High parasite virulence necessary for the maintenance of host outcrossing via parasite-mediated selection

Biparental sex is widespread in nature, yet costly relative to uniparental reproduction. It is generally unclear why self-fertilizing or asexual lineages do not readily invade outcrossing populations. The Red Queen hypothesis predicts that coevolving parasites can prevent self-fertilizing or asexual lineages from invading outcrossing host populations. However, only highly virulent parasites are predicted to maintain outcrossing, which may limit the general applicability of the Red Queen hypothesis. Here, we tested whether the ability of coevolving parasites to prevent invasion of self-fertilization within outcrossing host populations was dependent on parasite virulence. We introduced wild-type Caenorhabditis elegans hermaphrodites, capable of both self-fertilization and outcrossing, into C. elegans populations fixed for a mutant allele conferring obligate outcrossing. Replicate C. elegans populations were exposed for 24 host generations to one of four strains of Serratia marcescens parasites that varied in virulence, under three treatments: a heat-killed (control, noninfectious) parasite treatment, a fixed-genotype (nonevolving) parasite treatment, and a copassaged (potentially coevolving) parasite treatment. As predicted, self-fertilization invaded C. elegans host populations in the control and fixed-parasite treatments, regardless of parasite virulence. In the copassaged treatment, selfing invaded host populations coevolving with low- to mid-virulence strains, but remained rare in hosts coevolving with highly virulent bacterial strains. Therefore, we found that only highly virulent coevolving parasites can impede the invasion of selfing.

Insect herbivores drive sex allocation in angiosperm flowers

Why sex has evolved and is maintained is an open question in evolutionary biology. The Red Queen hypothesis predicts that host lineages subjected to more intense parasite pressure should invest more in sexual reproduction to continuously create novel defences against their rapidly evolving natural enemies. In this comparative study across the angiosperms, we show that hermaphrodite plant species associated with higher species richness of insect herbivores evolved flowers with higher biomass allocation towards the male sex, an indication of their greater outcrossing effort. This pattern remained robust after controlling for key vegetative, reproductive and biogeographical traits, suggesting that long-term herbivory pressure is a key factor driving the selfing-outcrossing gradient of higher plants. Although flower evolution is frequently associated with mutualistic pollinators, our findings support the Red Queen hypothesis and suggest that insect herbivores drive the sexual strategies of flowering plants and their genetic diversity.

Run or Die in the Evolution of New MicroRNAs-Testing the Red Queen Hypothesis on De Novo New Genes

The Red Queen hypothesis depicts evolution as the continual struggle to adapt. According to this hypothesis, new genes, especially those originating from nongenic sequences (i.e., de novo genes), are eliminated unless they evolve continually in adaptation to a changing environment. Here, we analyze two Drosophila de novo miRNAs that are expressed in a testis-specific manner with very high rates of evolution in their DNA sequence. We knocked out these miRNAs in two sibling species and investigated their contributions to different fitness components. We observed that the fitness contributions of miR-975 in Drosophila simulans seem positive, in contrast to its neutral contributions in D. melanogaster, whereas miR-983 appears to have negative contributions in both species, as the fitness of the knockout mutant increases. As predicted by the Red Queen hypothesis, the fitness difference of these de novo miRNAs indicates their different fates.

Getting somewhere with the Red Queen: chasing a biologically modern definition of the hypothesis

The Red Queen hypothesis (RQH) is both familiar and murky, with a scope and range that has broadened beyond its original focus. Although originally developed in the palaeontological arena, it now encompasses many evolutionary theories that champion biotic interactions as significant mechanisms for evolutionary change. As such it de-emphasizes the important role of abiotic drivers in evolution, even though such a role is frequently posited to be pivotal. Concomitant with this shift in focus, several studies challenged the validity of the RQH and downplayed its propriety. Herein, we examine in detail the assumptions that underpin the RQH in the hopes of furthering conceptual understanding and promoting appropriate application of the hypothesis. We identify issues and inconsistencies with the assumptions of the RQH, and propose a redefinition where the Red Queen's reign is restricted to certain types of biotic interactions and evolutionary patterns occurring at the population level.

