Resource allocation to reproduction and soma in Drosophila: a stable isotope analysis of carbon from dietary sugar

Knockdown of the ZcVgR Gene Alters the Expression of Genes Related to Reproduction and Lifespan in Zeugodacus cucurbitae (Coquillett) Under Extreme Heat Conditions

Zeugodacus cucurbitae (Coquillett) is an important migratory vegetable pest. Previous research has demonstrated that short-term high temperatures induce differential expression of the vitellogenin receptor (ZcVgR) gene, reducing the number of eggs laid and the lifespan of female Z. cucurbitae. In this paper, we used Tandem Mass Tags (TMT) quantitative proteomics and Illumina high-throughput sequencing to determine the proteomic and transcriptomic information of female Z. cucurbitae after siRNA-mediated silencing of the target gene (ZcVgR) to gain a comprehensive understanding of the molecular mechanism of this gene in the regulation of reproduction and lifespan. The findings demonstrated that following the target gene's silencing, the ZcVgR gene's transcriptional expression was significantly downregulated, and there was no significant difference in protein level. The transcriptome and proteome had a low correlation; when the ZcVgR gene was silenced, vitellogenin-1 (ZcVg1), juvenile hormone epoxide hydrolase (JHEH), troponin C (TnC), heat shock protein 70 (HSP70), and other related genes were downregulated at the transcriptional level. By silencing the ZcVgR gene, transcriptionally level immune-related pathways were activated and energy metabolism-related pathways were inhibited; protein-level glycometabolism and phagosome pathways were activated, while phototransduction-fly and autophagy-animal pathways were inhibited. The findings of this study might offer a theoretical foundation for integrated management of Z. cucurbitae in the summertime.

Circulating fructose regulates a germline stem cell increase via gustatory receptor-mediated gut hormone secretion in mated Drosophila

Oogenesis is influenced by multiple environmental factors. In the fruit fly, Drosophila melanogaster, nutrition and mating have large impacts on an increase in female germline stem cells (GSCs). However, it is unclear whether these two factors affect this GSC increase interdependently. Here, we report that dietary sugars are crucial for the GSC increase after mating. Dietary glucose is required for mating-induced release of neuropeptide F (NPF) from enteroendocrine cells (EECs), followed by NPF-mediated enhancement of GSC niche signaling. Unexpectedly, dietary glucose does not directly act on NPF-positive EECs. Rather, it contributes to elevation of hemolymph fructose generated through the polyol pathway. Elevated fructose stimulates the fructose-specific gustatory receptor, Gr43a, in NPF-positive EECs, leading to NPF secretion. This study demonstrates that circulating fructose, derived from dietary sugars, is a prerequisite for the GSC increase that leads to enhancement of egg production after mating.

Antibody development to identify components of IIS and mTOR signaling pathways in lepidopteran species, a set of non-model insects

Nutritional stress impacts many insect species that have differing reproductive strategies and life histories, yet it is unclear how nutrient-sensing signaling pathways mediate tissue-specific responses to changes in dietary input. In Drosophila melanogaster , insulin/insulin-like growth factor (IIS) and mTOR-mediated signaling within adipocytes regulates oogenesis. To facilitate comparative study of nutrient-sensing pathway activity in the fat body, we developed antibodies to assess IIS (anti-FOXO) and mTOR signaling (anti-TOR) across three nymphalid species (Lepidoptera). By optimizing whole-mount fat body immunostaining, we find FOXO nuclear enrichment in adult adipocytes, like that observed in Drosophila . Additionally, we show a previously uncharacterized TOR localization pattern in the fat body.

Heightened immune surveillance in Drosophila melanogaster populations selected for faster development and extended longevity

Maximization of life-history traits is under constraints due to both, limitations of resource acquisition and the restricted pathways of resource allocation. Drosophila melanogaster has served as an excellent model organism to not only unravel various trade-offs among life history traits but also numerous aspects of host immune response. Drosophila larvae are semi-aquatic that live, feed and excrete inside the food source-often over-ripe fruits and vegetables that are rich in both commensal and pathogenic microbiota that can impact the larval survival. In this study, we have used six populations of D. melanogaster, three of which are selected for faster pre-adult development and extended adult longevity, and their three ancestral controls, to explore the impact of selection on the basal immune activity in the larval stage. The larvae from selected populations had nearly significantly upregulated plasmatocyte density, significantly higher percent phagocytosis, phagocytic index and higher transcript levels of Tep3, eater and NimC1. Selected populations also had significantly upregulated crystal cell number along with higher transcript of PPO2. Out of seven tested AMPs level, Drosomycin was significantly upregulated in selected populations while Drosocin was significantly higher in control populations. ROS levels were comparable in the selected and control populations. Our results strongly suggest that enhanced basal immune activity during larval stage manages the faster development and could be responsible for comparable larval survival of selected and control populations.

