Geographic variation in sterilizing parasite species and the Red Queen

The Red Queen hypothesis predicts that sexual reproduction should be favoured in locations where the risk of infection by virulent parasites is consistently high. When hosts are exposed to multiple parasites over their geographic range, the coevolving parasite species may vary among host populations. We surveyed 26 streams on the South Island of New Zealand to determine whether the frequency of snails ( Potamopyrgus antipodarum ) infected by various sterilizing trematode parasite species was correlated with the frequency of sexual individuals. We compared the results with a survey conducted over 20 years ago to determine whether the associations were consistent. We also evaluated different measures of parasite-mediated selection among populations, including prevalence of the most common local parasite (MCLP) species and parasite diversity to assess the best predictor of sexual reproduction among stream populations. The results showed that the relationship between male frequency and parasite infection is more geographically widespread than previously recorded. Additionally, we found that the prevalence of the MCLP was the best predictor of sex in habitats where hosts populations are infected with multiple parasites (approximately 15 trematode species). This study provides evidence that sexual snails occur more often in environments with high infection levels, and that the pattern of parasite-imposed selection is geographically variable. Support for the Red Queen may be strengthened by focussing on the MCLP, which may vary among host populations.     

Reciprocal Interaction Matrix Reveals Complex Genetic and Dose-Dependent Specificity among Coinfecting Parasites

Abstract Understanding genetic specificity in factors determining the outcome of host-parasite interactions is especially important as it contributes to parasite epidemiology, virulence, and maintenance of genetic variation. Such specificity, however, is still generally poorly understood. We examined genetic specificity in interactions among coinfecting parasites. In natural populations, individual hosts are often simultaneously infected by multiple parasite species and genotypes that interact. Such interactions could maintain genetic variation in parasite populations if they are genetically specific so that the relative fitness of parasite genotypes varies across host individuals depending on (1) the presence/absence of coinfections and/or (2) the genetic composition of the coinfecting parasite community. We tested these predictions using clones of fish eye flukes Diplostomum pseudospathaceum and Diplostomum gasterostei. We found that interactions among parasites had a strong genetic basis and that this modified genetic variation in infection success of D. pseudospathaceum between single and multiple infections as well as across multiply infected host individuals depending on the genetic identity of the coinfecting D. gasterostei. The relative magnitude of these effects, however, depended on the exposure dose, suggesting that ecological factors can modify genetic interactions between parasites.     

Experimental evolution of resistance in Paramecium caudatum against the bacterial parasite Holospora undulata

Host-parasite coevolution is often described as a process of reciprocal adaptation and counter adaptation, driven by frequency-dependent selection. This requires that different parasite genotypes perform differently on different host genotypes. Such genotype-by-genotype interactions arise if adaptation to one host (or parasite) genotype reduces performance on others. These direct costs of adaptation can maintain genetic polymorphism and generate geographic patterns of local host or parasite adaptation. Fixation of all-resistant (or all-infective) genotypes is further prevented if adaptation trades off with other host (or parasite) life-history traits. For the host, such indirect costs of resistance refer to reduced fitness of resistant genotypes in the absence of parasites. We studied (co)evolution in experimental microcosms of several clones of the freshwater protozoan Paramecium caudatum, infected with the bacterial parasite Holospora undulata. After two and a half years of culture, inoculation of evolved and naive (never exposed to the parasite) hosts with evolved and founder parasites revealed an increase in host resistance, but not in parasite infectivity. A cross-infection experiment showed significant host clone-by-parasite isolate interactions, and evolved hosts tended to be more resistant to their own (local) parasites than to parasites from other hosts. Compared to naive clones, evolved host clones had lower division rates in the absence of the parasite. Thus, our study indicates de novo evolution of host resistance, associated with both direct and indirect costs. This illustrates how interactions with parasites can lead to the genetic divergence of initially identical populations.     

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.     

