A Two-Locus System with Strong Epistasis Underlies Rapid Parasite-Mediated Evolution of Host Resistance

Parasites are a major evolutionary force, driving adaptive responses in host populations. Although the link between phenotypic response to parasite-mediated natural selection and the underlying genetic architecture often remains obscure, this link is crucial for understanding the evolution of resistance and predicting associated allele frequency changes in the population. To close this gap, we monitored the response to selection during epidemics of a virulent bacterial pathogen, Pasteuria ramosa, in a natural host population of Daphnia magna. Across two epidemics, we observed a strong increase in the proportion of resistant phenotypes as the epidemics progressed. Field and laboratory experiments confirmed that this increase in resistance was caused by selection from the local parasite. Using a genome-wide association study, we built a genetic model in which two genomic regions with dominance and epistasis control resistance polymorphism in the host. We verified this model by selfing host genotypes with different resistance phenotypes and scoring their F1 for segregation of resistance and associated genetic markers. Such epistatic effects with strong fitness consequences in host–parasite coevolution are believed to be crucial in the Red Queen model for the evolution of genetic recombination.

Coinfecting parasites can modify fluctuating selection dynamics in host-parasite coevolution

Genetically specific interactions between hosts and parasites can lead to coevolutionary fluctuations in their genotype frequencies over time. Such fluctuating selection dynamics are, however, expected to occur only under specific circumstances (e.g., high fitness costs of infection to the hosts). The outcomes of host-parasite interactions are typically affected by environmental/ecological factors, which could modify coevolutionary dynamics. For instance, individual hosts are often infected with more than one parasite species and interactions between them can alter host and parasite performance. We examined the potential effects of coinfections by genetically specific (i.e., coevolving) and nonspecific (i.e., generalist) parasite species on fluctuating selection dynamics using numerical simulations. We modeled coevolution (a) when hosts are exposed to a single parasite species that must genetically match the host to infect, (b) when hosts are also exposed to a generalist parasite that increases fitness costs to the hosts, and (c) when coinfecting parasites compete for the shared host resources. Our results show that coinfections can enhance fluctuating selection dynamics when they increase fitness costs to the hosts. Under resource competition, coinfections can either enhance or suppress fluctuating selection dynamics, depending on the characteristics (i.e., fecundity, fitness costs induced to the hosts) of the interacting parasites.

Parasite-driven replacement of a sexual by a closely related asexual taxon in nature

Asexual species are thought to suffer more from coevolving parasites than related sexuals. Yet a variety of studies do not find the patterns predicted by theory. Here, to shine light on this conundrum, we investigate one such case of an asexual advantage in the presence of parasites. We follow the frequency dynamics of sexual and asexual Daphnia pulex in a natural pond that was initially dominated by sexuals. Coinciding with an epidemic of a microsporidian parasite infecting both sexuals and asexuals, the pond was rapidly taken over by the initially rare asexuals. With experiments comparing multiple sexual and asexual clones from across the local metapopulation, we confirm that asexuals are less susceptible and also suffer less from the parasite once infected. These results are consistent with the parasite-driven, ecological replacement of dominant sexuals by closely related, but more resistant asexuals, ultimately leading to the extinction of the formerly superior sexual competitor. Our study is one of the clearest examples from nature, backed up by experimental verification, showing a parasite-mediated reversal of competition dynamics. The experiments show that, across the metapopulation, asexuals have an advantage in the presence of parasites. In this metapopulation, asexuals are relatively rare, likely due to their recent invasion. While we cannot rule out other reasons for the observed patterns, the results are consistent with a temporary parasite-mediated advantage of asexuals due to the fact that they are rare, which is an underappreciated aspect of the Red Queen Hypothesis.

Oomycete parasites in freshwater copepods of Patagonia: effects on survival and recruitment

Copepods are hosts to various oomycete parasite species, but the effects of pathogens on copepod populations have rarely been studied. This study aimed to characterize oomycete infection in the freshwater copepod Parabroteas sarsi in a temporary pond in Patagonia (Argentina). A complete hydroperiod was monitored, evaluating environmental variables as influencing factors in the oomycete infections. Laboratory experiments were performed to evaluate the susceptibility of infected copepods to consumption by predators. Although 5 species of copepods were present in the pond, only ovigerous P. sarsi females were parasitized by oomycetes. Two species of oomycetes were always found together: Aphanomyces ovidestruens and Pythium flevoense. Infections were detected at water temperatures >20°C, with a positive relationship between temperature and parasite prevalence. Infection occurred after a decrease in large filter-feeder densities. The pathogens were not lethal to P. sarsi females in the short-term, but did produce mortality of entire egg sacs, thus negatively impacting subsequent recruitment. Mean prevalence of infection in females was 53%, reaching 83% in December. Females have the capacity to release an infected egg sac and generate a new one in a few days. This infection does not affect the susceptibility of P. sarsi to the predator Notonecta vereertbruggheni. The decrease in female abundance registered towards the end of the hydroperiod could be related to a combination of factors that may have a differential effect on female survival, such as increasing temperature, the energy cost of egg sac development and carrying and oomycete infection.