Utility of carbon and nitrogen stable isotopes for inferring wild bee (Hymenoptera: Apoidea) use of adjacent foraging habitats

Isotope analysis has proven useful for understanding diets of animals that are difficult to track for extended periods. Bees are small yet highly mobile and often forage from multiple habitats. However, current methods of assessing diet are limited in scope. Efficient methods of tracking bee diets that integrate across life stages, distinguish habitat use, and are sensitive to taxonomic differences will inform conservation strategies. We evaluated the utility of stable isotope analysis for estimating contributions of adjacent habitats to bees’ diets. We also investigated taxonomic variation in bee and flower isotope composition. We measured natural abundance of carbon and nitrogen stable isotopes in two body regions from three wild bee genera, as well as in 25 species of flowers that likely comprised their diets. Bee ∂13C and ∂15N varied with habitat and taxonomic groups (conflated with month), but did not match spatial or seasonal trends in their food plants. Flower ∂13C was lowest in the forest and in April–June, as expected if driven by water availability. However, bee ∂13C was elevated in the spring, likely from overwintering nutritional stress or unpredictable food availability. Bumble bees (Bombus) were enriched in ∂15N compared to others, possibly reflecting differences in larval feeding. Bee diet mixing models had high variation and should be interpreted with caution. Models estimated similar habitat contributions to diets of spring Andrena and overwintered Bombus queens. Summer Bombus queens and workers were indistinguishable. Sweat bees (Halictus) were estimated to use comparatively more field flowers than others. Overall, taxon more strongly influenced isotope composition than either foraging habitat or month, likely because of associated differences in sociality and timing of annual activity. Future studies seeking to reveal bee diets by isotope analysis may gain better resolution in more isotopically distinct habitats, in conjunction with controlled feeding or isotope labeling experiments.

Fitness consequences of biochemical adaptation in Drosophila melanogaster populations under simultaneous selection for faster pre-adult development and extended lifespan

In holometabolous insects like Drosophila melanogaster, critical size is an important time point during larval life, for irreversible commitment to metamorphosis. Here, we studied the impact of restricted growth duration in terms of selection for faster pre-adult development in Drosophila melanogaster populations which resulted in the evolution of reduced critical size on adult life history traits. Selection for faster pre-adult development resulted in biochemical adaptation in larval physiology with no compromise in major biomolecules at critical size time point. The flies from the selected populations seem to not only commit to metamorphosis on the attainment of critical size but also seem to channelize resources to reproduction as indicated by similar life-time fecundity of CS and NS flies from selected populations, while the Control CS flies significantly lower life-time fecundity compared to Control NS flies. The flies from selected populations seem to achieve longevity comparable to control flies despite being significantly smaller in size-thus resource constrained due to faster pre-adult development.

The Drosophila model to interrogate triacylglycerol biology

The deposition of storage fat in the form of triacylglycerol (TAG) is an evolutionarily conserved strategy to cope with fluctuations in energy availability and metabolic stress. Organismal TAG storage in specialized adipose tissues provides animals a metabolic reserve that sustains survival during development and starvation. On the other hand, excessive accumulation of adipose TAG, defined as obesity, is associated with an increasing prevalence of human metabolic diseases. During the past decade, the fruit fly Drosophila melanogaster, traditionally used in genetics and developmental biology, has been established as a versatile model system to study TAG metabolism and the etiology of lipid-associated metabolic diseases. Similar to humans, Drosophila TAG homeostasis relies on the interplay of organ systems specialized in lipid uptake, synthesis, and processing, which are integrated by an endocrine network of hormones and messenger molecules. Enzymatic formation of TAG from sugar or dietary lipid, its storage in lipid droplets, and its mobilization by lipolysis occur via mechanisms largely conserved between Drosophila and humans. Notably, dysfunctional Drosophila TAG homeostasis occurs in the context of aging, overnutrition, or defective gene function, and entails tissue-specific and organismal pathologies that resemble human disease. In this review, we summarize the physiology and biochemistry of TAG in Drosophila and outline the potential of this organism as a model system to understand the genetic and dietary basis of TAG storage and TAG-related metabolic disorders.

