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Dieter Ebert

Publications and source records attributed to Dieter Ebert.

At least 19 recordsLinked to original sources
PubMed2006

The coexistence of hybrid and parental Daphnia: the role of parasites.

Parasite driven time-lagged negative frequency-dependent selection of hosts has been studied in natural populations by following changes in host genotype frequencies over time. However, such dynamics have not been considered at higher taxonomic levels, for example, between parental species and their hybrids. In a field study on a Daphnia hybrid system, we observed that one Daphnia taxon first was relatively under-infected, but became over-infected after a strong increase in frequency. This finding is consistent with the idea of parasite evolution towards the most frequent host taxon. In two experiments, we investigated whether the assumptions made by a model of negative frequency-dependent selection apply to our host taxa system. First, we showed that the parasite can change the outcome of taxa competition and secondly, we confirmed that the over-infection of one host taxon observed in the field has a genetic basis. Our results indicate that the incorporation of host-parasite interactions at the species level may allow us to gain a more complete picture of forces driving dynamic taxa coexistence in Daphnia hybrid systems. More generally, we suggest that if hybrids coexist in sympatry with parental taxa, the infection patterns as observed under natural conditions may be rather temporal and unstable.

Animals↗
PubMed2006

Persistence of host and parasite populations subject to experimental size-selective removal.

Predators have the potential to limit the spread of pathogens not only by selecting infected prey but also by shaping prey demographics. We tested this idea with an epidemiological experiment in which we simulated variable levels of size-selective predation on zooplankton hosts and monitored the persistence of host and parasite populations. In the absence of simulated predation, the virulent protozoan Caullerya mesnili frequently drove its host Daphnia galeata to extinction. Uninfected control populations showed lower extinction rates and higher average densities than infected populations in the absence of simulated predation (all of the latter went extinct or remained infected). With a weak removal rate of the largest hosts, the proportion of populations in which the parasite drove the host to extinction decreased, while the number of populations in which the host persisted and the parasite went extinct increased. Host-parasite coexistence was also observed in some cases. With intermediate levels of removal, most of the parasite populations went extinct, while the host populations persisted. With an even higher removal rate, Daphnia were driven to extinction as well. Thus, variation in one factor, size-selective mortality, resulted in four different patterns of population dynamics. Our results highlight the potential role of predation in shaping the epidemiology and community structure of host-parasite systems.

Animals↗
PubMed2006

Apparent seasonality of parasite dynamics: analysis of cyclic prevalence patterns.

Seasonal disease dynamics are common in nature, but their causes are often unknown. Our case study provides insight into the cyclic prevalence pattern of the horizontally and vertically transmitted microsporidium Octosporea bayeri in its Daphnia magna host. Data from several populations over a four year period revealed a regular prevalence increase during summer and a decrease over winter when hosts underwent diapause. Prevalence also decreased after summer diapause indicating that the decline is causally linked to diapause rather than to winter conditions. Experiments showed that host diapause itself can explain a certain proportion of the decline. The decline further depends on the environmental conditions during diapause: infected resting eggs suffered from higher mortality under experimental winter than under experimental summer diapause conditions. Investigating the mechanisms of prevalence increase after diapause, the parasite was found to survive winter outside its host, enabling horizontal infection of susceptible hosts in the following growing season. Allowing for horizontal transmission in experimental host populations resulted in a steep prevalence increase, while excluding it led to a pronounced decline. Thus, the apparent seasonality in O. bayeri prevalence is characterized by a decline during host diapause followed by horizontal spread of the parasite during the host's asexual growth phase.

Animals↗
PubMed2006

Brain activation during craving for alcohol measured by positron emission tomography.

OBJECTIVE: Craving for alcohol is probably involved in acquisition and maintenance of alcohol dependence to a substantial degree. However, the brain substrates and mechanisms that underlie alcohol craving await more detailed elucidation. METHOD: Positron emission tomography was used to map regional cerebral blood flow (CBF) in 21 detoxified patients with alcohol dependence during exposure to alcoholic and non-alcoholic beverages. RESULTS: During the alcohol condition compared with the control condition, significantly increased CBF was found in the ventral putamen. Additionally, activated areas included insula, dorsolateral prefrontal cortex and cerebellum. Cerebral blood flow increase in these regions was related to self-reports of craving assessed in the alcoholic patients. CONCLUSIONS: In this investigation, cue-induced alcohol craving was associated with activation of brain regions particularly involved in brain reward mechanisms, memory and attentional processes. These results are consistent with studies on craving for other addictive substances and may offer strategies for more elaborate studies on the neurobiology of addiction.

