The plastic fly: the effect of sustained fluctuations in adult food supply on life‐history traits

Many adult traits in Drosophila melanogaster show phenotypic plasticity, and the effects of diet on traits such as lifespan and reproduction are well explored. Although plasticity in response to food is still present in older flies, it is unknown how sustained environmental variation affects life‐history traits. Here, we explore how such life‐long fluctuations of food supply affect weight and survival in groups of flies and affect weight, survival and reproduction in individual flies. In both experiments, we kept adults on constant high or low food and compared these to flies that experienced fluctuations of food either once or twice a week. For these ‘yoyo’ groups, the initial food level and the duration of the dietary variation differed during adulthood, creating four ‘yoyo’ fly groups. In groups of flies, survival and weight were affected by adult food. However, for individuals, survival and reproduction, but not weight, were affected by adult food, indicating that single and group housing of female flies affects life‐history trajectories. Remarkably, both the manner and extent to which life‐history traits varied in relation to food depended on whether flies initially experienced high or low food after eclosion. We therefore conclude that the expression of life‐history traits in adult life is affected not only by adult plasticity, but also by early adult life experiences. This is an important but often overlooked factor in studies of life‐history evolution and may explain variation in life‐history experiments.     

Cancer brings forward oviposition in the fly Drosophila melanogaster

Hosts often accelerate their reproductive effort in response to a parasitic infection, especially when their chances of future reproduction decrease with time from the onset of the infection. Because malignancies usually reduce survival, and hence potentially the fitness, it is expected that hosts with early cancer could have evolved to adjust their life‐history traits to maximize their immediate reproductive effort. Despite the potential importance of these plastic responses, little attention has been devoted to explore how cancers influence animal reproduction. Here, we use an experimental setup, a colony of genetically modified flies Drosophila melanogaster which develop colorectal cancer in the anterior gut, to show the role of cancer in altering life‐history traits. Specifically, we tested whether females adapt their reproductive strategy in response to harboring cancer. We found that flies with cancer reached the peak period of oviposition significantly earlier (i.e., 2 days) than healthy ones, while no difference in the length and extent of the fecundity peak was observed between the two groups of flies. Such compensatory responses to overcome the fitness‐limiting effect of cancer could explain the persistence of inherited cancer‐causing mutant alleles in the wild.     

Evolutionary implications of mitochondrial genetic variation: mitochondrial genetic effects on OXPHOS respiration and mitochondrial quantity change with age and sex in fruit flies

The ancient acquisition of the mitochondrion into the ancestor of modern‐day eukaryotes is thought to have been pivotal in facilitating the evolution of complex life. Mitochondria retain their own diminutive genome, with mitochondrial genes encoding core subunits involved in oxidative phosphorylation. Traditionally, it was assumed that there was little scope for genetic variation to accumulate and be maintained within the mitochondrial genome. However, in the past decade, mitochondrial genetic variation has been routinely tied to the expression of life‐history traits such as fertility, development and longevity. To examine whether these broad‐scale effects on life‐history trait expression might ultimately find their root in mitochondrially mediated effects on core bioenergetic function, we measured the effects of genetic variation across twelve different mitochondrial haplotypes on respiratory capacity and mitochondrial quantity in the fruit fly, Drosophila melanogaster. We used strains of flies that differed only in their mitochondrial haplotype, and tested each sex separately at two different adult ages. Mitochondrial haplotypes affected both respiratory capacity and mitochondrial quantity. However, these effects were highly context‐dependent, with the genetic effects contingent on both the sex and the age of the flies. These sex‐ and age‐specific genetic effects are likely to resonate across the entire organismal life‐history, providing insights into how mitochondrial genetic variation may contribute to sex‐specific trajectories of life‐history evolution.     

