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.     

Sexual signalling in female crested macaques and the evolution of primate fertility signals

Background

Host-parasite coevolution can lead to local adaptation of either parasite or host if there is specificity (GxG interactions) and asymmetric evolutionary potential between host and parasite. This has been demonstrated both experimentally and in field studies, but a substantial proportion of studies fail to detect such clear-cut patterns. One explanation for this is that adaptation can be masked by counter-adaptation by the antagonist. Additionally, genetic architecture underlying the interaction is often highly complex thus preventing specific adaptive responses. Here, we have employed a reciprocal cross-infection experiment to unravel the adaptive responses of two components of fitness affecting both parties with different complexities of the underlying genetic architecture (i.e. mortality and spore load). Furthermore, our experimental coevolution of hosts (Tribolium castaneum) and parasites (Nosema whitei) included paired replicates of naive hosts from identical genetic backgrounds to allow separation between host- and parasite-specific responses.

Results

In hosts, coevolution led to higher resistance and altered resistance profiles compared to paired control lines. Host genotype × parasite genotype interactions (G_H × G_P) were observed for spore load (the trait of lower genetic complexity), but not for mortality. Overall parasite performance correlated with resistance of its matching host coevolution background reflecting a directional and unspecific response to strength of selection during coevolution. Despite high selective pressures exerted by the obligatory killing parasite, and host- and parasite-specific mortality profiles, no general pattern of local adaptation was observed, but one case of parasite maladaptation was consistently observed on both coevolved and control host populations. In addition, the use of replicate control host populations in the assay revealed one case of host maladaptation and one case of parasite adaptation that was masked by host counter-adaptation, suggesting the presence of complex and probably dynamically changing fitness landscapes.

Conclusions

Our results demonstrate that the use of replicate naive populations can be a useful tool to differentiate between host and parasite adaptation in complex and dynamic fitness landscapes. The absence of clear local adaptation patterns during coevolution with a sexual host showing a complex genetic architecture for resistance suggests that directional selection for generality may be more important attributes of host-parasite coevolution than commonly assumed.     

Host-parasite local adaptation after experimental coevolution of Caenorhabditis elegans and its microparasite Bacillus thuringiensis

Coevolving hosts and parasites can adapt to their local antagonist. In studies on natural populations, the observation of local adaptation patterns is thus often taken as indirect evidence for coevolution. Based on this approach, coevolution was previously inferred from an overall pattern of either parasite or host local adaptation. Many studies, however, failed to detect such a pattern. One explanation is that the studied system was not subject to coevolution. Alternatively, coevolution occurred, but remained undetected because it took different routes in different populations. In some populations, it is the host that is locally adapted, whereas in others it is the parasite, leading to the absence of an overall local adaptation pattern. Here, we test for overall as well as population-specific patterns of local adaptation using experimentally coevolved populations of the nematode Caenorhabditis elegans and its bacterial microparasite Bacillus thuringiensis. Furthermore, we assessed the importance of random interaction effects using control populations that evolved in the absence of the respective antagonist. Our results demonstrate that experimental coevolution produces distinct local adaptation patterns in different replicate populations, including host, parasite or absence of local adaptation. Our study thus provides experimental evidence of the predictions of the geographical mosaic theory of coevolution, i.e. that the interaction between parasite and host varies across populations.     

Experimental coevolution leads to a decrease in parasite-induced host mortality

Host-parasite coevolution can lead to a variety of outcomes, but whereas experimental studies on clonal populations have taken prominence over the last years, experimental studies on obligately out-crossing organisms are virtually absent so far. Therefore, we set up a coevolution experiment using four genetically distinct lines of Tribolium castaneum and its natural obligately killing microsporidian parasite, Nosema whitei. After 13 generations of experimental coevolution, we employed a time-shift experiment infecting hosts from the current generation with parasites from nine different time points in coevolutionary history. Although initially parasite-induced mortality showed synchronized fluctuations across lines, a general decrease over time was observed, potentially reflecting evolution towards optimal levels of virulence or a failure to adapt to coevolving sexual hosts.     