Adult↗
PubMed2006

Empirical support for optimal virulence in a castrating parasite.

The trade-off hypothesis for the evolution of virulence predicts that parasite transmission stage production and host exploitation are balanced such that lifetime transmission success (LTS) is maximised. However, the experimental evidence for this prediction is weak, mainly because LTS, which indicates parasite fitness, has been difficult to measure. For castrating parasites, this simple model has been modified to take into account that parasites convert host reproductive resources into transmission stages. Parasites that kill the host too early will hardly benefit from these resources, while postponing the killing of the host results in diminished returns. As predicted from optimality models, a parasite inducing castration should therefore castrate early, but show intermediate levels of virulence, where virulence is measured as time to host killing. We studied virulence in an experimental system where a bacterial parasite castrates its host and produces spores that are not released until after host death. This permits estimating the LTS of the parasite, which can then be related to its virulence. We exposed replicate individual Daphnia magna (Crustacea) of one host clone to the same amount of bacterial spores and followed individuals until their death. We found that the parasite shows strong variation in the time to kill its host and that transmission stage production peaks at an intermediate level of virulence. A further experiment tested for the genetic basis of variation in virulence by comparing survival curves of daphniids infected with parasite spores obtained from early killing versus late killing infections. Hosts infected with early killer spores had a significantly higher death rate as compared to those infected with late killers, indicating that variation in time to death was at least in part caused by genetic differences among parasites. We speculate that the clear peak in lifetime reproductive success at intermediate killing times may be caused by the exceptionally strong physiological trade-off between host and parasite reproduction. This is the first experimental study to demonstrate that the production of propagules is highest at intermediate levels of virulence and that parasite genetic variability is available to drive the evolution of virulence in this system.

Animals↗
PubMed2006

Parasite-host specificity: experimental studies on the basis of parasite adaptation.

Specificity in parasitic interactions can be defined by host genotypes that are resistant to only a subset of parasite genotypes and parasite genotypes that are infective on a subset of host genotypes. It is not always clear if specificity is determined by the genotypes of the interactors, or if phenotypic plasticity (sometimes called acclimation) plays a larger role. Coevolutionary outcomes critically depend on the pervasiveness of genetic interactions. We studied specificity using the bacterial parasite Pasteuria ramosa and its crustacean host Daphnia magna. First, we tested for short-term adaptation of P. ramosa lines that had been rapidly shifted among different host genotypes. Adaptation at this time-scale would demonstrate the contribution of phenotypic plasticity to specificity. We found that infectivity was stable across lines irrespective of recent passage history, indicating that in the short term infection outcomes are fixed by genetic backgrounds. Second, we studied longer-term evolution with two host clones and two parasite lines. In this experiment, P. ramosa lines had the possibility to evolve adaptations to the host genotype (clone) in which they were serially passaged, which allowed us to test for a genetic component to specificity. Substantial differences arose in the two passaged lines: one parasite line gained infectivity on the host clone it was grown on, but it lost infectivity on the other host genotype (this line evolved specificity), while the other parasite line evolved higher infectivity on both host clones. We crossed the two host genotypes used in the serial passage experiment and found evidence that the number of host genes that underlies resistance variation is small. In sum, our results show that P. ramosa specificity is a stably inherited trait, it can evolve rapidly, and it is controlled by few genes in the host. These findings are consistent with the idea of a rapid, ongoing arms race between the bacterium and its host.

Adaptation, Physiological↗
PubMed2005

Ecological implications of parasites in natural Daphnia populations.

