CUMULATIVE PARENTAL AGE EFFECTS IN DROSOPHILA SUBOBSCURA

ANTAGONISTIC PLEIOTROPY: AN INTERSPECIFIC DROSOPHILA COMPARISON

Wild-type flies of 12 Drosophila species and semispecies were examined to determine whether correlation patterns between early- and late-life fitness characters predicted for individuals within a population by the antagonistic-pleiotropy hypothesis are reflected in comparisons of related species and semispecies that are known to differ in lifespan. Our goal was to determine whether the hypothesis is relevant to the evolution of life-history differences beyond the population level. Two fitness traits, egg production and percentage mating success, were observed at three ages: onset of reproductive age, one week later, and one month later. Age-dependent patterns of these traits do not consistently conform to predictions of the hypothesis. Species or semispecies that show reproductive vitality early in life need not be short-lived, and long lifespan need not be accompanied by a cost in early reproductive vitality, as measured by mating success and egg production. The two fitness traits can show different age-dependent patterns in the same species or semispecies. Potential explanations for the frequent inconsistency of the data with predictions of the hypothesis are discussed. Results support the idea that the hypothesis is only relevant to the evolution of life-history differences among individuals in the same breeding population confronted by the same environmental constraints.

EVOLUTIONARY EFFECTS OF SELECTION ON AGE AT REPRODUCTION IN LARVAL AND ADULT: DROSOPHILA MELANOGASTER

Two sets of three replicate lines of Drosophila melanogaster were artificially selected by reproduction at either a 'young' or an 'old' age. The pure lines, the hybrids between the lines within a selection regimen and the base stock from which the lines were derived were compared for longevity, early and late fertility, development time, larval viability and adult thorax length. Comparison of hybrid with pure lines showed some evidence for inbreeding depression in the lines from both selection regimes. Comparison of hybrid lines with the base stock did not provide evidence for any trade-off in either males or females between early fertility on the one hand and late life fertility and longevity on the other. Nor was there any clear evidence of a trade-off between pre-adult and adult fitness components. There was evidence of inadvertent selection for rapid development in both selection regimens, especially in the females of the 'young' lines, and this complicated the interpretation of the responses and correlated responses to selection. An improvement in adult performance in the 'old' line males relative to the base stock appeared to be attributable to reversal of mutation accumulation. Comparison of the hybrid 'young' and 'old' lines with the base stock did not support the idea that the superior longevity and late life fertility of the 'old' lines relative to the 'young' lines could be accounted for by the effects of mutation accumulation in the 'young' lines. The results point to the need to compare selected lines with their base stock when deducing responses and correlated responses to selection and to avoid unintentional selection. In this type of experiment, larval density should be standardized during selection, and adults should not be under pressure for rapid maturation.

Interactions between injury, stress resistance, reproduction, and aging in Drosophila melanogaster

An important aspect of the aging process in Drosophila melanogaster is the natural loss of antennae, legs, bristles, and parts of wings with age. These injuries lead to a loss of hemolymph, which contains water and nutrients. Stress-resistant lines of D. melanogaster are sometimes longer-lived than the populations from which they are derived. One hypothesis tested here is that increased stress-resistance fosters longevity because it allows fruit flies to cope with the loss of hemolymph due to injury to the aging fly. We tested the effects of surgically induced injury on the aging and reproduction of five replicate populations. We then tested the effects of injury on populations that had been selected for different levels of stress resistance and on control populations. Injury affected aging more in males than in females, in part because of a counter-balancing reduction in female reproduction brought about by injury. More specifically, injury reduced female fecundity and male virility. Injury significantly reduced the starvation resistance in some groups of flies, but not in others. These findings undermine any simple interpretation of the interactions between injury, reproduction, and aging based on stress resistance. But they do indicate the existence of significant interactions between these biological processes, interactions that should be resolved in greater mechanistic detail than has been managed here.

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.

Genetic approaches to study aging in Drosophila melanogaster

The process of aging can be described as a progressive decline in an organism's function that invariably results in death. This decline results from the activities of intrinsic genetic factors within an organism. The relative contributions of the biological and environmental components to senescence are hard to measure, however different strategies have been devised in Drosophila melanogaster to isolate and identify genetic influences on aging. These strategies include selective breeding, quantitative trait loci (QTL) mapping and single gene mutant analysis. Selective breeding effectively demonstrated a genetic, heritable component to aging while QTL mapping located regions within the Drosophila genome carrying loci that influence the aging process. Within the past decade, single gene mutant analysis has facilitated the identification of specific genes whose activities play a determinative role in Drosophila aging. This review will focus on the application of selective breeding, QTL mapping and single gene mutant analysis used in Drosophila to study aging as well as the results obtained through these strategies to date.