Parasite host range and the evolution of host resistance

Parasite host range plays a pivotal role in the evolution and ecology of hosts and the emergence of infectious disease. Although the factors that promote host range and the epidemiological consequences of variation in host range are relatively well characterized, the effect of parasite host range on host resistance evolution is less well understood. In this study, we tested the impact of parasite host range on host resistance evolution. To do so, we used the host bacterium Pseudomonas fluorescens SBW25 and a diverse suite of coevolved viral parasites (lytic bacteriophage Φ2) with variable host ranges (defined here as the number of host genotypes that can be infected) as our experimental model organisms. Our results show that resistance evolution to coevolved phages occurred at a much lower rate than to ancestral phage (approximately 50% vs. 100%), but the host range of coevolved phages did not influence the likelihood of resistance evolution. We also show that the host range of both single parasites and populations of parasites does not affect the breadth of the resulting resistance range in a naïve host but that hosts that evolve resistance to single parasites are more likely to resist other (genetically) more closely related parasites as a correlated response. These findings have important implications for our understanding of resistance evolution in natural populations of bacteria and viruses and other host–parasite combinations with similar underlying infection genetics, as well as the development of phage therapy.     

Antagonistic coevolution between a bacterium and a bacteriophage

Antagonistic coevolution between hosts and parasites is believed to play a pivotal role in host and parasite population dynamics, the evolutionary maintenance of sex and the evolution of parasite virulence. Furthermore, antagonistic coevolution is believed to be responsible for rapid differentiation of both hosts and parasites between geographically structured populations. Yet empirical evidence for host-parasite antagonistic coevolution, and its impact on between-population genetic divergence, is limited. Here we demonstrate a long-term arms race between the infectivity of a viral parasite (bacteriophage; phage) and the resistance of its bacterial host. Coevolution was largely driven by directional selection, with hosts becoming resistant to a wider range of parasite genotypes and parasites infective to a wider range of host genotypes. Coevolution followed divergent trajectories between replicate communities despite establishment with isogenic bacteria and phage, and resulted in bacteria adapted to their own, compared with other, phage populations.     

Balancing Selection for Pathogen Resistance Reveals an Intercontinental Signature of Red Queen Coevolution

The link between long-term host–parasite coevolution and genetic diversity is key to understanding genetic epidemiology and the evolution of resistance. The model of Red Queen host–parasite coevolution posits that high genetic diversity is maintained when rare host resistance variants have a selective advantage, which is believed to be the mechanistic basis for the extraordinarily high levels of diversity at disease-related genes such as the major histocompatibility complex in jawed vertebrates and R-genes in plants. The parasites that drive long-term coevolution are, however, often elusive. Here we present evidence for long-term balancing selection at the phenotypic (variation in resistance) and genomic (resistance locus) level in a particular host–parasite system: the planktonic crustacean Daphnia magna and the bacterium Pasteuria ramosa. The host shows widespread polymorphisms for pathogen resistance regardless of geographic distance, even though there is a clear genome-wide pattern of isolation by distance at other sites. In the genomic region of a previously identified resistance supergene, we observed consistent molecular signals of balancing selection, including higher genetic diversity, older coalescence times, and lower differentiation between populations, which set this region apart from the rest of the genome. We propose that specific long-term coevolution by negative-frequency-dependent selection drives this elevated diversity at the host''s resistance loci on an intercontinental scale and provide an example of a direct link between the host’s resistance to a virulent pathogen and the large-scale diversity of its underlying genes.     

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.     

Selection by parasites may increase host recombination frequency

Meiotic recombination destroys successful genotypes and it is therefore thought to evolve only under a very limited set of conditions. Here, we experimentally show that recombination rates across two linkage groups of the host, the red flour beetle Tribolium castaneum, increase with exposure to the microsporidian parasite, Nosema whitei, particularly when parasites were allowed to coevolve with their hosts. Selection by randomly varied parasites resulted in smaller effects, while directional selection for insecticide resistance initially reduced recombination slightly. These results, at least tentatively, suggest that short-term benefits of recombination--and thus the evolution of sex--may be related to parasitism.     

Evolution of reproductive isolation as a by‐product of divergent life‐history evolution in laboratory populations of Drosophila melanogaster

We show that two complementary asymmetric isolating mechanisms, likely mediated by divergence in body size, underlie the evolution of incipient reproductive isolation between a set of Drosophila melanogaster populations selected for rapid development and their ancestral controls. Selection has led to great reduction in body size in the fast developing lines. Small males belonging to fast developing lines obtain few matings with large control females, both in presence and absence of large control line males, giving rise to unidirectional, premating isolation caused by sexual selection. Conversely, small selected line females suffer greatly increased mortality following mating with large control males, causing unidirectional postcopulatory prezygotic isolation. We discuss preliminary evidence for evolution of reduced male harm caused to females upon mating in the fast developing lines, and speculate that the females from these lines have coevolved reduced resistance to male harm such that they can no longer resist the harm caused by males from control lines. This potentially implicates differing levels of sexual conflict in creating reproductive barrier between the selected line females and the control males. We also show that a large difference in development time is not sufficient to cause postzygotic incompatibilities in the two sets of populations reaffirming the belief that prezygotic isolation can evolve much earlier than postzygotic isolation.     

