Trade-off associated with selection for increased ability to resist parasitoid attack in Drosophila melanogaster

Shifted levels of sleep and activity under darkness as mechanisms underlying ectoparasite resistance

Parasites harm host fitness and are pervasive agents of natural selection to evolve host defense strategies Host defensive traits in natural populations typically show genetic variation, which may be maintained when parasite resistance imposes fitness costs on the host in the absence of parasites. Previously we demonstrated significant evolutionary responses to artificial selection for increasing behavioral immunity to Gamasodes queenslandicus mites in replicate lines of Drosophila melanogaster. Here, we report transcriptional shifts in metabolic processes between selected and control fly lines based on RNA-seq analyses. We also show decreased starvation resistance and increased use of nutrient reserves in flies from mite-resistant lines. Additionally, mite-resistant lines exhibited increased behavioral activity, such as, reduced sleep and elevated oxygen consumption under conditions of darkness. The link between resistance and sleep was confirmed in an independent panel of D. melanogaster genetic lines exhibiting variable sleep durations, showing a positive correlation between mite resistance and reduced sleep. Experimentally restraining the activity of artificially selected mite-resistant flies during exposure to parasites under dark conditions reduced their resistance advantage relative to control flies. The results suggest that ectoparasite resistance in this system involves increased dark-condition activity and metabolic gene expression at the expense of nutrient reserves and starvation resistance.

A novel and diverse family of filamentous DNA viruses associated with parasitic wasps

Large dsDNA viruses from the Naldaviricetes class are currently composed of four viral families infecting insects and/or crustaceans. Since the 1970s, particles described as filamentous viruses (FVs) have been observed by electronic microscopy in several species of Hymenoptera parasitoids but until recently, no genomic data was available. This study provides the first comparative morphological and genomic analysis of these FVs. We analyzed the genomes of seven FVs, six of which were newly obtained, to gain a better understanding of their evolutionary history. We show that these FVs share all genomic features of the Naldaviricetes while encoding five specific core genes that distinguish them from their closest relatives, the Hytrosaviruses. By mining public databases, we show that FVs preferentially infect Hymenoptera with parasitoid lifestyle and that these viruses have been repeatedly integrated into the genome of many insects, particularly Hymenoptera parasitoids, overall suggesting a long-standing specialization of these viruses to parasitic wasps. Finally, we propose a taxonomical revision of the class Naldaviricetes in which FVs related to the Leptopilina boulardi FV constitute a fifth family. We propose to name this new family, Filamentoviridae.

Activation of immune defences against parasitoid wasps does not underlie the cost of infection

Parasites reduce the fitness of their hosts, and different causes of this damage have fundamentally different consequences for the evolution of immune defences. Damage to the host may result from the parasite directly harming its host, often due to the production of virulence factors that manipulate host physiology. Alternatively, the host may be harmed by the activation of its own immune defences, as these can be energetically demanding or cause self-harm. A well-studied model of the cost of infection is Drosophila melanogaster and its common natural enemy, parasitoid wasps. Infected Drosophila larvae rely on humoral and cellular immune mechanisms to form a capsule around the parasitoid egg and kill it. Infection results in a developmental delay and reduced adult body size. To disentangle the effects of virulence factors and immune defences on these costs, we artificially activated anti-parasitoid immune defences in the absence of virulence factors. Despite immune activation triggering extensive differentiation and proliferation of immune cells together with hyperglycaemia, it did not result in a developmental delay or reduced body size. We conclude that the costs of infection do not result from these aspects of the immune response and may instead result from the parasite directly damaging the host.

Heritability and preadult survivorship costs of ectoparasite resistance in the naturally occurring Drosophila-Gamasodes mite system

Our understanding of the evolutionary significance of ectoparasites in natural communities is limited by a paucity of information concerning the mechanisms and heritability of resistance to this ubiquitous group of organisms. Here, we report the results of artificial selection for increasing ectoparasite resistance in replicate lines of Drosophila melanogaster derived from a field-fresh population. Resistance, as ability to avoid infestation by naturally co-occurring Gamasodes queenslandicus mites, increased significantly in response to selection and realized heritability (SE) was estimated to be 0.11 (0.0090). Deployment of energetically expensive bursts of flight from the substrate was a main mechanism of host resistance that responded to selection, aligning with previously documented metabolic costs of fly behavioral defenses. Host body size, which affects parasitism rate in some fly-mite systems, was not shifted by selection. In contrast, resistant lines expressed significant reductions in larva-to-adult survivorship with increasing toxic (ammonia) stress, identifying an environmentally modulated preadult cost of resistance. Flies selected for resistance to G. queenslandicus were also more resistant to a different mite, Macrocheles subbadius, suggesting that we documented genetic variation and a pleiotropic cost of broad-spectrum behavioral immunity against ectoparasites. The results demonstrate significant evolutionary potential of resistance to an ecologically important class of parasites.

Natural selection has driven the recurrent loss of an immunity gene that protects Drosophila against a major natural parasite

Polymorphisms in immunity genes can have large effects on susceptibility to infection. To understand the origins of this variation, we have investigated the genetic basis of resistance to the parasitoid wasp Leptopilina boulardi in Drosophila melanogaster. We found that increased expression of the gene lectin-24A after infection by parasitic wasps was associated with a faster cellular immune response and greatly increased rates of killing the parasite. lectin-24A encodes a protein that is strongly up-regulated in the fat body after infection and localizes to the surface of the parasite egg. In certain susceptible lines, a deletion upstream of the lectin-24A has largely abolished expression. Other mutations predicted to abolish the function of this gene have arisen recurrently in this gene, with multiple loss-of-expression alleles and premature stop codons segregating in natural populations. The frequency of these alleles varies greatly geographically, and in some southern African populations, natural selection has driven them near to fixation. We conclude that natural selection has favored the repeated loss of an important component of the immune system, suggesting that in some populations, a pleiotropic cost to lectin-24A expression outweighs the benefits of resistance.

