximal lifespan-extension by individual treatments, e.g. as a result of the use of hypomorphic rather than null alleles; (2) effects of a single treatment on survival through more than one mechanism, e.g. pleiotropic effects of lifespan mutants; (3) the difficulty of interpreting the magnitude of increases in lifespan in double treatments, and failure to measure and model age-specific mortality rates; (4) the non-specific effects of life extension suppressors; and (5) the possible occurrence of artefactual mutant interactions. When considered in the light of these problems, the conclusions of a number of recent lifespan interaction studies appear questionable. We suggest six rules for avoiding the pitfalls that can beset interaction studies.

Lifespan in captive baboons is heritable

The effects of aging are evident in multiple organ systems, tissues, cell types, and molecules; all complex phenotypes affected by multiple shared and unique environmental factors and genes, which makes identifying the role of genetics in human aging difficult. Researchers have used yeast, nematodes, fruit flies, and mice to search for genes that influence the aging process. Given the phylogenetic distance and anatomic and physiologic dissimilarities of these organisms from humans, directly extrapolating these results to our species is problematic. However, nonhuman primates have a high degree of genetic, anatomic and physiologic similarity with humans and, thus, they may assist in the detection, characterization, and identification of genetic and environmental influences on human aging. Our goal is to demonstrate that effects of genes on variation in lifespan, a surrogate measure of aging, can be detected in a nonhuman primate species. Using variance component analysis, heritability of age at death was estimated to be 0.23+/-0.08 (P=0.0003) in 674 baboons from the Southwest Foundation for Biomedical Research (SFBR). This research demonstrates that lifespan is under partial genetic control. Given these findings, we believe that the baboon has potential as a model of human aging.

Aging is a deprivation syndrome driven by a germ-soma conflict

Evolution through natural selection can be described as driven by a perpetual conflict of individuals competing for limited resources. Recently, I postulated that the shortage of resources godfathered the evolutionary achievements of the differentiation-apoptosis programming [Rev. Neurosci. 12 (2001) 217]. Unicellular deprivation-induced differentiation into germ cell-like spores can be regarded as the archaic reproduction events which were fueled by the remains of the fratricided cells of the apoptotic fruiting body. Evidence has been accumulated suggesting that conserved through the ages as the evolutionary legacy of the germ-soma conflict, the somatic loss of immortality during the ontogenetic segregation of primordial germ cells recapitulates the archaic fate of the fruiting body. In this heritage, somatic death is a germ cell-triggered event and has been established as evolutionary-fixed default state following asymmetric reproduction in a world of finite resources. Aging, on the other hand, is the stress resistance-dependent phenotype of the somatic resilience that counteracts the germ cell-inflicted death pathway. Thus, aging is a survival response and, in contrast to current beliefs, is antagonistically linked to death that is not imposed by group selection but enforced upon the soma by the selfish genes of the "enemy within". Environmental conditions shape the trade-off solutions as compromise between the conflicting germ-soma interests. Mechanistically, the neuroendocrine system, particularly those components that control energy balance, reproduction and stress responses, orchestrate these events. The reproductive phase is a self-limited process that moulds onset and progress of senescence with germ cell-dependent factors, e.g. gonadal hormones. These degenerate the regulatory pacemakers of the pineal-hypothalamic-pituitary network and its peripheral, e.g. thymic, gonadal and adrenal targets thereby eroding the trophic milieu. The ensuing cellular metabolic stress engenders adaptive adjustments of the glucose-fatty acid cycle, responses that are adequate and thus fitness-boosting under fuel shortage (e.g. during caloric restriction) but become detrimental under fuel abundance. In a Janus-faced capacity, the cellular stress response apparatus expresses both tolerogenic and mutagenic features of the social and asocial deprivation responses [Rev. Neurosci. 12 (2001) 217]. Mediated by the derangement of the energy-Ca(2+)-redox homeostatic triangle, a mosaic of dedifferentiation/apoptosis and mutagenic responses actuates the gradual exhaustion of functional reserves and eventually results in a multitude of aging-related diseases. This scenario reconciles programmed and stochastic features of aging and resolves the major inconsistencies of current theories by linking ultimate and proximate causes of aging. Reproduction, differentiation, apoptosis, stress response and metabolism are merged into a coherent regulatory network that stages aging as a naturally selected, germ cell-triggered and reproductive phase-modulated deprivation response.

