Evolving a photosynthetic organelle

The plastids, or chloroplasts, of algae and plants evolved from cyanobacteria by endosymbiosis. This landmark event conferred on eukaryotes the benefits of photosynthesis - the conversion of solar energy into chemical energy - and in so doing had a huge impact on the course of evolution and the climate of Earth [1]. From the present state of plastids, however, it is difficult to trace the evolutionary steps involved in this momentous development, because all modern-day plastids have fully integrated into their hosts. Paulinella chromatophora is a unicellular eukaryote that bears photosynthetic entities called chromatophores that are derived from cyanobacteria and has thus received much attention as a possible example of an organism in the early stages of organellogenesis. Recent studies have unlocked the genomic secrets of its chromatophore [2,3] and provided concrete evidence that the Paulinella chromatophore is a bona fide photosynthetic organelle [4]. The question is how Paulinella can help us to understand the process by which an endosymbiont is converted into an organelle.Photosynthetic eukaryotes are a tremendously diverse collection of organisms, from bacterium-sized unicells and giant kelp in the oceans to the plants and trees that inhabit dry land. Multiple rounds of eukaryote-eukaryote endosymbioses have resulted in a tangled web of plastid-bearing lineages [5]. Yet despite this complexity, all plastids appear to trace back to a single ancient endosymbiotic event between cyanobacteria and a heterotrophic host eukaryote. This so-called primary endosymbiosis probably occurred over one billion years ago [6]. The primary plastids of land plants, green algae, red algae and glaucophytes differ tremendously from their presumed cyanobacterial progenitors. What we know for certain is that the majority of the genes present in the endosymbiont were lost or transferred to the host nuclear genome and the protein products of many of these genes are now reimported into p