Virulence and local adaptation of a horizontally transmitted parasite

Host ploidy, parasitism and immune defence in a coevolutionary snail-trematode system

We studied the role of host ploidy and parasite exposure on immune defence allocation in a snail-trematode system (Potamopyrgus antipodarum-Microphallus sp.). In the field, haemocyte (the defence cell) concentration was lowest in deep-water habitats where infection is relatively low and highest in shallow-water habitats where infection is common. Because the frequency of asexual triploid snails is positively correlated with depth, we also experimentally studied the role of ploidy by exposing both diploid sexual and triploid asexual snails to Microphallus eggs. We found that triploid snails had lower haemocyte concentrations than did diploids in both parasite-addition and parasite-free treatments. We also found that both triploids and diploids increased their numbers of large granular haemocytes at similar rates after parasite exposure. Because triploid P. antipodarum have been shown to be more resistant to allopatric parasites than diploids, the current results suggest that the increased resistance of triploids is because of intrinsic genetic properties rather than to greater allocation to defence cells. This finding is consistent with recent theory on the advantages of increased ploidy for hosts combating coevolving parasites.

Local adaptation and population structure at a micro-geographical scale of a fungal parasite on its host plant

Local adaptation, which has been detected for several wild pathosystems is influenced by gene flow and recombination. In this study, we investigate local adaptation and population structure at a fine scale in wild populations of a plant-pathogen fungus. We sampled hierarchically strains of Colletotrichum lindemuthianum in a wild population of its host. The analysis of AFLP patterns obtained for 86 strains indicated that: (i) many different haplotypes can be discriminated, although occurrence of recombination could not be shown; (ii) migration between adjacent plants seemed rare during the season; and (iii) neutral diversity is structured according to groups of plants and individual host plants. Furthermore, we tested for the occurrence of local adaptation using a cross-inoculation experiment. Our results showed local adaptation at the scale of the individual host plant. These results indicate that fine-scale dynamics has evolutionary consequences in this pathosystem.

The effect of migration on local adaptation in a coevolving host-parasite system

Antagonistic coevolution between hosts and parasites in spatially structured populations can result in local adaptation of parasites; that is, the greater infectivity of local parasites than foreign parasites on local hosts. Such parasite specialization on local hosts has implications for human health and agriculture. By contrast with classic single-species population-genetic models, theory indicates that parasite migration between subpopulations might increase parasite local adaptation, as long as migration does not completely homogenize populations. To test this hypothesis we developed a system-specific mathematical model and then coevolved replicate populations of the bacterium Pseudomonas fluorescens and a parasitic bacteriophage with parasite only, with host only or with no migration. Here we show that patterns of local adaptation have considerable temporal and spatial variation and that, in the absence of migration, parasites tend to be locally maladapted. However, in accord with our model, parasite migration results in parasite local adaptation, but host migration alone has no significant effect.

The evolution of antifungal peptides in Drosophila

An essential component of the immune system of animals is the production of antimicrobial peptides (AMPs). In vertebrates and termites the protein sequence of some AMPs evolves rapidly under positive selection, suggesting that they may be coevolving with pathogens. However, antibacterial peptides in Drosophila tend to be highly conserved. We have inferred the selection pressures acting on Drosophila antifungal peptides (drosomycins) from both the divergence of drosomycin genes within and between five species of Drosophila and polymorphism data from Drosophila simulans and D. melanogaster. In common with Drosophila antibacterial peptides, there is no evidence of adaptive protein evolution in any of the drosomycin genes, suggesting that they do not coevolve with pathogens. It is possible that this reflects a lack of specific fungal and bacterial parasites in Drosophila populations. The polymorphism data from both species differed from neutrality at one locus, but this was not associated with changes in the protein sequence. The synonymous site diversity was greater in D. simulans than in D. melanogaster, but the diversity both upstream of the genes and at nonsynonymous sites was similar. This can be explained if both upstream and nonsynonymous mutations are slightly deleterious and are removed more effectively from D. simulans due to its larger effective population size.

Spatial scale of local adaptation in a plant-pathogen metapopulation

The rate and scale of gene flow can strongly affect patterns of local adaptation in host-parasite interactions. I used data on regional pathogen occurrence to infer the scale of pathogen dispersal and to identify pathogen metapopulations in the interaction between Plantago lanceolata and its specialist phytopathogen, Podosphaera plantaginis. Frequent extinctions and colonizations were recorded in the metapopulations, suggesting substantial gene flow at this spatial scale. The level of pathogen local adaptation was assessed in a laboratory inoculation experiment at three different scales: in sympatric host populations, in sympatric host metapopulations and in allopatric host metapopulations. I found evidence for adaptation to sympatric host populations, as well as evidence indicating that local adaptation may extend to the scale of the sympatric host metapopulation. There was also variation among the metapopulations in the degree of pathogen local adaptation. This may be explained by regional differences in the rate of migration.

