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

Examining adaptive evolution of immune activity: opportunities provided by gastropods in the age of 'omics'

Parasites threaten all free-living organisms, including molluscs. Understanding the evolution of immune defence traits in natural host populations is crucial for predicting their long-term performance under continuous infection risk. Adaptive trait evolution requires that traits are subject to selection (i.e. contribute to organismal fitness) and that they are heritable. Despite broad interest in the evolutionary ecology of immune activity in animals, the understanding of selection on and evolutionary potential of immune defence traits is far from comprehensive. For instance, empirical observations are only rarely in line with theoretical predictions of immune activity being subject to stabilizing selection. This discrepancy may be because ecoimmunological studies can typically cover only a fraction of the complexity of an animal immune system. Similarly, molecular immunology/immunogenetics studies provide a mechanistic understanding of immunity, but neglect variation that arises from natural genetic differences among individuals and from environmental conditions. Here, we review the current literature on natural selection on and evolutionary potential of immune traits in animals, signal how merging ecological immunology and genomics will strengthen evolutionary ecological research on immunity, and indicate research opportunities for molluscan gastropods for which well-established ecological understanding and/or 'immune-omics' resources are already available. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

Do Coinfections Maintain Genetic Variation in Parasites?

Host individuals are often infected with multiple, potentially interacting parasite species and genotypes. Such coinfections have consequences for epidemiology, disease severity, and evolution of parasite virulence. As fitness effects of coinfection can be specific to interacting parasite genotypes, coinfections may induce high fitness variation among parasite genotypes. We argue that such interactions can be an important mechanism maintaining genetic variation in parasite traits such as infectivity and virulence. We also argue that such interactions may slow coevolutionary dynamics between hosts and parasites. This is because, instead of depending only on host genotype, parasite fitness may be determined by average infection success across all coinfection scenarios.

Estimating Genetic and Maternal Effects Determining Variation in Immune Function of a Mixed-Mating Snail

Evolution of host defenses such as immune function requires heritable genetic variation in them. However, also non-genetic maternal effects can contribute to phenotypic variation, thus being an alternative target for natural selection. We investigated the role of individuals' genetic background and maternal effects in determining immune defense traits (phenoloxidase and antibacterial activity of hemolymph), as well as in survival and growth, in the simultaneously hermaphroditic snail Lymnaea stagnalis. We utilized the mixed mating system of this species by producing full-sib families in which each parental snail had produced offspring as both a dam and as a sire, and tested whether genetic background (family) and non-genetic maternal effects (dam nested within family) explain trait variation. Immune defense traits and growth were affected solely by individuals' genetic background. Survival of snails did not show family-level variation. Additionally, some snails were produced through self-fertilization. They showed reduced growth and survival suggesting recessive load or overdominance. Immune defense traits did not respond to inbreeding. Our results suggest that the variation in snail immune function and growth was due to genetic differences. Since immune traits did not respond to inbreeding, this variation is most likely due to additive or epistatic genetic variance.

The ecology, evolution, impacts and management of host-parasite interactions of marine molluscs

Molluscs are economically and ecologically important components of aquatic ecosystems. In addition to supporting valuable aquaculture and wild-harvest industries, their populations determine the structure of benthic communities, cycling of nutrients, serve as prey resources for higher trophic levels and, in some instances, stabilize shorelines and maintain water quality. This paper reviews existing knowledge of the ecology of host-parasite interactions involving marine molluscs, with a focus on gastropods and bivalves. It considers the ecological and evolutionary impacts of molluscan parasites on their hosts and vice versa, and on the communities and ecosystems in which they are a part, as well as disease management and its ecological impacts. An increasing number of case studies show that disease can have important effects on marine molluscs, their ecological interactions and ecosystem services, at spatial scales from centimeters to thousands of kilometers and timescales ranging from hours to years. In some instances the cascading indirect effects arising from parasitic infection of molluscs extend well beyond the temporal and spatial scales at which molluscs are affected by disease. In addition to the direct effects of molluscan disease, there can be large indirect impacts on marine environments resulting from strategies, such as introduction of non-native species and selective breeding for disease resistance, put in place to manage disease. Much of our understanding of impacts of molluscan diseases on the marine environment has been derived from just a handful of intensively studied marine parasite-host systems, namely gastropod-trematode, cockle-trematode, and oyster-protistan interactions. Understanding molluscan host-parasite dynamics is of growing importance because: (1) expanding aquaculture; (2) current and future climate change; (3) movement of non-native species; and (4) coastal development are modifying molluscan disease dynamics, ultimately leading to complex relationships between diseases and cultivated and natural molluscan populations. Further, in some instances the enhancement or restoration of valued ecosystem services may be contingent on management of molluscan disease. The application of newly emerging molecular tools and remote sensing techniques to the study of molluscan disease will be important in identifying how changes at varying spatial and temporal scales with global change are modifying host-parasite systems.

