Population specific fitness response of Drosophila subobscura to lead pollution

Abstract Differences in heavy metal tolerance among separate populations of the same species have often been interpreted as local adaptation. Persistence of differences after removing the stressor indicates that mechanisms responsible for the increased tolerance were genetically determined. Drosophila subobscura Collin (Diptera: Drosophilidae) populations were sampled from two localities with different history of heavy metal pollution, and reared for eight generations in the laboratory on a standard medium and on media with different concentrations of lead (Pb). To determine whether flies from different natural populations exposed to the Pb‐contaminated media in the laboratory show population specific variability in fitness components over generations, experimental groups with different concentrations of lead were assayed in three generations (F₂, F₅, and F₈) for fecundity, developmental time, and egg‐to‐adult viability. On the contaminated medium, fecundity was reduced in later generations and viability was increased, irrespective of the environmental origin of populations. For both populations, developmental time showed a tendency of slowing down on media with lead. Faster development was observed in later generations. Preadaptation to contamination, meaning higher fecundity, higher viability, and faster egg to adult development in all studied generations, was found in D. subobscura originating from the locality with a higher level of heavy metal pollution.     

Protein and carbohydrate composition of larval food affects tolerance to thermal stress and desiccation in adult Drosophila melanogaster

Larval nutrition may affect a range of different life history traits as well as responses to environmental stress in adult insects. Here we test whether raising larvae of fruit flies, Drosophila melanogaster, on two different nutritional regimes affects resistance to cold, heat and desiccation as well as egg production and egg-to-adult viability. We raised larvae on a carbohydrate-enriched and a protein-enriched growth medium. We found that flies developed on the high protein medium had increased heat and desiccation tolerance compared to flies developed on the carbohydrate-enriched medium. In contrast, flies developed on the carbohydrate-enriched growth medium recovered faster from chill coma stress compared to flies developed on a protein-enriched medium. We also found gender differences in stress tolerance, with female flies being more tolerant to chill coma, heat knockdown and desiccation stress compared to males. Egg production was highest in females that had developed on the protein-enriched medium. However, there was a sex-specific effect of nutrition on egg-to-adult viability, with higher viability for males developing on the sucrose-enriched medium, while female survival was highest when developing on the protein-enriched medium. Our study indicates that larval nutrition has a strong impact on the ability to cope with stress, and that the optimal nutrient composition varies with the type of stress.     

Dietary protein content affects evolution for body size, body fat and viability in Drosophila melanogaster

The ability to use different food sources is likely to be under strong selection if organisms are faced with natural variation in macro-nutrient (protein, carbohydrate and lipid) availabilities. Here, we use experimental evolution to study how variable dietary protein content affects adult body composition and developmental success in Drosophila melanogaster. We reared flies on either a standard diet or a protein-enriched diet for 17 generations before testing them on both diet types. Flies from lines selected on protein-rich diet produced phenotypes with higher total body mass and relative lipid content when compared with those selected on a standard diet, irrespective of which of the two diets they were tested on. However, selection on protein-rich diet incurred a cost as flies reared on this diet had markedly lower developmental success in terms of egg-to-adult viability on both medium types, suggesting a possible trade-off between the traits investigated.     

Genetic variation for rate of development in natural populations of Drosophila melanogaster

We have sampled wild chromosomes from two natural populations of Drosophila melanogaster and obtained flies fully homozygous for the second chromosome, the third chromosome, or both, as well as flies heterozygous for one or both wild chromosomes and balancer chromosomes. Rate of embryogenesis (egg laying to larval hatching) and rate of development from egg to adult are measured, by classifying the individuals into fast, intermediate, and slow developmental classes. The experiments indicate that variation for rate of embryogenesis and for rate of egg-to-adult development is plentiful in the natural populations. Various hypotheses are enunciated to account for the small range of phenotypic variation observed in wild-type individuals with respect to the two parameters (embryogenesis and egg-to-adult development) and for the difficulty in changing the mean rates by artificial selection. Appropriate experiments may decide among the hypotheses, helping us to understand the genetic control of rate of ontogenesis, which is an important fitness component.     

