"Resistance Is Futile": Weaker Selection for Resistance by Abundant Parasites Increases Prevalence and Depresses Host Density.

AbstractTheory often predicts that host populations should evolve greater resistance when parasites become abundant. Furthermore, that evolutionary response could ameliorate declines in host populations during epidemics. Here, we argue for an update: when all host genotypes become sufficiently infected, higher parasite abundance can select for lower resistance because its cost exceeds its benefit. We illustrate such a "resistance is futile" outcome with mathematical and empirical approaches. First, we analyzed an eco-evolutionary model of parasites, hosts, and hosts' resources. We determined eco-evolutionary outcomes for prevalence, host density, and resistance (mathematically, "transmission rate") along ecological and trait gradients that alter parasite abundance. With high enough parasite abundance, hosts evolve lower resistance, amplifying infection prevalence and decreasing host density. In support of these results, a higher supply of nutrients drove larger epidemics of survival-reducing fungal parasites in a mesocosm experiment. In two-genotype treatments, zooplankton hosts evolved less resistance under high-nutrient conditions than under low-nutrient conditions. Less resistance, in turn, was associated with higher infection prevalence and lower host density. Finally, in an analysis of naturally occurring epidemics, we found a broad, bimodal distribution of epidemic sizes consistent with the resistance is futile prediction of the eco-evolutionary model. Together, the model and experiment, supplemented by the field pattern, support predictions that drivers of high parasite abundance can lead to the evolution of lower resistance. Hence, under certain conditions, the most fit strategy for individual hosts exacerbates prevalence and depresses host populations.

Timescale reverses the relationship between host density and infection risk.

Host density shapes infection risk through two opposing phenomena. First, when infective stages are subdivided among multiple hosts, greater host densities decrease infection risk through 'safety in numbers'. Hosts, however, represent resources for parasites, and greater host availability also fuels parasite reproduction. Hence, host density increases infection risk through 'density-dependent transmission'. Theory proposes that these phenomena are not disparate outcomes but occur over different timescales. That is, higher host densities may reduce short-term infection risk, but because they support parasite reproduction, may increase long-term risk. We tested this theory in a zooplankton-disease system with laboratory experiments and field observations. Supporting theory, we found that negative density-risk relationships (safety in numbers) sometimes emerged over short timescales, but these relationships reversed to 'density-dependent transmission' within two generations. By allowing parasite numerical responses to play out, time can shift the consequences of host density, from reduced immediate risk to amplified future risk.

Host Controls of Within-Host Disease Dynamics: Insight from an Invertebrate System.

AbstractWithin-host processes (representing the entry, establishment, growth, and development of a parasite inside its host) may play a key role in parasite transmission but remain challenging to observe and quantify. We develop a general model for measuring host defenses and within-host disease dynamics. Our stochastic model breaks the infection process down into the stages of parasite exposure, entry, and establishment and provides associated probabilities for a host's ability to resist infections with barriers and clear internal infections. We tested our model on Daphnia dentifera and the parasitic fungus Metschnikowia bicuspidata and found that when faced with identical levels of parasite exposure, Daphnia patent (transmitting) infections depended on the strength of internal clearance. Applying a Gillespie algorithm to the model-estimated probabilities allowed us to visualize within-host dynamics, within which signatures of host defense could be clearly observed. We also found that early within-host stages were the most vulnerable to internal clearance, suggesting that hosts have a limited window during which recovery can occur. Our study demonstrates how pairing longitudinal infection data with a simple model can reveal new insight into within-host dynamics and mechanisms of host defense. Our model and methodological approach may be a powerful tool for exploring these properties in understudied host-parasite interactions.

Context-Dependent Host-Symbiont Interactions: Shifts along the Parasitism-Mutualism Continuum.

