Variation in costs of parasite resistance among natural host populations

Associating with kin selects for disease resistance and against tolerance

Behavioural and physiological resistance are key to slowing epidemic spread. We explore the evolutionary and epidemic consequences of their different costs for the evolution of tolerance that trades off with resistance. Behavioural resistance affects social cohesion, with associated group-level costs, while the cost of physiological resistance accrues only to the individual. Further, resistance, and the associated reduction in transmission, benefit susceptible hosts directly, whereas infected hosts only benefit indirectly, by reducing transmission to kin. We therefore model the coevolution of transmission-reducing resistance expressed in susceptible hosts with resistance expressed in infected hosts, as a function of kin association, and analyse the effect on population-level outcomes. Using parameter values for guppies, Poecilia reticulata, and their gyrodactylid parasites, we find that: (1) either susceptible or infected hosts should invest heavily in resistance, but not both; (2) kin association drives investment in physiological resistance more strongly than in behavioural resistance; and (3) even weak levels of kin association can favour altruistic infected hosts that invest heavily in resistance (versus selfish tolerance), eliminating parasites. Overall, our finding that weak kin association affects the coevolution of infected and susceptible investment in both behavioural and physiological resistance suggests that kin selection may affect disease dynamics across systems.

Host genetic identity determines parasite community structure across time and space in oyster restoration

Intraspecific variation in host susceptibility to individual parasite species is common, yet how these effects scale to mediate the structure of diverse parasite communities in nature is less well understood. To address this knowledge gap, we tested how host genetic identity affects parasite communities on restored reefs seeded with juvenile oysters from different sources-a regional commercial hatchery or one of two wild progenitor lines. We assessed prevalence and intensity of three micro- and two macroparasite species for 4 years following restoration. Despite the spatial proximity of restored reefs, oyster source identity strongly predicted parasite community prevalence across all years, with sources varying in their relative susceptibility to different parasites. Oyster seed source also predicted reef-level parasite intensities across space and through time. Our results highlight that host intraspecific variation can shape parasite community structure in natural systems, and reinforce the importance of considering source identity and diversity in restoration design.

The timings of host diapause and epidemic progression mediate host genetic diversity and future epidemic size in Daphnia-parasite populations

Epidemics commonly exert parasite-mediated selection and cause declines in host population genetic diversity. This can lead to evolution of resistance in the long term and smaller subsequent epidemics. Alternatively, the loss of genetic diversity can increase host vulnerability to future disease spread and larger future epidemics. Matters are made more complex by the fact that a great many host organisms produce diapausing life stages in response to environmental change (often as a result of sexual reproduction; e.g. plant seeds and invertebrate resting eggs). These diapausing stages can disrupt the relationship between past epidemics, host genetic diversity and future epidemics because they allow host dispersal through time. Specifically, temporally dispersing hosts avoid infection and thus selection from contemporary parasites, and also archive genetic variation for the future. We studied 80 epidemics in 20 semi-natural populations of the temporally dispersing crustacean Daphnia magna and its sterilizing bacterial parasite Pasteuria ramosa, and half of these populations experienced a simulated environmental disturbance treatment. We found that early initiation of diapause relative to the timing of the epidemic led to greater host genetic diversity and reduced epidemic size in the subsequent year, but this was unaffected by environmental disturbance.

Natural variation in host feeding behaviors impacts host disease and pathogen transmission potential

Animals ranging from mosquitoes to humans often vary their feeding behavior when infected or merely exposed to pathogens. These so-called "sickness behaviors" are part of the innate immune response with many consequences, including avoiding orally transmitted pathogens. Fully understanding the role of this ubiquitous behavior in host defense and pathogen evolution requires a quantitative account of its impact on host and pathogen fitness across environmentally relevant contexts. Here, we use a zooplankton host and fungal pathogen as a case study to ask if infection-mediated feeding behaviors vary across pathogen exposure levels and natural genetic variation in susceptibility to infection. Then, we connect these changes in behavior to pathogen transmission potential (spore yield) and fitness and growth costs to the host. Our results validate a protective effect of altered feeding behavior during pathogen exposure while also revealing significant variation in the magnitude of this response across host susceptibility and pathogen exposure levels. Across all four host genotypes, feeding rates were negatively correlated with susceptibility to infection and transmission potential. The most susceptible genotypes exhibited either strong anorexia, reducing food intake by 26%-42%, ("Standard") or pronounced hyperphagia, increasing food intake by 20%-54% ("A45"). Together, these results suggest that infection-mediated changes in host feeding behavior-which are traditionally interpreted as immunopathology- may in fact serve as crucial components of host defense strategies and warrant further investigation.

