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.     

Personality of hosts and their brood parasites

Brood parasites such as the common cuckoo Cuculus canorus exploit the parental abilities of their hosts, hosts avoid brood parasitism and predation by showing specific behavior such as loss of feathers, emission of fear screams and contact calls, displaying wriggle behavior to avoid hosts or potential prey, pecking at hosts and prey, and expressing tonic immobility (showing behavior like feigning death or rapid escape from predators and brood parasites). These aspects of escape behavior are consistent for individuals but also among sites, seasons, and years. Escape behavior expressed in response to a broad range of cuckoo hosts and prey are consistently used against capture by humans, but also hosts and brood parasites and predators and their prey. An interspecific comparative phylogenetic analysis of escape behavior by hosts and their brood parasites and prey and their predators revealed evidence of consistent behavior when encountering potential parasites or predators. We hypothesize that personality axes such as those ranging from fearfulness to being bold, and from neophobic to curiosity response in brood parasites constitute important components of defense against brood parasitism that reduces the overall risk of parasitism.     

寄生虫与宿主的相互作用

介绍了寄生虫对宿主的影响:降低宿主的适应性和繁殖能力;改变宿主的行为;影响宿主基因表达。同时还阐述了宿主对寄生虫的一系列抵抗行为包括:驱逐行为,梳理行为,逃避行为等。从而说明了寄生虫和宿主这对矛盾体间的相互作用关系。     

Interactions between monogenean parasites and their fish hosts

Parasite factors associated with recognition and selection of the host and the mechanisms in the host responsible for acceptance or rejection of the invading organism were evaluated. Sensory structures in parasites are able to detect differences between different fish species and this ability to discern between fishes may be based on both chemical and mechanical stimuli on the host surface. Complex glycoproteins, proteins, carbohydrates and simple molecules attract parasites or modify their behaviour. Furthermore, attachment of the monogenean parasite to a host is dependent on both mechanical structures and chemical factors in the parasite. These systems comprise anterior pads, posterior haptors, gland secretions, and muscular elements. The parasite needs access to appropriate nutrients which can be absorbed and used for reproduction and in this context signals from the host are needed for an optimal physiological response of the parasite. The innate and adaptive immune systems of the host are important elements in this question. Investigations have indicated that innate host factors (complement, lectins, acute phase reactants, macrophages) can bind to monogeneans and elicit severe damage to the parasites. The targets for these hostile products are not only the monogenean tegument, but may involve the gastrodermis and glands. However, the parasite's ability to avoid and even exploit the wide array of immunological elements of the host may be an important player in the dynamic interactions between host and monogenean determining host specificity. Even fish hosts susceptible to a certain parasite show an ability to mount a protective response at post-infection periods. Elevation of the host's production of adaptive and non-adaptive factors following monogenean infections of a certain duration may explain the acquired response.     

Spatial and genetic structure of directly-transmitted parasites reflects the distribution of their specific amphibian hosts

Parasite distributions depend on the local environment in which host infection occurs, and the surrounding landscape over which hosts move and transport their parasites. Although host and landscape effects on parasite prevalence and spatial distribution are difficult to observe directly, estimation of such relationships is necessary for understanding the spread of infections and parasite–habitat associations. Although parasite distributions are necessarily nested within host distributions, direct environmental influences on local infection or parasite effects on host dispersal could lead to distinct landscape or habitat relationships relative to their hosts. Our aim was to determine parasite spatial structure across a contiguous prairie by statistical modeling of parasite–landscape relationships combined with analysis of population genetic structure. We sampled northern leopard frogs (Lithobates pipiens) and wood frogs (L. sylvaticus) for host-specific lung nematodes (Rhabdias ranae and R. bakeri; respectively) across the Sheyenne National Grassland in southeastern North Dakota and developed primers for 13 microsatellite loci for Rhabdias. The two Rhabdias species exhibited different correlations with landscape characteristics that conformed with that of their hosts, indicating transmission is driven by host ecology, probably density, and not directly by the environment. There was evidence for localized, patchy spatial genetic structure, but no broader-scale geographic patterns, indicating no barriers to host and parasite dispersal. Nematodes cohabitating in an individual frog were most genetically similar. Worms within the same wetland were also genetically similar, indicating localized transmission and resulting wetland-scale patchiness are not completely obscured by broad-scale host–parasite dispersal. Beyond individual wetlands, we found no evidence of genetic isolation-by-distance or patchiness at the landscape-scale.     