Advice to an aging scientist

Fifty years ago, Peter Medawar and George Williams developed two now-classic theories for the evolution of senescence. In the past 20 years, evolutionary biologists studying aging have developed explicit mathematical models of these theories, used these models to derive explicit predictions, and tested these predictions using a variety of approaches. But, we argue here, our singular focus on these models may have hindered progress in evolutionary studies of aging. Research in this area has not kept pace with dramatic advances in evolutionary theory and molecular genetics. Progress in evolutionary studies of aging will depend on a bold, integrative approach, incorporating evolutionary and molecular advances from other fields, along with the powerful statistical and mathematical tools now available. We discuss several specific examples where we may gain new insight into the causes of aging by looking to other evolutionary phenomena, including sexual conflict and the evolution of social behavior. In addition, we present new results which suggest that the analysis of gene networks may lend particular insight into the genetic underpinnings of the aging process.

Circadian rhythm and time of mating in Bactrocera cucurbitae (Diptera: Tephritidae) selected for age at reproduction

Chrono-biological traits were changed by selecting for life-history traits via a genetic linkage controlling both time-related behavioural and life-history traits. Behavioural traits were compared between lines selected for young (Y-lines) and old (O-lines) age at reproduction in the melon fly, Bactrocera cucurbitae (Coquillett). Adults from O-lines, which survive longer than flies from Y-lines, mated later in the day and had a longer period of circadian rhythm in the locomotor activity than those from Y-lines. Flies from F(1) reciprocal crosses had an intermediate time of mating and periods of circadian rhythm between that of the parents, indicating a genetic basis to these traits. The presence of these behavioural differences across the selection lines indicates that chrono-biological traits exhibit correlated responses to selection on age at reproduction. The correlated responses in the behavioural traits to selection for life-history traits are discussed from two points of view: pleiotropy and inadvertent selection.

Mouse loci associated with life span exhibit sex-specific and epistatic effects

We have looked for genetic predictors of life span in a sibship of mice created as a four-way cross among inbred grandparental strains BALB/cJ, C57BL/6J, C3H/HeJ, and DBA/2J. To minimize the potential confounding effects of loci that influence early-life illnesses only, we conducted two analyses: one involving all the mice, and the other using a data set from which the first 20% of the deaths were excluded. The two strongest associations reach experimentwise significance levels (p <.01) when tested on the 80% of the mice with the longest life spans. Surprisingly, three of the four strongest associations showed sex-specific effects, with an influence on life span of either male or female mice, but not both. Epistatic interactions among the loci were also identified. The life-span effect of a locus on chromosome 10 (D10Mit15) exhibited epistatic interactions with loci on chromosomes 9 and 16 (D9Mit10 and D16Mit182). In a second example, a locus on chromosome 12 (D12Mit167) depended on the specific combination of alleles inherited from both male and female parents. Our results show that the common laboratory mouse strains are polymorphic at loci that produce substantial differences in life span and that these effects can be sex specific and conditional on alleles inherited at other loci.

Heterogeneity and its biodemographic implications for longevity and mortality

In the visible world, heterogeneity typically refers to the differences that exist among individuals in a defined population. These differences can arise from a variety of sources--biological, behavioral and social. Ever since Darwin, scientists have argued over the biological significance of differences observed at the individual, morphological, physiological, genetic, molecular and structural levels. A general consensus has been reached. Heterogeneity is ubiquitous, it is important, and it increases as observations are made at finer levels of biological resolution. Debates over the significance of heterogeneity have emerged once again as biologists and demographers work together in order to create the emerging field of biodemography. For these scientists, the debates center around the relative impact that individual heterogeneity has on population level statistics. It is argued here that in a world where the mortality barriers to long life for individuals have been dramatically weakened, the population consequences of heterogeneity are already visible and will grow in importance as biomedical technologies continue to usher progressively more people into the post-reproductive period of the lifespan.

Age-specific decrease in aerobic efficiency associated with increase in oxygen free radical production in Drosophila melanogaster

This study was designed to test the free radical theory of aging by using Drosophila melanogaster as a model system. Oxygen free radicals are generated by mitochondria during the process of normal oxidative metabolism. Age-specific measurements of oxygen consumption, heat production and anti-oxidant enzyme activity were obtained from two inbred lines of male flies, one selected for longevity and one normal-lived. The findings of this study demonstrate that although oxygen consumption remains relatively constant over the majority of the life span of each line of flies, aerobic efficiency declines with advancing age. This loss of aerobic efficiency manifests itself as a decline in total body metabolism as measured by heat production, and appears to be associated with an age-specific increase in damage inflicted upon mitochondria by oxygen free radicals.