Defensive symbiont genotype distributions are linked to parasitoid attack networks

Facultative symbionts are widespread in arthropods and can provide important services such as protection from natural enemies. Yet what shapes associations with defensive symbionts in nature remains unclear. Two hypotheses suggest that either interactions with antagonists, or host plants, may explain the prevalence of symbionts through shared selective pressures and routes of horizontal transmission. Here we investigate the factors driving similarities in the Hamiltonella defensa symbiosis shared among host species within field collected aphid communities. We show that, Hamiltonella’s genotype distribution strongly aligns with sharing the same parasitoids, rather than host plants, highlighting parasitoids as a key selective agent shaping the symbiosis across host species. Our data indicates parasitoid host-specificity drives the prevalence of specific aphid- Hamiltonella associations, suggesting defensive symbioses are maintained by the selective pressure imposed by dominant parasitoid species. These findings underscore the importance of interactions with natural enemies in explaining patterns of defensive symbiosis in nature.