Out of the 'host' box: Extreme off-host conditions alter the infectivity and virulence of a parasitic bacterium

Disease agents play an important role in the ecology and life history of wild and cultivated populations and communities. While most studies focus on the adaptation of parasites to their hosts, the adaptation of free-living parasite stages to their external (off-host) environment may tell us a lot about the factors that shape the distribution of parasites. Pasteuria ramosa is an endoparasitic bacterium of the water flea Daphnia with a wide geographic distribution. Its transmission stages rest outside of the host and thus experience varying environmental regimes. We examined the life history of P. ramosa populations from four environmental conditions (i.e., groups of habitats): the factorial combinations of summer-dry water bodies or not, and winter-freeze water bodies or not. Our goal was to examine how the combination of winter temperature and summer dryness affects the parasite's ability to attach to its host and to infect it. We subjected samples of the four groups of habitats to temperatures of 20, 33, 46 and 60˚C in dry and wet conditions, and exposed a susceptible clone of Daphnia magna to the treated spores. We found that spores that had undergone desiccation endured higher temperatures better than spores kept wet, both regarding attachment and subsequent infection. Furthermore, spores treated with heightened temperatures were much less infective and virulent. Even under high temperatures (60˚C), exposed spores from all populations were able to attach to the host cuticle, albeit they were unable to establish infection. Our work highlights the sensitivity of a host-free resting stage of a bacterial parasite to the external environment. Long heatwaves and harsh summers, which are becoming more frequent due to recent climate changes, may therefore pose a problem for parasite survival.We used R (version 4.1.2) for the analysis and the 'ggplot2' package for visualization. We fitted generalized linear models (glm) to our dependent variables of interest: parasite infectivity, spore production, and host reproduction.Funding provided by: Israel Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100003977Award Number:The data was obtained by two designed lab experiments. The infection experiment and the attachment experiment. For the analysis of the infection experiment, we used only individuals that survived long enough to inspect infectivity. For further information Please sea the README file attached