Changes in genetic architecture during relaxation in Drosophila melanogaster selected on divergent virgin life span

Artificial selection experiments often confer important information on the genetic correlations constraining the evolution of life history. After artificial selection has ceased however, selection pressures in the culture environment can change the correlation matrix again. Here, we reinvestigate direct and correlated responses in a set of lines of Drosophila melanogaster that were selected on virgin life span and for which selection has been relaxed for 10 years. The decrease in progeny production in long-lived lines, a strong indication of antagonistic pleiotropy, had disappeared during relaxation. This was associated with a higher cost of reproduction to long-lived flies in mated, but not in virgin life span. These data strongly suggest that genetic mechanisms of mated and virgin life span determination are partly independent. Furthermore, data on body weight, developmental time and viability indicated deleterious effects of longevity selection in either direction, giving rise to a nonlinear relationship with life span for these characters. In order to reclaim original patterns, we founded a new set of derived lines by resuming selection in mixed replicate lines of the original set. Although selection was successful, most patterns in correlated characters remained, showing that these new patterns are resistant to new episodes of selection.     

Ethanol extract of Cassia siamea L. increases life span in Drosophila melanogaster

The stem of Cassia siamea L. (Fabaceae) has been used in traditional Thai medicine as a longevity remedy. The objective of this study was to investigate the effect of ethanolic stem extract of C. siamea (CSE) on the life span of Drosophila melanogaster. The results showed that a diet containing 10 mg/mL CSE could significantly extend the mean life span of D. melanogaster by 14% compared with the control diet (P < 0.01). The maximum life span was 74, 78, and 84 days in control, CSE (5 mg/mL) and CSE (10 mg/mL) groups, respectively. Supplementation of CSE at 10 mg/mL also significantly increases the activity of superoxide dismutase (SOD) and catalase (CAT) at days 25 and 40 compared with the control diet. Treatment of CSE at 5 and 10 mg/mL significantly increased the climbing ability of D. melanogaster both on days 25 and 40 compared with the control flies. Paraquat and H₂O₂ challenge test showed that flies fed with CSE at 10 mg/mL had a longer survival time than the control flies (P < 0.01). This study provides supportive evidence that supplementation with CSE prolonged life span and reduced oxidative stress in D. melanogaster.     

Trade-off of energy metabolites as well as body color phenotypes for starvation and desiccation resistance in montane populations of Drosophila melanogaster

Storage of energy metabolites has been investigated in different sets of laboratory selected desiccation or starvation resistant lines but few studies have examined such changes in wild-caught populations of Drosophila melanogaster. In contrast to parallel selection of desiccation and starvation tolerance under laboratory selection experiments, opposite clines were observed in wild populations of D. melanogaster. If resistance to desiccation and starvation occurs in opposite directions under field conditions, we may expect a trade-off for energy metabolites but such correlated changes are largely unknown. We tested whether there is a trade-off for storage as well as actual utilization of carbohydrates (trehalose and glycogen), lipids and proteins in D. melanogaster populations collected from different altitudes (512-2500 m). For desiccation resistance, darker flies (> 50% body melanization) store more body water content and endure greater loss of water (higher dehydration tolerance) as compared to lighter flies (< 30% body melanization). Based on within population analysis, we found evidence for coadapted phenotypes i.e. darker flies store and actually utilize more carbohydrates to confer greater desiccation resistance. In contrast, higher starvation resistance in lighter flies is associated with storage and actual utilization of greater lipid amount. However, darker and lighter flies did not vary in the rate of utilization of carbohydrates under desiccation stress; and of lipids under starvation stress. Thus, we did not find support for the hypothesis that a lower rate of utilization of energy metabolites may contribute to greater stress resistance. Further, for increased desiccation resistance of darker flies, about two-third of total energy budget is provided by carbohydrates. By contrast, lighter flies derive about 66% of total energy content from lipids which sustain higher starvation tolerance. Our results support evolutionary trade-off for storage as well as utilization of energy metabolites for desiccation versus starvation resistance in D. melanogaster.     

