Stress experiments as a means of investigating age-specific mortality in Drosophila melanogaster

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.

Chill-coma recovery time, age and sex determine lipid profiles in Ceratitis capitata tissues

The remodeling of membrane composition by changes in phospholipid head groups and fatty acids (FA) degree of unsaturation has been associated with the maintenance of membrane homeostasis under stress conditions. Overall lipid levels and the composition of cuticle lipids also influence insect stress resistance and tissue protection. In a previous study, we demonstrated differences in survival, behavior and Cu/Zn superoxide dismutase gene expression between subgroups of Ceratitis capitata flies that had a reversible recovery from chill-coma and those that developed chilling-injury. Here, we analyzed lipid profiles from comparable subgroups of 15 and 30-day-old flies separated according to their recovery time after a chill-coma treatment. Neutral and polar lipid classes of chill-coma subgroups were separated by thin layer chromatography and quantified by densitometry. FA composition of polar lipids of chill-coma subgroups and non-stressed flies was evaluated using gas chromatography coupled to mass spectrometry. Higher amounts of neutral lipids such as triglycerides, diacylglycerol, wax esters, sterol esters and free esters were found in male flies that recovered faster from chill-coma compared to slower flies. A multivariate analysis revealed changes in patterns of storage and cuticle lipids among subgroups both in males and females. FA unsaturation increased after cold exposure, and was higher in thorax of slower subgroups compared to faster subgroups. The changes in neutral lipid patterns and FA composition depended on recovery time, sex, age and body-part, and were not specifically associated with the development of chilling-injury. An analysis of phospholipid classes showed that the phosphatidylcholine to lysophosphatidylcholine ratio (PC/LPC) was significantly higher, or showed a tendency, in subgroups that may have developed chilling-injury compared to those with a reversible recovery from coma.

Analysis of survival, gene expression and behavior following chill-coma in the medfly Ceratitis capitata: effects of population heterogeneity and age

The medfly Ceratitis capitata is an agricultural pest distributed worldwide thanks, in part, to its phenotypic plasticity of thermal tolerance. Cold exposure has been shown to reduce C. capitata survival, which may affect its distribution in areas with subfreezing temperatures. When insects are increasingly cooled, they attain a critical thermal threshold and enter a chill-coma state characterized by cessation of movement. It is not clear how a rapid cold exposure affects the physiological state of medflies, and how this is influenced by age and population heterogeneity. In order to approach these questions, C. capitata single-sex laboratory populations of 15 and 30 days old were subjected to a chill-coma recovery assay, and separated according to their recovery time in three subgroups: Fast-Subgroups, Intermediate-Subgroups, and Slow-Subgroups. Thereafter, we analyzed their survival, behavioral, and gene expression outputs. In female and old male populations, we found that flies with the slowest recovery time had a reduced life expectancy, a higher initial mortality rate, and a worse climbing performance compared with flies that recovered faster. Therefore, we were able to separate subgroups that developed chilling-injury from subgroups that had a reversible full recovery after cold exposure. The gene expression analysis of the heat shock protein genes hsp70 and hsp83 showed no clear association with the parameters studied. Interestingly, thorax expression levels of the Cu/Zn superoxide dismutase gene were elevated during the recovery phase in the Fast-Subgroups, but remained constant in the Slow-Subgroups that developed chilling-injury. On the other hand, none of the young male subgroups seemed to have suffered irreversible damage. Thus, we concluded that depending on age and population heterogeneity, chill-coma recovery time points out significant differences on individual cold tolerance. Moreover, the inability to properly induce the antioxidant defense system to counteract the oxidative damage caused by cold seems to contribute to the development of chilling-injury.

Decelerating mortality rates in older ages and its prospects through Lee-Carter approach

The present study attempts to study the age pattern mortality and prospects through Lee-Carter approach. The objectives of the study are to examine the trend of mortality decline and life expectancy. Contemporaneously, we have projected life expectancy up to 2025, projecting ASDR using Lee-Carter method. Life table aging rate (LAR) used to estimate the rate of mortality deceleration. Overtime, LAR increased and during recent decade it remained more or less unchanged. By age, LAR significant increased in the oldest of old. The slope is steepest in the oldest of old in the recent decade. The rates of mortality increased in oldest of old as the age group is more vulnerable to chronic disease and vulnerable to identifiable risk factors for virtually every disease, marked by senility. The analysis revealed that the level of mortality is not declining but rate of acceleration is declining and is further expected to decline. By the year 2025, the age specific death rates for the age group 5–9 and 10–14 will go below one per thousand.Life expectancy will attained as high as 73 and 79 years for male and female and is further expected to increase linearly. 71 percent of total female birth and 57 percent of total male birth will survive up to age 70+. Also the findings revealed that mortality rate is declining with constant rate up to age 70 and thereafter, the mortality rate accelerates and this holds true for both sexes.