Galba truncatula and Fasciola hepatica: Genetic costructures and interactions with intermediate host dispersal

Antagonistic interactions between hosts and parasites are key structuring forces in natural populations. Demographic factors like extinction, migration and the effective population size shape host-parasite metapopulational dynamics. Therefore, to understand the evolution of host-parasite systems it is necessary to study the distribution of the genetic variation of both entities simultaneously. In this paper, we investigate the population genetics co-structure of parasites and hosts within a metapopulation of the liver fluke, Fasciola hepatica, and two of its intermediate hosts, the main intermediate host in Europe, Galba truncatula, and a new intermediate host, Omphiscola glabra, in Central France. Our results reveal an absence of specificity of flukes as regard to the two alternative hosts though O. glabra shows higher prevalence of F. hepatica. Host and parasites displayed contrasting population genetics structure with very small, highly inbred (selfing) and strongly isolated G. truncatula populations and much bigger, panmictic and more dispersive F. hepatica. This could indicate a local adaptation of the parasite and a local maladaptation of the host. We also unveil a parasite-mediated biased population genetics structure suggesting that infected G. truncatula disperse more; have higher dispersal survival than uninfected snails or, more likely, that immigrant snails are infected more often than local snails (local parasites are less adapted to local hosts). Finally, an absence, or at least an ambiguous signature of isolation by distance was observed in both host and parasite population. A very weak migration rate for G. truncatula provides a reasonable explanation for this ambiguous result. Alternatively, smaller sample sizes combined with modest migration rates might explain the difficulties to unveil the signal in F. hepatica.

Ecology, Virulence, and Phylogeny of Blastulidium paedophthorum, a Widespread Brood Parasite of Daphnia spp

Parasitism is now recognized as a major factor impacting the ecology and evolution of plankton, including Daphnia. Parasites that attack the developing embryos of daphniids, known as brood parasites, were first described in the early 1900s but have received relatively little study. Here, we link previous morphological descriptions of the oomycete brood parasite Blastulidium paedophthorum with information on its phylogenetic placement, ecology, and virulence. Based on the morphology and phylogenetic relationship with other members of the Leptomitales, we show that a brood parasite observed in daphniids in the Midwestern United States is B. paedophthorum. We used morphology, DNA sequences, and laboratory infection experiments to show that B. paedophthorum is a multihost parasite that can be transmitted between species and genera. A field survey of six hosts in 15 lakes revealed that B. paedophthorum is common in all six host taxa (present on 38.3% of our host species-lake-sampling date combinations; the maximum infection prevalences were 8.7% of the population and 20% of the asexual adult female population). Although B. paedophthorum was observed in all 15 lakes, presence and infection prevalence varied among lakes. Infection with B. paedophthorum did not reduce host life span but significantly impacted host fecundity. Theory predicts that parasites that affect host fecundity without affecting host life span should have the strongest impact on host population dynamics. Based on its virulence and commonness in natural populations and on the central role of daphniids in freshwater food webs, we predict that B. paedophthorum will influence daphniid ecology and evolution, as well as the larger food web.

Population structure of a microparasite infecting Daphnia: spatio-temporal dynamics

Background

Detailed knowledge of spatial and temporal variation in the genetic population structure of hosts and parasites is required for understanding of host − parasite coevolution. As hot-spots of contemporary coevolution in natural systems are difficult to detect and long-term studies are restricted to few systems, additional population genetic data from various host − parasite systems may provide important insights into the topic. This is particularly true for parasites, as these players have been under-investigated so far due to the lower availability of suitable molecular markers. Here, we traced genetic variation (based on sequence variants in the internal transcribed spacer region, ITS) among seven geographically isolated populations of the ichthyosporean Caullerya mesnili, a common microparasite of the cladoceran Daphnia (here, the D. longispina hybrid complex). At three sites, we also studied parasite genetic variation over time (three to four sampling points) and tested for associations between parasite genotypes and host species.

Results

Parasite (and host) populations were significantly structured across space, indicating limited dispersal. Moreover, the frequency of parasite genotypes varied significantly over time, suggesting rapid evolutionary change in Caullerya. However, the distribution of parasite genotypes was similar across different host species, which might in turn have important consequences for parasite epidemiology.

