Integrity of the yeast mitochondrial genome, but not its distribution and inheritance, relies on mitochondrial fission and fusion

http://www.pnas.org/content/112/9/E947.full

Christof Osman, Thomas R. Noriega, Voytek Okreglak et al.

In this study, the authors develop a new means of imaging mtDNA nucleoids in budding yeast, without using dyes. They do this by introducing genes into the mitochondrial genome, whose protein products fluoresce. They constructed 3D images of mtDNA and mitochondrial content distributions. They found that the total length of the mitochondrial network correlates strongly with the number of nucleoids, with approximately 1 nucleoid per micron of network. They found that i) the inter-nucleoid distance, and ii) the distance between the network tip and the closest nucleoid, are significantly different to randomly distributed nucleoids. Indeed, they found that nucleoids tend to be preferentially distributed at the tips of the network, aiding mtDNA inheritance to daughter cells. 

After investigating the physiological case, the authors investigated the effect of knocking out mitochondrial fission (Dnm1) and fusion (fzo1). Remarkably, none of the above observations changed by a large magnitude, indicating that nucleoid distribution and inheritance in yeast is essentially fusion/fission independent. The authors show that respiratory-deficient cells accumulate in  fusion/fission knockouts. This suggests that the role of fusion and fission in yeast is for complementation of respiratory defects in individual mitochondria, rather than distribution and inheritance of mtDNA.