Host use of a hemiparasitic plant: no trade-offs in performance on different hosts

To examine putative specialization of a hemiparasitic plant to the most beneficial host species, we studied genetic variation in performance and trade-offs between performance on different host species in the generalist hemiparasite, Rhinanthus serotinus. We grew 25 maternal half-sib families of the parasite on Agrostis capillaris and Trifolium pratense and without a host in a greenhouse. Biomass and number of flowers of the parasite were the highest when grown on T. pratense. There were significant interactions between host species and R. serotinus seed-family indicating that the differences in performance on the two hosts and without a host varied among the families. However, we found no significant negative correlations between performance of R. serotinus on the host species or between performance on the two hosts and autotrophic performance. Thus, the genetic factors studied here are not likely to affect the evolution of specialization of R. serotinus to the most beneficial host.     

Differential impacts of shared parasites on fitness components among competing hosts

Effects of parasites on individual hosts can eventually translate to impacts on host communities. In particular, parasitism can differentially affect host fitness among sympatric and interacting host species. We examined whether the impact of shared parasites varied among host species within the same community. Specifically, we looked at the impacts of the acanthocephalan Acanthocephalus galaxii, the trematodes Coitocaecum parvum and Maritrema poulini, and the nematode Hedruris spinigera, on three host species: the amphipods, Paracalliope fluviatilis and Paracorophium excavatum, and the isopod, Austridotea annectens. We assessed parasite infection levels in the three host species and tested for effects on host survival, behavior, probability of pairing, and fecundity. Maritrema poulini and C. parvum were most abundant in P. excavatum but had no effect on its survival, whereas they negatively affected the survival of P. fluviatilis, the other amphipod. Female amphipods carrying young had higher M. poulini and C. parvum abundance than those without, yet the number of young carried was not linked to parasite abundance. Behavior of the isopod A. annectens was affected by M. poulini infection; more heavily infected individuals were more active. Paracorophium excavatum moved longer distances when abundance of C. parvum was lower, yet no relationship existed with respect to infection by both M. poulini and C. parvum. The differential effects of parasites on amphipods and isopods may lead to community‐wide effects. Understanding the consequences of parasitic infection and differences among host species is key to gaining greater insight into the role of parasite mediation in ecosystem dynamics.     

Host plant species affects virulence in monarch butterfly parasites

1. Studies have considered how intrinsic host and parasite properties determine parasite virulence, but have largely ignored the role of extrinsic ecological factors in its expression. 2. We studied how parasite genotype and host plant species interact to determine virulence of the protozoan parasite Ophryocystis elektroscirrha (McLaughlin & Myers 1970) in the monarch butterfly Danaus plexippus L. We infected monarch larvae with one of four parasite genotypes and reared them on two milkweed species that differed in their levels of cardenolides: toxic chemicals involved in predator defence. 3. Parasite infection, replication and virulence were affected strongly by host plant species. While uninfected monarchs lived equally long on both plant species, infected monarchs suffered a greater reduction in their life spans (55% vs. 30%) on the low-cardenolide vs. the high-cardenolide host plant. These life span differences resulted from different levels of parasite replication in monarchs reared on the two plant species. 4. The virulence rank order of parasite genotypes was unaffected by host plant species, suggesting that host plant species affected parasite genotypes similarly, rather than through complex plant species-parasite genotype interactions. 5. Our results demonstrate that host ecology importantly affects parasite virulence, with implications for host-parasite dynamics in natural populations.     

Host manipulation by parasites: a multidimensional phenomenon

The diversity of ways in which parasites manipulate the phenotype of their hosts to increase their transmission has been well‐documented during the past decades. Parasites clearly have the potential to alter a broad range of phenotypic traits in their hosts, extending from behaviour and colour to morphology and physiology. While the vast majority of studies have concentrated on few, often only one, host characters, there is increasing evidence that manipulative parasites alter multiple characteristics of their host's phenotype. These alterations can occur simultaneously and/or successively through time, making parasitically modified organisms undoubtedly more complex than traditionally viewed. Here, we briefly review the multidimensionality of host manipulation by parasites, discuss its possible significance and evolution, and propose directions for further research. This view should prove to be an extremely useful approach, generating a series of testable hypotheses regarding the ecology of parasitized hosts, and leading to a better comprehension of complex host–parasite relationships.     

