Population Density Affects Drosophila Male Pheromones in Laboratory-Acclimated and Natural Lines

In large groups of vertebrates and invertebrates, aggregation can affect biological characters such as gene expression, physiological, immunological and behavioral responses. The insect cuticle is covered with hydrocarbons (cuticular hydrocarbons; CHCs) which reduce dehydration and increase protection against xenobiotics. Drosophila melanogaster and D. simulans flies also use some of their CHCs as contact pheromones. In these two sibling species, males also produce the volatile pheromone 11-cis-Vaccenyl acetate (cVa). To investigate the effect of insect density on the production of CHCs and cVa we compared the level of these male pheromones in groups of different sizes. These compounds were measured in six lines acclimated for many generations in our laboratory - four wild-type and one CHC mutant D. melanogaster lines plus one D. simulans line. Increasing the group size substantially changed pheromone amounts only in the four D. melanogaster wild-type lines. To evaluate the role of laboratory acclimation in this effect, we measured density-dependent pheromonal production in 21 lines caught in nature after 1, 12 and 25 generations in the laboratory. These lines showed varied effects which rarely persisted across generations. Although increasing group size often affected pheromone production in laboratory-established and freshly-caught D. melanogaster lines, this effect was not linear, suggesting complex determinants.

Dietary sucrose determines the regulatory activity of lithium on gene expression and lifespan in Drosophila melanogaster

The amount of dietary sugars and the administration of lithium both impact the lifespan of the fruit fly Drosophila melanogaster. It is noteworthy that lithium is attributed with insulin-like activity as it stimulates protein kinase B/Akt and suppresses the activity of glycogen synthase kinase-3 (GSK-3). However, its interaction with dietary sugar has largely remained unexplored. Therefore, we investigated the effects of lithium supplementation on known lithium-sensitive parameters in fruit flies, such as lifespan, body composition, GSK-3 phosphorylation, and the transcriptome, while varying the dietary sugar concentration. For all these parameters, we observed that the efficacy of lithium was significantly influenced by the sucrose content in the diet. Overall, we found that lithium was most effective in enhancing longevity and altering body composition when added to a low-sucrose diet. Whole-body RNA sequencing revealed a remarkably similar transcriptional response when either increasing dietary sucrose from 1% to 10% or adding 1 mM LiCl to a 1% sucrose diet, characterized by a substantial overlap of nearly 500 differentially expressed genes. Hence, dietary sugar supply is suggested as a key factor in understanding lithium bioactivity, which could hold relevance for its therapeutic applications.

Effect of acetic acid bacteria colonization on oviposition and feeding site choice in Drosophila suzukii and its related species

Unlike many species of Drosophila flies that colonize decaying fruits, Drosophila suzukii lay eggs in ripening fruits. The oviposition and feeding site preferences for bacterial growth were quantified in multiple strains of D. suzukii and its closely related species, D. subpulchrella and D. biarmipes . A continuous degree of preference for oviposition sites with Acetobacter growth both within and across species suggested that the separation in resource usage is notable but not complete among these species. The lack of interspecific differences in feeding site preference for Acetobacter -containing media implied that the oviposition site preferences evolved independently from the feeding site preference.

The importance of time in nutrient regulation: a case study with spotted-wing Drosophila (Drosophila suzukii)

Introduction

The ability of living organisms to acquire the nutrients needed to carry out required physiological functions has important consequences for fitness. However, an organism must not simply meet the requirements for individual nutrients, but must ingest an optimal balance of multiple nutrients. Despite this, animals rarely consume truly balanced resources, and instead commonly feed selectively across multiple unbalanced resources to reach an optimal balance, i.e., intake target. Nutritional research has predominantly focused on the behavioral strategies employed during nutrient regulation, as well as the fitness consequence of failing to meet intake targets, but little work has been done on the temporal aspects of this process. For instance, within what timeframe must organisms reach their intake target before a fitness cost is incurred? Hours, days, weeks?

Methods

In this study, we investigated how nutrient regulation interval impacts consumption and performance in adult female spotted-wing Drosophila (Drosophila suzukii). Females were constrained to either a protein- orcarbohydrate-biased diet over different time intervals and at different schedules, while control flies were constrained to one diet for the entire feeding period.

Results

Regulation interval had a significant impact on feeding behavior and consumption. Total consumption was highest on the shorter interval treatments, where diets were alternated more frequently, and declined as the interval period increased. The relative consumption of both diets was statistically-different across intervals and was higher for the carbohydrate-biased diet. Consumption of the protein-biased diet was more variable across intervals and was more strongly impacted by the daily timing of diet switches. Performance data showed that shorter regulation intervals led to longer fly lifespans, a result commonly observed in studies exploring the impacts of diet macronutrient ratio variability on performance.

Discussion

These results show that the temporal aspects of nutrition, such as feeding intervals and the timing of resource availability, can have strong impacts on feeding behavior, nutrient regulation, and fitness. These results provide an insight into how consumers may deal with changes in host phenology, the availability of hosts, and changes in nutrient availability within hosts. Understanding these mechanisms will be important for predicting responses to changes in nutrient cycling and resource availability mediated by natural and anthropogenic habitat modifications, such as global climate change.