Diet can alter the cost of resistance to a natural parasite in Caenorhabditis elegans

Resistance to parasites confers a fitness advantage, yet hosts show substantial variation in resistance in natural populations. Evolutionary theory indicates that resistant and susceptible genotypes can coexist if resistance is costly, but there is mixed evidence that resistant individuals have lower fitness in the absence of parasites. One explanation for this discrepancy is that the cost of resistance varies with environmental context. We tested this hypothesis using Caenorhabditis elegans and its natural microsporidian parasite, Nematocida ironsii. We used multiple metrics to compare the fitness of two near-isogenic host genotypes differing at regions associated with resistance to N. ironsii. To quantify the effect of the environment on the cost associated with these known resistance regions, we measured fitness on three microbial diets. We found that the cost of resistance varied with both diet and the measure of fitness. We detected no cost to resistance, irrespective of diet, when fitness was measured as fecundity. However, we detected a cost when fitness was measured in terms of population growth, and the magnitude of this cost varied with diet. These results provide a proof of concept that, by mediating the cost of resistance, environmental context may govern the rate and nature of resistance evolution in heterogeneous environments.

Invasion of the body snatchers: the role of parasite introduction in host distribution and response to salinity in invaded estuaries

In dynamic systems, organisms are faced with variable selective forces that may impose trade-offs. In estuaries, salinity is a strong driver of organismal diversity, while parasites shape species distributions and demography. We tested for trade-offs between low-salinity stress and parasitism in an invasive castrating parasite and its mud crab host along salinity gradients of two North Carolina rivers. We performed field surveys every six to eight weeks over 3 years to determine factors influencing parasite prevalence, host abundance, and associated taxa diversity. We also looked for signatures of low-salinity stress in the host by examining its response (time-to-right and gene expression) to salinity. We found salinity and temperature significantly affected parasite prevalence, with low-salinity sites (less than 10 practical salinity units (PSU)) lacking infection, and populations in moderate salinities at warmer temperatures reaching prevalence as high as 60%. Host abundance was negatively associated with parasite prevalence. Host gene expression was plastic to acclimation salinity, but several osmoregulatory and immune-related genes demonstrated source-dependent salinity response. We identified a genetic marker that was strongly associated with salinity against a backdrop of no neutral genetic structure, suggesting possible selection on standing variation. Our study illuminates how selective trade-offs in naturally dynamic systems may shape host evolutionary ecology.

Ecology directs host-parasite coevolutionary trajectories across Daphnia-microparasite populations

Host-parasite interactions often fuel coevolutionary change. However, parasitism is one of a myriad of possible ecological interactions in nature. Biotic (for example, predation) and abiotic (for example, temperature) variation can amplify or dilute parasitism as a selective force on hosts and parasites, driving population variation in (co)evolutionary trajectories. We dissected the relationships between wider ecology and coevolutionary trajectory using 16 ecologically complex Daphnia magna-Pasteuria ramosa ponds seeded with an identical starting host (Daphnia) and parasite (Pasteuria) population. We show, using a time-shift experiment and outdoor population data, how multivariate biotic and abiotic ecological differences between ponds caused coevolutionary divergence. Wider ecology drove variation in host evolution of resistance, but not parasite infectivity; parasites subsequently coevolved in response to the changing complement of host genotypes, such that parasites adapted to historically resistant host genotypes. Parasitism was a stronger interaction for the parasite than for its host, probably because the host is the principal environment and selective force, whereas for hosts, parasite-mediated selection is one of many sources of selection. Our findings reveal the mechanisms through which wider ecology creates coevolutionary hotspots and coldspots in biologically realistic arenas of host-parasite interaction, and sheds light on how the ecological theatre can affect the (co)evolutionary play.