AbstractSymbiotic interactions can shift along a mutualism-parasitism continuum. While there are many studies examining dynamics typically considered to be mutualistic that sometimes shift toward parasitism, little is known about conditions underlying shifts from parasitism toward mutualism. In lake populations, we observed that infection by a microsporidian gut symbiont sometimes conferred a reproductive advantage and other times a disadvantage to its Daphnia host. We hypothesized that the microsporidian might benefit its host by reducing infection by more virulent parasites, which attack via the gut. In a laboratory study using field-collected animals, we found that spores of a virulent fungal parasite were much less capable of penetrating the guts of Daphnia harboring the microsporidian gut symbiont. We predicted that this altered gut penetrability could cause differential impacts on host fitness depending on ecological context. Field survey data revealed that microsporidian-infected Daphnia hosts experienced a reproductive advantage when virulent parasites were common while resource scarcity led to a reproductive disadvantage, but only in lakes where virulent parasites were relatively rare. Our findings highlight the importance of considering multiparasite community context and resource availability in host-parasite studies and open the door for future research into conditions driving shifts along parasitism to mutualism gradients.

A potential role for overdominance in the maintenance of colour variation in the Neotropical tortoise beetle, Chelymorpha alternans.

The presence of persistent polymorphisms within natural populations elicits the question of how such polymorphisms are maintained. All else equal, genetic drift and natural selection should remove genetic variants from populations. Disassortative mating and overdominance are potential mechanisms for maintaining variation within populations. Here, we consider the potential role of these mechanisms in maintaining variation in colour pattern in the tortoise beetle, Chelymorpha alternans. Five colour morphs distinguished by elytral and pronotal coloration are largely determined by a single locus of large effect with four segregating alleles. As many as four morphs co-occur in natural populations. We first assessed whether disassortative mating might maintain this polymorphism. To test for assortative and disassortative mating, we paired females with two males, one with the same colour pattern as the female and one with a different colour pattern and examined the colour patterns of the offspring. We found strong evidence for random mating as a function of colour pattern. We next assessed whether differences in offspring survival among assortative and disassortative male-female pairs maintain colour variation. Crosses involving disassortative pairings had significantly higher offspring survival during development and resulted in more adult progeny. This result is consistent with the effects of overdominance, whereby outcrossed individuals have higher fitness than their homozygous counterparts. Overall, differences in offspring survival appear to play a greater role in maintaining polymorphisms than nonrandom mating in species.

Genotypic variation in an ecologically important parasite is associated with host species, lake and spore size.

Genetic variation in parasites has important consequences for host­parasite interactions. Prior studies of the ecologically important parasite Metschnikowia bicuspidata have suggested low genetic variation in the species. Here, we collected M. bicuspidata from two host species (Daphnia dentifera and Ceriodaphnia dubia) and two regions (Michigan and Indiana, USA). Within a lake, outbreaks tended to occur in one host species but not the other. Using microsatellite markers, we identified six parasite genotypes grouped within three distinct clades, one of which was rare. Of the two main clades, one was generally associated with D. dentifera, with lakes in both regions containing a single genotype. The other M. bicuspidata clade was mainly associated with C. dubia, with a different genotype dominating in each region. Despite these associations, both D. dentifera- and C. dubia-associated genotypes were found infecting both hosts in lakes. However, in lab experiments, the D. dentifera-associated genotype infected both D. dentifera and C. dubia, but the C. dubia-associated genotype, which had spores that were approximately 30% smaller, did not infect D. dentifera. We hypothesize that variation in spore size might help explain patterns of cross-species transmission. Future studies exploring the causes and consequences of variation in spore size may help explain patterns of infection and the maintenance of genotypic diversity in this ecologically important system.

Intraspecific variation in resource use is not explained by population persistence or seasonality.