Effect of acetic acid bacteria colonization on oviposition and feeding site choice in Drosophila suzukii and its related species

Oviposition site choice has a large impact on offspring performance. Unlike other vinegar flies that colonize decaying fruits, Drosophila suzukii lay eggs into hard ripening fruits by using their enlarged and serrated ovipositors (oviscapts). This behavior has an advantage over other species by providing access to the host fruit earlier and avoiding competition. However, the larvae are not fully adapted to a low-protein diet, and the availability of intact healthy fruits is seasonally restricted. Thus, to investigate oviposition site preference for microbial growth in this species, we conducted an oviposition assay using single species of commensal Drosophila acetic acid bacteria, Acetobacter and Gluconobacter. The oviposition site preferences for media with or without bacterial growth were quantified in multiple strains of D. suzukii and its closely related species, D. subpulchrella and D. biarmipes, and a typical fermenting-fruit consumer, D. melanogaster. Our comparisons demonstrated a continuous degree of preference for sites with Acetobacter growth both within and across species, suggesting that the niche separation is notable but not complete. The preference for Gluconobacter showed large variations among replicates and no clear differences between the strains. In addition, the lack of interspecific differences in feeding site preference for Acetobacter-containing media implies that the interspecific divergence in oviposition site preference occurred independently from the feeding site preference. Our oviposition assays measuring the preference of multiple strains from each fly species for acetic acid bacteria growth revealed intrinsic properties of shared resource usage among these fruit fly species.

The Protein Paradox: Elucidating the Complex Nutritional Ecology of the Invasive Berry Pest, Spotted-Wing Drosophila (Diptera: Drosophila suzukii)

Spotted-wing drosophila (SWD), Drosophila suzukii, has become one of the most widely studied insect species over the last decade, largely due to its recent invasion and rapid expansion across the Americas and Europe. Unlike other drosophilid species, which colonize rotting fruit, SWD females possess a serrated ovipositor that allows them to lay eggs in intact ripening fruit, causing significant economic problems for fruit/berry producers worldwide. Though an impressive amount of research has been conducted on SWD's ecology and physiology, aspects of their nutritional ecology remain ambiguous. This review synthesizes the research to date to provide a more comprehensive view of SWD's nutritional relationship with its fruit hosts and associated microbes. Overall, data suggest that SWD's ability to utilize novel resources is likely due to changes in their ecological, rather than physiological, niche that are largely mediated by microbial associations. Studies show that SWD's nutrient intake is comparable to other drosophilid species, indicating limited adaptation to feeding on lower-protein resources. Instead, data show that fruit protein content is a reliable predictor of host suitability and that fruit-microbe dynamics have a strong impact on protein availability. In particularly, fruit protein increases after infestation with SWD-associated microbes, suggesting that initially-suboptimal intact fruits can become protein-rich on a timeframe that is relevant for larval nutrition. This body of work suggests that microbial associations between flies and their fruit hosts can compensate for the nutritional differences between intact and rotting fruit, and that these relationships are likely responsible for SWD's expanded nutritional niche.

Using Flies to Understand Social Networks

Many animals live in groups and interact with each other, creating an organized collective structure. Social network analysis (SNA) is a statistical tool that aids in revealing and understanding the organized patterns of shared social connections between individuals in groups. Surprisingly, the application of SNA revealed that Drosophila melanogaster, previously considered a solitary organism, displays group dynamics and that the structure of group life is inherited. Although the number of studies investigating Drosophila social networks is currently limited, they address a wide array of questions that have only begun to capture the details of group level behavior in this insect. Here, we aim to review these studies, comparing their respective scopes and the methods used, to draw parallels between them and the broader body of knowledge available. For example, we highlight how despite methodological differences, there are similarities across studies investigating the effects of social isolation on social network dynamics. Finally, this review aims to generate hypotheses and predictions that inspire future research in the emerging field of Drosophila social networks.

Poor adult nutrition impairs learning and memory in a parasitoid wasp

Animals have evolved cognitive abilities whose impairment can incur dramatic fitness costs. While malnutrition is known to impact brain development and cognitive functions in vertebrates, little is known in insects whose small brain appears particularly vulnerable to environmental stressors. Here, we investigated the influence of diet quality on learning and memory in the parasitoid wasp Venturia canescens. Newly emerged adults were exposed for 24 h to either honey, 20% sucrose solution, 10% sucrose solution, or water, before being conditioned in an olfactory associative learning task in which an odor was associated to a host larvae (reward). Honey fed wasps showed 3.5 times higher learning performances and 1.5 times longer memory retention than wasps fed sucrose solutions or water. Poor diets also reduced longevity and fecundity. Our results demonstrate the importance of early adult nutrition for optimal cognitive function in these parasitoid wasps that must quickly develop long-term olfactory memories for searching suitable hosts for their progeny.