Dietary consumption of monosodium L‐glutamate induces adaptive response and reduction in the life span of Drosophila melanogaster

Adaptive response is the ability of an organism to better counterattack stress‐induced damage in response to a number of different cytotoxic agents. Monosodium L‐glutamate (MSG), the sodium salt of amino acid glutamate, is commonly used as a food additive. We investigated the effects of MSG on the life span and antioxidant response in Drosophila melanogaster (D. melanogaster). Both genders (1 to 3 days old) of flies were fed with diet containing MSG (0.1, 0.5, and 2.5‐g/kg diet) for 5 days to assess selected antioxidant and oxidative stress markers, while flies for longevity were fed for lifetime. Thereafter, the longevity assay, hydrogen peroxide (H₂O₂), and reactive oxygen and nitrogen species levels were determined. Also, catalase, glutathione S‐transferase and acetylcholinesterase activities, and total thiol content were evaluated in the flies. We found that MSG reduced the life span of the flies by up to 23% after continuous exposure. Also, MSG increased reactive oxygen and nitrogen species and H₂O₂ generations and total thiol content as well as the activities of catalase and glutathione S‐transferase in D. melanogaster (P < .05). In conclusion, consumption of MSG for 5 days by D. melanogaster induced adaptive response, but long‐term exposure reduced life span of flies. This study may therefore have public health significance in humans, and thus, moderate consumption of MSG is advocated by the authors.     

Age‐related activity patterns are moderated by diet in Queensland fruit flies Bactrocera tryoni

Life‐history parameters and the fitness of tephritid flies are closely linked to diet. Studies of locomotor behaviour can provide insights to these links, although little is known about how locomotor behaviour is influenced by diet. In the present study, video recordings of Queensland fruit flies Bactrocera tryoni Froggatt (Diptera: Tephritidae) (‘Q‐flies’) that are maintained individually in cages are used to determine how diet affects the activity patterns (flight, walking, grooming, inactivity) of males and females at ages ranging from 4 to 30 days. The frequency and total duration of activities over 10‐min trials are affected by diet, age and sex. Supplementation of diet with hydrolysed yeast results in a higher frequency and duration of flight in flies of all ages and both sexes. The effect of diet on other activities varies with age. Q‐flies fed sugar only increase walking frequency steadily from 4 to 30 days post‐eclosion, whereas flies fed sugar + yeast have higher walking frequencies at 4 and 10 days than flies fed sugar only, although they then exhibit a sharp decline at 30 days post‐eclosion. The frequency and duration of inactivity remain consistent in flies fed sugar + yeast, whereas flies fed sugar only exhibit a marked increase in inactivity from 4 to 30 days post‐eclosion. Compared with older flies, 4 day‐old Q‐flies fed sugar only spend considerably more time grooming. The potential of activity monitoring as a quality control test for flies that are mass‐reared for use in sterile insect technique programmes is discussed.     

The development of a synthetic diet for investigating the effects of macronutrients on the development of Plodia interpunctella

The use of chemically defined artificial diets has allowed researchers to examine questions within nutritional ecology about how macronutrients affect life‐history traits and resource‐based trade‐offs. Using a chemically defined diet, it is possible to manipulate both the total nutritional content and the ratio of macronutrients (i.e., proteins, carbohydrates, or lipids) within the diet. Studies using the geometric framework have made use of these diets to examine lifespan, fecundity, and immune responses. Here, we develop an artificial diet suitable for rearing lepidopteran larvae. We created diets with three proportions of non‐nutritive material (30, 50, and 70% indigestible cellulose) relative to protein and carbohydrate macronutrients, and compared these to standard wheat bran laboratory diet. We then examined the effects of variable nutrient content on lifespan and development time in Plodia interpunctella Hübner (Lepidoptera: Pyralidae). The artificial diets supported development (almost) as well as bran‐based laboratory diets. Total nutrient content affected development time: females that fed on the diet with the highest nutrient content took the longest time to reach eclosion. We also found evidence to support dietary restriction, with larvae receiving the fewest nutrients having the longest lifespan as adults. These findings are indicative of the usefulness of this diet as a tool to further investigate the effects of nutrient content and macronutrient imbalance on resource‐based trade‐offs and life‐history traits.     