Populations of generalist grazers often contain genotypes with "powerful" and "efficient" strategies. Powerful genotypes grow rapidly on rich-quality resources, but slowly on poorer-quality ones, while efficient genotypes grow relatively better on poorer resources but cannot exploit richer resources as well. Via a "power-efficiency" trade-off, variation in resource quality could maintain genetic diversity. To evaluate this mechanism, we sampled six populations of the freshwater cladoceran Daphnia pulicaria. In persisting (year-round) populations, Daphnia consume resources that vary in quality, whereas in non-persisting (spring-only) populations, Daphnia primarily encounter rich-quality resources. We hypothesized that non-persisting populations harbor no efficient clones (hence should show lower growth on poor-quality resources). Although individuals from non-persisting populations remained smaller than individuals from persisting populations, no evidence arose for a trade-off between powerful and efficient strategies. In fact, growth rates on the two diets were positively correlated (instead of negatively, as predicted). Furthermore, in the persisting populations, we predicted that clonal selection from spring to summer should shift the distribution of genotypes from powerful (specialists on richer spring resources) to efficient (poorer, summer resources). Genetic composition of populations shifted from spring to summer, but not toward more efficient genotypes. Therefore, in these lakes, maintenance of variation among genotypes must stem from more complicated factors than population persistence patterns or seasonal shifts in resource quality alone.

The evolution of reproductive isolation in Daphnia.

BACKGROUND: The process by which populations evolve to become new species involves the emergence of various reproductive isolating barriers (RIB). Despite major advancements in understanding this complex process, very little is known about the order in which RIBs evolve or their relative contribution to the total restriction of gene flow during various stages of speciation. This is mainly due to the difficulties of studying reproductive isolation during the early stages of species formation. This study examines ecological and non-ecological RIB within and between Daphnia pulex and Daphnia pulicaria, two recently diverged species that inhabit distinct habitats and exhibit an unusual level of intraspecific genetic subdivision. RESULTS: We find that while ecological prezygotic barriers are close to completion, none of the non-ecological barriers can restrict gene flow between D. pulex and D. pulicaria completely when acting alone. Surprisingly, we also identified high levels of postzygotic reproductive isolation in 'conspecific' interpopulation crosses of D. pulex. CONCLUSIONS: While the ecological prezygotic barriers are prevalent during the mature stages of speciation, non-ecological barriers likely dominated the early stages of speciation. This finding indicates the importance of studying the very early stages of speciation and suggests the contribution of postzygotic isolation in initiating the process of speciation.

Genotypic variation in parasite avoidance behaviour and other mechanistic, nonlinear components of transmission.

Traditional epidemiological models assume that transmission increases proportionally to the density of parasites. However, empirical data frequently contradict this assumption. General yet mechanistic models can explain why transmission depends nonlinearly on parasite density and thereby identify potential defensive strategies of hosts. For example, hosts could decrease their exposure rates at higher parasite densities (via behavioural avoidance) or decrease their per-parasite susceptibility when encountering more parasites (e.g. via stronger immune responses). To illustrate, we fitted mechanistic transmission models to 19 genotypes of Daphnia dentifera hosts over gradients of the trophically acquired parasite, Metschnikowia bicuspidata. Exposure rate (foraging, F) frequently decreased with parasite density (Z), and per-parasite susceptibility (U) frequently decreased with parasite encounters (F×Z). Consequently, infection rates (F×U×Z) often peaked at intermediate parasite densities. Moreover, host genotypes varied substantially in these responses. Exposure rates remained constant for some genotypes but decreased sensitively with parasite density for others (up to 78%). Furthermore, genotypes with more sensitive foraging/exposure also foraged faster in the absence of parasites (suggesting 'fast and sensitive' versus 'slow and steady' strategies). These relationships suggest that high densities of parasites can inhibit transmission by decreasing exposure rates and/or per-parasite susceptibility, and identify several intriguing axes for the evolution of host defence.

Indirect effects in a planktonic disease system.

Indirect effects, both density- and trait-mediated, have been known to act in tandem with direct effects in the interactions of numerous species. They have been shown to affect populations embedded in competitive and mutualistic networks alike. In this work, we introduce a four-dimensional system of ordinary differential equations and investigate the interplay between direct density-effects and density- and trait-mediated indirect effects that take place in a yeast parasite-zooplankton host-incompetent competitor system embedded in a food web which also includes resources and predators. Among our main findings is the demonstration that indirect effects cause qualitative and quantitative changes almost indistinguishable from direct effects and the corroboration through our analysis of the fact that the effects of direct and indirect mechanisms cannot be disentangled. Our results underpin the conclusions of past studies calling for comprehensive models that incorporate both direct and indirect effects to better describe field data.

