CHROMOSOMAL POLYMORPHISM IN ISOFEMALE LINES AND CAGE POPULATIONS OF DROSOPHILA MELANOGASTER

Drosophila melanogaster populations in nature usually carry inversion polymorphisms. When they were transferred to and maintained in the laboratory as large cage populations, frequencies of polymorphic inversions were drastically decreased and finally eliminated. This “cage effect” was observed irrespective of the geographical origin of the population or the initial frequency of each inversion. The decrease and elimination of inversions in the cage was not overcome by changing conditions such as medium, temperature, or the number of isofemale lines (40-600) introduced. On the other hand, in the sets of isofemale lines derived from the same geographical origins as the cage populations, each of which was maintained as a small vial population, the inversion frequencies, though decreased from the initial frequencies, were kept at significantly high levels. The cage populations initiated with one or two isofemale lines also maintained the inversion polymorphisms that were as high as vial populations.     

EXPERIMENTAL EVOLUTION OF HSP70 EXPRESSION AND THERMOTOLERANCE IN DROSOPHILA MELANOGASTER

To examine whether recent evolutionary history affects the expression of Hsp70, the major heat-induced-heat shock protein in Drosophila melanogaster, we measured Hsp70 expression, thermotolerance, and hsp70 gene number in replicate populations undergoing laboratory evolution at different temperatures. Despite Hsp70's ancient and highly conserved nature, experimental evolution effectively and replicably modified its expression and phenotype (thermotolerance). Among five D. melanogaster populations founded from a common ancestral population and raised at three different temperatures (one at 18°C, two each at 25°C and 28°C) for twenty years, Hsp70 expression varies in a consistent pattern: the replicate 28°C lines expressed 30–50% less Hsp70 than the other lines at a range of inducing temperatures. This modification was refractory to acclimation, and correlated with thermotolerance: the 28°C lines had significantly lower inducible tolerance of 38.5°C and 39°C. We verified the presence of five hsp70 genes in the genome of each line, excluding copy number variation as a candidate molecular basis of the evolved difference in expression. These findings support the ability of Hsp70 levels in D. melanogaster populations to change over microevolutionary time scales and implicate constancy of environmental temperature as a potentially important selective agent.     

RAPID LABORATORY EVOLUTION OF ADULT LIFE-HISTORY TRAITS IN DROSOPHILA MELANOGASTER IN RESPONSE TO TEMPERATURE

Three replicate lines of Drosophila melanogaster were cultured at each of two temperatures (16.5°C and 25°C) in population cages for 4 yr. The lifespans of both sexes and the fecundity and fertility of the females were then measured at both experimental temperatures. The characters showed evidence of adaptation; flies of both sexes from each selection regime showed higher longevity, and females showed higher fecundity and fertility, than flies from the other selection regime when they were tested at the experimental temperature at which they had evolved. Calculation of intrinsic rates of increase under different assumptions about the rate of population increase showed that the difference between the lines from the two selection regimes became less the higher the rate of population increase, because the lines were more similar in early adulthood than they were later. Despite the increased adaptation of the low-temperature lines to the low temperature, like the high temperature lines they produced progeny at a higher rate at the higher temperature. The lines may have independently evolved adaptations to their respective thermal regimes during the experiment, or there may have been a trade-off between adaptation to the two temperatures, or mutation pressure may have lowered adaptation to the temperature that the flies no longer encountered.     

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.     

HERITABLE VARIATION FOR FECUNDITY IN FIELD-COLLECTED DROSOPHILA MELANOGASTER AND THEIR OFFSPRING REARED UNDER DIFFERENT ENVIRONMENTAL TEMPERATURES

