This review article is part of an entire issue dedicated to anaerobic protists. As an evolutionists, I don’t like the terms ‘opportunistic protists’ they use and ‘remnant mitochondria’ because the first is anthropomorph and teleological and the second implies that they once were mitochondria, which is far from proven (I think it is just plain wrong). I would change ‘remnant mitochondria’ with ’mitochondria-related’.
The data that is presented is in line with a gradual evolution of endomembranous structures that eventually evolved into mitochondria and started with an anaerobic variant that acquired more and more functions, including the use of oxygen as a teminal acceptor in the electron transport chain. So instead of gradually losing functions, they were gradually acquired.
This view has changed recently with the finding of mitochondrial remnant organelles termed mitosomes in Entamoeba (Mai et al., 1999; Tovar et al., 1999): this coupled with the presence of mitochondrial chaperonin genes (Arisue et al., 2002) confirms the secondary loss of mitochondria by this organism and this has been reviewed by Müller (2000). The presence of specialized membranes with electron transport functions has also been detected in Giardia (Lloyd et al., 2002). This organism once considered amitochondriate has mitochondrial-like chaperonin genes (Roger et al., 1998; Arisue et al., 2002), a nuclear coded valyl-tRNA synthetase (Hashimoto et al., 1998) and has recently been demonstrated to contain a fully functional mitochondrial iron–sulphur cluster assembly pathway involving the proteins IscS and IscU which are present in a double membrane-bound mitochondrial remnant organelle (Tovar et al., 2003). However, with the exception of Nyctotherus ovalis (Hackstein et al., 1999), all mitochondrial remnant organelles, including hydrogenosomes, lack an organelle genome, which was the major distinction between mitochondria and hydrogenosomes (Müller, 1993). The recently discovered Entamoeba remnant mitochondrial organelle, the mitosome, also lacks an organelle genome (León-Avila & Tovar, 2004), suggesting that reduction in organelle function was accompanied by loss of the genome. It is clear from several studies that these so-called anaerobes do encounter varying amounts of oxygen and therefore must have the ability to survive the effects of oxidative stress, and this is explored in the report by Lloyd et al. (2004). A mitochondrial relict organelle has also been described in cryptosporidia (Riordan et al., 2003). This observation was recently supported by the finding that Cryptosporidium parvum has genes (IscS and IscU) encoding a mitochondrial-type iron–sulphur cluster biosynthetic pathway and that these proteins target the proposed relict organelle. Thus, C. parvum is the latest to join the growing numbers that support the view that there are no truly amitochondriate extant eukaryotes (Müller, 2000). The report of mitochondrial-type hsp70 genes in two microsporidians strongly suggests that this group of amitochondriates has also undergone secondary mitochondrial loss (Arisue et al., 2002).