Different mechanisms of transmission of the microsporidium Octosporea bayeri: a cocktail of solutions for the problem of parasite permanence

Coinfection frequency in water flea populations is a mere reflection of parasite diversity

In nature, parasite species often coinfect the same host. Yet, it is not clear what drives the natural dynamics of coinfection prevalence. The prevalence of coinfections might be affected by interactions among coinfecting species, or simply derive from parasite diversity. Identifying the relative impact of these parameters is crucial for understanding patterns of coinfections. We studied the occurrence and likelihood of coinfections in natural populations of water fleas (Daphnia magna). Coinfection prevalence was within the bounds expected by chance and parasite diversity had a strong positive effect on the likelihood of coinfections. Additionally, coinfection prevalence increased over the season and became as common as a single infection. Our results demonstrate how patterns of coinfection, and particularly their temporal variation, are affected by overlapping epidemics of different parasites. We suggest that monitoring parasite diversity can help predict where and when coinfection prevalence will be high, potentially leading to increased health risks to their hosts.

Increased virulence due to multiple infection in Daphnia leads to limited growth in 1 of 2 co-infecting microsporidian parasites

Recent outbreaks of various infectious diseases have highlighted the ever-present need to understand the drivers of the outbreak and spread of disease. Although much of the research investigating diseases focuses on single infections, natural systems are dominated by multiple infections. These infections may occur simultaneously, but are often acquired sequentially, which may alter the outcome of infection. Using waterfleas (Daphnia magna) as a model organism, we examined the outcome of sequential and simultaneous multiple infections with 2 microsporidian parasites (Ordospora colligata and Hamiltosporidium tvaerminnensis) in a fully factorial design with 9 treatments and 30 replicates. We found no differences between simultaneous and sequential infections. However, H. tvaerminnensis fitness was impeded by multiple infection due to increased host mortality, which gave H. tvaerminnensis less time to grow. Host fecundity was also reduced across all treatments, but animals infected with O. colligata at a younger age produced the fewest offspring. As H. tvaerminnensis is both horizontally and vertically transmitted, this reduction in offspring may have further reduced H. tvaerminnensis fitness in co-infected treatments. Our findings suggest that in natural populations where both species co-occur, H. tvaerminnensis may evolve to higher levels of virulence following frequent co-infection by O. colligata.

Possible seasonal and diurnal modulation of Gammarus pulex (Crustacea, Amphipoda) drift by microsporidian parasites

In lotic freshwater ecosystems, the drift or downstream movement of animals (e.g., macroinvertebrates) constitutes a key dispersal pathway, thus shaping ecological and evolutionary patterns. There is evidence that macroinvertebrate drift may be modulated by parasites. However, most studies on parasite modulation of host drifting behavior have focused on acanthocephalans, whereas other parasites, such as microsporidians, have been largely neglected. This study provides new insight into possible seasonal and diurnal modulation of amphipod (Crustacea: Gammaridae) drift by microsporidian parasites. Three 72 h drift experiments were deployed in a German lowland stream in October 2021, April, and July 2022. The prevalence and composition of ten microsporidian parasites in Gammarus pulex clade E varied seasonally, diurnally, and between drifting and stationary specimens of G. pulex. Prevalence was generally higher in drifting amphipods than in stationary ones, mainly due to differences in host size. However, for two parasites, the prevalence in drift samples was highest during daytime suggesting changes in host phototaxis likely related to the parasite's mode of transmission and site of infection. Alterations in drifting behavior may have important implications for G. pulex population dynamics and microsporidians' dispersal. The underlying mechanisms are more complex than previously thought.

Bacteria-phage coevolution with a seed bank

Dormancy is an adaptation to living in fluctuating environments. It allows individuals to enter a reversible state of reduced metabolic activity when challenged by unfavorable conditions. Dormancy can also influence species interactions by providing organisms with a refuge from predators and parasites. Here we test the hypothesis that, by generating a seed bank of protected individuals, dormancy can modify the patterns and processes of antagonistic coevolution. We conducted a factorially designed experiment where we passaged a bacterial host (Bacillus subtilis) and its phage (SPO1) in the presence versus absence of a seed bank consisting of dormant endospores. Owing in part to the inability of phages to attach to spores, seed banks stabilized population dynamics and resulted in minimum host densities that were 30-fold higher compared to bacteria that were unable to engage in dormancy. By supplying a refuge to phage-sensitive strains, we show that seed banks retained phenotypic diversity that was otherwise lost to selection. Dormancy also stored genetic diversity. After characterizing allelic variation with pooled population sequencing, we found that seed banks retained twice as many host genes with mutations, whether phages were present or not. Based on mutational trajectories over the course of the experiment, we demonstrate that seed banks can dampen bacteria-phage coevolution. Not only does dormancy create structure and memory that buffers populations against environmental fluctuations, it also modifies species interactions in ways that can feed back onto the eco-evolutionary dynamics of microbial communities.

