Genetic variation in a host-parasite association: potential for coevolution and frequency-dependent selection

Abstract.— Models of host‐parasite coevolution assume the presence of genetic variation for host resistance and parasite infectivity, as well as genotype‐specific interactions. We used the freshwater crustacean Daphnia magna and its bacterial microparasite Pasteuria ramosa to study genetic variation for host susceptibility and parasite infectivity within each of two populations. We sought to answer the following questions: Do host clones differ in their susceptibility to parasite isolates? Do parasite isolates differ in their ability to infect different host clones? Are there host clone‐parasite isolate interactions? The analysis revealed considerable variation in both host resistance and parasite infectivity. There were significant host clone‐parasite isolate interactions, such that there was no single host clone that was superior to all other clones in the resistance to every parasite isolate. Likewise, there was no parasite isolate that was superior to all other isolates in infectivity to every host clone. This form of host clone‐parasite isolate interaction indicates the potential for coevolution based on frequency‐dependent selection. Infection success of original host clone‐parasite isolate combinations (i.e., those combinations that were isolated together) was significantly higher than infection success of novel host clone‐parasite isolate combinations (i.e., those combinations that were created in the laboratory). This finding is consistent with the idea that parasites track specific host genotypes under natural conditions. In addition, correspondence analysis revealed that some host clones, although distinguishable with neutral genetic markers, were susceptible to the same set of parasite isolates and thus probably shared resistance genes.     

The evolutionary significance of parasitism: do parasite‐driven genetic dynamics occur ex silico?

It has long been recognized that reciprocal antagonism might lock host and parasite populations into a process of constant change, adapting and reacting in open‐ended coevolution. A significant body of theory supports this intuition: dynamic genetic polymorphisms are a common outcome of computer simulations of host–parasite coevolution. These in silico experiments have also shown that dynamical interactions could be responsible for high levels of genetic diversity in host populations, and even be the principle determinant of rates of genetic recombination and sexuality. The evolutionary significance of parasitism depends on the strength and prevalence of parasite‐mediated selection in nature. Here I appraise whether parasitism is a pervasive agent of evolutionary change by detailing empirical evidence for selection. Although there is considerable evidence of genetic variation for resistance, and hence the potential for selection, direct observation of parasite‐driven genetic change is lacking.     

THE DYNAMICS OF RECIPROCAL SELECTIVE SWEEPS OF HOST RESISTANCE AND A PARASITE COUNTER‐ADAPTATION IN DROSOPHILA

Host–parasite coevolution can result in consecutive selective sweeps of host resistance alleles and parasite counter‐adaptations. To illustrate the dynamics of this important but little studied form of coevolution, we have modeled an ongoing arms race between Drosophila melanogaster and the vertically transmitted sigma virus, using parameters we estimated in the field. We integrate these results with previous work showing that the spread of a resistance allele of the ref(2)P gene in the host was followed by the spread of a virus genotype, which overcomes this resistance. In line with these observations, our model predicts that there can be rapid selective sweeps in both the host and parasite, which can drive large changes in the prevalence of infection. The virus will tend to be ahead in the arms race, as incomplete dominance slows down host adaptation and selection for host resistance is weaker than selection for parasites to overcome resistance—the “life‐dinner” principle. This asymmetry in the adaptation rates results in a partial sweep of the host resistance allele, as it loses its advantage part way through the selective sweep. This well‐understood natural system illustrates how the outcome of host–parasite coevolution is determined by different population genetic parameters in the field.     

POSITIVE GENETIC CORRELATION BETWEEN PARASITE RESISTANCE AND BODY SIZE IN A FREE-LIVING UNGULATE POPULATION

Abstract Parasite resistance and body size are subject to directional natural selection in a population of feral Soay sheep (Ovis aries) on the island of St. Kilda, Scotland. Classical evolutionary theory predicts that directional selection should erode additive genetic variation and favor the maintenance of alleles that have negative pleiotropic effects on other traits associated with fitness. Contrary to these predictions, in this study we show that there is considerable additive genetic variation for both parasite resistance, measured as fecal egg count (FEC), and body size, measured as weight and hindleg length, and that there are positive genetic correlations between parasite resistance and body size in both sexes. Body size traits had higher heritabilities than parasite resistance. This was not due to low levels of additive genetic variation for parasite resistance, but was a consequence of high levels of residual variance in FEC. Measured as coefficients of variation, levels of additive genetic variation for FEC were actually higher than for weight or hindleg length. High levels of additive genetic variation for parasite resistance may be maintained by a number of mechanisms including high mutational input, balancing selection, antagonistic pleiotropy, and host‐parasite coevolution. The positive genetic correlation between parasite resistance and body size, a trait also subject to sexual selection in males, suggests that parasite resistance and growth are not traded off in Soay sheep, but rather that genetically resistant individuals also experience superior growth.     