Environmental influences on for-mediated oviposition decisions in Drosophila melanogaster

Deciding whether or not to lay an egg on a given substrate is an important task undertaken by females of many arthropods. It involves perceiving the environment (e.g. quality of the substrate, temperature, and humidity), formulating a decision, and then conducting the appropriate behaviours to oviposit. This oviposition site selection (OSS) provides a useful system for studying simple decision-making. OSS in fruit flies, Drosophila melanogaster, is influenced by both genetic and environmental variation. Naturally occurring allelic variation in the foraging gene (for) is known to affect OSS. Given a choice of high- and low-nutrient oviposition substrates, groups of rovers (for^R) are known to lay significantly more of their eggs on low-nutrient sites than sitters (for^s) and sitter mutants (for^s2). Here we ask three questions: (1) Is the role of for in OSS affected by the availability of alternate oviposition sites? (2) Is the role of for in OSS sensitive to the density of ovipositing females? and (3) Does the gustatory sensation of yeast play a role in for-mediated variation in OSS? We find a role of choice and female density in rover/sitter differences in OSS, as well as a role of for in response to glycerol, an indicator of yeast. The role of for in OSS decision-making is complex and multi-faceted and should prove fertile ground for further research into the factors affecting decision-making behaviours.

Sex-Dependent Effects of the Microbiome on Foraging and Locomotion in Drosophila suzukii

There is growing evidence that symbiotic microbes can influence multiple nutrition-related behaviors of their hosts, including locomotion, feeding, and foraging. However, how the microbiome affects nutrition-related behavior is largely unknown. Here, we demonstrate clear sexual dimorphism in how the microbiome affects foraging behavior of a frugivorous fruit fly, Drosophila suzukii. Female flies deprived of their microbiome (axenic) were consistently less active in foraging on fruits than their conventional counterparts, even though they were more susceptible to starvation and starvation-induced locomotion was notably more elevated in axenic than conventional females. Such behavioral change was not observed in male flies. The lag of axenic female flies but not male flies to forage on fruits is associated with lower oviposition by axenic flies, and mirrored by reduced food seeking observed in virgin females when compared to mated, gravid females. In contrast to foraging intensity being highly dependent on the microbiome, conventional and axenic flies of both sexes showed relatively consistent and similar fruit preferences in foraging and oviposition, with raspberries being preferred among the fruits tested. Collectively, this work highlights a clear sex-specific effect of the microbiome on foraging and locomotion behaviors in flies, an important first step toward identifying specific mechanisms that may drive the modulation of insect behavior by interactions between the host, the microbiome, and food.

On the use of private versus social information in oviposition site choice decisions by Drosophila melanogaster females

Individuals are faced with decisions throughout their lifetimes, and the choices they make often have important consequences toward their fitness. Being able to discern which available option is best to pursue often incurs sampling costs, which may be largely avoided by copying the behavior and decisions of others. Although social learning and copying behaviors are widespread, much remains unknown about how effective and adaptive copying behavior is, as well as the factors that underlie its expression. Recently, it has been suggested that since female fruit flies (Drosophila melanogaster) appear to rely heavily on public information when selecting oviposition sites, they are a promising model system for researching patch-choice copying, and more generally, the mechanisms that control decision making. Here, we set out to determine how well female distinguish between socially produced cues, and whether females are using “relevant” signals when choosing an oviposition site. We found that females showed a strong preference for ovipositing on media patches that had been previously occupied by ovipositing females of the same species and diet over other female outgroups. However, in a separate assay, we observed that females favored ovipositing on media patches that previously housed virgin males over those exhibiting alternative conspecific signals. Our results confirm that females use cues left behind by other flies when choosing between potential oviposition sites, though their prioritization of these signals raises serious questions as to whether fruit flies are employing copying behavior, or are instead responding to signals that may not be of relevance to oviposition site suitability.

Development of fly tolerance to consuming a high-protein diet requires physiological, metabolic and transcriptional changes

Mortality in insects consuming high-protein-and-low-carbohydrate diets resembles a type III lifespan curve with increased mortality at an early age and few survivors that live a relatively long lifespan. We selected for a Drosophila line able to live for a long time on an imbalanced high-protein-low-carbohydrate diet by carrying out five rounds of breeding to select for the most long-lived survivors. Adaptation to this diet in the selected line was studied at the biochemical, physiological and transcriptomic levels. The selected line of flies consumed less of the imbalanced food but also accumulated more storage metabolites: glycogen, triacylglycerides, and trehalose. Selected flies also had a higher activity of alanine transaminase and a higher urea content. Adaptation of the selected line on the transcriptomic level was characterized by down-regulation of genes encoding serine endopeptidases (Jon25i, Jon25ii, betaTry, and others) but up-regulation of genes encoding proteins related to the immune system, such as antimicrobial peptides, Turandot-family humoral factors, hexamerin isoforms, and vitellogenin. These sets of down- and up-regulated genes were similar to those observed in fruit flies with suppressed juvenile hormone signaling. Our data show that the physiological adaptation of fruit flies to a high-protein-low-carbohydrate diet occurs via intuitive pathways, namely a decrease in food consumption, conversion of amino acids into ketoacids to compensate for the lack of carbohydrate, and accumulation of storage metabolites to eliminate the negative effects of excess amino acids. Nevertheless, transcriptomic adaptation occurs in a counter-intuitive way likely via an influence of gut microbiota on food digestion.