The parasites of my rival are my friends

The competitive exclusion principle asserts that two species cannot stably coexist in the same habitat. However, the presence of a parasite can facilitate temporary coexistence between two host species occupying the same habitat. Studies of parasite-mediated interspecific competition typically use two host species that are both susceptible to a single parasite species, as it is rare to find a resistant host species that requires a parasite to enable coexistence with a competitively superior susceptible host. We therefore investigated how two host species characterized by different susceptibility profiles affect each other when they coexist in the same habitat, by conducting two long-term mesocosm experiments in the laboratory. We followed populations of Daphnia similis coexisting with Daphnia magna, in either the presence or absence of the microsporidium Hamiltosporidium tvaerminnensis and then the bacterium Pasteuria ramosa. We found that in the absence of parasites, D. magna competitively excluded D. similis within a short period of time. However, in the presence of either parasites, the competitive ability of D. magna decreased dramatically. Our results emphasize the importance of parasites in shaping community structure and composition, by allowing coexistence of a resistant host species that would otherwise become extinct.

Infection phenotypes of a coevolving parasite are highly diverse, structured, and specific

Understanding how diversity is maintained in natural populations is a major goal of evolutionary biology. In coevolving hosts and parasites, negative frequency-dependent selection is one mechanism predicted to maintain genetic variation. While much is known about host diversity, parasite diversity remains understudied in coevolutionary research. Here, we survey natural diversity in a bacterial parasite by characterizing infection phenotypes for over 50 isolates in relation to 12 genotypes of their host, Daphnia magna. We find striking phenotypic variation among parasite isolates, and we discover the parasite can infect its host through at least five different attachment sites. Variation in attachment success at each site is explained to varying degrees by host and parasite genotypes. A spatial correlation analysis showed that infectivity of different isolates does not correlate with geographic distance, meaning isolates from widespread populations are equally able to infect the host. Overall, our results reveal that infection phenotypes of this parasite are highly diverse. Our results are consistent with the prediction that under Red Queen coevolutionary dynamics both the host and the parasite should show high genetic diversity for traits of functional importance in their interactions.

An alternative route of bacterial infection associated with a novel resistance locus in the Daphnia-Pasteuria host-parasite system

To understand the mechanisms of antagonistic coevolution, it is crucial to identify the genetics of parasite resistance. In the Daphnia magna–Pasteuria ramosa host–parasite system, the most important step of the infection process is the one in which P. ramosa spores attach to the host’s foregut. A matching-allele model (MAM) describes the host–parasite genetic interactions underlying attachment success. Here we describe a new P. ramosa genotype, P15, which, unlike previously studied genotypes, attaches to the host’s hindgut, not to its foregut. Host resistance to P15 attachment shows great diversity across natural populations. In contrast to P. ramosa genotypes that use foregut attachment, P15 shows some quantitative variation in attachment success and does not always lead to successful infections, suggesting that hindgut attachment represents a less-efficient infection mechanism than foregut attachment. Using a Quantitative Trait Locus (QTL) approach, we detect two significant QTLs in the host genome: one that co-localizes with the previously described D. magna PR locus of resistance to foregut attachment, and a second, major QTL located in an unlinked genomic region. We find no evidence of epistasis. Fine mapping reveals a genomic region, the D locus, of ~13 kb. The discovery of a second P. ramosa attachment site and of a novel host-resistance locus increases the complexity of this system, with implications for both for the coevolutionary dynamics (e.g., Red Queen and the role of recombination), and for the evolution and epidemiology of the infection process.

Parasite-mediated selection in a natural metapopulation of Daphnia magna

Parasite-mediated selection varying across time and space in metapopulations is expected to result in host local adaptation and the maintenance of genetic diversity in disease-related traits. However, nonadaptive processes like migration and extinction-(re)colonization dynamics might interfere with adaptive evolution. Understanding how adaptive and nonadaptive processes interact to shape genetic variability in life-history and disease-related traits can provide important insights into their evolution in subdivided populations. Here we investigate signatures of spatially fluctuating, parasite-mediated selection in a natural metapopulation of Daphnia magna. Host genotypes from infected and uninfected populations were genotyped at microsatellite markers, and phenotyped for life-history and disease traits in common garden experiments. Combining phenotypic and genotypic data a Q_ST -F_ST -like analysis was conducted to test for signatures of parasite mediated selection. We observed high variation within and among populations for phenotypic traits, but neither an indication of host local adaptation nor a cost of resistance. Infected populations have a higher gene diversity (Hs) than uninfected populations and Hs is strongly positively correlated with fitness. These results suggest a strong parasite effect on reducing population level inbreeding. We discuss how stochastic processes related to frequent extinction-(re)colonization dynamics as well as host and parasite migration impede the evolution of resistance in the infected populations. We suggest that the genetic and phenotypic patterns of variation are a product of dynamic changes in the host gene pool caused by the interaction of colonization bottlenecks, inbreeding, immigration, hybrid vigor, rare host genotype advantage and parasitism. Our study highlights the effect of the parasite in ameliorating the negative fitness consequences caused by the high drift load in this metapopulation.

The role of spore morphology in horizontal transmission of a microsporidium of Daphnia

The microsporidian parasite Hamiltosporidium tvaerminnensis can infect Daphnia magna both horizontally (through environmental spores) and vertically (through parthenogenetic and sexually produced eggs). The spores of H. tvaerminnensis come in three distinguishable morphologies, which are thought to have different roles in the transmission of the parasite. In this study, we examined the role of the two most common spore morphologies (i.e. oval-shaped spores and pear-shaped spores) in horizontal transmission of H. tvaerminnensis. To this end, we infected hosts with solutions consisting of either mostly oval- or mostly pear-shaped spores, and quantified infection rates, parasite-induced host mortality and mean number of parasite spores produced per host. We found that spore morphology by itself did not influence infection rates and parasite-induced host mortality. Instead, host clone and parasite isolate interacted with spore morphology in shaping infection outcome and mortality. Thus, there appear to be strong genotype-by-genotype (G × G) interactions in this system. While there is no dispute that H. tvaerminnensis can transmit both vertically and horizontally, our findings do not support theoretical predictions that different spore morphologies hold different roles in horizontal transmission of H. tvaerminnensis.