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.

Mechanisms of ageing: public or private?

Ageing--the decline in survival and fecundity with advancing age is caused by damage to macromolecules and tissues. Ageing is not a programmed process, in the sense that no genes are known to have evolved specifically to cause damage and ageing. Mechanisms of ageing might therefore not be expected to be as highly conserved between distantly related organisms as are mechanisms of development and metabolism. However, evidence is mounting that modulators of the rate of ageing are conserved over large evolutionary distances. As we discuss in this review, this conservation might stem from mechanisms that match reproductive rate to nutrient supply.

Response of Sod-2 enzyme activity to selection for high voluntary wheel running

The objective of this study was to examine the correlated response of anti-oxidant enzyme activity to selective breeding for increased voluntary wheel running in house mice. Activity of liver superoxide dismutase-2 (Sod-2), a free radical scavenger, was measured in four groups of mice. 'Active' individuals were housed in cages with attached wheels for 8 weeks beginning at weaning; 'sedentary' individuals were housed in cages with attached wheels that were prevented from rotating. Both of these treatments were applied to male and female mice from generation 14 of a replicated artificial selection experiment, which is composed of four lines selected for high wheel running and four randomly bred lines that serve as controls. In females, Sod-2 activity was significantly lower in selected vs control animals, regardless of presence/absence of a free-turning wheel. This difference suggests a trade-off between early-age voluntary wheel-running activity and Sod-2 activity. In males, Sod-2 activity was significantly affected by an interaction between selection group and activity group, with males from selected lines having lower Sod-2 activity relative to control males only in the sedentary treatment. These negative correlated responses of Sod-2 activity to selection on wheel running are discussed in the context of antagonistic pleiotropy models of aging and with respect to potential effects on lifespan.

Maternal and grandmaternal age influence offspring fitness in Drosophila

The influence of maternal and grandmaternal age on progeny egg-to-adult viability was assessed in Drosophila serrata. Viability in progeny decreased with increasing maternal age. The potential for cumulative age effects was investigated in two environments, one of which involved nutrient and cold stress. Environment influenced viability and female age influenced progeny egg-to-adult viability across one generation. The influence on viability was cumulative across two generations. Females from old mothers, who also had old grandmothers, had the lowest viability in both environments. Grandmaternal effects were associated with a decrease in egg hatch rate whereas maternal effects also involved larval-to-adult viability. The age of the mother and grandmother should be taken into account when evaluating life-history traits in Drosophila.

The evolutionary genetics of ageing and longevity

Evolutionary theories of ageing are based on the observation that the efficacy of natural selection decreases with age. This is because, even without ageing, individuals will die of environmental causes, such as predation, disease and accidents. Ageing is thought to have evolved as the result of optimising fitness early in life. A second process, namely the progressive accumulation of mutations with effects late in life, will reinforce this result. Longevity of a species is therefore determined by the amount of environmental mortality caused by the ecology of a species. The experimental data concerning the relative roles of both processes are reviewed here. Recent discoveries of the levelling of mortality curves, and of age specific mutations in mutation accumulation lines of Drosophila melanogaster, require adjustments to the original models of the evolution of ageing and species longevity. These adjustments do not invalidate the underlying rationale of evolutionary theories of ageing. With current developments in QTL mapping and genetic association studies, the unravelling of the ageing process has the potential to progress rapidly.

Another set of responses and correlated responses to selection on age at reproduction in Drosophila melanogaster

Ageing is the decline in survival probability and fertility later in adult life. It can evolve through mutation accumulation and pleiotropy. Artificial selection by age at reproduction is a useful method for detecting the effects of pleiotropy, and for producing lines that differ in their rate of ageing for further analysis. However, the approach has encountered difficulties from gene-environment interaction and inadvertent selection. We have produced a new set of selection lines in Drosophila melanogaster, breeding from either 'young' or 'old' adults, and avoiding some of the difficulties present in previous studies. Breeding from older adults resulted in an evolutionary increase in survival but, contrary to all previous studies using this method, in no increase in late-life fertility. The increase in survival was accompanied by an evolutionary decline in fertility early in adult life, confirming the importance of pleiotropy in the evolution of ageing. Contrary to previous studies, there were no correlated responses to selection in the pre-adult period; development time, larval competitive ability and adult size achieved did not differ between the lines from the two selection regimes.

