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.

Life history variation in an artificially selected population of Drosophila melanogaster: pleiotropy, superflies, and age-specific adaptation.

We measured age-specific fecundity and survival in recombinant inbred lines of Drosophila melanogaster that were derived from an artificial selection experiment for delayed reproduction. Age at peak oviposition is highly heritable (h(2) (B) = 0.55). We find three qualitative categories of peak oviposition: early-, midlife-, and bimodal. Genetic correlations between life span and early fecundity are not significantly different from zero, but correlations with midlife fecundity are positive and statistically significant. Long-lived genotypes exhibit a midlife fecundity peak. There is no evidence for a shift of reproductive effort from early to later stages. The existence of qualitatively recombinant phenotypes, including "superflies" that exhibit both enhanced survival and high levels of early fecundity, argues against the widespread idea that life history evolution in Drosophila is dominated by negative pleiotropy. There is clear evidence of age-specific adaptation in the timing of oviposition.

Genetic variation for life span, resistance to paraquat, and spontaneous activity in unselected populations of Drosophila melanogaster: implications for transgenic rescue of life span.

Genetic variation in adult life span, resistance to paraquat, resistance to DDT, and spontaneous flying activity were measured in 138 recombinant inbred lines of Drosophila melanogaster. We find that the phenotypic correlation between life span and resistance to an exogenous oxidizing agent is positive, though weak, and that there is little correlation between the two traits at the level of quantitative trait loci (QTLs). The sign of the life span-resistance correlation is haplotype-specific, suggesting a high degree of statistical interaction and dependence on genetic background. Because of the genotype-specificity in the relationship between life span and resistance phenotypes, interventions to extend life span by overexpression of antioxidant enzymes are likely to produce strain-specific results. These observations are in general agreement with the "genetic rescue" hypothesis of Sohal et al. [Sohal, R.S., Mockett, R.J., Orr, W.C., 2002. Mechanisms of aging: an appraisal of the oxidative stress hypothesis. Free Radic. Biol. Med. 33, 575-586.], though we emphasize that such statistical interaction is a normal feature of standing genetic variation, and does not imply that some genotypes are pathological. Ad hoc observation of spontaneous flying activity 5 days after emergence proved to be a much better predictor of life span than resistance to an exogenous oxidant in these populations.

The relationship between life span and adult body size is highly strain-specific in Drosophila melanogaster.

Among mammals, body size and life span tend to vary inversely within species, but the pattern is less clear in invertebrates. Here, we report on survival and weight of male flies from 29 laboratory strains of Drosophila melanogaster. Natural variation in body mass was enhanced by rearing larvae under normal and limited food conditions. Strain, weight, and larval treatment have significant effects on survival, but higher order interactions are also significant, indicating strain specificity. For pooled data the overall relationship between mass and life span is slight, positive, and statistically significant, but mass explains < or =1% of the variation in survival. This result is opposite to the common prediction of an inverse relationship between longevity and body size. Effects of artificially reduced body size vary substantially in both sign and magnitude from strain to strain, though long-lived strains generally retain their enhanced survival characteristics. Within-line regressions of life span on mass also vary dramatically from strain to strain; in Canton-S, the most extreme case, mass explains >40% of the variation in survival. For long-lived 'O' lines reared under normal larval conditions, smaller flies live 16% longer than larger flies; the latter are significantly underrepresented in the most advanced age class. We conclude that the body size-life span relationship is highly strain-specific; that inconsistencies in the literature probably reflect real biological variation; and that variation in body size can be a significant source of experimental noise in survival studies.

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.

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.

Testing the "rate of living" model: further evidence that longevity and metabolic rate are not inversely correlated in Drosophila melanogaster.

