Abstract

Bacteria are divided primarily into monoderms and diderms. Photosynthetic species are spread among the taxonomic groups, some having type I reaction centers, others with type II reaction centers, and some containing both. In most bacterial phylograms, photosystem types and diderm taxa are polyphyletic. A more parsimonious arrangement, which is supported by photosystem evolution, as well as additional sets of molecular characters, suggests that endosymbiotic events resulted in the formation of the diderms. In the model presented, monoderms readily form a monophyletic group, while diderms are produced by at least two endosymbiotic events, followed by additional evolutionary changes.