Don't go out there! How do bacteria-phages interactions in the soil impact the rhizobia-legume symbiosis

Rhizobia (nitrogen-fixing bacteria) are a classic model for mutualistic interactions - they form intimate intracellular symbioses with legumes where they exchange nitrogen for nutrients and shelter. The rhizobia-legume symbiosis has been studied near-exclusively as a pairwise interaction, but in reality

is embedded within a complex species community with consequences for each partner and thus the symbiosis. Between plant-hosts rhizobia live as free-living bacteria within a diverse soil microbial community including phages (bacterial viruses) - the primary predators of bacteria. Phage predation could shape the rhizobia-legume symbiosis in numerous ways;

phages drive continuous selection for phage resistance (antagonistic coevolution) which may result in pleiotropic trade- offs in the symbiosis. Conversely, higher mortality outside may drive greater symbiotic investment by increasing the value of protection within the plant, i.e. altering the 'economics' of the

interaction. In this project we will combine experimental evolution, next-generation sequencing and mathematical modelling to ask a critical overlooked question - how do interactions outside the host shape the rhizobia-legume symbiosis