Host cell invasion by malaria parasites

The complex life cycle of the malaria parasite includes three specialized invasive stages, distinct both in terms of their cellular architecture and in their choice of target host cell. Despite the dissimilarities between these forms, there are clear parallels in the manner by which they enter their respective host cells. Advances in the area of erythrocyte invasion by the malaria merozoite, outlined here by Chetan Chitnis and Mike Blackman and discussed at the Molecular Approaches to Malaria conference, Lorne, Australia, 2-5 February 2000, will undoubtedly impact on our understanding of mechanisms of cell entry by the other invasive forms. Similarly, recent progress in dissecting the functional role of surface proteins expressed by sporozoite and ookinete stages has provided fascinating insights into general aspects of invasion by all invasive stages of apicomplexan parasites.     

Physiology of the interaction of angiosperm parasites and their higher plant hosts

Abstract. Interactions between parasitic angiosperms and their hosts occur at the level of seed germination, haustorial development and resource transfer. Chemicals released from the host function as cues for host recognition, and trigger germination as well as haustorial initiation. Transpiration is a key process regulating solute transfer from host to parasite, and some parasitie plants have unusual stomatal characteristics. Although solute transfer is apoplastic, the haustorium appears to play a role in regulating solute composition. Host responses to infection are reviewed, and it is concluded that competition for water and solutes are unlikely to play a major role in determining reductions in host productivity: metabolic incompatability is suggested to be the major cause of this.     

Innate recognition of malarial parasites by mammalian hosts

Innate immunity plays a central role in combating infections. However, the importance of innate immune sensors in detecting intracellular parasites, such as Plasmodium spp., has only recently emerged as a central topic in the field of host-pathogen interactions. Genetic dissection of innate immune pathways has uncovered a complex relationship between the host innate immune system and Plasmodium blood-stage parasites. In fact, recognition molecules of the innate immune system, such as toll-like receptors, might not only be implicated in host defense but also in the pathogenesis of the disease. Whether Plasmodium liver stage parasites are recognised and controlled by the host innate immune system remains to be discovered. In this review we discuss recent findings on how the host innate immune system may sense and fight the different forms of Plasmodium and how the latter may have evolved mechanisms to escape host detection and/or to manipulate the defensive reaction of the host.     

Host plant use by the Heath fritillary butterfly, Melitaea athalia: plant habitat, species and chemistry

We present a study of habitat use, oviposition plant choice, and food plant suitability for the checkerspot butterfly Melitaea athalia Rottemburg (Lepidoptera: Nymphalidae) in Åland, Finland. We found that in Åland, unlike in the mainland of Finland and many parts of its range, M. athalia flies mainly in open meadows. When offered an array of plants in a large (32 × 26 m) field cage, they predominately oviposited upon Veronica chamaedrys L., V. spicata L. and Plantago lanceolata L. (Plantaginaceae), which grow in open meadows. The relative abundance of the butterfly in Åland, and its habitat and host plant use there, may reflect local adaptation to land use practices and geology that maintain clusters of small open meadows with little successional change. At the scale of a plant patch, preferred species were used as frequently in mixed species patches as in mono-specific patches, and more oviposition occurred in open than in grassy patches. All of the host plants used by M. athalia are defended by iridoid glycosides (IGs). However, oviposition choice among species and among individual plants within species was largely independent of IG concentration. This contrast with the more discerning congener, M. cinxia, supports the idea that host discrimination decreases with increasing host range. Finally, although the adult butterflies chose specific plant species for oviposition, as larvae they performed well on twelve out of thirteen species of plants, including both known hosts and related novel plants that occur in Åland, indicating a much wider range of larval food plant species than adult oviposition species.     