Temperature Drives Epidemics in a Zooplankton-Fungus Disease System: A Trait-Driven Approach Points to Transmission via Host Foraging.

Climatic warming will likely have idiosyncratic impacts on infectious diseases, causing some to increase while others decrease or shift geographically. A mechanistic framework could better predict these different temperature-disease outcomes. However, such a framework remains challenging to develop, due to the nonlinear and (sometimes) opposing thermal responses of different host and parasite traits and due to the difficulty of validating model predictions with observations and experiments. We address these challenges in a zooplankton-fungus (Daphnia dentifera-Metschnikowia bicuspidata) system. We test the hypothesis that warmer temperatures promote disease spread and produce larger epidemics. In lakes, epidemics that start earlier and warmer in autumn grow much larger. In a mesocosm experiment, warmer temperatures produced larger epidemics. A mechanistic model parameterized with trait assays revealed that this pattern arose primarily from the temperature dependence of transmission rate (ß), governed by the increasing foraging (and, hence, parasite exposure) rate of hosts (f). In the trait assays, parasite production seemed sufficiently responsive to shape epidemics as well; however, this trait proved too thermally insensitive in the mesocosm experiment and lake survey to matter much. Thus, in warmer environments, increased foraging of hosts raised transmission rate, yielding bigger epidemics through a potentially general, exposure-based mechanism for ectotherms. This mechanistic approach highlights how a trait-based framework will enhance predictive insight into responses of infectious disease to a warmer world.

Parasite rearing and infection temperatures jointly influence disease transmission and shape seasonality of epidemics.

Seasonal epidemics erupt commonly in nature and are driven by numerous mechanisms. Here, we suggest a new mechanism that could determine the size and timing of seasonal epidemics: rearing environment changes the performance of parasites. This mechanism arises when the environmental conditions in which a parasite is produced impact its performance-independently from the current environment. To illustrate the potential for "rearing effects", we show how temperature influences infection risk (transmission rate) in a Daphnia-fungus disease system through both parasite rearing temperature and infection temperature. During autumnal epidemics, zooplankton hosts contact (eat) fungal parasites (spores) reared in a gradually cooling environment. To delineate the effect of rearing temperature from temperature at exposure and infection, we used lab experiments to parameterize a mechanistic model of transmission rate. We also evaluated the rearing effect using spores collected from epidemics in cooling lakes. We found that fungal spores were more infectious when reared at warmer temperatures (in the lab and in two of three lakes). Additionally, the exposure (foraging) rate of hosts increased with warmer infection temperatures. Thus, both mechanisms cause transmission rate to drop as temperature decreases over the autumnal epidemic season (from summer to winter). Simulations show how these temperature-driven changes in transmission rate can induce waning of epidemics as lakes cool. Furthermore, via thermally dependent transmission, variation in environmental cooling patterns can alter the size and shape of epidemics. Thus, the thermal environment drives seasonal epidemics through effects on hosts (exposure rate) and the infectivity of parasites (a rearing effect). Presently, the generality of parasite rearing effects remains unknown. Our results suggest that they may provide an important but underappreciated mechanism linking temperature to the seasonality of epidemics.

Rapid evolution rescues hosts from competition and disease but-despite a dilution effect-increases the density of infected hosts.