Combined effects of temperature and macronutrient balance on life-history traits in Drosophila melanogaster: implications for life-history trade-offs and fundamental niche

Temperature and nutrition are amongst the most influential environmental determinants of Darwinian fitness in ectotherms. Since the ongoing climate warming is known to alter nutritional environments encountered by ectotherms, a precise understanding of the integrated effects of these two factors on ectotherm performance is essential for improving the accuracy of predictions regarding how ectotherms will respond to climate warming. Here we employed response surface methodology to examine how multiple life-history traits were expressed across a grid of environmental conditions representing full combinations of six ambient temperatures (13, 18, 23, 28, 31, 33 °C) and eight dietary protein:carbohydrate ratios (P:C = 1:16, 1:8, 1:4, 1:2, 1:1, 2:1, 4:1, 8:1) in Drosophila melanogaster. Different life-history traits were maximized in different regions in the two-dimensional temperature-nutrient space. The optimal temperature and P:C ratio identified for adult lifespan (13 °C and 1:16) were lower than those for early-life female fecundity (28 °C and 4:1). Similar divergence in thermal and nutritional optima was found between body mass at adult emergence (18 °C and P:C 1:1) and the rate of pre-adult development (28 °C and P:C 4:1). Pre-adult survival was maximized over a broad range of temperature (18-28 °C) and P:C ratio (1:8-8:1). These results indicate that the occurrence of life-history trade-offs is regulated by both temperature and dietary P:C ratio. The estimated measure of fitness was maximized at 23 °C and P:C 2:1. Based on the shape of the response surface constructed for this estimated fitness, we characterized the fundamental thermal and nutritional niche for D. melanogaster with unprecedented detail.

Olfactory Neurons and Brain Centers Directing Oviposition Decisions in Drosophila

The sense of smell influences many behaviors, yet how odors are represented in the brain remains unclear. A major challenge to studying olfaction is the lack of methods allowing activation of specific types of olfactory neurons in an ethologically relevant setting. To address this, we developed a genetic method in Drosophila called olfactogenetics in which a narrowly tuned odorant receptor, Or56a, is ectopically expressed in different olfactory neuron types. Stimulation with geosmin (the only known Or56a ligand) in an Or56a mutant background leads to specific activation of only target olfactory neuron types. We used this approach to identify olfactory sensory neurons (OSNs) that directly guide oviposition decisions. We identify 5 OSN-types (Or71a, Or47b, Or49a, Or67b, and Or7a) that, when activated alone, suppress oviposition. Projection neurons partnering with these OSNs share a region of innervation in the lateral horn, suggesting that oviposition site selection might be encoded in this brain region.

Larval mannitol diets increase mortality, prolong development and decrease adult body sizes in fruit flies (Drosophila melanogaster)

The ability of polyols to disrupt holometabolous insect development has not been studied and identifying compounds in food that affect insect development can further our understanding of the pathways that connect growth rate, developmental timing and body size in insects. High-sugar diets prolong development and generate smaller adult body sizes in Drosophila melanogaster We tested for concentration-dependent effects on development when D. melanogaster larvae are fed mannitol, a polyalcohol sweetener. We also tested for amelioration of developmental effects if introduction to mannitol media is delayed past the third instar, as expected if there is a developmental sensitive-period for mannitol effects. Both male and female larvae had prolonged development and smaller adult body sizes when fed increasing concentrations of mannitol. Mannitol-induced increases in mortality were concentration dependent in 0 M to 0.8 M treatments with mortality effects beginning as early as 48 h post-hatching. Larval survival, pupariation and eclosion times were unaffected in 0.4 M mannitol treatments when larvae were first introduced to mannitol 72 h post-hatching (the beginning of the third instar); 72 h delay of 0.8 M mannitol introduction reduced the adverse mannitol effects. The developmental effects of a larval mannitol diet closely resemble those of high-sugar larval diets.This article has an associated First Person interview with the first author of the paper.

A clinal polymorphism in the insulin signaling transcription factor foxo contributes to life-history adaptation in Drosophila

A fundamental aim of adaptation genomics is to identify polymorphisms that underpin variation in fitness traits. In Drosophila melanogaster, latitudinal life-history clines exist on multiple continents and make an excellent system for dissecting the genetics of adaptation. We have previously identified numerous clinal single-nucleotide polymorphism in insulin/insulin-like growth factor signaling (IIS), a pathway known from mutant studies to affect life history. However, the effects of natural variants in this pathway remain poorly understood. Here we investigate how two clinal alternative alleles at foxo, a transcriptional effector of IIS, affect fitness components (viability, size, starvation resistance, fat content). We assessed this polymorphism from the North American cline by reconstituting outbred populations, fixed for either the low- or high-latitude allele, from inbred DGRP lines. Because diet and temperature modulate IIS, we phenotyped alleles across two temperatures (18°C, 25°C) and two diets differing in sugar source and content. Consistent with clinal expectations, the high-latitude allele conferred larger body size and reduced wing loading. Alleles also differed in starvation resistance and expression of insulin-like receptor, a transcriptional target of FOXO. Allelic reaction norms were mostly parallel, with few GxE interactions. Together, our results suggest that variation in IIS makes a major contribution to clinal life-history adaptation.

