Toward experimental endosymbiosis – Expendo

Metabolic integration of established endosymbionts into novel organelles, such as the mitochondrion or plastids, defines events of the outmost rarity that have had far reaching consequences in the forging of the first eukaryotes and of all their photosynthetic derivatives. While the endosymbiont theory states that mitochondria and plastids derive from alpha-proteobacteria and cyanobacteria respectively, metabolic integration of these endosymbionts has evidently implied the massive participation of genes whose ancestry cannot be traced back to these two sole clades. In particular, plastid endosymbiosis is known to correlate with a phylogenomic imprint from intracellular chlamydia pathogens specific and selective to all lineages derived from this unique event. We have recently shown that enzymes, that are thought to have been responsible for photosynthetic carbon assimilation in the host cytosol, define metabolic effector proteins secreted, in this compartment, by intracellular chlamydiales pathogens. Hence, the intracellular pathogens and incipient cyanobacterium were tied together with their host in a tripartite symbiosis where the three partners coded essential components of a common photosynthetic carbon assimilation pathway. This suggests that intracellular bacteria living as temperate pathogens or symbionts within eukaryotes may define major players down the path of metabolic integration of future organelles. Such bacteria are usually viewed as degenerate genomes that evolved from free living sister lineages by selective gene losses. However, the intracellular life style also implied the evolution of hundreds of bacterial protein effectors secreted in the host, that ensure intracellular life either within phagocytosis derived vacuoles, or more rarely in the cytosol. Because direct microinjection of free living bacteria in the eukaryotic cytosol fails to yield any multiplication of the injected organisms unless they already define intracellular pathogens or symbionts, we propose to drive free living cyanobacteria into endosymbiosis thanks to helper intracellular symbionts. Our recent collaborative work on the impact of chlamydia in plastid endosymbiosis has yielded an unexpectedly detailed molecular description of the early events that may have triggered plastid endosymbiosis, including the molecular nature of the symbiotic gene, and the precise nature of the major carbon and ATP transporters involved. This speculative scenario is presently well sustained by a series of distinct phylogenetic and biochemical observations that together make a strong case for the implication of chlamydiales in the initial steps of plastid endosymbiosis. We believe that recapitulating the very first steps of plastid endosymbiosis may be therefore feasible and defines a worthwhile long-term scientific objective. The present short term proposal aims at providing an experimental model system in the social amoeba Dictyostelium discoideum to investigate the initial steps of endosymbiosis in general, and of plastid endosymbiosis in particular. In this grant proposal, our major goal will be to turn an intracellular Chlamydial pathogen that we have very recently isolated into an established symbiont. This organism named Estrella lausanensis defines the only Chlamydiale able to replicate in this model phagotroph. This will be achieved by complementing defects of host glycogen metabolism by the secretion of the corresponding effector enzyme from the pathogen. A first attempt will be made to use the chlamydial symbiont as a helper genome to drive bacteria unaccustomed to intracellular life into stable endosymbionts. The research proposal is in addition specifically designed to generate sufficient high impact knowledge in glycogen metabolism and chlamydial biology to overcome the risk involved in tackling such a novel and important topic.