The genetic basis for mating-induced sex differences in starvation resistance in Drosophila melanogaster

Multiple genetic and environmental factors interact to influence starvation resistance, which is an important determinant of fitness in many organisms, including Drosophila melanogaster. Recent studies have revealed that mating can alter starvation resistance in female D. melanogaster, but little is known about the behavioral and physiological mechanisms underlying such mating-mediated changes in starvation resistance. In the present study, we first investigated whether the effect of mating on starvation resistance is sex-specific in D. melanogaster. As indicated by a significant sex × mating status interaction, mating increased starvation resistance in females but not in males. In female D. melanogaster, post-mating increase in starvation resistance was mainly attributed to increases in food intake and in the level of lipid storage relative to lean body weight. We then performed quantitative genetic analysis to estimate the proportion of the total phenotypic variance attributable to genetic differences (i.e., heritability) for starvation resistance in mated male and female D. melanogaster. The narrow-sense heritability (h²) of starvation resistance was 0.235 and 0.155 for males and females, respectively. Mated females were more resistant to starvation than males in all genotypes, but the degree of such sexual dimorphism varied substantially among genotypes, as indicated by a significant sex × genotype interaction for starvation resistance. Cross-sex genetic correlation was greater than 0 but less than l for starvation resistance, implying that the genetic architecture of this trait was partially shared between the two sexes. For both sexes, starvation resistance was positively correlated with longevity and lipid storage at genetic level. The present study suggests that sex differences in starvation resistance depend on mating status and have a genetic basis in D. melanogaster.     

Seasonal trade-off between starvation resistance and cold resistance in temperate wild-caught Drosophila simulans

Abstract  Selection experiments with Drosophila have revealed a possible evolutionary trade-off between cold resistance and starvation resistance that may be controlled by lipid metabolism. To test this trade-off in naturally occurring Drosophila simulans populations, flies were simultaneously collected from two temperate locales experiencing contrasting seasons. Flies from a tropical locale served as a control. Cold coma recovery, starvation resistance and lipid proportion were assayed on adult males and females from each locale. Compared with the summer-collected flies from Canberra, the winter-collected flies from San Diego recovered from cold coma more quickly, were less starvation resistant and had lower lipid levels. These results support an evolutionary trade-off between cold resistance and starvation resistance. Combined, these data also suggest that differences in lipid metabolism may be an underlying mechanism for this trade-off.     

Evidence for a robust sex-specific trade-off between cold resistance and starvation resistance in Drosophila melanogaster

In insects changes in lipid metabolism may underlie a trade-off between cold resistance and starvation resistance. To test this we examined correlated responses in independent sets of Drosophila melanogaster lines selected for increased cold resistance and increased starvation resistance. The starvation lines showed correlated patterns found in other D. melanogaster populations selected for this trait, including higher lipid levels and increased resistance to desiccation, although the selected lines did not show a longer development time as found in some other studies. Consistent with the trade-off hypothesis, selected lines with increased starvation resistance showed decreased resistance to a cold stress as measured by mortality, whereas selected lines with increased cold resistance showed a decrease in starvation resistance. To counter the possibility of inadvertent selection accounting for these patterns, selected and control lines from both selection regimes were crossed to form mass bred populations, which were left for four generations prior to establishing isofemale lines. By scoring starvation and cold resistance in these lines derived from both sets of selection regimes, we confirmed the negative association between resistance to these stresses in females but not in males. Potential implications of this trade-off for surviving cold conditions when food resources are limiting are discussed.     

Within-population variation in cytoplasmic genes affects female life span and aging in Drosophila melanogaster

It has been suggested that mitochondrial DNA (mtDNA) may play an important role in aging. Yet, few empirical studies have tested this hypothesis, partly because the degree of sequence polymorphism in mtDNA is assumed to be low. However, low sequence variation may not necessarily translate into low phenotypic variation. Here, we report an experiment that tests whether there is within-population variation in cytoplasmic genes for female longevity and senescence. To achieve this, we randomly selected 25 "mitochondrial founders" from a single, panmictic population of Drosophila melanogaster and used these founders to generate distinct "mt" lines in which we controlled for the nuclear background by successive backcrossing. Potential confounding effects of cytoplasmically transmitted bacteria were eliminated by tetracycline treatment. The mt lines were then assayed for differences in longevity, Gompertz intercept (frailty), and demographic rate of change in mortality with age (rate-of-senescence) in females. We found significant cytoplasmic effects on all three variables. This provides evidence that genetic variation in cytoplasmic genes, presumably mtDNA, contributes to variation in female mortality and aging.     