Sex-specific transcriptomic responses to changes in the nutritional environment

Males and females typically pursue divergent reproductive strategies and accordingly require different dietary compositions to maximise their fitness. Here we move from identifying sex-specific optimal diets to understanding the molecular mechanisms that underlie male and female responses to dietary variation in Drosophila melanogaster. We examine male and female gene expression on male-optimal (carbohydrate-rich) and female-optimal (protein-rich) diets. We find that the sexes share a large core of metabolic genes that are concordantly regulated in response to dietary composition. However, we also observe smaller sets of genes with divergent and opposing regulation, most notably in reproductive genes which are over-expressed on each sex's optimal diet. Our results suggest that nutrient sensing output emanating from a shared metabolic machinery are reversed in males and females, leading to opposing diet-dependent regulation of reproduction in males and females. Further analysis and experiments suggest that this reverse regulation occurs within the IIS/TOR network.

"You are what you eat" is a popular saying that can often make scientific sense. Everything an animal eats gets broken down into smaller molecules that fuel the many biological processes required to survive, move and reproduce. However, the food that the sexes need to maximize their fertility may not be exactly the same, as males make lots of small, mobile sperm cells while females create a small number of large eggs. In fruit flies for example, females benefit most from foods that contain lots of protein, while males are more fertile when they eat foods that are rich in carbohydrates. However, it remained unclear how these differences have evolved.

Here, Camus et al. examine the genes that are active in male and female fruit flies which eat a diet rich in either carbohydrates or in proteins. Their experiments showed that both sexes share a large collection of genes which respond to the two diets in the same way. However, the type of food had opposite effects on the activity of certain genes involved in male and female reproduction. When the fruit flies had a protein-rich diet, for example, genes that promoted reproduction got turned on in females, but switched off in males. The opposite pattern was observed when the insects were exposed to carbohydrate-rich diets. Further analyses suggested that these different responses might be linked to a molecular network called IIS/TOR, which is a specific cascade of reactions that responds to nutrient availability.

The findings of Camus et al. suggest that male and female flies produce different signals in reaction to food, which helps them to reproduce when they are able to meet their particular nutritional needs. Armed with a better understanding of the fundamental differences between the sexes, it may be possible to improve research into human health and animal keeping.

Oviposition Preference and Larval Performance of Drosophila suzukii (Diptera: Drosophilidae), Spotted-Wing Drosophila: Effects of Fruit Identity and Composition

A better understanding of the factors affecting host plant use by spotted-wing drosophila (Drosophila suzukii) could aid in the development of efficient management tools and practices to control this pest. Here, proxies of both preference (maternal oviposition behavior) and performance (adult emergence) were evaluated for 12 different fruits in the form of purees. The effect of the chemical composition of the fruits on preference and performance traits was then estimated. We synthesized the literature to interpret our findings in the light of previous studies that measured oviposition preference and larval performance of D. suzukii. We show that fruit identity influences different parts of the life cycle, including oviposition preference under both choice and no-choice conditions, emergence rate, development time, and number of emerging adults. Blackcurrant was always among the most preferred fruit we used, while grape and tomato were the least preferred fruits. Larvae performed better in cranberry, raspberry, strawberry, and cherry than in the other fruits tested. We found that fruit chemical compounds can explain part of the effect of fruit on D. suzukii traits. In particular, oviposition preference under choice conditions was strongly influenced by fruit phosphorus content. In general, the consensus across studies is that raspberry, blackberry, and strawberry are among the best hosts while blackcurrant, grape and rose hips are poor hosts. Our results generally confirm this view but also suggest that oviposition preferences do not necessarily match larval performances. We discuss opportunities to use our results to develop new approaches for pest management.

Evidence of dietary protein restriction regulating pupation height, development time and lifespan in Drosophila melanogaster

Fitness and behavioral traits are optimized according to the rearing environment to ensure survival of most organisms including fruit flies Drosophila melanogaster Fruit flies are known to uphold various trade-offs in their lifespan, development time, fecundity, etc., to confer better survival in the particular exposed environment. The diet of D. melanogaster plays a major role between larval and adult fitness or fitness related traits; its role in the regulation of correlations between pupation height, pre-adult development and adult fitness has not been studied empirically. In our study, we assayed the effect of restricting dietary protein alone from the larval stage to adult stage in fruit flies and studied development time, pre-adult survivorship, pupation height, larval feeding rate and their corresponding lifespan under a light/dark cycle (LD12:12 h). We found that under very low protein concentration in diet, development time and lifespan of the flies increased significantly, along with decreased pupation height and vice versa, while pre-adult survivorship remained unchanged across diets. The results from our study can be taken to suggest that development time is negatively and positively correlated with pupation height and adult lifespan respectively. Thus, a higher protein restriction decreases pupation height and increases development time and vice versa, thereby emphasizing differential alterations taken up by various fitness traits, probably to enhance the overall organismal fitness.This article has an associated First Person interview with the first author of the paper.