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.     

Nonadditive genetic components in resistance of the red flour beetle Tribolium castanaeum against parasite infection

Genetically coupled antagonistic coevolution between host and parasites can select for the maintenance of recombination in the host. Mechanistically, maintenance of recombination relies on epistatic interactions between resistance genes creating linkage disequilibria (LD). The role of epistasis in host resistance traits is however only partly understood. Therefore, we applied the joint scaling principle to assess epistasis and other nonadditive genetic components of two resistance traits, survival, and parasite spore load, in population crosses of the red flour beetle Tribolium castanaeum under infections with the microsporidian Nosema whitei. We found nonadditive components only in infected populations but not in control populations. The genetic architecture underlying survival under parasite infection was more complex than that of spore load. Accordingly, the observed negative correlation between survival and spore load was mainly based on a correlation between shared additive components. Breakdown of resistance was especially strong in F2 crosses between resistant lines indicating that multiple epistatic routes can lead to the same adaptation. In general, the wide range of nonoverlapping genetic components between crosses indicated that parasite resistance in T. castanaeum can be understood as a multi peaked fitness landscape with epistasis contributing substantially to phenotypic differentiation in resistance.     

The evolution of reduced antagonism—A role for host–parasite coevolution

Why do some host–parasite interactions become less antagonistic over evolutionary time? Vertical transmission can select for reduced antagonism. Vertical transmission also promotes coevolution between hosts and parasites. Therefore, we hypothesized that coevolution itself may underlie transitions to reduced antagonism. To test the coevolution hypothesis, we selected for reduced antagonism between the host Caenorhabditis elegans and its parasite Serratia marcescens. This parasite is horizontally transmitted, which allowed us to study coevolution independently of vertical transmission. After 20 generations, we observed a response to selection when coevolution was possible: reduced antagonism evolved in the copassaged treatment. Reduced antagonism, however, did not evolve when hosts or parasites were independently selected without coevolution. In addition, we found strong local adaptation for reduced antagonism between replicate host/parasite lines in the copassaged treatment. Taken together, these results strongly suggest that coevolution was critical to the rapid evolution of reduced antagonism.     

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.     

The impact of migration from parasite-free patches on antagonistic host-parasite coevolution

Natural populations of hosts and parasites are often subdivided and patchily distributed such that some regions of a host species' range will be free from a given parasite. Host migration from parasite-free to parasite-containing patches is expected to alter coevolutionary dynamics by changing the evolutionary potential of antagonists. Specifically, host immigration can favor parasites by increasing transmission opportunities, or hosts by introducing genetic variation. We tested these predictions in coevolving populations of Pseudomonas fluorescens and phage Phi2 that received immigrants from phage-free populations. We observed a negative quadratic relationship between sympatric resistance to phage and host immigration rate (highest at intermediate immigration) but a positive quadratic relationship between coevolution rate and host immigration rate (lowest at intermediate immigration). These results indicate that for a wide range of rates, host immigration from parasite-free patches can increase the evolutionary potential of parasites, and increase the coevolutionary rate if parasite adaptation is limiting in the absence of immigration.     

The impact of parasite dispersal on antagonistic host-parasite coevolution

Coevolving populations of hosts and parasites are often subdivided into a set of patches connected by dispersal. Higher relative rates of parasite compared with host dispersal are expected to lead to parasite local adaptation. However, we know of no studies that have considered the implications of higher relative rates of parasite dispersal for other aspects of the coevolutionary process, such as the rate of coevolution and extent of evolutionary escalation of resistance and infectivity traits. We investigated the effect of phage dispersal on coevolution in experimental metapopulations of the bacterium Pseudomonas fluorescens SBW25 and its viral parasite, phage SBW25Phi2. Both the rate of coevolution and the breadth of evolved infectivity and resistance ranges peaked at intermediate rates of parasite dispersal. These results suggest that parasite dispersal can enhance the evolutionary potential of parasites through provision of novel genetic variation, but that high rates of parasite dispersal can impede the evolution of parasites by homogenizing genetic variation between patches, thereby constraining coevolution.     

