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.     

Selection on age at reproduction in Drosophila melanogaster: female mating frequency as a correlated response

We examined the effect of selection for age at reproduction on female mating frequency and fertility in female Drosophila melanogaster. Selection for increased age at reproduction (and hence increased lifespan) resulted in higher late life female mating frequencies, while females selected for younger ages at reproduction showed increased early life mating frequencies. These results indicate that the response to selection on age at reproduction has involved changes in the scheduling of female reproductive behavior.     

Correlated responses to selection for stress resistance and longevity in a laboratory population of Drosophila melanogaster

Laboratory studies on Drosophila have revealed that resistance to one environmental stress often correlates with resistance to other stresses. There is also evidence on genetic correlations between stress resistance, longevity and other fitness-related traits. The present work investigates these associations using artificial selection in Drosophila melanogaster. Adult flies were selected for increased survival after severe cold, heat, desiccation and starvation stresses as well as increased heat-knockdown time and lifespan (CS, HS, DS, SS, KS and LS line sets, respectively). The number of selection generations was 11 for LS, 27 for SS and 21 for other lines, with selection intensity being around 0.80. For each set of lines, the five stress-resistance parameters mentioned above as well as longevity (in a nonstressful environment) were estimated. In addition, preadult developmental time, early age productivity and thorax length were examined in all lines reared under nonstressful conditions. Comparing the selection lines with unselected control revealed clear-cut direct selection responses for the stress-resistance traits. Starvation resistance increased as correlated response in all sets of selection lines, with the exception of HS. Positive correlated responses were also found for survival after cold shock (HS and DS) and heat shock (KS and DS). With regard to values of resistance across different stress assays, the HS and KS lines were most similar. The resistance values of the SS lines were close to those of the LS lines and tended to be the lowest among all selection lines. Developmental time was extended in the SS and KS lines, whereas the LS lines showed a reduction in thorax length. The results indicate a possibility of different multiple-stress-resistance mechanisms for the examined traits and fitness costs associated with stress resistance and longevity.     

Direct and correlated responses to selection for desiccation resistance: a comparison of Drosophila melanogaster and D. simulans

Replicate lines of Drosophila melanogaster and D. simulans originating from the same location in Australia were selected at two selection intensities (50%, 85% mortality) for increased resistance to desiccation, and scored for correlated responses to see if similar physiological changes were associated with the selection responses. Realized heritabilities were much higher in D. melanogaster. Selected lines of both species were more resistant than control lines to starvation and a toxic ethanol concentration. Both species also showed similar correlated responses for traits underlying the selection response: selected lines lost water at a slower rate and had reduced activity levels in a dry environment, but they did not differ in wet or dry body weight or in water content. For D. melanogaster, realized heritabilities for lines selected at 85% mortality were higher than for lines selected at 50% mortality, but there was no effect of selection intensity for D. simulans. Comparative studies of this nature may be useful in predicting the extent to which species can adapt to stress in the wild.     

Dissecting chill coma recovery as a measure of cold resistance: evidence for a biphasic response in Drosophila melanogaster

Cold resistance in insects has traditionally been measured in terms of survival following a stress, but alternative methods are increasingly being used because of their relevance to the ecology of organisms and their utility in characterizing variation among species, populations and individuals. One such method capable of discriminating among Drosophila species and conspecific Drosophila populations from different environments is adult chill coma recovery time, the time taken for adults to become active again after being knocked down by a cold stress. Here we characterized the chill coma response of D.melanogaster in detail. Adults were exposed to a range of temperatures and stressful periods prior to measuring recovery. Recovery from chill coma in D.melanogaster was biphasic; as flies were stressed under cooler temperatures, recovery times leveled off and then decreased before sharply increasing again as mortality starts to occur. This biphasic response has previously been observed in D.subobscura where it has a somewhat different shape. A second mechanism therefore acts at relatively lower temperatures to ameliorate the effects of the cold stress. When D.melanogaster were reared at 19 and 25 °C for two generations, the shape of the curve relating temperature to recovery time was similar, but flies from the warmer temperature had longer recovery times and showed responses that leveled off and then decreased at relatively higher temperatures. As exposure time to cold stress was increased, recovery times also increased except at mild stress levels. Chill coma recovery in D.melanogaster is a complex trait and likely to reflect multiple underlying components.     

