Pervasive effects ofWolbachiaon host activity

Heritable symbionts have diverse effects on the physiology, reproduction and fitness of their hosts. Maternally transmittedWolbachia are one of the most common endosymbionts in nature, infecting about half of all insect species. We test the hypothesis thatWolbachia alter host behaviour by assessing the effects of 14 differentWolbachia strains on the locomotor activity of nineDrosophila host species. We find thatWolbachia alter the activity of six different host genotypes, including all hosts in our assay infected withw Ri-likeWolbachia strains (w Ri,w Suz andw Aur), which have rapidly spread amongDrosophila species in about the last 14 000 years. WhileWolbachia effects on host activity were common, the direction of these effects varied unpredictably and sometimes depended on host sex. We hypothesize that the prominent effects ofw Ri-likeWolbachia may be explained by patterns ofWolbachia titre and localization within host somatic tissues, particularly in the central nervous system. Our findings support the view thatWolbachia have wide-ranging effects on host behaviour. The fitness consequences of these behavioural modifications are important for understanding the evolution of host–symbiont interactions, including howWolbachia spread within host populations.