New technologies to study helminth development and host-parasite interactions

How parasites develop and survive, and how they stimulate or modulate host immune responses are important in understanding disease pathology and for the design of new control strategies. Microarray analysis and bulk RNA sequencing have provided a wealth of data on gene expression as parasites develop through different life-cycle stages and on host cell responses to infection. These techniques have enabled gene expression in the whole organism or host tissue to be detailed, but do not take account of the heterogeneity between cells of different types or developmental stages, nor the spatial organisation of these cells. Single-cell RNA-seq (scRNA-seq) adds a new dimension to studying parasite biology and host immunity by enabling gene profiling at the individual cell level. Here we review the application of scRNA-seq to establish gene expression cell atlases for multicellular helminths and to explore the expansion and molecular profile of individual host cell types involved in parasite immunity and tissue repair. Studying host-parasite interactions in vivo is challenging and we conclude this review by briefly discussing the applications of organoids (stem-cell derived mini-tissues) to examine host-parasite interactions at the local level, and as a potential system to study parasite development in vitro. Organoid technology and its applications have developed rapidly, and the elegant studies performed to date support the use of organoids as an alternative in vitro system for research on helminth parasites.     

Coevolutionary interactions between host and parasite genotypes

More than 20 years after Dawkins introduced the concept of "extended phenotype" (i.e. phenotypes of hosts and parasites result from interactions between the two genomes) and although this idea has now reached contemporary textbooks of evolutionary biology, most studies of the evolution of host-parasite systems still focus solely on either the host or the parasite, neglecting the role of the other partner. It is important to consider that host and parasite genotypes share control of the epidemiological parameters of their relationship. Moreover, not only the traits of the infection but also the genetic correlations among these and other traits that determine fitness might be controlled by interactions between host and parasite genotypes.     

Multiscale determinants of parasite abundance: A quantitative hierarchical approach for coral reef fishes

During recent decades, there have been numerous attempts to identify the key determinants of parasite communities and several influential variables have been clarified at either infra-, component or compound community scales. However, in view of the possible complexity of interactions among determinants, the commonly-used exploratory and statistical modelling techniques have often failed to find meaningful ecological patterns from such data. Moreover, quantitative assessments of factors structuring species richness, abundance, community structure and species associations in parasite communities remain elusive. Recently, because they are ideally suited for the analysis of complex and highly interactive data, there has been increasing interest in the use of classification and regression tree analyses in several ecological fields. To date, such approaches have never been used by parasitologists for field data. This study aims to both introduce and illustrate the use of multivariate regression trees in order to investigate the determinants of parasite abundance in a multi-scale quantitative context. To do this, we used new field epidemiological data from 1489 coral reef fishes collected around two islands in French Polynesia. We evaluated the relative effect and interactions of several host traits and environmental factors on the abundance of metazoan parasite assemblage at several scales and assessed the impact of major factors on each parasite taxon. Our results suggest that the islands sampled, the host species and host size are equal predictors of parasite abundance at a global scale, whereas other factors proved to be significant predictors of a local pattern, depending on host family. We also discuss the potential use of regression trees for parasitologists as both an explorative and a promising predictive tool.     

The role of phylogeny and ecology in experimental host specificity: insights from a eugregarine-host system

The degree to which parasites use hosts is fundamental to host-parasite coevolution studies, yet difficult to assess and interpret in an evolutionary manner. Previous assessments of parasitism in eugregarine-host systems suggest high degrees of host specificity to particular host stages and host species; however, rarely have the evolutionary constraints on host specificity been studied experimentally. A series of experimental infections were conducted to determine the extent of host stadium specificity (larval vs. adult stage) and host specificity among 6 tenebrionid host species and 5 eugregarine parasite species. Eugregarines from all host species infected both the larva and adult stages of the host, and each parasite taxa colonized several host species (Tribolium spp. and Palorus subdepressus). Parasite infection patterns were not congruent with host phylogeny, suggesting that host phylogeny is not a significant predictor of host-parasite interactions in this system. However, the 2 host stages produced significantly different numbers of parasite propagules, indicating that ecological factors may be important determinants of host specificity in this host-parasite system. While field infections reflect extant natural infection patterns of parasites, experimental infections can demonstrate potential host-parasite interactions, which aids in identifying factors that may be significant in shaping future host-parasite interactions.     

