Host sexual dimorphism and parasite adaptation

Host specificity and seasonality of helminth component communities in central European grebes (Podicipediformes) and loons (Gaviiformes)

Structure and pattern of helminth component communities parasitizing grebes and loons are poorly understood. Here we analyze the prevalence, intensity and diversity of helminths in 505 Czech grebes (Podiceps cristatus, Podiceps nigricollis, Tachybaptus ruficollis) and loons (Gavia arctica, Gavia stellata) collected between 1962 and 2014. The species richness of helminth component communities ranged from 31±8 (P. cristatus) to 50±4 (G. stellata) species, with helminth load similar in all five host species, but with strong differences in prevalence and intensity of infection at a helminth species-specific level. The dominance was low, ranging from 0.11 (P. cristatus and G. stellata) to 0.21 (P. nigricollis). Dominant species (>25% prevalence) in P. cristatus were Confluaria sp., Tylodelphys clavata, Echinochasmus coaxatus, Petasiger neocomense and Ligula colymbi; in P. nigricollis and T. ruficollis: Confluaria sp. and Tatria sp.; in G. arctica and G. stellata: Tetrabothrius microcephalus, Stephanoprora denticulata, Cryptocotyle concava, Diplostomum gavium and Ichthyocotylurus erraticus; in G. stellata only: Echinochasmus (Monilifer) spinulosus. Four (12%) of the grebe digenean species were not locally acquired (with non-sympatric intermediate hosts) despite the fact that they represented 25% of digenean individuals found. In loons, five (28%) of digenean species and striking 38% of individuals found used the non-sympatric intermediate host species. Component communities of grebes were similar to each other (Sørensen similarity index 0.47-0.65) but differed strongly from those hosted by loons (0.00-0.20). We present the first systematically collected evidence of intra-annual changes of helminth component communities in grebes, and provide 20 new host records.

Parasite evolution in response to sex-based host heterogeneity in resistance and tolerance

Heterogenity between sexes in terms of both the level and the type of immune response to infection is documented in many species, but its role on parasite evolution is only beginning to be explored. We adopt an evolutionary epidemiology approach to study how the ability of a host to respond to infection through active immunity (resistance) or through minimizing deleterious effects of a given parasite load (tolerance) affects the evolution of parasite virulence. Consistently with earlier models, we find that increases in host resistance and tolerance both favour more virulent parasite strains. However, we show that qualitatively different results can be obtained if dimorphism between the sexes occurs through resistance or through tolerance depending on the contact pattern between the sexes. Finally, we find that variations in host sex ratio can amplify the consequences of heterogeneity for parasite evolution. These results are analysed in the light of several examples from the literature to illustrate the prevalence of sexually dimorphic immune responses and the potential for further study of the role of sexual dimorphism on parasite evolution. Such studies are likely to be highly relevant for improving treatment of chronic infections and control of infectious diseases, and understanding the role of sex in immune function.

Sex-specific effects of parasitism on survival and reproduction of a rodent host in a subtropical montane region

Parasites can generate complex life history trade-offs in a host. In this study, we experimentally reduced the infection level of intestinal helminth parasites in the Taiwan field mouse (Apodemus semotus) to test (1) whether parasite richness and load are biased towards male or female mice (sex-biased parasitism) and (2) whether the effects of parasitism on the host's survival and reproduction are different between the sexes (sex-specific effects of parasitism). Our findings indicate that neither parasite richness (number of helminth taxa found in a fecal sample) nor parasite load (number of helminth eggs per gram of fecal material) was sexually biased in our A. semotus study population. These results are in agreement with those of previous studies on endoparasites in Apodemus spp., but are in contrast to those on ectoparasites in Apodemus spp. Parasite removal reduced the survival rate of reproducing females, possibly by allowing reproducing females to increase maternal investment in their current litters at the cost of their own future survival. Single-litter mothers with reduced parasitism had a higher body mass than the untreated single-litter mothers, suggesting an increased maternal investment. In addition, the reproductively more active A. semotus, particularly the females, carried higher parasite loads, suggesting a trade-off between reproduction and parasite defense. By demonstrating that parasites can affect life history trade-offs in A. semotus, our results highlight the importance of maintaining variation in life history traits under parasitism risks and illustrate the subtle demographic processes (e.g. reduced future survival among healthy reproducing females) that might be driven by parasitism.

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.