Heritable variation for fitness components is normally measured under favorable laboratory conditions, but organisms in the field experience variable conditions that are often stressful and may affect the expression of heritable variation. We examined heritable variation for early fecundity in three samples of Drosophila melanogaster from the field. Flies were obtained from a rotting fruit pile in summer, autumn, and spring, and progeny were reared under laboratory conditions. Field parents were tested for fecundity at 14°C or 28°C depending on ambient temperatures. Wing/thorax length ratios measured on flies from the spring collection suggested that flies had developed at around 20°C. Progeny were reared and tested at 14°C, 25°C, and 28°C. In the summer collection, parent-offspring regression coefficients were high and significant, compared to nonsignificant values obtained in two of three autumn comparisons. In the spring collection, parent-offspring regressions were negative regardless of testing temperature, suggesting that field females with a high fecundity produced offspring with low scores. Comparisons of F₁ and F₂ laboratory generations indicated intermediate heritabilities for fecundity in the laboratory. The lower bound heritability estimate for fecundity in field individuals was 37% in summer and 59% in autumn. Estimates of field heritability and evolvability for wing length measured in the spring collection were lower than in the laboratory. The results indicate that heritabilities and additive genetic variances for fecundity can be high in field-reared flies, but that results may vary between field collections.     

EVOLUTION AND DEVELOPMENT OF BODY SIZE AND CELL SIZE IN DROSOPHILA MELANOGASTER IN RESPONSE TO TEMPERATURE

We examined the evolutionary and developmental responses of body size to temperature in Drosophila melanogaster, using replicated lines of flies that had been allowed to evolve for 5 yr at 25°C or at 16.5°C. Development and evolution at the lower temperature both resulted in higher thorax length and wing area. The evolutionary effect of temperature on wing area was entirely a consequence of an increase in cell area. The developmental response was mainly attributable to an increase in cell area, with a small effect on cell number in males. Given its similarity to the evolutionary response, the increase in body size and cell size resulting from development at low temperature may be a case of adaptive phenotypic plasticity. The pattern of plasticity did not evolve in response to temperature for any of the traits. The selective advantage of the evolutionary and developmental responses to temperature is obscure and remains a major challenge for future work.     

EXPERIMENTAL EVOLUTION OF FEMALE TRAITS UNDER DIFFERENT LEVELS OF INTERSEXUAL CONFLICT IN DROSOPHILA MELANOGASTER

A number of studies have documented the evolution of female resistance to mate‐harm in response to the alteration of intersexual conflict in the populations. However, the life‐history consequence of such evolution is still a subject of debate. In this study, we subjected replicate populations of Drosophila melanogaster to different levels of sexual conflict (generated by altering the operational sex ratio) for over 45 generations. Our results suggest that females from populations experiencing higher level of intersexual conflict evolved increased resistance to mate‐harm, in terms of both longevity and progeny production. Females from the populations with low conflict were significantly heavier at eclosion and were more susceptible to mate‐harm in terms of progeny production under continuous exposure to the males. However, these females produced more progeny upon single mating and had significantly higher longevity in absence of any male exposure—a potential evidence of trade‐offs between resistance‐related traits and other life‐history traits, such as fecundity and longevity. We also report tentative evidence, suggesting an increased male cost of interacting with more resistant females.     

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.     

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.     

DESICCATION AND STARVATION TOLERANCE OF ADULT DROSOPHILA: OPPOSITE LATITUDINAL CLINES IN NATURAL POPULATIONS OF THREE DIFFERENT SPECIES

Desiccation and starvation tolerance were measured along latitudinal transects in three Drosophilid species (Drosophila ananassae, D. melanogaster, and Zaprionus indianus) of the Indian subcontinent. In each case, significant latitudinal clines were observed; desiccation tolerance increased with latitude while starvation tolerance decreased. Such field observations suggest that desiccation and starvation tolerance are fitness related traits that are independently selected in nature and genetically independent. It was, however, difficult to relate these genetic changes with precise climatic variables, except winter temperature. The overall negative correlation between the two traits, which was evidenced in natural populations, contrasts with a positive correlation generally observed in various laboratory selection experiments and that also seems to exist between different species. These observations point to the difficulty of interpreting correlations among fitness-related traits when different evolutionary levels are compared, and also different sets of data, that is, field versus laboratory studies.     

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.     

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.     

