Sex-Specific Among-Individual Covariation in Locomotor Activity and Resting Metabolic Rate in Drosophila melanogaster

Reporting guidelines for terrestrial respirometry: Building openness, transparency of metabolic rate and evaporative water loss data

Respirometry is an important tool for understanding whole-animal energy and water balance in relation to the environment. Consequently, the growing number of studies using respirometry over the last decade warrants reliable reporting and data sharing for effective dissemination and research synthesis. We provide a checklist guideline on five key sections to facilitate the transparency, reproducibility, and replicability of respirometry studies: 1) materials, set up, plumbing, 2) subject conditions/maintenance, 3) measurement conditions, 4) data processing, and 5) data reporting and statistics, each with explanations and example studies. Transparency in reporting and data availability has benefits on multiple fronts. Authors can use this checklist to design and report on their study, and reviewers and editors can use the checklist to assess the reporting quality of the manuscripts they review. Improved standards for reporting will enhance the value of primary studies and will greatly facilitate the ability to carry out higher quality research syntheses to address ecological and evolutionary theories.

Lagging Brain Gene Expression Patterns of Drosophila melanogaster Young Adult Males Confound Comparisons Between Sexes

Many species, including fruit flies (Drosophila melanogaster), are sexually dimorphic. Phenotypic variation in morphology, physiology, and behavior can affect development, reproduction, health, and aging. Therefore, designating sex as a variable and sex-blocking should be considered when designing experiments. The brain regulates phenotypes throughout the lifespan by balancing survival and reproduction, and sex-specific development at each life stage is likely. Changes in morphology and physiology are governed by differential gene expression, a quantifiable molecular marker for age- and sex-specific variations. We assessed the fruit fly brain transcriptome at three adult ages for gene expression signatures of sex, age, and sex-by-age: 6698 genes were differentially expressed between sexes, with the most divergence at 3 days. Between ages, 31.1% of 6084 differentially expressed genes (1890 genes) share similar expression patterns from 3 to 7 days in females, and from 7 to 14 days in males. Most of these genes (90.5%, 1712) were upregulated and enriched for chemical stimulus detection and/or cilium regulation. Our data highlight an important delay in male brain gene regulation compared to females. Because significant delays in expression could confound comparisons between sexes, studies of sexual dimorphism at phenotypically comparable life stages rather than chronological age should be more biologically relevant.

Context-dependent effects of thermal acclimation on physiological correlates of animal personality in Asiatic toads

Persistent individual variation in behaviour, or 'personality', is a widespread phenomenon in animals, and understanding the evolution of animal personality is a key task of current biology. Natural selection has been proposed to promote the integration of personality with animal 'intrinsic states', such as metabolic or endocrine traits, and this integration varies with ecological conditions. However, these external ecological modulatory effects have rarely been examined. Here, we investigate the effects of thermal acclimation on between-individual covariations between physiology and behaviour in Asiatic toads (Bufo gargarizans) along an altitudinal gradient. Our results reveal that the thermal modulatory effects on the covariations depend on the altitudinal population. Specifically, at low altitudes, between-individual covariations are highly plastic, with risk-taking behaviour covarying with baseline glucocorticoids (GCs) under warm acclimation, but risk-taking and exploration behaviour covarying with resting metabolic rate (RMR) under cold acclimation. In contrast, between-individual covariations are relatively fixed at high altitudes, with risk-taking behaviour consistently covarying with baseline GCs. Furthermore, at low altitudes, changes in covariations between RMR and personality are associated with adjustment of energy management models. Evidently, animal physiological states that determine or covary with personality can adapt according to the seasonal thermal environment and the thermal evolutionary background of populations. Our findings highlight the importance of a multi-system physiological approach to understand the evolution of animal personality.

Sexual Selection Increases Male Behavioral Consistency in Drosophila melanogaster

AbstractSexual selection has been suggested to influence the expression of male behavioral consistency. However, despite predictions, direct experimental support for this hypothesis has been lacking. Here, we investigated whether sexual selection altered male behavioral consistency in Drosophila melanogaster-a species with both pre- and postcopulatory sexual selection. We took 1,144 measures of locomotor activity (a fitness-related trait in D. melanogaster) from 286 flies derived from replicated populations that have experimentally evolved under either high or low levels of sexual selection for >320 generations. We found that high sexual selection males were more consistent (decreased within-individual variance) in their locomotor activity than male conspecifics from low sexual selection populations. There were no differences in behavioral consistency between females from the high and low sexual selection populations. Furthermore, while females were more behaviorally consistent than males in the low sexual selection populations, there were no sex differences in behavioral consistency in high sexual selection populations. Our results demonstrate that behavioral plasticity is reduced in males from populations exposed to high levels of sexual selection. Disentangling whether these effects represent an evolved response to changes in the intensity of selection or are manifested through nongenetic parental effects represents a challenge for future research.