Navigating infection risk during oviposition and cannibalistic foraging in a holometabolous insect

Deciding where to eat and raise offspring carries important fitness consequences for all animals, especially if foraging, feeding, and reproduction increase pathogen exposure. In insects with complete metamorphosis, foraging mainly occurs during the larval stage, while oviposition decisions are made by adult females. Selection for infection avoidance behaviors may therefore be developmentally uncoupled. Using a combination of experimental infections and behavioral choice assays, we tested if Drosophila melanogaster fruit flies avoid infectious environments at distinct developmental stages. When given conspecific fly carcasses as a food source, larvae did not discriminate between carcasses that were clean or infected with the pathogenic Drosophila C Virus (DCV), even though cannibalism was a viable route of DCV transmission. When laying eggs, DCV-infected females did not discriminate between infectious and noninfectious carcasses, and laying eggs near potentially infectious carcasses was always preferred to sites containing only fly food. Healthy mothers, however, laid more eggs near a clean rather than an infectious carcass. Avoidance during oviposition changed over time: after an initial oviposition period, healthy mothers stopped avoiding infectious carcasses. We interpret this result as a possible trade-off between managing infection risk and maximizing reproduction. Our findings suggest infection avoidance contributes to how mothers provision their offspring and underline the need to consider infection avoidance behaviors at multiple life-stages.

Sensory deficiencies affect resource selection and associational effects at two spatial scales

Many insect species have limited sensory abilities and may not be able to perceive the quality of different resource types while approaching patchily distributed resources. These restrictions may lead to differences in selection rates between separate patches and between different resource types within a patch, which may have consequences for associational effects between resources. In this study, we used an oviposition assay containing different frequencies of apple and banana substrates divided over two patches to compare resource selection rates of wild-type Drosophila melanogaster at the between- and within-patch scales. Next, we compared the wild-type behavior with that of the olfactory-deficient strain Orco 2 and the gustatory-deficient strain Poxn ΔM22-B5 and found comparable responses to patch heterogeneity and similarly strong selection rates for apple at both scales for the wild-type and olfactory-deficient flies. Their oviposition behavior translated into associational susceptibility for apple and associational resistance for banana. The gustatory-deficient flies, on the other hand, no longer had a strong selection rate for apple, strongly differed in between- and within-patch selection rates from the wild-type flies, and caused no associational effects between the resources. Our study suggests that differences in sensory capabilities can affect resource selection at different search behavior scales in different ways and in turn underlie associational effects between resources at different spatial scales.

Social nutrition: an emerging field in insect science

Nutrition is thought to be a major driver of social evolution, yet empirical support for this hypothesis is scarce. Here we illustrate how conceptual advances in nutritional ecology illuminate some of the mechanisms by which nutrition mediates social interactions in insects. We focus on experiments and models of nutritional geometry and argue that they provide a powerful means for comparing nutritional phenomena across species exhibiting various social ecologies. This approach, initially developed to study the nutritional behaviour of individual insects, has been increasingly used to study insect groups and societies, leading to the emerging field of social nutrition. We discuss future directions for exploring how these nutritional mechanisms may influence major social transitions in insects and other animals.

Adaptive learning in non-social insects: from theory to field work, and back

We review the evidence that learning affects fitness in non-social insects. Early accounts date back from the 1970s and were based on field-based observational and experimental work, yet exploration of the ways in which various forms of learning increase fitness remains limited in non-social insects. We highlight the concerns that arise when artificial laboratory settings, which do not take the ecology of the species into account, are used to estimate fitness benefits of learning. We argue that ecologically-relevant experimental designs are most useful to provide fitness estimates of learning, that is, designs that include: firstly, offspring of wild-caught animals producing newly established stocks under relevant breeding conditions, combined with common-garden and reciprocal transplant experiments; secondly, the spatio-temporal dynamics of key ecological resources; and thirdly, the natural behaviours of the animals while searching for, and probing, resources. Finally, we provide guidelines for the study of fitness-learning relationships in an eco-evolutionary framework.

Evolutionary compromises to metabolic toxins: Ammonia and urea tolerance in Drosophila suzukii and Drosophila melanogaster