A parasite-mediated life-history shift in Daphnia magna

The impact of parasitism on host populations will be modulated by both genetic variation for susceptibility, and phenotypically plastic-life-history traits that are altered to lessen the fitness consequences of infection. In this study we tested for life-history shifts in the crustacean Daphnia magna following exposure to the horizontally transmitted microsporidian, Glugoides intestinalis. In two separate experiments, we exposed hosts to parasite spores and measured their fecundity relative to controls. We show that host exposed G. intestinalis show fecundity compensation, i.e. hosts shift their life-history strategy towards early production. Our experiments included multiple host genotypes, and subtle differences among them indicated that fecundity compensation could be subject to parasite-mediated natural selection.

The evolution of parasitic diseases

Parasites are characterized by their fitness-reducing effect on their hosts. Studying the evolution of parasitic diseases is an attempt to understand these negative effects as an adaptation of the parasite, the host, both or neither. Dieter Ebert and E. Allen Herre here discuss how the underlying concepts are general and are applicable for all types of disease-producing organisms, broadly defined here as parasites. The evolutionary processes that lead to the maintenance of the harmful effects are believed to be characterized by genetic correlations with other fitness components of the parasite. Depending on the shape of these correlations, any level of virulence can evolve.

Emergence of a convex trade-off between transmission and virulence

Most models of virulence evolution assume that a parasite cannot raise its transmission rate without causing more harm to its host. However, the existence of such trade-off relationships has recently been challenged. Here, we study how a trade-off can emerge from a model that explicitly incorporates within-host dynamics. We find that the existence and the convexity of the trade-off are robust, which implies a definite level of evolutionarily stable virulence (ESV) for the parasite. However, we also show that the dependence of the ESV on parameter values may be very strong. One possible consequence of this sensitivity is that relationships between transmission and virulence observed across populations need not conform to the patterns expected on the basis of a common (fixed) trade-off. We discuss possible experiments and implications of our results for the development of virulence management strategies.

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.

Survival costs of reproduction in the blue tit (Parus caeruleus): a role for blood parasites?

One of the central tenets in life-history theory is that there is a trade-off between current and future reproduction (i.e. a cost of reproduction). The mechanism for this cost of reproduction is, however, largely unknown. One hypothesis is that the high workload during reproduction compromises resistance to parasites and that the resulting increase in parasitaemia has negative effects on the prospects of future survival. Although empirical evidence for a negative relationship between reproductive effort and parasite resistance exists, the causal relationships between reproductive effort, parasite resistance and future reproduction are still unclear. We use a path analytical approach to investigate whether a change in parasite resistance (as measured by intensities of infections by the blood parasite Haemoproteus) after manipulation of reproductive effort, translates into altered survival in female blue tits. Our results show a negative relationship between reproductive effort and parasite resistance, although evident only in first-year breeders. Moreover, we found survival costs of reproduction in first-year breeders. These costs were, however, not mediated by the blood parasite studied.

Parasite-mediated selection in experimental metapopulations of Daphnia magna

In metapopulations, only a fraction of all local host populations may be infected with a given parasite species, and limited dispersal of parasites suggests that colonization of host populations by parasites may involve only a small number of parasite strains. Using hosts and parasites obtained from a natural metapopulation, we studied the evolutionary consequences of invasion by single strains of parasites in experimental populations of the cyclical parthenogen Daphnia magna. In two experiments, each spanning approximately one season, we monitored clone frequency changes in outdoor container populations consisting of 13 and 19 D. magna clones, respectively. The populations were either infected with single strains of the microsporidian parasites Octosporea bayeri or Ordospora colligata or left unparasitized. In both experiments, infection changed the representation of clones over time significantly, indicating parasite-mediated evolution in the experimental populations. Furthermore, the two parasite species changed clone frequencies differently, suggesting that the interaction between infection and competitive ability of the hosts was specific to the parasite species. Taken together, our results suggest that parasite strains that invade local host populations can lead to evolutionary changes in the genetic composition of the host population and that this change is parasite-species specific.

