Specific and non-specific defense against parasitic attack

Could care giving have altered the evolution of human immune strategies?

Life history theory indicates that individuals/species with a slow pace of life invest more in acquired than innate immunity. Factors that decrease the pace of life and predict greater investment in acquired immunity include increased nutritional resources, increased pathogen exposure and decreased risk of extrinsic mortality. Common care behaviors given to sick individuals produce exactly these effects: provisioning increases nutritional resources; hygiene assistance increases disease exposure of carers; and protection can reduce the risk of extrinsic mortality to sick individuals. This study, therefore, investigated under what conditions care giving behaviors might impact immune strategy and pace of life. The study employed an agent-based model approach that simulated populations with varying levels of care giving, disease mortality, disease transmissibility, and extrinsic mortality, enabling measurements of how the immune strategy and age structure of the populations changed over evolutionary time. We used multiple regressions to examine the effects of these variables on immune strategy and the age structure of the population. The findings supported our predictions that care was selected for an acquired immunity. However, the pace of life did not slow as expected. Instead, the population shifted to a faster, but also more cost-intensive reproductive strategy in which care improved child survival by subsidizing the development of acquired immune responses.

Host-pathogen coevolution promotes the evolution of general, broad-spectrum resistance and reduces foreign pathogen spillover risk

Genetic variation for disease resistance within host populations can strongly impact the spread of endemic pathogens. In plants, recent work has shown that within-population variation in resistance can also affect the transmission of foreign spillover pathogens if that resistance is general. However, most hosts also possess specific resistance mechanisms that provide strong defenses against coevolved endemic pathogens. Here we use a modeling approach to ask how antagonistic coevolution between hosts and their endemic pathogen at the specific resistance locus can affect the frequency of general resistance, and therefore a host's vulnerability to foreign pathogens. We develop a two-locus model with variable recombination that incorporates both general (resistance to all pathogens) and specific (resistance to endemic pathogens only). We find that introducing coevolution into our model greatly expands the regions where general resistance can evolve, decreasing the risk of foreign pathogen invasion. Furthermore, coevolution greatly expands which conditions maintain polymorphisms at both resistance loci, thereby driving greater genetic diversity within host populations. This genetic diversity often leads to positive correlations between host resistance to foreign and endemic pathogens, similar to those observed in natural populations. However, if resistance loci become linked, the resistance correlations can shift to negative. If we include a third, linkage modifying locus into our model, we find that selection often favors complete linkage. Our model demonstrates how coevolutionary dynamics with an endemic pathogen can mold the resistance structure of host populations in ways that affect its susceptibility to foreign pathogen spillovers, and that the nature of these outcomes depends on resistance costs, as well as the degree of linkage between resistance genes.

Specific resistance prevents the evolution of general resistance and facilitates disease emergence

Host-shifts, where pathogens jump from an ancestral host to a novel host, can be facilitated or impeded by standing variation in disease resistance, but only if resistance provides broad-spectrum general resistance against multiple pathogen species. Host resistance comes in many forms and includes both general resistance, as well as specific resistance, which may only be effective against a single pathogen species or even genotype. However, most evolutionary models consider only one of these forms of resistance, and we have less understanding of how these two forms of resistance evolve in tandem. Here, we develop a model that allows for the joint evolution of specific and general resistance and asks if the evolution of specific resistance drives a decrease in the evolution of general resistance. We also explore how these evolutionary outcomes affect the risk of foreign pathogen invasion and persistence. We show that in the presence of a single endemic pathogen, the two forms of resistance are strongly exclusionary. Critically, we find that specific resistance polymorphisms can prevent the evolution of general resistance, facilitating the invasion of foreign pathogens. We also show that specific resistance polymorphisms are a necessary condition for the successful establishment of foreign pathogens following invasion, as they prevent the exclusion of the foreign pathogen by the more transmissible endemic pathogen. Our results demonstrate the importance of considering the joint evolution of multiple forms of resistance when evaluating a population's susceptibility to foreign pathogens.

Bet-hedging in innate and adaptive immune systems

Immune system evolution is shaped by the fitness costs and trade-offs associated with mounting an immune response. Costs that arise mainly as a function of the magnitude of investment, including energetic and immunopathological costs, are well-represented in studies of immune system evolution. Less well considered, however, are the costs of immune cell plasticity and specialization. Hosts in nature encounter a large diversity of microbes and parasites that require different and sometimes conflicting immune mechanisms for defense, but it takes precious time to recognize and correctly integrate signals for an effective polarized response. In this perspective, we propose that bet-hedging can be a viable alternative to plasticity in immune cell effector function, discuss conditions under which bet-hedging is likely to be an advantageous strategy for different arms of the immune system, and present cases from both innate and adaptive immune systems that suggest bet-hedging at play.

