Résumé
Parasites with exclusive vertical transmission from host parent to
offspring are an evolutionary puzzle. With parasite fitness entirely
linked to host reproduction, any fitness cost for infected hosts risks
their selective elimination. Environmental conditions likely influence
parasite impact, and thereby the success of purely vertical transmission
strategies. We tested for temperature-dependent virulence of Caedibacter
taeniospiralis, a vertically transmitted bacterial symbiont of the
protozoan Paramecium tetraurelia. We compared growth of infected and cured
host populations at five temperatures (16-32°C). Infection reduced host
density at all temperatures, with a peak at 28°C. These patterns were
largely consistent across five infected Paramecium strains. Similar to
Wolbachia symbionts, C. taeniospiralis may compensate fitness costs by
conferring to the host a 'killer trait', targeting uninfected
competitors. Considerable loss of infection at 32°C suggests that killer
efficacy is not universal and that limited heat tolerance restricts the
conditions for persistence of C. taeniospiralis.