Physiological basis of starvation resistance in Drosophila leontia: analysis of sexual dimorphism

Improvement of starvation resistance via periodic fasting is genetically variable in Drosophila melanogaster

Organisms subjected to periodic nutrient limitation early in life exhibit improvements in aspects of survival, including resistance to some environmental stressors. Recent findings indicate that forms of periodic fasting such as intermittent fasting and time restricted feeding can improve starvation resistance. However, it remains unclear to what extent this survival improvement persists across different genetic backgrounds. In this study, we examine fasting-induced starvation resistance across a broad survey of wild-derived lineages and document genetic variation within this trait. We adopt a standard dietary intervention and show improvement to starvation resistance within a common laboratory lineage, replicating previous results. Next, we examine fasting-induced starvation resistance across isofemale lines collected across latitudes and in different seasons, and among inbred lines derived from flies collected on different continents. We discover genetic variation of fasting-induced starvation resistance, and show that fasting improved starvation resistance as often as it worsened starvation resistance. Fasted flies generally showed reduced fat concentration, and their starvation survival varied with sex, season of collection, and geographic origin. While specific lineages common to the laboratory can show a specific fasting-induced phenotype, we show that this result is not consistent across genetic backgrounds, reinforcing the idea that phenotypes observed in historic laboratory strains may not be conserved across a species.

Exposure to polystyrene microplastic beads causes sex-specific toxic effects in the model insect Drosophila melanogaster

The toxicity of MPs on aquatic creatures has been extensively studied, but little attention was paid to terrestrial organisms. To fill this gab, we conducted a series of experiments using Drosophila as a model organism to understand whether exposure to different concentrations (0.005, 0.05, 0.5 µg/ml) of polystyrene microplastics (PS-MPs) beads (2 µm in size) can impact flies feeding activity, digestion and excretion. The ability of flies to distinguish between normal and PS-MPs treated food media was tested first, and then we evaluated the effects of a 7-day short-term exposure to PS-MPs on food intake, mortality, starvation resistance, fecal pellet count, and the cellular structure of mid gut cells. The results revealed that flies can really differentiate and ignore MPs-treated food. We discovered sex-specific effects, with male flies being more sensitive to PS-MPs, with all males dying after 14 days when exposed to 0.5 µg/ml of PS-MPs, whereas female flies survived more. All male flies exposed to PS-MPs died after 24 h of starvation. Midgut cells showed concentration-dependent necrosis and apoptosis in response to PS-MPs. Our findings provide new insights into MPs toxicity on terrestrial organisms and giving a warning that management measures against MPs emission must be taken.

An analysis of direct and indirect effects in Drosophila melanogaster undergoing a few cycles of experimental evolution for stress-related traits

The physiological mechanisms underpinning adaptations to starvation and cold stresses have been extensively studied in Drosophila, yet the understanding of correlated changes in stress-related and life-history traits, as well as the energetics of stress tolerance, still remains elusive. To answer the questions empirically in this context, we allowed D. melanogaster to evolve for either increased starvation or cold tolerance (24-generations / regime) in an experimental evolution system, and examined whether selection of either trait affects un-selected stress trait, as well as the impacts potential changes in life-history and mating success-related traits. Our results revealed remarkable changes in starvation/cold tolerance (up to 1.5-fold) as a direct effect of selection, while cold tolerance had been dramatically reduced (1.26-fold) in the starvation tolerant (ST) lines compared to control counterparts, although no such changes were evident in cold-tolerant (CT) lines. ST lines exhibited a higher level of body lipids and a reduced level of trehalose content, while CT lines accumulated a greater levels of body lipid and trehalose contents. Noticeably, we found that selection for starvation or cold tolerance positively correlates with larval development time, longevity, and copulation duration, indicating that these traits are among the most common targets of selection trajectories shaping stress tolerance. Altogether, this study highlights the complexity of mechanisms evolved in ST lines that contribute to enhanced starvation tolerance, but also negatively impact cold tolerance. Nevertheless, mechanisms foraging enhanced cold tolerance in CT lines appear not to target starvation tolerance. Moreover, the parallel changes in life history/mating success traits across stress regimes could indicate some generic pathways evolved in stressful environments, targeting life-history and mating success characteristics to optimize fitness.

