Parasitism shaping host life-history evolution: adaptive responses in a marine gastropod to infection by trematodes

Strigea robusta (Digenea: Strigeidae) infection effects on the gonadal structure and limb malformation in toad early development

The anomaly P is a mass morphological anomaly reported in some water frog populations across Europe. It was found that polydactyly is only a mild attenuated form of heavy cases of the anomaly P syndrome, which have strong deformations of the hindlimbs and, partly, forelimbs. It was shown that the anomaly P is caused by the trematode Strigea robusta and this syndrome can be considered as a special case of strigeosis in amphibians. The anomaly P for a long time considered to be specific for water frogs of the genus Pelophylax. Herein, we describe polydactyly and heavy forms of the anomaly P syndrome in toads of the genera Bufo and Bufotes, as a result of exposure to S. robusta cercariae. A total of 150 tadpoles of Bufo bufo, 60 tadpoles of Bufotes viridis, and 60 tadpoles of Bufotes baturae were divided into five experimental and four control groups (30 tadpoles in each group). All anomalies in the toads were similar to those observed in water frogs. The survival of tadpoles in the experimental groups was 76%. The anomaly P was observed in 57.9% of toad tadpoles (51.8% of mild forms and 6.1% of heavy forms). The occurrence of the anomaly P varied among groups from 19% to 78%. Heavy forms of the anomaly P were found in all experimental groups. We described rare asymmetrical cases of the anomaly P. According to severe modification of limb morphology, we supposed changes of gonadal morphology (any modifications of the germ and somatic cells). The gonadal development of infected tadpoles was however the same as in uninfected toad tadpoles, and heterochromatin distribution within gonocytes had no differences as well. It seems like the parasite doesn't have any effect on the gonadal development of the toads. The lack of heavy forms in natural populations of toads, as well as a development of gonads were discussed.

A role of flowering genes in the tolerance of Arabidopsis thaliana to cucumber mosaic virus

The genetic basis of plant tolerance to parasites is poorly understood. We have previously shown that tolerance of Arabidopsis thaliana to its pathogen cucumber mosaic virus is achieved through changes in host life-history traits on infection that result in delaying flowering and reallocating resources from vegetative growth to reproduction. In this system we analyse here genetic determinants of tolerance using a recombinant inbred line family derived from a cross of two accessions with extreme phenotypes. Three major quantitative trait loci for tolerance were identified, which co-located with three flowering repressor genes, FLC, FRI, and HUA2. The role of these genes in tolerance was further examined in genotypes carrying functional or nonfunctional alleles. Functional alleles of FLC together with FRI and/or HUA2 were required for both tolerance and resource reallocation from growth to reproduction. Analyses of FLC alleles from wild accessions that differentially modulate flowering time showed that they ranked differently for their effects on tolerance and flowering. These results pinpoint a role of FLC in A. thaliana tolerance to cucmber mosaic virus, which is a novel major finding, as FLC has not been recognized previously to be involved in plant defence. Although tolerance is associated with a delay in flowering that allows resource reallocation, our results indicate that FLC regulates tolerance and flowering initiation by different mechanisms. Thus, we open a new avenue of research on the interplay between defence and development in plants.

Temporal and spatial dynamics of gastrointestinal parasite infection in Père David's deer

Background

The Père David's deer (Elaphurus davidianus) population was established from only a small number of individuals. Their genetic diversity is therefore relatively low and transmissible (parasitic) diseases affecting them merit further attention. Parasitic infections can affect the health, survival, and population development of the host. However, few reports have been published on the gastrointestinal parasites of Père David's deer. The aims of this study were: (1) to identify the intestinal parasites groups in Père David's deer; (2) to determine their prevalence and burden and clarify the effects of different seasons and regions on various indicators of Père David's deer intestinal parasites; (3) to evaluate the effects of the Père David's deer reproductive period on these parasites; (4) to reveal the regularity of the parasites in space and time.

Methods

In total, 1,345 Père David's deer faecal samples from four regions during four seasons were tested using the flotation (saturated sodium nitrate solution) to identify parasites of different genus or group, and the McMaster technique to count the number of eggs or oocysts.

Results

Four groups of gastrointestinal parasites were found, of which strongyles were dominant; their prevalence and burden were significantly higher than other groups. Significant temporal and spatial effects on gastrointestinal parasitic infection were found. Parasite diversity, prevalence, parasite burden, and aggregation were the highest in summer. Among the four regions, parasite diversity, prevalence, and burden were the highest in the Dongting Lake area. In addition, parasite diversity and burden during the reproductive period of Père David's deer was significantly higher than during the post-reproductive period.

Conclusions

The summer season and the reproductive period of Père David's deer had great potential for parasite transmission, and there is a high risk of parasite outbreaks in the Dongting Lake area.

