DIRECT AND CORRELATED RESPONSES TO SELECTION ON AGE AT REPRODUCTION IN DROSOPHILA MELANOGASTER

Aging may be a consequence of mutation accumulation or of negative pleiotropic correlations between performance late and earlier in the lifespan. This study used artificial selection on flies derived from two different base stocks to produce “young” and “old” lines, propagated by breeding from young and old adults respectively. Virgin and mated adults of both sexes from the “old” lines lived longer than “young” line flies. “Young” and “old” mated females did not differ in fecundity or fertility early in the lifespan, but “old” line females had higher fecundity and fertility late in life. The results therefore suggested either that the response to selection had revealed the effect of mutation accumulation, or that pleiotropy involving characters other than early fecundity must have been involved. Development time from egg to adult was longer in the “old” lines. Competition of selected line larvae from one base stock against mutant marked larvae from the same base stock revealed that, at a wide range of larval densities, “old” line larvae showed lower survival rates than “young” line larvae. Thorax length and wet weight were significantly greater in the “old” line flies from one base stock. The results may imply that the selection regime in the “old” lines favored extended growth during development to produce a more durable adult soma, despite the cost in increased larval mortality and delayed reproduction, because the potential reproductive benefits later in life were increased. However, the differences between larvae from “old” and “young” lines could also be attributable to density differences, and this possibility needs systematic investigation.     

SELECTION FOR KNOCKDOWN RESISTANCE TO HEAT IN DROSOPHILA MELANOGASTER AT HIGH AND LOW LARVAL DENSITIES

Responses to short-term selection for knockdown resistance to heat (37°C) in Drosophila melanogaster reared under stressful (high larval density) and nonstressful (low larval density) conditions were compared. No difference in selection response between density treatments was found. A test of heat resistance (39°C) after pretreatment (37°C) did not reveal an increase in survival for selected lines as compared to controls. Flies reared at high density had higher knockdown resistance throughout the experiment. Resistance to heat was not associated with body size.     

RESPONSES AND CORRELATED RESPONSES TO ARTIFICIAL SELECTION ON THORAX LENGTH IN DROSOPHILA MELANOGASTER

Two sets of four replicate lines of Drosophila melanogaster were selected for large and small thorax with controls. F, progeny of crosses between the selected lines within each size category showed (a) a reduction in preadult viability in large lines relative to control and small lines when they were cultured at medium or high density in competition with a standard mutant marked competitor stock, and (b) an increase in larval development time in large lines relative to control and small lines. Natural selection for increased body size in adults may therefore be opposed by adverse effects on larval viability. The results are discussed in terms of the developmental mechanisms probably responsible for the change in body size. The preadult survival of the large and control lines was measured at three different temperatures, and there was no evidence for a significant interaction between size and temperature. The observed evolutionary increase in body size in response to reduced temperature in Drosophila must therefore involve either different genes from those subject to selection for size at a single temperature, or a fitness component other than preadult survival. There was no significant asymmetry in response to selection, and thorax length showed heterosis in crosses between the selected lines.     

THE RESPONSES OF DROSOPHILA MELANOGASTER TO ARTIFICIAL SELECTION ON BODY WEIGHT AND ITS PHENOTYPIC PLASTICITY IN TWO LARVAL FOOD ENVIRONMENTS

To investigate the potential response to natural selection of reaction norms for age and size at maturity, fresh body weight at eclosion was mass selected under rich and poor larval food conditions in Drosophila melanogaster. The sensitivity of dry weight at eclosion to the difference between rich and poor larval food was selected using differences in sensitivities among families. For both experiments, the correlated response to selection of age at eclosion was examined. The flies were derived from wild populations and had been mass cultured in the lab for more than six months before the experiments started. These flies responded to selection on body weight upwards and downwards on both rich and poor larval food. Selection on increased or decreased sensitivity of body weight was also successful in at least one direction. Sensitivity was reduced by selection upwards in a poor environment and downwards in a rich environment.     