Partitioning variance in immune traits in a zooplankton host-Fungal parasite system

Host immune traits arise from both genetic and environmental sources of variation. When immune traits have a strong genetic basis, the presence and severity of disease in a population may influence the distribution of those traits. Our study addressed how two immune-related traits (gut penetrability and the hemocyte response) are shaped by genetic and environmental sources of variation, and how the presence of a virulent disease altered the relative frequency of these traits in natural populations. Daphnia dentifera hosts were sampled from five Indiana lakes between June and December 2017 before and during epidemics of their fungal pathogen, Metschnikowia bicuspidata. Collected Daphnia were experimentally exposed to Metschnikowia and assayed for their gut penetrability, hemocyte response, and multi-locus genotype. Mixed-effects models were constructed to partition variance in immune traits between genetic and environmental sources. We then isolated the genetic sources to produce genotype-specific estimates of immune traits for each multi-locus genotype. Finally, we assessed the relative frequency and dynamics of genotypes during epidemics and asked whether genotypes with more robust immune responses increased in frequency during epidemics. Although genotype was an important source of variation for both gut penetrability and the hemocyte response, environmental factors (e.g., resource availability, Metschnikowia prevalence, and co-infection) still explained a large portion of observed variation, suggesting a high degree of flexibility in Daphnia immune traits. Additionally, no significant associations were detected between a genotype's immune traits and its frequency in a population. Our study highlights the power of variance partitioning in understanding the factors driving variation in Daphnia traits and motivates further research on immunological flexibility and the ecological drivers of immune variation.

Transcriptomic analysis provides insights into the immune responses and nutrition in Ostrinia furnacalis larvae parasitized by Macrocentrus cingulum

Macrocentrus cingulum is a principal endoparasite of Ostrinia furnacalis larvae. M. cingulum larvae repress host immune responses for survival and ingest host nutrients for development until emerging. However, most investigations focused on the mechanisms of how wasps repress the host immunity, the triggered immune responses and nutrient status altered by wasps in host are neglected. In this study, we found that parasitized O. furnacalis larvae activated fast recognition responses and produced some effectors such as lysozyme and antimicrobial peptides, along with more consumption of trehalose, glucose, and even lipid to defend against the invading M. cingulum. However, the expression of peroxidase 6 and superoxide dismutase 2 (SOD 2) was upregulated, and the messenger RNA (mRNA) levels of cellular immunity-related genes such as thioester-containing protein 2 (TEP 2) and hemocytin were also reduced, suggesting that some immune responses were selectively shut down by wasp parasitization. Taken together, all the results indicated that parasitized O. furnacalis larvae selectively activate the immune recognition response, and upregulate effector genes, but suppress ROS reaction and cellular immunity, and invest more energy to fuel certain immune responses to defend against the wasp invading. This study provides useful information for further identifying key components of the nutrition and innate immune repertoire which may shape host-parasitoid coevolutionary dynamics.

Impact of Temperature on the Immune Interaction between a Parasitoid Wasp and Drosophila Host Species

Temperature is particularly important for ectotherms, including endoparasitoid wasps that develop inside another ectotherm host. In this study, we tested the impact of three temperatures (20 °C, 25 °C and 30 °C) on the host-parasitoid immune interaction using two Drosophila host species (Drosophila melanogaster and D. yakuba) and two parasitoid lines of Leptopilina boulardi. Drosophila's immune defense against parasitoids consists of the formation of a melanized capsule surrounding the parasitoid egg. To counteract this response, Leptopilina parasitoids rely on the injection of venom during oviposition. Here, we tested the effect of temperature on parasitic success and host encapsulation capacity in response to a parasitoid egg or other foreign body. Increased temperature either promoted or did not affect the parasitic success, depending on the parasitoid-host pairs considered. The mechanisms behind the higher success seemed to vary depending on whether the temperature primarily affected the host immune response or also affected the parasitoid counter-immune response. Next, we tested the effect of parasitoid rearing temperature on its success and venom composition. Venom composition varied strongly with temperature for both parasitoid lines, partially consistent with a change in their parasitic success. Overall, temperature may have a significant impact on the host-parasitoid immune interaction.

Constitutive activation of cellular immunity underlies the evolution of resistance to infection in Drosophila

Organisms rely on inducible and constitutive immune defences to combat infection. Constitutive immunity enables a rapid response to infection but may carry a cost for uninfected individuals, leading to the prediction that it will be favoured when infection rates are high. When we exposed populations of Drosophila melanogaster to intense parasitism by the parasitoid wasp Leptopilina boulardi, they evolved resistance by developing a more reactive cellular immune response. Using single-cell RNA sequencing, we found that immune-inducible genes had become constitutively upregulated. This was the result of resistant larvae differentiating precursors of specialized immune cells called lamellocytes that were previously only produced after infection. Therefore, populations evolved resistance by genetically hard-wiring the first steps of an induced immune response to become constitutive.

Rejection of the beneficial acclimation hypothesis (BAH) for short term heat acclimation in Drosophila nepalensis

Beneficial acclimation hypothesis (BAH) is the phenotypic plasticity in response to changing environments which enables organisms to enhance their fitness. In recent years, however, BAH has received vigorous criticism and is still debatable. In this study, we tested thermal hardiness phenotypes (melanization, chill coma recovery, heat knockdown and percentage survival) on adult and pre-adult stages of Drosophila nepalensis, reared in different thermal environments (14, 17, 21 and 25 °C) to check whether increasing natural surrounding temperature and acclimation limit towards environmental change is detrimental or beneficial. Results showed that rearing D. nepalensis at higher temperatures (21 and 25 °C) reduces its melanization and cold hardiness but improves heat knockdown times. When temperature was raised to 26.2 °C (0.6 °C above the upper thermal maxima), to determine the short-term acclimation effects, survival and fitness of adults diminished approximately 1.5 to 2 folds. These results suggest that D. nepalensis has long-term developmental acclimation to both heat and cold which would be extremely beneficial as temperatures and climates alter in the region due to global warming. However, a lack of short-term heat acclimation suggests that rapid shifts in thermal extreme could be detrimental to D. nepalensis.