Geographic and within-population structure in variable resistance to parasite species and strains in a vertebrate host

Host resistance to parasites and parasite infectivity may be subject to significant genetically determined variation within species. However, relatively little is known of how this variability is structured in natural vertebrate populations and their macroparasites. A laboratory experiment on host susceptibility-parasite infectivity variation in a wildlife host-parasite system (subspecies of the anuran X. laevis and their polystome flatworms), including 33 pairwise allopatric and sympatric host-parasite combinations (three parasite geographical isolates x 11 host full-sibling families, n=600), revealed a complex pattern of infection success. Results amongst host sibships from different localities suggested that infection success was subject to a highly significant locality x parasite isolate interaction. Within localities, a highly significant sibship x isolate interaction also occurred in one of two groups of sibships examined. The existence of such interactions suggests a potential for frequency-dependent, Red Queen-like selection. Interaction between locality and isolate was partly due to higher infection levels in sympatric combinations, consistent with a general pattern of host-specific adaptation. However, some allopatric combinations produced unpredictably high infection levels, resulting in very asymmetrical cross-infectivity patterns (where the reciprocal cross-infections produced negligible infection). This phylogeographically structured host-parasite system may, therefore, sometimes generate local parasite strains with high infectivity to allopatric hosts. Secondary contact between populations could thus result in significant, and unequal, transfer of parasites.

DIFFERENTIAL PROPAGATION OF THE METAZOAN PARASITE MYXOBOLUS CEREBRALIS BY LIMNODRILUS HOFFMEISTERI, ILYODRILUS TEMPLETONI, AND GENETICALLY DISTINCT STRAINS OF TUBIFEX TUBIFEX

Whirling disease, caused by the parasite Myxobolus cerebralis, has infected rainbow trout (Oncorhynchus mykiss) and other salmonid fish in the western United States, often with devastating results to native populations but without a discernible spatial pattern. The parasite develops in a complex 2-host system in which the aquatic oligochaete Tubifex tubifex is an obligate host. Because substantial differences in whirling disease severity in different areas of North America did not seem explainable by environmental factors or features of the parasite or its fish host, we sought to determine whether ecological or genetic variation within oligochaete host populations may be responsible. We found large differences in compatibility between the parasite and various laboratory strains of T. tubifex that were established from geographic regions with different whirling disease histories. Moreover, 2 closely related species of tubificids, Limnodrilus hoffmeisteri and Ilyodrilus templetoni, which occur naturally in mixed species assemblages with T. tubifex, were incompatible with M. cerebralis. Virulence of the parasite was directly correlated with the numbers of triactinomyxon spores that developed within each strain of T. tubifex. Thus, the level of virulence was directly related to the compatibility between the host strain and the parasite. Genetic analyses revealed relationships that were in agreement with the level of parasite production. Differences in compatibilities between oligochaetes and M. cerebralis may contribute to the spatial variance in the severity of the disease among salmonid populations.

Mixed inoculations of a microsporidian parasite with horizontal and vertical infections

Mixed infections, where more than one parasite genotype is present in a single host, have been suggested to be an important factor in host-parasite interactions. As the host represents a limited resource, co-infecting parasite genotypes are expected to be under resource competition. Competition will not only modify the survival of the co-infecting genotypes, but is also likely to affect total within-host parasite growth as well as host survival and reproduction. We measured parasite infectivity and spore production of seven isolates of Octosporea bayeri (Microsporidia) and their effect on the reproduction and longevity of its host Daphnia magna (Cladocera), after single- or double-isolate inoculations through vertical and horizontal transmission. Double-isolate inoculations increased parasite infectivity and total spore production in horizontal infections, but had no significant effect on host reproduction or longevity. The increase in spore production in double-isolate inoculations was not found when infections occurred vertically. Our results suggest that, depending on the way the infection was acquired, within-host reproduction can increase as a result of parasite genetic diversity, without necessarily affecting the host's fitness. Whether this challenges the current views of virulence evolution depends on the definitions used.

Density-dependence and within-host competition in a semelparous parasite of leaf-cutting ants

Background

Parasite heterogeneity and within-host competition are thought to be important factors influencing the dynamics of host-parasite relationships. Yet, while there have been many theoretical investigations of how these factors may act, empirical data is more limited. We investigated the effects of parasite density and heterogeneity on parasite virulence and fitness using four strains of the entomopathogenic fungus, Metarhizium anisopliae var. anisopliae, and its leaf-cutting ant host Acromyrmex echinatior as the model system.