The genomic basis of Red Queen dynamics during rapid reciprocal host-pathogen coevolution

Pathogens are omnipresent and by definition detrimental to their hosts. Pathogens thus exert high selection on their hosts, which, if adapting, can exert similar levels of selection on the pathogen, resulting in ongoing cycles of reciprocal adaptation between the antagonists. Such coevolutionary interactions have a central influence on the evolution of organisms. Surprisingly, we still know little about the exact selection dynamics and the genome regions involved. Our study uses a controlled experimental approach with an animal host to dissect coevolutionary selection. We find that distinct selective processes underlie rapid coadaptation in the two antagonists, including antagonistic frequency-dependent selection on toxin gene copy number in the pathogen, while the host response is likely influenced by changes in multiple genome regions.

Red Queen dynamics, involving coevolutionary interactions between species, are ubiquitous, shaping the evolution of diverse biological systems. To date, information on the underlying selection dynamics and the involved genome regions is mainly available for bacteria–phage systems or only one of the antagonists of a eukaryotic host–pathogen interaction. We add to our understanding of these important coevolutionary interactions using an experimental host–pathogen model, which includes the nematode Caenorhabditis elegans and its pathogen Bacillus thuringiensis. We combined experimental evolution with time-shift experiments, in which a focal host or pathogen is tested against a coevolved antagonist from the past, present, or future, followed by genomic analysis. We show that (i) coevolution occurs rapidly within few generations, (ii) temporal coadaptation at the phenotypic level is found in parallel across replicate populations, consistent with antagonistic frequency-dependent selection, (iii) genomic changes in the pathogen match the phenotypic pattern and include copy number variations of a toxin-encoding plasmid, and (iv) host genomic changes do not match the phenotypic pattern and likely involve selective responses at more than one locus. By exploring the dynamics of coevolution at the phenotypic and genomic level for both host and pathogen simultaneously, our findings demonstrate a more complex model of the Red Queen, consisting of distinct selective processes acting on the two antagonists during rapid and reciprocal coadaptation.

Periodic, Parasite-Mediated Selection For and Against Sex

Asexual lineages should rapidly replace sexual populations. Why sex then? The Red Queen hypothesis proposes that parasite-mediated selection against common host genotypes could counteract the per capita birth rate advantage of asexuals. Under the Red Queen hypothesis, fluctuations in parasite-mediated selection can drive fluctuations in the asexual population, leading to the coexistence of sexual and asexual reproduction. Does shifting selection by parasites drive fluctuations in the fitness and frequency of asexuals in nature? Combining long-term field data with mesocosm experiments, we detected a shift in the direction of parasite selection in the snail Potamopyrgus antipodarum and its coevolving parasite, Microphallus sp. In the early 2000s, asexuals were more infected than sexuals. A decade later, the asexuals had declined in frequency and were less infected than sexuals. Over time, the mean infection prevalence of asexuals equaled that of sexuals but varied far more. This variation in asexual infection prevalence suggests the potential for parasite-mediated fluctuations in asexual fitness. Accordingly, we detected fitness consequences of the shift in parasite selection: when they were less infected than sexuals, asexuals increased in frequency in the field and in paired mesocosms that isolated the effect of parasites. The match between field and experiment argues that coevolving parasites drive temporal change in the relative fitness and frequency of asexuals, potentially promoting the coexistence of reproductive modes in P. antipodarum.