Mesenchymal Stromal/Stem Cells-Derived Exosomes as an Antimicrobial Weapon for Orodental Infections

The oral cavity as the second most various microbial community in the body contains a broad spectrum of microorganisms which are known as the oral microbiome. The oral microbiome includes different types of microbes such as bacteria, fungi, viruses, and protozoa. Numerous factors can affect the equilibrium of the oral microbiome community which can eventually lead to orodental infectious diseases. Periodontitis, dental caries, oral leukoplakia, oral squamous cell carcinoma are some multifactorial infectious diseases in the oral cavity. In defending against infection, the immune system has an essential role. Depending on the speed and specificity of the reaction, immunity is divided into two different types which are named the innate and the adaptive responses but also there is much interaction between them. In these responses, different types of immune cells are present and recent evidence demonstrates that these cell types both within the innate and adaptive immune systems are capable of secreting some extracellular vesicles named exosomes which are involved in the response to infection. Exosomes are 30-150 nm lipid bilayer vesicles that consist of variant molecules, including proteins, lipids, and genetic materials and they have been associated with cell-to-cell communications. However, some kinds of exosomes can be effective on the pathogenicity of various microorganisms and promoting infections, and some other ones have antimicrobial and anti-infective functions in microbial diseases. These discrepancies in performance are due to the origin of the exosome. Exosomes can modulate the innate and specific immune responses of host cells by participating in antigen presentation for activation of immune cells and stimulating the release of inflammatory factors and the expression of immune molecules. Also, mesenchymal stromal/stem cells (MSCs)-derived exosomes participate in immunomodulation by different mechanisms. Ease of expansion and immunotherapeutic capabilities of MSCs, develop their applications in hundreds of clinical trials. Recently, it has been shown that cell-free therapies, like exosome therapies, by having more advantages than previous treatment methods are emerging as a promising strategy for the treatment of several diseases, in particular inflammatory conditions. In orodental infectious disease, exosomes can also play an important role by modulating immunoinflammatory responses. Therefore, MSCs-derived exosomes may have potential therapeutic effects to be a choice for controlling and treatment of orodental infectious diseases.

The Evolutionary Consequences of Stepwise Infection Processes

Molecular and cellular studies reveal that the resistance of hosts to parasites and pathogens is a cascade-like process with multiple steps required to be passed for successful infection. By contrast, much of evolutionary reasoning is based on strongly simplified, one- or two-step infection processes with simple genetics or on resistance being a quantitative trait. Here we attempt a conceptual unification of these two perspectives with the aim of cross-fostering research and filling some of the gaps in our concepts of the ecology and evolution of disease. This conceptual unification has a profound impact on the way we understand the genetics and evolution of host resistance, ecological immunity, evolution of virulence, defence portfolios, and host-pathogen coevolution.

Do Coinfections Maintain Genetic Variation in Parasites?

Host individuals are often infected with multiple, potentially interacting parasite species and genotypes. Such coinfections have consequences for epidemiology, disease severity, and evolution of parasite virulence. As fitness effects of coinfection can be specific to interacting parasite genotypes, coinfections may induce high fitness variation among parasite genotypes. We argue that such interactions can be an important mechanism maintaining genetic variation in parasite traits such as infectivity and virulence. We also argue that such interactions may slow coevolutionary dynamics between hosts and parasites. This is because, instead of depending only on host genotype, parasite fitness may be determined by average infection success across all coinfection scenarios.

Coevolutionary feedback elevates constitutive immune defence: a protein network model

Background

Organisms have evolved a variety of defence mechanisms against natural enemies, which are typically used at the expense of other life history components. Induced defence mechanisms impose minor costs when pathogens are absent, but mounting an induced response can be time-consuming. Therefore, to ensure timely protection, organisms may partly rely on constitutive defence despite its sustained cost that renders it less economical. Existing theoretical models addressing the optimal combination of constitutive versus induced defence focus solely on host adaptation and ignore the fact that the efficacy of protection depends on genotype-specific host-parasite interactions. Here, we develop a signal-transduction network model inspired by the invertebrate innate immune system, in order to address the effect of parasite coevolution on the optimal combination of constitutive and induced defence.

Results

Our analysis reveals that coevolution of parasites with specific immune components shifts the host’s optimal allocation from induced towards constitutive immunity. This effect is dependent upon whether receptors (for detection) or effectors (for elimination) are subjected to parasite counter-evolution. A parasite population subjected to a specific immune receptor can evolve heightened genetic diversity, which makes parasite detection more difficult for the hosts. We show that this coevolutionary feedback renders the induced immune response less efficient, forcing the hosts to invest more heavily in constitutive immunity. Parasites diversify to escape elimination by a specific effector too. However, this diversification does not alter the optimal balance between constitutive and induced defence: the reliance on constitutive defence is promoted by the receptor’s inability to detect, but not the effectors’ inability to eliminate parasites. If effectors are useless, hosts simply adapt to tolerate, rather than to invest in any defence against parasites. These contrasting results indicate that evolutionary feedback between host and parasite populations is a key factor shaping the selection regime for immune networks facing antagonistic coevolution.

Conclusion

Parasite coevolution against specific immune defence alters the prediction of the optimal use of defence, and the effect of parasite coevolution varies between different immune components.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0667-3) contains supplementary material, which is available to authorized users.

Trade-offs between acquired and innate immune defenses in humans

Immune defenses provide resistance against infectious disease that is critical to survival. But immune defenses are costly, and limited resources allocated to immunity are not available for other physiological or developmental processes. We propose a framework for explaining variation in patterns of investment in two important subsystems of anti-pathogen defense: innate (non-specific) and acquired (specific) immunity. The developmental costs of acquired immunity are high, but the costs of maintenance and activation are relatively low. Innate immunity imposes lower upfront developmental costs, but higher operating costs. Innate defenses are mobilized quickly and are effective against novel pathogens. Acquired responses are less effective against novel exposures, but more effective against secondary exposures due to immunological memory. Based on their distinct profiles of costs and effectiveness, we propose that the balance of investment in innate versus acquired immunity is variable, and that this balance is optimized in response to local ecological conditions early in development. Nutritional abundance, high pathogen exposure and low signals of extrinsic mortality risk during sensitive periods of immune development should all favor relatively higher levels of investment in acquired immunity. Undernutrition, low pathogen exposure, and high mortality risk should favor innate immune defenses. The hypothesis provides a framework for organizing prior empirical research on the impact of developmental environments on innate and acquired immunity, and suggests promising directions for future research in human ecological immunology.