Radiation on Medfly Larvae of tsl Vienna-8 Genetic Sexing Strain Displays Reduced Parasitoid Encapsulation in Mass-Reared Diachasmimorpha longicaudata (Hymenoptera: Braconidae)

Improvements in the mass rearing of Diachasmimorpha longicaudata (Ashmead) on larvae of the Vienna-8 temperature-sensitive lethal genetic sexing strain of Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) (= GSS Vienna-8) at the San Juan biofactory, Argentina, are currently under way. Lowering cost production is a key factor regarding parasitoid rearing. Thus, the variation in mass-reared parasitoid encapsulation levels and the incidence of superparasitism were determined; also, the gamma radiation dose-effect relation on host larvae and the influence of Mediterranean fruit fly strain were considered. Naked Mediterranean fruit fly larvae of both GSS Vienna-8 and a wild bisexual strain (= WBS) aged 6-d-old were irradiated at 0, 20, 40, 60, 80, 100, and 120 Gy, and exposed to parasitoid females. Melanization level was tested for encapsulated parasitoid larval first-instars (= L1). Non-irradiated and irradiated WBS larvae at 20-40 Gy displayed a significantly higher incidence of encapsulation when compared with GSS Vienna-8 larvae. The low melanized level in encapsulated parasitoid L1 was the most common melanization process at 72 h puparium dissection. A high melanized level was only found in non-irradiated WBS larvae. Irradiated GSS Vienna-8 larvae can neutralize the host immunological reactions over irradiated WBS larvae much more quickly. Superparasitism intensity in both Mediterranean fruit fly strains was not affected by radiation doses. High levels of superparasitism seemingly helped to overcome the host's immune reaction by the surviving parasitoid larva. Parasitoid emergence increased from 60 Gy onwards in both Mediterranean fruit fly strains. Radiation in GSS Vienna-8 larvae may favor host's antagonistic reactions decrease in relation with D. longicaudata development.

Recurrent Loss-of-Function Mutations Reveal Costs to OAS1 Antiviral Activity in Primates

Immune responses counteract infections but also cause collateral damage to hosts. Oligoadenylate synthetase 1 (OAS1) binds double-stranded RNA from invading viruses and produces 2'-5' linked oligoadenylate (2-5A) to activate ribonuclease L (RNase L), which cleaves RNA to inhibit virus replication. OAS1 can also undergo autoactivation by host RNAs, a potential trade-off to antiviral activity. We investigated functional variation in primate OAS1 as a model for how immune pathways evolve to mitigate costs and observed a surprising frequency of loss-of-function variation. In gorillas, we identified a polymorphism that severely decreases catalytic function, mirroring a common variant in humans that impairs 2-5A synthesis through alternative splicing. OAS1 loss-of-function variation is also common in monkeys, including complete loss of 2-5A synthesis in tamarins. The frequency of loss-of-function alleles suggests that costs associated with OAS1 activation can be so detrimental to host fitness that pathogen-protective effects are repeatedly forfeited.

Transstadial transmission of larval hemocoelic infection negatively affects development and adult female longevity in the mosquito Anopheles gambiae

During all life stages, mosquitoes are exposed to pathogens, and employ an immune system to resist or limit infection. Although much attention has been paid to how adult mosquitoes fight infection, little is known about how an infection during the larval stage affects the biology of the resultant adult. In this study, we investigated whether a bacterial infection in the hemocoel of the African malaria mosquito, Anopheles gambiae, is transstadially transmitted from larvae to adults (both females and males), and whether immune stimulation in the hemocoel as a larva alters development or biological traits of the adult. Specifically, larvae were injected in the hemocoel with either fluorescent microspheres or Escherichia coli, and the following traits were examined: transstadial transmission, larval development to adulthood, adult survival, and adult body size. Our results show that transstadial transmission of hemocoel contents occurs from larvae to pupae and from pupae to adults, but that bacterial prevalence and intensity varies with age. Injury, immune stimulation or infection decreases the proportion of larvae that undergo pupation and eclosion, infection decreases the longevity of adult females, and treatment has complex effects on the body size of the resultant adults. The present study adds larval hemocoelic infection to the known non-genetic factors that reduce overall fitness by negatively affecting development and adult biological traits that influence mosquito vector competence.

Parallel and costly changes to cellular immunity underlie the evolution of parasitoid resistance in three Drosophila species

A priority for biomedical research is to understand the causes of variation in susceptibility to infection. To investigate genetic variation in a model system, we used flies collected from single populations of three different species of Drosophila and artificially selected them for resistance to the parasitoid wasp Leptopilina boulardi, and found that survival rates increased 3 to 30 fold within 6 generations. Resistance in all three species involves a large increase in the number of the circulating hemocytes that kill parasitoids. However, the different species achieve this in different ways, with D. melanogaster moving sessile hemocytes into circulation while the other species simply produce more cells. Therefore, the convergent evolution of the immune phenotype has different developmental bases. These changes are costly, as resistant populations of all three species had greatly reduced larval survival. In all three species resistance is only costly when food is in short supply, and resistance was rapidly lost from D. melanogaster populations when food is restricted. Furthermore, evolving resistance to L. boulardi resulted in cross-resistance against other parasitoids. Therefore, whether a population evolves resistance will depend on ecological conditions including food availability and the presence of different parasite species.