Results

The relationship between parasite density and infection was sigmoidal, with there being an invasion threshold for an infection to occur (an Allee effect). Although spore production was positively density-dependent, parasite fitness decreased with increasing parasite density, indicating within-host scramble competition. The dynamics differed little between the four strains tested. In mixed infections of three strains the infection-growth dynamics were unaffected by parasite heterogeneity.

Conclusions

The strength of within-host competition makes dispersal the best strategy for the parasite. Parasite heterogeneity may not have effected virulence or the infection dynamics either because the most virulent strain outcompeted the others, or because the interaction involved scramble competition that was impervious to parasite heterogeneity. The dynamics observed may be common for virulent parasites, such as Metarhizium, that produce aggregated transmission stages. Such parasites make useful models for investigating infection dynamics and the impact of parasite competition.

Do Hosts and Parasites Coevolve? Empirical Support from the Schistosoma System

Coevolution between host and parasite is, in principle, a powerful determinant of the biology and genetics of infection and disease. However, coevolution is difficult to demonstrate rigorously in practice and therefore has rarely been observed empirically, particularly in animal-parasite systems. Research on host-schistosome interactions has the potential for making an important contribution to the study of coevolution or reciprocal adaptation. This may be particularly pertinent because schistosomes represent an indirectly transmitted macroparasite, so often overlooked among both theoretical and empirical studies. Here we present ideas and experiments on host-schistosome interactions, in part reviewed from published work but focusing in particular on preliminary novel data from our ongoing studies of potential host-schistosome evolution and coevolution in the laboratory. The article is split into three main sections: we first focus on the evidence for evolution in the host, then in the parasite, before combining both to illustrate the gathering evidence of host-parasite coevolution in the snail-schistosome system. In particular, we demonstrate that genetic architecture, variability, and selective pressures are present for the evolution of resistance and susceptibility, virulence, and infectivity to occur, the mechanisms allowing such polymorphisms to be maintained, and that hosts and parasites appear to have reciprocal effects on each other's phenotype and genotype.

Variation in phenoloxidase activity and its relation to parasite resistance within and between populations of Daphnia magna

Estimates of phenoloxidase (PO) activity have been suggested as a useful indicator of immunocompetence in arthropods, with the idea that high PO activity would indicate high immunocompetence against parasites and pathogens. Here, we test for variation in PO activity among clones of the planktonic crustacean Daphnia magna and its covariation with susceptibility to infections from four different microparasite species (one bacterium and three microsporidia). Strong clonal variation in PO activity was found within and among populations of D. magna, with 45.6% of the total variation being explained by the clone effect. Quantitative measures of parasite success in infection correlated negatively with PO activity when tested across four host populations. However, these correlations disappeared when the data were corrected for population effects. We conclude that PO activity is not a useful measure of resistance to parasites or of immunocompetence within populations of D. magna. We further tested whether D. magna females that are wounded to induce PO activity are more resistant to infections with the bacterium Pasteuria ramosa than non-wounded controls. We found neither a difference in susceptibility nor a difference in disease progression between the induced group and the control group. These results do not question the function of the PO system in arthropod immune response, but rather suggest that immunocompetence cannot be assessed by considering PO activity alone. Immune response is likely to be a multifactorial trait with various host and parasite characteristics playing important roles in its expression.

Virulence in malaria: an evolutionary viewpoint

Malaria parasites cause much morbidity and mortality to their human hosts. From our evolutionary perspective, this is because virulence is positively associated with parasite transmission rate. Natural selection therefore drives virulence upwards, but only to the point where the cost to transmission caused by host death begins to outweigh the transmission benefits. In this review, we summarize data from the laboratory rodent malaria model, Plasmodium chabaudi, and field data on the human malaria parasite, P. falciparum, in relation to this virulence trade-off hypothesis. The data from both species show strong positive correlations between asexual multiplication, transmission rate, infection length, morbidity and mortality, and therefore support the underlying assumptions of the hypothesis. Moreover, the P. falciparum data show that expected total lifetime transmission of the parasite is maximized in young children in whom the fitness cost of host mortality balances the fitness benefits of higher transmission rates and slower clearance rates, thus exhibiting the hypothesized virulence trade-off. This evolutionary explanation of virulence appears to accord well with the clinical and molecular explanations of pathogenesis that involve cytoadherence, red cell invasion and immune evasion, although direct evidence of the fitness advantages of these mechanisms is scarce. One implication of this evolutionary view of virulence is that parasite populations are expected to evolve new levels of virulence in response to medical interventions such as vaccines and drugs.

