Long-term and short-term effects of a unicellular symbiont on its beetle host

Gregarines are often host-specific, share a long coevolution with their host and can be viewed on a parasitism-mutualism spectrum. However, little is known about whether impacts on their hosts depend on the infection history over generations and thus transgenerational effects, or rather on the recent infection status. In the mustard leaf beetle, Phaedon cochleariae, more negative than positive effects of the gregarine species Gregarina cochlearium on its host were found in individuals infected over several generations compared to recently gregarine-freed individuals. Here, we tested the effects of the infection history of previous generations and the current infection status (not infected or infected with gregarines) on the performance, consumption, behavior and surface profiles of P. cochleariae, using a full factorial design. The current infection alone and in interaction with the infection history affected some performance parameters; developmental time was most prolonged in short-term infected individuals, while survival was highest in long-term infected individuals. Under a current gregarine infection adult female body mass was reduced, while reproduction was neither affected by the current infection nor infection history. With regard to consumption-related traits, the current infection status affected the growth rate of both sexes (in females also in interaction with infection history) and the consumption index of males, being lowest in currently infected individuals. Currently infected individuals also showed slightly higher activity with regard to some of the measured traits, while boldness-related traits were unaffected. No changes were found in surface profiles in dependence of the gregarine infection status. Overall, under the used conditions, the past experience with gregarines over generations seems to have only little impact on P. cochleariae, while the current infection status causes plastic responses, with negative consequences mostly on development. The study highlights that gregarines modulate the phenotypes of their host in different traits.