The cartoon illustrates the proposed mechanism of evolutionary contingency that, due to shared pathways of lipid metabolism and defense against reactive oxygen species, whichever organelle starts to degenerate first places a constraint on the reductive evolution of the other until such time that these metabolic requirements become lifted through parasitism or full anaerobiosis. In the case of P. lacertae/Blastocystis the degeneration of the peroxisomal organelle manifests as a more conserved MRO. In other lineages the opposite may be the case. The span of organelle reduction is shown for peroxisomes (above the line) and MROs (below the line) with the line linking them representing their shared metabolic burden. I-V, respiratory complexes 1-5.