CROSS-RESISTANCE FOLLOWING ARTIFICIAL SELECTION FOR INCREASED DEFENSE AGAINST PARASITOIDS IN DROSOPHILA MELANOGASTER

An increase in resistance to one natural enemy may result in no correlated change, a positive correlated change, or a negative correlated change in the ability of the host or prey to resist other natural enemies. The type of specificity is important in understanding the evolutionary response to natural enemies and was studied here in a Drosophila-paxasitoid system. Drosophila melanogaster lines selected for increased larval resistance to the endoparasitoid wasps Asobara tabida or Leptopilina boulardi were exposed to attack by A. tabida, L. boulardi and Leptopilina heterotoma at 15°C, 20°C, and 25°C. In general, encapsulation ability increased with temperature, with the exception of the lines selected against L. boulardi, which showed the opposite trend. Lines selected against L. boulardi showed large increases in resistance against all three parasitoid species, and showed similar levels of defense against A. tabida to the lines selected against that parasitoid. In contrast, lines selected against A. tabida showed a large increase in resistance to A. tabida and generally to L. heterotoma, but displayed only a small change in their ability to survive attack by L. boulardi. Such asymmetries in correlated responses to selection for increased resistance to natural enemies may influence host-parasitoid community structure.

Personality, immune response and reproductive success: an appraisal of the pace-of-life syndrome hypothesis

The pace-of-life syndrome (POLS) hypothesis is an extended concept of the life-history theory that includes behavioural traits. The studies challenging the POLS hypothesis often focus on the relationships between a single personality trait and a physiological and/or life-history trait. While pathogens represent a major selective pressure, few studies have been interested in testing relationships between behavioural syndrome, and several fitness components including immunity. The aim of this study was to address this question in the mealworm beetle, Tenebrio molitor, a model species in immunity studies. The personality score was estimated from a multidimensional syndrome based of four repeatable behavioural traits. In a first experiment, we investigated its relationship with two measures of fitness (reproduction and survival) and three components of the innate immunity (haemocyte concentration, and levels of activity of the phenoloxidase including the total proenzyme and the naturally activated one) to challenge the POLS hypothesis in T. molitor. Overall, we found a relationship between behavioural syndrome and reproductive success in this species, thus supporting the POLS hypothesis. We also showed a sex-specific relationship between behavioural syndrome and basal immune parameters. In a second experiment, we tested whether this observed relationship with innate immunity could be confirmed in term of differential survival after challenging by entomopathogenic bacteria, Bacillus thuringiensis. In this case, no significant relationship was evidenced. We recommend that future researchers on the POLS should control for differences in evolutionary trajectory between sexes and to pay attention to the choice of the proxy used, especially when looking at immune traits.

Immune function trade-offs in response to parasite threats

Immune function is often involved in physiological trade-offs because of the energetic costs of maintaining constitutive immunity and mounting responses to infection. However, immune function is a collection of discrete immunity factors and animals should allocate towards factors that combat the parasite threat with the highest fitness cost. For example, animals on dispersal fronts of expanding population may be released from density-dependent diseases. The costs of immunity, however, and life history trade-offs in general, are often context dependent. Trade-offs are often most apparent under conditions of unusually limited resources or when animals are particularly stressed, because the stress response can shift priorities. In this study we tested how humoral and cellular immune factors vary between phenotypes of a wing dimorphic cricket and how physiological stress influences these immune factors. We measured constitutive lysozyme activity, a humoral immune factor, and encapsulation response, a cellular immune factor. We also stressed the crickets with a sham predator in a full factorial design. We found that immune strategy could be explained by the selective pressures encountered by each morph and that stress decreased encapsulation, but not lysozyme activity. These results suggest a possible trade-off between humoral and cellular immunity. Given limited resources and the expense of immune factors, parasite pressures could play a key factor in maintaining insect polyphenism via disruptive selection.

Evolution of increased larval competitive ability in Drosophila melanogaster without increased larval feeding rate

Multiple experimental evolution studies on Drosophila melanogaster in the 1980s and 1990s indicated that enhanced competitive ability evolved primarily through increased larval tolerance to nitrogenous wastes and increased larval feeding and foraging rate, at the cost of efficiency of food conversion to biomass, and this became the widely accepted view of how adaptation to larval crowding evolves in fruitflies.We recently showed that populations of D. ananassae and D. n. nasuta subjected to extreme larval crowding evolved greater competitive ability without evolving higher feeding rates, primarily through a combination of reduced larval duration, faster attainment of minimum critical size for pupation, greater efficiency of food conversion to biomass, increased pupation height and, perhaps, greater urea/ammonia tolerance. This was a very different suite of traits than that seen to evolve under similar selection in D. melanogaster and was closer to the expectations from the theory of K-selection. At that time, we suggested two possible reasons for the differences in the phenotypic correlates of greater competitive ability seen in the studies with D. melanogaster and the other two species. First, that D. ananassae and D. n. nasuta had a very different genetic architecture of traits affecting competitive ability compared to the long-term laboratory populations of D. melanogaster used in the earlier studies, either because the populations of the former two species were relatively recently wild-caught, or by virtue of being different species. Second, that the different evolutionary trajectories in D. ananassae and D. n. nasuta versus D. melanogaster were a reflection of differences in the manner in which larval crowding was imposed in the two sets of selection experiments. The D. melanogaster studies used a higher absolute density of eggs per unit volume of food, and a substantially larger total volume of food, than the studies on D. ananassae and D. n. nasuta. Here, we show that long-term laboratory populations of D. melanogaster, descended from some of the populations used in the earlier studies, evolve essentially the same set of traits as the D. ananassae and D. n. nasuta crowding-adapted populations when subjected to a similar larval density at low absolute volumes of food. As in the case of D. ananassae and D. n. nasuta, and in stark contrast to earlier studies with D. melanogaster, these crowding-adapted populations of D. melanogaster did not evolve greater larval feeding rates as a correlate of increased competitive ability. The present results clearly suggest that the suite of phenotypes through which the evolution of greater competitive ability is achieved in fruitflies depends critically not just on larval density per unit volume of food, but also on the total amount of food available in the culture vials. We discuss these results in the context of an hypothesis about how larval density and the height of the food column in culture vials might interact to alter the fitness costs and benefits of increased larval feeding rates, thus resulting in different routes to the evolution of greater competitive ability, depending on the details of exactly how the larval crowding was implemented.