DIRECT SELECTION ON LIFE SPAN IN DROSOPHILA MELANOGASTER

An important issue in the study of the evolution of aging in Drosophila melanogaster is whether decreased early fecundity is inextricably coupled with increased life span in selection experiments on age at reproduction. Here, this problem has been tackled using an experimental design in which selection is applied directly to longevity. Selection appeared successful for short and long life, in females as well as males. Progeny production of females selected for long life was lower than for short-lived females throughout their whole life. No increase of late-life reproduction in long-lived females occurred, as has been found in selection experiments on age at reproduction. This discrepancy is explained in terms of the inadequacy of the latter design to separate selection on life span from selection on late-life fecundity. Moreover, starvation resistance and fat content were lower for adults selected for short life. In general, the data support the negative-pleiotropy–disposable-soma theory of aging, and it is hypothesized that the pleiotropic allocation of resources to maintenance versus to reproduction as implicated in the theory might involve lipid metabolism. It is argued that further research on this suggestion is urgent and should certainly comprise observations on male reproduction because these are for the greater part still lacking. In conclusion, the longevity of D. melanogaster can be genetically altered in a direct-selection design, and such an increase is accompanied by a decreased general reproduction and thus early reproduction.     

ARTIFICIAL SELECTION FOR DEVELOPMENTAL TIME IN DROSOPHILA MELANOGASTER IN RELATION TO THE EVOLUTION OF AGING: DIRECT AND CORRELATED RESPONSES

A wild-type strain of Drosophila melanogaster was successfully selected for both fast and slow larval development. The realized heritabilities (h²) ranged from 0.20 to 0.30 for the fast lines and 0.35 to 0.60 for the slow lines. The selection applied is relevant in relation to the evolution of aging. The longevity of adults, either virgin or mated, was not affected by selection for developmental time, indicating that developmental time is not a causal determinant of life span, thus confirming the results of the studies on environmental effects on aging (Zwaan et al. 1991, 1992). However, adult body weights were higher in the slow developmental lines and lower in the fast lines, relative to the control flies. Furthermore, slow females showed relatively high early fecundity and low late fecundity, as compared with control and fast females. Mated longevities and total lifetime progeny productions were not statistically different. Previous results obtained by other authors from selection experiments on age at reproduction either supported the mutation accumulation or the negative pleiotropy theory of aging (Luckinbill et al. 1984; Rose 1984b). The impact of the reported results on the interpretation of these studies is discussed, and it is noted that direct selection on adult longevity is needed to settle this issue.     

THE EVOLUTION OF DEVELOPMENT IN DROSOPHILA MELANOGASTER SELECTED FOR POSTPONED SENESCENCE

The role of development in the evolution of postponed senescence is poorly understood despite the existence of a major gerontological theory connecting developmental rate to aging. We investigate the role of developmental rate in the laboratory evolution of aging using 24 distinct populations of Drosophila melanogaster. We have found a significant difference between the larval developmental rates of our Drosophila stocks selected for early (B) and late-life (O) fertility. This larval developmental time difference of approximately 12% (O > B) has been stable for at least 5 yr, occurs under a wide variety of rearing conditions, responds to reverse selection, and is shown for two other O-like selection treatments. Emerging adults from lines with different larval developmental rates show no significant differences in weight at emergence, thorax length, or starvation resistance. Long-developing lines (O, CO, and CB) have greater survivorship from egg to pupa and from pupa to adult, with and without strong larval competition. Crosses between slower developing populations, and a variety of other lines of evidence, indicate that neither mutation accumulation nor inbreeding depression are responsible for the extended development of our late-reproduced selection treatments. These results stand in striking contrast to other recent studies. We argue that inbreeding depression and inadvertent direct selection in other laboratories' culture regimes explain their results. We demonstrate antagonistic pleiotropy between developmental rate and preadult viability. The absence of any correlation between longevity and developmental time in our stocks refutes the developmental theory of aging.     

