, 2011; Pecoraro et al., 2011). Synechocystis PCC 6803 adds to this list, but is extraordinary in that the genome copy number is already down-regulated in linear growth phase. The genome copy numbers of 218 and 142 in exponentially growing cells of the two Synechocystis strains are considerably higher than the 120 genome copies per cell that have been reported for Buchnera,

a symbiotic bacterium with a reduced genome Anti-diabetic Compound Library molecular weight size (Komaki & Ishikawa, 1999). To our knowledge, a higher value has been reported only for Epulopiscium sp. that contains tens of thousands of genome copies (Mendell et al., 2008). However, Epulopiscium sp. is a giant bacterium exhibiting cell lengths in excess of 600 μm. Therefore, Synechocystis PCC 6803 has the highest ploidy level of any ‘normal’ sized prokaryote. However, it is unclear whether such high ploidy levels also exist in natural habitats, or whether this is an artifact of decades of cultivation in the laboratory. Synechocystis PCC 6803 was isolated 40 years ago and has been cultivated in the laboratory since then (Stanier et al., 1971). Therefore, an

in-depth analysis (see above) should also include fresh isolates of Synechocystis PCC6803 as well as samples from different culture collections that had been kept frozen since their submission. A literature search was performed to identify (hopefully) all cyanobacterial species with experimentally determined see more ploidy levels. Table 3 summarizes the results together with selected features. Three species are polyploid and contain at least 10 genome copies. They belong to different genera and grow either as single cells or as check details filaments. More than ten species are oligoploid and contain between three and nine genome copies. Again, among them are unicellular and filamentous species of several genera. Four species are monoploid, and thus

monoploidy is not the rule, but an exception in cyanobacteria. The ploidy level is highly variable in cyanobacteria similar to the proteobacteria (Pecoraro et al., 2011). One genus can harbor monoploid and oligoploid species (Synechococcus) or oligoploid and polyploid species (Anabaena). There is no obvious correlation between the number of genome copies and any of the listed features, i.e. genome size, growth temperature, and doubling time. Various evolutionary advantages of oligo- and polyploidy for prokaryotes exist. As has been extensively studied with D. radiodurans, one of the advantages is resistance against double strand breaks that can be induced by X-ray irradiation (an artificial situation) and desiccation (regularly occurring in natural habitats). In fact, it could be shown that the resistance of polyploid Synechocystis PCC 6803 against X-ray irradiation is much higher than that of the oligoploid Synechococcus PCC 7942 (Domain et al., 2004).