Direct and correlated effects of selection on flight after exposure to thermal stress in Drosophila melanogaster

To demonstrate how insects may adapt to ecologically relevant levels of heat stress, we performed artificial selection on the ability of Drosophila melanogaster to fly after an exposure to a high but non-lethal thermal stress. Both tolerance and intolerance to heat stress arose very quickly, as only a few generations of selection were necessary to cause significant separation between high and low lines for heat tolerance. Estimates of heritability based on the lines artificially selected for increased flight ability ranged from 0.024 to 0.052, while estimates of heritability based on the lines selected for the inability to fly after heat stress varied between 0.035 and 0.091. Reciprocal F₁ crosses among these lines revealed strong additive effects of one or more autosomes and a weaker X-chromosome effect. This variation apparently affected flight specifically; neither survival to a more extreme stress nor knockdown by high temperature changed between lines selected for high and low heat tolerance as measured by flight ability. As the well-studied heat-shock response is associated with heat tolerance as measured by survival and knockdown, the aspects of the stress physiology that actually affect flight ability remains unknown.     

Direct and correlated responses to selection for larval ethanol tolerance in Drosophila melanogaster

Ethanol is an important larval resource and toxin for natural Drosophila melanogaster populations, and ethanol tolerance is genetically variable within and among populations. If ethanol‐tolerant genotypes have relatively low fitness in the absence of ethanol, as suggested by the results of an earlier study, genetic variation for ethanol tolerance could be maintained by variation in ethanol levels among breeding sites. I selected for ethanol tolerance in large laboratory populations by maintaining flies on ethanol‐supplemented media. After 90 generations, the populations were compared with control populations in egg‐to‐adult survival and development rate on ethanol‐supplemented and unsupplemented food. When compared on ethanol‐supplemented food, the ethanol‐selected populations had higher survival and faster development than the control populations, but on unsupplemented food, the populations did not differ in either trait. These results give no evidence for a ‘trade‐off’ between the ability to survive and develop rapidly in the presence of ethanol and the ability to do so in its absence. The effect of physiological induction of ethanol tolerance by exposing eggs to ethanol was also investigated; exposing eggs to ethanol strongly increased subsequent larval survival on ethanol‐supplemented food, but did not affect survival on regular food, and slowed development on both ethanol‐supplemented and regular food, partly by delaying egg hatch.     

Resource limitation and responses to rivals in males of the fruit fly Drosophila melanogaster

Diet has a profound direct and indirect effect on reproductive success in both sexes. Variation in diet quality and quantity can significantly alter the capacity of females to lay eggs and of males to deliver courtship. Here, we tested the effect of dietary resource limitation on the ability of male Drosophila melanogaster to respond adaptively to rivals by extending their mating duration. Previous work carried out under ad libitum diet conditions showed that males exposed to rivals prior to mating significantly extend mating duration, transfer more ejaculate proteins and achieve higher reproductive success. Such adaptive responses are predicted to occur because male ejaculate production may be limited. Hence, ejaculate resources require allocation across different reproductive bouts, to balance current vs. future reproductive success. However, when males suffer dietary limitation, and potentially have fewer reproductive resources to apportion, we expect adaptive allocation of responses to rivals to be minimized. We tested this prediction and found that males held on agar‐only diets for 5–7 days lost the ability to extend mating following exposure to rivals. Interestingly, extended mating was retained in males held on low yeast/sugar: no sugar/yeast diet treatments, but was mostly lost when males were maintained on ‘imbalanced’ diets in which there was high yeast: no sugar and vice versa. Overall, the results show that males exhibit adaptive responses to rivals according to the degree of dietary resource limitation and to the ratio of individual diet components.     