Sexual selection reveals a cost of pathogen resistance undetected in life-history assays

Mechanisms of resistance to pathogens and parasites are thought to be costly and thus to lead to evolutionary trade‐offs between resistance and life‐history traits expressed in the absence of the infective agents. On the other hand, sexually selected traits are often proposed to indicate “good genes” for resistance, which implies a positive genetic correlation between resistance and success in sexual selection. Here I show that experimental evolution of improved resistance to the intestinal pathogen Pseudomonas entomophila in Drosophila melanogaster was associated with a reduction in male sexual success. Males from four resistant populations achieved lower paternity than males from four susceptible control populations in competition with males from a competitor strain, indicating an evolutionary cost of resistance in terms of mating success and/or sperm competition. In contrast, no costs were found in larval viability, larval competitive ability and population productivity assayed under nutritional limitation; together with earlier studies this suggests that the costs of P. entomophila resistance for nonsexual fitness components are negligible. Thus, rather than indicating heritable pathogen resistance, sexually selected traits expressed in the absence of pathogens may be sensitive to costs of resistance, even if no such costs are detected in other fitness traits.

Temperature and clone-dependent effects of microplastics on immunity and life history in Daphnia magna

Microplastic (MP) pollution is potentially a major threat to many aquatic organisms. Yet we currently know very little about the mechanisms responsible for the effects of small MPs on phenotypes, and the extent to which effects of MPs are modified by genetic and environmental factors. Using a multivariate approach, we studied the effects of 500 nm polystyrene microspheres on the life history and immunity of eight clones of the freshwater cladoceran Daphnia magna reared at two temperatures (18 °C/24 °C). MP exposure altered multivariate phenotypes in half of the clones we studied but had no effect on others. In the clones that were affected, individuals exposed to MPs had smaller offspring at both temperatures, and more offspring at high temperature. Differences in response to MP exposure were unrelated to differences in particle uptake, but were instead linked to an upregulation of haemocytes, particularly at high temperature. The clone-specific, context-dependent nature of our results demonstrates the importance of incorporating genetic variation and environmental context into assessments of the impact of plastic particle exposure. Our results identify immunity as an important mechanism underpinning genetically variable responses to MP pollution and may have major implications for predicting consequences of MP pollution.

Genotypic variation in parasite avoidance behaviour and other mechanistic, nonlinear components of transmission

Traditional epidemiological models assume that transmission increases proportionally to the density of parasites. However, empirical data frequently contradict this assumption. General yet mechanistic models can explain why transmission depends nonlinearly on parasite density and thereby identify potential defensive strategies of hosts. For example, hosts could decrease their exposure rates at higher parasite densities (via behavioural avoidance) or decrease their per-parasite susceptibility when encountering more parasites (e.g. via stronger immune responses). To illustrate, we fitted mechanistic transmission models to 19 genotypes of Daphnia dentifera hosts over gradients of the trophically acquired parasite, Metschnikowia bicuspidata. Exposure rate (foraging, F) frequently decreased with parasite density (Z), and per-parasite susceptibility (U) frequently decreased with parasite encounters (F × Z). Consequently, infection rates (F × U × Z) often peaked at intermediate parasite densities. Moreover, host genotypes varied substantially in these responses. Exposure rates remained constant for some genotypes but decreased sensitively with parasite density for others (up to 78%). Furthermore, genotypes with more sensitive foraging/exposure also foraged faster in the absence of parasites (suggesting 'fast and sensitive' versus 'slow and steady' strategies). These relationships suggest that high densities of parasites can inhibit transmission by decreasing exposure rates and/or per-parasite susceptibility, and identify several intriguing axes for the evolution of host defence.

Host traits and competitive ability jointly structure disease dynamics and community assembly

Parasitism and competition are both ubiquitous interactions in ecological communities. The ability of host species to interact directly via competition and indirectly through shared parasites suggests that host traits related to competition and parasitism are likely important in structuring communities and disease dynamics. Specifically, those host traits affecting competition and those mediating parasitism are often correlated either because of trade-offs (in resource acquisition or resource allocation) or condition dependence, yet the consequences of these trait relationships for community and epidemiological dynamics are poorly understood. We conducted a literature review of parasite-related host traits-competitive ability relationships. We found that transmission-competitive ability relationships were most often reported, and that superior competitors exhibited elevated transmission relative to their less-competitive counterparts in nearly 80% of the cases. We also found a significant number of virulence-competitive ability and parasite shedding-competitive ability relationships. We investigated these links by altering the relationship between host competitive ability and three parasite-related traits (transmission, virulence and parasite shedding rates) in a simple model, incorporating competitive asymmetries in a multi-host community. We show that these relationships can lead to a range of different communities. For example, depending on the strength and direction of these distinct trait relationships, we observed communities with anywhere from high parasite prevalence to complete parasite extinction, and either one, two or the maximum of three host species coexisting. Our results suggest that parasite-competitive ability relationships may be common in nature, that further integration of these relationships can produce novel and unexpected community and disease dynamics, and that generalizations may allow for the prediction of how parasitism and competition jointly structure disease and diversity in natural communities.