Adaptation to larval crowding in Drosophila ananassae and Drosophila nasuta nasuta: increased larval competitive ability without increased larval feeding rate

The standard view of adaptation to larval crowding in fruitflies, built on results from 25 years of multiple experimental evolution studies on Drosophila melanogaster, was that enhanced competitive ability evolves primarily through increased larval feeding and foraging rate, and increased larval tolerance to nitrogenous wastes, at the cost of efficiency of food conversion to biomass. These results were at odds from the predictions of classical K-selection theory, notably the expectation that selection at high density should result in the increase of efficiency of conversion of food to biomass, and were better interpreted through the lens of α-selection. We show here that populations of D. ananassae and D. n. nasuta subjected to extreme larval crowding evolve greater competitive ability and pre-adult survivorship at high density, primarily through a combination of reduced larval duration, faster attainment of minimum critical size for pupation, greater time efficiency of food conversion to biomass and increased pupation height, with a relatively small role of increased urea/ammonia tolerance, if at all. This is a very different suite of traits than that seen to evolve under similar selection in D. melanogaster, and seems to be closer to the expectations from the canonical theory of K-selection. We also discuss possible reasons for these differences in results across the three species. Overall, the results reinforce the view that our understanding of the evolution of competitive ability in fruitflies needs to be more nuanced than before, with an appreciation that there may be multiple evolutionary routes through which higher competitive ability can be attained.

Climatic and evolutionary drivers of phase shifts in the plague epidemics of colonial India

Immune heterogeneity in wild host populations indicates that disease-mediated selection is common in nature. However, the underlying dynamic feedbacks involving the ecology of disease transmission, evolutionary processes, and their interaction with environmental drivers have proven challenging to characterize. Plague presents an optimal system for interrogating such couplings: Yersinia pestis transmission exerts intense selective pressure driving the local persistence of disease resistance among its wildlife hosts in endemic areas. Investigations undertaken in colonial India after the introduction of plague in 1896 suggest that, only a decade after plague arrived, a heritable, plague-resistant phenotype had become prevalent among commensal rats of cities undergoing severe plague epidemics. To understand the possible evolutionary basis of these observations, we developed a mathematical model coupling environmentally forced plague dynamics with evolutionary selection of rats, capitalizing on extensive archival data from Indian Plague Commission investigations. Incorporating increased plague resistance among rats as a consequence of intense natural selection permits the model to reproduce observed changes in seasonal epidemic patterns in several cities and capture experimentally observed associations between climate and flea population dynamics in India. Our model results substantiate Victorian era claims of host evolution based on experimental observations of plague resistance and reveal the buffering effect of such evolution against environmental drivers of transmission. Our analysis shows that historical datasets can yield powerful insights into the transmission dynamics of reemerging disease agents with which we have limited contemporary experience to guide quantitative modeling and inference.

A newly evolved Drosophila Cytorace-9 shows trade-off between longevity and immune response

Species with an efficient immune system would be at an advantage to evade pathogenic challenges and adapt to an ever changing ecological niche. The upkeep of immunity is a costly affair, thus trade-offs between immunity and other life history traits are expected. However, studies on the relation between immunity and life span have yielded paradoxical results. Drosophila Cytoraces, being at different stages of evolutionary divergence, provide an excellent experimental model system to study how evolving populations gain novel traits in the absence of selection. We found that in the absence of pathogenic infections, the Cytorace-9 flies lived longer than those of Cytorace-3. However, when these Cytoraces were challenged with different pathogenic microbes, the trend was opposite. After infection with pathogens, the long-lived Cytorace-9 survived worse than the short lived Cytorace-3, which can be attributed to a reduction in its immune response. This study provides evidence to support the existence of a trade-off between life span and immunity.

Evolution of behavioural and cellular defences against parasitoid wasps in the Drosophila melanogaster subgroup

It may be intuitive to predict that host immune systems will evolve to counter a broad range of potential challenges through simultaneous investment in multiple defences. However, this would require diversion of resources from other traits, such as growth, survival and fecundity. Therefore, ecological immunology theory predicts that hosts will specialize in only a subset of possible defences. We tested this hypothesis through a comparative study of a cellular immune response and a putative behavioural defence used by eight fruit fly species against two parasitoid wasp species (one generalist and one specialist). Fly larvae can survive infection by melanotically encapsulating wasp eggs, and female flies can potentially reduce infection rates in their offspring by laying fewer eggs when wasps are present. The strengths of both defences varied significantly but were not negatively correlated across our chosen host species; thus, we found no evidence for a trade-off between behavioural and cellular immunity. Instead, cellular defences were significantly weaker against the generalist wasp, whereas behavioural defences were similar in strength against both wasps and positively correlated between wasps. We investigated the adaptive significance of wasp-induced oviposition reduction behaviour by testing whether wasp-exposed parents produce offspring with stronger cellular defences, but we found no support for this hypothesis. We further investigated the sensory basis of this behaviour by testing mutants deficient in either vision or olfaction, both of which failed to reduce their oviposition rates in the presence of wasps, suggesting that both senses are necessary for detecting and responding to wasps.

