Evolution of Photosynthetic Organelles Based on Studies of the Thecamoeba Paulinella chromatophora

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The thecamoeba Paulinella chromatophora (Rhizaria) harbors two conspicuous photosynthetic units, termed chromatophores, which represent cyanobacterial endosymbionts likely to be en route to becoming ‘typical’ photosynthetic organelles. The chromatophores evolved independently from plastids and likely represent a much more recent endosymbiotic event. The size of the chromatophore genome is intermediate between that of a free-living cyanobacterium and a plastid, suggesting that integration of endosymbiont and host functionalities is ongoing. The first evidence for gene transfer from the chromatophore genome to the nuclear genome of the host was recently reported. This project aims at exploring the occurrence and mode of further gene transfer from the chromatophore to the host nuclear genome, clarifying the question of whether protein import into the chromatophore occurs as in canonical organelles, examining the functionality of the proteins encoded by the transferred genes using a heterologous cyanobacterial system, and exploring mechanisms by which control over chromatophore growth and division was established. Another important aim of my work will be to generate an axenic culture of P. chromatophora, determine its nutritional requirements and improve its growth rate with the broader goal of making this unique organism a model for elucidating processes central to the evolution of a photosynthetic organelle.