Genetic slippage after sex maintains diversity for parasite resistance in a natural host population

Phylogeny, morphology, virulence, ecology, and host range of Ordospora pajunii (Ordosporidae), a microsporidian symbiont of Daphnia spp

The impacts of microsporidia on host individuals are frequently subtle and can be context dependent. A key example of the latter comes from a recently discovered microsporidian symbiont of Daphnia, the net impact of which was found to shift from negative to positive based on environmental context. Given this, we hypothesized low baseline virulence of the microsporidian; here, we investigated the impact of infection on hosts in controlled conditions and the absence of other stressors. We also investigated its phylogenetic position, ecology, and host range. The genetic data indicate that the symbiont is Ordospora pajunii, a newly described microsporidian parasite of Daphnia. We show that O. pajunii infection damages the gut, causing infected epithelial cells to lose microvilli and then rupture. The prevalence of this microsporidian could be high (up to 100% in the lab and 77% of adults in the field). Its overall virulence was low in most cases, but some genotypes suffered reduced survival and/or reproduction. Susceptibility and virulence were strongly host-genotype dependent. We found that North American O. pajunii were able to infect multiple Daphnia species, including the European species Daphnia longispina, as well as Ceriodaphnia spp. Given the low, often undetectable virulence of this microsporidian and potentially far-reaching consequences of infections for the host when interacting with other pathogens or food, this Daphnia-O. pajunii symbiosis emerges as a valuable system for studying the mechanisms of context-dependent shifts between mutualism and parasitism, as well as for understanding how symbionts might alter host interactions with resources.

IMPORTANCE

The net outcome of symbiosis depends on the costs and benefits to each partner. Those can be context dependent, driving the potential for an interaction to change between parasitism and mutualism. Understanding the baseline fitness impact in an interaction can help us understand those shifts; for an organism that is generally parasitic, it should be easier for it to become a mutualist if its baseline virulence is relatively low. Recently, a microsporidian was found to become beneficial to its Daphnia hosts in certain ecological contexts, but little was known about the symbiont (including its species identity). Here, we identify it as the microsporidium Ordospora pajunii. Despite the parasitic nature of microsporidia, we found O. pajunii to be, at most, mildly virulent; this helps explain why it can shift toward mutualism in certain ecological contexts and helps establish O. pajunii is a valuable model for investigating shifts along the mutualism-parasitism continuum.

Inconsistent dilution: experimental but not field evidence for a dilution effect in Daphnia-bacteria interactions

The dilution effect hypothesis, which suggests greater host biodiversity can reduce infectious disease transmission, occurs in many systems but is not universal. Most studies only investigate the dilution of a single parasite in a community, but many host communities have multiple parasites circulating. We studied a zooplankton host community with prior support for a dilution effect in laboratory- and field-based studies of a fungal parasite, Metschnikowia bicuspidata. We used paired experiments and field studies to ask whether dilution also occurred for a bacterial parasite, Pasteuria ramosa. We hypothesized that the similarities between the parasites might mean the dilution pattern seen in Metschnikowia would also be seen in Pasteuria. However, because Daphnia-Pasteuria interactions have strong host-parasite genotype specificity, dilution may be less likely if diluter host genotypes vary in their capacity to dilute Pasteuria. In a lab experiment, Pasteuria prevalence in susceptible Daphnia dentifera was reduced strongly by higher densities of D. pulicaria and marginally by higher densities of D. retrocurva. In a second experiment, different D. pulicaria genotypes had a similar capacity to dilute both Metschnikowia and Pasteuria, suggesting that Pasteuria's strong host-parasite genotype specificity should not prevent dilution. However, we found no evidence of an impact of the dilution effect on the size of Pasteuria epidemics in D. dentifera in Midwestern U.S. lakes. Our finding that a second parasite infecting the same host community does not show a similar dilution effect in the field suggests the impact of biodiversity can differ even among parasites in the same host community.

Genetic Drift Shapes the Evolution of a Highly Dynamic Metapopulation

The dynamics of extinction and (re)colonization in habitat patches are characterizing features of dynamic metapopulations, causing them to evolve differently than large, stable populations. The propagule model, which assumes genetic bottlenecks during colonization, posits that newly founded subpopulations have low genetic diversity and are genetically highly differentiated from each other. Immigration may then increase diversity and decrease differentiation between subpopulations. Thus, older and/or less isolated subpopulations are expected to have higher genetic diversity and less genetic differentiation. We tested this theory using whole-genome pool-sequencing to characterize nucleotide diversity and differentiation in 60 subpopulations of a natural metapopulation of the cyclical parthenogen Daphnia magna. For comparison, we characterized diversity in a single, large, and stable D. magna population. We found reduced (synonymous) genomic diversity, a proxy for effective population size, weak purifying selection, and low rates of adaptive evolution in the metapopulation compared with the large, stable population. These differences suggest that genetic bottlenecks during colonization reduce effective population sizes, which leads to strong genetic drift and reduced selection efficacy in the metapopulation. Consistent with the propagule model, we found lower diversity and increased differentiation in younger and also in more isolated subpopulations. Our study sheds light on the genomic consequences of extinction-(re)colonization dynamics to an unprecedented degree, giving strong support for the propagule model. We demonstrate that the metapopulation evolves differently from a large, stable population and that evolution is largely driven by genetic drift.