Virulent parasites can depress the densities of their hosts. Taxa that reduce disease via dilution effects might alleviate this burden. However, 'diluter' taxa can also depress host densities through competition for shared resources. The combination of disease and interspecific competition could even drive hosts extinct. Then again, genetically variable host populations can evolve in response to both competitors and parasites. Can rapid evolution rescue host density from the harm caused by these ecological enemies? How might such evolution influence dilution effects or the size of epidemics? In a mesocosm experiment with planktonic hosts, we illustrate the joint harm of competition and disease: hosts with constrained evolutionary ability (limited phenotypic variation) suffered greatly from both. However, populations starting with broader phenotypic variation evolved stronger competitive ability during epidemics. In turn, enhanced competitive ability-driven especially by parasites-rescued host densities from the negative impacts of competition, disease, and especially their combination. Interspecific competitors reduced disease (supporting dilution effects) even when hosts rapidly evolved. However, this evolutionary response also elicited a potential problem. Populations that evolved enhanced competitive ability and maintained robust total densities also supported higher densities of infections. Thus, rapid evolution rescued host densities but also unleashed larger epidemics.

Allocation, not male resistance, increases male frequency during epidemics: a case study in facultatively sexual hosts.

Why do natural populations vary in the frequency of sexual reproduction? Virulent parasites may help explain why sex is favored during disease epidemics. To illustrate, we show a higher frequency of males and sexually produced offspring in natural populations of a facultative parthenogenetic host during fungal epidemics. In a multi-year survey of 32 lakes, the frequency of males (an index of sex) was higher in populations of zooplankton hosts with larger epidemics. A lake mesocosm experiment established causality: experimental epidemics produced a higher frequency of males relative to disease-free controls. One common explanation for such a pattern involves Red Queen (RQ) dynamics. However, this particular system lacks key genetic specificity mechanisms required for the RQ, so we evaluated two other hypotheses. First, individual females, when stressed by infection, could increase production of male offspring vs. female offspring (a tenant of the "Abandon Ship" theory). Data from a life table experiment supports this mechanism. Second, higher male frequency during epidemics could reflect a purely demographic process (illustrated with a demographic model): males could resist infection more than females (via size-based differences in resistance and mortality). However, we found no support for this resistance mechanism. A size-based model of resistance, parameterized with data, revealed why: higher male susceptibility negated the lower exposure (a size-based advantage) of males. These results suggest that parasite-mediated increases in allocation to sex by individual females, rather than male resistance, increased the frequency of sex during larger disease epidemics.

Initial genetic diversity enhances population establishment and alters genetic structuring of a newly established Daphnia metapopulation.

When newly created habitats are initially colonized by genotypes with rapid population growth rates, later arriving colonists may be prevented from establishing. Although these priority effects have been documented in multiple systems, their duration may be influenced by the diversity of the founding population. We conducted a large-scale field manipulation to investigate how initial clonal diversity influences temporal and landscape patterns of genetic structure in a developing metapopulation. Six genotypes of obligately asexual Daphnia pulex were stocked alone (no clonal diversity) or in combination ('high' clonal diversity) into newly created experimental woodland ponds. We also measured the population growth rate of all clones in the laboratory when raised on higher-quality and lower-quality resources. Our predictions were that in the 3 years following stocking, clonally diverse populations would be more likely to persist than nonclonally diverse populations and exhibit evidence for persistent founder effects. We expected that faster growing clones would be found in more pools and comprise a greater proportion of individuals genotyped from the landscape. Genetic composition, both locally and regionally, changed significantly following stocking. Six of 27 populations exhibited evidence for persistent founder effects, and populations stocked with 'high' clonal diversity were more likely to exhibit these effects than nonclonally diverse populations. Performance in the laboratory was not predictive of clonal persistence or overall dominance in the field. Hence, we conclude that although laboratory estimates of fitness did not fully explain metapopulation genetic structure, initial clonal diversity did enhance D. pulex population establishment and persistence in this system.

Parasites destabilize host populations by shifting stage-structured interactions.

Should parasites stabilize or destabilize consumer-resource dynamics? Recent theory suggests that parasite-enhanced mortality may confer underappreciated stability to their hosts. We tested this hypothesis using disease in zooplankton. Across both natural and experimental epidemics, bigger epidemics correlated with larger--not smaller--host fluctuations. Thus, we tested two mechanistic hypotheses to explain destabilization or apparent destabilization by parasites. First, enrichment could, in principle, simultaneously enhance both instability and disease prevalence. In natural epidemics, destabilization was correlated with enrichment (indexed by total phosphorous). However, an in situ (lake enclosure) experiment did not support these links. Instead, field and experimental results point to a novel destabilizing mechanism involving host stage structure. Epidemics pushed hosts from relatively more stable host dynamics with less-synchronized juveniles and adults to less stable dynamics with more-synchronized juveniles and adults. Our results demonstrate how links between host stage structure and disease can shape host/consumer-resource stability.

