Because the hypothesized lateral gene transfer would probably have occurred after the acquisition of the fe-hydrogenase gene by the diplomonad lineage, this particular result does not disprove the hydrogen hypothesis (Martin and Muller, 1998).
Phylogenetic analyses suggested that the short Fe-hydrogenase genes of entamoeba and diplomonads share a common ancestor, while the long Fe-hydrogenase gene of entamoeba appears to have been laterally transferred from a bacterium.
Second, entamoebas and diplomonads have long been thought not to produce hydrogen gas in culture, although a recent report suggests giardias may produce hydrogen under anaerobic conditions (Lindmark and Muller, 1973; Reeves, 1984; Brown et al., 1998; Muller, 1998; Lloyd and Harris, 2002).
With the goal of understanding better the biochemistry and evolution of Fe-hydrogenases of entamoebas and diplomonads, we performed the following studies.
The newly recognized relationship between mitochondria and hydrogenosomes, as well as the research on microsporidia and diplomonads, pushes the origin of these two organelles back to the earliest eukaryotes, if not earlier.
Finally, the eukaryotes that became intracellular parasites, with little need of making ATP of their own, would ultimately have shed the endosymbiont almost completely Microsporidia and diplomonads would have been their descendants.
If this is true, intestinal parasitism is clearly plesiomorphic for the diplomonads, having arisen long before the divergence of this group from the parasitic ancestor shared with the retortomonads.
Though parasitism has been abandoned by some diplomonads, mitochondria have not reappeared.
Examination of some of the character state transformations in the phylogenetic analysis of the Diplomonadida (see Siddall et al., 1992) illuminates how some diplomonads are able to pursue a free-living mode without mitochondria.
Among the early-branching amitochondriate protists, diplomonads
remained as possible candidates for being primitively amitochondriate.
There is evidence that many archezoan groups are secondarily amitochondriate (4-8) although the diplomonads and parabasalids have clung more resolutely to the base of the eukaryotic tree than other taxa.
CAVALIER-SMITH: Mitch Sogin presented data at another conference in which he had a retortamonad as a sister group to diplomonads on a ribosomal RNA tree, and unless I am mistaken, that would be molecular evidence for two of the three Metamonad groups actually belonging together.