Paradoxical physiological transitions from aging to late life in Drosophila

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

A new cultivation system for studying chemical effects on the lifespan of the fruit fly

A side-by-side comparison was made between a conventional vial system and a novel bottle system for cultivating flies and testing the effect of chemical exposure on the lifespan of the flies. While the two cultivation systems yielded very similar results for the effect of DEHP (di[2-ethylhexyl] phthalate) on reducing the lifespan of fruit fly Drosophila melanogaster, the new bottle system has many advantages over the conventional vial system. The bottle system allowed long-term cultivation of flies in the same bottle and thus eliminated the need for transferring of flies between vials. Foods/nutrients were provided as fresh moisture medium coated on a glass slide vertically hanged in the center of the bottle. Fly discharges and dead flies were collected onto a draw horizontally inserted into the bottom of the bottle. These features have resulted in great convenience for cultivating flies and reduced labor and media cost. The effective separation of food from discharge may allow accurate mass balance measurement and thus yield more definitive observations for understanding the actual role of calorie restriction (CR) or dietary-restriction (DR) in fly metabolism and longevity.

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.

Biodemographic trajectories of age-specific reproliferation from stationary phase in the yeast Saccharomyces cerevisiae seem multiphasic

Ageing is usually seen as a monotonic decline of functions and survival. However, recent studies reported that age-specific mortality rates increased and then leveled off or even declined at later ages in several species including humans. Preliminary data using the yeast, Saccharomyces cerevisiae, demonstrated an even more complicated, non-monotonic pattern of reproliferation after stationary phase (i.e. the ability of a cell to exit stationary phase and form a colony). In the present article, we conducted a study of the age-specific reproliferation rates of yeast populations. Stationary phase yeast cells were maintained in water and the reproliferation rates were estimated by the number of yeast able to exit stationary phase on rich growth media. We showed that the age-specific reproliferation rates in yeast seem to rise, fall and rise again. Furthermore, we observed this pattern in different experiments and in different genotypes and established that this pattern was not due to genetic heterogeneity of the populations.

Doxycycline-regulated over-expression of hsp22 has negative effects on stress resistance and life span in adult Drosophila melanogaster

Drosophila hsp22 is a member of the small heat shock proteins family (shsps). The hsp22 is expressed in a tissue-general pattern in response to heat stress and during normal aging, and localizes to the mitochondrial matrix, however, its exact function and targets are unknown. Hsp22 was found to be rapidly induced in response to oxidative stress, indicating that hsp22 is also an oxidative stress response gene. To assay for effects of hsp22, a ubiquitous pattern of hsp22 gene expression was generated in young flies using the "tet-on" doxycycline-regulated promoter system. The hsp22 over-expression made flies more sensitive to heat and oxidative stress, while resistance to coumarin poisoning was not affected. Life span was also reduced, particularly at higher culture temperatures. Members of other hsp families have been shown to feedback-inhibit their own expression by interacting with the heat shock transcription factor (HSF) and preventing binding to the HSEs. Induction of hsp22:lacZ and hsp70:lacZ reporter transgenes in response to acute stress was normal in the presence of hsp22 protein over-expression and in old flies, indicating that the negative effects of hsp22 are downstream of the HSF/HSE pathway and the transcriptional heat shock response. The data demonstrate a specific over-expression phenotype for hsp22 and suggest that hsp22 interacts with heat and oxidative stress resistance pathways.