QTL mapping of a natural genetic polymorphism for long-term parasite persistence in Daphnia populations

Knowing the determinants of the geographic ranges of parasites is important for understanding their evolutionary ecology, epidemiology and their potential to expand their range. Here we explore the determinants of geographic range in the peculiar case of a parasite species – the microsporidian Hamiltosporidium tvaerminnensis – that has a limited geographic distribution in a wide-spread host – Daphnia magna. We conducted a quantitative trait loci (QTLs) analysis with monoclonal F2D. magna populations originating from a cross between a susceptible northern European genotype and a resistant central European genotype. Contrary to our expectations, long-term persistence turned out to be a quantitative trait across the F2 offspring. Evidence for two QTLs, one epistatic interaction and for further minor QTL was found. This finding contrasts markedly with the previously described bimodal pattern for long-term parasite persistence in natural host genotypes across Europe and leaves open the question of how a quantitative genetic trait could determine the disjunct geographic distribution of the parasite across Europe.

Host Diet Affects the Morphology of Monarch Butterfly Parasites

Understanding host-parasite interactions is essential for ecological research, wildlife conservation, and health management. While most studies focus on numerical traits of parasite groups, such as changes in parasite load, less focus is placed on the traits of individual parasites such as parasite size and shape (parasite morphology). Parasite morphology has significant effects on parasite fitness such as initial colonization of hosts, avoidance of host immune defenses, and the availability of resources for parasite replication. As such, understanding factors that affect parasite morphology is important in predicting the consequences of host-parasite interactions. Here, we studied how host diet affected the spore morphology of a protozoan parasite ( Ophryocystis elektroscirrha ), a specialist parasite of the monarch butterfly ( Danaus plexippus ). We found that different host plant species (milkweeds; Asclepias spp.) significantly affected parasite spore size. Previous studies have found that cardenolides, secondary chemicals in host plants of monarchs, can reduce parasite loads and increase the lifespan of infected butterflies. Adding to this benefit of high cardenolide milkweeds, we found that infected monarchs reared on milkweeds of higher cardenolide concentrations yielded smaller parasites, a potentially hidden characteristic of cardenolides that may have important implications for monarch-parasite interactions.

Genetic architecture of resistance in Daphnia hosts against two species of host-specific parasites

Understanding the genetic architecture of host resistance is key for understanding the evolution of host-parasite interactions. Evolutionary models often assume simple genetics based on few loci and strong epistasis. It is unknown, however, whether these assumptions apply to natural populations. Using a quantitative trait loci (QTL) approach, we explore the genetic architecture of resistance in the crustacean Daphnia magna to two of its natural parasites: the horizontally transmitted bacterium Pasteuria ramosa and the horizontally and vertically transmitted microsporidium Hamiltosporidium tvaerminnensis. These two systems have become models for studies on the evolution of host-parasite interactions. In the QTL panel used here, Daphnia's resistance to P. ramosa is controlled by a single major QTL (which explains 50% of the observed variation). Resistance to H. tvaerminnensis horizontal infections shows a signature of a quantitative trait based in multiple loci with weak epistatic interactions (together explaining 38% variation). Resistance to H. tvaerminnensis vertical infections, however, shows only one QTL (explaining 13.5% variance) that colocalizes with one of the QTLs for horizontal infections. QTLs for resistance to Pasteuria and Hamiltosporidium do not colocalize. We conclude that the genetics of resistance in D. magna are drastically different for these two parasites. Furthermore, we infer that based on these and earlier results, the mechanisms of coevolution differ strongly for the two host-parasite systems. Only the Pasteuria-Daphnia system is expected to follow the negative frequency-dependent selection (Red Queen) model. How coevolution works in the Hamiltosporidium-Daphnia system remains unclear.

Exploiting the architecture and the features of the microsporidian genomes to investigate diversity and impact of these parasites on ecosystems

Fungal species play extremely important roles in ecosystems. Clustered at the base of the fungal kingdom are Microsporidia, a group of obligate intracellular eukaryotes infecting multiple animal lineages. Because of their large host spectrum and their implications in host population regulation, they influence food webs, and accordingly, ecosystem structure and function. Unfortunately, their ecological role is not well understood. Present also as highly resistant spores in the environment, their characterisation requires special attention. Different techniques based on direct isolation and/or molecular approaches can be considered to elucidate their role in the ecosystems, but integrating environmental and genomic data (for example, genome architecture, core genome, transcriptional and translational signals) is crucial to better understand the diversity and adaptive capacities of Microsporidia. Here, we review the current status of Microsporidia in trophic networks; the various genomics tools that could be used to ensure identification and evaluate diversity and abundance of these organisms; and how these tools could be used to explore the microsporidian life cycle in different environments. Our understanding of the evolution of these widespread parasites is currently impaired by limited sampling, and we have no doubt witnessed but a small subset of their diversity.

