Second genesis of a plastid organelle

Abstract

One of the more remarkable discoveries of 20th century biology is that all eukaryotes are chimeras of two or more different cells (1). The tree of life does not just bifurcate through successive speciation events; its branches also graft, one onto another, merging lineages to produce novel fruit. Our mitochondria, and the photosynthetic plastids of plants and algae, are proof of the success of these new combinations. Both organelles were acquired from separate bacterial lineages, where a useful bacterium was internalized and maintained within the host cell—a process known as endosymbiosis. These organelles are no mere hitchhikers. They have been elaborately integrated with their hosts, so intimately that even their genes have been relinquished to the host cell to allow these partnerships to truly operate as one. To achieve this, the host cell learned to take responsibility for expressing the organelle genes and delivering the protein products back into the organelle according to its requirements. Our understanding of how these seminal achievements of organellogenesis occurred, however, is obscured by their antiquity, both organelles arising ∼1+ billion years ago (2). It is akin to studying modern jet aircraft in the hope of reconstructing the Wright brothers’ first daring attempts at flight. Iterative leaps of technology often mask the formative innovations. However, now we have a chance to revisit this process, as Nowack and Grossman in PNAS (3) show that the new photosynthetic endosymbiont of Paulinella chromatophora has also commenced this journey of genetic integration.