Relative helminth size in crustacean hosts: in vivo determination, and effects of host gender and within-host competition in a copepod infected by a cestode

Crustaceans are important hosts for a number of helminth parasites, and they are increasingly used as models for studying the physiology, ecology and evolution of parasite-host interactions. In ecological studies, this interaction is commonly described only in terms of prevalence and number of larvae per infected host. However, the volume of helminth parasites can vary greatly, and this variation can potentially give important insights into the nature of a parasite-host relationship. It may influence and be influenced, for example, by within-host competition, host size, growth, and life history. Here we present a simple method that allows rapid approximation of the absolute and relative volumes of cestode larvae within copepod hosts of various developmental stages (nauplii, copepodites and adults). The measurements are taken in vivo without much disturbance of the animals, i.e. the technique allows study of growth and development of the parasites in relation to that of their hosts. The principles of this technique can be adopted to other helminth parasites and other crustacean hosts. Using this method in the copepod Macrocyclops albidus infected with the cestode Schistocephalus solidus, we found that the relative parasite size (= `parasite index') ranged from 0.5% to 6.5% of host size 14 days after infection. It was greater in male than in female hosts. With increasing number of parasites per host, the total parasite volume increased while the mean volume of the individual parasites decreased. The magnitude of the observed parasite indices, the large variation that was found within a sample of 46 infected adult copepods, and the observed correlates suggest that this new index can indeed be an important measure of parasite success and its pathogenecity.     

Diversification and host switching in avian malaria parasites

The switching of parasitic organisms to novel hosts, in which they may cause the emergence of new diseases, is of great concern to human health and the management of wild and domesticated populations of animals. We used a phylogenetic approach to develop a better statistical assessment of host switching in a large sample of vector-borne malaria parasites of birds (Plasmodium and Haemoproteus) over their history of parasite-host relations. Even with sparse sampling, the number of parasite lineages was almost equal to the number of avian hosts. We found that strongly supported sister lineages of parasites, averaging 1.2% sequence divergence, exhibited highly significant host and geographical fidelity. Event-based matching of host and parasite phylogenetic trees revealed significant cospeciation. However, the accumulated effects of host switching and long distance dispersal cause these signals to disappear before 4% sequence divergence is achieved. Mitochondrial DNA nucleotide substitution appears to occur about three times faster in hosts than in parasites, contrary to findings on other parasite-host systems. Using this mutual calibration, the phylogenies of the parasites and their hosts appear to be similar in age, suggesting that avian malaria parasites diversified along with their modern avian hosts. Although host switching has been a prominent feature over the evolutionary history of avian malaria parasites, it is infrequent and unpredictable on time scales germane to public health and wildlife management.     

An evaluation of lipid metabolism in the insect trypanosomatid Herpetomonas muscarum uncovers a pathway for the uptake of extracellular insect lipoproteins

Lipid uptake and metabolism by trypanosomatid parasites from vertebrate host blood have been well established in the literature. However, there is a lack of knowledge regarding the same aspects concerning the parasites that cross the hemolymph of their invertebrate hosts. We have investigated the lipid composition and metabolism of the insect trypanosomatid Herpetomonas muscarum by ³H- palmitic acid and phosphate (³²Pi) and the parasite interaction with Lipophorin (Lp) the main lipid carrying protein of insect hemolymph. Gas chromatography-mass spectrometry (GC–MS) analyses were used to identify the fatty acids and sterols composition of H.muscarum. Furthermore, we investigated the Lp binding site in the plasma membrane of parasite by Immunolocalization. We showed that H. muscarum incorporated 3H-palmitic acid and inorganic phosphate (32Pi) which were readily used as precursor molecules of lipid biosynthetic pathways. Furthermore, H. muscarum was able to take up both protein and lipid moieties of Lp which could be used as nutrient sources. Moreover, we have also demonstrated for the first time the presence of a Lp binding site in the membrane of a parasite. Such results point out the role of describing the metabolic pathways of trypanosomatids in order to provide a better understanding of parasite-host interaction peculiarities. Such studies may enhance the potential form the identification of novel chemotherapeutic targets in harmful parasites.     

