Genotype-by-environment interactions and adaptation to local temperature affect immunity and fecundity in Drosophila melanogaster

No apparent cost of evolved immune response in Drosophila melanogaster

Maintenance and deployment of the immune system are costly and are hence predicted to trade-off with other resource-demanding traits, such as reproduction. We subjected this longstanding idea to test using laboratory experimental evolution approach. In the present study, replicate populations of Drosophila melanogaster were subjected to three selection regimes-I (Infection with Pseudomonas entomophila), S (Sham-infection with MgSO4 ), and U (Unhandled Control). After 30 generations of selection flies from the I regime had evolved better survivorship upon infection with P. entomophila compared to flies from U and S regimes. However, contrary to expectations and previous reports, we did not find any evidence of trade-offs between immunity and other life history related traits, such as longevity, fecundity, egg hatchability, or development time. After 45 generations of selection, the selection was relaxed for a set of populations. Even after 15 generations, the postinfection survivorship of populations under relaxed selection regime did not decline. We speculate that either there is a negligible cost to the evolved immune response or that trade-offs occur on traits such as reproductive behavior or other immune mechanisms that we have not investigated in this study. Our research suggests that at least under certain conditions, life-history trade-offs might play little role in maintaining variation in immunity.

Reproduction-Immunity Trade-Offs in Insects

Immune defense and reproduction are physiologically and energetically demanding processes and have been observed to trade off in a diversity of female insects. Increased reproductive effort results in reduced immunity, and reciprocally, infection and activation of the immune system reduce reproductive output. This trade-off can manifest at the physiological level (within an individual) and at the evolutionary level (genetic distinction among individuals in a population). The resource allocation model posits that the trade-off arises because of competition for one or more limiting resources, and we hypothesize that pleiotropic signaling mechanisms regulate allocation of that resource between reproductive and immune processes. We examine the role of juvenile hormone, 20-hydroxyecdysone, and insulin/insulin-like growth factor-like signaling in regulating both oogenesis and immune system activity, and propose a signaling network that may mechanistically regulate the trade-off. Finally, we discuss implications of the trade-off in an ecological and evolutionary context.

Reaction norms of host immunity, host fitness and parasite performance in a mouse--intestinal nematode interaction

The outcome of the encounter between a host and a parasite depends on the synergistic effects of the genetics of the two partners and the environment (sensulato) where the interaction takes place. Reaction norms can depict how host and parasite traits vary across environmental ranges for different genotypes. Here, we performed a large scale experiment where three strains of laboratory mice (SJL, BALB/c and CBA) were infected with four doses of the intestinal nematode Heligmosomoides polygyrus. An increasing infective dose can be considered as a proxy for the environment-dependent risk incontracting the infection. We looked at the fitness traits of hosts and parasites, and assessed the underlying immunological functions likely to affect the observed pattern of resistance/susceptibility/tolerance. We found that the infective dose had a strong effect on both host fitness and parasite performance. Interestingly, for most traits, host genotypes did not rank consistently across the increasing infective doses and according to the expected pattern of strain-specific resistance/susceptibility/tolerance. Analyses of cytokine production allowed better understanding of the mechanistic basis underlying variations in fitness-linked traits. The infective dose affected the shape of the reaction norms of the cytokines IL-4, IL-10 and IL-6. Dose-dependent variation in cytokine production explained, moreover, the strain-specific pattern of infection cost, host resistance and parasite performance. As long as the infective dose increased, there was a marked shift towards a pro-inflammatory status in the SJL strain of mice that was positively correlated with cost of the infection and parasite performance. Overall, our study strongly suggests that the notion of host resistance is labile and depends on the environmental conditions where the interaction takes place. Moreover, integrating information on fitness-linked traits and the underlying mechanisms seems essential for a better understanding of host and parasite adaptations across variable environments.

Cold-seeking behaviour mitigates reproductive losses from fungal infection in Drosophila

Animals must tailor their life‐history strategies to suit the prevailing conditions and respond to hazards in the environment. Animals with lethal infections are faced with a difficult choice: to allocate more resources to reproduction and suffer higher mortality or to reduce reproduction with the expectation of enhanced immunity and late‐age reproduction. However, the strategies employed to mediate shifts in life‐history traits are largely unknown.

Here, we investigate the temperature preference of the fruit fly, Drosophila melanogaster, during infection with the fungal pathogen, Metarhizium robertsii, and the consequence of temperature preference on life‐history traits.

We have measured the temperature preference of fruit flies under different pathogen conditions. We conducted multiple fitness assays of the host and the pathogen under different thermal conditions. From these data, we estimated standard measures of fitness and used age‐specific methodologies to test for the fitness trade‐offs that are thought to underlie differences in life‐history strategy.

