Abstract 289 13.1 Introduction 289 13.2 Derived Mitochondrial Organelles 290

13.2.1 Hydrogenosomes 291 13.2.2 Mitosomes 292

13.3 Morphology 293 13.4 Metabolic Capacity 294 13.5 Biogenesis 297

13.5.1 Protein Import 299 13.5.2 Organellar Dynamics 300

13.6 Evolutionary Considerations: A Common Selective Force? 300 13.7 Conclusions 301 Acknowledgments 302 References 302

Mitochondria are considered defining features of eukaryotic cells, but many eukaryotic microorganisms do not possess these cellular organelles. Research on amitochondriate eukaryotes has revealed significant compartmentalization of metabolic functions in distinctive organelles that at first inspection appear to have little in common with mitochondria. However, more careful investigations have clearly linked these organelles to mitochondria and there is increasing evidence that they all likely represent evolutionary variants of a single microbial endosymbiosis that occurred before the divergence of all extant eukaryotes. The apparent reluctance of eukaryotic organisms to part with these seemingly valuable organelles points to the existence of a hitherto-unidentified organellar function essential for cell survival in both aerobic and anaerobic environments.