"Harnessing chemoautotrophic symbionts is a recurring evolutionary strategy. Eukaryotes from six phyla as well as one archaeon have acquired sulfur-oxidizing bacteria from three groups, Alpha-, Gamma- and Epsilonproteobacteria. The intracellular ‘Cand. Riegeria’ endosymbionts of the mouth and gutless flatworms Paracatenula are the only alphaproteobacterial representatives. They form a family-level clade within Rhodospirillales and have been co-evolving with their hosts for the last 500 million years. The aim of this project is to study the evolution and ecophysiology of Cand. Riegeria galateiae (CRG). We will sequence the genome of CRG and analyze its evolution in a symbiosis-related context by comparing it to its closest free-living relatives. Comparison of the CRG genome to the genomes of other thiotrophic symbionts will reveal genes/pathways which are shared between these symbionts or are specific to CRG. We will screen the CRG genome and other intracellular Alphaproteobacteria for genes with a potential function in their intracellular lifestyle. Using RNA-seq we will analyze the transcriptomic reaction of CRG to fluctuations in its chemical environment and trace the transcriptional response and the carbon flow through the symbiotic association on a single cell level. The main training aspects for the researcher will be a multidisciplinary application of single cell level techniques in genomics, transcriptomics and ecophysiology. This research will contribute to our knowledge on the diversity and the function of chemosynthetic microbes, both free-living and symbiotic. This project on an ancient clade of intracellular endosymbionts with a phylogenetic position distant to other symbiotic Alphaproteobacteria will illuminate the common genetic predispositions that have allowed several members of this class to become successfully incorporated into eukaryotic cells as mutualists or pathogens."