The microbial world remains largely unexplored. The goal of this project is to study microbial genome diversity and understand how the genomes of microbial pathogens evolve, using the alpha-proteobacteria as a model system. The alpha-proteobacteria are a bacterial subdivision of much interest from both a medical and an ecological standpoint. Many species establish chronic host-infections, with higher vertebrates (e.g. human, cattle and rodents) as well as with plants and insects. Well-known alpha-proteobacterial infections include trench fever, cat-scratch disease (Bartonella sp.) and formation of tumour-like structures in host tissues, such as in plants by Agrobacterium tumefaciens. Moreover, other alpha-proteobacterial species are known to establish tight symbiotic interactions with plants, such as is the case with the formation of root nodules that are involved nitrogen fixation. Furthermore, they represent a genetic group that is predominant in the environment, as suggested by environmental shotgun genome sequencing of the Sargasso Sea. Because of the broad diversity in genome sizes and structures, the alpha-proteobacteria offer an excellent model system for studies of the forces, mechanisms and rates whereby bacterial genomes evolve. At the date of this writing, more than 20 alpha-proteobacterial genomes have been completely sequenced, ranging in size from 1.1 to 9.1 Megabases, and many more will be available in the near future. The aim of this project is to study the evolution of the alpha-proteobacteria by analyzing their genomes, and as such gain insight in how specific alpha-proteobacterial lineages have evolved into the present-day pathogens and (endo)symbionts. This will be achieved by reconstructing ancestral alpha-proteobacterial genomes and to specifically study the flow of genes along the nodes towards the different pathogenic and symbiotic lineages, including the ultimate endosymbionts: the eukaryotic mitochondria.