In natural host populations, parasitism is considered to be omnipresent and to play an important role in shaping host life history and population dynamics. Here, we study parasitism in natural populations of the zooplankton host Daphnia magna investigating their individual and population level effects during a 2-year field study. Our results revealed a rich and highly prevalent community of parasites, with eight endoparasite species (four microsporidia, one amoeba, two bacteria and one nematode) and six epibionts (belonging to five different taxa: Chlorophyta, Bacillariophyceae, Ciliata, Fungi and Rotifera). Several of the endoparasites were associated with a severe overall fecundity reduction of the hosts, while such effects were not seen for epibionts. In particular, infections by Pasteuria ramosa, White Fat Cell Disease and Flabelliforma magnivora were strongly associated with a reduction in overall D. magna fecundity. Across the sampling period, average population fecundity of D. magna was negatively associated with overall infection intensity and total endoparasite richness. Population density of D. magna was negatively correlated to overall endoparasite prevalence and positively correlated with epibiont richness. Finally, the reduction in host fecundity caused by different parasite species was negatively correlated to both parasite prevalence and the length of the time period during which the parasite persisted in the host population. Consistent with epidemiological models, these results indicate that parasite mediated host damages influence the population dynamics of both hosts and parasites.

Animals↗
PubMed2005

Increased prefrontal and hippocampal glutamate concentration in schizophrenia: evidence from a magnetic resonance spectroscopy study.

BACKGROUND: Glutamatergic dysfunction has been implicated in the pathophysiology of schizophrenia. However, so far there is limited direct evidence of altered in vivo glutamate concentrations in the brains of patients with schizophrenia. To test the hypothesis that altered glutamatergic neurotransmission might play a role in the pathogenesis of schizophrenia, we measured glutamate and glutamine concentrations in the prefrontal cortex and the hippocampus of patients with chronic schizophrenia using high-field magnetic resonance spectroscopy. METHODS: Twenty-one patients with schizophrenia and 32 healthy volunteers were examined clinically and by means of short echo time single voxel magnetic resonance spectroscopy of the dorsolateral prefrontal cortex and the hippocampus. Absolute concentrations of neurometabolites were calculated. RESULTS: Absolute concentrations of glutamate were significantly higher in the prefrontal cortex and the hippocampus in the patient group. Factorial analysis of variance (ANOVA) revealed no significant interactions between duration of schizophrenia, number of hospitalizations, or type of antipsychotic medication and glutamate concentrations. Increased prefrontal glutamate concentrations were associated with poorer global mental functioning. CONCLUSIONS: This is the first study that reports increased levels of glutamate in prefrontal and limbic areas in patients with schizophrenia. Our data support the hypothesis of glutamatergic dysfunction in schizophrenia.

Adult↗
PubMed2005

Genetic diversity and genetic differentiation in Daphnia metapopulations with subpopulations of known age.

If colonization of empty habitat patches causes genetic bottlenecks, freshly founded, young populations should be genetically less diverse than older ones that may have experienced successive rounds of immigration. This can be studied in metapopulations with subpopulations of known age. We studied allozyme variation in metapopulations of two species of water fleas (Daphnia) in the skerry archipelago of southern Finland. These populations have been monitored since 1982. Screening 49 populations of D. longispina and 77 populations of D. magna, separated by distances of 1.5-2180 m, we found that local genetic diversity increased with population age whereas pairwise differentiation among pools decreased with population age. These patterns persisted even after controlling for several potentially confounding ecological variables, indicating that extinction and recolonization dynamics decrease local genetic diversity and increase genetic differentiation in these metapopulations by causing genetic bottlenecks during colonization. We suggest that the effect of these bottlenecks may be twofold, namely decreasing genetic diversity by random sampling and leading to population-wide inbreeding. Subsequent immigration then may not only introduce new genetic material, but also lead to the production of noninbred hybrids, selection for which may cause immigrant alleles to increase in frequency, thus leading to increased genetic diversity in older populations.

Aging↗
PubMed2005

Sleep in adults with attention-deficit/hyperactivity disorder: a controlled polysomnographic study including spectral analysis of the sleep EEG.