Coevolutionary genetics of Plasmodium malaria parasites and their human hosts

Malaria has been invoked, perhaps more than any other infectious disease, as a force for the selection of human genetic polymorphisms. Evidence for genome-shaping interactions can be found in the geographic and ethnic distributions of the hemoglobins, blood group antigens, thalassemias, red cell membrane molecules, human lymphocyte antigen (HLA) classes, and cytokines. Human immune responses and genetic variations can correspondingly influence the structure and polymorphisms of Plasmodium populations, notably in genes that affect the success and virulence of infection. In Africa, where the burden from Plasmodium falciparum predominates, disease severity and manifestations vary in prevalence among human populations. The evolutionary history and spread of Plasmodium species inform our assessment of malaria as a selective force. Longstanding host-pathogen relationships, as well as recent changes in this dynamic, illustrate the selective pressures human and Plasmodium species place on one another. Investigations of malaria protection determinants and virulence factors that contribute to the complexity of the disease should advance our understanding of malaria pathogenesis.     

Lipid transport proteins in malaria,from Plasmodium parasites to their hosts

Eukaryotic unicellular pathogens from the genus Plasmodium are the etiological agents of malaria, a disease that persists over a wide range of vertebrate species, including humans. During its dynamic lifecycle, survival in the different hosts depends on the parasite's ability to establish a suitable environmental milieu. To achieve this, specific host processes are exploited to support optimal growth, including extensive modifications to the infected host cell. These modifications include the formation of novel membranous structures, which are induced by the parasite. Consequently, to maintain a finely tuned and dynamic lipid environment, the organisation and distribution of lipids to different cell sites likely requires specialised lipid transfer proteins (LTPs). Indeed, several parasite and host-derived LTPs have been identified and shown to be essential at specific stages. Here we describe the roles of LTPs in parasite development and adaptation to its host including how the latest studies are profiting from the improved genetic, lipidomic and imaging toolkits available to study Plasmodium parasites. Lastly, a list of predicted Plasmodium LTPs is provided to encourage research in this field.     

Population structure and the co-evolution between social parasites and their hosts

Co-evolutionary trajectories of host-parasite interactions are strongly affected by the antagonists' evolutionary potential, which in turn depends on population sizes as well as levels of recombination, mutation, and gene flow. Under similar selection pressures, the opponent with the higher evolutionary rate is expected to lead the co-evolutionary arms race and to develop local adaptations. Here, we use mitochondrial DNA sequence data and microsatellite markers to assess the amount of genetic variability and levels of gene flow in two host-parasite systems, each consisting of an ant social parasite--the European slavemaker Harpagoxenus sublaevis and the North American slavemaker Protomognathus americanus--and its two main host species. Our population genetic analyses revealed limited gene flow between individual populations of both host and parasite species, allowing for a geographic mosaic of co-evolution. In a between-system comparison, we found less genetic variability and more pronounced structure in Europe, where previous behavioural studies demonstrated strong local adaptation. Within the European host-parasite system, the larger host species Leptothorax acervorum exhibited higher levels of both genetic variability and gene flow, and previous field data showed that it is less affected by the social parasite H. sublaevis than the smaller host Leptothorax muscorum, which has genetically depleted and isolated populations. In North America, the parasite P. americanus showed higher levels of gene flow between sites, but overall less genetic diversity than its hyper-variable main host species, Temnothorax longispinosus. Interestingly, recent ecological and chemical studies demonstrated adaptation of P. americanus to local host populations, indicating the importance of migration in co-evolutionary interactions.     

Rodent and nonrodent malaria parasites differ in their phospholipid metabolic pathways

Malaria, a disease affecting humans and other animals, is caused by a protist of the genus Plasmodium. At the intraerythrocytic stage, the parasite synthesizes a high amount of phospholipids through a bewildering number of pathways. In the human Plasmodium falciparum species, a plant-like pathway that relies on serine decarboxylase and phosphoethanolamine N-methyltransferase activities diverts host serine to provide additional phosphatidylcholine and phosphatidylethanolamine to the parasite. This feature of parasitic dependence toward its host was investigated in other Plasmodium species. In silico analyses led to the identification of phosphoethanolamine N-methyltransferase gene orthologs in primate and bird parasite genomes. However, the gene was not detected in the rodent P. berghei, P. yoelii, and P. chabaudi species. Biochemical experiments with labeled choline, ethanolamine, and serine showed marked differences in biosynthetic pathways when comparing rodent P. berghei and P. vinckei, and human P. falciparum species. Notably, in both rodent parasites, ethanolamine and serine were not significantly incorporated into phosphatidylcholine, indicating the absence of phosphoethanolamine N-methyltransferase activity. To our knowledge, this is the first study to highlight a crucial difference in phospholipid metabolism between Plasmodium species. The findings should facilitate efforts to develop more rational approaches to identify and evaluate new targets for antimalarial therapy.     