Immunity, suicide or both? Ecological determinants for the combined evolution of anti-pathogen defense systems

Background

Parasite-host arms race is one of the key factors in the evolution of life. Most cellular life forms, in particular prokaryotes, possess diverse forms of defense against pathogens including innate immunity, adaptive immunity and programmed cell death (altruistic suicide). Coevolution of these different but interacting defense strategies yields complex evolutionary regimes.

Results

We develop and extensively analyze a computational model of coevolution of different defense strategies to show that suicide as a defense mechanism can evolve only in structured populations and when the attainable degree of immunity against pathogens is limited. The general principle of defense evolution seems to be that hosts do not evolve two costly defense mechanisms when one is sufficient. Thus, the evolutionary interplay of innate immunity, adaptive immunity and suicide, leads to an equilibrium state where the combination of all three defense strategies is limited to a distinct, small region of the parameter space. The three strategies can stably coexist only if none of them are highly effective. Coupled adaptive immunity-suicide systems, the existence of which is implied by the colocalization of genes for the two types of defense in prokaryotic genomes, can evolve either when immunity-associated suicide is more efficacious than other suicide systems or when adaptive immunity functionally depends on the associated suicide system.

Conclusions

Computational modeling reveals a broad range of outcomes of coevolution of anti-pathogen defense strategies depending on the relative efficacy of different mechanisms and population structure. Some of the predictions of the model appear compatible with recent experimental evolution results and call for additional experiments.

Lotka-Volterra dynamics kills the Red Queen: population size fluctuations and associated stochasticity dramatically change host-parasite coevolution

BACKGROUND

Host-parasite coevolution is generally believed to follow Red Queen dynamics consisting of ongoing oscillations in the frequencies of interacting host and parasite alleles. This belief is founded on previous theoretical work, which assumes infinite or constant population size. To what extent are such sustained oscillations realistic?

RESULTS

Here, we use a related mathematical modeling approach to demonstrate that ongoing Red Queen dynamics is unlikely. In fact, they collapse rapidly when two critical pieces of realism are acknowledged: (i) population size fluctuations, caused by the antagonism of the interaction in concordance with the Lotka-Volterra relationship; and (ii) stochasticity, acting in any finite population. Together, these two factors cause fast allele fixation. Fixation is not restricted to common alleles, as expected from drift, but also seen for originally rare alleles under a wide parameter space, potentially facilitating spread of novel variants.

CONCLUSION

Our results call for a paradigm shift in our understanding of host-parasite coevolution, strongly suggesting that these are driven by recurrent selective sweeps rather than continuous allele oscillations.

Parasite burden and the insect immune response: interpopulation comparison

The immune response affects host's survival and reproductive success. Insurmountable immune function has not evolved because it is costly and there is a trade-off between other life-history traits. In previous studies several factors such as diet and temperature have been proposed to cause interpopulation differences in immune response. Moreover, the insect immune system may be functionally more protective upon secondary exposure, thus infection history may associate with the immune response. Here we measured how geographical location and parasite burden is related to variation in immune response between populations. We included 13 populations of the Northern Damselfly Coenagrion hastulatum (Odonata: Coenagrionidae) in Finland over a latitudinal range of 880 km to this study. We found that water mites associated strongly with the immune response at interpopulation level: the more the mites, the higher the immune response. Also, in an alternative model based on AIC, latitude and individual size associated with the immune response. In turn, endoparasitic gregarines did not affect the immune response. To conclude, a positive interpopulation association between the immune response and the rate of water mite infection may indicate (i) local adaptation to chronic parasite stress, (ii) effective 'induced' immune response against parasites, or (iii) a combined effect of both of these.

Two-step infection processes can lead to coevolution between functionally independent infection and resistance pathways

There is growing evidence that successful infection of hosts by pathogens requires a series of independent steps. However, how multistep infection processes affect host-pathogen coevolution is unclear. We present a coevolutionary model, inspired by empirical observations from a range of host-pathogen systems, where the infection process consists of the following two steps: the first is for the pathogen to recognize and locate a suitable host, and the second is to exploit the host while evading immunity. Importantly, these two steps conform to different models of infection genetics: inverse-gene-for-gene (IGFG) and gene-for-gene (GFG), respectively. We show that coevolution under this scenario can lead to coupled gene frequency changes across these two systems. In particular, selection often favors pathogens that are infective at the first, IGFG, step and hosts that are resistant at the second, GFG, step. Hence, there may be signals of positive selection between functionally independent systems whenever there are multistep processes determining resistance and infectivity. Such multistep infection processes are a fundamental, but overlooked feature of many host-pathogen interactions, and have important consequences for our understanding of host-pathogen coevolution.