Experimental evaluation of the relationship between lethal or non-lethal virulence and transmission success in malaria parasite infections

Background

Evolutionary theory suggests that the selection pressure on parasites to maximize their transmission determines their optimal host exploitation strategies and thus their virulence. Establishing the adaptive basis to parasite life history traits has important consequences for predicting parasite responses to public health interventions. In this study we examine the extent to which malaria parasites conform to the predicted adaptive trade-off between transmission and virulence, as defined by mortality. The majority of natural infections, however, result in sub-lethal virulent effects (e.g. anaemia) and are often composed of many strains. Both sub-lethal effects and pathogen population structure have been theoretically shown to have important consequences for virulence evolution. Thus, we additionally examine the relationship between anaemia and transmission in single and mixed clone infections.

Results

Whereas there was a trade-off between transmission success and virulence as defined by host mortality, contradictory clone-specific patterns occurred when defining virulence by anaemia. A negative relationship between anaemia and transmission success was found for one of the parasite clones, whereas there was no relationship for the other. Notably the two parasite clones also differed in a transmission phenotype (gametocyte sex ratio) that has previously been shown to respond adaptively to a changing blood environment. In addition, as predicted by evolutionary theory, mixed infections resulted in increased anaemia. The increased anaemia was, however, not correlated with any discernable parasite trait (e.g. parasite density) or with increased transmission.

Conclusions

We found some evidence supporting the hypothesis that there is an adaptive basis correlating virulence (as defined by host mortality) and transmission success in malaria parasites. This confirms the validity of applying evolutionary virulence theory to biomedical research and adds support to the prediction that partially effective vaccines may select for increasingly virulent malaria parasite strains. By contrast, there was no consistent correlation between transmission and sub-lethal anaemia, a more common outcome of malaria infection. However, overall, the data are not inconsistent with the recent proposal that sub-lethal effects may impose an upper limit on virulence. Moreover, clone specific differences in transmission phenotypes linked to anaemia do suggest that there is considerable adaptive potential relating anaemia and transmission that may lead to uncertain consequences following intervention strategies.

Within-host dynamics of a microsporidium with horizontal and vertical transmission: Octosporea bayeri in Daphnia magna

The fresh-water crustacean Daphnia magna may acquire an infection with the microsporidium Octosporea bayeri either by ingesting spores from the water (horizontally), or directly from its mother (vertically). Due to differences in the time and mechanisms of transmission, horizontal and vertical infections may lead to differences in the growth of the parasite within the host. This may influence parasite virulence, transmission to new hosts, and, consequently, epidemiology and evolution. Here we describe the within-host dynamics of 3 spore-types of O. bayeri from infections that were acquired either horizontally or vertically. In all treatments the number of spores increased exponentially until spore density reached a plateau, suggesting density-dependent within-host growth. The spore types seen differ in their growth dynamics, suggesting different roles in the parasite life-cycle. Horizontally-infected hosts harboured significantly fewer spores than vertically-infected hosts. Further, host survival was affected by infection route, with mortality being higher in horizontal infections than in vertical infections. Our results suggest that different routes of infection have an immediate effect on within-host parasite growth and thus on parasite fitness and epidemiology.

A general theory for the evolutionary dynamics of virulence

Most theory on the evolution of virulence is based on a game-theoretic approach. One potential shortcoming of this approach is that it does not allow the prediction of the evolutionary dynamics of virulence. Such dynamics are of interest for several reasons: for experimental tests of theory, for the development of useful virulence management protocols, and for understanding virulence evolution in situations where the epidemiological dynamics never reach equilibrium and/or when evolutionary change occurs on a timescale comparable to that of the epidemiological dynamics. Here we present a general theory similar to that of quantitative genetics in evolutionary biology that allows for the easy construction of models that include both within-host mutation as well as superinfection and that is capable of predicting both the short- and long-term evolution of virulence. We illustrate the generality and intuitive appeal of the theory through a series of examples showing how it can lead to transparent interpretations of the selective forces governing virulence evolution. It also leads to novel predictions that are not possible using the game-theoretic approach. The general theory can be used to model the evolution of other pathogen traits as well.

Mitochondrial DNA variation in North American populations of Daphnia obtusa: continentalism or cryptic endemism?