Dietary fatty acids promote sleep through a taste‐independent mechanism

Consumption of foods that are high in fat contribute to obesity and metabolism‐related disorders. Dietary lipids are comprised of triglycerides and fatty acids, and the highly palatable taste of dietary fatty acids promotes food consumption, activates reward centers in mammals and underlies hedonic feeding. Despite the central role of dietary fats in the regulation of food intake and the etiology of metabolic diseases, little is known about how fat consumption regulates sleep. The fruit fly, Drosophila melanogaster, provides a powerful model system for the study of sleep and metabolic traits, and flies potently regulate sleep in accordance with food availability. To investigate the effects of dietary fats on sleep regulation, we have supplemented fatty acids into the diet of Drosophila and measured their effects on sleep and activity. We found that flies fed a diet of hexanoic acid, a medium‐chain fatty acid that is a by‐product of yeast fermentation, slept more than flies starved on an agar diet. To assess whether dietary fatty acids regulate sleep through the taste system, we assessed sleep in flies with a mutation in the hexanoic acid receptor Ionotropic receptor 56D, which is required for fatty acid taste perception. We found that these flies also sleep more than agar‐fed flies when fed a hexanoic acid diet, suggesting the sleep promoting effect of hexanoic acid is not dependent on sensory perception. Taken together, these findings provide a platform to investigate the molecular and neural basis for fatty acid‐dependent modulation of sleep.     

A genetics‐based approach confirms immune associations with life history across multiple populations of an aquatic vertebrate (Gasterosteus aculeatus)

Understanding how wild immune variation covaries with other traits can reveal how costs and trade‐offs shape immune evolution in the wild. Divergent life history strategies may increase or alleviate immune costs, helping shape immune variation in a consistent, testable way. Contrasting hypotheses suggest that shorter life histories may alleviate costs by offsetting them against increased mortality, or increase the effect of costs if immune responses are traded off against development or reproduction. We investigated the evolutionary relationship between life history and immune responses within an island radiation of three‐spined stickleback, with discrete populations of varying life histories and parasitism. We sampled two short‐lived, two long‐lived and an anadromous population using qPCR to quantify current immune profile and RAD‐seq data to study the distribution of immune variants within our assay genes and across the genome. Short‐lived populations exhibited significantly increased expression of all assay genes, which was accompanied by a strong association with population‐level variation in local alleles and divergence in a gene that may be involved in complement pathways. In addition, divergence around the eda gene in anadromous fish is likely associated with increased inflammation. A wider analysis of 15 populations across the island revealed that immune genes across the genome show evidence of having diverged alongside life history strategies. Parasitism and reproductive investment were also important sources of variation for expression, highlighting the caution required when assaying immune responses in the wild. These results provide strong, gene‐based support for current hypotheses linking life history and immune variation across multiple populations of a vertebrate model.     

On the fate of seasonally plastic traits in a rainforest butterfly under relaxed selection

Many organisms display phenotypic plasticity as adaptation to seasonal environmental fluctuations. Often, such seasonal responses entails plasticity of a whole suite of morphological and life‐history traits that together contribute to the adaptive phenotypes in the alternative environments. While phenotypic plasticity in general is a well‐studied phenomenon, little is known about the evolutionary fate of plastic responses if natural selection on plasticity is relaxed. Here, we study whether the presumed ancestral seasonal plasticity of the rainforest butterfly Bicyclus sanaos (Fabricius, 1793) is still retained despite the fact that this species inhabits an environmentally stable habitat. Being exposed to an atypical range of temperatures in the laboratory revealed hidden reaction norms for several traits, including wing pattern. In contrast, reproductive body allocation has lost the plastic response. In the savannah butterfly, B. anynana (Butler, 1879), these traits show strong developmental plasticity as an adaptation to the contrasting environments of its seasonal habitat and they are coordinated via a common developmental hormonal system. Our results for B. sanaos indicate that such integration of plastic traits – as a result of past selection on expressing a coordinated environmental response – can be broken when the optimal reaction norms for those traits diverge in a new environment.     