Variation in chronic radiation exposure does not drive life history divergence among Daphnia populations across the Chernobyl Exclusion Zone

Ionizing radiation is a mutagen with known negative impacts on individual fitness. However, much less is known about how these individual fitness effects translate into population-level variation in natural environments that have experienced varying levels of radiation exposure. In this study, we sampled genotypes of the freshwater crustacean, Daphnia pulex, from the eight inhabited lakes across the Chernobyl Exclusion Zone (CEZ). Each lake has experienced very different levels of chronic radiation exposure since a nuclear power reactor exploded there over thirty years ago. The sampled Daphnia genotypes represent genetic snapshots of current populations and allowed us to examine fitness-related traits under controlled laboratory conditions at UK background dose rates. We found that whilst there was variation in survival and schedules of reproduction among populations, there was no compelling evidence that this was driven by variation in exposure to radiation. Previous studies have shown that controlled exposure to radiation at dose rates included in the range measured in the current study reduce survival, or fecundity, or both. One limitation of this study is the lack of available sites at high dose rates, and future work could test life history variation in various organisms at other high radiation areas. Our results are nevertheless consistent with the idea that other ecological factors, for example competition, predation or parasitism, are likely to play a much bigger role in driving variation among populations than exposure to the high radiation dose rates found in the CEZ. These findings clearly demonstrate that it is important to examine the potential negative effects of radiation across wild populations that are subject to many and varied selection pressures as a result of complex ecological interactions.

Context-dependent costs and benefits of tuberculosis resistance traits in a wild mammalian host

Disease acts as a powerful driver of evolution in natural host populations, yet individuals in a population often vary in their susceptibility to infection. Energetic trade-offs between immune and reproductive investment lead to the evolution of distinct life history strategies, driven by the relative fitness costs and benefits of resisting infection. However, examples quantifying the cost of resistance outside of the laboratory are rare. Here, we observe two distinct forms of resistance to bovine tuberculosis (bTB), an important zoonotic pathogen, in a free-ranging African buffalo (Syncerus caffer) population. We characterize these phenotypes as "infection resistance," in which hosts delay or prevent infection, and "proliferation resistance," in which the host limits the spread of lesions caused by the pathogen after infection has occurred. We found weak evidence that infection resistance to bTB may be heritable in this buffalo population (h 2 = 0.10) and comes at the cost of reduced body condition and marginally reduced survival once infected, but also associates with an overall higher reproductive rate. Infection-resistant animals thus appear to follow a "fast" pace-of-life syndrome, in that they reproduce more quickly but die upon infection. In contrast, proliferation resistance had no apparent costs and was associated with measures of positive host health-such as having a higher body condition and reproductive rate. This study quantifies striking phenotypic variation in pathogen resistance and provides evidence for a link between life history variation and a disease resistance trait in a wild mammalian host population.

A genotypic trade-off between constitutive resistance to viral infection and host growth rate

Genotypic trade‐offs are fundamental to the understanding of the evolution of life‐history traits. In particular, the evolution of optimal host defense and the maintenance of variation in defense against infectious disease is thought to be underpinned by such evolutionary trade‐offs. However, empirical demonstrations of these trade‐offs that satisfy the strict assumptions made by theoretical models are rare. Additionally, none of these trade‐offs have yet been shown to be robustly replicable using a variety of different experimental approaches to rule out confounding issues with particular experimental designs. Here, we use inbred isolines as a novel experimental approach to test whether a trade‐off between viral resistance and growth rate in Plodia interpunctella, previously demonstrated by multiple selection experiments, is robust and meets the strict criteria required to underpin theoretical work in this field. Critically, we demonstrate that this trade‐off is both genetic and constitutive. This finding helps support the large body of theory that relies on these assumptions, and makes this trade‐off for resistance unique in being replicated through multiple experimental approaches and definitively shown to be genetic and constitutive.