RESOURCE ACQUISITION AND THE EVOLUTION OF STRESS RESISTANCE IN DROSOPHILA MELANOGASTER

Resistance to environmental stress is one of the most important forces molding the distribution and abundance of species. We investigated the evolution of desiccation stress resistance using 20 outbred Drosophila melanogaster populations directly selected in the laboratory for adult desiccation resistance (D), postponed senescence (O), and their respective controls (C and B). Both aging and desiccation selection increased desiccation resistance relative to their controls, creating a spectrum of desiccation resistance levels across selection treatments. We employed an integrative approach, merging data on the life histories of these populations with a detailed physiology of water balance. The physiological basis of desiccation resistance may be mechanisms enhancing either resource conservation or resource acquisition and allocation. Desiccation-resistant populations had increased water and carbohydrate stores, and showed age-specific patterns of desiccation resistance consistent with the resource accumulation mechanism. A significant proportion of the resources relevant to resistance of the stress were accumulated in the larval stage. Males and females of desiccation-selected lines exhibited distinctly different patterns of desiccation resistance and resource acquisition, in a manner suggesting intersexual antagonism in the evolution of stress resistance. Preadult viability of stress-selected populations was lower than that of controls, and development was slowed. Our results suggest that there is a cost to preadult resource acquisition, pointing out a complex trade-off architecture involving characters distributed across distinct life-cycle stages.     

THE EFFECT OF TEMPERATURE ON BODY SIZE AND FECUNDITY IN FEMALE DROSOPHILA MELANOGASTER: EVIDENCE FOR ADAPTIVE PLASTICITY

The reaction norm linking rearing temperature and size in Drosophila melanogaster results in progressively larger flies as the temperature is lowered from 30°C to 18°C, but it has remained unclear whether this phenotypic plasticity is part of an adaptive response to temperature. We found that female D. melanogaster reared to adulthood at 18°C versus 25°C showed a 12% increase in dry weight. Measurements of the fecundity of these two types of fly showed that the size change had no effect on lifetime fecundity, regardless of the adult test temperature. Thus the phenotypic plasticity breaks the usual positive correlation between body size and fecundity. However, at a given temperature, early fecundity (defined as productivity for days 5 through 12 after eclosion at 25°C and days 7 through 17 at 18°C) was highest when the rearing and test temperatures were the same. The early fecundity advantage due to rearing at the test temperature was 25% at 18°C and 16% at 25°C, a result consistent with the overall phenotypic response to temperature being adaptive. This conclusion is further supported by the finding that the temperature treatments resulted in a trade-off between early fecundity and longevity, a trade-off that parallels the known genetic correlation. Another parallel is that both the temperature-induced and genetic effects are independent of total fecundity. By contrast, within the temperature treatments, the phenotypic correlation between early fecundity and longevity was positive, illustrating the danger of assuming that phenotypic and genetic correlations are similar, or even of the same sign.     

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.     

HERITABILITY OF EXPRESSION OF THE 70KD HEAT-SHOCK PROTEIN IN DROSOPHILA MELANOGASTER AND ITS RELEVANCE TO THE EVOLUTION OF THERMOTOLERANCE

The principle inducible heat-shock protein of Drosophila melanogaster, Hsp70, contributes to thermotolerance throughout the entire life cycle of the species but may also reduce fitness in some life stages. In principle, selection might maximize the benefits of Hsp70 expression relative to its costs by adjusting the magnitude of Hsp70 expression for each life-cycle stage independently. Therefore we examined whether the magnitude of Hsp70 expression varied during the life cycle and the relationship of this variation to several life-history traits. For 28 isofemale lines derived from a single natural population, estimates of heritable variation in Hsp70 expression ranged between 0.25 and 0.49, and the association among variation in first- and third-instar larvae and in adults correlated highly. Thus, Hsp70 expression is genetically coupled at these developmental stages. A line engineered with extra copies of the hsp70 gene produced more Hsp70 and survived heat shock much better than did a control strain. Among natural lines, Hsp70 expression was only weakly related to tolerance of heat shock and to larva-to-adult survival and developmental time at permissive temperatures. Additionally, lines with high adult survival developed slowly as larvae, which is a possible trade-off. These and other findings suggest that trade-offs may maintain quantitative variation both in heat-shock protein expression and in life-history traits that associate with thermotolerance.     