The Transcriptome in Transition: Global Gene Expression Profiles of Young Adult Fruit Flies Depend More Strongly on Developmental Than Adult Diet

Developmental diet is known to exert long-term effects on adult phenotypes in many animal species as well as disease risk in humans, purportedly mediated through long-term changes in gene expression. However, there are few studies linking developmental diet to adult gene expression. Here, we use a full-factorial design to address how three different larval and adult diets interact to affect gene expression in 1-day-old adult fruit flies (Drosophila melanogaster) of both sexes. We found that the largest contributor to transcriptional variation in young adult flies is larval, and not adult diet, particularly in females. We further characterized gene expression variation by applying weighted gene correlation network analysis (WGCNA) to identify modules of co-expressed genes. In adult female flies, the caloric content of the larval diet associated with two strongly negatively correlated modules, one of which was highly enriched for reproduction-related processes. This suggests that gene expression in young adult female flies is in large part related to investment into reproduction-related processes, and that the level of expression is affected by dietary conditions during development. In males, most modules had expression patterns independent of developmental or adult diet. However, the modules that did correlate with larval and/or adult dietary regimes related primarily to nutrient sensing and metabolic functions, and contained genes highly expressed in the gut and fat body. The gut and fat body are among the most important nutrient sensing tissues, and are also the only tissues known to avoid histolysis during pupation. This suggests that correlations between larval diet and gene expression in male flies may be mediated by the carry-over of these tissues into young adulthood. Our results show that developmental diet can have profound effects on gene expression in early life and warrant future research into how they correlate with actual fitness related traits in early adulthood.

Experimental evolution of post-ingestive nutritional compensation in response to a nutrient-poor diet

The geometric framework of nutrition predicts that populations restricted to a single imbalanced diet should evolve post-ingestive nutritional compensation mechanisms bringing the blend of assimilated nutrients closer to physiological optimum. The evolution of such nutritional compensation is thought to be mainly driven by the ratios of major nutrients rather than overall nutritional content of the diet. We report experimental evolution of divergence in post-ingestive nutritional compensation in populations of Drosophila melanogaster adapted to diets that contained identical imbalanced nutrient ratios but differed in total nutrient concentration. Larvae from 'Selected' populations maintained for over 200 generations on a nutrient-poor diet with a 1 : 13.5 protein : carbohydrate ratio showed enhanced assimilation of nitrogen from yeasts and reduced assimilation of carbon from sucrose than 'Control' populations evolved on a diet with the same nutrient ratio but fourfold greater nutrient concentration. Compared to the Controls, the Selected larvae also accumulated less triglycerides relative to protein. This implies that the Selected populations evolved a higher assimilation rate of amino acids from the poor imbalanced diet and a lower assimilation of carbohydrates than Controls. Thus, the evolution of nutritional compensation may be driven by changes in total nutrient abundance, even if the ratios of different nutrients remain unchanged.

A developmental checkpoint directs metabolic remodelling as a strategy against starvation in Drosophila

Steroid hormones are crucial regulators of life-stage transitions during development in animals. However, the molecular mechanisms by which developmental transition through these stages is coupled with optimal metabolic homeostasis remains poorly understood. Here, we demonstrate through mathematical modelling and experimental validation that ecdysteroid-induced metabolic remodelling from resource consumption to conservation can be a successful life-history strategy to maximize fitness in Drosophila larvae in a fluctuating environment. Specifically, the ecdysteroid-inducible protein ImpL2 protects against hydrolysis of circulating trehalose following pupal commitment in larvae. Stored glycogen and triglycerides in the fat body are also conserved, even under fasting conditions. Moreover, pupal commitment dictates reduced energy expenditure upon starvation to maintain available resources, thus negotiating trade-offs in resource allocation at the physiological and behavioural levels. The optimal stage-specific metabolic shift elucidated by our predictive and empirical approaches reveals that Drosophila has developed a highly controlled system for ensuring robust development that may be conserved among higher-order organisms in response to intrinsic and extrinsic cues.

Cellular metabolic reprogramming controls sugar appetite in Drosophila

Cellular metabolic reprogramming is an important mechanism by which cells rewire their metabolism to promote proliferation and cell growth. This process has been mostly studied in the context of tumorigenesis, but less is known about its relevance for nonpathological processes and how it affects whole-animal physiology. Here, we show that metabolic reprogramming in Drosophila female germline cells affects nutrient preferences of animals. Egg production depends on the upregulation of the activity of the pentose phosphate pathway in the germline, which also specifically increases the animal's appetite for sugar, the key nutrient fuelling this metabolic pathway. We provide functional evidence that the germline alters sugar appetite by regulating the expression of the fat-body-secreted satiety factor Fit. Our findings demonstrate that the cellular metabolic program of a small set of cells is able to increase the animal's preference for specific nutrients through inter-organ communication to promote specific metabolic and cellular outcomes.

