Evolutionary and ecological feedbacks across tripartite interactions in the rhizobial community

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Many organisms engage in symbioses; intimate partnerships with another species that can be mutually beneficial to both partners. However, these relationships do not occur in isolation, but are embedded within a network of interacting species, which can shape the evolution of symbiotic partnerships. The relationship between nitrogen-fixing bacteria (rhizobia) and legumes (e.g. peas and beans) is arguably one of the most economically important examples of symbiosis. Rhizobia take nitrogen from the air and convert it into a form that plants can use. This is exchanged with the plant in return for nutrients and shelter in specialized organs called root nodules. By providing the plant with a supply of nitrogen these bacteria effectively act as 'biofertilisers', reducing the need for expensive and environmentally damaging applications of chemical fertiliser to legumes as well as the non-legume crops grown alongside them. Rhizobia do not only interact with their plant hosts however. When not living in the plant's root nodules, rhizobia live freely in the soil, interacting with the diverse microbial community that exists there. Key members of all soil microbe communities are the temperate phages, viruses that infect bacteria. These viruses can have widespread and various effects on their bacterial hosts. On the one hand they can be antagonistic, killing their host in order to produce more viral particles that will go on to infect new cells. Surprisingly however these viruses can also be beneficial. Every so often, upon infecting a bacterial cell, rather than reproducing to make more viral particles, they will integrate their own DNA into the bacterial genome, giving the bacteria access to novel genes and effectively becoming viral weapons that bacteria can use against their competitors. Rhizobia therefore engage in two important and highly contrasting interactions, with their legume hosts and with temperate phages. These separate interactions will place different selection pressures on the bacterial population and consequently are likely to have important implications for one another. This project will unpick this temperate phage - rhizobia - legume relationship in order to understand how this tripartite community shapes, and is shaped by, its constituent interactions.This project will combine studies using highly controlled lab experiments with studies of natural and semi-natural communities. Lab experiments allow hypotheses to be directly tested and, due to the high turnover of generations in bacteria, bacteria-phage coevolution to be observed in real time. Meanwhile, experiments conducted in the 'wild' or by bringing natural communities into the lab demonstrate how real communities are structured and behave. In combination these approaches offer a powerful way of exploring these complex interactions. This work will provide the first comprehensive examination of the role of temperate phages in rhizobial populations. With the rise in global food demand, understanding what shapes the ecology and evolution of symbiotic microbes such as rhizobia will be increasingly important. This work will therefore lay the foundation for exploring what roles the wider microbial community plays in these processes. This will lead to future work examining how we can apply these lessons to sustainable agricultural practices. For example, by using temperate phages to enhance the effectiveness of biofertilisers.