|MadSci Network: Evolution|
When people ask about eukaryotic organisms that lack mitochondria, the obvious candidate is Giardia intestinalis, a single celled intestinal parasite. For a long time, Giardia was thought to be a basal eukaryote, representing a survivor from the days before mitochondrial endosymbosis had occured.
However, as discussed by Mike Klymkowsky in this answer (1069520850.Cb), even Giardia contain remnants of mitochondria.
Eukaryotes that do not have obvious mitochondria are known as amitochondrial organisms. This group includes Giardia and other parasites such as Trachipleistophora hominis and Entamoeba histolytica. However, all of these appear to have remnants of mitochondria in their cytoplasms (as well as mitochondrial genes in their nuclear genomes), and in most cases these remants seem to retain some sort of function.
Take a look at the following papers for more information about the cytoplasmic and genomic mitochondrial remnants in these organisms.
Chan KW, Slotboom DJ, Cox S, Embley TM, Fabre O, van der Giezen M, Harding M, Horner DS, Kunji ER, Leon-Avila G, Tovar J. (2005) A novel ADP/ATP transporter in the mitosome of the microaerophilic human parasite Entamoeba histolytica. Curr Biol. 15(8):737-42.
Regoes A, Zourmpanou D, Leon-Avila G, van der Giezen M, Tovar J, Hehl AB. (2005) Protein import, replication, and inheritance of a vestigial mitochondrion. J Biol Chem. 280 (34):30557-63.
Williams BA, Hirt RP, Lucocq JM, Embley TM. (2002) A mitochondrial remnant in the microsporidian Trachipleistophora hominis. Nature. 418 (6900):865-9.
I thought that this quote from the end of the abstract in the Williams paper was particularly interesting, "The finding of relictual mitochondria in [amitochondrial organisms] provides further evidence of the reluctance of eukaryotes to lose the mitochondrial organelle, even when its canonical function of aerobic respiration has been apparently lost."
So, all eukaryotes appear to have mitochondria (or to be descended from an organism that had a functional mitochondrion). As Michael Onken points out in this answer (887302952.Ge), all mitochondria seem to be descended from the same ancestral endosymbiont. It seems likely that the first eukaryotes to incorporate endosymbiont organisms were wildy successful, and out-competed any nucleated non-endosymbiont organisms. With smaller, less complicated genomes and fewer internal structures to maintain, bacteria and archaea have been able to compete successfully against eukaryotes, where nucleated non-endosymbionts failed.
You can find much more information about research into amitochondrial organisms by searching for the word 'amitochondrial' on PubMed.
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