In fused cells, there's little mixing between the two kinds of mitochondrial DNA (red and green).

Mitochondria stow their DNA in structures called nucleoids. As Gilkerson et al. reveal, nucleoids are selfish and don't share their DNA with each other. The results might help explain some quirks of mitochondrial inheritance and support a proposed treatment strategy for illnesses caused by defects in the organelles.

A nucleoid can house up to 10 copies of a mitochondrion's genome. One reason that the structures intrigue researchers is that they might help control how the DNAs get parceled out when mitochondria divide. To understand mitochondrial DNA inheritance, researchers need to resolve whether nucleoids swap DNA. The question has remained unanswered because of the difficulty of tracking individual mitochondrial DNAs.

To overcome that problem, Gilkerson et al. fused two kinds of cells, each of which carried a different mitochondrial genome. In the merged cells, the researchers found, labeled versions of the two types of mitochondrial DNA rarely appeared together, suggesting that the nucleoids weren't mingling their contents. Stingy nucleoids could explain why cells that harbor a variety of mitochondrial genomes sometimes lose DNA diversity as they divide and sometimes don't. The outcome might depend on whether the DNAs in a particular nucleoid are uniform or varied. The results also offer support for plans to treat mitochondrial diseases by nudging cells to eliminate defective DNA. The lack of swapping will make it easier to purge faulty mitochondrial genomes, the researchers say.

Gilkerson, R.W., et al.
J. Cell Biol.