Composition, structure and pattern of helminth assemblages associated with central European herons (Ardeidae)

Helminths parasitizing the ardeid birds are poorly understood, and the majority of studies are limited to checklists and records of novel host-parasite interactions. Here we analyzed the prevalence, intensity and diversity of the helminth component communities associated with an extensive cohort of the five most common Czech herons (Ardea cinerea, Ardea alba, Nycticorax nycticorax, Botaurus stellaris and Ixobrychus minutus) collected in the years 1962-2013. Comparison with Ukrainian datasets supports the existence of local helminth component communities, subject to strong geographic variation. The diversity of the component communities ranged between 37.3±9.6 (A. cinerea) and 2.5±1.1 (I. minutus) species. Similarly, the frequency of particular helminths differed by over one order of magnitude, whereas the helminth load differed by over two orders of magnitude. Typically, the dominant species (Echinochasmus beleocephalus, Uroproctepisthmium bursicola, Posthodiplostomum cuticola, Apharyngostrigea cornu, Desmidocercella numidica and Neogryporhynchus cheilancristrotus) were considered local, with intermediate host species available onsite, as represented by freshwater mollusks. Of the digeneans, 52% of the species likely infected their definitive hosts outside the study area, frequently utilizing invertebrates of salt or brackish waters. For A. cinerea, the largest number of species was in adult males; however the helminth load of the adults was lower than in their juvenile counterparts. This study provides the first systematically collected evidence for the intra-annual changes of the helminth assemblages in herons.

Evolution of Plastic Transmission Strategies in Avian Malaria

Malaria parasites have been shown to adjust their life history traits to changing environmental conditions. Parasite relapses and recrudescences—marked increases in blood parasite numbers following a period when the parasite was either absent or present at very low levels in the blood, respectively—are expected to be part of such adaptive plastic strategies. Here, we first present a theoretical model that analyses the evolution of transmission strategies in fluctuating seasonal environments and we show that relapses may be adaptive if they are concomitant with the presence of mosquitoes in the vicinity of the host. We then experimentally test the hypothesis that Plasmodium parasites can respond to the presence of vectors. For this purpose, we repeatedly exposed birds infected by the avian malaria parasite Plasmodium relictum to the bites of uninfected females of its natural vector, the mosquito Culex pipiens, at three different stages of the infection: acute (∼34 days post infection), early chronic (∼122 dpi) and late chronic (∼291 dpi). We show that: (i) mosquito-exposed birds have significantly higher blood parasitaemia than control unexposed birds during the chronic stages of the infection and that (ii) this translates into significantly higher infection prevalence in the mosquito. Our results demonstrate the ability of Plasmodium relictum to maximize their transmission by adopting plastic life history strategies in response to the availability of insect vectors.

Seasonal fluctuations in the environment affect dramatically the abundance of insect species. These fluctuations have important consequences for the transmission of vector-borne diseases. Here we contend that malaria parasites may have evolved plastic transmission strategies as an adaptation to the fluctuations in mosquito densities. First, our theoretical analysis identifies the conditions for the evolution of such plastic transmission strategies. Second, we show that in avian malaria Plasmodium parasites have the ability to increase transmission after being bitten by uninfected Culex mosquitoes. This demonstrates the ability of Plasmodium parasites to adopt plastic transmission strategies and challenges our understanding of malaria epidemiology.

The maintenance of hybrids by parasitism in a freshwater snail

Hybrids have often been labelled evolutionary dead-ends due to their lower fertility and viability. However, there is growing awareness that hybridisation between different species may play a constructive role in animal evolution as a means to create variability. Thus, hybridisation and introgression may contribute to adaptive evolution, for example with regards to natural antagonists (parasites, predators, competitors) and adaptation to local environmental conditions. Here we investigated whether parasite intensity contributes to the continuous recreation of hybrids in 74 natural populations of Melanopsis, a complex of freshwater snails with three species. We also examined, under laboratory conditions, whether hybrids and their parental taxa differ in their tolerance of low and high temperatures and salinity levels. Infections were consistently less prevalent in males than in females, and lower in snails from deeper habitats. Infection prevalence in hybrids was significantly lower than in the parental taxa. Low hybrid infection rates could not be explained by sediment type, snail density or geographic distribution of the sampling sites. Interestingly, infected hybrid snails did not show signs of parasite-induced gigantism, whereas all parental taxa did. We found that hybrids mostly coped with extreme temperatures and salinity levels as well as their parental taxa did. Taken together, our results suggest that Melanopsis hybrids perform better in the presence of parasites and environmental stress. This may explain the widespread and long-term occurrence of Melanopsis hybrids as evidenced by paleontological and biogeographic data. Hybridisation may be an adaptive host strategy, reducing infection rates and resisting gigantism.