The costs of evolving resistance in heterogeneous parasite environments

The evolution of host resistance to parasites, shaped by associated fitness costs, is crucial for epidemiology and maintenance of genetic diversity. Selection imposed by multiple parasites could be a particularly strong constraint, as hosts either accumulate costs of multiple specific resistances or evolve a more costly general resistance mechanism. We used experimental evolution to test how parasite heterogeneity influences the evolution of host resistance. We show that bacterial host populations evolved specific resistance to local bacteriophage parasites, regardless of whether they were in single or multiple-phage environments, and that hosts evolving with multiple phages were no more resistant to novel phages than those evolving with single phages. However, hosts from multiple-phage environments paid a higher cost, in terms of population growth in the absence of phage, for their evolved specific resistances than those from single-phage environments. Given that in nature host populations face selection pressures from multiple parasite strains and species, our results suggest that costs may be even more critical in shaping the evolution of resistance than previously thought. Furthermore, our results highlight that a better understanding of resistance costs under combined control strategies could lead to a more 'evolution-resistant' treatment of disease.

Resolving the infection process reveals striking differences in the contribution of environment, genetics and phylogeny to host-parasite interactions

Background

Infection processes consist of a sequence of steps, each critical for the interaction between host and parasite. Studies of host-parasite interactions rarely take into account the fact that different steps might be influenced by different factors and might, therefore, make different contributions to shaping coevolution. We designed a new method using the Daphnia magna - Pasteuria ramosa system, one of the rare examples where coevolution has been documented, in order to resolve the steps of the infection and analyse the factors that influence each of them.

Results

Using the transparent Daphnia hosts and fluorescently-labelled spores of the bacterium P. ramosa, we identified a sequence of infection steps: encounter between parasite and host; activation of parasite dormant spores; attachment of spores to the host; and parasite proliferation inside the host. The chances of encounter had been shown to depend on host genotype and environment. We tested the role of genetic and environmental factors in the newly described activation and attachment steps. Hosts of different genotypes, gender and species were all able to activate endospores of all parasite clones tested in different environments; suggesting that the activation cue is phylogenetically conserved. We next established that parasite attachment occurs onto the host oesophagus independently of host species, gender and environmental conditions. In contrast to spore activation, attachment depended strongly on the combination of host and parasite genotypes.

Conclusions

Our results show that different steps are influenced by different factors. Host-type-independent spore activation suggests that this step can be ruled out as a major factor in Daphnia-Pasteuria coevolution. On the other hand, we show that the attachment step is crucial for the pronounced genetic specificities of this system. We suggest that this one step can explain host population structure and could be a key force behind coevolutionary cycles. We discuss how different steps can explain different aspects of the coevolutionary dynamics of the system: the properties of the attachment step, explaining the rapid evolution of infectivity and the properties of later parasite proliferation explaining the evolution of virulence. Our study underlines the importance of resolving the infection process in order to better understand host-parasite interactions.

Rapid genetic diversification and high fitness penalties associated with pathogenicity evolution in a plant virus

Under the gene-for-gene model of host-pathogen coevolution, recognition of pathogen avirulence factors by host resistance factors triggers host defenses and limits infection. Theory predicts that the evolution of higher levels of pathogenicity will be associated with fitness penalties and that the cost of higher pathogenicity must be much smaller than that of not infecting the host. The analysis of pathogenicity costs is of academic and applied relevance, as these are determinants for the success of resistance genes bred into crops for disease control. However, most previous attempts of addressing this issue in plant pathogens yielded conflicting and inconclusive results. We have analyzed the costs of pathogenicity in pepper-infecting tobamoviruses defined by their ability to infect pepper plants with different alleles at the resistance locus L. We provide conclusive evidence of pathogenicity-associated costs by comparison of pathotype frequency with the fraction of the crop carrying the various resistance alleles, by timescaled phylogenies, and by temporal analyses of population dynamics and selection pressures using nucleotide sequences. In addition, experimental estimates of relative fitness under controlled conditions also provided evidence of high pathogenicity costs. These high pathogenicity costs may reflect intrinsic properties of plant virus genomes and should be considered in future models of host-parasite coevolution.

Sympatric and allopatric divergence of MHC genes in threespine stickleback

Parasites can strongly affect the evolution of their hosts, but their effects on host diversification are less clear. In theory, contrasting parasite communities in different foraging habitats could generate divergent selection on hosts and promote ecological speciation. Immune systems are costly to maintain, adaptable, and an important component of individual fitness. As a result, immune system genes, such as those of the Major Histocompatibility Complex (MHC), can change rapidly in response to parasite-mediated selection. In threespine stickleback (Gasterosteus aculeatus), as well as in other vertebrates, MHC genes have been linked with female mating preference, suggesting that divergent selection acting on MHC genes might influence speciation. Here, we examined genetic variation at MHC Class II loci of sticklebacks from two lakes with a limnetic and benthic species pair, and two lakes with a single species. In both lakes with species pairs, limnetics and benthics differed in their composition of MHC alleles, and limnetics had fewer MHC alleles per individual than benthics. Similar to the limnetics, the allopatric population with a pelagic phenotype had few MHC alleles per individual, suggesting a correlation between MHC genotype and foraging habitat. Using a simulation model we show that the diversity and composition of MHC alleles in a sympatric species pair depends on the amount of assortative mating and on the strength of parasite-mediated selection in adjacent foraging habitats. Our results indicate parallel divergence in the number of MHC alleles between sympatric stickleback species, possibly resulting from the contrasting parasite communities in littoral and pelagic habitats of lakes.