Rapid evolution rescues hosts from competition and disease but-despite a dilution effect-increases the density of infected hosts

Virulent parasites can depress the densities of their hosts. Taxa that reduce disease via dilution effects might alleviate this burden. However, 'diluter' taxa can also depress host densities through competition for shared resources. The combination of disease and interspecific competition could even drive hosts extinct. Then again, genetically variable host populations can evolve in response to both competitors and parasites. Can rapid evolution rescue host density from the harm caused by these ecological enemies? How might such evolution influence dilution effects or the size of epidemics? In a mesocosm experiment with planktonic hosts, we illustrate the joint harm of competition and disease: hosts with constrained evolutionary ability (limited phenotypic variation) suffered greatly from both. However, populations starting with broader phenotypic variation evolved stronger competitive ability during epidemics. In turn, enhanced competitive ability-driven especially by parasites-rescued host densities from the negative impacts of competition, disease, and especially their combination. Interspecific competitors reduced disease (supporting dilution effects) even when hosts rapidly evolved. However, this evolutionary response also elicited a potential problem. Populations that evolved enhanced competitive ability and maintained robust total densities also supported higher densities of infections. Thus, rapid evolution rescued host densities but also unleashed larger epidemics.

Proximity to parasites reduces host fitness independent of infection in a Drosophila-Macrocheles system

Parasites are known to have direct negative effects on host fitness; however, the indirect effects of parasitism on host fitness sans infection are less well understood. Hosts undergo behavioural and physiological changes when in proximity to parasites. Yet, there is little experimental evidence showing that these changes lead to long-term decreases in host fitness. We aimed to determine if parasite exposure affects host fitness independent of contact, because current approaches to parasite ecology may underestimate the effect of parasites on host populations. We assayed the longevity and reproductive output of Drosophila nigrospiracula exposed or not exposed to ectoparasitic Macrocheles subbadius. In order to preclude contact and infection, mites and flies were permanently separated with a mesh screen. Exposed flies had shorter lives and lower fecundity relative to unexposed flies. Recent work in parasite ecology has argued that parasite-host systems show similar processes as predator-prey systems. Our findings mirror the non-consumptive effects observed in predator-prey systems, in which prey species suffer reduced fitness even if they never come into direct contact with predators. Our results support the perspective that there are analogous effects in parasite-host systems, and suggest new directions for research in both parasite ecology and the ecology of fear.

Cost of resistance to trematodes in freshwater snail populations with low clonal diversity

BACKGROUND

The persistence of high genetic variability in natural populations garners considerable interest among ecologists and evolutionary biologists. One proposed hypothesis for the maintenance of high levels of genetic diversity relies on frequency-dependent selection imposed by parasites on host populations (Red Queen hypothesis). A complementary hypothesis suggests that a trade-off between fitness costs associated with tolerance to stress factors and fitness costs associated with resistance to parasites is responsible for the maintenance of host genetic diversity.

RESULTS

The present study investigated whether host resistance to parasites is traded off with tolerance to environmental stress factors (high/low temperatures, high salinity), by comparing populations of the freshwater snail Melanoides tuberculata with low vs. high clonal diversity. Since polyclonal populations were found to be more parasitized than populations with low clonal diversity, we expected them to be tolerant to environmental stress factors. We found that clonal diversity explained most of the variation in snail survival under high temperature, thereby suggesting that tolerance to high temperatures of clonally diverse populations is higher than that of populations with low clonal diversity.

CONCLUSIONS

Our results suggest that resistance to parasites may come at a cost of reduced tolerance to certain environmental stress factors.

Simulated climate change, epidemic size, and host evolution across host-parasite populations

Climate change is causing warmer and more variable temperatures as well as physical flux in natural populations, which will affect the ecology and evolution of infectious disease epidemics. Using replicate seminatural populations of a coevolving freshwater invertebrate-parasite system (host: Daphnia magna, parasite: Pasteuria ramosa), we quantified the effects of ambient temperature and population mixing (physical flux within populations) on epidemic size and population health. Each population was seeded with an identical suite of host genotypes and dose of parasite transmission spores. Biologically reasonable increases in environmental temperature caused larger epidemics, and population mixing reduced overall epidemic size. Mixing also had a detrimental effect on host populations independent of disease. Epidemics drove parasite-mediated selection, leading to a loss of host genetic diversity, and mixed populations experienced greater evolution due to genetic drift over the season. These findings further our understanding of how diversity loss will reduce the host populations' capacity to respond to changes in selection, therefore stymying adaptation to further environmental change.