Fungal endophytes: common host plant symbionts but uncommon mutualists

Fungal endophytes are extremely common and highly diverse microorganismsthat live within plant tissues, but usually remain asymptomatic.Endophytes traditionally have been considered plant mutualists,mainly by reducing herbivory via production of mycotoxins, suchas alkaloids. However, the vast majority of endophytes, especiallyhorizontally-transmitted ones commonly found in woody plants,apparently have little or no effect on herbivores. For the systemic,vertically-transmitted endophytes of grasses, mutualistic interactionsvia increased resistance to herbivores and pathogens are morecommon, as predicted by evolutionary theory. However, even inthese obligate symbioses, endophytes are often neutral or evenpathogenic to the host grass, depending on endophyte and plantgenotype and environmental conditions. We present a graphical model based upon variation in nitrogenflux in the host plant. Nitrogen is a common currency in endophyte/hostand plant/herbivore interactions in terms of limitations tohost plant growth, enhanced uptake by endophytes, demand forsynthesis of nitrogen-rich alkaloids, and herbivore preferenceand performance. Our graphical model predicts that low alkaloid-producingendophytes should persist in populations when soil nutrientsand herbivory are low. Alternatively, high alkaloid endophytesare favored under increasing herbivory and increasing soil nitrogen,at least to some point. At very high soil nitrogen levels, uninfectedplants may be favored over either type of infected plants. Thesepredictions are supported by patterns of infection and alkaloidproduction in nature, as well by a manipulative field experiment.However, plant genotype and other environmental factors, suchas available water, interact with the presence of the endophyteto influence host plant performance.     

Interactions between parasites and their hosts: Metabolic aspects

Anaerobic mechanisms are the predominant energy-yielding pathways of endoparasites. However, all parasitic helminths use oxygen when it is present, and appear to be capable of oxidative phosphorylation. Bryant (1982) has argued that anaerobic pathways are primitive and that aerobic pathways are a more recent adaptation. The question as to why oxygen is not used more efficiently by parasitic helminths has still not been answered satisfactorily.The phosphoenolpyruvate branchpoint is critical in helminth metabolism, and the controlling factors influencing PK and PEPCK are still not clearly understood. This is further compounded by differences between strains (Mettrick & Rahman, 1984).The effect of size of an intestinal parasitic infection may, at low levels of infection, stimulate net glucose and ion flexes, while high levels of infection produce malabsorption. It is of particular interest that the washings from a low level intestinal infection may stimulate glucose and ion fluxes in an uninfected animal; this appears to be a new component in the compensatory mechanisms of the host response.5-hydroxytryptamine both influences helminth glycolysis and has a role in helminth neurophysiology. Current evidence indicates that 5-HT is synthesised in very small amounts by H. diminuta and is also taken up from the intestinal luminal pool. There is evidence that 5-HT and acetylcholine are antagonistic; 5-HT appears to be a modulator of longitudinal muscle contraction with glutamate acting as the neurostimulator.     

Host sex‐specific parasites in a functionally dioecious fig: a preference way of adaptation to their hosts

Host–parasites interaction is a common phenomenon in nature. Diffusive coevolution might maintain stable cooperation in a fig–fig wasps system, in which the exploiter might diversify their genotype, phenotype, or behavior as a result of competition with pollinator, whereas the figs change flower syconia, fruits thickness, and syconia structure. In functionally dioecious Ficus auriculata, male figs and female figs contain two types of florets on separate plant, and share high similarities in outside morphology. Apocryptophagus (Sycophaginae, Chalcidoidea, Hymenoptera) is one of few groups of nonpollinating fig wasps that can reproduce within both male and female figs. On the basis of the morphology and DNA barcoding, evidence from partial sequences of mitochondrial cytochrome c oxidase I and nuclear internal transcribed spacer 2, we found that there are two nonsibling Apocryptophagus species living on male and female F. auriculata figs, respectively. We estimated that these two species diverged about 19.2 million years ago. Our study suggests that the host shift from Ficus variegate or Ficus prostrata fig species to male figs is a preference way for Apocryptophagus wasps to adapt to the separation of sexual function in diecious figs. Furthermore, to escape the disadvantage or sanction impact of the host, the exploiter Apocryptophagus wasps can preferably adapt to exploiting each sex of the figs, by changing their oviposition, niche shift, and habitat.     