Life history shifts in an exploited African fish following invasion by a castrating parasite

Evolutionary theory predicts that infection by a parasite that reduces future host survival or fecundity should select for increased investment in current reproduction. In this study, we use the cestode Ligula intestinalis and its intermediate fish host Engraulicypris sardella in Wissman Bay, Lake Nyasa (Tanzania), as a model system. Using data about infection of E. sardella fish hosts by L. intestinalis collected for a period of 10 years, we explored whether parasite infection affects the fecundity of the fish host E. sardella, and whether host reproductive investment has increased at the expense of somatic growth. We found that L. intestinalis had a strong negative effect on the fecundity of its intermediate fish host. For the noninfected fish, we observed an increase in relative gonadal weight at maturity over the study period, while size at maturity decreased. These findings suggest that the life history of E. sardella has been shifting toward earlier reproduction. Further studies are warranted to assess whether these changes reflect plastic or evolutionary responses. We also discuss the interaction between parasite and fishery-mediated selection as a possible explanation for the decline of E. sardella stock in the lake.

Size and survival of two freshwater snail species in relation to shedding of cercariae of castrating Echinostoma spp

Trematode-induced castration of snails is widespread and can lead to other life history changes of snails such as changes in trajectories of size and growth or survival. The changes produced likely depend on whether the parasite or host controls allocation of host resources remaining after partial or complete cessation of host current reproduction by castrating trematodes. Documenting host life history changes, like changes in host size in response to castration, is a first step in assessing whether these changes are beneficial to the parasite (increasing transmission success) or to the host (outliving the infection) or to neither. Herein, we test for differences in size and survival among individuals of two snail species in relation to infection by Echinostoma spp. trematodes. Active shedding of Echinostoma spp. was associated with castration of all Stagnicola elodes snails from a site in Eastern Ontario. Snails actively shedding cercariae were not different in size from non-shedding, egg-laying snails but had a higher mortality than egg-laying snails. Active shedding of Echinostoma spp. cercariae was also associated with castration of nearly all Helisoma trivolvis monitored, from a site in Southwestern Ontario. Actively shedding, non-laying H. trivolvis hosts were smaller on average than non-shedding egg-laying hosts, but both non-laying and egg-laying snails survived equally well. We discuss these results in light of what is known about effects of castration on snail hosts in terms of growth and survival for these and other trematode species and speculate on whether changes in size or survival benefits parasite or host.

Small-mammal characteristics affect tick communities in southwestern Tennessee (USA)

Life histories can influence the degree of parasite infestations on a host. Pressures exerted on hosts based on age and sex convey varying degrees of parasite prevalence due to differences in host lifestyles, but it is not known how interactions between different host traits affect tick numbers. The objective of this study was to determine if host characteristics (e.g., age, sex, weight, and their interactions) affect the mean number of ticks found on small mammals regardless of host species or habitat. Sherman live traps were placed in forest and grass/forb habitats representative of the southeastern United States. After capture, host characteristics were recorded, and hosts were then searched for ticks. A total of 281 small mammals (148 Peromyscus leucopus, 34 P. maniculatus, 76 Sigmodon hispidus, 16 Microtus pinetorum, and 7 Ochrotomys nuttalli) and 610 ticks (488 Dermacentor variabilis, 114 Ixodes scapularis, 1 Amblyomma americanum, and 7 A. maculatum) were collected in this study. Host's age, sex, and weight affected the number of ticks collected from small mammals and significant interaction effects between host traits occurred (weight by sex, weight by age, and sex by age). For instance, female subadult rodents had significantly more ticks compared to female adults, male subadults had significantly fewer ticks compared to male adults, and the number of ticks on a host increased as host body mass increased. These results support the hypothesis that the number of ticks vary on rodent hosts based on life histories and trait interactions. Therefore, understanding the behavioral mechanisms of a host can aid in the management of parasites in the environment.

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Host life histories predict the number of ticks present on a rodent.

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Male and subadult rodents are infested with an overall greater number of ticks than females and adults.

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Rodents have more ticks as their weight increases.

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Interactions between sex and age predict the number of ticks present on a rodent.

Effects of vertical position on trematode parasitism in larval anurans

Spatial distributions of animals can affect interactions with their natural enemies, such as parasites, and thus have important implications for host–parasite dynamics. While spatial variation in infection risk has been explored in many systems at the landscape scale, less attention has been paid to spatial structure at smaller scales. Here, we explore a hypothesized relationship between a common spatial variable, vertical position, and risk of parasite infection in a model aquatic system, larval frogs (Rana) and trematode (Digenea) parasites. Vertical position is relevant to this system given evidence that the densities of snail first intermediate hosts, tadpole second intermediate hosts, and trematode infective stages can vary with depth. To test the effects of depth on infection risk of larval frogs by trematodes, we performed two enclosure experiments, one in the laboratory and one in the field, in which larval frogs in cages just below the water surface or near the bottom of the water column were exposed to parasites. Compared with near-surface cages, mean infection load (number of cysts) in tadpoles in near-bottom cages was 83% higher after 48-h exposures in the laboratory and 730% higher after 10-day exposures in the field. Our findings thus indicate that infection risk depends on depth, which may have adaptive significance, as tadpoles have previously been shown to change vertical position in response to parasite presence. These results motivate future work examining vertical variation in infection risk and may have broader implications for host–parasite dynamics and evolution of host and parasite behavior.