Global warming will reshuffle the areas of high prevalence and richness of three genera of avian blood parasites

The importance of parasitism for host populations depends on local parasite richness and prevalence: usually host individuals face higher infection risk in areas where parasites are most diverse, and host dispersal to or from these areas may have fitness consequences. Knowing how parasites are and will be distributed in space and time (in a context of global change) is thus crucial from both an ecological and a biological conservation perspective. Nevertheless, most research articles focus just on elaborating models of parasite distribution instead of parasite diversity. We produced distribution models of the areas where haemosporidian parasites are currently highly diverse (both at community and at within‐host levels) and prevalent among Iberian populations of a model passerine host: the blackcap Sylvia atricapilla; and how these areas are expected to vary according to three scenarios of climate change. On the basis of these models, we analysed whether variation among populations in parasite richness or prevalence are expected to remain the same or change in the future, thereby reshuffling the geographic mosaic of host‐parasite interactions as we observe it today. Our models predict a rearrangement of areas of high prevalence and richness of parasites in the future, with Haemoproteus and Leucocytozoon parasites (today the most diverse genera in blackcaps) losing areas of high diversity and Plasmodium parasites (the most virulent ones) gaining them. Likewise, the prevalence of multiple infections and parasite infracommunity richness would be reduced. Importantly, differences among populations in the prevalence and richness of parasites are expected to decrease in the future, creating a more homogeneous parasitic landscape. This predicts an altered geographic mosaic of host‐parasite relationships, which will modify the interaction arena in which parasite virulence evolves.     

Effects of resource availability and social aggregation on the species richness of raccoon endoparasite infracommunities

Within populations the contact rate of hosts and infectious parasites is mediated by the interactions of resource availability, host density, and host behavior. Fluctuations in host density can result in the loss or extinction of a parasite population as contact rates between parasites and susceptible individuals drop below thresholds of parasite population persistence. Less understood is how changes in resources and the behavioral ecology of host populations affect parasites. We used food provisioning to experimentally assess the effects of resource availability and of inducing host aggregation on the endoparasite community of free‐ranging raccoons. Twelve independent raccoon populations were subjected to differential resource provisioning for two years: a clumped food distribution to aggregate hosts (n = 5 populations), a dispersed food distribution to add food without aggregating hosts (n = 3), and a no food treatment (n = 4). Remote cameras indicated that aggregation sizes were three to four times greater in aggregated versus non‐aggregated populations. We considered endoparasites with direct and indirect life cycles separately and determined the best‐fit models of parasite species richness in relation to host aggregation, food supplements, and host age and sex. Social aggregation had a negligible impact on the species richness of directly or indirectly transmitted parasites. However, food additions decreased the number of indirectly transmitted parasite species by 35% in the oldest age classes. These results suggest that while resource availability can influence the transmission of indirectly transmitted parasites, an examination of additional factors will be necessary to understand the role of host contact and factors that shape the community structure of endoparasites in natural environments.     

HOST-PARASITE COEVOLUTION: EVIDENCE FOR RARE ADVANTAGE AND TIME-LAGGED SELECTION IN A NATURAL POPULATION

In theory, parasites can create time-lagged, frequency-dependent selection in their hosts, resulting in oscillatory gene-frequency dynamics in both the host and the parasite (the Red Queen hypothesis). However, oscillatory dynamics have not been observed in natural populations. In the present study, we evaluated the dynamics of asexual clones of a New Zealand snail, Potamopyrgus antipodarum, and its trematode parasites over a five-year period. During the summer of each year, we determined host-clone frequencies in random samples of the snail to track genetic changes in the snail population. Similarly, we monitored changes in the parasite population, focusing on the dominant parasite, Microphallus sp., by calculating the frequency of clones in samples of infected individuals from the same collections. We then compared these results to the results of a computer model that was designed to examine clone frequency dynamics for various levels of parasite virulence. Consistent with these simulations and with ideas regarding dynamic coevolution, parasites responded to common clones in a time-lagged fashion. Finally, in a laboratory experiment, we found that clones that had been rare during the previous five years were significantly less infectible by Microphallus when compared to the common clones. Taken together, these results confirm that rare host genotypes are more likely to escape infection by parasites; they also show that host-parasite interactions produce, in a natural population, some of the dynamics anticipated by the Red Queen hypothesis.     