The effects of catalase gene overexpression on life span and resistance to oxidative stress in transgenic Drosophila melanogaster.

Oxygen free radicals and hydroperoxides have been postulated to play a causal role in the aging process, implying that antioxidant enzymes may act as longevity determinants. Catalase (H2O2:H2O2 oxidoreductase; EC1.11.1.6) is the sole enzyme involved in the elimination of H2O2 in Drosophila melanogaster; glutathione peroxidase being absent. A genomic fragment containing the Drosophila catalase gene was used to construct transgenic Drosophila lines by means of P element-mediated transformation. Enhanced levels of catalase (up to 80%) did not prolong the life span of flies, nor did they provide improved protection against oxidative stress induced by hyperoxia or paraquat treatment. However, enhanced resistance to hydrogen peroxide was observed in the overexpressors.     

Stress specific correlated responses in fat content, Hsp70 and dopamine levels in Drosophila melanogaster selected for resistance to environmental stress

Studies of adaptation to stressful environments have frequently encountered cross resistance. This has prompted the hypothesis that certain adaptations confer resistance to multiple stressors. Some of the genes and mechanisms conferring stress resistance have been identified, however, the generality and basis of stress adaptation and cross resistance is still unclear. We investigated several physiological traits that have been previously linked to increased stress resistance: Hsp70 expression, fat content and dopamine levels. Additionally, we studied a behavioural trait, locomotor activity, as a proxy for the physiological state of the organisms. Physiology is the mechanistic link between resistance phenotype and underlying genetic background, and provides insights into the background and generality of cross resistance and correlated responses to selection for stress resistance. We assessed the relationship between the measured traits and stress resistance in a set of lines selected for increased resistance to several environmental stressors. We found that, although all physiological traits displayed significant differentiation among selection regimes, none were consistently associated with increased general stress resistance. This demonstrates that directional changes in Hsp70 expression level, dopamine level and fat content occur in response to the specific requirements of the different stress regimes, rather than as a general response to stress.     

Host life span and the evolution of resistance characteristics

There is a wide variety of resistance mechanisms that hosts may evolve in response to their parasites. These can be functionally classified as avoidance (lower probability of becoming infected), recovery (faster rate of clearance), tolerance (reduced death rate when infected), or acquired immunity. It is commonly thought that longer lived organisms should invest more in costly resistance. We show that due to epidemiological feedbacks the situation is often more complex. Using evolutionary theory we examine how the optimal investment in costly resistance varies with life span in a broad range of scenarios. In the absence of acquired immunity, longer lived populations do generally invest more in resistance. If hosts have acquired immunity, the optimal resistance may either increase or decrease with increasing life span. In addition, there may be evolutionary bistability with high and low investments in avoidance or tolerance. The optimal investment in the duration of acquired immunity always increases with life span, and due to bistability, shorter lived hosts may commonly not evolve any immunity. In contrast, the optimal investment in the probability of acquiring immunity initially increases and then decreases with life span. Our results have important implications for the evolution of invertebrate and vertebrate immunity, and for the evolution of acquired immunity itself.     