EXPERIMENTAL EVOLUTION OF ACCELERATED DEVELOPMENT IN DROSOPHILA. 1. DEVELOPMENTAL SPEED AND LARVAL SURVIVAL

Developmental time is a trait of great relevance to fitness in all organisms. In holometabolous species that occupy ephemeral habitat, like Drosophila melanogaster, the impact of developmental time upon fitness is further exaggerated. We explored the trade-offs surrounding developmental time by selecting 10 independent populations from two distantly related selection treatments (CB_1-5 and CO_1-5) for faster development. After 125 generations, the resulting accelerated populations (ACB_1-5 and ACO_1-5) displayed net selection responses for development time of -33.4 hours (or 15%) for ACB and -38.6 hours (or 17%) for ACO. Since most of the change in egg-to-adult developmental time was accounted for by changes in larval duration, the “accelerated” larvae were estimated to develop 25-30% faster than their control/ancestor populations. The responses of ACB and ACO lines were remarkably parallel, despite being founded from populations evolved independently for more than 300 generations. On average, these “A” populations developed from egg to adult in less than eight days and produced fertile eggs less than 24 hours after emerging. Accelerated populations showed no change in larval feeding rate, but a reduction in pupation height, the latter being a trait relating to larval energetic expenditure in wandering prior to pupation. This experiment demonstrates the existence of a negative evolutionary correlation between preadult developmental time and viability, as accelerated populations experienced a severe cost in preadult survivorship. In the final assay generation, viability of accelerated treatments had declined by more than 10%, on average. A diallel cross demonstrated that the loss of viability in the ACO lines was not due to inbreeding depression. These results suggest the existence of a rapid development syndrome, in which the fitness benefits of fast development are balanced by fitness costs resulting from reduced preadult survivorship, marginal larval storage of metabolites, and reduced adult size.     

GENETICS OF HOST-CACTUS RESPONSE AND LIFE-HISTORY EVOLUTION AMONG ANCESTRAL AND DERIVED POPULATIONS OF CACTOPHILIC DROSOPHILA MOJAVENSIS

The extent of host-specific genetic variation for two life-history traits, egg to adult developmental time and viability, and one morphological trait closely tied to fitness, adult thorax size, was exposed by employing a nested half-sib/full-sib breeding design with Baja and mainland populations of Drosophila mojavensis recently extracted from nature. This study was motivated by the presence of substantial variation in life histories among populations of D. mojavensis that use the fermenting tissues of particular species of columnar cacti for feeding and breeding in the Sonoran Desert. Full-sib progeny from all sire-dam crosses were split into cultures of agria cactus, Stenocereus gummosus, and organ pipe cactus, S. thurberi, to examine patterns of genotype-by-environment interaction for these fitness components. Baja flies expressed shorter egg-to-adult developmental times, higher viabilities, and smaller body sizes than mainland flies consistent with previous studies. Significant sire and dam components of variance were exposed for developmental time and thorax size. Genotype-by-environment interactions were significant at the level of dams for developmental time and nearly significant for viability (P = 0.09). Narrow- and broad-sense heritabilities were influenced by host cactus, sex, and population. No strong pattern of genetic correlation emerged among fitness components suggesting that host-range expansion has not been accompanied by formation of coadapted life histories, yet the ability to estimate genetic correlations and their standard errors was compromised by the unbalanced nature of the data set. Genetic correlations in performance across cacti were slightly positive, evidence for ecological generalism among populations explaining the observed pattern of multiple host cactus use within the species range of D. mojavensis.     

The effect of lead on fitness components and developmental stability in Drosophila subobscura

We analyzed the developmental time, egg-to-adult viability, and developmental stability (fluctuating wing size asymmetry) in Drosophila subobscura, maintained for six generations on different concentrations of lead. Development time is significantly affected by generation and lead concentration, but interaction of these factors is not a significant source of variability for this fitness component. Generation and the interaction generation x concentration of lead significantly affect egg-to-adult viability. Levene's test of heterogeneity of variance showed that variability of FA is not significant in any of the samples. Within both lead concentrations females showed significantly higher FA indices for the wing width than males. Within sexes, a significantly higher FA was found only in females for wing width FA between the control and the lower concentration of lead. The results show that if strong relationship between FA and the studied fitness components exists, it results in a stronger selection of unstable genotypes under lead as a stress factor and, consequently, FA needs to be used with caution as a biomarker in natural populations under environmental stress.     

