The trans-generational impact of population density signals on host-parasite interactions

Background

The density of a host population is a key parameter underlying disease transmission, but it also has implications for the expression of disease through its effect on host physiology. In response to higher densities, individuals are predicted to either increase their immune investment in response to the elevated risk of parasitism, or conversely to decrease their immune capacity as a consequence of the stress of a crowded environment. However, an individual’s health is shaped by many different factors, including their genetic background, current environmental conditions, and maternal effects. Indeed, population density is often sensed through the presence of info-chemicals in the environment, which may influence a host’s interaction with parasites, and also those of its offspring. All of which may alter the expression of disease, and potentially uncouple the presumed link between changes in host density and disease outcomes.

Results

In this study, we used the water flea Daphnia magna and its obligate bacterial parasite Pasteuria ramosa, to investigate how signals of high host density impact on host-parasite interactions over two consecutive generations. We found that the chemical signals from crowded treatments induced phenotypic changes in both the parental and offspring generations. In the absence of a pathogen, life-history changes were genotype-specific, but consistent across generations, even when the signal of density was removed. In contrast, the influence of density on infected animals depended on the trait and generation of exposure. When directly exposed to signals of high-density, host genotypes responded differently in how they minimised the severity of disease. Yet, in the subsequent generation, the influence of density was rarely genotype-specific and instead related to ability of the host to minimise the onset of infection.

Conclusion

Our findings reveal that population level correlations between host density and infection capture only part of the complex relationship between crowding and the severity of disease. We suggest that besides its role in horizontal transmission, signals of density can influence parasite epidemiology by modifying mechanisms of resistance across multiple generations, and elevating variability via genotype-by-environment interactions. Our results help resolve why some studies are able to find a positive correlation between high density and resistance, while others uncover a negative correlation, or even no direct relationship at all.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0828-4) contains supplementary material, which is available to authorized users.

The effect of methylphenidate intake on brain structure in adults with ADHD in a placebo-controlled randomized trial

BACKGROUND

Based on animal research several authors have warned that the application of methylphenidate, the first-line drug for the treatment of attention-deficit/hyperactivity disorder (ADHD), might have neurotoxic effects potentially harming the brain. We investigated whether methylphenidate application, over a 1-year period, results in cerebral volume decrease.

METHODS

We acquired structural MRIs in a double-blind study comparing methylphenidate to placebo. Global and regional brain volumes were analyzed at baseline, after 3 months and after 12 months using diffeomorphic anatomic registration through exponentiated lie algebra.

RESULTS

We included 131 adult patients with ADHD into the baseline sample, 98 into the 3-month sample (54 in the methylphenidate cohort and 44 in the placebo cohort) and 76 into the 1-year sample (37 in the methylphenidate cohort and 29 in the placebo cohort). Methylphenidate intake compared with placebo did not lead to any detectable cerebral volume loss; there was a trend toward bilateral cerebellar grey matter increase.

LIMITATIONS

Detecting possible neurotoxic effects of methylphenidate might require a longer observation period.

CONCLUSION

There is no evidence of grey matter volume loss after 1 year of methylphenidate treatment in adult patients with ADHD.

A high-density genetic map reveals variation in recombination rate across the genome of Daphnia magna

BACKGROUND

Recombination rate is an essential parameter for many genetic analyses. Recombination rates are highly variable across species, populations, individuals and different genomic regions. Due to the profound influence that recombination can have on intraspecific diversity and interspecific divergence, characterization of recombination rate variation emerges as a key resource for population genomic studies and emphasises the importance of high-density genetic maps as tools for studying genome biology. Here we present such a high-density genetic map for Daphnia magna, and analyse patterns of recombination rate across the genome.

RESULTS

A F2 intercross panel was genotyped by Restriction-site Associated DNA sequencing to construct the third-generation linkage map of D. magna. The resulting high-density map included 4037 markers covering 813 scaffolds and contigs that sum up to 77 % of the currently available genome draft sequence (v2.4) and 55 % of the estimated genome size (238 Mb). Total genetic length of the map presented here is 1614.5 cM and the genome-wide recombination rate is estimated to 6.78 cM/Mb. Merging genetic and physical information we consistently found that recombination rate estimates are high towards the peripheral parts of the chromosomes, while chromosome centres, harbouring centromeres in D. magna, show very low recombination rate estimates.

CONCLUSIONS

Due to its high-density, the third-generation linkage map for D. magna can be coupled with the draft genome assembly, providing an essential tool for genome investigation in this model organism. Thus, our linkage map can be used for the on-going improvements of the genome assembly, but more importantly, it has enabled us to characterize variation in recombination rate across the genome of D. magna for the first time. These new insights can provide a valuable assistance in future studies of the genome evolution, mapping of quantitative traits and population genetic studies.