Water fleas require microbiota for survival, growth and reproduction

Microbiota have diverse roles in the functioning of their hosts; experiments using model organisms have enabled investigations into these functions. In the model crustacean Daphnia, little knowledge exists about the effect of microbiota on host well being. We assessed the effect of microbiota on Daphnia magna by experimentally depriving animals of their microbiota and comparing their growth, survival and fecundity to that of their bacteria-bearing counterparts. We tested Daphnia coming from both lab-reared parthenogenetic eggs of a single genotype and from genetically diverse field-collected resting eggs. We showed that bacteria-free hosts are smaller, less fecund and have higher mortality than those with microbiota. We also manipulated the association by exposing bacteria-free Daphnia to a single bacterial strain of Aeromonas sp., and to laboratory environmental bacteria. These experiments further demonstrated that the Daphnia-microbiota system is amenable to manipulation under various experimental conditions. The results of this study have implications for studies of D. magna in ecotoxicology, ecology and environmental genomics.

The expression of virulence for a mixed-mode transmitted parasite in a diapausing host

Many parasites survive harsh periods together with their hosts. Without the possibility of horizontal transmission during host diapause, parasite persistence depends entirely on host survival. We therefore hypothesize that a parasite should be avirulent during its host's diapausing stage. In contrast, the parasite may express higher virulence, i.e. parasite-induced fitness reduction of the host, during host life stages with good opportunities for horizontal transmission. Here we study the effects of a vertically and horizontally transmitted microsporidium parasite, Hamiltosporidium tvaerminnensis, on the quantity and survival of resting eggs of its host Daphnia magna. We find that the parasite did not affect egg volume, hatching success and time to hatching of the Daphnia's resting eggs, although it did strongly reduce the number of resting eggs produced by infected females, revealing high virulence during the non-diapause phase of the host's life cycle. These results also explain another aspect of this system - namely the strong decline in natural population prevalence across diapause. This decline is not caused by mortality in infected resting stages, as was previously hypothesized, but because infected female hosts produce lower rates of resting eggs. Together, these results help explain the epidemiological dynamics of a microsporidian disease and highlight the adaptive nature of life stage-dependent parasite virulence.

(Cryptic) sex in the microsporidian Nosema granulosis--evidence from parasite rDNA and host mitochondrial DNA

Microsporidia are single-celled, intracellular eukaryotes that parasitise a wide range of animals. The Nosema/Vairimorpha group includes some putative asexual species, and asexuality is proposed to have originated multiple times from sexual ancestors. Here, we studied the variation in the ribosomal DNA (rDNA) of 14 isolates of the presumed apomictic and vertically transmitted Nosema granulosis to evaluate its sexual status. The analysed DNA fragment contained a part of the small-subunit ribosomal gene (SSU) and the entire intergenic spacer (IGS). The mitochondrial cox1 gene of the host Gammarus duebeni (Crustacea) was analysed to temporally calibrate the system and to test the expectation of cophylogeny of host and parasite genealogies. Genetic variability of the SSU gene was very low within and between the isolates. In contrast, intraisolate (within a single host) variability of the IGS felt in two categories, because 12 isolates possess a very high IGS genetic diversity and two isolates were almost invariable in the IGS. This difference suggests variable models of rDNA evolution involving birth-and-death and unexpectedly concerted evolution. An alternative explanation could be a likewise unattended mixed infection of host individuals by more than one parasite strain. Despite considerable genetic divergence between associated host mitochondrial haplotypes, some N. granulosis 'IGS populations' seem not to belong to different gene pools; the relevant tests failed to show significant differences between populations. A set of recombinant IGS sequences made our data incompatible with the model of a solely maternally inherited, asexual species. In line with recent reports, our study supports the hypothesis that some assumed apomictic Microsporidia did not entirely abstain from the evolutionary advantages of sex. In addition, the presented data indicate that horizontal transmission may occur occasionally. This transmission mode could be a survival strategy of N. granulosis whose host often populates ephemeral habitats.

Microsatellite and single-nucleotide polymorphisms indicate recurrent transitions to asexuality in a microsporidian parasite

Assessing the mode of reproduction of microparasites remains a difficult task because direct evidence for sexual processes is often absent and the biological covariates of sex and asex are poorly known. Species with geographically divergent modes of reproduction offer the possibility to explore some of these covariates, for example, the influence of life-history traits, mode of transmission and life-cycle complexity. Here, we present a phylogeographical study of a microsporidian parasite, which allows us to relate population genetic structure and mode of reproduction to its geographically diverged life histories. We show that in microsporidians from the genus Hamiltosporidium, that use the cladoceran Daphnia as host, an epidemic population structure has evolved, most probably since the last Ice Age. We partially sequenced three housekeeping genes (alpha tubulin, beta tubulin and hsp70) and genotyped seven microsatellite loci in 51 Hamiltosporidium isolates sampled within Europe and the Middle East. We found two phylogenetically related asexual parasite lines, one each from Fennoscandia and Israel, which share the unique ability of being transmitted both vertically and horizontally from Daphnia to Daphnia. The sexual forms cannot transmit horizontally among Daphnia, but presumably have a complex life cycle with a second host species. In spite of the similarities between the two asexual lineages, a clustering analysis based on microsatellite polymorphisms shows that asexual Fennoscandian parasites do not share ancestry with any other Hamiltosporidium that we have sampled. Moreover, allele sequence divergence at the hsp70 locus is twice as large in Fennoscandian than in Israeli parasites. Our results indicate that asexual reproduction evolved twice independently, first in Fennoscandian and more recently in the Israeli parasites. We conclude that the independent origin of asexuality in these two populations is associated with the altered parasite mode of transmission and the underlying dynamics of host populations.