Parasite-driven extinction in spatially explicit host-parasite systems

General host-parasite theory suggests that parasites may be implicated in the extinction of their hosts by causing instability that leads to increased risk of stochastic extinction. In contrast, spatially explicit models suggest that the parasite may directly drive the host population to extinction. Here we examine the ecological characteristics of host-parasite interactions that favor parasite-driven host extinction. Pair approximations and simulations show that parasites only drive their hosts to extinction when they significantly reduce host reproduction. As a matter of interest, parasites that have a relatively small effect on host death rate are more likely to cause host extinction. Parasite-driven host extinction occurs at any population size, whereas extinction caused by stochastic effects is less likely to occur in large host populations. Populations may therefore be under threat from parasites that stop host reproduction, and this type of parasite may prove to be the most effective biological pesticide.     

Food-environment mediates the outcome of specific interactions between a bumblebee and its trypanosome parasite

Specific host-parasite interactions, where the outcome of exposure to a parasite depends upon the genotypic identity of both parties, have implications for understanding host-parasite coevolution and patterns of genetic diversity. Thus, grasping the extent to which these interactions are mediated by environmental changes in a spatially and temporally heterogeneous world is vital. In this study, it is shown that the environment can influence specific host-parasite interactions in the well-studied system of the bumblebee Bombus terrestris and its trypanosome parasite Crithidia bombi. Naturally relevant variation in the quality of the food environment formed a three-way interaction with both host and parasite identity in determining the outcome of infection, with regard to the resistance of the host and the transmission of the parasite. The demonstration of such a host-genotype by parasite-genotype by environment interaction (G(H) x G(P) x E) shows the importance of considering environmental variation when investigating host-parasite interactions. Moreover, such interactions may to some extent explain levels of genetic diversity in natural host-parasite systems owing to the fact that they will create selection mosaics when environments are heterogeneous.     

Island and taxon effects in parasitism revisited: avian malaria in the Lesser Antilles

We identify and describe the distribution of 12 genetically distinct malaria parasite lineages over islands and hosts in four common passerine birds in the Lesser Antilles. Combined parasite prevalence demonstrates strong host effects, little or no island effect, and a significant host-times-island interaction, indicating independent outcomes of host-parasite infections among island populations of the same host species. Host- and/or island-specific parasite lineages do not explain these host-parasite associations; rather, individual lineages themselves demonstrate the same type of independent interactions. Unlike overall prevalence, individual parasite lineages show considerable geographic structure (i.e., island effects) as well as species effects indicating that parasite lineages are constrained in their ability to move between hosts and locations. Together, our results suggest an upper limit to the number of host individuals that malaria parasites, as a community, can infect. Within this limit, however, the relative frequency of the different lineages varies reflecting fine scale interactions between host and parasite populations. Patterns of host-parasite associations within this system suggest both historical co-evolution and ecologically dynamic and independent host-parasite interactions.     

Signals of demographic expansion in Drosophila virilis

Background

The antagonistic co-evolution of hosts and their parasites is considered to be a potential driving force in maintaining host genetic variation including sexual reproduction and recombination. The examination of this hypothesis calls for information about the genetic basis of host-parasite interactions – such as how many genes are involved, how big an effect these genes have and whether there is epistasis between loci. We here examine the genetic architecture of quantitative resistance in animal and plant hosts by concatenating published studies that have identified quantitative trait loci (QTL) for host resistance in animals and plants.

Results

Collectively, these studies show that host resistance is affected by few loci. We particularly show that additional epistatic interactions, especially between loci on different chromosomes, explain a majority of the effects. Furthermore, we find that when experiments are repeated using different host or parasite genotypes under otherwise identical conditions, the underlying genetic architecture of host resistance can vary dramatically – that is, involves different QTLs and epistatic interactions. QTLs and epistatic loci vary much less when host and parasite types remain the same but experiments are repeated in different environments.