Sexual dimorphism in the nutritional requirement for adult lifespan in Drosophila melanogaster

The nutritional requirements of Drosophila have mostly been studied for development and reproduction, but the minimal requirements for adult male and female flies for lifespan have not been established. Following development on a complete diet, we find substantial sex difference in the basic nutritional requirement of adult flies for full length of life. Relative to females, males require less of each nutrient, and for some nutrients that are essential for development, adult males have no requirement at all for lifespan. The most extreme (and surprising) sex differences were that chronic cholesterol and vitamin deficiencies had no effect on the lifespan of adult males, but they greatly decreased lifespan in females. Female oogenesis rather than chromosomal karyotype and mating status is the key cause of this gender difference in life‐sustaining nutritional requirements. These data are important to the way we understand the mechanisms by which diet modifies lifespan.

Life-History Evolution and the Genetics of Fitness Components in Drosophila melanogaster

Life-history traits or "fitness components"-such as age and size at maturity, fecundity and fertility, age-specific rates of survival, and life span-are the major phenotypic determinants of Darwinian fitness. Analyzing the evolution and genetics of these phenotypic targets of selection is central to our understanding of adaptation. Due to its simple and rapid life cycle, cosmopolitan distribution, ease of maintenance in the laboratory, well-understood evolutionary genetics, and its versatile genetic toolbox, the "vinegar fly" Drosophila melanogaster is one of the most powerful, experimentally tractable model systems for studying "life-history evolution." Here, I review what has been learned about the evolution and genetics of life-history variation in D. melanogaster by drawing on numerous sources spanning population and quantitative genetics, genomics, experimental evolution, evolutionary ecology, and physiology. This body of work has contributed greatly to our knowledge of several fundamental problems in evolutionary biology, including the amount and maintenance of genetic variation, the evolution of body size, clines and climate adaptation, the evolution of senescence, phenotypic plasticity, the nature of life-history trade-offs, and so forth. While major progress has been made, important facets of these and other questions remain open, and the D. melanogaster system will undoubtedly continue to deliver key insights into central issues of life-history evolution and the genetics of adaptation.

Distinct genomic signals of lifespan and life history evolution in response to postponed reproduction and larval diet in Drosophila

Reproduction and diet are two major factors controlling the physiology of aging and life history, but how they interact to affect the evolution of longevity is unknown. Moreover, although studies of large-effect mutants suggest an important role of nutrient sensing pathways in regulating aging, the genetic basis of evolutionary changes in lifespan remains poorly understood. To address these questions, we analyzed the genomes of experimentally evolved Drosophila melanogaster populations subjected to a factorial combination of two selection regimes: reproductive age (early versus postponed), and diet during the larval stage ("low," "control," "high"), resulting in six treatment combinations with four replicate populations each. Selection on reproductive age consistently affected lifespan, with flies from the postponed reproduction regime having evolved a longer lifespan. In contrast, larval diet affected lifespan only in early-reproducing populations: flies adapted to the "low" diet lived longer than those adapted to control diet. Here, we find genomic evidence for strong independent evolutionary responses to either selection regime, as well as loci that diverged in response to both regimes, thus representing genomic interactions between the two. Overall, we find that the genomic basis of longevity is largely independent of dietary adaptation. Differentiated loci were not enriched for "canonical" longevity genes, suggesting that naturally occurring genic targets of selection for longevity differ qualitatively from variants found in mutant screens. Comparing our candidate loci to those from other "evolve and resequence" studies of longevity demonstrated significant overlap among independent experiments. This suggests that the evolution of longevity, despite its presumed complex and polygenic nature, might be to some extent convergent and predictable.

RNA-seq analysis of gene expression changes during pupariation in Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

Background

The oriental fruit fly, Bactrocera dorsalis (Hendel) has been considered to be one of the most important agricultural pest around the world. As a holometabolous insect, larvae must go through a metamorphosis process with dramatic morphological and structural changes to complete their development. To better understand the molecular mechanisms of these changes, RNA-seq of B. dorsalis from wandering stage (WS), late wandering stage (LWS) and white puparium stage (WPS) were performed.

Results

In total, 11,721 transcripts were obtained, out of which 1914 genes (578 up-regulated and 1336 down-regulated) and 2047 genes (655 up-regulated and 1392 down-regulated) were found to be differentially expressed between WS and LWS, as well as between WS and WPS, respectively. Of these DEGs, 1862 and 1996 genes were successfully annotated in various databases. The analysis of RNA-seq data together with qRT-PCR validation indicated that during this transition, the genes in the oxidative phosphorylation pathway, and genes encoding P450s, serine protease inhibitor, and cuticular proteins were down-regulated, while the serine protease genes were up-regulated. Moreover, we found some 20-hydroxyecdysone (20E) biosynthesis and signaling pathway genes had a higher expression in the WS, while the genes responsible for juvenile hormone (JH) synthesis, degradation, signaling and transporter pathways were down-regulated, suggesting these genes might be involved in the process of larval pupariation in B. dorsalis. For the chitinolytic enzymes, the genes encoding chitinases (chitinase 2, chitinase 5, chitinase 8, and chitinase 10) and chitin deacetylase might play the crucial role in the degradation of insect chitin with their expressions significantly increased during the transition. Here, we also found that chitin synthase 1A might be involved in the chitin synthesis of cuticles during the metamorphosis in B. dorsalis.