We found that fungus‐infected fruit flies seek out cooler temperatures, which facilitates an adaptive shift in their life‐history strategy. The colder temperatures preferred by infected animals were detrimental to the pathogen because it increased resistance to infection. But, it did not provide net benefits that were specific to infected animals, as cooler temperatures increased lifetime reproductive success and survival whether or not the animals were infected. Instead, we find that cold‐seeking benefits infected animals by increasing their late‐age reproductive output, at a cost to their early‐age reproductive output. In contrast, naive control flies prefer warmer temperatures that optimize early‐age reproductive, at a cost to reproductive output at late ages.

These findings show that infected animals exhibit fundamentally different reproductive strategies than their healthy counterparts. Temperature preference can facilitate shifts in strategy, but not without inevitable trade‐offs.

Temperature-Related Reaction Norms of Gene Expression: Regulatory Architecture and Functional Implications

The environment has profound effects on the expression of many traits and reaction norms describe the expression dynamics of a trait across a broad range of environmental conditions. Here, we analyze gene expression in Drosophila melanogaster across four different developmental temperatures (13-29 °C). Gene expression is highly plastic with 83.3% of the genes being differentially expressed. We distinguished three components of plasticity: 1) Dynamics of gene expression intensity (sum of change), 2) direction of change, and 3) curvature of the reaction norm (linear vs. quadratic). Studying their regulatory architecture we found that all three plasticity components were most strongly affected by the number of different transcription factors (TFs) binding to the target gene. More TFs were found in genes with less expression changes across temperatures. Although the effect of microRNAs was weaker, we consistently noted a trend in the opposite direction. The most plastic genes were regulated by fewer TFs and more microRNAs than less plastic genes. Different patterns of plasticity were also reflected by their functional characterization based on gene ontology. Our results suggest that reaction norms provide an important key to understand the functional requirements of natural populations exposed to variable environmental conditions.

Effects of genotype, environment, and their interactions on honey bee health in Europe

There are several reports of honey bee populations in Europe which survive without treatment for Varroa. However, when evaluated outside their native area, higher survival and resistance traits were not observed in colonies of a survivor population. Varroa infestation is strongly influenced by environmental factors, probably affecting threshold levels on a European scale. In a Europe-wide experiment colonies of local origin survived significantly longer than colonies of non-local origin, clearly indicating the presence of genotype-environment interactions. Transmission by Varroa selects for virulent strains of DWV, but it is currently unknown how these may interact with different genotypes of bees. The distribution of Nosema ceranae is significantly affected by environment, but there is at least one Nosema-resistant population.

The complex contributions of genetics and nutrition to immunity in Drosophila melanogaster

Both malnutrition and undernutrition can lead to compromised immune defense in a diversity of animals, and "nutritional immunology" has been suggested as a means of understanding immunity and determining strategies for fighting infection. The genetic basis for the effects of diet on immunity, however, has been largely unknown. In the present study, we have conducted genome-wide association mapping in Drosophila melanogaster to identify the genetic basis for individual variation in resistance, and for variation in immunological sensitivity to diet (genotype-by-environment interaction, or GxE). D. melanogaster were reared for several generations on either high-glucose or low-glucose diets and then infected with Providencia rettgeri, a natural bacterial pathogen of D. melanogaster. Systemic pathogen load was measured at the peak of infection intensity, and several indicators of nutritional status were taken from uninfected flies reared on each diet. We find that dietary glucose level significantly alters the quality of immune defense, with elevated dietary glucose resulting in higher pathogen loads. The quality of immune defense is genetically variable within the sampled population, and we find genetic variation for immunological sensitivity to dietary glucose (genotype-by-diet interaction). Immune defense was genetically correlated with indicators of metabolic status in flies reared on the high-glucose diet, and we identified multiple genes that explain variation in immune defense, including several that have not been previously implicated in immune response but which are confirmed to alter pathogen load after RNAi knockdown. Our findings emphasize the importance of dietary composition to immune defense and reveal genes outside the conventional "immune system" that can be important in determining susceptibility to infection. Functional variation in these genes is segregating in a natural population, providing the substrate for evolutionary response to pathogen pressure in the context of nutritional environment.