STUDY OBJECTIVES: Previous studies in children with attention-deficit/hyperactivity disorder and attention deficit disorder (ADHD/ADD) have shown impaired sleep quality with increased nocturnal motor activity. However, polysomnographic findings are not unequivocal. Up to now, in adults with ADHD, only 1 case-control polysomnographic study with small sample size has been performed. We investigated objective and subjective sleep quality in adultADHD, including an electroencephalogram spectral power analysis. DESIGN: Single-blind comparative study. SETTING: University medical center. PARTICIPANTS: Twenty adult unmedicated ADHD patients without current comorbid major depression, drug abuse, or comorbid axis-II disorder and 20 sex- and age-matched control subjects. INTERVENTIONS: N/A. MEASUREMENTS: Conventional polysomnographic parameters and sleep electroencephalogram spectral power analysis was calculated for the 2 laboratory nights. Subjective sleep parameters were estimated by sleep questionnaires to assess the relationship between objective and subjective sleep measurements. RESULTS: Adult ADHD patients showed increased nocturnal motor activity (as indicated by heightened indexes of periodic leg movements in sleep), which was significantly inversely correlated with subjective total sleep time. Although ADHD patients displayed significantly increased objective total sleep time, the subjective ratings documented impaired sleep quality in those with ADHD. Other polysomnographic sleep patterns and spectral electroencephalogram parameters did not differ between ADHD patients and normal controls. CONCLUSIONS: Similar to children, adults with ADHD show increased nocturnal motor activity. Otherwise, sleep does not seem to be impaired in ADHD patients. However, the dissociation between objective and subjective sleep parameters points to a misinterpretation of sleep quality in patients with ADHD.

Adult↗
PubMed2004

Octosporea bayeri: fumidil B inhibits vertical transmission in Daphnia magna.

Microsporidia are a highly successful and ecologically diverse group of parasites, and thus represent interesting model systems for research on host-parasite interactions. However, such research often requires the ability to cure hosts of infections, a difficult task, given the short lifespan of most invertebrates and the efficient vertical transmission of some parasites. To our knowledge, few treatments are available to cure microsporidiosis in invertebrate hosts, and protocols have not yet been developed to inhibit vertical transmission and thereby cure host lines. We present a protocol for inhibiting vertical transmission of the microsporidian parasite Octosporea bayeri in the freshwater crustacean Daphnia magna. We used 100 mg/L Fumidil B dissolved in the culture medium of the host. This technique allowed Daphnia to survive and reproduce and inhibited vertical transmission of the parasite. The method presented here may be of general interest for other aquatic host-parasite systems involving microsporidia.

Animals↗
PubMed2004

Mixed inoculations of a microsporidian parasite with horizontal and vertical infections.

Mixed infections, where more than one parasite genotype is present in a single host, have been suggested to be an important factor in host-parasite interactions. As the host represents a limited resource, co-infecting parasite genotypes are expected to be under resource competition. Competition will not only modify the survival of the co-infecting genotypes, but is also likely to affect total within-host parasite growth as well as host survival and reproduction. We measured parasite infectivity and spore production of seven isolates of Octosporea bayeri (Microsporidia) and their effect on the reproduction and longevity of its host Daphnia magna (Cladocera), after single- or double-isolate inoculations through vertical and horizontal transmission. Double-isolate inoculations increased parasite infectivity and total spore production in horizontal infections, but had no significant effect on host reproduction or longevity. The increase in spore production in double-isolate inoculations was not found when infections occurred vertically. Our results suggest that, depending on the way the infection was acquired, within-host reproduction can increase as a result of parasite genetic diversity, without necessarily affecting the host's fitness. Whether this challenges the current views of virulence evolution depends on the definitions used.

Animals↗
PubMed2004

Parasite-mediated selection in experimental metapopulations of Daphnia magna.

In metapopulations, only a fraction of all local host populations may be infected with a given parasite species, and limited dispersal of parasites suggests that colonization of host populations by parasites may involve only a small number of parasite strains. Using hosts and parasites obtained from a natural metapopulation, we studied the evolutionary consequences of invasion by single strains of parasites in experimental populations of the cyclical parthenogen Daphnia magna. In two experiments, each spanning approximately one season, we monitored clone frequency changes in outdoor container populations consisting of 13 and 19 D. magna clones, respectively. The populations were either infected with single strains of the microsporidian parasites Octosporea bayeri or Ordospora colligata or left unparasitized. In both experiments, infection changed the representation of clones over time significantly, indicating parasite-mediated evolution in the experimental populations. Furthermore, the two parasite species changed clone frequencies differently, suggesting that the interaction between infection and competitive ability of the hosts was specific to the parasite species. Taken together, our results suggest that parasite strains that invade local host populations can lead to evolutionary changes in the genetic composition of the host population and that this change is parasite-species specific.

