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 100days, 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 prediction

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 use

The impact of infection on host competition and its relationship to parasite persistence in a Daphnia microparasite system

Evolutionary studies often estimate fitness components with the aim to make predictions about the outcome of selection. Depending on the system and the question, different fitness components are used, but their usefulness for predicting the outcome of selection is rarely tested. Here we estimate host fitness components in different ways with the aim to test how well they agree with each other and how well they predict host fitness at the population level in the presence of the parasite. We use a Daphnia magna-microparasite system to study the competitive ability of host clones in the absence and presence of the parasite, the infection intensity of the parasite in individuals of twelve host clones (an estimate of both host resistance and parasite reproductive success), and parasite persistence in small host populations (an estimate of R 0 of the parasite). Analysis of host competitive ability and parasite persistence reveals strong host genotype effects, while none are found for infection intensity. Host competitive ability further shows a genotype-specific change upon infection, which is correlated with the relative persistence of the parasite in the competing hosts. Hosts in which the parasite persists better suffer a competitive disadvantage in the parasite's presence. This suggests that in this system, parasite-mediated selection can be predicted by parasite persistence, but not by parasite infection intensit

The influence of pool volume and summer desiccation on the production of the resting and dispersal stage in a Daphnia metapopulation

Dispersal is a key process in metapopulations, as migrants genetically connect populations and enable the colonization of empty habitat patches. Sub-populations may differ in their numerical contribution of migrants within a metapopulation. This has strong implications on evolutionary and ecological dynamics and has led to two different hypotheses about the Daphnia metapopulation studied here: the assessment by some authors is that sub-populations contribute equally to the production of migrants, while others have postulated long-lived core populations in large "mainland” habitat patches as the dominant source of migrants. We have studied the resting and dispersal stage (ephippium) in a natural Daphnia metapopulation and in mesocosm experiments, and tested for effects of habitat size and summer desiccation. We found that a 1000-fold increase in rock pool volume resulted on average in only in a 2.8-fold increase in ephippium production. Mesocosm experiments confirmed these results: a 1000-fold increase of the mesocosms' volume resulted in a 7.2-fold increase in ephippium production. Additionally, we showed that ephippium production did not depend on the initial population size. Thus, populations in small pools may contribute only marginal fewer potential migrants in the whole metapopulation than populations in large pools. In a second mesocosm experiment we found that summer desiccation, which is a typical occurrence in small pools, is not detrimental for the populations. Daphnia hatched out of ephippia that were produced earlier within the same season and built up viable populations again. The substantial production of ephippia by populations in small pools suggests that these populations might be important for both the dynamics and global stability of metapopulation

Covariation of Mitochondrial Genome Size with Gene Lengths: Evidence for Gene Length Reduction During Mitochondrial Evolution

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

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 Jirovec, 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 (4.9-5.6x2.2-2.3 mu m) and have their polar filament arranged in 12-13 coils. A second, elongated spore type (6.8-12.0x1.6-2.1 mu m) is rod-shaped with blunt ends and measures 6.8-12.0x1.6-2.1 mu m. 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

Parasites promote host gene flow in a metapopulation

Local adaptation is a powerful mechanism to maintain genetic diversity in subdivided populations. It counteracts the homogenizing effect of gene flow because immigrants have an inferior fitness in the new habitat. This picture may be reversed in host populations where parasites influence the success of immigrating hosts. Here we report two experiments testing whether parasite abundance and genetic background influences the success of host migration among pools in a Daphnia magna metapopulation. In 22 natural populations of D. magna, immigrant hosts were found to be on average more successful when the resident populations experienced high prevalences of a local microsporidian parasite. We then determined whether this success is due to parasitism per se, or the genetic background of the parasites. In a common garden competition experiment, we found that parasites reduced the fitness of their local hosts relatively more than the fitness of allopatric host genotypes. Our experiments are consistent with theoretical predictions based on coevolutionary host-parasite models in metapopulations. A direct consequence of the observed mechanism is an elevated effective migration rate for the host in the metapopulatio

Interactions between environmental stressors: the influence of salinity on host-parasite interactions between Daphnia magna and Pasteuria ramosa

Interactions between environmental stressors play an important role in shaping the health of an organism. This is particularly true in terms of the prevalence and severity of infectious disease, as stressors in combination will not always act to simply decrease the immune function of a host, but may instead interact to compound or even oppose the influence of parasitism on the health of an organism. Here, we explore the impact of environmental stress on host-parasite interactions using the water flea Daphnia magna and it is obligate parasite Pasteuria ramosa. Utilising an ecologically relevant stressor, we focus on the combined effect of salinity and P. ramosa on the fecundity and survival of the host, as well as on patterns of infectivity and the proliferation of the parasite. We show that in the absence of the parasite, host fecundity and survival was highest in the low salinity treatments. Once a parasite was introduced into the environment, however, salinity and parasitism acted antagonistically to influence both host survival and fecundity, and these patterns of disease were unrelated to infection rates or parasite spore loads. By summarising the form of interactions found in the broader Daphnia literature, we highlight how the combined effect of stress and parasitism will vary with the type of stressor, the trait used to describe the health of Daphnia and the host-parasite combination under observation. Our results highlight how the context-dependent nature of interactions between stress and parasitism inevitably complicates the link between environmental factors and the prevalence and severity of diseas

Evolution of sperm morphology in a crustacean genus with fertilization inside an open brood pouch

Sperm is the most fundamental male reproductive feature. It serves the fertilization of eggs and evolves under sexual selection. Two components of sperm are of particular interest, their number and their morphology. Mode of fertilization is believed to be a key determinant of sperm length across the animal kingdom. External fertilization, unlike internal, favors small and numerous sperm, since sperm density is thinned out in the environment. Here, we study the evolution of sperm morphology in the genus Daphnia, where fertilization occurs in a receptacle, the brood pouch, where sperm can constantly be flushed out by a water current. Based on microscopic observations of sperm morphologies mapped on a phylogeny with 15 Daphnia and 2 outgroup species, we found that despite the internal fertilization mode, Daphnia have among the smallest sperm recorded, as would be expected with external fertilization. Despite being all relatively small compared to other arthropods, sperm length diverged at least twice, once within each of the Daphnia subgenera Ctenodaphnia and Daphnia. Furthermore, species in the latter subgenus also lost the ability of cell compaction by extracellular encapsulation and have very polymorphic sperm with long, and often numerous, filopodia. We discuss the different strategies that Daphnia evolved to achieve fertilization success in the females’ brood pouch