Cryptic organelles in parasitic protists and fungi.

A number of parasitic protists and fungi have adopted extremely specialised characteristics of morphology, biochemistry, and molecular biology, sometimes making it difficult to discern their evolutionary origins. One aspect of several parasitic groups that reflects this is their metabolic organelles, mitochondria and plastids. These organelles are derived from endosymbiosis with an alpha-proteobacterium and a cyanobacterium respectively, and are home to a variety of core metabolic processes. As parasites adapted, new demands, or perhaps a relaxation of demands, frequently led to significant changes in these organelles. At the extreme, the organelles are degenerated and transformed beyond recognition, and are referred to as "cryptic". Generally, there is no prior cytological evidence for a cryptic organelle, and its presence is only discovered through phylogenetic analysis of molecular relicts followed by their localisation to organelle-like structures. Since the organelles are derived from eubacteria, the genes for proteins and RNAs associated with them are generally easily recognisable, and since the metabolic activities retained in these organelles are prokaryotic, or at least very unusual, they often serve as an important target for therapeutics. Cryptic mitochondria are now known in several protist and fungal parasites. In some cases (e.g., Trichomonas), well characterised but evolutionarily enigmatic organelles called hydrogenosomes were shown to be derived from mitochondria. In other cases (e.g., Entamoeba and microsporidia), "amitochondriate" parasites have been shown to harbour a previously undetected mitochondrial organelle. Typically, little is known about the functions of these newly discovered organelles, but recent progress in several groups has revealed a number of potential functions. Cryptic plastids have now been found in a small number of parasites that were not previously suspected to have algal ancestors. One recent case is the discovery that helicosporidian parasites are really highly adapted green alga, but the most spectacular case is the discovery of a plastid in the Apicomplexa. Apicomplexa are very well-studied parasites that include the malaria parasite, Plasmodium, so the discovery of a cryptic plastid in Apicomplexa came as quite a surprise. The apicomplexan plastid is now very well characterised and has been shown to function in the biosynthesis of fatty acids, isopentenyl diphosphate and heme, activities also found in photosynthetic plastids.