Maintenance of genetic variation in immune defense of a freshwater snail: role of environmental heterogeneity

Natural populations often show genetic variation in pathogen resistance, which is paradoxal because natural selection is expected to erode genetic variation in fitness-related traits. Several different factors have been suggested to maintain such variation, but their relative importance is still poorly understood. Here we examined if environmental heterogeneity and genetic trade-offs could contribute to the maintenance of genetic variation in immune function of a freshwater snail Lymnaea stagnalis. We assessed the immunocompetence of snails originating from different families and maintained in different feeding treatments (ad libitum feeding, no food) by measuring the density of circulating hemocytes, phenoloxidase activity, and antibacterial activity of snail hemolymph. Food limitation reduced snail immune function, and we found significant among-family variation in hemocyte concentration and PO activity, but not in antibacterial activity. Interestingly, food availability modified the family-level variation observed in PO activity so that the relative immunocompetence of different snail families changed over environmental conditions (G x E interaction). We found no evidence for genetic trade-offs between snail growth and immune defense nor among immune traits. Thus, our findings support the idea that environmental heterogeneity may promote maintenance of genetic variation in immune defense, but also suggest that different immune traits might not respond similarly to environmental variation.

Principles of ecological immunology

Defending self against nonself is a major problem in a world in which individuals are under constant pressure from parasites that gain fitness benefits at a cost to their host. Defences that have evolved are diverse, and range from behavioural adaptations to physiochemical barriers. The immune defence is a final line of protection and is therefore of great importance. Given this importance, variability in immune defence would seem counterintuitive, yet that is what is observed. Ecological immunology attempts to explain this variation by invoking costs and trade-offs, and in turn proposing that the optimal immune defence will vary over environments. Studies in this field have been highly successful in establishing an evolutionary ecology framework around immunology. However, in order enrich our understanding of this area, it is perhaps time to broaden the focus to include parasites as more than simply elicitors of immune responses. In essence, to view immunity as produced by the host, the environment, and the active involvement of parasites.

The evolution of virulence and pathogenicity in plant pathogen populations

The term virulence has a conflicting history among plant pathologists. Here we define virulence as the degree of damage caused to a host by parasite infection, assumed to be negatively correlated with host fitness, and pathogenicity the qualitative capacity of a parasite to infect and cause disease on a host. Selection may act on both virulence and pathogenicity, and their change in parasite populations can drive parasite evolution and host-parasite co-evolution. Extensive theoretical analyses of the factors that shape the evolution of pathogenicity and virulence have been reported in last three decades. Experimental work has not followed the path of theoretical analyses. Plant pathologists have shown greater interest in pathogenicity than in virulence, and our understanding of the molecular basis of pathogenicity has increased enormously. However, little is known regarding the molecular basis of virulence. It has been proposed that the mechanisms of recognition of parasites by hosts will have consequences for the evolution of pathogenicity, but much experimental work is still needed to test these hypotheses. Much theoretical work has been based on evidence from cellular plant pathogens. We review here the current experimental and observational evidence on which to test theoretical hypotheses or conjectures. We compare evidence from viruses and cellular pathogens, mostly fungi and oomycetes, which differ widely in genomic complexity and in parasitism. Data on the evolution of pathogenicity and virulence from viruses and fungi show important differences, and their comparison is necessary to establish the generality of hypotheses on pathogenicity and virulence evolution.

Latitudinal variation of immune defense and sickness behavior in the white-crowned sparrow (Zonotrichia leucophrys)

There is a general trend that parasitism risk declines as latitude increases. Host populations breeding at high latitudes should therefore invest less in costly immune defenses than populations breeding in temperate or tropical zones, although it is unknown if such an effect is mediated by environmental (photoperiodic) or genetic factors or both. Acquired immune function (humoral, cell-mediated) and behavioral sickness responses to lipopolysaccharide (LPS; mimics bacterial infection) were assessed in two subspecies of white-crowned sparrow (Zonotrichia leucophrys) that breed at different latitudes in western North America. Zonotrichia l. gambelii (GWCS) is a high-latitude breeder (47-68 degrees N) while Z. l. pugetensis (PWCS) breeds at temperate latitudes (40-49 degrees N). Captive males of each subspecies were acclimated to (1) a short day (non-breeding) photoperiod (8L:16D), (2) the breeding photoperiod of PWCS (16L:8D), or (3) the breeding photoperiod of GWCS (20L:4D). Photoperiod was manipulated because shorter day lengths may enhance immune function. In support of a genetic effect, humoral responses to diphtheria-tetanus vaccination were significantly higher in PWCS compared to GWCS, regardless of photoperiod. There were no differences in cell-mediated responses to phytohemagglutinin (PHA) between subspecies or among photoperiods. For sickness responses to LPS, a significant interaction between photoperiod and subspecies was found, with long day GWCS producing stronger sickness responses (losing more weight, eating less) than short day GWCS and PWCS on all day lengths. However, these effects were influenced by photoperiodic changes in body condition. In conclusion, we find evidence for genetic control of immune responses across latitude, but no support for environmental (photoperiodic) regulation.

Resistance against heterogeneous sequential infections: experimental studies with a tapeworm and its copepod host

Parasite heterogeneity is thought to be an important factor influencing the likelihood and the dynamics of infection. Previous studies have demonstrated that simultaneous exposure of hosts to a heterogeneous mixture of parasites might increase infection success. Here this view is extended towards the effect of parasite heterogeneity on subsequent infections. Using a system of the tapeworm Schistocephalus solidus and its copepod intermediate host, heterogeneity of the tapeworm surface carbohydrates is investigated, i.e. structures that are potentially recognized by the invertebrate host's immune system. With lectin labelling, a significant proportion of variation in surface carbohydrates is related to differences in worm sibships (i.e. families). Tapeworm sibships were used for experimental exposure of copepods to either homogeneous combinations of tapeworm larvae, i.e. worms derived from the same sibship or heterogeneous mixtures of larvae, and copepods were subsequently challenged with an unrelated larva to study re-infection. Contrary to expectation, neither an effect of parasite heterogeneity on the current infection, nor on re-infection were found. The effect of parasitic heterogeneity on host immunity is therefore complex, potentially involving increased cross-protection on the one hand, with higher costs of raising a more heterogeneous immune response on the other.