Niche breadth in parasites: an evolutionarily stable strategy model, with special reference to the protozoan parasite Leishmania.

A parasite's host range essentially defines its niche breadth, which, as foraging theory predicts, is influenced by resource availability. For parasites, the interaction of infection and transmission characteristics with host population dynamics determines host availability. An epidemiological model, involving two host types and describing competition between a "generalist" parasite strain and a related "specialist" strain, is used to examine the interplay among host range, relative host availabilities, and adaptational compromises engendered by increased host range. Results show that the generalist can predominate even when it cannot maintain itself in either host alone, but that the specialist can persist if its reproductive rate attains some threshold relative to either of the generalist's respective rates in its two hosts. The model is in rough, qualitative agreement with observed dynamics of two Leishmania parasite-host systems, and overall results suggest that infection of two species with a common parasite can lead to complex, indirect coevolutionary dynamics.     

Impact of microsporidia on hormonal balance in insect hosts

Microsporidia (M) is a phylum of protists parasitizing obligatory in animal cells. Long way of adaptation of M to intracellular parasitism resulted in establishment of quite close relationships between the parasite and its host. Different species of M induce in their hosts symptoms similar to those caused by misbalance of juvenile hormone (JH) and ecdysone. M infection leads to pathology of different hormone-dependent functions such as cell differentiation and specialization, molting, metamorphosis, diapause and reproduction of insects. The signs of hormonal dysfunction evidence for elevated titer of JH in M-infected insects. Two possible explanation of this could be offered: JH secretion by M or specific influence of the parasites on the insect endocrine systems. Impact on insect endogenous JH titer by M could be mediated by affection of secretory activity of corpora allata or by suppression of enzymatic degradation of JH. According to different hypotheses, insect hormonal status during microsporidiosis could be modified by a) insect host stress-reaction, b) exhaustion of insect host reserves, characteristic for acute phase of the disease, c) destruction of infected insect cells and tissues during mass sporogenesis of M. Data found in literature and provided by our experiments evidence for presence of JH analogues or juvenilizing substance in the extracts of M spores. From detailed examination of pathological process it is also seen that juvenilizing effect of M infection is usually restricted to the invaded regions of tissues (i.e. expressed locally) but not a systemic one. Ability of M to modify morpho-functional features of infected tissues at the level of hormonal regulation is undoubtfully a prominent adaptation for stabilizing "microsporidia-insect" parasite-host systems.     

Random parasite encounters coupled with condition-linked immunity of hosts generate parasite aggregation

Parasite aggregation is viewed as a natural law in parasite-host ecology but is a paradox insofar as parasites should follow the Poisson distribution if hosts are encountered randomly. Much research has focused on whether parasite aggregation in or on hosts is explained by aggregation of infective parasite stages in the environment, or by heterogeneity within host samples in terms of host responses to infection (e.g., through representation of different age classes of hosts). In this paper, we argue that the typically aggregated distributions of parasites may be explained simply. We propose that aggregated distributions can be derived from parasites encountering hosts randomly, but subsequently by parasites being 'lost' from hosts based on condition-linked escape or immunity of hosts. Host condition should be a normally distributed trait even among otherwise homogeneous sets of hosts. Our model shows that mean host condition and variation in host condition have different effects on the different metrics of parasite aggregation. Our model further predicts that as host condition increases, parasites become more aggregated but numbers of attending parasites are reduced overall and this is important for parasite population dynamics. The effects of deviation from random encounter are discussed with respect to the relationship between host condition and final parasite numbers.     

Microbial mediation of fruit fly–host plant interactions: is the host plant the “centre of activity”?

