Replication Recombination

Fig. 1. DNA-RRR pathways are responsible for the high copy number, uniformity, and stable maintenance of plastid genomes.

individual cell or multicellular plant. Highly effective DNA maintenance pathways in plastids must underpin the evolutionary stability and uniformity of plastid

DNA-RRR pathways must overcome two potential problems associated with the mode of inheritance and ploidy of plastid DNA. First, in sexual crosses plastid DNA often exhibits uniparental inheritance (Corriveau and Coleman 1988; Re-boud and Zeyl 1994) reducing the possibility of DNA recombination between parental plastid genomes (Chapter 3). Moreover, in flowering plants, when two plastid types are present in the same cell they rarely recombine (Medgyesy et al. 1985) and segregate away from each other to form cells with pure populations of each plastid type. Segregation of plastids during vegetative growth is known as cytoplasmic sorting or vegetative segregation (Birky 1994). Lack of DNA recombination between different plastid types means that plastids propagate asexually and do not have the benefits of sex and DNA recombination between parental alleles (applicable to nuclear genes) to eliminate deleterious mutations. Muller's ratchet (Muller 1964) would operate leading to an accumulation of mutations in plastid DNA. Second, because plastid DNA is present in multiple copies any new mutations in plastid genes would be masked by the wild type (WT) alleles present in the cell. Asexual propagation and a high degree of polyploidy are two features of plastid DNA that would be expected to promote the accumulation of mutations. Without effective plastid DNA-RRR pathways, plastid mutations would accumulate with time resulting in loss of fitness and death.

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