Ecological genetics of sediment browsing behaviour in a planktonic crustacean

Zooplankton can display complex habitat selection behaviours that influence the way they interact with their environments. Some species, although primarily pelagic, can exploit sediment-borne particles as a food source or use sediments as a refuge from pelagic predation. However, this strategy may increase the exposure to other risks such as benthic predation and infection from sediment-borne parasite transmission stages. The evolution of habitat selection behaviour in these species is thus expected to be influenced by multiple and possibly contrasting selective forces. Here, we study the browsing behaviour of the water flea Daphnia magna on bottom sediments. First, we demonstrated genetic variation for sediment browsing among D. magna genotypes from natural populations sampled across a broad geographic range. Next, we used an F2 recombinant panel to perform a QTL analysis and identified three regions in the D. magna genome contributing to variation in browsing behaviour. We also analysed the correlation between our data and previously published data on the phototactic behaviour of genotypes from the same F2 panel. Clonal means of the two behavioral traits were not correlated, suggesting that they may evolve independently. Browsing behaviour is likely to be a relevant component of habitat selection in D. magna, and its study may help to incorporate the interactions with the sediment into eco-evolutionary models of this key freshwater species.

Infections by Pasteuria do not protect its natural host Daphnia magna from subsequent infections

The existence of immunological memory in invertebrates remains a contentious topic. Exposure of Daphnia magna crustaceans to a noninfectious dose of the bacterium Pasteuria ramosa has been reported to reduce the chance of future infection upon exposure to higher doses. Using clonal hosts and parasites, we tested whether initial exposure of the host to the parasite (priming), followed by clearing of the parasite with antibiotic, protects the host from a second exposure (challenge). Our experiments included three treatments: priming and challenge with the same or with a different parasite clone, or no priming. Two independent experiments showed that both the likelihood of infection and the degree of parasite proliferation did not differ between treatments, supporting the conclusion that there is no immunological memory in this system. We discuss the possibility that previous discordant reports could result from immune or stress responses that did not fade following initial priming.

Host-parasite Red Queen dynamics with phase-locked rare genotypes

Interactions between hosts and parasites have been hypothesized to cause winnerless coevolution, called Red Queen dynamics. The canonical Red Queen dynamics assume that all interacting genotypes of hosts and parasites undergo cyclic changes in abundance through negative frequency-dependent selection, which means that any genotype could become frequent at some stage. However, this prediction cannot explain why many rare genotypes stay rare in natural host-parasite systems. To investigate this, we build a mathematical model involving multihost and multiparasite genotypes. In a deterministic and controlled environment, Red Queen dynamics occur between two genotypes undergoing cyclic dominance changes, whereas the rest of the genotypes remain subordinate for long periods of time in phase-locked synchronized dynamics with low amplitude. However, introduction of stochastic noise in the model might allow the subordinate cyclic host and parasite types to replace dominant cyclic types as new players in the Red Queen dynamics. The factors that influence such evolutionary switching are interhost competition, specificity of parasitism, and degree of stochastic noise. Our model can explain, for the first time, the persistence of rare, hardly cycling genotypes in populations (for example, marine microbial communities) undergoing host-parasite coevolution.

Temperature-dependent benefits of bacterial exposure in embryonic development of Daphnia magna resting eggs

The environments in which animals develop and evolve are profoundly shaped by bacteria, which affect animals both indirectly through their roles in biogeochemical processes and also through direct antagonistic or beneficial interactions. The outcomes of these activities can differ according to environmental context. In a series of laboratory experiments with diapausing eggs of the water flea Daphnia magna, we manipulated two environmental parameters, temperature and presence of bacteria, and examined their effect on development. At elevated temperatures (≥26 °C), resting eggs developing without live bacteria had reduced hatching success and correspondingly higher rates of severe morphological abnormalities compared to eggs with bacteria in their environment. The beneficial effect of bacteria was strongly reduced at 20 °C. Neither temperature nor presence of bacteria affected directly developing parthenogenetic eggs. The mechanistic basis of this effect of bacteria on development is unclear, but these results highlight the complex interplay of biotic and abiotic factors influencing animal development after diapause.