Conclusion

This pattern of variability of the genetic architecture is predicted by strong interactions between genotypes and corroborates the prevalence of varying host-parasite combinations over varying environmental conditions. Moreover, epistasis is a major determinant of phenotypic variance for host resistance. Because epistasis seems to occur predominantly between, rather than within, chromosomes, segregation and chromosome number rather than recombination via cross-over should be the major elements affecting adaptive change in host resistance.     

Sex-specific response of a mosquito to parasites and crowding

Host-parasite interactions are significantly influenced by the sex of the host and the environment in which the host is found. Sex-specific responses to parasite infection, however, may change according to the host environment. I examine the combined effect of parasite infection and crowding on males and females of the mosquito Aedes albopictus. At a high larval density, infected males experienced a greater relative reduction in body size than did infected females, whereas the pattern was reversed at low density. This experiment demonstrates the importance of the environment on sex-specific responses to parasites and contributes to a growing body of work examining sources of variation in host-parasite interactions.     

Notch1 binds and induces degradation of Snail in hepatocellular carcinoma

Background

Infection processes consist of a sequence of steps, each critical for the interaction between host and parasite. Studies of host-parasite interactions rarely take into account the fact that different steps might be influenced by different factors and might, therefore, make different contributions to shaping coevolution. We designed a new method using the Daphnia magna - Pasteuria ramosa system, one of the rare examples where coevolution has been documented, in order to resolve the steps of the infection and analyse the factors that influence each of them.

Results

Using the transparent Daphnia hosts and fluorescently-labelled spores of the bacterium P. ramosa, we identified a sequence of infection steps: encounter between parasite and host; activation of parasite dormant spores; attachment of spores to the host; and parasite proliferation inside the host. The chances of encounter had been shown to depend on host genotype and environment. We tested the role of genetic and environmental factors in the newly described activation and attachment steps. Hosts of different genotypes, gender and species were all able to activate endospores of all parasite clones tested in different environments; suggesting that the activation cue is phylogenetically conserved. We next established that parasite attachment occurs onto the host oesophagus independently of host species, gender and environmental conditions. In contrast to spore activation, attachment depended strongly on the combination of host and parasite genotypes.

Conclusions

Our results show that different steps are influenced by different factors. Host-type-independent spore activation suggests that this step can be ruled out as a major factor in Daphnia - Pasteuria coevolution. On the other hand, we show that the attachment step is crucial for the pronounced genetic specificities of this system. We suggest that this one step can explain host population structure and could be a key force behind coevolutionary cycles. We discuss how different steps can explain different aspects of the coevolutionary dynamics of the system: the properties of the attachment step, explaining the rapid evolution of infectivity and the properties of later parasite proliferation explaining the evolution of virulence. Our study underlines the importance of resolving the infection process in order to better understand host-parasite interactions.     

Analysis of host-parasite coevolution

When examining the historical context of co-existence between host and parasite lineages, we speak of coevolution. But the principles and methods discussed herein apply equally well to any ecological association. This is the basis of the emerging field of historical ecology (Brooks, 1985; Brooks &; Wiley, 1986). Implementing serious studies in historical ecology requires expertise in phylogenetic analysis, ecology and development biology. Traditionally, parasitologists have received broad training in all three of these areas. This places parasitologists in an excellent position to assume leadership roles in developing this new field of research.     

Evolution of host resistance to parasite infection in the snail–schistosome–human system

The evolutionary strategies that emerge within populations can be dictated by numerous factors, including interactions with other species. In this paper, we explore the consequences of such a scenario using a host-parasite system of human concern. By analyzing the dynamical behaviors of a mathematical model we investigate the evolutionary outcomes resulting from interactions between Schistosoma mansoni and its snail and human hosts. The model includes two types of snail hosts representing resident and mutant types. Using this approach, we focus on establishing evolutionary stable strategies under conditions where snail hosts express different life-histories and when drug treatment is applied to an age-structured population of human hosts. Results from this work demonstrate that the evolutionary trajectories of host-parasite interactions can be varied, and at times, counter-intuitive, based on parasite virulence, host resistance, and drug treatment.     