Experimental infection of Calomys callosus with atypical strains of Toxoplasma gondii shows gender differences in severity of infection

There is a significant genetic diversity of Toxoplasma gondii in Brazil. Two parasite isolates were recently obtained from chickens in Uberlândia, Minas Gerais state, Brazil, namely, TgChBrUD1 and TgChBrUD2. In this study, we investigated Calomys callosus susceptibility to these atypical T. gondii strains. Male and female animals were intraperitoneally infected with tachyzoites and monitored to evaluate body weight change, morbidity, and mortality. Immunohistochemical assay and qPCR were performed to determine the parasitism in liver, spleen, and brain. Our data showed that TgChBrUD2-infected males died earlier than TgChBrUD1-infected males and 100% of mortality was observed after 10 and 12 days of infection, respectively. Also, TgChBrUD1-infected females died earlier than TgChBrUD1-infected males and 100% of mortality was observed after 9 and 12 days of infection, respectively. Both strains were able to induce a decrease in body weight of males, but only the TgChBrUD1 strain induced an increase in body weight of females. TgChBrUD2-infected females had significantly higher parasite load in both liver and spleen in comparison to TgChBrUD1-infected females, but no significant difference was found between genders or strains when males were infected. There was higher parasitism in the liver than the brain from both males and females infected with either strain. In conclusion, C. callosus specimens are susceptible to both T. gondii atypical strains with differences between males and females in severity of infection. These findings open new prospects for understanding different aspects of T. gondii infection, including reinfection and vertical transmission with these atypical strains when utilizing this experimental model.

Parasitic castration by Xenos vesparum depends on host gender

Host castration represents a mechanism used by parasites to exploit energy resources from their hosts by interfering with their reproductive development or to extend host lifespan by removing risks associated with reproductive activity. One of the most intriguing groups of parasitic castrators is represented by the insects belonging to the order Strepsiptera. The macroparasite Xenos vesparum can produce dramatic phenotypic alterations in its host, the paper wasp Polistes dominula. Parasitized female wasps have undeveloped ovaries and desert the colony without performing any social task. However, very little attention has been given to the parasitic impact of X. vesparum on the male phenotype. Here, we investigated the effects of this parasite on the sexual behaviour and the morpho-physiology of P. dominula males. We found that, differently from female wasps, parasitized males are not heavily affected by Xenos: they maintain their sexual behaviour and ability to discriminate between female castes. Furthermore, the structure of their reproductive apparatus is not compromised by the parasite. We think that our results, demonstrating that the definition of X. vesparum as a parasitoid does not apply to infected males of P. dominula, provide a new perspective to discuss and maybe reconsider the traditional view of strepsipteran parasites.

When should a trophically and vertically transmitted parasite manipulate its intermediate host? The case of Toxoplasma gondii

Parasites with complex life cycles are expected to manipulate the behaviour of their intermediate hosts (IHs), which increase their predation rate and facilitate the transmission to definitive hosts (DHs). This ability, however, is a double-edged sword when the parasite can also be transmitted vertically in the IH. In this situation, as the manipulation of the IH behaviour increases the IH death rate, it conflicts with vertical transmission, which requires healthy and reproducing IHs. The protozoan Toxoplasma gondii, a widespread pathogen, combines both trophic and vertical transmission strategies. Is parasite manipulation of host behaviour still adaptive in this situation? We model the evolution of the IH manipulation by T. gondii to study the conflict between these two routes of transmission under different epidemiological situations. Model outputs show that manipulation is particularly advantageous for virulent strains and in epidemic situations, and that different levels of manipulation may evolve depending on the sex of the IH and the transmission routes considered. These results may help to understand the variability of strain characteristics encountered for T. gondii and may extend to other trophically transmitted parasites.

Sex-specific effects of a parasite evolving in a female-biased host population

Background

Males and females differ in many ways and might present different opportunities and challenges to their parasites. In the same way that parasites adapt to the most common host type, they may adapt to the characteristics of the host sex they encounter most often. To explore this hypothesis, we characterized host sex-specific effects of the parasite Pasteuria ramosa, a bacterium evolving in naturally, strongly, female-biased populations of its host Daphnia magna.

Results

We show that the parasite proliferates more successfully in female hosts than in male hosts, even though males and females are genetically identical. In addition, when exposure occurred when hosts expressed a sexual dimorphism, females were more infected. In both host sexes, the parasite causes a similar reduction in longevity and leads to some level of castration. However, only in females does parasite-induced castration result in the gigantism that increases the carrying capacity for the proliferating parasite.

Conclusions

We show that mature male and female Daphnia represent different environments and reveal one parasite-induced symptom (host castration), which leads to increased carrying capacity for parasite proliferation in female but not male hosts. We propose that parasite induced host castration is a property of parasites that evolved as an adaptation to specifically exploit female hosts.