Heat knockdown resistance and chill‐coma recovery as correlated responses to selection on mating success at high temperature in Drosophila buzzatii

Reproduction and related traits such as mating success are strongly affected by thermal stress. We tested direct and correlated responses to artificial selection in replicated lines of Drosophila buzzatii that were selected for mating success at high temperature. Knockdown resistance at high temperature (KRHT) and chill‐coma recovery (CCR) were tested as correlated selection responses. Virgin flies were allowed to mate for four hours at 33°C in three replicated lines (S lines) to obtain the selected flies and then returned at 25°C to lay eggs. Other three replicated lines were maintained at 25°C without any selection as control (C lines). After 15 selection generations, KRHT and CCR were measured. Both traits were assessed in flies that did not receive any hardening pretreatments as well as in flies that were either heat or cold hardened. Thermotolerance traits showed significant correlated responses with higher KRHT in S than in C lines, both with a heat‐hardening pretreatment and without a heat‐hardening pretreatment. CCR time was longer in S than in C lines both with a cold‐hardening pretreatment and without a cold‐hardening pretreatment. Hardening treatments improved both KRHT and CCR in all cases excepting KRHT in C lines. Overall, KRHT and CCR showed an antagonistic pattern of correlated responses to our selection regime, suggesting either pleiotropy or tightly linked trait‐specific genes partially affecting KRHT and CCR.     

Sperm transfer and paternity in the scorpionfly <Emphasis Type="Italic">Panorpa cognata</Emphasis>: large variance in traits favoured by post-copulatory episodes of sexual selection

Post-copulatory episodes of sexual selection can be a powerful selective force influencing the reproductive success of males. In order to understand variation in male fertilisation success, we first need to consider the pattern of sperm utilisation by females following matings with more than one male. Second, we need to study those traits responsible for male success in sperm competition. Here we study both male sperm transfer characteristics as well as offspring paternity of females mated to two males in the scorpionfly Panorpa cognata. By repeatedly mating males to virgin females and interrupting copulation at defined time points, we found for all males that sperm transfer set off after approximately 40 min. During the remaining copulation, sperm transfer of individual males was continuous and with constant rate. Yet the rate of sperm transfer differed between individual males from about one sperm per minute to more than eight sperm per minute for the most successful males. In addition, we measured the fertilisation success in sperm competition of males with known sperm transfer capability. The relative number of sperm transferred by males during copulation, estimated from copulation duration and the males’ individual sperm transfer rate, explained a large proportion of variation in offspring paternity. The mode of sperm competition in this species, thus, conforms largely to a fair raffle following complete mixing of sperm prior to fertilisation. Hence, male differences in both the ability to copulate for long and of rapid sperm transfer will translate directly into differences in reproductive success.     

Sexual selection and interacting phenotypes in experimental evolution: a study of Drosophila pseudoobscura mating behavior

Sexual selection requires social interactions, particularly between the sexes. When trait expression is influenced by social interactions, such traits are called interacting phenotypes and only recently have the evolutionary consequences of interacting phenotypes been considered. Here we investigated how variation in relative fitness, or the opportunity for sexual selection, affected the evolutionary trajectories of interacting phenotypes. We used experimentally evolved populations of the naturally promiscuous Drosophila pseudoobscura , in which the numbers of potential interactions between the sexes, and therefore relative fitness, were manipulated by altering natural levels of female promiscuity. We considered two different mating interactions between the sexes: mating speed and copulation duration. We investigated the evolutionary trajectories of means and (co)variances ( P ) and also the influence of genetic drift on the evolutionary response of these interactions. Our sexual selection treatments did not affect the means of either mating speed or copulation duration, but they did affect P . We found that the means of both traits differed among replicates within each selection treatment whereas the P s did not. Changes as a consequence of genetic drift were excluded. Our results show that although variable potential strengths of sexual interactions influence the evolution of interacting phenotypes, the influence may be nonlinear.     