Mistletoes as parasites: Host specificity and speciation

Recent research on parasite evolution has highlighted the importance of host specialization in speciation, either through host-switching or cospeciation. Many parasites show common patterns of host specificity, with higher host specificity where host abundance is high and reliable, phylogenetically conservative host specificity, and formation of races on or in different host species. Recent advances in our understanding of host specificity and speciation patterns in a variety of animal parasites provides valuable insights into the evolutionary biology of mistletoes.     

Host partitioning by parasites in an intertidal crustacean community

Patterns of host use by parasites throughout a guild community of intermediate hosts can depend on several biological and ecological factors, including physiology, morphology, immunology, and behavior. We looked at parasite transmission in the intertidal crustacean community of Lower Portobello Bay, Dunedin, New Zealand, with the intent of: (1) mapping the flow of parasites throughout the major crustacean species, (2) identifying hosts that play the most important transmission role for each parasite, and (3) assessing the impact of parasitism on host populations. The most prevalent parasites found in 14 species of crustaceans (635 specimens) examined were the trematodes Maritrema novaezealandensis and Microphallus sp., the acanthocephalans Profilicollis spp., the nematode Ascarophis sp., and an acuariid nematode. Decapods were compatible hosts for M. novaezealandensis, while other crustaceans demonstrated lower host suitability as shown by high levels of melanized and immature parasite stages. Carapace thickness, gill morphology, and breathing style may contribute to the differential infection success of M. novaezealandensis and Microphallus sp. in the decapod species. Parasite-induced host mortality appears likely with M. novaezealandensis in the crabs Austrohelice crassa, Halicarcinus varius, Hemigrapsus sexdentatus, and Macrophthalmus hirtipes, and also with Microphallus sp. in A. crassa. Overall, the different parasite species make different use of available crustacean intermediate hosts and possibly contribute to intertidal community structure.     

Host plant use in sympatric closely related flea beetles

Studies on strategies of host plant use in sympatric-related species are significant to the theory of sympatric speciation. Altica fragariae Nakane and Altica koreana Ogloblin are sympatric closely related flea beetles found in Beijing, northern China. All their recorded host plants are in the subfamily Rosoideae of the Rosaceae, so we regard them as a model system to study interactions between herbivorous insects and plant-insect co-evolution. We conducted a set of experiments on the host preference and performance of these flea beetles to study whether these closely related species have the ability to use sympatric novel host plants and whether monophagous and oligophagous flea beetles use the same strategy in host plant use. Oviposition preference experiments showed that A. koreana, a monophagous flea beetle, displayed high host fidelity. However, A. fragariae, which is oligophagous, often made "oviposition mistakes," ovipositing on nonhost plants such as Potentilla chinensis, the host plant of A. koreana, although normal host plants were preferred over novel ones. Larval performance studies suggested that A. fragariae was able to develop successfully on P. chinensis. Feeding experiences of larvae had no effect on feeding preference, oviposition preference, and fecundity of adults. However, females were impaired in their reproductive ability when fed on nonhost plants. Therefore, A. fragariae finished their development of larval stages on P. chinensis and came back to their primary host plant, Duchesnea indica, for feeding and reproduction after eclosion.     

Begging and cowbirds: brood parasites make hosts scream louder

Avian brood parasites have evolved striking begging abilitythat often allows them to prevail over the host progeny in competitionfor parental resources. Host young are therefore selected bybrood parasites to evolve behavioral strategies that reducethe cost of parasitism. We tested the prediction that the intensityof nestling begging displays functioning to attract parentalcare increases across species with the frequency of parasitismby the brown-headed cowbird (Molothrus ater). This was expectedbecause host young should try to prevail over highly competitiveparasitic broodmates in scramble interactions, act more selfishlywhen frequency of parasitism is high because brood parasitesoften affect more severely host condition than conspecific broodmates,and discount the kin selection costs of subtracting resourcesto unrelated parasites. Across 31 North American host species,begging loudness positively covaried with parasitism rate inPasserines, and such effect was stronger in species with smallcompared with large clutches. Begging loudness increased withbrood parasitism and nest predation among the most suitablehost species. These results held after controlling for concomitantecological factors and for common ancestry effects. Our resultssupport the hypothesis that avian brood parasitism has playeda role in the evolution of begging behavior of host young.     

