Diverse bacterial lineages form beneficial relationships with hosts. Identifying the conditions under which relationships with symbiotic microbes stabilize or breakdown is key to understanding the major evolutionary transitions that have arisen from two species joining to form a new higher-level organism.Theory predicts that bacterial mutualism is maintained when partners provide reciprocal fitness benefits. The first step in understanding the evolution of bacterial mutualism is to identify the conditions under which symbiotic microbes have gained and lost beneficial functions. I propose to explore the conditions that have lead to the gains and losses of beneficial functions in the aphid symbiont, Regiella insecticola. Regiella can protect its aphid host from the pathogenic fungus Pandora neoaphidis, however only certain symbiont lineages confer this beneficial function.The goal of this project is to 1) characterize the interactions between Regiella and its aphid host, and then 2) explore the ecological conditions under which the protective function have been gained and lost. My approach uses comparative genomics to identify genes involved in mutualism, and then using phylogenetic reconstructions I will trace the evolutionary gains and losses of the mutualistic phenotype.The project has all the elements to complete my training as an independent researcher and leader in the study of symbiosis. I intend to start my own research groups at the end of this fellowship, and this project is designed to form the basis of my future research programme. I have much of the skills necessary to begin my own research group, however, there are practical skills that I have yet to develop that will increase the overall strength of my research programme. In particular, I wish to expand my knowledge of genomic techniques and Prof. Ellers groups is the ideal environment to learn these techniques. This will add to my research portfolio and completing my early career development.