The endosymbiont Rickettsiella viridis increases the virulence of Diuraphis noxia but reduces alate frequency

Aphids are among the world's most economically damaging pests and carry a diverse range of bacterial endosymbionts. There is increasing interest in exploring potential applications of natural and novel strains of endosymbionts in aphid control. One endosymbiont, Rickettsiella viridis, has a large fitness cost following transfer from its natural aphid host Acyrthosiphon pisum into a novel host aphid Myzus persicae. Here, we investigated host impacts after transferring the same Rickettsiella strain to an important cereal aphid, the Russian wheat aphid Diuraphis noxia. Rickettsiella in this host resulted in modest fitness effects, with a minor increase in heat tolerance and a decrease in development time at 25C. The infection persisted in mixed caged populations under different temperatures. Surprisingly, Rickettsiella increased aphid virulence to wheat plants and to two non-crop hosts of D. noxia, barley grass and brome grass. This was evident from sharper decreases in leaf number and leaf area, as well as an increase in chlorotic streaking when plants were exposed to Rickettsiella-infected D. noxia. Rickettsiella also reduced the proportion of alates in aphids held in small cages and in larger mesocosms containing multiple wheat plants where short-distance dispersal of aphids was impacted. These results provide compelling evidence that Rickettsiella can affect virulence - the first case of an endosymbiont transfer directly influencing aphid virulence to host plants - and highlight the species-specific impacts of endosymbiont transfers on aphids which can involve multiple traits.