Chlorophyll biosynthesis gene evolution indicates photosystem gene duplication, not photosystem merger, at the origin of oxygenic photosynthesis.

An open question regarding the evolution of photosynthesis is how cyanobacteria came to possess the two reaction center (RC) types, Type I reaction center (RCI) and Type II reaction center (RCII). The two main competing theories in the foreground of current thinking on this issue are that either 1) RCI and RCII are related via lineage divergence among anoxygenic photosynthetic bacteria and became merged in cyanobacteria via an event of large-scale lateral gene transfer (also called "fusion" theories) or 2) the two RC types are related via gene duplication in an ancestral, anoxygenic but protocyanobacterial phototroph that possessed both RC types before making the transition to using water as an electron donor. To distinguish between these possibilities, we studied the evolution of the core (bacterio)chlorophyll biosynthetic pathway from protoporphyrin IX (Proto IX) up to (bacterio)chlorophyllide a. The results show no dichotomy of chlorophyll biosynthesis genes into RCI- and RCII-specific chlorophyll biosynthetic clades, thereby excluding models of fusion at the origin of cyanobacteria and supporting the selective-loss hypothesis. By considering the cofactor demands of the pathway and the source genes from which several steps in chlorophyll biosynthesis are derived, we infer that the cell that first synthesized chlorophyll was a cobalamin-dependent, heme-synthesizing, diazotrophic anaerobe.