Spatially variable coevolution between a haemosporidian parasite and the MHC of a widely distributed passerine

The environment shapes host-parasite interactions, but how environmental variation affects the diversity and composition of parasite-defense genes of hosts is unresolved. In vertebrates, the highly variable major histocompatibility complex (MHC) gene family plays an essential role in the adaptive immune system by recognizing pathogen infection and initiating the cellular immune response. Investigating MHC-parasite associations across heterogeneous landscapes may elucidate the role of spatially fluctuating selection in the maintenance of high levels of genetic variation at the MHC. We studied patterns of association between an avian haemosporidian blood parasite and the MHC of rufous-collared sparrows (Zonotrichia capensis) that inhabit environments with widely varying haemosporidian infection prevalence in the Peruvian Andes. MHC diversity peaked in populations with high infection prevalence, although intra-individual MHC diversity was not associated with infection status. MHC nucleotide and protein sequences associated with infection absence tended to be rare, consistent with negative frequency-dependent selection. We found an MHC variant associated with a ∽26% decrease in infection probability at middle elevations (1501-3100 m) where prevalence was highest. Several other variants were associated with a significant increase in infection probability in low haemosporidian prevalence environments, which can be interpreted as susceptibility or quantitative resistance. Our study highlights important challenges in understanding MHC evolution in natural systems, but may point to a role of negative frequency-dependent selection and fluctuating spatial selection in the evolution of Z. capensisMHC.

Environmental heterogeneity generates opposite gene-by-environment interactions for two fitness-related traits within a population

Theory predicts that environmental heterogeneity offers a potential solution to the maintenance of genetic variation within populations, but empirical evidence remains sparse. The live-bearing fish Xiphophorus variatus exhibits polymorphism at a single locus, with different alleles resulting in up to five distinct melanistic "tailspot" patterns within populations. We investigated the effects of heterogeneity in two ubiquitous environmental variables (temperature and food availability) on two fitness-related traits (upper thermal limits and body condition) in two different tailspot types (wild-type and upper cut crescent). We found gene-by-environment (G × E) interactions between tailspot type and food level affecting upper thermal limits (UTL), as well as between tailspot type and thermal environment affecting body condition. Exploring mechanistic bases underlying these G × E patterns, we found no differences between tailspot types in hsp70 gene expression despite significant overall increases in expression under both thermal and food stress. Similarly, there was no difference in routine metabolic rates between the tailspot types. The reversal of relative performance of the two tailspot types under different environmental conditions revealed a mechanism by which environmental heterogeneity can balance polymorphism within populations through selection on different fitness-related traits.

Genomic evidence of rapid and stable adaptive oscillations over seasonal time scales in Drosophila

In many species, genomic data have revealed pervasive adaptive evolution indicated by the fixation of beneficial alleles. However, when selection pressures are highly variable along a species' range or through time adaptive alleles may persist at intermediate frequencies for long periods. So called “balanced polymorphisms” have long been understood to be an important component of standing genetic variation, yet direct evidence of the strength of balancing selection and the stability and prevalence of balanced polymorphisms has remained elusive. We hypothesized that environmental fluctuations among seasons in a North American orchard would impose temporally variable selection on Drosophila melanogaster that would drive repeatable adaptive oscillations at balanced polymorphisms. We identified hundreds of polymorphisms whose frequency oscillates among seasons and argue that these loci are subject to strong, temporally variable selection. We show that these polymorphisms respond to acute and persistent changes in climate and are associated in predictable ways with seasonally variable phenotypes. In addition, our results suggest that adaptively oscillating polymorphisms are likely millions of years old, with some possibly predating the divergence between D. melanogaster and D. simulans. Taken together, our results are consistent with a model of balancing selection wherein rapid temporal fluctuations in climate over generational time promotes adaptive genetic diversity at loci underlying polygenic variation in fitness related phenotypes.

Herein, we investigate the genomic basis of rapid adaptive evolution in response to seasonal fluctuations in the environment. We identify hundreds of polymorphisms (seasonal SNPs) that undergo dramatic shifts in allele frequency – on average between 40 and 60% – and oscillate between seasons repeatedly over multiple years, likely inducing high levels of genome-wide genetic differentiation. We provide evidence that seasonal SNPs are functional, being both sensitive to an acute frost event and associated with two stress tolerance traits. Finally, we show that some seasonal SNPs are possibly ancient balanced polymorphisms. Taken together, our results suggest that environmental heterogeneity can promote the long-term persistence of functional polymorphisms within populations that fuels fast directional adaptive response at any one time.