Conclusions

Significant changes at transcriptional level were identified during the larval pupariation of B. dorsalis. Importantly, we also obtained a vast quantity of RNA-seq data and identified metamorphosis associated genes, which would all help us to better understand the molecular mechanism of metamorphosis process in B. dorsalis.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5077-z) contains supplementary material, which is available to authorized users.

Modeling dietary influences on offspring metabolic programming in Drosophila melanogaster

The influence of nutrition on offspring metabolism has become a hot topic in recent years owing to the growing prevalence of maternal and childhood obesity. Studies in mammals have identified several factors correlating with parental and early offspring dietary influences on progeny health; however, the molecular mechanisms that underlie these factors remain undiscovered. Mammalian metabolic tissues and pathways are heavily conserved in Drosophila melanogaster, making the fly an invaluable genetic model organism for studying metabolism. In this review, we discuss the metabolic similarities between mammals and Drosophila and present evidence supporting its use as an emerging model of metabolic programming.

Life-extending Dietary Restriction Reduces Oxidative Damage of Proteins in Grasshoppers but Does Not Alter Allocation of Ingested Nitrogen to Somatic Tissues

Dietary restriction (DR) extends life span and reduces reproduction in most animals. The disposable soma hypothesis suggests that this longevity is the result of reduced investment in reproduction and increased nutrient allocation to the soma, permitting an increase in cellular maintenance. To investigate the role of nutrient allocation upon life-extending DR, tissue-specific nitrogen allocation was tracked in grasshoppers (Romalea microptera) upon a full or restricted (60% of full) diet. In addition, carbonyl (oxidized protein) assays addressed tissue maintenance. To develop a labeled diet on which grasshoppers could thrive, hydroponically grown Romaine lettuce was enriched with 15N. This allowed quantification of nitrogen allocation upon a normal or restricted diet. There was a 50% decrease in reproductive investment upon DR. At the same time, relative allocation of 15N to the ovary did not change. Most important, relative allocation was similar between restricted and full diet grasshoppers for somatic tissues (ie, mandibular and femur muscle, dried hemolymph, gut, and fat body). Carbonyl assays of muscles, hemolymph, and gut revealed an overall reduction in protein oxidation upon DR. These data suggest that DR does not alter nutrient allocation but does reduce protein oxidation, an observation that is inconsistent with the basic predictions of the disposable soma hypothesis.

Quantifying Drosophila food intake: comparative analysis of current methodology

Food intake is a fundamental parameter in animal studies. Despite the prevalent use of Drosophila in laboratory research, precise measurements of food intake remain challenging in this model organism. Here, we compare several common Drosophila feeding assays: the Capillary Feeder (CAFE), food-labeling with a radioactive tracer or a colorimetric dye, and observations of proboscis extension (PE). We show that the CAFE and radioisotope-labeling provide the most consistent results, have the highest sensitivity, and can resolve differences in feeding that dye-labeling and PE fail to distinguish. We conclude that performing the radiolabeling and CAFE assays in parallel is currently the best approach for quantifying Drosophila food intake. Understanding the strengths and limitations of food intake methodology will greatly advance Drosophila studies of nutrition, behavior, and disease.

Increased allocation of adult-acquired carbohydrate to egg production results in its decreased allocation to sex pheromone production in mated females of the moth Heliothis virescens

Females of most species of moths produce a volatile sex pheromone that attracts conspecific males over distance. In females of the polyandrous moth Heliothis virescens, feeding on carbohydrate (e.g. nectar) supplies precursor, via hemolymph trehalose, for both sex pheromone and egg production. With limited carbohydrate acquisition these two reproductive physiologies might compete for hemolymph trehalose, resulting in an allocation deficit to either sex pheromone or egg production. Using virgin and mated females, which have low and high egg maturation rates, respectively, we fed females a limited diet of (13)C-labeled glucose daily and, using mass isotopomer distribution analysis, determined allocations of adult-acquired carbohydrate (AAC) to newly synthesized pheromone and ovarian and egg fats, our proxies for allocation to egg production. With increased number of feeds, AAC enrichment of hemolymph trehalose increased, as expected. This led to mated females increasing their proportional allocation of AAC to ovarian and egg fats, but decreasing their proportional allocation of AAC to pheromone production. By contrast, virgins increased their proportional allocation of AAC to pheromone production with increased feeds, consistent with increasing AAC enrichment of hemolymph trehalose. These results show that with limited AAC intake, enhanced egg maturation in mated females results in reduced AAC allocation to pheromone production; this does not occur in virgins because of their lower egg maturation rate. This physiological competition for AAC corresponded with decreased pheromone production in mated moths to levels unlikely to attract mates. Therefore, the availability and/or allocation of AAC may be a proximate mechanism underlying the incidence of polyandry in this and other species of moths.