Context-Dependence in parasite effects on keyhole limpets

Parasites alter the reproductive performance of their hosts, limit their growth, and thereby modify the energy budget of these hosts. Experimental studies and theoretical models suggest that the outcome of the host-parasite interactions could be determined by ecological factors such as food availability levels in the local habitats. Nutrient inputs may affect the host's food resource availability with positive or negative effects on parasite infection rates and tolerance of infection, however this has not been specifically evaluated in natural systems. In this study, we evaluate the effects of parasitism by Proctoeces humboldti on body size, gonadosomatic index (GSI), and metabolic rate (oxygen consumption) of their second intermediate host Fissurella crassa limpets, under contrasting natural conditions of productivity (upwelling center vs upwelling shadow sites). Our results evidenced that parasitized limpets collected from the intertidal habitat influenced by coastal upwelling site showed greater shell length, muscular foot biomass and GSI as compared to non-parasitized limpets collected in the same site, and compared to parasitized and non-parasitized limpets collected from the sites under the influence of upwelling shadow conditions. Oxygen consumption was lower in parasitized limpets collected from the upwelling-influenced site than in the other groups, independent of age, suggesting reduced metabolic stress in infected individuals inhabiting these productive sites. Our results suggest that increased productivity in upwelling sites could mitigate the conflict for resources in the P. humboldti - F. crassa system, influencing where such interaction is found in the continuum between parasitism and mutualism. Since parasitism is ubiquitous in natural systems, and play important roles in ecological and evolutionary processes, it is important to analyze host-parasite interaction across a variety of ecological conditions, especially in biological conservation.

Trade-offs between host tolerances to different pathogens in plant-virus interactions

Although accumulating evidence indicates that tolerance is a plant defence strategy against pathogens as widespread as resistance, how plants evolve tolerance is poorly understood. Theory predicts that hosts will evolve to maximize tolerance or resistance, but not both. Remarkably, most experimental works failed in finding this trade-off. We tested the hypothesis that the evolution of tolerance to one virus is traded-off against tolerance to others, rather than against resistance and identified the associated mechanisms. To do so, we challenged eighteen Arabidopsis thaliana genotypes with Turnip mosaic virus (TuMV) and Cucumber mosaic virus (CMV). We characterized plant life-history trait modifications associated with reduced effects of TuMV and CMV on plant seed production (fecundity tolerance) and life period (mortality tolerance), both measured as a norm of reaction across viral loads (range tolerance). Also, we analysed resistance-tolerance and tolerance-tolerance trade-offs. Results indicate that tolerance to TuMV is associated with changes in the length of the pre-reproductive and reproductive periods, and tolerance to CMV with resource reallocation from growth to reproduction; and that tolerance to TuMV is traded-off against tolerance to CMV in a virulence-dependent manner. Thus, this work provides novel insights on the mechanisms of plant tolerance and highlights the importance of considering the combined effect of different pathogens to understand how plant defences evolve.

Parasitism by an invasive nest fly reduces future reproduction in Galápagos mockingbirds

Organisms allocate limited resources to competing activities such as reproduction, growth, and defense against parasites and predators. The introduction of a novel parasite may create new life history trade-offs. As hosts increase their investment in self-maintenance or defense, the cost of parasitism may carry over to other aspects of host biology. Here, in an experimental field study, we document delayed effects of an introduced nest parasite, Philornis downsi, on reproduction of Galápagos mockingbirds (Mimus parvulus). Parasitism of first nests reduced both the number and size of chicks that parents hatched when they re-nested several weeks later. The delayed effect of P. downsi on future reproduction may have been mediated by behavioral shifts by the parents to avoid or resist parasitism. Our results demonstrate that effects of parasitism can persist even after immediate exposure ends. We draw attention to the potential implications that introduced parasites have for host reproductive strategies.

Do ectoparasites affect survival of three species of lizards of the genus Sceloporus?

The short-term effects that ectoparasites cause to their hosts, such as local wounds and secondary infections that occur within a few hours or days after infection, are well documented in a wide variety of taxa, whereas long-term negative effects on the fitness of hosts, which result from chronic infections and are evident after several months, are less understood. Lizards are hosts of distinct species of mites and ticks that cause short-term negative effects such as ulcers, sores and local inflammation. However, the negative effects that these ectoparasites may have on the long-term survival of lizards have not been evaluated. In this study, we collected two years of capture-mark-recapture data and implemented a multi-model inference framework to examine if high ectoparasite loads have negative effects on the long-term survival probability of three lizard species of the genus Sceloporus (S. grammicus, S. megalepidurus, and S. torquatus). In addition, we considered that the potential negative effect of ectoparasites on survival may vary depending on sex, body condition, reproductive season, or climatic season. Contrary to our expectations, our results did not support the hypothesis that high ectoparasite loads reduce the survival probability of these lizards. In S. grammicus and S. megalepidurus we found no evidence of an effect of ectoparasite load on host survival. In S. torquatus ectoparasites influenced survival probability, but the effect was opposite to what we predicted: survival increased substantially as ectoparasite load increased. This unexpected result might be explained by mites discriminating between hosts and attaching more frequently to lizards in better health status, or by high-quality lizards having greater chances of contracting ectoparasites, because these individuals move around large areas and frequently engage in social interactions.