Alterations in brain glycogen levels influence life-history traits and reduce the lifespan in female Drosophila melanogaster

Sexual dimorphism in lifespan, wherein females outlive males, is evident across all animal taxa. The longevity difference between sexes is controlled by multiple physiological processes with complex relationships to one another. In recent years, glycogen, the storage form of glucose, has been shown to cause rapid aging upon forced synthesis in healthy neurons. Glycogen in the form of corpora amylacea in the aging brain is also widely reported. While these studies did suggest a novel role for glycogen in aging, most of them have focused on pooled samples, and have not looked at sex-specific effects, if any. Given the widespread occurrence of sex-biased expression of genes and the underlying physiology, it is important to look at the sex-specific effects of metabolic processes. In the present study, using transgenic fly lines for the human glycogen synthase, we investigated the sex-specific effects of glycogen on stress resistance, fitness, and survival. We demonstrate that Drosophila melanogaster females with altered levels of glycogen in the brain display a shortened lifespan, increased resistance to starvation, and higher oxidative stress than male flies. The present study thus provides a novel insight into the sex-specific effect of glycogen in survival and aging and how differences in metabolic processes could contribute to sex-specific traits.

Phenotypic coupling of sleep and starvation resistance evolves in D. melanogaster

BACKGROUND

One hypothesis for the function of sleep is that it serves as a mechanism to conserve energy. Recent studies have suggested that increased sleep can be an adaptive mechanism to improve survival under food deprivation in Drosophila melanogaster. To test the generality of this hypothesis, we compared sleep and its plastic response to starvation in a temperate and tropical population of Drosophila melanogaster.

RESULTS

We found that flies from the temperate population were more starvation resistant, and hypothesized that they would engage in behaviors that are considered to conserve energy, including increased sleep and reduced movement. Surprisingly, temperate flies slept less and moved more when they were awake compared to tropical flies, both under fed and starved conditions, therefore sleep did not correlate with population-level differences in starvation resistance. In contrast, total sleep and percent change in sleep when starved were strongly positively correlated with starvation resistance within the tropical population, but not within the temperate population. Thus, we observe unexpectedly complex relationships between starvation and sleep that vary both within and across populations. These observations falsify the simple hypothesis of a straightforward relationship between sleep and energy conservation. We also tested the hypothesis that starvation is correlated with metabolic phenotypes by investigating stored lipid and carbohydrate levels, and found that stored metabolites partially contributed towards variation starvation resistance.

CONCLUSIONS

Our findings demonstrate that the function of sleep under starvation can rapidly evolve on short timescales and raise new questions about the physiological correlates of sleep and the extent to which variation in sleep is shaped by natural selection.

Evolution of starvation resistance in an invasive insect species, Aethina tumida (Coleoptera: Nitidulidae)

Starvation resistance, or the ability to survive periods without food, can shed light on selection pressure imposed by food scarcity, including chances to invade new regions as a result of human transport. Surprisingly, little information is known about starvation resistance for invasive insect species. Given that native and invasive populations differ in starvation resistance, this would suggest different selection scenarios and adaptive shifts fostering invasion success. Here, we show striking differences in starvation resistance of adult small hive beetles Aethina tumida (SHB) between native and invasive populations. In the laboratory, starvation resistance of freshly emerged laboratory-reared and field-collected adult females and males was evaluated in the beetle's native African range and in their invasive North American range. SHB in their native African range survived longer than SHB in their invasive North American range. Across ranges, females survived longer than males. Field-collected SHB survived in Africa longer than freshly emerged ones, but not in the invasive range. This suggests no selection for starvation resistance in the invasive range, possibly due to differences between African and European-derived honey bee hosts facilitating a trade-off scenario between reproduction and starvation resistance. The ability of adult females to survive up to two months without food appears to be one factor contributing to the invasion success of this species. Assuming food availability is usually high in the invasive ranges, and trade-offs between starvation resistance and fecundity/reproduction are common, it seems as if selection for starvation resistance during transport could set up potential trade-offs that enhance reproduction after invasion. It would be interesting to see if this is a possible general pattern for invasive insect species.