Conclusions

The approach proposed here can be applied to track spatial and temporal changes in the population structure of other microparasite species for which sequence variation in the ITS or other highly variable genome regions has been documented but other types of polymorphic markers are lacking. Screening of parasite sequence variants allows for reliable detection of cross-species infections and, using advanced sequencing techniques in the near future, for detailed studies of parasite evolution in natural host − parasite systems.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-014-0247-3) contains supplementary material, which is available to authorized users.

Capturing the population structure of microparasites: using ITS-sequence data and a pooled DNA approach

The internal transcribed spacer (ITS) region of nuclear ribosomal DNA is a common marker not only for the molecular identification of different taxa and strains, but also for the analysis of population structure of wild microparasite communities. Importantly, the multicopy nature of this region allows the amplification of low-quantity samples of the target DNA, a common problem in studies on unicellular, unculturable microparasites. We analysed ITS sequences from the protozoan parasite Caullerya mesnili (class Ichthyosporea) infecting waterflea (Daphnia) hosts, across several host population samples. We showed that analysing representative ITS-types [as identified by statistical parsimony network analysis (SPN)] is a suitable method to address relevant polymorphism. The spatial patterns were consistent regardless of whether parasite DNA was extracted from individual hosts or pooled host samples. Remarkably, the efficiency in detecting different sequence types was even higher after sample pooling. As shown by simulations, an easily manageable number of sequences from pooled DNA samples are sufficient to resolve the spatial population structure in this system. In summary, the ITS region analysed from pooled DNA samples can provide valuable insights into the spatial and temporal dynamics of microparasites. Moreover, the application of SPN analysis is a good alternative to the well-established neighbour-joining method (NJ) for the identification of representative ITS-types. SPN can even outperform NJ by joining most of the singleton sequences to representative sequence clusters.

The expression and evolution of virulence in multiple infections: the role of specificity, relative virulence and relative dose

Background

Multiple infections of the same host by different strains of the same microparasite species are believed to play a crucial role during the evolution of parasite virulence. We investigated the role of specificity, relative virulence and relative dose in determining the competitive outcome of multiple infections in the Daphnia magna-Pasteuria ramosa host-parasite system.

Results

We found that infections by P. ramosa clones (single genotype) were less virulent and produced more spores than infections by P. ramosa isolates (possibly containing multiple genotypes). We also found that two similarly virulent isolates of P. ramosa differed considerably in their within-host competitiveness and their effects on host offspring production when faced with coinfecting P. ramosa isolates and clones. Although the relative virulence of a P. ramosa isolate/clone appears to be a good indicator of its competitiveness during multiple infections, the relative dose may alter the competitive outcome. Moreover, spore counts on day 20 post-infection indicate that the competitive outcome is largely decided early in the parasite’s growth phase, possibly mediated by direct interference or apparent competition.

Conclusions

Our results emphasize the importance of epidemiology as well as of various parasite traits in determining the outcome of within-host competition. Incorporating realistic epidemiological and ecological conditions when testing theoretical models of multiple infections, as well as using a wider range of host and parasite genotypes, will enable us to better understand the course of virulence evolution.

Microsporidia and 'the art of living together'

Parasitism, aptly defined as one of the 'living-together' strategies (Trager, 1986), presents a dynamic system in which the parasite and its host are under evolutionary pressure to evolve new and specific adaptations, thus enabling the coexistence of the two closely interacting partners. Microsporidia are very frequently encountered obligatory intracellular protistan parasites that can infect both animals and some protists and are a consummate example of various aspects of the 'living-together' strategy. Microsporidia, relatives of fungi in the superkingdom Opisthokonta, belong to the relatively small group of parasites for which the host cell cytoplasm is the site of both reproduction and maturation. The structural and physiological reduction of their vegetative stage, together with the manipulation of host cell physiology, enables microsporidia to live in the cytosolic environment for most of their life cycle in a way resembling endocytobionts. The ability to form structurally complex spores and the invention and assembly of a unique injection mechanism enable microsporidia to disperse within host tissues and between host organisms, resulting in long-lasting infections. Microsporidia have adapted their genomes to the intracellular way of life, evolved strategies how to obtain nutrients directly from the host and how to manipulate not only the infected cells, but also the hosts themselves. The enormous variability of host organisms and their tissues provide microsporidian parasites a virtually limitless terrain for diversification and ecological expansion. This review attempts to present a general overview of microsporidia, emphasising some less known and/or more recently discovered facets of their biology.