The role of immune-mediated apparent competition in genetically diverse malaria infections

Competitive interactions between coinfecting genotypes of the same pathogen can impose selection on virulence, but the direction of this selection depends on the mechanisms behind the interactions. Here, we investigate how host immune responses contribute to competition between clones in mixed infections of the rodent malaria parasite Plasmodium chabaudi. We studied single and mixed infections of a virulent and an avirulent clone and compared the extent of competition in immunodeficient and immunocompetent mice (nude mice and T cell-reconstituted nude mice, respectively). In immunocompetent mice, the avirulent clone suffered more from competition than did the virulent clone. The competitive suppression of the avirulent clone was alleviated in immunodeficient mice. Moreover, the relative density of the avirulent clone in mixed infections was higher in immunodeficient than in immunocompetent mice. We conclude that immune-mediated interactions contributed to competitive suppression of the avirulent clone, although other mechanisms, presumably competition for resources such as red blood cells, must also be important. Because only the avirulent clone suffered from immune-mediated competition, this mechanism should contribute to selection for increased virulence in mixed infections in this host-parasite system. As far as we are aware, this is the first direct experimental evidence of immune-mediated apparent competition in any host-parasite system.     

Wild immunology

In wild populations, individuals are regularly exposed to a wide range of pathogens. In this context, organisms must elicit and regulate effective immune responses to protect their health while avoiding immunopathology. However, most of our knowledge about the function and dynamics of immune responses comes from laboratory studies performed on inbred mice in highly controlled environments with limited exposure to infection. Natural populations, on the other hand, exhibit wide genetic and environmental diversity. We argue that now is the time for immunology to be taken into the wild. The goal of 'wild immunology' is to link immune phenotype with host fitness in natural environments. To achieve this requires relevant measures of immune responsiveness that are both applicable to the host-parasite interaction under study and robustly associated with measures of host and parasite fitness. Bringing immunology to nonmodel organisms and linking that knowledge host fitness, and ultimately population dynamics, will face difficult challenges, both technical (lack of reagents and annotated genomes) and statistical (variation among individuals and populations). However, the affordability of new genomic technologies will help immunologists, ecologists and evolutionary biologists work together to translate and test our current knowledge of immune mechanisms in natural systems. From this approach, ecologists will gain new insight into mechanisms relevant to host health and fitness, while immunologists will be given a measure of the real-world health impacts of the immune factors they study. Thus, wild immunology can be the missing link between laboratory-based immunology and human, wildlife and domesticated animal health.     

Sexual signalling in female crested macaques and the evolution of primate fertility signals

Background

Host-parasite coevolution can lead to local adaptation of either parasite or host if there is specificity (GxG interactions) and asymmetric evolutionary potential between host and parasite. This has been demonstrated both experimentally and in field studies, but a substantial proportion of studies fail to detect such clear-cut patterns. One explanation for this is that adaptation can be masked by counter-adaptation by the antagonist. Additionally, genetic architecture underlying the interaction is often highly complex thus preventing specific adaptive responses. Here, we have employed a reciprocal cross-infection experiment to unravel the adaptive responses of two components of fitness affecting both parties with different complexities of the underlying genetic architecture (i.e. mortality and spore load). Furthermore, our experimental coevolution of hosts (Tribolium castaneum) and parasites (Nosema whitei) included paired replicates of naive hosts from identical genetic backgrounds to allow separation between host- and parasite-specific responses.

Results

In hosts, coevolution led to higher resistance and altered resistance profiles compared to paired control lines. Host genotype × parasite genotype interactions (G_H × G_P) were observed for spore load (the trait of lower genetic complexity), but not for mortality. Overall parasite performance correlated with resistance of its matching host coevolution background reflecting a directional and unspecific response to strength of selection during coevolution. Despite high selective pressures exerted by the obligatory killing parasite, and host- and parasite-specific mortality profiles, no general pattern of local adaptation was observed, but one case of parasite maladaptation was consistently observed on both coevolved and control host populations. In addition, the use of replicate control host populations in the assay revealed one case of host maladaptation and one case of parasite adaptation that was masked by host counter-adaptation, suggesting the presence of complex and probably dynamically changing fitness landscapes.