Aphid-encoded variability in susceptibility to a parasitoid

BACKGROUND

Many animals exhibit variation in resistance to specific natural enemies. Such variation may be encoded in their genomes or derived from infection with protective symbionts. The pea aphid, Acyrthosiphon pisum, for example, exhibits tremendous variation in susceptibility to a common natural enemy, the parasitic wasp Aphidius ervi. Pea aphids are often infected with the heritable bacterial symbiont, Hamiltonella defensa, which confers partial to complete resistance against this parasitoid depending on bacterial strain and associated bacteriophages. That previous studies found that pea aphids without H. defensa (or other symbionts) were generally susceptible to parasitism, together with observations of a limited encapsulation response, suggested that pea aphids largely rely on infection with H. defensa for protection against parasitoids. However, the limited number of uninfected clones previously examined, and our recent report of two symbiont-free resistant clones, led us to explicitly examine aphid-encoded variability in resistance to parasitoids.

RESULTS

After rigorous screening for known and unknown symbionts, and microsatellite genotyping to confirm clonal identity, we conducted parasitism assays using fifteen clonal pea aphid lines. We recovered significant variability in aphid-encoded resistance, with variation levels comparable to that contributed by H. defensa. Because resistance can be costly, we also measured aphid longevity and cumulative fecundity of the most and least resistant aphid lines under permissive conditions, but found no trade-offs between higher resistance and these fitness parameters.

CONCLUSIONS

These results indicate that pea aphid resistance to A. ervi is more complex than previously appreciated, and that aphids employ multiple tactics to aid in their defense. While we did not detect a tradeoff, these may become apparent under stressful conditions or when resistant and susceptible aphids are in direct competition. Understanding sources and amounts of variation in resistance to natural enemies is necessary to understand the ecological and evolutionary dynamics of antagonistic interactions, such as the potential for coevolution, but also for the successful management of pest populations through biological control.

Shoaling as an antiparasite defence in minnows (Pimephales promelas) exposed to trematode cercariae

1. Individuals that live in groups benefit from increased foraging success and decreased predation. Protection from some types of parasites may provide an additional benefit of group-living. For fish, the extent to which shoaling can reduce an individual's risk of exposure to the infective stages of parasites is unknown. 2. We tested for antiparasite benefits of shoaling in fathead minnows exposed to larvae (cercariae) of two of their most common species of trematode, Ornithodiplostomum ptychocheilus and Posthodiplostomum minimum. As developing stages (metacercariae) of these trematodes cause reductions in minnow activity, growth and survival, natural selection should favour the evolution of cercariae-avoidance behaviours. 3. We evaluated shoal dimensions in groups of minnows exposed to O. ptychocheilus and to other chemical/physical stimuli within aquaria. To compare risk of exposure in shoaling vs. non-shoaling fish, we confined groups of minnows into mesh cages in outdoor mesocosms, exposed them to cercariae, then compared mean worm numbers in grouped vs. solitary fish. Lastly, we tested whether fish located within the centre of an artificial shoal reduced their risk of cercariae exposure compared with those along peripheral edges. 4. Minnows distinguished infective cercariae from other potential aquatic threats and responded with activity that reduced the 2-dimensional area of their shoals 15-fold compared with water-only controls. Fish confined within artificial shoals had 3-fold fewer worms than single fish and minnows located within the centre of artificial shoals had significantly fewer worms than those without peripheral minnows. 5. These results show that shoaling reduces a minnows' risk of exposure to cercariae, either directly via detection of cercariae in the water column followed by behavioural avoidance or indirectly via behaviour-mediated differences in exposure between shoaling vs. non-shoaling fish.