The male silkworm moth (Antheraea pernyi) is a key ingredient in hu-bao and sheng-bao for specific prolongation of the life-span of the male fruit fly (Drosophila melanogaster)

It is well established in Traditional Chinese Medicine that certain natural products, such as male silkworm moths, have different therapeutic effects on men than on women. These natural products have been used as dietary supplements specifically formulated for men or for women. However, this presumed sex-specific effect of certain natural products has not yet been confirmed experimentally with animal models or in human clinical trials. Here, using the fruit fly (Drosophila melanogaster) as a longevity model, we examined the effect of hu-bao (HB) and seng-bao (SB), two marketed health products made from a mixture of natural ingredients. Our results convincingly demonstrate that the effect of HB and SB are indeed specific for the male fly. The life-span of the male was significantly increased when HB or SB was added to the culture medium. In contrast, neither HB nor SB had much effect on the female fly. Upon removal of the male silkworm moth ingredient from HB or SB, the life-span prolongation effect of HB and SB was drastically diminished. Only with the addition of the male silkworm moth did the culture medium show a statistically significant life-span prolongation effect. This result suggests that the male silkworm moth is a key ingredient, in combination with other components, for specific prolongation of the life-span of male flies.

The reliability theory of aging and longevity

Reliability theory is a general theory about systems failure. It allows researchers to predict the age-related failure kinetics for a system of given architecture (reliability structure) and given reliability of its components. Reliability theory predicts that even those systems that are entirely composed of non-aging elements (with a constant failure rate) will nevertheless deteriorate (fail more often) with age, if these systems are redundant in irreplaceable elements. Aging, therefore, is a direct consequence of systems redundancy. Reliability theory also predicts the late-life mortality deceleration with subsequent leveling-off, as well as the late-life mortality plateaus, as an inevitable consequence of redundancy exhaustion at extreme old ages. The theory explains why mortality rates increase exponentially with age (the Gompertz law) in many species, by taking into account the initial flaws (defects) in newly formed systems. It also explains why organisms "prefer" to die according to the Gompertz law, while technical devices usually fail according to the Weibull (power) law. Theoretical conditions are specified when organisms die according to the Weibull law: organisms should be relatively free of initial flaws and defects. The theory makes it possible to find a general failure law applicable to all adult and extreme old ages, where the Gompertz and the Weibull laws are just special cases of this more general failure law. The theory explains why relative differences in mortality rates of compared populations (within a given species) vanish with age, and mortality convergence is observed due to the exhaustion of initial differences in redundancy levels. Overall, reliability theory has an amazing predictive and explanatory power with a few, very general and realistic assumptions. Therefore, reliability theory seems to be a promising approach for developing a comprehensive theory of aging and longevity integrating mathematical methods with specific biological knowledge.

Ageing and immortality

The concept of the force of natural selection was developed to explain the evolution of ageing. After ageing, however, comes a period in which mortality rates plateau and some individual organisms could, in theory, live forever. This late-life immortality has no presently agreed upon explanation. Two main theories have been offered. The first is heterogeneity within ageing cohorts, such that only extremely robust individuals survive ageing. This theory can be tested by comparisons of more and less robust cohorts. It can also be tested by fitting survival data to its models. The second theory is that late-life plateaus in mortality reflect the inevitable late-life plateau in the force of natural selection. This theory can be tested by changing the force of natural selection in evolving laboratory populations, particularly the age at which the force plateaus. This area of research has great potential for elucidating the overall structure of life-history evolution, particularly the interrelationship between the three life-history phases of development, ageing and immortality.

The fractionation experiment: reducing heterogeneity to investigate age-specific mortality in Drosophila

Age-specific mortality rates decelerate at older ages in both genetically homogenous and heterogeneous populations of Drosophila. One explanation proposed for deceleration is population heterogeneity. This hypothesis suggests that a population consists of sub-populations that differ in mortality characteristics and that the deceleration is the result of selective survival of stronger individuals. Here we describe an experiment that fractionates populations into several sub-populations without changing the physiological characteristics of the post-fractionated populations. Through a careful process of selection of Drosophila eggs, larvae, pupae and adults, we attempt to reduce as much as possible the degree of pre-adult, environmentally induced heterogeneity among individuals of a genetically identical cohort. We then ask whether such cohorts, when compared to non-fractionated populations, exhibit a lesser degree of mortality deceleration at advanced ages. From a total of 106 fractionated and control populations, consisting of 51331 individuals, 101 populations (93% of the fractionated populations and 100% of the control populations) exhibit a significant amount of mortality deceleration late in life. These observations suggest that environmental heterogeneity accrued during larval development is not a major factor contributing to mortality deceleration at older ages.