Nectarine promotes longevity in Drosophila melanogaster

Fruits containing high antioxidant capacities and other bioactivities are ideal for promoting longevity and health span. However, few fruits are known to improve the survival and health span in animals, let alone the underlying mechanisms. Here we investigate the effects of nectarine, a globally consumed fruit, on life span and health span in Drosophila melanogaster. Wild-type flies were fed standard, dietary restriction (DR), or high-fat diet supplemented with 0-4% nectarine extract. We measured life span, food intake, locomotor activity, fecundity, gene expression changes, and oxidative damage indicated by the level of 4-hydroxynonenal-protein adduct in these flies. We also measured life span, locomotor activity, and oxidative damage in sod1 mutant flies on the standard diet supplemented with 0-4% nectarine. Supplementation with 4% nectarine extended life span, increased fecundity, and decreased expression of some metabolic genes, including a key gluconeogenesis gene, PEPCK, and oxidative stress-response genes, including peroxiredoxins, in female wild-type flies fed the standard, DR, or high-fat diet. Nectarine reduced oxidative damage in wild-type females fed the high-fat diet. Moreover, nectarine improved the survival of and reduced oxidative damage in female sod1 mutant flies. Together, these findings suggest that nectarine promotes longevity and health span partly by modulating glucose metabolism and reducing oxidative damage.     

Life history trait divergence among populations of a non‐palatable species reveals strong non‐trophic indirect effects of an abundant herbivore

When large herbivores exert selection on their prey plant species, co‐occurring, non‐prey species may experience selection through non‐trophic indirect effects. Such selection is likely common where herbivores are overabundant. Yet, empirical studies of non‐trophic indirect effects as drivers of non‐prey trait evolution are lacking. Here we test for adaptive shifts in life history traits in an unpalatable species, Arisaema triphyllum, a common forest perennial that is unique because it exhibits size‐dependent sex switching. We collected A. triphyllum from six sites that experience a gradient in abiotic stress caused by deer browse pressure on prey plant species that generate indirect effects. We grew A. triphyllum from these sites in a common garden for five years to evaluate life history predictions linking strong indirect effects and abiotic stress to changes in life history traits: flowering onset size threshold, female flowering size threshold, relative growth rate (RGR), biomass allocation, and asexual reproduction. Despite observed differences among phenotypes in the field, expression of flowering onset size threshold, biomass allocation, and asexual reproduction did not differ among the six populations in the garden, indicating common plastic responses. In contrast, A. triphyllum collected from sites experiencing the two highest deer impacts exhibited smaller female flowering size thresholds and the highest RGR. Responses in these traits support the predictions of adaptive divergence in response to indirect effects. Our results reinforce the idea that non‐trophic indirect effects of large herbivores can elicit evolutionary responses in some traits of non‐prey species. In general, life history traits of unpalatable species may be cryptically adapting to stressful indirect effects where large herbivores are overabundant.     

Effects of maternal diet on offspring fitness in the bird cherry‐oat aphid

1. Maternal and offspring diet effects on life‐history traits of the bird cherry‐oat aphid Rhopalosiphum padi were tested on three wheat varieties. Using nine reciprocal combinations of wheat varieties, the effects of previous experience (maternal diet effect) on the aphid's response to resistant and susceptible varieties (offspring diet effect) were tested. Batis was susceptible, and Xiaoyan22 and Ww2730 were both resistant, but with different mechanisms. 2. Aphids produced the most alatae in the treatments with the most resistant maternal diet variety Xiaoyan22. The fecundity (F) and intrinsic rate of increase (r_m) of these alatae were at their greatest in the most resistant offspring diet variety, but these traits were not influenced in the apterae. 3. There were significant interactions in the alatae production and apterae life‐history traits, such as r_m, development time, weight gain, and mean relative growth rate, between the maternal and offspring diet varieties. The interactions in apterae responses between varieties, some of which were reciprocal, indicated phenotypic plasticity in these parthenogenetic aphids. 4. Rhopalosiphum padi produced more alatae on the most resistant variety; the alatae would disperse and were more fecund. The growth responses of the apterae showed phenotypic plasticity to the different combinations of maternal and offspring diet varieties. The phenotypic plasticity would allow R. padi to better utilise the variable environments represented by the small wheat plots of different varieties in China.     

