Fig. 6. Phylogenetic tree of PsaA and PsaB.

Some of the key events in the evolutionary history of PSI are overlaid with PsaA and PsaB evolution. PsaA and PsaB originated from an ancient gene duplication when the photosystem was homodimeric. The most recent common ancestor (MRCA) of Cyanobacteria, represented by green circles, often thought to have originated before the Great Oxidation Event, inherited both PsaA and PsaB and already had heterodimeric PSI. All cyanobacteria retain subunits PsaC, PsaD, PsaE, PsaF, PsaI, PsaJ, PsaL, and PsaM, which implies that these were added to PSI before the MRCA of cyanobacteria and a capacity for trimerization (yellow triangle). On PsaA, Gloeobacterales, including A. panamensis, its close relative Candidatus C. vandensis, and species of Gloeobacter make the earliest branching clade. On PsaB, Gloeobacterales is separated into two distinct clades, a phenomenon that had been observed before (13), but likely represents a long branch attraction artifact triggered by the long branch that separates PsaA and PsaB. PsaK appears to have originated after the branching event leading to the Gloeobacterales (dark blue triangle), while PsaX appears to have originated close to or during the major cyanobacteria radiation leading to microcyanobacteria and macrocyanobacteria (light blue circle), as defined by Sanchez-Baracaldo et al. (1).