Insects utilize resources in their environment with the aid of mutualistic or symbiotic mediation by microorganisms. Some insect species such as ants and termites often have complex ecological and evolutionary associations with their symbionts, while the nature and functional significance of such associations in non-social insects is often unclear. In the Dacinae (Diptera: Tephritidae), specific Enterobacteriaceae ( Erwinia herbicola , Enterobacter cloacae , Klebsiella oxytoca ) are believed to mediate interactions between the adult fruit flies and the larval host plant. This bacterial mediation is hypothesized as being integral to the larval host plant being the "centre of activity" of the fly. Using a non-pest, monophagous fruit fly ( Bactrocera cacuminata [Hering]), we tested this hypothesis by manipulating the fruiting state of its larval host plant ( Solanum mauritianum Scopoli) and subsequently assessing insect behaviour and phylloplane microflora on those hosts. On host plants that had never fruited, few flies or bacterial colonies were recorded, consistent with hypothesis expectations. On fruiting host plants or plants that had had their fruit removed, bacterial colonies were present; again consistent with expectation. However, few flies were recorded on fruit-removed plants and all fly behaviours, other than resting or oviposition, were rare or absent on any hosts; inconsistent with expectation. The general pattern of results suggested that female flies coming to oviposit on fruiting hosts were spreading Enterobacteriaceae, but such spread was incidental and not part of some mutualistic interaction between fruit flies and bacteria.     

Cellular and molecular interactions between the apicomplexan parasites Plasmodium and Theileria and their host cells

Apicomplexan parasites of the genera Theileria and Plasmodium have complicated life cycles including infection of a vertebrate intermediate host and an arthropod definitive host. As the Plasmodium parasite progresses through its life cycle, it enters a number of different cell types, both in its mammalian and mosquito hosts. The fate of these cells varies greatly, as do the parasite and host molecules involved in parasite-host interactions. In mammals, Plasmodium parasites infect hepatocytes and erythrocytes whereas Theileria infects ruminant leukocytes and erythrocytes. Survival of Plasmodium-infected hepatocytes and Theileria-infected leukocytes depends on parasite-mediated inhibition of host cell apoptosis but only Theileria-infected cells exhibit a fully transformed phenotype. As the development of both parasites progresses towards the merozoite stage, the parasites no longer promote the survival of the host cell and the infected cell is finally destroyed to release merozoites. In this review we describe similarities and differences of parasite-host cell interactions in Plasmodium-infected hepatocytes and Theileria-infected leukocytes and compare the observed phenotypes to other parasite stages interacting with host cells.     

The role of Ca2+ in the process of cell invasion by intracellular parasites

In order to replicate, many parasites must invade host cells. Changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) of different parasites and tissue culture cells during their interaction have been studied. An increase in cytosolic Ca(2+) in Trypanosoma cruzi trypomastigotes occurs after association of the parasites with host cells. Ca(2+) mobilization in the host cells also takes place upon contact with T. cruzi trypomastigotes, Leishmania donovani amastigotes or Plasmodium falciparum merozoites. When Ca(2+) transients are prevented by intracellular Ca(2+) chelators, a decrease in parasite association to host cells is observed. This reveals the importance of [Ca(2+)](i) in the process of parasite-host cell interaction, as discussed here by Roberto Docampo and Silvia Moreno.     

Biochemical interaction between chelonine wasps and their lepidopteran hosts: after a decade of research - the parasite is in control

A decade of research on the biochemical interaction between chelonine wasps and their lepidopteran hosts has yielded considerable data on the underlying basis for the developmental, immunological and reproductive effects that these parasites inflict upon their hosts. These egg-larval parasites induce their immunologically compromised host larvae to precociously initiate metamorphosis, followed by suppression of development of the precocious prepupa, in addition to castration of the host. The results from numerous laboratories have shown that the parasite egg that is normally injected by the adult female into the host along with venom, polydnavirus and calyx fluid proteins need not hatch or even be present for the host to exhibit each of these alterations. In addition to these aspects the parasite larva, when present, itself releases hormones and proteins into the hemolymph of the host. A review of the data amassed to date leads inexorably to the conclusion that it is the chelonine wasp that is the biochemically dominant partner. Thus, after 10 years of research, it still appears that in chelonine-lepidopteran parasite-host systems, the parasite is in control of specific points of the biochemistry and development of its host.     