Potential Response to Selection of HSP70 as a Component of Innate Immunity in the Abalone Haliotis rufescens

Assessing components of the immune system may reflect disease resistance. In some invertebrates, heat shock proteins (HSPs) are immune effectors and have been described as potent activators of the innate immune response. Several diseases have become a threat to abalone farming worldwide; therefore, increasing disease resistance is considered to be a long-term goal for breeding programs. A trait will respond to selection only if it is determined partially by additive genetic variation. The aim of this study was to estimate the heritability (h²) and the additive genetic coefficient of variation (CV_A) of HSP70 as a component of innate immunity of the abalone Haliotis rufescens, in order to assess its potential response to selection. These genetic components were estimated for the variations in the intracellular (in haemocytes) and extracellular (serum) protein levels of HSP70 in response to an immunostimulant agent in 60 full-sib families of H. rufescens. Levels of HSP70 were measured twice in the same individuals, first when they were young and again when they were pre-harvest adults, to estimate the repeatability (R), the h² and the potential response to selection of these traits at these life stages. High HSP70 levels were observed in abalones subjected to immunostimulation in both the intracellular and extracellular haemolymph fractions. This is the first time that changes in serum levels of HSP70 have been reported in response to an immune challenge in molluscs. HSP70 levels in both fractions and at both ages showed low h² and R, with values that were not significantly different from zero. However, HSP70 induced levels had a CV_A of 13.3–16.2% in young adults and of 2.7–8.1% in pre-harvest adults. Thus, despite its low h², HSP70 synthesis in response to an immune challenge in red abalone has the potential to evolve through selection because of its large phenotypic variation and the presence of additive genetic variance, especially in young animals.

Evolution of Drosophila resistance against different pathogens and infection routes entails no detectable maintenance costs

Pathogens exert a strong selective pressure on hosts, entailing host adaptation to infection. This adaptation often affects negatively other fitness-related traits. Such trade-offs may underlie the maintenance of genetic diversity for pathogen resistance. Trade-offs can be tested with experimental evolution of host populations adapting to parasites, using two approaches: (1) measuring changes in immunocompetence in relaxed-selection lines and (2) comparing life-history traits of evolved and control lines in pathogen-free environments. Here, we used both approaches to examine trade-offs in Drosophila melanogaster populations evolving for over 30 generations under infection with Drosophila C Virus or the bacterium Pseudomonas entomophila, the latter through different routes. We find that resistance is maintained after up to 30 generations of relaxed selection. Moreover, no differences in several classical life-history traits between control and evolved populations were found in pathogen-free environments, even under stresses such as desiccation, nutrient limitation, and high densities. Hence, we did not detect any maintenance costs associated with resistance to pathogens. We hypothesize that extremely high selection pressures commonly used lead to the disproportionate expression of costs relative to their actual occurrence in natural systems. Still, the maintenance of genetic variation for pathogen resistance calls for an explanation.

Bacterial Exposure at the Larval Stage Induced Sexual Immune Dimorphism and Priming in Adult Aedes aegypti Mosquitoes

Gender differences in the immune response of insects are driven by natural selection for females and sexual selection for males. These natural forces entail a multitude of extrinsic and intrinsic factors involved in a genotype-environment interaction that results in sex-biased expression of the genes shared by males and females. However, little is known about how an infection at a particular ontogenetic stage may influence later stages, or how it may impact sexual immune dimorphism. Using Aedes aegypti mosquitoes, the aim of the present study was to analyze the effect of a bacterial exposure at the larval stage on adult immunity in males and females. The parameters measured were phenoloxidase activity, nitric oxide production, antimicrobial activity, and the antimicrobial peptide transcript response. As a measure of the immune response success, the persistence of injected bacteria was also evaluated. The results show that males, as well as females, were able to enhance survival in the adult stage as a result of being exposed at the larval stage, which indicates a priming effect. Moreover, there was a differential gender immune response, evidenced by higher PO activity in males as well as higher NO production and greater antimicrobial activity in females. The greater bacterial persistence in females suggests a gender-specific strategy for protection after a previous experience with an elicitor. Hence, this study provides a primary characterization of the complex and gender-specific immune response of male and female adults against a bacterial challenge in mosquitoes primed at an early ontogenetic stage.

Adenosine signaling and the energetic costs of induced immunity

Life history theory predicts that trait evolution should be constrained by competing physiological demands on an organism. Immune defense provides a classic example in which immune responses are presumed to be costly and therefore come at the expense of other traits related to fitness. One strategy for mitigating the costs of expensive traits is to render them inducible, such that the cost is paid only when the trait is utilized. In the current issue of PLOS Biology, Bajgar and colleagues elegantly demonstrate the energetic and life history cost of the immune response that Drosophila melanogaster larvae induce after infection by the parasitoid wasp Leptopilina boulardi. These authors show that infection-induced proliferation of defensive blood cells commands a diversion of dietary carbon away from somatic growth and development, with simple sugars instead being shunted to the hematopoetic organ for rapid conversion into the raw energy required for cell proliferation. This metabolic shift results in a 15% delay in the development of the infected larva and is mediated by adenosine signaling between the hematopoietic organ and the central metabolic control organ of the host fly. The adenosine signal thus allows D. melanogaster to rapidly marshal the energy needed for effective defense and to pay the cost of immunity only when infected.

Fruit flies infected by a parasitoid wasp use adenosine signaling to recruit energy away from tissue growth in order to support proliferation of defensive immune cells, thereby paying a deployment cost of inducible immunity. Read the Research Article.