Among- and Within-Individual Variance in Metabolic Thermal Reaction Norms

AbstractIn ectotherms, temperature has a strong effect on metabolic rate (MR), yet the extent to which the thermal sensitivity of MR varies among versus within individuals is largely unknown. This is of interest because significant among-individual variation is a prerequisite for the evolution of metabolic thermal sensitivity. Here, we estimated the repeatability (R) of the thermal sensitivity of MR in individual virgin, adult male Drosophila melanogaster (N = 316 ) by taking repeated overnight measures of their MRs at two temperatures (~24°C and ~27°C). At the population level, thermal sensitivity decreased with locomotor activity, and older individuals showed a higher thermal sensitivity of MR than younger individuals. Taking these effects (and body mass) into account, we detected significant repeatability in both the centered intercept (R int = 0.52 ± 0.04 ) and the slope (R slp = 0.21 ± 0.07 ) of the metabolic thermal reaction norms, which respectively represent average MR and thermal sensitivity of MR. Furthermore, individuals with a higher overall MR also displayed greater increases in MR as temperature increased from ~24°C to ~27°C (r ind = 0.32 ± 0.14 ). Average MR and thermal sensitivity of MR were also positively correlated within individuals (r e = 0.15 ± 0.07 ). Our study represents a point of departure for future larger studies, in which more complex protocols (e.g., wider temperature range, breeding design) can be applied to quantify the causal components of variation in thermal sensitivity that are needed to make accurate predictions of adaptive responses to global warming.

Is personality and its association with energetics sex-specific in yellow-necked mice Apodemus flavicollis?

For the last two decades, behavioral physiologists aimed to explain a plausible covariation between energetics and personality, predicted by the "pace-of-life syndrome" (POLS) hypothesis. However, the results of these attempts are mixed with no definitive answer as to which of the two most acknowledged models "performance" or "allocation" predicts covariation between consistent among-individual variation in metabolism and repeatable behavior (animal personality). The general conclusion is that the association between personality and energetics is rather context-dependent. Life-history, behavior, and physiology as well as its plausible covariation can be considered a part of sexual dimorphism. However, up to now, only a few studies demonstrated a sex-specific correlation between metabolism and personality. Therefore, we tested the relationships between physiological and personality traits in a single population of yellow-necked mice Apodemus flavicollis in the context of a plausible between-sexes difference in this covariation. We hypothesized that the performance model will explain proactive behavior in males and the allocation model will apply to females. Behavioral traits were determined using the latency of risk-taking and the open field tests, whereas the basal metabolic rates (BMR) was measured using indirect calorimetry. We have found a positive correlation between body mass-adjusted BMR and repeatable proactive behavior in male mice, which can support the performance model. However, the females were rather consistent mainly in avoidance of risk-taking that did not correlate with BMR, suggesting essential differences in personality between sexes. Most likely, the lack of convincing association between energetics and personality traits at the population level is caused by a different selection acting on the life histories of males and females. This may only result in weak support for the predictions of the POLS hypothesis when assuming that only a single model explaining the link between physiology and behavior operates in males and females. Thus, there is a need to consider the differences between sexes in behavioral studies to evaluate this hypothesis.

Increased Iron Levels and Oxidative Stress Mediate Age-Related Impairments in Male and Female Drosophila melanogaster

Aging is characterized by a functional decline in the physiological functions and organic systems, causing frailty, illness, and death. Ferroptosis is an iron- (Fe-) dependent regulated cell death, which has been implicated in the pathogenesis of several disorders, such as cardiovascular and neurological diseases. The present study investigated behavioral and oxidative stress parameters over the aging of Drosophila melanogaster that, together with augmented Fe levels, indicate the occurrence of ferroptosis. Our work demonstrated that older flies (30-day-old) of both sexes presented impaired locomotion and balance when compared with younger flies (5-day-old). Older flies also produced higher reactive oxygen species (ROS) levels, decreased glutathione levels (GSH), and increased lipid peroxidation. In parallel, Fe levels were augmented in the fly's hemolymph. The GSH depletion with diethyl maleate potentiated the behavioral damage associated with age. Our data demonstrated biochemical effects that characterize the occurrence of ferroptosis over the age of D. melanogaster and reports the involvement of GSH in the age-associated damages, which could be in part attributed to the augmented levels of Fe.