The morphological stasis of many freshwater crustaceans has resulted in the prior delineation of cosmopolitan species and has been explained by their capacity for long-distance dispersal. This study examines the phylogeography of Daphnia obtusa, a cladoceran thought to be widespread in North America. However, sequence variation of the mitochondrial cytochrome c oxidase subunit I gene indicates that this taxon is composed of two morphologically cryptic species, designated D. obtusa NA1 and NA2. NA2 is restricted to the east, whereas NA1 is broadly distributed across the United States, and is subdivided into four phylogroups that show weak genetic differentiation over broad geographical areas, which likely reflects recent long-distance dispersal. The current distributions of the four phylogroups in NA1 can be explained by recent range expansion from different refugia following the last Pleistocene glacial advance. Interestingly, the mitochondrial phylogroups identified in this study do not correspond to lineages detected in a previous allozyme analysis. However, the latter groups are associated with a habitat shift suggesting that natural selection may have played a role in their divergence. The results of this and previous studies illustrate the complicated biogeographical history of freshwater cladocerans.

Association with host mitochondrial haplotypes suggests that feminizing microsporidia lack horizontal transmission

The amphipod crustacean Gammarus duebeni hosts two feminizing microsporidian parasites, Nosema granulosis and Microsporidium sp. Samples of G. duebeni were collected from three sites on the Scottish island of Great Cumbrae and screened for microsporidia using polymerase chain reaction. Associations between the prevalence of the two feminizing parasites and haplotypes of the host mitochondrial gene cytochrome oxidase I (COI) were investigated. The prevalence of both parasites varied significantly among the host's COI haplotypes, suggesting that horizontal transmission is rare or absent in the life cycles of the feminizing microsporidia and that all transmission must therefore be vertical. Life cycles in which all transmission is vertical are common among bacterial parasites but have never before been demonstrated in Eukaryotic parasites.

Evolution of multiple components of virulence in Drosophila-nematode associations

Virulence is of central importance in host-parasite interactions, yet little is known about how it changes over extended evolutionary periods. In this study, all four species in the testacea species group of Drosophila were experimentally infected with sympatric and allopatric nematodes in the Howardula aoronymphium species complex, and the effect of parasite infection on three components of host fitness was determined. The Drosophila species show striking differences in their responses to infection, with reductions reaching 80% in adult lifespan, 100% in female fertility, and 90% in male fertility. Female sterility appears to be determined by the host; species that are sterilized by their local nematodes are also sterilized by the other allopatric nematodes in the H. aoronymphium complex. Host species that are not sterilized by their local parasite are not sterilized by other nematodes in the complex. In contrast, reductions in host adult lifespan and male fertility depend on both the host and the parasite. Whereas all nematodes reduced the survival of their local host species equally (about 40-45%), survival of two host species was drastically reduced (about 80%) when infected with an allopatric parasite. Thus, virulence is evolutionarily labile in associations between Drosophila testacea group species and their Howardula parasites. The data suggest that changes in the sterility component of virulence are due primarily to host evolution, whereas changes in the host mortality component are due in large part to parasite evolution.

Host prophenoloxidase expression in freshwater crayfish is linked to increased resistance to the crayfish plague fungus, Aphanomyces astaci

The crayfish plague (Aphanomyces astaci) susceptible freshwater crayfish Astacus astacus and the resistant species Pacifastacus leniusculus were compared with respect to differential haemocyte count and expression of prophenoloxidase and peroxinectin. A major difference found was that resistant crayfish continuously produced high levels of prophenoloxidase (proPO) transcripts and that these levels could not be further increased, whereas in susceptible crayfish proPO transcript levels and resistance were augmented by immunostimulants. In As. astacus this could be registered as higher proPO transcript levels in the semigranular population of haemocytes and to an increased survival time after experimental infections with A. astaci.

Evidence for strong host clone-parasite species interactions in the Daphnia microparasite system

Organisms are often confronted with multiple enemy species. Defenses against different parasite species may be traded off against each other. However, if resistance is based on (potentially costly) general defense mechanisms, it may be positively correlated among parasites. In an experimental study, we confronted 19 clones from one Daphnia magna population with two bacterial and three microsporidian parasite species. All parasites were isolated from the same pond as the hosts. Host clones were specific in their susceptibility towards different parasite species, and parasite species were host-clone specific in their infectivity, spore production, and virulence, resulting in highly significant host-parasite interactions. Since the Daphnia's resistance to different parasite species showed no obvious correlation, neither general defense mechanisms nor trade-offs in resistance explain our findings. None of the Daphnia clones were resistant to all parasite species, and the average level of resistance was quite similar among clones. This may reflect a cost of defense, so that the cumulative cost of being resistant to all parasite species might be too high.