The invasive pest Drosophila suzukii has evolved morphological and behavioural adaptations to lay eggs under the skin of fresh fruits. This results in severe damage to a wide range of small fruits. Drosophila suzukii females typically lay few eggs per fruit, preferring healthy fruits. Hence, larvae are exposed to a reduced amount of nitrogenous waste. Differently, the innocuous Drosophila melanogaster lays eggs on fermented fruits already infested by conspecifics, with larvae developing in a crowded environment with the accumulation of nitrogenous waste such as ammonia and urea. These compounds derive from nitrogen metabolism, protein degradation, and amino acids catabolism and are relatively toxic at high concentrations in an organism. The observed differences in oviposition site and larval ecological niche suggest that these species might differ in behavioural and physiological mechanisms used to cope with nitrogenous waste. We investigated how different concentrations of ammonia and urea affect oviposition and larval development in both species. Females and larvae of D. suzukii showed greater susceptibility to high concentrations of both compounds, with a dramatic decrease in the number of eggs laid and egg viability. Moreover, we tested the chemotactic response of third instar larvae to high concentrations of the compounds. Interestingly, ammonia resulted in a repulsive behaviour in respect of the control and urea groups. To better understand the pathways underlying these differences, we evaluated the effect on ornithine aminotransferase and glutathione-S-transferase, two enzymes involved in nitrogen metabolism and stress response that are expressed during larval development. Both ammonia and urea significantly reduced the expression of these enzymes in D. suzukii compared to D. melanogaster. This shows how the ecological shift of D. suzukii to fresh fruit is accompanied by less efficient detoxifying and excretory mechanisms, with important implications for evolutionary biology and applied research. Our data suggest that the ecological shift of D. suzukii to fresh fruit as oviposition substrate is accompanied by a reduced tolerance to metabolic toxins during larval development.

Physiological responses of insects to microbial fermentation products: Insights from the interactions between Drosophila and acetic acid

Acetic acid is a fermentation product of many microorganisms, including some that inhabit the food and guts of Drosophila. Here, we investigated the effect of dietary acetic acid on oviposition and larval performance of Drosophila. At all concentrations tested (0.34-3.4%), acetic acid promoted egg deposition by mated females in no-choice assays; and females preferred to oviposit on diet with acetic acid relative to acetic acid-free diet. However, acetic acid depressed larval performance, particularly extending the development time of both larvae colonized with the bacterium Acetobacter pomorum and axenic (microbe-free) larvae. The larvae may incur an energetic cost associated with dissipating the high acid load on acetic acid-supplemented diets. This effect was compounded by suppressed population growth of A. pomorum on the 3.4% acetic acid diet, such that the gnotobiotic Drosophila on this diet displayed traits characteristic of axenic Drosophila, specifically reduced developmental rate and elevated lipid content. It is concluded that acetic acid is deleterious to larval Drosophila, and hypothesized that acetic acid may function as a reliable cue for females to oviposit in substrates bearing microbial communities that promote larval nutrition.

Nutritional geometry and fitness consequences in Drosophila suzukii, the Spotted-Wing Drosophila

Since its arrival to North America less than a decade ago, the invasive Spotted-Wing Drosophila (Drosophila suzukii) has inflicted substantial economic losses on soft fruit agriculture due to its ability to oviposit into ripening fruits. More effective management approaches for this species are needed, but little is known about the factors that influence behavioral choices made by D. suzukii when selecting hosts, or the consequences that their offspring experience when developing in different environments. Using a nutritional geometry methodology, we found that the ratio of proteins-to-carbohydrates (P:C) present in media greatly influenced adult D. suzukii behavior and subsequent offspring development. Whereas adult flies showed a strong bias in their oviposition and association behaviors toward carbohydrate-rich foods, larval survival and eclosion rate were strongly dependent on protein availability. Here, we explore the preference-performance hypothesis (PPH), in which females are predicted to oviposit on medias that provide the greatest offspring benefits, in regard to its relevance in D. suzukii behavior and consequences for management. Our results provide valuable insight into the ecology and evolution of this species that may hopefully lead to more effective management strategies.

Fruit flies may face a nutrient-dependent life-history trade-off between secondary sexual trait quality, survival and developmental rate

Optimal life-history strategies are those that best allocate finite environmental resources to competing traits. We used the geometric framework for nutrition to evaluate life-history strategies followed by Drosophila melanogaster by measuring the condition-dependent performance of life-history traits, including the morphology of male secondary sexual characters, sex combs. We found that depending on their rearing environment flies faced different forms of trait trade-offs and accordingly followed different life-history strategies. High-energy, high-carbohydrate, low-protein diets supported development of the largest and most symmetrical sex combs, however, consistent with handicap models of sexual selection these foods were associated with reduced fly survival and developmental rate. Expressing the highest quality sex combs may have required secondary sexual trait quality to be traded-off with developmental rate, and our results indicated that flies unable to slow development died. As larval nutritional environments are predominantly determined by female oviposition substrate choice, we tested where mated female flies laid the most eggs. Mothers chose high-energy, high-protein foods associated with rapid larval development. Mothers avoided high-carbohydrate foods associated with maximal sex comb expression, showing they may avoid producing fewer 'sexy' sons in favour of producing offspring that develop rapidly.