Natural distribution and co-infection patterns of microsporidia parasites in the Daphnia longispina complex

Microsporidia are intracellular parasites, frequently infecting the planktonic crustacean Daphnia. Questioning the ability to detect and identify microsporidia with conventional microscopic techniques, we applied molecular methods in order to investigate the distribution and co-infection patterns of this parasite among 8 communities of the Daphnia longispina hybrid complex. Eight microsporidian taxa were detected, including 3 that previously had not been characterized genetically. Microsporidian communities from nearby lakes were found to be more similar to each other, apparently due to short distance dispersal via secondary hosts. Moreover, we detected seasonal (but not interannual) changes in microsporidian community structure. With some microsporidia being host-specific, these changes might have resulted from seasonal changes in host taxon and clonal composition. The 2 dominant and closely related parasite species were found mainly in single infections, whereas another pair of related microsporidians was found predominantly in co-infections; suggesting species-level differences in the ability to colonize infected hosts. By applying molecular methods, we were not only able to unambiguously identify parasite taxa but also to reveal multiple infections that otherwise would have remained undetected. Given the increased level of accuracy and sensitivity, we highly recommend molecular approaches in future parasite surveys of Daphnia infections.

Genetic variation for maternal effects on parasite susceptibility

The expression of infectious disease is increasingly recognized to be impacted by maternal effects, where the environmental conditions experienced by mothers alter resistance to infection in offspring, independent of heritability. Here, we studied how maternal effects (high or low food availability to mothers) mediated the resistance of the crustacean Daphnia magna to its bacterial parasite Pasteuria ramosa. We sought to disentangle maternal effects from the effects of host genetic background by studying how maternal effects varied across 24 host genotypes sampled from a natural population. Under low-food conditions, females produced offspring that were relatively resistant, but this maternal effect varied strikingly between host genotypes, i.e. there were genotype by maternal environment interactions. As infection with P. ramosa causes a substantial reduction in host fecundity, this maternal effect had a large effect on host fitness. Maternal effects were also shown to impact parasite fitness, both because they prevented the establishment of the parasites and because even when parasites did establish in the offspring of poorly fed mothers, and they tended to grow more slowly. These effects indicate that food stress in the maternal generation can greatly influence parasite susceptibility and thus perhaps the evolution and coevolution of host-parasite interactions.

Converging seasonal prevalence dynamics in experimental epidemics

Background

Regular seasonal changes in prevalence of infectious diseases are often observed in nature, but the mechanisms are rarely understood. Empirical tests aiming at a better understanding of seasonal prevalence patterns are not feasible for most diseases and thus are widely lacking. Here, we set out to study experimentally the seasonal prevalence in an aquatic host-parasite system. The microsporidian parasite Hamiltosporidium tvärminnensis exhibits pronounced seasonality in natural rock pool populations of its host, Daphnia magna with a regular increase of prevalence during summer and a decrease during winter. An earlier study was, however, unable to test if different starting conditions (initial prevalence) influence the dynamics of the disease in the long term. Here, we aim at testing how the starting prevalence affects the regular prevalence changes over a 4-year period in experimental populations.

Results

In an outdoor experiment, populations were set up to include the extremes of the prevalence spectrum observed in natural populations: 5% initial prevalence mimicking a newly invading parasite, 100% mimicking a rock pool population founded by infected hosts only, and 50% prevalence which is commonly observed in natural populations in spring. The parasite exhibited similar prevalence changes in all treatments, but seasonal patterns in the 100% treatment differed significantly from those in the 5% and 50% treatments. Populations started with 5% and 50% prevalence exhibited strong and regular seasonality already in the first year. In contrast, the amplitude of changes in the 100% treatment was low throughout the experiment demonstrating the long-lasting effect of initial conditions on prevalence dynamics.

Conclusions

Our study shows that the time needed to approach the seasonal changes in prevalence depends strongly on the initial prevalence. Because individual D. magna populations in this rock pool metapopulation are mostly short lived, only few populations might ever reach a point where the initial conditions are not visible anymore.

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.

Temperature effects on parasite prevalence in a natural hybrid complex

Both host susceptibility and parasite infectivity commonly have a genetic basis, and can therefore be shaped by coevolution. However, these traits are often sensitive to environmental variation, resulting in genotype-by-environment interactions. We tested the influence of temperature on host-parasite genetic specificity in the Daphnia longispina hybrid complex, exposed to the protozoan parasite Caullerya mesnili. Infection rates were higher at low temperature. Furthermore, significant differences between host clones, but not between host taxa, and a host genotype-by-temperature interaction were observed.