Random parasite encounters coupled with condition-linked immunity of hosts generate parasite aggregation

Parasite aggregation is viewed as a natural law in parasite-host ecology but is a paradox insofar as parasites should follow the Poisson distribution if hosts are encountered randomly. Much research has focused on whether parasite aggregation in or on hosts is explained by aggregation of infective parasite stages in the environment, or by heterogeneity within host samples in terms of host responses to infection (e.g., through representation of different age classes of hosts). In this paper, we argue that the typically aggregated distributions of parasites may be explained simply. We propose that aggregated distributions can be derived from parasites encountering hosts randomly, but subsequently by parasites being 'lost' from hosts based on condition-linked escape or immunity of hosts. Host condition should be a normally distributed trait even among otherwise homogeneous sets of hosts. Our model shows that mean host condition and variation in host condition have different effects on the different metrics of parasite aggregation. Our model further predicts that as host condition increases, parasites become more aggregated but numbers of attending parasites are reduced overall and this is important for parasite population dynamics. The effects of deviation from random encounter are discussed with respect to the relationship between host condition and final parasite numbers.     

Multiple infections by the anther smut pathogen are frequent and involve related strains

Population models of host-parasite interactions predict that when different parasite genotypes compete within a host for limited resources, those that exploit the host faster will be selected, leading to an increase in parasite virulence. When parasites sharing a host are related, however, kin selection should lead to more cooperative host exploitation that may involve slower rates of parasite reproduction. Despite their potential importance, studies that assess the prevalence of multiple genotype infections in natural populations remain rare, and studies quantifying the relatedness of parasites occurring together as natural multiple infections are particularly scarce. We investigated multiple infections in natural populations of the systemic fungal plant parasite Microbotryum violaceum, the anther smut of Caryophyllaceae, on its host, Silene latifolia. We found that multiple infections can be extremely frequent, with different fungal genotypes found in different stems of single plants. Multiple infections involved parasite genotypes more closely related than would be expected based upon their genetic diversity or due to spatial substructuring within the parasite populations. Together with previous sequential inoculation experiments, our results suggest that M. violaceum actively excludes divergent competitors while tolerating closely related genotypes. Such an exclusion mechanism might explain why multiple infections were less frequent in populations with the highest genetic diversity, which is at odds with intuitive expectations. Thus, these results demonstrate that genetic diversity can influence the prevalence of multiple infections in nature, which will have important consequences for their optimal levels of virulence. Measuring the occurrence of multiple infections and the relatedness among parasites within hosts in natural populations may be important for understanding the evolutionary dynamics of disease, the consequences of vaccine use, and forces driving the population genetic structure of parasites.     

Whether larval amphibians school does not affect the parasite aggregation rule: testing the effects of host spatial heterogeneity in field and experimental studies

Almost all macroparasites show over‐dispersed infections within natural host populations such that most parasites are distributed among a few heavily‐infected individuals. Despite the importance of parasite aggregation for understanding system stability, the potential for population regulation, and super‐spreading events, many questions persist about its underlying drivers. Theoretically, aggregation results from heterogeneity in host exposure, resistance, and tolerance. However, few studies have examined how host spatial arrangement – which likely affects both parasite encounter and density‐dependent interactions – influences infection and dispersion, representing a critical gap in our current knowledge regarding the possible drivers of parasite aggregation. Using field data from over 165 ponds and 8000 hosts, we evaluated how the spatial clustering of amphibian larvae within ponds 1) varied among different amphibian species, and 2), affected the distribution of parasites within the host population using Taylor's power law. A complementary mesocosm experiment used field‐guided manipulations of the spatial arrangement of larval amphibians to create a gradient in host clustering while controlling host density, thereby testing for spatial effects on both infection success and aggregation by three different trematode species. Our field data indicated that larval amphibians exhibited significant spatial clustering that was well captured by Taylor's power law (R² 0.92 to 0.97 for different host species), but the residual variation only weakly correlated with observed patterns of trematode parasite over‐dispersion. Correspondingly, experimental manipulation of host clustering had no effects on parasite infection success or the degree of parasite aggregation among cages or mesocosms. Given the importance of parasite over‐dispersion for host populations and disease dynamics, we advocate for further investigations of host and parasite spatial aggregation, particularly studies that incorporate and/or control for heterogeneity in exposure and susceptibility.     