Juvenile hormone as a regulator of the trade-off between reproduction and life span in Drosophila melanogaster

Trade-offs between reproduction and life span are ubiquitous, but little is known about their underlying mechanisms. Here we combine treatment with the juvenile hormone analog (JHa) methoprene and experimental evolution in Drosophila melanogaster to study the potential role of juvenile hormone (JH) in mediating such trade-offs at both the physiological and evolutionary level. Exposure to JHa in the larval medium (and up to 24 h posteclosion) increased early life fecundity but reduced life span of normal (unselected) flies, supporting the physiological role of JH in mediating the trade-off. This effect was much smaller for life span, and not detectable for fecundity, in fly lines previously bred for 19 generations on a medium containing JHa. Furthermore, these selection lines lived longer than unselected controls even in the absence of JHa treatment, without a detectable reduction in early life fecundity. Thus, selection for resistance to JHa apparently induced some evolutionary changes in JH metabolism or signaling, which led to longer life span as a correlated response. This supports the hypothesis that JH may mediate evolution of longer life span, but--contrary to our expectation-this apparently does not need to trade--off with fecundity.     

Evolution of starvation resistance in Drosophila melanogaster: measurement of direct and correlated responses to artificial selection

Laboratory selection for resistance to starvation has been conducted under relatively controlled conditions to investigate direct and correlated responses to artificial selection. With regard to starvation resistance, there are three physiological routes by which the trait can evolve: resource accumulation, energy conservation and starvation tolerance. A majority of energetic compounds and macromolecules including triglycerides, trehalose and other sugars, and soluble protein increased in abundance as a result of selection. Movement was additionally investigated with selected males moving less than control males and selected females exhibiting a similar response to selection. Results obtained from this study supported two of the possible evolutionary mechanisms for adaptation to starvation: energy compound storage and conservation. If the response to selection is based on an evolutionarily conserved pattern of genetic correlations (elevated lipid, elevated sugars and reduced movement), then the response to selection is medically relevant and the genetic architecture should be investigated in depth.     

Starvation resistance and adult body composition in a latitudinal cline of Drosophila melanogaster

Abstract Latitudinal geographic variation in Drosophila melanogaster is pervasive. Parallel clines in traits such as body size, egg size, ovariole number, and development time have been found on several continents throughout the world. However, a cline in starvation resistance and fat content in D. melanogaster has so far been found only in India. Here we investigate starvation resistance and fat content in 10 populations from South America, in which clines in body size, egg size, and development time have previously been found. We find no evidence for a cline in starvation resistance or fat content in South America. We therefore suggest that the cline in starvation resistance in India may have evolved in response to specific climatic variation found only in India.     

Carbon and nitrogen content based estimation of the fat content of animal carcasses in various species

The objective of this study was to explore whether the C and N content can be used to estimate the fat content of animal carcasses. Considering the mean C and N contents of body fat and body protein, the fat content (EE) [%] can be predicted from C and N values [%] according to the generally valid equation EE=1.3038·C – 4.237·N. The application of this equation to estimate the total fat content of all animal carcasses results in significant differences in fat content between predicted and measured values. Therefore, we derived specific equations for rats, pigs, cattle, sheep, broilers and mice to predict the fat content by dual linear regression analysis (y=EE [% DM], x₁=C [% DM], x₂=N [% DM]) based on measured fat, C, and N contents of animal body samples. The specific equations for different animals showed residual standard deviations of 1.55, 1.63, 1.12, 1.35, 1.85 and 0.92% fat for rats, pigs, cattle, sheep, broilers and mice, respectively.     

Multitrait evolution in lines of Drosophila melanogaster selected for increased starvation resistance: the role of metabolic rate and implications for the evolution of longevity

Starvation resistance is a trait often associated with longevity. Animals with increased longevity frequently show elevated starvation resistance and vice versa. Consequently, both life-history traits are thought to share genetic and physiological mechanisms, such as increased fat content and lowered metabolic rate. Here, we present results from 20 generations of selection on Drosophila melanogaster for increased starvation resistance at the time of adult eclosion. We observe that starvation resistance can be the result of more than one mechanism, all associated with an increase in fat resources. In general, metabolic rate is lowered under starved conditions relative to fed conditions. Metabolic rate in the starvation resistant lines is generally higher than in control lines under starved conditions. Starvation resistant flies are able to sustain a higher metabolic rate for a longer period of time when food is unavailable. This implies depletion of the increased fat reserves. However, longevity was not consistently affected by selection for increased starvation resistance. Similarly, paraquat resistance differed between selection lines and did not associate with starvation resistance, but rather with longevity. The results are discussed in relation to previous reported results on starvation resistance and its relation with mechanisms of aging and longevity.     