Conclusions

Our results demonstrate that the use of replicate naive populations can be a useful tool to differentiate between host and parasite adaptation in complex and dynamic fitness landscapes. The absence of clear local adaptation patterns during coevolution with a sexual host showing a complex genetic architecture for resistance suggests that directional selection for generality may be more important attributes of host-parasite coevolution than commonly assumed.     

Detecting local adaptation in a natural plant-pathogen metapopulation: a laboratory vs. field transplant approach

Antagonistic coevolution between hosts and parasites in spatially structured populations can result in local adaptation of parasites. Traditionally parasite local adaptation has been investigated in field transplant experiments or in the laboratory under a constant environment. Despite the conceptual importance of local adaptation in studies of (co)evolution, to date no study has provided a comparative analysis of these two methods. Here, using information on pathogen population dynamics, I tested local adaptation of the specialist phytopathogen, Podosphaera plantaginis, to its host, Plantago lanceolata at three different spatial scales: sympatric host population, sympatric host metapopulation and allopatric host metapopulations. The experiment was carried out as a field transplant experiment with greenhouse-reared host plants from these three different origins introduced into four pathogen populations. In contrast to results of an earlier study performed with these same host and parasite populations under laboratory conditions, I did not find any evidence for parasite local adaptation. For interactions governed by strain-specific resistance, field studies may not be sensitive enough to detect mean parasite population virulence. Given that parasite transmission potential may be mediated by the abiotic environment and genotype-by-environment interactions, I suggest that relevant environmental variation should be incorporated into laboratory studies of parasite local adaptation.     

A systematic analysis of the early transcribed membrane protein family throughout the life cycle of Plasmodium yoelii

The early transcribed membrane proteins (ETRAMPs) are a family of small, highly charged transmembrane proteins unique to malaria parasites. Some members of the ETRAMP family have been localized to the parasitophorous vacuole membrane that separates the intracellular parasite from the host cell and thus presumably have a role in host-parasite interactions. Although it was previously shown that two ETRAMPs are critical for rodent malaria parasite liver-stage development, the importance of most ETRAMPs during the parasite life cycle remains unknown. Here, we comprehensively identify nine new etramps in the genome of the rodent malaria parasite Plasmodium yoelii, and elucidate their conservation in other malaria parasites. etramp expression profiles are diverse throughout the parasite life cycle as measured by RT-PCR. Epitope tagging of two ETRAMPs demonstrates protein expression in blood and liver stages, and reveals differences in both their timing of expression and their subcellular localization. Gene targeting studies of each of the nine uncharacterized etramps show that two are refractory to deletion and thus likely essential for blood-stage replication. Seven etramps are not essential for any life cycle stage. Systematic characterization of the members of the ETRAMP family reveals the diversity in importance of each family member at the interface between host and parasite throughout the developmental cycle of the malaria parasite.     

Evolution of multiple components of virulence in Drosophila-nematode associations

Abstract.— Virulence is of central importance in host-parasite interactions, yet little is known about how it changes over extended evolutionary periods. In this study, all four species in the testacea species group of Drosophila were experimentally infected with sympatric and allopatric nematodes in the Howardula aoronymphium species complex, and the effect of parasite infection on three components of host fitness was determined. The Drosophila species show striking differences in their responses to infection, with reductions reaching 80% in adult lifespan, 100% in female fertility, and 90% in male fertility. Female sterility appears to be determined by the host; species that are sterilized by their local nematodes are also sterilized by the other allopatric nematodes in the H. aoronymphium complex. Host species that are not sterilized by their local parasite are not sterilized by other nematodes in the complex. In contrast, reductions in host adult lifespan and male fertility depend on both the host and the parasite. Whereas all nematodes reduced the survival of their local host species equally (about 40–45%), survival of two host species was drastically reduced (about 80%) when infected with an allopatric parasite. Thus, virulence is evolutionarily labile in associations between Drosophila testacea group species and their Howardula parasites. The data suggest that changes in the sterility component of virulence are due primarily to host evolution, whereas changes in the host mortality component are due in large part to parasite evolution.