Podocotyle atomon (Trematoda: Digenea) impacts reproductive behaviour, survival and physiology in Gammarus zaddachi (Amphipoda)

The Trematoda are a group of phylogenetically diverse metazoan parasites that exhibit complex life cycles that often pass through invertebrate and vertebrate hosts. Some trematodes influence their host's behaviour to benefit transmission. Their parasitic influence may impact host population size by inhibiting an individual's reproductive capacity. We assessed the impact of infection by Podocotyle atomon on the reproductive behaviour and fecundity of its amphipod intermediate host, Gammarus zaddachi, using laboratory and field studies. Parasite prevalence was high in the field, with males more likely to be infected (prevalence in males 64%, in females 39%). Males also suffered a higher parasite burden than females. Infected females were less active, but we found no evidence for a reduction in female reproductive success. Infected females also had comparable pairing success to uninfected females. In males, infection reduced survival and fecundity, with mortality being highest, and sperm numbers lowest, in heavily infected individuals. Trematode parasites are sometimes associated with altered host fecundity, but studies often lack the relevant experimental data to explore the evolution of the trait. We discuss this among information specific to the effect of P. atomon infection in G. zaddachi.

Effective tolerance based on resource reallocation is a virus-specific defence in Arabidopsis thaliana

Plant viruses often harm their hosts, which have developed mechanisms to prevent or minimize the effects of virus infection. Resistance and tolerance are the two main plant defences to pathogens. Although resistance to plant viruses has been studied extensively, tolerance has received much less attention. Theory predicts that tolerance to low-virulent parasites would be achieved through resource reallocation from growth to reproduction, whereas tolerance to high-virulent parasites would be attained through shortening of the pre-reproductive period. We have shown previously that the tolerance of Arabidopsis thaliana to Cucumber mosaic virus (CMV), a relatively low-virulent virus in this host, accords to these predictions. However, whether other viruses trigger the same response, and how A. thaliana copes with highly virulent virus infections remains unexplored. To address these questions, we challenged six A. thaliana wild genotypes with five viruses with different genomic structures, life histories and transmission modes. In these plants, we quantified virus multiplication, virulence, and the effects of infection on plant growth and reproduction, and on the developmental schedule. Our results indicate that virus multiplication varies according to the virus × host genotype interaction. Conversely, effective tolerance is observed only on CMV infection, and is associated with resource reallocation from growth to reproduction. Tolerance to the other viruses is observed only in specific host-virus combinations and, at odds with theoretical predictions, is linked to longer pre-reproductive periods. These findings only partially agree with theoretical predictions, and contribute to a better understanding of pathogenic processes in plant-virus interactions.

Metabolic rate evolves rapidly and in parallel with the pace of life history

Metabolic rates and life history strategies are both thought to set the “pace of life”, but whether they evolve in tandem is not well understood. Here, using a common garden experiment that compares replicate paired populations, we show that Trinidadian guppy (Poecilia reticulata) populations that evolved a fast-paced life history in high-predation environments have consistently higher metabolic rates than guppies that evolved a slow-paced life history in low-predation environments. Furthermore, by transplanting guppies from high- to low-predation environments, we show that metabolic rate evolves in parallel with the pace of life history, at a rapid rate, and in the same direction as found for naturally occurring populations. Together, these multiple lines of inference provide evidence for a tight evolutionary coupling between metabolism and the pace of life history.

The ‘pace of life’ depends on both metabolic rate and life history traits; however, whether these evolve similarly in response to the environment is not clear. Here, Auer et al. show parallel evolution of metabolic rate and a suite of life history traits in response to predator environment in Trinidadian guppies.

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

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

Cancer and life-history traits: lessons from host-parasite interactions

Despite important differences between infectious diseases and cancers, tumour development (neoplasia) can nonetheless be closely compared to infectious disease because of the similarity of their effects on the body. On this basis, we predict that many of the life-history (LH) responses observed in the context of host-parasite interactions should also be relevant in the context of cancer. Parasites are thought to affect LH traits of their hosts because of strong selective pressures like direct and indirect mortality effects favouring, for example, early maturation and reproduction. Cancer can similarly also affect LH traits by imposing direct costs and/or indirectly by triggering plastic adjustments and evolutionary responses. Here, we discuss how and why a LH focus is a potentially productive but under-exploited research direction for cancer research, by focusing our attention on similarities between infectious disease and cancer with respect to their effects on LH traits and their evolution. We raise the possibility that LH adjustments can occur in response to cancer via maternal/paternal effects and that these changes can be heritable to (adaptively) modify the LH traits of their offspring. We conclude that LH adjustments can potentially influence the transgenerational persistence of inherited oncogenic mutations in populations.

Trade-offs between acquired and innate immune defenses in humans

Immune defenses provide resistance against infectious disease that is critical to survival. But immune defenses are costly, and limited resources allocated to immunity are not available for other physiological or developmental processes. We propose a framework for explaining variation in patterns of investment in two important subsystems of anti-pathogen defense: innate (non-specific) and acquired (specific) immunity. The developmental costs of acquired immunity are high, but the costs of maintenance and activation are relatively low. Innate immunity imposes lower upfront developmental costs, but higher operating costs. Innate defenses are mobilized quickly and are effective against novel pathogens. Acquired responses are less effective against novel exposures, but more effective against secondary exposures due to immunological memory. Based on their distinct profiles of costs and effectiveness, we propose that the balance of investment in innate versus acquired immunity is variable, and that this balance is optimized in response to local ecological conditions early in development. Nutritional abundance, high pathogen exposure and low signals of extrinsic mortality risk during sensitive periods of immune development should all favor relatively higher levels of investment in acquired immunity. Undernutrition, low pathogen exposure, and high mortality risk should favor innate immune defenses. The hypothesis provides a framework for organizing prior empirical research on the impact of developmental environments on innate and acquired immunity, and suggests promising directions for future research in human ecological immunology.