LIFE-HISTORY CONSEQUENCES OF ADAPTATION TO LARVAL NUTRITIONAL STRESS IN DROSOPHILA

Many animal species face periods of chronic nutritional stress during which the individuals must continue to develop, grow, and/or reproduce despite low quantity or quality of food. Here, we use experimental evolution to study adaptation to such chronic nutritional stress in six replicate Drosophila melanogaster populations selected for the ability to survive and develop within a limited time on a very poor larval food. In unselected control populations, this poor food resulted in 20% lower egg‐to‐adult viability, 70% longer egg‐to‐adult development, and 50% lower adult body weight (compared to the standard food on which the flies were normally maintained). The evolutionary changes associated with adaptation to the poor food were assayed by comparing the selected and control lines in a common environment for different traits after 29–64 generations of selection. The selected populations evolved improved egg‐to‐adult viability and faster development on poor food. Even though the adult dry weight of selected flies when raised on the poor food was lower than that of controls, their average larval growth rate was higher. No differences in proportional pupal lipid content were observed. When raised on the standard food, the selected flies showed the same egg‐to‐adult viability and the same resistance to larval heat and cold shock as the controls and a slightly shorter developmental time. However, despite only 4% shorter development time, the adults of selected populations raised on the standard food were 13% smaller and showed 20% lower early‐life fecundity than the controls, with no differences in life span. The selected flies also turned out less tolerant to adult malnutrition. Thus, fruit flies have the genetic potential to adapt to poor larval food, with no detectable loss of larval performance on the standard food. However, adaptation to larval nutritional stress is associated with trade‐offs with adult fitness components, including adult tolerance to nutritional stress.     

Latitudinal variation in wild populations of Drosophila melanogaster: heritabilities and reaction norms

Large amounts of genetic variation for wing length and wing area were demonstrated both within and between Drosophila melanogaster populations along a latitudinal gradient in South America. Wing length and wing area showed a strong positive correlation with latitude in both wild flies and laboratory-raised descendants. Large population differences were observed for heritability and coefficient of variation of these two traits, whereas relatively small population differences were found for development time, viability, pupal mortality, sex ratio and their norms of reaction to four developmental temperatures. No clear-cut latitudinal clines were established for these life-history characters. These results are discussed in the light of Bergmann's Rule and the relation between larval development and adult body size.     

In vitro rearing Oestrus caucasicus third-instar larvae and pupae (Diptera: Oestridae) from naturally-infested Iberian ibex, Capra Pyrenaica (Artiodactyla: Bovidae)

Third-instar Oestrus caucasicus larvae (n = 236) obtained from Iberian ibex, Copra pyrenaica, were reared in a laboratory to obtain adult flies. They were maintained at a temperature of 21.9 +/- 2.7 degrees C and a relative humidity of 38.9 +/- 8.0 %. In all, 78 imagos emerged (33.1 %), with a sex-ratio at emergence not differing significantly from 1:1; 25 larvae did not complete pupariation. A total of 14 adult flies (17.9 % of the adults obtained) showed malformations, mainly in their wings. The pupariation period lasted around 30 hours and the pupal stage lasted on average 29.8 +/- 6.8 days. The success of pupation in both sexes was mainly determined by the weight of the larvae. Sexual dimorphism, with higher weights in females, was evident in third-instar arvae, pupae and adults. The mean longevity of adult flies was 224.8 +/- 91.4 hours and males generally survived for onger than the females.     

Selection for developmental time in bean weevil (Acanthoscelides obtectus): correlated responses for other life history traits and genetic architecture of line differentiation

In this article we investigate the direct and correlated responses to selection for developmental time in order to discern differences between lines in several preadult and adult life history traits of Acanthoscelides obtectus (Coleoptera, Bruchidae). Selection for fast development was about five times as effective as selection for slow development, as judged by realized heritabilities. The correlated responses on the following life‐history traits were studied: egg size, hatching success, embryonic developmental time, egg‐to‐adult viability, body weight, first day of egg laying, total fecundity, and longevity. Analyses of the terminal generation of selection showed that all life history traits examined, except for hatching success, were affected by selection. The findings suggest that body weight, total fecundity, and longevity traded off to preadult developmental time. Unlike the adult traits, none of the preadult traits showed negative correlations with developmental time. We also present data concerning the underlying genetic basis that produces changes in preadult developmental time, body weight, and egg‐to‐adult viability in the lines selected for fast and slow preadult developmental time. Additive‐dominance genetic architecture for both preadult developmental time and body weight was found. In addition, it appears that the responses to selection for preadult developmental time involved between 10 and 28 loci, which were correlated with at least one to four genes for body weight. Epistasis makes a significant contribution to genetic divergence between fast and slow selected lines only with respect to preadult viability. The observed levels of dominance and epistasis underscore the important role of nonadditive genetic effects to the adaptive diversifications of bean weevil populations.     