Effects of the winter temperature regime on survival, body mass loss and post-winter starvation resistance in laboratory-reared and field-collected ladybirds

Ongoing climate change results in increasing temperatures throughout the seasons. The effects of climate change on insect performance are less studied during the winter season than during the growing season. Here, we investigated the effects of various winter temperature regimes (warm, normal and cold) on the winter performance of the invasive ladybird Harmonia axyridis (Coleoptera: Coccinellidae). Winter survival, body mass loss and post-winter starvation resistance were measured for a laboratory-reared population as well as three populations collected from the field prior to overwintering. The warm winter regime increased the survival rate and body mass loss and reduced post-winter starvation resistance compared to those of the ladybirds in the cold winter regime. The effects of the temperature regime were qualitatively similar for the laboratory-reared and field-collected beetles; however, there were significant quantitative differences in all measured overwintering parameters between the laboratory-reared and field-collected populations. The winter survival of the laboratory-reared beetles was much lower than that of the field-collected beetles. The laboratory-reared beetles also lost a larger proportion of their body mass and had reduced post-winter starvation resistance. Winter survival was similar between the females and males, but compared to the males, the females lost a smaller proportion of their body mass and had better post-winter starvation resistance. The pre-overwintering body mass positively affected winter survival and post-winter starvation resistance in both the laboratory-reared and field-collected ladybirds. The significant differences between the laboratory-reared and field-collected individuals indicate that quantitative conclusions derived from studies investigating solely laboratory-reared individuals cannot be directly extrapolated to field situations.

Age, sex, adult and larval diet shape starvation resistance in the Mediterranean fruit fly: an ecological and gerontological perspective

The ability of an animal to withstand periods of food deprivation is a key driver of invasion success (biodiversity), adaptation to new conditions, and a crucial determinant of senescence in populations. Starvation resistance (SR) is a highly plastic trait and varies in relation to environmental and genetic variables. However, beyond Drosophila, SR has been studied poorly. Exploiting an interesting model species in invasion and ageing studies-the Mediterranean fruit fly (Ceratitis capitata)- we investigated how age, food and gender, shape SR in this species. We measured SR in adults feeding in rich and poor dietary conditions, which had been reared either on natural hosts or artificial larval diet, for every single day across their lifespan. We defined which factor is the most significant determinant of SR and we explored potential links between SR and ageing. We found that SR declines with age, and that age-specific patterns are shaped in relation to adult and larval diet. Females exhibited higher SR than males. Age and adult diet were the most significant determinants of SR, followed by gender and the larval diet. Starvation resistance proved to be a weak predictor of functional ageing. Possible underlying mechanisms, ecological and gerontological significance and potential applied benefits are discussed.

Starvation resistance is associated with developmentally specified changes in sleep, feeding and metabolic rate

Food shortage represents a primary challenge to survival, and animals have adapted diverse developmental, physiological and behavioral strategies to survive when food becomes unavailable. Starvation resistance is strongly influenced by ecological and evolutionary history, yet the genetic basis for the evolution of starvation resistance remains poorly understood. The fruit fly Drosophila melanogaster provides a powerful model for leveraging experimental evolution to investigate traits associated with starvation resistance. While control populations only live a few days without food, selection for starvation resistance results in populations that can survive weeks. We have previously shown that selection for starvation resistance results in increased sleep and reduced feeding in adult flies. Here, we investigate the ontogeny of starvation resistance-associated behavioral and metabolic phenotypes in these experimentally selected flies. We found that selection for starvation resistance resulted in delayed development and a reduction in metabolic rate in larvae that persisted into adulthood, suggesting that these traits may allow for the accumulation of energy stores and an increase in body size within these selected populations. In addition, we found that larval sleep was largely unaffected by starvation selection and that feeding increased during the late larval stages, suggesting that experimental evolution for starvation resistance produces developmentally specified changes in behavioral regulation. Together, these findings reveal a critical role for development in the evolution of starvation resistance and indicate that selection can selectively influence behavior during defined developmental time points.

