PlastidOrigin · Testing the paradigm of a single plastid origin in eukaryotes

Photosynthesis was acquired by eukaryotes through endosymbiosis with cyanobacteria, which resulted in new cellular organelles: the plastids. From the origin of plastids evolved the first eukaryotic algae, giving rise to land plants but also triggering the evolution of most photosynthetic eukaryotes by subsequent endosymbioses between these first algae and other eukaryotes. Thus, the origin of plastids profoundly changed the course of eukaryotic life by being the launching point that shaped the biological diversity of most primary producers. Despite this importance, our understanding of how plastids originated remains largely uncertain. The current paradigm describes this transformative event as a single primary endosymbiosis, but I argue here that critical data is lacking, notably from the vast hidden environmental diversity of microbes, to adequately test this hypothesis. In this project, I propose to gain insight into the origin of plastids by addressing the main questions: 1) What is the currently hidden diversity of high-ranked taxa related to primary photosynthetic lineages? 2) What are the feeding behaviors of these taxa and are they aplastidic? 3) Are some lineages genetically predisposed to establish plastids from the acquisition of foreign genes? 4) What was the composition, size, and origin of the ancestral primary plastid proteomes? To answer these questions, I will link third generation environmental sequencing, transcriptomics, and genomics to cell structure and behavior of novel key lineages related to primary algae, and produce crucially missing plastid proteomes to allow comprehensive comparative proteomic analysis. My project will not only have immediate implications on our understanding of the origin of plastids and more generally the fundamental process of endosymbiosis, but the approaches developed will be a test bed for future global studies aimed at understanding the evolution and ecology of the microbial majority of complex life.