EVOLUTION OF STARVATION RESISTANCE IN DROSOPHILA MELANOGASTER: ASPECTS OF METABOLISM AND COUNTER-IMPACT SELECTION

An artificial selection experiment for increased female starvation resistance employed five selected lines and five control lines of Drosophila melanogaster. Females responded to selection within the first five generations, but a substantial male response was not observed until starvation resistance was assessed at generation 15. By measuring respiration rate in selected and control lines, it was possible to test the hypothesis that reduced metabolic rate is a general mechanism for stress resistance. There was no association between starvation resistance and respiration rate and thus no support for the hypothesis. Studies using vertebrates have shown that starvation causes a decrease in intermediary metabolism enzyme activity, but this relationship is not well documented in invertebrates. In the present study, intermediary metabolism enzyme activities decreased in response to starvation in control-line females and males, and in selected-line males. However, the selected females showed no overall decrease in enzyme activities in response to starvation. One interpretation is that selected females evolved to resist the phenotypic impact of stress. The concept of “counter-impact selection” is discussed in relationship to the use of phenotypic manipulations for the study of evolution.     

THE EVOLUTIONARY GENETICS OF MALE LIFE-HISTORY CHARACTERS IN DROSOPHILA MELANOGASTER

Alternative models of the maintenance of genetic variability, theories of life-history evolution, and theories of sexual selection and mate choice can be tested by measuring additive and nonadditive genetic variances of components of fitness. A quantitative genetic breeding design was used to produce estimates of genetic variances for male life-history traits in Drosophila melanogaster. Additive genetic covariances and correlations between traits were also estimated. Flies from a large, outbred, laboratory population were assayed for age-specific competitive mating ability, age-specific survivorship, body mass, and fertility. Variance-component analysis then allowed the decomposition of phenotypic variation into components associated with additive genetic, nonadditive genetic, and environmental variability. A comparison of dominance and additive components of genetic variation provides little support for an important role for balancing selection in maintaining genetic variance in this suite of traits. The results provide support for the mutation-accumulation theory, but not the antagonistic-pleiotropy theory of senescence. No evidence is found for the positive genetic correlations between mating success and offspring quality or quantity that are predicted by “good genes” models of sexual selection. Additive genetic coefficients of variation for life-history characters are larger than those for body weight. Finally, this set of male life-history characters exhibits a very low correspondence between estimates of genetic and phenotypic correlations.     

果蝇伴性隐性致死突变试验中芥子碱的辐射保护作用

过去的工作证明一些十字花科植物的抗辐射性与其体内存在的天然辐射保护物质有关。芥子碱是存在于十字花科植物中的一种天然辐射保护物质。它对大麦和小麦以及小鼠均有辐射保护作用。本文报道芥子碱在果蝇伴性隐性致死(SLRL)突变试验系统中的辐射保护作用。供试果蝇并不拒食芥子碱水溶液。10 mg/ml芥子碱对果蝇无生理毒性,对生育力亦无不良影响。40 Gy X 射线诱发8.96% SLRL 突变。而40 Gy 辐照前喂10 mg/ml芥子碱的果蝇其SLRL 突变率降为0.4%。Oregon K 果蝇的自发突变率为0-0.4%。可见喂芥子碱后辐射诱发的SLRL 突变不复出现,说明芥子碱对果蝇有很强的辐射保护作用。也讨论了芥子碱作为潜在抗癌物质的可能性。     

DIRECTIONAL AND STABILIZING DENSITY-DEPENDENT NATURAL SELECTION FOR PUPATION HEIGHT IN DROSOPHILA MELANOGASTER

Six populations of Drosophila melanogaster have been kept at extreme population densities, three high and three low, for 175 generations. Larvae from the high density populations pupate 50%-100% higher than larvae from the low density populations. At high larval test densities there is both a directional and a stabilizing component to selection, with viabilities ranging from 0.14 to 0.992, depending on the choice of pupation site. The directional component is stronger on the populations which have evolved at low densities, while the stabilizing component is stronger on the populations which have evolved at high densities. There is no indication that the evolution of this trait, in response to density, has altered its phenotypic plasticity.     