Antibiotic-driven escape of host in a parasite-induced Red Queen dynamics

Winnerless coevolution of hosts and parasites could exhibit Red Queen dynamics, which is characterized by parasite-driven cyclic switching of expressed host phenotypes. We hypothesize that the application of antibiotics to suppress the reproduction of parasites can provide an opportunity for the hosts to escape such winnerless coevolution. Here, we formulate a minimal mathematical model of host-parasite interaction involving multiple host phenotypes that are targeted by adapting parasites. Our model predicts the levels of antibiotic effectiveness that can steer the parasite-driven cyclic switching of host phenotypes (oscillations) to a stable equilibrium of host survival. Our simulations show that uninterrupted application of antibiotic with high-level effectiveness (greater than 85%) is needed to escape the Red Queen dynamics. Interrupted and low level of antibiotic effectiveness are indeed useless to stop host-parasite coevolution. This study can be a guide in designing good practices and protocols to minimize the risk of further progression of parasitic infections.

Bloody-minded parasites and sex: the effects of fluctuating virulence

Asexual lineages can grow at a faster rate than sexual lineages. Why then is sexual reproduction so widespread? Much empirical evidence supports the Red Queen hypothesis. Under this hypothesis, coevolving parasites favour sexual reproduction by adapting to infect common asexual clones and driving them down in frequency. One limitation, however, seems to challenge the generality of the Red Queen: in theoretical models, parasites must be very virulent to maintain sex. Moreover, experiments show virulence to be unstable, readily shifting in response to environmental conditions. Does variation in virulence further limit the ability of coevolving parasites to maintain sex? To address this question, we simulated temporal variation in virulence and evaluated the outcome of competition between sexual and asexual females. We found that variation in virulence did not limit the ability of coevolving parasites to maintain sex. In fact, relatively high variation in virulence promoted parasite-mediated maintenance of sex. With sufficient variation, sexual females persisted even when mean virulence fell well below the threshold virulence required to maintain sex under constant conditions. We conclude that natural variation in virulence does not limit the relevance of the Red Queen hypothesis for natural populations; on the contrary, it could expand the range of conditions over which coevolving parasites can maintain sex.

Cost of resistance to trematodes in freshwater snail populations with low clonal diversity

BACKGROUND

The persistence of high genetic variability in natural populations garners considerable interest among ecologists and evolutionary biologists. One proposed hypothesis for the maintenance of high levels of genetic diversity relies on frequency-dependent selection imposed by parasites on host populations (Red Queen hypothesis). A complementary hypothesis suggests that a trade-off between fitness costs associated with tolerance to stress factors and fitness costs associated with resistance to parasites is responsible for the maintenance of host genetic diversity.

RESULTS

The present study investigated whether host resistance to parasites is traded off with tolerance to environmental stress factors (high/low temperatures, high salinity), by comparing populations of the freshwater snail Melanoides tuberculata with low vs. high clonal diversity. Since polyclonal populations were found to be more parasitized than populations with low clonal diversity, we expected them to be tolerant to environmental stress factors. We found that clonal diversity explained most of the variation in snail survival under high temperature, thereby suggesting that tolerance to high temperatures of clonally diverse populations is higher than that of populations with low clonal diversity.

CONCLUSIONS

Our results suggest that resistance to parasites may come at a cost of reduced tolerance to certain environmental stress factors.

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.

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

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

THE MAINTENANCE OF SEX BY PARASITISM AND MUTATION ACCUMULATION UNDER EPISTATIC FITNESS FUNCTIONS

The mutation accumulation hypothesis predicts that sex functions to reduce the population mutational load, while the Red Queen hypothesis holds that sex is adaptive as a defense against coevolving pathogens. We used computer simulations to examine the combined and separate effects of selection against deleterious mutations and host-parasite coevolution on the spread of a clone into an outcrossing sexual population. The results suggest that the two processes operating simultaneously may select for sex independent of the exact shape of the function that maps mutation number onto host fitness.