Longevity and resistance to cold stress in cold‐stress selected lines and their controls in Drosophila melanogaster

Thermal environments can influence many fitness‐related traits including life span. Here, we assess whether longevity in Drosophila melanogaster can experimentally evolve as a correlated response to cold‐stress selection, and whether genotype‐by‐temperature and sex‐by‐temperature interactions are significant components of variation in life span. Three replicated S lines were cold‐stress selected and compared with their respective unselected controls (Clines) in the 16^th generation of thermal selection. Cold‐stress resistance exhibited a substantial direct response to selection, and also showed a significant interaction between sex and type of line. Mean longevity exhibited a significant interaction between adult test temperature (14 and 25 °C) and line (with suggestive evidence for increased longevity of S lines when tested at 14 °C), but there was no evidence for increased longevity in S lines at normal temperatures (i.e. 25 °C). Another temperature‐dependent effect was sex‐specific, with males being the longer lived sex at 25 °C but the less long‐lived sex at 14 °C. Additionally, we tested in an exploratory way the relationship between longevity and cold‐stress resistance by also measuring resistance to a prefreezing temperature before and after one generation of longevity selection at 14 °C (selection intensity, i = 1.47 for S lines, and 1.42 for C lines). In this longevity selection, we found that cold‐stress resistance increased by about 6% in S lines and 18% in C lines. However, taken together, the results indicate no simple relationship between longevity and cold‐stress resistance, with genotype‐by‐sex interactions in both traits. Temperature dependent interaction in longevity is apparent between S and C lines, and sex‐specific variation in mean longevity also depends on temperature.     

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.     

Consequences of adaptation to larval crowding on sexual and fecundity selection in Drosophila melanogaster

Sexual selection is a major force influencing the evolution of sexually reproducing species. Environmental factors such as larval density can manipulate adult condition and influence the direction and strength of sexual selection. While most studies on the influence of larval crowding on sexual selection are either correlational or single-generation manipulations, it is unclear how evolution under chronic larval crowding affects sexual selection. To answer this, we measured the strength of sexual selection on male and female Drosophila melanogaster that had evolved under chronic larval crowding for over 250 generations in the laboratory, along with their controls which had never experienced crowding, in a common garden high-density environment. We measured selection coefficients on male mating success and sex-specific reproductive success, as separate estimates allowed dissection of sex-specific effects. We show that experimental evolution under chronic larval crowding decreases the strength of sexual and fecundity selection in males but not in females, relative to populations experiencing crowding for the first time. The effect of larval crowding in reducing reproductive success is almost twice in females than in males. Our study highlights the importance of studying how evolution in a novel, stressful environment can shape adult fitness in organisms.     

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.     

Quantitative genetics of learning ability and resistance to stress in Drosophila melanogaster

Even though laboratory evolution experiments have demonstrated genetic variation for learning ability, we know little about the underlying genetic architecture and genetic relationships with other ecologically relevant traits. With a full diallel cross among twelve inbred lines of Drosophila melanogaster originating from a natural population (0.75 < F < 0.93), we investigated the genetic architecture of olfactory learning ability and compared it to that for another behavioral trait (unconditional preference for odors), as well as three traits quantifying the ability to deal with environmental challenges: egg‐to‐adult survival and developmental rate on a low‐quality food, and resistance to a bacterial pathogen. Substantial additive genetic variation was detected for each trait, highlighting their potential to evolve. Genetic effects contributed more than nongenetic parental effects to variation in traits measured at the adult stage: learning, odorant perception, and resistance to infection. In contrast, the two traits quantifying larval tolerance to low‐quality food were more strongly affected by parental effects. We found no evidence for genetic correlations between traits, suggesting that these traits could evolve at least to some degree independently of one another. Finally, inbreeding adversely affected all traits.     

