Figure 3. Schematic representation of two alternative explanations of the evolutionary history of the pyridine nucleotide disulfide oxidoreductase gene.

Dashed arrows indicate the possible lateral transfer of the gene. First hypothesis: The gene encoding pyr-redox exists in the gamma-Proteobacteria species and is absent in D. discoideum and T. thermophila. This gene was acquired from Legionella spp. by Mycobacterium spp., D. discoideum and T. thermophila. Second hypothesis: The gene encoding pyr-redox was lost from Legionella spp., D. discoideum and T. thermophila and was acquired later from Mycobacterium spp. T0 corresponds to the time of observation and T-x to the time when the event occurred.