Individual phenotypic variation reduces interaction strengths in a consumer-resource system

Natural populations often show variation in traits that can affect the strength of interspecific interactions. Interaction strengths in turn influence the fate of pairwise interacting populations and the stability of food webs. Understanding the mechanisms relating individual phenotypic variation to interaction strengths is thus central to assess how trait variation affects population and community dynamics. We incorporated nonheritable variation in attack rates and handling times into a classical consumer–resource model to investigate how variation may alter interaction strengths, population dynamics, species persistence, and invasiveness. We found that individual variation influences species persistence through its effect on interaction strengths. In many scenarios, interaction strengths decrease with variation, which in turn affects species coexistence and stability. Because environmental change alters the direction and strength of selection acting upon phenotypic traits, our results have implications for species coexistence in a context of habitat fragmentation, climate change, and the arrival of exotic species to native ecosystems.

Insect host-parasite coevolution in the light of experimental evolution

The many ways parasites can impact their host species have been the focus of intense study using a range of approaches. A particularly promising but under-used method in this context is experimental evolution, because it allows targeted manipulation of known populations exposed to contrasting conditions. The strong potential of applying this method to the study of insect hosts and their associated parasites is demonstrated by the few available long-term experiments where insects have been exposed to parasites. In this review, we summarize these studies, which have delivered valuable insights into the evolution of resistance in response to parasite pressure, the underlying mechanisms, as well as correlated genetic responses. We further assess findings from relevant artificial selection studies in the interrelated contexts of immunity, life history, and reproduction. In addition, we discuss a number of well-studied Tribolium castaneum-Nosema whitei coevolution experiments in more detail and provide suggestions for research. Specifically, we suggest that future experiments should also be performed using nonmodel hosts and should incorporate contrasting experimental conditions, such as population sizes or environments. Finally, we expect that adding a third partner, for example, a second parasite or symbiont, to a host-parasite system could strongly impact (co)evolutionary dynamics.

Genomic changes under rapid evolution: selection for parasitoid resistance

In this study, we characterize changes in the genome during a swift evolutionary adaptation, by combining experimental selection with high-throughput sequencing. We imposed strong experimental selection on an ecologically relevant trait, parasitoid resistance in Drosophila melanogaster against Asobara tabida. Replicated selection lines rapidly evolved towards enhanced immunity. Larval survival after parasitization increased twofold after just five generations of selection. Whole-genome sequencing revealed that the fast and strong selection response in innate immunity produced multiple, highly localized genomic changes. We identified narrow genomic regions carrying a significant signature of selection, which were present across all chromosomes and covered in total less than 5% of the whole D. melanogaster genome. We identified segregating sites with highly significant changes in frequency between control and selection lines that fell within these narrow 'selected regions'. These segregating sites were associated with 42 genes that constitute possible targets of selection. A region on chromosome 2R was highly enriched in significant segregating sites and may be of major effect on parasitoid defence. The high genetic variability and small linkage blocks in our base population are likely responsible for allowing this complex trait to evolve without causing widespread erosive effects in the genome, even under such a fast and strong selective regime.

What determines host acceptance and suitability in tropical Asian Drosophila parasitoids?

For successful parasitism, parasitoid females must oviposit and the progeny must develop in individual hosts. Here, we investigated the determinants of host acceptance for oviposition and host suitability for larval development of Drosophila parasitoids from Bogor and Kota Kinabalu (≍1,800 km northeast of Bogor), Indonesia, in tropical Asia. Asobara pleuralis (Ashmead) from both localities oviposited frequently (>60%) in all of the drosophilid species tested, except the strain from Kota Kinabalu oviposited rarely (10%) in Drosophila eugracilis Bock & Wheeler. Leptopilina victoriae Nordlander from both localities only oviposited frequently (>77%) in species from the Drosophila melanogaster species group except D. eugracilis (<3.7%), whereas Leptopilina pacifica Novković & Kimura from Bogor oviposited frequently (>85%) only in species from the Drosophila immigrans species group. Thus, host acceptance appeared to be affected by host taxonomy, at least in Leptopilina species. Host suitability varied considerably, even among closely related drosophilid species, which suggests that the host suitability is at least in part independent of host taxonomy and that it has been determined via parasitoid-host coevolutionary interactions (i.e., arms race). Host acceptance did not always coincide with host suitability, i.e., parasitoids sometimes oviposited in unsuitable host species. Geographic origin strongly affected the host acceptance and suitability in the A. pleuralis-D. eugracilis parasitoid-host pair, whereas it only weakly affected the acceptability and suitability in other parasitoid-host combinations.

Current immunity markers in insect ecological immunology: assumed trade-offs and methodological issues

The field of ecological immunology currently relies on using a number of immune effectors or markers. These markers are usually used to infer ecological trade-offs (via conflicts in resource allocation), though physiological nature of these markers remains elusive. Here, we review markers frequently used in insect evolutionary ecology research: cuticle darkening, haemocyte density, nodule/capsule formation, phagocytosis and encapsulation/melanization via use of nylon filaments and beads, phenoloxidase activity, nitric oxide production, lysozyme and antimicrobial peptide production. We also provide physiologically based information that may shed light on the probable trade-offs inferred when these markers are used. In addition, we provide a number of methodological suggestions to improve immune marker assessment.

Natural variation in differentiated hemocytes is related to parasitoid resistance in Drosophila melanogaster

As a measure of parasitoid resistance, hemocyte load and encapsulation ability were measured in lines collected from natural populations of Drosophila melanogaster in Europe. Results show large geographic variation in resistance against the parasitoid wasp Asobara tabida among the field lines, but there was no clear correlation between resistance and total hemocyte load, neither before nor after parasitization. This was in contrast to the patterns that had been found in a comparison among species of Drosophila, where total hemocyte counts were positively correlated to encapsulation rates. This suggests that the mechanisms underlying between-species variation in parasitoid resistance do not extend to the natural variation that exists within a species. Although hemocyte counts did not correspond to encapsulation ability within D. melanogaster, the ratios of lamellocytes and crystal cells were very similar in lines with successful encapsulation responses. Apart from variation in the hemocytic response of the different hemocyte types, within-species variation was also observed for accurate targeting of the foreign body by the hemocytes. These results are discussed in the context of possible causes of variation in immune functions among natural populations.

