Giardia lamblia is familiar to hikers and campers as an intestinal parasite best avoided by filtering one's drinking water. To biologists, Giardia is of interest because it lacks mitochondria and has been thought to be representative of simple, modern-day eukaryotes (others of which lack peroxisomes or the Golgi apparatus) that are descended from early branches of the eukaryotic limb of the "tree of life."
Arkhipova and Morrison have looked for the presence of transposable elements; the issue here is that Giardia reproduce asexually and that transposition of regions of the genome would be deleterious. They find evidence of retrotransposons--which encode a reverse transcriptase--and locate these elements next to the telomeric regions of the Giardia chromosomes, revealing a similarity to Drosophila retrotransposons, which have assumed the role of maintaining telomeres in the absence of the telomeric repeats and telomerase enzyme found in other organisms. This unexpected coincidence of apparently functional transposons in an asexual organism adds to the surprises these deep-branching eukaryotes are anticipated to provide as their genome sequencing projects are completed, as recently reviewed by Dacks and Doolittle.
The genome of another minimalist parasite, Encephalitozoon cuniculi, has been sequenced by Katinka et al. In this case, metabolic functions of the missing mitochondrion appear to have been cobbled together from a mix of host and parasite components in the absence of the critical enzyme pyruvate:ferredoxin oxidoreductase (for more on this protein, see Chabriere et al., Reports, this issue). As it turns out, Giardia do contain this enzyme, whereas Encephalitozoon lack transposons. -- GJC
Proc. Natl. Acad. Sci. U.S.A. 98, 14497 (2001); Cell 107, 419 (2001); Nature 414, 450 (2001).
