Another acquisition of a primary photosynthetic organelle is underway in Paulinella chromatophora

Abstract

Summary The birth of plastids brought photosynthesis to eukaryotes and had a huge impact on their evolution. Despite its importance, details of the plastid acquisition process through primary endosymbiosis are not well understood. Recently, a cercozoan testate amoeba, Paulinella chromatophora, has received considerable attention because it may be able to provide insights into the transition from a cyanobacterial endosymbiont to a photosynthetic organelle [1–3]. The P. chromatophora cell contains two chromatophores that look like cylindrical cyanobacteria [4,5], and it has been debated whether these chromatophores are endosymbiotic cyanobacteria or photosynthetic organelles [4–7]. The chromatophore genome of P. chromatophora strain M0880/a was recently sequenced, revealing that its size (∼1 Mbp) has been reduced and that it lacks several genes important to cyanobacteria, including a few photosynthetic genes [3]. Here, we obtained concrete evidence that psaE , one of the photosynthetic genes, is expressed from the nuclear genome of P. chromatophora . This indicates that the psaE gene has been transferred into the nuclear genome from the chromatophore. Thus, another primary endosymbiotic acquisition of a photosynthetic organelle is under way.