Distribution of ploidy in prokaryotes and evolutionary advantages of polyploidy in prokaryotes

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Eukaryotes are typically diploid, and some eukaryotic groups contain many polyploid species. In contrast, prokaryotes are generally believed to be monoploid and contain one molecule of a circular chromosome. However, recently it was discovered that two species of different genera of halophilic archaea are highly polyploid (Breuert, Allers, Spohn and Soppa 2006, PLoS ONE, e92). A literature survey revealed that the ploidy level has been determined only for a very limited number of species. More than half of them have several copies of the chromosome, indicating that polyploidy might be more widespread in prokaryotes than anticipated. One part of the project will address the distribution of polyploidy. The ploidy level will be determined for about 15 model species representing different phylogentic groups of bacteria. In addition, new isolates from environmental samples will be characterized, and it will be attempted to select for polyploid bacterial species. A second part of the project will address the evolutionary advantage of polyploidy. First, ploidy mutants of halophilic archaea will be selected that can be compared with the wildtype. The resistance against X-ray irradiation and desiccation will be characterized. It will be proven that intracellular polymorphism can yield a selective advantage under specific conditions, and gene conversion will be studied. Taken together, the project challenges the textbook view that prokaryotes are typically monoploid, and it will yield insight into the evolutionary advantage of polyploidy in prokaryotes.