Converging seasonal prevalence dynamics in experimental epidemics

Background

Regular seasonal changes in prevalence of infectious diseases are often observed in nature, but the mechanisms are rarely understood. Empirical tests aiming at a better understanding of seasonal prevalence patterns are not feasible for most diseases and thus are widely lacking. Here, we set out to study experimentally the seasonal prevalence in an aquatic host-parasite system. The microsporidian parasite Hamiltosporidium tvärminnensis exhibits pronounced seasonality in natural rock pool populations of its host, Daphnia magna with a regular increase of prevalence during summer and a decrease during winter. An earlier study was, however, unable to test if different starting conditions (initial prevalence) influence the dynamics of the disease in the long term. Here, we aim at testing how the starting prevalence affects the regular prevalence changes over a 4-year period in experimental populations.

Results

In an outdoor experiment, populations were set up to include the extremes of the prevalence spectrum observed in natural populations: 5% initial prevalence mimicking a newly invading parasite, 100% mimicking a rock pool population founded by infected hosts only, and 50% prevalence which is commonly observed in natural populations in spring. The parasite exhibited similar prevalence changes in all treatments, but seasonal patterns in the 100% treatment differed significantly from those in the 5% and 50% treatments. Populations started with 5% and 50% prevalence exhibited strong and regular seasonality already in the first year. In contrast, the amplitude of changes in the 100% treatment was low throughout the experiment demonstrating the long-lasting effect of initial conditions on prevalence dynamics.

Conclusions

Our study shows that the time needed to approach the seasonal changes in prevalence depends strongly on the initial prevalence. Because individual D. magna populations in this rock pool metapopulation are mostly short lived, only few populations might ever reach a point where the initial conditions are not visible anymore.

The expression of virulence during double infections by different parasites with conflicting host exploitation and transmission strategies

In many natural populations, hosts are found to be infected by more than one parasite species. When these parasites have different host exploitation strategies and transmission modes, a conflict among them may arise. Such a conflict may reduce the success of both parasites, but could work to the benefit of the host. For example, the less-virulent parasite may protect the host against the more-virulent competitor. We examine this conflict using the waterflea Daphnia magna and two of its sympatric parasites: the blood-infecting bacterium Pasteuria ramosa that transmits horizontally and the intracellular microsporidium Octosporea bayeri that can concurrently transmit horizontally and vertically after infecting ovaries and fat tissues of the host. We quantified host and parasite fitness after exposing Daphnia to one or both parasites, both simultaneously and sequentially. Under conditions of strict horizontal transmission, Pasteuria competitively excluded Octosporea in both simultaneous and sequential double infections, regardless of the order of exposure. Host lifespan, host reproduction and parasite spore production in double infections resembled those of single infection by Pasteuria. When hosts became first vertically (transovarilly) infected with O. bayeri, Octosporea was able to withstand competition with P. ramosa to some degree, but both parasites produced less transmission stages than they did in single infections. At the same time, the host suffered from reduced fecundity and longevity. Our study demonstrates that even when competing parasite species utilize different host tissues to proliferate, double infections lead to the expression of higher virulence and ultimately may select for higher virulence. Furthermore, we found no evidence that the less-virulent and vertically transmitting O. bayeri protects its host against the highly virulent P. ramosa.

Cytological and molecular description of Hamiltosporidium tvaerminnensis gen. et sp. nov., a microsporidian parasite of Daphnia magna, and establishment of Hamiltosporidium magnivora comb. nov

We describe the new microsporidium Hamiltosporidium tvaerminnensis gen. et sp. nov. with an emphasis on its ultrastructural characteristics and phylogenetic position as inferred from the sequence data of SSU rDNA, alpha- and beta-tubulin. This parasite was previously identified as Octosporea bayeri Jírovec, 1936 and has become a model system to study the ecology, epidemiology, evolution and genomics of microsporidia - host interactions. Here, we present evidence that shows its differences from O. bayeri. Hamiltosporidium tvaerminnensis exclusively infects the adipose tissue, the ovaries and the hypodermis of Daphnia magna and is found only in host populations located in coastal rock pool populations in Finland and Sweden. Merogonial stages of H. tvaerminnensis have isolated nuclei; merozoites are formed by binary fission or by the cleaving of a plasmodium with a small number of nuclei. A sporogonial plasmodium with isolated nuclei yields 8 sporoblasts. Elongated spores are generated by the most finger-like plasmodia. The mature spores are polymorphic in shape and size. Most spores are pyriform (4·9–5·6×2·2–2·3 μm) and have their polar filament arranged in 12–13 coils. A second, elongated spore type (6·8–12·0×1·6–2·1 μm) is rod-shaped with blunt ends and measures 6·8–12·0×1·6–2·1 μm. The envelope of the sporophorous vesicle is thin and fragile, formed at the beginning of the sporogony. Cytological and molecular comparisons with Flabelliforma magnivora, a parasite infecting the same tissues in the same host species, reveal that these two species are very closely related, yet distinct. Moreover, both cytological and molecular data indicate that these species are quite distant from F. montana, the type species of the genus Flabelliforma. We therefore propose that F. magnivora also be placed in Hamiltosporidium gen. nov.