A consideration of patterns of virulence arising from host-parasite coevolution

In this article we explore how host survival and fecundity are affected by host-parasite coevolution. We examine a situation in which hosts upon being infected can mount a defensive response to clear the infection, but in which there is a fecundity cost to such immunological up-regulation. We also suppose that the parasite exploits the host and thereby causes an elevated host mortality rate. We determine the coevolutionary stable strategies of the parasite's level of exploitation and the host's level of up-regulation, and illustrate the patterns of reduced host fitness (i.e., virulence) that these produce. We find that counterintuitive patterns of virulence are often expected to arise as a result of the interaction between coevolved host and parasite strategies. In particular, despite the fact that the parasite imposes only a mortality cost on the host, coevolution by the host results in a pattern whereby infected hosts always have the same probability of death from infection, but they vary in the extent to which their fecundity is reduced. This contrasts with previous results and arises from our inclusion of two important factors absent from previous theory: costs of immunological up-regulation and a more suitable measure of parasite-induced mortality.

Polymorphism of a complex resistance gene candidate family in wild populations of common bean (Phaseolus vulgaris) in Argentina: comparison with phenotypic resistance polymorphism

Fifteen populations of wild bean (Phaseolus vulgaris), located in three provinces in Argentina, were analysed for their polymorphism for a complex resistance gene candidate (RGC) family clustered on the genome and for resistance phenotypes to strains of Colletotrichum lindemuthianum. Results indicate that RGC polymorphism is high. Population structure obtained for markers related to resistance was compared to population structure obtained for RAPD markers in order to infer the evolutionary forces driving polymorphism for resistance in wild populations at both molecular and phenotypic levels. Hierarchical analysis of differentiation showed that, within provinces, populations were differentiated for RAPD as well as for molecular and phenotypic markers of resistance. In contrast, provinces were differentiated only for RAPD and RGC markers and not for resistance phenotypes. The discrepancies found between diversity structures for molecular markers (RAPD and RGCs) and for resistance phenotypes suggest an effect of selection and indicate that diversity for resistance may not be driven by the same selective forces at the molecular and phenotypic levels. We further discuss whether specific selective forces are exerted on RGC markers and underline the importance of spatial scale of analysis for demonstrating an impact of selection.

Challenging the trade-off model for the evolution of virulence: is virulence management feasible?

Progress in understanding the evolution of infectious diseases has inspired proposals to manage the evolution of pathogen (including parasite) virulence. A common view is that social interventions that lower pathogen transmission will indirectly select lower virulence because of a trade-off between transmission and virulence. Here, we argue that there is little theoretical justification and no empirical evidence for this plan. Although a trade-off model might apply to some pathogens, the mechanism appears too weak for rapid selection of substantial changes in virulence. Direct selection against virulence itself might be a more rewarding approach to managing the evolution of virulence.

Biological and biomedical implications of the co-evolution of pathogens and their hosts

Co-evolution between host and pathogen is, in principle, a powerful determinant of the biology and genetics of infection and disease. Yet co-evolution has proven difficult to demonstrate rigorously in practice, and co-evolutionary thinking is only just beginning to inform medical or veterinary research in any meaningful way, even though it can have a major influence on how genetic variation in biomedically important traits is interpreted. Improving our understanding of the biomedical significance of co-evolution will require changing the way in which we look for it, complementing the phenomenological approach traditionally favored by evolutionary biologists with the exploitation of the extensive data becoming available on the molecular biology and molecular genetics of host-pathogen interactions.

On the evolution of virulence and the relationship between various measures of mortality

Smallpox causes roughly 20% mortality whereas chickenpox causes less than 0.1%. Most 'verbal' (i.e. non-mathematical) discussions using a mortality definition of virulence would therefore label smallpox as more virulent. Indeed, the virulence of many diseases is measured using such case mortalities, chi, or related measures such as expected host lifespan, T, or lethal dose, LD(x). But chi, T and LD(x) are only indirectly related to parasite-induced instantaneous mortality rate, alpha, which is the mortality measure used in much of the theory developed to explain virulence evolution. Here I point out that relatively deadly pathogens can actually have lower values of alpha than benign pathogens, demonstrating that alpha does not, by itself, reflect the extent to which a parasite causes host mortality. I present mathematical relationships between alpha and chi, T and LD(x), and use these to demonstrate that predictions about virulence evolution can be qualitatively altered depending upon which measure is used as the definition of virulence. Two simple examples are presented to illustrate this point, one of which demonstrates that the well-cited prediction that virulence should evolve to be higher when disease-independent host mortality increases need not hold. This prediction has been made in terms of parasite-induced instantaneous mortality, alpha, but if virulence is measured using case mortality (or T or LD(x)) then this prediction can easily be reversed. Theoretical and empirical researchers must use compatible mortality measures before a productive exchange between the two can take place, and it is suggested that case mortality (or lethal dose) is best suited as a single (mortality) measure of parasite virulence.