Ambient temperature and dietary supplementation interact to shape mosquito vector competence for malaria

The extent to which environmental factors influence the ability of Anopheles mosquitoes to transmit malaria parasites remains poorly explored. Environmental variation, such as change in ambient temperature, will not necessarily influence the rates of host and parasite processes equivalently, potentially resulting in complex effects on infection outcomes. As proof of principle, we used Anopheles stephensi and the rodent malaria parasite, Plasmodium yoelii, to examine the effects of a range of constant temperatures on one aspect of host defense (detected as alterations in expression of nitric oxide synthase gene - NOS) to parasite infection. We experimentally boosted mosquito midgut immunity to infection through dietary supplementation with the essential amino acid l-Arginine (l-Arg), which increases midgut nitric oxide (NO) levels by infection-induced NOS catalysis in A. stephensi. At intermediate temperatures, supplementation reduced oocyst prevalence, oocyst intensity, and sporozoite prevalence suggesting that the outcome of parasite infection was potentially dependent upon the rate of NOS-mediated midgut immunity. At low and high temperature extremes, however, infection was severely constrained irrespective of supplementation. The effects of l-Arg appeared to be mediated by NO-dependent negative feedback on NOS expression, as evidenced by depressed NOS expression in l-Arg treated groups at temperatures where supplementation decreased parasite infection. These results suggest the need to consider the direct (e.g. effects of mosquito body temperature on parasite physiology) and indirect effects (e.g. mediated through changes in mosquito physiology/immunity) of environmental factors on mosquito-malaria interactions in order to understand natural variation in vector competence.

A bayesian MCMC approach to assess the complete distribution of fitness effects of new mutations: uncovering the potential for adaptive walks in challenging environments

The role of adaptation in the evolutionary process has been contentious for decades. At the heart of the century-old debate between neutralists and selectionists lies the distribution of fitness effects (DFE)—that is, the selective effect of all mutations. Attempts to describe the DFE have been varied, occupying theoreticians and experimentalists alike. New high-throughput techniques stand to make important contributions to empirical efforts to characterize the DFE, but the usefulness of such approaches depends on the availability of robust statistical methods for their interpretation. We here present and discuss a Bayesian MCMC approach to estimate fitness from deep sequencing data and use it to assess the DFE for the same 560 point mutations in a coding region of Hsp90 in Saccharomyces cerevisiae across six different environmental conditions. Using these estimates, we compare the differences in the DFEs resulting from mutations covering one-, two-, and three-nucleotide steps from the wild type—showing that multiple-step mutations harbor more potential for adaptation in challenging environments, but also tend to be more deleterious in the standard environment. All observations are discussed in the light of expectations arising from Fisher’s geometric model.

Complex environmental drivers of immunity and resistance in malaria mosquitoes

Considerable research effort has been directed at understanding the genetic and molecular basis of mosquito innate immune mechanisms. Whether environmental factors interact with these mechanisms to shape overall resistance remains largely unexplored. Here, we examine how changes in mean ambient temperature, diurnal temperature fluctuation and time of day of infection affected the immunity and resistance of Anopheles stephensi to infection with Escherichia coli. We used quantitative PCR to estimate the gene expression of three immune genes in response to challenge with heat-killed E. coli. We also infected mosquitoes with live E. coli and ran bacterial growth assays to quantify host resistance. Both mosquito immune parameters and resistance were directly affected by mean temperature, diurnal temperature fluctuation and time of day of infection. Furthermore, there was a suite of complex two- and three-way interactions yielding idiosyncratic phenotypic variation under different environmental conditions. The results demonstrate mosquito immunity and resistance to be strongly influenced by a complex interplay of environmental variables, challenging the interpretation of the very many mosquito immune studies conducted under standard laboratory conditions.

Genetic and phenotypic relationships between immune defense, melanism and life-history traits at different temperatures and sexes in Tenebrio molitor

Insect cuticle melanism is linked to a number of life-history traits, and a positive relationship is hypothesized between melanism and the strength of immune defense. In this study, the phenotypic and genetic relationships between cuticular melanization, innate immune defense, individual development time and body size were studied in the mealworm beetle (Tenebrio molitor) using three different temperatures with a half-sib breeding design. Both innate immune defense and cuticle darkness were higher in females than males, and a positive correlation between the traits was found at the lowest temperature. The effect of temperature on all the measured traits was strong, with encapsulation ability and development time decreasing and cuticle darkness increasing with a rise in temperature, and body size showing a curved response. The analysis showed a highly integrated system sensitive to environmental change involving physiological, morphological and life-history traits.

Directional selection on cold tolerance does not constrain plastic capacity in a butterfly

Background

Organisms may respond to environmental change by means of genetic adaptation, phenotypic plasticity or both, which may result in genotype-environment interactions (G x E) if genotypes differ in their phenotypic response. We here specifically target the latter source of variation (i.e. G x E) by comparing plastic responses among lines of the tropical butterfly Bicyclus anynana that had been selected for increased cold tolerance and according controls. Our main aim here was to test the hypothesis that directional selection on cold tolerance will interfere with plastic capacities.