Host patch selection induced by parasitism: basic reproduction ratio r(0) and optimal virulence

The effects of parasites on the behavior of their hosts are well documented. For example, parasites may affect the habitat selection of the host individual. We used variables aggregation methods to investigate the way in which parasites affect the spatial pattern of susceptible hosts. We developed a simple epidemiological model, taking into account both the reproduction processes of hosts (density-dependent birth and death) and infection, considered separately on two different patches, and the migration of susceptible hosts between these two patches. We used the complete model of three equations to generate an aggregated model describing the dynamics of the combined susceptible and infected host populations. We obtained the basic reproduction ratio (R(0)) from the aggregated model, and then studied the effect of the migratory behavior of susceptible hosts on the ability of the parasite to invade the system. We also used the basic reproduction ratio to investigate the evolution of parasite virulence in relation to the migration decisions of susceptible hosts. We found that host investment in avoidance of the infected patch leads to an increase in optimal virulence if host investment is costly.     

From individual heterogeneity to population-level overdispersion: quantifying the relative roles of host exposure and parasite establishment in driving aggregated helminth distributions

In most host-parasite systems, variation in parasite burden among hosts drives transmission dynamics. Heavily infected individuals introduce disproportionate numbers of infective stages into host populations or surrounding environments, causing sharp increases in frequency of infection. Parasite aggregation within host populations may result from variation among hosts in exposure to infective propagules and probability of subsequent establishment of parasites in the host. This is because individual host heterogeneities contribute to a pattern of parasite overdispersion that emerges at the population level. We quantified relative roles of host exposure and parasite establishment in producing variation in parasite burdens, to predict which hosts are more likely to bear heavy burdens, using big brown bats (Eptesicus fuscus) and their helminths as a model system. We captured bats from seven colonies in Michigan and Indiana, USA, assessed their helminth burdens, and collected data on intrinsic and extrinsic variables related to exposure, establishment, or both. Digenetic trematodes had the highest prevalence and mean abundance while cestodes and nematodes had much lower prevalence and mean abundance. Structural equation modeling revealed that best-fitting models to explain variations in parasite burden included genetic heterozygosity and immunocompetence as well as distance to the nearest water source and the year of host capture. Thus, both differential host exposure and differential parasite establishment significantly influence heterogeneous helminth burdens, thus driving population-level patterns of parasite aggregation.     

Parasitic infection reduces dispersal of ciliate host

Parasitic infection can modify host mobility and consequently their dispersal capacity. We experimentally investigated this idea using the ciliate Paramecium caudatum and its bacterial parasite Holospora undulata. We compared the short-distance dispersal of infected and uninfected populations in interconnected microcosms. Infection reduced the proportion of hosts dispersing, with levels differing among host clones. Host populations with higher densities showed lower dispersal, possibly owing to social aggregation behaviour. Parasite isolates that depleted host populations most had the lowest impact on host dispersal. Parasite-induced modification of dispersal may have consequences for the spatial distribution of disease, host and parasite genetic population structure, and coevolution.     

Parasite preferences for large host body size can drive overdispersion in a fly-mite association

Body size generally correlates intraspecifically with insect fitness but can also correlate with parasite abundance (number of parasites). Host preferences by parasites, and variation in host immunity, could contribute to this trend. We investigated the effect of host size on mite-fly interactions (Macrocheles subbadius and Drosophila nigrospiracula). Mites strongly preferred to infect larger flies in pair-wise choices, and larger flies were more likely to be infected and acquired more mites in infection microcosms. Preferences of parasites resulted in size-biased infection outcomes. We discuss the implications of this heterogeneity in infection on parasite overdispersion and fly populations.     

