Vertically transmitted symbiont reduces host fitness along temperature gradient
Résumé
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