Results

Plastic responses to temperature and feeding treatments were strong, with e.g. higher compared to lower temperatures reducing cold tolerance, longevity, pupal mass, and development time. We report a number of statistically significant genotype-environment interactions (i.e. interactions between selection regime and environmental variables), but most of these were not consistent across treatment groups. We found some evidence though for larger plastic responses to different rearing temperatures in the selection compared to the control lines, while plastic responses to different adult temperatures and feeding treatments were overall very similar across selection regimes.

Conclusion

Our results indicate that plastic capacities are not always constrained by directional selection (on cold tolerance) and therefore genetic changes in trait means, but may operate independently.

Rethinking vector immunology: the role of environmental temperature in shaping resistance

Recent ecological research has revealed that environmental factors can strongly affect insect immunity and influence the outcome of host-parasite interactions. To date, however, most studies examining immune function in mosquitoes have ignored environmental variability. We argue that one such environmental variable, temperature, influences both vector immunity and the parasite itself. As temperatures in the field can vary greatly from the ambient temperature in the laboratory, it will be essential to take temperature into account when studying vector immunology.

Complex effects of temperature on mosquito immune function

Over the last 20 years, ecological immunology has provided much insight into how environmental factors shape host immunity and host–parasite interactions. Currently, the application of this thinking to the study of mosquito immunology has been limited. Mechanistic investigations are nearly always conducted under one set of conditions, yet vectors and parasites associate in a variable world. We highlight how environmental temperature shapes cellular and humoral immune responses (melanization, phagocytosis and transcription of immune genes) in the malaria vector, Anopheles stephensi. Nitric oxide synthase expression peaked at 30°C, cecropin expression showed no main effect of temperature and humoral melanization, and phagocytosis and defensin expression peaked around 18°C. Further, immune responses did not simply scale with temperature, but showed complex interactions between temperature, time and nature of immune challenge. Thus, immune patterns observed under one set of conditions provide little basis for predicting patterns under even marginally different conditions. These quantitative and qualitative effects of temperature have largely been overlooked in vector biology but have significant implications for extrapolating natural/transgenic resistance mechanisms from laboratory to field and for the efficacy of various vector control tools.

Engineered anopheles immunity to Plasmodium infection

A causative agent of human malaria, Plasmodium falciparum, is transmitted by Anopheles mosquitoes. The malaria parasite is under intensive attack from the mosquito's innate immune system during its sporogonic development. We have used genetic engineering to create immune-enhanced Anopheles stephensi mosquitoes through blood meal-inducible expression of a transgene encoding the IMD pathway-controlled NF-kB Rel2 transcription factor in the midgut and fat-body tissue. Transgenic mosquitoes showed greater resistance to Plasmodium and microbial infection as a result of timely concerted tissue-specific immune attacks involving multiple effectors. The relatively weak impact of this genetic modification on mosquito fitness under laboratory conditions encourages further investigation of this approach for malaria control.

Some like it hot: the effects of climate change on reproduction, immune function and disease resistance in the cricket Gryllus texensis

In many parts of the world, climate change is increasing the frequency and severity of heat waves. How do heat waves impact short-lived poikilotherms such as insects? In the cricket, Gryllus texensis, 6 days of elevated temperatures (i.e. 7°C above the average field temperature and 5°C above their preferred temperature) resulted in increased egg laying, faster egg development and greater mass gain. The increased temperature also increased activity of phenoloxidase and lysozyme-like enzymes, two immune-related enzymes, and enhanced resistance to the Gram-negative bacterium Serratia marcescens. When given a sublethal S. marcescens infection, G. texensis maintained increased reproductive output at the elevated temperature (33°C). These data suggest that heat waves could result in more numerous, disease resistant, crickets. However, resistance to the Gram-positive bacterium, Bacillus cereus was lower at temperatures above or below the average field temperature (26°C). A sublethal infection with B. cereus reduced egg laying at all temperatures and suppressed the increase in egg laying induced by higher temperatures. These results suggest that for some species-pathogen interactions, increased temperatures can induce trade-offs between reproduction and disease resistance. This result may partly explain why G. texensis prefers temperatures lower than those that produce maximal reproductive output and enhanced immune function.

The impact of environmental change on host-parasite coevolutionary dynamics

Environmental factors are known to affect the strength and the specificity of interactions between hosts and parasites. However, how this shapes patterns of coevolutionary dynamics is not clear. Here, we construct a simple mathematical model to study the effect of environmental change on host-parasite coevolutionary outcome when interactions are of the matching-alleles or the gene-for-gene type. Environmental changes may effectively alter the selective pressure and the level of specialism in the population. Our results suggest that environmental change altering the specificity of selection in antagonistic interactions can produce alternating time windows of cyclical allele-frequency dynamics and cessation thereof. This type of environmental impact can also explain the maintenance of polymorphism in gene-for-gene interactions without costs. Overall, our study points to the potential consequences of environmental variation in coevolution, and thus the importance of characterizing genotype-by-genotype-by-environment interactions in natural host-parasite systems, especially those that change the direction of selection acting between the two species.