Genetic analysis of extended life span in Drosophila melanogaster I. RAPD screen for genetic divergence between selected and control lines

Using lines selected for long life by Luckinbill and his co-workers, we screened two selected and two control lines for allelic frequency differences at 1200 randomly chosen RAPD marker loci. Twenty-three marker loci showed frequency differences in excess of 80%, and five were greater than 90%. Age-specific effects of the five most differentiated loci were estimated by collecting complete survival data in segregating backcross populations. Alleles at four of the five marker loci were associated with significant extension of life span in males, while two marker loci had significant effects in females. Eighty percent of the total selection response in males can be explained by the identified QTL's, under the assumption of additivity. The N14+ marker allele accounted for a 12-day life span extension in males, but had little effect in females. Both sex-limited and sex-shared effects were observed. Analysis of age-specific mortality rates suggests that life span extension occurs by a combination of genetic factors that moderate both the level of mortality and the rate at which mortality increases with age. This revised version was published online in July 2006 with corrections to the Cover Date.     

Consistent effects of a major QTL for thermal resistance in field-released Drosophila melanogaster

Molecular genetic markers can be used to identify quantitative trait loci (QTL) for thermal resistance and this has allowed characterization of a major QTL for knockdown resistance to high temperature in Drosophila melanogaster. The QTL showed trade-off associations with cold resistance under laboratory conditions. However, assays of thermal tolerance conducted in the laboratory may not necessarily reflect performance at varying temperatures in the field. Here we tested if lines with different genotypes in this QTL show different thermal performance under high and low temperatures in the field using a release recapture assay. We found that lines carrying the QTL genotype for high thermal tolerance were significantly better at locating resources in the field releases under hot temperatures while the QTL line carrying the contrasting genotype were superior at cold temperatures. Further, we studied copulatory success between the different QTL genotypes at different temperatures. We found higher copulatory success in males of the high tolerance QTL genotype under hot temperature conditions, while there was no difference in females at cold temperatures. The results allow relating components of field fitness at different environmental temperatures with genotypic variation in a QTL for thermal tolerance.     

Effects of mating delay and nutritional signals on resource recycling in a cyclically breeding cockroach

Ovarian apoptosis has been found to occur in the female cockroach Nauphoeta cinerea in response to lack of mates. It has been proposed that ovarian apoptosis in continuously breeding insects is an adaptive mechanism for recouping resources in poor conditions (oosorption). However, N. cinerea is a cyclically breeding insect and ovarian apoptosis may represent ageing and clearance of old unused oocytes. To test the hypothesis that oocyte resorption via apoptosis reflects the reclamation of resources, we delayed mating in combination with positive and negative nutritional signals. Females without access to food during sexual maturation invested less in reproduction and had elevated rates of ovarian apoptosis in the terminal oocyte. Starvation also induced apoptosis in non-vitellogenic oocytes of the vitellarium and germinarium, which would be used for future reproductive events. This is paradoxical as theory states that oosorption is an adaptive means of rerouting resources into investment in future reproduction, yet these oocytes do not represent a cache of resources and their loss could limit future reproduction.     

Effects of starvation and desiccation on energy metabolism in desert and mesic Drosophila

Energy availability can limit the ability of organisms to survive under stressful conditions. In Drosophila, laboratory experiments have revealed that energy storage patterns differ between populations selected for desiccation and starvation. This suggests that flies may use different sources of energy when exposed to these stresses, but the actual substrates used have not been examined. We measured lipid, carbohydrate, and protein content in 16 Drosophila species from arid and mesic habitats. In five species, we measured the rate at which each substrate was metabolized under starvation or desiccation stress. Rates of lipid and protein metabolism were similar during starvation and desiccation, but carbohydrate metabolism was several-fold higher during desiccation. Thus, total energy consumption was lower in starved flies than desiccated ones. Cactophilic Drosophila did not have greater initial amounts of reserves than mesic species, but may have lower metabolic rates that contribute to stress resistance.