Optimality models of phage life history and parallels in disease evolution

Optimality models constitute one of the simplest approaches to understanding phenotypic evolution. Yet they have shortcomings that are not easily evaluated in most organisms. Most importantly, the genetic basis of phenotype evolution is almost never understood, and phenotypic selection experiments are rarely possible. Both limitations can be overcome with bacteriophages. However, phages have such elementary life histories that few phenotypes seem appropriate for optimality approaches. Here we develop optimality models of two phage life history traits, lysis time and host range. The lysis time models show that the optimum is less sensitive to differences in host density than suggested by earlier analytical work. Host range evolution is approached from the perspective of whether the virus should avoid particular hosts, and the results match optimal foraging theory: there is an optimal "diet" in which host types are either strictly included or excluded, depending on their infection qualities. Experimental tests of both models are feasible, and phages provide concrete illustrations of many ways that optimality models can guide understanding and explanation. Phage genetic systems already support the perspective that lysis time and host range can evolve readily and evolve without greatly affecting other traits, one of the main tenets of optimality theory. The models can be extended to more general properties of infection, such as the evolution of virulence and tissue tropism.

Local adaptation and enhanced virulence of Nosema granulosis artificially introduced into novel populations of its crustacean host, Gammarus duebeni

Local adaptation theory predicts that, on average, most parasite species should be locally adapted to their hosts (more suited to hosts from local than distant populations). Local adaptation has been studied for many horizontally transmitted parasites, however, vertically transmitted parasites have received little attention. Here we present the first study of local adaptation in an animal/parasite system where the parasite is vertically transmitted. We investigate local adaptation and patterns of virulence in a crustacean host infected with the vertically transmitted microsporidian Nosema granulosis. Nosema granulosis is vertically transmitted to successive generations of its crustacean host, Gammarus duebeni and infects up to 46% of adult females in natural populations. We investigate local adaptation using artificial horizontal infection of different host populations in the UK. Parasites were artificially inoculated from a donor population into recipient hosts from the sympatric population and into hosts from three allopatric populations in the UK. The parasite was successfully established in hosts from all populations regardless of location, infecting 45% of the recipients. Nosema granulosis was vertically (transovarially) transmitted to 39% of the offspring of artificially infected females. Parasite burden (intensity of infection) in developing embryos differed significantly between host populations and was an order of magnitude higher in the sympatric population, suggesting some degree of host population specificity with the parasite adapted to its local host population. In contrast with natural infections, artificial infection with the parasite resulted in substantial virulence, with reduced host fecundity (24%) and survival (44%) of infected hosts from all the populations regardless of location. We discuss our findings in relation to theories of local adaptation and parasite-host coevolution.

Asymmetry in host and parasitoid diffuse coevolution: when the red queen has to keep a finger in more than one pie

BACKGROUND: Coevolution between pairs of antagonistic species is generally considered an endless "arms race" between attack and defense traits to counteract the adaptive responses of the other species. PRESENTATION OF THE HYPOTHESIS: When more than two species are involved, diffuse coevolution of hosts and parasitoids could be asymmetric because consumers can choose their prey whereas preys do not choose their predator. This asymmetry may lead to differences in the rate of evolution of the antagonistic species in response to selection. The more long-standing the coevolution of a given pair of antagonistic populations, the higher should be the fitness advantage for the consumer. Therefore, the main prediction of the hypothesis is that the consumer trophic level is more likely to win the coevolution race. TESTING THE HYPOTHESIS: We propose testing the asymmetry hypothesis by focusing on the tritrophic system plant/aphid/aphid parasitoid. The analysis of the genetic variability in the virulence of several parasitoid populations and in the defenses of several aphid species or several clones of the same aphid species could be compared. Moreover, the analysis of the neutral population genetic structure of the parasitoid as a function of the aphid host, the plant host and geographic isolation may complement the detection of differences between host and parasitoid trophic specialization. IMPLICATIONS OF THE HYPOTHESIS: Genetic structures induced by the arms race between antagonistic species may be disturbed by asymmetry in coevolution, producing neither rare genotype advantages nor coevolutionary hotspots. Thus this hypothesis profoundly changes our understanding of coevolution and may have important implications in terms of pest management.

Evolutionary ecology of insect immune defenses

Evolutionary ecology seeks to understand the selective reasons for the design features of the immune defense, especially with respect to parasitism. The molecular processes thereby set limitations, such as the failure to recognize an antigen, response specificity, the cost of defense, and the risk of autoimmunity. Sex, resource availability, and interference by parasites also affect a response. In turn, the defense repertoire consists of different kinds of immune responses--constitutive or induced, general or specific--and involves memory and lasting protection. Because the situation often defies intuition, mathematical analysis is typically required to identify the costs and benefits of variation in design, but such studies are few. In all, insect immune defense is much more similar to that of vertebrates than previously thought. In addition, the field is now rapidly becoming revolutionized by molecular data and methods that allow unprecedented access to study evolution in action.