Interspecific interactions explain variation in the duration of paternal care in the burying beetle

Why is there so much variation within species in the extent to which males contribute to offspring care? Answers to this question commonly focus on intraspecific sources of variation in the relative costs and benefits of supplying paternal investment. With experiments in the laboratory on the burying beetle, Nicrophorus vespilloides, and its phoretic mite Poecilochirus carabi, we investigated whether interactions with a second species might also account for intraspecific variation in the extent of paternal care, and whether this variation is due to adaptation or constraint. In our first experiment we bred beetles in the presence or absence of phoretic mites, using a breeding box that mimicked natural conditions by allowing parents to leave the breeding attempt at a time of their choosing. We found that males abandoned their brood sooner when breeding alongside mites than when breeding in their absence. Female patterns of care were unchanged by the mites. Nevertheless, in this experiment, no correlates of beetle fitness were affected by the presence of the mites during reproduction (neither paternal life span after reproduction nor brood size or average larval mass). In a second experiment, we again bred beetles with or without mites but this time we prevented parents from abandoning the brood. This time we found that both parents and the brood suffered fitness costs when breeding alongside mites, compared with families breeding in the absence of mites. We conclude that males adaptively reduce their contributions to care when mites are present, so as to defend their offspring's fitness and their own residual fitness. Interspecific interactions thus account for intraspecific variation in the duration of paternal care.

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The extent of paternal care varies greatly within species.

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Some of this variation might be due to interspecific interactions.

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We investigated how mites influence paternal care in the burying beetle.

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We found that males leave their brood earlier when mites are present.

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We show that this is adaptive because it enhances offspring fitness.

Patterns of ectoparasitism in North American red squirrels (Tamiasciurus hudsonicus): Sex-biases, seasonality, age, and effects on male body condition

Within many species, males are often more heavily parasitised than females. Several hypotheses have been proposed to explain this phenomenon, including immunocompetence handicaps, sexual size dimorphism and behavioural differences. Here we set out to test the latter two hypotheses and make inferences about the former by assessing patterns of ectoparasitism across various life-history stages in a population of North American red squirrels (Tamiasciurus hudsonicus). We also conducted an ectoparasite removal experiment to investigate the effects of ectoparasites on male body condition. We found that males were more intensely parasitized than females, but only during the mating period. There was no difference in ectoparasite intensity between male and female juveniles at birth or at emergence, suggesting that ectoparasites do not exploit male red squirrels for longer-range natal dispersal. Male red squirrels in our population were slightly heavier than females, however we did not find any evidence that this dimorphism drives male-biased ectoparasitism. Finally, we could not detect an effect of ectoparasite removal on male body mass. Our results lend support to the hypothesis that ectoparasites exploit their male hosts for transmission and that male red squirrels are important for the transmission dynamics of ectoparasites in this population; however, the mechanisms (i.e., immunocompetence, testosterone) are not known.

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Host life-history stage and sex modulate observed patterns of ectoparasitism.

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Red squirrels experienced highest flea intensities as young juveniles in the nest.

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Sex-biases in ectoparasite intensity were only observable in adult hosts.

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Ectoparasitism did not appear to affect the mass of adult male red squirrels.

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Patterns of parasitism should be considered within the context of host life-history.

Female host sex-biased parasitism with the rodent stomach nematode Mastophorus muris in wild bank voles (Myodes glareolus)

Abundance and prevalence of helminth infections often differ between host sexes, and are usually biased in favor of males. Relatively few cases of female-biased parasitism have been reported. We sampled bank voles in three woodland sites in N.E. Poland over 11 years at 3-4-year intervals, and assessed their parasite burdens. Prevalence and abundance of the stomach nematode Mastophorus muris were consistently higher among females. Among adult female bank voles from the two sites that showed the highest prevalence with M. muris, both prevalence and abundance were significantly higher in lactating bank voles, but not pregnant animals, and the effect of lactation was evident in both sites, in all four surveys, and in both age classes. Although the magnitude of the effect of lactation varied between years, it was not confounded by any significant interactions with other factors. We hypothesize that mature and reproductively active female bank voles are subject to higher exposure compared with males of similar age, as a consequence of the increased content of invertebrates in their diet, including the intermediate hosts of M. muris, required to meet the higher increased energy and protein demands of nursing litters throughout the summer months.