Epidemiological, evolutionary, and coevolutionary implications of context-dependent parasitism

Abstract Victims of infection are expected to suffer increasingly as parasite population growth increases. Yet, under some conditions, faster-growing parasites do not appear to cause more damage, and infections can be quite tolerable. We studied these conditions by assessing how the relationship between parasite population growth and host health is sensitive to environmental variation. In experimental infections of the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa, we show how easily an interaction can shift from a severe interaction, that is, when host fitness declines substantially with each unit of parasite growth, to a tolerable relationship by changing only simple environmental variables: temperature and food availability. We explored the evolutionary and epidemiological implications of such a shift by modeling pathogen evolution and disease spread under different levels of infection severity and found that environmental shifts that promote tolerance ultimately result in populations harboring more parasitized individuals. We also find that the opportunity for selection, as indicated by the variance around traits, varied considerably with the environmental treatment. Thus, our results suggest two mechanisms that could underlie coevolutionary hotspots and coldspots: spatial variation in tolerance and spatial variation in the opportunity for selection.

Non-immunological defense in an evolutionary framework

After parasite infection, invertebrates activate immune system-based defenses such as encapsulation and the signaling pathways of the innate immune system. However, hosts are often able to defend against parasites without using these mechanisms. The non-immunological defenses, such as behaviors that prevent or combat infection, symbiont-mediated defense, and fecundity compensation, are often ignored but can be important in host-parasite dynamics. We review recent studies showing that heritable variation in these traits exists among individuals, and that they are costly to activate and maintain. We also discuss findings from genome annotation and expression studies to show how immune system-based and non-immunological defenses interact. Placing these studies into an evolutionary framework emphasizes their importance for future studies of host-parasite coevolution.

Successfully resisting a pathogen is rarely costly in Daphnia magna

BACKGROUND

A central hypothesis in the evolutionary ecology of parasitism is that trade-offs exist between resistance to parasites and other fitness components such as fecundity, growth, survival, and predator avoidance, or resistance to other parasites. These trade-offs are called costs of resistance. These costs fall into two broad categories: constitutive costs of resistance, which arise from a negative genetic covariance between immunity and other fitness-related traits, and inducible costs of resistance, which are the physiological costs incurred by hosts when mounting an immune response. We sought to study inducible costs in depth using the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa.

RESULTS

We designed specific experiments to study the costs induced by exposure to this parasite, and we re-analysed previously published data in an effort to determine the generality of such costs. However, despite the variety of genetic backgrounds of both hosts and parasites, and the different exposure protocols and environmental conditions used in these experiment, this work showed that costs of exposure can only rarely be detected in the D. magna-P. ramosa system.

CONCLUSIONS

We discuss possible reasons for this lack of detectable costs, including scenarios where costs of resistance to parasites might not play a major role in the co-evolution of hosts and parasites.

Haplotype structure and expression divergence at the Drosophila cellular immune gene eater

The protein Eater plays an important role in microbial recognition and defensive phagocytosis in Drosophila melanogaster. We sequenced multiple alleles of the eater gene from an African and a North American population of D. melanogaster and found signatures of a partial selective sweep in North America that is localized around the second intron. This pattern is consistent with local adaptation to novel selective pressures during range expansion out of Africa. The North American sample is divided into two predominant haplotype groups, and the putatively selected haplotype is associated with a significantly higher gene expression level, suggesting that gene regulation is a possible target of selection. The eater alleles contain from 22 to 40 repeat units that are characterized by the presence of a cysteine-rich NIM motif. NIM repeats in the structural stalk of the protein exhibit concerted evolution as a function of physical location in the repeat array. Several NIM repeats within eater have previously been implicated in binding to microbial ligands, a function which in principle might subject them to special evolutionary pressures. However, we find no evidence of elevated positive selection on these pathogen-interacting units. Our study presents an instance where gene expression rather than protein structure is thought to drive the adaptive evolution of a pathogen recognition molecule in the immune system.

Sex-specific heritability of cell-mediated immune response in the blue tit nestlings (Cyanistes caeruleus)

Here, we aimed at estimating sex-specific heritabilities of cell-mediated immune response (CMI) in the blue tit nestlings (Cyanistes caeruleus). To separate genetic and environmental components of the phenotypic variance in CMI (measured using phytohaemagglutinin assay), we performed a cross-fostering experiment. Additionally, controlled environmental variation was introduced by enlarging some broods. Our analyses revealed a significant genetic component (as approximated by the nest-of-origin term) of the phenotypic variance in immune response. More importantly, these genetic effects differed between sexes and experimentally manipulated brood sizes, as indicated by significant genotype-by-sex and genotype-by-environment interactions. We discuss possible causes of such sexual dimorphism in gene expression and suggest that sex- and environment-specific genetic interactions may contribute to the maintenance of genetic variability in traits related to immune functions.