THE EFFECTS OF HOST GENOTYPE AND SPATIAL DISTRIBUTION ON TREMATODE PARASITISM IN A BIVALVE POPULATION

A basic assumption underlying models of host-parasite coevolution is the existence of additive genetic variation among hosts for resistance to parasites. However, estimates of additive genetic variation are lacking for natural populations of invertebrates. Testing this assumption is especially important in view of current models that suggest parasites may be responsible for the evolution of sex, such as the Red Queen hypothesis. This hypothesis suggests that the twofold reproductive disadvantage of sex relative to parthenogenesis can be overcome by the more rapid production of rare genotypes resistant to parasites. Here I present evidence of significant levels of additive genetic variance in parasite resistance for an invertebrate host-parasite system in nature. Using families of the bivalve mollusc, Transennella tantilla, cultured in the laboratory, then exposed to parasites in the field, I quantified heritable variation in parasite resistance under natural conditions. The spatial distribution of outplanted hosts was also varied to determine environmental contributions to levels of parasite infection and to estimate potential interactions of host genotype with environment. The results show moderate but significant levels of heritability for resistance to parasites (h² = 0.36). The spatial distribution of hosts also significantly influenced parasite prevalence such that increased host aggregation resulted in decreased levels of parasite infection. Family mean correlations across environments were positive, indicating no genotype-environment interaction. Therefore, these results provide support for important assumptions underlying coevolutionary models of host-parasite systems.

LOWER MITE INFESTATIONS IN AN ASEXUAL GECKO COMPARED WITH ITS SEXUAL ANCESTORS

What advantage do sexually reproducing organisms gain from their mode of reproduction that compensates for their twofold loss in reproductive rate relative to their asexual counterparts? One version of the Red Queen hypothesis suggests that selective pressure from parasites is strongest on the most common genotype in a population, and thus genetically identical clonal lineages are more vulnerable to parasitism over time than genetically diverse sexual lineages. Our surveys of the ectoparasites of an asexual gecko and its two sexual ancestral species show that the sexuals have a higher prevalence, abundance, and mean intensity of mites than asexuals sharing the same habitat. Our experimental data indicate that in one sexual/asexual pair this pattern is at least partly attributable to higher attachment rates of mites to sexuals. Such a difference may occur as a result of exceptionally high susceptibility of the sexuals to mites because of their low genetic diversity (relative to other more-outbred sexual species) and their potentially high stress levels, or as a result of exceptionally low susceptibility of the asexuals to mites because of their high levels of heterozygosity.

The escalatory Red Queen: Population extinction and replacement following arms race dynamics in poplar rust

Host-parasite systems provide convincing examples of Red Queen co-evolutionary dynamics. Yet, a key process underscored in Van Valen's theory - that arms race dynamics can result in extinction - has never been documented. One reason for this may be that most sampling designs lack the breadth needed to illuminate the rapid pace of adaptation by pathogen populations. In this study, we used a 25-year temporal sampling to decipher the demographic history of a plant pathogen: the poplar rust fungus, Melampsora larici-populina. A major adaptive event occurred in 1994 with the breakdown of R7 resistance carried by several poplar cultivars widely planted in Western Europe since 1982. The corresponding virulence rapidly spread in M. larici-populina populations and nearly reached fixation in northern France, even on susceptible hosts. Using both temporal records of virulence profiles and temporal population genetic data, our analyses revealed that (i) R7 resistance breakdown resulted in the emergence of a unique and homogeneous genetic group, the so-called cultivated population, which predominated in northern France for about 20 years, (ii) selection for Vir7 individuals brought with it multiple other virulence types via hitchhiking, resulting in an overall increase in the population-wide number of virulence types and (iii) - above all - the emergence of the cultivated population superseded the initial population which predominated at the same place before R7 resistance breakdown. Our temporal analysis illustrates how antagonistic co-evolution can lead to population extinction and replacement, hence providing direct evidence for the escalation process which is at the core of Red Queen dynamics.