The host specificity of Gyrodactylus salaris Malmberg (Platyhelminthes, Monogenea): susceptibility of Oncovhynchus mykiss (Walbaum) under experimental conditions

An experimental epidemiological approach was chosen to study the survival and infection dynamics of Gyrodactylus salaris on juvenile rainbow trout, Oncorhynchus mykiss , in the laboratory. A marked heterogeneity in the host stock was apparent. The rainbow trout could be divided into three groups on the basis of parasite survival and infection pattern on individually isolated fish: (1) hosts receptive to initial parasite attachment, but unreceptive to parasite establishment and reproduction; (2) hosts moderately susceptible to parasite establishment and reproduction, but which, after a period of restricted parasite population growth, responded, recovered and eliminated the parasites; and (3) hosts very susceptible to parasite infection and reproduction, but which, after a period of significant parasite population growth, responded, recovered and eliminated the parasites. These different patterns are considered to reflect genetic differences between host individuals. Parasite aggregation was also shown to be an important factor in the outcome of the host-parasite association. The parasites were finally eliminated on the individually isolated hosts, but not on hosts maintained in batches and so host population size and immigration of fresh. previously unexposed, hosts appeared to be important for growth and maintenance of the parasite population. The parasite was not found to cause host mortality. Rainbow trout was a suitable host for G. salaris , capable of transmitting the parasite to new localities as a consequence of stocking programmes or migratory behaviour.     

Parasite-mediated selection in experimental Daphnia magna populations

Abstract It has been suggested that parasites are a strong selecting force for their hosts and therefore may alter the outcome of competition among host genotypes. We tested the extent to which parasite-mediated selection by different parasite species influenced competition among clones of the cyclic parthenogen Daphnia magna . We monitored clone frequency changes in laboratory microcosm populations consisting of 21 D. magna clones. Parasite treatments (two microsporidians, Glugoides intestinalis and Ordospora colligata ) and a parasite-free control treatment were followed over a nine-month period. A further treatment with the bacterium Pasteuria ramosa failed. We found significant differences in clonal success among the treatments: the two parasite treatments differed from the control treatment and from each other. Additionally, we measured the clone-specific population carrying capacity, competitive ability against tester clones, and reproductive success of infected and uninfected females to test whether they correlate with clonal success in the microcosms. The clone-specific competitive ability was a good predictor of clonal success in the microcosms, but clonal carrying capacity and host reproductive success were not. Our study shows that parasite-mediated selection can strongly alter the outcome of clonal competition. The results suggest that parasites may influence microevolution in Daphnia populations during periods of asexual reproduction.     

Strong density-dependent competition and acquired immunity constrain parasite establishment: implications for parasite aggregation

The vast majority of parasites exhibit an aggregated frequency distribution within their host population, such that most hosts have few or no parasites while only a minority of hosts are heavily infected. One exception to this rule is the trophically transmitted parasite Pterygodermatites peromysci of the white-footed mouse (Peromyscus leucopus), which is randomly distributed within its host population. Here, we ask: what are the factors generating the random distribution of parasites in this system when the majority of macroparasites exhibit non-random patterns? We hypothesise that tight density-dependent processes constrain parasite establishment and survival, preventing the build-up of parasites within individual hosts, and preclude aggregation within the host population. We first conducted primary infections in a laboratory experiment using white-footed mice to test for density-dependent parasite establishment and survival of adult worms. Secondary or challenge infection experiments were then conducted to investigate underlying mechanisms, including intra-specific competition and host-mediated restrictions (i.e. acquired immunity). The results of our experimental infections show a dose-dependent constraint on within-host-parasite establishment, such that the proportion of mice infected rose initially with exposure, and then dropped off at the highest dose. Additional evidence of density-dependent competition comes from the decrease in worm length with increasing levels of exposure. In the challenge infection experiment, previous exposure to parasites resulted in a lower prevalence and intensity of infection compared with primary infection of naïve mice; the magnitude of this effect was also density-dependent. Host immune response (IgG levels) increased with the level of exposure, but decreased with the number of worms established. Our results suggest that strong intra-specific competition and acquired host immunity operate in a density-dependent manner to constrain parasite establishment, driving down aggregation and ultimately accounting for the observed random distribution of parasites.     