Genetics of anti-parasite resistance in invertebrates

This review summarizes and compares available data on genetic and molecular aspects of resistance in four well-described invertebrate host-parasite systems: snail-schistosome, mosquito-malaria, mosquito-filarial worm, and Drosophila-wasp associations. It underlies that the major components of the immune reaction, such as hemocyte proliferation and/or activation, and production of cytotoxic radicals are common to invertebrate hosts. Identifying genes responsible for naturally occurring resistance will then be helpful to understand the mechanisms of invertebrate immune defenses and to determine how virulence factors are used by parasites to overcome host resistance. Based on these four well-studied models, invertebrate resistance appears as generally determined by one major locus or a few loci, displaying at least partial dominance. Interestingly, specificity of resistance is highly variable and would involve processes other than simple recognition mechanisms. Finally, resistance was shown to be generally costly but is nevertheless observed at high frequencies in many natural populations, suggesting a high potential for host parasite coevolution.

Concurrent Evolution of Resistance and Tolerance to Pathogens

Recent experiments on plant defenses against pathogens or herbivores have shown various patterns of the association between resistance, which reduces the probability of being infected or attacked, and tolerance, which reduces the loss of fitness caused by the infection or attack. Our study describes the simultaneous evolution of these two strategies of defense in a population of hosts submitted to a pathogen. We extended previous approaches by assuming that the two traits are independent (e.g., determined by two unlinked genes), by modeling different shapes of the costs of defenses, and by taking into account the demographic and epidemiological dynamics of the system. We provide novel predictions on the variability and the evolution of defenses. First, resistance and tolerance do not necessarily exclude each other; second, they should respond in different ways to changes in parameters that affect the epidemiology or the relative costs and benefits of defenses; and third, when comparing investments in defenses among different environments, the apparent associations among resistance, tolerance, and fecundity in the absence of parasites can lead to the false conclusion that only one defense trait is costly. The latter result emphasizes the problems of estimating trade-offs and costs among natural populations without knowledge of the underlying mechanisms.

Immunological investments reflect parasite abundance in island populations of Darwin's finches

The evolution of parasite resistance can be influenced by the abundance of parasites in the environment. However, it is yet unresolved whether vertebrates change their investment in immune function in response to variation in parasite abundance. Here, we compare parasite abundance in four populations of small ground finches (Geospiza fuliginosa) in the Galapagos archipelago. We predicted that populations exposed to high parasite loads should invest more in immune defence, or alternatively use a different immunological defence strategy. We found that parasite prevalence and/or infection intensity increased with island size. As predicted, birds on large islands had increased concentrations of natural antibodies and mounted a strong specific antibody response faster than birds on smaller islands. By contrast, the magnitude of cell-mediated immune responses decreased with increasing parasite pressure, i.e. on larger islands. The data support the hypothesis that investments into the immune defence are influenced by parasite-mediated selection. Our results are consistent with the hypothesis that different immunological defence strategies are optimal in parasite-rich and parasite-poor environments.

Evolution of the innate immune system: the worm perspective

Simple model organisms that are amenable to comprehensive experimental analysis can be used to elucidate the molecular genetic architecture of complex traits. They can thereby enhance our understanding of these traits in other organisms, including humans. Here, we describe the use of the nematode Caenorhabditis elegans as a tractable model system to study innate immunity. We detail our current understanding of the worm's immune system, which seems to be characterized by four main signaling cascades: a p38 mitogen-activated protein kinase, a transforming growth factor-beta-like, a programed cell death, and an insulin-like receptor pathway. Many details, especially regarding pathogen recognition and immune effectors, are only poorly characterized and clearly warrant further investigation. We additionally speculate on the evolution of the C. elegans immune system, taking into special consideration the relationship between immunity, stress responses and digestion, the diversification of the different parts of the immune system in response to multiple and/or coevolving pathogens, and the trade-off between immunity and host life history traits. Using C. elegans to address these different facets of host-pathogen interactions provides a fresh perspective on our understanding of the structure and complexity of innate immune systems in animals and plants.

Apparent competition and recovery from infection

We use mathematical models to analyse how the recovery rate from infection influences the fitness of a host in a setting of interspecific competition. We show that sub-optimal immunity against pathogens can be advantageous for the host in the presence of cross-species infection. Weaker immunity allows the parasite to be used as a biological weapon, and this increases the fitness of the host relative to a competitor. A parameter region is observed in which the outcome of competition depends on the initial conditions. We extend this model and consider the dynamics in a spatial setting and find that the outcome depends on the migration rate of the host species. At low migration rates, coexistence of the host species is possible across space. For higher migration rates, the host species characterized by a lower recovery rate can invade the territory of its competitor. Finally, we study these dynamics in an evolutionary setting. Although a lower recovery rate from infection can increase the competitive ability of a species, we find that evolution maximizes the recovery rate and minimizes parasite burden. The models presented are related to the concept of apparent competition, and our results are discussed in relation to both theoretical and empirical studies.

