Virulence and transmission biology of the widespread, ecologically important pathogen of zooplankton, .

Unlabelled: ( hereafter) is a widely distributed bacterial pathogen that has significant impacts on the population dynamics of zooplankton (.) particularly in months when are asexually reproducing. However, little is known about virulence, transmission mode, and dynamics. As a result, we cannot explain the dynamics of epidemics in nature or use as a model pathogen, despite tractability as a model host. Here, we work to fill these knowledge gaps experimentally. We found that is among the most virulent of pathogens, killing its host within a week and reducing host fecundity. We further found that did not transmit horizontally among hosts unless the host died or was destroyed (i.e., it is an "obligate killer"). In experiments aimed at quantifying the dynamics of horizontal transmission among asexually reproducing , we demonstrated that transmits poorly in the laboratory. In mesocosms, failed to generate epidemics; in experiments wherein individual were exposed, transmission success was low. In the (limited) set of conditions we considered, transmission success did not change with host density or pathogen dose and declined following environmental incubation. Finally, we conducted a field survey of prevalence within egg cases (ephippia) made by sexually reproducing . We found DNA in ~40% of ephippia, suggesting that, in addition to transmitting horizontally among asexually reproducing , may transmit vertically from sexually reproducing . Our work fills critical gaps in the biology of and illuminates new hypotheses vis-à-vis its life history.

Importance: is a bacterial pathogen of zooplankton, first described in the 19 century and recently placed in a new family of bacteria, the . causes large epidemics in lake zooplankton populations and increases the probability that zooplankton will be eaten by predators. However, little is known about how transmits among hosts, to what extent it reduces host survival and reproduction (i.e., how virulent it is), and what role virulence plays in ' life cycle. Here, we experimentally quantified virulence and showed that must kill its host to transmit horizontally. We also found evidence that may transmit vertically via s seed-like egg sacks. Our work will help scientists to (i) understand epidemics, (ii) use as a model pathogen for the study of host-parasite interactions, and (iii) better understand the unusual group of bacteria to which belongs.