Locomotion and brain gene expression exhibit sex-specific non-monotonic dose-response to HFPO-DA during Drosophila melanogaster lifespan

BACKGROUND

Legacy per- and polyfluoroalkyl substances (PFAS), known for their environmental persistence and bio-accumulative properties, have been phased out in the U.S. due to public health concerns. A newer polymerization aid used in the manufacture of some fluoropolymers, hexafluoropropylene oxide-dimer acid (HFPO-DA), has lower reported bioaccumulation and toxicity, but is a potential neurotoxicant implicated in dopaminergic neurodegeneration.

OBJECTIVE

We investigated HFPO-DA's bio-accumulative potential and sex-specific effects on lifespan, locomotion, and brain gene expression in fruit flies.

METHODS

We quantified bioaccumulation of HFPO-DA in fruit flies exposed to 8.7×10⁴µg/L of HFPO-DA in the fly media for 14 days via UHPLC-MS. Long-term effect on lifespan was determined by exposing both sexes to 8.7×10² - 8.7×10⁵µg/L of HFPO-DA in media. Locomotion was measured following 3, 7, and 14 days of exposures at 8.7×10¹ - 8.7×10⁵µg/L of HFPO-DA in media, and high-throughput 3'-end RNA-sequencing was used to quantify gene expression in fly brains across the same time points.

RESULTS

Bioaccumulation of HFPO-DA in fruit flies was not detected. HFPO-DA-induced effects on lifespan, locomotion, and brain gene expression, and lowest adverse effect level (LOAEL) showed sexually dimorphic patterns. Locomotion scores significantly decreased in at least one dose at all time points for females and only at 3-day exposure for males, while brain gene expression exhibited non-monotonic dose-response. Differentially expressed genes correlated to locomotion scores revealed sex-specific numbers of positively and negatively correlated genes per functional category.

CONCLUSION

Although HFPO-DA effects on locomotion and survival were significant at doses higher than the US EPA reference dose, the brain transcriptomic profiling reveals sex-specific changes and neurological molecular targets; gene enrichments highlight disproportionately affected categories, including immune response: female-specific co-upregulation suggests potential neuroinflammation. Consistent sex-specific exposure effects necessitate blocking for sex in experimental design during HFPO-DA risk assessment.

Evidence for Prepulse Inhibition of Visually Evoked Motor Response in Drosophila melanogaster

Prepulse inhibition (PPI) is a widely investigated behavior to study the mechanisms of disorders such as anxiety, schizophrenia, and bipolar mania. PPI has been observed across various vertebrate and invertebrate species; however, it has not yet been reported in adult Drosophila melanogaster. In this study, we describe the first detection of PPI of visually evoked locomotor arousal in flies. To validate our findings, we demonstrate that PPI in Drosophila can be partially reverted by the N-methyl D-aspartate (NMDA) receptor antagonist MK-801, known for inducing sensorimotor gating deficits in rodent models. Additionally, we show that the visually evoked response can be inhibited by multiple stimuli presentation, which can also be affected by MK-801. Given the versatility of Drosophila as a model organism for genetic screening and analysis, our results suggest that high-throughput behavioral screenings of adult flies can become a valuable tool for investigating the mechanisms behind PPI.

Spatial variation in the evolutionary potential and constraints of basal metabolic rate and body mass in a wild bird

An organism's energy budget is strongly related to resource consumption, performance, and fitness. Hence, understanding the evolution of key energetic traits, such as basal metabolic rate (BMR), in natural populations is central for understanding life-history evolution and ecological processes. Here we used quantitative genetic analyses to study evolutionary potential of BMR in two insular populations of the house sparrow (Passer domesticus). We obtained measurements of BMR and body mass (M_b ) from 911 house sparrows on the islands of Leka and Vega along the coast of Norway. These two populations were the source populations for translocations to create an additional third, admixed 'common garden' population in 2012. With the use of a novel genetic group animal model concomitant with a genetically determined pedigree, we differentiate genetic and environmental sources of variation, thereby providing insight into the effects of spatial population structure on evolutionary potential. We found that the evolutionary potential of BMR was similar in the two source populations, whereas the Vega population had a somewhat higher evolutionary potential of M_b than the Leka population. BMR was genetically correlated with M_b in both populations, and the conditional evolutionary potential of BMR (independent of body mass) was 41% (Leka) and 53% (Vega) lower than unconditional estimates. Overall, our results show that there is potential for BMR to evolve independently of M_b , but that selection on BMR and/or M_b may have different evolutionary consequences in different populations of the same species.