The effects of selection for cold tolerance on cross-tolerance to other environmental stressors in Drosophila melanogaster

Abstract  Cross tolerance, whereby tolerance to one environmental stress is correlated with tolerance to other stressors, is thought to be widespread in insects. We used lines of Drosophila melanogaster Meigen (Diptera: Drosophilidae) selected for survival at a 1-h exposure to −5°C to examine the extent to which this selection results in increased tolerance to other stresses, including high and low temperatures, desiccation and starvation. While selection improved tolerance to acute cold exposure and survival at −5°C, there was little effect of selection regime on tolerance to other stressors. There was no correlation between tolerances to any of the stressors, suggesting different mechanisms of tolerance. This supports arguments that correlations between stress tolerances during selection experiments with D. melanogaster may be coincidental. The magnitude of heat-hardening was apparently constrained by basal tolerance among lines, but the magnitude of the rapid cold-hardening response was not correlated with basal cold tolerance, implying that the relationship between inducible and basal tolerances differs at high and low temperatures.     

The influence of developmental stage on cold shock resistance and ability to cold-harden in Drosophila melanogaster

Thermal sensitivity and ability to rapidly cold- and heat-harden may change during ontogeny. This study reports how inherent cold tolerance and ability to rapidly cold-harden change across eight developmental stages in both genders of Drosophila melanogaster using a similar experimental approach for all stages. Inherent cold tolerance was estimated as LT50 by assaying cold shock survival over a wide range of temperatures (-16 to 5 degrees C). Rapid cold-hardening (RCH) was applied by cooling from 25 to 0 degrees C at -0.25 degrees C min(-1) followed by 1 h at 0 degrees C. Individuals were cold shocked either directly or after RCH to estimate the effect of RCH. We found large variation in cold tolerance among developmental stages and minor differences between genders. Eggs were most tolerant followed by adults, pupae and larvae. In the light of this and other studies it is suggested that there is a general pattern of stage specific thermal stress resistance in Drosophila. The capacity to rapidly cold-harden was found in both sexes of larval, pupal and adult stages, though some developmental stages showed negative or neutral effects of RCH which was probably due to the cost associated with the hardening treatment in these cold susceptible stages. The early presence of RCH indicates that the mechanisms behind hardening are not stage specific and that RCH may be an ecologically important trait in early stages of ontogeny.     

Natural selection in the laboratory for a change in resistance byDrosophila melanogaster to the parasitoid waspAsobara tabida

The selection response of the polymorphic hostD. melanogaster (Meigen) to the braconid waspA. tabida (Nees) is addressed. Cages of flies with and without wasps were initiated with a population ofD. melanogaster that exhibited variation both in larval foraging behavior and in encapsulation ability. Encapsulation ability was measured as the proportion of parasitized larvae that produce a hardened capsule which encapsulates the wasp egg and ultimately kills the wasp larva. We determined whether the host population changed its encapsulation ability and/or its foraging behavior in response to the wasp. Both species were collected from a local orchard whereA. tabida is the only wasp known to parasitizeD. melanogaster larvae. The naturally occurring genetic polymorphism for rover and sitter larval foraging behavior inD. melanogaster is also found in this field population.A. tabida's vibrotactic search behavior enables it to detect rover more frequently than sitter larvae. Rover larvae move significantly more while feeding than do sitter larvae. In this field population, rover larvae also show higher encapsulation abilities than do sitter larvae. Six cage populations, three without wasps and three with wasps, each containing an equal mixture of rover and sitter flies, were established in the laboratory and maintained for 19 fly generations. Selection pressure in the laboratory was similar to that found in the field population from which the flies and wasps were derived. We found that larvae from cages with wasps developed a significantly higher frequency of encapsulation than those reared without wasps. We were, however, unable to detect a change in larval movement (rover or sitter behavior) in larvae from cages subject to selection from wasps compared to larvae from cages containing no wasps. This may have resulted from a balance between two selective forces, selection against rovers by the wasps' use of vibrotaxis, and selection for rovers resulting from their increased encapsulation abilities