Parasites and the regional distribution of bumblebee species

Parasites and regional processes may be important to structure local species assemblages In particular, it has been hypothesized that widely distributed and abundant species should harbour more parasite species which could give them a competitive advantage in local species assemblages Empirical evidence bearing on these points are scarce and mainly restricted to vertebrate hosts or plants The aim of this study was to provide data in insect hosts and to test whether the patterns in field populations conform with those correlates expected from the parasite-host distribution hypothesis We investigated species assemblages of bumblebees at 12 different sites in a mesoscale region with their parasites over two consecutive years Parasites included dipteran and hymenopteran parasitoids. nematodes, mites, and protozoa The mean number of parasite species per host species ranged from 1 to 8 To account for sampling effort, all data were corrected for sample size effects The number of parasite species per average host individual (parasite load) ranged from 0 09 to 0 75 In cross-species comparisons, the number of parasite species per host species was positively correlated with regional distribution, i e the number of sites a host species occupied m the region, and with the average local host abundance The same relationships were found for parasite load In addition, parasite load correlated positively with average colony size of the host species, but not with body size of the individuals Bumblebee species were bimodally distributed When separated into widely-distributed and locally-occurring species, common hosts harboured more parasite species than rare ones Moreover, workers of common species individually had higher parasite loads From these results, we conclude that some of the necessary preconditions for parasites being able to affect the distribution and occurrence of their hosts are met in bumblebees The findings support a general pattern that parasite loads correlate positively with local abundance and geographical distribution of their hosts, also on mesoscales usually considered in ecological studies     

Nematode endoparasites do not codiversify with their stick insect hosts

Host–parasite coevolution stems from reciprocal selection on host resistance and parasite infectivity, and can generate some of the strongest selective pressures known in nature. It is widely seen as a major driver of diversification, the most extreme case being parallel speciation in hosts and their associated parasites. Here, we report on endoparasitic nematodes, most likely members of the mermithid family, infecting different Timema stick insect species throughout California. The nematodes develop in the hemolymph of their insect host and kill it upon emergence, completely impeding host reproduction. Given the direct exposure of the endoparasites to the host's immune system in the hemolymph, and the consequences of infection on host fitness, we predicted that divergence among hosts may drive parallel divergence in the endoparasites. Our phylogenetic analyses suggested the presence of two differentiated endoparasite lineages. However, independently of whether the two lineages were considered separately or jointly, we found a complete lack of codivergence between the endoparasitic nematodes and their hosts in spite of extensive genetic variation among hosts and among parasites. Instead, there was strong isolation by distance among the endoparasitic nematodes, indicating that geography plays a more important role than host‐related adaptations in driving parasite diversification in this system. The accumulating evidence for lack of codiversification between parasites and their hosts at macroevolutionary scales contrasts with the overwhelming evidence for coevolution within populations, and calls for studies linking micro‐ versus macroevolutionary dynamics in host–parasite interactions.     

The influence of parasitic water mites on the instantaneous death rate of their hosts

Summary The effect of 2 species of water mites on the instantaneous death rate of their hosts was measured on the basis of laboratory experiments. In both parasite-host association — the parasitic water mite Hydryphantes tenuabilis on the aquatic insect host Hydrometra australis and the parasitic water mite Arrenurus pseudotenuicollis on the mosquito Anopheles crucians — the effect of mite load on the instantaneous death rate of the host appeared to be linear. Also, the impact of a single parasite on the host's death rate was apparently related to the ratio of parasite to host body weight. The results of this study are in general agreement with recent theoretical investigations of the regulation of host populations by parasites.     

Host range and local parasite adaptation

Parasites may be expected to become locally adapted to their hosts. However, while many empirical studies have demonstrated local parasite adaptation, others have failed to demonstrate it, or have shown local parasite maladaptation. Researchers have suggested that gene flow can swamp local parasite-host dynamics and produce local adaptation only at certain geographical scales; others have argued that evolutionary lags can account for both null and maladaptive results. In this paper, we use item response theory (IRT) to test whether host range influences the likelihood of parasites locally adapting to their hosts. We collated 32 independent experiments testing for local adaptation, where parasites could be assigned as having either broad or narrow host ranges (BHR and NHR, respectively). Twenty-five tests based on BHR parasites had a significantly lower average effect size than seven NHR tests, indicating that studies based on BHR parasites are less likely to demonstrate local parasite adaptation. We argue that this may relate to evolutionary lags during diffuse coevolution of BHR parasites with their hosts, rather than differences in experimental approaches or other confounds between BHR and NHR studies.     

