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Plant mitochondrial dynamics (Logan, 2006)

Another recent review from Logan with an emphasis on evolution of the mitochondrial division apparatus. It is full of the usual assumptions and deductions derived from the endosymbiotic theory, and difficult to grasp what is really observed and what are the speculations.

The next paragraph shows that we would have expected bacterial division proteins but they lack in most euakaryotes, basically falsifying the endosymbiotic hypothesis. Instead, it is claimed that they all lost these essential proteins although a mechanism is not offered. Observe how the first sentence is used as the truth and that all deductions are based on this (speculative) assumption. 

It is now generally agreed that mitochondria evolved from free-living alpha-proteobacteria following a single endosymbiotic event around two billion years ago [15–18]. Since that time, eukaryotic cells have diversified into the many forms seen today. The evolution of mitochondria has resulted in the loss from yeast and all higher eukaryotes of the prokaryotic genes originally involved in division of the symbiont. Bacterial cytokinesis involves a suite of proteins and the earliest-acting and most phylogenetically widespread of these is FtsZ [19]. FtsZ is found in nearly all prokaryotes and assembles into a large oligomeric structure forming a contractile ring around the interior surface of the cell membrane [20]. However, while FtsZ is likely to be ubiquitous amongst prokaryotes it is absent from the mitochondria of yeast and higher eukaryotic cells [21–23]. Indeed, up until 2000, no eukaryote had been identified that contained mitochondria that used FtsZ as part of the division apparatus.

But homologues were found in some organisms, claimed to be the missing links. It is not clear [ToDo, find out how the homology was] however, how homologous they were and what the function of these homologous proteins was. In any case, there are many homologies between eukaryotes and bacteria so that is not so surprising. More interesting is that it is claimed (without proof) that 1) all proteins were transferred to the nucleus and 2) they were subsequently transferred back.

The evolutionary missing-link, a eukaryote with FtsZ-using mitochondria, was discovered by Beech and colleagues [24]. By screening a cDNA library of the unicellular chromophyte (stramenopile/heterokont) alga Mallomonas splendens with a fragment of the ftsZ1 gene of the alpha-proteobacterium Sinorhizobium meliloti [25], Beech et al. identified a single-copy nuclear gene named MsFtsZ-mt [24]. Phylogenetic analysis confirmed that MsFtsZ-mt is closely related to FtsZs of alpha-proteobacteria [24]. Later in 2000, a mitochondrial-type FtsZ homologue was reported in the genome of the red alga Cyanidioschyzon merolae [26]. Phylogenetic analysis suggested that this gene, named CmftsZ1, is a mitochondrial-type FtsZ gene and immunoblotting with antisera raised against bacterial-expressed CmftsZ1 demonstrated that the protein was located within mitochondria [26]; subsequent research has identified a second C. merolae mitochondrial-type FtsZ gene (the genes are now named CmftsZ1-1 and CmftsZ1-2) [27]. Two mitochondrial-type ftsZ genes (named fszA and fszB) have also been discovered recently in the amoeboid protist Dictyostelium discoideum [28].

During the course of evolution all mitochondrial-type FtsZ genes identified to date transferred from the mitochondrial genome to the nucleus and are now targeted back to mitochondria.[…]The discovery of mitochondrial FtsZ genes in M. splendens, C. merolae and D. discoideum has plugged a gap in the evolution of the mitochondrion from its free-living bacterial past to its present-day central role in mitochondriate eukaryotes [21] but still leaves unanswered the question: Why have higher eukaryotes lost mitochondrial FtsZ?

 

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