Host–parasite coevolution favours parasite genetic diversity and horizontal gene transfer

Host–parasite coevolution is predicted to favour genetic diversity and the underlying mechanisms (e.g. sexual reproduction and, more generally, genetic exchange), because diversity enhances the antagonists' potential for rapid adaptation. To date, this prediction has mainly been tested and confirmed for the host. It should similarly apply to the parasite. Indeed, our previous work demonstrated that experimental coevolution between the nematode Caenorhabditis elegans and its microparasite Bacillus thuringiensis selects for genetic diversity in both antagonists. For the parasite, the previous analysis was based on plasmid‐encoded toxin gene markers. Thus, it was restricted to a very small part of the bacterial genome and did not cover the main chromosome, which harbours a large variety of virulence factors. Here, we present new data for chromosomal gene markers of B. thuringiensis and combine this information with the previous results on plasmid‐encoded toxins. Our new results demonstrate that, in comparison with the control treatment, coevolution with a host similarly leads to higher levels of genetic diversity in the bacterial chromosome, thus indicating the relevance of chromosomal genes for coevolution. Furthermore, the frequency of toxin gene gain is significantly elevated during coevolution, highlighting the importance of horizontal gene transfer as a diversity‐generating mechanism. In conclusion, our study emphasizes the strong influence of antagonistic coevolution on parasite genetic diversity and gene exchange.     

Evolution of host resistance and trade‐offs between virulence and transmission potential in an obligately killing parasite

Standard epidemiological theory predicts that parasites, which continuously release propagules during infection, face a trade‐off between virulence and transmission. However, little is known how host resistance and parasite virulence change during coevolution with obligate killers. To address this question we have set up a coevolution experiment evolving Nosema whitei on eight distinct lines of Tribolium castaneum. After 11 generations we conducted a time‐shift experiment infecting both the coevolved and the replicate control host lines with the original parasite source, and coevolved parasites from generation 8 and 11. We found higher survival in the coevolved host lines than in the matching control lines. In the parasite populations, virulence measured as host mortality decreased during coevolution, while sporeload stayed constant. Both patterns are compatible with adaptive evolution by selection for resistance in the host and by trade‐offs between virulence and transmission potential in the parasite.     

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.     

A geographical mosaic of coevolution in a slave-making host-parasite system

Three different isolated populations of the slave‐making ant Rossomyrmex minuchae, sympatric with its obligate host Proformica longiseta, are known from the high mountains of southern Spain. To test the prediction that the slave‐maker and its host represent a coevolutionary geographical mosaic, we studied the variation in the cuticular hydrocarbons (CHCs) as the trait most likely to show the selection mosaic, plus trait remixing by the gene flow in the populations of each species by means of microsatellites. We found within populations, host and parasite had more similar CHC profiles than between the populations or between parasites and allopatric hosts. The differences between the CHC profiles of the host and parasite, which may be responsible for the level of tolerance towards the parasite, varied between the populations suggesting the existence of a selection mosaic of coevolution. Furthermore, P. longiseta showed higher gene flow than R. minuchae, which would allow local variation in the coevolution of the host and parasite while allowing some trait remixing.     

SELECTION AND STRAIN SPECIFICITY OF COMPATIBILITY BETWEEN SNAIL INTERMEDIATE HOSTS AND THEIR PARASITIC SCHISTOSOMES

Many theoretical models of host-parasite coevolution assume that variation in host resistance to parasite infection is, at least partially, genetically determined and specific to the strain of infecting parasite. However, very few experimental studies have been conducted to test this assumption in animal-parasite systems. Biomphalaria glabrata snails serve as the intermediate hosts of Schistosoma mansoni. Although some snails are resistant to infection, there is no evidence of fixation of resistance in field populations. Two possible explanations for this are high fitness costs associated with resistance and a dynamic coevolution between parasite and host, perhaps involving matching alleles or gene-for-gene interactions. Two strains of B. glabrata were artificially selected for either resistance or susceptibility to each of two strains of S. mansoni parasite for three generations. Third-generation snails were then were exposed to either the parasite strain to which they had been selected or to a different parasite strain. In both host strains, resistance and susceptibility (compatibility) were found to be heritable. Moreover, compatibility to one parasite strain was not associated with compatibility to another strain, implying no genetic trade-off. Our results are discussed in terms of potential mechanisms of resistance in this host-parasite system and their implications to general coevolutionary theory.     

