Mitochondria and primary plastids evolved more than 1 billion years ago from free-living bacteria via endosymbiosis. Organelle evolution was accompanied by a massive size reduction of the endosymbiont genome. Thousands of endosymbiont genes were transferred into the host nuclear genome and mechanisms for trafficking nuclear-encoded proteins into the endosymbiont-derived organelle evolved. Providing a multitude of new bioenergetic and biosynthetic abilities to the host, organellogenesis has been one of the most transformative forces during evolution of the eukaryotic cell. However, although the acquisition of mitochondria and plastids has been evolutionary very successful and has had a profound impact on ecosystems on this planet, organellogenesis has remained a very rare event.

Our group is fascinated by the evolutionary processes that enable the merger of two physiologically and genetically very distinct cells. In the resulting chimeric organism, complex interactions between prokaryote-derived organelles and the surrounding eukaryotic cell fine tune organelle metabolism, growth, and division in response to the state of the host cell and environmental factors. Our projects aim at gaining insight into the molecular mechanisms that underlie the formation of a chimeric organism. Due to the ancient origin of mitochondria and plastids, however, it is challenging to reconstruct evolution of the complex interaction networks between host and endosymbiont, based on studies of these organelles. Therefore, we study more recently evolved endosymbiotic systems: the amoeba Paulinella chromatophora, which harbors nascent photosynthetic organelles of cyanobacterial origin that are termed ‘chromatophores’, and the trypanosomatid Angomonas deanei, which harbors the endosymbiotic beta-proteobacterium Candidatus Kinetoplastibacterium crithidii.

Our research addresses the following key questions:

• To what extent resemble bacterial endosymbionts early eukaryotic cell organelles?
• How readily do bacterial symbionts become dependent on the import of host-encoded proteins and what are the functions of these imported proteins?
• Through which pathways can cytoplasmically-synthesized proteins traffic into the endosymbiont (or endosymbiont-derived organelle)?
• Are some of these mechanisms conserved across eukaryotic phyla?
• What is the role of the association of the endosymbiont with specific host cell compartments for the metabolic integration of the endosymbiont?

These questions are addressed by phylogenetic, molecular, biophysical, structural, and protein biochemical approaches.