In vitro encystment of Himasthla elongata cercariae (Digenea, Echinostomatidae) in the haemolymph of blue mussels Mytilus edulis as a tool for assessing cercarial infectivity and molluscan susceptibility

Infectivity of Himasthla elongata cercariae to mussels, their second intermediate hosts, and resistance by these hosts to infection were assessed on the basis of the cercariae's ability to encyst in mussel haemolymph in vitro. A series of experimental in vivo infections of mussels with batches of cercariae, each batch released from a different single infected mollusc and referred to as a clone (due to their shared genotype), demonstrated that the results of the in vitro tests corresponded to the actual indices of infectivity/susceptibility of the parasites and their hosts. Most cercarial clones had high infectivity, with a few clones having very high or, at the other extreme, very low infectivity. A similar pattern was revealed in mussel resistance to cercarial infection. Most of the molluscs tested were moderately susceptible to cercarial infection, but at each extreme a small fraction (less than 10%) displayed very high or very low susceptibility. It was shown that there were no totally compatible or totally incompatible 'cercaria clone/mussel' combinations. Results obtained are compared with the data on intra-population variability using the characters parasite infectivity/host compatibility for trematode/mollusc-first intermediate host associations. Results are made relevant to actual infection levels in mussel settlements at the White Sea.

Transmission dynamics of two strains of Schistosoma mansoni utilizing novel intermediate and definitive hosts

The intimate host-parasite relationship mandates adaptation to the genetic and phenotypic variability of their counterparts. Here, inbred and outcrossed strains of Schistosoma mansoni were challenged with "local" and "novel" intermediate and definitive hosts to examine effects of genetic variability and novelty on infection success and dynamics. Genetically distinct lines of Biomphalaria glabrata intermediate hosts exposed to inbred and outcrossed S. mansoni larvae were assessed for differences in both snail and parasite life-history parameters. Cercariae from each parasite-snail treatment were used to infect "local" and "novel" Mus musculus definitive hosts to assess parasite infectivity and fitness. Outcrossed parasites significantly reduced snail growth, were more productive, and induced greater host mortality than inbred parasites. Mouse strain did not influence parasite infectivity or reproduction, but parasite and snail host genetic background did, affecting both sex-specific infectivity and parasite productivity. Overall, genetic background of S. mansoni and its intermediate snail host altered life history traits and transmission dynamics of the parasite throughout its life cycle.

Food makes you a target: disentangling genetic, physiological, and behavioral effects determining susceptibility to infection

Genetics, physiology, and behavior are all expected to influence the susceptibility of hosts to parasites. Furthermore, interactions between genetic and other factors are suggested to contribute to the maintenance of genetic polymorphism in resistance when the relative susceptibility of host genotypes is context dependent. We used a maternal sibship design and long- and short-term food deprivation treatments to test the role of family-level genetic variation, body condition, physiological state, and foraging behavior on the susceptibility of Lymnaea stagnalis snails to infection by a trematode parasite that uses chemical cues to locate its hosts. In experimental exposures, we found that snails in the long-term food deprivation treatment contracted fewer parasites than snails that were continuously well-fed, possibly because well-fed snails grew larger and attracted more transmission stages. When we kept the long-term feeding rates the same, but manipulated the physiological state and foraging behavior of the snails with short-term food deprivation treatment, we found that snails that were fed before the exposure contracted more parasites than snails that were fed during the exposure. This suggests that direct physiological effects of food processing, but not foraging behavior, predisposed snails to infection. Feeding treatments also affected the family-level variation in snail susceptibility, suggesting that the relative susceptibility of host genotypes was context dependent.

Maintenance of genetic variation in immune defense of a freshwater snail: role of environmental heterogeneity

Natural populations often show genetic variation in pathogen resistance, which is paradoxal because natural selection is expected to erode genetic variation in fitness-related traits. Several different factors have been suggested to maintain such variation, but their relative importance is still poorly understood. Here we examined if environmental heterogeneity and genetic trade-offs could contribute to the maintenance of genetic variation in immune function of a freshwater snail Lymnaea stagnalis. We assessed the immunocompetence of snails originating from different families and maintained in different feeding treatments (ad libitum feeding, no food) by measuring the density of circulating hemocytes, phenoloxidase activity, and antibacterial activity of snail hemolymph. Food limitation reduced snail immune function, and we found significant among-family variation in hemocyte concentration and PO activity, but not in antibacterial activity. Interestingly, food availability modified the family-level variation observed in PO activity so that the relative immunocompetence of different snail families changed over environmental conditions (G x E interaction). We found no evidence for genetic trade-offs between snail growth and immune defense nor among immune traits. Thus, our findings support the idea that environmental heterogeneity may promote maintenance of genetic variation in immune defense, but also suggest that different immune traits might not respond similarly to environmental variation.

