The evolutionary transition from prokaryotes to eukaryotes—characterized by eukaryotic cells obtaining a nucleus and two membrane compartments—is still subject to considerable debate. It has been suggested that a nucleus developed first, followed by the acquisition of mitochondria, a theory that has partly relied on the single-celled eukaryote Giardia intestinalis because it has a nucleus and no mitochondria, suggestive of an intermediate step. The recent identification of mitochondrial remnants in other amitochondrial protists suggests this may not be true. In the November 13 Nature, Jorge Tovar and colleagues at Royal Holloway, University of London report that Giardia intestinalis possesses highly reduced mitochondria remnants called mitosomes (Nature, 426:172-176, November 13, 2003).

The work was stimulated by the cloning of a Giardia gene known in other organisms to be involved in the biosynthesis of Fe-S clusters, a critical function of the mitochondria. It was the second such gene to be identified in Giardia (the proteins of which are named IscU and IscS), suggesting the potential for a mitochondrion-derived organelle. Tovar et al. generated antibodies to IscU and IscS and demonstrated that both proteins were present in a high-speed centrifugation pellet containing membranes and membrane-bound organelles, but not in the cytosol. This high-speed sediment was also capable of assembling Fe-S clusters in vitro, signifying the presence of this mitochondrial biosynthetic pathway in the amitochondrial Giardia cells.

They then attempted to identify the cellular location of the IscU and IscS. Confocal scanning fluorescence microscopy and anti-IscS and anti-IscU antibodies suggested that the proteins were localized to small cellular structures distributed throughout the cytoplasm. A more detailed view of these structures was provided by immunoelectron microscopy, which localized IscS and IscU to these small organelles and revealed them to be “bounded by two limiting membranes,” a distinctive feature of mitochondria (since Giardia contains no chloroplast-related biochemistry).

“Our data provide direct physical evidence for the presence of mitochondrial remnant organelles (mitosomes) in Giardia, as postulated on the basis of previous phylogenetic and biochemical studies,” conclude the authors.

In an accompanying preview article, Katrin Henze and William Martin from the University of Dusseldorf comment, “These findings mark a turning point for views of early eukaryotic and mitochondrial evolution: Giardia's place as an intermediate stage in standard schemes of eukaryotic evolutionary history is no longer tenable.”

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