Notably these values are much higher than the value of 12 genome copies published for the ‘Kazusa’ strain more than 20 years ago. The results reveal that for SynechocystisPCC 6803 strain differences exist and that the ploidy level is highly growth phase-regulated. A compilation of the ploidy levels of all investigated cyanobacterial species gives an overview of the genome copy number distribution and shows that monoploid, oligoploid, and polyploid cyanobacteria exist. Many eukaryotic species including ciliates, fish, flowering plants, and even some cell types of humans are polyploid, and advantages as well as disadvantages of polyploidy have been discussed in various reviews

(e.g. Wendel, 2000; Osborn et al., 2003; Comai, 2005; Thorpe et al., 2007; Hegarty & Hiscock, 2008). In contrast, it is generally believed that prokaryotes typically contain a single copy of their chromosome. This is usually called ‘haploidy’, Dabrafenib but as the term ‘haploid’ does not seem to make much sense in species without a diploid stage, the term ‘monoploid’ will be used throughout this contribution. Galunisertib concentration The idea that prokaryotes are typically monoploid is a generalization from the results obtained with Escherichia coli, the best studied bacterium. Escherichia coli is monoploid when it is grown very slowly, e.g. with a doubling time of 16 h (Skarstad et al., 1983). When

the doubling time becomes shorter than the time to replicate and segregate the chromosome, E. coli starts a new round of replication before the previous round had been terminated, and thus the gene dosage of regions near the replication origin becomes very higher than of regions near the terminus. This unequal gene dosage is called merodiploidy or mero-oligoploidy. Under optimal conditions, E. coli grows with a doubling time of 20 min and contains on average 6.8 origins and nearly two termini (Bremer & Dennis, 1996; Pecoraro et al., 2011). The dependence

of DNA content and growth rate shows that E. coli ‘tries’ to grow as monoploid as possible. Several other species of bacteria are truely monoploid, e.g. Bacillus subtilis, Caulobacter crescentus, and Wolinella succinogenes (Webb et al., 1998; Pecoraro et al., 2011). However, several species of prokaryotes also have been described to be oligoploid or polyploid. A prominent example is Deinococcus radiodurans, which contains 5–8 genome copies (Hansen, 1978). It is long known that D. radiodurans can restore intact chromosomes from heavily fragmented chromosomes, which is not possible in monoploid species. Recently, it has been shown that this is a two-stage mechanism involving a high induction of DNA repair synthesis followed by recombination (Slade et al., 2009). The efficient repair of a high number of double strand breaks (induced by irradiation or, more naturally, desiccation) is one evolutionary advantage of polyploidy for prokaryotes.