Heat and humidity induced plastic changes in body lipids and starvation resistance in the tropical Zaprionus indianus of wet - dry seasons

Insects from tropical wet or dry seasons are likely to cope starvation stress through plastic changes (developmental as well as adult acclimation) in energy metabolites. Control and experimental groups of flies of Zaprionus indianus were reared under wet or dry conditions but adults were acclimated at different thermal or humidity conditions. Adult flies of control group were acclimated at 27°C and low (50% RH) or high (60% RH) humidity. For experimental groups, adult flies were acclimated at 32℃ for 1 to 6 days and under low (40% RH) or high (70% RH). For humidity acclimation, adult flies were acclimated at 27°C but under low (40% RH) or high (70% RH) for 1 to 6 days. Plastic changes in experimental groups as compared to control group (developmental as well as adult acclimation) revealed significant accumulation of body lipids due to thermal or humidity acclimation of wet season flies but low humidity acclimation did not change the level of body lipids in dry season flies. Starvation resistance and body lipids were higher in the males of dry season but in the females of wet season. Adult acclimation under thermal or humidity conditions exhibited changes in the rate of utilization of body lipids, carbohydrates and proteins. Adult acclimation of wet or dry season flies revealed plastic changes in mean daily fecundity; and a reduction in fecundity under starvation. Thus, thermal or humidity acclimation of adults revealed plastic changes in energy metabolites to support starvation resistance of wet or dry seasons flies.

Adaptation to fluctuating environments in a selection experiment with Drosophila melanogaster

A fundamental question in life‐history evolution is how organisms cope with fluctuating environments, including variation between stressful and benign conditions. For short‐lived organisms, environments commonly vary between generations. Using a novel experimental design, we exposed wild‐derived Drosophila melanogaster to three different selection regimes: one where generations alternated between starvation and benign conditions, and starvation was always preceded by early exposure to cold; another where starvation and benign conditions alternated in the same way, but cold shock sometimes preceded starvation and sometimes benign conditions; and a third where conditions were always benign. Using six replicate populations per selection regime, we found that selected flies increased their starvation resistance, most strongly for the regime where cold and starvation were reliably combined, and this occurred without decreased fecundity or extended developmental time. The selected flies became stress resistant, displayed a pronounced increase in early life food intake and resource storage. In contrast to previous experiments selecting for increased starvation resistance in D. melanogaster, we did not find increased storage of lipids as the main response, but instead that, in particular for females, storage of carbohydrates was more pronounced. We argue that faster mobilization of carbohydrates is advantageous in fluctuating environments and conclude that the phenotype that evolved in our experiment corresponds to a compromise between the requirements of stressful and benign environments.

Larval starvation improves metabolic response to adult starvation in honey bees (Apis mellifera L.)

Environmental changes during development have long-term effects on adult phenotypes in diverse organisms. Some of the effects play important roles in helping organisms adapt to different environments, such as insect polymorphism. Others, especially those resulting from an adverse developmental environment, have a negative effect on adult health and fitness. However, recent studies have shown that those phenotypes influenced by early environmental adversity have adaptive value under certain (anticipatory) conditions that are similar to the developmental environment, though evidence is mostly from morphological and behavioral observations and it is still rare at physiological and molecular levels. In the companion study, we applied a short-term starvation treatment to fifth instar honey bee larvae and measured changes in adult morphology, starvation resistance, hormonal and metabolic physiology and gene expression. Our results suggest that honey bees can adaptively respond to the predicted nutritional stress. In the present study, we further hypothesized that developmental starvation specifically improves the metabolic response of adult bees to starvation instead of globally affecting metabolism under well-fed conditions. Here, we produced adult honey bees that had experienced a short-term larval starvation, then we starved them for 12 h and monitored metabolic rate, blood sugar concentrations and metabolic reserves. We found that the bees that experienced larval starvation were able to shift to other fuels faster and better maintain stable blood sugar levels during starvation. However, developmental nutritional stress did not change metabolic rates or blood sugar levels in adult bees under normal conditions. Overall, our study provides further evidence that early larval starvation specifically improves the metabolic responses to adult starvation in honey bees.

