Ploidy and gene conversion in Archaea

The genome copy numbers of seven crenarchaeal species of four genera have been reported. All of them are monoploid and thus this seems to be a characteristic feature of Crenarchaeota. In stark contrast, none of six species representing six euryarchaeal genera is monoploid. Therefore Euryarchaea are typically oligoploid or polyploidy and their genome copy numbers are tightly regulated in response to growth phase and/or growth rate. A theoretical consideration called ‘Muller's ratchet’ predicts that asexually reproducing polyploid species should not be able to exist. An escape from Muller's ratchet would be a mechanism leading to the equalization of genome copies, such as gene conversion. Using two species of methanogenic and halophilic archaea, it was shown that heterozygous cells containing different genomes simultaneously can be selected, exemplifying gene redundancy as one possible evolutionary advantage of polyploidy. In both cases, the genomes were rapidly equalized in the absence of selection, showing that gene conversion operates at least in halophilic and methanogenic Euryarchaea.