Effects of intermediate host genetic background on parasite transmission dynamics: a case study using Schistosoma mansoni

For parasites that require multiple hosts to complete their development, genetic interplay with one host may impact parasite transmission and establishment in subsequent hosts. In this study, we used microsatellite loci to address whether the genetic background of snail intermediate hosts influences life-history traits and transmission patterns of dioecious trematode parasites in their definitive hosts. We performed experimental Schistosoma mansoni infections utilizing two allopatric populations of Biomphalaria glabrata snails and assessed intensities and sex ratios of adult parasites in mouse definitive hosts. Our results suggest that the genetic background of hosts at one point in a parasite's life cycle can influence the intensities and sex ratios of worms in subsequent hosts.

Recruitment rate of gymnophallid metacercariae in the New Zealand cockle Austrovenus stutchburyi: an experimental test of the hitch-hiking hypothesis

The rate at which host organisms accumulate parasites is affected by a number of intrinsic and extrinsic factors. The New Zealand cockle Austrovenus stutchburyi is frequently parasitised by trematodes comprising of two species of echinostomes and a species of gymnophallid that use it as a second intermediate host for trophic transmission to avian definitive hosts. The echinostomes are capable of manipulating the burrowing behaviour of the cockle to enhance their transmission success, whereas the gymnophallid is not capable of host manipulation. Previous studies have found patterns of positive associations between the echinostomes and the gymnophallid. Thus, it is possible that the latter is a "hitch-hiking" parasite that preferentially infects cockles already heavily infected by echinostome metacercariae to enhance its own transmission rate. A field experiment involving cockles forced to remain either above or below the sediment surface to simulate manipulated and non-manipulated cockles was conducted to test the hitch-hiking hypothesis. The gymnophallid was not found to display any preference for either surfaced or buried cockles; therefore, it cannot be considered as a hitch-hiking parasite. Possible alternative reasons for the pattern of positive association between the gymnophallid and the echinostomes are proposed.

Spatial heterogeneity in parasite infections at different spatial scales in an intertidal bivalve

Spatial heterogeneities in the abundance of free-living organisms as well as in infection levels of their parasites are a common phenomenon, but knowledge on parasitism in invertebrate intermediate hosts in this respect is scarce. We investigated the spatial pattern of four dominant trematode species which utilize a common intertidal bivalve, the cockle Cerastoderma edule, as second intermediate host in their life cycles. Sampling of cockles from the same cohort at 15 sites in the northern Wadden Sea (North Sea) over a distance of 50 km revealed a conspicuous spatial heterogeneity in infection levels in all four species over the total sample as well as among and within sampling sites. Whereas multiple regression analyses indicated the density of first intermediate upstream hosts to be the strongest determinant of infection levels in cockles, the situation within sites was more complex with no single strong predictor variable. However, host size was positively and host density negatively correlated with infection levels and there was an indication of differential susceptibility of cockle hosts. Small-scale differences in physical properties of the habitat in the form of residual water at low tide resulted in increased infection levels of cockles which we experimentally transferred into pools. A complex interplay of these factors may be responsible for within-site heterogeneities. At larger spatial scales, these factors may be overridden by the strong effect of upstream hosts. In contrast to first intermediate trematode hosts, there was no indication for inter-specific interactions. In other terms, the recruitment of trematodes in second intermediate hosts seems to be largely controlled by pre-settlement processes both among and within host populations.

Associations between blood parasite infection and a microsatellite DNA allele in an Australian scincid lizard (Egernia stokesii)

We used blood samples from 175 individuals of the Australian lizard Egernia stokesii to determine infection status of three apicomplexan blood parasites from the genera Hemolivia, Schellackia, and Plasmodium and to determine genotypes at 12 microsatellite DNA loci. We found one significant association between genotype and infection status. For locus Est4, individuals carrying allele 159 had lower prevalence of infection with Hemolivia (14.3% of 28 lizards) than individuals that did not carry the allele (58.4% of 89 lizards). We interpret this as a linkage to a functional gene associated with parasite resistance. We found no evidence among seven lizard populations that the frequency of allele 159 was related to the population prevalence of Hemolivia infection and discuss several explanations of that pattern.