Genetics and evolution of parasites and hosts: some comments

We provide a brief commentary on aspects of the analysis of the genetics and evolution of malaria parasites. Any attempt to understand the nature and manifestations of an infectious disease requires an understanding of the genetics of both pathogen and host. The outcome of a malaria infection, i.e. whether it is asymptomatic, mild, severe or causes cerebral malaria, is due to a complex interaction between the products of parasite and host genes. In general terms, genes in the parasite determine its ability to infect the host, its virulence, etc., while host genes will determine resistance or susceptibility to infection. More than this, however, genetics is about the spread of genes in populations, how they mutate and recombine to produce novel genotypes, and how the parasite and its hosts co-evolve with changing environments. This is a complex subject, and we present some discussion of a few aspects of its analysis.     

Host discrimination in modular mutualisms: a theoretical framework for meta-populations of mutualists and exploiters

Plants in multiple symbioses are exploited by symbionts that consume their resources without providing services. Discriminating hosts are thought to stabilize mutualism by preferentially allocating resources into anatomical structures (modules) where services are generated, with examples of modules including the entire inflorescences of figs and the root nodules of legumes. Modules are often colonized by multiple symbiotic partners, such that exploiters that co-occur with mutualists within mixed modules can share rewards generated by their mutualist competitors. We developed a meta-population model to answer how the population dynamics of mutualists and exploiters change when they interact with hosts with different module occupancies (number of colonists per module) and functionally different patterns of allocation into mixed modules. We find that as module occupancy increases, hosts must increase the magnitude of preferentially allocated resources in order to sustain comparable populations of mutualists. Further, we find that mixed colonization can result in the coexistence of mutualist and exploiter partners, but only when preferential allocation follows a saturating function of the number of mutualists in a module. Finally, using published data from the fig–wasp mutualism as an illustrative example, we derive model predictions that approximate the proportion of exploiter, non-pollinating wasps observed in the field.     

Experimental test of biotic resistance to an invasive herbivore provided by potential plant mutualists

Understanding the influence of resident species on the success of invaders is a core objective in the study and management of biological invasions. We asked whether facultative food-for-protection mutualism between resident, nectar-feeding ants and extrafloral nectar-bearing plants confers biotic resistance to invasion by a specialist herbivore. Our research focused on the South American cactus-feeding moth Cactoblastis cactorum Berg (Lepidopetra: Pyralidae) in the panhandle region of Florida. This species has been widely and intentionally redistributed as a biological control agent against weedy cacti (Opuntia spp.) but arrived unintentionally in the southeast US, where it attacks native, non-target cacti and is considered a noxious invader. The acquired host-plants of C. cactorum in Florida secrete extrafloral nectar, especially on young, vegetative structures, and this attracts ants. We conducted ant-exclusion experiments over 2 years (2008 and 2009) at two sites using potted plants of two vulnerable host species (O. stricta and O. ficus-indica) to evaluate the influence of cactus-visiting ants (total of eight species) at multiple points in the moth life cycle (oviposition, egg survival, and larval survival). We found that the presence of ants often increased the mortality of lab-reared C. cactorum eggsticks (stacks of cohered eggs) and larvae that we introduced onto plants in the field, although these effects were variable across sites, years, host-plant species, ant species, and/or between old and young plant structures. In contrast to these “staged” encounters, we found that ants had little influence on the survival of cactus moths that occurred naturally at our field sites, or on moth damage and plant growth. In total, our experimental results suggest that the influence of cactus-visiting ants on C. cactorum invasion dynamics is weak and highly variable.