Phylogenetic position of Octosporea muscaedomesticae (Microsporidia) and its relationship to Octosporea bayeri based on small subunit rDNA analysis

Comparative phylogenetic analysis of the small subunit rDNA sequence of Octosporea muscaedomesticae (Flu, 1911) (type species) (Microsporidia) isolated from the blowfly Phormia regina (Diptera:Calliphoridae) is presented. Neighbor Joining bootstrap, Maximum Parsimony and Maximum Likelihood analyses with 38 microsporidian taxa representing five major clades of Microsporidia placed O. muscaedomesticae on a separate branch within a clade containing parasites of freshwater hosts. O. muscaedomesticae differed from Octosporea bayeri, a parasite of the microcrustacean, Daphnia magna (Cladocera:Daphniidae) by 29% demonstrating that the latter microsporidium is not closely related to the type species at the generic level, and should not be placed within the genus Octosporea, a conclusion that is further supported by morphological and developmental differences. Considering the number of disparately related hosts from which Octosporea species have been previously described based mostly on developmental and morphological characters it is likely that many will not fit the current definition of the genus, and it is possible that molecular analysis of these species will show that this genus as defined represents a polyphyletic grouping of unrelated taxa.

Draft genome sequence of the Daphnia pathogen Octosporea bayeri: insights into the gene content of a large microsporidian genome and a model for host-parasite interactions

BACKGROUND

The highly compacted 2.9-Mb genome of Encephalitozoon cuniculi placed the microsporidia in the spotlight, encoding a mere 2,000 proteins and a highly reduced suite of biochemical pathways. This extreme level of reduction is not universal across the microsporidia, with genomes known to vary up to sixfold in size, suggesting that some genomes may harbor a gene content that is not as reduced as that of Enc. cuniculi. In this study, we present an in-depth survey of the large genome of Octosporea bayeri, a pathogen of Daphnia magna, with an estimated genome size of 24 Mb, in order to shed light on the organization and content of a large microsporidian genome.

RESULTS

Using Illumina sequencing, 898 Mb of O. bayeri genome sequence was generated, resulting in 13.3 Mb of unique sequence. We annotated a total of 2,174 genes, of which 893 encodes proteins with assigned function. The gene density of the O. bayeri genome is very low on average, but also highly uneven, so gene-dense regions also occur. The data presented here suggest that the O. bayeri proteome is well represented in this analysis and is more complex that that of Enc. cuniculi. Functional annotation of O. bayeri proteins suggests that this species might be less biochemically dependent on its host for its metabolism than its more reduced relatives.

CONCLUSIONS

The combination of the data presented here, together with the imminent annotated genome of Daphnia magna, will provide a wealth of genetic and genomic tools to study host-parasite interactions in an interesting model for pathogenesis.

Expression of parasite virulence at different host population densities under natural conditions

It has recently been suggested that the expression of parasite virulence depends on host population density, such that infected hosts have a higher sensitivity to density, and thus reach their carrying capacity earlier than uninfected hosts. In this scenario, parasite-induced reduction in fitness (i.e., virulence) increases with host density. We tested this hypothesis experimentally, using outdoor mesocosm populations of Daphnia magna infected by the microsporidian Octosporea bayeri. Contrary to the prediction, virulence was independent of host density. In a competition experiment with initial prevalence of 50%, O. bayeri reduced the competitive ability of infected Daphnia within the asexual growth phase independent of initial host population density. In an additional experiment we set up populations with 100% and 0% prevalence and followed their population dynamics over the whole season. Consistent with the competition experiment, we found no difference in population dynamics within the asexual growth phase of the host, suggesting that infected hosts are not more sensitive to density than uninfected hosts. The additional experiment, however, included more than the initial growth phase as did the competition experiment. Eventually, after 100 days, 100% infected populations assumed a reduced carrying capacity compared to uninfected populations. We identify and discuss three reasons for the discrepancy between our experiment and the predictions.

Biology and life-cycle of the microsporidium Kneallhazia solenopsae Knell Allan Hazard 1977 gen. n., comb. n., from the fire ant Solenopsis invicta

Thelohania solenopsae is a unique microsporidium with a life-cycle finely tuned to parasitizing fire ant colonies. Unlike other microsporidia of social hymenopterans, T. solenopsae infects all castes and stages of the host. Four distinctive spore types are produced: diplokaryotic spores, which develop only in brood (Type 1 DK spores); octets of octospores within sporophorous vesicles, the most prominent spore type in adults but never occurring in brood; Nosema-like diplokaryotic spores (Type 2 DK spores) developing in adults; and megaspores, which occur occasionally in larvae 4, pupae, and adults of all castes but predominantly infect gonads of alates and germinate in inseminated ovaries of queens. Type 2 DK spores function in autoinfection of adipocytes. Proliferation of diplokaryotic meronts in some cells is followed by karyogamy of diplokarya counterparts and meiosis, thereby switching the diplokaryotic sequence to octospore or megaspore development. Megaspores transmit the pathogen transovarially. From the egg to larvae 4, infection is inapparent and can be detected only by PCR. Type 1 DK spore and megaspore sequences are abruptly triggered in larvae 4, the key stage in intra-colony food distribution via trophallaxis, and presumably the central player in horizontal transmission of spores. Molecular, morphological, ultrastructural and life-cycle data indicate that T. solenopsae must be assigned to a new genus. We propose a new combination, Kneallhazia solenopsae.