Caste-specific expression of genetic variation in the size of antibiotic-producing glands of leaf-cutting ants

Social insect castes represent some of the most spectacular examples of phenotypic plasticity, with each caste being associated with different environmental conditions during their life. Here we examine the level of genetic variation in different castes of two polyandrous species of Acromyrmex leaf-cutting ant for the antibiotic-producing metapleural gland, which has a major role in defence against parasites. Gland size increases allometrically. The small workers that play the main role in disease defence have relatively large glands compared with larger workers, while the glands of gynes are substantially larger than those of any workers, for their body size. The gland size of large workers varies significantly between patrilines in both Acromyrmex echinatior and Acromyrmex octospinosus. We also examined small workers and gynes in A. echinatior, again finding genetic variation in gland size in these castes. There were significant positive relationships between the gland sizes of patrilines in the different castes, indicating that the genetic mechanism underpinning the patriline variation has remained similar across phenotypes. The level of expressed genetic variation decreased from small workers to large workers to gynes. This is consistent with the hypothesis that there is individual selection on disease defence in founding queens and colony-level selection on disease defence in the worker castes.

Molecular identification and hidden diversity of novel Daphnia parasites from European lakes

Parasites play important roles in local population dynamics and genetic structure. However, due to insufficient diagnostic tools, detailed host-parasite interactions may remain concealed by hidden parasite diversity in natural systems. Microscopic examination of 19 European lake Daphnia populations revealed the presence of three groups of parasites: fungi, microsporidia, and oomycetes. For most of these parasites no genetic markers have been described so far. Based on sequence similarities of the nuclear small-subunit and internal transcribed spacer (ITS) rRNA gene regions, one fungus, four microsporidian, and nine oomycete taxa were discovered in 147 infected Daphnia (and/or three other zooplankton crustaceans). Additionally, cloning of rRNA gene regions revealed parasite sequence variation within host individuals. This was most pronounced in the ITS region of one microsporidian taxon, where the within-host sequence variation ranged from 1.7% to 5.3% polymorphic sites for parasite isolates from 14 different geographical locations. Interestingly, the parasite isolates from close locations grouped together based on sequence similarities, suggesting that there was parasite dispersal. Taken together, the data obtained in this study revealed hidden diversity of parasite communities in Daphnia lake populations. Moreover, a higher level of resolution for identifying parasite strains makes it possible to test new hypotheses with respect to parasite dispersal, transmission routes, and coinfection.

An ancient immunity gene duplication in Daphnia magna: RNA expression and sequence analysis of two nitric oxide synthase genes

NO (nitric oxide) is a highly reactive free radical gas thought to play a major role in the invertebrate immune response by harming pathogens and limiting their growth. Here we report on studies of nitric oxide synthase (NOS) genes in the crustacean Daphnia, one of the few non-insect arthropod models used to study host-pathogen interactions. While the NOS gene is found as a single copy in other invertebrates, we found two copies (NOS1 and NOS2), which a phylogenetic reconstruction showed to be the result of an ancient duplication event. Both genes bear features commonly found in invertebrate NOS, however, the two genes differ in their rate of evolution, intraspecific polymorphism and expression level. We tested whether the more rapid evolution of NOS2 could be due to positive selection, but found the rate of amino-acid substitutions between Daphnia species to be compatible with a neutral model. To associate NOS or NO activity with infection, we performed infection experiments with Daphnia magna and one of its natural pathogens (the bacterium Pasteuria ramosa). In one set of experimental infections, we supplemented D. magna with L-arginine, the NOS substrate, or with L-NAME, a NOS antagonist, and found this to result in lower and higher infection levels, respectively, which is at least compatible with the notion that NO may aid defence against Pasteuria. A second experiment indicated that NOS transcription does not increase following exposure to Pasteuria. Thus, the function of NOS in Daphnia immunity remains uncertain, but the pattern of gene duplication and subsequent divergence suggests evolution via neo- or subfunctionalization.

Environment can alter selection in host-parasite interactions

Characteristics of hosts and parasites have a genetic basis, and thus can be shaped by coevolution. Infections measured under laboratory conditions have shown that the environment in which hosts and parasites interact might substantially affect the strength and specificity of selection. In addition, various components of host-parasite fitness are differentially altered by the environment. Despite this, environmental fluctuations are often excluded from experimental coevolutionary studies and theoretical models as 'noise'. Because most host-parasite interactions exist in heterogeneous environments, we argue that there is a need to incorporate fluctuating environments into future empirical and theoretical work on host-parasite coevolution.

The cost of being common: evidence from natural Daphnia populations

The Red Queen coevolutionary hypothesis predicts that parasites drive oscillations in host genotype frequencies due to frequency-dependent selection where common hosts are at disadvantage. However, examples of this phenomenon in natural populations are scarce. To examine if the Red Queen theory operates in the wild, we studied the genetic structure of populations of the crustacean waterflea (Daphnia), in relation to their infection levels, for which we collected multiple samples from a variety of lakes. The most common clone in a given population was often underinfected. This advantage, however, did not remain stable over time. Instead, the most common clone decreased in frequency over subsequent generations, indicating that parasites can track common clones. Such decreases were not observed in uninfected populations. Moreover, host clonal evenness was higher across the set of infected lakes compared to uninfected lakes; suggesting that any common clone is selected against when parasites are present. These results strongly suggest that Red Queen dynamics do operate in the wild.