Parasite-mediated selection in experimental metapopulations of Daphnia magna

In metapopulations, only a fraction of all local host populations may be infected with a given parasite species, and limited dispersal of parasites suggests that colonization of host populations by parasites may involve only a small number of parasite strains. Using hosts and parasites obtained from a natural metapopulation, we studied the evolutionary consequences of invasion by single strains of parasites in experimental populations of the cyclical parthenogen Daphnia magna. In two experiments, each spanning approximately one season, we monitored clone frequency changes in outdoor container populations consisting of 13 and 19 D. magna clones, respectively. The populations were either infected with single strains of the microsporidian parasites Octosporea bayeri or Ordospora colligata or left unparasitized. In both experiments, infection changed the representation of clones over time significantly, indicating parasite-mediated evolution in the experimental populations. Furthermore, the two parasite species changed clone frequencies differently, suggesting that the interaction between infection and competitive ability of the hosts was specific to the parasite species. Taken together, our results suggest that parasite strains that invade local host populations can lead to evolutionary changes in the genetic composition of the host population and that this change is parasite-species specific.

Life-history plasticity in hosts (Lymnaea elodes) exposed to differing resources and parasitism

Assessing phenotypic expression across environments is essential for understanding the evolution of life histories, yet relatively few studies have empirically determined the role that multiple environmental factors play in altering animal phenotypes. We used a multifactorial approach to investigate the effects of both infection of Echinostoma revolutum (Frölich, 1802) and nutrient availability on phenotypic expressions in lines of the snail Lymnaea elodes (Say, 1821). Lines were initially established via selfing events followed by breeding (within lines) over the next four generations. Juveniles from each line were then size-matched and randomly exposed to parasite (exposed / sham-exposed) and diet (high protein / low protein) treatments, generating a 2 (line) × 2 (diet) × 2 (exposure) factorial design. Snail growth, reproduction, and survival were monitored over 5 weeks. Analyses revealed an interactive effect of host line and infection status on host growth. Main effects of both snail line and diet also significantly influenced host growth. Reproductive patterns differed between lines, with snails from one line producing egg masses and eggs in all treatments, and snails from the second line producing minimal eggs in only a single treatment. Snail survival remained similar between snail lines. Results from this study suggest that snail life-history traits can vary dramatically as a result of host genetics, the environment, and the interaction between these factors. Reasons for the occurrence and maintenance of this variability in life-history traits are discussed.

Spatiotemporal heterogeneity in recruitment of larval parasites to shore crab intermediate hosts: the influence of shorebird definitive hosts

Parasitism is a major biotic determinant of animal population dynamics and community structure. Temporal and spatial heterogeneity in parasitism is commonly observed in intermediate host populations. Understanding the causes of temporal and spatial variation in the recruitment of parasites is crucial if we are to manage host populations and animal communities effectively. Here, the temporal and spatial dynamics of Profilicollis antarcticus and Profilicollis novaezelandensis (Acanthocephala) infections in three species of shore crabs (Macrophthalmus hirtipes, Hemigrapsus edwardsii, and Hemigrapsus crenulatus) are examined in relation to the distribution and abundance of shorebird definitive hosts. Temporal patterns of infection were observed in M. hirtipes but not the other two species. Spatial heterogeneity in recruitment of acanthocephalan larvae to M. hirtipes and H. edwardsii populations was found both within and between locations. Weak evidence is found that infection levels in crab populations are related to the distribution and abundance of shorebird hosts both temporally and spatially. In this system, abiotic factors seem to be at least as important in determining how infection levels vary in time and space as the input of parasite eggs from bird definitive hosts.

Clonal variation and covariation in aphid resistance to parasitoids and a pathogen

The potential rate of evolution of resistance to natural enemies depends on the genetic variation present in the population and any trade-offs between resistance and other components of fitness. We measured clonal variation and covariation in pea aphids (Acyrthosiphon pisum) for resistance to two parasitoid species (Aphidius ervi and A. eadyi) and a fungal pathogen (Erynia neoaphidis). We found significant clonal variation in resistance to all three natural enemies. We tested the hypothesis that there might be trade-offs (negative covariation) in defensive ability against different natural enemies, but found no evidence for this. All correlations in defensive ability were positive, that between the two parasitoid species significantly so. Defensive ability was not correlated with fecundity. A number of aphid clones were completely resistant to one parasitoid (A. eadyi), but a subset of these failed to reproduce subsequently. We discuss the factors that might maintain clonal variation in natural enemy resistance.

