Phenotypic shift in virulence towards its native host across serial horizontal passages.

Vertical transmission mode is predicted to decrease the virulence of symbionts. However, , 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 to exhibit various phenotypes depending on the prevailing environmental constraints. To study the ability of 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 -infected to naive individuals of the isopod Across passages, we observed phenotypic changes in the symbiotic relationship: (i) The titre increased in both haemolymph and nerve cord but remained stable in ovaries; (ii) 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 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 in arthropods.