Secondary bacterial symbiont community in aphids responds to plant diversity.

Biodiversity is important for ecosystem functioning and biotic interactions. In experimental grasslands, increasing plant species richness is known to increase the diversity of associated herbivores and their predators. If these interactions can also involve endosymbionts that reside within a plant or animal host is currently unknown. In plant-feeding aphids, secondary bacterial symbionts can have strong fitness effects on the host, e.g. resistance to natural enemies or fungal pathogens. We examined the secondary symbiont community in three species of aphid, each feeding on a unique host plant across experimental plots that varied in plant species richness. Aphids were collected in May and June, and the symbiont community identified using species-specific PCR assays. Aphis fabae aphids were found to host six different symbiont species with individual aphids co-hosting up to four symbionts. Uroleucon jaceae and Macrosiphum rosae hosted two and three symbiont species, respectively. We found that, at the aphid population level, increasing plant species richness increased the diversity of the aphid symbiont community, whereas at the individual aphid level, the opposite was found. These effects are potentially driven by varying selective pressures across different plant communities of varying diversities, mediated by defensive protection responses and a changing cost-benefit trade-off to the aphid for hosting multiple secondary symbionts. Our work extends documented effects of plant diversity beyond visible biotic interactions to changes in endosymbiont communities, with potentially far-reaching consequences to related ecosystem processes.