Genetic variance and genotype-by-environment interaction of immune response in Aedes aegypti (Diptera: Culicidae)

Immune response can be negatively affected by resource limitation, so it is expected that organisms evolve strategies to minimize the impact of this environmental outcome. Phenotypic plasticity in immune response could represent a genetic response to face such situations. We investigated the effects of high and low quality and quantity of food at the larval stage on two important immune components, phenoloxidase activity (PO) and nitric oxide production (NO) measured in adults of the Dengue vector, Aedes aegypti. We reared families to determine the magnitude and pattern of expression of genetic variance, environmental variance and genotype-by-environment interaction (GEI). In addition, we quantified whether there were differences in plastic immune responses in both sexes. Our results indicated additive variance for PO and NO, but rearing environment did not produce differences among individuals. For NO and PO in males, there were large differences among families in plasticity, as indicated by the different slopes produced by each reaction norm. Therefore, there is additive genetic variation in plasticity for NO production and PO activity. One possible interpretation of these results is that different genotypes may be favored to fight pathogens under the different food quality situations. Males and females showed similar overall GEI strategies but there were differences in PO and NO. Males showed a phenotypic correlation between PO and NO, but we did not find genetic correlations between immune parameters in both sexes.

Evolutionary forces on Anopheles: what makes a malaria vector?

In human malaria, transmission intensity is highly dependent on the vectorial capacity and competence of local mosquitoes. Most mosquitoes are dead ends for the parasite, and only limited ranges of Anopheles are able to transmit Plasmodium to humans. Research to understand the determinants of vectorial capacity and competence has greatly progressed in recent years; however, some aspects have been overlooked and the evolutionary pressures that affect them often neglected. Here, we review key factors of vectorial capacity and competence in Anopheles, with a particular focus on the most important malaria vector Anopheles gambiae. We aim to point out selection pressures exerted by Plasmodium on Anopheles to improve its own transmission and discuss how the parasite might shape the vector to its benefit.

Thermoregulatory behaviour affects prevalence of chytrid fungal infection in a wild population of Panamanian golden frogs

Predicting how climate change will affect disease dynamics requires an understanding of how the environment affects host-pathogen interactions. For amphibians, global declines and extinctions have been linked to a pathogenic chytrid fungus, Batrachochytrium dendrobatidis. Using a combination of body temperature measurements and disease assays conducted before and after the arrival of B. dendrobatidis, this study tested the hypothesis that body temperature affects the prevalence of infection in a wild population of Panamanian golden frogs (Atelopus zeteki). The timing of first detection of the fungus was consistent with that of a wave of epidemic infections spreading south and eastward through Central America. During the epidemic, many golden frogs modified their thermoregulatory behaviour, raising body temperatures above their normal set point. Odds of infection decreased with increasing body temperature, demonstrating that even slight environmental or behavioural changes have the potential to affect an individual's vulnerability to infection. The thermal dependency of the relationship between B. dendrobatidis and its amphibian hosts demonstrates how the progression of an epidemic can be influenced by complex interactions between host and pathogen phenotypes and the environments in which they are found.

Drosophila melanogaster as a model host for studying Pseudomonas aeruginosa infection

Conservation of host signaling pathways and tissue physiology between Drosophila melanogaster and mammals allows for the modeling of human host-pathogen interactions in Drosophila. Here we present the use of genetically tractable Drosophila models of bacterial pathogenesis to study infection with the human opportunistic pathogen Pseudomonas aeruginosa. We describe and compare two protocols commonly used to infect Drosophila with P. aeruginosa: needle-pricking and injector-pumping. Each model has relevance for examining host components and bacterial factors in host defense and virulence. Fly survival and bacterial proliferation within host flies can be assessed as a measure of host susceptibility and pathogen virulence potential. The profiles of host responses toward P. aeruginosa virulent and non-virulent strains can be determined, enabling the identification of interaction-specific genes that could potentially favor or limit the initiation and progression of infection. Both of the protocols presented herein may be adapted for the inoculation and study of other microbial pathogens. P. aeruginosa cell preparation requires 24 h, fly inoculation 1 h, and fly survival and bacterial proliferation 1-4 d.