Adaptation to divergent larval diets in the medfly,Ceratitis capitata

Variation in diet can influence the timing of major life‐history events and can drive population diversification and ultimately speciation. Proximate responses of life histories to diet have been well studied. However, there are scant experimental data on how organisms adapt to divergent diets over the longer term. We focused on this omission by testing the responses of a global pest, the Mediterranean fruitfly, to divergent selection on larval diets of different nutritional profiles. Tests conducted before and after 30 generations of nutritional selection revealed a complex interplay between the effects of novel larval dietary conditions on both plastic and evolved responses. There were proximate‐only responses to the larval diet in adult male courtship and the frequency of copulation. Males on higher calorie larval diets consistently engaged in more bouts of energetic courtship. In contrast, following selection, larval development time, and egg to adult survival showed evidence of evolved divergence between diet regimes. Adult body size showed evidence for adaptation, with flies being significantly heavier when reared on their “own” diet. The results show the multifaceted responses of individuals to dietary selection and are important in understanding the extreme generalism exhibited by the medfly.     

Morpho-functional changes of fat body in bacteria fed Drosophila melanogaster strains

We have examined the addition of Escherichia coli to the diet at day 0 of adult life of females from two Oregon R Drosophila melanogaster strains, selected for different longevities: a short-life with an average adult life span of 10 days and a long-life standard R strain with an average adult life span of 50 days. The addition of bacteria to the diet significantly prolonged the fly longevity in both strains and affected the structure and histochemical reactivity of the fat body. The increased survival was characterized by great amount of glycogen accumulated in fat body cells from both strains. In aged control animals, fed with standard diet, lipid droplets were seen to be stored in fat body of short-lived, but not long-lived, flies. On the whole, our data indicate that exogenous bacteria are able to extend the survival of Drosophila females, and suggest that such a beneficial effect can be mediated, at least in part, by the fat body cells that likely play a role in modulating the accumulation and mobilization of reserve stores to ensure lifelong energy homeostasis.     

The effect of photoperiod on life history and blood‐feeding activity in Aedes albopictus and Aedes aegypti (Diptera: Culicidae)

Several studies have examined how climatic variables such as temperature and precipitation may affect life history traits in mosquitoes that are important to disease transmission. Despite its importance as a seasonal cue in nature, studies investigating the influence of photoperiod on such traits are relatively few. This study aims to investigate how photoperiod alters life history traits, survival, and blood‐feeding activity in Aedes albopictus (Skuse) and Aedes aegypti (Linnaeus). We performed three experiments that tested the effects of day length on female survival, development time, adult size, fecundity, adult life span, and propensity to blood feed in Ae. albopictus and Ae. aegypti. Each experiment had three photoperiod treatments: 1) short‐day (10L:14D), 2) control (12L:12D), and 3) long‐day (14L:10D). Aedes albopictus adult females were consistently larger in size when reared in short‐day conditions. Aedes aegypti adult females from short‐day treatments lived longer and were more likely to take a blood meal compared to other treatments. We discuss how species‐specific responses may reflect alternative strategies evolved to increase survival during unfavorable conditions. We review the potential impacts of these responses on seasonal transmission patterns, such as potentially increasing vectorial capacity of Ae. aegypti during periods of shorter day lengths.     