The cause of parasitic infection in natural populations of Daphnia (Crustacea: Cladocera): the role of host genetics

Disease patterns in nature may be determined by genetic variation for resistance or by factors, genetic or environmental, which influence the host-parasite encounter rate. Elucidating the cause of natural infection patterns has been a major pursuit of parasitologists, but it also matters for evolutionary biologists because host resistance genes must influence the expression of disease if parasite-mediated selection is to occur. We used a model system in order to disentangle the strict genetic component from other causes of infection in the wild. Using the crustacean Daphnia magna and its sterilizing bacterial parasite Pasteuria ramosa, we tested whether genetic variation for resistance, as determined under controlled conditions, accounted for the distribution of infections within natural populations. Specifically, we compared whether the clonally produced great-granddaughters of those individuals that were infected in field samples (but were subsequently 'cured' with antibiotics) were more susceptible than were the great-granddaughters of those individuals that were healthy in field samples. High doses of parasite spores led to increased infection in all four study populations, indicating the importance of encounter rate. Host genetics appeared to be irrelevant to natural infection patterns in one population. However, in three other populations hosts that were healthy in the field had greater genetic-based resistance than hosts that were infected in the field, unambiguously showing the effect of host genetic factors on the expression of disease in the wild.

Life history of a malaria parasite (Plasmodium mexicanum) in its host, the western fence lizard (Sceloporus occidentalis): host testosterone as a source of seasonal and among-host variation?

The course of infection of a malaria parasite (Plasmodium mexicanum) is highly variable in its host, the fence lizard (Sceloporus occidentalis). However, a seasonal trend is superimposed on this variation such that gametocyte production is intensified during mid- to late summer. Host testosterone levels follow a similar seasonal fluctuation and are variable among individual lizards. We sought to determine if testosterone levels affect seasonal and among-host variation in 11 P. mexicanum life history traits: rate of increase in level of infection (3 measures), peak parasitemia (3 measures), duration of increase (3 measures), time to detectable infection, and timing of production of gametocytes. We followed the course of infection in 125 male S. occidentalis, each randomly assigned to 1 of 4 treatment groups: castrated, castrated and implanted with exogenous testosterone, sham implanted, and unmanipulated controls. Median values for the 11 life history traits did not differ among treatment groups, and variances were homogeneous among the treatment groups for 10/11 traits. However, elevated testosterone significantly reduced the variation in timing of the onset of gametocyte production. Therefore, testosterone does not appear to be a primary regulator of P. mexicanum life history, yet testosterone may have some effect on when gametocytes first become detectable.

Sex, linkage disequilibrium and patterns of parasitism in three species of cyclically parthenogenetic Daphnia (Cladocera: Crustacea)

To gain insight into genetic variation for resistance to parasites, this study assayed clonal variation in cyclically parthenogenetic Daphnia magna with respect to parasitic infection. Samples were collected from natural populations, and the allozyme phenotypes of infected hosts were compared to those of uninfected hosts. Differences between the clonal composition of the infected and uninfected class were evident in only two of 16 populations examined. This result stands in contrast to a study of species in the D. pulex and D. longispina species complexes, where clonal variation for infection was found in 12 of 25 populations (Little & Ebert, 1999). Considering all populations from both studies, associations between host genotype and infection were typically evident only in populations that showed low genotypic diversity and evidence of genetic disequilibria, with D. magna showing the least amount of disequilibria. This pattern is compatible with at least two possibly overlapping hypotheses. First, it may be that those populations lacking clonal variation for infection experienced weaker parasite-mediated selection. We can not rule out variation in selection pressure as an explanation, but found no evidence that the prevalence or intensity of parasitism differed either among species, or between those populations which showed clonal variation for infection and those that did not. Second, it could be that some populations, especially those of D. magna, have more frequent sexual recruitment than others. Sexual recombination breaks up gene combinations which are in linkage disequilibrium, and our method to detect clonal variation for resistance relies on linkage between genetic markers (allozymes) and resistance loci. Past work on Daphnia has shown that the level of sexual recruitment (which is in turn mediated by habitat permanency) is indeed commonly linked to the occurrence of genetic disequilibria. Our results may thus underestimate the prevalence of clonal variation for infection (especially for D. magna), because most of the populations analysed appeared to have high levels of sexual recruitment and therefore lacked the linkage disequilibrium that underlies associations between allozymes and susceptibility.