MULTIPLE ORIGINS OF ANAEROBIC CILIATES WITH HYDROGENOSOMES WITHIN THERADIATION OF AEROBIC CILIATES

Some ciliates live anaerobically and lack mitochondria, but possess hy drogenosomes: organelles that contain hydrogenase and produce hydrogen . The origin of hydrogenosomes has been explained by two competing hyp otheses: (i) they are biochemically modified mitochondria; or (ii) the y are derived from endosymbiotic association(s) of ciliates and anaero bic eubacteria that possessed the hydrogenosome biochemistry. Phylogen etic analyses of representative aerobic, and anaerobic hydrogenosomal ciliates using host nuclear SSU rDNA sequences indicate a minimum of t hree, but more likely four, separate origins of hydrogenosomes. Wherea s this does not refute either hypothesis, the implausibility of multip le convergent endosymbioses gives further support to the view that hyd rogenosomes in ciliates derive from an existing organelle, which ultra structural evidence suggests is the mitochondrion. Our results indicat e a considerable potential for physiological-biochemical plasticity am ong a group of predominantly aerobic eucaryotes, and provide a phyloge netic framework to further refine and test hypotheses of the origins o f the hydrogenosomal enzymes.