Diverse genotypes of the amphibian‐killing fungus produce distinct phenotypes through plastic responses to temperature

Phenotypes are the target of selection and affect the ability of organisms to persist in variable environments. Phenotypes can be influenced directly by genes and/or by phenotypic plasticity. The amphibian‐killing fungus Batrachochytrium dendrobatidis (Bd) has a global distribution, unusually broad host range, and high genetic diversity. Phenotypic plasticity may be an important process that allows this pathogen to infect hundreds of species in diverse environments. We quantified phenotypic variation of nine Bd genotypes from two Bd lineages (Global Pandemic Lineage [GPL] and Brazil) and a hybrid (GPL‐Brazil) grown at three temperatures (12, 18 and 24°C). We measured five functional traits including two morphological traits (zoospore and zoosporangium sizes) and three life history traits (carrying capacity, time to fastest growth and exponential growth rate) in a phylogenetic framework. Temperature caused highly plastic responses within each genotype, with all Bd genotypes showing phenotypic plasticity in at least three traits. Among genotypes, Bd generally showed the same direction of plastic response to temperature: larger zoosporangia, higher carrying capacity, longer time to fastest growth and slower exponential growth at lower temperatures. The exception was zoospore size, which was highly variable. Our findings indicate that Bd genotypes have evolved novel phenotypes through plastic responses to temperature over very short timescales. High phenotypic variability likely extends to other traits and may facilitate the large host range and rapid spread of Bd.     

Localization of quantitative trait loci for diapause and other photoperiodically regulated life history traits important in adaptation to seasonally varying environments

Seasonally changing environments at high latitudes present great challenges for the reproduction and survival of insects, and photoperiodic cues play an important role in helping them to synchronize their life cycle with prevalent and forthcoming conditions. We have mapped quantitative trait loci (QTL) responsible for the photoperiodic regulation of four life history traits, female reproductive diapause, cold tolerance, egg‐to‐eclosion development time and juvenile body weight in Drosophila montana strains from different latitudes in Canada and Finland. The F2 progeny of the cross was reared under a single photoperiod (LD cycle 16:8), which the flies from the Canadian population interpret as early summer and the flies from the Finnish population as late summer. The analysis revealed a unique QTL for diapause induction on the X chromosome and several QTL for this and the other measured traits on the 4th chromosome. Flies’ cold tolerance, egg‐to‐eclosion development time and juvenile body weight had several QTL also on the 2nd, 3rd and 5th chromosome, some of the peaks overlapping with each other. These results suggest that while the downstream output of females’ photoperiodic diapause response is partly under a different genetic control from that of the other traits in the given day length, all traits also share some QTL, possibly involving genes with pleiotropic effects and/or multiple tightly linked genes. Nonoverlapping QTL detected for some of the traits also suggest that the traits are potentially capable of independent evolution, even though this may be restricted by epistatic interactions and/or correlations and trade‐offs between the traits.     

Rapid evolution of thermal plasticity in mountain lake Daphnia populations

Populations at risk of extinction due to climate change may be rescued by adaptive evolution or plasticity. Selective agents, such as introduced predators, may enhance or constrain plastic or adaptive responses to temperature. We tested responses of Daphnia to temperature by collecting populations from lakes across an elevational gradient in the presence and absence of fish predators (long‐term selection). We subsequently grew these populations at two elevations in field mesocosms over two years (short‐term selection), followed by a common‐garden experiment at two temperatures in the lab to measure life‐history traits. Both long‐term and short‐term selection affected traits, suggesting that genetic variation of plasticity within populations enabled individuals to rapidly evolve plasticity in response to high temperature. We found that short‐term selection by high temperature increased plasticity for growth rate in all populations. Fecundity was higher in populations from fishless lakes and body size showed greater plasticity in populations from warm lakes (long‐term selection). Neither body size nor fecundity were affected by short‐term thermal selection. These results demonstrate that plasticity is an important component of the life‐history response of Daphnia, and that genetic variation within populations enabled rapid evolution of plasticity in response to selection by temperature.