Patterns of gregarine parasitism in dragonflies: host, habitat, and seasonality

Sexual and developmental variations of ecto-parasitism in damselflies

The prevalence and intensity of parasitism can have different fitness costs between sexes, and across species and developmental stages. This variation could arise because of species specific sexual and developmental differences in body condition, immunity, and resistance. Theory predicts that the prevalence of parasitism will be greater in individuals with poor body condition and the intensity of parasitism will be greater in individuals with larger body size. These predictions have been tested and verified in vertebrates. In insects, however, contradictory evidence has been found in different taxa. Here, we tested these predictions on two species of Agriocnemis (Agriocnemis femina and Agriocnemis pygmaea) damselflies, which are parasitized by Arrenurus water mite ectoparasites. We measured body weight, total body length, abdomen area and thorax area of non-parasitized damselflies and found body condition varied between males and females, between immature females and mature females and between A. femina and A. pygmaea. Then, we calculated the parasite prevalence, i.e., the absence or presence of parasites and intensity, i.e., the number of parasites per infected damselfly in eleven natural populations of both species. In line to our predictions, we observed greater prevalence in immature females than mature females but found no difference in parasite prevalence between males and females. Furthermore, we found that parasite intensity was higher in females than males and in immature females than mature females. Our result also showed that the frequency and intensity of parasitism varied between the two studied species, being higher in A. pygmaea than A. femina. Our study provides evidence that parasitism impacts sexes, developmental stages and species differentially and suggests that variation may occur due to sex, developmental stage, and species-specific resistance and tolerance mechanism.

Fluctuating Starvation Conditions Modify Host-Symbiont Relationship Between a Leaf Beetle and Its Newly Identified Gregarine Species

Gregarines are ubiquitous endosymbionts in invertebrates, including terrestrial insects. However, the biodiversity of gregarines is probably vastly underestimated and the knowledge about their role in shaping fitness-related traits of their host in dependence of fluctuating environmental conditions is limited. Using morphological and molecular analyses, we identified a new gregarine species, Gregarina cochlearium sp. n., in the mustard leaf beetle, Phaedon cochleariae. Applying a full-factorial design, we investigated the effects of a gregarine infection in combination with fluctuating starvation conditions during the larval stage on the development time and fitness-related traits of adult beetles. Under benign environmental conditions, the relationship between gregarines and the host seemed neutral, as host development, body mass, reproduction and survival were not altered by a gregarine infection. However, when additionally exposed to starvation, the combination of gregarine infection and this stress resulted in the lowest reproduction and survival of the host, which points to a parasitic relationship. Furthermore, when the host experienced starvation, the development time was prolonged and the adult females were lighter compared to non-starved individuals, independent of the presence of gregarines. Counting of gregarines in the guts of larvae revealed a lower gregarine load with increasing host body mass under stable food conditions, which indicates a regulation of the gregarine burden in dependence of the host condition. Contrary, in starved individuals the number of gregarines was the highest, hence the already weakened host suffered additionally from a higher gregarine burden. This interactive effect between gregarine infection and fluctuating starvation conditions led to an overall reduced fitness of P. cochleariae. Our study emphasizes the need to study endosymbionts as important components of the natural environment and to investigate the role of host-symbiont relationships under fluctuating environmental conditions in an evolutionary and ecological context.

Gregarines modulate insect responses to sublethal insecticide residues

Throughout their lifetime, insects face multiple environmental challenges that influence their performance. Gregarines are prevalent endoparasites in most invertebrates that affect the fitness of their hosts, but are often overlooked in ecological studies. Next to such biotic factors, a current common challenge is anthropogenic pollution with pesticides, which causes a major threat to non-target organisms that are readily exposed to lethal or sublethal concentrations. In a laboratory study, we investigated whether the presence of gregarines modulates the food consumption and life history traits of a (non-target) leaf beetle species, Phaedon cochleariae, in response to sublethal insecticide exposure. We show that the larval food consumption of the herbivore was neither affected by gregarine infection nor sublethal insecticide exposure. Nevertheless, infection with gregarines led to a delayed development, while insecticide exposure resulted in a lower body mass of adult males and a reduced reproduction of females. Individuals exposed to both challenges suffered most, as they had the lowest survival probability. This indicates detrimental effects on the population dynamics of non-target insects infected with naturally occurring gregarines that face additional stress from agrochemical pollution. Moreover, we found that the infection load with gregarines was higher in individuals exposed to sublethal insecticide concentrations compared to unexposed individuals. To counteract the global decline of insects, the potential of natural parasite infections in modulating insect responses to anthropogenic and non-anthropogenic environmental factors should be considered in ecological risk assessment.