Effects of disodium octaborate tetrahydrate on survival, behavior, and egg viability of adult muscoid flies (Diptera: Muscidae).

Disodium octaborate tetrahydrate (Na2B8O13(.)4H2O) was mixed with sugar and fed to adult Musca domestica L. and Fannia canicularis (L.) to determine concentration-mortality relationships. LC50s (48-h exposure) were 5.7% for M. domestica and 1.0% for F. canicularis. Rates of 1 and 2% were used to test effects on M. domestica mortality and egg hatch over an 8-d period. Reduced egg hatch was evident after 1 d of feeding on the treated mixtures and was greatest (less than 10% egg hatch) after flies fed only on treated mixtures for 2 d. A partial rebound in egg hatch occurred after 3-4 d of feeding on treated diet. Sperm motility in females fed treated sugar was apparently normal. Fertile egg placed on treated poultry manure did not hatch, indicating embryonic death, which also may have been involved in the low hatch of eggs observed from treated flies. When flies were exposed to treated sugar for 2 d then returned to untreated diet, delayed mortality effects and reduced egg hatch persisted for at least 3 d. Behavioral assays (feeding) with M. domestica demonstrated that flies rejected borate-sugar mixtures in favor of sugar alone when the concentration of borate was greater than 2%. Given a choice of treated and untreated poultry manure for oviposition, flies also rejected the treated manure. The potential of borates in adult bait formulations or applied to developmental substrates for fly control is discussed.     

Variation in sensitivity to aflatoxin B1 among several strains of Drosophila melanogaster (Diptera).

Eggs of five different wild-type strains of fruit flies, Drosophila melanogaster, were allowed to develop into adults on media containing 0.27 and 0.40 ppm of the toxic and carcinogenic mycotoxin, aflatoxin B₁ (AFB₁). Egg-to-adult viability and development time, adult sex ratio and size, and pupa case size were measured for each treatment and compared to control values for each strain. Adult size and pupa size were not affected significantly by the AFB₁ treatments, while egg-to-adult development time increased for strains grown on AFB₁-containing media. Strain-specific changes in egg-to-adult viability and adult sex ratio were observed. The most probable explanation for these differences is genetic variation among the strains. Crimea, Hikone-R, Lausanne-S, Oregon-R, and Swedish-C were the strains tested.     

Extreme temperatures increase the deleterious consequences of inbreeding under laboratory and semi-natural conditions

The majority of experimental studies of the effects of population bottlenecks on fitness are performed under laboratory conditions, which do not account for the environmental complexity that populations face in nature. In this study, we test inbreeding depression in multiple replicates of inbred when compared with non-inbred lines of Drosophila melanogaster under different temperature conditions. Egg-to-adult viability, developmental time and sex ratio of emerging adults are studied under low, intermediate and high temperatures under laboratory as well as semi-natural conditions. The results show inbreeding depression for egg-to-adult viability. The level of inbreeding depression is highly dependent on test temperature and is observed only at low and high temperatures. Inbreeding did not affect the developmental time or the sex ratio of emerging adults. However, temperature affected the sex ratio with more females relative to males emerging at low temperatures, suggesting that selection against males in pre-adult life stages is stronger at low temperatures. The coefficient of variation (CV) of egg-to-adult viability within and among lines is higher for inbred flies and generally increases at stressful temperatures. Our results contribute to knowledge on the environmental dependency of inbreeding under different environmental conditions and emphasize that climate change may impact negatively on fitness through synergistic interactions with the genotype.     

Life history evolution and cellular mechanisms associated with increased size in high‐altitude Drosophila

Understanding the physiological and genetic basis of growth and body size variation has wide‐ranging implications, from cancer and metabolic disease to the genetics of complex traits. We examined the evolution of body and wing size in high‐altitude Drosophila melanogaster from Ethiopia, flies with larger size than any previously known population. Specifically, we sought to identify life history characteristics and cellular mechanisms that may have facilitated size evolution. We found that the large‐bodied Ethiopian flies laid significantly fewer but larger eggs relative to lowland, smaller‐bodied Zambian flies. The highland flies were found to achieve larger size in a similar developmental period, potentially aided by a reproductive strategy favoring greater provisioning of fewer offspring. At the cellular level, cell proliferation was a strong contributor to wing size evolution, but both thorax and wing size increases involved important changes in cell size. Nuclear size measurements were consistent with elevated somatic ploidy as an important mechanism of body size evolution. We discuss the significance of these results for the genetic basis of evolutionary changes in body and wing size in Ethiopian D. melanogaster.     