Parasite effects on isopod feeding rates can alter the host's functional role in a natural stream ecosystem

Changes to host behaviour as a consequence of infection are common in many parasite-host associations, but their effects on the functional role hosts play within ecosystems are rarely quantified. This study reports that helminth parasites significantly decrease consumption of detritus by their isopod hosts in laboratory experiments. Natural host and parasite densities across eight contiguous seasons were used to estimate effects on the amount of stream detritus-energy processed. Extrapolations using mass-specific processing rates from laboratory results to field patterns suggest that the effects of the parasites occur year round but the greatest impact on the amount of detritus processed by isopods occurs in the autumn when the bulk of leaf detritus enters the stream, and when parasite prevalence in the isopod population is high. Parasites have a lesser impact on the amount of detritus processed in spring and summer when isopods are most abundant, when parasite prevalence is not high, and when fish predation on isopods is high. These results support the idea that parasites can affect the availability of resources critical to other species by altering behaviours related to the functional role hosts play in ecosystems, and suggest that seasonality may be an important factor to consider in the dynamics of these parasite-host interactions.     

The geographic structure of selection on a coevolving interaction between social parasitic wasps and their hosts hampers social evolution

Social parasites exploit societies, rather than organisms, and rear their brood in social insect colonies at the expense of their hosts, triggering a coevolutionary process that may affect host social structure. The resulting coevolutionary trajectories may be further altered by selection imposed by predators, which exploit the abundant resources concentrated in these nests. Here, we show that geographic differences in selection imposed by predators affects the structure of selection on coevolving hosts and their social parasites. In a multiyear study, we monitored the fate of the annual breeding attempts of the solitary nesting foundresses of Polistes biglumis wasps in four geographically distinct populations that varied in levels of attack by the congeneric social parasite, P. atrimandibularis. Foundress fitness depended mostly on whether, during the long founding phase, a colony was invaded by social parasites or attacked by predators. Foundresses from each population differed in morphological traits and reproductive tactics that were consistent with selection imposed by their natural enemies and in ways that may affect host sociality. In turn, parasite traits were consistent with selection imposed locally by hosts, implying a geographic mosaic of coevolution in this brood parasitic interaction.     

EVOLUTIONARY ASSOCIATIONS OF BROOD PARASITIC FINCHES (VIDUA) AND THEIR HOST SPECIES: ANALYSES OF MITOCHONDRIAL DNA RESTRICTION SITES

The species-specific associations of the African brood parasitic finches Vidua with their estrildid finch host species may have originated by cospeciation with the host species or by later colonizations of new hosts. Predictions of these alternative models were tested in two species groups of brood parasites (indigobirds, paradise whydahs) and their hosts. Phylogenetic analyses suggested that the brood parasites and their hosts did not speciate in parallel. The parasitic indigobirds share mitochondrial haplotypes with each other, and species limits in both indigobirds and paradise whydahs do not correspond with their gene trees. Different parasite species within a region are more closely related to each other than any is to parasites that are associated with its same host species in other regions of Africa. There is little genetic difference between parasite species D?_i,j < 0.001 in the indigobirds, D?_i,j = 0.01 in the whydahs). Genetic distances D?_i,j between the parasite species are less than the genetic distances between their corresponding host species in all parasite-host comparisons, and average only 7.2% as large in the indigobirds as in their hosts and 42% as large in the paradise whydahs as in their hosts. A phylogenetic model that allows ancestral haplotype polymorphisms to be retained in descendant species was compared to a constraint model of species monophyly requiring all but the one ancestral haplotype to be independently derived within each species. The constraint model increases the length of the indigobird tree by 50% over that of the model of retained ancestral polymorphisms; the difference is statistically significant. Both phylogenetic and distance analyses indicate that the brood parasites have become associated with their host species through host switches and independent colonizations of the hosts, rather than through parallel cospeciation with them. The molecular genetic results are supported by recent discoveries of additional host species that are associated with the indigobirds in the field and by variation in the species-specific song behaviors of the brood parasites.     