Parasite transmission in a natural multihost-multiparasite community

Understanding the transmission and dynamics of infectious diseases in natural communities requires understanding the extent to which the ecology, evolution and epidemiology of those diseases are shaped by alternative hosts. We performed laboratory experiments to test how parasite spillover affected traits associated with transmission in two co-occurring parasites: the bacterium Pasteuria ramosa and the fungus Metschnikowia bicuspidata Both parasites were capable of transmission from the reservoir host (Daphnia dentifera) to the spillover host (Ceriodaphnia dubia), but this occurred at a much higher rate for the fungus than the bacterium. We quantified transmission potential by combining information on parasite transmission and growth rate, and used this to compare parasite fitness in the two host species. For both parasites, transmission potential was lower in the spillover host. For the bacterium, virulence was higher in the spillover host. Transmission back to the original host was high for both parasites, with spillover influencing transmission rate of the fungus but not the bacterium. Thus, while inferior, the spillover host is not a dead-end for either parasite. Overall, our results demonstrate that the presence of multiple hosts in a community can have important consequences for disease transmission, and host and parasite fitness.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.

Higher risk of gastrointestinal parasite infection at lower elevation suggests possible constraints in the distributional niche of Alpine marmots

Alpine marmots Marmota marmota occupy a narrow altitudinal niche within high elevation alpine environments. For animals living at such high elevations where resources are limited, parasitism represents a potential major cost in life history. Using occupancy models, we tested if marmots living at higher elevation have a reduced risk of being infected with gastro-intestinal helminths, possibly compensating the lower availability of resources (shorter feeding season, longer snow cover and lower temperature) than marmots inhabiting lower elevations. Detection probability of eggs and oncospheres of two gastro-intestinal helminthic parasites, Ascaris laevis and Ctenotaenia marmotae, sampled in marmot feces, was used as a proxy of parasite abundance. As predicted, the models showed a negative relationship between elevation and parasite detectability (i.e. abundance) for both species, while there appeared to be a negative effect of solar radiance only for C. marmotae. Site-occupancy models are used here for the first time to model the constrains of gastrointestinal parasitism on a wild species and the relationship existing between endoparasites and environmental factors in a population of free-living animals. The results of this study suggest the future use of site-occupancy models as a viable tool to account for parasite imperfect detection in eco-parasitological studies, and give useful insights to further investigate the hypothesis of the contribution of parasite infection in constraining the altitudinal niche of Alpine marmots.

Aphid symbionts and endogenous resistance traits mediate competition between rival parasitoids

Insects use endogenous mechanisms and infection with protective symbionts to thwart attacks from natural enemies. Defenses that target specific enemies, however, potentially mediate competition between rivals and thereby impact community composition. Following its introduction to North America to control pea aphids (Acyrthosiphon pisum), the parasitoid Aphidius ervi competitively displaced other parasitoids, except for the native Praon pequodorum. The pea aphid exhibits tremendous clonal variation in resistance to A. ervi, primarily through infection with the heritable bacterial symbiont Hamiltonella defensa, although some symbiont-free aphid genotypes encode endogenous resistance. Interestingly, H. defensa strains and aphid genotypes that protect against A. ervi, provide no protection against the closely related, P. pequodorum. Given the specificity of aphid defenses, we hypothesized that aphid resistance traits may contribute to the continued persistence of P. pequodorum. We conducted multiparasitism assays to determine whether aphid resistance traits mediate internal competition between these two solitary parasitoid species, but found this was not the case; P. pequodorum was the successful internal competitor across lines varying in susceptibility to A. ervi. Next, to determine whether resistance traits influence competitive interactions resulting in the stable persistence of P. pequodorum, we established replicated cages varying in the proportion of resistant aphids and recorded successful parasitism for each wasp species over time. As expected, A. ervi outcompeted P. pequodorum in cages containing only susceptible aphids. However, P. pequodorum not only persisted, but was the superior competitor in populations containing any proportion (20–100%) of resistant aphids (20–100%). Smaller scale, better replicated competition cage studies corroborated this finding, and no-competition and behavioral assays provide insight into the processes mediating competition. Genetic variation, including that acquired via infection with protective symbionts, may provide a supply of hosts susceptible only to particular enemies, mediating competition with effects on community richness and stability.

