The replication and segregation mechanisms in plastids prevent the persistence of two different plastid genomes in cells. Heteroplasmy can only be maintained for long periods of time by selection for both plastid genomes (Drescher et al. 2000; Shikanai et al. 2001; Kode et al. 2005). The fine details of heteroplasmy are not known and the two plastid DNA types might be mixed within single nucleoids, or localised to separate nucleoids within a plastid or be separated into two populations of plastids within a cell. Heteroplasmy within a plastid is required when a lethal mutation is plastid autonomous (Kode et al. 2005) and cannot be rescued by import of cytoplasmic metabolites.
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