Parasite richness and abundance within aquatic macroinvertebrates: testing the roles of host- and habitat-level factors

The importance of parasites as both members of biological communities and as structuring agents of host communities has been increasingly emphasized. Yet parasites of aquatic macroinvertebrates and the environmental factors regulating their richness and abundance remain poorly studied. Here we quantified parasite richness and abundance within 12 genera of odonate naiads and opportunistically sampled four additional orders of aquatic macroinvertebrates from 35 freshwater ponds in the San Francisco Bay Area of California, USA. We also tested the relative contributions of host- and habitat-level factors in driving patterns of infection abundance for the most commonly encountered parasite (the trematode Haematoloechus sp.) in nymphal damselflies and dragonflies using hierarchical generalized linear mixed models. Over the course of two years, we quantified the presence and intensity of parasites from 1,612 individuals. We identified six parasite taxa: two digenetic trematodes, one larval nematode, one larval acanthocephalan, one gregarine, and a mite, for which the highest infection prevalence (39%) occurred in the damselfly genus, Ishnura sp. Based on the hierarchical analysis of Haematoloechus sp. occurrence, infection prevalence and abundance were associated predominantly with site-level factors, including definitive host (frog) presence, nymphal odonate density, water pH and conductivity. In addition, host suborder interacted with the presence of fishes, such that damselflies had higher infection rates in sites with fish relative to those without, whereas the opposite was true for dragonfly nymphs. These findings offer insights into the potential interaction between host- and site-level factors in shaping parasite populations within macroinvertebrate taxa.

Parasitism, immune response, and egg production of the spearhead bluet (Coenagrion hastulatum) damselfly

Theoretical models predict that parasites reduce reproductive success of their hosts, but very few empirical studies have given support to this. Using the spearhead bluet (Coenagrion hastulatum (Charpentier, 1825)) damselfly, we tested how immune response, wing length, and the number of both endo- and ecto-parasites affect egg production of host damselflies. The study was conducted with four different populations in southwest Finland. We found a negative association between endoparasitic gregarines and number of host eggs. Furthermore, immune response increased with the number of water mites, but decreased with the number of eggs. Contrary to previous studies with other damselfly species, the number of ectoparasitic water mites did not affect the number of eggs. Moreover, wing length, used as an indicator of individual size, was not associated with egg numbers. The negative effect of gregarine parasites on egg numbers is likely to affect the composition of host populations, i.e., damselflies that show higher resistance to these endoparasites will have more of their offspring represented in subsequent generations. In future, more experimental research on the varying effects of different parasite species on the number of eggs is needed.

Temporal Community Structure in Two Gregarines (Rotundula gammari and Heliospora longissima) Co-Infecting the Amphipod Gammarus fasciatus

This study surveyed gregarine parasites that infect the amphipod, Gammarus fasciatus , to investigate temporal dynamics in infracommunity structure. We sampled a population of hosts for 2 yr from the north branch of the Raritan River in New Jersey. These hosts were infected with 2 direct life cycle gregarine parasites, Rotundula gammari and Heliospora longissima. Infections were separated temporally, with the prevalence of R. gammari peaking within the amphipod population in the fall (prevalence = 78% year 1 and 97% year 2) and H. longissima peaking in early spring (prevalence = 41% year 1 and 52% year 2). Increases in host population density did not significantly correlate with the abundance of these 2 parasites. However, H. longissima abundance was positively correlated with host body weight while R. gammari showed no significant relationship. The mean body mass of amphipods infected with H. longissima was 20.7 ± 1. 2 mg, and with R. gammari 8.1 ± 0.2 mg, which suggests a sized-based infection pattern. Mixed species infections were infrequent with an overall prevalence of 4.6%. When both gregarine species co-infected the same host, the R. gammari but not the H. longissima infrapopulation size was significantly lower when compared to single-species infections, suggesting asymmetric interactions. We conclude that the observed temporal patterns of infection by the 2 parasites are driven by a seasonal change in host demographics and size-dependent infections. We argue that specificity for host developmental stages may have arisen as a mechanism to avoid overlap between these gregarine species.

Fungal Infection Induces Sex-Specific Transcriptional Changes and Alters Sexual Dimorphism in the Dioecious Plant Silene latifolia

Sexual dimorphism, including differences in morphology, behavior and physiology between females and males, is widespread in animals and plants and is shaped by gene expression differences between the sexes. Such expression differences may also underlie sex-specific responses of hosts to pathogen infections, most notably when pathogens induce partial sex reversal in infected hosts. The genetic changes associated with sex-specific responses to pathogen infections on the one hand, and sexual dimorphism on the other hand, remain poorly understood. The dioecious White Campion (Silene latifolia) displays sexual dimorphism in floral traits and infection with the smut fungus Micobrotryum lychnidis-dioicae induces a partial sex reversal in females. We find strong sex-specific responses to pathogen infection and reduced sexual dimorphism in infected S. latifolia. This provides a direct link between pathogen-mediated changes in sex-biased gene expression and altered sexual dimorphism in the host. Expression changes following infection affected mainly genes with male-biased expression in healthy plants. In females, these genes were up-regulated, leading to a masculinization of the transcriptome. In contrast, infection in males was associated with down-regulation of these genes, leading to a demasculinization of the transcriptome. To a lesser extent, genes with female-biased expression in healthy plants were also affected in opposite directions in the two sexes. These genes were overall down-regulated in females and up-regulated in males, causing, respectively, a defeminization in infected females and a feminization of the transcriptome in infected males. Our results reveal strong sex-specific responses to pathogen infection in a dioecious plant and provide a link between pathogen-induced changes in sex-biased gene expression and sexual dimorphism.