Genetic diversity of Daphnia magna populations enhances resistance to parasites

The diversity-disease hypothesis states that decreased genetic diversity in host populations increases the incidence of diseases caused by pathogens (= monoculture effect) and eventually influences ecosystem functioning. The monoculture effect is well-known from crop studies and may be partially specific to the artificial situation in agriculture. The effect received little attention in animal populations of different diversities. Compared with plants, animals are mobile and exhibiting social interactions. We followed the spread of a microsporidian parasite in semi-natural outdoor Daphnia magna populations of low and high genetic diversity. We used randomly selected, naturally occurring host genotypes. Host populations of low diversity were initially monoclonal, while the host populations of high diversity started with 10 genotypes per replicate. We found that the parasite spread significantly better in host populations of low diversity compared with host populations of high diversity, independent of parasite diversity. The difference was visible over a 3-year period. Host genotypic diversity did not affect host population density. Our experiment demonstrated a monoculture effect in independently replicated semi-natural zooplankton populations, indicating that the monoculture effect may be relevant beyond agriculture.

Phylogenetic characterization and prevalence of "Spirobacillus cienkowskii," a red-pigmented, spiral-shaped bacterial pathogen of freshwater Daphnia species

Microscopic examination of the hemolymph from diseased daphniids in 17 lakes in southwestern Michigan and five rock pools in southern Finland revealed the presence of tightly coiled bacteria that bore striking similarities to the drawings of a morphologically unique pathogen, "Spirobacillus cienkowskii," first described by Elya Metchnikoff more than 100 years ago. The uncultivated microbe was identified as a deeply branching member of the Deltaproteobacteria through phylogenetic analyses of two conserved genes: the 16S rRNA-encoding gene (rrs) and the beta-subunit of topoisomerase (gyrB). Fluorescence in situ hybridization confirmed that the rRNA gene sequence originated from bacteria with the tightly coiled morphology. Microscopy and PCR amplification with pathogen-specific primers confirmed infections by this bacterium in four species of Daphnia: Daphnia dentifera, D. magna, D. pulicaria, and D. retrocurva. Extensive field surveys reveal that this bacterium is widespread geographically and able to infect many different cladoceran species. In a survey of populations of D. dentifera in lakes in Michigan, we found the bacterium in 17 of 18 populations studied. In these populations, 0 to 12% of the individuals were infected, with an average of 3% during mid-summer and early autumn. Infections were less common in rock pool populations of D. magna in southern Finland, where the pathogen was found in 5 of 137 populations. The broad geographic distribution, wide host range, and high virulence of S. cienkowskii suggest it plays an important role in the ecology and evolution of daphniids.

Male-biased sex-ratio distortion caused by Octosporea bayeri, a vertically and horizontally-transmitted parasite of Daphnia magna

Female-biased sex-ratio distortion is often observed in hosts infected with vertically-transmitted microsporidian parasites. This bias is assumed to benefit the spread of the parasite, because male offspring usually do not transmit the parasite further. The present study reports on sex-ratio distortion in a host-parasite system with both horizontal and vertical parasite transmission: the microsporidium Octosporea bayeri and its host, the planktonic cladoceran Daphnia magna. In laboratory and field experiments, we found an overall higher proportion of male offspring in infected than in uninfected hosts. In young males, there was no parasite effect on sperm production, but, later in life, infected males produced significantly less sperm than uninfected controls. This shows that infected males are fertile. As males are unlikely to transmit the parasite vertically, an increase in male production could be advantageous to the host during phases of sexual reproduction, because infected mothers may obtain uninfected grandchildren through their sons. Life-table experiments showed that, overall, sons harboured more parasite spores than their sisters, although they reached a smaller body size and died earlier. Male production may thus be beneficial for the parasite when horizontal transmission has a large pay-off as males may contribute more effectively to parasite spread than females.

A short term benefit for outcrossing in a Daphnia metapopulation in relation to parasitism

Because host-parasite interactions are often specific to the host and parasite genotype, it may be important whether a host reproduces by selfing or outcrossing. The latter is associated with higher genetic diversity among the offspring and may reduce parasite success. Here, we test whether outbred offspring of Daphnia magna have an advantage over selfed offspring in the presence of a parasite transmitted from mothers to offspring. Using outdoor mesocosms, we set up monoclonal and polyclonal host populations of D. magna infected with a prevalence of 100% with the horizontally and vertically transmitted microsporidian parasite Octosporea bayeri. These populations diapaused after sexual reproduction and hatchlings were screened for signs of O. bayeri. Parasite prevalence was 98.9% for hatchlings from the monoclonal treatment, but only 85.2% among the hatchlings from the polyclonal populations, indicating a short-term benefit for outbreeding. This benefit occurs, we hypothesize, not owing to inbreeding depression, but because the vertically transmitted parasite is less able to establish itself in the relatively new genetic environment of the outbred offspring, as compared to the more stable environment when transmitted to selfed offspring. To quantify the fitness consequences of this 14% prevalence difference, we studied the within-season epidemiology of O. bayeri, using an epidemiological model. We then examined whether descendants of outbred offspring produce more resting eggs than the descendants of selfed offspring. The data and our model show that Daphnia which are uninfected at the beginning of the growth season have a large advantage when the entire season is considered. Our data support the Red Queen hypothesis which states that in the presence of coevolving parasites, outbreeding is favoured in the host.