Genetic analysis of ageing: role of oxidative damage and environmental stresses

Evolutionary theory predicts substantial interspecific and intraspecific differences in the proximal mechanisms of ageing. Our goal here is to seek evidence for common ('public') mechanisms among diverse organisms amenable to genetic analysis. Oxidative damage is a candidate for such a public mechanism of ageing. Long-lived strains are relatively resistant to different environmental stresses. The extent to which these stresses produce oxidative damage remains to be established.

Use of recombinant inbred strains to map genes of aging

Recombinant inbred strains have been used in a number of organisms for segregation and linkage analysis of quantitative traits. One major advantage of the recombinant inbred (RI) methodology is that the genetic identity of individuals within a strain permits replicate measures of the same recombinant genotype. Such replicability is important for traits such as aging in Drosophila, where phenotypic expression is highly influenced by different environmental conditions. RI strain methodology has an added advantage for DNA marker-based linkage analysis of traits measured over the lifespan of the organism. The DNA can be extracted from individuals of the same genotype as those measured in a longevity study. In this paper an argument is presented for the use of a set of recombinant inbred strains to map the quantitative trait loci involved in the aging process in Drosophila. A unique use of a set of stable, transposable molecular markers to trace the quantitative trait loci involved is suggested.

The evolutionary history of Drosophila buzzatii. XXVII. Thorax length is positively correlated with longevity in a natural population from Argentina

The correlation between body size and longevity was tested in an Argentinian natural population of Drosophila buzzatii. Mean thorax length of flies newly emerging from rotting cladodes of Opuntia vulgaris was significantly smaller than that of two samples of flies caught at baits. The present results which might be interpreted as directional selection for longevity favoring larger flies are in agreement with previous results achieved in a Spanish natural population of D. buzzatii. Flies emerging from different substrates showed significant differences in thorax length, suggesting that an important fraction of phenotypic variance can be attributed to environmental variability. However, laboratory and field work in different populations of D. buzzatii showed a significant genetic component for thorax length variation.

Sex ratio, relative frequency, and mating success in two genotypes of Drosophila melanogaster

When two types of Drosophila are in competition, the frequency dependence of mating success is measured routinely in our laboratory and many others by direct observation of mating pairs in observation chambers. In these experiments, the Sex Ratio (number of males divided by the number of females) is usually 1:1. Various authors have shown that the male sexual activity depends on the Sex Ratio. We wished to investigate a possible influence of Sex Ratio on the frequency dependence of mating success and the "rare-type advantage." "General" Sex Ratio changes do affect mating success. The influence of the so-called "relative" Sex Ratio (number of males of the first type, A, divided by number of females of the second type, B) seems very low. Male Ratio and Female Ratio changes (i.e., changes of Genotype Ratio for one sex only, the number and type of the other sex flies remaining constant) have approximately the same influence: in these experiments the relative success of both types is also frequency dependent.

The role of glucose-6-phosphate dehydrogenase in the evolution of longevity in Drosophila melanogaster

Different polymorphic elements of the enzyme glucose-6-phosphate dehydrogenase (G6PD) are favoured under selection for long versus short life span. Replicate independently selected populations of short-lived individuals exhibit a more rapidly migrating and less actively staining allozyme, while long-lived populations have a slower migrating and more active allozyme. These correspond to the common ZwA and ZwB variants of the G6PD locus Zw. In vitro measurements show G6PD activity varies with allozymes and life span. Long-lived males have 64 per cent greater activity in G6PD, while females of long-lived strains are 108 per cent higher than those of short-lived strains. Previous studies of these strains have repeatedly demonstrated additivity of life span in F1 crosses. Activity of G6PD in reciprocal F1 populations is additive and intermediate between parents.

Direct observation of sexual competition in Drosophila melanogaster: the mutant white in competition with other genotypes

When two types of Drosophila are in competition, the frequency dependence of mating success routinely is measured in our laboratory by direct observation of mating pairs in "Elens-Wattiaux" observation chambers. The present experiments concern white mutants (used here as a reference standard) in competition with three other genotypes: wild-type Canton S, "giant" gt wa, and "giant" gt13z/w+ Y/C(1)Dx, y f. From the present observations, the frequency dependence of mating success seems a very common phenomenon: a rare-type sexual advantage exists for females and for males. Sexual activity of male and female white mutants is significantly higher when food is present in the mating chamber. Males of the other strains also are more active in the presence of food. Homogamic matings are more frequent among gt13z/w+ Y/C(1)Dx, y f flies.