Differences in intestinal microbiota between avian brood parasites and their hosts

The intestinal microbiota determines the effectiveness of digestion in vertebrates, and is influenced by the external environment (mainly the diet), gut characteristics, and phylogeny. Avian brood-parasitic nestlings of the sub-family Cuculinae develop in nests of phylogenetically distant passerines and can be fed with the host diet. If the shaping of bacterial communities is dominated by phylogenetic constraints, and therefore the microbiota of parasitic nestlings differs from that of host nestlings, the energy and micronutrients that parasites and hosts obtain from a similar amount of food would be different. In this case, the bacterial communities of parasitic and host nestlings would have important consequences with respect to brood parasite development. By experimentally creating mixed broods of magpies ( Pica pica ) and great spotted cuckoos ( Clamator glandarius ), we investigated their cloacal microbiota using ribosomal intergenic spacer analysis. We found significant differences in bacterial assemblages of the parasitic and host nestlings, although none of the phylotypes were specific in either great spotted cuckoos or magpies. Cuckoos presented more complex communities, which could help the brood parasitic life style and allow the digestion of food provided by different potential hosts. Moreover, the intestinal morphology is different between the two species due to phylogenetic differences in the two taxa, which would influence the dissimilar bacterial assemblages. The detected differences in microbiota of great spotted cuckoo and magpie nestlings, which might occur in other brood parasite–host systems, may imply a lower digestion efficiency in parasites. Thus, the higher level requirements of cuckoo nestlings may be explained, at least in part, by cuckoos having a suboptimal bacterial community for processing the host diet.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 406–414.     

Interactions between malaria parasites and their mosquito hosts in the midgut

This review examines what is presently known of the molecular interactions between Plasmodium and Anopheles that take place in the latter's midgut upon ingestion of the parasites with an infectious blood meal. In order to become 'established' in the gut and to transform into a sporozoite-producing oocyst, the malaria parasite needs to undergo different developmental steps that are often characterized by the use of selected resources provided by the mosquito vector. Moreover, some of these resources may be used by the parasite in order to overcome the insect host's defence mechanisms. The molecular partners of this interplay are now in the process of being defined and analyzed for both Plasmodium and mosquito and, thus, understood; these will be presented here in some detail.     

Museum specimens and phylogenies elucidate ecology's role in coevolutionary associations between mites and their bee hosts

Coevolutionary associations between hosts and symbionts (or parasites) are often reflected in correlated patterns of divergence as a consequence of limitations on dispersal and establishment on new hosts. Here we show that a phylogenetic correlation is observed between chaetodactylid mites and their hosts, the long-tongued bees; however, this association manifests itself in an atypical fashion. Recently derived mites tend to be associated with basal bee lineages, and vice versa, ruling out a process of cospeciation, and the existence of mites on multiple hosts also suggests ample opportunity for host shifts. An extensive survey of museum collections reveals a pattern of infrequent host shifts at a higher taxonomic level, and yet, frequent shifts at a lower level, which suggests that ecological constraints structure the coevolutionary history of the mites and bees. Certain bee traits, particularly aspects of their nesting behavior, provide a highly predictive framework for the observed pattern of host use, with 82.1% of taxa correctly classified. Thus, the museum survey and phylogenetic analyses provide a unique window into the central role ecology plays in this coevolutionary association. This role is apparent from two different perspectives--as (a) a constraining force evident in the historical processes underlying the significant correlation between the mite and bee phylogenies, as well as (b) by the highly nonrandom composition of bee taxa that serve as hosts to chaetodactylid mites.     

Laser-assisted microdissection for the study of the ecology of parasites in their hosts

The population biology of internal parasites is difficult to study because the adult parasites are often inaccessible, deep within the host's body. Developing stages, such as eggs in the faeces or larvae in the skin are more easily obtained, but are difficult to handle because they are often very small and with a tough cuticle. This has limited their use in molecular ecology for estimating population biology parameters of the adults (their parents). We have used Onchocerca ochengi (a filarial nematode parasite of cattle) to describe a novel and generally applicable method of easily and conveniently isolating individual larvae (microfilariae) from the host using laser‐assisted microdissection. Furthermore, we have been able to improve the isolation of DNA by using the laser to bisect the larva to release DNA from the tissues enclosed within the parasite cuticle, and in this way we have achieved amplification of fragments over 1400 bp, and routinely PCR‐amplified single‐copy sequences from 5% of the DNA from a single larva (the equivalent of approximately 15 nuclei), and regularly from 0.5%.     