Parasitic exploitation as an engine of diversity

Parasitic exploitation occurs within and between a wide variety of taxa in a plethora of diverse contexts. Theoretical and empirical analyses indicate that parasitic exploitation can generate substantial genetic and phenotypic polymorphism within species. Under some circumstances, parasitic exploitation may also be an important factor causing reproductive isolation and promoting speciation. Here we review research relevant to the relationship between parasitic exploitation, within species-polymorphism, and speciation in some of the major arenas in which such exploitation has been studied. This includes research on the vertebrate major histocompatibility loci, plant-pathogen interactions, the evolution of sexual reproduction, intragenomic conflict, sexual conflict, kin mimicry and social parasitism, tropical forest diversity and the evolution of language. We conclude by discussing some of the issues raised by comparing the effect of parasitic exploitation on polymorphism and speciation in different contexts.

Invertebrate ecological immunology

Ecological immunology is a rapidly expanding field that examines the causes and consequences of variation in immune function in the context of evolution and of ecology. Millions of invertebrate species rely solely on innate immunity, compared with only 45,000 vertebrate species that rely additionally on an acquired immune system. Despite this difference in diversity, most studies of ecological immunology focus on vertebrates. Here we review recent progress derived largely from the mechanistic analysis of invertebrate innate immunity. Using this empirical base, we pose general questions in areas that are of central importance for the development of ecological immunology.

Specific versus nonspecific immune defense in the bumblebee, Bombus terrestris L

Hosts vary in both their strength of response to a general immunological insult and in their specific susceptibility to different parasite species or different strains of the same parasite. The variation in the general immune response is considered a result of the costs imposed by selection on defended individuals. The variation in the specific response may originate from variation in host and parasite genotypes and is a requirement for frequency-dependent selection. The relationship between these two fundamental aspects of defense has only rarely been studied. Using the bumblebee Bombus terrestris and its gut trypanosomal parasite Crithidia bombi we found that the host's specific response profile toward different strains correlates negatively with its level of response to a general insult. This is the opposite result one would expect if the level of general response were simply a measure of immunological quality (immunocompetence). Rather, it suggests that there is some form of a trade-off between these two fundamental aspects of the immune system. These results, therefore, shed an important light on the possible constraints that affect the evolution of the immune system and particularly the trade-off between different arms of the immune system.

Shared control of epidemiological traits in a coevolutionary model of host-parasite interactions

Most models concerning the evolution of a parasite's virulence and its host's resistance assume that each component of the relationship (transmission, virulence, recovery, etc.) is controlled by either the host or the parasite but not by both. We present a model that describes the coevolution of host and parasite, assuming that the rate of transmission or the virulence depends on both genotypes. The evolution of these traits is constrained by trade-offs that account for costs of defense and attack strategies, in line with previous studies on the separate evolution of the host and the parasite. Considering shared control by the host and the parasite in determining the traits of the relationship leads to several novel predictions. First, the host should evolve maximal investment in defense against parasites with an intermediate replication rate. Second, the evolution of the parasite strongly depends on the way the host's defense is described. Third, the coevolutionary process may lead to decreasing the parasite's virulence as a response to a rise in the host's background mortality, contrary to classical predictions.

Variation in immune defence as a question of evolutionary ecology

The evolutionary-ecology approach to studying immune defences has generated a number of hypotheses that help to explain the observed variance in responses. Here, selected topics are reviewed in an attempt to identify the common problems, connections and generalities of the approach. In particular, the cost of immune defence, response specificity, sexual selection, neighbourhood effects and questions of optimal defence portfolios are discussed. While these questions still warrant further investigation, future challenges are the development of synthetic concepts for vertebrate and invertebrate systems and also of the theory that predicts immune responses based on a priori principles of evolutionary ecology.

Modelling infection as a two-step process combining gene-for-gene and matching-allele genetics

The genetic basis of infection determines the dynamics of host-parasite coevolution and associated phenomena such as local adaptation and the evolution of sex and recombination. Here, we present parasite resistance as a two-step process in which hosts must first detect parasites and then eradicate them; failure at either step results in infection. The model incorporates 'matching-allele' (MA) genetics for detection and 'gene-for-gene' (GFG) genetics for eradication. We found that the oscillatory dynamics were similar to pure GFG genetics when the cost of 'virulence' alleles was low, but resembled pure MA genetics when the cost was high. The magnitude of the cost that switched the dynamics from GFG dominated to MA dominated depended on the genetic architecture of defence (i.e. the number of GFG and MA loci).

Family-level covariation between parasite resistance and mating system in a hermaphroditic freshwater snail

Genetic compatibility, nonspecific defenses, and environmental effects determine parasite resistance. Host mating system (selfing vs. outcrossing) should be important for parasite resistance because it determines the segregation of alleles at the resistance loci and because inbreeding depression may hamper immune defenses. Individuals of a mixed mating hermaphroditic freshwater snail, Lymnaea ovata, are commonly infected by a digenetic trematode parasite, Echinoparyphium recurvatum. We examined covariation between quantitative resistance to novel parasites and mating system by exposing snail families from four populations that differed by their inbreeding coefficients. We found that resistance was unrelated to inbreeding coefficient of the population, suggesting that the more inbred populations did not carry higher susceptibility load than the less inbred populations. Most of the variation in resistance was expressed among the families within the populations. In the population with the lowest inbreeding coefficient, resistance increased with outcrossing rate of the family, as predicted if selfing had led to inbreeding depression. In the other three populations with higher inbreeding coefficients, resistance was unrelated to outcrossing rate. The results suggest that in populations with higher inbreeding some of the genetic load has been purged, uncoupling the predicted relationship between outcrossing rate and resistance. Snail families also displayed crossing reaction norms for resistance when tested in two environments that presented low and high immune challenge, suggesting that genotype-by-environment interactions are important for parasite resistance.