Success, failure and ambiguity of the dilution effect among competitors.

It remains challenging to predict variation in the magnitude of disease outbreaks. The dilution effect seeks to explain this variation by linking multiple host species to disease transmission. It predicts that disease risk increases for a focal host when host species diversity declines. However, when an increase in species diversity does not reduce disease, we are often unable to diagnose why. Here, we increase mechanistic and predictive clarity of the dilution effect with a general trait-based model of disease transmission in multi-host communities. Then, we parameterise and empirically test our model with a multi-generational case study of planktonic disease. The model-experiment combination shows that hosts that vary in competitive ability (R*) and potential to spread disease (R0 ) can produce three qualitatively disparate outcomes of dilution on disease: the dilution effect can succeed, fail, or be ambiguous/irrelevant.

Trait-mediated indirect effects, predators, and disease: test of a size-based model.

Increasing prevalence of wildlife disease accentuates the need to uncover drivers of epidemics. Predators can directly influence disease prevalence via density-mediated effects (e.g., culling infected hosts leading to reduced disease prevalence). However, trait-mediated indirect effects (TMIEs) of predators can also strongly influence disease--but predicting a priori whether TMIEs should increase or decrease disease prevalence can be challenging, especially since a single predator may elicit responses that have opposing effects on disease prevalence. Here, we pair laboratory experiments with a mechanistic, size-based model of TMIEs in a zooplankton host, fungal parasite, multiple predator system. Kairomones can either increase or decrease body size of the host Daphnia, depending on the predator. These changes in size could influence key traits of fungal disease, since infection risk and spore yield increase with body size. For six host genotypes, we measured five traits that determine an index of disease spread (R 0). Although host size and disease traits did not respond to kairomones produced by the invertebrate predator Chaoborus, cues from fish reduced body size and birth rate of uninfected hosts and spore yield from infected hosts. These results support the size model for fish; the birth and spore yield responses should depress disease spread. However, infection risk did not decrease with fish kairomones, thus contradicting predictions of the size model. Exposure to kairomones increased per spore susceptibility of hosts, countering size-driven decreases in exposure to spores. Consequently, synthesizing among the relevant traits, there was no net effect of fish kairomones on the R 0 metric. This result accentuates the need to integrate the TMIE-based response to predators among all key traits involved in disease spread.

Heterocypris incongruens maintains an egg bank in stormwater habitats and influences the development of larval mosquito, Culex restuans.

Dormant propagules can provide a rapid colonization source for temporary aquatic habitats and set the trajectory for community dynamics, yet the egg banks of stormwater management systems have received little attention. We asked which species hatched from the sediment of drainage ditches in Champaign County, IL, and found bdelloid rotifers and ostracods (Heterocypris incongruens) to be the most common taxa. These sites also are colonized by mosquitoes, and we established laboratory experiments to examine interspecific interactions between common co-occurring taxa. Culex restuans larvae were reared in the presence or absence of H. incongruens at two intra- and interspecific densities (20 or 40 total individuals) and their survivorship to adulthood, development time to adulthood, adult body size, and sex ratio were determined. Survival for Cx. restuans was significantly lower at high larval density than at low larval density in both treatments. Culex restuans larvae reared in the presence of H. incongruens had a shorter development time to adulthood and emerged as larger adults compared to those reared in the absence of H. incongruens. The sex ratios in the H. incongruens treatments were female-biased whereas those in the Culex-only treatments were male-biased. These differences may have epidemiological implications, as only female mosquitoes serve as disease vectors. Our results emphasize the importance of understanding interspecific interactions in influencing larval mosquito development traits.