Life-history strategy defends against disease and may select against physiological resistance

Host ecological traits may limit exposure to infectious disease, thereby generating the wide variation in disease incidence observed between host populations or species. The exclusion of disease by ecological traits may then allow selection to act against physiological defenses when they are costly to maintain in the absence of disease. This study investigates ecological resistance in the Silene-Microbotryum pathosystem. An estimated 80% of perennial Silene species host the anther-smut disease while no annuals harbor the disease in nature. Artificial inoculations of annual and perennial Silene plants, obtained from both natural and horticultural populations, demonstrate that the absence of disease in annuals is not explained by elevated physiological resistance. The annual habit is thus a powerful form of ecological defense against anther smut. Moreover, the higher susceptibility of annual species to anther smut relative to perennials supports the hypothesis of a loss of costly physiological resistance under ecological protection. The observation in annuals that physiological susceptibility is correlated with lower rates of flowering (i.e., lower fitness) suggests that variation in physiological resistance is costly in the absence of disease, even in a naїve Silene species. The absence of disease in natural populations of annuals combined with their high physiological susceptibility attest to the strength of host ecology in shaping the distribution of disease and to the dynamic nature of disease resistance.

PCR-based molecular discrimination between Maritrema eroliae and Probolocoryphe uca (Digenea: Microphallidae) in Kuwait Bay

Microphallid trematodes are common parasites in marine snails and crustacean hosts at Kuwait Bay. The larval stages of two microphallids, Maritrema eroliae and Probolocoryphe uca, are difficult to differentiate morphologically. In this study, two PCR-based techniques were established for quick and accurate discrimination between the larval stages of the two microphallid species, employing restriction fragment length polymorphism (PCR-RFLP) and species-specific primers. Both techniques utilized nucleotide differences in the second internal transcribed region (ITS2) of the ribosomal DNA (rDNA) in the two species. For the PCR-RFLP technique, restriction enzyme AvaII was selected and it generated different restriction profiles among the two microphallids. In addition, species-specific primers were prepared for each microphallid species that amplified distinctive fragments. Both techniques showed that the larval stages of the two microphallid species can be identified accurately. However, direct PCR amplification using species-specific primers was more advantageous than the PCR-RFLP technique since it allowed rapid and specific discrimination between the two species. This technique provides a useful tool that can be used in future studies for the study of the distribution of microphallid species and their definitive hosts at different localities of Kuwait Bay.

Pathogen and host genotype differently affect pathogen fitness through their effects on different life-history stages

BACKGROUND

Adaptation of pathogens to their hosts depends critically on factors affecting pathogen reproductive rate. While pathogen reproduction is the end result of an intricate interaction between host and pathogen, the relative contributions of host and pathogen genotype to variation in pathogen life history within the host are not well understood. Untangling these contributions allows us to identify traits with sufficient genetic variation for selection to act and to identify mechanisms of coevolution between pathogens and their hosts. We investigated the effects of pathogen and host genotype on three life-history components of pathogen fitness; infection efficiency, latent period, and sporulation capacity, in the oat crown rust fungus, Puccinia coronata f.sp. avenae, as it infects oats (Avena sativa).

RESULTS

We show that both pathogen and host genotype significantly affect total spore production but do so through their effects on different life-history stages. Pathogen genotype has the strongest effect on the early stage of infection efficiency, while host genotype most strongly affects the later life-history stages of latent period and sporulation capacity. In addition, host genotype affected the relationship between pathogen density and the later life-history traits of latent period and sporulation capacity. We did not find evidence of pathogen-by-host genotypic (GxG) interactions.

CONCLUSION

Our results illustrate mechanisms by which variation in host populations will affect the evolution of pathogen life history. Results show that different pathogen life-history stages have the potential to respond differently to selection by host or pathogen genotype and suggest mechanisms of antagonistic coevolution. Pathogen populations may adapt to host genotypes through increased infection efficiency while their plant hosts may adapt by limiting the later stages of pathogen growth and spore production within the host.

Migration as an escape from parasitism in New Zealand galaxiid fishes

Parasite avoidance is increasingly considered to be a potential driving factor in animal migrations. In many marine and freshwater benthic fish, migration into a pelagic environment by developing larvae is a common life history trait that could reduce exposure to parasites during a critical window of developmental susceptibility. We tested this hypothesis on congeneric fish (family Galaxiidae, genus Galaxias) belonging to a closely related species complex sampled from coastal streams in southeastern New Zealand. Migratory Galaxias have larvae that migrate to pelagic marine environments, whereas the larvae of non-migratory species rear close to adult habitats with no pelagic larval phase. Both migratory and non-migratory fish are hosts to two species of skin-penetrating trematodes that cause spinal malformations and high mortality in young fish. Using generalized linear models within an Akaike information criterion and model averaging framework, we compared infection levels between migratory and non-migratory fish while taking into account body size and several other local factors likely to influence infection levels. For one trematode species, we found a significant effect of migration: for any given body length, migratory fish harboured fewer parasites than non-migratory fish. Also, no parasites of any kind were found in juvenile migratory fish sampled in spring shortly after their return to stream habitats. Our results demonstrate that migration spares juvenile fish from the debilitating parasites to which they would be exposed in adult stream habitats. Therefore, either the historical adoption of a migratory strategy in some Galaxias was an adaptation against parasitism, or it evolved for other reasons and now provides protection from infection as a coincidental side-effect.