我国东北地区黑腹果蝇P-M品系分布及P因子的入侵途径

对我国东北地区及毗邻的北京、呼和浩特和烟台地区近两年收集的黑腹果蝇（Drosophila melanogaster)自然群体进行了有关P－M杂种劣育的研究。用两类标准诊断杂交的方法测定了15个地方108个单雌系的P因子活性和细胞调控能力。结果表明,在15个地方自然群体中,大连、烟台、延吉、长白及哈尔滨5个地区为Q型,其余的均为M′型,没有P型。同时,这些自然群体的杂交A^*的不育率也存在着地理性差异：东南部地区,特别是沿渤海和中朝边境地区的品系以Q型为主,而中部和北部扩大地区则以M′型为主。据此推断我国境内的P因子是由朝鲜半岛和日本分两路侵入：一路从日本、朝鲜半岛经海路传递到沿海的烟台和大连地区,再由此向东北和南部地区入侵和扩散;另一路由朝鲜半岛侵入中朝边境的长白和延吉地区后,再向东北地区及内地入侵。两条路线在哈尔滨地区会合,使该地区品系的杂交A^*的不育率升高。     

AN INTERACTION BETWEEN ENVIRONMENTAL TEMPERATURE AND GENETIC VARIATION FOR BODY SIZE FOR THE FITNESS OF ADULT FEMALE DROSOPHILA MELANOGASTER

Abstract. — Drosophila and other ectotherms show geographic genetic variation in body size, with larger individuals at higher latitudes and altitudes. Temperature is implicated as an important selective agent because long-term laboratory culture of Drosophila leads to the evolution of larger body size at lower temperatures. In this paper, we tested the hypothesis that, in Drosophila melanogaster, larger size is favored at lower temperatures in part because of selection on adult females. We used replicated lines of D. melanogaster artificially selected for increased and decreased wing area with constant cell area. The resulting size differences between the selected lines were due solely to differences in cell number, and thereby were similar to the cellular basis of clinal variation in body size in nature. We examined life-history traits of adult females at 18 and 25°C. Rearing for two generations at the two temperatures did not affect the extent of the size differences between lines from the different selection regimes. There was a strong interaction between temperature and size selection for both survival and lifetime reproductive success, with larger females living significantly longer and producing more offspring over their lifetime only when reared and tested in the colder environment. There was also an increase in average daily progeny production in large-line females relative to the control and small lines again, only in the colder environment. Thus, the females from the large selection lines were relatively fitter at the colder temperature. At both experimental temperatures, especially the lower one, the small- line females rescheduled their progeny production to later ages. Larger body size may have evolved at higher latitudes and altitudes because of the advantages to the adult female of being larger at lower temperatures.     

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.     

NATURAL VARIATION IN THE EXPRESSION OF THE HEAT-SHOCK PROTEIN HSP70 IN A POPULATION OF DROSOPHILA MELANOGASTER AND ITS CORRELATION WITH TOLERANCE OF ECOLOGICALLY RELEVANT THERMAL STRESS

Although Hsp70, the principal inducible heat-shock protein of Drosophila melanogaster, has received intense scrutiny in laboratory strains, its variation within natural populations and the consequences of such variation for thermotolerance are unknown. We have characterized variation in first-instar larvae of 20 isofemale lines isolated from a single natural population of D. melanogaster, in which larvae are prone to thermal stress in nature. Hsp70 expression varied more than twofold among lines after induction by exposure to 36°C for one hour, with an estimated proportion of the variation due to genetic differences of 0.24 ± 0.08. Thermotolerance with and without a Hsp70-inducing pretreatment, survival at 25°C, and developmental time also varied significantly. As expected, expression of Hsp70 correlated positively with larval thermotolerance. By contrast, lines in which larval survival was high in the absence of heat stress showed lower than average Hsp70 expression and lower than average inducible thermotolerance. This conditional performance suggests an evolutionary trade-off between thermotolerance and the ability to produce higher concentrations of Hsp70, and survival in a benign environment.