EVIDENCE FOR NEGATIVE FREQUENCY-DEPENDENT SELECTION DURING EXPERIMENTAL COEVOLUTION OF A FRESHWATER SNAIL AND A STERILIZING TREMATODE

Host–parasite coevolution is often suggested as a mechanism for maintaining genetic diversity, but finding direct evidence has proven difficult. In the present study, we examine the process of coevolution using a freshwater New Zealand snail ( Potamopyrgus antipodarum ) and its common parasite (the sterilizing trematode, Microphallus sp.) Specifically, we test for changes in genotypic composition of clonal host populations in experimental populations evolving either with or without parasites for six generations. As predicted under the Red Queen model of coevolution, the initially most common host genotype decreased in frequency in the presence, but not the absence, of parasitism. Furthermore, the initially most common host genotype became more susceptible to infection by the coevolving parasite populations over the course of the experiment. These results are consistent with parasite-meditated selection leading to a rare advantage, and they indicate rapid coevolution at the genotypic level between a host and its parasite.     

The role of defensive symbionts in host–parasite coevolution

Understanding the coevolution of hosts and parasites is a long‐standing goal of evolutionary biology. There is a well‐developed theoretical framework to describe the evolution of host–parasite interactions under the assumption of direct, two‐species interactions, which can result in arms race dynamics or sustained genotype fluctuations driven by negative frequency dependence (Red Queen dynamics). However, many hosts rely on symbionts for defence against parasites. Whilst the ubiquity of defensive symbionts and their potential importance for disease control are increasingly recognized, there is still a gap in our understanding of how symbionts mediate or possibly take part in host–parasite coevolution. Herein we address this question by synthesizing information already available from theoretical and empirical studies. First, we briefly introduce current hypotheses on how defensive mutualisms evolved from more parasitic relationships and highlight exciting new experimental evidence showing that this can occur very rapidly. We go on to show that defensive symbionts influence virtually all important determinants of coevolutionary dynamics, namely the variation in host resistance available to selection by parasites, the specificity of host resistance, and the trade‐off structure between host resistance and other components of fitness. In light of these findings, we turn to the limited theory and experiments available for such three‐species interactions to assess the role of defensive symbionts in host–parasite coevolution. Specifically, we discuss under which conditions the defensive symbiont may take over from the host the reciprocal adaptation with parasites and undergo its own selection dynamics, thereby altering or relaxing selection on the hosts' own immune defences. Finally, we address potential effects of defensive symbionts on the evolution of parasite virulence. This is an important problem for which there is no single, clear‐cut prediction. The selection on parasite virulence resulting from the presence of defensive symbionts in their hosts will depend on the underlying mechanism of defence. We identify the evolutionary predictions for different functional categories of symbiont‐conferred resistance and we evaluate the empirical literature for supporting evidence. We end this review with outstanding questions and promising avenues for future research to improve our understanding of symbiont‐mediated coevolution between hosts and parasites.     

Relative number of generations of hosts and parasites does not influence parasite local adaptation in coevolving populations of bacteria and phages

A potential consequence of host-parasite coevolution in spatially structured populations is parasite local adaptation: local parasites perform better than foreign parasites on their local host populations. It has been suggested that the generally shorter generation times of parasites compared with their hosts contributes to parasites, rather than hosts, being locally adapted. We tested the hypothesis that relative generation times of hosts and parasites affect local adaptation of hosts and parasites, using the bacterium Pseudomonas fluorescens and a lytic phage as host and parasite, respectively. Generation times were not directly manipulated, but instead one of the coevolving partners was regularly removed and replaced with a population from an earlier time point. Thus, one partner underwent more generations than the other. Manipulations were carried out at both early and later periods of coevolutionary interactions. At early stages of coevolution, host and parasites that underwent relatively more generations displayed higher levels of resistance and infectivity, respectively. However, the relative number of generations that bacteria and phages underwent did not change the level of local adaptation relative to control populations. This is likely because generalist hosts and parasites are favoured during early stages of coevolution, preventing local adaptation. By contrast, at later stages manipulations had no effect on either average levels of resistance or infectivity, or alter the level of local adaptation relative to the controls, possibly because traits other than resistance and infectivity were under strong selection. Taken together, these data suggest that the relative generation times of hosts and parasites may not be an important determinant of local adaptation in this system.     

The costs of evolving resistance in heterogeneous parasite environments

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