A single plant cell contains many plastids and each plastid contains numerous (identical) plastome copies. Thus, in contrast to the two copies of each gene in the nucleus of a diploid plant, the cell is highly polyploid for its plastid genome. Depending on species, tissue, developmental stage and environmental conditions, the ploidy level can easily reach more than 10,000 identical copies of the plastid genome per cell (Bendich 1987). In land plants, plastome copy numbers are usually highest in photosynthetically active cells, where plastids are present as green chloroplasts. In contrast, non-green plastid types often possess fewer plastomes. The copy number in root plastids, for example, is only about one fifth of that in chloroplasts (Aguettaz et al. 1987; Isono et al. 1997). Likewise, chloroplast development from proplastids and etioplasts is associated with an increase in plastome copy number (from about 2000 to more than 8000 copies per cell in barley; Baumgartner et al. 1988). Changes in plastid genome copy numbers per cell during plastid differentiation and plant development most likely come from the combined action of two processes: changes in organelle number per cell and changes in the plastome copy number per plastid. For example, the copy number per plas-tids almost doubles during etioplast to chloroplast differentiation in barley leaf development (Baumgartner et al. 1988). Once plastid differentiation is completed, plastome copy numbers remain remarkably constant and do not vary significantly with leaf age or the plant's developmental stage (Li et al. 2006; Zoschke et al. 2007).
As plastids are asexual genetic systems and, in most species, excluded from sexual recombination (see 4), an intriguing question has been how plastid ge nomes can avoid evolutionary deterioration. Asexual reproduction is believed to be detrimental because of the accumulation of deleterious mutations over time, a hypothesis known as Muller's ratchet (Muller 1964). Since the vast majority of mutations are deleterious, an asexual genetic system is expected to suffer a continuous decline in fitness. Surprisingly, in spite of their asexual mode of reproduction, plastid genomes even have considerably lower mutation rates than nuclear genomes (Wolfe et al. 1987). A recent study has provided experimental evidence that it is the plastid's high degree of polyploidy which, together with a very active mutation-correcting activity by gene conversion, counteracts the detrimental effects of Muller's ratchet and keeps mutation rates in plastid genomes very low (Khakhlova and Bock 2006). These findings suggest a molecular link between asexual reproduction, high genome copy numbers, and low mutation rates.
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