Contribution of larval nutrition to adult reproduction in Drosophila melanogaster

Within the complex life cycle of holometabolous insects, nutritional resources acquired during larval feeding are utilized by the pupa and the adult. The broad features of the transfer of larval resources to the pupae and the allocation of larval resources in the adult have been described by studies measuring and tracking macronutrients at different developmental stages. However, the mechanisms of resource transfer from the larva and the factors regulating the allocation of these resources in the adult between growth, reproduction and somatic maintenance are unknown. Drosophila melanogaster Meigen presents a tractable system to test cellular/tissue mechanisms of resource acquisition and allocation, because of the detailed understanding of D. melanogaster development and the experimental tools to manipulate its tissues across developmental stages. In previous work, we demonstrated that the fat body of D. melanogaster larval is important for surviving starvation stress in the young adult and suggested that programmed cell death of the larval fat cells in the adult is important for allocation of resources for female reproduction. Here, we describe the temporal uptake of larval-derived carbon by the ovaries, and demonstrate the importance of larval fat-cell death in the maturation of the ovary and in fecundity. Larvae and adults were fed stable carbon isotopes to follow the acquisition of larval-derived carbon by the adult ovaries. We determined that over half of the nutrients acquired by the ovaries in 2-day old adult females are dependent upon the death of the fat cells. Furthermore, when programmed cell death is inhibited in the larval fat cells, ovarian development was depressed and fecundity reduced.

Ovariectomy in grasshoppers increases somatic storage, but proportional allocation of ingested nutrients to somatic tissues is unchanged

Reduced reproduction increases storage and extends lifespan in several animal species. The disposable soma hypothesis suggests this life extension occurs by shifting allocation of ingested nutrients from reproduction to the soma. A great deal of circumstantial evidence supports this hypothesis, but no direct tracking of nutrients has been performed in animals that are long-lived because of direct reduction in reproduction. Here, we use the stable isotopes to track carbon and nitrogen from ingestion to somatic organs in long-lived, ovariectomized grasshoppers. Three estimates of somatic storage (viz., quantity of hemolymph storage proteins, amount of femur muscle carbohydrates, and size of the fat body) all doubled upon ovariectomy. In stark contrast, ovariectomy did not increase the proportion of these tissues that were made from recently ingested foods. In other words, the physiology underlying relative allocation to these somatic tissues was not affected by ovariectomy. Thus, at the level of whole tissue storage, these results are consistent with a trade-off between reproduction and longevity. In contrast, our stable isotope data are inconsistent with the prediction that enhanced storage in ovariectomized females results from a physiological shift in allocation of ingested nutrients.

Energetics of metamorphosis in Drosophila melanogaster

We measured the energetic cost of metamorphosis in the fruitfly, Drosophila melanogaster. Metabolic rates decreased rapidly in the first 24h and remained low until shortly before eclosion, when the rates increased rapidly, thus creating a U-shaped metabolic curve. The primary fuel used during metamorphosis was lipid, which accounted for >80% of total metabolism. The total energy consumed during metamorphosis was lowest at 25°C, compared to 18 and 29°C, due to differences in metabolic rates and the length of pupal development. Temperature differentially affected metabolic rates during different stages of metamorphosis. Prepupal and late pupal stages exhibited typical increases in metabolic rate at high temperatures, whereas metabolic rates were independent of temperature during the first 2/3 of pupal development. We tested two hypotheses for the underlying cause of the U-shaped metabolic curve. The first hypothesis was that pupae become oxygen restricted as a result of remodeling of the larval tracheal system. We tested this hypothesis by exposing pupae to hypoxic and hyperoxic atmospheres, and by measuring lactic acid production during normoxic development. No evidence for oxygen limitation was observed. We also tested the hypothesis that the U-shaped metabolic curve follows changes in metabolically active tissue, such that the early decrease in metabolic rates reflects the histolysis of larval tissues, and the later increase in metabolic rates is associated with organogenesis and terminal differentiation of adult tissues. We assayed the activity of a mitochondrial indicator enzyme, citrate synthase, and correlated it with tissue-specific developmental events during metamorphosis. Citrate synthase activity exhibited a U-shaped curve, suggesting that the pattern of metabolic activity is related to changes in the amount of potentially active aerobic tissue.