Genetics of increased lifespan in Drosophila

Natural selection in the laboratory has been used to produce populations of Drosophila with genetically increased lifespan. These populations have been used to determine the physiological basis of postponed ageing and its pleiotropic concomitants. It appears that many loci and a number of physiological alterations are involved in increased lifespan.

Influence of food on frequency-dependent sexual activity of Drosophila melanogaster

When two types of Drosophila are in competition, the frequency dependence of mating successes frequently is measured by direct observation of copulating pairs in "Elens-Wattiaux" observation chambers, the relative frequency of both types being varied. The present experiments, concerning white-ebony mutants in competition with wild-type Canton-S flies, show that the presence of food in the mating chamber influences the sexual activity of flies, this influence differing in the two types when food is present. Thus, in order to have a realistic estimation of frequency dependence, it would seem prudent to conduct these experiments with food in the chamber. Three methods are used to analyze observation data: K coefficients of Petit and Ehrman, regression equations of Ayala and Campbell, and regression equations of Wattiaux and Lichtenberger. These three methods are compared and discussed.

Selection for life span in Drosophila melanogaster

Selection for reproduction at an early or a late age in life was applied to populations of D. melanogaster for 21 to 29 generations, with two experimental treatments of larval density. Populations with high and uncontrolled numbers of competing larvae responded strongly to selection for late-reproduction with the length of adult life increasing by as much as 50 per cent. In this treatment, selection produced true breeding long- and short-lived lines. When populations of developing larvae were held low, however, longevity fluctuated wildly during selection, showing little overall response, as several previous tests of senescence theory have also found. These experiments suggest that life span is either physiologically limited in that environment, or populations are unable to respond because either phenotype/genotype correlations are reversed, or genetic variation is suppressed. The inability of former studies to obtain a response to selection appears to have resulted from the artifactual introduction of strong gene-environment interactions through the use of a competition-free environment.

A computer model of the evolution of specific maximum lifespan

To answer the question of why organisms have evolved finite and specific maximum lifespans, I have built and experimentally studied a discrete-event simulation model of the evolution of lifespan. Through natural selection, the model evolves an apparent plateau in maximum lifespan, the height of which is a decreasing function of both the intensity of niche fluctuations and specific fecundity. Evolved lifespan is therefore finite (small and essentially constant over accessible time intervals) and specific. Experiments demonstrate that the plateau is not due to group selection. Instead, it occurs because the rate of increase of maximum lifespan by natural selection - in an environment presenting a finite probability that death will occur prior to reaching the genetically specified maximum - is a decreasing function of maximum lifespan itself and asymptotically approaches zero. This supports in part a class of existing hypotheses that finite lifespan is due to an equilibrium between weak selection, as in the model, and various lifespan-decreasing processes, which however were not simulated in the present experiments. Although the model shows that such counter processes are not strictly necessary for the evolution of finite and specific maximum lifespan, my interpretation of the model's correspondence to organic evolution does imply a counter process, a bias in random genetic drift toward shorter lifespan, that is more general than those previously hypothesized.

A test of evolutionary theories of senescence

Senescence is the post-maturation decline in survivorship and fecundity that accompanies advancing age. Two main evolutionary theories have been proposed to account for senescence. (1) The mutation-accumulation theory. Deleterious mutations exerting their effects only late in life would tend to accumulate, because of their minimal effects on fitness. More precisely, exclusively late-acting deleterious mutations will attain higher equilibrium frequencies under mutation--selection balance than will mutations that act early, resulting in lower mean values for fitness components late in life (ref. 3, p. 218). Medawar emphasized the possibility that this effect would be enhanced by selection of modifiers that postpone the age of onset of genetic diseases. (2) The pleiotropy theory. Williams suggested that many of the genes with beneficial effects on early fitness components have pleiotropic deleterious effects on late fitness components, but are nevertheless favoured by natural selection. (These theories are based on the decline with age in the effect of age-specific fitness-component changes on total fitness (ref. 3, pp. 206--214 and refs 4, 5). Either or both of these theories could apply in any particular population.) Selection experiments in Drosophila and Tribolium support the pleiotropy theory, although one such experiment gave results that only bordered on significance, but the mutation--accumulation theory has never been tested. The present results provide evidence for the pleiotropy theory, but do not support the mutation--accumulation theory.