The role of anorexia in resistance and tolerance to infections in Drosophila

Most infections induce anorexia but its function, if any, remains unclear. Because this response is common among animals, we hypothesized that infection-induced diet restriction might be an adaptive trait that modulates the host's ability to fight infection. Two defense strategies protect hosts against infections: resistance, which is the ability to control pathogen levels, and tolerance, which helps the host endure infection-induced pathology. Here we show that infected fruit flies become anorexic and that diet restriction alters defenses, increasing the fly's tolerance to Salmonella typhimurium infections while decreasing resistance to Listeria monocytogenes. This suggests that attempts to extend lifespan through diet restriction or the manipulation of pathways mimicking this process will have complicated effects on a host's ability to fight infections.

Environment can alter selection in host-parasite interactions

Characteristics of hosts and parasites have a genetic basis, and thus can be shaped by coevolution. Infections measured under laboratory conditions have shown that the environment in which hosts and parasites interact might substantially affect the strength and specificity of selection. In addition, various components of host-parasite fitness are differentially altered by the environment. Despite this, environmental fluctuations are often excluded from experimental coevolutionary studies and theoretical models as 'noise'. Because most host-parasite interactions exist in heterogeneous environments, we argue that there is a need to incorporate fluctuating environments into future empirical and theoretical work on host-parasite coevolution.

Cross-generational fitness effects of infection in Drosophila melanogaster

Activation of the immune system is beneficial in defending against pathogens, but may also have costly side effects on an organism's fitness. In this study we examine the fitness consequences of immune challenge in female Drosophila melanogaster by examining both direct (within generation) and indirect (between generations) costs and benefits of immune challenge. Though passing immunity to offspring has been studied in mammals for many years, only recently have researchers found evidence for a cross-generational priming response in invertebrates. By examining both potential fitness costs and benefits in the next generation, we were able to determine what effect immune challenge has on fitness. In agreement with other studies, we found a direct cost to infection, where immune challenged females laid fewer eggs than unchallenged females in two of the three lines we examined. In addition, we found some evidence for indirect costs. Offspring from immune challenged mothers had shorter lifespans than those from unchallenged mothers in two of the three lines. Interestingly, we do not see any effect of maternal immune challenge on offspring's ability to overcome an infection, nor do we see an effect on other fitness traits measured, including egg size, egg-adult viability and offspring resistance to oxidative stress. While previous studies in bumblebees and beetles have demonstrated cross-generation priming, our results suggest that it may not be a general phenomenon, and more work is needed to determine how widespread it is.

Introduction. Ecological immunology

An organism's fitness is critically reliant on its immune system to provide protection against parasites and pathogens. The structure of even simple immune systems is surprisingly complex and clearly will have been moulded by the organism's ecology. The aim of this review and the theme issue is to examine the role of different ecological factors on the evolution of immunity. Here, we will provide a general framework of the field by contextualizing the main ecological factors, including interactions with parasites, other types of biotic as well as abiotic interactions, intraspecific selective constraints (life-history trade-offs, sexual selection) and population genetic processes. We then elaborate the resulting immunological consequences such as the diversity of defence mechanisms (e.g. avoidance behaviour, resistance, tolerance), redundancy and protection against immunopathology, life-history integration of the immune response and shared immunity within a community (e.g. social immunity and microbiota-mediated protection). Our review summarizes the concepts of current importance and directs the reader to promising future research avenues that will deepen our understanding of the defence against parasites and pathogens.

Immunity in a variable world

Immune function is likely to be a critical determinant of an organism's fitness, yet most natural animal and plant populations exhibit tremendous genetic variation for immune traits. Accumulating evidence suggests that environmental heterogeneity may retard the long-term efficiency of natural selection and even maintain polymorphism, provided alternative host genotypes are favoured under different environmental conditions. 'Environment' in this context refers to abiotic factors such as ambient temperature or availability of nutrient resources, genetic diversity of pathogens or competing physiological demands on the host. These factors are generally controlled in laboratory experiments measuring immune performance, but variation in them is likely to be very important in the evolution of resistance to infection. Here, we review some of the literature emphasizing the complexity of natural selection on immunity. Our aim is to describe how environmental and genetic heterogeneities, often excluded from experimentation as 'noise', may determine the evolutionary potential of populations or the potential for interacting species to coevolve.

Anopheles mosquitoes: not just flying malaria vectors... especially in the field

The polymorphism of genes involved in the immunity of malaria vectors has been the subject of several recent studies with mosquitoes from natural populations. Most of the genes examined are known for their role against Plasmodium berghei and not necessarily for their role against Plasmodium falciparum. It seems, therefore, to be highly important not only to be cautious when linking natural selection with malaria epidemiology but also to consider the importance of other parasites and the environment on the mosquito genome.