Resource allocation to testes in walnut flies and implications for reproductive strategy

Testes size often predicts the winner during episodes of sperm competition. However, little is known about the source of nutrients allocated to testes development, or testes plasticity under varying nutrient availability. Among many holometabolous insects, metabolic resources can derive from the larval or adult diet. Distinguishing the source of nutrients allocated to testes can shed light on life history factors (such as maternal influences) that shape the evolution of male reproductive strategies. Here we used an experimental approach to assess resource allocation to testes development in walnut flies (Rhagoletis juglandis) from differing nutritional backgrounds. We fed adult male walnut flies on sugar and yeast diets that contrasted with the larval diet in carbon and nitrogen stable isotope ratios. This design allowed us to assess the dietary source of testes carbon and nitrogen and its change over time. We found significant incorporation of adult dietary carbon into testes, implying that walnut flies are income breeders for carbon (relying more on adult resources). In contrast, we found little evidence that walnut flies incorporate adult dietary nitrogen into testes development. We discuss the implications of these allocation decisions for life history evolution in this species.

Allocation from capital and income sources to reproduction shift from first to second clutch in the flesh fly, Sarcophaga crassipalpis

Understanding how resources are allocated between survival and reproduction is fundamental to the study of the evolution of life histories. Reproductive resources can come from two intrinsic resource pools, stored reserves (capital) acquired before reproduction or income acquired during reproduction. The variety of reproductive strategies in insects is remarkable and reproductive allocation encompasses the complete range of allocation strategies from pure capital breeders to pure income breeders. However, most organisms probably use a blend of capital and income and this blend is likely dynamic, changing between reproductive bouts in response to internal and external conditions. We used stable isotopes to quantify the allocation of capital and income resources to reproduction in the flesh fly, Sarcopha crassipalpis and assessed how allocation patterns change over multiple bouts of reproduction. Sarcophaga crassipalpis shifts from a slight investment of capital in the first clutch to an almost pure income breeder in the second clutch. We discuss the relationship between activity and allocation, and the potential for this system to understand how allocation patterns change in response to environmental stress.

Allocation of nutrients to somatic tissues in young ovariectomized grasshoppers

The disposable soma hypothesis predicts that when reproduction is reduced, life span is increased because more nutrients are invested in the soma, increasing somatic repair. Rigorously testing the hypothesis requires tracking nutrients from ingestion to allocation to the soma or to reproduction. Fruit flies on life-extending dietary restriction increase allocation to the soma "relative" to reproduction, suggesting that allocation of nutrients can be associated with extension of life span. Here, we use stable isotopes to track ingested nutrients in ovariectomized grasshoppers during the first oviposition cycle. Previous work has shown that ovariectomy extends life span, but investment of protein in reproduction is not reduced until after the first clutch of eggs is laid. Because ovariectomy does not affect investment in reproduction at this age, the disposable soma hypothesis would predict that ovariectomy should also not affect investment in somatic tissues. We developed grasshopper diets with distinct signatures of ¹³C and ¹⁵N, but that produced equivalent reproductive outputs. These diets are, therefore, appropriate for the reciprocal switches in diet needed for tracking ingested nutrients. Incorporation of stable isotopes into eggs showed that grasshoppers are income breeders, especially for carbon. Allocation to the fat body of nitrogen ingested as adults was slightly increased by ovariectomy; this was our only result that was not consistent with the disposable soma hypothesis. In contrast, ovariectomy did not affect allocation of nitrogen to femoral muscles. Further, allocation of carbon to the fat body or femoral muscles did not appear to be affected by ovariectomy. Total anti-oxidant activities in the hemolymph and femoral muscles were not affected by ovariectomy. These experiments showed that allocation of nutrients was altered little by ovariectomy in young grasshoppers. Additional studies on older individuals are needed to further test the disposable soma hypothesis.

Protein accumulation underlying lifespan extension via ovariectomy in grasshoppers is consistent with the disposable soma hypothesis but is not due to dietary restriction

Reduced reproduction extends lifespan in many experimental animals, but the mechanism by which this occurs is unclear. The disposable soma hypothesis suggests that when reproduction is reduced, more nutrients are allocated to the soma and lifespan is extended. Alternatively, the reproductive tissues or the process of reproduction may have a direct (i.e., non-nutritional) negative effect on lifespan. We used ovariectomized grasshoppers to examine the effects of reduced reproduction throughout the lifespan at the physiological level. We focused on protein, the limiting nutrient for egg production. Ovariectomized females lived significantly longer than sham females. Because both groups ingested similar amounts, the effect was independent of dietary restriction. Despite this, ovariectomized females gained less body mass than sham females. Ovariectomized grasshoppers produced the egg yolk-precursor protein vitellogenin. At the time sham females laid their first clutch, cumulative reproductive protein was similar in ovariectomized and sham females. By advanced ages, however, ovariectomized females had produced about five-fold less cumulative reproductive protein than sham females. In contrast, old ovariectomized females had at least two-fold more hemolymph storage protein. These results are consistent with ovariectomy extending lifespan in part via enhanced protein allocation to storage at the expense of reproduction.