Serotonin distinctly controls behavioral states in restrained and freely moving Drosophila

When trapped in a physical restraint, animals must select an escape strategy to increase their chances of survival. After falling into an inescapable trap, they react with stereotypical behaviors that differ from those displayed in escapable situations. Such behaviors involve either a wriggling response to unlock the trap or feigning death to fend off a predator attack. The neural mechanisms that regulate animal behaviors have been well characterized for escapable situations but not for inescapable traps. We report that restrained vinegar flies exhibit alternating flailing and immobility to free themselves from the trap. We used optogenetics and intersectional genetic approaches to show that, while broader serotonin activation promotes immobility, serotonergic cells in the ventral nerve cord (VNC) regulate immobility states majorly via 5-HT7 receptors. Restrained and freely moving locomotor states are controlled by distinct mechanisms. Taken together, our study has identified serotonergic switches of the VNC that promote environment-specific adaptive behaviors.

Light/dark phase influences intra-individual plasticity in maintenance metabolic rate and exploratory behavior independently in the Asiatic toad

Background

It is well-known that light/dark phase can affect energy expenditure and behaviors of most organisms; however, its influences on individuality (inter-individual variance) and plasticity (intra-individual variance), as well as their associations remain unclear. To approach this question, we repeatedly measured maintenance metabolic rate (MR), exploratory and risk-taking behaviors across light/dark phase four times using wild-caught female Asiatic toads (Bufo gargarizans), and partitioned their variance components with univariate and bivariate mixed-effects models.

Results

The group means of maintenance MR and risk-taking behavior increased at night, while the group mean of exploratory behavior remained constant throughout the day. At night, the intra-individual variances were elevated in maintenance MR but reduced in exploration, suggesting that phenotypic plasticity was enhanced in the former but constrained in the latter. In addition, maintenance MR was not coupled with exploratory or risk-taking behaviors in daytime or at night, neither at the inter-individual nor intra-individual levels.

Conclusions

Our findings suggest that these traits are independently modulated by the light/dark phase, and an allocation energy management model may be applicable in this species. This study sheds new insights into how amphibians adapt nocturnal lifestyle across multiple hierarchy levels via metabolic and behavioral adjustments.

Can Sex-Specific Metabolic Rates Provide Insight Into Patterns of Metabolic Scaling?

Females and males can exhibit striking differences in body size, relative trait size, physiology and behavior. As a consequence the sexes can have very different rates of whole-body energy use, or converge on similar rates through different physiological mechanisms. Yet many studies that measure the relationship between metabolic rate and body size only pay attention to a single sex (more often males), or do not distinguish between sexes. We present four reasons why explicit attention to energy-use between the sexes can yield insight into the physiological mechanisms that shape broader patterns of metabolic scaling in nature. First, the sexes often differ considerably in their relative investment in reproduction which shapes much of life-history and rates of energy use. Second, males and females share a majority of their genome but may experience different selective pressures. Sex-specific energy profiles can reveal how the energetic needs of individuals are met despite the challenge of within-species genetic constraints. Third, sexual selection often pushes growth and behavior to physiological extremes. Exaggerated sexually selected traits are often most prominent in one sex, can comprise up to 50% of body mass and thus provide opportunities to uncover energetic constraints of trait growth and maintenance. Finally, sex-differences in behavior such as mating-displays, long-distance dispersal and courtship can lead to drastically different energy allocation among the sexes; the physiology to support this behavior can shape patterns of metabolic scaling. The mechanisms underlying metabolic scaling in females, males and hermaphroditic animals can provide opportunities to develop testable predictions that enhance our understanding of energetic scaling patterns in nature.

Weak evidence that asset protection underlies temporal or contextual consistency in boldness of a terrestrial crustacean

Abstract

In recent years, many studies have investigated the potential state dependence of individual differences in behaviour, with the aim to understand the proximate and ultimate causes and consequences of animal personality. Among the potential state variables that could affect behavioural expression is size and mass, but few studies have found associations at the among-individual levels. Insufficient sampling and incorrect analysis of data are cited as impediments to detecting correlations, if they exist. Here, we conducted a study using 100 pillbugs (Armadillidium vulgare) and assayed their defensive behaviour 24 times each over time and across familiarity contexts, to test the asset protection hypothesis that predicts a negative correlation between boldness and mass, and with increases in mass over time. Multivariate mixed models revealed that despite mostly consistent individual behavioural differences over time (modest slope variance) and across contexts (near-parallel reaction norms), and 18-fold range in starting mass, there was no correlation between individual mean mass and boldness. However, individuals that gained more mass over time may have been more ‘shy’ compared to those gaining less mass, but the correlation was weak and observed variation in mass gain was small. There was also a mean level trend of increasing shyness over time that was coincident with mean level mass increases over time. Together, our study provides weak evidence for the asset protection hypothesis, whereby individuals that accumulate more resources are thought to protect them through risk averse behaviour.

