Abstract
Bacteria are generally assumed to be monoploid (haploid). This assumption is mainly based on generalization of the results obtained with the most intensely studied model bacterium, Escherichia coli (a gamma-proteobacterium), which is monoploid during very slow growth. However, several species of proteobacteria are oligo- or polyploid, respectively. To get a better overview of the distribution of ploidy levels, genome copy numbers were quantified in four species of three different groups of proteobacteria. A recently developed Real Time PCR approach, which had been used to determine the ploidy levels of halophilic archaea, was optimized for the quantification of genome copy numbers of bacteria. Slow-growing (doubling time 103 minutes) and fast-growing (doubling time 25 minutes) E. coli cultures were used as a positive control. The copy numbers of the origin and terminus region of the chromosome were determined and the results were in excellent agreement with published data. The approach was also used to determine the ploidy levels of Caulobacter crescentus (an alpha-proteobacterium) and Wolinella succinogenes (an epsilon-proteobacterium), both of which are monoploid. In contrast, Pseudomonas putida (a gamma-proteobacterium) contains 20 genome copies and is thus polyploid. A survey of the proteobacteria with experimentally-determined genome copy numbers revealed that only three to four of 11 species are monoploid and thus monoploidy is not typical for proteobacteria. The ploidy level is not conserved within the groups of proteobacteria, and there are no obvious correlations between the ploidy levels with other parameters like genome size, optimal growth temperature or mode of life.
Highlights
The existence of multiple copies of the genome in one cell is called polyploidy
One species was oligoploid or polyploid, depending on the growth rate, while the other had with about 50 genome copies the highest ploidy level found for any archaeon [22]
The ploidy levels of haloarchaea were determined by quantitative Southern blotting, and the genome copies of methanogenic archaea were quantified using a spectroscopic method, and in all cases the results of the independent method were in excellent agreement with the results of the Real Time PCR method [21,22]
Summary
The existence of multiple copies of the genome in one cell is called polyploidy. If the genomes originate from several species, the resulting species is allopolyploid, the multiplication of the chromosomes of one species leads to autopolyploidy. Many eukaryotes are polyploid, including species of ciliates, flowering plants, amphibians, fish and even some types of cells in humans. The advantages are more obvious for allopolyploids, in which alleles of two or more species are combined. They typically outperform their parent strains (heterosis effect). Gene redundancy can be accompanied by higher resistance against DNA damaging agents and it offers the possibility of mutating one copy while the wildtype information still remains available
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