Résumé
Vertical transmission mode is predicted to decrease the virulence of
symbionts. However, Wolbachia, a widespread vertically transmitted
endosymbiont, exhibits both negative and beneficial effects on arthropod
fitness. This ‘Jekyll and Hyde’ behaviour, as well as its ability to live
transiently outside host cells and to establish new infections via
horizontal transmission, may reflect the capacity of Wolbachia to exhibit
various phenotypes depending on the prevailing environmental constraints.
To study the ability of Wolbachia to readily cope with new constraints, we
forced this endosymbiont to spread only via horizontal transmission. To
achieve this, we performed serial horizontal transfers of haemolymph from
Wolbachia-infected to naive individuals of the isopod Armadillidium
vulgare. Across passages, we observed phenotypic changes in the symbiotic
relationship: (i) The Wolbachia titre increased in both haemolymph and
nerve cord but remained stable in ovaries; (ii) Wolbachia infection was
benign at the beginning of the experiment, but highly virulent, killing
most hosts after only a few passages. Such a phenotypic shift after
recurrent horizontal passages demonstrates that Wolbachia can rapidly
change its virulence when facing new environmental constraints. We
thoroughly discuss the potential mechanism(s) underlying this phenotypic
change, which are likely to be crucial for the ongoing radiation of
Wolbachia in arthropods.