Rapid evolution of a bacterial parasite during outbreaks in two Daphnia populations

A common multi-host parasite shows genetic structuring at the host species and population levels

Although individual parasite species commonly infect many populations across physical space as well as multiple host species, the extent to which parasites traverse physical and phylogenetic distances is unclear. Population genetic analyses of parasite populations can reveal how parasites move across space or between host species, including helping assess whether a parasite is more likely to infect a different host species in the same location or the same host species in a different location. Identifying these transmission barriers could be exploited for effective disease control. Here, we analysed population genetic structuring of the parasite Pasteuria ramosa in daphniid host species from different lakes. Outbreaks occurred most often in the common host species Daphnia dentifera and Daphnia retrocurva. The genetic distance between parasite samples tended to be smaller when samples were collected from the same lake, the same host species and closer in time. Within lakes, the parasite showed structure by host species and sampling date; within a host species, the parasite showed structure by lake and sampling date. However, despite this structuring, we found the same parasite genotype infecting closely related host species, and we sometimes found the same genotype in nearby lakes. Thus, P. ramosa experiences challenges infecting different host species and moving between populations, but doing so is possible. In addition, the structuring by sampling date indicates potential adaptation to or coevolution with host populations and supports prior findings that parasite population structure is dynamic during outbreaks.

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.

How does parasite environmental transmission stage concentration change before, during, and after disease outbreaks?

Outbreaks of environmentally transmitted parasites require that susceptible hosts encounter transmission stages in the environment and become infected, but we also know that transmission stages can be in the environment without triggering disease outbreaks. One challenge in understanding the relationship between environmental transmission stages and disease outbreaks is that the distribution and abundance of transmission stages outside of their hosts have been difficult to quantify. Thus, we have limited data about how changes in transmission stage abundance influence disease dynamics; moreover, we do not know whether the relationship between transmission stages and outbreaks differs among parasite species. We used digital PCR to quantify the environmental transmission stages of five parasites in six lakes in southeastern Michigan every 2 weeks from June to November 2021. At the same time, we quantified infection prevalence in hosts and host density. Our study focused on eight zooplankton host species (Daphnia spp. and Ceriodaphnia dubia) and five of their parasites from diverse taxonomic groups (bacteria, yeast, microsporidia, and oomycete) with different infection mechanisms. We found that parasite transmission stage concentration increased prior to disease outbreaks for all parasites. However, parasites differed significantly in the relative timing of peaks in transmission stage concentration and infection outbreaks. The "continuous shedder" parasites had transmission stage peaks at the same time as or slightly after the outbreak peaks. In contrast, parasites relying on host death for transmission ("obligate killers") had transmission stage peaks before outbreak peaks. For most parasites, lakes with outbreaks had higher spore concentrations than those without outbreaks, especially once an outbreak began; the exception was for a parasite, Pasteuria ramosa, with very strong genotypic specificity of infection. Overall, our results show that disease outbreaks are tightly linked to transmission stage concentration; outbreaks were preceded by increases in transmission stage concentration in the environment and then were fueled by the production of more transmission stages during the outbreak itself, with concentrations decreasing to pre-outbreak levels as outbreaks waned. Thus, tracking transmission stages in the environment improves our understanding of the drivers of disease outbreaks and reveals how parasite traits may affect these dynamics.