Significance statement

Individual variation in ‘state’, such as mass or energy reserves, is thought to be a predictor of individual differences in behaviour that are consistent over time. However, few studies reveal such links, and several studies suggest insufficient sampling may explain null results in most studies. We studied 100 animals sampled 24 times each in a controlled setting to reveal stable individual differences in mean behaviour over time and across contexts; however, individual behaviour was unrelated to large differences in individual mass but weakly related to increases in mass through time whereby individuals became more shy and those growing faster were somewhat more shy. Our results provide little evidence for the asset protection hypothesis.

Variation in mitochondrial DNA affects locomotor activity and sleep in Drosophila melanogaster

Mitochondria are organelles that produce cellular energy in the form of ATP through oxidative phosphorylation, and this primary function is conserved among many taxa. Locomotion is a trait that is highly reliant on metabolic function and expected to be greatly affected by disruptions to mitochondrial performance. To this end, we aimed to examine how activity and sleep vary between Drosophila melanogaster strains with different geographic origins, how these patterns are affected by mitochondrial DNA (mtDNA) variation, and how breaking up co-evolved mito-nuclear gene combinations affect the studied activity traits. Our results demonstrate that Drosophila strains from different locations differ in sleep and activity, and that females are generally more active than males. By comparing activity and sleep of mtDNA variants introgressed onto a common nuclear background in cytoplasmic hybrid (cybrid) strains, we were able to quantify the among-line variance attributable to mitochondrial DNA, and we establish that mtDNA variation affects both activity and sleep, in a sex-specific manner. Altogether our study highlights the important role that mitochondrial genome variation plays on organismal physiology and behaviour.

Sex-specific regulation of development, growth and metabolism

Adult females and males of most species differ in many aspects of their morphology, physiology and behavior, in response to sex-specific selective pressures that maximize fitness. While we have an increasingly good understanding of the genetic mechanisms that initiate these differences, the sex-specific developmental trajectories that generate them are much less well understood. Here we review recent advances in the sex-specific regulation of development focusing on two models where this development is increasingly well understood: Sexual dimorphism of body size in the fruit fly Drosophila melanogaster and sexual dimorphism of horns in the horned beetle Onthophagus taurus. Because growth and development are also supported by metabolism, the regulation of sex-specific metabolism during and after development is an important aspect of the generation of female and male phenotypes. Hitherto, the study of sex-specific development has largely been independent of the study of sex-specific metabolism. Nevertheless, as we discuss in this review, recent research has begun to reveal considerable overlap in the cellular and physiological mechanisms that regulate sex-specific development and metabolism.

Experimental sexual selection affects the evolution of physiological and life-history traits

Sexual selection and sexual conflict are expected to affect all aspects of the phenotype, not only traits that are directly involved in reproduction. Here, we show coordinated evolution of multiple physiological and life-history traits in response to long-term experimental manipulation of the mating system in populations of Drosophila pseudoobscura. Development time was extended under polyandry relative to monogamy in both sexes, potentially due to higher investment in traits linked to sexual selection and sexual conflict. Individuals (especially males) evolving under polyandry had higher metabolic rates and locomotor activity than those evolving under monogamy. Polyandry individuals also invested more in metabolites associated with increased endurance capacity and efficient energy metabolism and regulation, namely lipids and glycogen. Finally, polyandry males were less desiccation- and starvation resistant than monogamy males, suggesting trade-offs between resistance and sexually selected traits. Our results provide experimental evidence that mating systems can impose selection that influences the evolution of non-sexual phenotypes such as development, activity, metabolism and nutrient homeostasis.

Individual Variation in Thermal Reaction Norms Reveals Metabolic-Behavioral Relationships in an Ectotherm

