Parasitism and host dispersal plasticity in an aquatic model system

Dispersal is a central determinant of spatial dynamics in communities and
ecosystems, and various ecological factors can shape the evolution of
constitutive and plastic dispersal behaviours. One important driver of
dispersal plasticity is the biotic environment. Parasites, for example,
influence the internal condition of infected hosts and define external
patch quality. Thus state-dependent dispersal may be determined by
infection status and context-dependent dispersal by the abundance of
infected hosts in the population. A prerequisite for such dispersal
plasticity to evolve is a genetic basis on which natural selection can
act. Using interconnected microcosms, we investigated dispersal in
experimental populations of the freshwater protist Paramecium caudatum in
response to the bacterial parasite Holospora undulata. For a collection of
20 natural host strains, we found substantial variation in constitutive
dispersal, and to a lesser degree in dispersal plasticity. First,
infection tended to increase or decrease dispersal relative to uninfected
controls, depending on strain identity, potentially indicative of
state-dependent dispersal plasticity. Infection additionally decreased
host swimming speed compared to the uninfected counterparts. Second, for
certain strains, there was a weak negative association between dispersal
and infection prevalence, such that uninfected hosts tended to disperse
less when infection was more frequent in the population, indicating
context-dependent dispersal plasticity. Future experiments may test
whether the observed differences in dispersal plasticity are sufficiently
strong to react to natural selection. The evolution of dispersal
plasticity as a strategy to mitigate parasite effects spatially may have
important implications for epidemiological dynamics.