A 13C isotope labeling strategy reveals the influence of insulin signaling on lipogenesis in C. elegans

Although studies in C. elegans have identified numerous genes involved in fat storage, the next step is to determine how these factors actually affect in vivo lipid metabolism. We have developed a (13)C isotope assay to quantify the contribution of dietary fat absorption and de novo synthesis to fat storage and membrane lipid production in C. elegans, establishing the means by which worms obtain and process fatty acids. We applied this method to characterize how insulin signaling affects lipid physiology. Several long-lived mutations in the insulin receptor gene daf-2 resulted in significantly higher levels of synthesized fats in triglycerides and phospholipids. This elevation of fat synthesis was completely dependent upon daf-16/FoxO. Other long-lived alleles of daf-2 did not increase fat synthesis, however, suggesting that site-specific mutations in the insulin receptor can differentially influence longevity and metabolism, and that elevated lipid synthesis is not required for the longevity of daf-2 mutants.

Analysing body condition: mass, volume or density?

1. Body condition (defined as the relative amount of energy reserves in the body) is an animal trait with strong ecological implications. In some animal taxa (e.g. arthropods), the external volume of the body part in which most nutrients are stored (e.g. abdomen) is used interchangeably with body mass to estimate body condition, making the implicit assumption that abdomen residual volume is a good surrogate of residual mass. However, the degree of correlation between these two measures should largely depend on the density of the nutrients stored. 2. We simulated two food-supplemented experimental groups of animals, each storing a slightly different amount of lipids either in their abdomens or in their entire bodies, and explored (i) how different estimates of condition were able to detect fixed differences between the groups; and (ii) how the amount of lipids stored could affect the outcome of non-intrusive measures of condition on a dichotomous variable (e.g. survival, mating success). We found that density body condition (body mass statistically controlled for structural body size and body volume) has much greater power to detect differences between experimental groups or effects on binary response variables than do classic mass/size or volume/size condition indices. 3. Using data on Lycosa tarantula (L.), a burrowing wolf spider, we report dramatic differences among these three indices in their ability to detect sex differences in the effect of feeding treatment on body condition at maturity. In particular, a plot of residual mass against residual volume reflecting nutrient density suggests that poorly fed spiders are nutritionally unbalanced, since well-fed spiders invest in nutrients of very different density. 4. Furthermore, using data on Scathophaga stercoraria (L.), the yellow dung fly, we found that an index of density condition was better at distinguishing condition differences among three populations than were mass or volume condition estimates alone. 5. We propose that including these three surrogates of condition (mass, volume and density) will substantially improve the accuracy of non-intrusive estimates of body condition, thus providing more powerful tools with direct application in a wide range of disciplines.

Seminal influences: Drosophila Acps and the molecular interplay between males and females during reproduction

Successful reproduction requires contributions from both the male and the female. In Drosophila, contributions from the male include accessory gland proteins (Acps) that are components of the seminal fluid. Upon their transfer to the female, Acps affect the female's physiology and behavior. Although primary sequences of Acp genes exhibit variation among species and genera, the conservation of protein biochemical classes in the seminal fluid suggests a conservation of functions. Bioinformatics coupled with molecular and genetic tools available for Drosophila melanogaster has expanded the functional analysis of Acps in recent years to the genomic/proteomic scale. Molecular interplay between Acps and the female enhances her egg production, reduces her receptivity to remating, alters her immune response and feeding behavior, facilitates storage and utilization of sperm in the female and affects her longevity. Here, we provide an overview of the D. melanogaster Acps and integrate the results from several studies that bring the current number of known D. melanogaster Acps to 112. We then discuss several examples of how the female's physiological processes and behaviors are mediated by interactions between Acps and the female. Understanding how Acps elicit particular female responses will provide insights into reproductive biology and chemical communication, tools for analyzing models of sexual cooperation and/or sexual conflict, and information potentially useful for strategies for managing insect pests.

Counting calories in Drosophila diet restriction

The extension of life span by diet restriction in Drosophila has been argued to occur without limiting calories. Here we directly measure the calories assimilated by flies when maintained on full- and restricted-diets. We find that caloric intake is reduced on all diets that extend life span. Flies on low-yeast diet are long-lived and consume about half the calories of flies on high-yeast diets, regardless of the energetic content of the diet itself. Since caloric intake correlates with yeast concentration and thus with the intake of every metabolite in this dietary component, it is premature to conclude for Drosophila that calories do not explain extension of life span.