The origin of oxygenic photosynthesis in the ancestor of algae and plants was a turning point for our planet, ultimately laying the foundation for the rise of humans and other multicellular life. The widespread photosynthetic organelle in eukaryotes, the plastid, originated >1 billion years ago in a heterotrophic protist via primary endosymbiosis. In my talk, I will summarize some key aspects of genome evolution in these taxa to set the stage for our recent work with the only other known case of primary plastid origin, in the clade of the photosynthetic amoeba Paulinella. Members of this genus contain a photosynthetic organelle, referred to as a chromatophore, which is derived from an alpha-cyanobacterial endosymbiont. I will present the results of our analysis of Paulinella cultures, its physiology and symbionts, and genome and transcriptome data from two species, P. chromatophora and P. micropora. This information provides the first direct insights into what was once considered an unreachable goal: to reconstruct with confidence early events that lead from symbiont to organelle.
Debashish Bhattacharya is a Distinguished Professor in the Department of Biochemistry and Microbiology at Rutgers, The State University of New Jersey (USA). Work in the Bhattacharya lab primarily addresses the origin and evolution of photosynthetic eukaryotes. We use a variety of techniques ranging from algal culture and physiology, standard and single-cell genome sequencing, and functional genomics to study how algae gained and orchestrate the functions of their photosynthetic organelle (the plastid) and how they adapt to changing environmental conditions. We have also recently initiated a large-scale genomic and functional genomic analysis of corals and their symbionts to understand the basis of biomineralization, the genetic structure of coral populations, and their pathways of stress tolerance.