COMPLEX TRADE-OFFS AND THE EVOLUTION OF STARVATION RESISTANCE IN DROSOPHILA MELANOGASTER

The measurement of trade-offs may be complicated when selection exploits multiple avenues of adaptation or multiple life-cycle stages. We surveyed 10 populations of Drosophila melanogaster selected for increased resistance to starvation for 60 generations, their paired controls, and their mutual ancestors (a total of 30 outbred populations) for evidence of physiological and life-history trade-offs that span life-cycle stages. The directly selected lines showed an impressive response to starvation selection, with mature adult females resisting starvation death 4–6 times longer than unselected controls or ancestors—up to a maximum of almost 20 days. Starvation-selected flies are already 80% more resistant to starvation death than their controls immediately upon eclosion, suggesting that a significant portion of their selection response was owing to preadult growth and acquisition of metabolites relevant to the stress. These same lines exhibited significantly longer development and lower viability in the larval and pupal stages. Weight and lipid measurements on one of the starvation-selected treatments (SB_1–5), its control populations (CB_1–5), and their ancestor populations (B_1–5) revealed three important findings. First, starvation resistance and lipid content were linearly correlated; second, larval lipid acquisition played a major role in the evolution of adult starvation resistance; finally, increased larval growth rate and lipid acquisition had a fitness cost exacted in reduced viability and slower development. This study implicates multiple life-cycle stages in the response to selection for the stress resistance of only one stage. Our starvation-selected populations illustrate a case that may be common in nature. Patterns of genetic correlation may prove misleading unless multiple pleiotropic interconnections are resolved.     

THE RESPONSE TO SELECTION FOR FAST LARVAL DEVELOPMENT IN DROSOPHILA MELANOGASTER AND ITS EFFECT ON ADULT WEIGHT: AN EXAMPLE OF A FITNESS TRADE-OFF

A selection experiment using Drosophila melanogaster revealed a strong trade-off between adult weight and larval development time (LDT), supporting the view that antagonistic pleiotropy for these two fitness traits determines mean adult size. Two experimental lines of flies were selected for a shorter LDT (measured from egg laying to pupation). After 15 generations LDT was reduced by an average of 7.9%. The response appeared to be controlled primarily by autosomal loci. A correlated response to the selection was a reduction in adult dry weight: individuals from the selected populations were on average 15.1% lighter than the controls. The lighter females of the selected lines showed a 35% drop in fecundity, but no change in longevity. Thus, there is no direct relationship between LDT and adult longevity. The genetic correlation between weight and LDT, as measured from their joint response to selection, was 0.86. Although there was weak evidence for dominance in LDT, there was none for weight, making it unlikely that selection acting on this antagonistic pleiotropy could lead to a stable polymorphism. In all lines, sex differences in weight violated expectations based on intrasex genetic correlations: Females, being larger than males, ought to require a longer LDT, whereas there was a slight trend in the opposite direction. Because the sexual dimorphism in size was not significantly altered by selection, it appears that the controlling loci are either invariant or have very limited pleiotropic effect on developmental time. It is suggested that they probably control some intrinsic, energy-intensive developmental process in males.     

EFFECTS ON FITNESS COMPONENTS OF P-ELEMENT INSERTS IN DROSOPHILA MELANOGASTER: ANALYSIS OF TRADE-OFFS

We analyzed the trade-offs between fitness components detected in four experiments in which traits were manipulated by inserting small (control) and large (treatment) P-elements into the Drosophila melanogaster genome. Treatment effects and the interactions of treatment with temperature, experiment, and line were caused by the greater length and different positions of the treatment insert. In inbred flies, the treatment decreased early and total fecundity. Whether it increased the lifespan of mated females depended upon adult density. Analysis of line-by-treatment-by-temperature interactions revealed hidden trade-offs that would have been missed by other methods. They included a significant trade-off between lifespan and early fecundity. At 25°C high early fecundity was associated with decreased reproductive rates and increased mortality rates 10–15 days later and persisting throughout life, but not at 29.5°C. Correlations with Gompertz coefficients suggested that flies that were heavier at eclosion also aged more slowly and that flies that aged more slowly had higher fecundity late in life at 25°C. The results support the view that lifespan trades off with fecundity and that late fecundity trades off with rate of aging in fruitflies. Genetic engineering is an independent method for the analysis of trade-offs that complements selection experiments.     