THE GEOGRAPHY OF COEVOLUTION: COMPARATIVE POPULATION STRUCTURES FOR A SNAIL AND ITS TREMATODE PARASITE

Gene flow and the genetic structure of host and parasite populations are critical to the coevolutionary process, including the conditions under which antagonistic coevolution favors sexual reproduction. Here we compare the genetic structures of different populations of a freshwater New Zealand snail (Potamopyrgus antipodarum) with its trematode parasite (Microphallus sp.) using allozyme frequency data. Allozyme variation among snail populations was found to be highly structured among lakes; but for the parasite there was little allozyme structure among lake populations, suggesting much higher levels of parasite gene flow. The overall pattern of variation was confirmed with principal component analysis, which also showed that the organization of genetic differentiation for the snail (but not the parasite) was strongly related to the geographic arrangement of lakes. Some snail populations from different sides of the Alps near mountain passes were more similar to each other than to other snail populations on the same side of the Alps. Furthermore, genetic distances among parasite populations were correlated with the genetic distances among host populations, and genetic distances among both host and parasite populations were correlated with “stepping-stone” distances among lakes. Hence, the host snail and its trematode parasite seem to be dispersing to adjacent lakes in a stepping-stone fashion, although parasite dispersal among lakes is clearly greater. High parasite gene flow should help to continuously reintroduce genetic diversity within local populations where strong selection might otherwise isolate “host races.” Parasite gene flow can thereby facilitate the coevolutionary (Red Queen) dynamics that confer an advantage to sexual reproduction by restoring lost genetic variation.     

Genetic diversity,population structure and sex‐biased dispersal in three co‐evolving species

Genetic diversity and spatial structure of populations are important for antagonistic coevolution. We investigated genetic variation and population structure of three closely related European ant species: the social parasite Harpagoxenus sublaevis and its two host species Leptothorax acervorum and Leptothorax muscorum. We sampled populations in 12 countries and analysed eight microsatellite loci and an mtDNA sequence. We found high levels of genetic variation in all three species, only slightly less variation in the host L. muscorum. Using a newly introduced measure of differentiation (Jost’s D_est ), we detected strong population structuring in all species and less male‐biased dispersal than previously thought. We found no phylogeographic patterns that could give information on post‐glacial colonization routes – northern populations are as variable as more southern populations. We conclude that conditions for Thompson’s geographic mosaic of coevolution are ideal in this system: all three species show ample genetic variation and strong population structure.     

Spatial population genetic structure of a bacterial parasite in close coevolution with its host

Knowledge of a species’ population genetic structure can provide insight into fundamental ecological and evolutionary processes including gene flow, genetic drift and adaptive evolution. Such inference is of particular importance for parasites, as an understanding of their population structure can illuminate epidemiological and coevolutionary dynamics. Here, we describe the population genetic structure of the bacterium Pasteuria ramosa, a parasite that infects planktonic crustaceans of the genus Daphnia. This system has become a model for investigations of host–parasite interactions and represents an example of coevolution via negative frequency‐dependent selection (aka “Red Queen” dynamics). To sample P. ramosa, we experimentally infected a panel of Daphnia hosts with natural spore banks from the sediments of 25 ponds throughout much of the species range in Europe and western Asia. Using 12 polymorphic variable number tandem repeat loci (VNTR loci), we identified substantial genetic diversity, both within and among localities, that was structured geographically among ponds. Genetic diversity was also structured among host genotypes within ponds, although this pattern varied by locality, with P. ramosa at some localities partitioned into distinct host‐specific lineages, and other localities where recombination had shuffled genetic variation among different infection phenotypes. Across the sample range, there was a pattern of isolation by distance, and principal components analysis coupled with Procrustes rotation identified congruence between patterns of genetic variation and geography. Our findings support the hypothesis that Pasteuria is an endemic parasite coevolving closely with its host. These results provide important context for previous studies of this model system and inform hypotheses for future research.     

