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The investigated haloarchaeal species, Halobacterium salinarum, Haloferax mediterranei, and H. volcanii, have all been shown to be polyploid. They contain several replicons that have independent copy number regulation, and most have a higher copy number during exponential growth phase than in stationary phase. The possible evolutionary advantages of polyploidy for haloarchaea, most of which have experimental support for at least one species, are discussed. These advantages include a low mutation rate and high resistance toward X-ray irradiation and desiccation, which depend on homologous recombination. For H. volcanii, it has been shown that gene conversion operates in the absence of selection, which leads to the equalization of genome copies. On the other hand, selective forces might lead to heterozygous cells, which have been verified in the laboratory. Additional advantages of polyploidy are survival over geological times in halite deposits as well as at extreme conditions on earth and at simulated Mars conditions. Recently, it was found that H. volcanii uses genomic DNA as genetic material and as a storage polymer for phosphate. In the absence of phosphate, H. volcanii dramatically decreases its genome copy number, thereby enabling cell multiplication, but diminishing the genetic advantages of polyploidy. Stable storage of phosphate is proposed as an alternative driving force for the emergence of DNA in early evolution. Several additional potential advantages of polyploidy are discussed that have not been addressed experimentally for haloarchaea. An outlook summarizes selected current trends and possible future developments.
Gene conversion is defined as the non-reciprocal transfer of genetic information from one site to a homologous, but not identical site of the genome. In prokaryotes, gene conversion can increase the variance of sequences, like in antigenic variation, but can also lead to a homogenization of sequences, like in the concerted evolution of multigene families. In contrast to these intramolecular mechanisms, the intermolecular gene conversion in polyploid prokaryotes, which leads to the equalization of the multiple genome copies, has hardly been studied. We have previously shown the intermolecular gene conversion in halophilic and methanogenic archaea is so efficient that it can be studied without selecting for conversion events. Here, we have established an approach to characterize unselected intermolecular gene conversion in Haloferax volcanii making use of two genes that encode enzymes involved in carotenoid biosynthesis. Heterozygous strains were generated by protoplast fusion, and gene conversion was quantified by phenotype analysis or/and PCR. It was verified that unselected gene conversion is extremely efficient and it was shown that gene conversion tracts are much longer than in antigenic variation or concerted evolution in bacteria. Two sites were nearly always co-converted when they were 600 bp apart, and more than 30% co-conversion even occurred when two sites were 5 kbp apart. The gene conversion frequency was independent from the extent of genome differences, and even a one nucleotide difference triggered conversion.