THE RATE AND EFFECTS DISTRIBUTION OF VIABILITY MUTATION IN DROSOPHILA: MINIMUM DISTANCE ESTIMATION

The empirical distribution of the mean viability of mutation accumulation lines, obtained from three published experiments, was analyzed using minimum-distance estimation. In two cases (Mukai et al. 1972; Ohnishi 1977), mutations were allowed to accumulate in copies of chromosome II protected from natural selection and recombination. In the other one (Fernández and López-Fanjul 1996), they accumulated in inbred lines derived from an isogenic stock. In contrast with currently accepted hypotheses, we consistently estimated low (about 0.01) genomic viability mutation rates, λ, and a small kurtosis of the distribution of mutational effects on viability (a) in the three datasets. Minimum-distance estimates of the per-generation mean viability change due to mutation (λE[a]) were also obtained. These were very similar for both chromosomal datasets, their absolute values being about five times smaller than estimates obtained from the observed change in mean viability during the mutation process. It must be noted that, in both experiments, viability was measured relative to the Cy chromosome of a Cy/Pm stock. Thus, an unnoticed viability increase in this Cy chromosome may have resulted in overestimation of the mean viability reduction in the lines. In parallel, minimum-distance estimation of λE(a) from inbred lines data (where the selective pressure during the accumulation process was larger) was even somewhat smaller, in absolute value, and very close to the estimate obtained by comparing the mean viability of the lines with that of the control isogenic line. The evolutionary importance of these results, as well as their relevance to the solution of the mutational load paradox, is discussed.     

POLYGENIC MUTATION IN DROSOPHILA MELANOGASTER: ESTIMATES FROM DIVERGENCE AMONG INBRED STRAINS

A highly inbred line of Drosophila melanogaster was subdivided into 25 replicate sublines, which were independently maintained for 100 generations with 10 pairs of unselected flies per generation. The polygenic mutation rate (V_M) for two quantitative traits, abdominal and sternopleural bristle number, was estimated from divergence among sublines at 10 generation intervals from generations 30-100, and from response of each line to divergent selection after more than 65 generations of mutation accumulation. Estimates of V_M averaged over males and females both from divergence among lines and from response to selection within lines were 3.3 × 10^-3 V_E for abdominal bristles and 1.5 × 10^-3 V_E for sternopleural bristles, where V_E is the environmental variance. The actual rate of production of mutations affecting these traits may be considerably higher if the traits are under stabilizing selection, and if mutations affecting bristle number have deleterious effects on fitness. There was a substantial component of variance for sex × mutant effect interaction and the sublines evolved highly significant mutational variation in sex dimorphism of abdominal bristle number. Pleiotropic effects on sex dimorphism may be a general property of mutations at loci determining bristle number.     

WITHIN- AND BETWEEN-GENERATION EFFECTS OF TEMPERATURE ON THE MORPHOLOGY AND PHYSIOLOGY OF DROSOPHILA MELANOGASTER

We investigated the effects of developmental and parental temperatures on several physiological and morphological traits of adult Drosophila melanogaster. Flies for the parental generation were raised at either low or moderate temperature (18°C or 25°C) and then mated in the four possible sex-by-parental temperature crosses. Their offspring were raised at either 18°C or 25°C and then scored as adults for morphological (dry body mass, wing size, and abdominal melanization [females only]), physiological (knock-down temperature, and thermal dependence of walking speed), and life history (egg size) traits. The experiment was replicated, and the factorial design allows us to determine whether and how paternal, maternal, and developmental temperatures (as well as offspring sex) influence the various traits. Sex and developmental temperature had major effects on all traits. Females had larger bodies and wings, higher knock-down temperatures, and slower speeds (but similar shaped performance curves) than males. Development at 25°C (versus at 18°C) increased knock-down temperature, increased maximal speed and thermal performance breadth, decreased the optimal temperature for walking, decreased body mass and wing size, reduced abdominal melanization, and reduced egg size. Parental temperatures influenced a few traits, but the effects were generally small relative to those of sex or developmental temperature. Flies whose mother had been raised at 25°C (versus at 18°C) had slightly higher knock-down temperature and smaller body mass. Flies whose father had been raised at 25°C had relatively longer wings. The effects of paternal, maternal, and developmental temperatures sometimes differed in direction. The existence of significant within- and between-generation effects suggests that comparative studies need to standardize thermal environments for at least two generations, that attempts to estimate “field” heritabilities may be unreliable for some traits, and that predictions of short-term evolutionary responses to selection will be difficult.