Deceleration in the age pattern of mortality at olderages

The rate of mortality increase with age tends to slow down at very old ages. One explanation proposed for this deceleration is the selective survival of healthier individuals to older ages. Data on mortality in Sweden and Japan are generally compatible with three predictions of this hypothesis: (1) decelerations for most major causes of death; (2) decelerations starting at younger ages for more “selective” causes; and (3) a shift of the deceleration to older ages with declining levels of mortality. A parametric model employed to illustrate the third prediction relies on the distinction between senescent and background mortality. This dichotomy, though simplistic, helps to explain the observed timing of the deceleration.

Invertebrate gerontology: the age mutations of Caenorhabditis elegans

Ageing is a complex phenomenon which remains a major challenge to modern biology. Although the evolutionary biology of ageing is well understood, the mechanisms that limit lifespan are unknown. The isolation and analysis of single-gene mutations which extend lifespan (Age mutations) is likely to reveal processes which influence ageing. Caenorhabditis elegans is the only metazoan in which Age mutations have been identified. The Age mutations not only prolong life, but also confer a complex array of other phenotypes. Some of these phenotypes provide clues to the evolutionary origins of these genes while others allude to mechanisms of lifespan-extension. Many of the Age genes interact and share a second common phenotype, that of stress resistance. Rather than invertebrate ageing being determined by a 'clock mechanism', a picture is emerging of ageing as a non-adaptive process determined, in part, by resistance to intrinsic stress mediated by stress-response genes.

Age-specific mortality costs of exposure to inbred Drosophila melanogaster in relation to longevity selection

This article address the hypotheses that selection for early- or late-life fitness changes patterns of reproductive behavior, that this behavior may be dependent on the genetic makeup of the females, and that patterns of male mortality are strongly dependent on the type of females to which they are exposed. Flies selected for late-life reproduction and their associated stocks selected for early reproduction were exposed to flies of the opposite sex from either the same stock or a highly inbred stock. Males of both long- and short-lived stocks showed an increase in early mortality when exposed to inbred females. In addition, when males were exposed to inbred females early in life they showed a lower age-specific mortality rate late in life than males exposed to females from their own stock. Interestingly, females exposed to inbred males showed a significant reduction in mean longevity. Analysis of age-specific mortality revealed that this reduction was brought about as a result of increased early mortality. Interpretation of the results from an analysis of mean longevity not only fails to identify important information--as shown from a demographic analysis of age-specific mortality--but also presents a misleading description of mortality costs.

Deceleration of age-specific mortality rates in chromosomal homozygotes and heterozygotes of Drosophila melanogaster

Age-specific mortality trajectories were estimated in mixed-sex cohorts of D. melanogaster. We studied 22,000 flies that were either second-chromosome homozygotes or heterozygotes with a randomized genetic background. Broad-sense heritabilities for longevity were estimated to be 6% for males and 9% for females. Heterozygotes lived longer than homozygotes on average, but there were exceptions to the usual heterotic pattern; in several crosses parental homozygotes had average life spans as long as that of their F1 heterozygotes. Estimated age-specific mortality rates were found to decelerate at advanced ages in both homozygotes and heterozygotes. The mortality models that best fit the data are the logistic model and the two-stage Gompertz model, both of which produce mortality trajectories that level off at advanced ages. Old-age mortality deceleration is not peculiar to inbred Drosophila.

Thermotolerance and extended life-span conferred by single-gene mutations and induced by thermal stress

We have discovered that three longevity mutants of the nematode Caenorhabditis elegans also exhibit increased intrinsic thermotolerance (Itt) as young adults. Mutation of the age-1 gene causes not only 65% longer life expectancy but also Itt. The Itt phenotype cosegregates with age-1. Long-lived spe-26 and daf-2 mutants also exhibit Itt. We investigated the relationship between increased thermotolerance and increased life-span by developing conditions for environmental induction of thermotolerance. Such pretreatments at sublethal temperatures induce significant increases in thermotolerance and small but statistically highly significant increases in life expectancy, consistent with a causal connection between these two traits. Thus, when an animal's resistance to stress is increased, by either genetic or environmental manipulation, we also observe an increase in life expectancy. These results suggest that ability to respond to stress limits the life expectancy of C. elegans and might do so in other metazoa as well.