Recent evolution of extreme cestode growth suppression by a vertebrate host

Parasites can be a major cause of natural selection on hosts, which consequently evolve a variety of strategies to avoid, eliminate, or tolerate infection. When ecologically similar host populations present disparate infection loads, this natural variation can reveal immunological strategies underlying adaptation to infection and population divergence. For instance, the tapeworm Schistocephalus solidus persistently infects 0-80% of threespine stickleback (Gasterosteus aculeatus) in lakes on Vancouver Island. To test whether these heterogeneous infection rates result from evolved differences in immunity, we experimentally exposed laboratory-reared fish from ecologically similar high-infection and no-infection populations to controlled doses of Schistocephalus We observed heritable between-population differences in several immune traits: Fish from the naturally uninfected population initiated a stronger granulocyte response to Schistocephalus infection, and their granulocytes constitutively generate threefold more reactive oxygen species in cell culture. Despite these immunological differences, Schistocephalus was equally successful at establishing initial infections in both host populations. However, the no-infection fish dramatically suppressed tapeworm growth relative to high-infection fish, and parasite size was intermediate in F1 hybrid hosts. Our results show that stickleback recently evolved heritable variation in their capacity to suppress helminth growth by two orders of magnitude. Data from many natural populations indicate that growth suppression is widespread but not universal and, when present, is associated with reduced infection prevalence. Host suppression of helminth somatic growth may be an important immune strategy that aids in parasite clearance or in mitigating the fitness costs of persistent infection.

Eco-Evolutionary Theory and Insect Outbreaks

Eco-evolutionary theory argues that population cycles in consumer-resource interactions are partly driven by natural selection, such that changes in densities and changes in trait values are mutually reinforcing. Evidence that the theory explains cycles in nature, however, is almost nonexistent. Experimental tests of model assumptions are logistically impractical for most organisms, while for others, evidence that population cycles occur in nature is lacking. For insect baculoviruses in contrast, tests of model assumptions are straightforward, and there is strong evidence that baculoviruses help drive population cycles in many insects, including the gypsy moth that we study here. We therefore used field experiments with the gypsy moth baculovirus to test two key assumptions of eco-evolutionary models of host-pathogen population cycles: that reduced host infection risk is heritable and that it is costly. Our experiments confirm both assumptions, and inserting parameters estimated from our data into eco-evolutionary insect-outbreak models gives cycles closely resembling gypsy moth outbreak cycles in North America, whereas standard models predict unrealistic stable equilibria. Our work shows that eco-evolutionary models are useful for explaining outbreaks of forest insect defoliators, while widespread observations of intense selection on defoliators in nature and of heritable and costly resistance in defoliators in the lab together suggest that eco-evolutionary dynamics may play a general role in defoliator outbreaks.

If you see one, have you seen them all?: Community-wide effects of insecticide cross-resistance in zooplankton populations near and far from agriculture

The worldwide use of pesticides has led to increases in agricultural yields by reducing crop losses. However, increased pesticide use has resulted in pesticide-resistant pest species and recent studies have discovered pesticide-resistance in non-target species living close to farms. Such increased tolerance not only affects the species, but can alter the entire food web. Given that some species can evolve not only resistance to a single pesticide, but also cross-resistance to other pesticides that share the same mode of action, one would predict that cross-resistance to pesticides would also have effects on the entire community and affect community stability. To address this hypothesis, we conducted an outdoor mesocosm experiment comprised of 200 identical aquatic communities with phytoplankton, periphyton, and leopard frog (Lithobates pipiens) tadpoles. To these communities, we added one of four Daphnia pulex populations that we previously discovered were either resistant or sensitive to the insecticide of chlorpyrifos as a result of living close to or far from agriculture, respectively. We then exposed the communities to either no insecticide or three different concentrations of AChE-inhibiting insecticides (chlorpyrifos, malathion or carbaryl) or sodium channel-inhibiting insecticides (permethrin or cypermethrin). We discovered that communities containing sensitive Daphnia pulex experienced phytoplankton blooms and subsequent cascades through all trophic groups including amphibians at moderate to high concentrations of all five insecticides. However, communities containing resistant D. pulex were buffered from these effects at low to moderate concentrations of all AChE-inhibiting insecticides, but were not buffered against the pyrethroid insecticides. These data suggest that a simple change in the population-level resistance of zooplankton to a single insecticide can have widespread consequences for community stability and that the effects can be extrapolated to a wide variety of pesticides that share the same mode of action.