Do different parasite species interact in their effects on host fitness? A case study on parasites of the amphipod Paracalliope fluviatilis

There is a gap in our understanding of the relative and interactive effects of different parasite species on the same host population. Here we examine the effects of the acanthocephalan Acanthocephalus galaxii, an unidentified cyclophyllidean cestode, and the trematodes Coitocaecum parvum and Microphallus sp. on several fitness components of the amphipod Paracalliope fluviatilis, using a combination of infection surveys and both survival and behavioural trials. In addition to significant relationships between specific parasites and measures of amphipod survival, maturity, mating success and behaviour, interactions between parasite species with respect to amphipod photophilia were also significant. While infection by either A. galaxii or C. parvum was associated with increased photophilia, such increases were negated by co-infection with Microphallus sp. We hypothesize that this is due to the more subtle manipulative effect of A. galaxii and C. parvum being impaired by Microphallus sp. We conclude that the low frequency at which such double infections occur in our sampled population means that such interactions are unlikely to be important beyond the scale of the host individual. Whether or not this is generally true, implying that parasitological models and theory based on single parasite species studies do generally hold, requires cross-species meta-analytical studies.

Variation of parasite load and immune parameters in two species of New Zealand shore crabs

While parasites are likely to encounter several potential intermediate hosts in natural communities, a parasite's actual range of compatible hosts is limited by numerous biological factors ranging from behaviour to immunology. In crustaceans, two major components of immunity are haemocytes and the prophenoloxidase system involved in the melanisation of foreign particles. Here, we analysed metazoan parasite prevalence and loads in the two sympatric crab species Hemigrapsus crenulatus and Macrophthalmus hirtipes at two sites. In parallel, we analysed the variation in haemocyte concentration and amount of circulating phenoloxidase (PO) in the haemolymph of the same individuals in an attempt to (a) explain differences in parasite prevalence and loads in the two species at two sites and (b) assess the impact of parasites on these immune parameters. M. hirtipes harboured more parasites but also exhibited higher haemocyte concentrations than H. crenulatus independent of the study site. Thus, higher investment in haemocyte production for M. hirtipes does not seem to result in higher resistance to parasites. Analyses of variation in immune parameters for the two crab species between the two sites that differed in parasite prevalence showed common trends. (a) In general, haemocyte concentrations were higher at the site experiencing higher parasitic pressure while circulating PO activity was lower and (b) haemocyte concentrations were influenced by microphallid trematode metacercariae in individuals from the site with higher parasitic pressure. We suggest that the higher haemocyte concentrations observed in both crab species exposed to higher parasitic pressure may represent an adaptive response to the impact of parasites on this immune parameter.

Pathogen-induced rapid evolution in a vertebrate life-history trait

Anthropogenic factors, including climate warming, are increasing the incidence and prevalence of infectious diseases worldwide. Infectious diseases caused by pathogenic parasites can have severe impacts on host survival, thereby altering the selection regime and inducing evolutionary responses in their hosts. Knowledge about such evolutionary consequences in natural populations is critical to mitigate potential ecological and economic effects. However, studies on pathogen-induced trait changes are scarce and the pace of evolutionary change is largely unknown, particularly in vertebrates. Here, we use a time series from long-term monitoring of perch to estimate temporal trends in the maturation schedule before and after a severe pathogen outbreak. We show that the disease induced a phenotypic change from a previously increasing to a decreasing size at maturation, the most important life-history transition in animals. Evolutionary rates imposed by the pathogen were high and comparable to those reported for populations exposed to intense human harvesting. Pathogens thus represent highly potent drivers of adaptive phenotypic evolution in vertebrates.

Mortality affects adaptive allocation to growth and reproduction: field evidence from a guild of body snatchers

BACKGROUND

The probability of being killed by external factors (extrinsic mortality) should influence how individuals allocate limited resources to the competing processes of growth and reproduction. Increased extrinsic mortality should select for decreased allocation to growth and for increased reproductive effort. This study presents perhaps the first clear cross-species test of this hypothesis, capitalizing on the unique properties offered by a diverse guild of parasitic castrators (body snatchers). I quantify growth, reproductive effort, and expected extrinsic mortality for several species that, despite being different species, use the same species' phenotype for growth and survival. These are eight trematode parasitic castrators-the individuals of which infect and take over the bodies of the same host species-and their uninfected host, the California horn snail.

RESULTS

As predicted, across species, growth decreased with increased extrinsic mortality, while reproductive effort increased with increased extrinsic mortality. The trematode parasitic castrator species (operating stolen host bodies) that were more likely to be killed by dominant species allocated less to growth and relatively more to current reproduction than did species with greater life expectancies. Both genders of uninfected snails fit into the patterns observed for the parasitic castrator species, allocating as much to growth and to current reproduction as expected given their probability of reproductive death (castration by trematode parasites). Additionally, species differences appeared to represent species-specific adaptations, not general plastic responses to local mortality risk.

CONCLUSIONS

Broadly, this research illustrates that parasitic castrator guilds can allow unique comparative tests discerning the forces promoting adaptive evolution. The specific findings of this study support the hypothesis that extrinsic mortality influences species differences in growth and reproduction.

Tapeworm (Hymenolepis diminuta) infection in flour beetles (Tribolium confusum): does it cause a trade-off between host fecundity and egg size?