Coevolutionary interactions with parasites constrain the spread of self-fertilization into outcrossing host populations

Given the cost of sex, outcrossing populations should be susceptible to invasion and replacement by self-fertilization or parthenogenesis. However, biparental sex is common in nature, suggesting that cross-fertilization has substantial short-term benefits. The Red Queen hypothesis (RQH) suggests that coevolution with parasites can generate persistent selection favoring both recombination and outcrossing in host populations. We tested the prediction that coevolving parasites can constrain the spread of self-fertilization relative to outcrossing. We introduced wild-type Caenorhabditis elegans hermaphrodites, capable of both self-fertilization, and outcrossing, into C. elegans populations that were fixed for a mutant allele conferring obligate outcrossing. Replicate C. elegans populations were exposed to the parasite Serratia marcescens for 33 generations under three treatments: a control (avirulent) parasite treatment, a fixed (nonevolving) parasite treatment, and a copassaged (potentially coevolving) parasite treatment. Self-fertilization rapidly invaded C. elegans host populations in the control and the fixed-parasite treatments, but remained rare throughout the entire experiment in the copassaged treatment. Further, the frequency of the wild-type allele (which permits selfing) was strongly positively correlated with the frequency of self-fertilization across host populations at the end of the experiment. Hence, consistent with the RQH, coevolving parasites can limit the spread of self-fertilization in outcrossing populations.

Within-population covariation between sexual reproduction and susceptibility to local parasites

Evolutionary biology has yet to reconcile the ubiquity of sex with its costs relative to asexual reproduction. Here, we test the hypothesis that coevolving parasites maintain sex in their hosts. Specifically, we examined the distributions of sexual reproduction and susceptibility to local parasites within a single population of freshwater snails (Potamopyrgus antipodarum). Susceptibility to local trematode parasites (Microphallus sp.) is a relative measure of the strength of coevolutionary selection in this system. Thus, if coevolving parasites maintain sex, sexual snails should be common where susceptibility is high. We tested this prediction in a mixed population of sexual and asexual snails by measuring the susceptibility of snails from multiple sites in a lake. Consistent with the prediction, the frequency of sexual snails was tightly and positively correlated with susceptibility to local parasites. Strikingly, in just two years, asexual females increased in frequency at sites where susceptibility declined. We also found that the frequency of sexual females covaries more strongly with susceptibility than with the prevalence of Microphallus infection in the field. In linking susceptibility to the frequency of sexual hosts, our results directly implicate spatial variation in coevolutionary selection in driving the geographic mosaic of sex.

Red Queen dancing in the lek: effects of mating skew on host-parasite interactions

The RQH (Red Queen hypothesis), which argues that hosts need to be continuously finding new ways to avoid parasites that are able to infect common host genotypes, has been at the center of discussions on the maintenance of sex. This is because diversity is favored under the host–parasite coevolution based on negative frequency‐dependent selection, and sexual reproduction is a mechanism that generates genetic diversity in the host population. Together with parasite infections, sexual organisms are usually under sexual selection, which leads to mating skew or mating success biased toward males with a particular phenotype. Thus, strong mating skew would affect genetic variance in a population and should affect the benefit of the RQH. However, most models have investigated the RQH under a random mating system and not under mating skew. In this study, I show that sexual selection and the resultant mating skew may increase parasite load in the hosts. An IBM (individual‐based model), which included host–parasite interactions and sexual selection among hosts, demonstrates that mating skew influenced parasite infection in the hosts under various conditions. Moreover, the IBM showed that the mating skew evolves easily in cases of male–male competition and female mate choice, even though it imposes an increased risk of parasite infection on the hosts. These findings indicated that whether the RQH favored sexual reproduction depended on the condition of mating skew. That is, consideration of the host mating system would provide further understanding of conditions in which the RQH favors sexual reproduction in real organisms.

Running with the Red Queen: the role of biotic conflicts in evolution

What are the causes of natural selection? Over 40 years ago, Van Valen proposed the Red Queen hypothesis, which emphasized the primacy of biotic conflict over abiotic forces in driving selection. Species must continually evolve to survive in the face of their evolving enemies, yet on average their fitness remains unchanged. We define three modes of Red Queen coevolution to unify both fluctuating and directional selection within the Red Queen framework. Empirical evidence from natural interspecific antagonisms provides support for each of these modes of coevolution and suggests that they often operate simultaneously. We argue that understanding the evolutionary forces associated with interspecific interactions requires incorporation of a community framework, in which new interactions occur frequently. During their early phases, these newly established interactions are likely to drive fast evolution of both parties. We further argue that a more complete synthesis of Red Queen forces requires incorporation of the evolutionary conflicts within species that arise from sexual reproduction. Reciprocally, taking the Red Queen's perspective advances our understanding of the evolution of these intraspecific conflicts.