Virulence, cultivating conditions, and phylogenetic analyses of oomycete parasites in Daphnia

We describe the infectivity, virulence, cultivating conditions, and phylogenetic positions of naturally occurring oomycete parasites of Daphnia, invertebrates which play a major role in aquatic food webs. Daphnia pulex individuals were found dead and covered by oomycete mycelia when exposed to pond sediments. We were able to extract 4 oomycete isolates from dead Daphnia and successfully cultivate them. Using the ITS and LSU rDNA sequences, we further showed these isolates to be distinct species. The isolates were experimentally demonstrated to be parasitic and not saprobic. After exposure to the parasites, Daphnia mortality was much higher than that reported for Daphnia infected with other known parasite species. Therefore, it is likely that oomycete parasites are important selective pressures in natural Daphnia populations. Moreover, their close phylogenetic relationship to parasites of fish and algae suggests that the stability of aquatic food webs (i.e. fish-Daphnia-algae) might be influenced by the shared parasite communities.

Temperature-dependent costs of parasitism and maintenance of polymorphism under genotype-by-environment interactions

The maintenance of genetic variation for infection-related traits is often attributed to coevolution between hosts and parasites, but it can also be maintained by environmental variation if the relative fitness of different genotypes changes with environmental variation. To gain insight into how infection-related traits are sensitive to environmental variation, we exposed a single host genotype of the freshwater crustacean Daphnia magna to four parasite isolates (which we assume to represent different genotypes) of its naturally co-occurring parasite Pasteuria ramosa at 15, 20 and 25 degrees C. We found that the cost to the host of becoming infected varied with temperature, but the magnitude of this cost did not depend on the parasite isolate. Temperature influenced parasite fitness traits; we found parasite genotype-by-environment (G x E) interactions for parasite transmission stage production, suggesting the potential for temperature variation to maintain genetic variation in this trait. Finally, we tested for temperature-dependent relationships between host and parasite fitness traits that form a key component of models of virulence evolution, and we found them to be stable across temperatures.

Parasite-driven genetic change in a natural population of Daphnia

A substantial body of theory indicates that parasites may mould the population genetic structure of their hosts, but few empirical studies have directly linked parasitism to genetic dynamics. We used molecular markers (allozymes) to investigate genotype frequency changes in a natural population of the crustacean Daphnia magna in relation to an epidemic of the bacterial pathogen Pasteuria ramosa. The population experienced a severe epidemic during the study period in which parasite prevalence reached 100% of the adult portion of the population. The parasite epidemic was associated with genetic change in the host population. Clonal diversity was observed to decrease as parasite prevalence increased in the population, and tests for differences in the clonal composition of the population before, during, and after the epidemic indicated that significant change had occurred. A laboratory infection experiment showed that the genotypes which were more common following the peak of the parasite epidemic were also the most resistant to parasite infection. Thus, this study provides an illustration of parasite-mediated selection in the wild.

Epidemiology of a Daphnia brood parasite and its implications on host life-history traits

Parasites influence host life-history traits and therefore might crucially shape host populations in natural systems. In a series of laboratory experiments, we studied the impact of an oomycete brood parasite on its Daphnia (waterflea) host. We asked whether Daphnia dump the infected brood and subsequently are able to reproduce again as was occasionally observed in a preliminary study. No viable offspring developed from infected clutches, but 78% of the infected females produced healthy offspring after releasing the infected brood while molting. Neither those offsprings' development success nor their mothers' reproductive potential was affected by the brood parasite. However, infected Daphnia had a reduced life-span and suffered an increased susceptibility to another parasite, an unidentified bacterium. Additionally, we studied the prevalence of this brood parasite and the unidentified bacterium in a natural Daphnia assemblage in a pre-alpine lake, across changing demographic and environmental conditions. The brood parasite epidemic seemed to be host-density dependent. Our results show that the brood parasite's impact on the host population is enhanced when combined with the unidentified bacterium.

