BACKGROUND: Mitochondria of opisthokonts undergo permanent fission and fusion 
throughout the cell cycle. Here, we investigated the dynamics of the mitosomes, 
the simplest forms of mitochondria, in the anaerobic protist parasite Giardia 
intestinalis, a member of the Excavata supergroup of eukaryotes. The mitosomes 
have abandoned typical mitochondrial traits such as the mitochondrial genome and 
aerobic respiration and their single role known to date is the formation of 
iron-sulfur clusters.
RESULTS: In live experiments, no fusion events were observed between the 
mitosomes in G. intestinalis. Moreover, the organelles were highly prone to 
becoming heterogeneous. This suggests that fusion is either much less frequent 
or even absent in mitosome dynamics. Unlike in mitochondria, division of the 
mitosomes was absolutely synchronized and limited to mitosis. The association of 
the nuclear and the mitosomal division persisted during the encystation of the 
parasite. During the segregation of the divided mitosomes, the subset of the 
organelles between two G. intestinalis nuclei had a prominent role. 
Surprisingly, the sole dynamin-related protein of the parasite seemed not to be 
involved in mitosomal division. However, throughout the cell cycle, mitosomes 
associated with the endoplasmic reticulum (ER), although none of the known 
ER-tethering complexes was present. Instead, the ER-mitosome interface was 
occupied by the lipid metabolism enzyme long-chain acyl-CoA synthetase 4.
CONCLUSIONS: This study provides the first report on the dynamics of mitosomes. 
We show that together with the loss of metabolic complexity of mitochondria, 
mitosomes of G. intestinalis have uniquely streamlined their dynamics by 
harmonizing their division with mitosis. We propose that this might be a 
strategy of G. intestinalis to maintain a stable number of organelles during 
cell propagation. The lack of mitosomal fusion may also be related to the 
secondary reduction of the organelles. However, as there are currently no 
reports on mitochondrial fusion in the whole Excavata supergroup, it is possible 
that the absence of mitochondrial fusion is an ancestral trait common to all 
excavates.