A quantitative test of the relationship between parasite dose and infection probability across different host-parasite combinations

Epidemiological models generally assume that the number of susceptible individuals that become infected within a unit of time depends on the density of the hosts and the concentration of parasites (i.e. mass-action principle). However, empirical studies have found significant deviations from this assumption due to biotic and abiotic factors, such as seasonality, the spatial structure of the host population and host heterogeneity with respect to immunity and susceptibility. In this paper, we examine the effect of the dose level of the bacterial endoparasite Pasteuria ramosa on the infection rate of its host, the water flea Daphnia magna. Using seven host clones and two parasite isolates, we measure the fraction of infected hosts after exposure to eight different parasite doses to determine whether there is variation in the infection process across different host clone-parasite isolate combinations. In five combinations, a pronounced dose-dependent infection pattern was found. Using a likelihood approach, we compare the infection data of these five combinations to the fit of three mathematical models: a mass-action model, a parasite antagonism model (i.e. an increase in the parasite dose leads to an under-proportionate increase in the infection rate per host) and a heterogeneous host model. We found that the host heterogeneity model, in which we assumed the existence of non-inherited phenotypic differences in host susceptibilities to the parasite, provides the best fit. Our analysis suggests that among 5 out of the 14 host clone-parasite isolate combinations that resulted in appreciable infections, non-genetic host heterogeneity plays an important role.     

Aggregation patterns of helminth populations in the introduced fish, Liza haematocheilus (Teleostei: Mugilidae): disentangling host–parasite relationships

A number of hypotheses exist to explain aggregated distributions, but they have seldom been used to investigate differences in parasite spatial distribution between native and introduced hosts. We applied two aggregation models, the negative binomial distribution and Taylor’s power law, to study the aggregation patterns of helminth populations from Liza haematocheilus across its native (Sea of Japan) and introduced (Sea of Azov) distribution ranges. In accordance with the enemy release hypothesis, we predicted that parasite populations in the introduced host range would be less aggregated than in the native host area, because aggregation is tightly constrained by abundance. Contrary to our expectation, aggregation of parasite populations was higher in the introduced host range. However, the analyses suggested that the effect of host introduction on parasite aggregation depends on whether parasite species, or higher level taxonomic groups, were acquired in or carried into the new area. The revealed similarity in the aggregation parameters of co-introduced monogeneans can be attributed to the repeatability and identity of the host–parasite systems. In contrast, the degree of aggregation differed markedly between regions for higher level taxa, which are represented by the native parasites in the Sea of Japan versus the acquired species in the Sea of Azov. We propose that the host species plays a crucial role in regulating infra-population sizes of acquired parasites due to the high rate of host-induced mortality. A large part of the introduced host population may remain uninfected due to their resistance to native naïve parasites. The core concept of our study is that the comparative analysis of aggregation patterns of parasites in communities and populations, and macroecological relationships, can provide a useful tool to reveal cryptic relationships in host–parasite systems of invasive hosts and their parasites.     

Antagonistic coevolution with parasites increases the cost of host deleterious mutations

The fitness consequences of deleterious mutations are sometimes greater when individuals are parasitized, hence parasites may result in the more rapid purging of deleterious mutations from host populations. The significance of host deleterious mutations when hosts and parasites antagonistically coevolve (reciprocal evolution of host resistance and parasite infectivity) has not previously been experimentally investigated. We addressed this by coevolving the bacterium Pseudomonas fluorescens and a parasitic bacteriophage in laboratory microcosms, using bacteria with high and low mutation loads. Directional coevolution between bacterial resistance and phage infectivity occurred in all populations. Bacterial population fitness, as measured by competition experiments with ancestral genotypes in the absence of phage, declined with time spent coevolving. However, this decline was significantly more rapid in bacteria with high mutation loads, suggesting the cost of bacterial resistance to phage was greater in the presence of deleterious mutations (synergistic epistasis). As such, resistance to phage was more costly to evolve in the presence of a high mutation load. Consistent with these data, bacteria with high mutation loads underwent less rapid directional coevolution with their phage populations, and showed lower levels of resistance to their coevolving phage populations. These data suggest that coevolution with parasites increases the rate at which deleterious mutations are purged from host populations.