Costs of resistance and infection by a generalist pathogen

Pathogen infection is typically costly to hosts, resulting in reduced fitness. However, pathogen exposure may also come at a cost even if the host does not become infected. These fitness reductions, referred to as “resistance costs”, are inducible physiological costs expressed as a result of a trade‐off between resistance to a pathogen and aspects of host fitness (e.g., reproduction). Here, we examine resistance and infection costs of a generalist fungal pathogen (Metschnikowia bicuspidata) capable of infecting a number of host species. Costs were quantified as reductions in host lifespan, total reproduction, and mean clutch size as a function of pathogen exposure (resistance cost) or infection (infection cost). We provide empirical support for infection costs and modest support for resistance costs for five Daphnia host species. Specifically, only one host species examined incurred a significant cost of resistance. This species was the least susceptible to infection, suggesting the possibility that host susceptibility to infection is associated with the detectability and size of resistance cost. Host age at the time of pathogen exposure did not influence the magnitude of resistance or infection cost. Lastly, resistant hosts had fitness values intermediate between unexposed control hosts and infected hosts. Although not statistically significant, this could suggest that pathogen exposure does come at some marginal cost. Taken together, our findings suggest that infection is costly, resistance costs may simply be difficult to detect, and the magnitude of resistance cost may vary among host species as a result of host life history or susceptibility.

Understanding the ecology and evolution of host-parasite interactions across scales

Predicting the emergence, spread and evolution of parasites within and among host populations requires insight to both the spatial and temporal scales of adaptation, including an understanding of within-host up through community-level dynamics. Although there are very few pathosystems for which such extensive data exist, there has been a recent push to integrate studies performed over multiple scales or to simultaneously test for dynamics occurring across scales. Drawing on examples from the literature, with primary emphasis on three diverse host-parasite case studies, we first examine current understanding of the spatial structure of host and parasite populations, including patterns of local adaptation and spatial variation in host resistance and parasite infectivity. We then explore the ways to measure temporal variation and dynamics in host-parasite interactions and discuss the need to examine change over both ecological and evolutionary timescales. Finally, we highlight new approaches and syntheses that allow for simultaneous analysis of dynamics across scales. We argue that there is great value in examining interplay among scales in studies of host-parasite interactions.

Plasticity, not genetic variation, drives infection success of a fungal parasite

Hosts strongly influence parasite fitness. However, it is challenging to disentangle host effects on genetic vs plasticity-driven traits of parasites, since parasites can evolve quickly. It remains especially difficult to determine the causes and magnitude of parasite plasticity. In successive generations, parasites may respond plastically to better infect their current type of host, or hosts may produce generally 'good' or 'bad' quality parasites. Here, we characterized parasite plasticity by taking advantage of a system in which the parasite (the yeast Metschnikowia bicuspidata, which infects Daphnia) has no detectable heritable variation, preventing rapid evolution. In experimental infection assays, we found an effect of rearing host genotype on parasite infectivity, where host genotypes produced overall high or low quality parasite spores. Additionally, these plastically induced differences were gained or lost in just a single host generation. Together, these results demonstrate phenotypic plasticity in infectivity driven by the within-host rearing environment. Such plasticity is rarely investigated in parasites, but could shape epidemiologically important traits.

Assessing the energetic costs and trade-offs of a PHA-induced inflammation in the subterranean rodent Ctenomys talarum: immune response in growing tuco-tucos

A traditional approach used to assess whether immune defense is costly is to explore the existence of trade-offs between immunity and other functions; however, quantitative studies of the energetic costs associated with the activation of the immune system are scarce. We assessed the magnitude of a PHA-triggered immune response and the associated energetic costs in 60-day old Ctenomys talarum. We expected that the magnitude of the macroscopic inflammatory response to PHA is lower in young tuco-tucos compared with that of adults, given the allocation of substantial energy to growth, and that the magnitude of the inflammation is lower in male pups compared to females, due to the higher investment in growth of the larger sex. Concomitantly, we expected that the pups challenged with PHA show an increase in oxygen consumption compared to control animals and that a positive association exists between magnitude of the PHA-induced inflammation and oxygen consumption. Contrary to what was expected, young tuco-tucos mounted a higher inflammatory response compared with adults and there were no differences in the magnitude of this response between sexes. The inflammatory response induced by a PHA injection did not represent a significant energetic cost for young tuco-tucos. There were no differences in oxygen consumption between PHA-injected and control animals, and tuco-tucos that mounted a higher inflammatory response to PHA did not show higher oxygen consumption. Energy expenditure, however, is not the only physiological cost involved in trade-offs between immune response and various functions of the organism, and other currencies are discussed.