The evolution of virulence when parasites cause host castration and gigantism

It has been suggested that the harm parasites cause to their hosts is an unavoidable consequence of parasite reproduction with costs not only for the host but also for the parasite. Castrating parasites are thought to minimize their costs by reducing host fecundity, which may minimize the chances of killing both host and parasite prematurely. We conducted a series of experiments to understand the evolution of virulence of a castrating bacterium in the planktonic crustacean Daphnia magna. By manipulating food levels during the infection of D. magna with the bacterium Pasteuria ramosa, we showed that both antagonists are resource-limited and that a negative correlation between host and parasite reproduction exists, indicating resource competition among the antagonists. Pasteuria ramosa also induces enhanced growth of its hosts (gigantism), which we found to be negatively correlated with host fecundity but positively correlated with parasite reproduction. Because infected hosts never recovered from infections, we concluded that gigantism is beneficial only for the parasite. Hosts, however, have evolved counteradaptations. We showed that infected hosts have enhanced reproduction before castration. This shift to earlier reproduction increases overall host fecundity and compromises parasite reproduction. Finally, we showed that this resource conflict is subject to genetic variation among host and parasite genotypes within a population and is therefore likely to be an important force in the coevolution of virulence in this system. A verbal model is presented and suggests that the adaptive value of gigantism is to store host resources, which are liberated after parasitic castration for later use by the growing parasite. This hypothesis assumes that infections are long lasting, that is, that they have a high life expectancy.

Experimental coevolution with bacteria and phage. The Pseudomonas fluorescens--Phi2 model system

Parasites are ubiquitous in biological systems and antagonistic coevolution between hosts and parasites is thought be a major ecological and evolutionary force. Recent experiments using laboratory populations of bacteria and their parasitic viruses, phage, have provided the first direct empirical evidence of antagonistic coevolution in action. In this article we describe this model system and synthesise recent findings that address the causes and consequences of antagonistic coevolution.

Parasite survey of a Daphnia hybrid complex: host-specificity and environment determine infection

1. Hybridization between species is a common phenomenon in plants and animals. If parasite prevalence differs for hybrids and parental species (i.e. taxa) there may be considerable consequences for relative hybrid fitness. Some studies have investigated hybrid complexes for infection, and complex-specific differences in parasite prevalence have been detected. 2. Based on the results of a field study on a hybridizing Daphnia population from a single lake, it has been hypothesized that permanently over- or under-infected hybrids do not exist. The observed field-patterns can only be temporal because taxa, in addition to single genotypes, might be the subject of parasite driven host frequency-dependent selection. Thus, parasites will track any common taxon within a hybrid complex. 3. In the present study, hybridizing Daphnia populations from 43 lakes were screened for parasite infections to obtain indirect evidence for coevolutionary cycles. It was hypothesized that, due to time lags between the evolution of resistance in host populations and the evolution of the parasite towards tracking of a common host taxon, the same Daphnia taxon will be over-infected in some lakes, while being under-infected in others. 4. Two of the four parasite species were specialists: their prevalence differed among coexisting Daphnia taxa. The varying infection patterns detected across spatially segregated hybridizing Daphnia populations are consistent with theoretical predictions for coevolutionary cycles. Thus the infection patterns, as observed under natural conditions, are temporal and unstable. 5. Additionally, the spatial distribution of the four parasite species was analysed with respect to habitat differences. The results show that the presence of a particular parasite on a host taxon was determined not only by the host-specificity of the parasite, but also by host-habitat relations.

The coexistence of hybrid and parental Daphnia: the role of parasites

Parasite driven time-lagged negative frequency-dependent selection of hosts has been studied in natural populations by following changes in host genotype frequencies over time. However, such dynamics have not been considered at higher taxonomic levels, for example, between parental species and their hybrids. In a field study on a Daphnia hybrid system, we observed that one Daphnia taxon first was relatively under-infected, but became over-infected after a strong increase in frequency. This finding is consistent with the idea of parasite evolution towards the most frequent host taxon. In two experiments, we investigated whether the assumptions made by a model of negative frequency-dependent selection apply to our host taxa system. First, we showed that the parasite can change the outcome of taxa competition and secondly, we confirmed that the over-infection of one host taxon observed in the field has a genetic basis. Our results indicate that the incorporation of host-parasite interactions at the species level may allow us to gain a more complete picture of forces driving dynamic taxa coexistence in Daphnia hybrid systems. More generally, we suggest that if hybrids coexist in sympatry with parental taxa, the infection patterns as observed under natural conditions may be rather temporal and unstable.

Can overwintering versus diapausing strategy in Daphnia determine match–mismatch events in zooplankton–algae interactions?