Females and males differ from each other in many traits, including morphology, behavior and physiology. Differences in gene expression between the sexes, known as sex-biased gene expression, contribute to such sexual dimorphism. Here we characterize the responses of females and males of the dioecious plant Silene latifolia to infection with the anther smut fungus Micobrotryum lychnidis-dioicae. This fungus sterilizes the plant and induces a partial sex reversal in female hosts that form rudimentary stamens, thus allowing the fungus to transmit its spores via pollinators. Our comparisons of gene expression in healthy and infected plants reveal strong sex-specific responses to anther smut infection. Expression changes in females and males are in opposite directions and are associated with reduced sexual dimorphism between infected females and males. Our study reveals that infection with the anther smut fungus alters the extent of sex-biased gene expression in S. latifolia in a sex-specific manner and highlights how transcriptomic changes in females and males shape sexual dimorphism.

Effect of gregarines (Apicomplexa: Sporozoa) on survival and weight loss of Victorwithius similis (Arachnida: Pseudoscorpiones)

Gregarines are common intestinal parasites of numerous invertebrate groups. Their effects on host viability and development have been a matter of debate. Although they may not be lethal to the host, they can be harmless commensals, by affecting adaptive traits, or have a beneficial relationship with the host. This study focused on determining prevalence, intensity, and change in infection intensity over time by septate gregarines, and monitoring the effects on survival and weight loss in the pseudoscorpion Victorwithius similis. Individuals (n=24 females, n=55 males and n=41 tritonymphs) were captured in the field, transported to the vivarium and bred under laboratory conditions. A high prevalence of infection was found, with 77.27% of females, 62.50% of males and 73.53% of tritonymphs harboring intense infections. Of the infected pseudoscorpions, 62% of females, 58% of males and 71% of tritonymphs did not show changes in infection intensity over time. The group that maintained intense infections survived longer than those with less intense infections (χ(2)=8.642; p=0.035). Most of the results obtained indicate that relationship studied between gregarines and the pseudoscorpion V. similis might be a case of commensalism. This would explain why the infection level and prevalence was very high, as well as the apparent lack of direct costs to highly infected individuals those with infections.

Host sexual dimorphism and parasite adaptation

Disease expression and prevalence often vary in the different sexes of the host. This is typically attributed to innate differences of the two sexes but specific adaptations by the parasite to one or other host sex may also contribute to these observations.

In species with separate sexes, parasite prevalence and disease expression is often different between males and females. This effect has mainly been attributed to sex differences in host traits, such as immune response. Here, we make the case for how properties of the parasites themselves can also matter. Specifically, we suggest that differences between host sexes in many different traits, such as morphology and hormone levels, can impose selection on parasites. This selection can eventually lead to parasite adaptations specific to the host sex more commonly encountered, or to differential expression of parasite traits depending on which host sex they find themselves in. Parasites adapted to the sex of the host in this way can contribute to differences between males and females in disease prevalence and expression. Considering those possibilities can help shed light on host–parasite interactions, and impact epidemiological and medical science.

Biogeographical survey identifies consistent alternative physiological optima and a minor role for environmental drivers in maintaining a polymorphism

The contribution of adaptive mechanisms in maintaining genetic polymorphisms is still debated in many systems. To understand the contribution of selective factors in maintaining polymorphism, we investigated large-scale (>1000 km) geographic variation in morph frequencies and fitness-related physiological traits in the damselfly Nehalennia irene. As fitness-related physiological traits, we investigated investment in immune function (phenoloxidase activity), energy storage and fecundity (abdomen protein and lipid content), and flight muscles (thorax protein content). In the first part of the study, our aim was to identify selective agents maintaining the large-scale spatial variation in morph frequencies. Morph frequencies varied considerably among populations, but, in contrast to expectation, in a geographically unstructured way. Furthermore, frequencies co-varied only weakly with the numerous investigated ecological parameters. This suggests that spatial frequency patterns are driven by stochastic processes, or alternatively, are consequence of highly variable and currently unidentified ecological conditions. In line with this, the investigated ecological parameters did not affect the fitness-related physiological traits differently in both morphs. In the second part of the study, we aimed at identifying trade-offs between fitness-related physiological traits that may contribute to the local maintenance of both colour morphs by defining alternative phenotypic optima, and test the spatial consistency of such trade-off patterns. The female morph with higher levels of phenoloxidase activity had a lower thorax protein content, and vice versa, suggesting a trade-off between investments in immune function and in flight muscles. This physiological trade-off was consistent across the geographical scale studied and supports widespread correlational selection, possibly driven by male harassment, favouring alternative trait combinations in both female morphs.