Endoparasitism in colonial hosts: patterns and processes

This study begins to redress our lack of knowledge of the interactions between colonial hosts and their parasites by focusing on a novel host-parasite system. Investigations of freshwater bryozoan populations revealed that infection by myxozoan parasites is widespread. Covert infections were detected in all 5 populations studied and were often at high prevalence while overt infections were observed in only 1. Infections were persistent in populations subject to temporal sampling. Negative effects of infection were identified but virulence was low. Infection did not induce mortality in the environmental conditions studied. However, the production of statoblasts (dormant propagules) was greatly reduced in bryozoans with overt infections in comparison to uninfected bryozoans. Overtly-infected bryozoans also grew more slowly and had low fission rates relative to colonies lacking overt infection. Bryozoans with covert infections were smaller than uninfected bryozoans. High levels of vertical transmission were achieved through colony fission and the infection of statoblasts. Increased fission rates may be a strategy for hosts to escape from parasites but the parasite can also exploit the fragmentation of colonial hosts to gain vertical transmission and dispersal. Our study provides evidence that opportunities and constraints for host-parasite co-evolution can be highly dependent on organismal body plans and that low virulence may be associated with exploitation of colonial hosts by endoparasites.

Microsporidiosis in a flock of tricolor parrot finches (Erythrura tricolor)

The lesions caused by a microsporidian infection in a flock of tricolor parrot finches (Erythrura tricolor) are described. Affected birds had a widespread nodular to diffuse granulomatous inflammation of the serosal surfaces of the gastrointestinal tract, peritoneum, perirenal airsacs and connective tissue, bone marrow, dura, and conjunctiva. This was composed predominantly of foamy macrophages containing numerous intracytoplasmic microsporidia measuring 1 to 2 microm. Ultrastructural features consistent with microsporidia were the presence of a coiled polar filament and an electrode-dense outer surface and thick electron-lucent capsule. Differential diagnoses included infection with intracellular organisms, including coccidian and other apicomplexan parasites, such as lsospora, Eimeria, and blood parasites; Chlamydophilosis; disseminated mycobacteriosis; and other bacterial and fungal species.

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.

Quantitative PCR to detect, discriminate and quantify intracellular parasites in their host: an example from three microsporidians in Daphnia

Reliable detection, discrimination and quantification of parasites are important for host-parasite studies and diagnostics. Microsporidial infections are problematic in this respect. Their discrimination and quantification using light microscopy is difficult because spores are the only light microscopically visible form of the parasite and they offer few distinct characters. We developed a quantitative PCR (qPCR) assay based on SYBR Green chemistry to quantify the microsporidia Glugoides intestinalis, Octosporea bayeri and Ordospora colligata in their host, the freshwater crustacean Daphnia magna. The assay allows the quantification of infection intensities in whole animals and is more than an order of magnitude more sensitive than light microscopy. Sampling and DNA extraction account for more than 90% of the residual variance in infection intensity data and this variance considerably impairs the resolution of qPCR. Where higher resolution is required, we propose using the ratio of parasite to host DNA as the measure of infection intensity. We show that this measure is robust and greatly improves resolution of qPCR. Additionally, this method can be applied to compare samples of unequal volume.

Transmission and burden and the impact of temperature on two species of vertically transmitted microsporidia

Microsporidia are unusual amongst eukaryotic parasites in that they utilize both vertical and horizontal transmission and vertically transmitted species can cause sex ratio distortion in their host. Here we study vertical transmission in two species of feminising microsporidia, Nosema granulosis and Dictyocoela duebenum, infecting a single population of the crustacean host Gammarus duebeni and measure the effect of temperature on parasite transmission and replication. N. granulosis was vertically transmitted to 82% of the host embryos and D. duebenum was transmitted to 72% of host embryos. For both parasites, we report relatively low parasite burdens in developing host embryos. However, the parasites differ in their pattern of replication and burden within developing embryos. Whilst N. granulosis undergoes replication during host development, the burden of D. duebenum declines, leading us to propose that parasite dosage and feminisation efficiency underlie the different parasite frequencies in the field. We also examine the effect of temperature on parasite transmission and replication. Temperature does not affect the percentage of young that inherit the infection. However, low temperatures inhibit parasite replication relative to host cell division, resulting in a reduction in parasite burden in infected embryos. The reduced parasite burden at low temperatures may underpin reduced feminization at low temperatures and so limit the spread of sex ratio distorters through the host population.

Phenotypic plasticity of host-parasite interactions in response to the route of infection

The microsporidium Octosporea bayeri can infect its host, the planktonic crustacean Daphnia magna, vertically and horizontally. The two routes differ greatly in the way the parasite leaves the harbouring host (transmission) and in the way it enters a new, susceptible host (infection). Infections resulting from each route may thus vary in the way they affect host and parasite life-histories and, subsequently, host and parasite fitness. We conducted a life-table experiment to compare D. magna infected with O. bayeri either horizontally or vertically, using three different parasite isolates. Both the infection route and the parasite isolate had significant effects on host life-history. Hosts matured at different ages depending on the parasite isolate, and at a size that varied with infection route. The frequency of host sterility and the host's life-time reproductive success were affected by both the infection route and the parasite isolate. The infection route also affected parasite life-history. The production of parasite spores was much higher in vertically than in horizontally infected hosts. We found a trade-off between the production of spores (the parasite's horizontal fitness component) and the production of infected host offspring (the parasite's vertical fitness component). This study shows that hosts and parasites can react plastically to different routes of infection, suggesting that ecological factors that may influence the relative importance of horizontal and vertical transmission can shape the evolution of host and parasite life histories, and, consequently, the evolution of virulence.

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.