Relative Importance of Sex, Pre-Starvation Body Mass and Structural Body Size in the Determination of Exceptional Starvation Resistance of Anchomenus dorsalis (Coleoptera: Carabidae)

In nature, almost all animals have to cope with periods of food shortage during their lifetimes. Starvation risks are especially high for carnivorous predatory species, which often experience long intervals between stochastic prey capturing events. A laboratory experiment using the common predatory carabid beetle Anchomenus dorsalis revealed an exceptional level of starvation resistance in this species: males survived up to 137 days and females up to 218 days without food at 20°C. Individual starvation resistance was strongly positively affected by pre-starvation body mass but only slightly by beetle structural body size per se. Females outperformed males even when the effect of gender was corrected for the effects of structural body size and pre-starvation body mass. The better performance of females compared to males and of beetles with higher relative pre-starvation body mass could be linked to higher fat content and lean dry mass before starvation, followed by a greater decrease in both during starvation. There was also a difference between the sexes in the extent of body mass changes both during ad libitum feeding and following starvation; the body masses of females fluctuated more compared to males. This study stresses the need to distinguish between body mass and structural body size when investigating the ecological and evolutionary consequences of body size. Investigation of the net effects of body size and sex is necessary to disentangle the causes of differences in individual performances in studies of species with significant sexual size dimorphism.

Obesity-associated cardiac dysfunction in starvation-selected Drosophila melanogaster

There is a clear link between obesity and cardiovascular disease, but the complexity of this interaction in mammals makes it difficult to study. Among the animal models used to investigate obesity-associated diseases, Drosophila melanogaster has emerged as an important platform of discovery. In the laboratory, Drosophila can be made obese through lipogenic diets, genetic manipulations, and adaptation to evolutionary stress. While dietary and genetic changes that cause obesity in flies have been demonstrated to induce heart dysfunction, there have been no reports investigating how obesity affects the heart in laboratory-evolved populations. Here, we studied replicated populations of Drosophila that had been selected for starvation resistance for over 65 generations. These populations evolved characteristics that closely resemble hallmarks of metabolic syndrome in mammals. We demonstrate that starvation-selected Drosophila have dilated hearts with impaired contractility. This phenotype appears to be correlated with large fat deposits along the dorsal cuticle, which alter the anatomical position of the heart. We demonstrate a strong relationship between fat storage and heart dysfunction, as dilation and reduced contractility can be rescued through prolonged fasting. Unlike other Drosophila obesity models, the starvation-selected lines do not exhibit excessive intracellular lipid deposition within the myocardium and rather store excess triglycerides in large lipid droplets within the fat body. Our findings provide a new model to investigate obesity-associated heart dysfunction.

Desiccation tolerance as a function of age, sex, humidity and temperature in adults of the African malaria vectors Anopheles arabiensis and Anopheles funestus

Adult mosquito survival is strongly temperature and moisture dependent. Few studies have investigated the interacting effects of these variables on adult survival and how this differs among the sexes and with age, despite the importance of such information for population dynamic models. For these reasons, the desiccation tolerance of Anopheles arabiensis Patton and Anopheles funestus Giles males and females of three different ages was assessed under three combinations of temperature and humidity. Females were more desiccation tolerant than males, surviving for longer periods than males under all experimental conditions. In addition, younger adults were more tolerant of desiccation than older groups. Both species showed reduced water loss rate (WLR) as the primary mechanism by which they tolerate desiccation. Although A. arabiensis is often considered to be the more arid-adapted of the two species, it showed lower survival times and higher WLR than A. funestus. The current information could improve population dynamic models of these vectors, given that adult survival information for such models is relatively sparse.