Defense traits of larval Drosophila melanogaster exhibit genetically based trade-offs against different species of parasitoids

Populations of Drosophila melanogaster face significant mortality risks from parasitoid wasps that use species-specific strategies to locate and survive in hosts. We tested the hypothesis that parasitoids with different strategies select for alternative host defense characteristics and in doing so contribute to the maintenance of fitness variation and produce trade-offs among traits. We characterized defense traits of Drosophila when exposed to parasitoids with different host searching behaviors (Aphaereta sp. and Leptopilina boulardi). We used host larvae with different natural alleles of the gene Dopa decarboxylase (Ddc), a gene controlling the production of dopamine and known to influence the immune response against parasitoids. Previous population genetic analyses indicate that our focal alleles are maintained by balancing selection. Genotypes exhibited a trade-off between the immune response against Aphaereta sp. and the ability to avoid parasitism by L. boulardi. We also identified a trade-off between the ability to avoid parasitism by L. boulardi and larval competitive ability as indicated by differences in foraging and feeding behavior. Genotypes differed in dopamine levels potentially explaining variation in these traits. Our results highlight the potential role of parasitoid biodiversity on host fitness variation and implicate Ddc as an antagonistic pleiotropic locus influencing larval fitness traits.

Drosophila melanogaster Selection for Survival of Bacillus cereus Infection: Life History Trait Indirect Responses

To study evolved resistance/tolerance in an insect model, we carried out an experimental evolution study using D. melanogaster and the opportunistic pathogen B. cereus as the agent of selection. The selected lines evolved a 3.0- to 3.3-log increase in the concentration of spores required for 50% mortality after 18–24 generations of selection. In the absence of any treatment, selected lines evolved an increase in egg production and delayed development time. The latter response could be interpreted as a cost of evolution. Alternatively, delayed development might have been a target of selection resulting in increased adult fat body function including production of antimicrobial peptides, and, incidentally, yolk production for oocytes and eggs. When treated with autoclaved spores, the egg production difference between selected and control lines was abolished, and this response was consistent with the hypothesis of a cost of an induced immune response. Treatment with autoclaved spores also reduced life span in some cases and elicited early-age mortality in the selected and wound-control lines both of which were consistent with the hypothesis of a cost associated with induction of immune responses. In general, assays on egg production yielded key outcomes including the negative effect of autoclaved spores on egg production.

The evolution of disease resistance and tolerance in spatially structured populations

THE UBIQUITOUS CHALLENGE FROM INFECTIOUS DISEASE HAS PROMPTED THE EVOLUTION OF DIVERSE HOST DEFENSES, WHICH CAN BE DIVIDED INTO TWO BROAD CLASSES: resistance (which limits pathogen growth and infection) and tolerance (which does not limit infection, but instead reduces or offsets its negative fitness consequences). Resistance and tolerance may provide equivalent short-term benefits, but have fundamentally different epidemiological consequences and thus exhibit different evolutionary behaviors. We consider the evolution of resistance and tolerance in a spatially structured population using a stochastic simulation model. We show that tolerance can invade a population of susceptible individuals (i.e., neither resistant nor tolerant) with higher cost than resistance, even though they each provide equivalent direct benefits to the host, because tolerant hosts impose higher disease burden upon vulnerable competitors. However, in spatially structured settings, tolerance can invade a population of resistant hosts only with lower cost than resistance due to spatial genetic structure and the higher local incidence of disease around invading tolerant individuals. The evolution of tolerance is therefore constrained by spatial genetic structure in a manner not previously revealed by nonspatially explicit models, suggesting mechanisms that could maintain variation or limit the occurrence of tolerance relative to resistance.

The mode of host-parasite interaction shapes coevolutionary dynamics and the fate of host cooperation

Antagonistic coevolution between hosts and parasites can have a major impact on host population structures, and hence on the evolution of social traits. Using stochastic modelling techniques in the context of bacteria–virus interactions, we investigate the impact of coevolution across a continuum of host–parasite genetic specificity (specifically, where genotypes have the same infectivity/resistance ranges (matching alleles, MA) to highly variable ranges (gene-for-gene, GFG)) on population genetic structure, and on the social behaviour of the host. We find that host cooperation is more likely to be maintained towards the MA end of the continuum, as the more frequent bottlenecks associated with an MA-like interaction can prevent defector invasion, and can even allow migrant cooperators to invade populations of defectors.

Evolutionary determinants of genetic variation in susceptibility to infectious diseases in humans

Although genetic variation among humans in their susceptibility to infectious diseases has long been appreciated, little focus has been devoted to identifying patterns in levels of variation in susceptibility to different diseases. Levels of genetic variation in susceptibility associated with 40 human infectious diseases were assessed by a survey of studies on both pedigree-based quantitative variation, as well as studies on different classes of marker alleles. These estimates were correlated with pathogen traits, epidemiological characteristics, and effectiveness of the human immune response. The strongest predictors of levels of genetic variation in susceptibility were disease characteristics negatively associated with immune effectiveness. High levels of genetic variation were associated with diseases with long infectious periods and for which vaccine development attempts have been unsuccessful. These findings are consistent with predictions based on theoretical models incorporating fitness costs associated with the different types of resistance mechanisms. An appreciation of these observed patterns will be a valuable tool in directing future research given that genetic variation in disease susceptibility has large implications for vaccine development and epidemiology.