Comparing the impacts of macronutrients on life-history traits in larval and adult Drosophila melanogaster: the use of nutritional geometry and chemically defined diets

Protein and carbohydrate are the two major macronutrients that exert profound influences over fitness in many organisms, including Drosophila melanogaster. Our understanding of how these macronutrients shape the components of fitness in D. melanogaster has been greatly enhanced by the use of nutritional geometry, but most nutritional geometric analyses on this species have been conducted using semi-synthetic diets that are not chemically well-defined. Here we combined the use of nutritional geometry and chemically defined diets to compare the patterns of larval and adult life-history traits expressed across 34 diets systematically varying in protein:carbohydrate (P:C) ratio and in protein plus carbohydrate (P+C) concentration. The shape of the response surfaces constructed for all larval and adult traits differed significantly from one another, with the nutritional optima being identified at P:C 1:4 for lifespan (P+C 120 g l−1), 1:2 for egg-to-adult viability (120 g l−1), 1:1 for female body mass at adult eclosion (240 g l−1) and lifetime fecundity (360 g l−1), 2:1 for larval developmental rate (60 g l−1), and 8:1 for egg production rate (120 g l−1). Such divergence in nutritional optima among life-history traits indicates that D. melanogaster confined to a single diet cannot maximize the expression of these traits simultaneously and thus may face a life-history trade-off. Our data provide the most comprehensive and nutritionally explicit analysis of the impacts of macronutrients on life-history traits in D. melanogaster and support the emerging notion that the fundamental trade-offs among life-history traits are mediated by macronutrients.

A Drosophila female pheromone elicits species-specific long-range attraction via an olfactory channel with dual specificity for sex and food

Background

Mate finding and recognition in animals evolves during niche adaptation and involves social signals and habitat cues. Drosophila melanogaster and related species are known to be attracted to fermenting fruit for feeding and egg-laying, which poses the question of whether species-specific fly odours contribute to long-range premating communication.

Results

We have discovered an olfactory channel in D. melanogaster with a dual affinity to sex and food odorants. Female flies release a pheromone, (Z)-4-undecenal (Z4-11Al), that elicits flight attraction in both sexes. Its biosynthetic precursor is the cuticular hydrocarbon (Z,Z)-7,11-heptacosadiene (7,11-HD), which is known to afford reproductive isolation between the sibling species D. melanogaster and D. simulans during courtship. Twin olfactory receptors, Or69aB and Or69aA, are tuned to Z4-11Al and food odorants, respectively. They are co-expressed in the same olfactory sensory neurons, and feed into a neural circuit mediating species-specific, long-range communication; however, the close relative D. simulans, which shares food resources with D. melanogaster, does not respond to Z4-11Al.

Conclusion

The Or69aA and Or69aB isoforms have adopted dual olfactory traits. The underlying gene yields a collaboration between natural and sexual selection, which has the potential to drive speciation.

Gut Microbiota Modifies Olfactory-Guided Microbial Preferences and Foraging Decisions in Drosophila

The gut microbiota affects a wide spectrum of host physiological traits, including development [1-5], germline [6], immunity [7-9], nutrition [4, 10, 11], and longevity [12, 13]. Association with microbes also influences fitness-related behaviors such as mating [14] and social interactions [15, 16]. Although the gut microbiota is evidently important for host wellbeing, how hosts become associated with particular assemblages of microbes from the environment remains unclear. Here, we present evidence that the gut microbiota can modify microbial and nutritional preferences of Drosophila melanogaster. By experimentally manipulating the gut microbiota of flies subjected to behavioral and chemosensory assays, we found that fly-microbe attractions are shaped by the identity of the host microbiota. Conventional flies exhibit preference for their associated Lactobacillus, a behavior also present in axenic flies as adults and marginally as larvae. By contrast, fly preference for Acetobacter is primed by early-life exposure and can override the innate preference. These microbial preferences are largely olfactory guided and have profound impact on host foraging, as flies continuously trade off between acquiring beneficial microbes and balancing nutrients from food. Our study shows a role of animal microbiota in shaping host fitness-related behavior through their chemosensory responses, opening a research theme on the interrelationships between the microbiota, host sensory perception, and behavior.

Adaptation to new nutritional environments: larval performance, foraging decisions, and adult oviposition choices in Drosophila suzukii

Background

Understanding how species adapt to new niches is a central issue in evolutionary ecology. Nutrition is vital for the survival of all organisms and impacts species fitness and distribution. While most Drosophila species exploit rotting plant parts, some species have diversified to use ripe fruit, allowing earlier colonization. The decomposition of plant material is facilitated by yeast colonization and proliferation. These yeasts serve as the main protein source for Drosophila larvae. This dynamic rotting process entails changes in the nutritional composition of the food and other properties, and animals feeding on material at different stages of decay are expected to have behavioural and nutritional adaptations.

Results

We compared larval performance, feeding behaviour and adult oviposition site choice between the ripe fruit colonizer and invasive pest Drosophila suzukii, and a closely-related rotting fruit colonizer, Drosophila biarmipes. Through the manipulation of protein:carbohydrate ratios in artificial diets, we found that D. suzukii larvae perform better at lower protein concentrations and consume less protein rich diets relative to D. biarmipes. For adult oviposition, these species differed in preference for substrate hardness, but not for the substrate nutritional composition.

Conclusions

Our findings highlight that rather than being an exclusive specialist on ripe fruit, D. suzukii’s adaptation to use ripening fruit allow it to colonize a wider range of food substrates than D. biarmipes, which is limited to soft foods with higher protein concentrations. Our results underscore the importance of nutritional performance and feeding behaviours in the colonization of new food niches.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-017-0131-2) contains supplementary material, which is available to authorized users.