Animals↗
PubMed2004

Variation in phenoloxidase activity and its relation to parasite resistance within and between populations of Daphnia magna.

Estimates of phenoloxidase (PO) activity have been suggested as a useful indicator of immunocompetence in arthropods, with the idea that high PO activity would indicate high immunocompetence against parasites and pathogens. Here, we test for variation in PO activity among clones of the planktonic crustacean Daphnia magna and its covariation with susceptibility to infections from four different microparasite species (one bacterium and three microsporidia). Strong clonal variation in PO activity was found within and among populations of D. magna, with 45.6% of the total variation being explained by the clone effect. Quantitative measures of parasite success in infection correlated negatively with PO activity when tested across four host populations. However, these correlations disappeared when the data were corrected for population effects. We conclude that PO activity is not a useful measure of resistance to parasites or of immunocompetence within populations of D. magna. We further tested whether D. magna females that are wounded to induce PO activity are more resistant to infections with the bacterium Pasteuria ramosa than non-wounded controls. We found neither a difference in susceptibility nor a difference in disease progression between the induced group and the control group. These results do not question the function of the PO system in arthropod immune response, but rather suggest that immunocompetence cannot be assessed by considering PO activity alone. Immune response is likely to be a multifactorial trait with various host and parasite characteristics playing important roles in its expression.

Animals↗
PubMed2004

Covariation of mitochondrial genome size with gene lengths: evidence for gene length reduction during mitochondrial evolution.

Reduction of genome size and gene shortening have been observed in a number of parasitic and mutualistic intracellular symbionts. Reduction of coding capacity is also a unifying principle in the evolutionary history of mitochondria, but little is known about the evolution of gene length in mitochondria. The genes for cytochrome c oxidase subunits I-III, cytochrome b, and the large and small subunit rRNAs are, with very few exceptions, always found on the mitochondrial genome. These resident mitochondrial genes can therefore be used to test whether the reduction in gene lengths observed in a number of intracellular symbionts is also seen in mitochondria. Here we show that resident mitochondrial gene products are shorter than their corresponding counterparts in alpha-proteobacteria and, furthermore, that the reduction of mitochondrial genome size is correlated with a reduction in the length of the corresponding resident gene products. We show that relative genomic AT content, which has been identified as a factor influencing gene lengths in other systems, cannot explain gene length/genome size covariance observed in mitochondria. Our data are therefore in agreement with the idea that gene length evolves as a consequence of selection for smaller genomes, either to avoid accumulation of deleterious mutations or triggered by selection for a replication advantage.

Bacteria↗
PubMed2004

The evolution of virulence when parasites cause host castration and gigantism.

It has been suggested that the harm parasites cause to their hosts is an unavoidable consequence of parasite reproduction with costs not only for the host but also for the parasite. Castrating parasites are thought to minimize their costs by reducing host fecundity, which may minimize the chances of killing both host and parasite prematurely. We conducted a series of experiments to understand the evolution of virulence of a castrating bacterium in the planktonic crustacean Daphnia magna. By manipulating food levels during the infection of D. magna with the bacterium Pasteuria ramosa, we showed that both antagonists are resource-limited and that a negative correlation between host and parasite reproduction exists, indicating resource competition among the antagonists. Pasteuria ramosa also induces enhanced growth of its hosts (gigantism), which we found to be negatively correlated with host fecundity but positively correlated with parasite reproduction. Because infected hosts never recovered from infections, we concluded that gigantism is beneficial only for the parasite. Hosts, however, have evolved counteradaptations. We showed that infected hosts have enhanced reproduction before castration. This shift to earlier reproduction increases overall host fecundity and compromises parasite reproduction. Finally, we showed that this resource conflict is subject to genetic variation among host and parasite genotypes within a population and is therefore likely to be an important force in the coevolution of virulence in this system. A verbal model is presented and suggests that the adaptive value of gigantism is to store host resources, which are liberated after parasitic castration for later use by the growing parasite. This hypothesis assumes that infections are long lasting, that is, that they have a high life expectancy.

Adaptation, Physiological↗