Heteroplasmy

Heteroplasmy describes the presence of different copies of organellar DNA (mitochondrial DNA (mtDNA) or plastid DNA) within a single cell or individual. Although previously considered a transient, and often, deleterious state, persistent populations of heteroplasmic individuals have been recorded across plants, animals, and fungi. In animals and fungi, heteroplasmy can be found in mtDNA while plants can exhibit heteroplasmy in mtDNA and ptDNA. Heteroplasmy exists at various degrees of severity and can be caused by various processes such as somatic mutation, DNA recombination, and paternal mtDNA leakage. It is hypothesized to also be caused by the incorporation of DNA from endosymbionts, though this is a relatively recent hypothesis and remains to be tested.

Although low levels of heteroplasmy are common among human populations, severe heteroplasmy can cause mitochondrial diseases when deleterious mtDNA mutants reach a threshold beyond which the proportion of wild-type mitochondria can no longer compensate for the dysfunctional mutants. Heteroplasmy is also linked to aging as new mutations accumulate and as the frequencies of different copies of mtDNA fluctuate under various selective pressures.