Ectothermic organisms respond to rapid environmental change through a combination of behavioral and physiological adjustments. As behavioral and physiological traits are often functionally linked, an effective ectotherm response to environmental perturbation will depend on the direction and magnitude of their association. The role of various modifiers in behavioral-physiological relationships remains largely unexplored. We applied a repeated-measures approach to examine the influence of body temperature and individual variation on the link between resting metabolic rate (RMR) and exploratory locomotor activity (ELA) in juvenile Alpine newts, Ichthyosaura alpestris. We analyzed trait relationships at two body temperatures separately and as parameters, intercepts and slopes, of thermal reaction norms for both traits. Body temperature affected the level of detectable among-individual variation in two different directions. Among-individual variation in ELA was detected at 12°C, while RMR was repeatable at 22°C. We found no support for a link between RMR and ELA at either temperature. While analysis of intercepts revealed among-individual variation in both traits, among-individual variation in slopes was detected in RMR only. Intercepts were positively associated at the individual, but not the whole-phenotypic, level. For ELA, the target of selection should be individual trait values across temperatures, rather than their thermal sensitivities. The positive association between intercepts of thermal reaction norms for ELA and RMR suggests that phenotypic selection acts on both traits in a correlated fashion. Measurements at one body temperature and within-individual variation hide the metabolic-behavioral relations. We conclude that correlative studies on flexible behavioral and physiological traits in ectotherms require repeated measurement at two or more body temperatures in order to avoid misleading results. This approach is needed to fully understand ectotherm responses to environmental change and its impact on their population dynamics.

Sex-specific covariance between metabolic rate, behaviour and morphology in the ground beetle Carabus hortensis

BACKGROUND

Individuals within the same species often differ in their metabolic rates, which may covary with behavioural traits (such as exploration), that are consistent across time and/or contexts, and morphological traits. Yet, despite the frequent occurrence of sexual dimorphisms in morphology and behaviour, few studies have assessed whether and how sexes differ in metabolic trait covariances.

METHODS

We investigated sex-specific relationships among resting or active metabolic rate (RMR and AMR, respectively) with exploratory behaviour, measured independently of metabolic rate in a novel environment, body size and body mass, in Carabus hortensis ground beetles.

RESULTS

RMR, AMR and exploratory behaviour were repeatable among individuals across time, except for male RMR which was unrepeatable. Female RMR neither correlated with exploratory behaviour nor body size/body mass. In contrast, AMR was correlated with both body size and exploratory behaviour. Males with larger body sizes had higher AMR, whereas females with larger body sizes had lower AMR. Both male and female AMR were significantly related to exploratory behaviour, though the relationships between AMR and exploration were body mass-dependent in males and temperature-dependent in females.

DISCUSSION

Differences between sexes exist in the covariances between metabolic rate, body size and exploratory behaviour. This suggests that selection acts differently on males and females to produce these trait covariances with potentially important consequences for individual fitness.

Endosymbiotic male-killing Spiroplasma affect the physiological and behavioural ecology of Macrocheles-Drosophila interactions

While many arthropod endosymbionts are vertically transmitted, phylogenetic studies reveal repeated introductions of hemolymph-dwelling Spiroplasma into Drosophila. Introductions are often attributed to horizontal transmission via ectoparasite vectors. Here, we test if mites prefer to infect Spiroplasma poulsonii MSRO infected flies, and if MSRO infection impairs fly resistance against secondary mite (Macrocheles subbadius) attack. First we tested if mites prefer MSRO+ or MSRO- flies using pair-wise-choice tests across fly ages. We then tested whether mite preferences are explained by changes in fly physiology, specifically increased metabolic rate (measured as CO₂ production). We hypothesize that this preference is due in part to MSRO+ flies expressing higher metabolic rates. However, our results showed mite preference depended on an interaction between fly age and MSRO status: mites avoided 14-days old MSRO+ flies relative to MSRO- flies (31% infection), but prefered MSRO+ flies (64% infection) among 26-day old flies. Using flow-through respirometry, we found 14 day-old MSRO+ flies had higher CO₂ emissions than MSRO- flies (32% greater), whereas at 26 days old the CO₂ production among MSRO+ flies was 20% lower than MSRO- flies. Thus, mite preferences for high metabolic rate hosts did not explain the infection biases in this study. To assess changes in susceptibility to infection, we measured fly endurance using geotaxis assays. Older flies had lower endurance consistent with fly senescence, and this effect was magnified among MSRO+ flies. Given the biological importance of male-killing Spiroplasma, potential changes in the interactions of hosts and potential vectors could impact the ecology and evolution of host species. Importance Male-killing endosymbionts are transmitted mother to daughter and kill male offspring. Despite these major ecological effects, how these endosymbionts colonize new host species is not always clear. Mites are sometimes hypothesized to transfer these bacteria between hosts/host species. Here we test if 1) if mites prefer to infect flies that harbour Spiroplasma poulisoni MSRO and 2) if flies infected with MSRO are less able to resist mite infection. Our results show that flies infected with MSRO have weaker anti-mite resistance but the mite preference/aversion for MSRO+ flies varied with fly age. Given the fitness and population impacts of male-killing Spiroplasma, changes in fly-mite interactions have implications for the ecology and evolution of these symbioses.

