Dinotoms: a novel model system for plastidogenesis studies between kleptoplasty and endosymbiosis

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It is widely accepted that all plastids were originally independent microalgae and arose by endosymbiotic events; however, many processes of plastidogenesis remain a mystery as almost all phototrophs already passed through the evolutionary developmental stages. Many model organisms are used to shed light on the developmental processes of plastids, e.g., sea slugs Elysia spp., ciliates Mesodinium spp., or the amoeba Paulinella chromatophore. The yet most unexplored question of plastidogenesis is: how do free-living microalgae transform into early-stage endosymbionts. However, none of the present model organisms can be applied to answer this question, because they are either still in the transient (“kleptoplastic”), or already in the permanent endosymbiotic stage. Dinotoms, dinoflagellates that harbour diatoms as their photosynthetic endosymbionts, are ground-breaking organisms that allow us to experimentally test these elsewise hidden processes. Their endosymbiotic diatoms exhibit three successive evolutionary stages: a kleptoplastic stage (D. capensis), in which ingested diatoms are maintained for approximately two months; a stage in which multiple endosymbiotic diatoms are permanently maintained (D. kwazulunatalensis); and a stage where a single diatom is permanently maintained (other known dinotoms). To establish D. capensis, D. kwazulunatalensis and the diatom Nitzschia cf. agnita, the plastidial source of D. capensis, as a model system that is explorable with modern molecular biology techniques, I and my collaborators are currently generating their genomic and transcriptomic information as well as genetic modification tools. During the first DFG-funded period, I focused on establishing the D. capensis strain to clarify its evolutionary stage and to investigate the bioactivity of N. cf. agnita after ingestion by D. capensis. This proposal aims to use the newly-established model system to elucidate the molecular evolutionary processes from kleptoplasty to early permanent endosymbiosis. I propose three hypotheses, based on my achievements of the first DFG period. These hypotheses are A) host dinoflagellates tightly control their endosymbiotic diatoms metabolically already at the kleptoplastic stage; B) the presence or absence of two systems, i.e., the control of diatom cell cycle gene expression and the control of nitrate supply for endosymbiotic diatoms, separates the evolutionary stages between D. capensis and D. kwazulunatalensis; and C) diatom mitochondria reduce their activity already at the kleptoplastic stage and the functions are partially replaced by dinoflagellate mitochondria. These projects will be conducted with the already-established N. cf. agnita transformation tools and the analysed dinotom transcriptomic data. My projects aim to elucidate how deeply endosymbiotic diatoms are integrated and controlled by the host at the different evolutionary stages of D. capensis and D. kwazulunatalensis.