MICROBIAL SYMBIOSES Symbiosis, the living together of two dissimilar organisms (De Bary, 1898), is a key principle in nature which contributed to the origin of eukaryotes around two billion years ago and continues to have a profound impact on the ecology and evolution of extant organisms. My lab is interested in microbial symbioses in which eukaryotes serve as hosts for intracellular bacteria. By focusing on selected model systems, we are studying molecular and evolutionary mechanisms underlying and driving these associations. Chlamydiae are important pathogens of humans but also occur as symbionts of environmental, free-living amoebae. Analysis of only recently discovered chlamydial symbionts such as Protochlamydia amoebophila, Parachlamydia acanthamoebae, and Simkania negevensis, and comparison with their pathogenic counterparts allows us to investigate the evolution of the intracellular life style of chlamydiae and their adaptation to different eukaryotic hosts. This is complemented by the analysis of other, phylogenetically distant bacterial symbionts of amoebae, including Amoebophilus asiaticus.