Pervasive effects of on host activity.

Heritable symbionts have diverse effects on the physiology, reproduction and fitness of their hosts. Maternally transmitted are one of the most common endosymbionts in nature, infecting about half of all insect species. We test the hypothesis that alter host behaviour by assessing the effects of 14 different strains on the locomotor activity of nine host species. We find that alter the activity of six different host genotypes, including all hosts in our assay infected with Ri-like strains (Ri, Suz and Aur), which have rapidly spread among species in about the last 14 000 years. While 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 of Ri-like may be explained by patterns of titre and localization within host somatic tissues, particularly in the central nervous system. Our findings support the view that 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 how spread within host populations.