In a recent study examining the relationship between longevity and metabolism in a large number of recombinant inbred Drosophila melanogaster lines, we found no indication of the inverse relationship between longevity and metabolic rate that one would expect under the classical "rate of living" model. A potential limitation in generalizing from that study is that it was conducted on experimental material derived from a single set of parental strains originally developed over 20 years ago. To determine whether the observations made with those lines are characteristic of the species, we studied metabolic rates and longevities in a second, independently derived set of recombinant inbred lines. We found no correlation in these lines between metabolic rate and longevity, indicating that the ability to both maintain a normal metabolic rate and have extended longevity may apply to D. melanogaster in general. To determine how closely our measurements reflect metabolic rates of flies maintained under conditions of life span assays, we used long-term, flow-through metabolic rate measurements and closed system respirometry to examine the effects of variables such as time of day, feeding state, fly density, mobility of the flies, and nitrogen knockout on D. melanogaster metabolic rate. We found that CO2 production estimated in individual flies accurately reflects metabolic rates of flies under the conditions used for longevity assays.

Stress resistance declines with age: analysis of data from a survival experiment with Drosophila melanogaster.

An approach towards analyzing survivorship data is proposed for the study of changes in stress resistance with age in the population of Drosophila melanogaster. This is based on the model of heterogeneous mortality (frailty model). Results of the data analysis show that observed populations of flies are heterogeneous and the accelerated selection, debilitative effect and changes in individual frailties are the aftermath of stress. These results also reveal that debilitative effect and accelerated selection are much better pronounced in survivals of flies that are stressed at an older age. Mild stress, when applied at both ages, produced a reduction in frailty variance. Stress of greater magnitude produced higher frailty variance in the young-treated flies. Among the old-treated insects, stress of longer duration led to a reduction of both the mean and the variance of frailty distribution. Population of young-treated flies became more heterogeneous, population of old-treated flies became less heterogeneous, and both populations became more robust in average after stress.

Lack of correlation between body mass and metabolic rate in Drosophila melanogaster.

We examined the association between body mass and metabolic rate in Drosophila melanogaster under a variety of conditions. These included comparisons of body mass and metabolic rate in flies from different laboratory lines measured at different ages, over different metabolic sampling periods, and comparisons using wet versus dry mass data. In addition, the relationship between body mass and metabolic rate was determined for flies recently collected from wild populations. In no case was there a significant correlation between body mass and metabolic rate. These results indicate that care must be taken when attempting to account for the effects of body mass on metabolic rate. Expressing such data in mass-specific units may be an inappropriate method of attempting to control for the effects of differences in body mass.

Selected contribution: long-lived Drosophila melanogaster lines exhibit normal metabolic rates.

The use of model organisms, such as Drosophila melanogaster, provides a powerful method for studying mechanisms of aging. Here we report on a large set of recombinant inbred (RI) D. melanogaster lines that exhibit approximately a fivefold range of average adult longevities. Understanding the factors responsible for the differences in longevity, particularly the characteristics of the longest-lived lines, can provide fundamental insights into the mechanistic correlates of aging. In ectothermic organisms, longevity is often inversely correlated with metabolic rate, suggesting the a priori hypothesis that long-lived lines will have low resting metabolic rates. We conducted approximately 6000 measurements of CO2 production in individual male flies aged 5, 16, 29, and 47 days postemergence and simultaneously measured the weight of individual flies and life spans in populations of each line. Even though there was a wide range of longevities, there was no evidence of an inverse relationship between the variables. The increased longevity of long-lived lines is not mediated through reduction of metabolic activity. In Drosophila, it is possible to both maintain a normal metabolic rate and achieve long life. These results are evaluated in the context of 100 years of research on the relationship between metabolic rate and life span.

Lifespan, QTLs, age-specificity, and pleiotropy in Drosophila.

We have constructed a set of 120 recombinant inbred lines for use in studies of the genetics of lifespan in Drosophila. The lines are derived from Luckinbill and Clare's (Heredity 55 (1985) 9) artificial selection experiment for increased lifespan. Inbred lines retain the relative lifespan characteristics of the experimental and control stocks from which they are derived. Mapping experiments suggest that a small number of QTLs accounts for much of the selection response. The age-specificity of genetic effects is best visualized in three-dimensional QTL maps of age-specific mortality. QTLs are shared by males and females, and have effects on age-specific mortality that are positively correlated across ages, with different times of onset. There is evidence for positively correlated pleiotropic effects of lifespan QTLs on mid-life fertility and resistance to an oxidizing chemical, and a striking lack of evidence for negative pleiotropy.