Inter- and intra-specific comparisons commonly reveal an inverse relationship between fecundity and offspring size. Many animals also vary egg size in response to environmental conditions. Infection of flour beetles Tribolium confusum Jaquelin Du Val, 1868 with the rat tapeworm Hymenolepis diminuta (Rudolphi, 1819) causes a major reduction in host fecundity. This study tested if this fecundity reduction was associated with changes in host egg size. Age-matched beetles were either fasted and then exposed to parasites, fasted only, or neither, and egg production and egg length determined for 5 weeks postexposure. Control beetles that were neither fasted nor exposed to parasites had steady egg production, but produced smaller eggs as they aged. Beetles that were fasted only, produced fewer but larger eggs for 1–2 weeks after the fast ended. Then, fecundity and egg size returned to control levels. Infected beetles also produced fewer, larger eggs for 1–2 weeks, but at levels indistinguishable from beetles that had been fasted only. After 2 weeks, while fecundity of infected beetles remained low, egg size became similar to noninfected hosts. Beetles appeared to trade-off fecundity and egg size in response to reduced feeding, but not to the presumed nutritional stress of parasitic infection.

Parasitic castration: the evolution and ecology of body snatchers

Castration is a response to the tradeoff between consumption and longevity faced by parasites. Common parasitic castrators include larval trematodes in snails, and isopod and barnacle parasites of crustaceans. The infected host (with its many unique properties) is the extended phenotype of the parasitic castrator. Because an individual parasitic castrator can usurp all the reproductive energy from a host, and that energy is limited, intra- and interspecific competition among castrators is generally intense. These parasites can be abundant and can substantially depress host density. Host populations subject to high rates of parasitic castration appear to respond by maturing more rapidly.

Cryptic species complexes in manipulative echinostomatid trematodes: when two become six

SUMMARY

Recent studies have shown that some digenean trematodes previously identified as single species due to the lack of distinguishing morphological characteristics actually consist of a number of genetically distinct cryptic species. We obtained mitochondrial 16S and nuclear ITS1 sequences for the redial stages of Acanthoparyphium sp. and Curtuteria australis collected from snails and whelks at various locations around Otago Peninsula, New Zealand. These two echinostomes are well-known host manipulators whose impact extends to the entire intertidal community. Using phylogenetic analyses, we found that Acanthoparyphium sp. is actually composed of at least 4 genetically distinct species, and that a cryptic species of Curtuteria occurs in addition to C. australis. Molecular data obtained for metacercariae dissected from cockle second intermediate hosts matched sequences obtained for Acanthoparyphium sp. A and C. australis rediae, respectively, but no other species. The various cryptic species of both Acanthoparyphium and Curtuteria also showed an extremely localized pattern of distribution: some species were either absent or very rare in Otago Harbour, but reached far higher prevalence in nearby sheltered inlets. This small-scale spatial segregation is unexpected as shorebird definitive hosts can disperse trematode eggs across wide geographical areas, which should result in a homogeneous mixing of the species on small geographical scales. Possible explanations for this spatial segregation of the species include sampling artefacts, local adaptation by first intermediate hosts, environmental conditions, and site fidelity of the definitive hosts.

Host responses in life-history traits and tolerance to virus infection in Arabidopsis thaliana

Knowing how hosts respond to parasite infection is paramount in understanding the effects of parasites on host populations and hence host–parasite co-evolution. Modification of life-history traits in response to parasitism has received less attention than other defence strategies. Life-history theory predicts that parasitised hosts will increase reproductive effort and accelerate reproduction. However, empirical analyses of these predictions are few and mostly limited to animal-parasite systems. We have analysed life-history trait responses in 18 accessions of Arabidopsis thaliana infected at two different developmental stages with three strains of Cucumber mosaic virus (CMV). Accessions were divided into two groups according to allometric relationships; these groups differed also in their tolerance to CMV infection. Life-history trait modification upon virus infection depended on the host genotype and the stage at infection. While all accessions delayed flowering, only the more tolerant allometric group modified resource allocation to increase the production of reproductive structures and progeny, and reduced the length of reproductive period. Our results are in agreement with modifications of life-history traits reported for parasitised animals and with predictions from life-history theory. Thus, we provide empirical support for the general validity of theoretical predictions. In addition, this experimental approach allowed us to quantitatively estimate the genetic determinism of life-history trait plasticity and to evaluate the role of life-history trait modification in defence against parasites, two largely unexplored issues.

Hosts have developed a variety of mechanisms to compensate for the negative impact of parasite infection. Modification of life-history traits in response to parasitism has received less attention than other defence strategies. Life-history theory assumes trade-offs between resource allocation to different fitness components, and predicts that hosts under parasitism will allocate more resources to reproduction, subtracting them from those dedicated to growth and survival. Empirical support for predictions is not abundant, and derives mostly from the analysis of animal-parasite systems. We have analysed the modification of various life-history traits in the plant Arabidopsis thaliana infected by Cucumber mosaic virus. Life-history trait modification upon virus infection depended on the host genotype and on the developmental stage at infection. All plant genotypes delayed flowering, but only the more tolerant ones allocated more resources to reproduction, and reduced the length of reproductive period. These results agree with reports from parasitised animals and with predictions from life-history theory, providing empirical support for the general validity of theoretical predictions. In addition, results allow for the more precise evaluation of the role of life-history trait modification in defence against parasites by taking into account plant–virus interactions where life-history traits were differentially modified.