Chlamydiales implication in the evolution of Archaeplastida: Testing the Ménage à Trois Hypothesis

Among the Eukaryota Kingdom, many lineages emerged as a result of symbiosis. The most described and most important for the establishment of life on Earth as we know it  was a unique symbiosis between a heterotrophic eukaryote and a cyanobacteria, which became a new organelle called plastid. This major event in evolution, also known as the plastidial primary endosymbiosis, gave birth to the Archaeplastida, composed of Rhodophyta (red algae), Glaucophyta and Chloroplastida (green algae and plants).
Recently, a paradigm shift in the acquisition of photosynthesis has proposed the implication of an intracellular obligate pathogen in plastid establishment. This hypothesis, dubbed the Menage-a-trois Hypothesis (MATH), suggests an active and direct role of Chlamydiales in primary endosymbiosis, which would have provided many critical genes to the cyanobiont in the common inclusion vesicle. The expression and efficient localization of these genes, such as key transporters and glucan transferases, would have initiated the biochemical fluxes of symbiosis. Even if still controversial, the MATH is supported by molecular, biochemical and phylogenetic evidence. Hence, studies performed more than a decade ago concluded that 30-100 genes would have been transferred from Chlamydiales pathogens to the ancestor of Archaeplastida.