Local adaptation and enhanced virulence of Nosema granulosis artificially introduced into novel populations of its crustacean host, Gammarus duebeni

Local adaptation theory predicts that, on average, most parasite species should be locally adapted to their hosts (more suited to hosts from local than distant populations). Local adaptation has been studied for many horizontally transmitted parasites, however, vertically transmitted parasites have received little attention. Here we present the first study of local adaptation in an animal/parasite system where the parasite is vertically transmitted. We investigate local adaptation and patterns of virulence in a crustacean host infected with the vertically transmitted microsporidian Nosema granulosis. Nosema granulosis is vertically transmitted to successive generations of its crustacean host, Gammarus duebeni and infects up to 46% of adult females in natural populations. We investigate local adaptation using artificial horizontal infection of different host populations in the UK. Parasites were artificially inoculated from a donor population into recipient hosts from the sympatric population and into hosts from three allopatric populations in the UK. The parasite was successfully established in hosts from all populations regardless of location, infecting 45% of the recipients. Nosema granulosis was vertically (transovarially) transmitted to 39% of the offspring of artificially infected females. Parasite burden (intensity of infection) in developing embryos differed significantly between host populations and was an order of magnitude higher in the sympatric population, suggesting some degree of host population specificity with the parasite adapted to its local host population. In contrast with natural infections, artificial infection with the parasite resulted in substantial virulence, with reduced host fecundity (24%) and survival (44%) of infected hosts from all the populations regardless of location. We discuss our findings in relation to theories of local adaptation and parasite-host coevolution.     

Big bang in the evolution of extant malaria parasites

Malaria parasites (genus Plasmodium) infect all classes of terrestrial vertebrates and display host specificity in their infections. It is therefore assumed that malaria parasites coevolved intimately with their hosts. Here, we propose a novel scenario of malaria parasite-host coevolution. A phylogenetic tree constructed using the malaria parasite mitochondrial genome reveals that the extant primate, rodent, bird, and reptile parasite lineages rapidly diverged from a common ancestor during an evolutionary short time period. This rapid diversification occurred long after the establishment of the primate, rodent, bird, and reptile host lineages, which implies that host-switch events contributed to the rapid diversification of extant malaria parasite lineages. Interestingly, the rapid diversification coincides with the radiation of the mammalian genera, suggesting that adaptive radiation to new mammalian hosts triggered the rapid diversification of extant malaria parasite lineages.     

The scaling of total parasite biomass with host body mass

The selective pressure exerted by parasites on their hosts will to a large extent be influenced by the abundance or biomass of parasites supported by the hosts. Predicting how much parasite biomass can be supported by host individuals or populations should be straightforward: ultimately, parasite biomass must be controlled by resource supply, which is a direct function of host metabolism. Using comparative data sets on the biomass of metazoan parasites in vertebrate hosts, we determined how parasite biomass scales with host body mass. If the rate at which host resources are converted into parasite biomass is the same as that at which host resources are channelled toward host growth, then on a log-log plot parasite biomass should increase with host mass with a slope of 0.75 when corrected for operating temperature. Average parasite biomass per host scaled with host body mass at a lower rate than expected (across 131 vertebrate species, slope=0.54); this was true independently of phylogenetic influences and also within the major vertebrate groups separately. Since most host individuals in a population harbour a parasite load well below that allowed by their metabolic rate, because of the stochastic nature of infection, it is maximum parasite biomass, and not average biomass, that is predicted to scale with metabolic rate among host species. We found that maximum parasite biomass scaled isometrically (i.e., slope=1) with host body mass. Thus, larger host species can potentially support the same parasite biomass per gram of host tissues as small host species. The relationship found between maximum parasite biomass and host body mass, with its slope greater than 0.75, suggests that parasites are not like host tissues: they are able to appropriate more host resources than expected from metabolically derived host growth rates.