Sex determination gene transformer regulates the male-female difference in Drosophila fat storage via the adipokinetic hormone pathway

Sex differences in whole-body fat storage exist in many species. For example, Drosophila females store more fat than males. Yet, the mechanisms underlying this sex difference in fat storage remain incompletely understood. Here, we identify a key role for sex determination gene transformer (tra) in regulating the male-female difference in fat storage. Normally, a functional Tra protein is present only in females, where it promotes female sexual development. We show that loss of Tra in females reduced whole-body fat storage, whereas gain of Tra in males augmented fat storage. Tra's role in promoting fat storage was largely due to its function in neurons, specifically the Adipokinetic hormone (Akh)-producing cells (APCs). Our analysis of Akh pathway regulation revealed a male bias in APC activity and Akh pathway function, where this sex-biased regulation influenced the sex difference in fat storage by limiting triglyceride accumulation in males. Importantly, Tra loss in females increased Akh pathway activity, and genetically manipulating the Akh pathway rescued Tra-dependent effects on fat storage. This identifies sex-specific regulation of Akh as one mechanism underlying the male-female difference in whole-body triglyceride levels, and provides important insight into the conserved mechanisms underlying sexual dimorphism in whole-body fat storage.

Sex-specific genetic (co)variances of standard metabolic rate, body mass and locomotor activity in Drosophila melanogaster

A longstanding focus in evolutionary physiology concerns the causes and consequences of variation in maintenance metabolism. Insight into this can be gained by estimating the sex-specific genetic architecture of maintenance metabolism alongside other, potentially correlated traits on which selection may also act, such as body mass and locomotor activity. This may reveal potential genetic constraints affecting the evolution of maintenance metabolism. Here, we used a half-sibling breeding design to quantify the sex-specific patterns of genetic (co)variance in standard metabolic rate (SMR), body mass and daily locomotor activity in Drosophila melanogaster. There was detectable additive genetic variance for all traits in both sexes. As expected, SMR and body mass were strongly and positively correlated, with genetic allometry exponents (b_A  ± SE) that were close to 2/3 in females (0.66 ± 0.16) and males (0.58 ± 0.32). There was a significant and positive genetic correlation between SMR and locomotor activity in males, suggesting that alleles that increase locomotion have pleiotropic effects on SMR. Sexual differences in the genetic architecture were largely driven by a difference in genetic variance in locomotor activity between the sexes. Overall, genetic variation was mostly shared between males and females, setting the stage for a potential intralocus sexual conflict in the face of sexually antagonistic selection.

Pervasive effects of Wolbachia on host activity

Heritable symbionts have diverse effects on the physiology, reproduction and fitness of their hosts. Maternally transmitted Wolbachia are one of the most common endosymbionts in nature, infecting about half of all insect species. We test the hypothesis that Wolbachia alter host behaviour by assessing the effects of 14 different Wolbachia strains on the locomotor activity of nine Drosophila host species. We find that Wolbachia alter the activity of six different host genotypes, including all hosts in our assay infected with wRi-like Wolbachia strains (wRi, wSuz and wAur), which have rapidly spread among Drosophila species in about the last 14 000 years. While Wolbachia effects on host activity were common, the direction of these effects varied unpredictably and sometimes depended on host sex. We hypothesize that the prominent effects of wRi-like Wolbachia may be explained by patterns of Wolbachia titre and localization within host somatic tissues, particularly in the central nervous system. Our findings support the view that Wolbachia have wide-ranging effects on host behaviour. The fitness consequences of these behavioural modifications are important for understanding the evolution of host-symbiont interactions, including how Wolbachia spread within host populations.

Territoriality in Drosophila: indirect effects and covariance with body mass and metabolic rate

Territoriality (i.e., defense of a resource) is the outcome of behavioral interactions that can result in selective advantages in many vertebrates and invertebrates. Since territoriality is expressed in a social context, an individuals’ territoriality may change according to the phenotype of the opponents that they are confronted with (termed “indirect effects”). Defending a territory may also confer energetic costs to individuals, which could be reflected in their standard metabolic rate (SMR), a key component of an ectotherms’ energy budget. Here, we measured territoriality using dyadic contests, body mass, and SMR using flow-through respirometry, twice in each of 192 adult male Drosophila melanogaster. Territoriality, body mass, and (whole-animal) SMR were all significantly repeatable. However, essentially all the among-individual variation in SMR was shared with body mass, as indicated by a very strong among-individual correlation (rind) between body mass and SMR. The among-individual correlation between territoriality and SMR also tended to be positive, suggesting the presence of underlying metabolic costs to territoriality. Although indirect effects on territoriality were present but weak (accounting for 8.4% of phenotypic variance), indirect effects on territoriality were negatively and significantly correlated with body mass. This indicates that larger individuals tended to suppress their opponents territoriality. Variation among individuals in their ability to suppress territoriality in others was not associated with their own territoriality or SMR.

