Abstract
Parasites with exclusive vertical transmission from host parent to offspring arean evolutionary puzzle. With parasite fitness entirely linked to host reproduc-tion, any fitness cost for infected hosts risks their selective elimination. Envi-ronmental conditions likely influence parasite impact and thereby the successof purely vertical transmission strategies. We tested for temperature-depen-dent virulence of Caedibacter taeniospiralis, a vertically transmitted bacterialsymbiont of the protozoan Paramecium tetraurelia. We compared growth ofinfected and cured host populations at five temperatures (16–32 °C). Infec-tion reduced host density at all temperatures, with a peak of 30% at 28 °C.These patterns were largely consistent across five infected Paramecium strains.Similar to Wolbachia symbionts, C. taeniospiralis may compensate fitness costsby conferring to the host a ‘killer trait’, targeting uninfected competitors.Considerable loss of infection at 32 °C suggests that killer efficacy is not uni-versal and that limited heat tolerance restricts the conditions for persistenceof C. taeniospiralis