Host–parasite genotypic interactions in the honey bee: the dynamics of diversity

Parasites are thought to be a major driving force shaping genetic variation in their host, and are suggested to be a significant reason for the maintenance of sexual reproduction. A leading hypothesis for the occurrence of multiple mating (polyandry) in social insects is that the genetic diversity generated within‐colonies through this behavior promotes disease resistance. This benefit is likely to be particularly significant when colonies are exposed to multiple species and strains of parasites, but host–parasite genotypic interactions in social insects are little known. We investigated this using honey bees, which are naturally polyandrous and consequently produce genetically diverse colonies containing multiple genotypes (patrilines), and which are also known to host multiple strains of various parasite species. We found that host genotypes differed significantly in their resistance to different strains of the obligate fungal parasite that causes chalkbrood disease, while genotypic variation in resistance to the facultative fungal parasite that causes stonebrood disease was less pronounced. Our results show that genetic variation in disease resistance depends in part on the parasite genotype, as well as species, with the latter most likely relating to differences in parasite life history and host–parasite coevolution. Our results suggest that the selection pressure from genetically diverse parasites might be an important driving force in the evolution of polyandry, a mechanism that generates significant genetic diversity in social insects.     

Experimental evolution with a multicellular host causes diversification within and between microbial parasite populations—Differences in emerging phenotypes of two different parasite strains

Host‐parasite coevolution is predicted to have complex evolutionary consequences, potentially leading to the emergence of genetic and phenotypic diversity for both antagonists. However, little is known about variation in phenotypic responses to coevolution between different parasite strains exposed to the same experimental conditions. We infected Caenorhabditis elegans with one of two strains of Bacillus thuringiensis and either allowed the host and the parasite to experimentally coevolve (coevolution treatment) or allowed only the parasite to adapt to the host (one‐sided parasite adaptation). By isolating single parasite clones from evolved populations, we found phenotypic diversification of the ancestral strain into distinct clones, which varied in virulence toward ancestral hosts and competitive ability against other parasite genotypes. Parasite phenotypes differed remarkably not only between the two strains, but also between and within different replicate populations, indicating diversification of the clonal population caused by selection. This study highlights that the evolutionary selection pressure mediated by a multicellular host causes phenotypic diversification, but not necessarily with the same phenotypic outcome for different parasite strains.     

The impact of parasitism on resource allocation in a fungal host: the case of Cryphonectria parasitica and its mycovirus,Cryphonectria Hypovirus 1

Parasites are known to profoundly affect resource allocation in their host. In order to investigate the effects of Cryphonectria Hypovirus 1 (CHV1) on the life‐history traits of its fungal host Cryphonectria parasitica, an infection matrix was completed with the cross‐infection of six fungal isolates by six different viruses. Mycelial growth, asexual sporulation, and spore size were measured in the 36 combinations, for which horizontal and vertical transmission of the viruses was also assessed. As expected by life‐history theory, a significant negative correlation was found between host somatic growth and asexual reproduction in virus‐free isolates. Interestingly this trade‐off was found to be positive in infected isolates, illustrating the profound changes in host resource allocation induced by CHV1 infection. A significant and positive relationship was also found in infected isolates between vertical transmission and somatic growth. This last relationship suggests that in this system, high levels of virulence could be detrimental to the vertical transmission of the parasite. Those results underscore the interest of studying host–parasite interaction within the life‐history theory framework, which might permit a more accurate understanding of the nature of the modifications triggered by parasite infection on host biology.     

Strong specificity in the interaction between parasitoids and symbiont‐protected hosts

Coevolution between hosts and parasites may promote the maintenance of genetic variation in both antagonists by negative frequency‐dependence if the host–parasite interaction is genotype‐specific. Here we tested for specificity in the interaction between parasitoids (Lysiphlebus fabarum) and aphid hosts (Aphis fabae) that are protected by a heritable defensive endosymbiont, the γ‐proteobacterium Hamiltonella defensa. Previous studies reported a lack of genotype specificity between unprotected aphids and parasitoids, but suggested that symbiont‐conferred resistance might exhibit a higher degree of specificity. Indeed, in addition to ample variation in host resistance as well as parasitoid infectivity, we found a strong aphid clone‐by‐parasitoid line interaction on the rates of successful parasitism. This genotype specificity appears to be mediated by H. defensa, highlighting the important role that endosymbionts can play in host–parasite coevolution.     