Development of the solitary endoparasitoid Microplitis demolitor: host quality does not increase with host age and size

Abstract.  1. Many studies examining the relationship between host size, an index of host quality, and parasitoid fitness use development time and/or adult parasitoid size as currencies of fitness, while ignoring pre-adult mortality. Because the physiological suitability of the host may vary in different stages, sizes, or ages of hosts, a misleading picture of host quality may therefore be obtained in cases where fitness is based on only one or two developmental traits.
2. The development of the solitary koinobiont endoparasitoid Microplitis demolitor is examined in different larval age-classes of its host the soybean looper Pseudoplusia includens . Hosts were parasitised on days 1–8 after hatching from the egg, and development time, adult body size, and mortality of the parasitoid were compared.
3. A comparison of larval growth trajectories (using dry body mass) of M. demolitor revealed that parasitoid larvae attained over twice as much body mass in old hosts than in younger hosts. Similarly, adult parasitoid size at eclosion generally increased with host size, although parasitoids developing in smaller hosts lost a much lower proportion of mass between pupation and eclosion.
4. Overall egg-to-adult development was most rapid in intermediate-aged hosts, and longer in hosts at opposite ends of the age continuum. Moreover, parasitoid mortality varied non-linearly with host stage, and was generally higher in very young and older hosts.
5. Based on these results and other empirical data for koinobionts, it is argued that fitness functions in this group of parasitoids are not simply a positive function of host size or age, but instead may be distinctly dome-shaped, both patterns reflecting the degree of physiological and nutritional compatibility between the two organisms.     

Constant, cycling, hot and cold thermal environments: strong effects on mean viability but not on genetic estimates

It has frequently been suggested that trait heritabilities are environmentally sensitive, and there are genetic trade-offs between tolerating different environments such as hot and cold or constant and fluctuating temperatures. Future climate predictions suggest an increase in both temperatures and their fluctuations. How species will respond to these changes is uncertain, particularly as there is a lack of studies which compare genetic performances in constant vs. fluctuating environments. In this study, we used a nested full-sib/half-sib breeding design to examine how the genetic variances and heritabilities of egg-to-adult viability differ at high and low temperatures with and without daily fluctuations in temperatures using Drosophila melanogaster as a model organism. Although egg-to-adult viability was clearly sensitive to developmental temperatures, heritabilities were not particularly sensitive to developmental temperatures. Moreover, we found that egg-to-adult viabilities at different developmental temperatures were positively correlated, suggesting a common genetic background for egg-to-adult viability at different temperatures. Finding both a uniform genetic background coupled with rather low heritabilities insensitive to temperatures, our results suggest evolutionary responses are unlikely to be limited by temperature effects on genetic parameters or negative genetic correlations, but by the direct effects of stressful temperatures on egg-to-adult viability accompanied with low heritabilities.     

Chill-coma temperature in Drosophila: effects of developmental temperature, latitude, and phylogeny

We modify and apply a nonlethal technique for rapidly quantifying the cold tolerance of large numbers of Drosophila and other small insects. Flies are transferred to individual vials, cooled in groups in progressive 0.5 degrees C steps, and checked for loss of righting response (chill-coma temperature [T(cc)]). Flies recover quickly when transferred to ambient temperature, and thus this technique potentially can be used in selection experiments. We applied this technique in several experiments. First, we examined the sensitivity of T(cc) to developmental temperature. Drosophila melanogaster (Congo, France), Drosophila subobscura (Spain, Denmark), and Drosophila ananassae (India) were reared from egg to adult at 15 degrees, 18 degrees, 25 degrees, or 29 degrees C, transferred to 15 degrees C for several days, and then progressively chilled: T(cc) was positively related to developmental temperature, inversely related to latitude of the population, but independent of sex. The sensitivity of T(cc) to developmental temperature (acclimation flexibility) was marked: T(cc) shifted on average 1 degrees for each 4 degrees C shift in developmental temperature. Among 15 species of the obscura group of Drosophila, T(cc) varied from -0.1 degrees to 4.5 degrees C; T(cc) was inversely related to latitude in both nonphylogenetic and phylogenetically based ANCOVA (standardized independent contrasts) and was unrelated to body size.     

Maternal and grandmaternal age influence offspring fitness in Drosophila.

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