A Population Biology Perspective on the Stepwise Infection Process of the Bacterial Pathogen Pasteuria ramosa in Daphnia

The infection process of many diseases can be divided into series of steps, each one required to successfully complete the parasite's life and transmission cycle. This approach often reveals that the complex phenomenon of infection is composed of a series of more simple mechanisms. Here we demonstrate that a population biology approach, which takes into consideration the natural genetic and environmental variation at each step, can greatly aid our understanding of the evolutionary processes shaping disease traits. We focus in this review on the biology of the bacterial parasite Pasteuria ramosa and its aquatic crustacean host Daphnia, a model system for the evolutionary ecology of infectious disease. Our analysis reveals tremendous differences in the degree to which the environment, host genetics, parasite genetics and their interactions contribute to the expression of disease traits at each of seven different steps. This allows us to predict which steps may respond most readily to selection and which steps are evolutionarily constrained by an absence of variation. We show that the ability of Pasteuria to attach to the host's cuticle (attachment step) stands out as being strongly influenced by the interaction of host and parasite genotypes, but not by environmental factors, making it the prime candidate for coevolutionary interactions. Furthermore, the stepwise approach helps us understanding the evolution of resistance, virulence and host ranges. The population biological approach introduced here is a versatile tool that can be easily transferred to other systems of infectious disease.

Expression of parasite genetic variation changes over the course of infection: implications of within-host dynamics for the evolution of virulence

How infectious disease agents interact with their host changes during the course of infection and can alter the expression of disease-related traits. Yet by measuring parasite life-history traits at one or few moments during infection, studies have overlooked the impact of variable parasite growth trajectories on disease evolution. Here we show that infection-age-specific estimates of host and parasite fitness components can reveal new insight into the evolution of parasites. We do so by characterizing the within-host dynamics over an entire infection period for five genotypes of the castrating bacterial parasite Pasteuria ramosa infecting the crustacean Daphnia magna. Our results reveal that genetic variation for parasite-induced gigantism, host castration and parasite spore loads increases with the age of infection. Driving these patterns appears to be variation in how well the parasite maintains control of host reproduction late in the infection process. We discuss the evolutionary consequences of this finding with regard to natural selection acting on different ages of infection and the mechanism underlying the maintenance of castration efficiency. Our results highlight how elucidating within-host dynamics can shed light on the selective forces that shape infection strategies and the evolution of virulence.

Increased hippocampal volumes in adults with high functioning autism spectrum disorder and an IQ>100: A manual morphometric study

Previous studies concerning the volumes of the amygdala and the hippocampus in autism spectrum disorders (ASD) show inconsistent results. We acquired magnetic resonance images of 30 individuals with ASD and individually matched controls. All participants had an IQ>100 to increase the likelihood of including non-syndromal forms of ASD. Manually defined amygdala volumes showed no significant group difference, while hippocampi were significantly enlarged in ASD. This finding is discussed with regard to the 'intense world hypothesis'.

Normal Neurochemistry in the Prefrontal and Cerebellar Brain of Adults with Attention-Deficit Hyperactivity Disorder

Attention-deficit hyperactivity disorder (ADHD) is a common neurodevelopmental disorder. In an attempt to extend earlier neurochemical findings, we organized a magnetic resonance spectroscopy (MRS) study as part of a large, government-funded, prospective, randomized, multicenter clinical trial comparing the effectiveness of specific psychotherapy with counseling and stimulant treatment with placebo treatment (Comparison of Methylphenidate and Psychotherapy Study). We report the baseline neurochemical data for the anterior cingulate cortex (ACC) and the cerebellum in a case–control setting. For the trial, 1,480 adult patients were contacted for participation, 518 were assessed for eligibility, 433 were randomized, and 187 were potentially eligible for neuroimaging. The control group included 119 healthy volunteers. Single-voxel proton MRS was performed. In the patient group, 113 ACC and 104 cerebellar spectra fulfilled all quality criteria for inclusion in statistical calculations, as did 82 ACC and 78 cerebellar spectra in the control group. We did not find any significant neurometabolic differences between the ADHD and control group in the ACC (Wilks’ lambda test: p = 0.97) or in the cerebellum (p = 0.62). Thus, we were unable to replicate earlier findings in this methodologically sophisticated study. We discuss our findings in the context of a comprehensive review of other MRS studies on ADHD and a somewhat skeptical neuropsychiatric research perspective. As in other neuropsychiatric disorders, the unclear nosological status of ADHD might be an explanation for false-negative findings.