The evolution of parasites from their hosts: intra- and interspecific parasitism and Emery's rule

In some taxa of Hymenoptera, fungi, red algae and mistletoe, parasites and their hosts are either sibling species or at least closely related (Emery's rule). Three evolutionary mechanisms have been proposed for this phenomenon: (i) intraspecific parasitism is followed by sympatric speciation; (ii) allopatric speciation is followed by secondary sympatry and the subsequent parasitism of one sibling species by the other; and (iii) allopatric speciation of a species with intraspecific parasitism is followed by secondary sympatry, in which one species becomes an obligate parasite of the other. Mechanisms (i) and (ii) are problematic, while mechanism (iii) has not, to our knowledge, been analysed quantitatively. In this paper, we develop a model for single- and two-species evolutionary stable strategies (ESSs) to examine the basis for Emery's rule and to determine whether mechanism (iii) is consistent with ESS reasoning. In secondary sympatry after allopatric speciation, the system's evolution depends on the relative abundances of the two sibling species and on the proportional damage wrought by parasites of each species on non-parasitic members of the other. Depending on these interspecific effects, either the rarer or the commoner species may become the parasite and the levels of within-species parasitism need not determine which evolves to obligate parasitism.     

Arms races between social parasites and their hosts: geographic patterns of manipulation and resistance

The evolutionary interactions between permanently social parasiticspecies and their hosts are of special interest because socialparasites are not only closely dependent on, but are also closelyrelated to, their hosts. The small European slavemaker Harpagoxenussublaevis has evolved several characters that help manipulateits host. In this study we investigated adaptations of thissocial parasite to its local hosts and the geographic patternof host resistance in two main host species from three differentpopulations. In behavioral experiments, we examined whetherhost colonies from three geographically distant Leptothoraxacervorum populations varied in their ability to defend thenest against social parasites. Naive colonies from the unparasitizedEnglish population killed attacking slavemakers more often thandid host colonies from two parasitized populations. We alsofound strong interpopulation variation in the ability of theslavemaker to manipulate host behavior. H. sublaevis uses theDufour gland secretion to induce intracolonial fights and, ingeneral, this "propaganda" substance was most effective againstlocal hosts. Our results suggest that the social parasite isleading the arms race in this aspect. Similar experiments uncovereddifferences between two populations of the second host speciesL. muscorum and could demonstrate that nest defense in bothhost species is similarly efficient. In L. acervorum, monogynouscolonies were more successful in nest defense, whereas socialstructure had no impact in L. muscorum. Colony size did notaffect the efficacy of nest defense in either host species.The caste of the slavemaker had a strong influence on the successof an attack.     

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.     

Long‐term coevolution between avian brood parasites and their hosts

Coevolutionary theory predicts that the most common long‐term outcome of the relationships between brood parasites and their hosts should be coevolutionary cycles based on a dynamic change selecting the currently least‐defended host species, given that when well‐defended hosts are abandoned, hosts will be selected to decrease their defences as these are usually assumed to be costly. This is assumed to be the case also in brood parasite‐host systems. Here I examine the frequency of the three potential long‐term outcomes of brood parasite–host coevolution (coevolutionary cycles, lack of rejection, and successful resistance) in 182 host species. The results of simple exploratory comparisons show that coevolutionary cycles are very scarce while the lack of rejection and successful resistance, which are considered evolutionary enigmas, are much more frequent. I discuss these results considering (i) the importance of different host defences at all stages of the breeding cycle, (ii) the role of phenotypic plasticity in long‐term coevolution, and (iii) the evolutionary history of host selection. I suggest that in purely antagonistic coevolutionary interactions, such as those involving brood parasites and their hosts, that although cycles will exist during an intermediate phase of the interactions, the arms race will end with the extinction of the host or with the host acquiring successful resistance. As evolutionary time passes, this resistance will force brood parasites to use previously less suitable host species. Furthermore, I present a model that represents the long‐term trajectories and outcomes of coevolutionary interactions between brood parasites and their hosts with respect to the evolution of egg‐rejection defence. This model suggests that as an increasing number of species acquire successful resistance, other unparasitized host species become more profitable and their parasitism rate and the costs imposed by brood parasitism at the population level will increase, selecting for the evolution of host defences. This means that although acceptance is adaptive when the parasitism rate and the costs of parasitism are very low, this cannot be considered to represent an evolutionary equilibrium, as conventional theory has done to date, because it is not stable.