Mismatches between predator and prey due to climate change have now been documented for a number of systems. Ultimately, a mismatch may have far-reaching consequences for ecosystem functioning as decoupling of trophic relationships results in trophic cascades. Here, we examine the potential for climate change induced mismatches between zooplankton and algae during spring succession, with a focus on Daphnia and its algal food. Whereas the development of an overwintering population of daphnids may parallel shifts in phytoplankton phenology due to climate warming, changes in the photoperiod-temperature interaction may cause the emerging population of daphnids to hatch too late and mismatch their phytoplankton prey. A decoupling of the trophic relationship between the keystone herbivore Daphnia and its algal prey can result in the absence of a spring clear water phase. We extended an existing minimal model of seasonal dynamics of Daphnia and algae and varied the way the Daphnia population is started in spring, i.e., from free swimming individuals or from hatching resting eggs. Our model results show that temperature affects the timing of peak abundance in Daphnia and algae, and subsequently the timing of the clear water phase. When a population is started from a small inoculum of hatching resting eggs, extreme climate warming (+6 degrees C) results in a decoupling of trophic relationships and the clear water phase fails to occur. In the other scenarios, the trophic relationships between Daphnia and its algal food source remain intact. Analysis of 36 temperate lakes showed that shallow lakes have a higher potential for climate induced match-mismatches, as the probability of active overwintering daphnids decreases with lake depth. Future research should point out whether lake depth is a direct causal factor in determining the presence of active overwintering daphnids or merely indicative for underlying causal factors such as fish predation and macrophyte cover.

Does genetic diversity hinder parasite evolution in social insect colonies?

Polyandry is often difficult to explain because benefits of the behaviour have proved elusive. In social insects, polyandry increases the genetic diversity of workers within a colony and this has been suggested to improve the resistance of the colony to disease. Here we examine the possible impact of host genetic diversity on parasite evolution by carrying out serial passages of a virulent fungal pathogen through leaf-cutting ant workers of known genotypes. Parasite virulence increased over the nine-generation span of the experiment while spore production decreased. The effect of host relatedness upon virulence appeared limited. However, parasites cycled through more genetically diverse hosts were more likely to go extinct during the experiment and parasites cycled through more genetically similar hosts had greater spore production. These results indicate that host genetic diversity may indeed hinder the ability of parasites to adapt while cycling within social insect colonies.

Parasite-mediated selection and the role of sex and diapause in Daphnia

To gain insight into parasite-mediated natural selection, we studied a natural population of the crustacean Daphnia magna during a severe epidemic of the bacterial parasite Pasteuria ramosa. We also investigated the relationship between susceptibility and the production of resting eggs, which are only produced during the sexual phase of reproduction. Live host samples were taken before and after this epidemic and resistance to P. ramosa was examined in the laboratory. Host clones collected after the epidemic were more resistant to P. ramosa than were those collected pre-epidemic, which is consistent with parasite-mediated selection. In our study population, asexually reproducing females were observed across the entire study period, but females carrying resting eggs were observed only prior to the epidemic. For hosts isolated in this pre-epidemic period, we found evidence that those carrying resting eggs (at the time of collection) were more susceptible than those that were reproducing asexually. This was especially apparent for measures of parasite growth, although not all measures of infection success conclusively supported this pattern. Nevertheless, the data suggest that some genotypes invest heavily in diapause at the expense of immunocompetence. Sex could therefore inhibit the evolution of resistance because each spring new genotypes will hatch from resting eggs that are relatively susceptible as they were not exposed to the previous years bout of parasite-mediated selection.

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.

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.

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.

Genetic diversity and disease resistance in leaf-cutting ant societies

Multiple mating by females (polyandry) remains hard to explain because, while it has substantial costs, clear benefits have remained elusive. The problem is acute in the social insects because polyandry is probably particularly costly for females and most material benefits of the behavior are unlikely to apply. It has been suggested that a fitness benefit may arise from the more genetically diverse worker force that a polyandrous queen will produce. One leading hypothesis is that the increased genetic diversity of workers will improve a colony's resistance to disease. We investigated this hypothesis using a polyandrous leaf-cutting ant and a virulent fungal parasite as our model system. At high doses of the parasite most patrilines within colonies were similarly susceptible, but a few showed greater resistance. At a low dose of the parasite there was more variation between patrilines in their resistance to the parasite. Such genetic variation is a key prerequisite for polyandry to result in increased disease resistance of colonies. The relatedness of two hosts did not appear to affect the transmission of the parasite between them, but this was most likely because the parasite tested was a virulent generalist that is adapted to transmit between distantly related hosts. The resistance to the parasite was compared between small groups of ants of either high or low genetic diversity. No difference was found at high doses of the parasite, but a significant improvement in resistance in high genetic diversity groups was found at a low dose of the parasite. That there is genetic variation for disease resistance means that there is the potential for polyandry to produce more disease-resistant colonies. That this genetic variation can improve the resistance of groups even under the limited conditions tested suggests that polyandry may indeed produce colonies with improved resistance to disease.