Red Queen dynamics in multi-host and multi-parasite interaction system

In host-parasite systems, dominant host types are expected to be eventually replaced by other hosts due to the elevated potency of their specific parasites. This leads to changes in the abundance of both hosts and parasites exhibiting cycles of alternating dominance called Red Queen dynamics. Host-parasite models with less than three hosts and parasites have been demonstrated to exhibit Red Queen cycles, but natural host-parasite interactions typically involve many host and parasite types resulting in an intractable system with many parameters. Here we present numerical simulations of Red Queen dynamics with more than ten hosts and specialist parasites under the condition of no super-host nor super-parasite. The parameter region where the Red Queen cycles arise contracts as the number of interacting host and parasite types increases. The interplay between inter-host competition and parasite infectivity influences the condition for the Red Queen dynamics. Relatively large host carrying capacity and intermediate rates of parasite mortality result in never-ending cycles of dominant types.

[Well Educated Unemployed--On Education, Employment and Comorbidities in Adults with High-Functioning Autism Spectrum Disorders in Germany]

BACKGROUND

Based on clinical experience there is a discrepancy between the educational records and vocational performance in patients with high functioning autism spectrum disorder (ASD).

METHOD

In order to assess psychosocial and vocational specificities of adult ASD patients we analyzed the demographic and hospital data of consecutively diagnosed patients employing descriptive statistics.

RESULTS

We were able to include 255 patients into our sample who were consecutively diagnosed between October 2009 and October 2011. The gender ratio was 162:93 in favor of male patients. The educational records of our patient sample was comparatively good [50 % highest level of German schooling system (allgemeine Hochschulreife), 39 % university degree], however, the vocational records were poor with 58 % of our patients being unemployed. The psychiatric comorbidity was high, 57 % of the patients suffered from depression.

CONCLUSION

There is a high need for special support programs for adult high functioning ASD patients focusing on adaptive vocational skills to avoid unemployment and secondary psychiatric problems.

Elevated background noise in adult attention deficit hyperactivity disorder is associated with inattention

Background

Inattention and distractibility are core symptoms of attention deficit hyperactivity disorder (ADHD). Still the neuronal organization is largely unknown. Previously we studied the electrophysiological activity of a distinct neuronal network—the retina—and found no change in stimulus-driven neural activity in patients with ADHD. However there is growing evidence for an elevated non stimulus-driven neural activity, or neuronal background noise, as underlying pathophysiological correlate. To further examine the biological bases that might underlie ADHD and problems with inattention, we performed a new analysis to test the hypothesis of an elevated background noise as underlying neuronal correlate for ADHD and problems with inattention in humans. A direct measure of background noise in patients with ADHD has not been described yet.

Methods

The retinal background noise was assessed based on pattern electroretinogram (PERG) data in 20 unmedicated ADHD patients and 20 healthy controls. The PERG is an electrophysiological measure for retinal ganglion cell function. ADHD severity was assessed by interview and questionnaire.

Results

Noise amplitude was significantly higher (138%) in patients with ADHD compared to the control group (p = 0.0047). Noise amplitude correlated significantly with psychometric measures for ADHD (CAARS) especially inattention (r = 0.44, p = 0.004).

Conclusions

The data provide evidence that an elevated background noise is associated with symptoms of inattention in ADHD and support the use of therapeutic interventions that reduce noise and distraction in patients with ADHD.

Freiburg Questionnaire of linguistic pragmatics (FQLP): psychometric properties based on a psychiatric sample

BACKGROUND

Asperger's Syndrome (AS) is an autism spectrum disorder that is characterized by significant difficulties in social interaction and nonverbal communication, and restricted and repetitive patterns of behavior and interests. Difficulties with respect to pragmatic speech, reading emotional and social cues, differentiating between fact and fiction, and taking into account the influence of context on a statement are commonly described features. However, hitherto established questionnaires did not focus on these symptoms.

METHODS

In this study we present a short (11 questions) questionnaire which focuses on self-rated pragmatic speech abilities, the Freiburg Questionnaire of linguistic pragmatics (FQLP). Psychometric properties of the questionnaire were explored in a sample of 57 patients with Asperger's Syndrome, 66 patients with other psychiatric disorders, and a convenience sample of 56 people.

RESULTS

Reliability analysis showed a high Cronbach's α. Strong correlations could be demonstrated for the FQLP with the Autism Quotient and the Empathy Quotient. Concerning divergent validity a moderate correlation was found between the FQLP and self-rated symptoms of personality disorders. No significant correlation was found between the FQLP and the vocabulary skills. The receiver operating characteristics curve showed an excellent diagnostic accuracy of the FQLP (.97).

CONCLUSIONS

As the control group consisted of people without mental disorder and patients with different psychiatric disorders, the results indicate that the construct examined by the FQLP is quite specific to the peculiarities of AS. The FQLP is a reliable, brief and valid instrument. First results regarding sensitivity and specificity are highly promising.