Local adaptation in the Linum marginale-Melampsora lini host-pathogen interaction

The potential for local adaptation between pathogens and their hosts has generated strong theoretical and empirical interest with evidence both for and against local adaptation reported for a range of systems. We use the Linum marginale-Melampsora lini plant-pathogen system and a hierarchical spatial structure to investigate patterns of local adaptation within a metapopulation characterised by epidemic dynamics and frequent extinction of pathogen populations. Based on large sample sizes and comprehensive cross-inoculation trials, our analyses demonstrate strong local adaptation by Melampsora to its host populations, with this effect being greatest at regional scales, as predicted from the broader spatial scales at which M. lini disperses relative to L. marginale. However, there was no consistent trend for more distant pathogen populations to perform more poorly. Our results further show how the coevolutionary interaction between hosts and pathogens can be influenced by local structure such that resistant hosts select for generally virulent pathogens, while susceptible hosts select for more avirulent pathogens. Empirically, local adaptation has generally been tested in two contrasting ways: (1) pathogen performance on sympatric versus allopatric hosts; and (2) sympatric versus allopatric pathogens on a given host population. In situations where no host population is more resistant or susceptible than others when averaged across pathogen populations (and likewise, no pathogen population is more virulent or avirulent than others), results from these tests should generally be congruent. We argue that this is unlikely to be the case in the metapopulation situations that predominate in natural host-pathogen interactions, thus requiring tests that control simultaneously for variation in plant and pathogen populations.

Virulence in rodent malaria: host genotype by parasite genotype interactions

In an effort to understand what limits the virulence of malaria parasites, we infected inbred mice of three genotypes (C57Bl/6J, CBA/Ca and DBA/2) with one of two parasite lines of the rodent malaria Plasmodium chabaudi. One of these parasite lines had been serially passaged through C57Bl/6J mice and had evolved higher asexual growth rate, virulence and transmission in the process. The other parasite line was the unadapted ancestral line which had low virulence. In all three host genotypes, the C57Bl/6J-adapted parasite line was more virulent than the ancestral line thus indicating that trade-offs in virulence between alternative host genotypes had not placed strong constraints on the evolution of high virulence in this system. By examining the infection dynamics for fitness-related components-asexual parasite population growth, transmission and virulence-we revealed alternative possible explanations for what sets the upper limit to virulence in nature. The total number of transmission forms (gametocytes) produced during the infection, a measure of parasite Darwinian fitness, was four-fold higher in mice that survived the infection than those which died. Among mice that survived, total gametocyte production was greatest in the host genotype that suffered intermediate levels of morbidity (anaemia and weight loss). Thus, there were transmission costs of high virulence that were partly due to host death (as most theoretical models of virulence evolution assume), but perhaps partly due to some factor related to high morbidity. Both mortality and morbidity-related factors might therefore influence the upper limit on virulence of malaria parasites.

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.

Timing of transmission and the evolution of virulence of an insect virus

We used the nuclear polyhedrosis virus of the gypsy moth, Lymantria dispar, to investigate whether the timing of transmission influences the evolution of virulence. In theory, early transmission should favour rapid replication and increase virulence, while late transmission should favour slower replication and reduce virulence. We tested this prediction by subjecting one set of 10 virus lineages to early transmission (Early viruses) and another set to late transmission (Late viruses). Each lineage of virus underwent nine cycles of transmission. Virulence assays on these lineages indicated that viruses transmitted early were significantly more lethal than those transmitted late. Increased exploitation of the host appears to come at a cost, however. While Early viruses initially produced more progeny, Late viruses were ultimately more productive over the entire duration of the infection. These results illustrate fitness trade-offs associated with the evolution of virulence and indicate that milder viruses can obtain a numerical advantage when mild and harmful strains tend to infect separate hosts.