Genotype specificity among hosts,pathogens, and beneficial microbes influences the strength of symbiont‐mediated protection

The microbial symbionts of eukaryotes influence disease resistance in many host‐parasite systems. Symbionts show substantial variation in both genotype and phenotype, but it is unclear how natural selection maintains this variation. It is also unknown whether variable symbiont genotypes show specificity with the genotypes of hosts or parasites in natural populations. Genotype by genotype interactions are a necessary condition for coevolution between interacting species. Uncovering the patterns of genetic specificity among hosts, symbionts, and parasites is therefore critical for determining the role that symbionts play in host‐parasite coevolution. Here, we show that the strength of protection conferred against a fungal pathogen by a vertically transmitted symbiont of an aphid is influenced by both host‐symbiont and symbiont‐pathogen genotype by genotype interactions. Further, we show that certain symbiont phylogenetic clades have evolved to provide stronger protection against particular pathogen genotypes. However, we found no evidence of reciprocal adaptation of co‐occurring host and symbiont lineages. Our results suggest that genetic variation among symbiont strains may be maintained by antagonistic coevolution with their host and/or their host's parasites.     

GENETIC VARIATION IN ASTERIONELLA FORMOSA (BACILLARIOPHYCEAE): IS IT LINKED TO FREQUENT EPIDEMICS OF HOST‐SPECIFIC PARASITIC FUNGI?1

Understanding of the genetic basis for susceptibility and resistance is still lacking for most aquatic host–parasite systems, for instance, for phytoplankton and their fungal parasites. Fungal parasites can have significant effects on phytoplankton populations, mainly through their ability to decimate algal host populations during epidemics. We used random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) analysis to study levels of genetic variation within a population of the freshwater diatom Asterionella formosa Hassall in relation to parasitism by the obligate, host‐specific, fungal parasite Zygorhizidium planktonicum Canter. The level of genetic variation within the A. formosa population in Lake Maarsseveen, The Netherlands was found to be high despite the presumed absence or very low frequency of sexual reproduction in this species, the limited gene flow, and the severity of parasite attack that would purge the population from susceptible genotypes. RAPD analysis revealed four distinct banding patterns, with 3 of 21 markers (14%) being polymorphic. In AFLP analysis, every single isolate of A. formosa showed a unique banding pattern, and 120 of the 210 AFLP markers (57%) were found to be polymorphic. Furthermore, character compatibility analysis revealed that sexual reproduction may be one of the mechanisms that generates and maintains genetic variation in the A. formosa population in Lake Maarsseveen. The presence of genetic variation in A. formosa was reflected in infection experiments, which showed that genetically different A. formosa strains differed in their susceptibility to various Z. planktonicum strains and that parasite strains differed in their ability to infect particular host strains.     

Genotypic and phenotypic variation in transmission traits of a complex life cycle parasite

Characterizing genetic variation in parasite transmission traits and its contribution to parasite vigor is essential for understanding the evolution of parasite life‐history traits. We measured genetic variation in output, activity, survival, and infection success of clonal transmission stages (cercaria larvae) of a complex life cycle parasite (Diplostomum pseudospathaceum). We further tested if variation in host nutritional stage had an effect on these traits by keeping hosts on limited or ad libitum diet. The traits we measured were highly variable among parasite genotypes indicating significant genetic variation in these life‐history traits. Traits were also phenotypically variable, for example, there was significant variation in the measured traits over time within each genotype. However, host nutritional stage had no effect on the parasite traits suggesting that a short‐term reduction in host resources was not limiting the cercarial output or performance. Overall, these results suggest significant interclonal and phenotypic variation in parasite transmission traits that are not affected by host nutritional status.     

Within‐ and among‐population variation in infectivity,latency and spore production in a host–pathogen system

In spatially structured populations, host–parasite coevolutionary potential depends on the distribution of genetic variation within and among populations. Inoculation experiments using the plant, Silene latifolia, and its fungal pathogen, Microbotryum violaceum, revealed little overall differentiation in infectivity/resistance, latency or spore production among host or pathogen populations. Within populations, fungal strains had similar means, but varied in performance across plant populations. Variation in resistance among seed families indicates the potential for parasite‐mediated selection, whereas there was little evidence for local pathogen genotype × plant genotype interactions assumed by most theoretical coevolution models. Lower spore production on sympatric than allopatric hosts confirmed local fungal maladaptation already observed for infectivity. Correlations between infectivity and latency or spore production suggest a common mechanism for variation in these traits. Our results suggest low variation available to this pathogen for tracking its coevolving host. This may be caused by random drift, breeding system or migration characteristic of metapopulation dynamics.