Figure 2.

Phylogeny of Type 2 phototrophic reaction center proteins made from concatenated sequences of PufL and PufM. Lineages with a fused pufLM gene are highlighted in pink while lineages with unfused reaction center genes are shown in green. The phylogeny of reaction center proteins is incongruent with the organismal tree (Figure 1), suggesting a history of horizontal gene transfer. However, the monophyly of fused pufLM genes (pink) is consistent with a singular gene fusion event.