The ecology of sexual reproduction

Sexual reproduction is widely regarded as one of the major unexplained phenomena in biology. Nonetheless, while a general answer may remain elusive, considerable progress has been made in the last few decades. Here, we first review the genesis of, and support for, the major ecological hypotheses for biparental sexual reproduction. We then focus on the idea that host-parasite coevolution can favour cross-fertilization over uniparental forms of reproduction, as this hypothesis currently has the most support from natural populations. We also review the results from experimental evolution studies, which tend to show that exposure to novel environments can select for higher levels of sexual reproduction, but that sex decreases in frequency after populations become adapted to the previously novel conditions. In contrast, experimental coevolution studies suggest that host-parasite interactions can lead to the long-term persistence of sex. Taken together, the evidence from natural populations and from laboratory experiments point to antagonistic coevolution as a potent and possibly ubiquitous force of selection favouring cross-fertilization and recombination.

A mean field model for competition: from neutral ecology to the Red Queen

Individual species are distributed inhomogeneously over space and time, yet, within large communities of species, aggregated patterns of biodiversity seem to display nearly universal behaviour. Neutral models assume that an individual's demographic prospects are independent of its species identity. They have successfully predicted certain static, time-independent patterns. But they have generally failed to predict temporal patterns, such as species ages or population dynamics. We construct a new, multispecies framework incorporating competitive differences between species, and assess the impact of this competition on static and dynamic patterns of biodiversity. We solve this model exactly for the special case of a Red Queen hypothesis, where fitter species are continually arising. The model predicts more realistic species ages than neutral models, without greatly changing predictions for static species abundance distributions. Our modelling approach may allow users to incorporate a broad range of ecological mechanisms.

Exposure to parasites increases promiscuity in a freshwater snail

Under the Red Queen hypothesis, outcrossing can produce genetically variable progeny, which may be more resistant, on average, to locally adapted parasites. Mating with multiple partners may enhance this resistance by further increasing the genetic variation among offspring. We exposed Potamopyrgus antipodarum to the eggs of a sterilizing, trematode parasite and tested whether this altered mating behaviour. We found that exposure to parasites increased the number of snail mating pairs and the total number of different mating partners for both males and females. Thus, our results suggest that, in host populations under parasite-mediated selection, exposure to infective propagules increases the rate of mating and the number of mates.

Clonal diversity driven by parasitism in a freshwater snail

One explanation for the widespread abundance of sexual reproduction is the advantage that genetically diverse sexual lineages have under strong pressure from virulent coevolving parasites. Such parasites are believed to track common asexual host genotypes, resulting in negative frequency-dependent selection that counterbalances the population growth-rate advantage of asexuals in comparison with sexuals. In the face of genetically diverse asexual lineages, this advantage of sexual reproduction might be eroded, and instead sexual populations would be replaced by diverse assemblages of clonal lineages. We investigated whether parasite-mediated selection promotes clonal diversity in 22 natural populations of the freshwater snail Melanoides tuberculata. We found that infection prevalence explains the observed variation in the clonal diversity of M. tuberculata populations, whereas no such relationship was found between infection prevalence and male frequency. Clonal diversity and male frequency were independent of snail population density. Incorporating ecological factors such as presence/absence of fish, habitat geography and habitat type did not improve the predictive power of regression models. Approximately 11% of the clonal snail genotypes were shared among 2-4 populations, creating a web of 17 interconnected populations. Taken together, our study suggests that parasite-mediated selection coupled with host dispersal ecology promotes clonal diversity. This, in return, may erode the advantage of sexual reproduction in M. tuberculata populations.