Mother's curse is pervasive across a large mitonuclear Drosophila panel

The maternal inheritance of mitochondrial genomes entails a sex‐specific selective sieve, whereby mutations in mitochondrial DNA can only respond to selection acting on females. In theory, this enables male‐harming mutations to accumulate in mitochondrial genomes as long as they are neutral, beneficial, or only slightly deleterious to females. Ultimately, this bias could drive the evolution of male‐specific mitochondrial mutation loads, an idea known as mother's curse. Earlier work on this hypothesis has mainly used small Drosophila panels, in which naturally sourced mitochondrial genomes were coupled to an isogenic nuclear background. The lack of nuclear genetic variation in these designs has precluded robust generalization. Here, we test the predictions of mother's curse using a large Drosophila mitonuclear genetic panel, comprising nine isogenic nuclear genomes coupled to nine mitochondrial haplotypes, giving a total of 81 different mitonuclear genotypes. Following a predictive framework, we tested the mother's curse hypothesis by screening our panel for wing size. This trait is tightly correlated with overall body size and is sexually dimorphic in Drosophila. Moreover, growth is heavily reliant on metabolism and mitochondrial function, making wing size an ideal trait for the study of the impact of mitochondrial variation. We detect high levels of mitonuclear epistasis, and more importantly, we report that mitochondrial genetic variance is larger in male than female Drosophila for eight out of the nine nuclear genetic backgrounds used. These results demonstrate that the maternal inheritance of mitochondrial DNA does indeed modulate male life history traits in a more generalisable way than previously demonstrated.

Quantifying selection on standard metabolic rate and body mass in Drosophila melanogaster

Standard metabolic rate (SMR), defined as the minimal energy expenditure required for self-maintenance, is a key physiological trait. Few studies have estimated its relationship with fitness, most notably in insects. This is presumably due to the difficulty of measuring SMR in a large number of very small individuals. Using high-throughput flow-through respirometry and a Drosophila melanogaster laboratory population adapted to a life cycle that facilitates fitness measures, we quantified SMR, body mass, and fitness in 515 female and 522 male adults. We used a novel multivariate approach to estimate linear and nonlinear selection differentials and gradients from the variance-covariance matrix of fitness, SMR, and body mass, allowing traits specific covariates to be accommodated within a single model. In males, linear selection differentials for mass and SMR were positive and individually significant. Selection gradients were also positive but, despite substantial sample sizes, were nonsignificant due to increased uncertainty given strong SMR-mass collinearity. In females, only nonlinear selection was detected and it appeared to act primarily on body size, although the individual gradients were again nonsignificant. Selection did not differ significantly between sexes although differences in the fitness surfaces suggest sex-specific selection as an important topic for further study.

Mitochondria, sex and variation in routine metabolic rate

Variation in the metabolic costs associated with organismal maintenance may play a key role in determining fitness, and thus these differences among individuals are likely to be subject to natural selection. Although the evolvability of maintenance metabolism depends on its underlying genetic architecture, relatively little is known about the nature of genetic variation that underlies this trait. To address this, we measured variation in routine metabolic rate (ṀO_{2 routine} ), an index of maintenance metabolism, within and among three populations of Atlantic killifish, Fundulus heteroclitus, including a population from a region of genetic admixture between two subspecies. Polygenic association tests among individuals from the admixed population identified 54 single nucleotide polymorphisms (SNPs) that were associated with ṀO_{2 routine} , and these SNPs accounted for 43% of interindividual variation in this trait. However, genetic associations with ṀO_{2 routine} involved different SNPs if females and males were analysed separately, and there was a sex-dependent effect of mitochondrial genotype on variation in routine metabolism. These results imply that there are sex-specific genetic mechanisms, and potential mitonuclear interactions, that underlie variation in ṀO_{2 routine} . Additionally, there was evidence for epistatic interactions between 17% of the possible pairs of trait-associated SNPs, suggesting that epistatic effects on ṀO_{2 routine} are common. These data demonstrate not only that phenotypic variation in this ecologically important trait has a polygenic basis with considerable epistasis among loci, but also that these underlying genetic mechanisms, and particularly the role of mitochondrial genotype, may be sex-specific.