Analysis of the genome of Cyanophora paradoxa: An algal model for understanding primary endosymbiosis

Algae and plants rely on the plastid (e.g., chloroplast) to carry out photosynthesis. This organelle traces its origin to a cyanobacterium that was captured over a billion years ago by a single-celled protist. Three major photosynthetic lineages (the green algae and plants [Viridiplantae], red algae [Rhodophyta], and Glaucophyta) arose from this primary endosymbiotic event and are putatively united as the Plantae (also known as Archaeplastida). Glaucophytes comprise a handful of poorly studied species that retain ancestral features of the cyanobacterial endosymbiont such as a peptidoglycan cell wall. Testing the Plantae hypothesis and elucidating glaucophyte evolution has in the past been thwarted by the absence of complete genome data from these taxa. Furthermore, multigene phylogenetics has fueled controversy about the frequency of primary plastid acquisitions during eukaryote evolution because these approaches have generally failed to recover Plantae monophyly and often provide conflicting results. Here, we review some of the key insights about Plantae evolution that were gleaned from a recent analysis of a draft genome assembly from Cyanophora paradoxa (Glaucophyta). We present results that conclusively demonstrate Plantae monophyly. We also describe new insights that were gained into peptidoglycan biosynthesis in glaucophytes and the carbon concentrating mechanism (CCM) in C. paradoxa plastids.