Abstract
Polyploidy is common in higher eukaryotes, especially in plants, but it is generally assumed that most prokaryotes contain a single copy of a circular chromosome and are therefore monoploid. We have used two independent methods to determine the genome copy number in halophilic archaea, 1) cell lysis in agarose blocks and Southern blot analysis, and 2) Real-Time quantitative PCR. Fast growing H. salinarum cells contain on average about 25 copies of the chromosome in exponential phase, and their ploidy is downregulated to 15 copies in early stationary phase. The chromosome copy number is identical in cultures with a twofold lower growth rate, in contrast to the results reported for several other prokaryotic species. Of three additional replicons of H. salinarum, two have a low copy number that is not growth-phase regulated, while one replicon even shows a higher degree of growth phase-dependent regulation than the main replicon. The genome copy number of H. volcanii is similarly high during exponential phase (on average 18 copies/cell), and it is also downregulated (to 10 copies) as the cells enter stationary phase. The variation of genome copy numbers in the population was addressed by fluorescence microscopy and by FACS analysis. These methods allowed us to verify the growth phase-dependent regulation of ploidy in H. salinarum, and they revealed that there is a wide variation in genome copy numbers in individual cells that is much larger in exponential than in stationary phase. Our results indicate that polyploidy might be more widespread in archaea (or even prokaryotes in general) than previously assumed. Moreover, the presence of so many genome copies in a prokaryote raises questions about the evolutionary significance of this strategy.
Highlights
Polyploidy, the existence of multiple copies of the normal set of chromosomes, is widely distributed in eukaryotes
It has been reported that H. cutirubrum may contain 6–10 genome copies [16], and we have previously shown that in H. salinarum, intracellular localization of the DNA is highly regulated in the course of the cell cycle [17]
It was previously reported that H. cutirubrum contains six to ten copies of the chromosome [16]
Summary
Polyploidy, the existence of multiple copies of the normal set of chromosomes, is widely distributed in eukaryotes. Polyploids are common among fish and amphibian and very common among plants. The advantages and disadvantages of being polyploid have recently been reviewed [2,3]. The advantages of polyploidy are heterosis (hybrid vigour, an increased perfomance of the allopolyploid compared with the inbred parents), loss of selfincompatibility leading to the gain of asexual reproduction, and gene redundancy. Disadvantages of being polyploid are 1) a higher frequency of mitotic or meiotic problems leading to aneuploidy and 2) epigenetic instability. Several of these points like heterosis apply only to eukaryotic species with sexual reproduction, while others like gene redundancy are relevant for archaea, bacteria and eukaryotes
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